“My concern, however, is that decision makers are too often caught in traditional, linear (and non-disruptive) thinking or [are] too absorbed by immediate concerns to think strategically about the forces of disruption and innovation shaping our future.”

— Klaus Schwab, The Fourth Industrial Revolutionⁱ

I would like to begin this article by assuring all of CoatingsTech’s readers that “all is well—the raw material supply chains have returned to normal,” but I cannot. What I can say is that the statement is more like 85–90% true. There are still a few issues, typically with special materials that are not in broad general use, and also with certain alkyd and acrylic emulsion resins, but the majority of the bread-and-butter resins, solvents, pigments and additives of the U.S. paint and coatings industry are generally available, albeit with slightly longer lead times (at least as of early March, as this article is being written), and significantly higher selling prices than was the case three years ago. The supply chains are being helped, of course, by the fact that large portions of the paint and coatings industry, especially architectural, were seasonally down during Q4 2022, and because of the hoarding that had taken place between 2021 and Q3 2022, paint and coatings production during Q4 was largely made with the expensive raw materials that the industry collectively had in stock already.

Activity in January 2023 was nothing to write home about, but the de-stocking of raw materials held in the paint companies’ warehouses continued well into January, with a certain amount of spillover into February. Things began picking up for raw material orders in February, and it is reasonable to assume that the overall industry, along with its supply chain partners, will exhibit solid, but certainly not outstanding, performance by the end of Q1. My personal crystal ball doesn’t yield a lot of insight beyond Q1, although there seems to be a general consensus among both the raw material suppliers and the coatings producers in most segments that Q2-Q4 will give solid, although unexciting, performance. There is uncertainty, rather than fear, about the future among formulators. We are not seeing, at least as of early March, layoffs in paint, coatings, and inks manufacturers, although RPM announced in January that it was cutting production amid falling orders.ⁱⁱ

No one among the broad array of raw material suppliers, distributors, and coatings producers with whom I regularly have contact expects to see significant supply chain disruptions during 2023, but they tend to temper this feeling by adding one or more of the following qualifying phrases, “Assuming that:

• We do not have a recession.”
• “Neither the Russians nor the Chinese do anything to disrupt the global supply chains.”
• “We don’t have additional devastating earthquakes like the ones that hit Türkiye and Japan.”
• “Interest rates drop.”

That is, of course, asking a lot, but it is probably as realistic as it is possible to be in early 2023. As is generally the case, people in the industry tend to be thinking tactically, and there is certainly nothing wrong with that . . . unless it keeps them from learning the most important lesson taught by COVID-19: tactics alone aren’t sufficient as we move into the future. They must be accompanied by strategy, because the future is likely to be increasingly uncertain for global industry, and the only way in which we can be as prepared as possible to deal with future uncertainties is by putting strategic plans and systems in place that are able to anticipate future disruptions of various types, and create long-term plans for both avoiding such disruptions, and for dealing with them if they cannot be avoided.

Continue reading in the�����Ǵ���CoatingsTech.

If you are a raw material supplier, you probably don’t need me to tell you that Khaos (“Chaos”), the first of the Primeval Greek Gods, reigns around the world at the present time. As of the end of February 2021, the entire supply chain is a mess:

• Titanium dioxide is difficult-to-impossible to obtain from China.
• Raw materials that have historically had lead times that ranged from just-in-time to two–three weeks for imports have increased to multiple weeks for domestic materials and three–four months for many imported materials.
• Production for virtually everything cannot keep up with demand.
• Shipping costs doubled and, in some cases, tripled.
• The great American seaports such as Los Angeles, Long Beach, New York/New Jersey, Savannah, and Seattle are being taxed beyond their capabilities—ships are waiting weeks to be unloaded.
• When ships are finally unloaded, the lack of gate appointments on the docks for trucking companies to load goods and transfer them to other parts of the country creates even more delays moving goods from the seaports to the interior of the country.
• Even when trucking companies can obtain gate appointments, and goods are finally ready to be loaded for land transport, there are insufficient trucks (especially tankers) and drivers to handle the volume of goods.
• S. Consumer spending created a very significant increase in demand for imported goods—January 2021 marked the sixth month of year-over-year increases, with no end in sight.
• At the same time, U.S. exports declined for 25 of the previous 27 months, creating, in the words of Port of Los Angeles Executive Director Gene Seroka, “one-way trade, which has created challenges for the entire supply chain.”¹
• The cost of lumber, a major component of the booming U.S. residential housing industry, rose 180% during the 10-month period from April 17, 2020, to February 25, 2021. Per the National Association of Home Builders (NAHB), this added $24,000 to the cost of an average new single-family home.²
• Lead times for lumber went from two weeks or less pre-pandemic, to 12 weeks in mid-February, and the situation did not suggest an early resolution.
• On Monday, February 15, temperatures dropped as low as -17 °C (0 °F) in Midland, Texas, and production across the Permian Basin dropped by an average of more than 2 million barrels/day during the next three days—a significant dip in average, anticipated production.³
• Predictions were that, with rising temperatures, petrochemical facilities on the Gulf Coast would be up and running in mid-March. That’s the good news. The bad news is that the few weeks needed to get the oil and gas wells back to normal production will translate into many, many weeks getting downstream chemical products back to their pre-freeze levels.
• Raw material price increases for a basket of paint and coatings components appeared to be increasing by 2–5% in January 2021, but by mid-February price increases, in the range of 5–9% and often higher, were flooding the offices of coatings manufacturers’ purchasing departments.
• Also flooding these same offices, between mid- and late-February, were force majeure notifications on a broad variety of raw materials, the majority of which were sourced from the Gulf Coast.
• After a strong start of $64/bbl in January 2020, West Texas Intermediate (WTI), plummeted to -$37/bbl (not a typo) in April, then averaged roughly $40/bbl for the remainder of the year. After hearing most the globe’s most illustrious economic prognosticators confidently declare, as recently as Q3 of 2020, that the price of WTI would average roughly $35/bbl in 2020 and $40/bbl in 2021, we were sitting in February 2021 with the price of WTI climbing from $54/bbl on February 1 to $64/bbl on February 25, before slipping down to $62/bbl on February 28. The projections for WTI from the U.S. Energy Information Administration (EIA) on February 9 indicated an anticipated average price during 2021 of $53/bbl. Clearly, the future price of crude is an open question, with multiple—and varied—implications for the future.⁴

None of this news is good, and most of the problems are being blamed on some combination of COVID-19, the “Great Freeze of 2021,” and increasing consumer demand. To an extent, it is reasonable to lay the current problems at the feet of these three sources of disruption, but I would posit that all three of these sources of shortages, compounded with late deliveries; force majeure declarations; the sharp and rapid increases in the price of raw materials; the resultant price increases in finished products, such as paints, coatings, varnishes, stains et al., have merely decreased the timeframe in which our current problems manifested themselves, rather than caused them.

The handwriting has been on the wall for quite some time. Erratic oil prices during the period 2014–2020⁴; rising consumer demand during this same period of time⁵; historically low mortgage rates⁶, which have fallen steadily from a high in 16+% in 1981 to ~2.5% in early 2021, before rising to ~3% in early March, fueling a major residential housing boom; serious problems, during the past decade, with insufficient personnel and equipment in the trucking sector, that is galloping into the current decade with a vengeance⁷; an extremely high savings rate; high consumer confidence; and the list goes on, have all been acting, over the past several years, on the economy to bring us to the point where we are today.

There is no point blaming COVID-19, the Gulf Freeze, or any other factor or set of factors. The problem is that U.S. industry, in general, and the paint and coatings industry, specifically, have failed to build resilience into their supply chains, and are now paying the price for their negligence. The United States is now putting an additional $1.9 trillion in new stimulus funds into circulation at a time when many experts feel that we are on cusp of a consumer boom, and this will only exacerbate the situation for the most prepared of all manufacturers and potentially wreak havoc on less-prepared producers of raw materials and finished goods, who have failed to build sufficient resilience into their supply chain philosophies and practices.

Introduction

Over the last 40-plus years, newer coatings technologies, such as high solids, powder, waterborne, and radiation-curable coatings have had significant market share growth. These coatings technologies have been developed to meet the challenges of: (a) governmental regulations in the areas of ecology (volatile organic compounds (VOC) emission); (b) long-term increasing costs of energy and petroleum based solvents; (c) more active public consumerism; and (d) the continual need for cost-effective high-performance coatings in a highly competitive and global business environment. These new coatings technologies require the use of water as the major solvent with water-soluble or high molecular weight latex polymers or the use of strategically designed low-molecular weight polymers, oligomers, and reactive additives which, when further reacted, produce high molecular weight and crosslinked polymers. This has led to a need for improved methods of materials characterization in diverse areas, which include molecular weight distribution analysis, particle size distribution (PSD) measurement and assessment and characterization, rheology of coatings, film formation and cure process characterization, morphological surface and bulk characterization, and spectroscopic analysis, as well as a need for improved methods for modeling and predicting materials properties and processes.

Concurrent with the major technological changes in the coatings industry was the significant increase in the rate of change in instrumentation technology. This change was driven by significant advances in electronics and computer and sensor technologies to produce computer-aided, more user-friendly, reliable, and cost-effective instrumentation. The advances in instrumentation and computer technology are filling the need for improved polymer and coatings characterization methods in the context of the newer coatings technologies.

The newer coatings technologies have driven the need for improved methods of PSD assessment and characterization for each of the coatings technologies in terms of size ranges and component particulates as follows:

Waterborne coatings (0.01–50 mm)

• Latex
• Pigments (size and shape)
• Emulsions and dispersions

Powder coatings (0.1–100 mm)

• Resin-pigment composite particle:
  (a) dry powder;
  (b) wet concentrated dispersions
• High solids
• Pigment grind particle size analysis (0.1–100 μm)

The particle size assessment and characterization needs resulting from the challenges presented by the newer coatings technologies and their component materials leads to the following measurement requirements:

• Wide dynamic particle size range
• Improved resolution
• Measurements made in the concentrated dispersion regime
• Assessment of dispersion stability
• Description of structural and textural morphology

The changes in the field of PSD characterization and measurement, aided by�� the advances in electronics computer
and sensor technologies, can be characterized by at least
four activities:

• End user innovation
• Revitalization of older instruments
• Evolution of research grade instrumentation into
  low-cost, user-friendly instrumentation
• Attempts to meet extremely difficult technical challenges

The advances in PSD measurement methods will be discussed in the context of the four above-mentioned activities in terms of measurement principles and the strengths and weaknesses of these methods for characterizing PSDs.

End User Innovation—Commercial Development

The first activity in the changes in PSD characterization and measurement is the commercial development of instrumental methods originally developed in a few academic and industrial laboratories by individuals with high levels of skill and expertise. Innovations in instrumentation and methods usually are driven by the customers who are the technological and scientific leaders and prototype developers, while the instrument vendors are the technological followers who have the engineering skills to do cost-effective commercial development. Examples of such methods are hydrodynamic chromatography (HDC), capillary hydrodynamic fractionation (CHDF), and field flow fractionation methods involving the use of sedimentation, flow, and thermal fields (e.g., SdFFF, FIFFF, and ThFFF).

Hydrodynamic Chromatography (HDC)

HDC, first reported in 1976, was invented in an industrial laboratory by Hamish Small¹ to fulfill an analysis need of The Dow Chemical Company. It took 10 years to be transferred from a laboratory method requiring a high degree of skill into a commercial instrument requiring a moderate degree of skill. The commercialization was carried out by Micromeritics Corporation. However, the commercialization failed in the marketplace because of technological limitations inherent in the method and because of competitive market pressures.

HDC instrumentation is comprised of a liquid chromatograph with an accurate and precise pumping system. Detection was accomplished with a sensitive fixed wavelength ultraviolet (UV) detector. Fractionation and separation of particles occurred in a column packed with uniform, non-porous beads. The separation of particles by size took place in the interstices between the beads and was primarily a function of the bead size and the ionic strength of the medium. The separation mechanism is shown in Figure 1. The interstices between the beads can be treated as capillaries of varying sizes. Larger particles ride higher up on the parabolic flow profile and are eluted first while some smaller particles hug the walls of the capillary experiencing slower flow streamlines and exit later. Other contributing factors to the separation mechanism include electrical double layer effects and Van der Waals attraction. The fatal technical flaw in the methodology was the unpredicted occurrence and amount of particle deposition, which took place in the packed columns. The overall features, benefits, and limitations of this methodology are shown in Figure 2.

Capillary Hydrodynamic Fractionation (CHDF)

The “Tubular Pinch Effect” mechanism operative in CHDF was first discovered by Segre and Silberberg² in 1962 and applied to particle size analysis in 1979 by Regnier and Ball³. However, it did not develop into a viable commercial instrument until innovative experimental and theoretical work was carried out by Silebi and Dos Ramos.^4,5 The commercial embodiment of CHDF had become available from MATEC Applied Sciences. CHDF instrumentation is quite analogous to HDC in that a liquid chromatograph with precise, accurate, and reproducible flow is required. In this instrument, the separation does take place in capillaries of defined dimension using split-flow injection. The mechanism of this separation method is essentially the same as that postulated for HDC. However, now the column is an empty capillary, so there is much less propensity for the particles to deposit in the column. The factors influencing the particle size fractionation are shown in Figure 3, and the associated features, benefits, and limitations of CHDF are shown in Figure 4.

Field Flow Fractionation (FFF)

Field Flow Fractionation methods were invented by J. Calvin Giddings and co-workers and reported in 1967.^6-7 The technology transfer of FFF methods from an academic laboratory method to viable commercial instrument has taken a minimum of 20 years to occur. The DuPont Instrument Company commercialized a sedimentation FFF instrument (SdFFF) in 1986 based upon the pioneering work of J. Calvin Giddings and the subsequent research of Kirkland and Yau.⁸ Unfortunately, the DuPont Instrument Company’s SdFFF instrument was a commercial failure and unavailable by 1991. However, a small entrepreneurial company known as Fractionation, Inc. began making SdFFF instruments commercially available in 1988, followed by FIFFF (flow FFF) instruments in 1991 and, more recently, ThFFF (thermal FFF) instruments.

Field flow fractionation is a form of one-phase chromatography. The instrument is a liquid chromatographic system in which the separation takes place in a flat, narrow channel (typically 100 to a few hundred micrometers in width). A parabolic flow profile is created in this narrow channel by the mobile phase. An external field is applied across the face of the channel such that the field extends over the channel’s thin dimensions and is perpendicular to the flow field. Particles are driven toward the accumulation wall of the channel and form a diffuse cloud, which has an exponential concentration distribution as a result of Brownian motion acting against the field. For particles less than 1–2 µm, smaller particles are displaced farther from the accumulation wall. When the flow field is turned on, the smaller particles, which ride higher on the parabolic flow profiles elute first. The fields that have been used include SdFFF, ThFFF consisting of a temperature gradient across the channel walls, cross flow (FIFFF), electric (ElFFF), and magnetic (MFFF). Commercial instruments are available to perform SdFFF, ThFFF, and FIFFF.

A schematic of the instrumentation separation mechanism and some channel configurations are shown in Figure 5. When the particles are greater than 1 µm, effects of Brownian motion become negligible, and the larger particles ride higher on the parabolic flow streamlines because the velocity-dependent lift forces increase relative to the driving forces produced by the perpendicular external field. In the ~1–100 µm size range, the larger particles elute first, analogous to CHDF. This mechanism is termed the steric mode of FFF (StFFF). In the commercially available instrumentation, field programming options are available for extending the size separation range, reducing the analysis time and optimizing the resolution per unit of time. Examples of separation by SdFFF, FIFFF, ThFFF, and StFFF are shown in Figure 6. The features, benefits, and limitations of FFF chromatography methods are shown in Figure 7. For a review of FFF applied to particle characterization, the reader is referred to the many papers and review articles of Giddings and co-workers.⁹

The commercialization of the instrumental techniques of HDC, CHDF, and FFF have attempted to address the market pull by many industries for PSD analysis methods, primarily in the 0.01–1.0 µm size range. The coatings industry material characterization needs, as a result of the growth of waterborne coatings technology for improved particle size distribution analysis of latex, pigments, and emulsions, is a component of the overall market pull for these techniques.

Revitalization of Older Instrumental Methods

Gravitational and Centrifugal Sedimentation

The second activity in the renaissance of PSD characterization and measurement is characterized by the revitalization of older instrumental methods such as gravitational and centrifugal sedimentation methods. Redesign, modernization with advanced electronics, and user-friendly computer-aided analysis have extended the instrument product life cycle. A good example is disc centrifuge photosedimentometry (DCP).^10,11 The basic technology has been around since the late 1950s and was embodied into a commercial instrument in the 1960s by the Joyce Loebel Company. During the 1970s, the Joyce Loebl Disc Centrifuge Photosedimentometer was the only commercially available instrument for obtaining PSD information by the DCP technique. This technique was particularly well suited for the particle size analysis requirements of the coatings industry for the analysis of latexes, pigments, and emulsions.^12-14 However, it had deficiencies that made it unsuitable as a plant quality control instrument. Over a period of about 10 years, industrial scientists at The Glidden Company developed an improved instrument¹¹, method of use^15,16 and user-friendly data analysis system. The Glidden Company’s DCP technology was licensed to the Brookhaven Instruments Corporation in 1986, which made additional engineering improvements and successfully commercialized the enhanced and revitalized DCP technology. The current instrument can operate in both the line start and homogeneous start modes.

In addition, Brookhaven Instruments Corporation extended the technology by developing an X-ray detection system that facilitated the analysis of heavy small inorganic particles. The X-ray disc centrifuge photosedimentometer (X-DCP) became commercially available in 1991 and was based upon a prototype developed by Terry Allen at DuPont. The X-DCP can operate both in the gravitational and sedimentation modes.

The range of variations in sedimentation instrumentation mode, detection, and experimental method is shown in Figure 8. Also shown in Figure 9 are Stokes’ laws for gravitational and centrifugal sedimentation, which govern the fractionation of particles by size in a gravitational or centrifugal force field. The line start mode^11,12 can produce very high resolution separations. An example of such a separation is shown in Figure 10 for a mixture of nine Duke polystyrene latex standards covering a size range of 107–993 nm in approximately 100-nm increments. Baseline resolution was achieved in seven out of the nine standards. Another major advance in the method of operation is protecting the aqueous meniscus from evaporative cooling and disruption of the density gradient in the fluid by sealing the surface with a small amount of dodecane (usually 1 ml of n-dodecane, injected a few minutes after the density gradient has formed, in about 15–20 ml of an aqueous spin fluid). The sealing of the fluid surface inhibits evaporative cooling and thereby maintains a stable density gradient for several hours and extends the analysis size range. The homogeneous start mode is faster, covers a wider dynamic size separation range, but has less resolution than the line start mode for multi-modal separations. The features, benefits, and limitations of sedimentation methods are shown in Figure 11.

An instrument designed for particle size analysis by sedimentation in a gravitational field was patented by Oliver, Hicken, and Orr and commercially embodied, in 1969, by Micromeritics into an instrument known as the Sedigraph. This instrument produces a cumulative distribution of particle sizes and has been used heavily for the analysis of pigments such as titanium dioxide. Starting in 1988, the Sedigrap’s design and function were revitalized by adding an automatic sample introduction system for unattended operation as well as improving the data analysis system.

Evolution of A Research Instrument Into A Routine User-Friendly Instrument

Dynamic Light Scattering (DLS, PCS, QELS)

The third activity is the evolution of a research instrument into a low-cost instrument that requires a minimum degree of skill to use. An excellent example of this process is the transformation of research-grade photon correlation spectrometers into low-cost, easy to use, limited-function instruments for routine analysis applications.

Dynamic light scattering (DLS) was first known as quasielastic light scattering (QELS) derived from the fact that when photons are scattered by mobile particles, the process is quasielastic. This gave rise to the acronym DLS, since QELS gave information on the dynamics of the scatterer. Since measurements are made with a digital correlator, the acronym PCS (photon correlation spectroscopy) is widely used. The first QELS measurements were made in 1964 by Cummins et al.¹⁷ The first commercial instruments were available circa 1976 and only suitable for use by experts. The early measurements were concerned with obtaining translational diffusion coefficients of macromolecules and particles. The technique was used to gain information about particle size by relating the diffusion coefficient to particle size through the Stokes Einstein equation for spheres.¹⁸

By the early 1970s, the fundamentals had been established. From the mid-1970s onward, improvements occurred in the technology for digital autocorrelators as well as in automation and integration of advances in microprocessors and laser technology. From the early 1980s onward, the PCS instrumentation was increasingly used for particle size analysis. By 1985, low-cost routine 90° instruments were commercially available for routine analyses. The PCS instrumentation was made more automatic and useful for the analysis of particle size and estimating PSD with the commercialization of an automatic sample dilution system invented by Nicoli and Elings.¹⁹

The DLS method utilizes the fluctuations in light scattering intensity observed over a range of time intervals. The autocorrelation function of light scattering intensity at time t��is compared to time zero, C(τ) = ( l(t)-I(0)), for a range of Δt intervals. The autocorrelation function can be described by an exponential decay function of Δt. This process is schematically shown in Figure 12. For a monodisperse sphere, the correlation function is represented by an exponential decay function as shown in Figure 12 where D_T��is the translational diffusion coefficient, q is the scattering wave vector. The particle size d is related to D_T��through the Stokes-Einstein equation as shown in Figure 12. It has been shown that particle sizes obtained on monodisperse spheres are accurate, precise, and highly reproducible. In addition, the PCS instrument is fast and has high throughput.

A schematic of a PCS instrument with auto-dilution is shown in Figure 13. This type of instrument also has been shown to have potential for online measurement of particle size for an emulsion polymerization reactor.²⁰

For polydisperse distributions, an average size and estimate of dispersity can be obtained from the method of cumulants. Inversion of correlograms to obtain PSDs for multimodal distributions has had limited success. If particle size modes are in a ratio of 3:1 or greater, then it should be possible to extract the peak modes from the data. This is a low-resolution method compared to techniques such as CHDF, FFF, or sedimentation in a centrifugal force field.

The features, benefits, and limitations of DLS are summarized in Figure 14.

Attempts to Meet Extremely Difficult Technical Challenges

The fourth activity characterizing the changes in PSD characterization and measurement methods involve attempts to meet extremely difficult technological challenges as a result of advances in electronics and computing and sensor technologies. These technological challenges will be discussed below and can be cataloged as follows:

• Measurements in concentrated dispersions
• Wide dynamic particle size range measurement capability in a single instrument
• On-line and at line analyses
• Structural and textural morphology characterization of particle shape

Concentrated Dispersion Measurements

Fiber Optic Quasi-Elastic Light Scattering {FOQELS)

The need to measure particles in concentrated dispersions has led to the development of fiber optics QELS, given the acronym FOQELS by Brookhaven Instruments, for making measurements in concentrated dispersions up to 40% by weight. This commercial development is based on the work of Dhadwal et al.^21,22 Visible light from a laser diode is focused into the sample by a monomode fiber, and the scattered light is collected by a second monomode fiber at an angle of 153°. The fluctuations in scattered light are analyzed by the photon correlation technique. As long as the dispersion has observable fluidity, translational diffusion coefficients can be measured and transformed into particle size information. The fiber optics technology enables remote measurement of processes. For small particles less than 100 nm, accurate measurements of size in concentrated dispersion can be obtained. As the particle size increases, deviations from absolute size are observed as the concentration increases due to multiple scattering effects. The precision, reproducibility, and size range of measurement associated with PCS measurements applies to the FOQELS measurements. FOQELS applications reported include particle size growth in emulsion polymerization, nucleation processes in metallic oxide manufacture and monitoring protein crystal growth.²³

Laser Light Diffraction

Over the last 30 years, there has been intense activity to revitalize Fraunhofer light diffraction technology to measure concentrated dispersions over a wide dynamic particle size range (0.1–800 mm) by combining Fraunhofer diffraction technology with light scattering detectors to generate a hybrid instrument. There are at least 10 instrument vendors involved in this marketplace. The market pull for this type of instrument has been the need to provide particle size distribution information above and below 1.0 mm with a single instrument. Fraunhofer diffraction physics dates back to 1840. The advent of laser technology coupled with advances in computer technology has made this a viable commercial method of measuring particle size since 1972. The measurement is fast and can be used with dry powders and often the instrument of choice for powder coatings PSD measurements. Combining Fraunhofer diffraction with light scattering detection via Mie theory has enabled measurements to be made down to 0.06 mm, as reported by some vendors. The technique is a moderate to low-resolution technique for extracting multimodal distribution information. Below 2 mm, one has to carefully check the validity of the specific instrument vendor’s software to extract multimodal distributions.

Electroacoustic Efforts

There have been at least three vendors (Matec Applied Sciences, Sympatec, and Malvern Instruments) that have produced instruments which take advantage of electroacoustic techniques in concentrated dispersion to measure both the particle size and the electrophoretic mobility distributions. These techniques are well suited for dense, inorganic materials such as titanium dioxide.²⁴ Exploratory experimentation has been underway to evaluate the technique for organic (low-density) dispersions. The technique is limited with respect to bimodal or multimodal distribution analysis in the same way as is the DLS technique.

Other Methods

Dielectric spectroscopy has some promise for particle size characterization in concentrated colloids, as reported by Sauer et al.²⁵ The authors showed that unique Cole-Cole plots could be obtained as a function of concentration and ionic strength for monodisperse latex particles.

Rheological flow curve analysis using the Cross equation for concentrated dispersions shows correlations of one of the rheological parameters to particle size and distribution.

Both dielectric spectroscopy and rheological flow curve analysis merit further study as possible methods for characterizing the PSD of concentrated dispersions.

Wide Dynamic Particle Size Range Measurement Capability

There has been a lot of activity in developing this capability manifested by the development of hybrid instruments to cover a broad particle size range, some of which has been previously mentioned.

• Fraunhofer Diffraction/Mie Scattering (0.06–8000 µm)
• High resolution gravitational/centrifugal sedimentation
  (0.05–100 µm)
• Combination of FFF modes (e.g., StFFF with FIFFF or SdFFF)
• Single-particle optical sensing/DLS (0.1–300 µm)

The combination of single-particle optical sensing (SPOS) with DLS offers the possibility of analyzing a broad particle size range conservatively from 0.1–300 µm. The single particle optical sensing method is a photozone sensing method in which particles are counted by their ability to obscure light as they move through an orifice. This technique is automated through an auto-diluter. The sensing size range of SPOS is from about 1–300 µm. The SPOS technique is a high-resolution technique with respect to extracting multimodal distributions and is extremely sensitive to low levels of large particles in the presence of small particles (contaminant analysis). A schematic of the auto-dilution SPOS apparatus is shown in Figure 15 with a separation of a hexamodal mixture of monodisperse particles shown in Figure 16 to demonstrate the resolution of the method. When coupled with DLS, this hybrid instrument can characterize the full PSD of particulate systems in which the main contributor to the PSD are small particles.

DLS provides characterization of the small particles while SPOS characterizes the small level of larger particles, barely discernible in the DLS size distribution. An example of this type of analysis is shown in Figure 17.

On-Line, At-line Analysis

The main techniques available for on-line and at-line analysis of particle size distributions have been discussed to some degree. These techniques are:

• Fiber optic QELS (FOQELS) for concentrated dispersions
• Automatic dilution PCS
• Automatic dilution turbidity analysis
• Laser sensor-backscatter measurements

Angular scattering and absorbance measurements as a function of wavelength with photo diode array instruments and automatic dilution coupled with sophisticated mathematical analysis of PSDs is revitalizing the classical turbidity measurement and has been the subject of research by Prof. Luis Garcia-Rubio at the University of South Florida at Tampa.^26-28

Lasentech has developed a laser backscatter instrument to measure effective particle size (chord length across a particle) in concentrated dispersion from several microns to larger sizes and has found practical application for monitoring particle size changes in many processes.

Structural and Textural Morphology Characterization of Particle Shape

The need for structural and textural morphology information has led to cost/performance improvements in automated image analyzers that now provide shape information using signature wave form characterization and fractal dimension characterization of shape and texture.²⁹

Conclusions

As we look forward toward the future, we can expect further advances in PSD characterization and measurement. It is anticipated that additional hybrid type instruments will be produced to cover a wide dynamic particle size range with improved resolution. The quest for PSD analysis methods for concentrated dispersions with good resolution over a wide dynamic range will continue and will be extremely difficult to achieve. However, advances will continue to be made in providing structural and textural information as a result of advances in computer technology. Another major technical challenge which will probably be met is the development of in-line or in-situ particle size monitoring technology for latex reactors.

In addition, we expect the instruments to be more user-friendly, easier to use, more automated and smarter with respect to programmed intelligence.

Acknowledgement

This article is based partly on an American Chemical Society (ACS) workshop on “Modern Methods of Particle Size Distribution: Assessment and Characterization,” which originated as part of the ACS Division of Polymeric Materials: Science and Engineering technical programming. The author acknowledges the contributions of many of the workshop speakers over the last 30 years for their technical contributions to this evolving workshop and for numerous stimulating discussions on the subject.

Specifically, the author wishes to acknowledge the following individuals: Dr. J. Gabriel Dos Ramos, Dr. David Fairhurst, Mr. Peter Faraday, Dr. J. Calvin Giddings, Mr. Kerry Hasapidas, Mr. Richard Karun, Dr. Brian Kaye, Dr. David Nicoli, Dr. Remi Trottier, Dr. Bruce Weiner, Dr. Kim (Ratanathanawongs) Williams, and Dr. Stewart Wood.

Reprinted from Progress in Organic Coatings, 32 /1-4, Theodore Provder, Challenges in particle size distribution measurement past, present and for the 21st century, Pages 143-153, Copyright (1997), with permission from Elsevier.

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13. M. J. Devon, T. Provder, and A. Rudin, “Measurement of Particle Size Distribution with A Disc Centrifuge: Data Analysis Considerations”, ACS Symposium Series, 472, 134 (1991).

14. M. J. Devon, E. Meyer, T. Provder, A. Rudin, and B. B. Weiner, “Detector Slit Width Error in Measurement of Latex Particle Size Distribution with a Disc Centrifuge”, ACS Symposium Series, 472 154 (1991).

15 Holsworth, R. M., Provder, T., and Stansbrey, J. J., ACS Symposium Series No. 332, T. Provder, Ed., 1987, p. 191.

16. Holsworth, R. M. and Provder, T., US Patent 4,478,073, 1985.

17. Cummins, H. Z., Knable, N., and Yeh, Y., Phys. Rev. Let., 1964, 12, 150.

18. Foord, R., et al., Nature, 1970, 227, 242.

19. Nicoli, D. F. and Elings, V. B., US Patent 4,794,806, Jan. 3, 1989, “Automatic Dilution Systems.”

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CoatingsTech | Vol. 17, No. 10 | October 2020

The field of coatings technology has utilized many forms of silica-based particles in the last 70 years. This large, varied class of fillers is generically broken into two categories of crystalline and amorphous morphology. With ongoing scrutiny and sensitivity in the coatings industry to move towards less hazards in the workplace, greater emphasis is placed on suitable amorphous technology to replace crystalline silica technology. Amorphous silica is highly adaptable and flexible to be modified in both powder and pre-dispersed forms, and numerous engineered types of technologies have been developed to provide functional solutions to many coatings problems.

Amorphous silica technology has been developed to address functionalities including: rheological control, suspension of pigments and fillers, and reinforcement of coatings film; to impart scratch resistance, hydrophobicity / anti-corrosion benefits, and oleophobicity; as a carrier of trace actives into coatings for homogenous distribution; for flow control, charge, and fluidization enhancement of powdered coatings; and gloss reduction of liquid systems. Particle technology and modification will be addressed along with performance attributes highlighted for each of the types of tailor-made modifications. The importance of proper dispersion and homogenous distribution within a coating matrix will be reviewed.

This article will address how amorphous silica technology is differentiated and engineered to create specially tailored solutions to enhance the performance of coatings and will highlight the latest technical developments in this field.

Introduction

Silica, or silicon dioxide, is one of the most abundant minerals present on earth. It is estimated that quartz, the most stable form of this complex family of materials, makes up more than 10% of the earth’s crust and, as a major component of the natural sands widely used in the construction industry, is a key raw material to produce glass and silicon.¹

Silica is also an important raw material for the coatings industry, as it can provide a wide range of functionalities and benefits. These include rheological control, enhanced film formation, improved mechanical properties of the final coating film, free flow and fluidization enhancement of powders, and control of gloss. Silica is also an important raw material for the formulation and production of defoamers. The silica grades used in the coatings industry are produced synthetically and typically meet greater quality control standards, often having tighter physical-chemical requirements, such as color and brightness. The enormous variety of performance properties is achieved by adjusting the particle size and morphology during production as well as via surface treatment and densification of the silica particles in downstream processes.

A summary of the main methods for producing synthetic silica is shown in Figure 1. The most common types of silica used in modern coatings are produced either by a liquid phase process of precipitation or gas phase process of flame hydrolysis. Precipitated silica is produced by the controlled reaction of sodium silicate (“water glass”) and sulfuric acid similar to the production of silica gels. The silica is precipitated, filtered, washed, and dried before milling and classification (Figure 1).

The production of fumed silica began with the discovery of the flame hydrolysis of silicon tetrachloride by Harry Klöpfer in 1943. This discovery was part of a wartime effort to produce silica that could act as a white reinforcing filler to modify rubber, which was then much needed for tire production to replace oil used to make carbon black. A simple diagram of the process is shown in Figure 2. The overall chemistry of the process is efficient and versatile. A vaporizable metal precursor is fed into a hydrogen/air flame, and the hydrolysis product, silicic acid for instance, rapidly condenses to the metal oxide. Multiple pathways to particle formation are possible, such as particle growth through deposition, particle evaporation, aggregation, and aggregate coagulation. The elegant efficiency of the overall chemistry makes the process very amenable to variation. A diverse array of metal oxides beyond silica has been produced, including mixed metal systems and surface modified and doped particles that can be used for a wide variety of industries and applications.

Fumed silica consists of three conceptual levels of structure (Figure 3). The primary particle only exists for a short time in the flame. Primary particles fuse together to form an aggregate, which is the secondary particle structure. Isolated primary particles, in this model, are rare. The tertiary structure is an agglomeration of the secondary structures. This collection of particle aggregates can be disrupted by the introduction of shear, and then reform over time after the shear is removed from the system. This mechanism is the means by which fumed silica imparts pseudo-plastic rheological properties to formulations.

A comparison of the different physical properties of synthetic silica is shown in Table 1. It is important to note that all of these synthetic silica types are amorphous and do not contain crystalline silica. This has been confirmed by X-ray diffraction.

The second element of particle design is surface modification to render the hydrophilic particles hydrophobic in character. This is achieved by the reaction of the surface silanol groups with different silanes. These treatments create different grades of fumed silicas that vary in hydrophobicity, tribo-electrostatic charge, and thickening efficiency. A summary of the typical surface treatments with corresponding attributes is shown in Table 2. The level of treatment, which can be measured by carbon content and methanol wettability (Figure 4), indicates the consistency of treatment and the balance of hydrophilic to hydrophobic surface.

The Multipoint Methanol Wettability method is a quantitative test method to measure the level and consistency of hydrophobic treatment. The 0.2 g of the treated silica is added to a series of graduated test tubes containing 8 ml of dilutions of methanol in water made in 5% increments, starting with 100% water, 95% water, and 5% methanol up to 100% methanol. The silica/solutions mixtures are shaken and then centrifuged under controlled and defined conditions. Depending on the level of hydrophobicity and consistency of surface treatment, the silica will wet differently into each water. Methanol mixtures and the amount of wetted silica in each solution mixture is recorded and plotted to a curve known as the methanol wettability fingerprint. Silicas requiring higher methanol amounts for wetting are more hydrophobic. Consistently treated silica shows a steep rise in wet-in, whereas a more gradual curve indicates a wider range in the consistency of treatment. Precipitated silicas can also be surface treated, typically with waxes and reactive oligomers, to improve product and formulation stability and reduce viscosity impact.

The third element of particle design is structure modification via one of several proprietary processes. Granulation results in larger, individual spherical particles in the range of 20–30 μm that are porous; their main function is to act as free-flowing carriers of liquid-based actives and oils. Other chemical and mechanical post-processes reduce structure (i.e., the level of aggregation or agglomeration). Products resulting from post-processing can have significantly higher bulk densities and dramatically reduced thickening efficiency due to reduced levels of aggregation at the primary aggregate level. The functional benefit resulting from such grades are enhanced scratch and abrasion resistance, as higher loading can be achieved with minimal impact to formulation viscosity. This higher loading results in reinforced domains, which drives the scratch and abrasion resistance.

Rheology and Film Formation

Fumed silica, in various grades and modifications, has been used for decades in coating formulations to impart thixotropy, anti-settling, and anti-sag properties. The main requirements for good performance are proper selection and adequate dispersion to homogenously distribute aggregates throughout the coating matrix. Proper grade selection can be loosely correlated to dosage, particle size, structure, and surface treatment. Untreated, hydrophilic fumed silica grades give the best performance in non-polar environments, whereas hydrophobically modified grades, such as those treated with DDS, TMOS, and HMDS (Table 2) are more efficient as polarity increases. This trend is shown in Figure 5.

Grades treated with TMOS and HMDS are highly effective for high solids and radiation-cure systems. Polydimethylsiloxane-treated grades are the most hydrophobic. This technology can also be considered in high solids and 100% solids systems, where it is very effective. However, care must be taken, as this surface modification is not fully reacted to the surface, and migration of the free PDMS may cause surface defects or adhesion problems.

Proper dispersion of the fumed silica is critical to good performance. When optimizing a dispersion for thickening efficiency and rheological enhancement, several parameters should be considered including shear rate. Dispersion time, temperature control, and sequence of addition are all important. High-speed dispersion using a saw-type blade at a shear rate > 10 m/s is recommended. Longer dispersion time will not compensate for inadequate shear rate. The consequences of poor dispersion are typically larger agglomerates that remain visible in the final coating, reduced thickening efficiency, poor thixotropic stability over time, lower gloss and transparency, and possibly film defects.

Fumed silica has excellent thermal stability, but as the temperature of the coating environment increases, particularly upon shear, the wetting properties of the coating typically improve. This can lead to over-dispersion, whereby the aggregates are reduced past their optimum association level. Once this occurs, reduced thickening efficiency results, sometimes to the point where it appears as if no thickener was added. Fumed silica is one of the smallest particle size materials added to a coating formulation. This component should be added early in the formulation, preferably to the resin or binder, rather than into non-film-forming components like solvents for optimal effect. Caution should be used when post-adjusting batches with powder, as only minimal shear that is inadequate for homogeneous incorporation can often be used at this stage. Post addition with low shear may bring formulations to their desired rheology, but this can deteriorate over time, as larger agglomerates slowly wet-out.

Lab evaluations have demonstrated that multiple performance attributes can also be enhanced using fumed silica dispersions. These include improved suspension of pigments, fillers and matting agents, reduced tack, improved dirt pick-up resistance, enhanced film strength, and even improved film formation. These features are achieved without compromising gloss and other appearance attributes. An example is shown in Figure 6, where a pre-made aqueous fumed silica dispersion improves the film formation in combination with reduced coalescent solvent levels. These improvements have been seen with many different film-forming resins and using less or no coalescing solvents. This can help reduce the overall volatile organic compounds of the formulation, in addition to providing the other properties referenced above. This enhanced film formation is a result of reduced stress propagation due to the reinforcing effects of the very finely dispersed fumed silica.⁹

Anti-Corrosion/Water Repellency

Hydrophobically modified grades of fumed silica have also been used together with anticorrosive pigments to improve corrosion performance and water repellency of coatings. These grades are not considered to be anti-corrosion pigments, but they work effectively with many classes of anti-corrosive pigments such as modified barium metaborate, calcium phosphosilicate, and zinc dust. Loadings between 1.0% and 3.0% by weight of total formulation are used to ensure that there are enough particles in the coating matrix to support a hydrophobic barrier, improve the mechanical properties of the film, and increase hydrophobicity. Water repellency measured by improved blister resistance can be improved at lower loading levels starting at 0.5% by total formulation weight. Two examples of this effect are shown in Figure 7.

Proper dispersion is again needed to homogeneously distribute the silica throughout the coating matrix for maximum effect. It is suggested that the treated fumed silica be dispersed together with the anti-corrosion pigments to ensure optimal dispersion. The best results have been obtained using DDS, TMOS, and HMDS treatments, although the impact on formulation rheology must also be considered.

Scratch Resistance

The development of silica particles for the specific purpose of improving scratch resistance came from the creation of particles for high reinforcement of elastomers and composites. The critical factor needed to achieve high reinforcement is to be able to fill the polymer matrices with significantly higher levels of fumed silica without increasing the viscosity to unworkable levels. This is achieved by structure modification through a proprietary post-processing to achieve a highly reduced structure and a low level of aggregation. This results in a material that can be used as a reinforcing filler to increase the mechanical strength of the coating and impart scratch resistance. The corresponding physical property observed is a significant increase in the bulk density (Figure 8) and a dramatic reduction in the ability of silica to increase viscosity. When compared to other reinforcing fillers, such as alumina, silica has the advantage of a lower refractive index of 1.46, more closely aligned with many polymer systems, which results in improved transparency and clarity. These materials can also be hydrophobically modified with DDS, HMDS, and TMOS for improved water resistance.

The main consideration for successful use of surface-treated, structure-modified fumed silica particles is adequate loading level. Optimum loading levels start at 5% by weight on total formulation and can approach 15%. Inorganic particle load must be high enough to attain a homogenous density through the polymer to achieve a consistent, reinforced matrix. This can be seen in the scanning electron microscopy (SEM) analysis of a cross-section of a high-solids coating (Figure 9), which shows a homogenous distribution through the film with no surface enrichment resulting from higher particle density.

Five percent loading of an easy-to-disperse (E2D) structured modified silica particle treated with DDS achieved improved scratch resistance in a high-solids system, tested by a dry scratch method using a Crock meter (abrasive paper) and wet scratching using an Elcometer (40 double strokes, bristle brush, and 0.15% quartz in water slurry). Improved scratch resistance and higher gloss retention of the coating was observed after using�� both scratching methods, and reduced haze was observed after the panels were subjected to Elcometer testing (Figure 10). The addition of 5% silica did slightly reduce gloss, but the silica significantly improved scratch resistance.

Results in Figure 10 show three variants of DDS-treated structure-modified silica. Variant 1 is the original that requires milling, variant 2 the same DDS structure modified silica pre-dispersed in methoxypropyl acetate (MPA), and variant 3 is the newest version, which is easy-to-disperse.

Free Flow, Fluidization, Transfer Efficiency

Powder coatings, whether conventional, fine, thermosetting, thermoplastic, tribo, or UV-cured, all require good flow, reduced moisture pick-up, good package stability (no caking), efficient fluidization, and high-transfer efficiency as well as reduced Faraday cage effects for even film thickness and optimized appearance during application. Hydrophilic- and hydrophobic-treated fumed silica and alumina can be used to improve flow, storage stability with reduced moisture pick-up, and improved fluidization and transfer efficiency.

In practice, flow additives used in powder coatings can be added in one of three places during the powder coating manufacturing process: 1) directly in the hopper, 2) dosed into the powder during chipping, or 3) post-added after pulverization. Flow additives can be dry blended into problematic powdered components before they are charged into the hopper to help them feed more consistently and homogeneously into the extruder. The typical loading of flow additives used in this step is 0.1–0.3%. Flow additives used to pre-treat ingredients are extruded into the powder matrix and do not influence the bulk flow properties after compounding and pulverization.

When additives are used to influence the final powder coating properties, they must be added after extrusion and be oriented on the outside of the powder coating particles. There are typically two places where silica or alumina (or a combination of the two) can be added into the process to achieve this: 1) prior to chipping where the additive is cut into the powder coating particle or 2) after pulverization and classification. Care should be taken when dosing the additive before pulverization, as classification systems can remove the additive out of the powder and reduce the final dose remaining in the powder coating. The typical dosage level used in the chipping or as post add is also 0.1–0.3% by weight.

A study was organized with the University of Western Ontario to assess specific performance attributes associated with four classes of additives in different powder coatings. The first two powder coatings were corona applied. The first was a conventional polyester with d50 of 31.5 μm and the second was a finer particle size, polyester with d50 of 21.5 μm. The additive dosage level was adjusted based on the particle size of the powder. A 0.3% dosage level was used for the coarse powder and a 0.5% dosage level for the fine powder. A third powder was a tribo-applied polyester powder coating. The attributes tested were angle of repose (flowability), bed expansion (flowability and fluidity), transfer efficiency, Faraday cage effects, gloss, and gel time. Four types of silica were tested: untreated hydrophilic silica with a surface area of 200 m²/g, HDMS- and aminosilane-treated silica with a surface area of 200 m²/g, DDS-treated silica with a surface area of 130 m²/g, and HDMS-treated silica with a surface area of 300 m²/g.

Surface-treated alumina was most effective at improving transfer efficiency and reducing Faraday cage effects due to its neutral to slightly positive electrostatic charge character. This is shown in Figure 11 where a disk applied with coarse powder containing 0.3% high surface area alumina (130 m²/g) has a more consistent level of jetness than the disk applied with powder containing no additive. This test measures how much powder is transferred and the consistency of coverage of the disk (by weight) under controlled application conditions.

Faraday cage effects were measured by determining how much coating is deposited in the inner trough of a test specimen. The interior parts of the trough have three removable panels under controlled applications conditions. After application, these inner panels are removed and weighed. Reduced Faraday cage effects (improvement) are associated with higher, more consistent weights of powder deposited on these inner removable panels. The 0.3% alumina treated with TMOS was effective in reducing Faraday cage effects in the coarse, black powder coating.

Fluidization efficiency was also assessed in this study. The results in coarse and fine powder show that particle size of the powder coating significantly affects the type of additive most effective for improving fluidization. Alumina was more effective in improving fluidization in the coarse powder coating as measured by lower air velocities needed to obtain 20% bed expansion, while silica was more effective in the fine powder coating (Figure 12). This trend suggests that additive packages may need to be adjusted based on their particle sizes.

Gloss Control

Gloss is defined, according to DIN EN ISO 4618, as the human perception of the more-or-less directed reflection of light rays from a surface. Glossy surfaces appear shiny and reflect most light in the��specular��(mirror-like) direction, while matte surfaces diffuse��most of the light in a range of angles.��Gloss level can be characterized by the angular distribution of light scattered from a surface, measured with a��glossmeter or reflectometer, and it is dependent upon the viewing angle (Figure 13).

There is no common or globally accepted definition of the term “matte.” It is always measured based on a comparative measurement of the gloss against a standard.^2-5 For coating surfaces, the term “gloss” means almost complete reflection in the sense that the surface reflects and scatters incident light in a wide-angle cone. The greater the cone angle, the less gloss is generally observed (Figure 14).⁶

Lin and Biesiada demonstrated that matting is a function of both the silica particle size and degree of coating shrinkage during drying (either through solvent evaporation, chemical reaction, or coalescence).⁷ Larger particles are more efficient at reducing gloss as a function of silica dosage, but the larger particles can lead to a rough surface and increased dirt pickup over time. Most silica grades used for matting coatings are produced via wet and gas phase processes and are classically larger than the grades used for rheology. Some of these grades can influence thickening to a lesser or greater extent. Surface treatment, either with wax or reactive oligomers, can help reduce viscosity build-up, prevent hard settling, and improve transparency and stability. Recent technology developments have produced silicas with improved haptic effects like soft-feel.

Recent Developments in Silica Technology

While considered a mature technology, the use of silica in coatings continues to benefit through innovation. Romer described a new process for producing precipitated silica that allows greater control of particle morphology during the precipitation process to produce highly spherical particles with narrow-sized distributions (Figure 15).⁸ The spherical shape imparts high apparent hardness to improve scrub, abrasion, and burnish resistance of the formulated coating, with low binder demand and minimal impact on coating rheology. These spherical silica particles also can provide matting properties, depending on particle size, and they have excellent transparency for use in deep colors and clear coats.

Both hydrophilic (200 m²/g) and some hydrophobically treated grades (e.g., DDS, HDS, HMDS, 130–300 m²/g) of fumed silica can be used in water-based coatings when it is possible to adequately disperse the powder into water dispersible resins and solutions. However, the low viscosity and high dielectric constant makes water a poor grinding medium for fumed silica, and it is difficult to achieve the degree of de-aggregation and dispersion of particles needed to achieve optimum benefits in water-based coatings. Incorporation of hydrophobically treated grades can also be difficult due to the poor wetting properties of water-based formulations, especially when resin solids drop below 35% non-volatile content. Additives, such as acetylenic diols, can help to improve the wetting and dispersion of fumed silica into water, but care must be taken to ensure that the additives do not disrupt the silica network formation, reducing rheology control.

When adding the silica directly into water, or when using a shear-sensitive, film-forming resin, it is recommended to use a pre-dispersed form of fumed silica to overcome these challenges. A new aqueous dispersion of a functionalized fumed silica has been developed using a new production process and carefully selected additives. The silica is already dispersed, so it can be easily stirred into water-based formulations without requiring high shear dispersion.

The new dispersion WF 7620, contains 20% functionalized silica with a high surface area of 300 m²/g and demonstrates outstanding rheological effectiveness in waterborne coatings, especially those applied via spray application where anti-sagging properties are critical while maintaining excellent flow and levelling properties.

This is demonstrated in the jump-curve rheology graph shown in Figure 16. The jump curve simulates a spray application where the coating is sheared at high shear (500s^-1) continuously to simulate the spraying process and then the shear is suddenly reduced to 0.1s^-1 simulating the coating on the substrate after application. A fast build-up of viscosity is desired to prevent sagging after application onto vertical surfaces. This enables the formulator designing perfect finishes for three-dimensional parts including general industrial coatings, transportation coatings, plastic coatings, and wood coatings. The jump curve demonstrates the new dispersion WF 7620 and gives a significant increase and improvement in rheological efficiency, compared to an existing water-based dispersion of fumed silica based on a fumed silica core of 130 m²/g. This improved performance is due to the use of a higher surface area silica, combined with a novel functionalization in-situ. The use of a higher surface area fumed silica in the dispersion also helps to improve clarity and transparency and helps to reduce haze in the final coating.

Conclusion

Modified grades of silica have been used for many years to improve a variety of performance attributes in many different coating applications. These include rheology, film formation, and mechanical properties as well as surface appearance. The morphology of the silica particles, size distribution, and surface treatment are critical to the broad range of properties that can be attained using these materials. Recent advances in the manufacturing and post-treatment processing of silica have continued to develop new grades of silica that offer new and or improved performance for the coatings industry.

References

1. https://en.wikipedia.org/wiki/Silicon_dioxide.
2. Ryde, J.W., Proceedings of the Royal Society (London), 131 A, 451–464 (1931).
3. Brockes, A. and W. Helm, W. Farbe und Lack, 66, 53 (1960).
4. Zorll, U., Farbe und Lack, 67, 426 (1961).
5. Becker, Noven, H. and Rechmann, H., Farbe und Lack, 73, 625 (1967).
6. H. Haussühl and H. Hamann, Farbe und Lack, 64, 642 (1958).
7. Lin, B.T., and Biesiada, C., “Novel Synthetic Silica Matting Agents for Polyaspartic Coatings” Proceedings of 2016 Waterborne Symposium, 2016.
8. Romer, R., “Spherical Precipitated Silica: Next Generation Particle Morphology for Performance in Coatings,” Paints and Coatings Industry, January 2017.
9. “The Use of AERODISP® Fumed Silica Dispersions to
   enhance Waterborne Coatings,” Evonik Technical Bulletin TI1371, September 2009.

CoatingsTech | Vol. 17, No. 6 | June 2020

Energy consumption impacts not only costs, but sustainability. Reducing energy consumption, therefore, reduces costs and lowers greenhouse gas emissions. The coatings industry is helping to reduce energy consumption in many ways. Manufacturers are developing more efficient processes for the production of coating ingredients and formulated paints. Resin manufacturers are developing new technologies that cure at lower temperatures, and equipment manufacturers are boosting the performance of curing equipment to expand the applicability of more energy-efficient alternative curing mechanisms. Coatings that contain novel resin and pigment chemistries are increasing the energy efficiency of buildings, cars, and airplanes.

Manufacturing Efficiency

In the manufacture of key ingredients of basic raw materials—resins, additives, pigments, fillers and solvents—and formulated paints and coatings, there are numerous opportunities to improve energy efficiency and achieve energy savings, according to Robertino Chinellato, global R&D director for Powder Coating Resins with allnex. “The specific actions that can be taken depend, of course, on the different manufacturing processes involved, but approaches for heat recovery, achieving greater energy efficiency, and general process optimization to improve yields and reduce cycle times can be considered for all industrial processes used in the paint and coatings industry,” he says. Options include the use of alternative energy production technologies such as tri-generation technology, which provides three forms of output energy using natural gas as the fuel: electrical power, heat, and cooling.

As examples, James Martin, global operational excellence manager at allnex, points to the use of variable frequency drives on motors powering fans and centrifugal pumps to match the power demand, avoiding the use of excess energy, and reducing the use of distillation processes by implementing more efficient solvent recovery systems. Advanced process modeling has also helped the company reduce energy consumption in highly integrated distillation columns. In addition, Martin comments that operational analytics have helped allnex to better understand and monitor energy systems and energy flows, enabling the company to identify opportunities to reduce energy consumption more quickly.

Covestro is highly committed to the United Nations Sustainable Development Goals, according to Steven Reinstadtler, infrastructure marketing manager at Covestro LLC. “By 2025, 80% of Covestro’s R&D project spending will be targeted in areas that contribute to achieving these goals, which include realizing energy savings,in our own production of coating raw materials as well as in processes related to their use on the customer side,” he says. Reinstadtler adds that Covestro’s efforts to develop more energy-efficient solutions are often pursued in close collaboration with partners.

Most coating companies also have sustainability initiatives that include reduction of energy consumption. AkzoNobel, for instance, recently announced its intention to move the company towards zero waste and to cut its carbon emission in half by 2030. “This initiative is part of the ‘Planet’ element of our new ‘People. Planet. Paint.’ approach to sustainability,” notes Rinske van Heiningen, AkzoNobel’s director of Sustainability. Targets for 2030 include a 30% reduction in energy use, 100% electricity derived from renewable sources, 100% water reuse at the company’s most water consumption-intensive sites, and zero non-reusable waste. Electricity from renewable sources is already in use at 33 locations across eight countries, including the installation of solar panels at 14 sites. “To help us achieve our energy reduction goals, we see the greatest opportunities for decreasing energy consumption with utilities, such as compressors, chillers, cooling systems, and LEV and HVAC solutions,” van Heiningen says. The company has already seen significant reduction of energy consumption thanks to modern, energy-efficient equipment in utilities, but also in motors, pumps, and drives. Achieving “right first time” improvement has had a huge impact on energy savings, according to van Heiningen.

PPG is committed to reducing energy consumption to minimize greenhouse gases (GHGs), reduce costs, and create more efficient facilities, according to PPG executive vice president, Tim Knavish. “While the vast majority of our coatings are manufactured at ambient temperatures and pressures and, therefore, are not energy-intensive, we are focusing on making the milling step more energy-efficient,” he observes. One way to increase energy efficiency in coating manufacturing, agrees Mark Ryan, marketing manager for The Shepherd Color Co., is by using easily dispersed pigments, which reduce the need for time and energy-
intensive pigment dispersion steps.��He notes that Shepherd Color’s Dynamix pigments eliminate the small-media milling step while also reducing the re-work needed for off-spec batches.

PPG is also shortening cycle times and making other relevant changes to reduce the energy required to manufacture its coating technologies. The company’s Natural Resources and Climate Change Subcommittee of its Sustainability Committee oversees PPG’s energy-reduction strategy, determining the greatest opportunities to decrease energy use. Key energy efficiency initiatives include creating a culture of energy conservation through communication and awareness-building; developing a PPG standard for energy management based on ISO^® 50001 and ENERGY STAR^®; identifying and focusing resources on locations with the highest energy use; sharing best practices across the organization; exploring partnerships with energy solution firms; and increasing the company’s use of renewable energy. Specific goals set in 2018 include a 15% reduction of energy consumption intensity by 2025 from a 2017 baseline and increasing renewable energy to 25% of total electricity usage, exclusive of GHG reductions by 2025.

“Our global locations have been successful in decreasing their individual energy usage by following the ENERGY STAR guidelines and through the commitment of our people to reduce energy in the workplace, from small steps such as turning off the lights to installing energy-efficient equipment,” Knavish asserts. He describes, as one example, the installation of more than 180 solar panels on three buildings at the PPG Aerospace Applications Support Center (ASC) in Tullamarine, Australia. The solar panels provide 70,000 kWh of power, reducing the center’s annual electricity consumption by 27% and related costs by 38%, as well as GHG intensity.

Cool Cure

A second significant area with potential to dramatically reduce energy consumption—this time during coating application—is the development of resin technologies that cure at lower temperatures. “The trend to reduce temperatures during curing of paints has been an important driver over the last couple of years,” notes Chinellato. A variety of opportunities exist, including lower-temperature powder coatings; switching to ultraviolet (UV) curing systems; faster-curing, two-component (2K) polyurethane systems; and new technology systems with inherently faster chemistry, according to Robert Skarvan, global marketing director for Liquid Resins and Additives with allnex.

From the perspective of powder coatings, there has been a drive to increase the reactivity of binder chemistries so that coatings can be cured either at lower temperatures or over shorter periods of time, notes Robert Watson, global marketing and business development manager for Powder Coating Resins at allnex. “The current state of art is that some powder coatings can be cured at temperatures of 130–160°C, depending on technology type. Having lower-curing coatings means massive metal componentry, which is slow to heat, can reach curing temperature faster, and thus increase throughput, saving energy,” he explains. He adds that while powder coatings are typically associated with metal finishing, new developments in lower-temperature curing are now starting to open up wood substrates such as medium-density fiberboard (MDF), commonly used in furniture, to powder. Another goal for powder coating is to target a wider range of thermally sensitive substrates, including plastics and composites. Allnex is currently developing a novel binder for powder coatings based on new chemistry that enable reduced curing temperatures, Chinellato adds.

Interpon Low-E powder coatings from AkzoNobel, meanwhile, are specially engineered for curing at temperatures lower than the current standard of 180–190°C for triglycidyl isocyanurate (TGIC)-free polyesters. “These products can help our customers reduce their energy bill by up to 20% or increase their output by up to 25%,” says van Heiningen. The coatings are designed for use in a wide range of applications, including industrial steel products, street and garden furniture, and agricultural and construction equipment. The coatings are also suitable for use in interior or exterior environments.

In the future, Watson also expects lower-temperature curing of powder coatings to be increasingly important for electric vehicles (EVs). “As EVs become more prevalent, there is a greater tendency to light-weighting to increase range. Traditional metals could be replaced with thinner metals or non-metallic substrates.��As the nature of these substrates change, the curing characteristics of coatings will also have to change, and this trend will drive greater use of lower-temperature curing systems and, therefore, lower energy,” he explains.

In the liquid coating space, allnex offers ACURE^® technology, a new non-isocyanate chemistry with inherently faster cure speed vs conventional 2K urethane systems. “The main driver for the development of this system was, in fact, faster cure. It is the non-isocyanate nature of the chemistry that provides that faster cure,” Skarvan observes. “Whether ACURE or other faster-curing 2K systems are used, curing oven temperatures may be reduced,” he adds. The company has also developed a line of faster-curing acrylic and polyester polyols. One current need is for faster-curing primer systems that can be paired with fast-curing topcoats. Wet-on-wet systems that eliminate a baking step are currently common in automotive and industrial OEM applications, according to Skarvan, and their expanded use will also lead to further decreases in energy consumption.

PPG, in order to decrease energy consumption for automotive OEM customers, has introduced a low-cure paint technology to the automotive market in 2018. “Before our low-cure paint technology, approximately 70% of total energy consumption at an automotive assembly facility took place in the paint shop. Our low-cure technology uses up to 39% less energy through a next-generation clearcoat that cures at about 175°F (80°C) compared to nearly 285°F (140°C) for current systems. Added benefits include a simplified manufacturing process and smaller paint shop footprint,” Knavish observes. From a UV coatings application perspective, primarily in packaging coatings and inks, but also for industrial wood and construction and industrial plastics, Michela Fusco, global marketing director for Radcure at allnex indicates that the greatest energy efficiency opportunities are connected to the adoption of UV light-emitting diode (LED) ��lamp technologies as replacements for traditional mercury lamps. “LED lamps used for UV curing of coatings enable higher energy efficiency in cycling operations due to the ability to switch the systems on and off instantly. “Recent advances in UV LED lamps and their use in combination with UV LED curing systems is enabling broader applicability of UV LED technology in the coatings industry,” she observes. She notes that these developments have been driven by limitations with respect to surface cure when using earlier versions of LED curing lamps, which has prevented wider adoption of this coating technology. “Further improvements are still needed in this area to make the most of the energy efficiency potential of LED curing technology across different coatings applications,” Fusco adds. The use of LED lamps and the challenges with surface cure are, in fact, driving the development of LED-fit resins and boosters. For instance, allnex recently launched EBECRYL LED 03 oligomer, which has enhanced reactivity when used as a co-resin in UV-curable formulations, according to Fusco. She also notes that allnex is optimizing LED boosters for ink technologies and improving the LED cured surface performance in wood applications.

Pigment technology can impact the energy efficiency of some coating applications. Shepherd Color’s patented Niobium Tin Pyrochlore (NTP) Yellow pigment in the middle-yellow color space has excellent durability, chromaticity, and opacity that organic pigments cannot match, according to Ryan. This innovative pigment expands the envelope of durable color and serves as a bridge between the high chromaticity of organic pigments and the durability and opacity of inorganic pigments. “The increased opacity allows for single-coat color matches, eliminating the need for multiple coats of passes through the paint application process, thus saving time, energy, and materials,” he remarks.

Textile manufacturers are also achieving greater energy savings and reducing GHG emissions due to a new coating technology developed by Covestro. When a transfer coating process using INSQIN^®�� water-based technology is used instead of the conventional solvent-based coagulation process, the relative GHG potential of synthetic-coated textiles produced is 45% lower, according to Bob Saunders, head of Textile Coatings North America at Covestro LLC. “If the entire textile industry were to switch to water-based technology from Covestro, for example, the resulting greenhouse gas reduction would be comparable to the elimination of all cars driving on the streets of London, Hong Kong, and Los Angeles,” he comments.

Coatings that Contribute

Coatings, once applied to substrates, are also helping to reduce energy consumption in several ways. Coatings such as those from PPG provide environmental and sustainability benefits to end users. “Our innovations contribute to a healthier, greener environment without sacrificing appearance and color flexibility. They reduce corrosion and extend the customer product lifetimes. They help reduce energy usage and emissions, protect customer employees, and minimize waste and water consumption,” Knavish says.

One example of an energy-saving technology from PPG is PPG POWERCRON^® 160 anionic epoxy e-coat, a significant technical advance in electrocoat technology that enables high film builds (greater than 6 mils) over multiple substrates and pretreatment chemistries. Previously, according to Knavish, anionic e-coats had to be matched to specific pretreatments and metal substrates, which added complexity, cost, and energy use to the coatings process.” PPG Powercron 160 coating cures at lower temperatures than conventional anionic e-coats, which further reduces energy use and related carbon emissions,” he notes. The technology was developed to meet the unique corrosion protection requirements of the pipe industry and is engineered for manufacturers who finish complex cast profiles in the castings, automotive, heavy-duty equipment, and other industries.

While not necessarily energy-saving, PPG has also developed novel technology for aircraft windows to significantly block harmful UV radiation and high-energy visible (HEV) light to help protect aircrews, passengers, and aircraft interiors from solar radiation. PPG windows with PPG SOLARON BLUE PROTECTION™ UV+ blocking technology block 99% of UVA and UVB radiation and more than 50% of HEV (blue) light. The new technology is incorporated into PPG’s windows at the time of manufacture and can be applied to cockpit and cabin windows.

Driving Efficiency

In the automotive sector, light-weighting has not been restricted to EVs. In all passenger and commercial automobiles, light-weighting has become a well-known way to increase fuel efficiency and save energy. Dow’s ACCOUSTICRYL™ Copolymer Emulsions are one type of coating technology designed to help with light-weighting. These coatings, according to Ramesh Iyer, sustainability director for Dow Coatings and Performance Monomers, are increasingly used in automobiles to reduce structure-borne noise for a quieter interior by replacing incumbent heavier bitumen pads, reducing weight by up to 35%. “In 2018, about 96 million vehicles (passengers and commercial vehicles) were produced. This translates to an opportunity to reduce over half a million tons of CO₂ emissions, enough to take approximately 108,000 cars off our roads for a year,” he comments.

In addition to sound-dampening coatings, the light-weighting trend is also leading to greater demand for specialty elastomerics that offer benefits such as vibration reduction, according to Sanjay Luthra, business development and marketing manager at Arkema. “Advances in raw materials that offer lower coat weights and high-performance attributes are critical to ongoing success in the industry,” he asserts. Additionally, with the growing prevalence of plastics in vehicle construction, he notes that there is greater need for adhesion to low-energy substrates. “Many of our customers are using elastomeric coatings formulated using specialty waterborne acrylic and styrene acrylic copolymers, as well as hybrid, silane terminated urethane and polyether polymers to address this need,” Luthra says. He adds that incorporating foam into the coating systems allows for additional light-weighting that ultimately leads to lower energy consumption.

Coatings In Supporting Roles

Coatings also provide protection to wind energy towers and solar panels, indirectly aiding in energy efficiency, according to Luthra. “Furthermore, for these applications, the myriad coating technologies that allow for reduced numbers of paint layers or layer thicknesses all contribute to reduced overall object weight and, thereby, reduced energy consumption,” he states.

Light-weighting is also important in the building and construction sector, where use of higher-performance acrylic copolymers with increased hydrophobicity and tensile strength is allowing the reduction of coating weights for functional applications, Luthra says. Such applications include impregnation of glass fibers for dry-wall facers��and higher-performance coated, prefabricated concrete panels for use in tilt-up walls and modern building fabrication methods. “These coatings allow for thinner and lighter-weight fabrications that result in reduced energy consumption during transportation without compromising the durability,” he explains.

Newer air and vapor barrier coatings that constitute a component of advanced building envelope systems also allow buildings to conserve energy during their use. These coatings have to be regionally engineered, however, to meet local codes and building specifications, according to Luthra. They��contain a balance of hydrophobicity and permeability to allow air, water vapor, and moisture flow as well as contribute to the insulation and fire resistance properties of the walls. “Organizations such as the Air Barrier Association of America are instrumental in creating and managing the specifications for such coating systems,” he observes.

Cool roof coatings are perhaps one of the best-known ways in which coatings help conserve energy. Elastomeric cool roof coatings have been important for decades. “These coatings provide benefit by reflecting sunlight and lowering air-conditioning costs of buildings and warehouses, as well as imparting waterproofing protection that allows prevention of costly maintenance costs due to water damage,” Luthra explains. Recent innovation focuses on regional and specific needs driving reductions in energy consumption, such as newer systems that, rather than being white, have slight color imparted to them to address specific application and community color requirements, according to Luthra. He adds that a modified ASTM D-6083 Specification (ASTM D-6083 type 2) for elastomeric roof coatings is allowing coating manufacturers to address roof coating needs more for the sunbelt states. “These coatings do not need the low-temperature flexibility required in the snow belt states, but do need excellent early water, dirt pick-up, and UV resistance, as well as adhesion to multiple substrates,” he says.

Another emerging coating technology for high-end roof coatings is aqueous water-based fluoropolymer roof coatings. While these coatings have a higher unit cost, they offer significantly higher performance with respect to durability, Luthra comments. They are applied to a wide variety of substrates as thinner films compared to elastomeric acrylic roof coatings but provide the traditional unparalleled color and UV stability for which fluoropolymers are well renowned.

Another trend in the fluid-applied roof coating market is a switch from aromatic polyurethane coatings, which have helped to address market needs for a longer-lasting, durable and seamless waterproofing membrane on low-slope commercial roofs, to one-component (1K) and 2K aliphatic polyurethane roof coating systems with long-term UV and weather resistance. The driver, Reinstadtler says, is the increased awareness of the energy-saving benefits of reflective roof assemblies. Aliphatic polyurethane roof coatings provide the hail resistance, wind uplift, rain resistance, and seamless long-term flexibility of aromatic systems while also ensuring that surfaces retain their original color (often white) and, thus, their high-infrared (IR) reflectance for a longer period. “The UV resistance of the employed resins and aliphatic hardeners keeps the membrane from yellowing or darkening over time, which can impact the solar reflectance. Additionally, the excellent resistance to weathering keeps the coating surface smooth and non-porous longer, reducing dirt pick-up and mildew growth that can impact solar reflectance and ultimately, the energy savings of the reflective roof, explains Reinstadtler.

Advances in IR-reflective pigment technology, meanwhile, are enabling the formulation of colored cool roof coatings for residential and other sloped-roof applications. For instance, Ryan notes that Shepherd Color’s Arctic IR reflective pigments are used around the world in building and construction applications to extend product lifetime and reduce the amount of solar energy absorbed.��In addition to reducing cooling loads for specific buildings with cool roofs, the reduction of sunlight absorption can also reduce the urban heat island effect. “Shepherd Color is on our 5^th generation of products that maximize a jet masstone, high tint-strength, and high Total Solar Reflectance (TSR).��We like to say that we have a range of products to fit your specific need—a veritable ‘Black Rainbow’ of products,” he observes. The company’s IR-reflective pigments are combined with its Dynamix easily dispersed technology for further energy savings during production.

Reflectance can also be important in interior spaces. “Often overlooked in the built environment is the effect of gloss and color on the overall lighting requirements,” remarks Reinstadtler. He observes that in industrial, warehouse, and retail spaces, the requirements are typically in the 200- to 100-foot candle range. But in critical task areas or showrooms, the requirements can rise to over 250-foot candles. “In these areas, floor coatings can contribute to significant energy reductions by reflecting natural and artificial light in a space due to their inherent gloss level and pigmentation,” he says. “In particular, a light-stable aliphatic polyurethane or polyaspartic is preferred due to their ability to retain a high gloss over time, ensuring continued light reflection. Additionally, a lighter pigmented floor designed to reflect ambient light will benefit from the long-term color stability imparted by the use of aliphatic hardeners in polyurethane and polyaspartic floor coatings,” Reinstadtler adds. For instance, in a large warehouse example, the owner was able to reduce the artificial lighting requirements and, therefore, the energy consumption in the space by roughly 20% by specifying a gloss white aliphatic polyurethane topcoat.

Also, in the building and construction segment, polyaspartic coating technology is helping to reduce energy consumption. When used as a concrete floor coating, there are clear, tangible energy savings, according to Reinstadtler. In northern climates, floor-coating can be challenging in early spring and late fall due to permeation of colder outdoor temperatures into buildings. The current solution, says Reinstadtler, is to turn up the heat on the existing HVAC system or to deploy electric or propane torpedo heaters to raise the temperature of the air and concrete, thereby expending energy. “By using polyaspartic floor-coating technology, contractors are able to complete more jobs in the shoulder season without the need to raise the ambient and concrete temperatures, which reduces energy consumption,” he notes. The faster working time of polyaspartics can create challenges, though, particularly for less-experienced contractors who are not familiar with polyaspartics. “As a workaround, they may use a longer working time polyaspartic coating on a cold-weather job and turn the ambient heat up even more to get a faster cure rate,” Reinstadtler says. One area of innovation, therefore, is designing new polyaspartic resins that offer both a longer working time and a quicker return-to-service time, even in colder weather.

There are opportunities for reducing energy consumption in many other industries as well. Dow has developed CANVERA™ Polyolefin Dispersion for metal cans, which provides significant sustainability advantages over traditional epoxy-acrylic coatings, according to Iyer. “Every 1000 cans coated with CANVERA Polyolefin Dispersion lowers the global warming potential of each can by 20% on a relative basis versus conventional BPA-based epoxy-acrylics,” he says. The company’s ROPAQUE™ Opaque Polymers and EVOQUE™ Pre-Composite Polymers, meanwhile, reduce the need for titanium dioxide (TiO₂) extenders in architectural coatings. While TiO₂ is good at imparting hiding and brightening, it requires large amounts of water and energy to mine and purify to make it suitable for use in paints, Iyer explains. The combination of Dow’s two technologies can partially replace TiO₂ in paints and utilize the remaining TiO₂ more efficiently. “Our lifecycle analysis of ROPAQUE Opaque Polymer and reported industry information about TiO₂ indicate that the carbon footprint��for a typical ROPAQUE Opaque Polymer is nearly half that reported for TiO₂ on a ‘dry’ per-kg basis. Therefore, partially replacing and more efficiently using TiO₂ present a large opportunity to reduce the carbon footprint of selected paints in their lifecycle,” Iyer asserts.

Separately, Ryan notes that the ultimate efficiency for a coating is a long lifecycle.��“The repainting process is time-consuming, expensive, and uses more materials, leading to greater energy consumption. A coating needs to keep its performance properties along with its aesthetic appeal,” he says. The company’s Complex Inorganic Color Pigments (CICPs) are inert and durable with outstanding weathering properties, according to Ryan. While high-temperature calcination is required to produce them, he strongly believes the investment in energy needed to make the pigments is more than balanced by the reduction in overall energy usage achieved due to the long lifetimes of the paints and coatings formulated with those pigments.

Going forward, Luthra notes that the paint and coatings industry will be challenged to meet the demands of an increasingly circular economy. “Recyclability and incorporating greater use of renewable content in coating raw materials are ongoing challenges for everyone in the industry. However, with increased focus and commitment to this area, innovation is sure to follow and help drive the growth of the global coatings industry,” he concludes.

CoatingsTech | Vol. 17, No. 5 | May 2020

In nature, bright white shades are created by a foamy, Swiss cheese-like structure made of a solid, interconnected network and air, according to Andrew Parnell, a researcher from the University of Sheffield’s Department of Physics and Astronomy. How these structures form and develop—and how they have evolved light-scattering properties—has, until recently, not been understood.

Parnell and colleagues from the University of Sheffield have solved the mystery by investigating the white scales of the Cyphochilus beetle, which are recognized to be one of the brightest white objects in nature. The team was provided access to x-ray imaging facilities at the European Synchrotron Research Facility in Grenoble, France, where they performed x-ray tomography of individual beetle scales. This detailed analysis enabled them to develop a better understanding of the structure of these scarabs’ scales and how they scatter light. The x-ray technology was also used to observe the mechanisms by which layers of paint dry and become structured.

Using this knowledge, the researchers created similar nanostructures using plastic and incorporated them, with the assistance of scientists from AkzoNobel, into a paint with a color that mimics the bright white color of the Cyphochilus beetle’s scales. The synthetic white nanostructures created by the team were also evaluated using the instrument Larmor at the ISIS Spallation Neutron Source at the Rutherford Appleton Laboratory in Oxfordshire—part of the Science and Technologies Facilities Council.

The ultimate goal, according to Parnell, is to use plastic waste that would otherwise be burned or deposited in a landfill. “By restructuring such materials for use in super-white paint, we would reduce the carbon footprint of these coatings and help tackle the challenge of recycling single-use plastics,” he says. Adds Stephanie Burg, a Ph.D. researcher at the University of Sheffield: “This research answers long-standing questions about how the structure inside these scales actually form, and we hope these lessons from nature will help inform the future of sustainable manufacturing for paint.”

The research was initially published in Nature Communications Chemistry.

Siloxane-based additives are critical tools in coating applications because their structures can be varied to provide a broad range of performance benefits in many types of formulations and chemistries. Surfactants and defoamers are some of the more commonly recognized additive classes, but many other functionalities can be derived from siloxane chemistries, particularly attributes related to surface control such as flow and leveling, slip, scratch resistance, and haptic properties. This extensive range of performance attributes is achievable due to the broad flexibility inherent in siloxane chemistry, allowing a fine-tuned balance of compatibility, incompatibility, and surface activity.

As with many additive types, a broad range of functionalities creates many options for improvement and innovation but also presents challenges in finding the right additive and optimizing to achieve the desired performance. This article will attempt to clarify the general structure-
property relationships that drive the performance attributes of siloxane additives and detail the continuum that exists between wetting, leveling, defoaming, and slip within this chemistry class. Surface control properties and testing will be reviewed and related to recent evaluation work conducted in developing novel siloxane surface control additives.

Introduction

Siloxane Chemistry

Siloxanes are molecules that contain Si–O–Si linkages, and their structures can be tailored to create materials with a broad spectrum of properties. While siloxanes can be derivatized with many different organo-functional groups to create a variety of small molecules and polymers, the siloxane backbone itself contributes some of the most unique attributes. The structure of underivatized polydimethylsiloxane (Figure 1) illustrates how this backbone, comprised of repeating –S��(���₃)₂–O– units, results in a molecule that is both highly methylated and extremely flexible. It is this feature of siloxanes that enables their high level of surface activity because the surface energy of a “methyl-
saturated” surface is ~20 mN/m.^1-3

Siloxane-based wetting agents are generally smaller molecules with relatively short siloxane backbones. As shown in Figure 2, they are typically either trisiloxanes (n = 0, m = 1) or have comb structures with very few –S��(���₃)₂–O– units (n) and only a very few organo-modifications (m); generally, n + m is less than 5. Hydrophilicity can be tuned by varying the nature and length of the pendant organic groups and by modifying the ratio of hydrophilic and hydrophobic moieties present in the molecule. In aqueous systems, these amphiphilic siloxanes are driven to the air–water interface and can lower surface tension within a time frame consistent with their molecular mobility. Molecules with longer siloxane backbones can also be rendered oleophobic and may also be driven to the air–liquid interface in nonaqueous, organic-based systems. Siloxane-based wetting agents are most often used to lower the surface tension of aqueous and solventborne, as well as high-solids and 100% solids, formulations to improve substrate wetting. Occasionally, these wetting agents are also used to compatibilize insoluble oils—particularly silicone contaminants—to prevent problems such as dewetting and defects.

Siloxane defoamers can also be created using comb siloxane structures like that shown in Figure 2; however, siloxane defoamer molecules are higher in molecular weight than siloxane wetting agents, and they usually have longer
–S��(���₃)₂–O– segments (larger n) as well as more pendant groups (m) that are more hydrophobic in nature (i.e., c = 0 or d > c). A wide variety of molecular structures can be synthesized via formation of Si–O–C or Si–C linkages. Linear siloxanes like that shown in Figure 3 (also known as a, w- or bolaform structures) can be used to create defoamers and deaerators that range in performance attributes as the length of the siloxane backbone and lengths and chemical natures of organo-modifications are varied. In general, defoamers are designed to be incompatible in the surrounding media because this enables them to be driven to the lamellae of foam bubbles where they can spread and destabilize them, causing bubble rupture.

As one increases the length of the siloxane block within a molecule, the interfacial activity of the molecule also increases. Therefore, polyether-modified siloxanes that can improve flow and leveling, increase surface slip, and impact other surface properties are also possible. These surface control additives are generally of moderate molecular weight (between 1,000–20,000 g/mol), and compatibility within the intended liquid matrix is achieved through a diversity of organo-modifications. Thus, surface control additives that function well in waterborne, solvent-based, and 100% active (solvent-free) systems now exist. Three examples of organo-modified siloxane-based surface control additive structures are shown in Figure 4. Additionally, it is possible to crosslink and emulsify or disperse siloxanes to create emulsions or dispersions of even higher molecular weight and more hydrophobic moieties. Unlike the 100% active liquid siloxanes that exist as nanometer-scale polymers, these siloxane emulsions or dispersions exist as micrometer-sized droplets or particles, as shown in Figure 5.

Siloxane-based Surface Control Additives in Coating Applications

Due to their unique interfacial activity, siloxane-based surface control additives have become invaluable tools in the coating formulator’s tool box. Many surface control additives are capable of mitigating surface energy gradients that can exist within a liquid film immediately after application; thus, they can improve flow, prevent retraction, minimize cratering, enable better surface leveling, and ensure a flawless surface appearance. As their surface activity increases, surface control additives can also render special properties to the surface of the coating. For example, depending on their chemical structures, certain surface control additives can impact surface slip, affect haptic properties (surface “feel”), impart scratch resistance, act as antiblocking agents, and even create a release effect on the surface of the cured coating.

The property of “slip” is characteristic of a smooth sliding motion across the coating that results from a reduced coefficient of friction. It is quantified as the force needed to slide a mass across the coating surface. The material properties of both the substrate and the object to be moved are reflected in the static and dynamic coefficients of friction, and the chemical composition of the coating and the interactions arising from it, as well as surface roughness, all contribute. Surface control additives with large polydimethylsiloxane segments and a high degree of surface activity can ensure particularly slippery surfaces. Additionally, coatings with a high slip often feel smooth and silky.⁴

Coatings may resist scratching when the scraping object slips off the surface rather than penetrating the coating film; however, force-dependent scratch resistance of a coating can only be significantly improved if the surface control additive used contains functional groups that do not interact strongly with one another.^4,5 Organo-modified siloxanes, with a high percentage of polydimethylsiloxane domains, exhibit particularly weak interactions, both with each other and with other materials. This can make them ideal for this purpose. Moreover, during the drying process, organo-modified siloxanes continually migrate to the air–liquid interface, producing a lubricating film that significantly reduces the coefficient of friction of the coating. When a cured coating surface has a strong polydimethylsiloxane character due to the use of a surface control additive it is also more likely to resist blocking—adhesion of the dried coating to another freshly coated surface or other substrates. Liles has described the vast differences in, as well as the unique features of, silicone chemistry in two review articles; the reader is encouraged to consult these for additional background information on this fascinating topic. ^6,7

The Need for Recoatability: The Balancing Act

While it is possible to achieve a variety of desired benefits by using siloxane-
based surface control additives that have large polydimethylsiloxane segments, the need for most coatings to be able to be eventually recoated remains. The downside to employing these incredibly hydrophobic and oleophobic surface control additives is the tendency for the resultant coating to have a very low surface energy and to be very difficult—if not impossible–to recoat. Even if a second coating layer can be applied to such a surface, lack of adhesion and the tendency for the second coating layer to show craters are true problems.^6,7

To address these seemingly conflicting requirements, a series of new organo-modified siloxane-based surface control additives has been developed. This article will describe both the chemical attributes and the achievable benefits of these novel surface control additives.

Results and Discussion

Experimental

Three different waterborne self-crosslinking acrylic wood coating formulations were prepared using the formulations presented in Tables 1 through 4. Siloxane-based surface control additives were commercial products sold by Evonik Corporation under the TEGO® Glide brand, and they were used as received. The 35% active wax emulsion (AQUACER® 539) used as a control surface control additive in the formulation in Table 1 was a commercial product supplied by BYK USA. The water-based oil-modified polyurethane clear semi-gloss (MINWAX, Product code: 63020/71032) was purchased from a local retail store.

The coating formulations in Tables 1 and 2 were applied to plain black, sealed Leneta charts that measured 8 5/8 x 11 1/4 in.
(219 x 286 mm) using a #28 wire wound rod. After seven days of drying at ambient temperature and humidity, the coatings were inspected and evaluated for visual defects and ranked on a scale of 1 to 10 with 10 being perfect and 1 having many defects (Figure 6). Defects observed included pinholes, craters, de-wetting, orange peel, and smearing. Gloss measurements at 20°, 60°, and 85° were taken on the coated black Leneta charts using a Micro-TRI-gloss (Cat. No. 4446 Ser. No. 1083656) from BYK Additives & Instruments.

Relative Qualitative Slip was assessed by taping a Leneta chart coated (once) with the formulation containing no surface control additive and a second Leneta chart coated (once) with the comparative coating onto a single glass plate. A clean coin was placed on each chart and the glass plate was lifted relatively slowly until one of the coins slid off. This test was repeated several times to help assess whether the coated surface was truly more slippery than the blank. The coins were switched, and testing was repeated. The blank (no additive) coating was assigned a “0” and scores of “+” and “++” were given for increasing degrees of slipperiness while a “–” rating was assigned to coatings that were less slippery than the blank. Similar ratings were used for Relative Qualitative Smooth Feel. In these evaluations, the technician compared the feel of each coating and rendered a judgment based on her subjective opinion.

Red oak panels were lightly sanded using 220-grit fine sandpaper and they were brushed off with a clean high quality synthetic bristle brush before each application. Three panels were used for the blank formulation: two in the beginning and one at the end. The remaining panels were coated with the formulations containing 0.5 wt% of the surface control additives. The order of the coating process was randomized to reduce the effect of the operator. The brush was dipped into the coating sample and the entire panel was coated for the first coat. After two hours, the panel was lightly sanded, and 1 mL of the coating was pipetted to the panel and evenly distributed using the designated brush in a back and forth motion. This process was repeated after another two hours but 2 mL were added in the final coat. The brush was placed in aluminum foil and tightly sealed between each coat to avoid drying. The panels were left to dry for about five days before scratch and mar testing.

Scratch resistance was evaluated using an internally developed method (Figure 7). A plastic cuvette stirrer (5-in. long with a flat rounded edge) was used to quickly and with moderate pressure scrape the coated wood panel perpendicular to the grain pattern all the way across the short length of the panel and back. This motion was repeated rapidly 10 times and then the panel was inspected for any scratching or marring. Ratings were assigned using the following scale: 0 = no scratching, 1 = mild scratching, 2 = moderate scratching (like blank), and 3 = severe scratching (worse than blank). This method was found to be more reproducible than a fingernail scratch test.

For blocking studies, coatings prepared according to the formulation in Table 4 and containing 0.3 wt% surface control additives were applied as 200 mm wet drawdowns on primed beech panels. The coatings were dried for 5 h at room temperature. Blocking conditions used were: 1.6 kg/cm² with Osimeter, 20 h at room temperature, one hour recovering; two coated wood panels were positioned coating-to-coating.

Design of the New Surface Control Additives

With the ambitious goal of designing organo-modified polydimethylsiloxane surface control additives that could provide the required surface effects while being recoatable, synthetic efforts turned toward the variety of molecular structures achievable with this chemistry. Not only are linear, comb, and branched structures possible, but variations in the length of the siloxane backbone, the position of polyether groups (organo-modification), and the length and chemical nature of these polyether groups are also possible.

To meet all objectives of this study, several key molecular attributes would need to be realized. First, the polydimethylsiloxane segments would need to be large enough to drive the molecule to the coating–air and coating–substrate interfaces in order to provide substrate wetting and adhesion as well as the benefits for which surface control additives are typically used (slip, scratch resistance, antiblocking, etc.). Second, there would need to be a means of compatibilizing the siloxane surface control additive in coatings formulations. In the case of waterborne coatings, one can achieve this by designing a water-based emulsion containing the active surface control additive. However, a 100% active surface control additive that is compatible in both aqueous and nonaqueous systems requires the introduction of a high degree of organo-modification in order to impart this compatibility.

After many iterations within this synthetic space, four new surface control additives were developed; these include three siloxane emulsions (SCA#1, SCA#2, and SCA#3) as well as one 100% active siloxane (SCA#4). The characteristics of these four new materials are shown in comparison to two benchmark surface control additives (SCA#5 and SCA#6) in Table 5.

Evaluations of New Surface Control Additives in Waterborne Wood Coatings

To better understand the performance of these new surface control additives, they were evaluated in several different waterborne wood coating formulations. These included a commercial water-based oil-modified polyurethane clear semi-gloss as well as three internally prepared waterborne wood coating formulations—two clears and one pigmented—based on different self-crosslinking acrylic resins (see Experimental). It should be noted that the original objectives for each study differed; some were simple screens of the new additives with minimal comparison to other surface control additives, while others were more thorough comparative studies. As a result, SCA#5 and SCA#6 were not evaluated in all systems and some of the surface control additives were evaluated at fewer use levels than others.

The first study undertaken was a screen of the surface control additives at 0.25 wt%, 0.5 wt%, and 0.75 wt% as post-additions to a commercial do-it-yourself (DIY) wood coating.�� Results for coatings applied to Leneta charts are shown in Table 6 and those for coatings applied on red oak are shown in Table 7. Clearly, all the surface control additives at either 0.5 or 0.75 wt% significantly improve the appearance of the initial coating; however, SCA#1 and SCA#2 impart considerable hydrophobicity to the coating surface and the appearance of the second coating layer is very poor, resulting in moderate dewetting. All the surface control additives increase the gloss as well as the relative qualitative slip and smooth, slippery feel. They all improve scratch resistance of this coating when post-added at 0.5 wt%, and SCA#1 and SCA#2—the most incompatible additives—provide the greatest improvement in scratch resistance. Overall, at 0.5 wt%, SCA#6 provides more benefits than the other surface control additives in this formulation; it increases gloss, slip, and scratch resistance of the coating while rendering it recoatable. SCA#3 and SCA#4 at higher use levels also improve the coating’s recoatability.

The second formulation studied was a water-based self-crosslinking modified acrylic clear wood coating based on NeoCryl XK-12 (Table 1). Results of post-adding 0.5 wt% and 0.75 wt% of the surface control additives to this coating are shown in Table 8. At 0.75 wt%, SCA#2 outperforms the other surface control additives with regard to improving blocking and scratch resistance; however, it seems to introduce more foam to the formulation. SCA#1 provides the best appearance when the coating is applied on red oak, and it provides a moderate improvement in scratch resistance at both 0.5 wt% and 0.75 wt%. The 100% active organo-modified siloxane SCA#4 appears to be too compatible in this coating formulation; therefore, it is not able to significantly improve scratch resistance of the coating when used at use levels up to 0.75 wt%.

The third coating formulation studied was a water-based self-crosslinking modified acrylic clear wood coating based on Alberdingk AC 3630 (Table 2). Results of post-adding 0.3 wt% and 0.5 wt% of the surface control additives to this coating are shown in Table 9. Again, 0.5 wt% SCA#1 and SCA#2 provide the best scratch resistance, while SCA#3 hurt scratch resistance at 0.3 wt% and did not improve it at a 0.5 wt% use level. While SCA#4 was only tested at 0.3 wt% (the same actives level as was delivered by using 0.5 wt% of the three emulsions), it did not improve scratch resistance at this use level.

The final coating studied was a water-based pigmented acrylic wood coating based on Acronal LR 9014 (Table 4). Figure 8 shows the results of blocking tests performed using primed beech panels that had been coated (200 mm wet) using coatings containing 0.3 wt% active surface control additives. The Competitive SCA is an ultra-high molecular weight silicone dispersion in water that is promoted as a slip and antiblocking agent for waterborne wood coatings; it was used at 0.5 wt% in order to deliver 0.3 wt% actives on total formulation.

The effects of the new 100% active organo-modified siloxane surface control additive SCA#4 on recoatability in this pigmented water-based acrylic wood coating were also studied.�� A comparison of the cross-cut adhesion test panels is shown in Figure 9. At 0.5 wt%, SCA#4 shows excellent recoatability and no delamination of the second coating layer. However, the panels prepared using the coatings containing 0.5 wt% SCA#5 and 0.5 wt% Competitive SCA both show some delamination in regions near the cross cuts.

Discussion

This article has uncovered a few findings that should be mentioned. First, SCA#5 is extremely hydrophobic and proved to be quite incompatible in the two waterborne formulations in which it was evaluated. While SCA#5 can be used to provide a high-surface slip and silky feel, it is difficult to recoat with a waterborne coating; therefore, it might be more easily used in waterborne formulations that are intended as topcoats or overprint varnishes, and it is probably very advantageous in solvent-based and high solids coatings where its oleo-phobicity should render it very surface active. If a coating that contains SCA#5 requires recoating with a second waterborne formulation, the second coating should include an organo-modified siloxane designed to provide anticratering benefits. Also, intercoat sanding to improve adhesion and surface appearance should be considered if possible.

The more compatible 100% active organo-modified surface control additives, SCA#4 and SCA#6, can perform well—being sufficiently compatible with and providing significant surface benefits—in certain waterborne formulations. SCA#4 appears to be slightly more compatible than SCA#6 in the commercial oil-modified polyurethane wood coating, and SCA#4 showed better slip and feel at higher concentrations while SCA#6 provided better scratch resistance. With that said, SCA#6 was probably too compatible in the pigmented water-based acrylic wood coating because it was unable to significantly improve blocking. SCA#4, on the other hand, was able to provide antiblocking when used in the same formulation.

A comparison of the three new siloxane emulsion surface control additives has shown that, in general, SCA#1 appears to be most incompatible, SCA#2 a bit more compatible, and SCA#3 the most compatible in the waterborne coatings studied here. The additive that performs “best” in a formulation will really depend on what specific benefits are needed. For example, in a coating that employs a binder that has excellent blocking properties but is susceptible to scratching, a surface control additive that improves scratch resistance but has less of an impact on blocking might be the ideal choice. Other properties such as foam stabilization and anticratering properties may become important in coatings that require perfect mirror-like surfaces. Therefore, the formulator should consider several different surface control additives in order to identify the optimum one for the formulation under development.

Conclusions

Clearly there is no “universal” surface control additive. Each formulation and the application in which it will be used have different demands and the compatibility and properties of a surface control additive need to be matched to the specific system and its performance needs. The ideal surface control additive should provide the optimal balance between system compatibility and interfacial activity that minimizes issues and maximizes the benefits that these powerful tools can provide. Improvements in surface properties, including scratch and abrasion resistance, blocking behavior, and slip and haptic qualities, can be significantly enhanced with siloxane surface control additives. While compatibility and recoatability are significant concerns with this additive chemistry, these concerns can be readily overcome with a properly designed product.

The work described in this article highlights the fact that surface control additives can be engineered to provide a range of benefits in waterborne coating formulations. Of the four new surface control additives introduced in this work, two of the emulsions of crosslinked, high molecular weight polydimethylsiloxanes (SCA#1 and SCA#2) were found to provide very good scratch resistance while being less compatible in the water-based wood coatings. Interestingly, the third siloxane surface control additive emulsion (SCA#3) proved to be more compatible, resulting in very good surface appearance and haptic feel but little scratch resistance in the four wood coatings used in this study. While probably not the ideal characteristics for a wood coating, these properties are sought after in other coatings applications like leather coatings and graphic arts. The new 100% active organo-modified siloxane has good compatibility in the waterborne wood coatings studied as well as in other coating formulations. As a result, coatings containing SCA#4 may be recoatable and properties like antiblocking may be achieved in certain systems.

Clearly, siloxane-based surface control additives offer the formulator useful tools that can be employed to create high-performance coatings. This work has merely scratched the surface of what is undoubtedly an area that deserves further exploration, and additional fundamental studies to better elucidate chemical structure-property relationships are warranted.

References

1. Owen, M.J., “Interfacial Activity of Polydimethylsiloxane,” in Surfactants in Solution, 6, Mittal, K.L. and Botherel, P. (Eds.), New York: Plenum Press, 1557–1569, 1986.
2. Hill, R.M., “Siloxane Surfactants,” in Specialist Surfactants, Robb, I.D. (Ed.), London: Chapman & Hall, 143–168, 1997.
3. Hill, R.M., Silicone Surfactants, New York: Marcel Dekker, Inc., 1999.
4. The Big TEGO: Products—Services—Data Sheets, 4th ed., Evonik Industries AG, Essen, Germany, 68–75, 2014.
5. Chen, H.H., “Scratch Resistant Low Friction/Low Surface Energy Coating for Silicon Substrate,” J. Appl. Polym. Sci., 37, 349–364 (1989).
6. Liles, D.T., “The Fascinating World of Silicones and Their Impact on Coatings: Part 1,” CoatingsTech, 9, No. 4, 58–66 (2012).
7. Liles, D.T., “The Fascinating World of Silicones and Their Impact on Coatings: Part 2,” CoatingsTech, 9, No. 5, 34–46 (2012).

CoatingsTech | Vol. 17, No. 3 | March 2020

Introduction

When it comes to color testing, there are many precise instruments, color correction software packages, automated spray machines, etc., in the marketplace. Colorimetry is a science of great importance to the paint industry, especially in meeting the high-quality standards of customers. However, the high scientific level and the precision of the instruments cannot compensate for the challenges presented, because conventional color testing takes place at the very end of the paint production process and requires a great deal of time due to the drying process. In addition, it has become state-of-the-art to correct colors instead of producing them in the required quality in the first run.

Liquid Color Measurement: An Approach for Color Measurement of Liquid Paint and Pigment Preparations

The liquid color measurement (LCM) system discussed here is based on a machine layout, where an opaque liquid (paint or pigment preparation) is put on a moving part and measured touchless with a spectrophotometer. In a three-angle layout, this is done on a rotating disk and the measurement is carried out at 25°, 45°, and 75° angles (Figure 1a).��In a one-angle layout, the liquid is placed on a rotating cylinder, and the measurement is carried out at a 45°angle (Figure 1b).

The setups are characterized as follows:

1. The liquid is held in a reservoir (approximately 30 mL), from where the paint is permanently re-mixed and the film is freshly applied continuously on the moving part (disk or cylinder).
2. A film thickness of approximately 800 mm allows for an opaque film for several materials.
3. The optical components of the spectrophotometer measure touchless through the air, which avoids influences, such as glass barriers, that are experienced with other methods, such as with cuvettes or glass probe measurements.

The test process, including the cleaning of the equipment, takes less than three minutes. The integrated spectrophotometer is calibrated automatically without any need for manual calibration.

Integrated software controls the equipment and generates, stores, and evaluates the measured data. All values are stored as spectral data in an SQL database, either locally or hosted on the intranet or on a cloud system.

Color Metrics

Measuring color in liquid has several characteristic aspects, including the correlation of liquid and dry measurements, the repeatability of measurements, and usability.

Correlation between Liquid and Dry Measurements

If a supplier and a customer define a color to be used, it is related to the color of the dry film. Color metrics on a liquid film, as generated by the LCM system, can be used as a relative measurement setup for the detection of differences between a standard and a sample of a paint system.

Figure 2 shows the correlation of measured differences in the L*, a*, b* space of liquid and dry samples against a (liquid and dry) standard in a production setup with correction steps 0 (initial batch) to 8. The black line represents the differences in liquid against the liquid standard, and the red line represents the differences in dry against the dry standard. Several tests have proven that a liquid measurement shows differences that are equal or similar to what a dry measurement would show.

Repeatability and Reproducibility

Usually, repeatability of a spectrophotometer is specified on a white tile. This represents the repeatability of a measurement, not a test process. The setup of an LCM consists of a spectrophotometer and a machine. Carrying out measurements on an LCM device is not limited to the measurement of a color; it also includes a part of the test process. A conventional, i.e., dry test process, for a color sample consists of the following steps: sample taking, sample preparation, application, drying, eventually tempering, and the measurement. With LCM, the process is streamlined so the application, drying, and tempering steps are not necessary.

Sample taking and sample preparation are the same in both liquid and dry testing methods. The application, drying, and tempering process adds variance to the color of the sample, which can be generated by flocculation, sedimentation, or—in case of effect pigments—orientation of the pigments depending on the drying conditions. The dry testing process adds deviations from the pigmentation of the liquid paint and deviations of the application process.

The application of the LCM is limited to handling the material in a liquid condition and generating a liquid film. This influence is minimized due to its low complexity, low shear, and constant conditions, such as the speed of the disk/cylinder and the time between the filling and the measurement.

Table 1 shows an example of a silver metallic paint, where 12 samples were prepared. Out of the same sample, four panels were generated and measured three times each. From the same sample, 12 measurements were carried out in an LCM device. In this example, the repeatability of the liquid test process was down to 1/10 of the repeatability of the dry process.

The repeatability depends on the material characteristics of the paint system. Rheology and interactions of the colloids can influence the repeatability. The values, which have been found in white liquid paint systems, range between dE = 0.01 to dE = 0.04 as an average value of three measurements, including cleaning and refill of the material.

Color Strength

Color strength is a relative measure and is used in the paint industry as a quality index, e.g., for tinting pastes. There are several formulas available, as described in EN ISO 787-24.

In a production scenario, the information about color strength of intermediates can help to reduce color variations of the finished products.

The test method (color measurement) and the mathematics for calculating a color strength value show a good correlation to the concentration of a tinting paste in the formula. If such a correlation can be mathematically described, it could be used to compensate for color strength variations in a mixed paint by reducing or increasing the amount of tinting paste against the original recipe, which is usually formulated with a color strength of 100%.

In a study, the following questions were addressed:

• Is there a possibility of describing a linear correlation between the color strength of a tinting paste and its concentration with a weighted function based on the spectrum?
• How does the LCM perform against a drawdown process, which is usually used for color strength testing?

Linear Correlation between Color Strength and Concentration of a Tinting Paste

According to DIN EN ISO 787-24, color strength is defined in a white reduction. For the study, different concentrations of a tinting paste were generated by mixing the paste in different ratios with white. Table 2 shows the different mixtures of a tinting paste with white. Assuming that there is a linear relationship with a slope of 1, there would be a theoretical concentration in percent as shown in “CS_theo.”

Two different methods were used for the measurement:

Drawdown application, drying, tempering, and measuring with a conventional laboratory spectrophotometer

LCM with direct measurement of the liquid mixture

The mixture 5.00 g/95.00 g was set as the standard as the reference to the relative color strength calculation. The calculation itself was made with the standardized color strength calculation formula according to DIN EN ISO 787-24.

In a second step, the differences between the measured color strength and the theoretical (expected) color strength were compared in a graph. For example, the expected color strength difference of the 5.25 g/95.00 g mixture would be 5%.

Figure 3 shows the results of the dry process. The red line shows the theoretical plot in the case of an ideal correlation between dCS_measured and dCS_theo. It can be seen that the points are a certain distance from this ideal plot, which leads to a confidence range, shown as green lines.

Figure 4 shows the results for the liquid process. The values are much closer to the ideal line and the confidence range.

In Table 3, the results are compared at the point of a dCS_theo of zero. The difference between the upper and lower confidence interval is 4.65% for the dry process vs 0.85% for the liquid process.

Table 4 shows the minimally acceptable difference. Even if the tolerance of 5% is set to the process, the dry process is not capable with this material.

Effect Pigments and Their Preparation

The three-angle version of the LCM system is suitable for the measurement of liquids containing effect pigments. These can be pigment preparations, e.g., for raw material control, slurries as semi-finished products in a production, or finished paint systems. The multi-
angle capability is due to the design of the system, because the flow conditions between the reservoir and the disk allow an orientation of the particles, which can then be measured by the spectrophotometer in a defined sphere.

This possibility enables paint and raw material producers to test paint systems and pigment preparations without the influence of the application, which is usually very high in connection with effect pigments. As seen in Table 1, the liquid color measurement displays better repeatability than the conventional dry process connected with a spray application.

At the interface between a pigment supplier and a paint manufacturer, the LCM system can help to harmonize the specification of a material. Figure 5 shows an example of an aluminum slurry of batches A and B, where the spectra of the three angles (25°, 45°, and 75°) are shown.

Both batches were specified by the supplier as being the same product quality. Using the LCM system revealed that there was a significant difference between both batches, especially visible in the difference of the reflection in the 25° angle. Without using the LCM system, both batches would have been used in the same way for the same finished product and would have led to much effort for correction.

Black and Dark Colors

The spectra of black and dark colors show ranges of very low reflection. For a spectrophotometer, this leads to a low signal-to-noise ratio and thus to a fluctuation of values at the different sampling points of the spectrum. The LCM system uses a feature called a “black booster” to increase the energy used for the illumination of the sample, which increases the signal-to-noise ratio and decreases the fluctuation at the sample points. Figure 6 shows this in the three angles of the measurement of a black paint, comparing the values with and without using the black booster.

Stability Test

Using the LCM system can also indicate the stability of a paint system or a pigment preparation. For such a test, several measurements are repeated while the film is left on the rotating part (disk or cylinder). In normal operation, the paint is removed from this rotating part after each measurement.

The color drift, which is calculated as the dE* against the first measurement of the test, is an indication of how stable the material is formulated. This stability is not specified as a specific physical behavior, it can be caused by flocculation, separation, and other phenomenon, which disturb the distribution of colloids in the matrix. In practice, this method is used as a differential method for the formulation of paint, e.g., in connection with different surfactant types.

Figure 7 shows the different compositions of a pigment paste when tested. The only difference between the samples was the surfactant. Obviously, the mixture represented by the blue line was the worst formulation. However, the black and green lines show significant differences as well.

Process

The LCM system reduces the amount of time for color testing. In a batch production, the testing time for color equals production time, because the next step in the process depends on either the approval for release or correction instruction. A reduction of testing time leads to a reduction of the cycle time of a batch production process.

In many production scenarios, a parallel production of batches takes place. Batches are initiated, stirred, and mixed, and in the case of mobile tanks, taken away from the stirrer. This process is repeated with more batches and several mobile tanks are handled in the shop floor.

A significant reduction in the testing time leads to a “one piece flow” operation of the production, which means that initiated batches are manufactured in one process instead of several parallel processes. Thus, using a quicker test method not only reduces the cycle times by a total reduction of the test time, but also changes the production sequence from parallel to serial production.

The possibility of standardizing semi-finished products reduces the total amount of corrections, because the initial batch is already closer to the target if each of the used components is closer to a standard.

The LCM system leads to a broader use of color testing in the supply chain, as illustrated in Figure 8. Starting from the testing of pigments, it also helps to test semi-finished products and, finally, finished products.

This leads to a reduced color variance that can occur in the different steps in the supply chain.

Prospects

The LCM system is a tool for manufacturers at different steps of the supply chain—reducing testing time with a possibility for process optimization. It enables a harmonization of quality standards in these steps and accelerates the communication and management of the supply chain process. A data exchange can be managed, e.g., by a cloud system, which supplies data for the IT systems of the manufacturer.

In addition, faster generation of color data in different production steps supports the idea of a big data analysis, which finally helps in understanding the process and leads to improved tools for process control.

Together with software tools, the system contributes to the possibility of a “smart” paint factory. This is a vision of paint manufacturing in the future, where all value-added processes, including manufacturing and customer service, are characterized by a high degree of flexibility and self-learning processes. The latter, as a discipline of artificial intelligence (AI), requires a high degree of digitalization of all relevant processes plus valid data. Using the LCM system is a suitable approach for generating precise and relevant digital data, with the possibility for feedback deviations into a control system for manual or software supported corrections in the beginning of the process. For the future, it will be suitable for generating data for training a machine learning algorithm, which will help in avoiding errors and deviations.

Liquid Color Measurement—A Benchmark

Several attempts have been undertaken to build devices that make liquid color measurement possible. Colorimetric methods are state-of-the-art in chemical analytics, where the color of a (transparent) liquid functions as an indicator of the concentrations of chemicals. Using these setups for opaque paint systems causes some problems, because there are several physical effects which reduce the repeatability of such a measurement.

There are four basic methods available as technical solutions, as follows (see Figure A):

1. Touchless, Dynamic Measurement:

In the touchless, dynamic measurement setup (e.g., the LCM described in this article), a film is applied on an applicator (disk or cylinder) through the slit of a cup. During the measurement, this cup is held on the applicator, and the liquid is remixed in the cup during the rotation of the applicator. In the case of a disk as an applicator and laminar flow conditions in the slit of the cup, aspheric pigments like metallic pigments or pearls are forced into the direction of the rotation. A spectrophotometer measures the color of the film at the surface in a touchless way. In the case of effect pigmented liquids, a multi-angle measurement is possible. By repeating the measurements over a certain time while the material stays on the applicator, it
is possible to quantify the stability of a paint system.

2. Touchless, Static Measurement:

Setups are normally a glass cup, where the liquid is filled in, and a spectrophotometer, which measures the color from the top in a touchless way. When compared with method #1, the disadvantage is that in mixtures of different pigments, the composition on the surface changes because of settling or coagulation or flocculation. Especially for mixtures of pigments with very different density, this process takes place very quickly. This method is not capable of measuring any effect pigmented liquids.

3. Measurement through a Medium:

There are systems available where a glass cup is put on a hole of a spectrophotometer, which measures the color from the bottom. This method has the same deficit as #2 due to the change in pigment composition at the surface—in this case, at the bottom of the cup. The refraction difference between the liquid and the material of the cup (normally glass) has an influence on the color reading. However, this can usually be considered as constant. A more significant influence comes from the tendency of some pigments to have a selective adhesion to the material of the cup (normally glass). This means that in a mixture of different pigments, one pigment has a higher adhesion than the other(s).

4. Measurement by Dive Probe:

This method was taken from the plastic industry, where color measurements are carried out in extruders. A probe with a glass lens at the end is used as the light source and the sensor for the spectrophotometer at the same time. It can be used for inline measurements of solid colors, e.g., in the batch or in pipes, but the repeatability suffers from the adhesion effect (as in method #3) and the cleaning needs a great deal of effort. Additionally, the method requires a high investment and can only be installed at one production aggregate at a time.

Method # 1 is the only one capable of measuring paint systems with effect pigments.

CoatingsTech | Vol. 17, No. 2 | February 2020

BACKGROUND

In a seminal paper, Asbeck and Van Loo¹ described the importance of pigment volume concentration (PVC) on many of the performance aspects of coatings. These include color, gloss, and as studied in this work, corrosion and humidity resistance. While PVC is of fundamental importance, it is the ratio of coating PVC to the critical pigment volume concentration, PVC/CPVC, referred to as the reduced pigment volume concentration and denoted as λ, that is the parameter of most interest in highly filled films.^2,3 CPVC is directly related to the oil absorption of the pigments utilized in the coating and this relationship may be used to calculate an empirical CPVC.⁴ The calculation is simple and with today’s coatings software the value of λ may be displayed among the coating physical properties on the computer screen as the formulator considers variations in composition. This empirical value is subject to some inherent error. Oil absorption values are determined for pigments dispersed at closest (tetrahedral) packing, while in paint films, pigment is packed in a random configuration that includes the more widely spaced cubic packing. Further, oil absorption is derived for individual pigments; the calculation cannot allow for packing of pigments of dissimilar size. With sufficient knowledge of the pigment particle size distribution a pigment packing factor can be calculated. However, obtaining this information can be problematic and the associated mathematics become cumbersome.^5,6 Finally, a spatula rub-up oil absorption determination (itself subject to considerable error) wets the particles with a thin film of low molecular weight linseed oil. In formulated coatings the spacing between pigments is influenced by the considerably higher molecular weight of synthetic polymers and/or latex particles. It is this aspect of pigment packing and its influence on PVC/CPVC that is a key difference between solution and traditional latex polymers. As a general rule, the Tg and particle size of a vinyl, (meth) acrylic, styrene, or other emulsion polymer are determining factors in the film former’s ability to allow close pigment packing. High Tg particles cannot deform readily to allow close pigment packing. Similarly, large latex particles do not allow for the pigment packing density of smaller particles. That these polymers are essentially solid at room temperature is a key consideration in this lack of deformability. A binder index or latex binder efficiency factor4 describes this relationship as CPVC actual/CPVC calculated. This index is always less than 1.0; values in the range of 0.70 are common for architectural lattices. Alkyd latex polymers have a Tg of 0–4°C and a minimum film forming temperature at or below 1°C. These polymers are liquid at room temperature and build Tg and molecular weight post-application through auto-oxidation reactions with molecular oxygen. This liquid state allows considerable deformation in response to the osmotic pressures associated with film formation. The literature is rife with methods to determine the CPVC for a given coating composition. Measurement of pore volume via very low angle microtome sampling and fluorescence microscopy is described by Wang et al.⁷ Tensile strength measured on free films as well as changes in color and gloss are also possibilities, but each of these determinations requires considerable time and effort. An expedient and practical method is described by Asbeck that takes advantage of the porosity of films above CPVC and is preferred for this work.⁸ This film porosity method is referred to as a salt-water spot test in this article. The CPVC is a function of the property being measured and thus the critical point for corrosion resistance may not be the same as the CPVC for fracture strength, hiding power, permeability or other parameters.⁹

EXPERIMENTAL

Two alkyd latex polymers were used for the majority of this work. Polymer A was specifically designed for use in direct-to-metal primers. Development of this polymer and the associated paint formulations was described previously.¹⁰ The key performance attributes for Polymer A were wet adhesion to metal and stability in the presence of anticorrosive pigments. Polymer B was specifically designed for road markings; its development was detailed by Dziczkowski at the American Coatings Conference in 2014.¹¹ A key performance attribute of Polymer B was sufficient shear stability to tolerate addition into a pigment grind and subsequent processing in high-speed dispersion equipment. Polymer A was formulated as a metal primer as shown in the generic Formulation 1. Pigment substitutions were made on an oil-absorption basis to arrive at the desired λ value for each experimental series. Dispersant/surfactant levels were adjusted for each formulation based on the total pigment surface area. Each formulation was adjusted to ca. 41% volume solids. The use level of an associative thickener was varied as needed to produce finished coatings with a viscosity of approximately 90 KU. Where used, styrene-acrylic direct-to-metal latex polymers were substituted on a weight solids basis and the appropriate coalescing solvent(s) were added. Note that when using even the lower Tg styrene-acrylic latex, the coating VOC exceeded the 100 g/L limit for rust-preventative coatings set by the South Coast Air Quality Management District (SCAQMD). Polymer B was formulated into white road marking paint per Federal Specification TT-P-1952 E. This formulation, Formulation 2, used one pound of TiO2 per gallon and the required loading of calcium carbonate to achieve the specified 60% pigment by weight. The coating was 60% volume solids and was formulated with a blend of methanol and water for a maximum VOC of 150 g/L. The specification allows for the coalescent required for pure acrylic latex polymers as part of the VOC, but this was replaced with water in the model formulation as alkyd latexes do not require film-forming aids. Note the formulated VOC of 75 g/L. Metal primers were evaluated using films of nominal 37.5 µm thickness applied to cold-rolled steel panels. Films were air dried for seven days prior to testing. Exposure to salt fog was per ASTM B117 and exposure to humidity was per ASTM D 1735. The ratings for degree of blistering and scribe creep were per ASTM standards. Permeability was carried out in triplicate with cups of 25 cm2 using free films of nominal 87.5 µm (3.5 mils) thickness measured prior to testing. Per ASTM 1653 protocol, films were applied as two coats to release paper. Exposure was done in a walk-in chamber maintained at nominal 25°C and 50% RH. Most of this work utilized the wet-cup method with 10 ml of water placed in the cup below the film. Testing films at equilibrium against the 100% relative humidity environment afforded by the wet-cup method was considered more representative for evaluating films to be used in metal protection. Data reported in the tables was for this wet-cup method unless otherwise noted. For comparative purposes some films were tested using the dry-cup method; desiccant was placed in the cup below the film to allow equilibration at 50% relative humidity from the test chamber. Given the consistent dry film thickness utilized in these studies, the data was reported as mg of water lost/hour; this is denoted as “perms” for simplicity in the text. As noted previously, the fast method of Asbeck was one method used to determine if a given pigmentation exceeded CPVC. In this test, the film was applied to the lower half of a steel panel and allowed to dry. A spot of 3% salt water was placed on the film and the electrical resistance was measured through the film. Films above CPVC will rapidly absorb salt water and the resistance will drop by several decades in a matter of minutes while films below CPVC will maintain much of their near-infinite resistance for extended periods of time. A standard volt-ohm meter with 1.5 VDC potential was used for resistance measurements between the film and the uncoated area of the panel.

RESULTS AND DISCUSSION

One part of this work involved a project to develop novel alkyd latex polymers for metal protection that offer very low VOC and contain bio-renewable resources. These polymers must produce coatings formulations below 100 g/L VOC to meet the requirements of the SCAQMD. Note that VOC is calculated differently in the United States than in the EU. The VOC reported as 16.8 g/L in the red oxide formula (Formulation 1) is per U.S. protocols where water is excluded in the volume of paint. The VOC of this coating is 9.0 g/L per the EU calculation where water is included in the volume of paint. The coatings used to evaluate experimental polymers were formulated based on the industry’s understanding, that metal primers exhibit the best performance when pigmented in the vicinity of the critical point.4,9 An initial set of formulations was made to determine the relative pigment binding ability of the preferred experimental polymer. A formulation made with red iron oxide, anticorrosive pigment, and calcium carbonate as the extender was used to vary the PVC. Films were cast and tested via the method of Asbeck for pigment loading relative to the critical point. Films cast on metal were subjected to both salt fog and humidity exposure. Data are presented in Table 1. The data show good correlation between λ and performance. Porosity as measured by permeability and the salt water spot test suggest that CPVC is around 0.95. The critical point for a film is a function of the property being tested. Blister resistance in humidity testing improves above the critical point as water is able to be transported through the film without building hydrostatic pressure. The converse is seen in corrosion resistance testing where films above the critical point allow the ingress of water leading to rapid failure. These tests suggest a critical point between 0.90 and 0.95, the same range as found by measures of permeability. Note that the failure mechanism in humidity testing of the film at λ=1.0 was corrosion, not blistering. Blistering seen in the humidity resistance of the red oxide primer reported in Table 1 was unacceptable. Subsequent formulation studies indicated that inclusion of platy talc provided the desired performance. The model direct-to-metal formulation (Formulation 1) was followed with respect to the loading of prime pigment, anticorrosive pigment and talc. The extender pigment loading was varied to achieve the desired PVC and λ value. Optimum performance was found at λ=0.80-0.85, sufficiently below critical to preclude loss of film integrity. Further, performance was acceptable through a range of PVCs and with multiple pigmentations. Representative data are provided in Table 2. The data demonstrates the robustness of the polymer in its ability to bind pigment. Film permeability remained low across the tested range of pigmentation in red oxide primer formulations and indicates that, as was seen in the PVC ladder reported in Table 1 and relative to the white enamel, decreases with increased PVC up to the critical point. Performance failure of the white enamel was due to the lack of proper pigmentation and is shown to further illustrate the PVC/permeability relationship. An improvement in corrosion resistance was noted for coatings containing calcium silicate in conjunction with calcium carbonate. Two mechanisms for this result are offered: improved pigment packing due to the small difference in particle size/shape and the more alkaline nature of calcium silicate relative to calcium carbonate. Figure 1 is a 336-h salt fog exposure for the red primer containing talc and CaCO₃. The need to include talc for optimal humidity resistance was one in keeping with most of the supplier-published starting formulations for metal primers. Comparing perms at similar λ from Tables 1 and 2 suggests a small, if any, change in permeability with the inclusion of talc. Three formulations to evaluate the influence of talc on performance were made. The test data are summarized in Table 3. Perms were consistent with the data collected in earlier experiments. Permeability testing for metal primer pigment combinations showed a small variation with changes to the pigmentation. This is consistent with the thermodynamics of water movement through a film. Films have an intrinsic porosity due to the random nature of polymers. Molecular vibrations create pores through which small molecules like water can move. The hydrophobicity of the polymer plays an important role in water uptake; the polymer’s structure is largely responsible for the quantity of water a film will absorb. The relative humidity in the permeability cup (100% for the wet cup method) in conjunction with the nature of the polymer determines the equilibrium water content at steady-state conditions. These factors result in the small variation in permeability versus pigmentation type at equal λ value. This concept was demonstrated by using the dry-cup permeability method. In this method the cup was filled with desiccant and the film reached steady-state conditions at 50% relative humidity from the air space above the cup. Films from the PVC ladder presented in Table 1 were used to compare these test methods; data are given in Table 4. This shows that polymer structure controls steady-state moisture content, PVC is a measure of polymer content in the film and controls transport. The data in Table 3 show that with calcium carbonate as the sole extender the film developed few blisters through 336 h of testing. The degree of blistering was rated as very few in 96 h, similar to the results reported in Table 1. Inclusion of talc reduced or prevented blistering, and this result cannot be solely attributed to the difference in permeability. The harsh environments utilized in salt fog and humidity testing force much higher concentrations of water into the films. Under these conditions the pigments exhibit a very strong influence on water transport. Particles with a strong affinity for water will limit movement by creating an energy barrier. Talc binds strongly to water ⁹ and the platy nature of the particles creates a long and tortuous path for water movement. Thus, while the permeability of these films is similar at the lower equilibrium water concentration found in this test, performance under saturated conditions is not. Talc serves to limit the transport of water to the substrate, in turn preventing the buildup of hydrostatic pressure and the resultant blisters. Calcium carbonate is slightly soluble in water (0.0013 g/L) and its alkaline nature may aid in corrosion protection, but this was not sufficient to prevent the buildup of black corrosion products and/or blisters at the film/substrate interface. Given the excellent pigment binding demonstrated for these alkyd latex formulations at high pigment loading, a comparison to styrene-acrylic latexes that are traditionally formulated to much lower PVC was dictated. Two in-house styrene-acrylic lattices designed for metal protection were formulated using the same pigmentation as shown in Table 1. The formulations were as similar to the alkyd latex as practical; the key difference being the use of coalescing solvent required by the higher Tg latexes. Data are presented in Table 5. Given the failure of the styrene-acrylic polymers in this experiment, the work was repeated through a narrow range of PVC/λ with the lower Tg styrene-acrylic latex. In addition to the salt-water spot test, the films were evaluated for wet-cup permeability. Data is presented in Table 6. Perm values for the styrene-acrylic latex are similar to the alkyd latex for films near the critical point, in this case at λ=.40 to.50. As PVC increases above CPVC, film permeability increased as expected. The change from λ=0.50 to 0.60 suggests that the latter is above CPVC, and λ=0.55 from Table 5 is most likely just above critical. The data for the styrene-acrylic latex indicates a reduced pigment binding efficiency relative to the alkyd latex and provides insight into the traditionally low PVC of formulations using this polymer chemistry. The much smaller particle size of the products was not sufficient to allow these hard, very high viscosity polymers to readily deform and allow for close pigment packing. A second ongoing program was to develop alkyd latex polymers specific to road marking paints. A VOC under 100 g/L along with bio-renewable content was desired. Formulations for existing products based on pure acrylic latex per Federal Specification TT-P-1952-E were used to evaluate experimental alkyd latex products; in this article Polymer B was used. Control coatings were made with a commercially available acrylic latex meeting the requirements of the Type 2 paint, the so-called “quick dry” product. The formulation was presented as a Model White Road Marking Paint. Note that the pure acrylic latex formulation requires a coalescing solvent; this was eliminated in the alkyd latex formulation and replaced with water. Also note that methanol was used for fast evaporation and freeze–thaw stability, not film formation. The Federal Specification required certain minimum performance standards in a battery of tests including dry opacity and water resistance. Dry opacity was measured from films of a specific thickness applied over black and white test charts. Reflectance was calculated as Y (black)/Y (white) with a minimum of 0.92 required. Water resistance was tested on a film applied to concrete. These were air-dried for three days and then soaked in deionized water for 18 h. No blistering, adhesion loss, or excessive softening was allowed. The data from comparisons of alkyd latex Polymer B and the pure acrylic latex from this and other testing are given in Table 7. The salt water spot test and permeability data show that the alkyd latex is below CPVC while the pure acrylic latex is above critical. The relationship of these two coatings relative to CPVC is also evidenced in the performance data. As a formulation exceeds CPVC air voids develop in the film; these are additional sites for light refraction and thus generate the increase in contrast ratio shown in the data for the acrylic latex. As noted previously, films above CPVC are not prone to blistering in humidity and water-soak tests; ready transport of water allowed blister-free performance for the pure acrylic. The alkyd latex relied on film integrity to prevent blisters. Retained water did cause some minimal softening, but this recovered in a matter of minutes after exposure, as is allowed in the test protocol. Given two films below CPVC, the significant difference in perms for Polymer B in a white road marking paint versus that seen in Table 1 for Polymer A in metal primer pigmentation may be attributed to two factors. Most importantly, the metal primer latex is considerably more hydrophobic,10 so does not adsorb as much water. Also, the pigmentation is very different; the road marking paint uses a very high concentration of large particle size calcium carbonate. This does not allow for efficient pigment packing and the hydrophilic nature of calcium carbonate is considered influential. While pigmentation was not seen as the dominant factor in film permeability in red oxide primer formulations, the smaller particle size and lower concentration of the calcium carbonate in this formulation, in conjunction with the hydrophobic red iron oxide and anticorrosive�� pigment, produces a pigmentation that is tightly packed and inherently less permeable to water.^2,6

CONCLUSION

The relationship between PVC and CPVC is dependent on both the binder and pigments. The fluidity of the binder is a key factor in discussions of influence on CPVC with viscosity and molecular weight being the critical parameters. The data presented demonstrate that the liquid, or fluid, nature of alkyd binders allows for higher pigment loading than their extremely high viscosity acrylic counterparts. This pigment binding ability is not compromised when the alkyd is supplied as a surfactant stabilized latex rather than as a solution in solvent. The critical point for alkyd latex polymers can be determined by any of the classic measures.

References

1.���� ��Asbeck, W.K. and Van Loo, M., “Critical Pigment Volume Relationships,” Ind. Eng. Chem., 41 (7) 1470-1475 (1949). 2.���� ��Bierwagen, G.P. and Rich, D.C., “The Critical Pigment Volume Concentration in Latex Coatings,” Prog. Org. Coat., 11, 339 (1983). 3.���� ��Bierwagen, G.P. and Hay, T.H., “The Reduced Pigment Volume Concentration as an Important Parameter in Interpreting and Predicting the Properties of Organic Coatings,” Prog. Org. Coat., 281, 3 (1975). 4.���� ��Patton, T.C., Paint Flow and Pigment Dispersion, John Wiley and Sons, 1979. 5.���� ��Lee, Do Ik, “Packing of Spheres and its Effect on the Viscosity of Suspensions,” J. Paint Technol., 42 (550), 579 (November 1970). 6.���� ��Bierwagen, G.P. and Saunders, T.E., “Studies of the Effects of Particle Size Distribution on the Packing Efficiency of Particles,” Powder Technol., 10 (1974). 7.���� ��Wang, J., Xu, H., Battocchi, D., and Bierwagen, G., “The Determination of Critical Pigment Volume Concentration in Organic Coatings with Fluorescence Microscopy,” Prog. Org. Coat., 77, (12) 2085 (2014). 8.���� ��Asbeck, W.K., “Critical Pigment Volume Concentration Measurements, a Very Fast Method,” CoatingsTech, 2 (12) 2005. 9.���� ��Wicks, Z. Jr., Jones, F., Pappas, S.P., Wicks, Z.W., Organic Coatings Science and Technology, John Wiley and Sons, 2007. 10.���� ��Danneman, J.H., “Optimizing Anti-Corrosion,” European Coat. J., 12 (2014). 11.���� ��Dziczkowski, J.,“Breakthrough Waterborne Technology Brings Alkyd Back to the Road,” American Coatings CONFERENCE, April 2014. This article was reprinted from the September 2015 issue of China Coatings Journal (CCJ), published by Sinostar-ITE Int’l Ltd., Hong Kong.

by Partha Majumdar, David Conner, Yuanqin Liu, Carol Hawkins, Gary W. Dombrowski, and Paul Doll, Dow Coating Materials

From flat to eggshell to semi-gloss, architectural paints have made incredible gains in ultra-low VOC paint performance, but achieving a smooth, hard, glossy finish without the help of solvent has been much more elusive across the color spectrum of deep tones. In these polymer-rich paint formulations, ultra-low VOC options typically employ a soft polymer or a hard polymer with a nonvolatile coalescent. Both result in poor block and tack resistance. As titanium dioxide- and extender-rich neutrals and pastels make way for bolder, brighter accent colors, there is growing consumer demand for deep-tone paints that go on smoothly, dry quickly, and maintain a glossy, hard finish without cracking. A controlled polymer morphology has been developed which employs hard, soft, and functional monomers in a single waterborne acrylic binder.* When compared to both low-VOC (< 50 g/L) and ultra-low VOC (< 5 g/L) commercial binders, test results demonstrate that this polymer technology offers improved film formation, hardness, and tack and print resistance when used in ultra-low VOC, deep-base paints.

*Dow Coating Materials

Hard Facts about Soft Binders

Neutral tones have dominated the interior color spectrum for many years, with gray scoring back-to-back honors as the top paint color of 2014 and 2015 in surveys by the Paint Quality Institute (PQI). Today, fashion forward trends are calling for more vibrant interiors: purples, reds and other saturated hues are topping the list of recommended colors for accent areas such as doors, railings, window trim, furniture, entryways, and cornices. These deep tones are notably absent of titanium dioxide (TiO2), the primary white pigment in neutral through mid-tone paints.

Titanium dioxide, and to a lesser extent calcium carbonate, are used to provide hiding through light refraction. They also contribute to film hardness. Deep and accent tone paints, by contrast, do not contain the aforementioned scattering pigments which can reduce the chromaticity of the paint film. These paint formulations derive most of their film hardness properties from the polymeric binder. When used with a volatile coalescent, high Tg polymers are preferred because they promote faster hardness development and overall harder finishes. Thus, challenges emerge in this chromatic ultra-low VOC space due to the combined lack of volatile coalescent and the absence of pigments to promote hardness.

To lower the VOC content of aqueous paints, a nonvolatile coalescent-like plasticizer may be used in place of volatile ones to enhance film formation; however, these plasticizers do not volatilize out of the paint film and the resulting finish remains soft and tacky, with poor block and print resistance. Using a higher Tg binder without plasticizer generally improves hardness, but takes away from film formation and results in poor scrape and crack resistance. A third option is to use a low Tg polymer. While this alternative generally improves scrape and crack resistance, it does so at the expense of hardness and tack.

The Goldilocks Solution

Research was conducted to achieve an optimal balance between the soft and hard phases during film formation for ultra-low VOC, deep-base paints.�� The research focused on post film formation crosslinking chemistries used in combination with latex particle morphology optimization. Initial experiments were conducted to establish structure-property relationships between binder compositions and formulation variables. Then, predictive models from the experimental designs were used to develop an optimized binder for ultra-low VOC, deep-tone paints.

As demonstrated in Figure 1, the optimized binder is based on controlled morphology that combines hard and soft monomers plus a functional ambient crosslinking monomer (see Table 1 for typical binder properties). When used in low-pigmented and zero-pigmented test formulations, this controlled polymer morphology with functionality exhibits excellent film formation as well as faster hardness development, harder final films, equal block resistance, and improved print and tack resistance compared to commercial low-VOC and ultra-low VOC offerings.

TESTING AND RESULTS

Binder technology based on controlled polymer morphology with functionality was evaluated against three competitive ultra-low VOC (< 5 g/L) binders and a leading low-VOC (< 50 g/L) binder in a deep-base gloss test formulation and a gloss accent base formulation (see Tables 2 and 3 for starting point test formulation details).

Improved Hardness Profile

Hardness properties as affected by binder selection were evaluated over a seven-day period per ASTM D-4366 method for a deep-base gloss paint (4% PVC/30% VS) tinted with 12 oz/gal of black colorant and applied to aluminum panels via 5-mil drawdown bar. As demonstrated in Figure 2, the deep-tone test paint formulated with the optimized binder demonstrated significant improvement in hardness development compared to test paints formulated with competitive binders at equal VOC level�� (< 5 g/L) and a leading commercial binder at higher VOC level (< 50 g/L).

Table 5—Performance of Ultra-Low VOC Binder Technology Optimized for ?Deep-base Paints in 0% PVC/30% VS Accent Base (Tinted with 12 oz/gal of Black Colorant) Relative to Commercial Gloss Binder in a Low-VOC (

Reduced Tack

Tack resistance is a key performance benefit derived from harder paint films. This performance property was evaluated using 5 mil drawdowns of deep-base gloss test paint formulations (4% PVC/30% VS) tinted with 12 oz/gal of black colorant applied to aluminum panels and dried for 24 hr. Cotton balls were placed on each panel and subjected to 15 min of pressure from a 1000-g weight. Resistance to tack was measured by the amount of cotton ball fibers remaining after the cotton balls were removed from the coated panels. As shown in Figure 3 and presented in Table 4, the binder technology based on controlled morphology with functionality demonstrated excellent tack resistance versus competitive binders at equal VOC (< 5 g/L) and the commercial binder at higher VOC (< 50 g/L).

Improved Print Resistance

Print resistance is influenced by film hardness and is particularly important where deep-base coatings are used on window sills, accent tables, shelves, and other horizontal surfaces. Longer-term pressures from flower pots, cups/saucers, and similar household items can lead to permanent deformations in the paint film. This key property was evaluated by a method based off ASTM D-2064 using 5 mil drawdowns of 4% PVC/30% VS paints tinted with 12 oz/gal of black colorant. Formulations were applied to aluminum panels and dried for 24 hr before lids were placed on films with 1000 g weight for 20 hr. As shown in Figure 4, the optimized binder technology for deep-base paints demonstrated excellent resistance against imprinting compared to a test paint formulated with a commercial binder and the high gloss commercial binder.

Superior Scrape Resistance

A paint’s scrape resistance is influenced by both its mar resistance and its adhesion to the substrate. This performance property was evaluated using a scrape adhesion test method based on ASTM D-2197. Five mil drawdowns of 4% PVC/30% VS deep-base gloss paints tinted with 12 oz/gal of black colorant were made on aluminum panels and dried for 24 hr before a controlled metal loop stylus was slid across the films with increasing amounts of weight and film damage was observed and rated. As shown in Figure 5, the binder technology based on controlled morphology with functionality demonstrated excellent scrape resistance far exceeding the other competitive binders tested.

Overall Performance

In addition to extensive testing in low-pigmented paint formulations, the new binder technology optimized for ultra-low VOC (< 5 g/L), deep-base paints was compared to a commercial low-VOC (< 50 g/L) gloss binder in a zero-PVC gloss accent base. As demonstrated in Table 5, the ultra-low VOC binder technology performed as well or better than the higher VOC
commercial offering in gloss and hardness, as well as tack, print, and block resistance.

SUMMARY

A 100% acrylic binder has been developed that offers early hardness development in ultra-low VOC, deep-base architectural paints. The binder technology is based on controlled morphology that combines hard and soft polymers with ambient crosslinking functionality. Test results demonstrate that this binder technology offers world-class scrape adhesion and hardness, better or comparable print resistance to all other binders tested, and better block and tack resistance versus both low- and ultra-low-VOC alternatives. It can be used to formulate ultra-low VOC, binder-rich, highly colored architectural paints that still have excellent hardness, and excellent tack, block, print, and scrape resistance.