By Clifford K. Schoff, Schoff Associates

This is my last Coatings Clinic, after 17 years and 160 articles on surface defects and other aspects of coatings. I am getting old (81) and crotchety (ask my wife) and decided to take things a little easier. I began this series in April 2004 with an article on craters and will finish with a few more thoughts on that subject. However, I still owe CoatingsTech a longer article on wetting, surface tension, and contact angles. I may write a few other items in the future, but not on a regular basis.

For a contaminant to cause cratering, it must have certain properties; these “requirements” will vary depending on whether the contaminant is under the paint, in the paint, or falls on it. These comments apply mainly to cratering during application and early in the flash or air-dry, areas where almost all the craters that I have observed in the factory or field occurred. I have rarely seen cratering that occurred in a baked coating in the oven.

One reason why certain additives are very effective in preventing craters may be that they allow rapid and extensive diffusion of contaminates as well as the fact that they provide a homogeneous, low-surface-tension surface.

Substrate or underfilm contaminants cause craters because the paint does not wet the contaminated area or immediately dewets. The latter may be a pulling apart of a film bridge over an area that has never been wet rather than a true dewetting. The contaminant is a low-surface-tension material, either a liquid or a soluble solid. There may be diffusion into the paint that it touches, but this is not necessary for cratering to occur.

One test for the ability of a coating to cover contaminants involves spraying the coating over a panel with a series of drops of solvents and/or wet paint of different surface tensions (such as 30-50 dynes/cm) and looking for dewetting over the spots. This test can be used to get a sense of the robustness of the applied coating to possible contaminants. It is important to also test a control coating noted for its crater resistance for comparison. In addition, if cratering is occurring with a customer application but not in the lab, then customer substrate and panels with customer-applied primer or other layers should be used in lab testing. Prevention of such craters is often a matter of working with the customer to improve substrate cleaning.

Contaminants in the paint involve a different situation. For a contaminant to cause craters, the material must be sufficiently incompatible with the paint to come to the surface as droplets or some other cohesive form larger than dissolved molecules. Then, the contaminant must be able to diffuse enough into the surrounding paint to give a gradient in surface tension outward from the contaminant. If diffusion did not occur, there would be a surface-tension jump and the paint would pull away, but a small gap would be sufficient to remove the driving force for flow. There must be a gradient for a crater to form. Over the years, many people have added contaminants to paint, but very few have produced craters. I believe that the contaminants dissolved or dispersed so well that they did not come to the surface. Occasionally, a contaminant will cause craters initially, but after the paint is stirred or circulated for a period (minutes to hours), no more craters will form when the paint is applied. Rerunning pigment paste through the mill can have the same effect. Again, I believe that dissolution or dispersion of the contaminant removes it from the picture. Using filtration, possibly multiple times, may also remove a contaminant.

I also believe that it is impossible for solids to act as in-paint cratering agents unless they have a certain degree of solubility in the paint, or they are semisolids (such as resin gels) which can absorb and slowly release solvents or other low surface tension components. This would preclude Teflon from causing craters in the flash and might explain why most additions of Teflon pieces or particles to paint do not cause craters. Liquids must be incompatible enough to form droplets or centers of concentration at the surface, but they must be soluble enough to slowly diffuse into the paint after they make contact. Testing for sensitivity to in-paint contaminants is extremely complicated because it is more of a test of the ability of a coating to assimilate contaminants than it is a crater test. A solvent or additive replacement (probably for other reasons) may completely change the resistance of a paint to a given contaminant.

Contaminants that fall on the paint are somewhat like those that are in the paint. However, fall-on contaminants are more dangerous because they are on the surface from the start and are much less apt to be dissolved or dispersed into the paint. As with in-paint contaminants, fall-in contaminants must be incompatible, but diffuse into the surrounding paint to form a surface tension gradient for craters to occur. I do not know if there is a critical envelope of diffusion rates for cratering to occur, but I suspect there is.

One reason why certain additives are very effective in preventing craters may be that they allow rapid and extensive diffusion of contaminates as well as the fact that they provide a homogeneous, low-surface-tension surface. Rapid diffusion would mean that the driving force for cratering acts for a short time. Diffusion over a longer distance would reduce the driving force itself. I believe that this would be most likely to happen with silicone contaminants falling on silicone additives that have come to the paint surface. A test like the dewetting test previously described can be used to see whether applying drops of solvents with a range of surface tensions after application can differentiate between formulations. If a coating does show less crater resistance to any of contaminant situations than is acceptable, then surfactants or low-surface-tension solvents can be added to improve wetting of substrates and contaminants. A thixotrope also can be used to increase viscosity after application. That plus solvent evaporation may prevent craters from starting.

CoatingsTech | Vol. 18, No. 4 | April 2021

Additives modify paint or film properties or behavior, preferably (but not always) improving them. They are critical to successful formulation and to solving all kinds of paint problems, but choosing the right one can be exceedingly difficult and usually involves extensive trial and error.

Many different types of additives are available and there are many different products within a given type. Detailed coverage would take many pages, rather than just one, but here are comments regarding a variety of additive types.

Surfactants: These additives are surface active agents that have groups of opposite polarity or solubility on the same molecule. The long list of surfactants includes a wide range of chemistries, structures, and properties. They have many applications, including modifications of coating surfaces to reduce surface tension and make the surface more homogeneous. They can improve flow and leveling, increase crater resistance, and prevent other defects such as dewetting, flooding and floating, and BĂ©nard cells. Other surfactants can provide wetting and solubilization for cleaning agents, improve pigment wetting, and emulsify paint components. The latter is especially useful for waterborne coatings.

Use additives only if necessary and add small amounts in steps, testing along the way.

Rheology Modifiers: These are additives that enable formulators to control the viscosity of a paint during manufacture, on application, and during drying and baking. Rheology modifiers include thixotropes and thickeners that provide structure and raise low-shear viscosity. Examples of such products for solventborne coatings are fumed silicas, microgels, organoclays, and hydrogenated castor oils. Modifiers for waterborne coatings include cellulose derivatives, acrylates, and associative thickeners. Wetting agents improve pigment dispersion and tend to reduce low-shear viscosity. There also are “flow control” agents that often are polymeric surfactants. They tend to give smoother, more homogeneous coatings.

Defoamers/Antifoams: These additives often combine a surfactant with fine particles (a silicone and silica, for example). The surfactant covers the surface and destabilizes the foam. The particles act as bubble breakers. Removing or preventing foam is system-dependent; a product that works with one formulation may not work with another. Such products must be used at extremely low levels and can cause craters if too much is added.

Adhesion Improvers: Inclusion of polar groups such as hydroxyls, carboxyls, and amines in coating resins promotes the formation of hydrogen bonds with metals, which improves adhesion. Epoxy resins have hydroxyl and ether groups that bond to steel and interact with other molecules in the coating. Many plastics are difficult to adhere to because they have low solid surface tensions and few or no polar groups. A tie coat or adhesion promotor is necessary—usually a thin layer of a chlorinated olefin or chlorinated rubber.

Slip Aids: It may be useful to reduce the coefficient of friction of a coating surface to make it slippery. Scratch resistance likely will be improved. Since most things will not stick to the surface, blocking resistance will be raised. Waxes are often used for this purpose, as are fluorinated surfactants.

Solvents as Additives: Solvents can reduce the surface tension of a paint for better substrate wetting and defect reduction during air dry or flash (it can act as a fugitive surfactant). They can improve pigment wetting and dispersion. Non-water miscible solvents in waterborne coatings can aid coalescence and reduce defects (“oil on troubled waters”). Solvents can act as defoamers and pop-reducers (e.g., 2-ethyl hexanol, tributyl phosphate). A low-evaporation-rate water-miscible solvent can keep the surface of a waterborne coating surface open longer to allow better release of volatiles.

Coalescing Solvents: Such solvents are especially important for waterborne coatings. They plasticize the paint mixture so that the glass transition (T_g) is lower. This lowers the minimum film-forming temperature (MFT) so that the paint will coalesce to be a homogeneous film at lower temperatures. High T_g polymers are difficult to coalesce (high MFT)—addition of sufficient coalescing solvent to allow low-temperature film formation may mean a slow dry and a soft and possibly tacky surface. Characteristics of an effective coalescing solvent are low evaporation rate, good solvent for the resins, and a greater tendency to be in the polymer phase than in the aqueous phase (i.e., greater resin compatibility than water compatibility).

Resins as Additives: Polyacrylates such as polybutylacrylate and polyethylacrylate/ethylhexyl acrylate copolymer can act as flow-control agents and surface homogenizers. Addition of a small amount of an epoxy can improve adhesion. Various acrylics (especially with imino groups) can be used as pigment dispersants or as additives for dispersants. Very small amounts of very high molecular-weight (MW) resins or latexes may improve atomization on spraying. Polystearylmethacrylates and high MW latexes have been used as anti-pop additives.

Recommendations: When you have a problem or defect, make sure you understand what it is before you begin throwing in additives. Use additives only if necessary and add small amounts in steps, testing along the way. Some additives will work in one paint system, but not another. Additives often cause as many problems as they solve. For example, surfactants stop many defects, but also can cause coating water sensitivity. Where do you find help with choosing additives? Ask colleagues what they have used to solve problems or for certain types of coatings. Look at specific additive types online. That will show general information and suppliers of such additives. Ask suppliers for help.

FEBRUARY 2021 | VOL. 18, NO. 2

This is a continuation of the article begun in the September 2020 issue of CoatingsTech. Part 1 ended with film formation but had not discussed paint flow, which occurs during making of the paint, its application, and through film formation. The formulation undergoes a wide range of shear stresses on pigment dispersion, mixing, storage, application, and post-application. It responds to these stresses with variable flowability (viscosity), which depends on solids (resin and pigment), resin molecular weight, pigment dispersion quality, flow additives, wetting agents, etc.

The viscosity level during the different operations affects those operations. For example, too low a viscosity during pigment dispersion means poor shearing action and inadequate dispersion; too high a viscosity may cause shear thickening of the paste and jamming of the mill. Too low a viscosity during application (a high shear process) can cause spattering, drips, and sags; too high a viscosity makes brushing exceedingly difficult and prevents flow and leveling. In a spray application, it makes atomization impossible so that there is no spray pattern as such. Viscosity measurements at high shear stress/rate are useful in predicting application properties, and measurements at low shear can help evaluate flow and leveling and tendencies to sag. This should be done with viscometers that can measure viscosity over a range of shear stresses or shear rates. One-point measurements (Ford cup, Zahn cup) offer snapshots at one shear stress/rate, but do not indicate overall flow behavior.

Viscosity measurements at high shear stress/rate are useful in predicting application properties, and measurements at low shear can help evaluate flow and leveling and tendencies to sag.

Let us go back and look at the step right after film formation, which is drying. In the drying period, solvent evaporation changes from surface control (wet surface, rapid loss) to diffusion control (dry surface, slow loss). The film is dry to the touch, but not completely dry due to retention of solvent. The coating is likely to be soft enough that it is easily damaged, collects dirt, and probably has poor block resistance. It may continue to flow (particularly true for high solids coatings) and show poor chemical and water resistance. Retained solvent and additives can cause blistering and reduce corrosion resistance. Solvent retention can be prevented or reduced by using faster, more linear solvents that leave the coating more easily and/or softer more linear polymers that are less apt to trap solvent. Solvents can be forced from air dry coatings with IR lamps or hot air blasts. Baked coatings lose most solvents but may hold on to very low evaporation rate solvents. Yet another way to enable solvents to escape is to keep the coating surface open by delaying or slowing cure. This can be done by using a less active catalyst, less crosslinker, or a crosslinker with lower functionality.

Now that I have mentioned cure, what is it and how does it relate to formulation? One definition of cure is “the removal of solvents and achieving of sufficient molecular weight to give properties that meet the user’s needs.” The customer decides whether the coating will be air-dried or baked along with a cure specification, such as a certain number of MEK rubs or a combination of physical properties. Depending on the customer and the end use, the formulator may have considerable latitude in designing chemistry to produce a coating that meets that specification. Cure may or may not involve crosslinking. It may just involve the loss of solvents (including water). Lacquers and most latexes have high enough MW to not need crosslinking. However, crosslinking provides many advantages. After application, it causes increases in molecular weight and viscosity that give sag resistance yet allow flow and leveling. Ultimately, a solid film is formed that has good film integrity, hardness, toughness, and solvent resistance. Crosslinking allows the use of low MW polymers and oligomers that make high solids coatings practical due to low application viscosities yet moderate to high viscosities during and after the curing process. Thermosetting resins such as acrylic- and polyester-melamines and the more reactive epoxy-amines, polyol-isocyanates, and alkyds (oxidative cure) build molecular weight through crosslinking. Except for oxidative cure, the curing process takes energy, which usually is heat, but can be UV, electron beam, or microwave radiation. The degree of cure depends on the amount of energy, although catalysts can increase the rate of reaction.

Cure can be monitored and evaluated in the laboratory by a variety of techniques, including dynamic mechanical analysis (DMA), thermal mechanical analysis (TMA or indentometer), differential scanning calorimetry (DSC), oscillatory rheological measurements, tensile and elongation measurements, Fourier transform infrared spectroscopy (FTIR), dielectric spectroscopy, and measurement of extractables/gel content. Less esoteric methods are more common. Examples are solvent rubs (ASTM D 5402), pencil hardness (ASTM D 3363), indentation hardness (ASTM D 1474), and rocker or pendulum hardness (Sward – ASTM D 2134; König and Persoz – ISO 1522). Unfortunately, such techniques are subjective, have poor precision, and are difficult to relate to material properties or performance. However, if used carefully, they can give rapid estimations of whether the coating has adequate properties for its end use.

When finished, the formulator should have a fine coating that passes all the tests and has excellent properties. Now, it must be scaled up in a pilot plant or go directly to the paint plant. In both cases, shear stresses/rates, addition and letdown rates, and temperatures are different from the lab. The increase in batch volume changes shear and flow patterns and affects mixing. This is where developing relationships with the engineers in the pilot plant and/or paint plant can be beneficial and enable manufacturing problems to be headed off before they happen. A formulator can learn valuable lessons about what works in the plant and what does not without finding this out the hard way.

CoatingsTech | Vol. 17, No. 11 | November-December 2020

I have never been a formulator, but I have worked with formulators for nearly 50 years and learned a lot from them. In this article and the one that will follow, I will set out what I think are the things that need to be understood to formulate superior coatings. Readers may say that much here is obvious, but I have seen failures at one time or another in every one of the requirements, properties, and processes listed. People make mistakes.

Readers will know that the overarching requirements for coatings involve appearance and protection. Nearly all coatings
must look good, i.e. have a pleasing appearance, and continue to do so for many years. Nearly all coatings are designed to protect something—a substrate, another coating, themselves—and be capable of doing so for many years. A lot of things must occur for these things to happen. The basic requirements are that the coating must be able to be applied, that it must wet the substrate, form a film, and flow and level. It must dry and cure. It must release air and other volatiles. It must have good appearance (which means no or very few surface defects). There are physical and mechanical properties, the requirements of which vary with the application and customer and are negotiable: adhesion, hardness, flexibility, scratch resistance, impact resistance, etc. Protection and durability needs also are variable and negotiable. They depend on what is being coated and where it will be used. One thing must be pointed out. Coatings cannot cover or fix everything. They cannot make up for poor customer design, rough or damaged substrates, or other problems with the ware to be painted. It always is a good idea to do some testing on customer substrates and parts, not just standard panels.

Paint application affects both the appearance and protective abilities of paint films. Common application techniques are spraying, brushing, and roll coating. These are high-shear stress processes, and their effectiveness can be predicted from paint viscosity measured at high-shear stress (or shear rate) such as a high-shear cone and plate viscometer. The viscosity depends on polymer molecular weight (MW), paint solids, and application temperature. If the viscosity is too high, ugly films will result. Therefore, higher solids require lower MW resins.

After application, the wet coating continues to flow and leveling occurs. Sagging may begin. These are low-stress processes, and viscosity measured at low stress (or shear rate) with a Brookfield or other low-shear viscometer tells us what to expect. Flow and leveling as well as sag resistance can be built into the coating by including fillers, thickeners, and thixotropes such as fumed silica and Bentonite clays so that after an initial low viscosity, the viscosity builds to a moderate or high level. Baked coatings may suffer from hot sag and need special additives such as microgels that flatten the viscosity-temperature curve.

It always is a good idea to do some testing on customer substrates and parts, not just standard panels.

Unfortunately for the formulator, the customer controls application on the paint line or in the field. The paint formulator must maximize “workability,” i.e., make application as foolproof as possible under a range of conditions. Different application methods and equipment (and the settings used) can produce different results in terms of color, flow, sag, film thickness, and uniformity. I have been involved in many cases where having different application equipment in the lab compared to that of the customer caused difficulty and much frustration in meeting requirements.

Appearance may be in the eye of the beholder, but our industry has standard tests to quantify appearance: color, gloss, haze, and distinctness of image. We also expend much effort in trying to prevent ugly surface defects by using surfactants and flow additives to control surface tension and viscosity. Dirt, the most common defect of all, can be prevented by filtering of paint and ambient air in labs and paint plants and pushing customers to do the same in their plants.

Good adhesion is critical to coatings performance. We all know that a coating is no good if it does not stick, but what lab test relates best to field performance? My experience, plus results of ASTM interlaboratory testing, says crosshatch and tape (ASTM D3359) is a better predictor if rankings are compared, not absolute values, and a known good adhesion control is included to validate the testing.

Dirt, the most common defect of all, can be prevented by filtering of paint and ambient air in labs and paint plants and pushing customers to do the same in their plants.

Film formation involves going from a wet layer to a smooth, continuous solid film. It begins with solvent evaporation where there is an induction period followed by rapid loss under surface resistance control (like evaporation from an open dish). Solventborne coatings experience slow, steady solidification, forming a semi-solid of around 70% weight solids (higher for high solids products). Waterborne coatings show a range of behaviors, depending on the technology, T_g of the polymer(s), and coalescing solvents. Latexes (emulsions) are made in water, have high molecular weight, and little or no crosslinking is needed to achieve properties. Dispersion resins are made in solvent, then neutralized and dispersed in water. Depending on acid content, neutralizing amine and solvents, they may be milky white like a latex or clear like a solution. Dispersion resins are low in MW and must undergo considerable crosslinking for good film properties. Film formation is affected by temperature. If the coating is applied at a temperature below that of the T_g of the polymers (as might happen outdoors on a cold day), the result will be a discontinuous powdery layer. Solventborne coatings are plasticized by the solvents and rarely suffer this problem. With waterborne coatings, poor films are more likely to happen in cold weather and a low evaporation rate coalescing solvent may have to be added. The temperature below which a coating will not form a continuous, cohesive film is called the minimum film forming temperature (MFT or MFFT). Applying the paint to an instrument with a bar that has a range of temperatures from low to high along it will show how low a temperature the wet paint can tolerate and still provide an acceptable film.

Part II of this column will appear in the November/December issue of CoatingsTech.

CoatingsTech | Vol. 17, No. 9 | September 2020

Pigment dispersion quality in pastes and paints may be tested by one of several different techniques. Probably the most common method in labs and plants is taking readings with a grind gauge, usually the Hegman type (ASTM D1210). The main reason for doing this for pastes usually is to see whether the dispersion process has reached the required end point for that pigment. Another reason would be to find out whether the dispersion equipment was adequate to achieve the target reading in a reasonable length of time. Hegman readings of 6 and 7 correspond to particle sizes of 25 and 12.5 mm, respectively, yet the average pigment particle size will be a fraction of that, perhaps as little as 1/10th. With a grind gauge, we only see the boulders, the largest particles (aggregates, agglomerates). Beginners find grind gauges frustrating to use and their precision is poor, but experienced users can produce rapid, repeatable, and meaningful results.

Tint strength (ASTM D387, ISO 787/16) is a useful test that measures the color strength of a colored paste let down into a white base by comparison with a standard. Formulators use tint strength to test the color development of the pigment being dispersed, which depends on the degree of deagglomeration and freedom from flocculation of the paste as well as the quality of the pigment itself. The test may be used for quality control and to evaluate pigment from different suppliers, batches of pigment, effect of process changes and equipment, etc.

There are several techniques for measuring the particle size and size distribution in pigment pastes and paints, but my favorite device is an optical microscope with a digital camera connected to a computer with image analysis software. I like to see the pigment particles and be able to record and save what I have seen. I recommend a compound (conventional) optical microscope at 200–500X with transmitted light and dark field. This combination also can be used to look for flocculation and seeds. Pastes usually are difficult to evaluate because there are so many particles jammed together. It is possible to dilute the paste (with the paint vehicle, not just solvent), but this must be done with great care to prevent flocculation. It usually is better to observe the paint made from the paste rather than the paste itself. A paste-paint comparison is shown in Figures 1 and 2. The specimens were produced by placing a small drop of pigment paste or paint on a microscope slide. The picture of the paste does not supply useful information, whereas that of the paint shows what turns out to be loose flocculation, but otherwise a homogeneous mixture.

Regarding pigment flocculation, most production paints show this phenomenon to some extent. In fact, a little flocculation in a paint is good. It prevents pigment settling and reduces sag and the tendency to form surface defects.
In multi-pigment paints, slight co-flocculation of the pigments improves color uniformity and reduces the possibility of convection flow related defects on drying or baking. Co-flocculation with extender pigments can be particularly effective in these respects. How can we tell if there is too much flocculation? That situation usually causes problems such as off color, flooding and floating, BĂ©nard cells, or rough film. For example, a blue paint may be a lighter shade than specified. A mistake may have been made in its preparation (too much white pigment), or the paint may be flocculated. We can use a simple test called rub-up to determine which scenario is more likely. The technique involves rubbing the wet paint film in a circular motion with a gloved forefinger and observing the result. If the problem is one of flocculation, the color in the rubbed area will be the correct one. The rubbing action dispersed the pigment in the paint.

There are other, instrument-based particle size methods such as dynamic light scattering, laser diffraction, and dynamic imaging (ASTM D8090). Light scattering techniques have been around for many years and can provide excellent data. However, the specimen must be diluted considerably, and larger particles may be filtered out during the process. I have seen where microscopy showed a bimodal particle size distribution (large, small) and light scattering did not. The cause of the paint problem turned out to be the large particles.

Viscosity is an important aspect of pigment dispersion, yet we rarely measure it or need to do so. Paste viscosity varies with the pigment, its loading, its vehicle demand, the pigment/binder ratio, the wetting efficiency of the dispersant, the order of addition of the components and the temperature. High viscosity causes poor mixing, a poor flow rate, poor pumping, high temperature, and packing of the media (sometimes to the point of clogging the mill). Low viscosity produces poor dispersion quality and efficiency due to a lack of shearing action. There is a possibility of splashing and cavitation in the mill and settling is likely in the resultant pigment paste. Viscosity problems may occur where there are changes in the paste formula such as raising pigment loadings (which may cause shear thickening leading to jamming of pumps and mills), changing dispersants, adding a wetting agent (likely to lower viscosity), etc. When such changes are made, it is a good idea to measure the viscosity vs shear rate behavior of the new paste and compare it to that of a control paste. Adjustments can then be made to bring the new paste into line.

CoatingsTech | Vol. 17, No. 7 | July 2020

A friend recently wanted to know about pigment grinding, which made me realize that I had better begin this article by saying that we do not grind pigments, we disperse them. A bag of pigment contains clumps that must be separated into smaller clumps or individual particles. This is necessary in order to produce paint that provides good appearance on application and cure and does it economically. Pigment is expensive. Depending on the color, it may be the most expensive component in a paint.

The pigment dispersion process involves replacing air-solid interfaces in the dry powder with liquid-solid interfaces and separating the clumps of pigment particles so that they are dispersed in the liquid. The dispersed particles must be separated, or they will flocculate to form new clumps. The dispersant may be a polymer solution (possibly of a surfactant-like polymer), a surfactant solution or dispersion, a solvent or a combination of these. Wetting agents often are added to reduce interfacial tension and improve initial formation of the liquid-solid interfaces. The resulting dispersion commonly is called a pigment paste.

Pigment dispersion can be divided into three overlapping steps:

1. Wetting—for good wetting, the dispersant must have a lower surface tension than that of the pigment. This enables the dispersant to displace air and water from pigment surfaces and penetrate pores, gaps, and channels between particles and adsorb onto them. Wetting agents often are used to facilitate this process. Another strategy is to modify the pigment surface to make it more wettable and, therefore, more easily dispersed. This usually is done by the pigment manufacturer by adsorbing a surfactant on the pigment to produce an “easy-disperse” grade.
2. Deagglomeration—the wetted clumps are sheared by a blade, roller, sand, or beads, which break the pigment into smaller agglomerates or primary particles. Additional wetting occurs where particles have been freshly separated. Wetting of a pigment surface allows better stress transfer which improves shearing and provides more efficient breaking up of clumps.
3. Stabilization—dispersants help with wetting and deagglomeration, but their main purpose is to stabilize the pigment particles. Dispersants adsorb on the particles and either impart a charge to the surface so that particles repel each other (charge
   stabilization—waterborne pastes and paints) or build up an adsorbed layer to repel other particles (steric stabilization—
   solventborne pastes and paints). The most important requirement is that the dispersant must stay on the pigment! This is not as simple as it may seem. The paste must be mixed with the portion of the formula called the letdown; whose solvents may strip the dispersant from the pigment. Paste-letdown temperature or viscosity differences also may destabilize the dispersion.

Dispersants contain anchoring groups that strongly adsorb on the pigment surface. Anchors are designed to be insoluble in the paint solvent package or have low solubility in it. Anchors have chains (also called tails) attached to them that that are soluble in the solvent (or the aqueous phase of a waterborne system) and have little or no affinity for the pigment surface. Dispersants used to be made by random polymerization that gave irregular structures which sometimes adsorbed on multiple pigment particles resulting in bridging flocculation. Most dispersants now contain block or graft copolymers with special anchoring groups and controlled structures. See Figure 1. Anchors usually are strong polar groups (which may be charged in waterborne systems) such as amines, carboxylic acids, sulfonates, phosphates, and ureas. Chains (tails) in solvent systems may be fatty groups C18H37 or longer, fatty polyesters or poly(isobutylene). In waterborne systems, they may be polyethylene oxide, block copolymers of ethylene oxide/propylene oxide, polyvinyl alcohol, or polyvinyl pyrrolidone.

Pigment dispersion requirements in the lab include good color development, a homogeneous paste, sufficient opacity or transparency and excellent stability as a paste and in a paint. The lab dispersion then must be scaled up to make paste batches in the paint plant. This often is difficult. The increase in volume changes shear and flow patterns and affects mixing. Velocities, shear stresses and shear rates often are less in the plant, sometimes much less, which can affect particle size and size distribution. Manufacturing requirements include consistent product, a definite end point (completion of dispersion), reasonable cost, ease of manufacturing, including the time required to produce a batch, the power requirement, equipment wear, ease of filtration, ease of clean-up, etc. The dispersion process rarely is optimized in either the lab or the plant, but that must not stop the technician or engineer from continuing to try to accomplish that objective.

I will cover the testing of pigment dispersions in the second part of this article.

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

In June/July of 2019, I was irradiated with Bremsstrahlung X-rays to knock out prostate cancer. The radiation did its job, and the cancer appears to be gone. Being fair-skinned, I also have experienced skin cancer. My experiences have made me think about radiation effects on coatings for good or ill, as well as on me. We use UV and IR radiation for curing coatings and in analytical testing. Electron-beam curing uses high energy electrons to crosslink polymeric materials including coatings. IR and EB tend to be lumped together under the label “radiation cure.” Coatings that are used outdoors are exposed to UV radiation from the sun, and real or simulated solar exposure is used to test such coatings. We include additives in some formulations, particularly for automotive topcoats, to provide UV resistance. Coatings for military, nuclear, and space applications may encounter ionizing radiation, which causes serious problems for electronic devices. Radiation shielding is necessary—a possible job for coatings.

Many coatings are designed to dry and cure in ambient air, but even these coatings can benefit from raised temperatures via sunlamp radiation or other heating devices. Waterborne automotive basecoats are partially dehydrated by IR lamps or other relatively low heating. Some baking ovens include an IR section to “set” the surface of the coating and make it more resistant to dirt pick-up in the main thermal sections of the oven. Most ovens are dirty, and convection air flow blows that dirt around.

Radiation cure is used on a range of products, including coatings for plastics and wood flat stock: doors, paneling, flooring, etc. In most cases, the wet coatings contain no solvent (100% solids). They cure rapidly, and the cured films have outstanding toughness and scratch resistance. Because of these properties, there long has been interest in UV-cure coatings for automobile applications. Car bodies are not like flat stock, but complicated shapes can be cured by attaching IR lamps to robots. So far, UV-cured coatings are only used for certain parts, not for auto bodies.

IR analysis is a valuable technique for analyzing resins, wet paints, and coatings to see whether the expected product has been produced, to identify contaminants, to see how competitors’ products compare to yours, etc. Cure may be measured, even followed, from the appearance or disappearance and intensity of specific bands in the IR spectra. UV analysis is less commonly used for coatings, but is a valuable tool to test the effect of UV on resins and coatings, the effectiveness of UV-resistance packages, whether any of the UV package is left after baking, how long it lasts in the field, etc. In addition, UV fluorescent dyes can be added to solvents and used to test the surface uptake of solvent by primers to explain why solvent “bite” by topcoats is necessary for good topcoat–primer adhesion and to explain why primer over-baking causes topcoat adhesion failures.

Exposure to sunlight causes coatings to degrade over time and where topcoats are clear, underlying coatings can be damaged as well. Automotive coating failures have occurred when UV radiation has reached all the way through the clearcoat and the basecoat to an electrodeposition primer underneath. Outdoor exposure of coated panels in South Florida for multiple years is a common test procedure to evaluate resistance of coatings to the combination of UV, high temperatures, and high times of wetness that occurs there. Accelerated weathering can be used to speed up degradation, but the question always is: how does it compare to natural weathering? See the recent article by Mark Nichols in CoatingsTech, 17 (1), January 2020, pp. 18–25 for an excellent discussion of accelerated weathering and comparisons with putting panels on fences outdoors.

Because of the degradation of coatings by sunlight, protective additives such as UV absorbers and light stabilizers have been developed. UV absorbers do just what their name indicates. They absorb UV radiation, which they change to heat, which then dissipates. However, with time and exposure, absorbers are used up. Hindered amine light stabilizers (HALS) work by scavenging free radicals initiated by the degradation process, thereby inhibiting degradation of the polymers in the coating. Unlike UV absorbers, HALS are regenerated rather than consumed during the stabilization process. Unfortunately, baking of the coating, especially over-baking, has been seen to drive off a significant portion of the additives and loss may also occur during natural weathering. Pigments can protect coatings as well. TiO₂ and carbon black are UV absorbers, but other pigments are blockers rather than absorbers. However, highly pigmented coatings provide protection for the polymers in them and for underlying coatings. The thicker the coating, the better the protection. Most automotive basecoats have high-pigment loadings that protect the primer below from UV that passes through the clear.

One form of radiation that rarely is faced by coatings is ionizing radiation, such as gamma rays, X-rays, radioactive decay particles, and the higher UV part of the electromagnetic spectrum. Electronics used in space exploration, military equipment, and at nuclear sites must be shielded from ionizing radiation. This normally is done by putting aluminum boxes around sensitive equipment. However, research at North Carolina State University has shown that coatings containing metal oxide powder provide effect shielding from gamma rays and neutron radiation at lower weight and volume than conventional shielding (M. Devanzo and R.B. Hayes., J. Rad Phys Chem, 2020: 171: 108685 DOI/j.radphyschem.2020.108685).

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

I have published many of the thoughts and recommendations in this article previously in CoatingsTech. However, since paint problems continue to occur and there may be new readers facing them, I thought that this was a good time to discuss problem solving yet again. Surface defects probably are the most common type of paint problem, but it is not always clear what the defect is. That must be established before the investigator can look for causes and consider possible solutions. That may seem obvious to the reader, but I have seen many cases where it was not done, and the wrong countermeasures were taken. Although this article is aimed primarily at surface defects and failures, the basic process presented applies to nearly all coatings problems.

The keys to problem solving are using your eyes (often augmented by a microscope) and your mind. Look and think! Careful observations of the problem by the investigator are critical. Be sure to see the defect or failure first-hand. Be skeptical of other people’s opinions until after you have seen the problem yourself. The observations begin with looking at the defect or failure with your own eyes, but a hand lens or microscope is always better. If you are dealing with surface defects on a paint application line or in the field, a 40–60X shop microscope allows a good look. The latter device has moved into the 21st Century with the availability of new handheld digital microscopes that are just as portable, but can interface with mini-recorders, computers, and monitors. The resultant images can be stored and shared as well as being viewed. If possible, bring an example of the defect back to the lab and examine it with a microscope, first at low power (2–10X), then with greater magnification (100–300X). Ideally the microscope should be connected to a camera and/or a computer, so that images can be saved, pictures printed or e-mailed, etc. You should be able to tell what the defect is, although it sometimes can be difficult to distinguish craters from pops. It may be necessary to cross-section the defect to accomplish this. If you are dealing with a flow problem, besides making viscosity measurements, look at the wet paint at higher powers to see the quality of the pigment dispersion, whether there is flocculation, whether a waterborne resin is showing phase separation, etc. [See my article in JCT CoatingsTech, 3 (2), February 2006, pp. 36–43 for more details on microscope use and applications.] For many problems, a scanning electron microscope is a very valuable tool for characterization.

Once that you are certain what the problem is, the next step is establishing its cause. This may be possible via further analysis of the defects themselves. Pick a piece of dirt out of a dirt defect, or cross-section it to make the identification. Examination of a cross-section of a pop in a coating on plastic with a microscope will show whether the volatiles came from the plastic or from the coating. The back of a paint chip may show a layer of wood fibers, indicating possible substrate degradation before painting. If the defects are dirt or craters, then look for possible sources where the paint is applied. Observe operations at the customer—substrate preparation (including the pretreatment line), paint application, work attitudes and practices, cleanliness, and possible sources of contamination such as oil in the compressed air used for spraying. If the paint is applied outdoors or in a small shop, (auto refinish, maintenance, or architectural), then observe everything that you can—defects, substrate preparation or the lack of it, signs of contamination, whether the correct paint is being applied, etc. You may have to check your own plant for pigment dispersion quality, housekeeping, effectiveness of cleaning of tanks and other equipment, filtration, and sources of contamination. Ask a lot of questions—of people working on the line or at the work site, service people, and formulators. Obtain as much background information as possible. Colleagues may have dealt with the same or a similar defect. Be a detective!

Looking in the paint literature can be useful. Journal of Coatings Technology, JCTR, and CoatingsTech have published papers and articles on defects and other problems over the years. Do not ignore older papers. Much excellent work was published in the 60s, 70s, and 80s as well as more recently. Keep files of problems, causes, and solutions and refer to them as needed. The internet also can be very useful. Search engines such as Google will turn up much material, but you need to be careful because there may be false or antiquated information as well as good papers and articles.

Try to avoid making assumptions. I still make that mistake after all these years. Do not assume that the paint was formulated correctly and tested properly. Do not assume that the batch with the flow problem really had the same viscosity as the control batch. Measure both their viscosities. Formulation mistakes can happen, and testing may have been inadequate. I have encountered problems where customer conditions and/or substrates were different from those used in lab testing. Do not take the customer’s word that the cleaning and pretreatment line is working well. I have been in many customer plants where it was not, and the operators were quite willing to tell me so and why. Running tests on paints applied to the customer’s substrate rather than standard panels often shows why there is a difference between lab results and performance online or in the field.

Good luck with your problem-solving efforts!

The coating system almost always is blamed for field defects, but there are other possibilities: design of the painted object, poor substrate quality or surface preparation, and the process, including application and cure. Was the painting done in the sun? Or in the rain? Were the freshly painted rail cars pushed out into fog? Were all the required coats applied? Was there an underbake? Overbake? Do the field defects occur where application defects were sanded and repainted?  Regarding design, much of the great improvement in corrosion resistance of auto bodies is due to improved coatings, but if the auto companies had not finally changed their designs to prevent the trapping of salt water and mud-salt poultices in rocker panels and other interior spaces, corrosion would not have been reduced nearly as much.

However, what if evidence indicates the coating is not sufficiently resistant to outdoor exposure in places where it is to be used or is being used? Many chemical changes can occur on weathering, often in concert with each other. Most of them are bad for the appearance and effective lifetime of a coating. Examples include photo-oxidation, shrinkage, hydrolysis, bond breaking and loss of crosslinks, additional crosslinking, and loss of UV absorbers and stabilizers. Chemical changes can lead in turn to changes in physical and mechanical properties: lower strength, lower toughness (decreased elongation as well as lower tensile strength), softening (especially due to contact with water), higher water absorption and transport, increased internal stress due to shrinkage and/or higher crosslink density, increased or decreased glass transition or softening point, and increased brittleness (the energy to cause fracture decreases) leading to cracking.

Some field defects are due less to changes in the coating on weathering than to additives or replacement components that change properties. For example, low evaporation rate solvents help film formation, flow-out, and leveling, but they may remain in a coating on drying or even baking, producing a soft film. This increases the likelihood of defects such as poor scratch and abrasion resistance, blistering, water spotting, and dirt pick-up. It is doubly bad if the slow solvent is highly hydrophilic and attracts moisture in solventborne coatings and does not allow it to be released in waterbornes. The result may be popping/pinholing on baking and blushing on air dry as well as softening of the film. Surfactants can cause similar problems as can the retention of amines in waterborne coatings.

When defects begin to show up on panels at the exposure site or during accelerated weathering, or even worse on houses, oil tanks, or cars with your paint on them, there will be pressure to quickly figure out how to fix the problem so that future batches or formulations do not suffer the same defect or group of defects. However, it is important to establish what the defect is before jumping to conclusions as to what to do about it. Effective field defect identification requires a part, piece, or chip so that you can see the problem, then a light microscope to get a better look. Depending on what you see, you may need to measure or test physical properties such as adhesion, hardness, impact strength, water sensitivity, solvent resistance (especially with solvent rubs), softening point, or glass transition. Be sure to run controls and compare results to those of the problem coatings.

Recreation of field defects is difficult because it rarely is possible to recreate the application and cure history of the coating or the field conditions that produced the defect. Experiments such as water soak, exposure to 100% humidity, cyclic corrosion testing, accelerated weathering, temperature, and/or humidity cycling, and measuring the effect of high and low bake temperatures can be run on lab panels to force the occurrence of the problem or defect.

Strategies for prevention of coatings-related field defects include development of tough coatings (high tensile strength, but also flexible) that have wide cure windows (i.e., excellent properties over a wide range of cure temperatures and times). Solvent blends must be optimized to avoid high levels of low evaporation rate solvent, especially those that are highly hydrophilic. Use surfactants sparingly. Coatings must apply well even under conditions that are less than optimum with a minimum of application defects so that repairs and repainting are not necessary. This reduces the likelihood of defects on outdoor exposure.

Good luck in solving field defect problems!

How can we get more effective pigment dispersion and letdown? Let us begin with dispersion. Many readers will know that the dispersing of pigment involves three steps: wetting, deagglomeration, and stabilization. Good wetting depends on the dispersing medium (solvent, resin solution, or surfactant dispersion) being compatible with and having a lower surface tension than the pigments. Deagglomeration is done by shearing the pigment-dispersant mixture. The process usually is not difficult, but attention must be paid to the design of the mill base, particularly in terms of viscosity. Too low a viscosity results in little or no shearing and a poor dispersion. Too high a viscosity leads to the mill heating up and can cause kick-out of the paste and clogging of the mill. High solids paints can be a problem because the formulator is allowed so little solvent to work with that it may not be possible to truly optimize the mill base.

The key step is to stabilize the pigment properly so that it does not flocculate in the paste, on letdown, or when the paint is shipped and stored. Dispersants are used to help with wetting and deagglomeration, but their main purpose is to stabilize the particles (to do that they must stay on the pigment—no matter what). Dispersants adsorb on the particles and either impart a charge to the surface so that particles repel each other (charge stabilization—waterborne pastes and paints) or build up an adsorbed layer to repel other particles (steric stabilization—solventborne systems). A wide range of dispersants for solventborne and waterborne coatings are described in the literature.

The letdown process is deceptively simple. You either stir paste into the letdown solution or dispersion or the letdown is stirred into the paste. Occasionally, the two components are not compatible with each other, and they separate into two phases instead of mixing. I recall working on a dispersion problem where I wanted to observe letdown effects under the microscope. I put a small drop each of paste and letdown next to each other on a microscope slide and placed a cover slip over them. Normally that would result in mixing. In this case, they immediately ran away from each other, ending up on opposite sides of the drop like girls and boys at a 7th grade dance. It was quite a shock to see it. Solvent changes had to be made in the mill base vehicle and letdown to make them more compatible.

A letdown solvent blend can cause another problem if it is such a good solvent for the dispersing resin that it strips it from the pigment surface resulting in flocculation. The result can be a wrong color on application due to floating of one pigment to the surface. For example, a medium blue coating spray applied as a light blue. Rub-up on the wet surface gave the correct color; light microscopy showed pigment flocculation, particularly of the blue. Much of the white pigment was tending to float to the top of the film. Initial attempts to solve the problem produced dark blue (blue float) or even lighter blue (white float), but not the right color. Eventually, measuring the solubility parameters of the dispersing and film forming resins, identifying solvents and nonsolvents for each one, and working up a letdown solvent package that did not remove dispersing resin from either of the pigments solved the problem.

The technique that I use for characterizing the quality of pigment dispersions in terms of particle size, size distribution, and degree of flocculation is optical microscopy at 200–500x with transmitted light and dark field. Pastes can be examined directly but are difficult to evaluate because there are so many particles jammed together. Pastes can be diluted for viewing (dilution with the dispersant vehicle is highly recommended), but if the diluted specimen turns out to be flocculated, it is not possible to tell whether the dilution caused the flocculation or if that is the true state of the paste. It is more useful to view a paint made with that paste since the quality of the dispersion in the paint rather than the paste is the key to performance anyway. For more on optical microscopy, see JCT CoatingsTech, 4 (8) 72, (2007).

Good luck with upgrading pigment dispersion to improve sustainability!

CoatingsTech | Vol. 16, No. 4 | April 2019