For many years, aqueous hybrid dispersions of poly(vinylidene fluoride) (PVDF) and acrylic resins have been formulated into waterborne topcoats because of their exceptional weatherability and ability to meet stringent volatile organic compound (VOC) requirements and sustainability targets. For applications requiring strong chemical resistance, hydroxy-functional PVDF-acrylic dispersions are available which allow formulators to build resistance properties with isocyanate crosslinking. However, isocyanate not only presents respiratory hazards to paint applicators, but it can cause microfoaming in the resultant films due to CO₂ generation from the reaction of isocyanate with water. Recently, new one-component self-crosslinkable PVDF-acrylic hybrid latexes were successfully developed, and can be formulated below 100 g/L VOC. Detailed properties, such as detergent resistance, chemical resistance, and dirt pick-up resistance, in target applications such as architectural façade coatings and protective coatings are presented.

Introduction

Poly(vinylidene fluoride) (PVDF)-based topcoats¹ have been used for decades on monumental buildings around the world, and they have become the gold standard for decorative property durability and substrate protection. This performance is attributed to PVDF’s outstanding UV and chemical resistance, and its good barrier properties against moisture and oxygen. The superior UV and chemical resistance of PVDF, relative to other common coating polymer chemistries, is demonstrated by its extremely low absorbance in the UV, as can be seen in Figure 1. The UV absorbance spectra were normalized to 25 μm dry film thickness. PVDF copolymer with hexafluoropropylene (HFP) comonomer was used in this study.

FIGURE 1: UV absorbance spectra of PVDF and other polymer chemistries. UV-B band (280-313 nm) from sunlight is highlighted in yellow.

PVDF-based coatings have historically been limited to factory-baked coatings on metal because of the need for a high-temperature bake (230-250 °C) to alloy the polymer components. These baked PVDF coatings, commonly used as topcoat finishes on monumental buildings and other architectural applications, typically contain a blend of 70-80 wt % PVDF resin with 20-30 wt % of a miscible acrylic resin. Around the year 2000, water-based hybrid resin technology became commercially available,² combining PVDF copolymers and acrylic resins in pre-alloyed form.³ The new water-based technology allows for the application of durable PVDF-based coatings under field-applied and low-temperature bake OEM conditions, with dramatically lower levels of emitted volatile organic compounds (VOCs). Figure 2 shows some South Florida weathering panels for a commercial waterborne PVDF-acrylic binder, after 20 years of exposure. Minimal visible differences can be noted between the unexposed and exposed areas of the panels.

FIGURE 2: Twenty years’ South Florida field exposure weathering panels of one-component PVDF-acrylic-based waterborne coatings (as shown in left for each color) vs legacy solventborne baked PVDF-acrylic coatings (right for each color). Panels are exposed at 45° South-facing. The substrates of these panels are chromated aluminum Q-Panels (AL-412 from Q-Lab Corporation).

More recently, hydroxyl-functional PVDFacrylic hybrid products in this class were developed, which could be used with crosslinkers such as water-reducible polyisocyanates.^4,5 These systems show enhancements in certain key properties such as early hardness, barrier properties, solvent resistance, and adhesion, with performance contributions coming both from the entangled polymer network and from the network formed by crosslinking reactions.⁶

However, similar to other two-component systems with isocyanate crosslinking, isocyanate not only poses respiratory hazards and carcinogenic potential to paint applicators, but it can also cause microfoaming in the resultant films due to CO₂ generation from the reaction of isocyanate with water in the paints. This is particularly noticeable when applying paint with a brush or roller outdoors (Figure 3).

FIGURE 3: Microfoaming issue with two-component waterborne paint with isocyanate crosslinking.

In this article, we present studies for developing a one-component selfcrosslinkable PVDF-acrylic hybrid latex and waterborne formulations based on this hybrid latex for architectural façade coatings and protective coatings.

Experimental

Synthesis of Self-crosslinkable PVDF-acrylic Hybrid Latex

A two-stage process was used to make the PVDF-acrylic hybrid latex.^7,8 PVDF latex was made in the first stage through emulsion polymerization of vinylidene difluoride monomer and an optional HFP comonomer in a pressure reactor. The second stage of the polymerization process, also called the acrylation stage, incorporated the acrylic monomers into PVDF latex to produce the intimate micromolecular mixture of PVDF and acrylic polymers. A wide variety of acrylic monomers with different T_g and functionalities can be chosen to tailor the coatings properties to the desired requirements. In this work, diacetone acrylamide (DAAM) monomer was introduced into the PVDF-acrylic hybrid latex along with other acrylic monomers and was copolymerized within the acrylic copolymers, creating well-dispersed pendant ketone crosslinking sites. Adipic dihydrazide (ADH) acted as a difunctional crosslinking agent, remaining partitioned in the water phase outside of the emulsion particles. ADH could be either added to the latex after emulsion polymerization was complete or during the paint formulation stage. Acrylic emulsions based on DAAM with ADH were initially nonreactive, ensuring emulsions maintained good long-term stability during shipping and storage. One-component keto-hydrazide self-crosslinking can occur at ambient temperature, facilitated by water evaporation during the film drying process and a simultaneous reduction in pH resulting from the loss of ammonia (a neutralization agent added in emulsion polymerization and paint formulation) (Scheme 1).

SCHEME 1: Keto-hydrazide self-crosslinking mechanism. P denotes the polymer chain.

The water resistance of waterborne all-acrylic and styrene acrylic dispersions is investigated at different T_gs. The performance of films obtained from latexes stabilized using conventional surfactants (SLS and Surf 1) is compared to that of latexes stabilized using polymerizable surfactants (PolySurf 1 and 2). The effect of the surfactant concentration and surfactant chemistry on the water whitening and water uptake of the latex films is studied. The surfactant chemistry and concentration, as well as the pH of the final latex have a significant influence on the water resistance results. The water whitening phenomenon is directly linked to the latex colloidal stability during drying and the excess of surfactant in the continuous water phase. Atomic Force Microscopy (AFM) images revealed that labile surfactants can migrate within the film during drying and form hydrophilic pathways or pockets, making the film more susceptible to water at higher concentration of surfactants. Proton nuclear magnetic resonance (1H-NMR) analyses, combined with AFM imaging, showed that when polymerizable surfactant is used, due to its incorporation into the polymer backbone, the residual amount of surfactant in the continuous phase is low, resulting in films with superior water resistance. Moreover, the controlled distribution of the polymerizable surfactant on the latex particles results in a more homogeneous distribution of the surfactant in the latex film lowering the water sensitivity of the films. Overall, for T_gs between -5 °C and 10 °C in acrylic latexes, the PolySurf2 exhibits superior water whitening and water intake performances as compared to conventional surfactants. PolySurf2 is designed to provide superior water resistance performances in clearcoats.

Introduction

Clearcoats are widely used in coatings markets such as industrial and architectural paints as an invisible top layer to protect the underlying paint coats from UV rays, dirt, and water. Indeed, paints in direct contact with rain or a high-humidity environment require extra protection against water penetration. Because of additional environmental regulations and recent market trends toward more sustainable technologies, solvent-based formulations that provide excellent water resistance are under scrutiny, and waterborne clearcoats have become more desirable.

Waterborne polymers used in clearcoats are environmentally friendly, easy to handle, relatively low in cost, and can be made with high-solids content through emulsion polymerization. Although surfactants and hydrophilic monomers are necessary during the latex synthesis to provide steric and/or electrostatic stabilization to the particles formed, their distribution during drying can impede the water resistance of the polymer films. The phenomenon of water whitening in polymer films has been mainly attributed to water molecules “clustering” near the polar constituents of the films, such as the surfactants and the hydrophilic monomers.^1-4 Moreover, during the film formation, surfactants can migrate toward interfaces, forming hydrophilic pockets within the polymer film or concentrate near the surface of the particles to create hydrophilic pathways throughout the film.^1-5 The water-swollen pockets scatter light and make the film appear cloudy or white.^6,7 To reduce the negative effects of conventional ionic surfactants on the water sensitivity of the latex films, polymerizable surfactants are used. These reactive surfactants polymerize with the monomers and are incorporated into the backbone of the polymer chains of the latex.

Polymerizable surfactants are then locked on the latex particles, and a better control of their distribution within the polymer film is achieved during drying because of the lack of both desorption from the latex particles and migration in the polymer film.^8-10 Although the mechanisms of water sensitivity of polymer films have been extensively studied before, to the best of our knowledge, a broad analysis of the water whitening and the water uptake of clear latex films as a function of T_g has not been reported to this date. The effect of the latex pH on the water whitening resistance of the final films was also studied. In addition, the effect of surfactant concentration on the latex films’ water-resistance properties was investigated. Finally, it was demonstrated that the polymerizable surfactants used in this study are almost fully converted during latex syntheses leaving very little unreacted surfactant in the dried polymer films, which enhances the water-resistance properties of thus films.

Materials and Methods

Materials

Methyl methacrylate (MMA), butyl acrylate (BA), and methacrylic acid (MAA) (99%, Sigma-Aldrich) were used as is. Surf1 (Syensqo), SLS (Syensqo), PolySurf1 (Syensqo), and PolySurf2 (Syensqo) were used as emulsifiers in the latex syntheses. Sodium bicarbonate (99.7%, Sigma-Aldrich) was used as a buffer in the preparation of the latex surfactant control. Ammonium persulfate (99.8%, Sigma) was used as an initiator in all latex synthesis. Deionized (DI) water was used throughout the work. Figure 1 presents the main characteristics of the surfactants used throughout this study.

FIGURE 1 (a) Surfactants’ physico-chemical properties; (b) concentrations.