Waterborne coatings significantly reduce VOC emissions and improve air quality compared to traditional solvent-based systems. They offer strong performance, durability, and versatility across architectural, industrial, and automotive applications. CoatingsTech talks with Dr. James W. Rawlins from the University of Southern Mississippi about the future of waterborne coatings both in the industry and at the university level.

Q: What are the most recent noteworthy advancements in waterborne performance?

A: In the last few years, exterior-durable latex platforms enabled by reactive polymer-bound surfactants have been a consequential step forward. By copolymerizing reactive/polymerizable surfactants into the latex backbone, concerns associated with surfactant migration, water uptake, and sensitivity have been mitigated, which have historically been issues with waterborne films. Additionally, modern waterborne polyurethanes and polyurethane/acrylic hybrids are now becoming available from experimental/laboratory and commercial materials.

Q: Where is the next big leap likely to come from?

A: There is a lot of research activity happening right now. Innovations in dynamic covalent networks show near-urethane mechanical energy absorption methods, resulting in circularity, repairability, and potential recyclability, in addition to higher crosslink network control and tunable mechanical deformation processes. There is also significant work being done on designed stratification and hybrid lattices in PU-acrylic and silicone-acrylic systems.

In the short term, there is strong progress coming from advancements in additives. Reactive and anchored additives are particularly promising. Another strong area in additives is platelet barriers and rare-earth/organophosphate inhibitors, but the fastest acceleration is happening in testing and digital technology. New methods are being developed for early detection and quantification of failure before they become visible macroscopically.

Q: How can formulators meet sustainability targets without sacrificing durability?

A: In several ways. Water uptake could be reduced at the source by using reactive surfactants and polymer-bound stabilizers, and free, mobile surfactants could be avoided wherever possible. Crosslink density should be driven down with low volatile organic compound chemistry: driven mainly through carbodiimide, blocked-isocyanate, or self-crosslinking mechanisms in acrylic/polyurethane dispersion hybrids. Barriers could be built but understanding and quantifying water, electrolyte, and oxygen barrier differences for common chemistries is needed.

Q: How do university–industry collaborations accelerate innovation?

A: Learning is best achieved through immersion. The combination of total immersion and timeline-driven projects, which is something industry needs, with solid fundamentals such as scientific goals and objectives in an integrated team, drive real depth and real-knowledge gains through necessity. These combined teams are driven by student enthusiasm, industry support, and passionate scientist and engineering personnel with experience. Flagship consortia move students from working with the theoretical to the practical, with fundamental concepts moving into products, as well trained and developed students drive new paradigms.

Q: What excites you most about the future of waterborne coatings?

A: There are many. One is one-pass which are self-organizing films, stratifying hybrids, that can deliver stain resistance and direct-to-metal (DTM) corrosion without extra layers. Another important contribution is novel material compositions that are sustainable and circular, and dynamic networks enabling repairable, recyclable waterborne films with high chemical resistance. Lastly, data-driven developments such as AI data gathering for machine learning that is linked to accelerated tests in a context of real test results are beginning to remove blind spots for unquantified or poorly connected concepts and this improves upon our scientific rationale moving forward and shrinks lab-to-field translation cost and time.

James W. Rawlins is a professor of Polymer Science and Engineering at the University of Southern Mississippi, where he has directed an 11-member research group focused on Surface Coatings and Circular Materials since 2004. Rawlins, chairman of The Waterborne Symposium, has published 61 peer-reviewed articles and holds 17 U.S. and European patents. Earlier, he served as technical director at Highland International and held R&D and European technical marketing roles at Bayer (now Covestro) in Pittsburgh and Leverkusen.

Wood, by virtue of being a renewable material, and when sourced from responsibly managed forests, constitutes a sustainable and responsible building material. If continuously exposed to water, wind, and sunlight, though, wood substrates are subject to damage, manifested as discoloration, cracking, and decay. Protective coatings can delay or prevent damage, improving the durability and prolonging the life of this vital natural resource. There is a demand for wood coatings with low-volatile organic compound (VOC) content, a requirement often achieved with “VOC exempt” solvents.

Recognizing that the use of certain, widely used VOC exempt solvents may be permanently restricted in many applications because of proposed state-level environmental regulations—for example, by the South Coast Air Quality Management District in California as early as 2026— creative solutions to match VOC and performance targets are critical. A proprietary polymer has been developed that enables the replacement of these VOC exempt solvents with water. Using this unique technology, a low-VOC waterborne coating for exterior wood substrates has been formulated to provide exceptional protection, which meets or exceeds that of conventional wood coatings.

Introduction

As an exterior substrate, wood has both practical and aesthetic advantages. Not only does wood offer an organic, warm look, it readily accepts color, ranging from semi-transparent stains to fully opaque coatings. Notably, wood can act as a natural insulator. Wood substrates, therefore, are an attractive and environmentally sound alternative to brick, concrete and vinyl.

Wood substrates, however, can be an expensive option, because regular maintenance is required to prevent decay, damage from insects, and deterioration caused by weather. Wood is particularly susceptible to fluctuations in environmental moisture. Continuous high humidity and periodic severe and wet weather can result in warping, cracking, and rot, which, if not addressed by timely repairs, can lead to irreversible structural damage.^1,2

Protective coatings are therefore critical to maintaining exterior wood substrates. One such coating, spar varnish, is particularly effective. Spar varnish, also known as “marine varnish” or “yacht varnish,” was originally developed for use on the wooden poles that support the sails (“spars”) of sailing ships.³ Generally, a spar varnish is composed of an oil, like tung oil or linseed oil, which penetrates the substrate, and a resin, such as an alkyd or a polyurethane, which provides hardness. These components are solubilized in a compatible solvent, which contributes to ease of application. Spar varnishes are formulated to be flexible, allowing the coating to expand and contract in concert with changes occurring within the wood substrate, and water resistant, allowing the coating to provide a barrier to environmental moisture. Spar varnishes are also typically formulated with UV-absorbing compounds, which both extend the lifetime of the coating and prevent the substrate underneath from degrading.

As of 2023, the global market size for marine spar varnish was valued at approximately USD 1.2 billion. This market is projected to reach around USD 2.5 billion by 2032, growing at a compound annual growth rate (CAGR) of 7.5%.⁴ This growth is the result of increasing demand for protective coatings in marine applications and outdoor wood furniture, as well as increased awareness about the importance of safeguarding wooden structures against harsh environmental conditions.

Solvents typically used to formulate a spar varnish include mineral spirits, aliphatic hydrocarbon-based solvents, such as naphtha, and aromatic solvents, such as xylene. While the volatile organic compound (VOC) content can vary, VOC content for a standard spar varnish using traditional solvents usually ranges close to 475 g/L. The VOC limit for spar varnishes under the South Coast Air Quality Management District (SCAQMD) in California and the Ozone Transport Commission (OTC; Mid-Atlantic and Northeast) Model Rule 2010, however, is 275 g/L.^5,6 For a spar varnish to meet lower VOC requirements, “VOC exempt” solvents, such as parachlorobenzotrifluoride (PCBTF) or tert-butyl acetate (t-BAc), must be incorporated.

The U.S. Environmental Protection Agency (EPA) granted PCBTF and t-BAc exempt status because these solvents have negligible photochemical reactivity and do not significantly contribute to ground-level ozone formation.^7,8 PCBTF is the most widely used VOC exempt solvent in the coatings and adhesives industry. Another VOC exempt solvent, tert-butyl acetate, has been promoted as a potential replacement for the halogenated PCBTF. In 2022, however, the SCAQMD included a provision in Rule 1168 (Industrial Adhesives and Sealants) that prohibited the use of PCBTF and t-BAc.^9,10 The District intends to follow through with identical rulings for other coating rules, including Rule 1136 (Wood Products Coatings) as early as 2026.¹¹ Recognizing that the use of VOC exempt solvents may be permanently restricted in many applications, creative solutions to match VOC and performance targets are needed.

By Brian Vest, Lichang Zhou, Linda Adamson, and Celine Burel, Syensqo

Perfluoro-and polyfluorinated-alkyl substances (PFAS) have been widely used for a variety of applications, including water-resistant fabrics, nonstick cookware, stain-resistant carpets, firefighting foams, food packaging, and some personal care products. Their unique properties of resisting heat, oil, grease, and water make them highly versatile across various sectors. One type of these chemicals, fluorocarbon surfactants (FCS), has been used specifically in waterborne coatings for improving early “hot block” resistance. However, risk management and upcoming regulations are driving formulators to eliminate the use of these chemicals from their formulations. This has led to a balancing act for formulators as they try to move to more sustainable additives without sacrificing performance.

This study will cover the efforts made in our group for the use of novel phosphate ester wetting agents which deliver improved early hot block resistance without the adverse environmental concerns of fluorocarbon chemistry. These novel alkyl phenol ethoxylate (APE)-free and very low volatile organic compound (VOC) specialty additives deliver improved colloidal stability, which helps provide a combination of wetting, dispersing, and compatibility properties to the finished water-based coating. Data will highlight overall paint performance and touch upon structure/property relationships that lead to improved anti-blocking performance.

Introduction

Manufacturers of waterborne coatings are increasingly facing stringent regulatory requirements to transition towards environmentally sustainable formulations. These formulations must not only exhibit low volatile organic compound (VOC) levels but also eliminate hazardous substances such as alkyl phenol ethoxylates (APEs) and fluorocarbon surfactants (FCS). This regulatory landscape presents a formidable challenge for formulators who are tasked with integrating eco-friendly additives while maintaining the integrity and performance of the coatings.

FCS pose a particular challenge for formulators due to their distinctive molecular structure and the comprehensive balance of properties they impart to formulations. A critical performance attribute provided by FCS is early-stage—specifically, 1-day dry—hot block resistance. This property is essential in waterborne semi-gloss to gloss formulations designed for low-VOC applications. For decades, FCS have been instrumental in achieving this performance characteristic without adversely impacting other application properties. However, FCS belong to a broader category of chemicals known as perfluoroalkyl or polyfluoroalkyl substances (PFAS), which are classified as substances of very high concern (SVHC). PFAS are of significant health concern due to their persistent nature, as they do not readily degrade and can accumulate in the environment and in the human body over time, earning them the moniker “forever chemicals.”¹ Consequently, there is mounting regulatory pressure globally to identify safe and environmentally benign alternatives to fluorosurfactants across various industries.

In response to the demands within architectural coatings, an extensive study of alternative technologies was undertaken, with a concentrated emphasis on waterbased architectural coatings. This rigorous investigation identified that modifications to a specific phosphate ester chemistry could provide an effective solution, yielding superior early-stage hot block resistance while maintaining a comprehensive balance of application properties. This research culminated in the development of an innovative anti-blocking additive tailored for waterborne coatings.

Background

Block resistance is the capability of a paint when applied to two surfaces to not stick upon contact when pressure is applied under various temperature and humidity conditions. For example, good block resistance helps keep a door from sticking to the jamb or a window from sticking to its frame. When the two painted surfaces are pressed together, chain diffusion and entanglement occur from the mobile polymer chain ends, resulting in poor block resistance. Several factors can influence the blocking resistance of a formulation, such as the polymer T_g, the formulation space, and the type of surface-active additives being used. Figure 1 is a cartoon schematic demonstrating good and poor block resistance.

FIGURE 1 Cartoon schematic of block resistance.

FCS are commonly used to improve blocking resistance in waterborne formulations by providing a “protective layer” at the surface, thereby preventing the polymer-to-polymer entanglement (Figure 1). However, due to the environmental challenges and regulations of fluorocarbon chemistry, formulators need a new additive solution to improve block resistance. The scope of this project was to identify an additive that could be easily used by the formulator, either as a post-add to an emulsion or added directly into the formulation, which could match the block resistance performance of fluorosurfactant chemistry without the environmental concerns.

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.