New Standard for Wood Coatings

Two-Component Water-Based Polyurethane Systems Without Compromising Drying Time

Approximately 40% of the global industrial wood coatings market is based on conventional solvent-based polyurethane technology. Water-based polyurethane systems are high-performance and environmentally compatible, but their inherently slower drying has limited the market's shift toward this low-emission alternative. A new hybrid hydrophilic polyisocyanate offers a promising solution by drying quickly.

The industrial wood coatings market, representing 5% of the global industrial coatings market [1], is still dominated by 2K solvent-based (SB) polyurethane technology [2] due to the combination of the following:

• High mechanical and chemical resistance, • A unique balance of flexibility and hardness, • Very rapid drying enabling high productivity. Sustainability is becoming more important, and there is a growing need for alternatives to reduce VOCs in these coating applications. Water-based (WB) solutions already exist on the market, which is why users must choose between fast-drying but lower-performance systems (1K polyacrylate (PAC) systems) or higher-performance but slower-drying systems (2K WB PU). Unlike other applications, SB wood coatings typically rely on aromatic or mixed aromatic/aliphatic polyisocyanate crosslinkers to accelerate drying. Yellowing caused by aromatic polyisocyanates is not an issue for clear coatings on wood because wood has a high tendency to yellow (at a greater rate than yellowing caused by aromatic polyisocyanates). For white-pigmented systems, aliphatic polyisocyanates (or blends with aromatic polyisocyanates) are used in combination with polyols to achieve low yellowing levels when required, but in this case drying occurs more slowly.

Rapid Crosslinking Hardener Development Is Key

Aliphatic hydrophilically modified polyisocyanates [3] that can be emulsified with water are used to formulate aqueous two-component polyurethane coatings. Modern WB 2K PU coatings are definitely comparable and sometimes even superior to solvent-based coating materials in terms of mechanical and chemical resistance. However, visibly slower drying prevents greater adoption in the wood sector, as finished parts cannot be sanded or stacked until they are dry. By selecting aqueous fast-drying polyols, the reactivity of 2K WB systems can be improved, but it has still not been possible to match the drying speed of 2K SB PU coatings. The hardener determines the system's curing speed, so improving hardeners is necessary to complete the most challenging part of developing rapid crosslinkers in water-based coatings.

New HDI-TDI Hardener for Faster Curing of 2K WB Systems

By selecting the right monomers and optimizing the amount and type of hydrophilization, we can develop the first mixed aliphatic-aromatic polyisocyanate based on HDI and TDI, suitable as a crosslinking component for 2K WB PU coating systems. Using this new crosslinker in combination with existing standard aqueous hardeners provides significantly faster drying compared to established water-emulsifiable polyisocyanates. This new hardener, Bayhydur® quix 306-70 (Table 1), enables formulation of fast-drying WB 2K PU wood coatings with high mechanical and chemical resistance, good film appearance, and long pot life.

Faster Curing With Minimal Trade-Offs

The new hardener has been tested in different systems to see the benefits of the new HDI/TDI trimers. Properties were compared by preparing coatings based on a primary (Prim. PAC) and a secondary (Sec. PAC) acrylic polyol combined with two different hardeners (the new PIC versus a standard anionic-hydrophilized HDI trimer (STD PIC)). The primary PAC is a 40% solids acrylic dispersion containing 1.5% OH on solids basis; the secondary PAC contains 3% OH on solids basis and 40% solids. The same NCO/OH ratio was used for comparison. When replacing the standard hardener with the new one, comparable gloss and haze values are achieved in the final films (see Figure 1). As expected, coatings containing the new hybrid hardener cure significantly faster than those prepared using standard aliphatic polyisocyanate. There is a notable improvement in T4 (overall drying) related to hardener reactivity. Sanding drying time is hardly affected by the hardener's aromatic character, because T1 is related to solvent and water evaporation rather than crosslinker reactivity (Figure 2).

Faster curing is also evident in faster hardness evolution of the system when the new PIC is used (Figure 3).

Despite the faster curing shown in the examples above, formulations based on the new PIC have sufficiently long pot life (the tested formulations lasted >7 hours). Depending on the formulation, some reduction in pot life was observed, but generally no significant reduction in pot life occurred. Since the new polyisocyanate is based on anionic hydrophilization, it provides very high chemical resistance even in white-pigmented coatings. The chemical resistance of a coating based on aliphatic/aromatic polyisocyanate (new PIC) is comparable to that achieved with a standard high-end hydrophilic polyisocyanate (STD PIC): the system based on the new hardener was found to pass DIN 68861-1B, meaning it passes 16-hour coffee and 6-hour red wine tests even in a white-pigmented formulation. The new hardener was also tested with a polyurethane polyol dispersion (OH-PUD). This polyurethane dispersion is 38% solids and contains 3% OH. This polyurethane has very high final hardness and very fast drying characteristics [6]. Figure 4 shows the drying and hardness evolution of this polyurethane polyol with the two polyisocyanate types. Even in such a fast system, it is possible to reduce curing time by up to 60% using the new HDI/TDI hydrophilic hardener. Another advantage of the new product is that matting for matte formulations is easier. It is well known that faster systems require less matting agent, so the faster curing of the new PIC saves on matting agent quantity. As an example, the OH-PUD formulation requires 20% less TS100 to achieve the same gloss values (1.8% used on STD PIC to achieve 18 GU gloss at 60°C, versus 1.4% on new PIC to achieve 15 GU gloss at 60°C).

Drying Time According to 2K SB Standard

In general, by optimizing the proportion of aromatic content in the hardener and combining it with the right polyol, a water-based system can achieve reactivity comparable to standard 2K SB PU systems at quite low VOC levels and yellowing behavior. Table 2 compares two standard solvent-based aromatic systems with two formulations based on the new hardener. The solvent-based systems are based on a polyester polyol crosslinked with aromatic polyisocyanates and are compared with two 2K WB coatings based on the new hardener. For water-based formulations we selected two different polyols; a primary acrylic polyol and an OH-functional polyurethane dispersion. The table shows that water-based coatings based on the new hardener can match and even improve upon standard solvent-based systems but at lower VOC levels. Using polyurethane polyol further reduces VOC levels because this product can be formulated without adding an auxiliary solvent. An additional benefit is that despite the new hydrophilic hardener's specific aromatic character (see Figure 5), the product's yellowing behavior is closer to an aliphatic product, meaning the product is suitable for white-pigmented formulations. A new hydrophilic HDI/TDI polyisocyanate crosslinker enables formulation of high-performance 2K WB PU coatings with reactivity comparable to standard solvent-based 2K PU systems, but with lower VOC levels and improved yellowing behavior. All other coating properties are at the same level as existing high-performance standard 2K WB systems. This new development will facilitate the transition from established 2K solvent-based technology to more environmentally friendly high-performance water-based solutions.

Summary

• Solvent-based polyurethane technology currently dominates the wood coatings market due to its high performance and rapid drying, • A new hybrid hydrophilic polyisocyanate offers a more environmentally friendly alternative, • The new aromatic-aliphatic crosslinker makes it possible to formulate high-performance 2K water-based coatings that dry as fast as standard solvent-based systems but with lower VOC, • Yellowing behavior is similar to that of an aliphatic product; this means the system is also suitable for white-pigmented formulations.  

Figure 2. Comparison of drying time of different 2K WB coatings (based on secondary and primary acrylic polyols and two different hardeners) at room temperature (23°C, 50% RH) on glass according to DIN 53150, 120 g/m2. The total drying time (T4) of 2K WB systems can be cut in half using the new hardener compared to a standard system.

Figure 3. Hardness development of coatings based on different polyols combined with both hardeners. The new development shows faster hardness development than the homologous HDI type

 

Figure 4. Drying and hardness of coatings based on OH-functional PUD and different hardeners. The new PIC shows faster hardness development compared to the homologous HDI type.

  Figure 5. Comparison of yellowing under natural light (under glass, south-facing at 45°C) from coatings based on the new hardener and standard types (aliphatic and aromatic)  

References [1]O. Menukhin, Irfab-Marktstudien, Polyurethanlacke, FARBE UND LACK, 04/2018, p. 15 [2]IrfabTM Global Industrial Coatings Markets (GICM), PRA World Ltd (2017) [3]Meier-Westhues U., Danielmeier K., Kruppa P., Squiller E. P., Polyurethanes: Coatings, Adhesives and Sealants, 2nd Revised Edition Hanover: Vincentz Network 2019, p. 48 - 51 [4]Maria Almatö, Martin Melchiors, Eva Tejada "The solventless solution" European Coatings Journal 07 | 08 l 2010, p.38