Eva Tejada – COVESTRO
ABSTRACT
Stricter regulatory requirements on chemicals, emissions and workplace hygiene; the competitive pressure to minimize production costs while remaining innovative; ecological demands fueled by increasing environmental awareness among end-users, particularly in Europe and the U.S.: these key trends in the global wood coatings market are presenting wood coating and furniture manufacturers with new challenges. As a result, OEMs and furniture part manufacturers are looking for more sustainable coating solutions that still keep productivity high. UV curing technologies are a possible alternative if the high investment is justified.
On the other hand, waterborne systems can be easily implemented in existing lines. 1K waterborne (1K WB) coating systems come with good drying times but are inferior in performance compared to solventborne systems.
As for 2K waterborne (2K WB) systems, wood coating manufacturers see them as a low-VOC solution with a similar performance to solventborne systems but are unhappy about their slow drying performance.
A newly developed fast-curing hydrophilic polyisocyanate hardener from Covestro, Bayhydur® quix ultra 306-70, uniquely solves this slow-drying problem while maintaining the required highperformance properties and in particular, lowering VOC emission levels to less than 100 g per liter.
MARKET BACKGROUND AND KEY TRENDS
Around 32% of the global industrial wood coatings market is dominated by traditional 2K solventborne polyurethane technology. Up to now, the strong presence of solventborne technology has been based on the high mechanical and chemical resistance properties of such coatings, their unique balance of flexibility and hardness, and their very fast drying capability.
The latter is of course critical for high productivity, especially in the furniture segment. Though originally developed to protect and enhance the beauty of wood surfaces, wood coatings are increasingly being required to show haptic qualities and even display smart functions. In furniture applications, the surface of an item has become its main image carrier and there is clear evidence of a natural look and feel trend in interior design. This trend is linked to increasing consumer interest in comfort, well-being and more environmentally friendly solutions.
THE SLOW-DRYING ISSUE
Unlike in other applications, solventborne polyurethane wood coatings often rely mainly on aromatic polyisocyanate crosslinkers to speed up drying. The yellowing tendency caused by aromatic polyisocyanates is not a problem for clear wood coatings, as the wood itself tends to yellow to a greater extent than the yellowing caused by the aromatic polyisocyanates in the coating.
For white pigmented systems aliphatic polyisocyanates or blends with aromatic polyisocyanates are used in combination with polyols to achieve low yellowing levels. Not surprisingly, wood coating manufacturers using 2K solventborne polyurethane technology have been skeptical about switching to high-performance 2K waterborne systems because of the slow-drying issue. Wood coating manufacturers who are already using 2K waterborne polyurethane technology are now looking for ways to increase productivity. For both these groups of customers, Covestro has come up with a solution to the slow-drying problem that bridges the gap between sustainability and productivity.
Bayhydur® quix ultra is a novel hydrophilic hardener that allows 2K waterborne polyurethane to dry as fast as solventborne aromatic wood coatings, but with one decisive difference: 80% reduction of VOC emissions, from >500 g per liter for a standard solventborne system to <100 g per liter for a waterborne system.
NOVEL CHEMISTRY BREAKS NEW GROUND
As the hardener determines the curing speed of a wood coating system, efforts have focused on overcoming the hurdle of developing fast crosslinkers that work in waterborne coatings. There have been various attempts to reduce the drying times of waterborne 2K polyurethane-based coatings by hydrophillizing highly reactive polyisocyanates with aromatic isocyanate groups like toluene diisocyanate (TDI).
However, the waterborne systems formulated using these pure aromatic polyisocyanates do not produce coatings with a sufficiently high aesthetic appeal.
Combining hexamethylene diisocyanate (HDI) and TDI did not solve the problem either. Additionally, the pure aromatic and mixed aliphatic aromatic non-ionic hydrophilized crosslinkers contain considerable amounts of hydrophilic polyether alcohols. These are required for adequate emulsification, but this results in a permanently high coating hydrophilicity and reduced hardness.
Consequently, Covestro had to invest a great deal of research into selecting the right monomers and optimizing the amount and type of hydrophilization. Different internal hydrophilization systems were tested to develop the new crosslinker. The best results were achieved using anionic hydrophilization in the polymers. This achieved rapid drying combined with good gloss, film transparency, and outstanding chemical resistance of comparable standard to the best currently available solution. Different percentages of TDI were also tested in the polymer before the optimal ratio of TDI/HDI was found.
The outcome has been the development of the first-ever mixed aliphatic-aromatic polyisocyanate based on HDI and TDI which is suitable as a crosslinker component for 2K waterborne polyurethane coating systems.This new aromatic-aliphatic crosslinker makes it possible to formulate high-performance 2K waterborne coatings that dry as fast as solventborne systems but come with a significant reduction in VOC emissions.
The new hardener, which features a patented technology and is marketed as Bayhydur® quix ultra 306-70, allows the formulation of fast-curing 2K waterborne wood coatings of polyurethane quality, with high mechanical and chemical resistance, good film appearance and a long pot life of up to seven hours. Once Covestro’s chemists had reached this stage in the product development, the novel hardener was put to the test.
The new product was tested in different systems to determine what benefits it brought in wood coating applications. The results revealed up to 60% shorter drying times than standard 2K waterborne systems, which thus matched the drying times of aromatic solventborne systems. With 7-hour workability, a figure that meets the industry’s requirements, the waterborne coatings based on this novel hardener can be formulated to have a similar pot life to solventborne coatings. Last but not least, in both clear and pigmented systems Bayhydur® quix ultra 306-70 was shown to be highly resistant to aggressive substances such as coffee, wine, mustard, and ethanol.
An additional benefit is that despite the specific aromatic character of the hydrophilic hardener, its yellowing behavior is closer to that of an aliphatic product and is thus suitable for white pigmented formulations as well.
Faster drying also has a consequent benefit, the product allows easier down-glossing, making it possible to reduce the amount of matting agent in matt formulations. Example: Formulation based on Bayhydrol® A 2651 and UH 2593/1 with a 1.3% Acematt TS100 leads to lower gloss levels when using Bayhydur® quix ultra as hardener.
SUSTAINABILITY IN PRACTICE
Nowadays, sustainability is an overused buzzword that often hides a multitude of ‘greenwashing’ sins. However, the highperformance 2K waterborne coating technology based on Bayhydur® quix ultra 306-70 does indeed lead to a genuine improvement in the sustainability of wood coatings compared to traditional 2K solventborne polyurethane systems. One important ‘anti-greenwashing’ argument is the positive contribution this novel hardener makes to the following UN Sustainable Development Goals:
– substantial reduction in air pollution during the application phase through much lower VOC emissions (Target 3.9);
– improvement in the safety and well-being of coating manufacturers’ personnel and end-product applicators in their working environment using chemicals that are labeled more favorably and reduce their exposure to solvents (Target 8.8);
– better chemical management through a significant reduction of waste-to-air formation (i.e. VOCs) along the life cycle (Targets 12.4 and 12.5).
The product contributes with the SDG’s 3 thanks to a residual monomer content below 0,1 percent. Drying time reduction has an impact on C-footprint during the drying step, as has been proved by an LCA study done with Hesse, where we have evaluated the C-footprint of a standard solventborne PU system vs. a standard waterborne PU system and a waterborne PU system based on this hardener.
When tracking the lifecycle carbon footprint from cradle to application gate, including application and drying, standard 2K waterborne polyurethane systems (2K WB PU) showed a slight improvement of around 10% in CO2 emissions in comparison to 2K solventborne polyurethane (2K SB PU).
This improvement can be increased up to 25% simply by using Bayhydur® quix ultra, the new patented low VOC waterbased hardener that dries as quickly as aromatic solventborne crosslinkers. The lifecycle assessment study also showed that using this hardener as a drop-in alternative solution to solventborne coatings, the carbon footprint can be improved significantly.
CONCLUSIONS
This new development from Covestro will both facilitate and speed up the shift from the well-established 2K solventborne technology towards more environmentally friendly, highperforming 2K waterborne wood coating systems. The difference Bayhydur® quix ultra 306-70 makes may be invisible in terms of look and feel but is of decisive benefit to both wood coating manufacturers and end-users. Wood coating manufacturers can now offer products that combine high allround coating qualities with a reassuringly high ecological and economic performance. End-users in furniture, joinery and parquet applications can look forward to a fast-drying, high-performing product that is safe and easy to handle.
