This technology is expected to increase its share in industrial applications
As more and more companies focus on the issue of sustainability, water-based PU coatings are increasingly coming into focus. We talked about this with Eugénie Charrière, Global marketing & strategy director Vencorex.
What is the state of affairs with water-borne PU coatings?
Eugénie Charrière: Since the 1990s, when the first polyisocyanate for water-borne polyurethanes was introduced, we have invested a lot in the application science and development of new grades. To reduce VOC content as much as possible, solvents are replaced with water. But isocyanates react with water, which could lead to poor coating appearance, with CO2 bubbles and micro-gels of urea trapped into the film. Besides, isocyanates being hydrophobic, they are difficult to mix with water-
borne resins and poor film appearance can be observed in case of heterogenous mixtures.
To overcome these challenges, we have developed a proprietary unique technology: the isocyanate is modified to be self-emulsifiable but temporarily protected from water by a hydrophilic shell. This allows a good dispersion of the crosslinker into the water. As the crosslinking reaction happens after the majority of the water is evaporated, a smooth film with good appearance and high gloss is obtained. Various grades are available, depending on market requirements, including low viscosity easy-to-mix crosslinker for matt finishes; product for high gloss and enhanced heat and humidity resistance coatings and fast drying hardeners, providing early stackability resistance and a long pot-life.
In parallel, resin manufacturers have also been developing new water-borne polymers, based on different chemistries and production processes. Thanks to this toolbox, water-borne PU formulations with high performance can be obtained, without compromising on quality and durability. This is why this technology continues to grow faster than the market and is expected to increase its share in industrial applications.
Which novel application fields for PU coatings do you consider especially promising and why?
Charrière: Rising awareness of climate change and demands for more sustainable goods influence buying trends. Not only have new regulations been introduced, but major brand owners have started to push their suppliers to move to more environmentallyfriendly products. As a result, the switch from low-solids solvent-based coatings to water-borne and high solids PU formulations accelerated. Aliphatic polyurea (PUA) or polyaspartic (PAS) formulations have started to grow all over the world, since they can be formulated at very low or even zero VOC. In PUA, polyisocyanates are reacted with polyamines, instead of polyols in PU.
It is a PAS if the amine is a secondary amine with high steric hindrance. When using aliphatic polyisocyanates, PUA/PAS formulations show excellent UV resistance. PUA/PAS have been used in the USA for a while, especially for floor coatings, as they allow high film build, together with time savings and fast return to service.
Fast curing and quick hardness development also occur at low temperatures. Thus, PUA/PAS are of high interest for cold climate countries, like Northern Europe or Russia. But until recently, the growth of this technology has been limited by film brittleness and short pot-life linked to its high reactivity, necessitating specific application tools.
We have launched several low viscosity, flexible and elastic aliphatic polyisocyanates for PUA/PAS, showing superior mechanical and weathering resistance. They provide pot life long enough for standard equipment, good hardness and acceptable working time allowing their use in protective or sport flooring coatings. This expands the possibilities to general industry or refinish coatings where PAS starts to be used for spot repair.
These answers are part of a longer article that can be found in the European Coatings Journal 01/2021. The issue is available digitally in our 360° online library.
