In Part 1 of this post, I opened a discussion on the growing popularity of waterborne 2K polyurethanes, also commonly called “2K WB’s,” “2K urethanes,” “2K-PU,” “two-part” or “2-pack” urethanes.
These are waterborne coating formulations that use a secondary additive in their design, making the coating a 2-component system.
With the use of 2K polyurethanes trending on social media channels, often with NOTHING being said about their potential hazards…
I decided to drill deeper into the topic, starting with the history of 2K waterborne coatings, how they are made, and why they are still available when self-crosslinking 1K and externally crosslinked waterborne technology is gaining popularity in the wood finishing markets.
What I found was some very interesting — and sometimes troubling — approaches to the 2K trend, both in what the formulators are branding as “two-component” in functionality, and the type of catalyst chemicals being marketed and sold to the unsuspecting tradesperson and DIY customer.
But, to better understand the current situation, let’s first take a brief step back and learn a bit more about the history of 2K coatings specific to the wood finishing trades.
First, let’s look at the terminology of “2K” in the realm of water-based coatings being marketed in 2021 to woodworkers and cabinet finishers.
Similar to the use of the term 2K in solvent-based coatings, as it is being used today in water-based formulas, a 2K system is simply a two-component formulation composed of a Part-A and a Part-B.
This coating system requires the blending of the two components together, in premeasured volumes, into a single, functional reaction that allows the resin to through-cure and not remain soft or tacky.
Common 2K systems from the 20th Century that are still in use today are solvent-based acid-catalyzed varnishes, and isocyanate catalyzed solvent-based urethanes.
Again, these older examples of 2-Component coating technologies require the use of an external additive (the “Part B”) to complete the chemical reaction. If the external additive is not blended into the resin in the correct proportions the reaction will not be complete, and the resin will not attain its intended physical properties.
What I see trending in the waterborne coatings market is the modification of the term 2K for use with water based polyurethane and acrylic lacquer finishes that will dry and cure to a film without the use of an actual catalyst — but are fortified with a blocked isocyanate in rather low percentages, much in the way a crosslinker is used.
The “Part-A” side of these 2K products are fully capable of drying and curing completely on their own accord without the addition of their respective “Part-B” affiliate. Whereas the traditional solvent 2K systems mentioned above absolutely require the addition of their allied catalyst to perform the curing mechanism.
Like the trendy 2K WB’s, modern waterborne urethane and acrylic resin systems being marketed today will form a film and cure when exposed to oxygen alone. The addition of a less toxic chemical compound, correctly called a crosslinker, will enhance and fortify the final physical characteristics of the curing WB resin — also at much lower percentages by volume. An example of a crosslinking agent common in the early development of WB technology is aziridine – which is now out of favor due to its dangerous effects on regular users. Other, and far less hazardous crosslinkers are carbodiimides, siloxanes and specialty ester diols such CL100 Crosslinker.
Yes, catalysts and crosslinkers function differently on a chemical level, but this functionality (and incorrectly using the terms interchangeably) is NOT what has me, and others, concerned.
Rather, it’s that the additives being sold as part of a “modern” water based 2K polyurethane system containing highly toxic modified isocyanate blends, and in some rarer instances polyfunctional aziridine, are being promoted as user and environmentally safe.
The use of aziridines and perhaps more importantly, isocyanate compounds at low but still hazardous percentages in waterborne paint and clear coat systems being marketed today leads to a further blurring of definitions between catalyst and crosslinker, as well as user and environment safety…
And one must question why these very toxic additives are being sold to a customer base that should not be exposed to these chemicals and/or doesn’t realize the higher levels of safety protocols required to use them.
We’ll go into detail about these additives in Part 3 of this post, so stay tuned.
For now, please keep your questions and comments coming about 2K polyurethanes. Please share your thoughts or read what others are saying in the comments section below.