Previous UL Prospector articles have discussed typical crosslinking chemistries1,2,3,4. These were predominantly polyurethane-based, relying on the reaction between a polyisocyanate (PI) and a polyol in a two-part system. There are also chemistries that rely on heat to initiate a reaction, such as hydroxyl-functional acrylics that utilize melamine as the crosslinker, or ambient-cure using polyisocyanates.
In the former, lower levels of –OH are ideal for the formulation of hot melt adhesives, while at higher hydroxyl content, acrylic emulsions such as ROSHIELD™ 3275 react with polyisocyanates. This is helpful for wood coating applications where early hardness and sandability are important, as either an ambient-cure system or heat-cure to accelerate processing.
A one- or two-part (or -component) system is also referred to as 1K or 2K. That means that the paint consists of one-component which may be self-reactive (crosslink) or non-reactive, or two components which are two parts that are mixed together prior to application.
There are many factors that are driving the industry to develop a 1K self-crosslinking system, as well as isocyanate-free chemistries. First and foremost, there is a high level of regulatory pressure to eliminate polyisocyanates, although only about 15 percent are used in coatings, adhesives, sealants, and elastomers (CASE) applications, with the rest in rigid and flexible foams.
A second motivation is the 2K system somewhat limits applications, in that a “kit” consisting of typically a part-filled gallon and a quart, has to be well-mixed prior to the time of application. This requires the reaction of the entire gallon, unless the applicator blends less material, back-calculating based on weights obtained from the MSDS’ density reporting.
Unless from the coatings industry, it is unlikely someone can prepare less than the full kit. On-site mixing is an issue as well, as poorly homogenized coating will provide less-than-ideal performance and may affect dry and cure. Open time is also a concern, in that the pot life may not be sufficient to complete a project.
Therefore, at the least, isocyanate-free crosslinking is desired. A recent article in Coatings Tech; “Isocyanate-Free Polyurethane Technology for Automotive Refinish Applications”5, outlines such an approach to achieve that goal.
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The team from The Dow Chemical Company presents an ambient-cure, two-component isocyanate-free polyurethane coating technology based on the reaction of polycarbamates with polyaldehydes. The technology provides the benefit to decouple pot life from cure speed. This achieves a goal stated previously: faster return to service, higher production throughput, and less material waste.
Research in 1K ambient cure systems has existed for over 30 years, but some approaches suffered from performance-degradation due to long-term shelf stability and the inability to effectively keep the components from reacting until it was desired. In 1985, Rhone-Poulenc/Rhodia developed a product trade named “Ambicure”, which was a 1K ambient-cure aqueous resin. It is cited in United States Patent 5,439,952 issued to The Thompson Minwax Company (Sherwin-Williams) in 1995. Ambicure utilized a water-soluble amine and a reactive acrylic polymer that could be formulated into high-performance coatings but reacted slowly in the can, which degraded properties.
Blocked polyisocyanates are another approach that requires heat for activation. A blocked polyisocyanate is an isocyanate reaction product that is stable at room temperature but dissociates to regenerate isocyanate functionality when heated. Temperatures between 120° and 200°C are needed to release the blocking agents, which usually volatilize from the coating. The resulting polyisocyanates can react with other active hydroxyl-containing compounds to form thermally stable urethane or urea linkages.6
Blocked polyisocyanates based on aromatic polyisocyanates dissociate at comparable temperatures to those based on aliphatic polyisocyanates. For technical and economic reasons, blocked polyisocyanates are most preferred for a heat curing coating system that requires 1K technology and free from isocyanates. Key advantages include ease of handling, broad choices of baking temperature and less sensitivity to atmospheric humidity.
Via sol-gel chemistry, The Dow Chemical Company7 utilizes epoxy silane and triflates to produce a 1K system that crosslinks at elevated temperatures below the melting point of polycarbonate (PC) and polymethylmethacrylate (PMMA), such that the coating is suitable to coat plastics used for transportation windows, eyewear, etc.
A United States patent application from Columbia Insurance Company (Benjamin Moore & Co.)8 presents an invention relating to storage-stable film forming latex particles that crosslink under ambient conditions during or after drying. Monomers for the formation of the crosslinkable moieties in the latex particles include diacetone acrylamide (DAAM) or the likes and methacrylamide (MAM) or the likes with or without styrene. A paint composition comprising the storage-stable latex particles that may include a small amount of crosslinking agent such as adipic acid dihydrazide (ADH) in the aqueous phase is also described. This is a far different approach than what had been tried in the past.
With the continued regulatory pressure to move away from polyisocyanates, as well as the need to decouple potlife from cure speed, we will see sustained efforts to develop 1K systems that perform as well as their 2K counterparts and provide other benefits.
References:
- “Industrial Coatings Resins – Polyurethanes, Part 1”
- “Industrial Coatings Resins – Polyurethanes, Part 2”
- “Dynamic Polyurethanes” (Infographic)
- “Polyisocyanates Deep Dive”
- CoatingsTech: “Isocyanate-Free Polyurethane Technology for Automotive Refinish Applications”
- Covestro: “Prepolymers for crosslinking solventborne, waterborne resins”
- U.S. Patent 6586502 B2
- U.S. Patent Application 20160186000 A1
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Pity that the coatings industry missed the low hazard 1k coating forming Nylon self arranging coatings from 2010 with the ability to form self arranging heterogeneous structures.
It was the binder for the paint on the roads during the Olympics in London
Polymer and Method of Forming a Polymer
Publication number: 20120004338
Abstract: The present invention provides a method of forming a polymer, the method comprising reacting a first species comprising at least one isocyanate group with a second species comprising at least one aziridine group to form the polymer.
Type: Application
Filed: December 18, 2009
Publication date: January 5, 2012
Applicant: Adbruf Ltd.
Inventor: Duncan Hywel-Evans
The non-isocyanate coating technology already exist with a company called Hybrid Coating Technologies. The tech has already received an EPA award. This will be the future for a greener, safer chemical industry.
Aziridine is a great cross-linker for PU’s, and I seem to recall Flecto sold a coating that included a small container of it that you mixed in with the coating right before application. However, it is toxic and not used much if at all any longer (at least in North America)
Our company specializes in the production of crosslinked catalyst polyurethane catalyst.DBTL MBTO
Thanks for posting this information