Paint & Coatings Industry - February 2009 - (Page 34) 1K Polyurethane for Conventional 2K Applications olyurethanes are a unique class of polymers due to their combination of both hard and soft segment blocks within the same polymer chain. This combination results in polyurethanes being able to demonstrate high surface hardness, very good chemical resistance and toughness along with excellent flexibility, particularly at low temperatures where other polymers may become brittle.1-3 Moreover, this combination of polymer characteristics leads to polyurethanes displaying excellent durability and abrasion resistance. The hard segment of the polymer, made up of urethane and urea linkages, provides the hardness and chemical resistance through the formation of strong hydrogen bonds; the hydrogen bonding can be considered a weak form of self-crosslinking (compared to covalent bond type crosslinking). The soft segment, determined by the polyol from which the polyurethane is made from, contributes to the low temperature flexibility, impact resistance and, to a large extent, the film forming capabilities of the coatings. This combination of properties has led to polyurethanes being the polymer of choice for wood floor coating applications, particularly for on-site application or refinish. Initially, these coatings were solvent, solution-based polyurethanes, however, with government regulations limiting the coating VOC, waterborne polyurethanes have gained an everincreasing market share, particularly as their performance has been demonstrated to be comparable to or better than their solventborne counterparts in many applications. Waterborne polymers are more user friendly than their typical solventborne counterpart and provide reduced flammability, health and odor concerns. Despite the excellent properties exhibited by polyurethanes they typically need to have some degree of crosslinking to provide the required performance needed for certain applications such as wood floor coatings. The required crosslinking is often conventionally done by the introduction of an added crosslinking component to the polyurethane. This leads P to concerns on handling toxic crosslinkers, limited pot life of the mixture, the need to mix components prior to application, and the potential for waste due to exceeded pot life or mixing mistakes. This paper discusses furthering the inherent advantages of waterborne polyurethanes including their user friendliness by way of a newly developed one-component (1K) self-crosslinking polyurethane technology that performs on par with conventional commercial two-component (2K) waterborne PUDs for wood floor coatings. Waterborne Polyurethanes 2K Crosslinkers There are several options available to formulators for improving the performance of waterborne thermoplastic polyurethanes by crosslinking of the polymer. Among these, polyaziridines and polyisocyanates have found significant commercial application in contractor-applied wood floor coatings, mainly due to their cure effectiveness at ambient conditions in a reasonable length of time. Polyaziridines can give very effective crosslinking in carboxylic acid functionalized polyurethane dispersions with good property development after as little as 1 day curing time at room temperature. Moreover, they are readily incorporated into waterborne polymers due to their high degree of water solubility and low viscosity. Polyaziridines have a reasonably long pot life of at least one working day, or 8 hours, and purportedly can be “re-inoculated” with a second dose after the recommended pot life has expired. But polyaziridines have issues with skin irritation and are a suspected carcinogen/mutagen primarily based on the fact that they give a positive result in the Ames bacterial assay test. The use of water-dispersible polyisocyanates as a crosslinking component of 2K PUD for floor coating applications is a somewhat newer technology that is growing in popularity. It has gained some favor over polyaziridine crosslinkers due to reduced toxicological concerns, however, they are still of concern for allergic reaction, skin irritation and asthma.4 By Anthony Pajerski and George Ahrens | Lubrizol Advanced Materials, Inc. Brecksville, OH 34 FE BRUARY 2009 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM
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