Like any other reactive adhesive and sealant, polyurethane systems may contain certain hazardous or regulated components where reasonable caution must be exerted to avoid inhalation or contact. The most notorious are isocyanates and mercury or lead based catalysts. Significant effort has been made to reduce or even eliminate such materials in conventional polyurethane adhesives and sealants. This editorial will attempt to summarize some of the more recent activities in this area.
Health and safety information on polyurethane products can be found through several organizations. In the US resources include the Polyurethane Manufacturers Association (PMA) and the Center for the Polyurethane Industry (CPI). In Europe, this sort of information is available through ISOPA, the European Diisocyanate and Polyol Producers Association. Of course, important information is also available through the manufacturers' MSDS (Material Safety Data Sheet).
Silyl polyurethane prepolymers or MS polymers offer moisture curing adhesives and sealants that do not contain any free isocyanate component, but isocyanates are used in the preparation of the prepolymer. Solutions that eliminate the use of isocyanate at any point in the production processes are preferred.
Cyclocarbonates are a relatively new class of compound attracting research interest due to their potential use in the preparation of materials not utilizing highly toxic isocyanates or their predecessors (phosgene) in their production. Cyclocarbonates can be synthesized from corresponding epoxy precursors. Network or linear non-isocyanate based polyurethanes can be formed from the reaction of cyclocarbonate resins and amines.
Several companies are commercializing this technology:
Ecotech, Ltd. (Fairfax, VA), for example, has developed NIPUs in which an intramolecular hydrogen bond is formed during synthesis that improves hydrolytic stability well above that of conventional polyurethanes1.
Huntsman has used a model cyclocarbonate resin (Laprolate-803) for formulation of non-isocyanate based solventless polyurethane derived from the reaction of amines2.
Nanotech Industries is offering Green Polyurethane™ for use in coatings and pilot samples are being evaluated for adhesives and sealants. Nanotech Industries network non-isocyanate polyurethanes are formed as a result of the reaction between cyclocarbonate oligomers and primary amine oligomers.
In another effort to reduce isocyanate components in polyurethane adhesives, Rohm and Haas, the Virginia Polytechnic Institute (VPI), and the USDA have been partnering to develop new biobased, non-isocyanate based chemistries for various uses including flexible packaging applications. This development involves using biomass derived materials for Carbon Michael reactants. Carbon Michael based compositions exceed the performance of existing polyurethane adhesives with faster cure, lower toxicity, and less by-product formation.
Catalysts play a major part in determining the cure rate and final properties of polyurethane adhesives and sealants. Tertiary amines catalyze by activation the carbonyl carbon of the isocyanate, while organo-metallic catalysts work differently by complexing the incoming hydroxyl group with the isocyanate group.
Mercury catalysts such as mercury carboxylates and phenylmercuric neodeconate are particularly effective polyurethane catalysts since they are highly selective toward the polyol-isocyanate reaction. Mercury catalyst can be used at low levels and remain in the final product at concentrations of 0.1-0.6% by weight. They give adhesive and sealant systems a long pot life while still providing excellent cure rates. However, Mercury and lead catalysts wastes must be treated as hazardous materials, and therefore formulators are searching for alternatives. Bismuth and zinc carboxylates have been used but have several shortcomings such as inferior early strength development.
Huntsman has developed and commercialized several new mercury-free adhesives that replace previous mercury-containing versions3. The mix ratio, cure rate, and ultimate performance are claimed to be equivalent to mercury containing versions. The formulated adhesives are well suited for use in rapid setting, high strength assembly of thermoplastic components.