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Polyurethane Adhesives General Description Polyurethane adhesives are supplied as one- and two-part systems which range in viscosity from self-leveling liquids to non-slumping pastes. They cure to form thermoset polymers with good solvent and chemical resistance. They are extremely versatile and can range in cured form from extremely soft elastomers to rigid, extremely hard plastics. Polyurethanes offer a good blend of cohesive strength and flexibility which makes them very tough, durable adhesives. Polyurethanes bond well to most unconditioned substrates, but may require the use of solvent-based primers to achieve high bond strengths. They offer good toughness at low temperatures, but typically degrade in strength after long-term exposure over 302°F (150°C). Since the cure of one-part, moisture-curing polyurethanes is dependent on moisture diffusing through the polymer, the maximum depth of cure that can be achieved in a reasonable time is limited at approximately 0.375" (9.5 mm). Two-part systems, on the other hand, offer unlimited depth of cure. Chemistry One-part polyurethane adhesives can react with moisture to polymerize. Another cure mechanism involves the evolution of species that inhibit the cure of the polyurethane. In either case, cure is dependent on a chemical species diffusing through the polyurethane matrix, so the depth of cure is limited. Two-part polyurethanes, which generally cure through the reaction of an isocyanate and a polyol, avoid this limitation and offer superior depth of cure. In either case, the polyurethane polymer forms rigid and soft domains that give the polymer its balance of flexibility and high strength. Advantages • Extremely tough • Good resistance to solvents • High cohesive strength • Good impact resistance • Good abrasion resistance Disadvantages • Limited depth of cure for one-part polyurethanes • Mixing required for two-part polyurethanes • Primer may be needed for adhesion to some substrates • Limited high temperature use 10 The Loctite® Design Guide for Bonding Plastics, Volume 6PDF Image | Design Guide for Bonding Plastics Volume 6 LT-2197
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