Polyurethane

Extremely wide variations in forms and in physical and mechanical properties are available in polyurethanes. Grades can range in density from 0.5 lb/ft3 in cellular form, to over 70 lb/ft3 in solid form, and in hardness from rigid solids at 85 Shore D to soft, elastomeric compounds. Polyurethane polymers, produced by the reaction of polyisocyanates with polyester or polyether-based resins can be either thermoplastic or thermosetting. They have outstanding flex life, cut resistance, and abrasion resistance. Some formulations are as much as 20 times more resistant to abrasion than metals. The noncellular grades - millable gums and viscous, castable, liquid urethanes are elastomeric thermoset types, processed by conventional rubber methods. These are discussed in the chapter, Thermoset rubber. Grades processed by thermoplastic methods are covered in the chapter, Thermoplastic elastomers. This chapter discusses the cellular polyurethane materials. Polyurethane foams are thermoset materials that can be made soft and flexible or firm and rigid at equivalent densities. These foams, made from either polyester or polyether-type compounds, are strong, even at low density, and have good chemical resistance. Polyether-based foams havi greater hydrolysis resistance, are easier to process, and cost less. Polyester-based foams have higher mechanical propertie.% better oil resistance, and more uniform Cill structure. Both types can be rayed, sP' molded, foamed in place, or furnished'ir sheets cut from slab stock - buns, 30 to-A in. high, to 80 in. wide, and to 2013 ft 10 ft A low-pressure molding process reaction-injection molding (RIM) is us most exclusively to produce ureth~ane some weighing as much as 100 lb. In the process, two or more highly reactive liquid systems are injected with high-pressure impingement mixing into a closed mold at low pressure, where they react to form a fin~ ished polymer. Depending on formulation, the polymer can be a rigid, integral-skin, microcellular urethane