Foams are made by forming gas bubbles in the polymerising mixture, with the use of a blowing agent. Foam manufacture can be carried out continuously, to produce continuous laminates or slabstock, or discontinuously, to produce moulded items or free-rise blocks.
Flexible foams can be produced easily in a variety of shapes by cutting or molding. Widely used in the furniture and automotive markets as comfortable and durable seating foam, they are also popular for mattresses and pillows.
Flexible foams can be produced in high and low densities. Low-density flexible foams are materials of densities 10-80kg/m3, composed of lightly cross-linked, open cells, which enables air to flow through the structure very easily. Semi-rigid variants also have an open cell structure but different chemical formulations.
High density flexible foams have densities above 100kg/m3. The range includes moulded self-skinning foams and microcellular elastomers. Self-skinning foam systems are used to make moulded parts with a cellular core and a relatively dense, decorative skin. The biggest applications of self-skinning foams and microcellular elastomers are in moulded parts for upholstery and vehicle trim, and for shoe soling.
Low-density rigid foams (30kg/m3 - 80kg/m3) are highly cross-linked polymers with a closed cell structure - each bubble within the material has unbroken walls so that gas movement is impossible. These materials offer good structural strength in relation to their weight, combined with outstanding thermal insulation properties. This has led to their widespread use as an insulant in buildings, refrigerators, freezers and refrigerated transport vehicles.
An HCFC (hydrochlorofluorocarbon), HFC (hydrofluorocarbon) or other blowing agent is usually contained within the cells, and as these substances have a much lower thermal conductivity than air, such closed-cell foams have a significantly lower conductivity than any open-celled foam. To retain this low thermal conductivity the gas must not leak out. Consequently, rigid polyurethane foam insulation must have at least 90% closed cells and a density above 30kg/m3.
Although foamed polyurethanes form some 90% by weight of the total market for polyurethanes, there is a wide range of solid polyurethanes used in many, diverse applications.
Solid polyurethane elastomers
Most polyurethane elastomers have excellent resistance to abrasion and attack by oil, petrol and many common solvents. They can be tailored to meet the needs of specific applications, as they may be soft or hard, of high or low resilience, solid or cellular.
Adhesives, binders and coatings
Polyurethanes are also used in flexible coatings for textiles and adhesives for film and fabric laminates. Polyurethane coatings give the highest wear resistance to surfaces such as floors and the outer skins of aircraft. Polyurethane binders are used to bind waste to produce new materials ranging from construction boards to sports surfaces.