The polyurethane foam world is very large and diverse—chances are good you are sitting on some kind of flexible polyurethane foam right now—but the useful products for structural and composite-core applications are rigid polyurethane foams.
The term “rigid polyurethane foam” comprises of two polymer types: Polyisocyanurate formulations, and polyurethane formulas. There are distinct differences between the two, both in the manner in which they are produced, and in the performance of the results.
Polyisocyanurate foams (or “trimer foams”) are generally low density, insulation-grade foams, usually made in large blocks via a continuous extrusion process. These blocks are then put through cutting machines to make sheets and other shapes.
Polyisocyanurate foams have excellent insulating value, good compressive-strength properties, and temperature resistance up to 300 degrees F. They are made in high volumes at densities between 1.8 and 6 lbs per cubic foot, and are reasonably inexpensive. Their stiff, brittle consistency, and their propensity to shed dust (friability) when abraded can serve to identify these foams.
Other uses include under-slab insulation in cold-storage buildings, and below-grade insulation for other building structures.
Polyurethane foams, on the other hand, are considerably different, and more useful in composite constructions. These foams are made in large blocks in either a continuous-extrusion process, or in a batch-process. The blocks are then cut to make sheets or other shapes. They are sometimes also individually molded into discrete part-shapes. Isocyanate foam polymers, while not as heavily cross-linked as polyisocyanurate materials, offer many cost-efficient advantages for users.
Foam densities range from approximately 2 pounds per cubic foot, up to 50 pounds per cubic foot. Unlike thermoplastic foams (PVC, SAN), the unit cost of polyurethane foam increases in a more linear fashion with density; e.g., a 20-pound per cubic foot polyurethane foam will be approximately twice the cost of a 10-pound foam.
There can be considerable differences in foam strength, at the same density, depending on the foam production process used. This results from differences in chemical formulation required to make foams via different production methods, and the curing temperature of the foam while in production. Also, if flammability is a concern, it is useful to know what kind of blowing-agent is employed to create cells in the foam. Many producers use carbon-dioxide (a by-product of the foam-making chemical reaction) to create cells in their foams. Other producers have switched from chlorofluorocarbon (HCFC, HFC) blowing agents to pentane in low-density foam manufacturing processes, which can have a deleterious effect on flame-resistance.
Polyurethane polymer foams can be made considerably tougher and less-friable than the polyisocyanurate foams, mostly at the expense of some modulus and high-temperature strength properties. Nevertheless, these foams can be useful (depending on formulation) to temperatures as high as 275 degrees F, while retaining a substantial portion of their strength and toughness. This allows them also to be used in panel applications along with high-temperature curing pre-pregs, cured in ovens or autoclaves.
Typical applications include use as an edge close-out for honeycomb aircraft-interior panels, structural shapes (transom cores, bulkhead core, stringers, motor-mounts etc.) in FRP boat building, impact-limiters and crash-pads, RTM cores, mold-patterns and plugs, sports-equipment core material, and composite tooling.
Polyurethane foams are becoming increasingly important as replacement products for wood in several applications where substrate consistency and resistance to moisture and decay are important. The excellent chemical and solvent resistance of urethane compounds makes them useful where many other resins might fail.