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 composite-core applications are rigid foams.
The term “rigid polyurethane foam” comprises 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.
It is this friability that limits the utility of polyisocyanurate foams in composite panel applications, as this lack of toughness at the foam surface can cause failure of the foam-to-laminate bond under conditions of vibration or flexure. For this reason, structural use of these foams is often limited to internal-mould shapes for stringers and hat-section reinforcements in FRP boat construction. Here the foam has no supportive function but to provide a form for the fiber-and-resin composite laid over it.
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 moulded 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.
There are producers of polyisocyanurate/polyurethane foams, a blending of the two foam types, trying to get the best of both worlds. These foams offer some improvements in strength values (compared to polyurethane foams) and a reduction of friability (compared to polyisocyanurate foams) with a sacrifice in temperature resistance.
Still, the result of this combination is a compromise, and may not present the best properties of both polymers in some applications. These foams are limited to densities of 2-8 pounds per cubic foot.
Published courtesy of Ted Hile, General Plastics Mfg. Co.
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