Rigid polyurethane foam primer

In this sidebar to CT's feature story entitled "Structural polyurethanes: Bearing bigger loads," the author defines the differences between the two major types of polyurethane foam.

There are two types of rigid polyurethane foam: polyisocyanurate formulations (PIR) and polyurethane (PUR). These designations are misleading, however, because both are polyurethanes — that is, they are polymers formed by reacting a monomer containing at least two isocyanate functional groups with another monomer having at least two hydroxyl groups. The resulting polymer is a chain of organic units joined by urethane (carbamate) linkages. In PIR the proportion of methylene diphenyl diisocyanate (MDI) is greater than for PUR. The PIR reaction also uses a polyester-derived polyol in place of a polyether polyol. PIR foam typically has an MDI/polyol ratio (also called its index) of between 200 and 500, whereas PUR indices are normally about 100. The net result is that PIR foams are more crosslinked and, therefore are, stiffer but more friable than PUR foams.

PIR foams, sometimes called “trimmer” foams, generally are low density (1.8 to 6 lb/ft³) with high insulating values and good compressive strength. Friability, however, limits their utility in sandwich panel applications, because this lack of toughness can result in a failure of the foam-to-laminate bond under dynamic loads. As a result, structural use of these foams is usually limited to two roles: 1) as core-carrier material for glass-fiber reinforcement or 2) to provide internal mold shape for laminate overlays.

Considerably tougher (less friable) than PIR foams, PUR foams are produced in densities ranging from 2 to 50 lb/ft³, depending on the formulation, can retain a substantial strength and toughness up to 275°F/135°C, which allows their use with high-temperature-cure prepregs in ovens and autoclaves. Cost is usually directly correlated to density — a 20-lb/ft³ foam is twice the cost of a 10-lb/ft³ foam. PUR foam is widely in structural shapes, such as transoms and bulkheads, in RTM cores and composite tooling, and as an edge-close out in honeycomb aircraft interior panels.

A third category, a blend of PIR and PUR foams, attempts to get the best of both worlds. These foams provide some improvement in strength compared to PUR foams, with reduced friability relative to PIR foams.

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