Polyisocyanurate foam (polyiso foam) is a cellular, thermoset plastic formed when two basic liquid chemicals, isocyanurate and polyol, are combined in the presence of a catalyst that helps the molecules to rearrange and join, a blowing agent to create closed cells in the structure, flame retardents, and possibly other agents. If the "art" of the process is correct, the result is an inert, non-nutritive, highly stable polyiso rigid foam that has highest strength-to-weight ratio as well as thermal insulating values superior to other composite substrate alternatives. Some inappropriately refer to polyiso as polyurethane (PUR), and they should not since polyiso's physical properties are much better, including k-factor, flame/smoke performance, and dimensional stability. [How does polyisocyanurate differ from polyurethane?]
Polyisocyanurate rigid foam cores (often referred to as PIR foam) has been used successfully across the array of different composite designs.
Polyisocyanurate foam cores in these applications is typically shaped as an insulated panel, typically consisting of a polyisocyanurate foam board that forms a rigid foam core, laminated on one or two sides with either metal, foil, fiber-reinforced felt, or coated-glass facers.
Due to its excellent thermal insulating efficiency at service temperatures ranging from -297°F to +300°F, polyiso foam has become the standard for structural composites where thermal insulating value has benefit.