1.1.1 Requirements of Foam Core Materials
Many unique properties are required for structural sandwich cores; thus, prudent selection of the polymer from which the foam is produced is most important. Not every polymer can be foamed commercially, and this is why the polymers available for making core materials are few in number. First of all, the foam should be sufficiently stiff. As the core in a structural sandwich, its shear modulus is a representative indication of sandwich stiffness. PVC foam core with density of 40 kg/m3, a general-purpose core material, has a shear modulus of 12 MPa, while PMI foam core, a high-performance core material, with a lower density of 30 kg/m3 has a shear modulus of 13 MPa. However, a rigid PUR foam with a density of 70 kg/m3 has only a shear modulus of 7.6 MPa, which is why PUR foam is a lower-performance core material.
Equally important are the surface properties of the foam core. It should have a sufficiently high surface energy so that it can be wetted out (compatible) with all the adhesives and liquid thermosetting resins that are used in composite industries, but also it should have good resistance against solvent attack so it cannot be softened by any solvents in the resin or adhesive. The close cell content of the foam core also is a critical property that should be, generally, more than 80%. If the open cell content is too high, the liquid resin will migrant into the core during laminating, increasing the weight of the sandwich laminate. In summary, the foam should have a high-enough surface energy to bond to with the adhesive or resin for laminating the facing but cannot be attacked by them physically or chemically. The bond strength must be high enough to withstand the constant bending, compressive, or tensile forces of dynamic loading, such as the forces on a boat hull or rotating turbine blade. The coefficient of thermal expansion (CTE) of the core, the laminate material, and resin or adhesive must be compatible to ensure that thermal cycling doesn’t cause unequal expansions and contractions, leading to de-bonding.
The hot and cold temperature resistance is another important property of the structural core materials. During lamination, the core will endure a heat history because of the need to cure the resin at elevated temperatures and by the heat released by adhesive or resin curing (exothermic). The bond layer between facing and foam surface will shrink or swell if the foam core does not have a sufficiently high heat resistance, reducing the bond strength and possibly causing a laminate to fail. For the specific laminate application or service, the sandwich structure could be exposed to extremes in temperature environment. The foam core should maintain mechanical properties within an acceptable design range to ensure that the composite sandwich product performs the intended function.
The foam core also needs to be fatigue-resistant. A product made by using the foam cored sandwich construction, such as a boat or a wind turbine blade, will endure cyclical forces. For example, a helicopter or wind blade may be designed for over 10 million cycles in its service life. If the core cannot maintain sufficient mechanical properties for its design li...
