Flexible spacer fabrics for reinforcement of rigid polyurethane foams in sandwich structures

The development and characterization of sandwich structures made from rigid polyurethane (PUR) foams reinforced with spacer fabrics offers a wide field of investigation. Due to the textile structure of warp-knitted spacer fabrics an intrinsic sandwich layer construction is given. The two textile surfaces are held by monofilaments at a defined distance. It is possible to manufacture spacer with a height of 65 mm. With its open diffusion thread architecture and the associated low mass and high dimensional stability, they can be used as reinforcement structure for fiber-reinforced composites. The polyurethane formulations must have the fluidity for penetration of the spacer fabric and has to be adjusted to the conditions of mechanical processing. The macro- and microstructure of textiles must be reconsidered and interpreted in terms of the function as reinforcement of foam molding. In this study the process for reaction injection molding of sandwich structures with warp-knitted spacer fabrics and polyurethane foam is investigated. A commercial PUR-foam formulation and different types of spacer fabrics, varying in the textile structure are used for the experiments. The impregnation of the fabrics with the PUR-foam is done in a closed mold with a rectangular cavity, thus molding a sandwich panel suitable for 3-point-bending test. For the textile reinforced polyurethane foams significant increases in both the absolute and the specific mechanical properties can be demonstrated. The textile structure achieves this with a fiber volume content of less than 5%. The reinforcement effect is caused by hindering the spacer threads in bending and buckling by the lack of space occupied by the PUR-foam. This results in a special deformation behavior of the composite structure.