Stress relaxation of glass‐fiber‐reinforced rigid polyurethane foam

Flexural stress relaxations were measured for rigid polyurethane foams (PUF) and glass‐fiber‐reinforced rigid polyurethane foams (FRU). The results were successfully analyzed in terms of the five element Maxwell model: (1) Samples reinforced with longer fibers exhibit reduced stress relaxation and reduced temperature dependency of stress relaxation; (2) The increased expansion ratio reduces the flexural modulus of both reinforced and non‐reinforced materials, but the stress relaxation tends to increase greatly at the higher temperature for PUF, while not so greatly for FRU; (3) The temperature dependency of E 1 decreases as longer fibers are used to reinforce the polyurethane. The dependency is minimal for the polyurethane reinforced with continuous fibers, where the reinforcing effect is maximal; and (4) The activation energy calculated from τ 2 according to the Arrhenius plot is smaller for the longer fiber reinforced polyurethane foams.