Combined effects of temperature and humidity on the mechanical properties of polyurethane foams

The effects of temperature and environmental moisture on the viscoelastic behavior of polyurethane foams were investigated both theoretically and experimentally. It was shown that the effect of the environmental parameters can be explained in terms of a variation of the free volume of the solid fraction of the foams, thus allowing the use of the superposition principle to predict their influence on the viscoelastic behavior of the materials. Dynamic mechanical analyses were performed to measure the dependence on frequency, temperature, and relative humidity of the complex modulus of two different polyurethane foams, differing in terms of their glass transition temperature. The time-temperature-humidity superposition principle was proved to be applicable for the tested materials. Next, the relaxation spectra and their dependence on the relative humidity were adopted to assess its effects on the large strain behavior of the foams.