The influence of environmental aging upon the loadbearing properties of polyurethane foams

The effects of environmental exposure upon the loadbearing properties of nine polyurethane foams commonly used for wheelchair cushion construction were studied. Test pieces with and without a stretch cloth cover were aged in the open air at Houston over the period April 26 to October 31, 1982. The indentation resistance of each piece was measured initially and at frequent intervals during the exposure period. Resistance changes between covered and uncovered specimens were not found to be significant. All foams displayed a sharp rise in indentation resistance within the first 2 weeks of aging followed by a gradual decrease to an average 68 percent of the initial value over a 6-month period. These hardness changes were found to be strongly correlated with the density, thickness, and initial indentation resistance of the test pieces. Foams of maximum density and minimum practicable indentation resistance are recommended for wheelchair cushion construction to minimize the adverse effects of environmental aging upon the support properties of these devices.