Thermal conductivity of nonporous polyurethane
Lars Nielsen, Hans-Peter Ebert, Frank Hemberger, Jochen Fricke, Anja Biedermann, Michael Reichelt, Udo Rotermund
An important parameter for the insulation capability of rigid polyurethane foam is the thermal conductivity of the nonporous polyurethane. This work focuses on ways to produce nonporous polyurethane materials and to determine their thermal conductivity. Additionally, the influence of the dissolved blowing agent, the flame retardant, and the chemical composition has been investigated. Two different methods were applied when determining the thermal conductivity: the hot-wire method and the guarded hot-plate method. Both methods gave similar results on samples of the same composition. Nonporous polyurethane samples were prepared either by reaction of isocyanate and polyol, which were dried under vacuum, or by a cold-cured reaction with zeolites for water removal. In independent trials the influence of different levels of dissolved blowing agent and different amounts of flame retardant on the thermal conductivity has been investigated. This was made possible by the addition of different amounts of cyclopentane and R141b when preparing the sample as a nonporous cold-cured resin with the use of zeolites for water removal. The influence of the dissolved level of cyclopentane and the presence of flame retardant on thermal conductivity has been found to be negligible. To investigate the influence of chemical composition on the thermal conductivity, polyurethane foams with density 70 kg m-3 have been crushed and compressed to higher densities. The thermal conductivity of the compressed foam versus density was extrapolated in order to obtain the nonporous polyurethane conductivity with vanishing porosity. The thermal conductivity obtained thereby was almost identical to the value of about 0.21 W m-1 K-1 determined on the nonporous polyurethane samples at 296 K.