Aging of polyurethane foam: Experimental analysis and modeling of cell gas composition and thermal conductivity

The aging behavior of closed-cell polyurethane (PUR) foam, a material widely used in household refrigeration, is studied by examining the variation of cell gas composition and thermal conductivity over time. Aging is primarily driven by gas permeation, wherein the initially present cell gases carbon dioxide and cyclopentane are progressively replaced by nitrogen and oxygen from the ambient, resulting in an increased thermal conductivity and reduced insulation performance. The cell gas composition is measured over 1400 days employing gas chromatography, and the thermal conductivity of the foam is measured over 190 days. Morphological foam characteristics, such as average cell diameter, are determined via scanning electron microscopy and barrier measurements are performed to estimate the effective diffusion coefficient of oxygen. To simulate the aging process, one-dimensional and three-dimensional models are developed for both diffusive mass transfer as well as heat transfer. The present model for the thermal conductivity explicitly accounts for condensation effects, i.e. partial condensation of cyclopentane and carbon dioxide occurring at around 12°C, which significantly influences the insulation behavior of the foam. Sensitivity analyses indicate that an initial cell gas pressure of approximately 0.7 bar yields results that closely coincide with the experimental measurements, where the three-dimensional model demonstrates better accuracy. These measurements and simulations provide valuable insights for evaluating and predicting the long-term degradation of the insulation performance of PUR foams.