(750h) Aging of Thermal Insulation Properties of Polyurethane Foams

Polyurethane foams are extensively used in many branches of industry. Our contribution is focused on the thermal insulation properties of rigid polyurethane foams, which have closed-cell structure. The practical use of rigid PU foams is based on their low thermal conductivity, low density, low water absorption and relatively good mechanical strength. Therefore, they are widely used as heat insulators in housebuilding, cooling systems, furniture, etc.

The good thermal insulation properties of rigid PU foams result from the combination of a fine, closed-cell foam structure and cell gases, which are poor heat conductors. Rigid PU foam is formed by the reaction of isocyanates with polyols in the presence of catalysts and foamed with chemical and physical blowing agents. As the chemical blowing agent, CO2 generated during the reaction is used; as the physical blowing agent, pentane or cyclopentane is often used. However, these blowing agents spontaneously diffuse out of the foam and are replaced by air. As a consequence, the heat conductivity of the foam increases. This phenomenon is called foam aging. A general understanding of this process would help to produce foams with better insulation properties over the whole lifespan of these products.

The mathematical modelling of aging is a challenging multiscale problem, which requires the combination of many models. We developed a new model, which is able to simulate the diffusion of gases through a series of successive polymer walls and gas cells. This model estimates the time evolution of the concentration profiles of individual gases inside the foam. Once the composition of gases in the foam is known, the gas conductivity and then the foam conductivity are calculated through connected models.

The mathematical modelling was complemented by experimental work. The solubility and the diffusivity of gases in PU were measured on gravimetric and pressure decay apparatuses. To properly simulate the morphology, PU foam samples prepared using carbon dioxide and cyclopentane as blowing agents were scanned by X-ray micro-tomography and the cell size distribution and porosity were determined. The evolution of foam conductivity during accelerated aging of these foam samples at elevated temperatures was used to validate the mathematical model. Then, this evolution was compared with the predictions and a good agreement was observed.

Acknowledgement: Financial support of the 7th FP EU project NMP4-SL-2013-604271 â??MODENAâ? is acknowledged.