Cell-gas Composition - An Important Factor in the Evaluation of Long-term Thermal Conductivity in Closed-cell Foamed Plastics

In addition to the initial thermal conductivity value, the thermal conductivity of closed-cell foamed plastics over long periods of time depends to a large degree on the gas exchange processes between the blowing agent and the ambient air. It is very important to know and to show whether the cell gas remains in the foam for a short time or, to a large extent, for the whole lifetime of the foam material. A lifetime of between c.20 to 50 years is assumed for thermal insulation materials in construction, and it will be shown that blowing agents remain to a large extent in the foam material over such periods. The method most extensively applied for measuring cell-gas composition is to take a sample using a gas-tight syringe and then analyse by gas chromatography. In addition to the measuring method, possible sources of sampling errors and measuring uncertainty are indicated. In order to show the relationship between thermal conductivity and cell-gas composition, thermal conductivity curves for PUR rigid foam materials blown with CFC 11 over a period of 30 years, and PUR foam materials blown with HCFC 141b or pentane over a period of 9 years are presented, with the corresponding measured values for cell-gas composition. Cell-gas measurement continues to be applied as proof of the blowing agent used for consumers and official bodies, and also for compliance with regulations.