Research On The Thermal Properties At Low Temperatures And Outgassing Under Vacuum Condition Of Environmental Friendly Polyurethane Foams

The newly developed closed-celled PU foams with blowing agent of HCFC-141b and HFC-245fa are environmental friendly, but the lack of thermal properties at low temperatures, such as mean apparent thermal conductivity, mean linear thermal expansion coefficient and specific heat capacity, has made it difficult to be used. In this paper, through a series of experiments, the thermal properties at low temperatures of the two newly developed PU foams were researched. On the other hand, in order to develop Vacuum Insulation Panel(VIP), outgassing rate of the newly developed open-celled PU foam, which will negatively influence the thermal insulating performance, was studied experimentally in order to optimize the thermal insulating performance and reduce the cost of VIP.At first, the mean apparent thermal conductivity was tested by means of evaporation method, in which relationship between foam density and the mean apparent thermal conductivity was studied, also, the influence of fullness rate of liquid nitrogen in the main calorimeter on the foam thermal conductivity was analyzed. Then, three components of the apparent thermal conductivity were studied, including polymer conducting, gas conducting and radiation heat transfer. In this part, foam aging was a major term to be researched through a diffusion model to describe the phenomenon of diffusion in the newly developed foams. In terms of thermal radiation, the influence of struts was considered and added to Williams model, through which the Rosseland mean extinction coefficient was optimized. At last radiative thermal conductivity was calculated by different models.In addition, the mean linear thermal expansion coefficient and specific heat capacity of the environmental friendly PU foam were researched experimentally by relative quartz method and adiabatic calorimeter method respectively. In the thermal expansion study, the influences of anisotropic properties and foam density were studied. In addition, the mechanism of foam expansion was analyzed through molecule potential energy and special structure of the foams. At last, a thermal-elastic model based on Kelvin foam structure was used to describe the mechanism of thermal expansion of the PU foams. In the process of the measurement of specific heat capacity, inner and outer adiabatic shields and two sets of heating system and two temperature control systems were adopted to ensure the excellent heat insulation environment for the experiment. The influences of foam density and blowing agents on the specific heat capacity were discussed. Based on heat capacity, enthalpy, entropy and Gibbs free energy were obtained through foam thermodynamics. The relationship between specific heat capacity and Grueneisen parameter was studied with the consideration of thermal expansion of the foams. A temperature-layered description about the specific heat capacity was introduced when the foam was used as thermal insulating layers at low temperature.Outgassing property of the open-celled PU foam was experimentally studied by means of throughput method, in which the factors of open cell rate and temperatures were researched respectively. Two types of foams with open cell rate of 95% and 70% were tested at temperatures of -25℃、5℃、45℃and 70℃. Based on outgassing rate, the active energy of foams was obtained. At last service life and thermal insulating performance of VIP were analyzed.