Effective Diffusivity of HCFC-141b,HFC-134a,HFC-245fa and Cyclopentane in PIR Foam by Thin-Slice Gravimetric Analysis

摘要

Thin slices of polyisocyanurate (FIR) slab-stock insulation,containing the blowing agent CFC-11,produced for a Joint Industry/Government Aging Study (JIGAS) were initially used to demonstrate their utility in insulation aging evaluations.Additional thin slices from the JIGAS were prepared using specimen T3B4 #3 in September 1992.These slices were placed into a controlled environment at 75 of and 50 percent relative humidity for inteml:ittent diffusion weight loss measurement to depletion in 1998.It was necessary to determine the air-filled,blowing agent depleted weights and physical dimensions of each foam slice before resaturation.The same JIGAS thin slices were resaturated in a sealed pressure vessel to a pressure of six psig with the current blowing agent HCfC-141b and each of the potential replacements HFC-134a,HFC-245fa and cyclopentane.The atmosphere into which the blowing agent diffuses will be diluted with afresh air exchange every five minutes,controlled at 23.8 deg C(75 deg f) and 50% relative humidity.One at a time,each of the blowing agent resaturated slices was removed from its saturation vessel and placed on the weighing pan of an electronic analytical balance inside the environmental chamber.The apparent weight of the slice was determined at a series of predetermined time intervals,representing approximately one percent change in the initial weight of saturated gas in the slice.The atmospheric pressure inside the environmental chamber was sensed by an atmospheric pressure transducer and monitored by the computer that controls the experiment.The atmospheric pressure,chamber temperature and relative humidity are used to compute the density of air in the chamber.The air density and closed-cell volume of the foam slice are used to evaluate the foam slice weight (in vacuum).By determining the slope of the thin slice weight (in vacuum) during the first 50 percent loss of blowing agent,the effective diffusivity of the blowing agent was determined using a short-term approximation of the one-dimensional diffusion equation.