INVESTIGATION OF FLUOROCARBON BLOWING AGENTS IN INSULATING POLYMER FOAMS BY F-19 NMR IMAGING

Currently, there is no reliable and readily accessible technique with which the distribution and diffusion of blowing agents in rigid insula ting foams can be detected and monitored, In this paper, we demonstrat e that F-19. NMR microscopic imaging together with F-19 solid-state MA S NMR spectroscopy is ideally suited for such measurements and yield q uantitatively reliable information that will be critical to the develo pment and fabrication of optimized insulating materials with alternati ve blowing agents, Polystyrene (PS) and polyurethane (PU) foam samples were investigated with the objective of determining quantitatively th e amount of blowing agents in the gaseous phase and dissolved in the p olymer phase, and to determine and monitor the distribution of the blo wing agents in aged foams as a function of time and temperature. The c oncentrations of the gaseous blowing agents in the cells and dissolved in the solid were simultaneously and quantitatively measured by F-19 MAS NMR spectroscopy, An unfaced 1-yr-old PS foam filled with CH3CF2Cl has about 13% of total HCFCs dissolved in the solid; while there is a bout 24% of HCFCs in the solid of a faced 3-mos-old PU foam filled wit h CH3CCl2F. The data from F-19 NMR imaging demonstrate that the distri butions of the blowing agents in an aged foam are quite uniform around the center part (2 cm away from any edge) of a foam board; however, a gradient in blowing agent concentration was found as a function of di stance from the initial factory cut edge. The effective diffusion coef ficients of the blowing agents can be directly calculated from the ima ging data, Quantitative diffusion constants and activation barriers we re determined. Additionally, a foam treated with a second blowing agen t was monitored with chemical shift selective imaging and the diffusio n of the second gas into the foam and the out-diffusion of the origina l gas were determined. Copyright (C) 1996 Elsevier Science Inc.