REACTIONS OF UNCONVERTED ISOCYANATE IN MOLDED RIM PARTS AND THEIR IMPLICATIONS TO OUTGASSING

A convenient, cheap, and reliable method was developed and evaluated t o measure the extent of conversion and subsequent water/diisocyanate r eaction kinetics for Infection molded (RIM) parts using cryogenic grin ding and conventional Fourier Transform infra-Red (FTIR) spectroscopy, The reliability of the method was independently confirmed by using in -mold fiber-optic FTIR and attenuated total reflectance spectroscopy ( Priester, private communication, The Dow Chemical Company 1994). The r eaction of isocyanate with water produces carbamic acid, which is beli eved to immediately decompose to carbon dioxide (CO2) and amine, leadi ng to outgassing during the paint cure. Given previous measurements of effective diffusion coefficients of water and CO2, if the molded RIM parts are stored under typical humid conditions for hours, most of the CO2 should form as a product of isocyanate/water reaction and diffuse out of the part, However, our results indicate that relatively little CO2 evolution occurs during ambient storage for up to 6 h, and signif icant evolution occurs for long times during subsequent high temperatu re post cure. Therefore, in contrast to the earlier expectations, the outgassing phenomenon seems to be. due to carbamic acids that decompos e slowly at ambient conditions and become unstable at higher temperatu res, Therefore. to assure minimum outgassing in this RIM system one mu st 1) operate the post cure oven at higher temperatures than the paint baking oven, or 2) keep parts in ambient storage at least for 24 h to allow slow decomposition of carbamic acids, or 3) transfer them to th e post cure oven immediately alter removal from the mold, thereby prev enting possible water/diisocyanate reactions.