Reactions of unconverted isocyanate in molded RIM parts and their implications to outgassing

Abstract

AbstractA convenient, cheap, and reliable method was developed and evaluated to measure the extent of conversion and subsequent water/diisocyanate reaction kinetics for injection molded (RIM) parts using cryogenic grinding 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 reaction of isocyanate with water produces carbamic acid, which is believed to immediately decompose to carbon dioxide (CO2) and amine, leading 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 significant evolution occurs for long times during subsequent high temperature post cure. Therefore, in contrast to the earlier expectations, the outgassing phenomenon seems to be due to carbamic acids that decompose slowly at ambient conditions and become unstable at higher temperatures. Therefore, to assure minimum outgassing in this RIM system one must (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 the post cure oven immediately after removal from the mold, thereby preventing possible water/diisocyanate reactions.