Reaction-induced crystallization and reaction injection molding process of nylon 6

Abstract

The anionic polymerization of $\varepsilon$-caprolactam to nylon 6 is carried out somewhat below the melting point of the polymerized product, and thus nonisothermal crystallization occurs as the reaction proceeds. Because the polymerization and crystallization occur at the same time, it is quite difficult to analyze the crystallization kinetics during polymerization. One must understand the nonisothermal crystallization behavior during polymerization in order to simulate the RIM process of nylon 6. The kinetics of crystallization during anionic polymerization of $\varepsilon$-caprolactam was studied by the adiabatic temperature rise method. A new kinetic model for the nonisothermal crystallization was derived and the possibility of its application was investigated. The experimental and predicted temperature rise showed pretty good agreement, which indicated that the model equation was applicable to describe the reaction-induced crystallization kinetics of nylon 6. By evaluating the model``s diffusion and nucleation parameters for the given system, the crystallization behavior during polymerization was analyzed. At low conversion the reaction mixture was less viscous, a condition which made transport to the liquid-crystal interface easier, and thus the activation energy for transfer decreased. On the other hand, the increasing mobility retarded the formation of the stable nucleus and the free energy of formation of a nucleus increased. A computer simulation model was developed to analyze the nylon 6 RIM. Balance equations calculating the temperature, conversion, and crystallinity change in a disc type mold were solved simultaneously with the aid of a computer. As an attempt to optimize the RIM process, the effects of molding parameters such as the feed temperature, the wall temperature, and the overall reaction constant on the process criteria of the eject time were analyzed.