Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling

Processing of polyamide 66 (PA 66) by injection molding includes cooling of the melt at rates between 10(0) and 10(3) K/s and its solidification at high supercooling. The kinetics of crystallization at such conditions is unknown and has been evaluated in this work using fast scanning chip calorimetry. Slow cooling of the melt of PA 66 leads to formation of crystals, with the maximum crystallinity being about 30%. It has been found by analysis of the crystallinity as a function of the cooling rate that crystallization is suppressed on cooling faster than about 300 K/s. Isothermal analysis of the crystallization rate revealed a bimodal temperature dependence, with maxima obtained at about 165 and 110 degrees C. It is suggested that the observation of two distinct crystallization-rate maxima is related to a change of the nucleation mechanism on temperature variation or the formation of different crystal polymorphs exhibiting different growth rate. The findings provide a fundamental step towards accurately predicting the solidification behavior, the skin core morphology, and properties of injection moldings of PA 66. (C) 2014 Elsevier B.V. All rights reserved.