Crystallization kinetics of new low viscosity polyamides 66 for thermoplastic composites processing

In this paper, we present the isothermal crystallization kinetics study of a new generation of polyamides 66 (PA 66) over a large temperature range [100-234 °C] using differential scanning calorimetry (DSC) to reach low crystallization temperature at very high cooling rates. This PA 66 with a low viscosity is of high industrial interest since it is dedicated to composite part manufacturing at low pressure (≤0.15 MPa) using new thermoplastic for Resin Transfer Molding (RTM) process with high fiber volume fraction (≈50 vol%). The low viscosity (≈15 Pa s), is one of the major difference between the new generation of PA 66 and the others. A crystallization model based on Avrami's theory is proposed for further simulation of this industrial process in commercial softwares. However, industrial process conditions require to know the crystallization kinetics over a large temperature domain. For this purpose, isothermal crystallization is quantified from Flash DSC 1 experiments using two methodologies. The measurements were compared to standard DSC data to validate the results. Then, the effects of the molar mass, additive and the sample size of the Flash DSC 1 on the crystallization kinetics were studied. As a result, crystallization of PA 66 without additive exhibits a bimodal temperature dependence. The molar mass has only an effect on the crystallization kinetics in the high temperature range, suggesting a change in the nucleation mechanism. However, the presence of additives suppresses this bimodal behavior. Finally, the effect of the sample mass on the Avrami parameters was also discussed. The results obtained in this study can be used to accurately compute heat transfer coupled to solidification during the manufacturing of PA 66 parts including processes with high cooling rates.