Determination of modified polyamide 6's foaming windows by bubble growth simulations based on rheological measurements

Abstract

ABSTRACTReactive extrusion was used to modify virgin polyamides 6 (v‐PA6) and to prepare chain extended PA6 (CE‐PA6) and long‐chain branched PA6 (LCB‐PA6) for the melt foaming process. This was done using a twin‐screw extruder and the following modifiers: a chain extender ADR‐4368 and a branching agent maleic anhydride grafted polypropylene. A reaction mechanism was proposed to explain the chain extension and long‐chain branching reactions and was verified by the Fourier transform infrared spectroscopy data. The analysis of the gel permeation chromatography data showed that LCB‐PA6 presented a strong increase in the molecular weight and in the dispersity index. Moreover, the rheological properties of the v‐PA6 and modified PA6 resins were characterized by a dynamic shear test. The LCB‐PA6 compared with CE‐PA6 showed much higher shear viscosity and longer characteristic relaxation times, indicating the presence of an LCB structure. A uniaxial elongation test showed that the LCB‐PA6 had the highest melt viscosity and melt strength as well as most obvious strain‐hardening behavior. A high‐pressure differential scanning calorimeter under compressed CO2 was used to investigate the PA6's crystallization properties so as to analyze its minimum temperature of foaming windows. The melt foamability of the CE‐PA6 and the LCB‐PA6 was verified by batch melt foaming experiments with CO2 as the blowing agent and maximum temperature of foaming windows was also quantitatively determined by numerical simulation of bubble growth based on the rheological measurements. The results showed that the LCB‐PA6 foams had a smaller cell diameter, a larger cell density, a greater expansion ratio, and wider foaming temperature window than the CE‐PA6. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48138.