New method of single liquid-phase reactor synthesis of high-impact polypropylene: Structure, morphology, and impact properties of copolymers

Abstract

High-impact polypropylene (hiPP) copolymer is a very important material in the industry because of its various applications. It is obtained in an industrial process involving two consecutive reactions either in the same or different phases. Unlike industrial processes that yield materials with a fixed amount of ethylene without the possibility of modifying the operation variables, we are presenting a new method to synthesize hiPP materials on a laboratory scale using two sequential steps in only one reactor. This approach permits the study of the influence of variables such as H2 addition, reaction time, and composition during the second step on the formation of propylene-ethylene copolymer materials with different properties. In the first step, the polypropylene matrix is synthesized in a liquid phase with or without addition of H2, and, in the second step, the elastomeric phase is also obtained in a liquid phase.The influence of the addition of hydrogen and ethylene and other operation conditions on the polypropylene matrix and the elastomeric phase distribution was studied and correlated to the mechanical and thermal properties of the material. Obtained materials were characterized by analytical temperature rising elution fractionation (TREF), calorimetric methods (DSC), 13C nuclear magnetic resonance (13C NMR), gel permeation chromatography (GPC), melt flow index (MFI), and scanning electron microscopy (SEM).The hydrogen addition remarkably enhanced the propylene polymerization while causing a significant decline in the ethylene polymerization rate. Based on these results, it seems that hydrogen has a complex effect on the catalyst behavior beyond its role as chain transfer agent. Furthermore, the presence of hydrogen in the polymerization caused a significant isotactic polypropylene (iPP) matrix molecular weight decay, which induced a modification of the viscosity ratio between the matrix and the rubbery phase produced in the second step. This yielded ethylene-propylene rubber (EPR) domains with higher average sizes and affected the phases’ morphology, resulting in a remarkable copolymer impact strength decline.The addition of ethylene to the second step in the absence of hydrogen affords an ethylene-propylene copolymer with high resistance to impact, which ranks it as a hiPP copolymer. Fractionation of this sample and the analysis of the obtained fractions provided knowledge about the microstructure of the copolymer and proved adequate distribution of the elastomeric phase into the iPP matrix.