Effect of the multiple injection process on the structural and mechanical properties of PP impact copolymers focusing on the deformation of ethylene-propylene copolymer

The influence of multiple injection on the structural and mechanical properties of PP impact copolymers (ICP) has been investigated, mainly focusing on the deformation of dispersed ethylene-propylene copolymer (EPC) particles in ICPs. Various properties of recycled ICPs throughout repetitive injection processes have been compared. The thermo-mechanical degradation by heat and shear stress imposed on ICPs several times via multiple injection triggered chemical changes, such as the generation of C[dbnd]C and C[dbnd]O bonds and chain scission in ICPs. These changes alter the crystallinity of the ICPs by shortening the chain length and narrowing the molecular weight distribution. To determine the effect of multiple injection on the deformation of EPC particles, the intrinsic viscosities (IV) of the polypropylene (PP) matrix and dispersed EPC were measured. Degradation by multiple injection was greater for PP than for EPC, thereby increasing the viscosity ratio (IV_EPC/IV_PP). At a high viscosity ratio, the PP matrix could not effectively transfer shear stress to the dispersed EPC particles, causing the EPC particles to agglomerate and become larger with poor dispersity. Regarding the mechanical properties, the agglomeration of EPC exerted a significant negative effect on the Izod impact strength. Finally, numerical calculations in the injection mold flow region confirmed that the re-injected product required a higher shear stress than the virgin product.