Miscibility and crystallisation of polypropylene–linear low density polyethylene blends

Abstract

Crystallisation, morphology and miscibility of polypropylene (PP) and linear low density polyethylene (LLDPE) blends were studied by polarised optical microscopy connected to a computer with digital image processing and analysis. In particular the effects of LLDPE and its melt flow index (MFI) on the kinetics of PP crystallisation was investigated through establishing a relationship between nucleation density, spherulitic growth rate and overall crystallisation growth rate. All the blends contained 20% by mass of PP and the LLDPEs used were of the similar grades. The crystallisation of PP was controlled to occur isothermally at temperatures where LLDPEs were in molten state. It was found that, the PP crystallised as open-armed diffuse spherulites, similar to those observed in the miscible blends, suggesting that the PP and the LLDPE may be miscible at some temperatures. The nuclei density, spherulite growth rate and overall crystallisation rate of PP decreased significantly in the blends, indicating that the LLDPE retarded crystallisation of PP, possibly due to various reasons such as the dilution of PP by LLDPE (LLDPE as a solvent in molten state), hindrance of viscous LLDPE to the PP crystallisation front, and decreased supercooling degree because of the miscibility between the PP and LLDPE. This provided further evidence that the PP and the LLDPE could be miscible at crystallisation temperatures selected. In addition, the spherulite growth rate of PP decreased with a decrease in MFI of LLDPE while the MFI of LLDPE had negligible effect on the nuclei density, showing that the diffusion process controlled overall crystallisation rate when the nucleation density were similar for blends with various MFI. This further confirmed that PP and LLDPE were miscible at elevated temperatures since the more viscous LLDPE (lower MFI) reduced the crystallisation rate of PP at a greater degree.