Miscibility and crystallisation of polypropylene-linear low density polyethylene blends

Crystallisation, morphology and miscibility of polypropylene (PP) and linea r low density polyethylene (LLDPE) blends were studied by polarised optical microscopy connected to a computer with digital image processing and analy sis. In particular the effects of LLDPE and its melt flow index (MFI) on th e kinetics of PP crystallisation was investigated through establishing a re lationship 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 molte n state. It was found that, the PP crystallised as open-armed diffuse spher ulites, similar to those observed in the miscible blends, suggesting that t he PP and the LLDPE may be miscible at some temperatures. The nuclei densit y, spherulite growth rate and overall crystallisation rate of PP decreased significantly in the blends, indicating that the LLDPE retarded crystallisa tion of PP, possibly due to various reasons such as the dilution of PP by L LDPE (LLDPE as a solvent in molten state), hindrance of viscous LLDPE to th e PP crystallisation front, and decreased supercooling degree because of th e miscibility between the PP and LLDPE. This provided further evidence that the PP and the LLDPE could be miscible at. crystallisation temperatures se lected. 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 den sity, 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 temper atures since the more viscous LLDPE (lower MFI) reduced the crystallisation rate of PP at a greater degree. (C) 2000 Elsevier Science Ltd. All rights reserved.