Preparation of skim natural rubber and polypropylene blends via melt blending: A study on processability

Processability of thermoplastic natural rubber (TPNR) blends is a vital aspect in the preparation of blends for industrial applications and is assessed using different techniques. In this study, skim natural rubber/Polypropylene (SNR/PP) blends were prepared using melt mixing and their processability was examined in terms of mixing torque development, long-term processability, moving die rheometer (MDR) torque, thermo-gravimetric analysis (TGA), melt flow index (MFI) and morphological studies. A series of unvulcanized (UV) and dynamically vulcanized (DV) blends having 70/30, 60/40, 50/50, 40/60, and 30/70 SNR/PP compositions were prepared. Standard Lanka Rubber (SLR) and PP blends having corresponding ratios were also studied for the purpose of comparison. The study reveals that a high percentage of non-rubbers present in the SNR has positively influenced the processability of both UV and DV SNR/PP blends compared to SLR/PP blends as assessed by the mixing torque values. As suggested by the mixing torque development and long-term processability, natural rubber dominant both DV blends cannot be processed. However, they can be processed under low shear rates as shown in the MDR studies irrespective of the rubber type. Therefore, it can be inferred that these rubber dominant blends can be processed into molded products via compression molding technique. TGA studies revealed that a similar degree of protection has been offered to the blends against thermal degradation by SNR and SLR. MFI studies showed that there is no significant difference in flowability between UV SNR/PP blends and SLR/PP blends. However, for DV blends, SNR/PP blends showed a higher flowability than SLR/PP blends. In addition, increase in PP percentage in the blend enhances the flowability while dynamic vulcanization reduces the flowability irrespective of the type of rubber used in the blends. Morphological studies suggest continuous or co-continuous phase structures for UV blends and two-phase structures for DV blends where the plastic phase acts as the continuous phase.