Short fiber‐reinforced thermoplastic elastomers from blends of natural rubber and polyethylene

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Abstract
Thermoplastic elastomer blends of natural rubber (NR) with high density polyethylene (HDPE) and with low density polyethylene (LDPE) were reinforced with short silk fiber. Processing characteristics such as torque and temperature developed during mixing and the effect of processing parameters such as nip gap and number of passes in the mill necessary to secure maximum orientation of the fibers in the blends were studied. A small nip gap and a single pass in the mill were found to give best results. Of the different mixing sequences studied, the sequence where short fibers followed by rubber were added to the molten thermoplastic was found to give a uniform dispersion of fibers. Fiber breakage and the change in aspect ratio of the fibers after mixing were also examined. It was observed that, as a direct consequence of the mixing sequence, each fiber was coated with a layer of thermoplastic. Although the properties improved on the addition of the dry bonding system of silica–resorcinol–hexamethylenetetramine, the comparatively long curing time required for full development of adhesion between the fibers and the matrix proved to be a major disadvantage associated with the incorporation of the bonding system. The tensile and tear properties were substantially enhanced, but the ultimate elongation decreased sharply with increasing loading of short fibers in the blends. The effect of fiber orientation and the development of anisotropy in the properties was also noted. Scanning electron microscopy (SEM) studies of the benzene‐extracted surfaces of the NR/HDPE (high density polyethylene) blends substantiated the theory of fibers behaving like “mechanical anchors” between the rubber and thermoplastic phase. The effect of fiber loading on the tear and tensile properties of the blends of NR/LDPE with varying blend ratios was studied. Most pronounced improvement in the properties on the addition of short fibers was observed in the high rubber blends. As the plastic content in the blends increased, the short fibers were found to have a lesser influence on the properties. SEM photomicrographs of the tensile and tear fracture surfaces indicated the fiber orientations and the effect of orientation, fiber loading, and blend ratios on the nature of fracture.

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