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Biocomposites were prepared from a kenaf core powder and recycled high‐density polyethylene/(natural rubber) blend by using an internal mixer at 165oC and 50 rpm. The effect of the filler content and the filler surface treatment was studied. Chemical modification of kenaf filler was performed with alkali pretreatment followed by treatment with silane. Scanning electron microscopy and infrared spectroscopy studies confirmed changes in the chemical compositions and structural characteristics induced through the modification. It was found that treated biocomposites offered higher tensile strength and tensile modulus, but lower elongation at break compared with untreated biocomposites. Lower water absorption and higher thermal stability of the resultant biocomposites were also obtained when treated fillers were used. J. VINYL ADDIT. TECHNOL., 20:218–224, 2014. © 2014 Society of Plastics Engineers

This work proposes a new understanding of the side effect of mineral fillers in polymer compounding that caused different levels of thermo‐mechanical degradation (TMD) during extrusion. Three mineral fillers, silica (random shape), kaolin (platy), and wollastonite (needle‐like), were selected and compounded with the polypropylene‐ethylene copolymer (PPcoE) using a twin‐screw extruder. Their influences on the TMD of PPcoE were examined by checking the substances in the extrudates using Fourier transform infrared spectroscopy. The built‐up shear stress causing by mineral filler in PPcoE composite (Δτcom) as the root of TMD was measured and estimated using capillary rheometer. In this study, TMD is classified as two levels: chain scission and oxidization. These fillers were found to accelerate the TMD mechanism and generate the oxidised products during extrusion. The silica filler caused the highest Δτcom, the lowest tensile strength, and elongation at break of PPcoE composites. The kaolin filler gave the most potent accelerating effect on oxidization, resulted in the formation of tertiary alcohol, and the highest ester contents in PPcoE composites; although, its composite had the lowest Δτcom among the fillers. Lastly, wollastonite filler provided higher Δτcom than kaolin composite, the second‐highest formation of alkene products, yet better retention in elongation at break compared to the silica‐filled PPcoE composite. In general, the side effect of mineral filler on TMD was a larger particle size always comes with higher chain scission, simpler particle morphology gives a smoother built‐up shear stress and minimizes TMD, and filler having hydroxyl groups catalyzes the oxidization TMD.

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