The effects of processing on natural fibres in starch acetate composites

Composite compounds of triethyl citrate plasticised starch acetate and natural fibres (hemp and flax) were prepared by melt processing in kg scale. Plasticiser contents from 20 to 35 wt% and fibre contents of 10 and 40 wt% were used. The material was injection moulded to tensile test specimens. The effect of process steps, melt viscosity and fibre type on the fibre length was investigated. The lengths of fully processed fibres were determined by dissolving the starch acetate matrix and measuring the length of the remaining fibres by optical microscopy and image analysis. A clear reductive effect of the pelletising and melt processing on the fibre length was noticed. Also a reduction of fibre length along the increasing fibre content and the decreasing plasticiser content was detected. This reduction was originated from the increasing shear forces during compounding, which again depended on the increased viscosity of the material. Comparison of the fully processed hemp and flax fibres showed that hemp fibres remained longer and fibrillated more than flax fibres, leading to higher aspect ratio. Thus, the reinforcement efficiency of hemp fibres by the processing was improved, on the contrary to the reduced reinforcement efficiency of flax fibres. In addition, the analysis of fibre dispersion and orientation showed a good dispersion of fibres in the matrix, and a predominant orientation in the melt flow direction.