A study of fiber length degradation during compounding of chopped glass fibers and polypropylene using a co-rotating self-wiping twin-screw extruder

An important processing method to produce short fiber reinforced polymers is twin-screw compounding using a co-rotating self-wiping extruder. The glass fibers undergo significant length degradation during this process. Influencing process parameters were studied in detail in the past. The scope of this experimental study was to investigate the influence of the mixing zone position on the resulting fiber length distribution in the extrudate and the fiber length degradation process along the screw. It was found that the mixing zone position had no influence on the resulting fiber length distribution in the observed range of mixing zone positions. The investigation of the average fiber length along the screw position showed that a zone of higher degradation was shifted with changes in the mixing zone position. Specific energy input is directly proportional to the product of the squared shear rate and the residence time. Replotting the mean fiber length along the screw over this product explained the differences to some extent. Remaining differences in the plot were contributed by viscosity differences and the simplified prediction of the mean shear rate and the mean residence time a partially filled kneading block. The overall result was evaluated as an indicator for the correlation between the fiber length degradation process and specific energy input, which has to be quantified in future investigations.