Highly-filled biocomposites: modelling, processing, and characterization

Licentiate thesis, 2022

The use of polymer with natural fiber reinforcements is expected to increase significantly. This can be noted in several industrial sectors of strategic importance for Sweden, such as automotive, packaging, construction, and furniture industries. In addition to the reinforcement effect, with improved stiffness and strength at a low price, part of the plastics is replaced by a renewable material. A current drive for, e.g., wood plastic composites (WPCs) is to maximize the use of renewable content in mass production systems. However, while varying processing conditions using existing processing elements, e.g., dies, screws, moulds, designed for conventional plastics may allow processability of WPCs to a certain extent, their full potential as reinforcement cannot be reached without the development of dedicated processing components. This task required a fundamental understanding of WPC flow dynamics during processing that can only be achieved through combined experimental-numerical work.

In this framework, this PhD project focuses on the processing, optimizing, and modeling of WPCs in single screw extrusion. For processing optimization, a study of inline extrudate analysis was carried out by analyzing temporal and spatial characteristic periodicities via a 2D Fourier transform. Spectral dynamics of space-time diagrams were determined as a function of the die shear rate and moisture content of the WPCs. Based on the spatio-temporal spectrograms, we assessed for the first time with unprecedented detail the onset and decay of surface instabilities in WPC. A combined numerical-experimental methodology for predicting orientation in WPCs flowing through a film die at three screw speeds of 21, 66, and 91 rpm is presented to take the initial steps toward extrusion modeling. In addition, material functions for validating the in-situ melt pressure and fiber orientation from SEM micrograph analysis were studied. The results show a promising agreement between the experimental and numerical data, but also challenges that need further exploration in terms of boundary conditions and constitutive models.