Wood-Plastic-Composites (WPC)

The composite material WPC, which consists of a polymer matrix, wood fibers, and suitable additives, has been an integral area of the KTP´s research for several years. In comparison to pure polymer, such compounds offer increased strength and stiffness; they are used, among other things, in outdoor living areas as a wood substitute because of their resistance to weather-related influences. One growing market for WPC is the furniture industry; consequently, there is an increased need for research in the area of WPC-injection molding.

The following gives an overview of research foci in this area:

WPC and its Components Enlarge image

Injection Molding 

  • back-injection molding
  • sandwich-molding
  • fill and flow behavior of WPC-melt 

Compounding Process 

  • process optimization
  • screw and barrel design
  • alternative compounding methods (internal mixers) 

Formula Development

  • optimization of mechanical properties based on application
  • targeted manipulation of absorption behavior 

Bonding

  • surface pretreatment (laser treatment, mechanical processing)
  • surface activation using plasma

Natural fibre-filled plastics, such as wood-plastic composites (WPC), are ideally suited for use as lightweight construction materials due to the lower density of wood compared to mineral fillers. Furthermore, they have a higher three-dimensional design freedom and moisture resistance compared to pure wood. Compared to mono-plastic, WPC has a higher stiffness and a lower coefficient of thermal expansion.


However, in addition to the positive properties, problems often occur in the processing of WPC with a filler content above 40 wt.% during the injection moulding process. Instead of a regular swelling flow, flow anomalies or segregation can be observed during mould filling. Such a degeneration is shown in the figure below.

The research project in cooperation with the SKZ in Würzburg aimed at optimising the flow behaviour of the highly filled WPCs. For this purpose, physical and chemical blowing agents were added to the WPC. The mould filling behaviour was recorded and evaluated using a mould with a special glass insert in the injection moulding process. In addition, the cell morphology and the specific mechanical properties for thick-walled test specimens up to 12 mm wall thickness were investigated in order to provide the injection moulding processor with a guideline catalogue for foaming WPC.

 

Wood-plastic composites (WPC) have been an integral part of the plastics processing industry for years. The composite material is a combination of wood fibres and a plastic matrix, so that improved mechanical and optical properties and increased weather resistance can be achieved. Basically, WPCs are extruded or injection moulded on the basis of a cost-effective thermoplastic matrix, such as polypropylene or polyethylene. The production of a WPC based on a thermoplastic elastomer (TPE) has only been researched to a very limited extent. Diestel et al. and Nagatani et. al. describe the moulding of WPC based on a matrix of thermoplastic polyurethane (TPU), or thermoplastic vulcanisates (TPV). To date, there is no known research on the injection moulding technology of WPC with a TPU matrix. This research gap is to be closed in this research project so that a cost-effective and easily processable WPC with good optical, haptic and mechanical properties (damping behaviour), as well as high recycling potential, can be generated and made available to the plastics processing industry.


The aim of the project is to develop a new material in the wood-plastic composite material class and to characterise the material properties for successful application in the injection moulding process. In particular, the flow behaviour of the wood fibre-reinforced, thermoplastic elastomer, as well as the adjustment of the fibre length distribution and the fibre orientation can be determined as characteristic variables for a successful injection moulding process. In addition, a comprehensive investigation is carried out with regard to the recycling of wood fibres (e.g. chipboard or similar), TPU, as well as WPC, and the resulting influence on the product quality is quantified. In order to finally enable a wood-WPC composite structure for application in the furniture sector, automotive or sports industry, an extensive investigation of the surface characteristics is indispensable.

 

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