Celfi: Development,production, application and recycling of Wood Plastic Composites (Celfi)

Results and conclusions

During the last decades, wood plastic composites (WPC) have become popular in the construction of deckings, sidings, but also in the automobile industry. The reason for this is twofold. First of all, they consist of low-cost plastics such as PVC, PP, PE…, which are easy processable due to their good melt flow at elevated temperatures. On the other hand, they contain wood fibres, which are cheaper, stronger and stiffer than the plastic component. Furthermore, compared to solid wood, the composite material is less susceptible towards water absorption. In other words, WPCs combine the best of two worlds and in this way, they offer unique properties, such as good mechanical strength, a natural wood look, low maintenance, slip resistance, splinter-free nature, …

Within the project Celfi, different aspects were studied regarding this class of materials.

In a first part, wood.be examined whether WPCs could offer opportunities in the world of construction, i.e. the building industry for more load-bearing applications. Since literature only mentions moduli of elasticity (MOE) and Moduli of rupture in 3-point bending (MOR) on small scale samples, wood.be explored the implementation of a new testing method to evaluate on a structural scale, the potential of market available WPC materials for long term loading purposes. Long term testing (up to 8 months) showed that WPC creep behaviour resembles the one of plastics instead of wood an it was shown that only one of the selected market samples showed some potential to be used for structural applications. It could be concluded that a valuable, low-tech and relatively fast method was developed to map long-term mechanical properties. Nevertheless, if promising results are obtained with this test method, a more rigorous assessment would be necessary to prove the implementation potential of the material in building applications.  

Apart from exploring new markets for this class of materials, solutions were explored by centexbel to address existing problems. A first problem that might arise with WPCs includes the creation of stains, which can be caused by mould and staining fungi. They develop more quickly on the material compared to decay fungi and mainly cause discolouration of the material. In this context, several biocides were tested to prevent fungal growth at the material surfaces. Different test methods were applied to prove the anti-fungal effect and based on the results, a good candidate could be selected.

A second problem that was tackled within the project includes the water sensitivity of WPC materials. First, an extensive study was done to compare the water absorption properties of different WPC materials, including PVC, HDPE , PP-based, plasticized PVC-based WPCs and WPCs consisting of different wood filling percentages. In a next step, different routes were explored to decrease the water absorption properties. In this context, wood fibres were incubated in SMA-based water emulsions, prior to their processing via compounding. It was shown that the wood fibres were transformed into hydrophobic (i.e. water repellent) materials, since water was not anymore absorbed by the fibres. Nevertheless, upon compounding the fibres in combination with a plastic matrix at elevated temperature, this effect was destroyed. The treatment even had an inverse effect on the water absorption properties. The addition of 50% of hydrophobic wood fibres increased the water uptake to the level of WPCs containing 70% of wood fibre. Apart from the SMA technology, an alternative wood fibre treatment was evaluated to decrease the water uptake of WPC materials. In this case humins were applied as potential hydrophobing agent for the wood fibres. Humins comprise a side-product of the PEF (polyethylene furanoate) production process, a bio-based polymer which can be used as a superior alternative for PET. Since significant amounts will become available upon PEF production, their use as hydrophobing agent for WPCs seemed an interesting route. Unfortunately, the use of a coupling agent seemed more successful to improve the water absorption properties compared to the humin strategy. Unfortunately, the project could not identify a successful route to decrease the water absorption properties.

Finally, it can be mentioned that the project explored different WPC compositions, but also different processing techniques, including the more standard ones such as extrusion, injection moulding and pressing techniques, but also 3D printing was evaluated. In this context PLA-based 3D print filament was produced, containing up to 30% of wood fibre, and was processed via a filament printer. Furthermore, PP-based WPC pellets were processed via the freeformer technology to obtain 3D samples. As post-processing technique, attention was paid to the welding of WPC materials and its impact on the mechanical properties.

Composition-wise, different additives were screened and discussed during the course of the project. Apart from fungicides, different coupling agents were compared to increase the compatibility between the wood and plastic part, as well as lubricants to improve the processability during compounding and injection moulding. A selection of wood and natural fillers were examined, including recycled wall paper and also foaming agents were introduced during this project.