Many possibilities are available for the avoidance and/or reduction of weather-related changes to WPC. These include the use of pigments and UV-protection substances in the overall formula for wood-plastic composites, pre-treatment of wood, co-extrusion, encasement in film as well as powder coating and wet coating. A liquid coating can be more effective and more flexible as regards meeting the requirements concerning durability, colouring and design when used for a façade. Therefore, only one WPC formula without pigments and UV-protection must be extruded, as the colouring and UV-protection can be achieved through the subsequently-applied coating. This enables a more economic manufacture of smaller production batches.
It is currently not possible to satisfactorily coat WPC surfaces with water-based paints. The reason for this is the low surface energy and the barely-existent functionality which result in bad wettability and lack of material anchorage for aqueous binders. The objective can not be obtained solely through the implementation of technology from the field of plastic coatings, as polypropylene and WPC differ in terms of the inhomogeneity of the material and the higher surface roughness. From an ecological point of view, it would be desirable for not only the top coat but also the primer to be solvent-free and non-halogen. With regard to the disposal and recycling of the materials, it is also desirable to favour products which are not only non-halogen in the plastic components in the composite material but also in the primer and top coat formulation.
The research results obtained through this project are intended to describe and explain the connection between surface properties, surface energy, surface functionality, wettability and adhesive strength. For this, special WPC formulations have been extruded at the Fraunhofer WKI, and industrially-manufactured sections have been used. These results should make it possible to select the optimal recommendation for different substrates as regards pre-treatment and primer choice. Furthermore, the research results serve to illustrate the bond between the substrate WPC and various binder classes as well as to explain the influence of surface pre-treatments.
The implementation of WPC with water-based paints could open up new application possibilities and markets in the construction sector, e.g. for façade and window sections. In the last few years, the WPC market in Europe has expanded rapidly and the outlook is viewed positively. In Germany, the current annual production volume is around 50,000 t, whilst Europe has an estimated capacity of around 120,000 t.
For medium-sized companies, significant advantages gained from the liquid coating of WPC include good application of the coating systems (also for smaller production batches), flexibility in the colouring, reduced inventory levels (as only one colour must be extruded which can then be diversely coated), and the avoidance of larger investments. The implementation of environmentally-friendly, water-based binder materials and the exclusion of halogenated source materials constitute a valuable contribution towards environmental protection.
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Fraunhofer Institute for Wood Research 
