Natural UV protection of wood surfaces through cellulose fibres
Wood surfaces become coarse and discoloured when exposed to sunlight and rain. If there were a way to avoid or slow down the weathering of wood, it would be more competitive for out-of-door use in comparison with other materials. This project investigated how wood could be rendered more resistant to UV light.
Project description (completed research project)
If wood is used on the exterior of buildings, its surface suffers most. Depending on the type and intensity of the exposure to the elements, substance is lost and the wood becomes grey and unattractive.
Central to this process is the photochemical transformation of lignin and other molecules that absorb UV light. In reaction to sunlight, the chromophore groups that give the wood its natural colour are transformed into substances that are water-soluble. They are subsequently washed out by rain or snow and the wood surface becomes coarse and grey. We searched for a process that could stop this weathering process.
Wood could be more desirable for outdoor use if its surfaces remained aesthetically appealing for longer. Because of the weathering process, wood is mostly not the first choice for exterior surfaces.
The aim of the project was to develop a natural protection against the harmful effects of ultraviolet radiation on the surface of wood. Such protection can be achieved through layers and wads of cellulose fibres. In the first phase of the project, we analysed various options of delignifying. In particular, we assessed the deep effects of lignin degradation and the mechanical and optical changes to the surface. In a second phase, we studied to what extent the delignified surface can be mechanically stabilised and made water-repellant. For this purpose, we tested the suitability of functionalised reagents and polymeric substances. We developed a method to create a natural, stable protective layer on wood which is to certain extent water-repellant and delays the weathering process.
A method that delignifies wood surfaces and therefore stabilises them, protects wood used on the exterior of buildings. This could increase the competitiveness of wood in relation to other building materials. It could also be an opportunity to promote the use of wood in areas that are exposed to the elements.
To create a cellulose-rich, fibrous layer on the surface of wood, we tested various methods to delignify surfaces. These methods transform the wood surface into a protective layer which stops or delays the photochemical degradation and protects the wood underneath from sunlight.
A good method of delignification is natural weathering but this approach is industrially not viable at the moment. In the course of this natural process, lignin degradation occurs according to a specific pattern and depending on the climate after only four weeks most of the lignin has degraded. The process affects wood to a depth of around 200μm. At this point, the surface can be stabilised with a acrylate dispersion; no additional agents are necessary to create the UV protection.
We explored various mechanical approaches to stabilise the modified surfaces. Stabilising the surfaces is necessary because the delignified surface is mechanically instable and very hygroscopic. To protect the surfaces from water, we used different polymers. Acrylate systems, natural and synthetic oils as well as functionalising substances were employed.
Development of a natural UV-protection of wood surfaces by cellulose-rich layers