Wood surface functionalisation using photoinitiators

The aim was to develop two strategies for the photochemical production of composite materials from cellulose components und organic polymers. In one procedure, the surface of the cellulose is modified such that it becomes possible to activate it with light, making a large organic molecule - a polymer - appear on the surface due to a chemical reaction. In the other procedure, a photoactive polymer is created and attached to the wood surface.

Coating and bonding of solid wood or wood particles with other materials depends on the elevated chemical reactivity of the involved surfaces. In this project, researchers developed new methods which bind the superficial lignocellulose, typical of ligneous plants, to new functional groups which render the surface more reactive and lend it new properties. This was achieved by using phosphorous-based photoinitiators which are transformed into reactive intermediate products (radicals). These radicals then trigger chemical reactions on the basis of which cellulose can be integrated into a polymeric material.

Treatments were developed for two forms of wood-based materials with a different application spectrum: for cellulose nanofibres (CNF) as a fibrillar, structural wood component and for solid wood surfaces in their native state. As a result of the different surface properties and the different application spectrum, researchers were able to pursue various modification strategies. When treating CNF, the challenge is to better adjust the surface properties, for example to improve adhesion or the integration of wood particles into polymer matrices of composite materials. The aim of solid wood treatment, on the other hand, was to make the surface more water repellent and insensitive to light. In the course of the project, the researchers were able to show that these strategies can be applied successfully.

The results gained in the project could enable the tailoring of the reactivity and functionality of wood surfaces. Such surface modifications have a high potential for innovations (e.g. application of nanotechnology) and are particularly important for high-value wood utilisation such as coated wood for external use, engineered wood products for timber constructions or wood-plastic composites.

The findings of this project have not yet been directly applied or commercially exploited.

Two strategies for the photochemical production of composite materials from cellulose components und organic polymers were successfully developed. In the first procedure, a photoinitiator molecule is covalently joined to a cellulose surface through a chemical reaction. After exposure to light in the presence of polymerisable monomers, an organic polymer begins to grow on the wood surface. This process consists of two steps, which can be described in simple terms as follows: (i) C + PI*  C-PI*; (ii) (C-PI*) + n M  (C-PI-(M)n), with: C = cellulose, PI* = active photoinitiator, PI = initiator molecule after exposure to light, M = polymerisable monomer and n = number of monomer molecules contained in the polymer (M)n after the reaction. Alternatively, one can first produce a photoactive polymer PI* -(M)n and join it to the cellulose under light exposure, which again produces (Z-PI-(Mn). Very good results have been achieved with both methods, with the weight of the cellulose increasing by up to 400%. Based on these results, it is now possible to produce and study other composite materials.

Wood surface functionalisation using photoinitiators