Dimensionsstabile und pilzresistente Furnierwerkstoffe durch Zellwandmodifizierung mit niedermolekularem Phenol-Formaldehyd

Abstract

The sorption and desorption of water within the cell wall of wood lead to swelling and shrinkage. Severe stresses occur under varying climatic conditions and damage of the wood structure can result, which eventually causes face checking and pronounced deterioration. Generally, high moisture contents lead to degradation of the wood by basidiomycetes. To protect wood and wood products against these corrupting influences, the modification of the wooden cell wall to inhibit the water uptake, is the best way to provide dimensional stability and durability. In this work rotary cut beech (Fagus sylvatica L.) veneers, 3.7 mm and 2.1 mm thick, were treated in with low molecular weight phenol formaldehyde in aqueous solution. The process contained a two-step impregnation, within a vacuum was followed by atmospheric or over-pressure and a pre-drying. The final curing took place while gluing the veneers in a heated press to form a laminated veneer lumber (LVL) of eight layers resp. 15 layers. It was the aim of the treatment to decrease the water uptake and the extent of dimensional changes by incorporating the phenol formaldehyde resin inside the cell wall and to achieve a permanent bulking. The used resins were water-soluble alkaline types. Related to the used PFconcentration in solution, medium, middle and high dry weight gains were achieved. The fixation was proven in a leaching test according to EN84 with an initial water impregnation under vacuum and with that a dimensional stability that was reached by the modification was found. The swelling in radial direction was reduced by 57% at moderate WPG and was fully eliminated at highest WPG. The durability of the LVL against white rot fungi has been tested for 16 weeks according to the standard ENV 12038. This test revealed that the mass loss had been decreased to almost 1% by the PF-treatment. Because the moisture content of the samples was above the fibre saturation point, fungal growth had been possible, by the way no constrains of fungal growth could be observed. As it is known that cell wall modification has an influence on the mechanical properties of wood, the modulus of rupture (MOR), modulus of elasticity (MOE) and impact bending strength have been assessed. It came out that the MOR and MOE can be increased by the modification and the pressing regime. Concerning the impact bending strength the energy that was absorbed by sample during the test was considerably lower. The weathering performance was tested both in a self-created lab test for LVL and in a field test. Both tests showed that weathering stability can be achieved, but under consideration of economical aspects, a coating is seen to be beneficial. Furthermore, an in-ground test over two years appears the modified LVL to be resistant against soft rot. It can be concluded that an appropriate process exists to produce LVL from the PF modified beech veneers, which is highly durable against white rot fungus Trametes versicolor, dimensional stable, weatherproof, usable in ground contact and with adequate mechanic...