VOLUME 57, ISSUE 4/2016

The production of unoriented strand boards (USB) as a new development was in focus, and the question was investigated to which extent “other deciduous trees with short life expectancy” (ALN) – like poplar, birch, willow or alder – are suitable to this purpose. USB were manufactured based on strands of the individual wood species with the size of 5 mm to 20 mm and tested according the relevant requirements. The physical-technological properties of USB reveal that willow and alder woods lead to boards with higher strength properties compared to reference boards made with pinewood. In addition, the VOC-emissions of the respective boards are very low, as demonstrated in part 1 of this article (Kraft et al., 2016). In view of the increasing shortage of conventionally used coniferous wood for panel production, it can be concluded that ALN, especially willow and alder, are well suitable for the production of wood based panels and can substitute coniferous woods.

For the establishment of natural-fibre composites and biocomposites the use of particularly cost-effective and high-performance fillers and reinforcing materials is of interest. With the development of a novel method for treating lignocellulosic residues from recovered paper processing it was managed to provide an inexpensive, alternative fibre for the plastics processing industry. The homogenized and the dry solids content can be adjusted to a processing-oriented level. Likewise, it was managed to separate unwanted foreign substances from the pulp. As a result, it was shown that the material properties of the fibre-reinforced plastic compounds produced could easily compete with higher-quality cellulose compounds in certain areas. Thus, the functionality of the novel method has been successfully demonstrated. It has been demonstrated that with about 0.50 € preparation costs per kg of dried pulp a cost-effective alternative to conventional methods for semi and medium sized enterprises (SME) could be offered.

The impregnation with low molecular weight phenol-formaldehyde (PF) resins is a way to improve the dimensional stability and durability of wood. The study discusses different treatment methods for chemical modification of wood fibres with PF resin for insulation boards. Firstly, the modification was carried out by pre-treatment of chips and post-treatment of fibres. Secondly, the modification was performed during hydro-thermal pulping. The way of fibre modification reveals a detectable effect on the resulting impregnation and fibre morphology. As a result, insulation boards can be produced with similar mechanical properties but a lower hygroscopicity in comparison with untreated insulation boards.

Barks of Silver fir (Abies alba [Mill.]), larch (Larix decidua [Mill.]), spruce (Picea abies [Karst.]), Douglas fir (Pseudotsuga menziesii [Mirb.]) and pine (Pinus sylvestris [L.] ) were extracted with hot water and the chemical composition of the extracts analysed. The influence of temperature on the composition of spruce and pine bark extracts was studied at the laboratory and the pilot scale. The development of the shear strength during the curing of adhesive formulations based on spruce bark extracts was studied using Adhesive Bond Evaluation System (ABES). Medium density fibreboards (MDF) were then produced with the same formulation and their mechanical properties were measured. Procyanidins were the most abundant class of tannins oligomers. A high amount of co-extracted saccharides was also detected in the extracts, which could be reduced using a multi-stage extraction processes leading to an improvement of adhesive properties. Adhesive formulations based on spruce bark tannins without any hardener showed comparable properties to those prepared with a hardener (hexamine), differently from other tannins type such as mimosa. The MDF bonded with spruce extracts showed similar bending modulus but lower strength than those produced with adhesives made of mimosa extracts. The results highlight the likelihood to formulate adhesives based on spruce bark extracts with a limited addition of additives.

This paper presents the results of investigations of the weathering stability of surfaces from compressed laminated beech wood modified with phenol formaldehyde resin (compreg). The material compreg – in the German language world also known as Kunstharzpressholz (KHP) – is characterized by its dimensional and mechanical stability as well as chemical resistance. Despite the fact that KHP is much more weathering resistant than non-impregnated compressed wood, phenolic components of the PF resin and lignin from wood can be degraded by UV light. Macroscopically, the resulting surface degradation becomes evident by increasing differentiation of its microstructure, formation of micro-cracks, and discolouration of weathered KHP surfaces. For an evaluation of these phenomena, KHP panels were prepared in laboratory scale and artificial weathering tests were performed. Resulting colour changes, loss of gloss and microscopic as well as surface topographical features were examined. It was found that the application of transparent overlays significantly improves the weather resistance of KHP surfaces. The extent of the weather-related surface damaging of KHP was shown to be strongly dependent on the veneer type used as top layers. Highest weather resistance was achieved when veneers from European maple combined with PF impregnated overlays were applied as top layers.

The production process and properties of WPC (wood-polymer composites) are strongly affected by the shape and size of the wood particles (e. g. length, diameter and ratio of length to diameter) as well as frequency distributions of these parameters and the percentage by weight of specific mass fractions. For the time being, characterization of compounds and of particles used for the preparation of WPC is mostly insufficient. Suitable are the sieve analysis or optical measurements. Methods were evaluated using materials which are typical for WPC. The measurement systems Fibre-Shape, QICPIC and additionally FibreCube were tested with wood flours with grain size varying from “very fine” to “comparatively coarse” and evaluated with respect to informational value, test duration, and applicability. All systems tested provided very similar results regarding the type of distribution and its characteristic percentiles for the particle length and a shape factor (length to width ratio) when optical resolution and weighting method were similar. However, methods show constraints with respect to particle dispersion, measurement range and resolution. Optical size measurements are suitable for the characterization of particle size distributions in the process monitoring for quality of raw materials as well as in trade and standardization.