VOLUME 58, ISSUE 2/2017

Fibre mats for medium-density fibreboards (MDF) are normally formed without dedicated fibre orientation. Simulations of the micro-structure of MDF based on real particle size distributions and micro-tomographic image data revealed that a horizontal fibre orientation results in an increase of macroscopic strength. Suitable methods for orienting fibres during the mat forming process were evaluated and proved. For MDF samples with equal amount of fibres and adhesive but with fibre orientation an increase of approximately 25 % for modulus of elasticity (MOE) and modulus of rupture (MOR) in the direction of orientation were measured while length variation and thickness swelling were comparable. An industrial implementation of this principle can contribute to a considerable increase of strength parameters while material input or board weight could be reduced.

One main objective of wood modification processes is the improvement of the durability against wood destroying organisms. Therefore, investigations of the resistance against wood destroying fungi have been one of the main focus within the last years. For the interpretation of the durability results, the selection of suitable test fungi is one of the main questions. A “wrong” fungus species may cause no attack whilst the same wood specimen would have been heavily attacked by another test fungus. Durability tests which were carried out at the University of Göttingen during the last years were evaluated concerning the activity of different fungi against modified wood. The results have shown that R. placenta and T. versicolor are relevant test fungi in general. Thereby R. placenta caused high mass losses also for hardwood species as well as T. versicolor caused high mass losses also for softwood species. Furthermore C. puteana caused high mass losses at DMDHEU treated wood and might be selected for modified wood. In general, it would be preferable to adapt the existing standards concerning the selection of relevant test fungi for modified wood.

A structurally optimized sandwich composite made from wood particles and bio-based particle foam has been developed. The production is based on a single-step process for multi-layered wood-based panels. Unlike agglutinating separate layers, this process can be performed continuously by using existing presses in the wood-based panel industry. For many mechanical properties the near-surface properties are more relevant than those in the core. Therefore, it is possible to reduce the density of the middle layer with only minor effects on material properties by replacing the wood by a foamed bio-based polymer. Polymer blends made of cellulose acetate butyrate (CAB) modified with a bio-based plasticizer and a biodegradable, partly bio-based polymer showed good suitability for this purpose. After loading with a blowing agent, the polymer granules are scattered between the face layers. Heat and pressure are then used to compress the face layers and subsequently the core layer forms due to the expansion of the foam. The production process is adjusted to the particle foams’ properties and to established processes.

The potential of treating thermally modified wood with melamine resin to improve the dimensional stabilization is tested in this research. One half of two poplar (Populus ssp.) boards were thermally modified (T1: 210°C; T2: 230°C) in a commercial process, the other two halves were used as untreated reference material. Ten specimens of each material were impregnated with a solution of a commercially available methylolated melamine resin and dry-cured in a laboratory oven. The anti-swelling efficiency (ASE) based on the swell rate was measured during ten cycles of repeated drying and wetting. The melamine treatment caused a higher bulking in the references than in thermally modified wood. The ASE of T1 was improved by secondary modification, whereas the ASE of T2 was higher than that of the secondary modified material. Reasons for the low bulking may be found in the same mechanisms providing good dimensional stability of thermally modified wood in the first place: The cell walls are hydrophobized by the thermal modification and thus less accessible for melamine oligomers.

The adhesive systems used for the production of wood-based products often must have large bandwidths in the property profile to ensure secure bonding despite the often highly inhomogeneous material properties (surface energy, capillarity, hygroscopicity, density, modulus of elasticity etc.). In not a few cases, this leads to an unwanted compromise formation between optimum and feasibility. The procedural success of an application-compatible adhesive bonding, in addition to the adhesive selection, can be seen in the preparation and adaptation of the adherent surfaces by modification. The precise set of surface properties of two especially contrasting joint partners on the requirements of adhesive chemistry allows the use of adhesives with properties which are precisely tuned for process and application, what the spectrum of useful adhesive systems significantly expanded, and it is advantageous out of the ecological and economic perspective. The article outlines the possibilities of a joint surface optimization of wood and its substitutes by a plasma treatment and effects of the plasma treatment intensity on the water contact angle using the example of bamboo surfaces.

Over the last years the popularity and approval of wood-polymer composites (WPC) are growing. Particularly in the field of terrace deckings the produced amounts have increased considerably. Outdoor applications lead to the questions of durability of these bio-composites. Therefore, in this work the determination of the oxidation induction time (OIT) as a possible quality criterion for WPC was evaluated. For this purpose, WPC-profiles were produced via direct-extrusion, with different amounts of additives. The produced profiles were treated by artificial weathering and examined continuously. In the course of the work, numerous influencing factors on the measurement were revealed and it was shown that an initial high OIT-value is no warranty for a long-lasting product. Parallelly conducted high-performance liquid chromatography (HPLC) measurements revealed what kind of additive has the biggest influence on the oxidation induction time and to what extent the amount of additives in the bio-composite is decreased by artificial weathering.

In order to utilize waste products such as sawdust and chips in the wood industry, the pellet making is a common practice using compaction pressures in the range of 100-150 MPa. During the compaction process the density and the modulus of elasticity of the material rapidly increase. Wood materials generally show non-linear viscoelastic-plastic behaviour and, therefore, the pressure-deformation relationship is dependent on the loading velocity and on the time during which the material is subjected to constant deformation or load. The energy requirement of pellet production depends on many influencing factors and in such cases the use of dimen-sionless numbers in the form of similarity equation facilitates the processing of experimental results and the obtained similarity relationship has a more general validity for the users. Various fractions of different wood species were used in these experiments and the pressure, pellet diameter, temperature were also varied. The proposed similarity equation shows a good correlation with the experimental results.