VOLUME 51, ISSUE 1/2010

Internal stresses in multilayered solid wood boards during climate change are caused by the crosswise gluing of their single layers. These stresses may partly be released in cracks during drying. In the current investigation we measured the free swelling, the swelling pressure and the internal stresses after a climate change from 20 °C/35 % to 20 °C/85 % RH. Therefore we tested single layers of spruce wood (Picea abies Karst.) and MDF as well as multilayered solid wood boards. Part 1 of the publication gives an overview of free and partial restrained swelling and describes methods and material. Furthermore the results of free swelling are presented. The investigation revealed lower swelling ratios for cross-laminated wooden panels compared to the single layers. The cross-laminated wooden panels showed the lowest swelling values in the tangential direction perpendicular to the grain direction of the top layer. This was similar to native wood which shows the same behaviour due to the anisotropic swelling in the anatomical directions. The equilibrium moisture content of the boards was about 2 % lower than in the single layers. This behaviour is caused by the restrained swelling of the cross-laminated wooden panels. How much the swelling pressure and the internal stresses are affected by the cross lamination is subject of part 2 of the article.

MDF of blowline glued pine pulp with different raw densities were tested on the physical-mechanical short-term properties as well as on the form stability and the creep behaviour. The results were evaluated regarding the influence of the raw density profile perpendicular to the panel surface. The creep behaviour has been tested with use-oriented load according to DIN 68874_1 as well as with strength-oriented load based on DIN ENV 1156. The MDF panels with a distinctive raw density profile occur to withstand the resulting compression- respectively tensilestrength of an applied bending strength better then the MDF panels with an even raw density profile. This results in the better creep behaviour of the panels with the distinctive raw density profile. The higher creepage of the mouldable MDF (even raw density profile) can be reduced by increasing the panel thickness. The strength-oriented loading of the MDF panels with 25 % of their breaking load, led to an disproportional high creepage with no practical relevance. The use-oriented load is advantageously related to the expectable working load in use. A direct comparison of the creep behaviour of all test specimen is applicable since all are applied with the same load. Regarding the level of load is to be proposed: Test load = safety factor x to be expected maximum working load, e.g. 5 x L 125 according to DIN 68874-1. It is to be proposed to incorporate the use-oriented testing method alternatively into DIN ENV 1156. It is to notice, that panels with a distinctive density profile perpendicular to the panel surface are dimensionally more stable than MDF with an even density profile.

According to DIN CEN/TS 15534-1 (2007), artificial weathering using xenon lamps (EN ISO 4892-2) is suggested as test method for uncoated WPC, however, at present, DIN EN 927-6 (2006) as well as EN ISO 4892-2 (2009) are being evaluated with regard to their usefulness for characterization of WPC. Colour changes of polyolefin-based WPC from different manufacturers following artificial weathering (QUV) during 4032 hours were small compared to Bangkirai wood whose colour changed strongly into grey. Colour fastness of the various WPC following artificial weathering varied strongly. Colour and gloss measurements are problematic due to the corrugation of the WPC decking profiles; potentially, the use of a template during measurements could be beneficial in order to retain the profile sector chosen for measurements. Results pertaining to changes in mechanical and physical properties of WPC after artificial weathering (QUV) and water storage will be presented in part 2 of this series.

The natural phenol “Cardanol” is a renewable raw material from cashew nut shells. Due to the good chemical and toxicological properties the use of cardanol as a “green” building block in PCF-resins for the wood industry is of increasing interest. For a successful handling of such resins a good understanding of the curing reaction is necessary. It was proved by differential scanning calorimetry and kinetic calculations that cardanol increases the speed of cure. An addition of only 3,5 mol-% cardanol to 100 mol-% phenol had a significant impact on the reactivity of the resin. The calculation based on a model of a one stage reaction of nth order without auto catalysis met best the experimental results. Because of the plasticizer effect of cardanol the gel-time cannot be used as an indicator for the reactivity of a Phenol-Cardanol- Formaldehyde-resin. The results of the kinetic calculations have been applied to the curing of a PCF-resin in the production of particle boards by hot pressing.

By applying a rotating abrasive tool to a moulder it is possible to remove the cyclic cutter marks generated in the surface of the work piece and thus improve the quality. Due to the required accuracy of alignment between the two processing tools it is not easy to set-up. By a process model using as input the geometry of the cyclic cutter marks and representing a model for the stock removal variables the moulder set-up can be determined.

Several techniques of micro area analysis can be used to determine the distribution of chemical components in the μm-scale, e.g. Raman spectroscopy. For understanding this technique its physical basics are explained and an application in the pulp chemistry is demonstrated. The Raman spectroscopy is based on inelastic scattering processes of monochromatic radiation at the electron shell of molecules. The scattering can be used for analyzing molecular changes as a result of thermal or chemical modification of wood.

Furfural is formed during the thermo-mechanical defibration of wood consuming the wood polyoses in acid dependent reactions. Thus, the formation of furfural requires the presence of organic acids like acetic or formic acid already present in the wood or formed during the thermal treatment, respectively. The formation of furfural is correlated with the temperature during cooking and defibration as well as the duration of the refining process. The higher the temperature and the longer the digestion time, the formation of furfural increases. Lowering of furfural contents in the wood fibres can be achieved by reducing pressure/temperature, digestion time and by neutralizing the acidity of the wood. Alkaline additives, however, increase the oxidative cleavage of fatty acids, which leads to the formation of malodorous aldehydes. Fatty acid esters are present in all woods, contents are especially high in pine, birch or poplar e. g. It was found that a pronounced suppression of the formation of these aldehydes is achieved by addition of a combination of alkaline and complexing agents. This technical solution can be used to minimize the health risk from the emissions of aldehydes, including furfural from wood fibers and insulation products made from it.

Evenly spreaded wood fibers before the press are important to save energy and raw material. The area weight distribution is measured to control the forming. The measurement system based on x-ray and moves across the mat. Detectors deliver a measurement value as voltage. The calibration procedure takes place during start up and delivers the relationship between measurement value and area weight. Up to now the calibration of the weight per unit area gauges was done with already pressed MDF with a known area weight. But the experience shows that the measurement results do not relate to laboratory measurements. A new calibration procedure was developed to increase measurement accuracy by calibration on the fiber mat. Therefore the system is calibrated with same material used in production. Even if the area weight of a panel and mat is same the different thickness of the material has an influence on the the absorption of x-ray beams.

For diagnosis and characterization of indoor wood-decay basidiomycetes, the second part of the review describes SDS-PAGE, immunological methods, MALDI-TOF MS and Pyrolysis-GC/MS.