VOLUME 54, ISSUE 2/2013

At present, spruce is the most important wood type used for construction purposes in Germany but reliable chemical wood protection is hard to achieve due to its mostly poor treatability. Several problems are associated with the currently applied technical solutions or processes being developed for the impregnation of spruce wood. Thus, freeze drying as an alternative drying process as well as the supportive application of ultrasound have been tested to overcome those problems. Freeze drying leads to an incomplete closure of bordered pits as well as to improved results of pressure impregnations. Drawbacks are the tendency of crack formation and high economic efforts. The application of ultrasound generates positive effects in pressure impregnations with highest levels of active compounds beneath the boundary zone if freeze dried spruce was used.

Norway spruce wood (Picea abies L.) was reacted with a commercial Trametes versicolor laccase in the presence of potassium iodide salt or the essential oils thymol and isoeugenol to impart the wood surface an antimicrobial property. In order to assess the efficacy of the wood treatment a leaching procedure of the iodinated and polymerized wood and biotests (blue stain and wood decay fungi) were performed. After laccase catalysed oxidation of the essential oil phenols the antimicrobial effect was significantly reduced. In contrast, the enzymatic oxidation of iodide (I-) to iodine (I2) in the presence of wood led to an enhanced resistance of the wood surface against all microorganisms, even after exposure to leaching. The efficiency of the enzymatic wood iodination was comparable to a common chemical wood preservative. The modification of the lignocellulose by laccase catalysed iodination was analysed by FTIR-ATR technique and the results indicated a structural change in the lignin matrix.

This publication presents a wood modification process, which combines a densification and a chemical modification process. Thereto the wood is impregnated with a solution of furfuryl alcohol and a catalyst (maleic acid anhydride) and subsequently densified in a heated press. The properties of the modified wood are depending on the densification degree and the amount of furfuryl alcohol in the impregnation solution. Different properties, like density, Brinell hardness and moisture related properties, are presented and evaluated. Wood modified with the presented modification process can be fabricated with different modification degrees. So different properties can be set in dependence on demands.

In this study, the Six Sigma define-measure-analyse-improve-control (DMAIC) approach was chosen, to enhance quality improvement by implementation of a measurement system capability in terms of kerf thickness accuracy in a sawmill. In a first step, the major possible defects were defined and the measurement system was tested by means of a gage repeatability and reproducibility study (GR&R). Using the GR&R both measurement devices, i. e. automatic and manual, were analysed. The results yielded an underperformance of the automatic system, which made the manual system, by means of caliper rule, for the first stage, the system of choice. A second GR&R was performed after the automatic system was recalibrated, but still the results were not convincing, leading to manual measurements. The operators were trained and improved methods for the data reporting were implemented. In terms of the analysed thickness accuracy, for each saw blade an optimum cutting speed section could be found. In comparison to the initial situation the saw blade deflection could be reduced by a rate of 23.5 %. The implemented process control, by means of manual measurements, allows a continuous quality control. In conclusion, adoption the DMAIC approach with GR&R procedure turns out to be an effective method in improving the quality involving measurements and allows a direct analysis of the final product if parameters in production are changed.

Three layer particleboards with densities of 400 kg/m³ and 500 kg/m³ were produced from residues of bagasse (Saccharum officinarum L.), canola (Brassica napus L.) and hemp (Cannabis sativa L.) as well as industrial wood chips. The proportions were 100 % woodchips in the surface layers and 100 % of one of the above mentioned annual plants in the middle layer. They were bonded with urea formaldehyde (UF) resin in the surface layers and UF and of wheat protein (1:1) in the middle layers. The mechanical and physical properties of these boards are measured. According to the European standard (EN 312, 2003), particleboards made from hemp chips and bagasse in a density of 500 kg/m³ were able to fulfill the minimum requirement for internal bond strength (IB), surface strength (AS) and module of elasticity (MOE) but they could not achieve adequate values of bending strength. In comparison with reference, the bagasse and hemp particleboards had higher values of bending strength in both densities (400 kg/m³ and 500 kg/m³). The TS value of bagasse with 500 kg/m³ density met the standard requirement. The WA in bagasse 400 kg/m³ and 500 kg/m³ was comparable with the reference values, which were made of wood and UF-resin. It was found that particleboards made from bagasse and hemp with 400 kg/m³ and 500 kg/m³ densities and 50 % of biological binder (wheat protein) in core could be successfully developed.

There are several concepts for the production of light-weight wood composite materials. A sandwich-type structure is applicable for the production of panels with a property profile similar to conventional wood composite panels but drastically reduced weight. Conventional wood-based sandwich panels are normally produced in a multi-step process using components which have been produced in separate processes. Thuenen Institute for Wood Research, in close cooperation with Department of Wood Science at University of Hamburg, has succeeded in producing light-weight wood composites with a foam core in one single production step. By in-situ foaming of the core material in a hot press sandwich-type foam core panels can be produced which have similar physical and mechanical properties as conventional wood-based panels but only 50 % of their weight. The specific, weight-related properties of these panels outperform their conventional alternatives in most cases.

Powder coating medium density fibreboards (MDF) is an emerging environmentally sustainable surface finishing/coating technology that is used today mainly in the furniture industry. A problem in powder coating MDF can be the development of cracks on the narrow face during the melting and curing processes in the oven. Generally, this failure is influenced by the strength (internal bond) and density profile of the MDF. Additionally, it is assumed that this defect could also be closely related to the moisture content of the raw MDF (immediately prior powder coating application). Especially, changes in climate conditions during transport und storage can lead to variation in panel moisture content prior powder coating. This variation might be a cause for coating defects. Commercially available MDF boards (optimised for powder coating applications) with various thicknesses were equilibrated in different climate conditions and tested in context with the boards edge cracking behaviour and their electrostatic applicability. The results indicate a distinct tendency towards the development of cracks increasing with panel moisture content. Additionally, the electrostatic powder application is influenced by the moisture content due to the increase of electrical resistance with decreasing panel moisture content.

The application of waterborne coating systems on wood or wood based materials causes a swelling process which requires high effort for sanding. This swelling process can be minimized by the use of forced drying e. g. HF-drying. A new HF-dryer by using stray field technology was developed in a research project. Using this prototype, the technological process was investigated. Thereby no damaging influences of the HF-irradiation on the quality of surfaces were detected. In comparison with another drying technology (microwave process), the drying time was decreased by 30 to 50 percent to approximately four minutes. Furthermore, the total energy consumption was decreased by 50 %. That causes a higher capacity with lower costs. As the drying procedure is quite sensitive, the main advantage of the new process is the tempering of the lacquer layer only. The determined surface properties were comparable to those when using conventional drying.