JAHRGANG 47, AUSGABE 3/2006

To widen up the range of application of solid wood panels it was the object of the project presented hereinafter to manufacture multi-layered solid wood panels under use of core layers made of wood fibre insulation boards. Whereas the first part of the publication („holztechnologie” issue 2/2006) described results of the lab-scale tests, the on hand second part deals with reasons of and possibilities to avoid edge-crackings mentioned before and shows the completion of industrials trials followed by the comparison of the resulting properties with those of common wood-based materials.

For the first time untreated Norway spruce (Picea abies Karst.) was densified in a common industrial scale process and afterwards thermally modified in an oilheat treatment process. Biological and mechanical properties were investigated on axially matched samples. Wetting and drying cycles were performed to determine the change in dimensional stability of densified and heat treated samples compared to references. Biological tests of resistance to basidiomycetes on malt agar according to EN 113 (1996) and soil block tests according to prEN 807 (2001) showed a substantially reduced mass loss of densified heat treated material compared to untreated controls. According to the classification of natural durability (EN 350-1, 1994) the densified and oilheat treated material was classified as "very durable" (durability class 1). The bending strength of densified and oilheat treated material was only slightly reduced compared to untreated Norway spruce. The MOE of densified and oilheat treated material was increased by 42 %, whereas the impact bending strength was decreased by 44 % compared to untreated Norway spruce controls. Densified and thermally modified samples showed improved dimensional stability compared to untreated densified material.

The second part of this work deals with the influences of a heat treatment in a drying oven on the colour variables of ash and beech wood in each case with and without coloured heartwood. With increasing temperature, the wood becomes darker and the differences in colour between wood with and without coloured heartwood become smaller. Beech shows after the treatment bright stripes in some areas of the coloured heartwood. The changed wood colour caused by the heat treatment shows greying after the exposure to UV rays. Specimens of the heat treated wood were coated with either water-based varnish or acrylic resins used in the parquet industry. After the exposure to UV rays the specimens coated with water-based varnish show a strong greyish appearance. With acrylic resins on the other hand, the colours get stabilized and become scarcely lighter after exposure to UV rays.

Medium density fibreboards (MDF) were prepared from thermomechanical (TMP) and chemo-thermomechanical (CTMP) pulps of pine using an alkaline curing phenolform- aldehyde resin (PF-resin), a tanninformaldehyde resin (TF-resin) and an adhesive based on polymeric diphenylmethan-diisocyanates (PMDI) as binders. The hydrolysis resistance of the differently bonded MDF was assessed by measuring the change in the transverse tensile strength, thickness swelling and water absorption of the boards due to soaking in water at 40 °C and 60 °C for 24 h and 3 weeks. The results reveal that MDF prepared from CTMP were in general of higher hydrolysis resistance compared to those prepared from TMP. Highly distinctive differences in the hydrolysis resistance between TMP- and CTMP-boards were found in PMDI-bonded boards. Moreover, the results indicate that post-curing of PF-resin seems to take place in the course of water treatment at 40 °C and 60 °C. Summing up, the results reveal that both the pulping method (TMP, CTMP) and the type of binder (PF-resin, TF-resin, PMDI) are highly relevant to the hydrolysis resistance of the MDF. Moreover, the interaction between binders and pulping method depends highly on the type of binder used.

Wood surfaces were coated with different thermoplastic coatings by means of a welding process. One side of the coating was melted with a wedge heater and combined with the wood surface with pressure. Three different thermoplastic foils, i.e. polyethylene terephthalate (PET), polyethylene (PE) and polypropylene (PP) were used. Additionally, gluing experiments with a cyanoacrylate adhesive were performed. The quality of the adhesion was evaluated by measuring the peeling strength. Only for the PET-foil a sufficient bonding could be achieved by using the welding process. The formation of adhesive forces between the wood surface and the PET-foil can be increased by increasing the pressure in the welding process.

The glue application system is one of the most important units regarding the edge processing. After heating the hot melts are applied on the board or the edge material using a special roll. This process is very intensive concerning energy and time. The melting of the glue with laser allows a direct positioned energy placement. So there is no energy necessary for the permanent heating of the glue container. The arranged investigations show that standard glues can be melt using laser and the adhesion of the coating material and the board can be realized under production conditions. Using the new technology very good strength values can be reached. Furthermore an increase of the process rate is possible.

The compression strength perpendicular to the grain was investigated at 12 variants of industrially manufactured thermally modified timber (TMT). This characteristic could be of interest at potential, but currently not yet possible applications, like as top plates in timber frame constructions. Due to the brittleness of TMT, caused by the thermal process, the results were only partly evaluable.

When using frameless lightweight boards with a paper honeycomb core and face sheets thicker than 3 mm, there is the problem of fixing fittings in furniture construction. In order to be able to use such panels industrially and continue to use conventional hardware technology, a new type of process can be used to generate force application points. The technology uses an undercut tool that clears a space under the top layer for the injection of resins or foams, thus opening up the possibility of fixing fittings. Two process variants are under discussion: After pre-drilling, undercutting and filling with a two-component polyurethane grouting compound, in variant 1 the hole for fixing the fittings is drilled, in variant 2 an adapter is pressed in at the same time as filling. Five compounds were tested for filling: Casting compounds based on polyurethane and polyester, these casting compounds stretched with fillers, foaming filling compounds. To characterize the load-bearing capacity of the joint, the screw extraction force was selected on the basis of DIN 320. The method allows the necessary local reinforcement to be applied in the area of the fittings at any point on frameless components.

The increase in the degree of automation in the wood processing industry requires the development of automated machine diagnostics. Online diagnostics of the wear of woodworking tools is particularly important. An acoustic sensor was used to check whether the noise emission during milling can be used to detect the blunting of the cutting edge. Frequencies above 50 kHz were used and electronically processed. Oak wood with a moisture content of 12 % and a density of 790 kg/m3 was processed. The tests were carried out in three phases: Data recording with sharp milling cutter, generating wear, data recording with blunt milling cutter. The measurement of the acoustic emission of high frequency (50 to 400 Hz) can be a useful source of information about the condition of a double-edged milling tool. The information obtained in this way has the same significance as measurements of low frequencies (20 to 500 Hz) with a microphone. A contact measurement is less sensitive to external influences. The change in the frequency spectrum of the signal due to the appearance of an additional peak of the frequency is a clear indication of asymmetry caused by gradual displacement of the main cutting edge during the processing of the wood.