The thin-veneer strip technique was applied to investigate the modifying effects of 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) on the tensile strength of wood. Pinewood veneers treated with solutions of DMDHEU alone and in combination with magnesium chloride (MgCl 2 ) as a catalyst showed considerable strength losses of up to 50% in zero-span and up to 70% in finite-span testing modes. The higher strength losses in the latter case are due to hemicellulose degradation, which cannot be assessed in zero-span testing. Strength loss observed after treatment with DMDHEU and MgCl 2 was approximately as high as the sum of the strength losses determined after individual treatments with MgCl 2 or DMDHEU. Micrographs of veneers after finite-span testing revealed that catalysed DMDHEU treatment changed the predominant failure mode from interfibre fracture (in controls) to intrafibre fracture. The mechanism of strength loss resulting from treatment with DMDHEU is discussed.

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Activation spectra of wood under natural irradiation were investigated in detail in this work. The main purpose was to study colour changes on the wood surface over time and into the depth during natural light exposure and thus to further contribute to the optimization of surface-protecting treatments. In a natural weathering test, three 80-lm-thick strips of fir wood forming the surface layer of a wood composite were exposed to light under a series of glass cut-off filters. Samples were withdrawn at intervals and tested for colour changes. Identification of the most detrimental wavebands of light causing photodegradation was performed based on recorded colour changes. With chronological development of exposure, the colour changes shifted ever deeper into the surface and further into the visible region of the spectrum. A relatively narrow waveband from 360 to 435 nm was identified in the activation spectra to be the most active band, causing the greatest proportion of recorded colour changes. However, also visible light of wavelengths up to 515 nm significantly contributed to colour changes of the surface layers.

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This paper describes an investigation into the relationship between the changes in tensile properties and structural integrity of wood during weathering. Investigated species were Scots pine (heartwood and sapwood, and separately earlywood and latewood), and Norway spruce. The photodegradation was monitored through the changes in microtensile strength of wood during natural and artificial UV-weathering (''thin strip'' method), and by subsequent FE SEM microscopical analysis of transverse-fracture surfaces of the strips. The tensile strength changes during exposure (initial increase and two following phases of decrease) were analysed and were shown to be consistent with fractographic evidence of the structural changes in wood. These include the breakdown of compound middle lamella (CML), the thinning of cell walls and the development of brittleness of the secondary walls. The mechanism of failure is essentially different in earlywood and latewood, and the microtensile strength is predominantly determined by the latewood portions within growth rings. Untersuchung des Photoabbaus von Holz mit Hilfe eines Mikro-ZugfestigkeitstestsTeil 4: Zug-und Brucheigenschaften von bewittertem Holz Diese Arbeit beschreibt Untersuchungen der Beziehung zwischen den Ä nderungen der Zugfestigkeit und des Feingefüges von Holz infolge Wetterbeanspruchung. Untersucht wurden Kiefernholz (Kern-und Splintholz und, getrennt voneinander, Früh-und Spätholz) sowie Fichtenholz. Der Lichtabbau des Holzes wurde charakterisiert durch die Veränderung der Mikrozugfestigkeit während einer natürlichen und einer künstlichen UV-und Wetterbeanspruchung von dünnen (mikrotomierten) Holzstreifen ('thin strip'-Methode). Anschliessend wurden Querbruchflächen der Holzstreifen mit dem FE-SEM (Feldemissions-Rasterelektronenmikroskop) mikroskopisch analysiert. Die Ä nderung der Zugfestigkeit der Streifen während der Wetterexposition -anfänglicher Anstieg und nachfolgende Phasen des Rückgangs -wurden analysiert und erwiesen sich als übereinstimmend mit den fraktografischen Anzeichen der Gefügeveränderungen. Diese zeigten sich als Abbau der zusammengesetzten Mittellamelle, im Dünnerwerden und in einer Versprö-dung der Zellwände. Die Schädigungsmechanismen sind merklich unterschiedlich in Früh-und Spätholz. Die Mikrozugfestigkeit ist dominant vom Spätholzanteil der Jahrringe bestimmt. IntroductionA series of investigations was performed to determine the influences of various factors, such as temperature, moisture, spectral light distribution and other physical aspects of exposures, on the weathering mechanisms and photodegradation rates in wood. The first two papers in this series (Derbyshire et al. 1995(Derbyshire et al. , 1996 demonstrated that the measurement of loss of microtensile strength of thin wood strips exposed to solar radiation offers a consistent and reliable means of determining photodegradation rates for wood. It was shown that artificial weathering regimes could provide good simulation of the effects of natural weathering and analytical procedures we...