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 Samples were withdrawn at intervals and tested for colour changes.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.

The biotechnological application of the white rot fungus Physisporinus vitreus named ''bioincising'' is currently being investigated for permeability improvement of Norway spruce (Picea abies (L.) Karst.) wood. During short-term (-9 weeks) incubation, fungal activity induces degradation of pit membranes and a simultaneous alteration of the tracheid cell wall structure. In Part 1 of this article series, the occurrence of selective delignification and simultaneous degradation was shown by UV-microspectrophotometry (UMSP). Moreover, significant reduction of Brinell hardness was recorded after 7 and 9 weeks incubation. For a better understanding of the chemical alterations in the wood constituents and the corresponding changes of mechanical properties due to fungal activity, we applied microtensile tests on thin strips that were prepared from the surface of incubated Norway spruce wood. Indications for the occurrence of selective delignification and simultaneous degradation were evident. Determination of lignin content and carbohydrate analysis by borate anion exchange chromatography confirmed the results. The present study verifies the findings from Part 1 of this article series and from previously conducted microscopic investigations. Now, the degradation characteristics of P. vitreus are established and the bioincising process can be further optimized with higher reliability.

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This paper presents the effects of properties of tested material and exposure conditions on the fi nal result of testing. These include density, uniformity of ring width, number of rings and latewood portions, as well as light source, presence of water and duration of exposure. Infl uences of these parameters in testing of several softwood species after exposure to different natural and artifi cial photodegradation regimes were monitored by means of changes in microtensile properties. The main fi ndings indicate that comparisons between various species should be made taking into account average material properties, predominantly latewood portion. The fact that strength loss changes follow the same pattern during different exposure conditions indicates that there is no difference in the nature of degradation process in various weathering machines. This forms a basis for the sound comparison between the artifi cial and natural weathering regimes.

In this study, beech wood (Fagus silvatica L.) has been chemically modified with citric acid (Acidum citricum) and sodium hypophosphate (SHP) as the catalyst and gradually thermo-condensed in the dryer. Afterwards, wetting angle, surface energy, and shear strength of glued joints of modified and unmodified wood were determined. Testing of the bond strength according to standard EN 204 and comparison between modified and unmodified samples were executed. The adhesive used for bonding samples was polyvinyl acetate (PVAC), commonly used for gluing solid wood panels. Testing material was divided into three groups (dry, wet, and wet conditioned samples), within which statistical analysis was performed, and the significance of the differences between the modified and unmodified samples was determined. Surface energy is correlated with the bond strength, indicating that modification with citric acid negatively affects the adhesive properties of beech wood. A reduction in the bond strength of modified wood glued with PVAC glue compared to unmodified wood was determined. All the results indicate that the modified samples do not meet the minimum requirements for EN 204 bonded with PVAC glue. Therefore, it will be necessary to conduct further studies using other types of adhesives to investigate whether modified wood might be suitable for gluing.

This paper presents the influence of natural surface ageing in indoor conditions on bonding quality of thermally modified wood used in structural laminated products. Two unmodified and thermally modified wood species were used for the experiment: oak and beech. Samples were planed and glued with MUF adhesive 2 hours, 1, 2, 6, 10, and 18 days after planing. Properties of laminated beech and oak beams, namely shear strength, delamination and contact angle, were measured in order to detect 1) suitability of wood species for lamination process and 2) influence of extended storage time after planing on properties of laminated wood. Generally, both native and thermally modified beech exhibited better results of shear strength and delamination and had lower contact angles compared to oak and thermally modified oak. Results of the delamination test (total delamination) indicate time dependence of surface ageing. Both unmodified and thermally modified beech may be successfully laminated at least up to 2 days after planing, whereas neither oak nor thermally modified oak are suitable for lamination process due to excessive delamination. Results of delamination may be related to contact angle measurements. Shear strength of glue lines did not show any influence on natural surface ageing. However, whereas beech and thermally modified beech samples exhibited almost the same values of the shear strength regardless of the duration of surface ageing, there is an obvious difference in shear strength of oak and thermally modified oak samples.

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