International Wood Products Journal

Acetylation appears suited to provide adequate protection against biological attack for materials derived from non-durable wood species. But still there are unanswered questions related to resistance against fungal decay. The paper summarises existing knowledge related to fungal deterioration of acetic anhydride modified wood and also highlights future research opportunities. In addition, statistical analyses based on previously published decay fungi studies were performed to quantify what factors contribute most to the performance (calculated as test sample/control). The results showed that weight per cent gain can explain approximately 50% of the performance for acetic anhydride treated wood. Others of the applied variables, like wood species or type of fungus, can reduce the variance in performance by additional 15%. Based on the surveyed literature the degree of cell wall bulking in combination with lowering of the equilibrium moisture content seems to be the primary mode of action.

In this study, solid Scots pine was surface densified in an open press using different moisture contents (9·6 and 12·4%), temperatures (150 and 200°C), press closing times (0·5 and 5 min), holding times (1 and 10 min) and compression ratios (6·7 and 25%). The characteristics of the formed density profiles were defined and their correlations to the process parameters and Brinell hardness and elastic recovery were analysed. Compression ratio and closing time had the strongest effect on the formation of the density profile, as well as the hardness and elastic recovery. The amount of localised deformation (peak area) did not have significant effect on the Brinell hardness or elastic recovery, whereas, the highest density achieved (peak density) and its location (peak distance) dominated the effect. Brinell hardness and elastic recovery were found to correlate well with each other.

Tannins are the natural substances that plants use to protect wood. Novel tannin based formulations were tested to evaluate their efficacy in preventing weathering degradation of the surfaces. Scots pine and European beech specimens were treated with flavonoid based wood preservatives and exposed to artificial and natural weathering. The surface properties of the samples before and after weathering were evaluated using contact angle and colour measurements. Tannin treated samples showed a moderate resistance against discoloration and greater wettability than untreated samples. ATR-FT-MIR and FT-NIR spectroscopies combined with principal component analysis (PCA) revealed that weathering mainly degrades the aromatic component of wood. Because the protective tannin network is nothing more than an aromatic polymer, it suffers a similar degradation of lignin. Although the higher amount of aromatics leads to higher degradations, it is still possible to observe via vibrational spectroscopies that the flavonoid enriched surfaces contain more aromatics also after weathering.

Colour change and/or mass loss due to thermal treatment are in direct relation to the performance of thermally modified wood. The application of techniques commonly used to characterise polymers may be used to study this relationship. A standard testing procedure in the field of plastics for the characterisation of bulk properties that directly affect material performance, is dynamic mechanical analysis (DMA). DMA indicates how a material will perform in a specific application. It can be used to identify the intensity of a thermal modification by evaluation of the viscoelastic response of wood. The paper presents the method together with results of preliminary tests with Norway spruce [Picea abies (L.) Karst.] and beech (Fagus sylvatica L.). The tests showed that it is possible to thermally modify small wood specimens directly inside the DMA equipment while, at the same time, getting real time mechanical and physicochemical information about the progress of the process.

Wood compositional changes during thermal modification follow a characteristic trajectory when mapped in a van Krevelen diagram. The trajectories of widely different wood species appear to merge into a single master curve, suggesting a common thermal modification chemistry shared by these wood species. The largest effect of thermal modification on the chemical composition can be explained by dehydration reactions, followed by decarboxylation reactions. A carbon valence electron donor–acceptor model is proposed, which relates the observed compositional changes to changes in polarity and redox character, which in turn are related to the characteristic hydrophobic and fungal resistance effects on wood by thermal modification.

The benefits from acetylation of wood to enhance resistance against fungal decay and dimensional stability have been known for many years. Since 2007 Accsys Technologies has been commercially producing Accoya wood that is based on acetylation of Radiata pine. Accoya has shown its potential for many applications, even for structural use. However, due to limited engineering data each project had to be evaluated on a case-by-case basis. Based on research at various universities and institutes, Accsys Technologies has in combination with TimberSolve and ARUP, developed a handbook to assist designers and structural engineers produce reliable, durable and consistent designs utilising Accoya wood in structural applications.

We examine the performance of clear coatings on wood treated to improve its photostability. The treatments tested were: benzoyl chloride; chromic acid; modified PF resins; vinyl benzoate; and primers containing different photostabilisers. Treated panels were finished with clear coatings containing different types/levels of photostabilisers and exposed outdoors in Australia for 2 years. The coatings performed better on treated than untreated panels, except for coatings on panels treated with a primer containing cerium oxide nanoparticles. Clear coatings performed best on wood treated with chromic acid, vinyl benzoate or PF resin containing a hindered amine light stabiliser. However, the performance of the coatings was also strongly influenced by coating type. We conclude that the performance of clear coatings can be increased if they are applied to photostable wood substrates. However, coating performance is also dependent on coating flexibility, its propensity to form surface defects, and the level/types of photostabiliser in the coatings.

Studying structure and chemistry of wood and wood-based materials is the backbone of all wood research and many techniques are at hand to do so. A very valuable modality is X-ray computed tomography (CT), able to non-destructively probe the three-dimensional (3D) structure and composition. In this paper, we elaborate on the use of Nanowood, a flexible multi-resolution X-ray CT set-up developed at UGCT, the Ghent University Centre for X-ray Tomography. The technique has been used successfully in many different fields of wood science. It is illustrated how 3D structural and microdensitometrical data can be obtained using different scan set-ups and protocols. Its potential for the analysis of modified wood is exemplified, e.g. for the assessment of wood treated with hydrophobing agents, localisation of modification agents, pathway analysis related to functional tissues, dimensional changes due to thermal treatment, etc. Furthermore, monitoring of transient processes is a promising field of activity too.

The work presented in this paper reports on the water sorption properties of thermally modified wood, densified wood and wood modified using a combination of the two methods. The paper presents new findings concerning the sorption isotherms and sorption kinetics behaviour. The sorption kinetics is analysed in terms of the parallel exponential kinetics (PEK) model which has recently been introduced to the wood science community as a better alternative to the often used Fickian models, at least where cell wall sorption behaviour is considered. The PEK model comprises two exponential sorption kinetic terms which are termed fast and slow sorption processes. The PEK model is then interpreted by using a relaxation-limited kinetics model consisting of two series-coupled Kelvin–Voigt elements.

The aim of this work was to achieve a better understanding of the wettability, i.e. liquids wetting and sorption characteristics (or penetrability), of acetylated Southern yellow pine (SYP) including probable differences in such characteristics between early- or latewood. Matched samples of acetylated and untreated SYP boards were prepared. The wettability of the samples were measured by the Wilhelmy technique using standard probe liquids as well as two different sample coatings, a cationic knot sealer and an acrylic based dye. The results showed that latewood regions of the acetylated wood had a noticeably lower uptake of the non-polar low surface tension liquid octane as well as the polar high surface tension liquid water compared with latewood of the untreated controls. Contact angle analysis based on the Lewis acid–base concept indicated that the acetylated wood is predominantly Lewis basic. A preferential wetting of the knot sealer was observed on the acetylated wood.