International Wood Products Journal

Modified wood shows resistance against wood deteriorating fungi, but the mechanisms are still not fully understood. The aim of this paper was to summarise the molecular studies performed on modified wood with regard to brown rot decay fungi. The DNA data showed that fungi are present inside the laboratory wood test samples already after two weeks of inoculation. Generally the fungal DNA content reflects mass loss and wood moisture content. The oxidative gene expression seems to be higher in modified wood than in untreated wood and it tend to increase during incubation. Based on the gene expression data we suggest that the hypothesis of lack of substrate recognition by the fungus should be rejected. In the reviewed studies, total wood moisture content in the samples was generally not low enough to inhibit fungal colonisation. Hence, moisture distribution within the wood should be studied more closely.

This paper presents the European initiatives, strategies, and directives that must be considered in future development of thermally modified timber and when optimising the existing, commercialised processes and discusses the next steps in developing thermally modified timber needed to meet the requirements of the European low carbon economy. The standards for assessment of environmental impacts of processes and products with the focus on Environmental Product Declarations (EPDs) for materials used in the built environment are presented and followed by discussion about the need to consider the environmental impacts on sustainably aspects of thermally modified timber. This includes: determination of environmental impacts of processing and resulting products; design of processes and products towards minimal environmental impacts; and acquisition of EPDs and of Product Category Rules (PCR) for products of thermally modified timber.

Oligomers of lactic and glycolic acid (OLA, OGA) and poly(butylene succinate/adipate) (OBS, OBA) were impregnated and heated into wood for dimensional stabilisation. These bio-polyesters variously provide bulking and lumen filling treatments. Solvent extraction of treated wood showed that ca. 50% of the polymer remains in wood with in situ polymerisation evidenced by GPC. OGA readily penetrates the wood cell wall, which occurred only on heating OLA, whereas OBS/OBA was lumen filling with any cell wall penetration only evident after water soaking. Stability of OLA/OGA treated wood was improved, but did not withstand extended leaching, whereas OBS/OBA treated wood presented higher stability once leached. Bending resistance was influenced by OLA/OGA treatments, but original MOE values were recovered after leaching. Overall, the results show the bio-polyester treatment can confer specific properties of the modified wood to suit selected end use applications.

A large number of different heating technologies has been put into use for industrial scale thermal modification of wood. A useful classification of these processes is by the level of water vapour pressure, which ranges from vacuum to high saturated steam pressures. Only high water vapour pressure systems can maintain a finite moisture content during the heat treatment, but little is known about the water vapour pressure dependence of the thermal modification chemistry and the resulting modified wood properties. It is concluded from our analysis that the thermal wood reaction chemistry at the molecular functional group level is quite independent of the process and wood species. Wood properties that are strongly determined by wood chemical composition, such as the fungal durability and the equilibrium moisture content (EMC), can hence be equally achieved by all processes and for all wood species. This finding cannot be transferred to every other thermally modified wood property.

The modification of wood with methylolated melamine formaldehyde resin belongs to the group of impregnation modifications. In the course of this study, Koto sapwood samples were impregnated with methylolated melamine formaldehyde resin solutions in a full cell vacuum pressure process. The samples were cured at a maximum temperature of 120°C for 24 h. To characterise the modification, the solution uptake and weight percent gain of the samples were calculated. The fixation of the melamine as parameter for the degree of curing was examined by C/N analysis. Areal UV-microspectrophotometry scans of ultra-thin transverse sections of an untreated control and samples impregnated with methylolated melamine formaldehyde resin at 240 nm were recorded. Additionally, photometric point measurements with a spot size of 1 μm⁻² in the range 230 and 350 nm were conducted. UV-microspectrophotometry was proven as suitable technique for the quantitative analysis of wood modified with methylolated melamine formaldehyde resin.

Dimensional stabilisation is a crucial property for using wood in most applications. Therefore an isocyanate-free route is proposed, based on the reaction of various diamines with dicarbonates obtained by addition of two glycerol carbonates moieties on a linker. For the initial experiments, the mildest conditions to perform the reaction are researched with model mono- and polyamines without wood: solvent, temperature, number of equivalent and regioselectivity were investigated. Subsequently, beech wood blocks were impregnated by a double vacuum process involving cyclic carbonates and then polyamines. The key step was a polycondensation performed in the beech blocks after the monomers had been impregnated. Resistance of treatment to leaching, anti-swelling efficiency and decay durability versus Poria placenta were evaluated. Results were promising with ASE values of about 70% for a weight gain of approximately 30% after leaching. Biological tests indicated also an improvement of resistance to fungal decay of beech wood.

Fungal cellulases have been shown to be less efficient in modified wood than in untreated wood (Lekounogou et al. 2008; Venås 2008). However, Verma and Mai (2010) showed that cellulase efficacy is partly restored in 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) treated wood by pre-treatment with Fenton's reagent, simulating the oxidative degradation phase in initial brown rot decay. In this study, we examined whether Fenton derived hydroxyl radicals (·OH) and cellulases are able to cleave polysaccharides in furfurylated and acetylated wood and to what extent enzyme efficacy is increased by oxidative pre-treatment of these materials. The results show that fungal cellulases were able to degrade acetylated wood and that the cellulase efficacy was increased by oxidative pre-treatment by 20%, which is half of the increase in untreated wood. Furthermore, the results indicate that poly(furfuryl alcohol) is degraded by Fenton derived ·OH. This indicates a possible route for the eventual degradation of modified wood.