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Mode of action of brown rot decay resistance in modified wood: A review
Holzforschung
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Chemically or physically modified wood materials have enhanced resistance to wood decay fungi. In contrast to treatments with traditional wood preservatives, where the resistance is caused mainly by the toxicity of the chemicals added, little is known about the mode of action of nontoxic wood modification methods. This study reviews established theories related to resistance in acetylated, furfurylated, dimethylol dihydroxyethyleneurea- treated, and thermally modified wood. The main conclusion is that only one theory provides a consistent explanation for the initial inhibition of brown rot degradation in modified wood, that is, moisture exclusion via the reduction of cell wall voids. Other proposed mechanisms, such as enzyme nonrecognition, micropore blocking, and reducing the number of free hydroxyl groups, may reduce the degradation rate when cell wall water uptake is no longer impeded.
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... Although there is no consensus on what the mode of protection in chemically modified wood is, existing data suggest that the degradation mechanism of brown rot and white rot decay follow the same path as for untreated wood, including similar effects on water relations [25]. For the brown rot fungi, a modus operandi is presumably the moisture exclusion via the reduction of cell wall voids [26,27], resulting in the reduction of maximum moisture capacity of the wood cell wall [28]. The reduction in available space for water molecules inside the wood cell wall inhibits the diffusion of fungal reductants, hydrogen peroxide and oxalic acid and, thereby, impeding the formation of hydroxyl radicals, which are essential for the oxidative degradation. ...
... The transport of cations in chemically modified wood is known to be restricted, depending on the treatment level (WPG) and on the relative humidity (RH) surrounding the specimen [29]. Therefore, if the oxidation is prevented, the hydrolyzing enzymes cannot penetrate the wood cell walls, as they are too large and require prior depolymerization of hemicelluloses by the hydroxyl radicals [26]. ...
... According to the most widely accepted mode of decay protection of chemically modified wood (moisture exclusion via reduction of cell wall voids [26]), the enhanced [30]. The following concentrations of the impregnation solution (in wt%) were tested and are given on the x-axis: 10%, 20%, 30%, 50% and 0% for the untreated reference. ...
This study aimed to investigate the relationship between moisture dynamics of sorbitol and citric acid (SorCA) modified wood and its biological durability. Specifically, the research aimed to determine the chemical loading needed for effective protection against wood-destroying basidiomycetes, while also improving the understanding of the moisture behavior of SorCA-modified wood. The SorCA modification process is relatively new, and thus, there is limited knowledge on its moisture behavior and its impact on biological durability. The research focused on Scots pine sapwood (Pinus sylvestris L.) and used the EN 113-2 standard to investigate its durability against wood-destroying basidiomycetes. Moisture behavior was analyzed through short-term water uptake and release tests, capillary water uptake and CEN/TS 16818. Results showed a significant reduction in liquid and water vapor uptake, likely due to a reduction in the maximum moisture capacity within the wood cell wall. The study confirmed that high chemical loadings (i.e., weight percent gain, WPG) are necessary for adequate decay protection.
... A large number of wood modification methods have been developed to date [2][3][4], and although the different modification processes affect the wood material in different ways, most of them improve the decay resistance of wood. The primary reason for the improved decay resistance is thought to be a reduction in the moisture content of the modified wood cell walls [5][6][7]. One theory is that modification reduces the rate of diffusion in the wood cell wall, interfering with the transport of fungal degradative agents [5,7]. ...
... Since the results gave no indication of substantial resin degradation in PF-or SCA-modified wood, their decay resistance can be assumed to be a function of a reduction in moisture content [5][6][7][8]. When mass losses due to C. puteana and R. placenta were plotted against ASE* and MEE* (Fig. 5), no uniform relationship was found between mass loss and ASE*, which is in agreement with previous results [8]. ...
Impregnation modifications improve the decay resistance of wood, but the mechanisms behind improved resistance are not yet fully understood. In this study, Scots pine sapwood samples were impregnation modified to investigate the relationship between moisture properties, decay resistance and chemical changes caused by decay. The samples were modified with phenol formaldehyde (PF) and sorbitol-citric acid (SCA) at different solids contents to study the effects of two different types of resins. The anti-swelling efficiency (ASE) and moisture exclusion efficiency (MEE) of the samples were measured, after which they were exposed to the brown rot fungi Coniophora puteana and Rhodonia placenta to determine their mass losses, moisture contents and chemical changes due to decay. The results showed that both modifications were able to increase ASE, MEE and decay resistance, and that neither modification was appreciably degraded by the fungi. However, no uniform relationship was found between mass loss and ASE or MEE for the two modifications, and there was a clear increase in the moisture contents of the decayed samples and sterile controls under decay test conditions with increasing modification degree. Overall, the results showed that modification with PF and SCA increases decay resistance, but the relationship between resistance and moisture properties requires further investigation.
... It is generally accepted that acetylated wood treated to an uptake of 15%−20% weight percent gain (WPG) is decay-resistant [6][7][8][9]. However, the mechanisms by which acetylation protects wood from fungi are still under debate [10][11][12] and it is possible that wood treated to 20% WPG is not decay-resistant, but instead the kinetics are retarded or delayed enough that decay is not a practical concern [10, [13][14][15]. In addition to improving the resistance of wood to fungal decay, acetylation also changes the physical properties of wood. ...
Acetylation is a type of commercial wood modification used to enhance the durability of wood. Despite its adoption, especially in outdoor environments, there are mixed data on how acetylation affects the combustion of wood. This paper evaluates the differences in acetylated and untreated wood using a cone calorimeter in combination with Fourier Transform Infrared Spectroscopy (FTIR) to look for acetic acid vapors in the combustion gases. Two thicknesses of acetylated pine boards were tested and compared against an untreated board from the same genus. No differences were observed between the peak heat release between the acetylated and untreated boards. Likewise, there were no trends in the time to ignition between the acetylated wood and the control group. Differences were observed however in the chemical composition of the combustion products. An increase in acetic acid in the products of combustion was observed for the acetylated samples that corresponded with the peak heat release of the sample.
... The enhanced biological durability of acetylated wood has been explained by the following potential modes of protective action: (1) wooden cell wall polymers are not recognized by fungal enzymes, (2) nutrients, such as hemicelluloses, are not easily accessible because they are blocked or modified, (3) micropores in the wood cells are blocked, hindering the fungi to pass and reach the nutrients, (4) hydrolysis of polysaccharides is inhibited because less -OH groups are available, and (5) the MC in the cell wall is insufficiently low for the transport of enzymes of decay fungi (Ringman et al. 2014(Ringman et al. , 2019Zelinka et al. 2016Zelinka et al. , 2022. The theory that acetylation renders wood polymers unrecognizable to fungi was recently debunked (Zelinka et al. 2022). ...
The overall aim of this study was to investigate the durability of acetylated beech (Fagus sylvatica L.) laminated veneer lumber (LVL) against wood-destroying basidiomycetes. The secondary objective was to test whether the specimen size affects the mass loss and durability assessment of wood-material under test. The durability test was based on the pre-standard prEN 113-3. Six materials (acetylated beech LVL, untreated beech LVL, beech solid wood, pine sapwood (Pinus sylvestris L.), larch (Larix decidua Mill.) 3-layer slab, larch solid wood) were tested using three specimen geometry designs (50 × 25 × 15 mm 3 as well as 50 × 50 × 19 mm 3 with and without sealed edges) against Coniophora puteana, Rhodonia placenta, Gloeophyl-lum trabeum, Trametes versicolor, and Pleurotus ostreatus. The durability assessment was made using the arithmetic mean and median percentage mass loss (ML), the relative ML (x-values), and the decay susceptibility index (DSI). It was found that mass loss was affected by the test fungus, the material, and the specimen size and design, with the latter being the most essential factor in this study. In addition, the assessment parameter had a significant effect on the durability classification. Furthermore, small differences in ML resulted in different durability classes (DC) in some cases, whereas large differences in ML did not. However, acetylated beech LVL was always considerably durable (DC 1) against all tested fungi independent of the specimen design and durability assessment method.
... The mechanisms by which these chemical modifications provide improved properties, particularly fungal decay resistance, is still an ongoing area of research. Most research indicates that lowering the moisture content of the end product is important for the decreased susceptibility of modified wood to wood-decay fungi [3][4][5][6]. Moreover, recent studies have shown that chemical modifications can lead to nanostructural changes [7,8] and alter the nanoscale moisture distribution inside wood cell walls [9][10][11][12], which in turn likely has an impact on the susceptibility of wood nanostructure to biodegradation. ...
Reactive chemical modifications have been shown to impart decay resistance to wood. These modifications change hydroxyl availability, water uptake, surface energy, and the nanostructure of wood. Because fungal action occurs on the micro and nano scale, further investigation into the nanostructure may lead to better strategies to prevent fungal decay. The aim of this article is to introduce our findings using small angle neutron scattering (SANS) to probe the effects of chemical modifications on the nanostructure of wood fibers. Southern pine wood fiber samples were chemically modified to various weight percentage gains (WPG) using propylene oxide (PO), butylene oxide (BO), or acetic anhydride (AA). After modification, the samples were water leached for two weeks to remove any unreacted reagents, homopolymers or by-products and then the equilibrium moisture content (EMC) was determined. Laboratory soil-block-decay evaluations against the brown rot fungus Gloeophyllum trabeum were performed to determine weight loss and decay resistance of the modifications. To assist in understanding the mechanism behind fungal decay resistance, SANS was used to study samples that were fully immersed in deuterium oxide (D2O). These measurements revealed that modifying the fibers led to differences in the swollen wood nanostructure compared to unmodified wood fibers. Moreover, the modifications led to differences in the nanoscale features observed in samples that were exposed to brown rot fungal attack compared to unmodified wood fibers and solid wood blocks modified with alkylene oxides.
... However, it is evident that thermal modification significantly improves the fungal decay resistance of panels produced with thermally modified strands, suggesting that more hydrophobic strands improved long-term durability of panels. Increased decay resistance of materials undergoing TM results from the reduction of hemicelluloses in wood fibers since it is known that modified wood reduces the amount of available water needed for fungi to colonize wood cells [86]. ...
Thin-strand composite panels and subsequent mass timber beams were produced using thermally modified
wood strands in a pressurized system. The effects of thermal modification (TM) temperature and dwell time on
the mechanical, moisture, and decay performance of panels were studied. TM reduced moisture sorption and
increased decay resistance. The thin-strand composites were evaluated in flexure and benchmarked against
commercially available structural products. Moreover, the mass timber beams’ out-of-plane bending was
accurately predicted with traditionally used laminated beam theory. The study shows that TM, under controlled
conditions, enables the production of high-performing wood-strand panels with improved dimensional stability
and decay resistance.
... Indeed, limiting access to, or modifying, hydroxyl groups already produces important changes: for example, highly specific enzymatic reactions cannot take place anymore, and the ability to form hydrogen bonds is impaired. 112 Compared to impregnation, wood modification involves the formation of relatively stable bonds between wood biopolymers (through their hydroxyl groups) and the functionalizing agents, reducing their tendency to leach out of wood. ...
Wood is a renewable resource with excellent qualities and the potential to become a key element of a future bioeconomy. The increasing environmental awareness and drive to achieve sustainability is leading to a resurgence of research on wood materials. Nevertheless, the global climate changes and associated consequences will soon challenge the wood-value chains in several regions (e.g., central Europe). To cope with these challenges, it is necessary to rethink the current practice of wood sourcing and transformation. The goal of this review is to address the intrinsic natural diversity of wood, from its origin to its technological consequences for the present and future manufacturing of wood products. So far, industrial processes have been optimized to repress the variability of wood properties, enabling more efficient processing and production of reliable products. However, the need to preserve biodiversity and the impact of climate change on forests call for new wood processing techniques and green chemistry protocols for wood modification as enabling factors necessary for managing a more diverse wood provision in the future. This article discusses the past developments that have resulted in the current wood value chains and provides a perspective about how natural variability could be turned into an asset for making truly sustainable wood products. After briefly introducing the chemical and structural complexity of wood, the methods conventionally adopted for industrial homogenization and modification of wood are discussed in relation to their evolution toward increased sustainability. Finally, a perspective is given on technological potentials of machine learning techniques and of novel functional wood materials. Here the main message is that through a combination of sustainable forestry, adherence to green chemistry principles and adapted processes based on machine learning, the wood industry could not only overcome current challenges but also thrive in the near future despite the awaiting challenges.
Heat treatment increases the decay resistance of wood by decreasing its hygroscopicity, but the wood material remains degradable by fungi. This study investigated the degradation of heat-treated wood by brown rot fungi, with the aim of identifying fungal-induced hygroscopicity changes that facilitate degradation. Scots pine sapwood samples were modified under superheated steam at 200 and 230 • C and then exposed to Coniophora puteana and Rhodonia placenta in a stacked-sample decay test to produce samples in different stages of decay. Sorption isotherms were measured starting in desorption from the undried, decaying state to investigate their hygroscopic properties. Although there were substantial differences in degradative ability between the two fungi, the results revealed that decay by both species increased the hygroscopicity of wood in the decaying state, particularly at high relative humidity. The effect was stronger in the heat-treated samples, which showed a steep increase in moisture content at low decay mass losses. The reference samples showed decreased hygroscopicity in absorption from the dry state, while the heat-treated samples still showed an increase at low mass losses. Near infrared spectroscopy showed that the early stages of decay were characterised by the degradation of hemicel-lulose and chemical changes to cellulose and lignin, which may explain the increase in hygroscopicity. The results provide a new perspective on brown rot decay and offer insight into the degradation of heat-treated wood.
The chemical modification of wood can be directed to improve various properties, e.g., the dimensional stability, hardness properties, and/or durability properties, against weathering. In this study, a Romanian softwood species Abies alba Mill. was treated and chemically modified using two cyclic acid anhydrides, i.e., maleic and succinic, to improve its interfacial properties relative to unmodified wood. Structural changes, with focus on the evolution of crystalline part in wood after chemical modification, the water absorption, and the water repellent efficiency, were determined. Maleic anhydride exhibited a lower reactivity towards wood substrate than succinic anhydride, presumably because of their different chemical structure (maleic anhydride is very sensitive to the presence of water). It was found that the percentage of water absorption was diminished, primarily after the succinic anhydride treatment. The chemically modified wood was characterized via Fourier transform-infrared spectroscopy and wide-angle X-ray diffraction methods. The crystalline part from wood structure was evidenced in relation to the employed anhydride in chemical modification approach.
Homobasidiomycetes include the majority of wood-decaying fungi. Two basic forms of wood decay are known in homobasidiomycetes:
white rot, in which lignin and cellulose are degraded, and brown rot, in which lignin is not appreciably degraded. An apparent
correlation has been noted between production of a brown rot, decay of conifer substrates, and possession of a bipolar mating
system (which has a single mating-type locus, in contrast to tetrapolar systems, which have two mating-type loci). The goals
of this study were to infer the historical pattern of transformations in decay mode, mating type, and substrate range characters,
and to determine if a causal relationship exists among them. Using nuclear and mitochondrial rDNA sequences, we performed
a phylogenetic analysis of 130 species of homobasidiomycetes and performed ancestral state reconstructions by using parsimony
on a range of trees, with various loss:gain cost ratios. We evaluated pairwise character correlations by using the concentrated
changes test (CCT) of Maddison and the maximum likelihood (ML) method of Pagel. White rot, tetrapolar mating systems, and
the ability to decay conifers and hardwoods appear to be plesiomorphic in homobasidiomycetes, whereas brown rot, bipolar mating
systems, and exclusive decay of conifers appear to have evolved repeatedly. The only significant correlation among characters
was that between brown rot (as the independent character) and exclusive decay of conifer substrates (P < 0.03). This correlation was supported by the CCT on a range of plausible trees, although not with every reconstruction
of ancestral states, and by the ML test. Our findings suggest that the evolution of brown rot has promoted repeated shifts
to specialization for conifer substrates.
The reasons for the increased decay resistance of thermally modified timber (TMT) are not yet fully understood and were therefore exemplarily examined for thermally modified spruce against Oligoporus placenta. Spruce specimens heat treated at 200 and 220°C were successively extracted and additionally treated with 2%-KOH solution to modify the moisture sorption properties. The extracts were surveyed regarding their inhibitory effect on fungal growth, while the extracted specimens were tested against O. placenta to determine the decay resistance. Furthermore, maximum swelling and equilibrium moisture content (EMC) were determined. The decrease in swelling and EMC as well as the increased decay resistance of TMT were not affected by extraction and further on, none of the extracts revealed an inhibitory effect. In contrast, the alkali treatment provoked enhanced moisture sorption of TMT specimens and lead to increased fungal decay at the same time. Thus it is assumed, that the reduced moisture sorption of TMT is mainly responsible for the increased decay resistance.
Furfurylated wood has proven to be a biological inert material and, thus, is an environmentally-benign alternative to wood treated with heavy metals or tropical hardwoods. The chapter will deal with the history, properties and uses of furfurylated wood to date.