Wood heat treatment has increased significantly in the last few years and is still growing as an industrial process to improve some wood properties. The first studies on heat treatment investigated mainly equilibrium moisture, dimensional stability, durability and mechanical properties. Mass loss, wettability, wood color, and chemical transformations have been subsequently extensively studied, while recent works focus on quality control, modeling, and study the reasons for the improvements. This review explains the recent interest on the heat treatment of wood and synthesizes the major publications on this subject on wood properties, chemical changes, wood uses, and quality control.

https://bioresources.cnr.ncsu.edu/BioRes_04/BioRes_04_1_0370_Esteves_P_Wood_Mod_Heat_Treatment_Rev_367.pdf

Wood modification by heat treatment: a review.

Properties of biochar

anisms by which lignin restrict fiber degradation are poorly understood. Many aspects of grass cell wall lignification and degradation Hatfield, 1997). These reductions in degradability are are successfully modeled by dehydrogenation polymer-cell wall partly related to the increased lignin content of cell (DHP-CW) complexes formed with primary walls of corn Zea mays walls; however, variations in three-dimensional strucL. This system was used to assess how variations in lignin composition, ture and composition of lignin and its hydrophobicity, structure, and cross-linking influence the hydrolysis of cell walls by encrustation, and cross-linking to other matrix compofungalenzymes.Alteringthenormalguaiacyl,syringyl,andp-hydroxy- nents also have been implicated (Chesson, 1993; Jung phenyl makeup of lignin did not influence cell wall degradability; each and Deetz, 1993). unit of lignin depressed cell wall degradability by two units. Plants Because of the anatomical, morphological, and develwith perturbed lignin biosynthesis often incorporate unusual precuropmental complexity of cell wall and lignin formation sors into lignin and one of these, coniferaldehyde, increased lignin in various plant tissues, studies with normal, mutant, hydrophobicity and further depressed degradability by up to 30%. In other studies, lignin formed by gradual “bulk” or rapid “end-wise” or transgenic germplasm often do not show consistent polymerization of monolignols had markedly different structures but relationships between lignin-matrix characteristics and similar effects on degradability. Reductions in cell wall cross-linking, cell wall degradability at the whole plant, plant part, or via oxidative coupling of feruloylated xylans to lignin or nucleophilic even tissue level (Grabber et al., 1992; Jung and Vogel, addition of cell wall sugars to lignin quinone-methide intermediates, 1992; Jung et al., 2000). Even when plant selection or increased the initial hydrolysis of cell walls by up to 46% and the enzyme downregulation is targeted at specific lignin extent of hydrolysis by up to 28%. Overall, these studies suggest that properties or lignin-matrix interactions, compensatory reductions in lignin concentration, hydrophobicity, and cross-linking or associative changes in other cell wall characteristics will improve the enzymatic hydrolysis and utilization of structural often occur, making it difficult to identify underlying polysaccharides for nutritional and industrial purposes. In ongoing mechanisms controlling cell wall degradability. Plants work, we are developing a DHP-CW system for dicots and are investigating how cross-linking and various acylated and unusual monolig- may, for example, respond to lower lignification by innols influence the formation of lignin and the degradation of cell walls creasing the amount of cross-linking (Chabannes et al., by rumen microflora. 2001), perhaps yielding no net change in digestibility. A common but faulty assumption is that correlations between a particular lignin trait and degradability dem

How Do Lignin Composition, Structure, and Cross-Linking Affect Degradability? A Review of Cell Wall Model Studies

X-ray microtomography

The complex structure of wood, one of the most abundant biomaterials on Earth, has been optimized over 270 million years of tree evolution. This optimization has led to the highly efficient water and nutrient transport, mechanical stability and durability of wood. The unique material structure and pronounced anisotropy of wood endows it with an array of remarkable properties, yielding opportunities for the design of functional materials. In this Review, we provide a materials and structural perspective on how wood can be redesigned via structural engineering, chemical and/or thermal modification to alter its mechanical, fluidic, ionic, optical and thermal properties. These modifications enable a diverse range of applications, including the development of high-performance structural materials, energy storage and conversion, environmental remediation, nanoionics, nanofluidics, and light and thermal management. We also highlight advanced characterization and computational-simulation approaches for understanding the structure–property–function relationships of natural and modified wood, as well as informing bio-inspired synthetic designs. In addition, we provide our perspective on the future directions of wood research and the challenges and opportunities for industrialization. The porous hierarchical structure and anisotropy of wood make it a strong candidate for the design of materials with various functions, including load bearing, multiscale mass transport, and optical and thermal management. In this Review, the composition, structure, characterization methods, modification strategies, properties and applications of natural and modified wood are discussed.

Structure-property-function relationships of natural and engineered wood

In this study the effect of high temperature on mechanical properties, dimensional stability and color of spruce was investigated. Wood specimens conditioned at different relative humidities (50, 65, 80 and 95 %) were subjected to heat treatment at 200 degreesC for 2, 4, 8, 10 and 24 h and at 100, 150 and 200degreesC for 24 h. Color changes were measured in the Minolta Croma-Meter CR-300 color system. Bending strength and modulus of elasticity were determined according to DIN 52186. The results show that heat treatment mainly resulted in a darkening of wood tissues, improvement of the dimensional stability of wood and reduction of its mechanical properties. The darkening accelerated generally when treatment temperature exceeded approximately 200degreesC. Most of the darkening occurred within the first 4 h of exposure. For the specimens heated to high temperatures, the average decrease in bending strength was about 44-50 %, while modulus of elasticity was reduced by only 4-9 %. We found that treatment time and temperature were more important than relative humidity regarding the color responses. Strong correlations between total color difference and both modulus of elasticity and bending strength were found. Thus, the color parameters can be estimated quantitatively and used as a prediction of wood strength.

Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood

In this work, densities and porosity parameters are determined on domestic and overseas soft- and hardwoods by application of pycnometric methods and mercury intrusion porosimetry (MIP). Great variability was found in bulk density, porosity and in the specific surface area. According to the pore size distribution, four pore size classes could be distinguished: macropores (radius 58–2 μm and 2–0.5 μm), mesopores (500–80 nm), and micropores (80–1.8 nm). The pore size distribution can vary even in the case of comparable pore volumes. The hardwoods, particularly the European diffuse-porous ones, show a higher amount of micropores, which represent the microvoids or cell wall capillaries. A high cumulative pore volume can also be the result of a high content of micropores with poorer accessibility. The value of the total specific surface area from MIP measurements is, generally, below those values obtained by the water vapour adsorption technique. These results can provide information for further investigations on the sorption behaviour and the fluid intake as technological characteristics in industrial processes of impregnation and penetration of coating materials or adhesives.

/pdf/porosity-and-pore-size-distribution-of-different-wood-types-3s3b7zox4i.pdf

Peter Niemz

Papers

Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood

Porosity and pore size distribution of different wood types as determined by mercury intrusion porosimetry

Physik des Holzes und der Holzwerkstoffe

Holzwerkstoffe und Leime

3D imaging of microstructure of spruce wood.