Published by De Gruyter

Volume 74 Issue 12

Issue of Holzforschung

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Contents
Journal Overview

Publicly Available October 22, 2020

Review

Publicly Available June 1, 2020

Thickness-dependent stiffness of wood: potential mechanisms and implications

Fei Guo, Clemens M. Altaner, Michael C. Jarvis

Page range: 1079-1087

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Abstract

When wood is split or cut along the grain, a reduction in tensile stiffness has been observed. The averaged mechanical properties of wood samples, veneers or splinters therefore change when their thickness is less than about 1 mm. The loss of stiffness increases as the thickness approaches that of a single cell. The mechanism of the effect depends on whether the longitudinal fission plane is between or through the cells. Isolated single cells are a model for fission between cells. Each cell within bulk wood is prevented from twisting by attachment to its neighbours. Separation of adjacent cells lifts this restriction on twisting and facilitates elongation as the cellulose microfibrils reorientate towards the stretching direction. In contrast when the wood is cut or split along the centre of the cells, it appears that co-operative action by the S 1 , S 2 and S 3 cell-wall layers in resisting tensile stress may be disrupted. Since much of what is known about the nanoscale mechanism of wood deformation comes from experiments on thin samples, caution is needed in applying this knowledge to structural-sized timber. The loss of stiffness at longitudinal fracture faces may augment the remarkable capacity of wood to resist fracture by deflecting cracks into the axial plane. These observations also point to mechanisms for enhancing toughness that are unique to wood and have biomimetic potential for the design of composite materials.

Original articles

June 22, 2020

Wood grain angles variations in Eucalyptus and their relationships to physical-mechanical properties

José Clailson Franco Coelho, Graziela Baptista Vidaurre, João Gabriel Missia da Silva, Maria Naruna Felix de Almeida, Ramon Ferreira Oliveira, Pedro Gutemberg de Alcântara Segundinho, Rejane Costa Alves, Paulo Ricardo Gherardi Hein

Page range: 1089-1097

Abstract

The relationship between grain angle and wood properties has not been focus of researches in wood industry. The aim of this study was to establish grain angle variations in commercial Eucalyptus logs and their effects on physical-mechanical wood properties. Wood maximum angular deviation (MAD) was correlated with density, volumetric shrinkage, compressive strength parallel to grain, flexural strength and stiffness as determined by bending and acoustic methods in wood of seven Eucalyptus grandis × Eucalyptus urophylla clones at 13 years old. The relationship between MAD at pith-bark and base-top positions and its effect on the physical and mechanical properties were evaluated. Amplitude of MAD values was small for the seven clones, and the mean was 6.2°. The grain deviation decreased by only 8% in base-top direction, and the correlations among MAD and three logs heights were small and negative ( r = −0.13). MAD values presented an increasing trend of 33% in pith-bark direction, with a small positive correlation ( r = 0.42). Basic density (BD) presented a significant correlation with the MAD ( r = 26). There was no significant correlation between the MAD and volumetric shrinkage, mechanical properties and modulus of elasticity dynamic (determined by stress wave timer, ultrasound or transverse vibration).

June 25, 2020

Thermal characteristics of birch and its cellulose and hemicelluloses isolated by alkaline solution

Chusheng Qi, Suyun Hou, Jianxiong Lu, Weiwei Xue, Ke Sun

Page range: 1099-1112

Abstract

Cellulose and hemicelluloses were isolated from birch wood using a dilute alkaline solution and then consolidated into pellets as model compounds of cellulose and hemicelluloses in the wood cell wall. The purity of isolated cellulose and hemicelluloses was examined by Fourier-transform infrared spectroscopy and thermogravimetric analysis. The density, thermal diffusivity, heat capacity, and thermal conductivity were experimentally determined for consolidated birch powder, cellulose, and hemicelluloses in over-dry condition. The thermal degradation kinetic parameters of these materials were successfully calculated using a conversion rate step of 0.01, and the relationship with conversion rate was established. The results show that cellulose and hemicelluloses consolidated under 25 MPa had densities of 1362 kg/m 3 and 1464 kg/m 3 , respectively. The cell wall of birch powder in the oven-dry state was not collapsed under 25 MPa. The thermal diffusivity of consolidated birch powder, cellulose, and hemicelluloses linearly decreased with temperature, with values of 0.08, 0.15, and 0.20 mm 2 /s at room temperature, respectively. The specific heat capacity (1104, 1209, and 1305 J/(kg·K) at 22 °C, respectively) and thermal conductivity (0.09, 0.24, and 0.38 W/(m·K) at 22 °C, respectively) linearly increased with temperature, except for those for hemicelluloses which exhibited a nonlinear relationship with temperature above 120 °C, and their linear experimental prediction equations were given. Birch cellulose was more thermally stable than hemicelluloses. The thermal degradation kinetic parameters including activation energy and pre-exponential factor of birch powder, cellulose, and hemicelluloses varied with the conversion rate and calculation methods, with average activation energy in a conversion rate range of 0.02–0.15 of 123.2, 159.0, and 147.2 kJ/mol, respectively (using the Flynn–Wall–Ozawa method), for average natural logarithm pre-exponential factors of 25.0, 33.1, and 28.7 min −1 , respectively. Linear and quadratic equations were fitted to describe the relationship between the kinetic parameters and conversion rates. These results give comprehensive thermal properties of the densified cellulose and hemicelluloses isolated from a specific wood.

May 11, 2020

Features of frequency dependence of electrical conductivity and dielectric properties in lignins from conifers and deciduous trees

Sergey Khviyuzov, Konstantin Bogolitsyn, Aleksandr Volkov, Gennadiy Koposov, Maria Gusakova

Page range: 1113-1122

Abstract

Lignins are among the most common plant polymers and demonstrate pronounced electrical conductivity properties due to their conjugated polymolecular aromatic structure and polyfunctional nature. Electrical conductivity and dielectric properties of lignins from conifers and deciduous trees in the range of electric field frequencies from 10 −2 to 10 7 Hz were investigated by means of dielectric spectroscopy. Characteristic parameters of static and high frequency electrical conductivity were calculated. To study the influence of the lignins functional nature on their electrophysical properties, the study determined three types of relaxators (separate charges or charge systems in the structure of a substance changing their position in space when exposed to an external alternating electric field) in the structure of the lignin macromolecule. Low-frequency relaxators are associated with oscillations of methoxyl groups. Mid-frequency relaxators correspond predominantly to phenolic hydroxyl groups and to hydroxyl groups of adsorbed water. High-frequency relaxators correspond to the hopping of π -electrons along the chain of conjugated bonds of a benzene ring. Differences in the structure and functional nature of lignins from conifers and deciduous trees cause different contributions of low-frequency relaxators. As a result, these features form differences in the electrophysical properties of lignins from conifers and deciduous trees.

June 1, 2020

Developing deep learning models to automate rosewood tree species identification for CITES designation and implementation

Tuo He, Yang Lu, Lichao Jiao, Yonggang Zhang, Xiaomei Jiang, Yafang Yin

Page range: 1123-1133

Abstract

The implementation of Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) to combat illegal logging and associated trade necessitates accurate and efficient field screening of wood species. In this study, a total of 10,237 images of 15 Dalbergia and 11 Pterocarpus species were collected from the transverse surfaces of 417 wood specimens. Three deep learning models were then constructed, trained, and tested with these images to discriminate between timber species. The optimal parameters of the deep learning model were analyzed, and the representative wood anatomical features that were activated by the deep learning models were visualized. The results demonstrated that the overall accuracies of the 26-class, 15-class, and 11-class models were 99.3, 93.7, and 88.4%, respectively. It is suggested that at least 100 high-quality images per species with minimum patch sizes of 1000 × 1000 from more than 10 wood specimens were needed to train reliable and applicable deep learning models. The feature visualization indicated that the vessel groupings and axial parenchyma were the main wood anatomical features activated by the deep learning models. The combination of the state-of-the-art deep learning models, parameter configuration, and feature visualization provide a time- and cost-effective tool for the field screening of wood species to support effective CITES designation and implementation.

May 28, 2020

Fabrication of highly stable and durable furfurylated wood materials. Part I: process optimization

Wanju Li, Minghui Liu, Hankun Wang, Yan Yu

Page range: 1135-1146

Abstract

In order to improve dimensional stability and durability of wood, furfurylation of poplar and Chinese fir wood using newly developed furfuryl alcohol (FA) formulation combined with a common vacuum and pressure impregnation process was studied. An orthogonal experiment was designed to optimize the furfurylation process for the two wood species. The weight percent gain (WPG), equilibrium moisture content (EMC), anti-swelling efficiency (ASE), modulus of rupture (MOR), modulus of elasticity (MOE), as well as resistance to mold, decay fungi, and termites were evaluated. The results showed that nearly all the properties of the furfurylated wood could be improved to various extents. The average ASE of the furfurylated Chinese fir and poplar could reach as high as 80, 71, 92% and 79, 90, 75% in tangential and radial directions, and by volume, respectively, higher than most previously reported wood modification processes. Furthermore, the modified wood had excellent biological durability, with nearly 100% mold resistance, strong decay and termite resistance. Finally, processing parameters with 50% FA, 105–115 °C curing temperature, and 5–8 h curing time were therefore recommended for pilot-scale production of furfurylated poplar and Chinese fir wood based on range analysis.

June 29, 2020

Fabrication of highly stable and durable furfurylated wood materials. Part II: the multi-scale distribution of furfuryl alcohol (FA) resin in wood

Wanju Li, Minghui Liu, Hankun Wang, Yan Yu

Page range: 1147-1155

Abstract

The aim of this investigation was mainly to evaluate the multi-scale distribution of furfuryl alcohol (FA) resin in modified Chinese fir and poplar wood. 13 C CP/MAS NMR, Scanning Electron Microscopy (SEM), Confocal Laser Scanning Microscopy (CLSM), Nanoindentation and Imaging Fourier transform infrared microscopy (Imaging FT-IR) were applied to describe the FA resin distribution in wood from bulk to cell wall scale. The results showed that FA resin were mainly located in the cell cavity of Chinese fir tracheids. For poplar, FA resin was mostly deposited in the cavity of fibers and ray cells, while little was found in the adjacent vessels. Lots of pits of wood cells were covered with FA resin which implied a higher risk of drying after wood furfurlation in practical production. Nanoindentation demonstrated that FA resin could easily infiltrate into the wood cell wall because both reduced modulus and hardness of the modified wood cell walls were significantly improved. This conclusion was further supported by the results of imaging FT-IR.

July 22, 2020

Changes in wheat straw cell walls during ozone pretreatment

Elena M. Ben’ko, Dmitriy G. Chukhchin, Valeriy V. Lunin

Page range: 1157-1167

Abstract

Treatment of plant biomass with ozone is a promising delignification method. It was shown that lignin removal from the cell wall during ozonation was limited by topochemical reactions and toke place in the secondary rather in the primary cell wall. The separation of cellulose microfibrils, the loss of cell wall stiffness and complete removal of intercellular substance during the delignification process were visualized by SEM. The dependence of the average diameter of the cellulose microfibril aggregates in the cell wall of ozonized straw on ozone consumption was studied. Lignin removal caused an increase of size of cellulose microfibrils aggregates. It was demonstrated that there was an optimal degree of delignification, at which cellulose became more accessible to enzymes in the subsequent bioconversion processes. The data on the ozone consumption, residual lignin content, and sugars yield in the enzymatic hydrolysis of ozonized wheat straw were obtained. It was also found that the optimum delignification degree for sugars yield was ≈10% of residual lignin content and optimum ozone consumption was 2 mol·О 3 /mol C 9 PPU (phenylpropane structural unit) of lignin in raw straw.

About this journal

Objective
Holzforschung (HF) is an international scholarly journal that publishes cutting-edge research on the biology, chemistry, physics and technology of wood and wood components. High quality papers about biotechnology and tree genetics are also welcome. Rated year after year as one of the top scientific journals in the category of Pulp and Paper (ISI Journal Citation Index), Holzforschung represents innovative, high quality basic and applied research. The German title reflects the journal's origins in a long scientific tradition, but all articles are published in English to stimulate and promote cooperation between experts all over the world. Ahead-of-print publishing ensures fastest possible knowledge transfer.

Topics

Biology, chemistry, physics and technology of wood and wood components
Biotechnology and tree genetics

Article formats
Original articles, Reviews, Short notes

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Volume 74 Issue 12

Issue of Holzforschung

Frontmatter

Thickness-dependent stiffness of wood: potential mechanisms and implications

Abstract

Wood grain angles variations in Eucalyptus and their relationships to physical-mechanical properties

Abstract

Thermal characteristics of birch and its cellulose and hemicelluloses isolated by alkaline solution

Abstract

Features of frequency dependence of electrical conductivity and dielectric properties in lignins from conifers and deciduous trees

Abstract

Developing deep learning models to automate rosewood tree species identification for CITES designation and implementation

Abstract

Fabrication of highly stable and durable furfurylated wood materials. Part I: process optimization

Abstract

Fabrication of highly stable and durable furfurylated wood materials. Part II: the multi-scale distribution of furfuryl alcohol (FA) resin in wood

Abstract

Changes in wheat straw cell walls during ozone pretreatment

Abstract