Theory of X-ray measurement of microfibril angle in wood

Summary The property of fibre symmetry as exhibited by wood cellulose can be used to derive an explicit relationship between the orientation of a cellulose microfibril and the orientation of the X-ray beam diffracted by any of its crystallographic planes. The solution applies to a microfibril of any orientation and so is well suited to evaluating the microfibril angle distribution in wood containing cells of any cross-sectional shape. The (002) and (040) reflections of cellulose have complementary properties that could be exploited to enable current problems associated with the use of each individually for evaluating the mean microfibril angle of the S2 layer to be overcome. It is expected that it will be possible to measure the microfibril angle distribution throughout the whole cell wall and also measure the average cell cross-section of a wood sample, by analysing (002) and (040) diffraction profiles in conjunction with each other.This work is supported by the NZ Foundation for Research, Science and Technology under contract # UOC 401     

Effect of tree age on the cellulose structure of Nalita wood (Trema orientalis)

The change of crystalline structure of Nalita cellulose with tree age has been studied using X-ray diffraction and FT-IR spectroscopy. The proportion of crystallinity and crystal size were increased with tree age. FT-IR spectroscopy showed that the Nalita cellulose was a monoclinic unit cell structure (I_β). The proportion of crystallinity and crystal size of the 30-month-old Nalita wood was higher as compared to aspen wood. The degree of polymerization (DP) of Nalita cellulose of different ages has also been studied. The DP of cellulose increased with tree age. The DP of Nalita cellulose was lower than that of aspen cellulose. The percentage of glucose in Nalita wood increased with tree age.     

高温炭化热处理对杉木XRD特征的影响规律

采用X射线粉末衍射仪研究高温炭化热处理对杉木XRD特征的影响规律。结果表明：高温炭化热处理对杉木纤维素结晶区002晶面衍射峰位置的影响不显著;不同处理温度水平下处理时间对木材X射线衍射峰强度的影响表现出不同的变化趋势;当处理时间相同、处理温度不同时,随着温度的升高,衍射峰强度呈现出先升高、后降低、再升高的趋势。     

An investigation on the transition characteristics of the wood cell walls during carbonization

The structural changes of the cell wall and crystalline cellulose of Quercus variabilis wood in a pyrolysis system at several temperatures ranging from 250 to 500°C were investigated to examine the wood carbonization characteristics. The volume of the wood sample was decreased and the weight loss was increased by increasing the carbonization temperature. Vessels collapsed severely in tangential direction during the charring process above 350°C. SEM observation indicated that the layering structure of the walls in wood fibers and parenchyma cells were retained below 300°C. However, the cell walls above 350°C changed to an amorphous-like structure without cell wall layering. X-ray diffraction confirmed that the cellulose crystalline substance was still remained at the carbonization temperature of 300°C but was not detected above 350°C. It can be concluded that the transition from Q. variabilis wood to charcoal might occur at approximately 350°C.     

The fibrillar orientation in the S2-layer of wood fibres as determined by X-ray diffraction analysis

Summary It is the fibrillar orientation in the S2-layer which to a great extent determines the mechanical properties of the wood fibre, with regard both to strength and stiffness and to swelling properties. Measurements of the average fibril angle of fibres are not however easy and the results differ between the methods used. In order to evaluate in more detail how the fibril angle varies in spruce wood, an X-ray method based on diffraction from the 040-plane was developed. By comparison with microscopic examination it is concluded that reliable results relating to the fibrillar orientation in the S2-layer are obtained with the X-ray technique. It is shown that the fibril angle of mature wood is rather constant with regard to both age of the annual ring and its position in the height of the tree. The fibril angle of the earlywood is found to be only slightly higher than that of latewood fibres. It is also shown that compression wood may be easily identified by virtue of the fact that its fibril angle is much higher than that of normal mature wood.The authors thank Ms Ulla Jonsson for the microscopic measurements and Dr Anthony Bristow for the linguistic revision     

An investigation of mercerization in decayed oak wood by a white rot fungus (Lentinula edodes)

The crystal transformation of cellulose I to cellulose II during alkali swelling was investigated in decayed oak wood that was used for shiitake mushroom cultivation and the results were compared with those of sound wood using X-ray diffraction analysis and ultraviolet microscopy. During mercerization, the sapwood cellulose of decayed wood was easily transformed into Na-cellulose I and then Na-cellulose I was easily converted into cellulose II after washing and drying. The sapwood cellulose of sound wood was converted more slowly to Na-cellulose I and very little Na-cellulose was converted to cellulose II. Na-cellulose I of sound wood can be reconverted to cellulose I during washing and drying. Therefore, it could be concluded that lignin prevented the alkali swelling of wood cellulose and the transformation from cellulose I to cellulose II. The decay of crystalline cellulose might cause an increase in the susceptibility of alkali swelling, so that the degree of mercerization may be also affected.     

The effect of axial strain on crystalline cellulose in Norway spruce

The effect of strain on dry, clear Norway spruce (Picea abies [L.] Karst.) wood was studied by tensile testing along the cell axis and by in situ X-ray diffraction measurements. The mean microfibril angle (MFA) was initially 3–12 degrees and did not decrease due to strain. Based on the positions of the reflections 200 and 004 of crystalline cellulose, cellulose chains elongated and the distance between the hydrogen bonded sheets of chains decreased due to the strain. The elongation of the unit cell parallel to the cellulose chains was twice as high in juvenile wood as in mature wood. The (X-ray) Poisson ratio ν _ca for crystalline cellulose in Norway spruce was calculated from the deformation of the unit cell. The average ν _ca of earlywood was 0.28 ± 0.10 in juvenile wood and 0.38 ± 0.23 in mature wood. In latewood, the average ν _ca was 0.48 ± 0.10 in juvenile wood and 0.82 ± 0.11 in mature wood. The average ν _ca values were not directly correlated to the crystallite dimensions or to the mean MFA in juvenile and mature earlywood and latewood. The results show that the amorphous matrix has a definite effect on the deformation of cellulose crystallites.     

Determination of the cellulose and lignin content on wood fibre surfaces of eucalypts as a function of genotype and site

We compared the chemical composition of wood fibres and fibre surfaces of several eucalypt species and hybrids originating from various growth sites in South Africa. The objective was to test for differences in chemical surface composition due to genetics or site with the ultimate aim to facilitate a tailor-made supply of wood for pulping that results in an optimal blend of fibres that can be pulped together with similar yields. This, however, requires a sound knowledge of the fibre properties. The surface functionality on the single fibre level is a key property, because it determines how good inter-fibre bonding will be when paper is formed, which depends amongst other fibre properties on the amount of free hydroxyl groups that are available and therefore on the cellulose content on the fibre surface. The cellulose and lignin content on the fibre surface were determined with chemical force microscopy, a variation of atomic force microscopy. Since the general bulk composition of the fibre and the surface composition might differ, both parameters were determined. We found significant differences in the cellulose and lignin content on fibre surfaces, with regard to genotype and site, respectively. In some, but not all, cases, the surface composition of wood fibres followed the bulk composition, and differences were generally more pronounced. Differences due to genotype were significant, especially with regard to the surface lignin content—but variation due to site was also distinctly recognisable. This variation in surface functionality could be the reason why some pulpwood blends result in a lower pulp yield and different quality.     

IMPRESSIONS OF FORESTRY IN SOUTHERN EUROPE

The bonding of wood by means of glue has been practised for many centuries.

Adhesion between an adhesive and wood is the result of unbalanced secondary valency forces (Van der Waal's forces) present on the interfaces. It is fundamental to good adhesion that the adhesive must (a) wet the surface it is required to adhere to and (b) penetrate the wood capillaries. The phenomenon of “wetting” is indicated by the contact angle the adhesive forms on the wood surface as well as its ability to penetrate the wood capillaries. Maximum penetration of the capillaries is inhibited in practice due to air becoming trapped in “inkpot” type capillaries caused by the sawblade tearing the wood fibres over in the direction of the cut. Several ways to increase capillary penetration are suggested.

The anisotropic chemical reactivity of wood is theorized in so far that a unit area of wood substance (excluding lumen openings) cut on the cross-sectional plane cannot be as effectively glued as a unit area of wood substance on the radial and tangential plane. This is due to the positioning of the chemically reactive groups on the cellulose chains which are predominantly oriented parallel to the fibre axis.

The engineered design of joints is briefly discussed and mathematical expression given as to how incorrect joint design can be detrimental to the ultimate joint strength.

The general character of the better-known synthetic adhesives is briefly discussed. Little detail is given as excellent hand books exist on this specific subject.     

Mechanical interaction between cellulose microfibril and matrix substance in wood cell wall determined by X-ray diffraction

We investigated mechanical interactions between the cellulose microfibril and the matrix substance in wood cell walls. X-ray diffraction measurements showed that the peak positions of (200) and (004) from cellulose crystals in wood cell walls tended to shift lower and higher toward 2θ, respectively, during water desorption in wood. From our simulations, it is shown that the peak shift of (200) during water desorption is not due to changes in the scattering pattern of the amorphous substance or to lateral expansion of the cellulose crystals due to the Poisson effect in the cellulose microfibril, which is compressed in the molecular chain direction as the amorphous substance shrinks. This suggests that the cellulose microfibril expands transversely during water desorption in the wood cell wall, and that there is a mechanical interaction between the cellulose microfibril and the matrix substance.     

A study of the structure of wood cells by x-ray diffraction

Summary An x-ray diffraction method was used to determine the values of the mean microfibrillar helical angles and to estimate quantitatively the amount of crystalline cellulose in the various cell wall layers of wood fibers. To interprete the intensity variation along the diffraction arcs a new curve fitting method based on Gaussian pairs was developed. As an application results are given for Scots pine (Pinus sylvestris) and Norway spruce (Picea abies).the authors acknowledge the financial support by the National Research Council for Science, Finland     

Orientation distributions of cellulose crystallites in Pinus densiflora woods

Summary Pole figures were described for (101), (101), (002) and (040) crystallographic planes of cellulose crystallites in opposite, normal and compression woods of Pinus densiflora. The orientation functions for these planes were plotted on the equilateral triangular coordinate. The orientation factors were calculated from the functions. It was found that the cellulose crystallites in the S₂ layer contributed to the orientation distribution although those in the other layers also contributed to some extent. From the equilateral triangular coordinate plots it was found that the orientation distributions of cellulose crystallites in wood varied in some kind of regular fashion. This was more clearly confirmed by the variations of the orientation factors.The authors are indebted to Dr. Keizo Okamura, Faculty of Agriculture, Kyoto University, Kyoto, Japan, and Dr. Misato Norimoto, Wood Research Institute, Kyoto University, Uji, Kyoto, Japan, for their valuable discussions and comments on this paper     

Ultrastructural features affecting mechanical properties of wood fibres

Abstract

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure–property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure–property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.     

Ultrastructural features affecting mechanical properties of wood fibres

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure-property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure-property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.     

Creep and stress relaxation behavior for natural cellulose crystal of wood cell wall under uniaxial tensile stress in the fiber direction

We investigated the temporal changes in creep and stress relaxation behavior in both microscopic crystalline cellulose and macroscopic strain of wood specimen using Japanese cypress (Chamaecyparis obtusa Endl.) to understand the viscoelastic properties of wood cell walls. Specimens 600 µm in thickness were observed by the X-ray diffraction and submitted to tensile load. The crystal lattice strain of (004) plane and macroscopic strain of specimen were continuously detected during creep and stress relaxation tests. It was found that the creep compliance based on macroscopic strain showed a gradual increase after instantaneous deformation due to loading and then the parts of creep deformation remained as permanent strain after unloading. On the other hand, crystal lattice strain showed a different behavior for macroscopic strain; it kept a constant value after instantaneous deformation due to loading and then increased gradually after a certain period of time. These differences between macroscopic and microscopic levels were never found in the stress relaxation tests in this study. Relaxation modulus at the macroscopic level only showed a decreasing trend throughout the relaxation process. However crystal lattice strain kept a constant value during the macroscopic relaxation process. In addition, the microfibril angle (MFA) of wood cell wall has a role of mechanical behavior at microscopic level; crystal lattice strains were smaller with increasing MFA at both creep and relaxation processes. Creep compliance and stress relaxation modulus at the macroscopic level decreased and increased with increasing MFA, respectively. Our results on the viscoelastic behavior at microscopic level evidenced its dependency on MFA.     

Eucalypt Wood Anatomical and Physical Properties and Their Effects on Plywood Veneer Quality

In order to better understand the reasons why eucalypt veneer checks easily and severely, wood samples of three eucalypt species were selected, and their anatomical and physical properties were examined according to conventional methods and the national standards. The effects of variances in cell wall thickness of wood fibre and vessel, and diameter of the cell lumen as well as the tissue ratio on the quality of plywood veneer were analysed. The results show that： 1） There is a great difference in fibre cell wall thickness and diameter of the cell lumen between early wood and late wood of Eucalyptus delegatensis. 2） E. obliqua has a high wood fibre tissue ratio and the thickest fibre cell wall, but the difference inthe fibre cell wall thickness between early wood and late wood is the smallest. 3） The wood fibre tissue ratio of E. regnans is smaller than that of E. obliqua, and its wood fibre cell wall isthe thinnest and there is only a very small difference in fibre cell wall thickness between early wood and late wood. The difference inthe diameter of wood fibre cell lumen among early wood, transition area and late wood is also small： 4） E. delegatensis has the highest tangential shrinkage rate and radial-tangential shrinkage rate, andE. obliqua has the lowest. It is the variability of wood anatomical properties of these species that cause the difference in the veneer shrinkagei and then affects plywood veneer quality.     

Mechanical behavior of the crystal lattice of natural cellulose in wood under repeated uniaxial tension stress in the fiber direction

This study investigated the relationship between the cellulose crystal lattice strain (crystalline region) and the macroscopic surface strain in specimens of Chamaecyparis obtusa wood under repeated uniaxial tension stress in the fiber direction. Changes in the strain of the crystal lattice were measured from the peak of (004) reflection using the transit X-ray method. The macroscopic surface strain of each specimen was measured with a strain gauge. In both loading and unloading, the surface strain changed linearly with changes in stress. However, crystal lattice strain was not linear but exhibited changes along a curve with changing stress. Under stressed conditions, the crystal lattice strain was always less than the surface strain, regardless of the frequency of repetition in the loading and unloading cycle. The ratio of the crystal lattice strain to the surface strain showed a negative correlation for stress in both loading and unloading. That is, the ratio decreased with increasing stress, and finally tend to converge to a specific value. The ratio (I/I ₀) between the diffracted intensity (I ₀) in the (004) plane in the unloaded condition and the diffracted intensity (I) in the (004) plane in the loaded condition tend to converge on a specific value with increasing frequency of repetition. When the substantial tension Young’s modulus of the wood in the longitudinal direction decreased, the ratio of the strain of the crystal lattice to the surface strain also decreased. Moreover, the ratio decreased with increasing microfibril angle of the specimen.     

Understanding and adding value to Eucalyptus fibre

Eucalyptus wood has become one of the most important hardwood resources for pulp mills worldwide. Furthermore, bleached Eucalyptus pulp is used extensively both in paper-making globally where it is included in such diverse products as tissue, packaging, as well as printing papers and in chemical cellulose products such as viscose, acetate and microcrystalline cellulose. This paper investigates and highlights the physical and chemical attributes of the wood and pulp fibre from Eucalyptus that contribute to its popularity in the pulp and paper-making industries and to suggest how these can be enhanced or conserved in the manufacturing process to add maximum value. The fibre properties of macerated wood samples from a range of Eucalyptus species used commercially in South Africa are compared with those of North American hardwoods such as birch, maple and aspen. In comparison to the American hardwoods, the Eucalyptus species were found to have short and thin fibres (on average, fibre length from 0.6 to 0.8 mm and fibre width between 15 and 17 μm, compared with 0.6–1.4 mm and 17–30 μm, respectively, for the American hardwoods. This particular combination of dimensions for the Eucalyptus fibre produces a low fibre coarseness, which is a highly desirable attribute for products such as coated and uncoated papers. The Eucalyptus fibre is therefore reasonably fragile and this makes it particularly vulnerable to damage during the pulp and bleaching processes. Fibre damage occurs throughout the pulp process but is most severe in the mechanical sections such as digester blowing, high shear mixers, medium- and high-consistency pumps as well as low-consistency refining. These areas are highlighted in this paper and possibilities for fibre conservation are discussed.     

Experimental micromechanical characterisation of wood cell walls

The properties of wood and wood-based materials are strongly dependent on the properties of the fibres, that is, the cell wall properties. It is thus highly important to be able to mechanically characterise cell walls in order to understand structure–property relationships. This article gives a brief overview of the state of the art in experimental techniques to characterise the mechanical properties of wood at both the level of the single cell and that of the cell wall. Challenges, opportunities, drawbacks and limitations of single fibre tensile tests and nanoindentation are discussed with respect to the wood material properties.     

Mechanical behavior of the crystalline region of wood and the piezoelectric response of wood in tension tests

We investigated the relationship between the crystal lattice strain and the piezoelectric response in Japanese cypress (Chamaecyparis obtusa Endl.) wood fibers subjected to tension stress in the fiber direction. As a result, the piezoelectric voltage was very sensitive to the mechanical behavior (deformation) of the wood crystalline regions obtained from the x-ray stress measurement. Thus, by investigating the behavior of piezoelectric voltage, it was possible to simply estimate the mechanical behavior of the crystalline regions in the wood.