Reaction wood in Pseudowintera colorata — A vessel-less dicotyledon

Summary Inclined branches of Pseudowintera colorata exhibit pronounced growth promotion to the lower (abaxial) side similar to that found in gymnosperms. The only other significant difference between the anatomy of the upper and lower regions is that the tracheids on the lower side have a larger microfibril angle. Other microscopic features normally associated with compression wood or tension wood are completely absent. The longitudinal shrinkage of samples from the upper and lower regions is shown to be related to the mean microfibril angle in a highly non-linear way, and a relatively small change in microfibril angle is associated with a large change in longitudinal shrinkage. This result is in agreement with the hypothesis that compression wood force generation arises during the lignification phase of secondary wall deposition and is critically dependent on mean microfibril angle.The author is indebted to Mr R. R. Exley of this laboratory who prepared the samples and made all the measurements in this project     

Behavior of the cellulose microfibril in shrinking woods

We measured the longitudinal and tangential shrinking processes in wood specimens from Chamaecyparis obtuse Endl. with different microfibril angles (MFAs). The shape of the shrinking curve was compared with the MFA. Only the longitudinal shrinking process of specimens with a small MFA clearly showed nonlinearity, and the degree of nonlinearity increased as the MFA decreased. In contrast, the tangential shrinking process and the longitudinal shrinking process of compression wood with a large MFA were linear. The nonlinearity is probably caused by the longitudinal shrinkage of the noncrystalline region of the cellulose microfibril (CMF) in regions of low moisture content during water desorption. When the moisture content is high, the matrix substance in the cell wall begins to dry; however, the shrinkage in the chain direction is restrained by the rigid CMF. As the wood dries further, the noncrystalline region of the CMF embedded in the matrix substance begins to shrink. Because the longitudinal mechanical behavior of wood with a small MFA is greatly affected by a rigid CMF, longitudinal shrinkage increases suddenly at about 10% moisture content; as a result, the shrinking process shows nonlinearity.     

Relationship between fibre morphology and shrinkage of wood

Summary This is a study on the shrinkage of wood representing the wide range of morphology variation in leaning trees. It involved 13 trees of Eucalyptus regnans, one of Eucalyptus sieberi and four of Pinus radiata, and specimens taken at close intervals around the circumference of each. Data indicated a systematic modulation, between extremes at upper and lower sides of each stem, in longitudinal growth strains, relative proportions of thin, medium and thick-walled fibres, microfibril angle in the S₂ layer of these, and both Klason and acid-soluble lignin content. Analyses indicated that the microfibril angle in S₂ was a prime factor in influencing both longitudinal and volumetric shrinkage reactions; proportion of thick-walled fibres in the tissue, thickness of S₂ relative to S₁, and variations in lignification also were involved. Unusually thick-walled fibres were associated with visco-elastic strain recovery effects, which could form a substantial part of dimensional changes apparently attributable to shrinkage.Relevant to 10 of the E. regnans trees in this study, appreciation is expressed to three colleagues for providing access to the detailed data on strain, cell wall thickness, and volumetric shrinkage involved in their published study [Nicholson, J. E., Hillis, W. E., and Ditchburne, N. 1975]. By prior arrangement to minimize duplication of effort, their data were derived from specimens from those trees that were involved in this study.     

Relationships of anatomical characteristics versus shrinkage and collapse properties in plantation-grown eucalypt wood from China

To explore the influence of the basic density on collapse-type shrinkage properties and to quantify the relationships of the main anatomical features with shrinkage and collapse properties, all above-mentioned parameters were determined and analyzed for three species of collapse-susceptible eucalypts, Eucalyptus urophylla, Eucalyptus grandis, and E. urophylla × E. grandis, planted in South China. The correlation coefficients were also determined and the corresponding regression equations were established with the anatomical parameters measured by using multiple linear regression. The results indicated that: (1) basic density was strongly positively linearly related to both unit tangential shrinkage (r = 0.970) and unit radial shrinkage (r = 0.959), weakly positively related to total shrinkage (r = 0.656 and 0.640 for tangential and radial, respectively), and weakly negatively related to residual collapse (r = 0.632 and 0.616 for tangential and radial, respectively). (2) The main factors affecting unit shrinkage were cell wall proportion (WP), microfibril angle (MFA), and double fiber cell wall thickness (DWT); factors playing an important role in total shrinkage were WP, ray parenchyma proportion (RP), and MFA, while RP had the highest effect on residual collapse (r = 0.949 and 0.860 for tangential and radial, respectively). (3) All corresponding regression models obtained were very suitable for the evaluation of relationships between the anatomical parameters and unit shrinkage, total shrinkage, and residual collapse, as measured using a moisture content of 28% as the fiber saturation point for all specimens.     

Longitudinal shrinkage variations within trees of sugi (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) cultivars

The longitudinal shrinkage variations within trees and the relationship with density, microfibril angle, and modulus of elasticity were examined for five sugi cultivars selected for different within-tree distributions of density, microfibril angle, and modulus of elasticity. The cultivars showed significant differences in longitudinal shrinkage and in its within-tree distribution. The within-tree distributions were categorized into two types: (1) large values of longitudinal shrinkage near the pith that decreased with height and from pith to bark, (2) small values of longitudinal shrinkage near the pith that increased slightly from pith to bark. There were strong relationships between longitudinal shrinkage and microfibril angle, and modulus of elasticity, with large values of longitudinal shrinkage associated with large microfibril angle and low modulus of elasticity. Sugi exhibited large variation in longitudinal shrinkage within stem and among cultivars, with the variation strongly affected by microfibril angle. Part of this article was presented at the 56th Annual Meeting of the Japan Wood Research Society, Akita, Japan, August 2006     

The distribution and interrelationship of collapse,volumetric shrinkage,moisture content and density in trees of Eucalyptus regnans F. Muell.

Summary The relationships among collapse, volumetric shrinkage, moisture content and basic density and their distribution within the stem were examined for trees of E. regnans. It was found that collapse and volumetric shrinkage each were significantly related to moisture content (positively), basic density (negatively), and (positively) to the derived values P (per cent of theoretical saturation) and Q (per cent of cell cavity volume containing water). Notwithstanding the inverse association evident between moisture content and density in the living tree, moisture content was shown to be independently related to each of collapse and volumetric shrinkage.Within the stem, collapse and volumetric shrinkage were negatively correlated with height in the tree although the significance of this relationship was qualified by the adjustment for other variables. Samples containing sapwood displayed collapse values significantly lower than heartwood samples. This was reflected in the presence of a positive relationship between collapse and distance from the periphery when all material was considered, but a negative relationship when sapwood-containing samples were omitted. Basic density was positively correlated with height in the tree and negatively correlated with distance from the periphery. Moisture content, P and Q were negatively correlated with both height in the tree and distance from the periphery although the latter relationship for moisture content was dependent on adjustment for density.The highly significant relationship between volumetric shrinkage and collapse suggested that the former could confidently be used in assessing collapse severity. Moisture content was determined to be an independent indirect indicator of collapse whereas basic density, P, Q and green density could all be of indicatory value, especially the last in view of its ease of measurement.     

Properties of cell wall constituents in relation to longitudinal elasticity of wood

 To predict the origin of longitudinal elasticity of the solid wood in relation to the composite structure of the wood cell wall, an analytical procedure was developed on the basis of the idea of “the reinforced-matrix hypothesis” originally introduced by Barber and Meylan (1964). A multi-layered circular cylinder, having the CML, the S1, and the S2 layers, was used as a model of the ligno-cellulosic (wood) fiber, and the elastic properties of an isolated wood fiber were formulated mathematically. In the formulation, not only the structural factors, such as the microfibril angle and the thickness of each layer, but also the environmental condition, e.g. the moisture content, were taken into consideration. The effects of the moisture content and the microfibril angle upon the longitudinal Young's modulus and the Poisson's ratio of the wood fiber were simulated by using the newly derived formulae. It is anticipated to give a start to estimate the fine structure and the internal properties of the cell wall constituents in relation to the macroscopic behaviors of the wood through simulating the mechanical behaviors of the wood fiber. Received 17 August 1999     

Rapid Estimation of Microfibril Angle of Increment Cores of Chinese Fir by Near Infrared Spectroscopy

Near infrared (NIR) spectroscope and X-ray diffractometry have been used for rapid prediction of the microfibril angle (MFA) which is one of the important factors affecting wood properties. Wood property evaluation in breeding and resource evaluation requires effective and rapid analysis methods for thousands of samples. In the experiment, all samples from increment cores with moisture content of 60% to 150% were used for measuring MFA by X-ray scanning diffractometry. Then, a partial least squares regressi...     

Helical fissures in compression wood cells: Causative factors and mechanics of development

Summary There is evidence showing that lignification causes both an increase in the thickness of the walls, and changes in the overall width or circumference of wood cells. Although data are not available on changes in length during lignification, it can be deduced that these must also tend to occur. As lignin occupies sites in the cell walls corresponding to those occupied by water, the theory of anisotropic shrinkage of wood may be used to predict the proportional dimensional changes tending to occur as each wall layer in a compression wood cell is lignified. Taking account of the microfibril angles in those layers, it is shown that if the angle for S₂ is more than about 45°, inevitably S₂ will tend to develop deep helical fissures or splits of the form of those reported in the literature.     

Stem damage of lodgepole pine clonal cuttings in relation to wood and fiber traits,acoustic velocity,and spiral grain

Eight clones from a 16-year-old field trial of clonal cuttings of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) were analyzed for growth, growth pattern, and stem damage in the field. In addition, wood and fiber traits (acoustic velocity [AcVel] and spiral grain) were analyzed and wood density, microfibril angle, modulus of elasticity, and radial fiber diameter (FibDR) determined from SilviScan analyzes. Two clones with considerably more bent, broken, and leaning stems differed from the other clones in terms of microfibril angle and modulus of elasticity (MOE) in the outermost annual rings. FibDR and, to some extent, MOE in the outermost annual rings were negatively correlated with the frequency of bent, leaning, or broken stems, while microfibril angle (MFA) was positively correlated. AcVel was negatively correlated with both MFA and the frequency of bent, broken, and leaning stems. We conclude that AcVel could be used as an effective tool to predict severe stem damage and determine stem strength in the field instead of using costly lab-based SilviScan measurements of microfibril angle. If developed further, this approach could be used for large-scale screening of progeny tests when selecting for stem strength.     

Wood tensile strength at temperatures and moisture contents simulating fire conditions

Summary The immediate tensile strength parameters for spruce parallel to the grain and for hardboard have been determined at equilibrium conditions at temperatures up to 250°C. Below 100°C the moisture content has been varied between 0 and 30%. Above 100°C dry samples have been studied. An increase in moisture content up to about 12% leads to a slight increase in the tensile strength of spruce whereas the modulus of elasticity remains constant. With a further increase in moisture content, both properties decrease significantly. At any given moisture content, both properties decrease with increasing temperature. The corresponding strain at rupture is constant. An increase in temperature leads to a more or less linear decrease in the tensile strength and in the modulus of elasticity up to about 200°C. Above 200°C there is a more rapid decrease due to thermal softening. It is most relevant to consider the relative strength decrease since the absolute levels may be quite high due to the fact that wood samples without any irregularities were used. Such relative strength data are compared with the small amount of similar data found in the literature. The effects on the modulus of elasticity are discussed in terms of thermal softening and of water as a softener for the cellulose/hemicellulose polymers. The glass transition temperature is determined as a function of the moisture content.A special thank to Ms. K. Bojadzijev for skillful experimental assistance, to Prof. E. L. Back for stimulating discussions and to the fund for research in woodworking industries for financial support     

Influence of moisture sorption on the tangential compression strength of Mahogany wood (<Emphasis Type="Italic">Swietenia macrophylla</Emphasis> King)

Moisture sorption tests and compression tangential tests at 25°C were carried out on specimens of mahogany (Swietenia macrophylla King) wood from Peru. The tests were performed over seven adsorption and five desorption moisture conditions, and differences in strength between adsorption and desorption curves at a given equilibrium moisture content were evaluated. The results showed that second-order effects in mahogany wood were not discernible in either the tangential compliance coefficient or for the stress at the proportional limit in tangential compression.     

Modelling moisture-related mechanical properties of wood Part II: Computation of properties of a model of wood and comparison with experimental data

Starting with simple concepts of the molecular structure and models of the stiffness and swelling behaviour of lignin, hemi-cellulose and cellulose and building up through the various levels of organisation in the wood cell wall a model has been constructed that simultaneously predicts the variation with moisture content change of both the longitudinal Young's modulus and longitudinal shrinkage of wood. The model closely predicts both longitudinal shrinkage and Young's modulus as they vary with the moisture content of the wood. The model also takes into account structural variations in the form of changes in cell wall layer thicknesses and mean cellulose microfibril orientation.     

Effect of mean moisture content on the steam reconditioning of collapsed <Emphasis Type="Italic">Eucalyptus regnans</Emphasis>

Sixteen quarter-sawn boards (100 × 40 mm²) of regrowth Eucalyptus regnans (Mountain Ash) were conditioned to various moisture contents to investigate the effect of mean moisture content on collapse recovery. The results support the recommendation that boards should be reconditioned at a mean moisture content of between 15 and 20%. It is likely that the actual amount of collapse recovery was nearly as good for moisture contents up to about 25%. The main disadvantage with reconditioning boards with a moisture content of between 20 and 25% was the additional normal shrinkage that occurs because of the early reduction or removal of drying stresses. The samples in this study were dried under mild conditions for long periods of time to minimise the presence of moisture gradients.     

Bending creep behavior of hot-pressed wood under cyclic moisture change conditions

This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK _E and the MS parameterK _Mincreased with compression and decreased as the press temperature increased. The MS parameterK _M was apparently greater than the elastic parameterK _E. The MS parameterK _M increased with swelling coefficient K_SW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK _SH during desorption.     

Variation in wood properties of six natural acacia hybrid clones in northern Vietnam

Wood properties of six 8-year-old natural acacia hybrid clones between Acacia mangium and Acacia auriculiformis, planted in Bavi, Vietnam, were studied. The hybrid clones possessed obvious heterosis in growth and in some wood properties. The characteristics of growth, air-dry specific gravity, lengths of fibers and vessel elements, S₂ microfibril angle, green moisture content, and shrinkage were examined to clarify the variation among clones. From the results, the differences among the clones in growth and in some wood properties were significant. The pattern of distribution of specific gravity showed that there were low and high specific gravity zones in the stem. Specific gravity at stump height or at 3.0 m was useful for prediction of specific gravity in the whole tree stem. Moreover, clones with high specific gravity can be predicted at a young age. There was no significant correlation between diameter growth and specific gravity. Of the six clones studied, clone BV5 was selected as the best based on its growth ability and specific gravity. Part of this report was presented at the 57th Annual Meeting of the Japan Wood Research Society, Hiroshima, August 2007     

Nonisothermal moisture movement in wood

In order to analyze the effect of temperature gradient on moisture movement during highly intensive drying, such as microwave-vacuum drying, the profile of the temperature and moisture content in sealed wood whose opposite faces were subjected to temperature gradient for a short time was measured. The ratio of the moisture content (MC) gradient to the temperature gradient (dM/dT) was calculated and the factors influencing moisture movement under nonisothermal conditions were discussed. The results indicate that moisture moved in wood from the warm surface to the cold one even if opposite faces of the sealed wood assembly were exposed continuously to different but constant temperatures for a short period. The moisture content on the cold surface was higher than that on the warm surface. The moisture content gradient opposite to the temperature gradient was established, and the dM/dT was below 0.9%/°C. The temperature in the sample and the distance from the hot surface of the sample was strongly linearly correlated. With an increase in temperature, initial moisture content and experimental time, the dM/dT was significantly increased. __________ Translated from Journal of Beijing Forestry University, 2005, 27(2): 96–100 [译自: 北京林业大学学报, 2005, 27(2): 96–100]     

Effects of fire-retardant treatment and wood grain on three-dimensional changes of sandwich panels made from bubinga decorative veneer

ABSTRACT

The effects of a fire-retardant treatment (FRT) and wood grain on three-dimensional changes of aircraft sandwich panels were evaluated. Unvarnished and varnished panels having the outer decorative layer made with bubinga (Guibourtia spp.) were studied. Half of the samples from each type of panel received an FRT (phosphate-based) on all three layers of the decorative plywood. The other half had the two inner layers treated and the outer layer untreated. Three different figures formed by the rotary cutting and grain orientation were identified and separately studied on veneer surfaces. Samples pre-conditioned to 20°C and 40% relative humidity (RH) underwent an adsorption (25°C, 90% RH) and then a desorption (25°C, 40% RH) treatments. Changes in moisture content (MC), swelling, shrinkage, roughness, and waviness were measured after each moisture exposure condition. The results showed that the FRT increased significantly MC, swelling, and shrinkage of unvarnished and varnished panels. This treatment as well as the type of wood figure affected roughness and waviness variations of unvarnished panels. However, the effects of these two factors were not noticeable once panels were varnished.     

Modelling the structure of the softwood cell wall for computation of mechanical properties

A means to quantitatively construct two layer models of the wood cell-wall utilising basic density and mean microfibril angle data is discussed. It is assumed that the lignin distribution is uniform in the secondary wall layers, that there is a fixed polysaccharide ratio throughout the wall and that variation in wall thickness arises only from variation in S2 layer thickness. It is shown that the relative thickness of those cell wall layers in which the cellulose is transversely oriented (M+P, S1 and S3) have a significant effect on longitudinal shrinkage and that variance between computed and measured shrinkage values is reduced when compared with earlier models if both basic density and mean microfibril angle are taken into account.     

Effect of growing site on the fundamental wood properties of natural hybrid clones of <Emphasis Type="Italic">Acacia</Emphasis> in Vietnam

We investigated clonal and site variations in wood fiber length, microfibril angle, and specific gravity of seven natural hybrid clones of Acacia (Acacia mangium × Acacia auriculiformis) grown in northern and southern Vietnam. Fiber length did not differ between clones or between sites. The microfibril angle of the S₂ layer did not significantly differ between clones but significantly differed between sites. Clone and site significantly affected specific gravity. The significant effects of genetic × environmental interactions on wood properties indicated the difference in the response of clones to different growing conditions. The trends of changes in fiber length, microfibril angle, and specific gravity from the vicinity of the pith to near the bark were similar for all clones at each site; however, variations in fiber length, microfibril angle, and specific gravity were more visible in northern Vietnam than in southern Vietnam, with a significant effect of genetic factors. This difference may be attributable to winter, which is experienced in northern Vietnam but not in southern Vietnam. For clone selection for plantation in the northern region, combining growth rate with wood properties was recommended. On the other hand, for plantation in the southern region, clone selection depends mainly on the growth rate, taking into consideration the specific gravity.