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     

Theory of X-ray measurement of microfibril angle in wood

Summary A diffraction intensity function for material bodies composed of arrays of crystalline fibres such as occurs with the cellulose of wood has been derived. It is implied in the analysis that the crystalline fibres making up the body have fibre symmetry- that there is a tendency for groups of fibres to have one set of crystal axes parallel while in the orthogonal direction the axes assume a low degree of order. It is further assumed that the patterns of the angular arrangement of the fibre groups relative to one axis of the body is independent of the direction about that axis. These conditions are believed to be compatible with the cellulosic structure found in wood. Thus it becomes possible to calculate the expected diffraction intensity profiles of realistic (and therefore complex) models of wood. This has aided the interpretation of the reflections from the (040) crystal planes of cellulose which are contaminated by low level reflections from other crystal planes, and it has been found that it might be possible by conjoint analysis of the paratropic (002) reflections and the diatropic (040) reflections to measure the complete cell wall planar microfibril angle distribution and the shape of the cell wall cross-section.This work is supported by the New Zealand Foundation for Research Science and Technology under contract # UOC 401     

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.     

Models for predicting microfibril angle variation in Scots pine

Context

Microfibril angle (MFA) is one of the key determinants of solid timber performance due to its strong influence on the stiffness, strength, shrinkage properties and dimensional stability of wood.

Aims

The aim of this study was to develop a model for predicting MFA variation in plantation-grown Scots pine (Pinus sylvestris L). A specific objective was to quantify the additional influence of growth rate on the radial variation in MFA.

Methods

Twenty-three trees were sampled from four mature Scots pine stands in Scotland, UK. Pith-to-bark MFA profiles were obtained on 69 radial samples using scanning X-ray diffractometry. A nonlinear mixed-effects model based on a modified Michaelis–Menten equation was developed using cambial age and annual ring width as explanatory variables.

Results

The largest source of variation in MFA (>90 %) was within trees, while between-tree variation represented just 7 % of the total. Microfibril angle decreased rapidly near the pith before reaching stable values in later annual rings. The effect of ring width on MFA was greater at higher cambial ages.

Conclusion

A large proportion of the variation in MFA was explained by the fixed effects of cambial age and annual ring width. The final model is intended for integration into growth, yield and wood quality simulation systems.     

Mechanism of mercerization revealed by X-ray diffraction

We studied the crystalline conversion of cellulose fiber from cellulose I to cellulose II (mercerization) by X-ray diffraction, focusing on the putative chain-polarity conversion from parallel to antiparallel. The structural change of Na-cellulose was examined during stepwise changes in NaOH concentration. Either Na-cellulose I or Na-cellulose II was formed depending on the initial NaOH concentration. Once formed, both structures were stable and did not inter-convert to each other when the NaOH concentration was changed. Such stability indicates that the parallel-to-antiparallel conversion is not likely to take place in the crystalline region of Na-cellulose. Regeneration of cellulose II from both forms of alkali cellulose proceeded with the formation of 0.44 nm lattice plane corresponding to the sheet of (1 ¯1 0) plane of cellulose II, showing that the molecular stacking due to van der Waals' interaction is the driving force of the formation of cellulose II. A mechanism was proposed whereby the geometry of the cellulose molecule allows close fitting of the hydrophobic faces only in the antiparallel arrangement, thus driving formation of the antiparallel structure of cellulose II.This study was presented in part at the 47th Annual Meeting of the Japan Wood Research Society, Kochi, April 1997; and at the 6th Annual Meeting of the Cellulose Society of Japan, Tokyo, June 1999     

The influence of microfibril angle on the longitudinal shrinkage-moisture content relationship

Summary Incremental longitudinal shrinkage has been measured on 44 samples of Pinus radiata at each 5% increment of moisture content from 0 to 25%. The samples range in mean microfibril angle from 10° to 40°. The data is presented in the form of a family of curves, of incremental shrinkage against microfibril angle, for each moisture content. This family of curves compares very closely with those derived theoretically by Barber [1968] and Cave [1972] based on considering the cell wall as a fibre composite of cellulose microfibrils embedded in a matrix which swells on wetting and whose shear modulus is a function of moisture content.     

木材微纤丝角两种测试方法的对比研究

木材微纤丝角是木材各项性质中最重要的指标之一,它的大小对木材的解剖、物理、力学性能和化学性能有着很大的影响。笔者使用两种方法测定分析了杉木的微纤丝角,为研究者选择合适的实验方法提供依据。分别利用X射线衍射法和偏振光显微镜法得到杉木的木材微纤丝角及其变异规律,并对这两种方法进行分析比较,结果表明：（1）X射线衍射法测定速度快,代表性强,适用于大量试样的变异研究,但对实验仪器要求高。（2）偏振光显微镜法测定速度较慢,但实验仪器成本相对较低,适合实验室试样的少数测定。     

Effect of microfibril angle on the longitudinal tensile creep behavior of wood

In this report, we undertook studies of the viscoelastic properties of wood from the viewpoint of the fine structure and properties of the constituent materials in the wood cell wall. To measure the mechanical properties of the wood as the behavior of the cell wall, it is required to perform the longitudinal tensile test using a homogeneous specimen. In this study, microtomed specimens of sugi (Cryptomeria japonica D.Don) earlywood were used for the creep test, which were conducted at the fiber saturation point. The substantial creep compliance of the cell wall was simulated using a simplified viscoelastic model consisting of a Voigt element and an independent spring in series. Based on the experimental results, the values of the parameters were optimized. The results were as follows: (1) the longitudinal tensile creep deformation tends to increase with the elapsed time, similar to the bending creep behavior; (2) the magnitude of the longitudinal creep function increases with MFA; and (3) each parameter in the simplified viscoelastic model is markedly affected by the MFA. Based on these results, the mechanism of the longitudinal tensile creep deformation of wood is discussed.     

杨树微纤丝角的变异及其与木材性质的相关关系

微纤丝角为细胞次生壁S2层微纤丝排列方向与细胞主轴所形成的夹角,与木材的物理性质、力学性质和化学性质都有着直接的关系。应用x射线衍射法测定了7个杨树无性系(14株样木)胸径处各年轮的微纤丝角,并对应分析和测定了各年轮的木材基本密度、纤维长度、纤维宽度和纤维素含量。研究结果表明,杨树微纤丝角在年轮间存在显著差异,其径向变异规律为从髓心向外以微纤丝角逐渐降低,年轮间的平均微纤丝角在7.8旱?8褐洌荒静幕久芏取⑾宋ざ取⑾宋矶群拖宋睾吭谀曷旨湟泊嬖谙灾钜臁O喙胤治霰砻鳎⑾怂拷怯肽静幕久芏取⑾宋ざ取⑾宋矶群拖宋睾看嬖谙灾母合喙毓叵??0.01),相关系数分别为-0.450、-0.586、-0.516和-0.660。回归分析结果表明,多项式方程可较好地描述杨树微纤丝角与所测定的木材性质的关系,相关系数均在-0.45以上(n=125)。本文的研究结果认为,在今后针对杨树材性改良的育种计划中,微纤丝角是一个重要的选育和改良指标。图3表3参34。     

Temporal variation of microfibril angle in Eucalyptus nitens grown in different irrigation regimes

In 1990, a 2-ha plantation of Eucalyptus nitens (Deane and Maiden) Maiden was established in southeastern Tasmania and subjected to different irrigation regimes. Point dendrometers were installed in March 1995 to monitor radial stem movement every 15 min over several growing seasons. In this study, data from two growing seasons (1996-1998) were considered. From these measurements, daily increments of stem radius were determined. At the end of the second growing season, we extracted 12-mm cores and measured microfibril angles (MFA) of the wood at high resolution. Microfibril angles were rescaled on a time axis and mapped to daily and distance-based elements. Among treatments, irrigated trees in particular formed higher MFA early in the growing season (September-November) and lower MFA later in the growing season. Trees subjected to cyclic droughts showed clear relationships between MFA and soil water deficits, with MFA increasing in response to water stress release. Increases in MFA were preceded by accelerations in daily increment of stem radius. Among treatments, trees subjected to severe drought had the smallest MFA and generally low fluctuations in MFA. Irrigated trees were susceptible to changes in climate, whereas growth of the trees in the other treatments was limited by water availability. Use of path-analysis showed that temperature had an effect on stem radius increment but not on MFA; wind speed was the only factor that influenced MFA directly. Microfibril angle was correlated with stem shrinking and expansion phases; growth period length and growth rates were positively related to MFA.     

Interpretation of X-ray diffractograms of wood for assessments of microfibril angles in fibre cell walls

Summary Graphical models have been developed to represent X-ray diffraction patterns for microfibril arrangements in each of the characteristic secondary wall layers of fibres in normal earlywood, latewood, compression wood, and tension wood. Models for usual combinations of typical layers S₁, S₂, and S₃, and for complex tissues including more than one S₂ layer class indicate a basis for a new analytical technique for diffractograms.Diffractograms of tissues from earlywood or latewood zones may involve effects of three to four S₂ layer variations, possibly including tension wood or compression wood. The new technique enables assessment of the microfibril angle for each. Corresponding probable experimental errors are considered. Thus it is demonstrated that, even without direct calibration by other methods for measuring microfibril angle, realistic comparative values may be obtained for all S₂ layer classes substantially represented. Such data constitute significantly more reliable indices of actual values than those provided by other techniques. Also, the data give qualitative information on other aspects of the variability of fibre types within each specimen.     

Simultaneous X-ray diffraction and X-ray fluorescence microanalysis on secondary xylem of Norway spruce

Secondary xylem of Norway spruce was studied by X-ray microanalysis. Average dimensions of cellulose crystallites, fraction of oriented cellulose, mean microfibril angle, and nutrient element concentrations of K, Ca, Mn, and Zn were simultaneously determined using microfocused synchrotron radiation and a combination of X-ray diffraction and X-ray fluorescence spectroscopy techniques. The variation of these quantities in the microscopic size scale was noticeable, and similar between samples taken from the same annual ring. The mean microfibril angle and the nutrient concentrations of Ca, Mn, and Zn showed a correlation. The mean values of the structural parameters and their variation as a function of the annual ring were similar as reported in previous studies on Norway spruce.     

Radial and between-family variations of the microfibril angle and the relationships with bending properties in Picea jezoensis families

With emphasis on tree breeding for wood quality in Picea jezoensis, we aimed to evaluate radial and between-family variations in the microfibril angle (MFA) of the S₂ layer in the latewood tracheids in 10 open-pollinated families of 43-year-old P. jezoensis trees. In addition, the relationships between MFA/wood density with the modulus of elasticity (MOE) or modulus of rupture (MOR) were investigated. Significant differences in MFA between families were found from the pith toward the bark. MFA showed higher values around the pith area, although some families showed relatively lower values than others around this area. In addition, due to a larger coefficient of variations of MFA near the pith, the potential for juvenile wood MFA improvement may be greater compared with mature wood. MOE was correlated with MFA in juvenile wood and with wood density in mature wood, whereas MOR was mainly correlated with wood density at radial positions in both woods. Therefore, to improve the MOE and MOR of P. jezoensis wood, both MFA and wood density would be factors to consider in both juvenile and mature woods. On the other hand, there are indications that, only wood density would be an important criterion for improving mature wood properties.     

The variation of microfibril angle in South African grown Pinus patula and its influence on the stiffness of structural lumber

Reduction in the rotation ages of softwood saw-log plantations in South Africa is causing increased proportions of low stiffness sawn lumber at final harvest. It has been shown for some species that the microfibril angle (MFA) of the S2 layer of tracheids is strongly related to the modulus of elasticity (MOE) of wood, even more so than wood density, especially in wood formed during juvenile growth. The objectives of this study were to describe the variation in MFA in young Pinus patula trees and to determine the relationship between MFA and the dynamic MOE of sawn P. patula lumber. Thirty 16- to 20-year-old trees from six compartments from the Mpumalanga escarpment were processed into discs and lumber. The MFA, density and ring width were measured at two height levels using Silviscan 3. The average annual ring MFA varied between 7° and 29°; the pattern of variation depended mainly on height level and the ring number from the pith. The MFA in P. patula followed the same within-tree variation trends as in New Zealand-grown Pinus radiata but the average MFA was lower in absolute terms and differences between height levels were less pronounced. The MFA and density exhibited highly significant Pearson correlations of 0.73 and 0.70, respectively, with board dynamic MOE. A multiple regression model, which included MFA, density and ring width, explained 71% of the variation in the dynamic MOE of boards. A sensitivity analysis on the model showed that MFA and density had approximately similar influences on predicting the dynamic MOE of Pinus patula boards.     

竹重组材微晶结构的X射线衍射分析

运用X射线衍射分析(XRD)的检测手段,对冷压热固化与热压法两种生产模式的竹重组材试样进行分析,结果表明竹材纤维素是以微晶结构为基础的无定型结构,仅在衍射角2θ为22.5°附近出现较突出的晶体衍射峰,在衍射角16.7°、35°附近出现对应强度较弱的晶体衍射峰。冷压热固化与热压过程中,竹材纤维素结晶区主体结构未见明显变化,但相对结晶度有不同程度下降,对竹重组材的物理力学性能有所影响。     

Evaluation of moisture content distribution in wood by soft X-ray imaging

A technique for nondestructive evaluation of moisture content distribution in Japanese cedar (sugi) during drying using a newly developed soft X-ray digital microscope was investigated. Radial, tangential, and cross-sectional samples measuring 100 × 100 × 10 mm were cut from green sugi wood. Each sample was dried in several steps in an oven and upon completion of each step, the mass was recorded and a soft X-ray image was taken. The relationship between moisture content and the average grayscale value of the soft X-ray image at each step was linear. In addition, the linear regressions overlapped each other regardless of the sample sections. These results showed that soft X-ray images could accurately estimate the moisture content. Applying this relationship to a small section of each sample, the moisture content distribution was estimated from the image differential between the soft X-ray pictures obtained from the sample in question and the same sample in the oven-dried condition. Moisture content profi les for 10-mm-wide parts at the centers of the samples were also obtained. The shapes of the profiles supported the evaluation method used in this study. Part of this work was presented at the 57th Annual Meeting of the Japan Wood Research Society, Hiroshima, August 2007     

Determining the transition from juvenile to mature wood microfibril angle in lodgepole pine: a comparison of six different two-segment models

? Context

The transition of microfibril angle (MFA) values from juvenile to mature wood marks the change from variable, low-quality wood to stronger and more consistent wood that can produce higher value products.

? Aims

We evaluate the utility of different statistical models that predict how much of a log is higher quality mature wood based on MFA.

? Methods

MFA was measured from pith to bark at breast height in six lodgepole pine stands in western Canada. Six different forms of two-segment regression models were assessed to determine the point of transition (TP) in MFA from juvenile to mature wood.

? Results

All six models provided useful and significant TP estimates. In the first segment (juvenile phase), the quadratic form produced the most conservative TPs, the linear form the least conservative, and the exponential form was intermediate. A linear second segment (mature phase) was only a minor improvement over a constant. There were significant differences in MFA TP among some sites. Analyses of the relationships between TP and tree variables, e.g., DBH, height, were inconclusive.

? Conclusions

Any of the six two-segment models can be used objectively to estimate MFA transition points; the choice of model will allow mill managers to manage risk in product out-turn.     

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