The values of the MFA and the standard deviation σ in the pith region for each age of
The orientation of the cellulose microfibrils in the S2 layers of the cell walls of softwood has a significant influence on the mechanical properties of wood. The angle between the cellulose fibrils and the longitudinal cell axis, the microfibril angle, MFA was found to be a critical factor in determining the physical and mechanical properties of wood (Cave, 1997). For this reason, considerable effort has been directed towards the measurement of the cellulose MFA. Direct measurement of MFA has been made by highlighting microfibrils in individual cell walls with iodine staining, but the most widely adopted techniques use either wide-angle X-ray diffraction or small-angle X-ray scattering The pioneering work of Cave (1966) and Meylan (1967) led to the use of the ‘
MFA has been found to influence shrinkage of wood (Harris and Meylan, 1965). The MFA of the S2 layer in the tracheid cell wall is known to be one of the main determinants of the mechanical properties of wood (Cave, 1968). Donaldson (1993) reported that the MFA also had a significant impact on paper properties. The MFA has a very strong influence on the stiffness of wood (Walter and Butterfield, 1996). MFA in the S2 layer of the cell wall of
1.1. Problem and background
Much of the future timber supply in Malaysia is expected to come from
1.2. Wood microstructure
The primary structural building block of wood is the tracheid or fibre cell. Cells vary from 16 to 42 microns in diameter and from 870 to 4000 microns long. Thus, the cubic centimeter of wood could contain more than 1.5 million wood cells. When packed together they form a strong composite. Each individual wood cell is even more structurally advanced because it is actually a multilayered, closed-end tube (Fig. 1) rather than just a homogeneous-walled, nonreinforced straw. Each individual wood cell is even more structurally advanced because it is actually a multilayered, closed-end tube (Fig. 1) rather than just a homogeneous-walled, nonreinforced straw. Each individual cell has four distinct cell wall layers (Primary, S1, S2, and S3). Each layer is composed of a combination of three chemical polymers: cellulose, hemicellulose, and lignin (Fig. 1). The cellulose and hemicellulose are linear polysaccharides (i.e., hydrophilic multiple-sugars), and the lignin is an amorphous phenolic (i. e., a three-dimensional hydrophobic adhesive). Cellulose forms long unbranched chain and hemicellulose forms short branched chains. Lignin encrusts and stiffens these polymers. Because carbohydrate and phenolic components of wood are assembled in a layered tubular or cellular manner with a large cell cavity, specific gravity of wood can vary immensely. Wood excels as a viable building material because the layered tubular structure provides a large volume of voids (void volume), it has an advantageous strength-to-weight ratio, and it has other inherent advantages, such as corrosion resistance, fatigue resistance, low cost, and ease-of modification at the job site.
1.4. Wood quality
Wood quality results largely from the chemical and physical structure of the cell walls of its component fibres. It can be defined in terms of attributes that make it valuable for a given end use (Jozsa and Middlleton, 1994). In general, density and MFA are indicators of strength and stiffness respectively. They are reputed to be the key determinants of wood quality. For the sawmiller, wood quality is reflected in the value of mill production and this depends on grade out-turn and the value for each grade (Addis Tsehaye et al.,1995). Wood quality for the structural engineer means wood with a high stiffness level, an attribute which is most important for beams, joists and purlins. Strong wood is required for studs and trusses (Addis Tsehaye et al., 1995). For the wood technologist, wood density is important, at higher timber strength and greater yield of pulp (Elliot, 1970). The paper and pulp mill requirements for quality wood are long fibre length with low lignin content (Zobel, 1961). A minimum fibre length of 2 mm is necessary to produce acceptable kraft pulp. A reduction in lignin content leads to a considerable savings during the production of bleached kraft pulp (Walker and Butterfield, 1996). The most important parameters of kraft pulping are basic density and fibre length (Cown and Kibblewhite, 1980).
1.5. Chemical composition of wood
Wood material is primarily composed of three organic polymeric components, namely:- cellulose, hemicelluloses and lignin. They primarily determine the chemical and physical properties of wood. Minor amounts of mainly organic inclusions are present collectively called extractives, which are present in wood but are not structural components. The amount of extractives such as gums, fats, resins, sugars, oils, starches, alkaloids and tannins, varies from less than 1% to more than 10% of the oven-dry weight of wood (Tsoumis, 1992).
Cellulose is the main constitutent of wood, occupying 40-45% of the dry wood substance in both softwoods and hardwoods (Watson and Dadswell, 1964). It consist of glucose (C6H12O6) linked together to form cellulose chain molecule. The structural formula of the cellulose chain molecule is shown in Figure 2 (Cave and Walker 1994). Each glucose molecule added to the repeating unit of the molecule chain is rotated 180°. The number of the glucose monomers (C6H10O5) in the cellulose chain is called the degree of the polymerization and in wood it is about 8000-10000, giving the cellulose molecules a length of 4-5 µm (Butterfield and Meylan, 1980). In wood cell walls, cellulose micromolecules are arranged into bundles consisting of a number of cellulose chain molecules called microfibrils, in which cellulose is mainly present in crystalline and in amorphous regions (Tsoumis, 1992). The crystalline regions of cellulose has been investigated by using x-ray scattering methods (Jakob
In addition to cellulose, a number of various polysaccharides called hemicelluloses are present in wood. Hemicelluloses are formed from glucose and other six-carbon and five-carbon sugar molecules and constitute 25-30 % of the dry wood substance in softwoods and 25-35 % in hardwoods (Meylan, 1967). With regard to degree of polymerization, hemicellulosees are quite small macromolecules comparied to cellulose since they seldom have more than 150 -200 monomer units (Wilson and Archer, 1979). However, the clear distinction between cellulose and hemicelluloses is that hemicelluloses are soluble in aqueous alkali but cellulose is not.
1.6. The Structure of the cell wall
Wood cell walls are structurally complex. It is has a hollow center called lumen and layered cell walls. The cell wall can be divided into separate distinguishable layers, namely, middle lamella (M), primary wall (P) and secondary wall (S1,S2 and S3). These layers differ from one another with respect to chemical composition as well as their structure. The middle lamella is located between the adjacent wood cells and serves the function of binding them together. Sometimes the combine region of the middle lamela and two adjacent primary walls are called the compound middle lamella (CML), which has the most lignin rich region(Batchelor et al., 1997).
The cell wall layers are divided on the basis of how the microfibrils are arranged on the specific layers. The differences in the orientation of the microfibrils help to distiguish the primary wall and the three layers of the secondary wall from each other. The primary wall is very thin, approximately only 0.1 µm, and the microfibrils are randomly orientated on the outer surface and more or less transversally orientated with respect to cell axis on the inner surface near the secondary wall (Cave and Walker, 1994).
In the secondary wall, the microfibrils are closely packed and the differences in the orientation of the microfibrils are quite distinctive. The thin outer S1 layer adjacent to the CML consist of a few lamella in which the orientation of the microfibrils have an alternating left - handed and right - handed helical arrangement, forming a crossed microfibrillar texture. In each lamela, the microfibrils spiral around the longitudinal cell axis (about 50 - 70°). The thickness of the S1 layer is about 0.1 – 0.2 µm. The inner S3 layer is usually even thinner than the S1 layer and consist of a similar microfibrillar orientation (Cave and Walker, 1994).
The S2 layer of the secondary cell wall is, by far, the thickest of the cell wall layers and has the most profound effect on the physical properties of wood. The microfibrils in the S2 layer show a high degree of parallelism in all lamella with only a small dispersion and, in general, are more or less parallel to the longitudinal cell axis. Figure 3 shows the different layers of the cell wall of thin sections through
1.7. Cellulose microfibrils
Microfibrils are the structural units of plant cell wall. Each microfibril contains a number of cellulose chain molecules bundled together, and is surrounded by low molecular weight hemicelluloses (Tsoumis, 1992). The hemicellulose act as connecting agents that link or of bond the microfibrils together (Hygreen and Bowyer, 1996). The cellulose chain molecules are generally arranged lengthways with regard to the microfibril axis, but run parallel to each other in portions. These portions are called crystalline regions. The molecules in these regions are strongly connected to each other by hydrogen bonding. The crystalline regions are followed by amorphous regions in which the cellulose molecules have no definite arrangement. The transition from crystalline to amorphous region is gradual. Approximately two thirds of the cellulose in the cell wall is crystalline in form while one third is amorphous (Tsoumis, 1992). Microfibrils vary in width from 1 µm in the primary walls to 10 µm in the secondary walls (Zobel, 1961). The angle that the cellulose microfibril make to the axis of the cell wall is known as the microfibril angle (MFA). Microfibrils are present in each of the cell wall layers (Butterfield, 1980). The microfibrils is the smallest component of the cell wall which can be visualized by transmission electron microscopy occurring along fibril 3-4 nm in diameter and it consists of a group of cellulose surrounded by a sheath of hemicellulose (Harris and Meylan, 1965). The cellulose microfibrils are wound helically around the cell wall in the S2 layer, as shown in Figure 4.
1.8. Microfibril Angle (MFA)
One of the primary importance in many investigations of the properties of natural cellulosic fibres is the knowledge of the orientation of the constituent cellulose microfibrils. The orientation of the elementary cellulose fibrils reinforcing the wood cell wall has been a subject of growing interest in recent years. Particular attention has been attracted by the tilt angle of the cellulose fibrils with respect to the longitudinal cell axis called the microfibril angle (MFA) that was found to influence the mechanical properties of wood (Cave, 1997) as well as shrinkage during drying (Meylan, 1967). MFA was also discussed with respect to influences on stiffness and tensile strength of fibres and paper (Cave, 1976). MFA varies from tree to tree, pith to cambium and with height in the stem. It also varies with speed of growth of the tree. Many methods, such polarizing, fluorescence and electron microscopy and iodine staining, used to estimate the MFA are tedious and time consuming because the extreme variability of biological material demands that large numbers of fibre elements be measured to give meaningful average values. In contrast, x-ray diffraction can provide a mean diffraction pattern of several hundred elements in a single exposure, a little cost in preparation and observation time. The main drawback to the x-ray method has been the interpretation of the diffraction patterns in terms of microfibril distribution. Methods to measure MFA include X-ray diffraction (Cave, 1966), Wide angle x-ray scattering (Boyd, 1977), and Small-angle X-ray scattering (Jakob et al., 1994). The recently increased attention on MFA in wood has encouraged researchers to compare different measuring methods (Bertaud and Holmbom, 2004). These approaches are important, as encountered discrepancies foster new research towards a better understanding of MFA in wood.
In the present study, x-ray diffraction and small-angle x-ray scattering techniques were used to determine MFA as a rapid technique for measuring microfibril angles in
1.9. Importance of MFA
The term microfibril angle, (MFA) in wood science refers to the angle between the direction of the helical windings of cellulose microfibrils in the secondary cell wall of fibres and the long axis of the cell wall (Cave, 1966). Technologically, it is usually applied to the orientation of the cellulose microfibrils in the S2 layer that makes up the greatest proportion of the wall thickness, since it is this which most affects the physical properties of wood (Senft Bendetsen, 1985). Figure 5 shows a confocal micrograph of wood fibre under cross-polarisers. It shows the fibre as bundle of helical wound microfibrils composing highly aligned molecules, the MFA in the S2 layer is the angle of the microfibrils relative to the long axis of the cell wall (Cave, 1966). The MFA of the S2 layer represent an important ultramicroscopical feature influencing the performance of wood products. It has a major effect on the stability of wood on drying and subsequent manufacturing processes (Zobel, 1961). Orientation of the S2 MFA has a significant influence on tensile strength, stiffness and shrinkage. Modeling suggests that the relative thickness of the P, S1 and S3 layers contributes significantly to the variability of longitudinal shrinkage (Cave, 1976). Both the longitudinal tensile strength and stiffness of wood have been shown to be markedly affected by MFA; as the MFA increases, tensile strength and stiffness quickly decrease (Mary Treacy
1.10. Environmental impacts on MFA
Growth rate influences MFA in two ways. Firstly, fast growing trees have the largest microfibril angles both in juvenile and mature wood, and secondly, narrow growth rings are formed in some trees when they are suppressed and these rings tend to have tracheids with a large MFA (Brändström, 2002). The results of a trial carried out by Mary
1.11. Methods of measuring MFA
There are essentially four methods for measuring MFA in the cell wall: X-ray diffraction (Cave, 1966; Boyd, 1977), polarized light microscopy (Meylan, 1969; Evans, 1999), direct or indirect observation (Senft and Bendtsen, 1985; Donaldson,1991) and small angle X-ray scattering ( SAXS), (Jakob
X-ray diffraction is the fastest and most modern method of measuring the MFA. This method enables large sample numbers to be measured in a short time. It has been used to determine MFA (Cave,1966; Harris and Meylan, 1965; Meylan,1967). Evans (1999) used XRD technique for determining the MFA in
1.12. The parameter
In general, the width of the (002) diffraction arc reflects the magnitude of the mean MFA and most methods in use are based on a measure of the width of the arc (Cave, 1966). The width of the diffraction arc in the method presented here is determined by the angular separation,
1.13. Angular distribution of microfibrils
The theoretical relation between
Tracheids may vary in shape from rectangular through irregular hexagonal to circular. The theory considers two extreme shapes, square and circular, in order to indicate the likely effect of cell cross-sectional shape on the diffraction diagram (Cave, 1966). Wherever possible, a general angular distribution of microfibrils has been assumed in the plane of the cell wall, subject to the following conditions:
The microfibril is essentially a single crystall.
All microfibrils are crystallographically identical
The cell wall consist of a single homogeneous layer of microfibrils called S2 layer embedded in a noncrystalline matrix.
The microfibrils lie strictly in the plane of the cell wall (Cave, 1976). e angular distribution of microfibrils in the plane of the wall. This taken to be approximately that of the normal probability function with a mean MFA, and standard deviation σ (provided σ ≤ 30°) (Meylan, 1972)
2. Materials and methods
The wood samples used in this study were selected from 3, 5, 7, 9, 10, 11, 13 and 15 year-old of
Radial slices 50 µm thick were cut by rotary microtome from each trunk and then used for Probe microscope.
Samples for Dynamic Mechanical Analyzer DMA testing were prepared using a table saw. They were further machined down to nominal thickness of 3.0 mm using vertical milling machine. The samples were held in place under controlled humidity and temperature. Care was taken to obtain samples from the same area of the impact region in the wood trunk. Each disc of wood was machined to produce a balance DMA samples desired thickness. The final samples dimensions were 50 mm × 13 mm × 3 mm.
3. Determining of MFA
Methods to determine the mean microfibril angle, MFA, crystallinity of wood and the average size of cellulose crystallites were presented. The mean MFA, the crystallinity of wood and the size of the cellulose crystallites were determined as a function of the tree age in
3.1. Calculation of Microfibril Angle (MFA) Using X-Ray Diffraction (XRD)
Microfibril angle, MFA can be defined as the angle between tracheid or fibre axis and microfibril orientation in the S2 layer. Evaluation of the mean MFA then involves an assumption of the form of the microfibril distribution. It is of interest to examine the differences of the intensity distributions diffracted between the different ages of the real cell wall structure. We can clearly observe the change in the diffraction pattern with increasing the tree age. Figure 14 schematically represents thin sample of wood, which has a rectangular cross-section. The fibre axis or the cell axis is vertical towards the radial direction of the tracheid, which represent the surface of the paper (Figure 8). Two cellulose MFA (Z helix) have been drawn, one in the front cell wall and the other on the back cell wall. When the MFA is determined using x-ray diffraction and a slice of wood, both the front and back cell wall contribute to the intensity curve (Figure 8).
The models, which will be presented in this study, related primarily to studies of wood cell wall structure. An SEM micrograph of a cell wall from
Figure 11, shows typically diffraction pattern arising from the pith region of
The (-) signal mean that the microfibrils orient in the back cell wall.
σ = 14.21°
Where σ = 14.21° represent the standard deviation of the intensity distribution arising from the fibril orientation about the mean value.
Figure 13 shows SEM micrograph for the wood slice of thickness 50.0 µm taken from the pith region of
MFA decreased as the tree age increase. most significant drop occurring from 21.45° at age 5 year-old to 16.14° at age 7 year-old, and from 9.80° at 10 year old to 4.96° at 11 year old at the pith region. The smallest value of MFA was found in the pith center, MFA = 0.20° ± 0.01°. An inverse relationship between MFA and tree age was evident in this study within the pith region (Figure 14). The MFA of
|1||3||26.13||14.21||Back cell wall|
|2||5||21.45||8.97||Back cell wall|
|3||7||16.14||5.47||Back cell wall|
|4||9||11.30||2.40||Back cell wall|
|5||10||9.80||1.40||Back cell wall|
|6||11||4.96||4.27||Front cell wall|
|7||13||0.26||0.08||Front cell wall|
|8||15||0.20||0.07||Front cell wall|
|9||15 (Pith center)||0.20||0.07||Front cell wall|
|1||3||31.62||15.13||Back cell wall|
|2||5||27.36||12.68||Back cell wall|
|3||7||17.83||6.60||Back cell wall|
|4||9||14.44||4.47||Front cell wall|
|5||10||9.87||1.43||Front cell wall|
|6||11||5.67||1.42||Front cell wall|
|7||13||3.17||3.16||Back cell wall|
|8||15||0.47||0.15||Back cell wall|
5. Thermal and dynamic-mechanical properties of wood
The complicated hierarchical and cellular structure of wood is well known to provide excellent mechanical properties such as stiffness and strength. Wood represents a natural composite with the ability to adapt its structural properties to external mechanical requirements in all hierarchical levels (Cave, 1997a). In the microscopic and nanoscopic scale, the structural optimization of the mechanical behavior of wood is closely related to the cell wall microstructure. The mechanical properties of wood are known to be greatly influenced by its anatomical structure (Boyd, 1977). It is well known that the stiffness of wood is mostly given by the semi-crystalline cellulose microfibrils.
Thermal analysis has been extensively applied to investigate the thermal behavior of various materials as a function of temperature. A number of researches on thermal properties of wood fiber and polymer composites (WFPCs) have been reported (Tamer and Fauziah, 2009). A number of different methods have been used to investigate thermal properties and vescoelastic properties of wood. One such method is dynamic mechanical thermal analysis (DMTA). This has been used to investigate wood from different trees species. The variations of MFA with the tree ages were studied for the real wood cell wall structure of
Study by (Tamer and Fauziah, 2009) using small angle x-ray scattering technique shows that MFA in
Results from the same study show a good inversely relationship between MFA and the
Differences in MFA have a profound effect on the properties of wood, in particular its stiffness. The large MFA in juvenile wood confers low stiffness and gives the sapling the flexibility it needs to survive high winds without breaking. It also means, however, that timber containing a high proportion of juvenile wood is unsuitable for use as high-grade structural timber. This fact has taken on increasing importance in view of the trend in forestry towards short rotation cropping of fast grown species. These trees at harvest may contain 50% or more of timber with low stiffness and therefore, low economic value. Although they are presently grown mainly for pulp, pressure for increased timber production means that ways will be sought to improve the quality of their timber by reducing juvenile wood MFA (Barnett et al., 2004). Glas transition (
5.1. Determination of glass transition of
Acacia mangiumWood by Dynamic Mechanical Thermal analyzer (DMTA)
In this study, the DMTA technique is used in glass transition,
|MFA (°)||Glass transition (°C)|
The general declining trend for all curves of
5.2. The relationship between microfibril angle and storage modulus, loss modulus and glass transition
The relationship between MFA and the thermal and mechanical properties in
|N Statistic||Range||Minimum||Maximum||Mean||Std. Error||Std.||Variance|
|Storage||9||4.2 ×108||4.8 ×108||9.0 ×108||6.9 ×108||3.8 ×107||1.2 ×108||1.3 ×1016|
|Loss||9||2.3 ×107||3.2 ×107||5.5 ×107||4.4 ×107||2187580||6562740||4.3 ×1016|
The relationship between MFA and the loss modulus was discussed. The statistical analysis showed that a straight line fit the data very well. Thus, as the MFA increases the
The effect of MFA on the glass transition is studied in this research. Figure 21 present the relationship between MFA and
As a result, the mechanical properties of
In this work, the variations of MFA with the tree ages were studied for the real wood cell wall structure of
The only noticeable variation can be detected in the case of MFA about 6.46° (Table 4). Thus, as the MFA increases the
We greatly appreciate the professional co-operation and assistance of Universiti Malaysia Sabah. We also wish to thank Universiti Putra Malaysia in accessing and using the X-Ray diffraction equipment. Finally, we would like to thank fully Universiti Kebangsaan Malaysia for the professional cooperation.