Hardwood fibres

Fibres make up a high proportion of the volume of most hardwoods. Fibres are imperforate, axially-elongated cells, with small lumens and ends that taper into pointed tips. The density of a hardwood is largely determined by the proportion of fibres to other cell types present in the wood. In a low density wood, the vessels occupy a major proportion of the wood volume, whereas denser woods have a larger proportion of thick-walled fibres. The secondary walls of fibres are usually sparsely pitted and the cells lack cell contents at functional maturity. 

ARBOFORM formulations show a dominant dependence of the mechanical properties on the fibre content (especially hardwood fibres), thus enabling material properties of the injection... 

Distribution of lignin in various cell wall layers of softwood tracheids and hardwood fibres... 

Cellulose fibres are obtained from softwood and hardwood. Softwood fibres are 30-45 pm in diameter and 3-7 mm long. Hardwood fibres are thinner (10-20 pm) and shorter (1-2 mm). Their tensile strength is by an order greater than that of the wood for example, lumber wood without macro-defects may have tensile strength equal to 70 MPa and a single fibre, 700 MPa. The fibres are extracted from wooden pulp by different chemical and mechanical processes. 

The extent of xylan labelling varied to some extent between individual fibres some fibres were labelled faintly and their neighbours more intensively. In the latter case the label was not distributed evenly on fibres, but more intense labelling was observed on deformed parts of fibres, e.g. around kinks and nodes (Fig. 4a). This might be due to true chemical variation on different parts of the fibres or to variation of accessibility of xylan to the antibody. Softwood kraft fibres were also labelled with anti-MeGIcAXyb, but the labelling intensity was weaker than with hardwood fibres (results not shown). This is in keeping with surface chemistry, because the surface of softwood kraft fibres contains less xylan than hardwood fibres (J8). 

The comparisons between different types of fibres that were treated similarly indicate that the composites produced may have a similar flexural strength, but may be quite different in toughness (Table 11.3) [26,29]. These differences cannot be explained on the basis of density (which is similar in all the specimens), or the geometry (aspect ratio) of the fibre, which is highest for the fibre which led to the composite of lowest toughness (Table 11.3). Similar trends were reported by Coutts [30] for softwood and hardwood fibres prepared with room temperature cured matrix. There, the better performance of the softwood fibres was attributed to the higher fibre aspect ratio. 

Cellulose fibres produced from hardwoods, with various chemical surface treatments to ensure that they are compatible with rubbers, can be used to produce high modulus vulcanisates. The bond between rubber and fibres is created during vulcanisation. These fibres can be used to reinforce extruded hoses gaining orientation in the direction of flow. There is a range of fibres available which are compatible with different rubber types. 

Figure 2.2 Light photomicrographs of fibre preparations illustrating the morphological differences between softwood and hardwood commercial pulps (a) bleached sulfate pine (softwood), (b) bleached sulfate eucalyptus (hardwood). Scale bar = 200 /an.

Figure 1.2. The transverse and tangential-longitudinal faces of the hardwood poplar, Populus sp. The wood comprises vessels arranged in radial groups and fibres both in the axial system of cells, as well as uniseriate parenchyma in the ray system. Magnification x 210.

Figure 1.4. Tire hardwood Knightia excelsa. Vessels provide largely uninterrupted conduits for the conduction of sap, with thick-walled fibres functioning in support earlywood with large vessels in the centre latewood fibres to the lower right, x 235.

Axial parenchyma cells (also called longitudinal parenchyma) are generally very abundant in hardwoods. Like vessel elements and fibres, axial parenchyma cells are derived from the axially-elongated fusiform initials of the vascular cambium but, whereas vessel elements and fibres (except septate fibres) remain unsegmented, axial parenchyma cells are formed by the transverse segmentation of the derivatives of fusiform initials. Axial parenchyma cells, therefore, tend to lie in vertical files... 

Bark has very little meehanieal strength on aeeount of its lack of fibres. The bark of softwoods has no thiekened axially-elongated cells, but the bark of some hardwoods do sometimes eontain small numbers of short fibres similar to wood fibres. 

These trends are valid for many species both hardwoods and softwoods. Bao et al. (2001) found for seven softwoods that tracheid length in corewood was 21-52% shorter than in outerwood, and for the three hardwoods studied the fibres were about 24% shorter in corewood. 

For over 40 years there have been intense efforts to develop improved eucalypt hybrids for 7 yr-old pulpwood crops. Elite clones of E. urophylla x grandis have both desired long fibre characteristics, i.e. long for a hardwood, and less variation despite consisting solely of variable corewood ... 

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