Microfibril angle

Thickness of Various Cell Wall Layers and Microfibril Angle Within the Layers... 

The accuracy of interference microscopy depends on the calibration standards used. For measurements in the secondary wall, these values are 1.604 for Nb and a range of values for Na depending on the specimen. The variation in Na arises presumably from variation in microfibril angle (Hermans 1946). The need to measure Na for each sample is one disadvantage of this technique that increases the amount of work necessary to perform the measurement. If Na is not measured for each sample, an error of 11% is introduced compared to the usual error of 2% (Donaldson 1985a). Na does not vary between earlywood... 

In normal wood tissue, the fiber secondary wall consists of three fairly distinct layers. The outermost layer or Si is very thin (0.1-0.2 jjim) and exhibits an average microfibril angle (for the layer as a whole) of about 50-70° (2). The bulk of the secondary wall is made up of the S2 layer, which is typically several micrometers thick (Figure 18). Here the microfibrils are usually oriented to the fiber axis at a relatively small angle (5-20°). The thickness and small microfibril angle... 

At the ultrastructural level, juvenile wood fibers have a much greater S2 microfibril angle than normal mature wood fibers. The net... 

Several quantitative models have been proposed for explaining the effect of fibril angle d on both axial and transverse wood shrinkage (10). Only the model of Barber (36) will be discussed here. In this model the typical wood cell is assumed to be circular in cross section and the cell wall is considered to consist of two layers. One of these is the thick Sg layer whose microfibril angle 0 is the principal independent variable (Figure 13). The second layer is the layer,... 

Figure 1.20. The S2 wall layer of a compression wood tracheid in Pinus radiata. Note the large microfibril angle, x 3750.

Figure 4.4. A cell waU element. The cell is aligned in the x direction, the y direction lies parallel to the wall (width), z is transverse to the wall (thickness) and 0 is the microfibril angle within the S2 layer (Barber and Meylan, 1964). The y and z directions are not defined relative to the radial and tangential directions they approximate to polar coordinates.

Figure 4.6. Relationship between microfibril angle and longitudinal and tangential shrinkage in Pinus jeffreyi (Meylan, 1968).

Clearly shrinkage anisotropy is a eomplex issue. A number of faetors ean contribute and the relative importance of each will vary between timbers. In some cases a large microfibril angle might be significant, as in corewood and in compression wood. Ray tissue will be important in species such as beech and oak. Contrasting earlywood and latewood densities is a likely cause in Douglas fir, but would be irrelevant for a tropical hardwood. The effects of elastic anisotropy would be more apparent in low density softwoods. 

Microfibril angle Furniture - quahty finish calling for... 

Until recently, few have recognized the enormous impact that the microfibril angle has on wood properties, especially in the corewood of plantation softwoods. In particular the microfibril angle strongly determines the stability and stiffhesss of wood within the first 10-15 growth rings of the pith (Barber and Meylan, 1964 Cave, 1968 Preston, 1974). 

Microfibril angle is a characteristic that has a profound influence on the wood properties of stiffness and stability. This is especially true where considering the... 

Figure 6.7. Variation in microfibril angle in cultivars of sugi, Cryptomeria japonica (Fujisaki, 1985).

Table 6.2. Influence of microfibril angle on mechanical properties essential for standing trees (Reiterer et al., 1999 2001). The data has been normalized, i.e. values have been adjusted by multiplying by cell wall density (1500 kg m ) and dividing by the acmal density of the sample. Maximum property values (highlighted in grey) occur at different MFAs.

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