Page 377 - Biosystems Engineering

354 Cha pte r T w e l v e

MFA
The cellulose microfibril angle of the S2 layer is a critical factor in
determining the mechanical properties of wood. The microfibril angle
is defined as the angle between the most probable cellulose microfi-
bril orientation and the long axis of the cell. Figure 12.2 shows an
average 4.17° microfibril angle in the cell wall of kwila wood. The
microfibril angle can be determined by a number of techniques. X-ray
diffraction has become a popular method in recent years.
The microfibril angle varies considerably between trees and within
the trunk of a tree with a large angle near pit and small angles in the
outerwood. Our group (Tze et al. 2007) measured the mechanical
properties of individual, native wood fibers for five different annual
rings of a loblolly pine, with microfibril angles (MFA) between 14 and
36°using continuous stiffness measurement techniques. Results showed
that a predictable pattern of stiffness values was found as a function of
MFA, and stiffness can at least be considered to be a relative measure
of the longitudinal stiffness properties of wood cell walls (Fig. 12.3).
To determine hardness values, a dependence on orientation was also

Microscope Acelerating Voltage Working Distance
Quanta FEG 5kv 10.2 mm —2 μm—
FIGURE 12.2 Radial section of kwila wood showing microfi brils aligning in the
direction of the fi ber axis (average MFA 4.17°).