Page 378 - Biosystems Engineering
P. 378
Evaluation of Transgenic Wood for Pr oductivity & Quality 355
(I) Nanoindentation
40 (II) Model for radiata pine
(II) (III) Model for spruce
35 (III) (IV) Model for radiata pine (horizontal shifted)
Longitudinal modulus (GPa) 25 (VI) (I)
(V) Model for spruce (horizontal shifted)
30
(VI) Vertical shifting of curve (V)
20
(IV)
15
10
5 (V)
0
5 10 15 20 25 30 35 40 45
Microfibril angle (°)
FIGURE 12.3 The S-2 cell wall longitudinal stiffness as a function of
microfi bril angles: experimental and modeled values (Tze et al. 2007).
observed, and there is a preliminary indication that the dependence
could be affected by cell wall extractives. In cell wall modification
studies, it is desirable to minimize any unintended variations by using
samples that are from the same growth ring, so that any treatment-
induced changes in cell wall hardness can be identified.
12.3.3 Chemical Composition
Wood tissue is composed of a group of polymeric components, which
make up the bulk of wood cell walls. All polymeric components are
mixed in the wall and determine the physical and mechanical proper-
ties of wood. A typical wood cell includes a primary wall, a second-
ary wall (S1, S2, S3), and a lumen.
There are two chemical components in a wood cell wall. The
primary components, which establish the chemical and physical
nature of the cell wall and constitute the bulk of materials in the
wood cell wall, include cellulose (40 to 50 percent), hemicellulose (20 to
35 percent), and lignin (15 to 35 percent). Cellulose is the most impor-
tant single component in the cell wall and has a primary relationship
to the physical and mechanical behavior of the wood. Hemicellulose
and lignin are also important in the wall because they act as matrixes
in the natural polymer composites. The secondary components in the
wall include ash (<1 percent) and extractives, such as tannins, volatile
oils and resins, gums, latex, alkaloids, and other compounds. The sec-
ondary components may not significantly affect the structure of the