Environmental Engineering Reference
In-Depth Information
mm
µ
m
Cell wall
Macrofibril
Microfibril
Molecule
nm
Å
Figure 6.4
Hierarchical structure of wood: from timber to cellulose.
the trunk of the tree, used for construction purposes due to its strength. World
production of sawlogs and veneer logs equates to 635 million m
3
. In Europe,
Sweden, Finland and Germany account for practically half of the EU-28 produc-
From the nano- to the macroscopic scale, wood is structured in a hierarchical
way. The molecules of cellulose (40-50%) are arranged first in microfibrils then
in macrofibrils, and they comprise the layers of the wall of every cell in the wood
(Figure 6.4).
The cell wall (Figure 6.5) is composed of a primary and a secondary wall sur-
rounding a void, the lumen (L). The primary wall (0.1-0.2 µm) presents no par-
ticular arrangement and comprises cellulose, hemicelluloses, pectin, proteins and
lignin. The secondary wall consists of three layers, namely S1, S2 and S3, all of
which are oriented layers of cellulose. In the S1 layer (0.2-0.3 µm) and in the S3
layer (0.1 µm), the microfibrils make an angle of 50-70° to the axis of the fibre. In
the S2 layer (1-5 µm), which comprises 90% of the weight of the cell wall, the
angle of microfibrils is between 10 and 30°. The shorter this angle, the higher the
modulus and the strength of the fibre (and the lower the elongation at break).
Two neighbouring cells are separated by the middle lamella (M), which is com-
posed of polysaccharides and lignin as depicted in Figure 6.3. Its thickness is
between 0.2 and 1 µm.
