Wood as a Biopolymer Composite

Objective: using the basic information we have learned, generate a three-dimensional picture of the woody cell wall from a chemical perspective.

Important Concepts
Heterogeneous distribution of polymeric components.
Compression wood
Tension wood
Juvenile wood
Knots
Lignin-Polysaccharide Interactions
Lignin-Tannin Interactions
Introduction
We have looked at wood by zooming in on the individual chemical components (cellulose, hemicellulose, lignin and extractives) and examined the structure and general reactivity of each component. Now we need to put the cell wall back together and see how the components are arranged within the cell wall.
Overall Chemical Composition
The chemical composition of most temperate zone woods fall into a relatively small range as can be seen in the Table below as well as the associated pdf file:
 

Chemical Composition of Softwoods and Temperate Zone Hardwoods (all values in percent)
Component Softwoods Hardwoods
Cellulose 40-45 40-45
Glucomannan 20 5
Xylan 10 25-30
Lignin 25-30 20-25

Chemical Composition of Some North American Woods.
Distribution of Chemical Components
The above components are not distributed evenly throughout the cell wall, and the chemical makeup of a wood cell can be considered to be quite heterogeneous. To understand why the polymeric constituents are distributed the way they are, it is useful to recall how a woody cell wall is biosynthesized....A polysaccharide matrix expands to final size and lignification begins at cell corners and ends up filling all voids. With this in mind, it makes perfect sense that the middle lamella is very high in lignin content (90%). The middle lamella is about 10% of wood mass, so about 10% of lignin in wood is found in the middle lamella. The bulk of lignin in wood is found in the secondary wall (S2) even though the concentration of lignin in this part of the cell wall is less than 25%. The pdf file below provides a tabular listing of polymeric components and their relative concentrations, depending on location in the cell wall.
Distribution of Chemical Components by Location in the Cell Wall.
 
Wood as a Biopolymer Composite
So how is all of this connected? This is not an easy question. Many wood chemists and biochemists feel that there is some covalent interaction between lignin and polysaccharide, but not direct evidence has yet been presented. There are many graphics displaying the interaction between lignin, hemicellulose and cellulose; but by and large, the most commonly held belief is that lignin has a few covalent bonds to hemicellulose and hemicellulose is hydrogen bonded to cellulose. The extractives would then be coating this structure providing resistance to pathogens.
Characteristics of Various Types of Woods in Hardwoods and Softwoods
 
A. Compression Wood
1. Found in Ginkgo and all conifers.
2. In leaning or bent stems, branches (knots), exposed roots. Typically on the lower side.
3. Wide growth rings on the lower side.
4. Chemical composition:
a. Low cellulose (30%) and high lignin (40%).
b. Less galactoglucomannans.
c. Lignin contains more p-coumaryl residues than normal wood.
5. Present in all forest and plantation trees. About 10-15% by volume.
6. Compression wood, by expanding along the grain when it is formed, makes it possible for conifer trees to perform movements of orientation. A displaced stem bends up again, a bent down branch moves back up. If the leader is destroyed, a branch below bends upward, replacing the leader nd developing a new stem.
7. Groundwood (mechanical) pulp cannot be made from compression wood. Very harmful in the sulfite process and undesirable in the kraft process. The wood is difficult to delignify but easy to defiber. The strength properties of the resulting pulp are quite poor.
 

Average Chemical Composition of Normal and Compression Woods (all values in percent of oven-dried wood)
Constituent Normal Wood Compression Wood
Cellulose 42 30
Lignin 30 40
Galactoglucomannan 20 10
Galactan -- 10
Xylan 8 8

 
B. Softwood Knots
In general, when a branch dies the wood begins to produce resins with resin acids reaching 40% content, whereas normal wood has about 3% resin. The resin prevents pulping liquors from penetrating the knots and therefore the knots remain largely unpulped. The portions that do pulp often provide dark resin spots on the paper. Groundwood pulps cannot be made, grinding provides a dark powder.
 
C. Softwood Juvenile Wood.
1. Formed in trees that are less than 10-20 years old. It is present at the core of the tree and the entire tree top.
2. The amount will decrease with the age of the tree. An 18-yr old southern pine stand will have 50% juvenile wood whereas a 48-yr old stand only 8%.
3. The wood is less dense with more compression wood and knots. It therefore has more lignin and less cellulose. The resulting pulp fibers are usually short which causes some strength losses in the sheet. Pulp yields are 5-10% less than mature wood due to the higher lignin and reduced cellulose contents.
 
D. Tension Wood.
1. Present in most hardwoods. In leaning or crooked trees, usually on the upper side.
2. An additional layer on the innermost side of the cell wall is called the gelatinous layer, and is composed of highly crystalline cellulose.
3. The lignin content is essentially the same, but due to the presence of the gelatinous layer, the weight percent of lignin is less. The lignin composition is the same in normal and tension woods.
4. Much less common than compression wood. By being on the upper side of a branch it pulls the branch upward by contracting as it develops.
5. Gives an excellent groundwood pulp. Chemical pulps are obtained in high yield due to the presence of the high cellulose content, but the paper has poor strength properties.

Chemical Composition of Normal and Tension Woods of Red Maple and White Birch
Constituent

Red Maple

Normal

Red Maple

Tension

White Birch

Normal

White Birch

Tension

Lignin 25 17 22 16
Cellulose 41 58 40 50
Glucomannan 4.9 2.7 4.0 2.6
Galactan 2.0 3.2 2.6 11
Xylan 25 17 33 24
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