Chapter 4 Chemical Components of Wood
木材的化学组成
Ⅰ. Polysaccharide fractions of the cell wall
木材细胞壁上的多糖
Ⅱ. Lignin
木素
Ⅲ. Secondary cell wall components
细胞壁上的次要成份
Ⅳ. Analytical data
分析数据
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All properties of wood arise from the nature and organization of the
chemical compounds.
Schematic Classification of the Chemical Components of Cell Wall Substance in Normal Wood
(Percentages Based on Ovendry Weight)
Ⅰ.
Primary components
A.
Total polysaccharide fractions, expressed as holocellulose—60-85%
1.
Cellulose—40-50%
Long-chain polymer with low solubility
2.
Hemicellulose—20-35%
Noncellulosic polysaccharides; these are readily soluble in dilute alkali and
hydrolysable by dilute acids to component sugars and uronic acids
B.
Ⅱ.
Lignin—15-30%
Secondary components
A.
Tannins
B.
Volatile oils and resins
C.
Gum, latex, alkaloids, and other complex organic compounds including dyes and coloring
materials
D.
Ash—usually less than 1%
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Ⅰ. Polysaccharide fractions of the cell wall
木材细胞壁上的多糖
The polysaccharide materials are separated into two fractions
by treatment with dilute alkali:
• The insoluble fraction — cellulose（纤维素）
• The soluble fraction — hemicelluloses（半纤维素）
• The combined cellulose and hemicelluloses in wood may be
expressed as the holocellulose（全纤维素）
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1. Cellulose / 纤维素
Cellulose is the most important single component in the
woody cell wall in terms of its volume and its effect on the
characteristics of wood.
1-1 Molecular formula: (C6H10O5)n, n =10000~15000
1-2 Structural formula:
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1-3 Characteristics of cellulose structure
• Linear polymers composed by D-glucose units, through
βoxygen linkage between 1- 4 carbon.
• Can be degraded into glucoses through hydrolysis, pyrolysis,
and enzyme.
• There is one –OH group on C2, C3 and C6 respectively, so it
could suffer chemical reactions such as oxidization（氧化）,
etherification（醚化） and esterification（酯化）.
• There is a concealed –CHO at one end, so the molecule bears
direction and polarity.
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1-4 crystalline structure（结晶结构）
Cellulose in woody cell walls occurs in a crystalline form.
Crystalline area
Non-crystalline area
• Microfibril structure
• Crystallite structure （晶胞结构）
Parameters: a, b, c, β.
One cellulose chain in the center
and one on each corner .
The center chain oriented in opposite
direction to those on the corner,
and is one half of glucose unit
ahead of those on the corner.
Chain units contained
in each crystallite
(4*1/4+1)=2
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1-5 Supermolecular structure
40 cellulose chains →Elementary fibril
16 elementary fibrils →microfibril
4 microfibrils → macrofibril
Several macrofibrils →lamella
Several lamellas → cell wall
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2. Hemicelluloses / 半纤维素
2-1 Sugar types in hemicellulses
xylose, arabinose, galactose, mannose, glucose
2-2 Plane structure of hemicelluloses
2-3 Degree of polymerization: n = 150~200
2-4 Chemical properties : much more active than cellulose
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Ⅱ. Lignin / 木素
1. Chemical structure of lignin
• Basic units: phenylpropane
• Linkages: C-O-C, C-C
• With 3-dimensional structure (cellulose 1 , hemicelluloses 2)
2. Function of lignin
• In the living tree, glue microfibrils and cells together to form
walls.
• In fiber board, glue wood fibers together to form boards.
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3. Lignin types in softwood and hardwood
• Softwood : composed of mainly guaiacyl units and a few Phydroxyl phenylpropane
• Hardwood: composed of mainly syringyl and guaiacyl units,
and a few p-hydroxyl phenylpropane.
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
H3CO
OCH3
OCH3
OCHH3 3CO
OH
OH
OH
guaiacyl-
C
C
C
C
C
C
C
C
C
OCH3
OCH3
OCH3
H
CO
3
OH
OH
OH
OH
OH
syringyl-
C
C
C
OH
para-hydroxyl-phenylpropane
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4. Distribution in the cell walls
• In compound middle lamella (PMP)
lignin content in PMP: ~ 80%
the amount in PMP: ~ 20% of the total amount in cell wall
• In the secondary walls (S-wall)
lignin content in S-walls: ~ 25%
the amount in S-walls: ~ 80% of the total amount in cell wall
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5. The thermoplastic nature of lignin
The thermoplastic temperature of lignin :
Softwood lignin : 170-175oC
Hardwood lignin : 160-165oC
6. The Maule reaction of lignin
The Maule test involves treatment with chlorine, followed
by a weak base, such as ammonia. A red color is produced
in hardwoods and a pale brown color in most softwood. The
Maule reaction can be used to distinguish wood chips or
powder of softwood and hardwood.
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Ⅲ. Secondary components in wood
1. Extraneous materials / 抽提物质
• Location：in cell lumens
• Content: 1-40%
• Types:
Wood extractives represent an extremely wide range of classes of
organic compounds.
polyphenols（多酚类): tannis（单宁), flavones（黄酮）
oleoresin（含油树脂): turpentine（松节油）, rosin（松香）
other types: gums, fats, fatty acids, waxes
• Effects to wood properties:
Color, odor, weight, permeability, shrinkage, durability, painting and
gluing.
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2. Ash content / 灰分含量
• The ash content is normally 0.1 to 0.5percent of the oven dry
weight of wood , but it may be much greater.
• The alkaline earths, i.e., calcium, potassium, and
magnesium, usually account for 70 percent of the total ash
present.
• Silicon occurs only in certain groups of trees and may
amount to more than 2 percent of the dry weight of wood.
The presence of silica in amounts less than 0.5 percent dry
weight of wood will cause marked dulling of machine tools.
• Those inorganic elements may be present in wood as
components of the extractives or in the form of crystals.
• Most crystals in wood are some form of calcium oxalate of
various shapes and sizes.
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Ⅳ. Analytical data 分析数据
In general terms, the chemical composition of wood on an
oven dry basis can be summarized as:
Cellulose:
40 - 50%
Hemicelluloses: 20 - 35%
Lignin:
15 - 35%
Extractives:
1 – 40%
Ash (inorganic components) : < 1%
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Reflection and practice：
1. Chemical composition of wood?
2. Chemical structure of cellulose?
3. Main differences between cellulose and hemi-celluloses?
4. Chemical structure of lignin?
5. The thermoplastic nature of lignin and its function in a
tree and in fiberboard?
6. Maule action and its mechanism?
7. Structural difference of lignin in softwoods and
hardwoods?
8. What is holocellulose?
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