Chemical composition of wood

1. WOOD CHEMISTRY:
“What is wood made of?”

2. Wood Microscopic Structure
 Imagine that wood is
made up of millions and
millions of toilet paper
rolls glued together.
These rolls are the fibers
that will make paper.
 Chemical Composition
 ~50% Cellulose
 ~25% Hemicellulose
 ~25% Lignin
 Variable amounts of
extractives

3. 3 Main Wood Chemicals
 Cellulose
 Hemicelluloses
 Lignin
They are all
POLYMERS

4. Softwood Hardwood
Cellulose 45 – 50% 45 – 50%
Hemicellul
ose(s)
18 – 25% 25 – 35%
Lignin(s) 25 – 35% 20 – 25%
Chemical composition of wood

5. 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

6. 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
Galactoglucoma
nnan
20 10
Galactan -- 10
Xylan 8 8

7. 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
Chemical Composition of Normal and Tension
woods of Red Maple and White Birch

8. 50-70
10-30
60-90

9. Fiber Cell Wall Structure
Lumen
Primary
Wall
Middle lamella
(almost all lignin
about 80 % )
Secondary
Wall

10. ‫طب‬ ‫من‬ ‫يتكون‬ ‫الخشبية‬ ‫الخلية‬ ‫جدار‬
‫قات‬
‫عديدة‬ ‫مختلفة‬
‫موضح‬ ‫هو‬ ‫كما‬
‫بالرســــم‬
‫تسم‬ ‫الخاليا‬ ‫بين‬ ‫الموجودة‬ ‫والمنطقة‬
‫ى‬
‫الوسطى‬ ‫بالصفيحة‬
( Middle
Lamella )
‫حوالى‬ ‫من‬ ‫تتكون‬ ‫وهذه‬
80
%
‫لجنين‬
.

11. ‫الميكروفبريلالت‬ ‫زاوية‬
( Microfibrile Angle )
‫الطولى‬ ‫المحور‬ ‫بين‬ ‫المحصورة‬ ‫الزاوية‬ ‫هى‬
‫الخشبية‬ ‫للخلية‬
(
‫قصيبة‬ ‫أو‬ ‫ليفة‬
)
‫و‬
‫إتجاة‬
‫الميكروفبريلالت‬
,
‫ط‬ ‫بأختالف‬ ‫وتختلف‬
‫بقات‬
‫بالشكل‬ ‫موضحا‬ ‫هو‬ ‫كما‬ ‫الخلوى‬ ‫الجدار‬
.

12. How is the cell wall put together?
 Cell wall is assembled by gluing together a
bunch of very small fibers called macrofibril
 Same technology used to produce
composite materials today
 The glue holding the macrofibrils together is
lignin
 Macrofibrils are made up of microfibrils
which in turn are made up of cellulose and
hemicellulose polymers
 The glue holding all this together is lignin

13. Representation of Cell Wall
Components
Cellulose
Hemicelluloses
Lignin

14. What is Cellulose?
 Cellulose is straight chain
polymer. In bead terms,
imagine a very very long
straight string of beads
with 2 ends and no
branching points.
 In wood, cellulose chains
contain typically 10,000
glucose molecules…quite
a long sting of beads. Source: World Book Encyclopedia

15. ‫للميكروفبريلله‬ ‫الدقيق‬ ‫التركيب‬

16. ‫السيلولوز‬ ‫تركيب‬
--- Glucose ------- Glucose ------- Glucose -----
Glucose ---

18. Cellulose For Chemists
 Very long straight chain polymer of glucose:
approximately 10,000 in a row in wood. Cotton is
nearly pure cellulose.
 Cellulose molecules link up in bundles and bundles
of bundles and bundles of bundles of bundles to
make fibers
 Cellulose forms tight bundles which are very
resistant to chemical attack

19. What are Hemicelluloses?
 Hemicelluloses are also sugar polymers but
different from cellulose because they are:
 Made up glucose and other sugars.
 Contain some molecules other than sugars.
 Branched little polymers
 The beads have Y’s in them
 Much smaller than cellulose as they are made
up of between 50-300 sugars
 There are lots of varieties of hemicelluloses.
 Not very resistant to chemical attack – many
easily break down to simple sugars

20. Hemicelluloses for
Chemists
 Branched little uncolored sugar
polymers (~ 50 to 300 sugar units)
 Composition varies between wood species
 5 carbon sugars: xylose, arabinose
 6 carbon sugars: mannose, galactose, glucose
 Uronic Acids: galacturonic acid, glucuronic acid
 Acetyl and methoxyl groups (acetic acid & methanol)

21. Hemicelluloses vs. Cellulose
• Additional sugar monomer types
• Lower degree of polymerization (150-
200)
• Additional chemical functional groups
• Branched chain structure
• Less crystalline
21

22. Structure of monosaccharides
commonly found in the cell wall

23. ‫اللجنين‬ ‫فى‬ ‫الفعالة‬ ‫المجاميع‬ ‫أنواع‬

24. ‫اللجنين‬ ‫فى‬ ‫الروابط‬ ‫أنواع‬

25. What is holding all these fibers
together in the tree?
 Lignin
 Three dimensional polymer
 No sugars in it
 Nature’s glue – very similar to phenolic
resin used in plywood. Holds cellulose and
hemicelluloses together
 Second most plentiful natural material
 Must be removed or weakened to
separate fibers; turn wood to pulp
 Dark in nature – especially after reacting
with alkali – must be de-colored or
removed to bleach pulp

26. Lignin – Phenyl propane unit
26

27. softwoods &
hardwoods
hardwoods

28. Lignin for Chemists

29. ‫اللجنين‬ ‫فى‬ ‫الروابط‬ ‫أنواع‬

30. Cell Wall Constituents
Polymer Cellulose Hemicellulose(s
)
Lignin(s)
Monomer(s) Glucose Various
monosaccharide
s
Phenyl
propane
Shape Straight chain Branched chain 3-D
Order Crystalline Semi-crystalline Amorphous
% of Wood 45-50 20-35 20-35
Degree of
Polymerization
10,000 150-200 50-500

31. Extractives
 The term extractives refers to a group of unique chemical
compounds which can be removed from plant materials
through extraction with various solvents
 Typically these chemicals constitute only a small portion of
the tree (<5%)
 In some tropical species this can be as high as 25%
 Extractives are produced by plants for a variety of uses
 The most common is protection
 Extractives can cause serious problems for processing
 Pitch is a term which is often used when describing some
groups of extractives
 Extractives are responsible for the characteristic color and
odor of wood

32. COOH
HO
HO
HO
Gallic ac.
C-O
O-C
O
O
OH
OH
HO
HO
Ellagic ac.
OH
OH
HO
OH
OH
O
flavan-3 ol
Gallotannins
The Families
of Tannins
Condensed
tannins
Proanthocyanidic
Tannins
Hydrolysable
tannins
Ellagitannins

33. Chemical Composition of Wood:
Hardwoods
0
10
20
30
40
50
60
%
Red Maple English
Oak
Iroko Balsa
Cellulose*
Hemicellulose*
Lignin*
Extractives
* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis

34. Chemical Composition of Wood:
Softwoods
0
5
10
15
20
25
30
35
40
45
50
%
Douglas Fir Redwood Yellow Pine Balsam Fir
Cellulose*
Hemicellulose*
Lignin*
Extractives
* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis

35. 35
Cellulose and Hemicellulose(s)
Glucose Mannose Mannose Mannose
Galactose Acetyl
Glucose Glucose Glucose Glucose
Cellulose
Xylose Xylose Xylose Xylose
Acetyl Glucoronic
acid
7
Hardwood xylan
Softwood galactoglucomannan

36. Cellulose
Hemicellulose (softwood)
Hemicellulose (hardwood)
Cellulose and Hemicellulose(s)