chapter 4 cellulose shujun li forest products chemical processing dept. northeast forestry...
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Chapter 4 Cellulose
Shujun Li
Forest Products Chemical Processing Dept.
Northeast Forestry University
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4.1 Existence &separation methods of nature cellulose
1. Existence of nature cellulose
Main constituent of wood
Approximately 40-45% of the dry substance in most wood species
Located predominantly in the secondary cell wall
2. Separation methods
Nitric acid- Ethanol cellulose: Nitric acid- Ethanol
(80:20 v/v)
α-cellulose: From Holocellusoe, 17.5%NaOH, residue
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(1) Main chain: Cellulose is a homopolysaccharide compo
sed of β-D-glucopyranose units which are linked together by (1 4)-glycosidic bonds.
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4.2 Chemical structure
4.2 Chemical structure
(2) Configration: β-D-glucopyranose units
(3) Repeating Unit: cellobiose (1.03 nm)
Every glucose residue is accordingly displaced 180º with respect to its neighbors.
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4.2 Chemical structure
(4) Basic unit: Except the two ends, every unit has three free hydroxyl groups at C2,
C3, and C6. Secondary alcohol, primary alcohol Different react activity Influence: moisture absorption, hydrogen bonds, swelling and
dissolution of cellulose
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4.2 Chemical structure
(5) Ends: Two ends are different. Non-reducing end: C4, secondary alcohol hydroxyl gr
oup. Reducing end: C1, glycosidic hydroxyl group, aldehy
de.
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4.2 Chemical structure
(6) Middle units: Except the reducing end, all other units exist in the cycle form. T
hey are very stable.
(7) Confirmation: The β-D-glucopyranose chain units are in chair conformation and
the substituents HO-2, HO-3, and CH2OH are oriented equatorially.
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4.3 Physical Structure
Crystalline regions: highly ordered
Amorphous regions: less ordered
Method for determining Crystallinity:
X-ray diffraction
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4.3 Physical Structure
X-ray diffractiom spectrum of defat cotton
Cr I(%)= (I002 –Iam)/I002 ×100%23/4/18 下午 12:04
4.3 Physical Structure
a=0.835nmb=1.03nmc=0.79nmβ=84°
Two cellobiose units
Four glucose units
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4.4 Molecular Weight & Degree of Polymerization
DP = MW of cellulose/MW of units =(M+18)/162 ≈M/162
Methods:Gel permeation chromatography
(GPC)viscosity method
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4.5 Chemical Properties
4.5.1 Chemical structure and reactivity1. Chemical reactionGlycosidic bonds : acetal, degradationAlcohol hydroxyl groups: esterification, et
herificationReducing ends: peeling reaction, stoppin
g reaction
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4.5 Chemical Properties
2.ReactivityEsterification : steric effect
primary>secondary alcohol hydroxyl group
Etherificaion : ionization C2>C3>C6
3. Accessibility
Higher crystallinity, lower accessibility
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4.5 Chemical Properties
4.5.2 Degradation
1. Types
Acid hydrolysis
Alkaline degradation
Oxidative degradation
Enzymatic degradation /biodegradation
Thermal degradation
Mechanical degradation
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4.5 Chemical Properties
2.Acid hydrolysis ( 1 ) Products: hydrocellulose
Eventually, D-glucose
( 2 ) Type:
Concentrated acid hydrolysis: homogeneous
Dilute acid hydrolysis: hyterogeneous
( 3 ) factors
Acid types
Acid concentration:
High concentration, faster hydrolysis
Temperature:
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4.5 Chemical Properties
3.Alkaline degradation
Alkaline hydrolysis, peeling reaction, stopping reaction
( 1 ) Alkaline hydrolysis
Occurring at high temperatures
β-alkoxy elimination
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4.5 Chemical Properties
(2)Peeling reaction
Form reducing end groups
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Isomerization β-alkoxy elimination isomerization rearrangement
4.5 Chemical Properties
(3) Stopping reaction
The reducing end group is converted to a stable carboxylic acid group.
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β-alkoxy elimination isomerization rearrangement
4.6 Physical Properties
4.6.1 Moisture absorption and desorption
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Absorption isothermal curve: “S” shape
Moisture sortion/desorption hysteresis
4.6 Physical Properties
4.6.2 Swelling and Dissolution
1. SwellingCellulose swells in different solventsIntercrystalline swelling:
The swelling agent enters only into the disordered (amorphous) regions
Intracrystalline swelling:
The ordered (crystalline) regions are penetrated
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4.6 Physical Properties
4.6.2 Swelling and Dissolution
2. Dissolution: limitless swelling
SolventsBreak hydrogen bondsForm a soluble complex with cellulose
Viscosity measurement
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4.7 Derivatives
Via esterification, etherification, cross-linking and graft copolymerization
Pretreatment: Swelling, discrystallination Xanthate:
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Processes: CelluloseImpregnationComminution
AgingReactiondissolutionfiltrationshaping
wash & desulfurproducts
4.7 Derivatives
3. Nitrate
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Factors:
(1)Raw materials: low pentosan
(2)Catalyst: H2SO4
(3) Water:
(4)Temperature
(5)Stabilization
4.7 Derivatives
4. (Sodium) Carboxymethyl Cellulose(CMC)the most widely used water soluble derivative of cellulose
[C6H7O2(OH)3]n + nClCH2COOH + 2nNaOH
[C6H7O2(OH)2(OCH2COONa)]n + nNaCl + nH2O
5. Chemical Modification
(1)Graft copolymerization: radical polymerization, ionic graft copolymerization
(2)Cross-linking: antiswell / shrink, antiwrinkle