For. 202 Harvesting and Utilization
III Chemical Composition and Conversion
1. Chemical Composition of Wood
A. Cellulose: Cellulose is a linear molecule, having over 8,000 glucoses units linked together end to end. Bundles cellulose molecules fasiculated laterally and tightly to form microfbrils.
These microfibrils are the framework or skeleton of the cell wall and are responsible for the strength of wood. Cellulose is not soluble in water and common organic solvents. Because cellulose molecules are held tightly by hydrogen bonding to each other, they are quite resistant against dilute acids, alkalis and other forms of attack.
B. Hemicellulose s : Hemicelluloses are a group of compounds made up of from 60 to 200 units of different simple sugars, including glucose. Most of them are branched. Because of their relatively small molecular sizes (compares to cellulose), they are not as resistant as cellulose against chemicals, radiation, and heat. Hemicellluloses together with cellulose are the carbohydrates of wood, called Holocellulose. Hemicelluloses and lignin form the metric system in the cell wall that bonds cells together. Most hemicelluloses are not soluble in water but they are soluble in aqueous alkali solutions and can be easily degraded with dilute acids.
C. Lignin: Lignin is a three-dimensional molecule, which is formed by linking 15 to 18 phenolic units ( phenylpropanes). Lignin contributes to the rigidity of wood.
D. Extractives: Extractives are substances in wood that can be extracted from wood with various solvents. These are a group of complex compounds, most of them are formed long after all wood cells, including tracheids and fiber tracheids, are dead. These compounds include simple sugars and high-molecular-weight carbohydrates, fatty acids, waxes, turpentine, resin acids, and complex phenolic compounds. Some extractives are natural fungicides and insecticides, which make wood naturally durable against fungal decay and insect attacks. The heartwood of some wood species may contain more than 20% (by weight) of extractives.
E. Average Composition of Extractive-free Wood:
Celluolose Hemicelluloses Lignin
Softwood 43% 28% 29%
Hardwood 45 34% 21%
Differences in hemicellulose and lignin contents between softwood and hardwood are significant.
2. Mechanical Pulping
(Stone) Groundwood Process:

Drum-debarked bolts are loaded into the chutes and pressed against revolving grinding stone to convert logs into pulp. A water shower is applied to cool the stone, towash the pulp off the stone into a vat, and to adjust the consistency of the pulp.
Pulp Quality: Over 90% pulp yield; no discoloration of the pulp; but suffer certain degree of fiber breakage; presence of fiber bundles due to incomplete separation .
Major use: Add 15 to 20% long-fiber chemical pulp (to increase the tensile strength) for the production of newsprint.
Bleaching Method: bleached with peroxides.
Species selection: prefer long fiber to compensate fiber breakage and light-color wood species to reduce bleaching costs.
2. Thermal Mechanical Processes (TMPs)
Basic Principle: Wood chips are first softened with intense heat and immediately followed by applying a mechanical force to grind heated wood chips into pulp.
A. Asplund Process: Wood chips about 50% moisture content are heated to about 200 o
C with 150-psi steam for about 2-4 minute, and immediately refined by a disk refiner (Figure) to obtain pulp.
Pulp quality: Over 90% pulp yield, pulp is somewhat darkened due to heat exposure, little fiber breakage, hardwoods preferred because of low lignin content.
Major use: medium-density fiberboard and hardboard.
B. Masonite (Explosion) Process: Wood chips about 50% moisture content is charged into a cylinder, the temperature is rapidly brought up to 200 o C and maintained for about 15 seconds. The softened chips are further heated to about 280 o
C and the temperature is kept for about 4-5 seconds. Then, the content of the digester cylinder is blown to the atmosphere in an explosion. Three levels of defibrization take place, within individual chips, chips hitting metal grate at the valve opening, and hitting the hard floor of the pulp vats.
Pulp quality: over 90% pulp yield, pulp is very dark due to exposure to intense heat, very little fiber breakage.
Major use: insulation board, hardboard.
C. Chemical Thermal Mechanical Process (CTMP): This process is used to produce newsprint from southern pine that is not light-colored enough for using the groundwood process. In this case, southern pine wood chips is impregnated with a solution containing 2%
(to dry wood) sodium bisulfite (NaHSO
3
) before processed by the Asplund process. Sodium bisulfite bleaches and softens wood chips, giving a pulp brighter than the wood.
3. Semichemcial Process: N eutral S ulfite S emi C hemical process is a typical example. In the process, wood chips are first charged into a tubular digester (Figure) with a pulping solution

containing 15 to 20% (to dry wood) sodium sulfite (Na
2
SO
3
) plus sodium carbonate
(Na
2
CO
3
) to adjust the pH of pulping liquor between 6 and 9 and heated to 150 to 165 o
C with steam for 1 to 3 hours. The chemical digestion is followed by disk refining partially digested chips into pulp.
Pulp quality: 85 to 60% pulp yields depending upon severity of chemical digestion of wood chips. Low yield pulps are good enough for the production of paper for printing, such as Xeroxing and computer printing paper.
Major uses: High yield pulps for corrugating medium and low yield pulps for printing and magazine.
Chemical Recovery: The spent liquor from chemical digestion of wood chips is processed as follows. Spent liquor is heated to about 280 o C to evaporate water until it contains 50% solids. Evaporation is followed by incinerating the concentrated spent liquor in a boiler furnace. Combustion of organic matters in the concentrated spent liquor produces heat, and inorganic matters, mainly Na
2
SO
4
(sodium sulfate)
, can be sold to a kraft mill or properly disposed.
4. Chemical Pulping: The kraft (sulfate) process is an example. The kraft process also is called the sulfate process in which wood chips are pulped by the following conditions:
Chemicals: 15 to 20% sodium hydroxide (NaOH, 3 parts) + sodium sulfide (Na
2
S,1 part).
Pulping time: 1.5 hours for heating to 170 o
C and heating at 170 o
C for another 1.5 hours.
Pulp quality: 45 to 50% pulp yield; pulp contains hemicelluloses to aid fiber to fiber bonding, resulting in strong paper products; the pulp is dark and is somewhat more costly to bleach.
Chemical Recovery for the kraft Process: About 98% of the chemicals used in pulping can be recovered by the following steps: a.
Evaporation of black liquor: The spent liquor is heat to about 285 o
C to evaporate water and concentrate the black liquor to over 50% solids content. b.
Incineration: Concentrated spent liquor is sprayed into the boiler furnace, burning the organics, which produces heat. Complex reactions occur in the furnace, resulting in molten sodium sulfide (Na
2
S) and sodium carbonate (Na
2
CO
3
). c.
Causticizing: Molten sodium sulfide (Na
2
S) and sodium carbonate (Na
2
CO
3
) are discharged from the furnace into a tank of water to obtain a green liquor. Lime is added to convert the green liquor to white liquor as indicated by the equations below.
CaO + H
2
O ----> Ca(OH)
2
Na
2
CO
3
+ Ca(OH)
2 + Na 2
S ------> 2NaOH + Na
2
S + CaCO
3
Precipitate

d. Lime recovery: The CaCO
3
Precipitate is heated to recover lime
CaCO
3 ---100 o
C--------> CaO + CO
2
Kraft pulping is most important process to produce full chemical pulps because it has the following advantages over the other processes: a.
Kraft pulps produce strong paper—most hemicelluloses are dissolved during pulping, which re-deposit onto pulp fiber surfaces during pulp washing stages and serve as natural glues for fiber-to fiber bonding. b.
About 98% of the chemicals used in the pulping process can be recovered and reused. c.
Can be used to pulp any types of lignicellulosic materials (materials containing lignin, cellulose and hemicelluloses) including agricultural residues. d.
Tolerate up to 10% bark in wood chips.
5. Relationships between Fiber morphology and Paper Strength
Fiber morphological characteristics: fiber length, cell-wall thickness, and cell lumen diameter.
Paper strength properties: Tensile, burst, and tearing strengths.
Degree of fiber-to-fiber bonding is important for both paper tensile and burst strength, and cell-wall thickness has a positive relation with paper tearing strength. A long fiber with thin cell wall and large lumen diameter makes the fiber very flexible and is described as ribbonlike. Fiber flexibility increases fiber-to-fiber bonding for tensile and burst strength. Fibers with thick walls and small lumen diameter are described as rod-like, and these stiff fibers hinder fiber to fiber contact. Fibers with thick walls, however, provide a high resistance to transverse severance (tearing). Therefore, long fibers with thick walls (such as latewood tracheids in softwoods) provide paper with a high tearing strength.
7. Sulfur-free pulping Methods: Most chemical pulping processes including the kraft process use sulfur-containing chemicals, which generate sulfur-containing pollutants. Therefore, there is an urgency to invent sulfur-free chemical pulping methods. Organic solvent pulping methods (oganosolv pulping) and biopulping (with fungi) are currently under investigation.
6. Bleaching of Wood Pulps
A. Bleaching High Yield (mechanical) Pulps: Use the lignin-preserving methods either by treating the pulps with reducing or oxidative agents.
Reducing agents: Sodium or zinc hydrosulfites (Na
2
S
2
O
4
, and ZnS
2
O
4
)
Oxidative agents: Peroxides such as hydrogen (H
2
O
2
) or sodium peroxide (Na
2
O
2
).
These agents are mostly used nowadays and are called chlorine-free bleaches.
The brightness of the pulp can be increase from 5 to 12%, depending on the initial pulp brightness.

B. Bleaching Chemical Pulps: Use the lignin-removing methods to remove residual lignin in the pulp. This is done in several stages. a.
Chlorination (C): Use Cl
2
dissolved in water to break down lignin molecules. b.
Extraction (E): Use NaOH solution to dissolve lignin segments and remove them from the cell walls. c.
Oxidative Bleaching: As some lignin will persist in the cell walls, this final process changes the chemical nature of lignin to obtain pulp brightness. Oxidative bleaching agents are:
Hypochlorites (H): same chemicals as the laundry bleach
Chlorine dioxide (D): Cl0
2
Peroxides (P): Na
2
0
2
or H
2
0
2
Therefore, complete chemical pulp bleaching operations may be indicated by CEH, CEHEH,
CED, CEHED, or CEHEDP or other sequences. It is possible to bleach kraft pulp to over
90% brightness. Wood pulp bleach involves the use of chlorine or chlorine containing bleaching agents, during which very potent carcinogens such as chlorinated phenolic compounds (dioxanes) are generated. There also is an urgent need to develop chlorine-free bleach methods to bleach chemical pulps. Biobleaching (with fungi) is one of the methods under investigation.
7. Dissolving Pulp and Cellulose Derivatives: Dissolving pulp is almost pure cellulose, containing very little hemicellulose and lignin. Dissolving pulp can be manufactured by a modified kraft pulping, in which wood chips are subjected to acid hydrolysis to remove hemicelluloses before the normal kraft cook. Then, the pulp is bleached to remove residual lignin to obtain dissolving pulp. Dissolving pulp is the starting materials to manufacture cellulose derivatives.
Rayon and cellophane: Dissolving pulp is treated with 18% NaOH (steeping), followed by treating with CS
2
, and the product is dissolved in lye to obtain viscose. Then, the viscose is extruded into an acid bath to obtain spun yarns for fabrics (rayon) or sheets of cellophane.
Cellulose Acetates: Cellulose reacts with acidic anhydride in strong acids to obtain different grades of cellulose acetates for photographic or X-ray films, fabrics, lacquers (wood finish), etc. Therefore, cellulose acetates can be described as “plastics” from a renewable material.
Cellulose nitrates: Cellulose reacts with nitric acid to obtain different grades of cellulose nitrates for explosives, lacquers (wood finishes), etc.