Wood is the hard, fibrous substance found beneath bark in the stems and branches of trees and shrubs. Practically all commercial wood, however, comes from trees. It is plentiful and replaceable. Since a new tree can be grown where one has been cut, wood has been called the world's only renewable natural resource.
Two most important properties of any papermaking cellulosic raw material are, how much cellulose fiber it has and how long the fibers are. The amount of cellulose fiber in wood determines the pulp yield, ease of pulping and cost of pulp produced. The importance of fiber length is explained in pulp properties. The maximum average fiber length pulp will have is that of wood because whatever pulping method, full chemical to full mechanical, fiber is going to damage. In mechanical pulping the damage is physical (cutting, bruising etc.) and in chemical pulping it is chemical degradation (lower degree of polymerization).
- CHEMICAL COMPOSITION OF WOOD
Average chemical contents of wood
|Elements||Share, % of dry matter weight|
Wood is mainly composed of cellulose, Hemicellulose, lignin and extractives. The following table provides main chemical components of some wood species.
|Constituents||Scot Pine||Spruce||Eucalyptus||Silver Burch|
|Total Extractive (%)||3.5||2.1||2.8||3.0|
- It is a high molecular weight, stereoregular, and linear polymer of repeating beta-D-glucopyranose units. Simply speaking it is the chief structural element and major constituents of the cell wall of trees and plants. The empirical formula for cellulose is (C6H10O5)n where 'n' is degree of polymerization (DP).
|Substance||Degree of Polymerization (DP)||Molecular Weight|
|Purified Cotton||1000 - 3000||150,000 - 500,000|
|Wood Pulp||600 - 1000||90,000 - 150,000|
|Commercial Regenerated Cellulose (e.g. Rayon)||200 - 600||30,000 - 150,000|
|β Cellulose||15 - 90||3000 - 15,000|
|Dynamite Nitro-Cellulose||3000 - 5000||750,000 - 875,000|
|Plastic Nitro-Cellulose||500 - 600||125,000 - 150,000|
|Commercial Cellulose Acetate||175 - 360||45,000 - 100,000|
- A constituent of woods that is, like cellulose, a polysaccharide, but less complex and easily hydrolysable. Hemicellulose have lower degree of polymerization (only 50 - 300) with side groups on the chain molecule and are essentially amorphous.
|Pulping Process||Yield (%)||% of Pulp||Papermaking Properties|
|b Cellulose||Hemicellulose||Lignin||Initial Tensile||Max. Tensile||Tear||Rate of Freeness Developed|
|Kraft||44||None||14||1 - 2||Low||Very High||Low||Very High|
|Sulfite||50||High||11||1 - 2||Medium||Medium||Medium||Medium|
|Alkaline Pretreatment With Sulfite Cook||52||Medium||17||1 - 2||Medium High||Medium||Very High||Low|
|High Yield Bi-Sulfite||60||Low||19||10||High||High||Low||Medium|
- A complex constituent of the wood that cement the cellulose fibers together. Lignin is brown in color. Lignin is largely responsible for the strength and rigidity of plants.
- Solvent Extractives
- Soluble materials or extractives in wood consist of those components that are soluble in neutral organic solvents. The di-chloromethane extractable content of wood is a measure of such substances such as waxes, fats, resins, photosterols and non-volatile hydrocarbons. The amount of extractives is highly dependent on seasoning or drying of wood.
- The ethanol-benzene extractable content of the wood consists of certain other di-chloromethane insoluble components such as low molecular weight carbohydrates, salts, and other water soluble substances.
- Most water soluble and volatile compounds are removed during pulping. The extractives reduce pulp yield, increase pulping and bleaching chemical consumption and create problems such as foaming during papermaking if not removed.
- The standard procedure of measuring solvent Extractive is laid out in TAPPI T204
|Wood Components||Hardwood (%)||Softwood (%)|
|Cellulose||40 - 50||40 - 50|
|Hemicellulose||25 - 35||25 - 30|
|Lignin||20 - 25||25 - 35|
|Pectin||1 - 2||1 - 2|
Chemical composition of wood is the determining factor of pulping yield for various pulping processes.
|Pulping Process/Pulp Grade||Wood Components Retained in Pulp||Wood Components Removed||Yield|
|Soft Chemical Cook and Bleached||Cellulose only||Lignin, Hemicellulose & Extractives||Less than 40%|
|Chemical Pulping & Bleached||Cellulose and partly Hemicellulose||Lignin, partly Hemicellulose & Extractives||45 - 55%|
|Chemical Pulping NO Bleaching||Cellulose, partly Hemicellulose & traces of Lignin||Partly Lignin & Hemicellulose & Extractives||45 - 55%|
|Semi-Chemical||Cellulose, mostly Hemicellulose & partly lignin||Partly lignin, some Hemicellulose &Extractives||50 - 65%|
|TMP, RMP & GW||Cellulose, Hemicellulose and Lignin||Extractives||More than 95%|
Non wood plant materials such as agricultural residue, grasses etc., contain lesser amount of cellulose compare to wood hence have lower pulp yield. On the other hand cotton which is almost pure cellulose has very high yield.
- TYPES OF WOOD
- Hard Wood
- Wood from trees of angiosperms class, usually with broad leaves. Trees grown in tropical climates are generally hardwood. Hardwood grows faster than softwood but have shorter fibers compared to softwood.
- The trees classified as softwoods have needle like or scale like leaves that, with a few exceptions, remain on the tree all through the year. Hence softwood trees are sometimes called evergreens. Botanically, they are known as gymnosperms, from the Greek word meaning "naked seeds." Instead of bearing seeds from flowers, gymnosperms have exposed seeds in cones.
- Generally grown in cold climates, softwood grows slower than hardwood but have longer fibers compared to hardwood.
Rotation and Yield Comparison of Hardwood Pulp Species (Source: Poyry)
|Species||Country||Rotation (Years)||Yield m3/ha/Year|
Rotation and Yield Comparison of Softwood Pulp Species (Source: Poyry)
|Species||Country||Rotation (Years)||Yield m3/ha/Year|
|Pinus Spp (Pine)||Brazil||15||38|
|Pinus Radiata (Pine)||Chile||25||22|
|Pinus Radiata (Pine)||New Zealand||25||22|
|Pinus Elliottii/Taeda (Pine)||USA||25||10|
|Douglas Fir||Canada (Coast)||45||7|
|Picea Abies (Spruce)||Sweden||70-80||4|
|Picea Abies (Spruce)||Finland||70-80||4|
|Picea glauca (Spruce)||Canada (Inland)||55||3|
|Picea Mariana (Spruce)||Canada (East)||90||2|
- TYPES OF WOOD WITHIN A TREE
- Heart Wood
- The dark colored , center of a tree, consisting of dormant wood. Heart wood of soft wood generally contain slightly less lignin and cellulose than the sap wood.
- Sap Wood
- The fluid part of the tree that moves up from the roots through the outer portion of the trunk and branches and contributes to its growth. The acetyl content is higher in sap wood compared to heart wood.
- Spring Wood (Early Wood)
- This is the wood tree produced early in the growing season of the year or spring. Composition and morphology of softwood's early and late wood fibers differs. The early wood fibers have thin walls and wide lumens. Late wood fibers have much thicker walls.
- Summer Wood (Late Wood)
- This is the wood tree produced late in the growing season of the year or summer. Late wood contains more cellulose and less lignin than early wood.
- Compression Wood
- This wood occurs on the lower side of the branches and leaning trunks in soft wood. Compression wood contains more lignin and less cellulose compared to normal wood. For a picture of compression wood please click
- Tension Wood
- This wood occurs on the upper side of the branches and leaning trunks of hard wood. Tension wood contains more cellulose and less lignin compared to normal wood. For a picture of tension wood please click
|Softwoods, earlywood vs. latewood||Hardwoods, earlywood vs. latewood|
- Core Wood
- The center of a tree stem.
- Slab Wood
- The outer part of tree stem.
|Compression Wood vs. Normal Wood||Tension Wood vs. Normal Wood|
|Location||lower side of stem||upper side of stem|
Juvenile vs. Mature Wood
- first 10-20 years of growth
- associated with proximity to crown
- not very good for pulping
- vascular cambium is not yet very good at reproducing
- softwoods and hardwoods behave the same with respect to juvenile vs. mature wood
- near the top of the tree, juvenile wood is in first 10 rings
- near the bottom of the tree, juvenile wood in first 20 rings
- has a short cooking time than mature wood since it is much lower in density than mature wood
|Summary of Effects||Softwood, juvenile wood vs.||Hardwood, juvenile wood vs.|
|mature wood||mature wood|
- PHYSICAL PROPERTIES OF WOOD (In reference to papermaking only)
Wood ash has following elements:
Carbon (5% to 30%), calcium (5% to 30%), carbon (7% to 33%), potassium (3% to 4%), magnesium (1% to 2%), phosphorus (0.3% to 1.4%) and sodium (0.2% to 0.5%).
The following compound composition limits are also reported:
SiO2 (4% to 60%), Al2O3 (5% to 20%), Fe2 O3 (10% to 90%), CaO (2% to 37%), MgO (0.7% to 5%), TiO2 (0% to 1.5%), K2O (0.4% to 14%), SO3 (0.1% to 15%), LOI (0.1% to 33%), moisture content (0.1% to 22%), and available alkalis (0.4% to 20%).
Structure and Properties of Wood A presentation by Canadian Wood Association