Properties of Wood (for Papermaking)

PROPERTIES OF WOOD

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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
Carbon	45-50%
Hydrogen	6.0-6.5%
Oxygen	38-42%
Nitrogen	0.1-0.5%
Sulphur	max 0.05

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
Cellulose (%)	40.0	39.5	45.0	41.0
Hemicellulose (%)	28.5	30.6	19.2	32.4
Lignin (%)	27.7	27.5	31.3	22.0
Total Extractive (%)	3.5	2.1	2.8	3.0

Cellulose: 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
Native Cellulose	>3500	>570,000
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
b Cellulose	15 - 90	3000 - 15,000
Y Cellulose	<15	<3000
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

Hemicellulose: 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

Lignin: 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

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.

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.

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

Core Wood: The center of a tree stem.
Slab Wood: The outer part of tree stem.

Ash Content: Ash is a solid, particulate, inorganic combustion residue left after the wood is burnt. Ash content varies between different components of trees. Stem wood contains 0.4-0.6%, stem bark 2.0-5.0% and 1.0-2.0% in branches. The ash content is highest in those parts of the tree where growth occurs. Ash contents in the leaves, needles and branches and leading shoot varied between 2 and 6%. As a mean value wood can be expected 1-2% ash content.

Moisture Content: All freshly cut wood contain moisture. Wood may contain around 50% moisture. Moisture in the wood increases handling weight. moist wood is elastic, while dried wood may be brittle.
: Wood moisture provide lubrication to ground stone and keep the temperature low in grinding zone. Wood moisture help in better chemical penetration during cooking due to diffusion.
: Knowing moisture content of wood is important as the useful part of wood is dry content and this is what the money paid for. The eliminate the role of moisture content, wood is normally traded by volume.
: Cunit: A term used in the measurement of pulpwood, i.e. 100 cubic feet of solid wood, bark excluded. One cunit corresponds to 2.83 cubic meter of wood.; Cord: A term used in the measurement of pulpwood, i.e. 128 cubic feet (4-foot logs, 8 feet across and 4 feet high) of gross volume. It is unreliable measurement as it include air volume between logs.; Unit: A term used in the measurement of wood chips, i.e. 200 cubic feet of bulk wood chips.
: The standard procedure of measuring moisture content by toluene distillation is laid out in TAPPI T208

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Wood Components	Hardwood (%)	Softwood (%)
Cellulose	40 - 50	40 - 50
Hemicellulose	25 - 35	25 - 30
Lignin	20 - 25	25 - 35
Pectin	1 - 2	1 - 2
Starch	Trace	Trace

	Compression Wood vs. Normal Wood	Tension Wood vs. Normal Wood
Location	lower side of stem	upper side of stem
Cellulose Content	lower	higher
Lignin Content	higher	lower
Fibril Angle	increase	decrease
Cooking Time	longer	longer
Chemical Requirements	higher	equal

	Softwoods, earlywood vs. latewood	Hardwoods, earlywood vs. latewood
Cell Length	shorter	shorter
Wall Thickness	thinner	thinner
Fibril Angle	higher	higher
Cellulose Content	lower	lower
Lignin Content	higher	higher
D.P. Cellulose	lower	lower
Cellulose Crystallinity	lower	lower

Summary of Effects	Softwood, juvenile wood vs.	Hardwood, juvenile wood vs.
	mature wood	mature wood
Cell Length	lower	lower
Fibril Angle	higher	higher
Cellulose Content	lower	lower
Lignin Content	higher	higher
Cooking Time	shorter	shorter
Chemical Requirements	higher	higher