PROPERTIES OF WOOD

 

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). Cellulose The DNA of Paper

Picture of cellulose

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
β Cellulose  15 - 90 3000 - 15,000
γ 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
 

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
 

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.
image of maple tree

Rotation and Yield Comparison of Hardwood Pulp Species (Source: Poyry)

Species Country Rotation (Years) Yield m3/ha/Year
Eucalyptus Brazil 7 44
Eucalyptus South Africa 8-10 20
Eucalyptus Chile 10-12 25
Eucalyptus Portugal 12-15 12
Eucalyptus Spain 12-15 10
Birch Sweden 35-40 6
Birch Finland 35-40 4
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.
image of pine tree

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


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  • TYPES OF WOOD WITHIN A TREE
image of wood annual rings
 
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.

image of sap wood and heart wood                                                            image of sap wood and 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.

  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
 
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


  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
Core Wood
The center of a tree stem.

Slab Wood 
The outer part of tree stem.

Juvenile vs. Mature Wood


Juvenile 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
Cell Length lower lower
Fibril Angle  higher higher
Cellulose Content lower lower
Lignin Content higher higher
Cooking Time shorter shorter
Chemical Requirements higher higher

  • PHYSICAL PROPERTIES OF WOOD (In reference to papermaking only)
 
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.

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%).

 
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
 

Structure and Properties of Wood  A presentation by Canadian Wood Association

Forest Facts by American Forest

  • A single tree can absorb 10 pounds of air pollutants per year.
  • The average healthy, mature tree produces roughly 260 pounds of oxygen annually. The average person consumes 386 pounds of oxygen per year. Two trees provide enough oxygen for one person per year.
  • Chicago’s urban forest (more than 3.5 million trees) removes about 888 tons of air pollution per year.
  • Seventy percent of the Earth’s surface is covered in water. About 2.5 percent of the Earth’s water is freshwater. Less than one percent is in the form of groundwater.
  • More than half of the country’s drinking water originates in forests. Approximately 180 million people depend on forests for their drinking water.
  • A single front-yard tree can intercept 760 gallons of rainwater in its crown, reducing runoff and flooding on your property.
  • On average, a mature tree can absorb 36 percent of the rainfall it comes in contact with.
  • Forests are the largest forms of carbon storage, or sinks, in the United States. Currently, plants absorb and store about 15 percent of the United States’ total carbon dioxide emissions from the transportation and energy sectors.
  • One mature tree absorbs CO2at the rate of 48 pounds per year.
  • Over a year, an acre of forest can consume the amount of CO2created by driving a car 26,000 miles, about twice the annual mileage for an average driver.
  • Deforestation accounts for up to 15 percent of global emissions of heat-trapping gases.
  • Trees properly placed around buildings can reduce air conditioning needs by 30 percent and save 20-50 percent in energy used for heating.
  • The net cooling effect of a young, healthy tree is equivalent to 10 room-size air conditioners operating 20 hours a day.
  • A mature tree can reduce peak summer temperatures by 2 º to 9 º Fahrenheit.
  • 100 million mature trees growing around residences in the U.S. can save about $2 billion annually in energy costs.
  • Trees properly placed around buildings can reduce air conditioning needs by 30 percent and save 20-50 percent in energy used for heating.
  • The net cooling effect of a young, healthy tree is equivalent to 10 room-size air conditioners operating 20 hours a day.
  • A mature tree can reduce peak summer temperatures by 2º to 9º Fahrenheit.
  • 100 million mature trees growing around residences in the U.S. can save about $2 billion annually in energy costs.
  • Recycling paper products is the most common way to save trees. Wheat, oak and barley left over from harvesting, or agri-pulp, are used as fillers with recycled paper. In 2010, 77 percent of all papermakers substituted some recycled paper for new wood.
  • Products such as paper towels, napkins, bathroom and facial tissue are made from 100 percent recycled paper.
  • Many designers and manufactures are using reclaimed wood from factory scraps, old furniture and sunken wood to design contemporary, earth-friendly furniture.


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