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Components of Wood

  • Cellulose consists of long, straight chains of glucose molecules. It forms the skeleton of the plant wall and has the most desired properties for making paper. These fibres are long, strong and translucent.

  • Hemicelluloses are short, branched chains of glucose and other sugar molecules. They fill in space in the plant wall. Hemicelluloses are more soluble in water and are thus often removed during the pulping process.

  • Lignin is a three dimensional phenolic polymer network. This "glue from hell" holds the cellulose fibres together and makes them rigid. Chemical pulping and bleaching processes selectively remove the lignin without significantly degrading the cellulose fibres.

  • Extractives account for 3(+/-2)% of softwoods. These materials include plant hormones, resin and fatty acids along with other substances that help the tree grow and resist disease and pests. These substances are highly toxic to aquatic life and account for much of the acute toxicity of pulp mill effluent.

                  
Average composition of softwoods (Smook)

Wood is a natural composite material consisting of hollow, flexible tubes of cellulos bonded together and rigidified by a glue called lignin (NC State, 1993).

Wood Types

Characteristics of softwood and hardwood fibres

 

Softwood

Hardwood

Cellulose content

42% +/- 2%

45% +/- 2%

Lignin content

28% +/- 3%

20% +/- 4%

Extractives content

3% +/- 2%

5% +/- 3%

Fibre length

2-6 mm

0.6-1.5 mm

Coarseness

15-35 mg/100 mm

5-10 mg/100m

Trees can be divided into two general classes - softwoods and hardwoods. Softwood trees are conifers - e.g., southern pine, Douglas fir, spruce. Hardwood trees lose their leaves every year. Examples include birch, aspen, red gum.

Softwood fibres with their length and coarseness are generally used to provide strength to a sheet of paper. Hardwood fibres, being finer and more conformable, give a sheet of paper its smooth printing surface and opacity. Hardwood fibres are also easier to bleach to high brightness because they have less lignin.

Paper generally consists of a blend of hardwood and softwood pulps to meet the strength and printing surface demands of the customer.

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