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Making Pulp from Wood
Woodpulp currently represents 44% on average of the
fibre used to manufacture paper and board worldwide, of which 11% is home
produced. None of these mills uses mature fully grown trees, but rather small
dimension timber, (which is no use to other commercial users such as furniture
makers and builders), saw mill waste and forest thinning.
In the past the industry used softwoods such as
spruce, pine, fir, larch and cedar almost exclusively, but hardwoods such as
birch and aspen are gaining in popularity. Fast growing eucalyptus have been
successfully cultivated in Northern Spain, Portugal and Brazil and provide the
papermaker with very high quality pulp. Softwoods provide long strong cellulose
fibres and are used to produce papers where strength is a requirement, for
example, packaging papers. The shorter hardwood fibres provide bulk, smoothness
and opacity and are used to produce fluting medium and printings and writings.
Trees vary enormously in the time they take to
reach full size. Much depends on climate and the soil, but these figures give a
rough comparison:
| Tree Type |
Life Expectancy (years) |
| Willow |
25 - 35 years |
| Sitka spruce |
50 years |
| Douglas fir |
55 - 60 years |
| Scots pine |
70 - 80 years |
| Norway spruce |
70 - 75 years |
| Oak |
>100 years (not used for papermaking) |
Oak trees usually live 200 - 300 years, but some
have been known to live for more than 1,000
When a tree trunk or thick branch is cut across,
a series of dark rings can be seen. These are annual rings which result for the
seasonal growth. Each ring represents one year's growth. The grain in timber is
caused by the annual rings which show when a log is cut. Teak, mahogany, oak,
beech and elm have attractive grain and are used for furniture making (rather
than papermaking).
Woodpulp comes from trees from managed forests
where more trees are planted than harvested to ensure that demand for timber
products will never outstrip supply. The forest industry has become more aware
over recent years that it has a responsibility, not only to ensure the economic
viability of its operations, but also to ensure that the requirements of local
communities are catered for. Greater care is taken to ensure that logged areas
blend in more readily with the landscape.
Certain pulp characteristics depend on the
process used to reduce the wood to its component fibres. There are three main
ones:
Mechanical
or ground wood
Chemical
Combination
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