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Chemical
Pulping
The chemical
pulping process was discovered in the nineteenth century, and allowed
wood to be pulped for strong, white, long-lasting paper. The goal
of chemical pulping is to remove all the parts of wood which are
not cellulose. Chief among these is lignin, a carbohydrate that
cements adjacent cells together in wood. This material can be made
more soluble by cooking the pulp either in a strong acid or a strong
base.
Originally,
wood pulp was cooked in lye
alone but this produced a rather weak paper. The addition of sodium
sulfide to the pulp produced a much stronger paper. This process
is called the kraft process, from the German word for strong. About
80 per cent of this kraft pulp is wood and the remaining 20 per
cent is lye and sodium sulfide. The pulp is cooked, or digested,
at 170°C (338°F) for 3 hours until most of the lignin is broken down.
The liquid is drained off and the pulp is washed to remove the chemicals.
The resulting
pulp is dark brown in color. If it is made directly into paper,
the resulting paper is strong but brown. This is the kind of paper
out of which grocery bags and corrugated cardboard are made. For
white paper, the pulp is bleached.
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