To make pulp from wood, a chemical pulping process
separates lignin from cellulose fibres. This is accomplished by dissolving lignin
in a cooking liquor, so that it may be washed from the cellulose; this
preserves the length of the cellulose fibres. Paper made from chemical pulps
are also known as wood-free papers–not to be confused with tree-free paper;
this is because they do not contain lignin, which deteriorates over time. The
pulp can also be bleached to produce white paper, but this consumes 5% of the fibres;
chemical pulping processes are not used to make paper made from cotton, which
is already 90% cellulose.
There are three main chemical pulping processes: the sulfite
process dates back to the 1840s and it was the dominant method extent before
the second world war. The kraft process, invented in the 1870s and first used
in the 1890s, is now the most commonly practiced strategy, one of its
advantages is the chemical reaction with lignin, that produces heat, which can
be used to run a generator. Most pulping operations using the kraft process are
net contributors to the electricity grid or use the electricity to run an
adjacent paper mill. Another advantage is that this process recovers and reuses
all inorganic chemical reagents. Soda pulping is another specialty process used
to pulp straws, bagasse
and hardwoods with high silicate content.
Much of the early paper made from wood
pulp contained significant amounts of alum, a variety of aluminium sulfate
salts that is significantly acidic. Alum was added to paper to assist in sizing,
making it somewhat water resistant so that inks did not "run" or
spread uncontrollably. Early papermakers did not realize that the alum they
added liberally to cure almost every problem encountered in making their
product would eventually be detrimental. The cellulose fibres that make up
paper are hydrolyzed by acid, and the presence of alum would eventually degrade
the fibres until the paper disintegrated in a process that has come to be known
as "slow fire". Documents written on rag paper were significantly
more stable. The use of non-acidic additives to make paper is becoming more
prevalent, and the stability of these papers is less of an issue. For More
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