34 Chapter 3
suspension, and then allowing the water to drain away. The paper is then
removed from the mould and allowed to dry. Machine-made paper is made by
allowing the fibre suspension to flow onto a moving mesh (the web), thus
forming a continuous strip of paper which may be dried and pressed on the
machine before being made into a roll for storage. One of the arts of paper-
making is to form the suspension of fibres in the first place and then stop the
fibres becoming entangled before they settle on the web. When clumps of fibres
occur in paper, they are called knots. Modern papermakers use synthetic sus-
pending agents to avoid knots, but Oriental artisans used mucilage made from
seaweed.
Papermaking fibres usually contain substances other than cellulose. There
are often carbohydrate polymers other than those that are polymers of glucose.
There are many sugars present in plants, which may polymerise to form carbo-
hydrate polymers and which become included in plant cells from which
fibres are obtained. Carbohydrates similar to cellulose, which are made out of
a mixture of different sugars, are called hemicelluloses. Often the hemicellu-
lose is named after the principal sugar that they contain, e.g.xylan, a hemi-
cellulose in wood, is named after the sugar xylose. The polymers may be
branched, unlike cellulose molecules. With all their branching and mixture
of different sugar monomers, hemicellulose molecules cannot pack closely
together and do not contain ordered regions. Cellulose is a comparatively lin-
ear polymer and by hydrogen bonding will form regularly packed, ordered
regions that have crystalline properties, e.g.they will diffract X-rays. Carbo-
hydrates in solids with crystalline areas are relatively resistant to chemical
agents trying to attack them because the attacking molecules find access
difficult. Hemicelluloses are more reactive than cellulose because of the
reactivity of the sugars they contain and because they have no crystallinity.
Hemicelluloses can be removed from impure cellulose fibres by treatment
with alkalis.
Some papermaking fibres, especially those derived from wood, contain
lignin which is a large three-dimensional polymer made of building blocks
containing rings of six carbon atoms called phenyl-propanes. The structures
of the phenyl-propanes in grasses and in coniferous and deciduous woods are
slightly different from one another, however, Figure 2 indicates how lignin is
typically constructed.
Lignin is the stiffening material that gives wood its characteristic mechan-
ical properties; it stops plants from falling over! Wood cellulose is so intim-
ately associated with lignin in natural products that the vigorous physical and
chemical treatments needed to ensure complete removal of lignin damage the
cellulose.
Wood can be ground to a powder and then with a little further processing
can be used for making a type of paper. These papers have low strength as