ISSN ………… - International Network for Bamboo and Rattan

termed ‘xylans’ or more loosely ‘pentosans’. Those contain mannose units linked to each other and to
glucose units have been referred as ‘mannans’.
The hemicelluloses (when freed from lignin) swell more than does cellulose and are in part dispersible in
water. They have adhesive properties not shared by cellulose. Whereas cellulose is fibrous,
hemicelluloses are non-fibrous. Whereas cellulose is quite insoluble in cold alkali, hemicelluloses are
quite soluble in dilute caustic soda. In any chemical pulping operations, some of the initial hemicelluloses
are retained in the pulp. A portion of the less resistant hemicelluloses is removed during digestion, and
their degradation products are then found in the spent liquors.
In the case of pulps freed from lignin by adequate and controlled bleaching, the hemicelluloses have been
shown repeatedly to contribute greatly to tensile and bursting strength and to folding endurance of the
pulp sheet. Both the quantity and the type of hemicelluloses in a pulp influence the pulp properties and
the type of paper that can be made from such a pulp. There are certain disadvantages also about their
presence. These are undesirable for dissolving grade pulp. In the case of certain bleached pulps, these are
responsible for a loss in brightness of the bleached pulp on storage or aging.
3.1.3. Lignin
Lignin is the cementing substance between fibres and tissues and is concentrated mainly in the region of
the middle lamella and imparts rigidity to wood tissue. Lignin exists in wood or bamboo as branchedchain
polymer molecules. The lignin may be separated from an associated wood component either by
preferentially dissolving lignin or by preferentially dissolving non-lignin components. Isolated lignins, in
general, are amorphous and non-crystalline, and show definite softening points at elevated temperatures.
The average molecular weight is in the range of 11,000. An important property of lignin is its capacity to
absorb ultra violet light. The chemical skeleton of lignin is a phenylpropane or a “C6 – C3” or a “C9” type.
Pulping is basically and mainly a delignification process employing inorganic acids or alkalies and other
compounds or organic solvents and compounds or by employing biological agents such as certain fungi
which will selectively attack on lignin, causing its degradation and consequent dissolution. The amount
and reactivity of lignin have a marked effect on the pulpability of the material. These differ depending
upon the raw material (softwoods, hardwoods, bamboos, etc.). During most pulping reactions,
components other than lignin are simultaneously removed. The character of pulp depends upon the form
and amount of energy supplied for accomplishing the separation. Chemical, mechanical or a combination
of the two forms of energy are utilized. In general, when chemical energy alone is supplied, completely
separated fibres are obtained; whereas in mechanical and semichemical pulping (combination of
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