Natural fibres can be classified according to their origin.
CLASSIFICATION AND PROPERTIES
Natural fibres can be classified according to their origin.
1. The vegetable, or cellulose-base,
class includes such important fibres as cotton, flax, and jute.
2. The animal, or protein-base, fibres
include wool, mohair, and silk.
3. Regenerated and synthetic fibres
include Nylon, Terylene, Orlon, Viscose, Alginate fibres, etc.
4. An important fibre in the mineral
class is asbestos.
The vegetable fibres can be divided into smaller groups,
based on their origin within the plant. Cotton, kapok, and coir are examples of
fibres originating as hairs borne on the seeds or inner walls of the fruit,
where each fibre consists of a single, long, narrow cell. Flax, hemp, jute, and
ramie are bast fibres, occurring in the inner bast tissue of certain plant
stems and made up of overlapping cells. Abaca, henequen, and sisal are fibres
occurring as part of the fibrovascular system of the leaves.
Chemically, all vegetable fibres consist mainly of cellu-lose,
although they also contain varying amounts of such substances as hemicellulose,
lignin, pectins, and waxes that must be removed or reduced by processing. The
animal fibres consist exclusively of proteins and, with the exception of silk,
constitute the fur or hair that serves as the protective epidermal covering of
animals. Silk filaments are extruded by the larvae of moths and are used to
spin their cocoons.
With the exception of mineral fibres, all natural fibres
have an affinity for water in both liquid and vapour form. This strong affinity
produces swelling of the fibres connected with the uptake of water, which
facilitates dyeing in watery solutions.
Unlike most synthetic fibres, all natural fibres are non-thermoplastic—that
is, they do not soften when heat is applied. At temperatures below the point at
which they will decompose, they show little sensitivity to dry heat, and there
is no shrinkage or high extensibility upon heating, nor do they become brittle
if cooled to below freezing. Natural fibres tend to yellow upon exposure to
sunlight and moisture, and extended exposure results in loss of strength.
All natural fibres are particularly susceptible to microbial
decomposition, including mildew and rot. Cellulosic fibres are decomposed by
aerobic bacteria (those that live only in oxygen) and fungi. Cellulose mildews
and decomposes rapidly at high humidity and high temperatures, especially in
the absence of light. Wool and silk are also subject to microbial decomposition
by bacteria and moulds. Animal fibres are also subject to damage by moths and
carpet beetles; termites and silverfish attack cellulose fibres. Protection
against both microbial damage and insect attacks can be obtained by chemical
modification of the fibre substrate; modern developments allow treatment of
natural fibres to make them essentially immune to such damage.
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