Textile fibers /classification of fiber.
Fibers and
Classifications of Fibres:
Fibers -units of
matter characterized by flexibility, fineness and high ratio of length to
thickness. Other necessary attribute for textiles are adequate strength and
resistance to conditions encountered during wears, as well as absence of
undesirable colour, and finally the property of dye ability.
In
generally, the steps in the manufacture of fabrics from raw material to
finished goods are as follows:
·
Fibre, which is either spun (or twisted) into yarn or else directly
compressed into fabric.
·
Yarn, which is woven, knitted, or otherwise made into fabric.
·
Fabric, which by various dyeing and finishing processed becomes
consumers’ goods.
v The
principle classifications of textile fibres
The principle
types of textile fibre in general use may be classified as in Table 2.1.
Animal
Wool and hair fibres
Cocoon fibres
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Sheep’s wool (many varieties)
Cashmere: hair of the cashmere goat Mohair: hair of the agora goat
(South Africa, turkey, and USA)
Camel
Rabbit fur
Cultivated silk (Asia, Europe)
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Vegetable
|
|
Seed fibres
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Cotton
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Bast fibres
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Flax
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Jute
|
|
Hemp
|
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Ramie etc.
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Man-made Fibres
Natural starting materiel
Rayon
Lyocell/Tencel Cellulose acetates
Synthetic material
Organic origin
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Viscose, including polysonic
Normal or secondary
acetate, and triacetate Polyacrylonitriles
Polyamides (nylon) Polyesters
(PET) Polyurethane (spandex/lycra)
|
For convenience
in studying their dyeing properties, fibers be broadly divided into three
classes as showed in Table 2.2.
Classification of fibers according to dyeing properties
Fiber Properties
The structure of fibres
Textile Fibres
are composed of molecules that are very long and flexible, as are the fibres
themselves. These molecules are polymeric, i.e. they are composed of a lame
number of small repeating units, which may he either all the same or of several
types, occurring at intervals along the length of the molecule. Such molecules
are usually referred to as molecular chains. The chemical structure of some
important fibres will be showed later in discussing their properties.
Fibres have
properties that make it appear that these molecular chains are ii places held
together lengthwise by various faces, fanning tightly packed bundles known as
crystalline regions, whereas in other places, known as the amorphous region,
they are less firmly held and can he readily separated, for example. Water is
strongly attracted by portions of the molecular chains of most fibres, and so
quickly enters the amorphous regions, forcing them apart so that the whole
fibre structure becomes interpenetrated b in minute pores and channels, into
which any substance dissolved in the water outside can freely enter. The fibre
in this water-swollen state is something in the nature of a sponge.
It is important
to understand the infinitesimally minute scale on which this molecular
architecture of fibres is constructed, Perhaps the most vivid illustration of
this is conveyed by the severed; the pores through which water enters may be
only about one- millionth of an inch in diameter
Cellulose and
its properties;
Cellulose
pertains to the class of carbohydrates. It contains 44.4% of carbon, 6.2% of
hydrogen, and 19.4% of oxygen. The elementary unit of a cellulose macromolecule
is anhydrous-d-glucose, which is repeated a great number of times in the
cellulose molecule; i.e. cellulose is a high-molecular compound.
In the cellulose
molecule, the d-glucose anhydrides of ß-form are interconnected the glucosidic
linkage 1-4, characterized by the following atomic structure:
The chemical
structure of cellulose is showed as follows, each elementary unit the
macromolecule (except the end ones) contains three (one primary and two
secondary) alcohol hydroxyls. In many reactions (mainly esterfication) the
primary hydroxyl groups have a greater reactivity; it is possible that in other
reactions, primary hydroxyls may have a lower reactivity than secondary
hydroxyls.
An important
characteristic of cellulose is its molecular weight. For cotton cellulose it
equals from 100,000 to 1-2 million; for flax, up to 6 million; for viscose
rayon, from 20.000 to 230,000.
The degree of
orientation of cellulose macromolecules in vegetable fibres varies greatly,
being the highest in flax and ramie fibres (as the degree of polymerization).
In cotton fibres it is lower and differs from one layer to another depending on
the maturity and on the kind of cotton; corresponding changes are observed in
the viscosity and specific density of cellulose, characterizing the length of
macromolecules and the closeness of their packing.
Cellulose fibres
are complex structures consisting of cellulose macromolecules, which are
arranged in certain, order and do not completely fill their geometrical volume.
Fibrillation is inherent not only in nature cellulose fibres, hut has also been
observed in high-tenacity viscose fibres, and the fibrils are considered to be
identical in both cases.
From the
investigation of the morphological structure of cellulose it may he concluded
that super molecular elements (cell layers, fibrils, etc.) do not form dense
sum dense in the fibre. They have a great number of pores and cracks through
which great reagent diffuses inside the structure in most heterogeneous
reactions. However, in sonic cases when almost no swelling of cellulose occurs,
the reaction may be localized on the surface of the fibres.
The action of
pH and chemical agents on cellulose
Effect of temperature:
The thermal
stability of cellulose is very limited and depends in the large measure on his
time during which it is subjected to the effect of temperature. Prolonged
heating even at a temperature of 100°C impairs the swelling properties of the
fibre and consequently its dye ability. Cellulose withstands short-term heating
at a temperature of 180-200°C. A temperature above 275°c, intensive
decomposition of cellulose lakes place with the formation of liquid and gaseous
products of different composition. At 400 – 450 degree centigrade all gaseous
decomposition products disappear and a hard residue (carbon) remains.
Notwithstanding
the great number of hydroxyl groups, which are hydrophilic in nature, cellulose
is not soluble in water, which is probably due to presence of hydrogen and
vanderwaals’ intermolecular bonds and is capable only of limited swelling.
On immersion in
water the cross section of cellulose cotton fibre increase at most by 15-50%
and its length by 1-2%. But water penetrates only into the parts with less
orientation. The moisture content and swelling at constant humidity somewhat
decrease with an increase in temperature.
The fiber
strength is reduced by prolonged action of steam wit the formation
oxycellulose. Cellulose is not soluble in any’ of the usual organic solvents,
such as alcohol, etc. The treatment of fibres with certain organic solvents,
however, considerably increases the reactivity cellulose.
Action of acids;
The glucosidic
linkages of cellulose molecules are highly unresisting to the action of mineral
acids and arc readily hydrolyzed, i.e. the break up, combining with water which
results in the disintegration of macromolecules.
The hydrolysis
equation may be written as follows:
The intermediate
product of hydrolysis is called hydrocellulose. The hydrolytic action of acids
depends on the nature of acid and temperature.
Action of salt solutions:
Solutions of acid
salts have the same effect on cellulose as acids. The ammonical solution of
copper oxide, the cupriaminohydrate [Cu(NH3)(OH)2, is the specific solvent for
cellulose.
Action of alkalies:
It is
characteristic of the glucosidic linkage of the cellulose molecule that it is
highly resistant to alkalies. At normal temperature weak solutions of caustic
soda have no effect on cellulose. But on boiling in a 1 % solution of caustic
soda, a small part of cellulose passes into solution. As the alkali
concentration increases, the cellulose solubility becomes considerably higher.
The action of alkali solutions on cellulose is particularly pronounced in air.
In this case, caustic soda promotes cellulose oxidation by atmospheric oxygen
and cellulose is transformed into oxycellulose.
In concentrated
solutions of caustic soda (over 10 per cent) at normal temperatures. the Fibre
swells, becomes elastic, and contracts in length. If shrinkage is impeded, the
fibre acquires luster which is retained after the alkali ha b washed out. From
the chemical standpoint, the essence of this is process is in the absorption of
alkali with the formation of alkali cellulose. Caustic soda combines with
cellulose forming a molecular compound according to following equation:
Alkali cellulose
is an unstable compound; it is easily decomposed by water to form cellulose
hydrate which is more hygroscopic than native cellulose, has a high swelling
capacity, is more liable to undergo hydrolysis and is characterized by a more
intensive dye acceptance. Gray cotton fibres contain on the average up to 7% of
moisture, while the moisture cont of cellulose hydrate amount to 9.5 -10.5%.
Using above
mentioned property, cellulose fibres is treated with strong alkali solution in
dyeing-finishing process which is cal led mercerization.
Action of reducing and oxidizing agents:
Reducing agents
have no effect on cellulose, while oxidizing agents readily convert it to
oxycellulose. For chemical treatment of fibrous materials, large use is made of
various oxidizing agents: sodium hypochlorite, hydrogen peroxide, sodium
chlorite etc, and such acids that are ca of oxidizing, as for instance, nitric acid.
These reagents
may cause more or less intensive oxidation of cellulose functional groups arid
breakage of chains as a result of glucosidic- linkage rupture. The oxidizing
agent first act on the functional groups located on the cellulose fibre surface
and then progressively penetrate into the depth of the fibre..
Bleaching
treatment can he applied to cellulose fibres according to above properties.
Effect of
light and atmospheric conditions:
Under the action
of light, cellulose is oxidized by atmospheric oxygen and due to photo
oxidation, oxycellulose is formed as a result of which the strength of the
cellulose is considerable reduced, the copper and iodine numbers are
correspondingly increased, and the viscosity of cuprammonium solution is reduced.
Effect of
microorganism on cellulose:
Cotton:
Table the
approximate composition of dry cotton
Effect of
microorganism on cellulose:
If the moisture
content in fibres is over 9% and the relative humidity is over 75- 85%. Some
bacteria and mildew fungi may cause cellulose decay.
Cotton:
Raw cotton
contains, in addition to cellulose, the usual constituents of vegetable cell.
These are oil and wax, pectoses and pectins, proteins and simple related
nitrogen compounds. Organic acids, mineral matter, arid natural
colouring-matter, or piece goods may contain in addition, adventitious dirt,
size, and machine oil; the approximate composition of raw dry cotton is as showed
in table 2.3:
Table the
approximate composition of dry cotton
Percentage
|
|
Cellulose
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94.0
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Oil and wax
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0.5
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Proteins, pectose, and colouring-matter
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4.5-5.0
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Mineral matter
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1.0
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If cotton is
properly purified before bleaching it must lose 6.5 % in weight. When all the
impurities have been removed the main constituent of the fibre, cellulose
remains. Cotton fabric is stable even in strong alkali solution, but it will be
hydrolysis in acidic solution as well as in the solution with strong oxidizing
agent. It must be very careful controlling the bleaching process to avoid the
cotton over oxidizing and hydrozing.
The wet strength
of cotton fabric is higher that that of dry. The cotton fabrics are very
absorbent due to the presence of countless polar-Oil groups. Because of this
hygroscopic nature, it prevents from developing static electricity.
Cotton posses
many desirable properties, especially hydrophilic and attractive handle. So
that cotton is suitable for comfort wear, such as underwear. However, its main
undesirable property is dimensional unstable, because it is not thermoplastic,
so it can not be permanent heat set, therefore it is easily to get wrinkles.
The cotton fabric is relatively inelastic because of its crystalline polymer
system. The finishing process for cotton mainly focuses on anti crease, i.e.
crease resisting finishing.
The main types of commercial
cotton are showed in table
Name
|
Characters
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Sealan cotton
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Gulf of florida
Spinning count is 1/200
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Egyptian cotton
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Two
types, brown and white Spinning count: 1/200 for brown
1/70 for white
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South American cotton
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Good for blends
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American cotton
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Most
abundant type Good natural colours
Cannot be use for fine counts
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India cotton
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Short fibres
Cannot be used of fine count
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China cotton
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Short fibres
Cannot be used for counts
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