Cotton is the oldest fibre used for textile purpose.
In tropical countries, it is the most important fibre.
India was the center for the world’s cotton industry as well as variety of fine fabrics till 1600 A.D.
The date of origin of cotton is unknown.
Now cotton is cultivated in almost every country in the world having a mild climate, for cotton cultivation.
American cotton dominates world market. Other countries, large producers of cotton are India, brazil, Mexico, Egypt and china.
COTTON PLANT
DEFINITION OF GRADING
The cotton qualities from place and plant differ; the difference in quality can be expressed in grading and staple length.
Grade generally determined from 3 factors i.e. colour, trash content and ginning quality.
In general, the grading indicates the trash and colour of cotton like LM, Tg, M Lt GY etc...
PHYSICAL PROPERTIES OF COTTON
STAPLE LENGTH
Staple length is one of the important primary properties of any textile fibres. The staple length of cotton varies from 1 cm to 8 cm for different classes, which is shown in table
FIBRE FINENESS
The wall thickness of different types of cotton ranges from 3.5 microns to 10 microns.
Ribbon width is said to range 12 microns to 25 microns.
The tip end is usually gently tapered.
FIBRE UNIFORMITY
Cotton cannot be considered a uniform material even though sufficiently large number of fibres may have a characteristics average behavior.
It has been observed that longer cotton tends to become uniform in length than the shorter ones. The varying percentage of immature fibres also indicates non-uniformity of wall thickness for the same variety of fibres. Also there are considerable difference between cotton grown from the same seed in the same location from time to time.
POROSITY
Cotton fibre is porous and exhibit capillary effects to a high degree. The fibrils themselves are dense as a result of the higher packing density of the molecules and so non-porous.
The arrangement of denser fibrils in the fibre may be visualized as analogous to the packing of fibres in well-made yarn.
So the porosity of the unoccupied space in the fibre ranges from 20-40% of the fibre volume.
The pore space is largely between the fibrils as capillaries of small diameter.
Pores is generally expressed in terms of average surface area.
The surface area of dry cotton is 0.6 to 0.7 sq.m/gm.
The internal surface area can be developed and can be made largely by immersing cotton fibre in water, acetic acid or ethanol.
The surface area of cotton in water is around 137 sq.m/gm.
LUSTRE
The natural lustre of cotton fibre is determined by two factors i.e fibre shape and fibre polish.
The lustre does not depend upon hair weight, length, diameter, fineness or convolutions.
The highest lustre is noticed in the fibre with circular cross section. So the dominating influence in lustre is noticed is the external fibre surface and the exact geometric shape is of secondary importance.
To manufacture a lustrous yarn, apart from the lustre of fibre, the fibre length is another important factor.
When two cottons of the same lustre are used, the longer fibre yields more lustrous yarn.
DENISTY
Cotton fibre has a density of 1.54 grams/cc, which corresponds to a specific volume of 0.64 cc/gm.
MOISTURE
Cotton fibres are composed of an assembly of fibrils. Due to the imperfections in the packing of fibrils, the fibre absorbs moisture.
The moisture absorption takes place on the surface of the fibrils.
The amount of moisture in cotton depends on the relative humidity and temperature of the air which it is exposed.
At higher temperature, there is a small change in moisture and cotton retain constant moisture over small change in temperature.
At 65% relative humidity (R.H) and 22-degree centigrade temperature, the moisture pick up is around 8.3%.
STRENGTH
The load required to break i.e. tensile strength of single cotton fibre varies widely. It depends upon the thickness of the wall, prior damage to the fibre and cellulose degradation.
Matured fibres with coarse and heavy wall are the strongest fibres. Their strength ranges from 9 gm to 13 gm per fibre.
The strength of the matured fibres of intermediate and fine types is between 4 gm to 9 gm per fibre.
On the other hand, immature fibre strength can be low as 0.5 gm 1.0 gm per fibre.
The strength of the fibre increases at higher humidity or at higher moisture. In general, the tensile strength increases upto a relative humidity of 60% and then it remains mostly constant.
At higher humidity or moisture pick-up, moisture or water penetrates inside amorphous region breaks the inter molecular forces, also internal stresses and improves its strength as well as deformability because of more uniform load transfer action.
ELONGATION
When load is applied, the length increases. The change in length with respect to the original length is defined as extension or elongation or strain.
Average fibre elongation at break is about exactly around 6% to 8%.
MODULLUS
Modulus is generally related to the resistance to deformation.
The proportionality between stress and strain is referred as modulus or elastic modulus.
The modulus of cotton fibre is about 500-525 G. wt./tex.
TORSIONAL RIGIDITY
The mean rigidity of cotton fibre is about 7.9 x 10^-4 g.wt.sq.cm.sq.tex.
Rigidity varies with the shape, conditions of growth and wall thickness of the fibre.
The high rigidity of thick walled fibres suggests why course cottons must be more highly twisted than fine cottons to produce yarns of the same size.
CHEMICAL PROPERTIES OF COTTON
The cotton fibre is an elongated cell, constructed from millions of cellulose molecules. Small amount of moisture, fatty materials, minerals are other constituents of cotton. So the chemical properties of cotton are mostly influenced by the chemical characteristics of cellulose.
ACTION OF HEAT
Cotton fibre ignites easily and burns with an odour similar to that of burning paper.
It burns with a bright flame, which continues even after the fibre is removed from fire.
After the flame has been extinguished, the fibre continues to smoulder and smoke. This typical test of cellulose.
Cotton can be heated in a dry state to 150 degrees centigrade without any decomposition. But if heating continues, a brown color on cotton develops gradually.
A slight brown discoloration can occur at temperatures lower than 150 degrees centigrade which does not deteriorate the fibre. However, it is sufficient to spoil the effects of bleaching.
So care should not exceed more than 93 degrees centigrade. Prolonged exposure at high temperature to an atmosphere containing oxygen causes tendering due to the formation of oxycellulose.
At about 170 degrees centigrade, cotton begins to scorch even in short time. If cotton is heated out of contact with air, the cotton cellulose molecules break down to form gaseous hydrocarbons, methyl alcohol, acetic acid and carbon di oxide.
ACTION OF LIGHT
Exposure of air in presence of sunlight for a long period will have an effect on cotton like that of heat.
Oxycellulose is gradually formed accompanied by tendering because of atmospheric oxygen. The tendering effect by light and air is accelerated by traces of metals like copper.
ACTION OF WATER
Raw cotton is very hard to wet because the wax present on the surface of the fibre i.e cuticle is difficult to wet.
Wax can be removed by scouring. So unscoured cotton will not absorb water so easily as scoured cotton.
ACTION OF ACIDS
Cold dilute solutions of mineral acids at boil have no effect on cotton cellulose, provided the acid are neutralized or washed out completely before drying.
However, if traces of mineral acids like 0.01% be allowed to dry in, tendering soon becomes apparent due to formation of hydrocellulose.
Cold concentrated sulphuric acid dissolves cellulose and forms cellulose hydrate.
If this solution is poured in cold water, the cellulose hydrate is precepted in a gelatinous form. This principle is used for parchmentizing paper to give a transparency effect with higher strength.
Hydrochloric acid affects cotton much more severely from sulphuric acid. Degradation is more rapid and severe in presence of hydrochloric acid than sulphuric acid.
ACTION OF ALKALIES
One of the main advantages of cotton is its resistance to alkali solutions. Mild alkalis like sodium carbonate have no action on cotton in the absence of air either at low temperature or at high temperature.
However, in presence of oxygen or air, oxycellulose is formed with gradual tendering in cotton.
Dilute solution of strong alkalis like sodium hydroxide with concentration of 2%-7% can be boiled without least tendering in absence of air.
Generally, dilute solution of sodium hydroxide is used for scouring i.e. removal of waxy and other impurities from cotton fibre. The scouring process purifies cellulose and imparts hydrophilic character and permeability to cotton fibre.
In this range, the fibre will have moderate swelling depending upon concentration of alkali used.
Strong alkalis with higher concentration include structural and physical changes in cotton fibre.
ACTION OF MICRO-ORGANISMS
Many micro-organisms attack cotton. Numerous fungi cause mildew. The mildew discolours, rots and weakens the fibre.
Most fungi reproduce by means of spores, largely present in air and are attracted to cotton wherever found.
Certain bacteria also micro- biological and appear to be set its cause under water logged conditions.
DEFINITION OF MERCERIZATION
A treatment of cotton yarn or fabric to increase its luster and affinity for dyes. The material is immersed under tension in a cold sodium hydroxide (caustic soda) solution in warp or skein form or in the piece, and is later neutralized in acid. The process causes a permanent swelling of the fibre and thus increases its luster.
It is the process of treatment of cellulosic material with cold or hot caustic conditions under specific conditions to improve its appearance and physical as well as chemical properties.
END USES OF COTTON
Fluff: absorbent cotton, padding.
Clothing: pants, shirts, blouses, dresses, suits, jackets, undergarments.
Home furnishings: Bed sheets, pillow covers, mattress, curtains, towels, table cloths, napkins.
REFERENCES
1. A Textbook of Fiber Science and Technology – S.P Mishra
2. Textiles: Fiber to Fabric by Bernard P Corban
By:
Mr. Kiran Kumar. P
(Asst. Senior Professor, Textiles)
Garden City University
Bengaluru
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