Natural Fibers

COTTON

STRUCTURE

Cotton is a seed hair. Each cotton bal containing 20-25 seeds enclosed of green pod. When fiber grows, the ball splits, the moisture inside it evaporates. This misshapes the fiber structure. As drying increase the wall of the fiber shrinks, collapse, lumen become smaller and the fiber develops convolution.

CUTICLE

Thin layer- attached outside of the primary wall. Cotton fiber enclosed of cuticle, it protect the fiber from the mechanical and chemical damages. It consist of cotton wax and a complex mixture of wax, oils and fats. During fiber growth it appears to be oily film. Later stage during drying cuticle becomes hard like a varnish.

PRIMARY WALL

            Primary wall builds up of cellulose. Cellulose is most widely spread natural compounds on the world. Cellulose has three alcoholic group and hydrolysis leads to the glucoseglucose.

                                             ( C6H10O5)n+ n H2O →[ C6H12O6]

Growth            – proportionally during fiber

Arrangement   – random manner

Cellulose         – form of fibril or  micro threads

Fibril axis        – not parallel to fiber axis

Angle              – 70 degree round the fiber axis

Angle                          -greater at tip and low at base

Fiber dia          – constant generally. Except base and tip

Dia                  – 15-20 micron

primary wall thickness = 1- 0.2 micron

SECONDARY WALL

                                                     - Contributes the most of the weight of the fiber

LUMEN

At the center of growing fiber there is a lumen. The area is about 30 -35 % 0f fiber area. The lumen content evaporate after the ball splits. After drying it reduced to 5%. Lumen contains coloring matter it decides the colour of the fiber.

PHYSICAL PROPERTY

Length : 

Length of cotton fibre varies form ½” to 2½” and depends on the variety.

Lustre : 

Due to the wax present in the raw cotton, it has little or no lustre.  Bleached cotton is slightly more lustrous, while mercerised cotton is quite lustrous, especially when stretch mercerised. (MERCERISATION BASED)

Strength : 

Cotton is moderately strong fibre.  Its strength is due to the presence of crystalline regions (65 - 70%), a fibrillar structure and a moderate degree of orientation.  The dry tenacity is 27 to 44 gf/tex (9 gf/tex = 1 gf/d).  The wet tenacity is 30 to 48 gf / tex.  Cotton thus shows a 10% increase in tenacity when wet.  This is an advantage as no special care in fiber handling is required during wet – treatment.

Elastic – Plastic Nature (Visco –elastic behavioru) :

 It is inelastic (poorly elastic) due to its crystalline polymer system; cotton fabric thus wrinkles and shrinks easily unless specially finished to overcome this tendency.

Elongation:  

 Cotton has an elongation – at – break of about 7%.  The elongation increases by about 10% when the fiber is wet.

AMORPHOUS AND CRYSTALLINE REGION

Hygroscopic nature : 

Cotton fibre is very absorbent.  This is due to the presence of polar –OH groups which attract water molecules

Moisture regain :  Cotton has a moisture regain of 7 – 6.5% in standard atmosphere (65% relative humidity and 200C).

Heat conductivity :  Cotton is a good conductor of heat.  It therefore feels cool to the touch and is ideally for summer wear.

Specific gravity :  Cotton has specific gravity 1.50 – 1.54

Color :  Its color varies from white of cream, depending on the variety and cultivation conditions.

Diameter :  The fibre diameter (ribbon width) various from 11mm to 22mm.  The finer cottons are generally longer and the coarser ones shorter.

Microscopic appearance :

Longitudinal view :  Mature cotton looks like a flat, twisted, ribbon or a collapsed twisted tube.  A large number of convolutions are usually found.

Immature cotton also looks like a flat twisted ribbon but has a lower number of convolutions. Mercerised mature cotton is almost cylindrical in shapes and has very few convolutions.

Cross – sectional view :  Mature cotton has been or kidney – shaped cross section, and a small lumen.  Immature cotton also has a bean – shaped cross section but a larger lumen.  Mercerised cotton looks almost round to oval, a small point at or near the centre representing the lumen.

Length of breath ratio : 

Cotton has a length to breath width ratio ranging from 1000 : 1 to 6000 : 1.

Porosity :  Cotton is a porous fibre.

Special Property :  Cotton fibre consists of characteristic convolutions or twists along its length that are randomly left handed or right handed and number 40 – 65/cm.

CHEMICAL PROPERTIES

Chemical composition of raw cotton : 

a)      Cellulose                     =88 -97 %

b)      Protein                         = 1 – 2%

c)      Oil and wax                = 0.4 – 1.5 %

d)     Pectin                          = 0.4 – 1.5 %

e)      Minerals                      = 0.7 – 1.6 %

f)       Others                         = 0.5 – 0.6%

Polymer (or) Fibre forming Substance :   

                                    Linear cellulose polymer –(C6H10O5)n =10000

Fine Structure :          

            Crystalline region=65 to 70%,     Amorphous region =35 to 30%

Effect of Acids : 

Cotton fibers are weakened and destroyed by acids.  This is due to the hydrolysis of cotton polymer at glucoside oxygen atoms and the formation of hydrocellulose.  The DP is lowered by such hydrolysis and this results in lower strength.

•         Concentrated acids like HNO3, H2SO4 and HCI destroy the fibre easily.

•         Cold dilute acids like HNO3, H2SO4 and HCL destroy the fibre easily.

Organic acids have little effect on cotton

Average degree of polymerization of cotton cellulose :  about 5000 celloblose units.

Effect of Alkalis : 

            Cotton fibres are much more resistant to alkalis. Concentrated alkalis do not normally harm cotton.  Concentrated sodium hydroxide in particular causes the cotton fibre to swell.  The fibre increases in thickness and contracts in length.  This process is known as mercerisation and may be effected with or without tension to the material.  A better alignment of molecules in the polymer system and an increased formation of hydrogen bonds due to the mercerisation leads to better properties in general.  Mercerisation under tension causes the fibre to become smoother, more regular and nearly cylindrical.  The cross sectional shape of mercerised cotton is round to oval and the almost absent.  The longitudinal view appears cylindrical and has only a few convolutions.  

Effect of Sunlight :  Cotton loses its strength and has a tendency to become yellow with prolonged exposure to sunlight.

Affinity of dyestuffs :  Cotton is considered to be a relatively easy fibre to due and print.  This is due to the presence of polar chemical groups in the polymer.  Direct, napthol, vat, reactive and sulphur dyes can be applied to cotton.

Resistance to Microbiological Attack :  Cotton is readily attacked by mildew and bacteria if stored in hot humid conditions.  Sized cotton is very readily attacked.  Mildew affected cotton shows yellowish stains, reduced strength and uneven dyeability.

Effect of bleaching agents :  If bleaching agents are used with care and control with regard to concentration, pH, temperature and time, cotton is not damaged.  If any or these variables goes out of control, chemical damage occurs, oxycellulise is formed and the fibre weakens.  Cotton is readily bleached by solutions of sodium and calcium hypochlorite peroxide and sodium chlorite.

Effect of Water :  Cotton is not affected by water even under boiling condition.  It can thus withstand normal washing and laundering treatments repeatedly.

Effect of Metallic salts :  Cotton is not affected by metallic salts.

Effect of heat :  Cotton can withstand dry heal up to 1400C.  At 2450C, it scorches and burns.  Cotton textiles can thus withstand domestic ironing without yellowing provided that their contact with the hot iron is kept to the required minimum

USES OF COTTON

Cotton is used extensively throughout the world in the form fibre, yarn and fabric.  Its used in various forms are given below:

1.  Fibre Form:

•         Surgical cotton used in the medical field:  Wadding (fibre-fill) used in pillows and mattresses.

•         Cotton Linters:  Cotton linters are the very short fibre bits removed from the seed during the second ginning process.  The linters are too short to be spun into yarn, but are useful in the manufacture of writing paper, varnishing celluloid, photographic films, regenerated celluosic and cellulose-ester fibres.

2.  Yarn Form:   Used as sewing threads, cords, tapes and twines.

Fabric Form:

•         Apparel Field: Dress wear and inner garments.

•         Household: Bed sheets, towels, carpet and bags.

•         Industrial: Used in the manufacture of tyre-cord fabric, canvas sheeting, book binding cloth handgloves, tarpaulins tent cloth, conveyor belts, tapes, non-wovens.

Some of the reasons for this are:

•         Cotton is considered as Nature’s most economical fibre.  Due to its relatively low price, cotton is still the cheapest fibre for clothing purposed.

•         Cotton fibres can withstand moderately rough handling and water at boiling temperature.  It can therefore be laundered and ironed regularly.

•         Though cotton textiles tend to soil easily due to the presence of fine protruding fibres, they can also be easily washed and cleaned.  Cotton is thus a very hygienic fibre.

•         Cotton has is increasingly preferred psychologically to man-made fibres for dresswear; a natural fibre feels better and ‘safer’ next to the skin than an ‘artificial’ fibre.

Other Fibers

        1. Jute

        2. Silk

        3. Wool

       4. Viscose Rayon