PHYSICAL PROPERTIES OF COTTON

Properties of cotton, cotton properties, properties of cotton fibre, physical properties of cotton fibre, physical properties of cotton


Some impotent properties of cotton


STAPLE LENGTH

Staple length is one of the important primary properties of any textile fibre. The staple length of cotton varries from 1 cm to 8 cm for d classes, which is shown in Table
Staple length of cotton
1. Sea Island 5.0 cm and more
2. Egyptian 3.8 cm-4.4 cm
3. Brazilian 2.5 cm-3.8 cm
4. American 2.5 cm-3.0 cm
5. Indian 2.0 cm-2.5 cm
6. China 1.5 cm-2.0 cm

FIBRE FINENESS

The wall thickness of different types of cotton ranges from 3.5 micron to 10 micron. Ribbon width is said to range from 12 micron to 25 micron. The thickness part of a fibre is not at the base but it is at the middle. The tip end is usually gently tapered. The base end is slightly finer than the middle portion.

PHYSICAL PROPERTIES OF COTTON


FIBRE UNIFORMITY

Cotton cannot be considered a uniform material even though sufficiently large number of fibres may have a characteristic average behavior. Each fibre must be regarded as an individual with its own characteristic length strength, fineness and other properties. For this reason, sampling methods are extremely important and test data must be handled by statistical method.
It has been observed that longer cotton tends to become uniform in length than the shorter ones. The varying percentage of immature fibre also indicates non-uniformity of wall thickness for the same variety of fibres. Also, there are considerable differences between cotton grown from the same seed in the same location from time to time

POROSITY

Cotton fibre is porous and exhibits capillary effects to a higher degree. The fibrils themselves are dense as a result of the higher packing density average
 of the molecules and so non-porous. This part of the structure constitute approximately 70 % or more of the fibre . The arrangement of denser fibrils in the fibre may be visualized as analogous to the packing of fibres in a well made yarn. So the porosity of the unoccupied space in the fibre ranges from 20-40 % of the fiber volume. The fine cottons are more compacted than the coarse variety. Also, the lumen is generally small, about one third of the unoccupied space.



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. It depends upon the ratio of semi- major and semi-minor axes of the elliptical fibre cross-section. If the ratio is be low, the lustre will be high. The highest lustre is noticed in the fibre with circular cross-section. So the dominating influence in lustre is the external fib re 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
.


DENSITY

Cotton fibre has a density of 1.54 gm/cc, which corresponds to a specific volume of 0.64 cc/gm.


MOISTURE

The amount of moisture in cotton depends on the relative humidity and temperature of the air to which it is exposed. The moisture is more sensitive to relative humidity than temperature. At higher temperature, there is a small change in moisture and cottons retain constant moisture over small change in temperature. The moisture pick - up is about 5.8 % at 40 % humidity ( R.H. ) , which can be increased to 120 % at 90 % RH and at 25 ° C . At 65 % RH and 22 ° C, the moisture pick - up is around 8.3 % . At higher humidity, the fibre absorbs more moisture as a result of breakage of hydrogen bonds in non-crystalline region and availability of more hydroxyl groups. Also, at higher relative humidities, the absorption occurs on top of the directly absorbed water. This is generally referred to as 'indirect sorption' or solid solution'. Also, at saturation, there is considerable swelling of the fibre



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 mature fibres of intermediate and fine types is between 4 gn to 9 gm per fibre. On the other hand, immature fibre strength can be as low as 0.5 gm to 1.0 gm per fibre.


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 5 % to 10 % , exactly around69 -to 8 % . In the structure of the cotton fibre , the fibrils spiral round at an angle , of about 20 to 30e to the fibre axis. In general, increasing the reduces the resistance for extension.

 MODULUS

 Modulus is generally related to the resistance to deformation. Up to a certain limit of deformation, the stress and strain follow Hooke's law i.e. strains is proportional to stress. The proportionality between stress and strain is referred to as modulus or elastic modulus. In case of cotton fibre, the elastic modulus means little unless the exact history of the sample is known. The modulus of cotton fibre is about 500-525 g.wt/tex. The stress-strain relation for a single fibre is roughly a straight line when the fibre contains little moisture and in this case, Hooke's law is valid up to the breaking point.

TOPSIONAL RIGIDITY


The mean rigidity of cotton fibre is about 7.9x10 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 coarse cottons must be more highly twisted than fine cottons to produce yarns of the same size.  

   




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