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.
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.
MODULUS
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 fibreSTRENGTH
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.
