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September 2016

Photo Credits: Wikimedia

Grading of Cotton Fibres

The quality of cotton fibre differ from place to place and plant to plant. The difference in quality can be expressed in grading and staple length. Grade is generally determined from three factors viz. (a) colour, (b) trash content and (c) ginning quality.

 

Colour

Best cotton is generally white in colour. White cotton loses its brightness and it becomes yellowish in nature Because of continued exposure to weathering and the action of microorganisms. Following are the colour groups which are present in cotton fibre:

White

 

 

Light Spotted

:

Lt Sp

Spotted

:

Sp

Tinged

:

Tg

Yellow Stained

:

YS

Light Grey

:

Lt Gy

Grey

:

Gy

 

Trash Content

Trash is the impurities which are present there in the cottonfibres. The trash includes materials such as leaf, stems, hulls, bark, seeds, shale, motes, grass, sand, oil and dust. Cottons which contain minimum amount of trash after ginning have highest spinning value. Depending upon the trash content, cotton can be graded as follows:

Strict Good Middling

:

DGM

Good Middling

:

GM

Strict Middling

:

SM

Middling

:

M

Strict Low middling

:

SLM

Low middling

:

LM

Strict good Ordinary

:

SGO

Good ordinary

:

GO

 

Sometimes, depending upon the trash content, plus (+) can be given to any grade like SLM+ or SGM+.

Quality of Ginning

In ginning process, we separate the cottonfibres from seed.  During this process there is a chance of formation of entanglements of fibres (neps). Neps are adversely affecting yarn and fabric appearance as well as quality. Presence of neps and naps are two important factors to determine the quality of cotton. Neps are small tangled knots of fibre that are visible as dots. This type of cotton is known as neppy cotton. Naps are large clumps or matted masses. of fibres that contribute to the rough appearance. This type of cotton is known as nappy cotton. The quality of ginning is considered as better if it produces zero or minimum number of neps.

 

In general, the grading indicates the trash and colour of the cottonlike LM Tg, M Lt Gy etc.

 

Photo Credits: Wikimedia
BOTANICAL CLASSIFICATION OF COTTON
CottonFibres belong to the botanical genus 'Gossypium'. Further Cotton is classified in four names as per the places of cultivation like,
Gossypium Arboreum
:
India
Gossypium Herbaceum
:
Middle East
Gossypium Barbadense
:
Peru, Egypt
Gossypium Hirsutum
:
America, West Indies

COMMERCIAL CLASSIFICATION OF COTTON
Cottonfibres are classified commercially according to the source they are; as their staple length. There are six types of cotton available. Those (b) Egyptian cotton, (c) Brazilian cotton (d) American cotton, (e) cotton.
Sea Island Cotton
Sea Island cotton comes originally from Barbadoes, hence it has the Barbadense'. It is the most important cotton and is grown in USA, Florida. It is a long, fine, soft and silky fibre. This cotton is in length or twist. The colour is of a light creamy tint. The staple length is around 5cm or more.
Egyptian Cotton
This cotton is available in Egypt and Middle East countries. These fibres are also long, fine, soft and silky like that of Sea Island cotton with slight inferior qualities. The staple length of this cotton fibre is in between 3.7 to 4.5 cm.
American Cotton
American cotton is grown in USA and south of North America. The staple length of this cotton fibre is in between 2.5 to 3.5 cm.
Indian Cotton
This cotton is available in India. The quality is generally poor with lower staple and coarser diameter. This cotton is generally white in color. The staple length is in the range of 2-3cm.
China Cotton
This type of cotton is generally available in china only. The quality of this cotton is poorest and it cannot be used for finest quality of fabrics. The staple length is 1.5-2cm only.

CLASSIFICATION OF FIBRES ACCORDING TO LENGTH OF THE FIBRE

Cottonfibres can also be classified according to its length. Being natural fibre, inherent variation exists the properties of fibre. Classification of fibres according to length of the fibre is as follows:



Cotton is a seed hair fibre. It grows in the form of a single cell merging from the epidermis or outer layer of cotton seed. Each flower of cotton plant may produce 20-25 seeds enclosed in a green boll. When the growth ceases (stops), the boll splits. The fibres grow in a tubular form, with a well-developed wall enclosing the lumens running down the centre. When the boll splits, the moisture inside it evaporates. As drying proceeds, the wall of the fibre shrinks and collapses, on drying and collapsing of the fibre, the cylindrical cross-section is converted in to convoluted ribbon form with the flattening of the ribbon Morphological structure of cotton consists of four parts. These are:
  • Cuticle
  • Primary Wall
  • Secondary Wall
  • Lumen
Following figure shows the morphological structure of cotton fibre.

Morphology of Cotton Fibre
Figure 1 Morphology of Cotton Fibre
Photo Credit: NPTEL
Figure 2 Morphology of Cotton Fibre
Figure 2 Morphology of Cotton Fibre

Cuticle

The cuticle of the cotton fibre is a very thin layer tightly attached to the outside of primary wall. More accurately, cotton fibre is enclosed in cuticle which protects the fibre from any mechanical and chemical damages. The cuticle consists of cotton wax, mixture of fats, waxes and oils. During initial stages of growth, the cuticle appears as an oily film. During the later stage, the cuticle becomes hard like a varnish.

Primary Wall

The primary wall is built up from cellulose. It also contains pectineus substances. The cellulose appears to concentrate from the growth period and increases proportionally later stage of cell elongation. On the surface, the molecular chains in the primary wall are arranged in a random manner without any orientation and definite order. However, cellulose present inside the primary wall is in the form of fine threads or fibrils, when observed through microscope. The fibrils are not parallel to the fibre axis but spiral at an angle of about 700 round the fibre axis. The spirals do not reverse in their direction; the spiral angle is greater at the tip and smaller at the base. The diameter of the cotton fibre is fairly constant throughout the length except at the base and the tip. The diameter of the fibre is in the order of 15-20 microns, whereas the primary wall is very thin and about 0.1-0.2 micron thick.

Secondary Wall

It is composed mainly of cellulose and contributes most of the weight to the fibre, in general, within the primary wall, the bulk of the fibre consists of secondary wall. Like primary wall, it consists of concentric layers of fibrils in spiral formation. The outer layers of secondary wall, deposited near the primary wall are built up of fibrils at spiral angle of about 200-300 The fibrils in the subsequent layers are finer than former and the spiraling angle is about 200- 450 The spiral angle changes slightly in magnitude between the outside and the inside. The spirals also change their direction of rotation at frequent intervals along the fibre length at the reversal point, they simply form a curve. Always the second set of fibril begins in the opposite direction. In all the layers, the fibrils tend to follow a closely similar pattern. Arrangement of chain molecules in different parts is shown in fig. 1.

Lumen

At the centre of the growing fibre, there is a lumen, which remains as cylindrical void at maturity. The area is about 30-35% of the total area of cross section. The lumen content evaporates after the boll splits. After drying and collapsing of the fibre, the area of lumen is reduced to about 5% of the total area. Of course, there is variation from fibre to fibre. In the dried state, lumen contains colouring matters apart from other impurities, which decides the colour of the fibre.

Convolutions

After bursting of the mature boll, the wall of the fibre shrinks and collapses, on drying and collapsing of the fibre, the cylindrical cross-section is converted in to convoluted ribbon form with the flattening of the ribbon. Due to the spiral structure, collapse results twisting of the fibre about axis. The direction of rotation convolution change at irregular interval along the length of the fibre at places determined by reversal point in pattern of fibrillar spiral in secondary walls. The convolution arises in order to relieve the internal stresses during the drying and collapse of cotton fibre and as shown in figure below:

Figure 3 Convoluted Cotton Fibres
Figure. 3 Convoluted Cotton Fibres

Maturity of Cotton Fibre

When the cotton fibre starts developing on cotton plant, initially it is like a hollow tube having only primary wall. As the time passes secondary wall depositions will go on taking place on inside pad of the primary wall and the hollow tube will start filling up.

When the cotton fibre is fully developed, ideally the secondary wall occupies 85-90% of the cross section of the fibre leaving a small capillary at the centre called as Lumen. The secondary wall gives the strength and rigidity to the fibres. In a fully developed fibre called as mature fibre the secondary wall development is complete (85-90% of the cross section) and lumen size is small thus these fibres are having more strength and rigidity and therefore better mechanical properties.

Many a times, due to certain reasons, the secondary wall development does not take place fully and then the fibres remain under developed or immature. Due less development oi the secondary wall in immature fibre the size of the lumen is bigger. The immature fibres therefore have lower strength and they are less rigid or more flexible. Due to this higher flexibility these fibres form more entanglements during processing which results in neps (entangled mass of fibres). Thus immature fibres will give yarns with lower strength and higher neps. Due to less secondary wall deposition the immature fibres will also pickup less dye during chemical processing.

1 Load
It is the weight applied to a specimen which causes a tension to be developed in the specimen.
2 Breaking load
 It is the load at which the specimen breaks, usually expressed in grams’ weight.
3 Stress
It is the ratio of the force applied to the cross-sectional area of the specimen.
4 Mass stress
The cross section of many fibres are irregular in shape and difficult to measure. To simplify matters, a dimension related to cross-section is used, such as the linear density of the specimen.
The linear density of a specimen may be expressed in denier or tex unit; then the mass stress becomes the ratio of the force applied to the mass per unit length. The units of mass stress, therefore, become gram per denier (gpd) or gram per tex (gpt).
5 Tenacity
The tenacity of a material is the mass stress at break. The units being, of course, grams per denier or grams per tex. An alternative term for tenacity is a specific strength. It is useful to note that by expressing the breaking strengths of different materials in terms of tenacity, a comparison can be made directly between specimens of varying fineness.
6 Strain
When a load is applied to a specimen, a certain amount of stretching takes place. The amount will vary with the initial length of the specimen. The strain is the term used to relate the stretch or elongation with the initial length. This is expressed in percentage.
7 Extension
It is the ratio of extended length to the original length of the specimen, expressed as a percentage. The extension is sometimes referred to as the strain percentage.
8 Breaking extension
The breaking extension is the extension of the specimen at the breaking point.
9 Initial modulus
It is the ratio of breaking stress to breaking strain. Young’s modulus gives a measure of the force required to produce a small extension. A high modulus indicates inextensibility and a low modulus indicates great extensibility. In textiles, the term initial modulus is used to describe the initial resistance to extension of a textile material. In other words, this is a measure of fibre’s resistance to small initial extension. When the fibre has high resistance to stretching, it will have high modulus. High modulus indicates inextensibility and brittleness. Low modulus fibre requires little force to stretch it. It is also an indication of flexibility. The initial modulus is a very important part of stress–strain curve. If the initial portion of the curve is straight, it indicates a linear relationship between stress and strain. The material behaves like spring and obeys Hook’s law. The significance is that when the load is removed the material recovers its original length. If the stress is in units of grams per denier, then the initial modulus will also be in grams per denier because the strain is a number and is dimensionless.
10 Elastic recovery
It is defined as that property of a body by which it tends to recover its original size and shape after deformation. Elastic recovery is often expressed as a percentage.


Perfectly elastic materials will have an elastic recovery of 100% while perfectly plastic materials will have a zero recovery (i.e. 0%). The elastic recovery of different fibres shows considerable variations. Also, the same fibre may show variable recovery depending upon the degree of extension and the relative humidity.
11 Elongation and elastic recovery
The amount of extension or stretch that a fibre accepts is referred to as elongation. Elongation at break is the amount of stretch a fibre can take before it breaks. Elastic recovery indicates the ability of fibres to return to their original length after being stretched. A fibre with 100% elastic recovery will come back to its original length after being stretched to a specific degree for specified period of time. After removing and re-measured.
12 Load-elongation curve
The load-elongation curve describes the mechanical response of the specimen from zero load and zero elongation up to the breaking point. From a close study of this curve important information on its mechanical behaviour, like elastic modulus, yield point, tenacity and elongation can be obtained.

Figure 1 Load-elongation curve of different textile fibres
13 Moisture regain
Moisture regain is defined as the weight of water in a material expressed as a percentage of its oven-dry weight.
14 Moisture content
Moisture content is the weight of water in a material expressed as percentage of its total weight.
15 Physical shape
Shape of a fiber include, its longitudinal sections, cross section, surface contour, irregularities and average length.
16 Luster
It refers to the sheen or gloss that a fiber possesses. It is directly proportional to the amount of light reflected by a fiber. This in turn is affected by their cross section shape. Among the natural fibers, silk has a high luster, and cotton has low.
17 Specific gravity/ Density
The specific gravity of a fiber is the density related to that of water (at 4 degree celsius ). The density of water at that temperature is 1. fiber density will affect their performance and laundering. If the specific gravity of a fiber is less than 1, it will float in water, making its washing and dyeing very difficult. E.g. Olefins fiber. A related property is density which is defined as the mass per unit volume and measured in g/cm3.
18 Absolute humidity
Absolute humidity is defined as the weight of water present in a unit volume of moist air.
19 Relative humidity (R.H.)
Relative humidity is defined as the ratio of the actual vapor pressure to the standard vapor pressure at the same temperature expressed as percentage.


Ashish Hulle

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