- 1 Fibre Structure
- 2 The Effect of Internal Fibre Structure on Textile Fibre Properties
Fibre structure is the main aspect which directly affects the majority of fibre properties. Textile fibre is the smallest visible unit in any textile material. An essential characteristic of fiber is its high ratio of length to diameter; the shape.
The constituted material alone does not define that it is a textile fibre. Because materials can exist in the form of both molded plastic and fibre.
Textile Fibre can be defined as “Units of matter characterize by fineness, flexibility and a high ratio of length to thickness.”
1 The shape of Textile Fibre
Some examples of length: diameter ratios are as follows.
|Type of fibre||Typical length||Typical diameter||Length: diameter|
To be used as a textile fibre for manufacturing clothes, length: diameter ratio should be more than 1000:1.
However materials with length: diameter ratios above 100:1 are considered as fibres. These coarser fibers are mostly used for cordages and packaging.
Natural fibres have unique characteristic cross-sectional shapes whereas man-made fibres have a variety of different shapes. Its influence on fibre properties.
Lustre, bulk, body, texture, hand feel, etc. depend on cross-sectional shape. Natural fibres have unique cross-sectional shapes. The shape of the man-made fibres is controlled by the spinnerette and the spinning method.
Generally, flat fibres give better luster and covering power but harsh, unpleasant hand feel. Circular fibres give better hand feel but poor covering power.
2 Internal Fibre Structure
Different textile fibres have different chemical constituents. Monomers polymerize to different degrees to make polymers. A chemical constituent of the polymer is mainly responsible for the chemical properties of textile fibers.
Within the fibre structure itself, Polymers are aligned more or less parallel to the longitudinal axis of the fibre. Bundles of polymers form microfibrils, fibrils and finally a fibre.
The physical arrangement of polymer chains in fiber structure and polymer chain length is mainly responsible for the physical properties of textile fibres.
3 Molecular Orientation in Fibre Structure
The molecular orientation of fibres is the alignment of long-chain molecules relative to the fibre axis. Molecular orientation governs many fibre properties.
The arrangement of molecules in natural fibres is characteristic of each fibre type and practically unalterable.
In Man-made fibres, it is possible to control the degree of orientation through stretching and hence modify fibre properties.
When fibres are extruded through the spinneret, they consist of randomly arranged molecules. If molecules are stretched in one direction, they slip over each other and arrange themselves in line with the fibre axis.
Upon stretching, fibre properties are modified as follows.
- Tenacity – the better the orientation, the better the tenacity.
- Elongation – higher the orientation, lower the extension at the break.
- Brittleness – many oriented fibres are brittle.
- Lustre – high degree of orientation is accompanied by increased lustre.
- Moisture absorption – higher the orientation, lower the moisture absorption.
- Chemical stability – higher the orientation, better the chemical stability.
- Dyeing affinity – Lower the orientation, better the dyeing affinity.
- Fabric handle – Oriented fibres sometimes have an unattractive harsh handle.
4 Crystalline and Amorphous Regions
An orderly arrangement of molecular chains can be seen in crystalline regions. Molecules are well packed and lesser internal spaces are available.
Crystalline regions impart higher strength due to the better orientation of molecules to the fibre axis but show poor water penetration properties due to lesser internal spaces.
In amorphous regions, molecular chains are randomly arranged and hence more free spaces. Therefore amorphous regions are easily penetrated by water but impart lower strength due to the poor orientation of molecules.
The Effect of Internal Fibre Structure on Textile Fibre Properties
1 Effect Of Arrangement Of Molecules On Tenacity
Long chain molecules are arranged more or less parallel to the longitudinal axis in the fibre. When tension is applied, molecules parallel to the longitudinal axis take their fair share of the load result in a high breaking load.
Molecules lying approximately at right angles to the longitudinal axis take little or none of the load hence result in low breaking load.
Higher the orientation, higher the tenacity.
2 Tenacities of Cotton and Flax
Both flax and cotton are cellulosic and chemically indistinguishable. Possibly average cellulose polymer chain length in flax is slightly greater than that of cotton.
However, the tenacity of cotton is about 2.5–3.0 g/denier whereas that of flax is about 5.5–6.0 g/denier. Elongation at the break for cotton is about 6-7% and for flax is about 1.5-2.0%.
This difference in physical properties is mainly due to the arrangement of the molecules. In flax, they are highly oriented, very well parallelized and lie side by side along the length of the fibre.
In cotton, some of the molecules lie parallel to the fibre axis, but quite a large proportion of them lie at an appreciable angle to the fibre axis.
When the load is applied to the fibre nearly all the molecules in flax take their fair share of the load and result in a high breaking load.
But in case of cotton, the load is taken by molecules which are roughly parallel to the fibre axis. Molecules lie approximately at right angles to the fibre axis take little or none of the load.
Therefore the tenacity of cotton is less. Probably the parallel molecules in cotton are actually ruptured by the time the other molecules get aligned with the axis.
In both cotton and flax molecules are arranged spirally along the fibre axis. The angle of spirality of cotton is about 300 and that of flax is about 50.
In case of cotton when the load is applied, molecules pull more into line with the axis of the fibre and result in higher elongation at break.
In case of flax when the load is applied, little energy is utilized for parallelization of molecules and hence lower elongation at break.
3 Effect of Molecular length on Strength
If the load required to break a molecule is less than the sum of the loads required to separate it from adjacent molecules, then the molecules will break.
If the load required to separate the molecules is lesser, then the molecules will slide over each other.
Area of attraction that has to be broken is greater in the long molecules than in short molecules. Therefore fibres consist of longer molecules are stronger than fibres consist of shorter molecules.
However, once the chain length has reached the stage at which the energy required to make a molecule slide over its adjacent molecules is greater than that required to break a molecule, then further increase in molecular chain length will be without effect on the fibre strength.
Therefore up to a certain point fibre strength will increase along with increasing molecular chain length.
The actual strength of the fibre is very much lower than the theoretically calculated strength of the fibre based on the known strengths of the bonds. This is due to faults in the fibre structure which create weak places.
Higher the molecular length, higher the tenacity.
4 The Effect of Side Chains
If the fibrous molecule is long, straight and flexible, upon stretch it can be orderly arranged. But if molecules have bulky side chains, these can considerably obstruct the orderly arrangement of molecules.
When fibres are stretched, some sort of order may arrive. But there will be no stability because the side chains will be under considerable strain.
So that when the tension is released, side chains will pull back the long chain molecules into their original positions; hence fibre acquires rubber-like elasticity.
Side chains impart better elasticity.
5 The Effect of Cross-Linking
Cross-linking of molecules prevents molecules sliding over each other and gives good recovery from strain; so that cross-linked fibres do not readily crease.
Wool and other natural hair fibres show better crease recovery due to cross-linking. In resin-finishing process also this principle is applied.
6 The Skin Effect
The surface of the fibre is different from the core or inside. This difference has been described as a greater degree of orientation or a greater degree of crystallinity obtains in the skin of the fibre.
When the spinning solution passes through the spinneret, the part which is in contact with the sides of the orifice will be subjected to more frictional forces than the solution in the center of the orifice.
Skin orientation protects the fibre from too easy wetting.
7 Anisotropic Swelling
Anisotropy – different properties in different directions. e.g. fibres.
Isotropy – similar properties in all directions. e.g. gelatin.
Anisotropic swelling – when a fibre is immersed in water, water molecules infiltrate into the fibre and find their way in between the long-chain molecules, so pushing them apart.
As chain molecules are oriented in line with the axis of the fibre and they have pushed apart, there will be a considerable increase in the diameter of the fibre, but very little increase in the length.