1. Introduction
It is also called anealing (Heat setting) of the drawn fibres in an important post spinning operations.It is carried out to bring the marerial to its thermal equilibrium at a perticular temperature of use.At thermal equilibrium,the material would not change its morphology even if kept at that temperature for a long time.The degree of set,a term often used to describe the extent of heat setting is the measure of how close the material has moved to a thermal equilibrium.A 100% set material is considered to be at its thermal equilibrium at a given temperature.
2. Types of Heat Setting
There are three types of heat settings :
- Temporary Heat Setting
- Semi Permanent Heat Setting
- Permanent Heat Setting
1. Temporary Heat Setting
This type of setting destroyed with regular use of the materials,for example , a steam pressed cotton textile.
2. Semi Permanent Heat Setting
In type of heat setting material raised above its its Tg and then set in to new form,this type of set is maintained with normal use of the material,however setting is lost when the material is subjected to severe conditions of use ex-hot washing or steaming above Tg.
3. Permanent Heat Setting
This type of heat setting involve change of internal morphology of the material in such a way that it would not reverse till the material is destroyed by taking it above its melting point.
3. Parameters of Heat Setting
The permanent heat setting is the most important for the three described above as this can be used to stablize the material in its final form.
a) Temperature of heat setting
Heat setting involves formation of new crystallites in the amorphous part of the fibre. The formation of crystals is a thermal process and maximizes at a certain temperature.Crystallization involves nucleation followed by a growth of the nucleated crystallite. At a high temperature oriented chains tend to relax fast and fibre can loose its orientation if left at that temperature for a sufficiently long time.
b) Time of heat setting
As stated above ,the rate of crystallization is dependent on the temperature of heat setting and the orientation of the drawn yarn,also the disorientation is a function of time.keeping in view the two,the time is decided so that thermal equilibrium is reached in a minimum time with minimum loss of the orientation.
c) Tension given to the material during heat setting
The drawn yarn may be subjected to heat in loose(free) or taut form.In free form the chains can disorient at a faster rate and fold to form amellar crystals at the heat setting temperature.Applying tension at the time of heat setting restrict the rate of folding of polymeric chains,which gives enough time for small crystallites to grow along the length of oriented chains.This improves connectivity among oriented chains as many chains are involve in a single crystallites. Heat setting under tesnsion is able to retain mechanical properties of starting material.
d) Moisture in the starting material and its sensitivity to it
Certain polymers like nylon are sensitive to moisture.Their Tg becomes lower as their moisture content increases,since moisture acts as a plastisizer for these materials,their chains may relax and fold easily when in wet state compared to the dry state.On the other hand dry heat setting increase in crystallinity and also closeness of packing in the amorphous region result in poor dyeability and diffusion coefficient of heat set material.
e) Morphology of starting material
The morphology of starting material has significant effect on the effectiveness of the heat setting process. If material is already crystalline, it may not undergo setting to retain a deformed state.
4. Effect of Heat Setting on Morphology of Fibre
Heat setting involves bringing the material to its thermal equilibrium. This is achieved when a material,which has tendency to crystallize can crystallize to the maximum extent possible with given structure. As crystallization proceeds, a time comes when further conversion of amorphous region to crystalline region result in increasing the overall energy of the system, the conversion stop and a thermal equilibrium s reached.
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