INTRODUCTION TO HEAT SETTING

Heat setting is carried out to bring the material to it’s thermal equilibrium the material wouldn’t changes it’s morphology it’s kept at that temperature for a long.

Heat setting is also known as ANNEALING.

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.

Mechanism of Heat Setting• The setting temperature used is

above Tg. In heat setting, inter-chain bonds, such as hydrogen and dipole bonds, break. This allows the molecular chains to move and adopt new, stress-free positions. New intermolecular bonds then form with the fabric in a relaxed condition at the setting temperature. After cooling, the polymer molecules in the filaments become frozen in place. The new bonds are stable up to the heat setting temperature. The reorganized internal polymer structure, and the material’s dimensions, will be stable.

Objectives

The objectives of heat setting processes include

structure homogenization and the elimination of

internal tensions within the fibre resulting in reduced

shrinkage, improved dimensional stability, reduced

creasing propensity and reduced edge-curl in woven

and knitted fabrics. To this extent, the process may be

better described as thermal relaxation. Heat setting

changes not only the mechanical, but also the dyeing

properties of man-made fibres. The principle is based

on heating the fibre within a fibre-specific temperature

range which is limited at the upper end by the melting

point (softening range) and the respective glass transition

temperature (necessary to break the secondary

bonds) at the lower end.

WHY HEAT SETTING IS DONE ….?

Heat setting is a heat treatment by which shape retention, crease resistance & elasticity are imparted to the fibres.

It also brings changes in strength, stretchability, softness, dyeability & sometimes on the colour of the material.

All these changes are connected with the structural & chemical modifications occurring in the fibres.

TYPES OF HEAT SETTING

Heat setting

Temporary heat setting

Semi-permanent heat setting

Permanent heat setting

TEMPORARY HEAT SETTING-

This type of heat setting is destroyed by regular use of material.

For ex:- A steam pressed cotton textiles.

SEMI-PERMANENT HEAT SETTING-

In this type of heat setting material is raised above their glass transition temperature & then set into a new form.

This type of set is maintain in normal use of material however the setting is lost when the material is subjected to severe condition of use.

For ex:- hot washing or steaming of material above Tg.

PERMANENT HEAT SETTING

• This type of heat setting involves change of internal morphology of the material in such a way that it would not revers till the material is destroyed by taking it above it’s melting point.

Heat Set Temperature of some Fibres

Fibre Min T. °C Max. T. °C Time (in seconds)

Polyester (PE) 170 210 15-50

Polyamide PA 6.6 170 210 15-40

Polyamide PA 6 160 180 15-40

Triacetate 160 180 15-40

Acrylic (PAC) 160 180-200 15-40

Elastomers 170 180-200 15-40

Stages of heat setting

Heat setting can be carried out at three different stages:

• In grey condition

• After scouring

• After dyeing

Heat setting at grey stage

• If heat setting is carried out in loom state, mineral oils and

non-ionic emulsifiers can modify the fiber.

• Structure and rubbing and perspiration fastness may be

reduced due to the solubility of disperse dye in the coning oil.

• However, grey heat setting is useful in warp knitting industry

as in that material carry less amount of lubricants.

• Yellow color due to heat setting can be removed by bleaching.

• Fabric is less sensitive to crease formation in next processes.

Heat setting after scouring

Heat setting is done after scouring if it is suspected that:

• goods will shrink .

• Cloth has developed stretch or other properties after scouring.

Heat setting after dyeing

These fabric shows considerable resistance to stripping compared

with dyeing on unset fabric.

Disadvantage of this are:

• Yellow color developed can not b removed.

• Handle of cloth may get altered.

• Risk of color to get fade.

Mechanism used for Heat SettingMachines used: Stenters.

Fluctuating temperatures inside the stenter cause a consistent variation of

crystallinity in the fibre structure, which leads to different affinity for dyes.

The moisture in the fibre produces soft hand, but variable moisture

percentages in the different fabric sections create the above mentioned defect

(variable crystallinity).

Too low temperatures do not allow a good setting while too high

temperatures and too long

setting times cause yellowing (PA and elastic fibres), stiff hand (acrylics), and

loss of elasticity (elastic fibres).

References

• Book- VB Gupta & VK Kothari.

• http://www.slideshare.net/sheshir/heat-setting?qid=51840e8d-4fdc-4c2f-a012-753c10478363&v=&b=&from_search=1

• file:///D:/Forms,%20Results%20&%20docs/SEMINAR/NPTEL%20__%20Textile%20Engineering%20-%20Textile%20chemical%20Processing_%20Theory%20and%20practice%20of%20Preparatory%20Processes.html

• Encyclopedia of textile finishing.

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