Heat Setting

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                           Heat Setting

Heat setting is a term used in the textile industry to describe a thermal process usually taking place in either a steam atmosphere or a dry heat environment. The effect of the process gives fibersyarns or fabric dimensional stability and, very often, other desirable attributes like higher volume, wrinkle resistance or temperature resistance. Very often, heat setting is also used to improve attributes for subsequent processes.

Heat setting can eliminate the tendency of undesirable torquing. At the winding, twisting, weaving, tufting and knitting processes, the increased tendency to torquing can cause difficulties in processing the yarn. When using heat setting for carpet yarns, desirable results include not only the diminishing of torquing but also the stabilization or fixing of the fiber thread. Both twist stabilization and stabilization of frieze effect are results of the heat setting process. Heat setting benefits staple yarns as well as bulked continuous filament (BCF) yarns. Heat setting often causes synthetic fibers to gain volume as well. This volume growth is commonly described as “bulk development”. All processes using temperature and/or moisture to give textiles one of the above-mentioned attributes are known as heat setting. The term “thermal fixation” is used less frequently. In the carpet industry, the process is exclusively called “heat setting”.

What is Heat Setting?

Heat-setting is a heat treatment by which shape retention, crease resistance, resilience and elasticity are imparted to the fibres. It also brings changes in strength, stretchability, softness, dyeability and sometimes on the colour of the material. All these changes are connected with the structural and chemical modifications occurring in the fibre.
This operation is crucial for fabrics made of synthetic fibres (PE, PA, elastomers), for triacetate, and partly for PAC fibres (setting), since it grants excellent dimensional stabilisation and creaseproof properties, maintained till the fabric is exposed (by air blowing) to temperatures exceeding the heat setting one (after being treated with water at a temperature above the second order glass transition temperature, i.e. 80-85°C for acrylics).

INTRODUCTION TO HEAT SETTING

  1. 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.
  2. 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.
  3. 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.

Working Process of Heat Setting

                                              Heat  Setting Process

Heat setting of Some Fibers

Fibre

Min T. °C

Max. T. °C

Time in sec

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 a processing sequence i.e. in grey condition; after scouring; and after dyeing. The stage of heat-setting depends on extent of contaminations and types of fibres or yams present in the fabric. Heat setting after dyeing could lead to the sublimation of disperse dyes (if not accurately selected).

Stenter Machine
Stenters are widely used for stretching, drying, heat-setting and finishing of Fabrics. The stenter frame is usually 80-100 feet long and 70-100 inches wide. The speed ranges from 10-45 m/min with a maximum setting time in the setting zone 30 sec at temperature ranging from 175 to 250~ depending upon the thickness and type of the material.

Methods of Heat Setting:

1.Contact method-
In this method the fabric is run in contact with a heated metal surface. Some machines are composed of metal rollers having gas fired cores and are filled with a liquid known as diatherm to uniformly distribute the heat. Sometimes enclosed rollers are heated with high temperature steam.

2.Steam-setting method-
Short staple polyester yarns including polyester/cotton blends are normally set by relaxation in saturated steam. The most effective means of stabilising these materials are to steam at 107~ on the ring spinners tube and soft dyeing packages under minimum tension. Sewing threads receive special setting treatments, designed to confer stability whilst preserving their high tensile properties. Polyester garments, garment lengths and hosiery are also stabilised by steaming in much the same way as for yams. Nylon can be set in saturated steam at temperatures above 1OO°C in an autoclave by batchwise process.

3.Hydro-setting method-
The hydro-setting or aqueous heat-setting of polyester is done with hot water in a high temperature liquor circulating machine at about 130°C. A typical cycle may require 30 min. Water (or steam) promote swelling of fibre and may cause some hydrolysis in the ester groups in polyester chain. Nylon fabric can be hydro-set in hot water since the swelling action assists in weakening or breaking intermolecular bonds.

Heat setting of polyester:
Heat-setting is a heat treatment by which shape retention, crease resistance, resilience and elasticity are imparted to the fibres. It also brings changes in strength, stretchability,  softness,  dyeability and sometimes on the colour of the material. All these changes are connected with the structural and chemical modifications occurring in the fibre.Heat setting is the process applicable to fabrics made from synthetic fibres like nylon, polyester in which the fabric is subjected to the action of high temperature for a short time to make it dimensionally stable so that the garments made from such fabrics retain their shape on washing and ironing.The main aim of heat setting process is ensure that fabric do not alter their dimensions during use.

Three different stages of heat setting:
Heat-setting can be carried out at three different stages in a processing sequence i.e.

  1. in grey condition (scarcely applied)
  2. after scouring (Frequently applied); and
  3. after dyeing (Scarcely applied) .

The stage of heat-setting depends on extent of contaminations and types of fibres or yams present in the fabric. Heat setting after dyeing could lead to the sublimation of disperse dyes (if not accurately selected).

The process grants excellent dimensional stability and good crease-proof properties. As far as operating conditions are concerned, the fabric must be treated in accurately controlled moisture and temperature conditions.

Heat setting of Some Fibers  

Stenter Machine
Stenters are widely used for stretching, drying, heat-setting and finishing of Fabrics. The stenter frame is usually 80-100 feet long and 70-100 inches wide. The speed ranges from 10-45 m/min with a maximum setting time in the setting zone 30 sec at temperature ranging from 175 to 250~ depending upon the thickness and type of the material.

Referance:https://en.wikipedia.org/wiki/Heatsetting