Heat Setting Process:
The most general reason for heat setting is to achieve the dimensional stability of a yarn or fabric containing thermoplastic fibers. Heat setting is a heat treatment by which shape retention, crease resistance, resilience and elasticity are imparted to the fibers. It also brings changes in strength, stretchability, softness, dyeability and sometimes on the color of the material. All these changes are connected with the structural and chemical modifications occurring in the fiber. Heatsetting also reduces tendency to develop creases in a fabric, for example, during washing and hot ironing.
Heat setting is carried out at high temperature, usually with hot water, steam, or dry heat. The selection of the heat setting method depends on the textile material itself and the desired setting effect, and very often, of course, on the equipment available. Relaxation of tensions within the textile material results in shrinkage.
Heat-setting process is only used for synthetic fabrics such as those made from polyester or their blends to make them dimensionally stable against subsequent hot processes. Other benefits of heat-setting include less fabric wrinkling, low fabric shrinkage and reduced pilling tendency. Heat-setting process involves subjecting the fabric to dry hot air or steam heating for a few minutes followed by cooling. The temperature of heat-setting is usually set above the glass transition temperature and below the melting temperature of the material comprising the fabric.
Fabrics made of polyester and polyamide can be heat-treated to eliminate the internal tensions within the fibers. These tensions are generally formed during manufacture and further processing such as weaving and knitting. The fibers’ new relaxed state is fixed (or set) by rapid cooling after the heat treatment. Without this setting, the fabrics might shrink and crease during later washing, dyeing and drying processes.
Heat-setting can be carried out by hot-air-setting, hydro-setting or steam-setting. Hot-air-setting is by far the most popular heat-setting method, especially for polyester fabric. The process is usually carried out in a pin stenter, which holds the fabric at a pre-determined width by two rows of moving pins and feeds it into a heated chamber at 180°C–220°C for 30–60s, where it is set.
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This operation is crucial for fabrics made of synthetic fibers (PE, PA, elastomers), for triacetate, and partly for PAC fibers (setting), since it grants excellent dimensional stabilization and crease proof 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).
Heat setting is carried out on gray fabrics (scarcely applied), on scoured fabrics (frequently applied) and on dyed fabrics (scarcely applied). 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.
In either routine heat setting may be carried out before dyeing, since it confers a very good set on the fabric and therefore reduces considerably the
possibilities of running marks when dyeing in rope form. The risk of dye sublimation is also eliminated. The above routines are independent of the yarn type and of the polyester content of the blend within the recommended blend compositions, including 100% polyester.
Heat Setting Parameters of Some Fibers:
|Fiber||Min T. °C||Max. T. °C||Time in sec|
|Polyamide PA 6.6||170||210||15-40|
|Polyamide PA 6||160||180||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 fibers or yams present in the fabric. Heatsetting after dyeing could lead to the sublimation of disperse dyes (if not accurately selected).
Grey heat-setting is useful in the warp knitting industry for materials that can carry only small amount of lubricants and for goods that are to be scoured and dyed on beam machines. The other advantages of grey heat-setting are: yellow color due to heat-setting can be removed by bleaching, fabric is less sensitive to crease formation during subsequent processing etc.
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Heat setting can be carried out after scouring if it is suspected that the goods will shrink or for the cloth in which ‘stretch’ or other properties are developed during a carefully controlled scouring process. However, this stage requires drying the cloth twice.
Heat-setting can be carried out after dyeing also. Post set fabrics show considerable resistance to stripping compared with the same dyeing on unset fabric. The disadvantages of post setting are: yellow color developed cannot be removed any more by bleaching, handle of the cloth may get altered and there is a risk of colors or optical brighteners be faded somewhat.
Heat Setting of Different Blended Fiber Fabrics:
New fibers and newer blended fabrics have been produced and these are also to be heat-set for successful dyeing and finishing operations.
1. Polyester / cotton blends:
Short staple polyester fibers are often blended with cotton or some other cellulosic fibers. Normally heat-setting in stenter with hot air is carried out at 180oC for 30 secs. Higher temperatures may discolour the cellulosic portion of the material. For increased stability, setting may be carried out even at 200oC for 30 secs without serious risk of damaging the cellulose. The free weft-shrinkage of a typical polyester/cotton shirting fabric is about 4% at its normal setting temperature, but this is restricted to 2-3% (i.e. 1-2% residual shrinkage is allowed) in order to ensure the removal of creases and to get control over weft straightness.
2. Polyester/wool blends:
Polyester/wool blended fabrics are normally heat-set before dyeing. Similar effect is obtained on the wool component on the blend by crabbing. Polyester/wool blended fabrics can be heat-set in a hot air stenter for 30 secs at 180±10oC.
For worsted-spun goods 3-5% relaxation shrinkage is allowed both warp and weft and for woollen-spun materials 1-2% is allowed. The wool should be allowed the normal moisture regain before heat-setting. For blends containing more than 40% polyester content, heatsetting is done with adequate overfeed to avoid shrinkage during dyeing operations. Top dyed wool rich blend qualities of polyester/wool fabrics having wool content above 67% need not be heat-set as adequate dimensional stability can be imparted by autoclave decatising popularly known as K.D.Finish. After setting, the goods may be damped or steamed to restore normal equilibrium moisture content of the wool as rapidly as possible. The sequence of operations is slightly different, according to whether a clear-cut or milled finish is required; for clear-cut finishes, heat-setting may either precede or follow dyeing.
For milled finish, it is recommended to carry out heat-setting after milling and dyeing. Singeing should follow heatsetting, unless the fabric is to be milled, when singeing precedes heat-setting.
3. Polyester/linen blends:
Polyester/long-staple fibers are also used in the linen industry, where the yarns may be of either the ‘stretch-broken’ or ‘unbroken’ type, but are more commonly of the latter. Heat-setting of such fabrics may be carried out at 180oC for 30 secs on the hot air pin stenter, allowing up to 2% weft shrinkage with overfeeding only to compensate for any warp shrinkage that has occurred in the previous process.
4. Polyester/silk blends:
Heat-setting of polyester/silk fibers blended fabrics can be carried out on stenter at 190oC for 30 secs. The introduction of up to 50% polyester does not effect the characteristics drape and handle associated with silk fibers.
5. Polyvinyl chloride/cellulosic fibers blends:
For blended fabrics with cellulosic fibers it is possible to produce heat-embossed blend fabrics with 25% polyvinyl chloride fibers. Heat treatment is carried out by one passage on pin-stenter without overfeed at 85-90oC and if for example a 15-17% shrinkage is to be achieved, the speed of travel is about 10 m/min.
Requirements of Heat Setting:
- Heat the fabric to within about 20–40°C of the fiber melting point.
- Hold at this temperature under tension for approximately 20 s.
- Cool fabric before removing tension.
The time–temperature relationship will vary depending on the polymer, the fabric weight and construction. The simple way to determine these is to look at the shrinkage of the finished product, if the material has been correctly heat set then it should show a residual shrinkage of less than 1% on a 5 min immersion in boiling water.
Heat-setting is exclusively done on stenter these days, since it permits good control of the width of the goods and can be operated with a given overfeed. Besides, stenters are widely used for stretching, drying, 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 250oC depending upon the thickness and type of the material. I have published a comprehensive article on Stenter Machine, you can read it.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.