What is Heat Setting?
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. Different methods of heat setting also help to get a flat fabric with required weight and standard width. All these changes are connected with the structural and chemical modifications occurring in the fiber. Normally heat setting done for lycra fabric.
Besides, Heat-setting process is 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.
Heat setting has to be done before any hot wet processing to avoid curling of the selvedge and discolouration (patches) during dyeing. Heat setting after wet processing may not give the required final fabric and will have problems of wet creases not cleared, less stable, less white than the pre-heat set fabric due to less smooth, flat surface of the finished fabric.
Once the fabric is adequately relaxed, it can be heat-set. Heat setting can be done before or after the fabric is scoured, bleached, or dyed; however, some shades of colour, including white, may yellow when subjected to high heat setting temperatures. This is especially true if greige fabric is heat-set. If heat setting is performed on greige goods, spinning oils, waxes, and knitting oils may cause discoloration or yellowing that cannot be removed in subsequent scouring and bleaching processes. In this article I will discuss different types and methods of heat setting.
Methods of Heat Setting:
Different methods of heat setting used in textile industry are descibed below.
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.
Steam-setting method
Short staple polyester yarns including polyester/cotton blends are normally set by relaxation in saturated steam. The most effective means of stabilizing 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 stabilized 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.
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 using stenter machine
Stenter machine are widely used for stretching, drying, heat-setting and finishing of fabrics. Woven and knitted fabrics of polyester and nylon fibers and their blends are normally heat-set on pin-stenter in hot air.
An alternative to the pin stenter is the clip machine. The fabric is held into the chains either by pins mounted into a base plate or by clips in which the fabric edge is clamped between two smooth surfaces. Stenters that are used for setting only have a light pin chains whereas stenters used for both drying and setting (finishing) are provided with a heavy combined pin and clip chain.
Selective infra-red emitters method
Polyester can be heat-set by exposing the material under selected areas of magnetic spectrum of infra-red rays. The wavelength of the radiation source must be chosen with respect to the absorption band of the fiber i.e. a particular infra-red wavelength is chosen for a particular fiber. For example, in the case of polyester the selective infra-red radiation wavelength is the region of 1 to 4 g.
References:
- Chemical Technology in the Pre-Treatment Processes of Textiles, Volume 12by S.R. Karmakar
- Textile Engineering – An Introduction Edited by Yasir Nawab
- Pretreatment of textile substrates by Mathews Kolanjikombil
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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.
