Singeing
The aim of singeing is to bum-off the protruding fibres and hairs from the fabric surface. The spinning process produces hairiness of the yam and lower the yarn counts (Ne 26/1 - 36/1) greater is the degree of hairiness. Therefore the reasons for singeing should be considered more closely.(i) Singeing improves the end use and wearing properties of texiles.
(ii) The burning-off of protruding fibres results in a clean surface which allows the suructure of the fabric more clear.
(iii) Singeing reduces the fogginess caused by differing reflection of light by the projecting fibre and the dyed fabrics appear brighter.
(iv) Singeing is an effective means of reducing pilling in blended fabrics containing synthetic fibres.
(v) Unsinged fabrics soil more easily than singed fabrics.
(vi) A closely singed fabric is essential for printing fine intricate patterns.
(vii) Singeing process facilitates and speeds up desizing, if the fabric is impregnated with desizing liquor immediately after singeing.
On the other hand there are singeing faults which are not visible and once occurred can no longer be repaired. They are :
(i) Uneven singeing effect can cause streaks when the fabric is dyed, or bubbles when the fabric is finished.
(ii) In the cotton system singeing is done on the grey cloth, but for blended fabrics containing synthetic fibres grey state singeing is not advisable because small globules of melted synthetic fibres absorb dye preferentially,giving cloth a speckled appearance.
(iii) There is a possibility of thermal damage to temperature sensitive fibres, for instance polyester.
(iv) Stop-offs can cause heat bars on fabrics. Creasing produces streaks which is magnified when dyed.
(v) Protruding fibres are firmly bound by singeing on the surface by the sizing agent by hardening of the size and can lead to difficulties in desizing.
(vi) When singeing is done after dyeing, heat can cause colour loss from polyester portion of the blend because of sublimation of dye.
(vii) There may be reduction of tear strength due to over singeing of the fabric.
Generally, singeing is done on both sides of the fabric. No chemical change occurs in the fabric during singeing and the reaction is basically one of oxidation. Singeing and desizing can be frequently combined by passing the singed cloth through the water bath which include enzymes. The enzymes digest the various sizing agents, making it easy to remove them during the scouring operation. The
combined process also prevents possible latent damage to fabric from the singeing flame or heated plates.
Singeing different kinds of fibres fabrics
In singeing the short fibres are burnt off from the surface of the fabric by direct or indirect heating systems without damage to the cloth by scorching or burning. The thermal behaviour of different kinds of fibres are different and singeing at higher temperature is naturally associated with greater hazards on excessive contact period and may cause thermal degradation of the fibre. In case of vegetable fibres, grey singeing is necessary as it leads to slight yel-
lowing which needs subsequent bleaching to get high degree of whiteness. Grey singeing is also economical as singeing at any other stages of processing requires additional washing and drying. Vegetable and regenerated fibres fabrics can be singed very strongly with maximum burner intensity to obtain good results. Regenerated fibres normally bum to a little less easily than natural fibres.Wool has poor combustion properties and are very sensitive to temperatures and hence woollen materials are not subjected to intense flame like cotton. In woollen fabric flame is not generally allowed to penetrate the material and this can be obtained by blowing air through the fabric from the opposite side of the flame so that the flame will be restricted only on the surface of the fabric. Alternatively, the fabric can be guided to water cooled guide rollers allowing the flame to heat the cloth. When the flame strikes the fabric it is reflected by air/steam cushion created within the material. Amongst the synthetic fibres polyester has the greatest significance. It melts at 280-290~ but does not burn till about 500~ 'Reflector' or 'refractory' singeing machines produce smears of fused polymer on the surface of the polyester cloth and therefore unsuitable for polyester material. Thus flame singeing machine with a powerful flame is needed and also helps in overcoming the problems of oligomers i.e. the small chain polymers that come to the surface. High temperature singeing process may sometimes change the glass-transition temperature (Tg) of synthetic fibres that lead to uneven dyeing.For blended fibre fabrics singeing conditions are to be selected depending on the sensitiveness of the kinds of fibres to heat, blend composition, weight of fabric and fabric geometry. For example, singeing should be carefully conducted to avoid heat damage of the acetate component of the accetate/viscose blended fibre fabrics.Though singeing improves the restistance to pilling of the polyester/wool blended fabrics, but should not be carried out on low weight fabrics because of risk of damage. In the case of 80/20 polyester/cotton blended fabrics or fabrics with dented pattern, sometimes, the cloth gets weekened only in the thinner or shadow portion. This is not detected until the material reaches to finish folding. If singeing is carried out after dyeing the sublimation fastness of disperse dyes used must be adequate to
withstand the singeing conditions. Singeing machineries are mainly based on direct and indirect singeing systems. The cloth is first passed over one or more steam-heated copper drying cylinders to
remove the moisture and to raise the nap. The direct singeing may be done either on a hot plate, or on a rotary cylinder, or on a gas singeing machine or on a machine combining plates and gas burners. The special features of indirect singeing systems
are no flame contact, uniform singeing, heat retention zone and singeing by means of heat radiations. The indirect system produces fabrics which have a softer touch as compared to other methods. Gas singeing is more convenient, more economical and more effective than other methods and is well accepted commercially. The plate singeing and roller singeing machines are now out of date. Clean singeing of potentially troublesome synthetic fibres can be achieved by using the scavenger tubes which are specifically designed to avoid thermoplastic melt (beading) prob-
lems.
Plate singeing machine
The machine consists of two copper plates of about 1-2" thick. The plates are 58 Preparation Before Chemical Processes heated to bright red from the opposite side. The furnaces are fired by heavy petroleum oil, which is vaporised through special nozzles, by the action of steam, into a fine spray. The properly dried cloth passes over the plates at speeds up to 200 yards per minute, according to quality. A special traversing arrangement for the cloth is supplied to prevent local cooling of the hot plate. The singed cloth is then run
through water to extinguish the spark, passed through a padding mangle and delivered to a winch or plaiter. The machine is driven by electric motor, steam engine orbelt pulleys.
Rotary cylinder machine
In this type of singeing machine the cloth passes over a hollow cylinder which revolves slowly in the opposite direction of the goods. The hollow cast iron or copper cylinder may be fired internally. The cloth is drawn through the machine by a powerful squeezer nip, singed, run through water bath and plaited.
Gas singeing machine
Old singeing machines are often used which in their time worked acceptably when only pure cotton materials were singed, but which, however are no longer suitable for singeing of newer fibres and their blends. New technologies and techniques have been developed to suit the modem requirements.