Yarns containing elastane are continuing to make significant inroads into the knitted fabric
market. The high stretch and rapid recovery properties of elastane provide unsurpassed elastic
recovery properties which have been utilised by fabric and garment makers to provide elastic
stretch and impart garment body clinging, shaping and shape retention, and silhouette shaping
and slimming properties. Indeed, it's now estimated that 35- 40% of all apparel in developed
markets may contain some elastane in filament form.
However fabrics containing polyester/elastane yarns present some formidable practical
processing problems in dyeing and finishing, and dyers and finishers must adhere to the
recommended guidelines from the fibre producers and from dyemakers in order to achieve the
best performance. With single wrapped or double wrapped elastane with polyester filament, the
dyeing process is designed to dye the polyester only, so careful dye selection is absolutely
imperative.
Elastane filaments are well-known to be sensitive to high temperatures, to chlorine bleaching,
and to biological attack from microorganisms. Heat setting is a vital stage in the wet processing
sequence. Polyester/elastane is generally heat set in the temperature range 182-188°C in order to
minimise the possibility of yellowing occurring at higher temperatures, while the heat setting
time of 20-35 seconds should not be greatly exceeded.
Elastane filaments are normally supplied with 2-5 per cent of lubricant mainly based upon low
molecular weight polydimethylsiloxane. Other lubricants are used on polyester so that the
polyester/elastane yarn requires scouring prior to dyeing. Pre-heat setting can remove some of
the fibre process lubricants through volatilisation, and these oils can condense and sometimes
give rise to oil spots when processing subsequent fabrics.
Aqueous or solvent scouring can be utilised for removal of the lubricants. Aqueous scouring in
soft water using special surfactants to prevent redeposition of the removed silicone lubricant is
widely used, although this does not remove all of the lubricant. Solvent scouring is more
thorough, using perchloroethylene, but this can remove not only more of the silicone, but also
some of the additives in the elastane which are present to improve the resistance to heat and
ultraviolet radiation.
An essential aspect of dyeing polyester/elastane weft knitted fabrics is to minimise the tension at
high temperatures. If this is not ensured then the fabric will be stretched and the stretch/recovery
properties will be adversely affected. For this reason jet dyeing machines of the soft-flow type
are normally used and the normal practical precautions during dyeing must be carried out to
prevent rope marks and creases.
Dye selection is the key to successful dyeing of polyester/elastane fabrics. Dyers should aim for
right-first-time dyeing, which may necessitate careful preparatory work in the dyehouse
laboratory on the specific polyester/elastane fabric to be dyed. It is generally advisable to avoid
making shading additions in dyeing because the extended dyeing time at the dyeing temperature
can adversely affect the fabric elastic properties.
Disperse dyes have a high substantivity for elastane fibres, and this staining is increased with
high dyeing temperatures. For this reason polyester/elastane fabrics are dyed at temperatures
below 130°C which is normally used for 100 per cent polyester fabrics. Even when using dyeing
temperatures of 115°C there will be severe disperse dye staining on the elastane and the elasticity
of the elastane can be decreased.
It is therefore essential to give an alkaline reduction clear after dyeing. Commonly this is carried
out at pH 10-11 and 70-80°C using either sodium hydrosulphite (dithionite) or alternatively
thiourea dioxide. The treated fabric must then be washed well to remove the dye decomposition
products and any other impurities to ensure that the highest standards of colour fastness to wet
treatments, light and rubbing (crocking) are attained.
When polyester microfibre/elastane fabrics are to be dyed to medium and heavy depths, the
fineness of the polyester microfibres leads to a large increase in the surface area of the polyester.
Consequently the dyed microfilaments appear lighter and much more disperse dyestuff is
required, particularly for heavy depths of colour. As a result the disperse dye staining on the
elastane can be intensified, requiring intensive after-treatment to remove staining and surface
dyestuff.
An alternative approach to the use of disperse dyes with their attendant heavy staining problems
on elastane is to use a more expensive polyester fibre, namely, cationic-dyeable polyester
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