Introduction

LYCRA is a registered trademark used for DUPONT’s elastane fibres. Elastane is a generic term, like polyamide or polyester. It defines a manmade fibre in which the fibre-forming substance is a synthetic chain polymer containing at least 85% segmented polyurethane.

LYCRA elastane yarn has outstanding elastic properties – very high stretch and recovery as well as high power at stretch (modulus). Its other important characteristics include good abrasion resistance, long wear life, strength, long flex life, resistance to daylight, to weathering, to mildew and to most chemicals. In addition, LYCRA is white or transparent in its natural state, and it can be dyed.

LYCRA added to fabrics of non-elastic or “hard” yarns gives them stretch and recovery from stretch. This property makes them eminently suitable for a wide range of garments, an advantage which is recognized by the garment users, assuring the success of LYCRA with knitters, weavers and cutters.

LYCRA in the stretch fabric is knitted or woven under tension. When relaxed, it trends to compact the fabric length and width to a “jam point”. The “hard” fibre buckles when the fabric is “jammed”, and it limits the extension of the fabric when it is being stretched. Elastic fabrics are very often required wider and lighter than their width and weight at “Jam point”. To ensure complete control of the desired physical changes, the processor must first determine the parameters of the “jammed” (fully relaxed) fabric, namely  

  narrowest width, highest weight,

  corresponding wale and course or end and pick counts,

  greatest shrinkage and stretch in length and width.

For this purpose a marked grey sample is boiled for 10-15 minutes, then it is dried relaxed, and measured. This check is especially important for the development of new fabric styles.

Fabrics containing LYCRA elastane yarn require careful control of processing conditions to preserve the intrinsic elastic properties of the fibre, while obtaining the required fabric characteristics. These dyeing and finishing conditions should be chosen with care, because the performance of LYCRA elastane can be changed by prolonged hot/wet treatments, certain chemicals, excessive tension and high temperature. It is, therefore, necessary to be fully aware of the physical changes required to develop the desired weight, width, stability and stretch.

Tension, temperature, concentration of process chemicals and the duration of treatments must be kept to a minimum, because they affect the elastic properties and the appearance of the finished goods. In particulars, the tension must be kept to a minimum during those steps which are carried out at elevated temperatures, i.e. relaxation, hot wet processing, drying and curing. Although the use of high temperatures and tensions does not degrade LYCRA, fabrics containing it lose some power when they are hot – stretched and this loss cannot be recovered.

Storage of Grey Fabrics

Most off-loom fabrics stay for some time in the grey room prior to wet processing. Storage conditions and the handling of greys affect heir quality. Good storage can help, while bad storage can reduce the quality of the finished fabric.

Grey goods are doffed either in batches or in pieces and these ar sent to the grey room. Stacking is undesirable, because : 

  it will obstruct handling of bottom pieces that were stored first,

  the weight and pressure of the stack can use undesired moiré, folds or creases.

It is best to keep the rolls of grey suspended horizontally, one be one, on metal axle-tubes. Light pieces can be stored individually in boxes.

Storage of grey fabrics must be short to prevent yellowing or decay of LYCRA elastane by hard yarn finish or machine lubricants that might contain unsaturated fatty acids or fatty esters. The knitter, weaver, fibre supplier or coning oil producer should make sure by tests that the lubricants used do not discolour or degrade LYCRA during the normal course of processing.

Before grey goods are put in stock for an extended period, they should be relaxed, thoroughly scoured and batched up again at controlled low tension (10-20% stretch over relaxed fabric) in order to remove the winding tension which might cold-set the grey fabric, and to prevent pleats and creases from setting and becoming permanent.

If lag time is expected between operations, the fabric should be wrapped airtight, chemically inert covers, ideally black, to protect it from discoloration. This discoloration mostly affects batch edges and the external upper layers of unprotected fabric.

Finally, it is recommended to avoid storage of grey fabrics for more than two months and to always dispatch the oldest fabric first, so that the first fabrics into the grey room are first out to the dyehouse. 

Chemical Resistance

LYCRA elastane yarn withstands most of the conditions required for wet processing the other fibres with which it may be combined. However elastane fibres may be sensitive to certain chemicals used for rigid fabrics.

LYCRA keeps its elastic properties almost intact when subjected to mercerizing, to carbonizing, to alkaline sourcing at 95OC, to dyeing in strongly acid baths, to peroxide bleaching, to weak hypochlorite bleaching, to dry cleaning. One type of LYCRA elastane, i.e. LYCRA type 128C, reacts differently to certain of these agents, as reported in Technical Information Bulletin L-515.

Fabrics containing LYCRA are sensitive to unsaturated oils, greases, fatty acids and their derivatives, which tend to discolour and to degrade elatane yarns. Such compounds sometimes serve as lubricants for hard fibres and they vary from one yarn producer to another. Pine oil, used in printing cutting or barding, can also affect stretch fabrics. It is essential to make sure that these lubricants, if present, will not discolour or degrade LYCRA during the usual course of processing. Greys containing LYCRA which need storage for an extended period of time should be thoroughly scoured and dried to prevent any discoloration or degradation by an unsaturated finish. Further, the fabrics should be protected from any contact with grease during processing.

In general, the duration of treatments and the concentration of chemicals applied in the wet process should not exceed the minimum necessary to attain the best performance of the treated goods. The duration and concentration of bleaching baths, the duration and pH of dye baths, the duration of solvent scouring, the treatments which involve caustic soda or acids, the selection of carriers for LYCRA/polyester dyeing, as well as resin curing catalysts and conditions, need particular attention.

Fumes and other atmospheric pollutants may decrease the initial whiteness of fabrics containing LYCRA. Long exposure to ultraviolet light has a similar effect. This discoloration occurs more quickly in some localities than in others, because air pollution and climate vary widely. Such yellowing does not alter the wear performance of LYCRA, but fabrics or displayed garments may lose their customer appeal. During storage all fabrics and garments should be wrapped in chemically inert, airtight packing in order to prevent this effect.

 

Process Steps

Selection of a particular processing route depends on the desired appearance, the required performance of the fabric in use and on fabric composition. Stretch textiles include a large variety of fabrics, each with their particular features, and it is impossible to advise one general process route for all of them.

A standard discontinuous process suitable for most stretch fabrics comprises :

  RelaxationHeat – SettingScouringBleaching / Dyeing / PrintingFinishing

The first two steps involve LYCRA elastane directly and govern the future performance of the stretch fabric or garment, while the remaining steps must take into account the specific features of LYCRA in order to retain the best benefits of it.

The above standard sequence may not apply to certain woven fabrics or to hosiery. Different

procedures also apply to continuous processes.

 

Relaxation

A grey fabric containing LYCRA must be exposed in a relaxed condition to steam, hot water or warm organic solvent before dyeing and finishing. Relaxation of the grey fabric relieves residual stresses caused by tension of the LYCRA yarns during kinitting or weaving. These stresses can distort the structure, deform the design, and cause puckering of the fabric. Such effects show up especially in clear faced or regularly patterned woven and knitted fabrics. The potential shrinkage in the grey fabric should also be removed to obtain an evenly dyed and uniformly finished product.

A stretch fabric is therefore relaxed at an early stage of its finishing by one of the following methods :

Passing over a steam table

Steam framing

Solvent scouring

Hot water treatment

Steam table relaxation is preferred because complete relaxation can be achieved. The time of relaxation will depend upon fabric construction and rate of steam generation.

Steam framing consists in overfeeding a fabric over a steam box fixed at the stenter inlet. The stenter should then only dry the steam relaxed fabric. Combined steam relaxation and heat setting gives less uniform results than relaxation and heat-setting in separate steps.

Continuous solvent scouring serves to relax and dry clean fabrics at the same time. It provides a good widthwise relaxation and a controlled lengthwise relaxation of treated fabrics.

Relaxation by hot water occurs when a non heat-set grey fabric is washed or dyed without stress. This is an efficient way to relax the fabric, but it may leave permanent creases or interfere with the subsequent heat-setting of the relaxed textile.

Fully relaxed fabrics are stable to washing, but in most cases they are too narrow, too heavy, and prone to creasing.

HEAT – SETTING

Most fabrics which contain fine LYCRA elastane yarns, in particular knitted fabrics, need heat-setting to make a textile of satisfactory appearance with an adequate with an adequate

dimensional stability and flatness at a given fabric weight and width.

Grey yard fabrics can be heat-set stretched, if their width is less than the desired finished width. The off-loom width often exceeds the requirements and the fabric must be narrowed by relaxation before heat-setting.

Fabrics should be heat-set prior to hot wet processing in order to reduce shrinkage, control curling, and prevent discolorarion of goods. Setting after the wet process yields fabrics which are less stable, more curly and less white than the preset ones. Therefore, it is advisable to preset fabrics with LYCRA.

Heat-setting requires a controlled exposure of fabrics to heat, and flat stenter driers with hot air blown on to the fabric are commonly used for this purpose. Suitable stenters should allow the fabric to be evenly heated up to around 200OC for at leat 45 seconds. Precise control of fabric temperature inside the stenter is vital for good, and consistent, setting. Indirect or electric heating of stenter air is preferred to heating by gas or oil, because these fuels from gases which can discolour the fabric. Stenters must comprise a large span overfeed / underfeed device, preferably coupled with an automatic weight control, to adjust fabric weight and stretch as required. In the inlet section, the stenter should have an efficient steam box.

The selection of setting conditions is a compromise between many factors, most of them independent of LYCRA, chosen for a particular fabric. They are

 

  required weight, width or structure of fabric,

  type of garment to be made,

 key feature of the finished fabric, e.g. whiteness, stability, flatness or holding power

  colour of fabric

  type, origin, content, composition and count of hard fibre.

  Lubricant of hard fibre and its sensitivity to heat

  Finishing process and its sequence

  Make, size, heat source and set-up of the stenter

Table 1 shows how the setting temperature and time affect some of these factors.Table 1. Effect on fabric of heat-set temperature or time. 

  Property Temp increase Time increaeStabilityShrinkage

 

WhitenessFlatnessPower àWidth ÞLubricant spots NeglUniformity (PA 6) NeglSetting Cost Negl Þ

 Negl. = negligible

A small amount of shrinkage still remains in a fabric with LYCRA after heat-setting. For this reason it should be set 5% to 15% wider than its desired finished width in order to compensate for some shrinkage expected during wet processing. The actual setting width can be assessed by mill testing the heat-set efficiency of a fabric (see below). An efficient fabric cooler at the stenter outlet should fix the set and prevent any unwanted irregular fabric after-setting.

Dry heat in excess of 180OC will be needed to set fabrics which contain LYCRA elastane. Tests show that such fabrics usually require pre-setting for 30 to 70 seconds at temperatures ranging from 185OC to 195OC.

Excessive heat exposure will overset the fabric whilst too little heat will leave it underset. Under-setting of a fabric may lead to more shrinkage and curling, higher weight and narrower width than expected, whereas oversetting may discolour it, reduce its power too much or impair the dye uniformity of companion hard fibres.

Saturated steam can also set fabrics with LYCRA. It is mainly applied to hosiery partly set by boarding, which requires milder conditions, e.g. 30 to 60 seconds at 110-120OC (see technical information bulletin L-527). Saturated steam applied for 10-15 minutes at 120-130OC will also set batched piece fabrics. As this technique requires vacuum in an autoclave, and does not ensure a constant fabric width, it is seldom used.Further details of heat-setting procedures are contained in specific bulletins (see Technical Information Bulletins L-523, L-524, L-525, L-528). 

Heat-Set efficiency

The quality of heat-setting is measured by the heat-set efficiency test. Heat-set efficiency(HSE), or the retention of fabric heat-set width (HSW), can be checked on the stenter frame at the working temperature and time, as follows :

Heat-set a fabric sample. Note the width of fabric leaving the stenter, i.e. its heat-set width (HSW). Boil the sample 5 to 10 minutes in water, then dry it relaxed and check its dry finished width (FW). The ratio of widths after and before wet relaxation (FW:HWS) indicates heat-set efficiency (HSE). This figure can then be used to estimate the heat-set width a greige fabric requires to obtain a given finished width (FW).

For example, a test sample shrinks from 160 cm HSW to 144 cm FW. The required fabric is 152cm. How wide should it be set?

Required HSW value is a good approximation, but will not predict the small width difference which will come from the subsequent wet processes. For example, the width shrinks more on a winch than on a beam. Each mill must therefore make its own trials to establish these precise details.

 

Sourcing

Efficient removal of yarn lubricants and mill soil by sourcing of grey goods is an important step towards good dyeing and finishing. It can be done either in aqueous baths or by solvent removal (dry cleaning).

Although a simple detergent scour is usually sufficient for LYCRA, the actual scour must also take into account the companion hard fibre.

A typical procedure for scouring synthetics fabrics with LYCRA is given below.

Required HSW value is a good approximation, but will not predict the small width difference which will come from the subsequent wet processes. For example, the width shrinks more on a winch than on a beam. Each mill must therefore make its own trials to establish these precise details. 

    Set bath at 50OC using soft water, a long liquor to goods ratio (20:1) and add:0.5g/1 sequestering agent (water softener)1g/1 trisodium phosphate (TSP) or 2g/1 soda ash

1-2g/1 non-ionic detergent

 

   

Circulate bath and heat it to 80OC over 15 minutes. Run 30-45 minutes at 80OC. Cool to 50OC, drop bath or overflow rinse. Rinse hot and cold until clear and neutral.

It may be necessary to adjust the amounts of chemicals according to the type of hard fibre (e.g. cotton ) and / or the nature of mill water.

Heavily soiled grey fabrics should be scoured before heat-setting in a bath containing TSP and about 5cc/1 of solvent emulsion. The fabric is then scoured again 20 minutes at 80OC in a bath containing about 1g/1 non-ionic detergent / emulsifier to remove residues of solvent from the fabric. Persistent solvent odours can be removed by heating this bath to boiling point. Adequate ventilation is essential to prevent unsafe concentration of solvent vapours in the dyehouse. Failure to remove all flammable solvent from the fabric will cause a fire hazard in subsequent drying.

Technical Information Bulletins L-523 and L-525 contain more details on scouring relevant to particular fabric types.

 

Bleaching and Whitening

Bleaching is often desirable for elastic fabrics containing LYCRA and other fibres. Since the base colour of LYCRA is satisfactory for most end-uses, bleaching is either a matter of whitening companion fibres without yellowing the LYCRA, or removing from LYCRA the discoloration caused by atmospheric fumes.

The whiteness of “LYCRA” fabrics can be enhanced by:    reducing scour    reducing bleach    optical whitening

Reducing scour

The method can be used to improve the natural whiteness of LYCRA in a grey fabrics. A typical procedure is as follows :

Set bath at 50OC with1-2g/1 non-ionic detergent1g/1 trisodium phosphate (or2g/1soda ash)3-5 g/1 sodium dithionite (hydrosulfite)

Heat to 75-80OC. Treat 45-60 minutes at this temperature. Cool and drop bath. Run 15 minutes in a fresh bath containing :

0.5 ml/1 hydrogen peroxide 35%

Rinse cold.

 

Reducing Bleach

This technique applies more reducing chemicals and exerts a stronger bleaching action on a fabric containing LYCRA than the reducing scour.

Set bath at 50OCAdd 5 to 10 g/1 sodium dithionite (hydrosulfite)5 to 10 g/1 sodium metabisulfite

Heat to 80-85OC. Treat 45-60 minutes at this temperature. Cool, drop bath and rinse. Treat 15 minutes in a fresh bath with :

0.5 ml / hydrogen peroxide 35%

The reducing scour or reducing bleach methods also provide a good base white for application of fluorescent brightening agents (FBA) to fabrics containing LYCRA.

Bleaching with peracetic acid, the use of chorine-containing bleaches such as sodium hypochlorite, calcium hypochlorite, or sodium chlorite could yellow and / or degrade the LYCRA elastane fibre and should therefore be avoided.

Hydrogen peroxide bleaching of cellulosic fabrics is current practice for LYCRA, except for type 128OC (see Technical Information Bulletin L-525)

 

Optical Whitening

Fluorescent brightening agents (FBA) alone, or combined with tints, will impart the desired “ high white” to fabrics finished in white or prepared for printing. The choice of FBA must be made with due care to provide fabrics : - with high whiteness- with adequate fastness to light, washing and pool water- which do not turn yellow due to light or fumes- withstand the conditions of fabric processing e.g. a reducing bath or dry heat.

The selection of brightening agent must suit both the LYCRA and the hard fibre in the fabric.

For best results the whitener should be applied from a reducing bath, in combination with a stabilized hydrosulfite, e.g. Blankit IN (BASF) or Clarit PS (Cica-Geigy) at a pH of about 5.5.

Table 2 lists fibres which can be bleached or whitened in the presence of LYCRA.

Table 2. Bleaching / Whitening fabrics containing LYCRA

 

Optimum whiteness retention

To obtain the most durable white on fabrics of LYCRA, it is recommended to use FBAs fast to light and air pollutants, as well as a maximum amount of tint commensurate with customer shade acceptance.

The following conditions may cause loss of whiteness retention, and should be avoided:- unsatisfactory fluorescent brightener- drying with air heated by combustion (gas or oil)- unsatisfactory softeners- noxious fumes in the dyehouse atmosphere- poor storage of white fabrics or garments, namely : 

  excessive fumes (nitrogen oxides, burnt gas fumes)

  excessive light, either natural or artificial

  lack of airtight and inert packaging

  packaging which releases phenolic derivatives.

DYEING

LYCRA has affinity for many types of dyes. Acid, chrome, premetallized, disperse, and leuco vat dyes, as well as some reactive dyes and a few direct, sulphur and vat dyes, can all be used for dyeing LYCRA to varying degrees. Premetallized, sulphur and vat dyes exhibit good light and wash fastness as well as good colour build-up. Chrome dyes lack colour brilliance and, in some shades, light fastness is inadequate for outerwear goods. Disperse dyes give good results in pale shades, but their wet fastness in full shades is limited. However, many clolur fastness standards can be met, when the percentage of LYCRA is low.

Table 3 lists various classes of dyes and their properties on LYCRA (see limitations for LYCRA T128C in table 5)

Fabrics which contain LYCRA T-128C must not be dyed with sulphur, vat or chrome dyes, most reactive and most cationic dyes, because this “LYCRA” type is sensitive to strongly acid (pH ≤ 4) or strongly alkaline baths (pH ≥ 10)

Table 3 Coloration of LYCRA by dye type

Rating : VG = Very good, G = Good , F = Fair, P = Poor

Many acid dyes give an acceptable all-round fastness on LYCRA, but the faster dye strike and heavier build-up on its companion yarn can cause the dull LYCRA to grin through in dyed fabric. Bright and clear LYCRA yarns have greatly improved the colour wet fastness of stretch fabrics. These yarns are virtually invisible and do not grin through in the finished fabric, hence they can be left undyed. The choice of dyestuffs can thus best suit the companion hard fibre, if these dyes are compatible with the chemical and physical properties LYCRA. Table 4 lists hard fibres used in stretch textiles with LYCRA and the dye classes suitable to colour such fabrics (not applicable to “LYCRA” T-128C)

Table – 4. Dyeing fabric containing LYCRA (except T-128 C)

X = Suitable, S = Selected dyes suitable

LYCRA T-128C does not withstand certain chemicals and certain dyes procedures compatible with other LYCRA types. Table 5 gives the dye / fibre combinations suitable for stretch fabrics which contain “LYCRA” T-128C.

Table – 4. Dyeing fabric containing LYCRA T-128 C

X = Suitable, S = Selected dyes suitable

Dyeing fabrics of LYCRA combined with nylon, cotton, cellulosic, wool or silk is straightforward because :

- many dyes are suitable to colour them with good fastnes,,- bath temperatures of 100OC or less, needed to dye these fibres, do not affect LYCRA,- There are LYCRA products suitable for either acid or alkaline dye baths.

Fabrics of acetate or chlorofibre and “LYCRA” are also easy to dye using disperse dyes, but the wet fastness of medium to dark shades is moderate to poor.

Regular acrylic / LYCRA fabrics can be dyed to a complete range of fast shades using :

- selected cationic dyes which reserve LYCRA- LYCRA of clear or bright luster

Good light fastness and easy application of certain disperse dyes make them useful for dyeing LYCRA / acrylics to pastel and light shades. Disperse dyes should not exceed 0.5% on the fabric weight to prevent bleeding and poor wash fastness.

Acid-dyeable acrylics require dyeing with chrome or 1:1 premetallized dyes for good wet fastenss in full shades. This precludes the use of LYCRA T-128 C in such fabrics.

Polyester, triacetate and LYCRA dye with the same disperse dyes, but under very different conditions. Fabrics of polyester and LYCRA or triacetate and LYCRA are therefore difficult to dye without affecting the elastic properties of the fabric.

Polyester need bath temperatures of 120-130OC for adequate dye depth, but such baths overset the elastane and excessively reduce the power of the fabric. Dye baths of about 100OC do not affect LYCRA, but the polyester then requires dye carriers which also tend to weaken the tenacity and power retention of LYCRA. Aromatic acid esters cause limited LYCRA degradation, chlorobenzes and biphenyls cause a little more, whilst phenolic carriers are the most aggressive.

Stretch textiles often contain three fibres, for example LYCRA elastane, nylon and cotton or LYCRA, polyester and wool. The chosen dye procedure then has to compromise between the dyeing properties of all the components, the available dyes, and the required colour fastness of the fabric. Tables 4 & 5 outline the existing choice of dye types.

Table – 6. Stripping fabrics containing LYCRA (except T – 128 C)

X = In general, can be stripped,  S = Only selected dyes can be stripped.

 

STRIPPING

Fabrics of LYCRA sometimes need stain removal or stripping if they are dyed unevenly, too dark or are stained. The elastane component of the fabric imposes restraints on the selection of suitable stripping agents.

Reducing agents do not degrade “LYCRA” elastane fibre and are therefore recommended. Alkaline reducing baths are preferred, whenever compatible with the type of “LYCRA” and the hard fibre in the fabric. Dye stripping by compounds whicyh release chlorine, such as hypochlorite or chlorite, will weaken and degrade the elastane fibre and is unsuitable.

Some recommended procedures are given below

Colour correction or light satin removal :

 

  1g / 1non – ionic surfactant

  1-2g / 1 trisodium phosphate (TSP)

  15-20 minutes at 85OC

   

Partial stripping or removal of medium stains :

  1-3 g/1 Sodium dithionite (hydrosulfite)

  1-2 g/1 TSP

  15-20 minutes at 85OC

   

Table – 7. Stripping fabrics containing LYCRA T-128C

 

X = In general, can be stripped, S = Only selected dyes can be stripped.

 

Alkaline stripping

 

  5% sodium sulphoxylate formaldehyde

  1% amphoteric dispersing agent

  Sodium hydroxide to pH 10-11

  45 minutes at 90-95OC, then soap and rinse the fabric to neutral pH.

Acid stripping

  1% zinc sulphoxylate formaldehyde

  1% amphoteric dispersing agent

  Adjust bath to pH 5 if needed

  Treat 45 minutes at 85OC, then soap and rinse till fabric pH is neutral.

  A complete colour strip, usually followed by redyeing, represents extra processing of the stretch fabric which may weaken it and impair its subsequent performance in use.

The suggested stripping procedures do not suit all dye types, hard fibres or LYCRA types, as shown in Tables 6 and 7.

 

PRINTING

A large proportion of nylon/LYCRA tricot fabrics for swimwear and activewear as well as various circular knits and wovens for outerwear are printed. Such prints are produced by flat bed, hand, rotary screen methods or by wet transfer techniques. Dry heat transfer printing is usually limited to fabrics of polyester and LYCRA.

LYCRA is always used as the lesser weight component accompanying the hard fibre, and is hidden in the fabric. The printing technique should be therefore chosen for the hard fibres only, providing they

- do not affect the stretch properties of the fabric- do not degrade the LYCRA not impair subsequent garment performance- do not distort the printing pattern- yield uniform colours.

 

Direct printing

Stretch fabrics are more difficult to handle mechanically than comparable “rigid” fabrics containing only hard fibres. Before printing, the fabric should be adequately scoured and heat-set as described earlier in this bulletin. Insufficient scouring and lack of stable dimensions can cause uneven colour development and pattern distortion. The fabric should be laid fully relaxed on to the printing table or machine, using an efficient adhesive to keep it securely fixed on the table during print colour application.

LYCRA can degrade with excessive steam ageing of printed fabrics. The temperature in cottage steamers should not exceed 104OC (equal to 15 kPa or 0.2 kg / cm2 steam pressure) during 30-60 minutes, to maintain the properties of LYCRA. These precautions also apply to continuous steaming which must be carried out tension – free in loop steamers. Continuous high temperature steaming is used for polyester or triacetates combined with LYCRA. Suitable conditions for such treatment (about 8 minutes at 160OC in loop steamer) must be established by preliminary trials.

Fabrics printed with substantive dyes need soaping to remove residual chemical and unfixed colour. Washing vessels must not be overloaded since this makes the scour inefficient and could cause pattern distortion or fabric deformation from tgling. The scoured fabric should be dried

with overfeed on a pin-stenter in order to finish it with stable dimensions. Its drying temperature should not exceed 150OC to avoid excessive exposure to heat.

 

Heat transfer printing

The technique is mainly confined to tricot fabrics of polyester and LYCRA. Preprinted paper transfer patterns to the fabric by sublimation of dyes during exposure to head in a calendar or press. Ease of application with no need for afterwash, quick chane of patterns, and absence of fabric waste make this method of printing attractive. However, lower colour wet fastness and duller shades than those of direct prints with acid dyes limit its suitability for tricot fabrics containing nylon.

Soaping of transfer printing fabric – although not essential – improves its handle, fastness, and physical properties. Time and temperature used for head transfer should be sufficient to transfer the colours, yet stay as short and low as possible to minimize changes in fabric properties. The best conditions for both satisfactory printing and adeque performance of the printed fabric should be established by testing.

 

FINISHING

Finishing concerns handle, look and dimensions, and can impart special properties to fabric containing “LYCRA”. It comprises both mechanical and chemical treatments which all require careful control applied tension, temperature time and chemicals. 

Mechanical finishing

Mechanical finishing includes the steps either before or after dyeing and printing which change the physical features of a fabric. The methods chosen to finish a fabric containing LYCRA should, in general involve little or no tension, particularly when the treated fabric is hot (except for heat-setting). The joint action of high tension and heat may reset or overset the fabric and cause loss of its final elasticity. Low tension allows the best relaxation, improves dimensional stability, and gives a supple handle to the goods. Fabrics that have been overstretched in processing tend to relax afterwards causing difficulties in cutting, and high or irregular shrinkage of garments.

LYCRA will withstand without undue damage most of the mechanical finishes applied to rigid textiles, namely : 

  mangle or spin dry

  dry

  steam relax or London shrink

  compressive – shrink

  semidecate or decate

  press or Palmer-press

  raise, sand or suede

  crop

  calendar or emboss

Key rules to follow are “do not stretch hot fabric” and “no tension for long periods”. Technical Information Bulletins L-523, L-525, L-528 give details concerning specific fabric types. Drying is considered in this bulletin, because this treatment applied to all piece fabrics.

 

DRYING

Hot air drying, usually on pin stenters with overfeed, removes non-extracted water and establishes the final width and weight of the fabric.

Temperatures should not exceed 150OC when drying stretch fabrics. The time to drying and the resultant speed of fabric passage, will depend on the weight and structure of the fabric, but should not be longer than the minimum needed to dry it. Heating should be uniform over the entire fabric and overdrying must be avoided s it may slightly yellow the fabric. The risk of yellowing is increased when the drying air is heated directly with oil or gas burners.

The same advise also applied to drying required to fix or develop and chemical finishes on fabrics of LYCRA.

 

Chemical finishing

Fabrics containing LYCRA withstand many chemical treatments or finishes which serve to change or improve their look, surface, handle, performance or their properties. LYCRA also withstands mercerizing and light corboniztion. LYCRA is compatible with many 

  antistats

  softeners

  water repellents

  resin finishes for easy care, when used with organic catalysts

  resin finishes to impart body, when used with organic catalysts

  rot-proofing agents

  coating finishes in emulsion

Since the range of finishing agents available and their combination in finishing formulae is enormous, all their possible applications and effects are beyond the scope of this bulletin. Different types of LYCRA elastane also have varying resistance to finishing chemicals, as shown in Table 8. Finishing agents should therefore be selected by careful preliminary testing.

Further information on chemical finishing of specific fabric types is contained in Technical Information Bulletins L-523, L-525 and L-528.

Table 8. Finishing agents applicable to fabrics containing LCYRA

 

Evaluation and Testing

In addition to all the requirements current for rigid fabrics, a stretch fabric must also provide an elastic performance which is measured by :

 Modulus or power developed by stretching the fabric (checked at certain extensions that most closely resemble the stress in garment use)

  Ultimate elongation

  Recovery from stretch

     

These properties reflect the performance expected from different types of stretch garments. DU PONT recommends guidelines for different elastic fabrics to ensure that stretch textiles containing LYCRA are of high quality and performance. Such guidelines giving the recommended elastic properties, and indicating the LYCRA content in fabric necessary to meet them, are included in Technical Information Bulletins L-521 to L-528.