By: D. Saravanan

Senior Lecturer
Department of Textile Technology
Bannari Amman Institute of Technology
Sathyamangalam 638 401 Erode Dist.

Introduction

In the recent years, the garment dyeing has been slowly emerging as a process to meet the ephemeral fashion trends adopted by the consumers and new machines and methods have made this process to wear a brighter future for it. However, the garment dyeing is not coming without any strings attached to its, the cautions are often pronounced in terms of fabrics and their preparation, accessories used in the garments for safe operation. Along with raw stock dyeing, yarn dyeing and piece dyeing, garment dyeing has also taken its place as another form of dyeing textiles. In the case of garment dyeing, the volume of popular and unpopular colours can be increased or decreased as envisaged from the market requirements.

Garment Dyeing

Garment dyeing may be defined [1] as the �application of colour to fully fashioned apparel articles, may be in the form of garments cut and sewn either prepared or unprepared knitted fabrics and then dyed, garments and / or components knitted from either prepared or unprepared yarn and then dyed, garments manufactured from either prepared or unprepared woven fabric and then dyed and mixed fabric garments i.e. woven and knitted fabrics manufactured from prepared fabrics and then dyed�. The benefits of processing good through garment dyeing procedure revolve around quick response and improved inventory control. Differentiation is made between garment dyeing and hosiery dyeing where socks and ladies stockings have been dyed in the prepared form [2]. Garment dyeing is the obvious way to meet the needs i.e. quick response to fashion change, lead times have been reduced to 4 to 5 days from 2 to 3 weeks as minimum with other processing routes (Fig. 1) [3].


Greige / PFD Garments

Garments are made from either �greige� or �prepared� fabrics. The technique involved in garments is the same as the standard exhaust procedures of the dyes, except for pigments. Garment dyeing articles include hosiery, socks, sportwear, shirts, T-shirts, trousers, bath robes, bath mats, throw rugs. Dyeing of garments made from polyamide and their blends [4 - 8], acrylics, polyesters and mixtures of these fibres have discussed in the past.

Dyeing of cotton is more suitable for relaxed look associated with casual wear and leisurewear garments. In greige fabric garments, a full scour / bleach procedure is recommended [9] with the following recipe shown in the Table 1.

In the case of PFD (prepared for dyeing) garments, a small amount of chemicals is used to enhance the processing such as softeners, anti-curls; lubricants must be removed to ensure proper dyeing. After 10 minutes run at 80 � 90o F, 5.0 � 8.0% of 50% NaOH is added and treated for 30 � 45 minute at 205o F.

The dyeing of the garments requires more care than the dyeing of the fabric, though there is very little difference in the dyeing procedures. This is due to the fact that processing of garments involves value added goods. The entire garment dyeing activities may be broken down in to four categories namely fully fashioned garment dyeing carried out by major dyers and finishers, cut and sewn garment covering woven and knitted fabrics, dyeing of 100% cotton goods for boutique trade suitable for hand washing and washing, desizing, bleaching denim goods, leading to stone wash, snow wash, over dyeing and highlighting effects.

Review on denim garment processing has been carried out many authors in the past [10]. A multi-colour splatter effect called �splatter dyeing� has been made possible on denim garments without necessity of tieing or knotting, using wash-fast reactive dyes combining, exhaust dyeing, pad-batch and printing technologies [10]. A technique for estimating the fraction of the total perceived colour depth in the denim fabrics has been developed where more than one colour is used in the same fabric [3]. Since the majority of garments, currently, being dyed are constructed from cotton fabrics, reactive and direct dyes are the most popular classes though other classes are also used to some extent e.g. sulphur and vat dyes [9, 11, 12, 13].

Reactive dyes appear to be very much attractive in the dyeing of cotton garments similar to fabric dyeing [14, 15]. Dyeing of cotton fabrics with sulphur dyes require more precautions to avoid damage to the garments; presence of impurities like iron up to 20 ppm may alter the yield [16] and also adversely affects copper containing buttons.

Exhaust dyeing with pigments is possible only with materials pretreated with a cationic agent which imparts substantivity to overcome the non-substantitive in nature of the pigments. Exhaust dyeing of garments with pigment involves prewash-pretreatment-exhaust dyeing-stone wash (optional)-fixation-thermo treatment [17]. Once the goods are prepared it is necessary to apply a chemical auxiliary that will impart substantivity to the pigment e.g. cationic agent and once the cationic charge is applied to the cotton piece or garments, the pigments can be applied together with an antifoaming agent and anionic dispersing agent. The exhaust method is followed by curing operation at 120 � 130o C for 20 min in presence of the catalyst [18]. Colour differences observed in the selvedges in the case of fabric dyeing can be sorted out using clustering techniques and shade sorting (555) methods [19].

Garment Dyeing Machines

Unlike fabric dyeing machines where rollers and jets are employed in transporting the fabric through out the machine and liquor, garment dyeing machine require special arrangements to move piece goods / garments through the liquor continuously. Also, care needs to be taken to avoid abrasion marks, stagnation of goods and to reduce tumbling action to reduce the damage to the fabrics. Salient features of drum type machines, extractors, paddle type and jet circulators have been discussed in the past by many authors [12, 13, 20, 21, 22]. Garment dyeing and wet processing equipment have evolved from the combination of traditional textile equipment and industrial laundry equipment. The attributes of ideal garment dyeing machine would include: automatic controller for cycle repetition and optimization, shade consistency, centrifugal extraction, heating facility to make the cycle faster, cooling facility, lint filter to give cleaner look to the garments, sampling device for better shade management, addition tank, tilting mechanism for faster unloading of the garments, variable speed for processing different garments, volume level control for shade reproducibility. The first means to ensure success in garment dyeing is to design the garment to withstand the rigorous to rotary processing.

The basic designs involved in the garment dyeing machines have been discussed earlier by many authors. In open pocket dye extractors, cylinder speed determines the amount of agitation and even at very low cylinder speeds, considerable agitation results from the tumbling action [22]. Cylinder speed influences the depth of the shade (by agitation), redistribution of the dyes. The availability of equipment with high temperature capability further expands the potential of garment processing for both PET and its blends. A variety of machines are available for garment processing can be classified as given in the Table 2

Paddle Dyeing Machines

Paddles are widely accepted for sweaters and loosely knitted goods due to their soft dyeing action, which avoids abrading and pilling the garments. Overhead paddle, lateral paddle and high temperature paddle machines serve the needs of the entire range of the garments.

Horizontal Paddle Machines (over head paddle machine) consist of a curved beck like lower suction to contain the materials and the dye liquor. The goods are moved by a rotating paddle, which extends across the width of the machine. Half immersed paddles cause the material to move upwards and downwards through out the liquor. The temperature can be raised to 98o C in such system.

In lateral / oval paddle machines consist of oval tank to enhance the fluid flow and the processing the goods. In the middle of this tank is a closed oval island. The paddle moves in a lateral direction and is not half submerged in the liquor and the temperature can be increased up to 98o C.

HT Paddle Machines work according to the principle of horizontal paddle machine, however, the temperature can be raised up to 140o C. PES articles are preferably dyed on HT paddles.

In paddle machines, the dyeing can be carried out with 30:1 to 40:1, lower ratios reduces optimum movement of the goods, lead to unlevel dyeing, crease formation. For gentleness, the blades of the paddle are either curved or have rounded edges and the rotating speed of the paddle can be regulated from 1.5 to 40 rpm. Circulation of the liquor should be strong enough to prevent goods from sinking to the bottom. Paddle machines are suitable for dyeing articles of all substrates in all forms of make ups. The goods are normally dyed using PP/PET bags.

Extractors

Reduction of the speed results in the reduced movements of the goods within the dyebath, and so the recirculation of dye liquor becomes important [22]. Recirculation of dye liquor does not create the abrasion caused by faster cylinder speeds and increased tumbling action. Dye extractors with multi-pocket design divide the cylinder into compartments to control garment movements, abrasion due to mechanical action and in the compartments the garments are carried through the dye liquor in the compartments. Depending upon the design and construction twin compartments, three or four compartments can be made in the extractors.

�D� type pocket extractors have two compartments while �Y� type pocket extractors have three compartments and four compartment types of extractors are also available. By making more compartments the tumbling effects are reduced and so is the abrasion associated with it. �D� type pocket reduces the drop or tumbling effect by 50% compared to open type pocket and Y type by 2/3. Side loading and end loading are the two options available for loading the multi-pocket designs.

Drum Type Dyers

In drum type machines, a perforated drum is suspended along lengthwise axis in a horizontal position, submerged in the dye liquor. The drums are divided into compartments and are capable rotating at 2 � 20 rpm. High temperature drum machines are capable of processing the garments up to 140o C.

Washing-Centrifuging Machine is similar in construction to a normal household washing machine. This machine has a perforated suspended inner drum, which is not divided into compartments and inner drum is located within an outer drum. Further developments in this machine have led to the drum dyeing centrifuging machine. In centrifuging type machines, non-foaming agents are to be selected. Wax used in the knitted yarns need to be removed above the melting point of the wax to avoid redeposition.

Drum dyeing-centrifuging machines are also called �multipurpose drum machines� or �multi-rapid dyeing-centrifuging machines� since these machines can perform scouring, dyeing, centrifuging and conditioning successively with automated controls. The goods are treated in a perforated inner drum housed within an outer drum (dyeing tank). Inner drums without dividing walls are provided with ribs that carry the goods along for a certain time, partially lifting them up out of the liquor. These machines can operate at very low liquor ratios and can dye the goods up to 98 -140o C. This is suitable for knits as well as other garments. Liquor circulation can be intensified using additional jets. Drums can be rotated in both the directions and some salient features of a drum dyeing - centrifuging machines along with processing speeds for various goods are given in the Table 3 and Table 4.

Jet circulation Dyeing machine

The dye liquor and goods are kept in motion by jet nozzles whose direction and force are adjustable. Nozzles located along the wall cause the liquor to move in circular motion. Turbulence nozzles at the bottom ensure liquor circulation, prevent goods from sinking and allow opening them. The machine capacity can vary from 25 � 125 kg of dry weight and does not require to pack the goods in bags and m:l ratio ranges 1:25 to 1:40.

Hydrodynamic circulation machines can work at temperature as high as up to 130o C. A pump conveys the liquor to the vertical distributor, which rests upon a specially constructed perforated bottom. With the combination of perforated bottom and liquor distributor, spiral movements of goods are achieved and so even with weak circulation, goods are prevented from sinking.

Mechanical Finishing and Related Processes

Mechanical finishing refers to the finishes, which must be given to made-up textiles before they are ready for delivery. In most cases, the process consists of a steaming treatment, to remove creases or to relax the goods. Large, open pocket tumblers are used for drying the garments after dyeing or processing. Small units are heated by electricity, steam or gas, while large units are steam / gas heated. Microprocessors are available for controlling functions like cycle time, temperature, cooling and maintaining the moisture levels. Variable speeds can be achieved for processing of knits or other delicate fabrics without excessive abrasion. Driers are often constructed using tumbler, made of perforated drum, equipped with heating unit, an air suction unit to draw fresh air, circulation unit and a lint sieve. In drying, the goods are carried up by the ribbed drum to its apex and then fall of their own weight through a current of air, which can be adjusted along with the direction of the movement.

Topper is used for simultaneous steaming and stretching trousers. Pressing machines are available in the forms of pull overs, jackets, sweat shirts, shirts and etc i.e. with dummies. The steam is blown outwards through the garment and carried out at low tensions. Ironing is carried out after toppers, pressing dummy operation, in areas like collar, cuffs, pocket and button flaps, etc.

Problems Related to Garment Dyeing

Though garment dyeing has attracted many processors to look forward, the process is attached with many stringent requirements [1, 2, 3, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32] related to seams, elasticated areas, waist band, cuffs and problems often occur due to shrink behaviour, chafe marks / creases, accessories, sewing threads, foreign substances, interlining and care labeling. Processing problems related to the garment size control and appearance normally result from variations in yarn size / twist, ends per inch, picks per inch or courses per inch in the knitted fabrics. Preparation is an area of textiles in which the least attention is paid to correct procedures. Most of all dyeing problems can be traced to improperly prepared cloth. Natural impurities in the cotton vary among the various locations across the world. Shade non-uniformity and the related appearance problems arise due to the differences in fabric preparation processes like, desizing, scouring and bleaching. Retention of shade, in the case of rayon fabrics, is unstable than that of cotton fabrics [33]. All these problems are further aggravated, if there is any inconsistency in the garment assembly. Though garment dyeing has been reviewed by many authors [34], the problems arising in the garment dyeing are discussed in detail.

Some of the problems that are difficult to correct after dyeing include: poor fabric preparation, improper tension of the sewing threads that may lead to seam puckering, garments prepared with panels taken from differently or inconsistently prepared fabrics and discoloration of button and closures caused by dyes and chemicals used in the process. Consistent shade replication depends on material to liquor ratio, salt concentration, heating rate, alkali concentration, fixation time and temperature. Chlorine emitted from bleaching or ice wash can lead to white specks. Also, the residual hydrogen peroxide in the bleached materials can interfere with dyes and spoil the colours.

Different machines used in the fabric preparation can result in differences due to the time and temperature differences. While sewing, it is necessary to keep sewing tensions minimum on lock stitch and chain stitch operations. About 90 � 95% of the garments that the dyer receives are cut and sewn from previously prepared cloth. Oils, softeners used in knitting needs to be removed. The various problems observed in the dyed garments include variation in shade / depth within the garment panels, between garments occurs due to mixing of panels cut from differently treated fabrics, white undyed / light seams happens mainly due to tight stitching, which further gets tighter due to the shrinkage during a higher temperature of dyeing, and prevents dye penetration on seams and underneath the stitches, variation in the shade and brightness differences within the garment panels happens due mixing fabrics treated with OBA and without OBA, finish, resin and etc has been added to the garments. Dimensionally stable thread with low elongation will help prevent seam puckering after dyeing. The dimensional stability of the stitching thread should be tested prior to stitching and thread with low elongation can prevent seam puckering after dyeing. Core spun thread also give excellent results. In the case of knitted fabrics, pre relaxation is employed to avoid the problems of seam pucker and garment distortion and such fabrics can also be used along with woven fabrics as fabric cut and sewn garment designs.

Size of the filaments also affects the depth of the shade, due to the fact that coarser filaments absorb more light than finer filaments. As the cross section of fibres becomes more and more non-circular, the fibres light scattering power increases. Trilobal fibres need about 50% extra dye than circular fibres for same depth. However, bulked multi-lobal fibres in the yarn �trap the light� within the body of the textile and increases the depth of the colour. Therefore, light scattering or light trapping can alter the depth as decided by cross section.

The randomness of orientation of the dyed fibres in a textile material strongly influences the depth of the colour. The apparent colour of yarns made from filaments tends to be highly dependent on the angle of viewing, whereas such sensitivity is not obvious in the case of staple fibre yarns. In the fabrics containing raised surfaces like velour, velvet, corduroy, difference in the side and end arise mainly due to the optical origin, which is traceable to the path length traversed by the light in the longitudinal and transverse directions in fibres. More light is absorbed in the longitudinal direction than in the cross sectional direction. Side and end effects can give rise to non-uniform colour appearance in the garments. Significant darkening of the fabrics occurs after subjecting it to calendaring because of the increased optical contact and reduced light scattering power. Similarly oil or water present in the fabrics also brings significant darkening effects in the fabrics.

Elasticated areas, waist bands and cuffs must be fairly slack and seams should not be too much bulky or tightly. Shrinkage behaviour becomes more important where knitted and woven fabrics are mixed. Pre-relaxed knitted fabrics, pretreated woven fabrics should be fully preshrunk through suitable process.

The various patterns present in the garments must be adjusted to compensate for shrinkage during dyeing process. It has been suggested many times to make sample garments before initiating large scale production to determine the adjustments required in the patterns. Establishing dyeing procedures for each garments style, dye system and set-controls can help to address the variations in load, water volume, temperature and cycle time. Garment to garment shade differences can be minimized with machine loads containing garments made from the same lot of fabric.

Chafe marks / creases are, particularly, related to drum dyeing machines and in many circumstances, garments are turned inside out and dyed with non-foaming lubricants. Tendency to entangle can be reduced by tacking and eliminated by bagging the articles which in turn reduces the abrasion, wear and tear of interior metallic surface. Once formed, a crease may then remain through out the finishing processes. They lead to unlevel treatment and dyeing effect and of irreparable surface damages to the goods from. Swollen cellulosic fibres are especially sensitive to mechanical friction, while thermoplastic synthetic fibres tend to form permanent creases. Fibre type (natural / thermoplastic) fabric construction (tightly woven crease prone), loading (higher loading-higher creases), addition of lubricant (special lubricants reduces friction) are some of the parameters that influence chafe marks.

The majority of the problems in dyeing of garments are attributed to pretreatment of garment prior to dyeing, quality and type of materials used in the making of garments viz fabric construction, sewing thread, buttons, zippers, pocket lining etc., dyeing process and the machine used for dyeing.

Problems related to electrolysis of ionic processing solutions resulting from galvanic action of bimetallic garment accessories have been discussed in the past. Anionic organic inhibitors are used to protect metal accessories such as button, zippers made of non-ferrous, white metals (non-aluminium) from oxidation and tarnishing. The relative merits and demerits of the garment processes have been well discussed in the past [1] and include the following aspects.

The major advantages include:

� Flexibility towards fast changing market trends

� Quick response and rapid turnaround

� Flexibility towards dye shades and finishes

� Flexibility of lot size

� Flexibility of items to be dyed

� Comparatively less rejection

� Low inventory

� Less capital investments

� Fancy effects

The disadvantage of garment dyeing include:


� Labour intensive process and requires thorough checking of every piece

� Higher �seconds� rate

� Poor appearance

� Poor reproducibility of shades

� Special care in the selection of fittings

� More material handling

Conclusion

Though the garment dyeing was started in those days, as a means to hide the shade variations in the garments, by the dry cleaners, the process has gone through a long way to establish itself as a viable one. In earlier 1980s, the efforts made to make the garment dyeing through scientific approaches, today, have resulted in many takers in this area, to react instantaneously to the market demands. The problems faced in dyeing of fabrics in terms of economy of operations and other cost factors can be, largely, optimized and reduced in the garment dyeing. At the surface level, the garment dyeing appears to be very much attractive nevertheless stringent conditions are essential while making the garments to address the problems related to the seams, closures, interlining and the pleated areas.

References

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About the author:

D. Saravanan is a Senior Lecturer in the Department of Textile Technology, Bannari Amman Institute of Technology Sathyamangalam 638 401 Erode Dist


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