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Chapter 8: Disperse Dyes and Their Application to Polyester
By J. R. ASPLAND, School of Textiles, Clemson University, Clemson, S. C.
ISPERSE dyes are nonionic, have
D very limited solubility in water a t
room temperature and have substantivity for one or more hydrophobic fibers; e.g., polyesters and nylons. They are usually applied from a fine aqueous dispersion containing some dissolved dye. This defi- nition changes the emphasis of that given
in the Colour Index (I) to stress the most
important fibers and to reflect the fact that it is the aqueous solution from which dyeing normally takes place, despite the 'low water solubility of the dye. Solutions are characterized by the pres- ence of the solute (dye) as single mole- cules; i.e., in a monomolecular form. Even in thermofixation and heat transfer print-
ing (Z), where dyeing takes place in the
absence of water, the dyes must first sublime into single molecules (dissolve) in air before diffusion into the fiber can take place. The AATCC Buyer's Guide (3) lists over 700 disperse dye products under more than 200 Colour Index names. The major- ity of these are blues, ca. 30%; reds, ca. 25% and yellows, ca. 20%. Oranges and violets both account for ca. 8%and browns account for ca. 3%. This leaves little room
ABSTRACT
The fiber, dye and dyebath variables which affect the batch dyeing of polyester with disperse dyes are discussed, as are some of the chemical properties of disperse dyes which influence the normal conditions of dye application, stripping and reduction clearing. Discussion of the continuous dyeing of polyester i s deferred to a iater chapter on dyeing polyester/cellulosic fiber blends.
KEY TERMS
Batch Dyeing Disperse Dyes Dye Classification Fastness Properties Glass Transition Temperature Heatsetting M icrofibers Oligomers Polyester Trimers
for blacks and greens but in no way indicates a problem in obtaining blackand green dyeings with disperse dyes; the color gamut obtainable is extremely large. It does reflect the difficulty of synthesizing green and black compounds, which are nonionic and of small enough molecular size (relative molecular mass, rmm) to have substantivity for and to be able to diffuse into hydrophobic fibers. C.I. Disperse Blacks 9 and 33 are actually navies which only become blacks after they have been diazotized and devel- oped (cf. the aftertreatment of direct dyes, Chapter 2 and azoic combinations, Chap- ter 6). But black disperse product mixtures are readily available, and they are almost invariably mixtures of a navy blue shaded with appropriate proportions of a red or rubine plus a yellow, yellow-brown or orange component. Because they have more than one principal chemical compo- nent, they cannot have a C.I. name. There is also no particular problem in obtaining a wide variety of green shades with disperse dyes, even though only one green disperse dye, C.I. Disperse Green 9, is listed (3). But the choice of blue and yellow components is in the hands of the user. Green disperse dye mixtures are not generally available.
Disperse Dye Classification Dyes may be classified in a variety of ways, some of which are unique to the particular application category. Disperse dyes are no exception. As might be anticipated, chem- ical classification by chromophore is gen- erally the least useful to the dyer. How- ever, there are some chemical differences between disperse dyes which affect their performance in dyeing. These include the ease with which they are reduced and the ease with which they are hydrolyzed.
Reduction Clearing Because disperse dyes have such limited solubility in water, some particulate dis- perse dye may still be occluded on fiber surfaces after the dyeing phase is com- plete. If this condition is suspected, the last stage of the dyeing process may need to be one where surface dye is removed. Adverse results of excess dye on the fiber surfaces include considerably reduced wetfastness, washfastness, sublimation and dryclean- ing fastness, as well as dulling of the shade. With experience, the presence of exces-
sive amounts of surface dye can be deter- mined by simply agitating a sample of the dyed goods in a little cold acetone for a few seconds, when surface dye will dissolve. The acetone will not extract dye from within the fiber, which remains unswollen, but will dissolve surface color. The usual practical procedure for re- moving this unwanted dye is called reduc- tion clearing and uses a bath of about two grams per liter of both caustic soda and sodium dithionite (hydro), as 100% solid products, plus about one gram per liter of a stable surfactant. A treatment for 20 minutes a t ca. 70C ( 1 60F) is often sufficient to clear the fiber surfaces, but the ease of removal varies from chromophore to chromophore and dye to dye. Provided the clearing tempera- ture is not above the dyeing transition temperature, no dye will be stripped from within the polyester fibers. The largest majority of disperse dyes
contain the azo group, -N = N-. This
group is easily split into two amino groups by treatment with reducing agent:
-N = N- - -NH2 + H2N- Eq. 1
which makes them particularly suitable for reduction clearing since the amino residues are virtually colorless unless de- liberately oxidized to form totally differ- ent products. Some bulky azo dye molecules, notably the navies as a group, are so sensitive to reduction that under conditions of too high pH they can cleave a t the azo group, even during the dyeing process, to give dyeings
of a characteristic lighter, duller and
greener appearance. The condition is more pronounced in polyester/cotton blends. Another chromophore still found in some of the brighter blue, pink and red disperse dyes, despite its cost, is anthraqui- none. This is more difficult to reduce and is not destroyed during normal reductive clearing. However, as in the case of vat dyes, the anthraquinone residue is a t least partially and reversibly reduced to a sob- ble sodium leuco form which can be washed away but which on subsequent. exposure to air becomes insoluble again.
Hydrolysis of Dye Esters Another chemical group frequently founi in disperse dyes is an ester group, often BI- acetyl group, -0-CO-CH3, and like tht:
red.
I ,^ acetyl groups in cellulose acetate it^ is
susceptible to hydrolysis in neutral and alkaline conditions: H2O
Dye-0-CO-CH3 -
Dye-OH + HO-CO-CH3 Eq. 2
The products are acetic acid and a differ- ent azo disperse dye whose color may be quite different from that of the parent dye. Usually the wavelength of maximum light absorption (minimum reflectance) is shifted to a longer wavelength. This is known as a bathochromic shift, in which colors change in the general direction: yellow-orange-ved-violet-blue- green. The subject will be discussed more fully in a^ section^ on^ color and constitution. However, two additional, widely different points are worth noting here. The presence of hydrolyzable groups in many disperse dyes and their protection is the principle reason dyeings are generally made on the slightly acid side. The pH has no fundamental role in the dyeing mecha- nism as such and some disperse dyes without ester groups do not need a weakly acidic dyebath. Dyes and dyeings in any application category have traditionally been present- ed-e.g., in shade cards, the Colour Index ( I ) and until recently in the Buyer’s Guide (3)- in the same bathochromic series order listed above. Blacks are shown last and browns (being dull oranges) are some- times presented after orange and some- times between green and black. To sim- plify processing by computer, the Buyer’s Guide now lists dyes alphabetically by color-Le., black, blue, brown, green, orange, red, violet and yellow-and the Colour Index is considering the possibility of doing the same. While it makes no coloristic sense, the alphabetical system simplifies the processing of the data. A group of disperse dyes utilizes the alkaline hydrolysis of esters in an inge- nious way. These products contain ester groups in the reverse orientation relative to the dye chromophore to that shown in Eq. 2. During alkaline scouring,without the need for a reducing agent, the ester link is hydrolyzed, the surface dye becomes a water soluble carboxylate salt and is easily removed by washing.
Dye -CO-O-CH3 +^ NaOH-
Dye -COO-Na+ + CH30H Eq. 3 Energy Level Most disperse dye classifications are based on some form of generalized grouping according to their rates of dyeing and resistance to sublimation. For example, a major company in the United Kingdom has chosen to classify its disperse dyes into four groups, A-D, where subgroup A contains thosedyes with the highest rateof dyeing on polyester and the lowest subli-
December 1992 033
mation fastness, while the dyes in sub- group D are just the reverse ( 4 , 5 ). The fact ofthematter is that therelative dyeing, physical and most fastness proper- ties of disperse dyes lie scattered around a line from generally small molecules (low rmm) with low polarity, poor heat and sublimation resistance, rapid rates of dye- ing and good leveling characteristics to generally much larger molecules (high rmm, but still not very large as dye molecules go) which are quite polar with- out being ionic, with good heat and subli- mation fastness, poor leveling and low rates of dyeing. Note that lightfastness is not a property which is dependent on the molecular size or polarity. A disperse dye’s suitability for dyeing cellulose acetate, carrier dyeing polyester, high temperature dyeing of polyester and dyeing of polyester/cotton blends by ther- mofixation runs along the same line from A to D. But the precise position of an individual dye relative to others on the line depends on the particular physical prop- erty selected and therefore any subdivision is somewhat arbitrary. In the U.S. it is normal to classify disperse dyes into three subgroups called low, medium and high energy (L,^ M and H). These cover the same range of proper- ties as the A-D classification mentioned earlier. Again the groups are somewhat arbitrary. But disperse dyes within anyone of the subgroups are much more likely to have similar dyeing and fastness charac- teristics (other than lightfastness) and are consequently more suitable for dyeing together than dyes outside the same sub- group. Because of the number of available dyes, there is still plenty of room for selecting dyes within any subgroup which can deliver the particular characteristics desirable in the final dyed product. As an illustration, one major manufac- turer carries a line of just over 30 disperse dyes principally for polyester. About 25% a professor of textile chemistry at are low energy dyes. Six are very suitable for carrier dyeing, a different six are very suitable for high temperature dyeing and only one is very suitable for dyeing the polyester in polyester/cotton blends by thermofixation. About 25% are medium energy dyes. Three are recommended for carrier dyeing, nine are very suitable for high temperature dyeing and five are very suitable for thermofixation. Nearly 50% are high energy dyes. Only one is recom- mended for carrier dyeing, four for high temperature dyeing and 11 for thermofix- ation. Fastness Properties on Polyester The fastness properties of disperse dyes on polyester cover a wide enough range for an adequate dye selection for most end uses. The same dyes generally show poorer fastness on^ nylon (see later). Lightfastness ratings at the I S 0 stan- dard depth (1/1 SD) can easily be in the 6-7 range on^ the Blue Wool Scale of 1-8, although they do drop slightly if the light source is a carbon arc as opposed to xenon lamp. As the depth of shade decreases, lightfastness drops, a phenomenon shared by dyeings of all application classes of dyes. If extremely high lightfastness is needed (automotive fabrics), a nonionic UV in- hibitor may be added to the dyebath and applied to the fiber along with the dye. These compounds, often benzotriazoles, work much like sunscreen, screening out and dissipating UV radiation to prevent sunburn. Wetfastness tests are frequently con- ducted after the goods have been reduction cleared and heat set; e.g., at 180C (356F) for 30 seconds. They are assessed in terms of the staining on^ multifiber or adjacent nylon piece goods. Ratings of 4+ out of 5 are readily achieved on regular denier fibers. What is interesting here is that the ratings are very dependent on the extent of clearing of the fiber surfaces, the duration and temperature of the heat treatment and whether the fabric has been treated with a finish of any kind. Heating disperse dyed goods causes the dyes to tend to migrate towards the fiber surfaces and some of the disperse dyes are quite soluble in hydro- phobic surface films; e.g., in softeners which may have been applied. Fastness to crocking or rubbing as well as drycleaning suffers if dye migrates to the fiber surface or surface layer. For those dealing in imports and exports of dyed goods, it is vitally important to be aware that the methods of fastness testing, and consequently the ratings for dyed goods, vary from country to country. The International Organization for Standard- ization (ISO) has developed a series of fastness tests which are often very differ- ent from test methods (6) used in the^ U.S. So11 (7) compared tests for 30 fastness properties as run in 22 countries. The U.S.
.
Fiber Fineness Much attention has been given recently
( I I) to dyeing microfibers. These are fibers
with fineness of less than one decitex (dtex); Le.. fibers for which 10,000meters would weigh less than a gram. One decitex equals 0.9 denier. It is important to appre- ciate that the division between micro and regular fibers is quite arbitrary. Wool dyers have been making allowances for differences in fiber fineness for more than 50 years. It is also important to note that these are not microdenier fibers, a misno- mer which relates to fibers of fineness ca. 1 0-6 denier. What has changed is that man-made fibers can now be made substantially finer than the finest natural fibers and that the magnitude of the differences in fineness possible between micro and regular man- made fibers, and which might confront a dyer, is much greater than it has ever been. This means that dyers will not simply need to adjust dye formulations slightly to compensate for fibers of different dtex, but they will need to make adjustments which could be several hundred percent in the amounts of dye used to achieve the same apparent depth of shade. A useful preliminary relationship be- tween the percentages of dye on weight of goods (C) needed to achieve a particular depth of shade on polyester fibers of two different finenesses (dtex) is given below:
C,/C, = (dtex,/dtex,)" Eq. Here subscripts m and r can be used to indicate micro and regular fibers but could also be used to designate any two fibers of different fineness (denier, dtex). Thevalue of n is normally taken to be 0.5, which means the right hand term of Eq. 4 is the square root of dtex,/dtex,. Putting arbitrary values for dtex, (4.5) and dtex, (0.5) into Eq. 4, gives:
C,/C, =^ (4.5/0.5)'.'^ =^0 =^3 It takes about three times as much dye to dye this microfiber to the same apparent depth as the regular fiber. Unfortunately, the relationship may not be exact, and in any case was only intended to apply (12) when all other fiber characteristics are identical; e,g., cross section, surface char- acter, delustrant percentage, morphology. Thestructureofthe yarnor fabricinwhich the fiber is used must also be held constant for the relationship to apply. Also, dye penetration must be uniform for Eq. 4 to
apply. Such marked denier differences can affect dyers in a number of ways, using the example above as an illustration. As seen already, the microfiber will need approximately three times as much dye to achieve the same apparent depth of shade (cf. Eq. 4). *The microfiber will dye approximately three times as fast which could lead to the need for procedural changes in dyeing to
counter possible unlevelness due to inade- quate circulation, particularly in pale shades. The dyeing rate increases because an equal weight of the microfiber will have three times the surface ar.ea. If the same apparent depth is dyed on both fibers, the wetfastness after heatset- ting of the shade on the microfiber will be significantly reduced. This is because of both the increased surface area and the greater percentage dye on the fiber. The lightfastness of the dyed microfi- ber will be significantly reduced a t equal apparent depth. Problems in uniformity when dyeing polyester microfiber fabrics could result from difficulties in keeping the fabrics properly wetted. The closeness of fiber packing possible with polyester microfiber fabrics makes them very suitable for rain- wear applications since the fibers are so difficult to wet.
Polyester Dyeing Procedures: General The continuous pad-thermofix process, which is of such enormous importance in dyeing polyester/cellulosic fiber blended fabrics, has proved unsuitable for all but narrow fabrics of 100% polyester, such as seatbelts and ribbons. The process can be used for sliver and continuous filament tows, but these are not major factors in the
U.S. Consequently, the polyester dyeing
procedures considered here will be batch procedures and further consideration of thermofixation will be deferred until the chapter on blended fibers and fabrics.
Preparation Besides the dyeing procedures there are several other processes involved in the production of dyed polyester goods. The first of these is preparation-a scouring and cleaning process. Polyester fibers can be dyed in a variety of physical forms anywhere in the chain from fiber to greige goods. Not all possible forms need a preparative step prior to dyeing. Loose stock or raw stock will have received only small amounts of applied processing aids, most of which should be self emulsifiable. Such products may be loaded directly into the dyebath although it might be desirable to give them a water rinse before doing so.
On the other hand, knitgoods might be
heavily oiled, yarns may be waxed and woven goods may contain sizing materials, all of which should be removed prior to dyeing. Any cationic materials applied during processing must be removed, since they will interfere with the anionic surfac- tants essential to insure good dye disper- sions. A simple anionic surfactant plus soda ash scour, with about two grams per liter of each component in a lukewarm bath (40- 50C (100-120F)], will often be satisfac- tory. Sized goods will need an appropriate
desizing, and heavily oiled or waxed goods may even require scouring in the presence of a solvent emulsion to be followed by rinsing. Problems should not arise unless the goods are contaminated by locally heavy stains. It is important to be sure that no residual fiber lubricants are carried over to contaminate the dyebath. In this same regard the use of solvent or surfactant- containing spotting agents should be avoided.
Batch Dyeing Procedures In general, batch dyeing procedures divide naturally into those conducted a t the boil in the presence of up to 10%owg of carrier and those conducted at temperatures above the boil in pressurized high temper- ature (HT)^ dyeing equipment. The H T procedures may be run with or without carrier. The choice of equipment will deperld on its availability and the nature of the goods to be dyed. The liquor ratio will be determined to a largeextent by the equipment to be used. I t could be as high 20-3O:l for becks, 5- 1O:l for jet dyeing machines and 3-5:l for package dyeing machines. Continuous padding applications use even less liquor,
The point of departure chosen here is a conservative, generalized procedure for dyeing piece goods in a jet dyeing machine at 20:l 1iquor:goods ratio. A typical stan- dard H T dyeing method might include x % disperse dye(s) owg; a suitable pH buffer- ingsystem tocontrol pH to4.5-5.5, (e.g., 1 gram per liter ammonium sulfate ad- justed, with the bath a t full volume, to pH 4.5-5.5 with formic acid); and about 0. grams per liter of a suitable anionic dispersing agent. Alternatively, acetic acid is frequently used to adjust the pH without the addition of ammonium sul- fate, although this is not a true buffer system. The dyes are dispersed separately in water and added to a^ bath at about 50-60C (120-140F) containing the ammonium sulfate, the dispersing agent and the goods. The pH is adjusted to 5-6 with formic acid axd the! i p x level to ?e:!, while the temperature is raised to 70C (1 58F) and the dye liquor is circulated through the goods. This temperature is still below the giass transition temperature and no dye- ing will have taken place. With good circulation, the bath temper- ature is raised to 13OC (266F) at 1.5C (3F) per minute and held there for 60 minutes. The dyebath is then dropped a t as high a temperature rate as possible. The goods are rinsed and given a reduc- tion clear in a bath containing the equiva- lent of two grams per liter of both 100% solid caustic soda (sodium hydroxide) and hydro (sodium hydrosulfite, properly known as sodium dithionite) at 70C for 20
December 1992 (^033) 21
6 ' minutes. The last steps in the procedure are rinsing and neutralization. The process outline is similar to the one given by Schuler (13) except that he used an atmospheric dyeing with c,a-rrier as an example. The last decade has seen carrier use in most highly developed countries fall dramatically for obvious environmental reasons. However, in some small commis- sion dyehouses, in garment dyehouses and in some principally cotton goods and single knitgoods dyehouses, the cost of purchas- ing pressure dyeing equipment is still considered prohibitive and a rear guard action is being fought against the elimina- tion of carriers.
High Temperature Procedure The components of the high temperature procedure will now be examined in detail to find the purpose of every chemical and every step, in order to be able to modify the procedure on demand for the purpose of fitting it to any given set of circumstances.
Water There is no category of dyes for which water quality' is unimportant. Traces of soluble copper and iron salts can affect the shade of some disperse dyes quite mark- edly due to the formation of co-ordina tion complexes. The presence of calcium and magnesium cations can interfere with the anionic dye dispersing agents or anionic leveling and wetting agents present in the dyebaths. For these reasons, small amounts of chelants might be routinely added to disperse dyebaths, particularly when problem dyes must be used. The effect of pH on some dyes has been mentioned earlier. Ideally pH should be buffered in the range of 4.5-5.5 to mini- mize the possibility of dye hydrolysis.
Surfactants There are a multitude of purposes for which surface active agents are added to dyebaths. They include the wetting, pene- trating and de-aerating of the goods to ensure that there is intimate, uninter- rupted contact between the dyebath and all the fiber surfaces. Since the terms wetting, penetrating and de-aerating are essentially synonymous, there is no need for more than one auxiliary product to fulfill these demands. Such products are normally anionic surfactants. Anionic sur- factants are also used to stabilize the diluted dispersions of the dye. This is necessary particularly when dyeing paler shades where the dyebath concentration of surfactant introduced with the dye itself would be small. The levelness of the dyeing can be improved by controlling the rate of ex- change of dye from the dyebath to the fiber. This is^ the function of other gener- ally nonionic surfactants which are se- lected for their ability to form surfactant micelles attractive to thedye. Thesecan be
22
thought of as competing with the fiber for the dye, thus slowing down the rate at which the dye would become available to the fiber. They are called leveling agents, or perhaps more correctly, retarders. All surfactants can generate consider- able foam, except when they are formu- lated from intrinsically low foaming mate- rials, or when they include defoaming agents for the purpose. Although silicone derivatives are wonderfully efficient de- foamers, they are capable of forming water resistant spots on the fiber surfaces if there is any cracking of the emulsions. Their use is to be avoided where practica- ble.
Temperature and Time Hypothetically the time and temperature parameters could be optimized for each individual machine, dyebath formulation and type of goods. Practically speaking, such detailed optmization is often out of the question. However, it is still worth- while to remember that the most cost effective choice of starting temperature, rate of temperature rise, dyeing tempera- ture and length of dyeing which will produce a well exhausted dyebath, with goods of satisfactory color uniformity and shade consistency batch to batch, can mean thedifference between a healthy and a marginal operation. A general rule of thumb has the starting temperature about 70-80C (1 60- 180F), the rate of tempera- ture rise at 1.5-2C (3-4F) per minute, the dyeing temperature between 115-130C (240-265F) and the time of dyeing a t temperature from 15-60 ,minutes. Note the absence of a carrier or atmospheric dyeing recommendation. The conditions should be chosen such that not more than 2-3% of the dye is exhausted for each full cycle of goods through the dyebath or dyebath through the goods. This is particularly important in the early stages of dyeing when the tem- perature is being raised and before the dyeing temperature has been reached. If dyeing times are to be minimized, it is clearly important that the dye goes onto the fiber uniformly in the first place and does not require lengthy leveling by des- orption and resorption to^ produce^ a^ iuni- form product. In these ideal conditions it is not necessary to prolong the dyeing long beyond the time of maximum exhaustion to achieve the maximum penetration and color yield. However, it is clear that the times and temperatures must be depen- dent on the individual dyes present in the formulation and their concentrations.
Dyes Some disperse dye manufacturers indicate in their shade cards the length of time necessary for individual dyes, at particular concentrations and under specified dyeing conditions, to reach effective equilibrium between the fiber and the bath. and also
maximum penetration and color yield.
Although dyes with similar dyeing charac-
teristics (high, medium or low energy) may be selected for a particular formula. tion, it is important to recognize that the actual dyeing rate of each component will be dependent on the concentration present in the formulation. The lower the concen- tration, the more rapidly it will dye relative to the other dyes in the bath. Care should be taken that the dyeing conditions allow the slowest dyeing component to reach effective equilibrium. Individual disperse dyes do not gener- ally interfere with one another when dyed as mixtures. Unlike acid dyes on nylon, disperse dyes do .not block one another from dyeing efficiently. The phenomenon of blocking will be treated in the chapter on acid dyeing nylon. A group of disperse dyes take advantage of the phenomena mentioned in the pre- ceding two paragraphs and offers a num- ber of disperse dyes, each of which is a mixture of several dyes of similar energy level and general hue. Dyeings with these mixtures, either singly or in combination, are rapid because each component of the formulation is present at a relatively low concentration. For example, if a red dis- perse dye product contains five red dis- perse components at equal concentration, then a 1% owg dyeing will be completed in the time it takes the slowest of the red components to dye to a 0.2% owg depth.
Oligomers/Trimers Polyester fibers invariably contain small percentages (1-4%) of relatively intracta- ble, oligometric material, mostly a cyclic trimer (three molecules joined end-to-end in a ring) of ethylene terephthalate. In powder form this trimer is white, has a melting point above that of dyebaths, ca. 320C (610F), and is just soluble enough for some of it to be extracted from tht polyester during dyeing. It can often be seen as a white dusty surface on packages or inside the dyeing machine. Because any of this material dissolved during dyeing can precipitate during cool- ing, it has been found desirable to mini- mize the problem by dropping dyebaths at temperatures above the boil into specia!!y built expansion tanks. This method is not practicable in the case of piece goods in rope form where creases and crack marks might develop, but it is still desirable to lower the temperature and drop the bath as quickly as possible. Nonetheless, it is periodically necessary to wash out pres- sure dyeing vessels a t high temperatures and with some added alkali to assist in hydrolyzing and solubilizing this oligo- meric trimer. Suitable cleaning conditions might be ca. 130C (265F) and five grams per liter caustic soda (8).
Miscellaneous Other materials which show up in polyes-
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