Properties of Dyes for Transfer Printing

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Roland Nash
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1 Properties of Dyes for Transfer Printing By F. Schlaeppi Dyestuffs & Chemicals Division Ciba-Geigy Corporation THE growth in industrial use of most technical innovations is usually a slow process, directly related to their increasing simplicity due to continuing developments. There is also the necessary and expensive training of personnel to supervise production, not to mention plant installation and modification. All of these factors help to retard the acceptance of a new competitive process. But in these respects, transfer printing is an anomaly and so has been its growth pattern. The converter need know nothing of a technical nature, nor does he have to learn about new chemicals or processes. The machinery is relatively inexpensive and limited space is hardly a problem. Because of these factors, the industrial use of the process has far outstripped the technical understanding of transfer printing. A major part of the original development work was concentrated on the testing of available dyes for polyester to determine which would transfer. However, as transfer printing grew from a novelty to a viable process, problems arose in both printing and transferring operations. It became evident that more advanced technology and understanding were necessary in dye selection if transfer printing was to continue to grow. Most of this paper will deal with the varioys factors of dye selection and show how each affects some stage of the transfer printing process. The remainder will treat the research work which is being conducted in order to broaden the scope of transfer printing in the textile industry. The selection of a dye is based on specific physical and chemical properties. Once these are determined and the dye is found suitable, it must also work in conjunction with other dyes and in fact be compatible with the entire printing system. Compatibility involves several problems which will be discussed. Finally, the dye also should have fastness properties adequate for the end use of the article. Only after meeting all of these requirements can a dyestuff be considered acceptable for transfer printing. Chemical nature Since practically all of the transfer printing performed today is on polyester, the requirements for those dyes applicable to this fiber will be discussed here in some detail. Being a hydrophobic fiber, polyester requires dyes which are insoluble or only sparingly soluble in water., Therefore, they do not contain solubilizing groups such as are found in acid or direct dyes. This narrows the dyes available for transfer printing of polyester to those classified as disperse and solvent dyes. Although there are several chemical classes of disperse dyes, those used in transfer printing are either monoazo or anthroquinone dyes. These dyes derive their ability to produce color from groups of atoms 55

2 which are called chromophores. For azo and anthraquinone dyes, these chromophores are, shown on Figure 1. Chemical groups which are then attached to these chromophores and which alter the color of the dye are called auxochromes. By changing the number and types of auxochromes, the dye chemist can synthesize thousands of dyes of differing hues and properties, all based on the same chromophoric system. Figure 2 demonstrates how, by adding simple auxochromes to the anthraquinone nucleus, dyes of various colors are produced. Besides changing the coloristic properties of the dye, it has been shown in many studies that the substituents also determine fastness and application properties of the dye. While a substituent may increase one fastness property, such as sublimation, it may very well decrease other fastness properties, eg, fastness to light. Through manipulation of these factors, hundreds of dyestuffs are synthesized in laboratories every year, but after considering problems with intermediates, production costs and other factors, only a handful will reach the market place. The paramount property of a dye to be considered for transfer printing is its sublimation, i.e. its transformation from the solid to the gaseous state without going through the liquid phase. In addition, the dye also must diffuse into the substrate. Diffusion takes place above the glass transition ter~peratt~re ~f the fiber, which is 17OoC for polyesters. However, the temperature must remain considerably below the melting point of the fiber in order to avoid undesirable effects such as stiffening and yellowing of the fabric. For practical purposes, the temperature of the transfer ma- chine is O C for processing of polyester. Therefore, the dyes required must sublime at temperatures below 200OC. On the other hand, the sublimation temperature of a dye must not be too low, causing it to sublime in subsequent manufacturing processes (pressing, pleating), or even in domestic use (ironing or drying). The temperature at which a dye 1 1 is "gay determined by its molecular weight. Dyes which have been found suitable for transfer printing under practical conditions have molecular weights between 250 and 400. Molecular weight is not the only factor influencing sublimation. Molecules have a property called polarity which causes them to act like magnets with each other. The greater the polarity in a chromophoric system, the more intense the color of the dye, but also the greater are the inter-molecular forces, i.e. the forces which hold the molecules together. Hence, dyes of low molecular weight but high polarity may require higher sublimation tempera tu res. Even if a dye sublimes easily, it may not be adequate for transfer printing due to a property called vapor pressure. When the dye sub1imes;only a specific amount will become a gas. For any given temperature, a dynamic equilibrium is established, i.e. the rate of sublimation equals the rate of condensation. This quantity is known as the vapor pressure of the dye and is a result of the chemical and physical nature of the dye molecule. It is apparent in Figure 3 that in the case of a dye with low vapor pressure, there are not as many dye molecule? available for diffusion into the fabric as there are in the case of a dye with high vapor pressure. Therefore, assuming equal diffusion rates into the fibers, 56

3 C. 1. Number Disperse Yellow 54 Disperse Yellow 33 Disperse Yellow 3 TABLE I Structure Quinoline Nitrodiphenylamine HO \ Disperse Red 60 HO \ Disperse Red 65 Cl HO Disperse Orange 25 Disperse Violet 27 Anthraquinone NH, 0. Disperse Violet 28 Disperse Blue 60 0 NHZ Anthraquinone Disperse Blue 3 Disperse Blue 35 Anthraquinone the dye with a low vapor pressure requires a much longer dwell time to achieve the desired transfer from the paper to the substrate. Examples of dyes which have been found suitable for transfer printing 57

4 FIGURE 1 FIGURE 2 FIGURE 3 FIGURE 4 58

and which are currently in use are shown in Table 1. Compat a bility in transfer Adequate sublimation and vapor pressure are not sufficient requisites to use a dye in a transfer printing process.

5 and which are currently in use are shown in Table 1. Compat a bility in transfer Adequate sublimation and vapor pressure are not sufficient requisites to use a dye in a transfer printing process. When several dyes are applied on a paper, as self shades and especially in combination.shades, they must exhibit a similar rate of transfer. It is generally accepted practice to classify dyes used in transfer printing as high, medium or low energy dyes, depending on the temperature required for 90 per cent development at a dwell time of 30 seconds. Figure 4 shows energy curves for a high energy blue dye and two medium energy oranges. Anywhere along these curves, the dye will give 90 per cent development if the transfer conditions are as stated on the axes. For the blue dye shown, the same transfer effect will be obtained at 250 C for 30 seconds as is obtained at 180 C for 60 seconds. Both oranges give 90 per cent development at 30 seconds dwell time and at 225 C. But it is apparent from their slopes that as the dwell time or temperature changes, they begin to differ greatly. Figure 5 shows per cent transfer of the same dyes at a dwell time of 30 seconds as a function over a practical temperature operating range. As the temperature is increased, the amount of transfer increases for all dyes, but there is a significant difference in the magnitude of change between the two oranges. While the blue and orange 2 change in approximately the same proportion, orange 1 does not. When the temperature accidentally fluctuates on the transfer machine, or when it is increased intentionally to get stronger colors on the fabric, the hue of a combination of blue and or- ange 2 will remain constant, but that of blue and orange 1 will change drastically to the blue side. Figure 6 illustrates the changing proportions of blue and orange 1 compared to the constant proportion of blue and orange 2 on the fabric as the transfer temperature is varied. If the mixtures at 22OOC gave a jet black, at 19O0C the combination with orange 1 would become a brown, but that with orange 2 would remain black. It is evident that the task of matching a four color design could become impossible to the transfer printer, if all of his dye combinations displayed an incompatibility similar to that of blue and orange 1. This incompatibility, besides causing fluctuations in hue, may also give rise to haloes, as shown in Figure 7. In this case the blue component used is of much lower energy than the other dyes in the mixture. It is subliming so profusely, that it is moving sideways. Figure 8 shows the correction of this problem by using a compatible blue, i.e. a higher energy blue of similar sublimation properties to the other components in the black mixture. Other physical requirements While compatibility in regard to rate of transfer is a primary consideration, other physical requirements are equally important. One of the factors is solubility of the dye in the vehicle employed by the paper printer. The top left of Figure 9 shows paper printed with an ink containing the dye in dispersed form. On the right is the same dye in the same concentration, but in solution (in this case the ink contains a solvent for this dye). Below are the resulting transfer prints. The dye in solution penetrated deeply into the paper and does not reach the textile during the short dwell time. 59

6 FIGURE 5 FIGURE 6 FIGURE 7 FIGURE 8 60

7 The problem of solubility in the solvent system also affects the printer, especially in gravure printing. A dye which dissolves in the solvent can recrystallize upon temperature changes of the ink or due to saturation caused by the evaporation of solvents during the printing. Figure 10 shows the dispersion of a dye in a proper solvent system after several days in storage. Here the dispersion has not been destroyed. Figure l l shows the same dye in the same concentration, but dispersed in a solvent in which it was slightly soluble. After several days of storage, large crystals of dye have formed that may not only affect the tinctorial strength and lead to irregularities in the final print, but may also damage cylinders and doctor blades causing streaks in the print. Another problem in transfer printing inks is agglomeration of disperse dye particles, causing specks in the print. It is also possible that dyes which do not agglomerate when used individually, will on mixing interact to cause agglomeration and spotty prints. This can be seen in Figure 12 where a blue dye and a yellow dye have been used together to form a green print. Although the individual dyes give level prints, the combination shows dye specks. This may not be apparent on the printed paper, but only show up after the transfer. Trichromatic printing systems A complete series of dyes meeting the considerations for their application by the transfer technique as well as giving adequate end-use fastness, is not available. However, paper printers employing a process printing system, have long been acquainted with the use of three primary colors, a magenta, a cyan and a yellow, to produce a complete color range. In transfer printing, the same results are attainable with a pink, such as C.I. Disperse Red 60, one or two blues, and a yellow, such as C.I. Disperse Yellow 54. A trichromatic system of this nature allows the production of a wide range of shades, including black, and gives a maximum of reproducibility of shade on the transfer machine. Additional dyes can be added for novelty shades, but their use in combination should be avoided. Rotary screen printing machines Rotary screen printing promises to be an economical process for the production of transfer paper, particularly because of the high cost of gravure cylinder engraving. Hence, there is considerable interest in this approach by conventional textile printers. However, the design versatility of gravure printing cannot be matched. One of the significant technical differences between rotary screen print machines and gravure printing is the following : In gravure printing the paper is dried after each print cylinder, whereas this is not the case for rotary screen printing. This means that the preceding colors on the paper are crushed by subsequent screens. Unlike textiles, paper cannot absorb large quantities nf nrimt nncta nma nriich;-n r ha V I PI IllL JJUJLW, UIIU -1 UJlIIll~ lllay UG detrimental to design definition. It is, therefore, desirable to deposit less print paste on the paper, but to increase the dye concentration.to obtain the same depth of shade. Special, highly concentrated disperse dyes are already on the market for this purpose. 61

E 9 FIGURE 11 FIGURE 14 FIGURE 12 62 New developments Thus far, we have dealt with the problems in dye chemistry which have been overcome and

8 E 9 FIGURE 11 FIGURE 14 FIGURE New developments Thus far, we have dealt with the problems in dye chemistry which have been overcome and therefore made transfer printing possible. But there are still many problems which remain unsolved. Of these, the most important limitations are:

0 Fastness to sublimation on polyester. 0 Wash fastness on polyamide. 0 Wet fastness and crocking on acrylics. 0 Use on cotton/polyester blends. 0 Transferring on cotton and protein fibers.

9 0 Fastness to sublimation on polyester. 0 Wash fastness on polyamide. 0 Wet fastness and crocking on acrylics. 0 Use on cotton/polyester blends. 0 Transferring on cotton and protein fibers. These problems come as no surprise tn tho he range of usable dyes is considered. If appreciable improvements are to be made, then slight modifications of the disperse dyestuffs are not sufficient: something fundamental has to be changed. At this point I should emphasize that the following are merely results from research described in patent applications of CIBA-GEIGY and of some others. As in all research, to make any valid predictions as to if and when such developments come to practical Ciuition is not possible. First, let us consider the sublimation fastness on polyester. A possibility for improvement is provided by the development of the vacuum technique. Apart from the fact that it gives better penetration and that it has been used for transfer printing of carpets, a vacuum promotes sublimation of the dyestuffs at a lower temperature. By ming normal temperatures and a vacuum, it is possible to transfer high energy dyes having better sublimation properties. This technology also may offer the possibility of faster transfer rates or lower temperatures for fragile r nn1vmm-s -- J ~ g 2s ~ acrylics. h Hoy+re\~e:, it is possible that application of a vacuum may cause dyes to transfer faster than they can diffuse into the fiber. This would bring about an increase in surface dye and an overall decrease in fast ness properties. Another method of increasing the sublimation fastness involves chemical reaction between the dye and a second compound. Such a compound can be applied either before or after the transfer of the dyestuff by transfer from a second paper or by a wet pretreatment of the textile. A typical reaction is seen in Figure 13. The process requires a dye having reactive groups, such as amino or hydroxy substituents, capable of reacting with a polyfunctional fixing agent. Particularly suitable for this purpose are polyisocyanates which can be used in the masked form, for example, as reaction products with malonic acid, and which are not released until heating occurs. This avoids reaction with the paper or moisture in the atmosphere. The new dye formed in the fiber is then very fast to sublimation due to its increased molecular weight. Figure 14 demonstrates the improvement in sublimation fastness of a conventional yellow dye on treatment with a second transfer paper containing the fixing agent. Although it is not possible to obtain adequate fastness to wet processing on polyamide in transfer printing with available disperse dyestuffs, considerable yardage of material is already being printed by this process. An even greater potential could well be expected if it should be possible to improve the wet fastness. Again this can be achieved by chemistry. Figure 15 shows the result of a water bleeding test of two red dyes on polyamide. The stain caused by the conventional disperse dye is unacceptab!e. PLr! experimental disperse dye with a reactive moiety in its structure is capable of reacting with amino groups in the polyamide substrate. Being chemically attached to the fiber, it cannot bleed. Acrylic fibers cannot be transfer printed with satisfactory wash and 63

10 FlGURE 16 FIGURE 17 FIGURE 18 as monoethanol amine in place of sodium hydroxide. Incidentally, this paper won first prize in the International Technical Paper Cnmpet_it_icJn; The development of a transfer printing process for cotton and especially for the important cotton/polyester blends would have an enormous commerical impact. Unfortunately, there is no known successful method at this time. An interesting approach is shown in Figure 17. When a polyester/cotton blend is printed by the

11 transfer process with disperse dyestuffs, then, as shown on the upper left, the cotton constituent remains white. When the textile material is pre-treated with aqueous solutions of suitable swelling agents and dried before transfer printing, then a perfect solid shade can be obtained, as shown on the right. Unfortunately, however, most of the dyestuffs on the cotton are bv All attempts to improve the fastness to washing on the cotton portion by condensation with finishing agents, etc., have so far failed to give satisfactory results. Perhaps the most promising approach to transfer printing of natural, as well as certain synthetic fibers, involves the use of different types of reactive disperse dyes, such as described in CIBA-GEIGY S U.S. Patent 3,632,291. So far we have mentioned new chemical approaches to transfer printing. However, apart from present limitations in regard to suitable fibers and/or adequate fastness, the transfer printing process has one inherent disadvantage. It requires a support from which the dye is transferred to the textile. At the present state of the art, the support is a paper sheet. Paper is expensive, in short supply and poses a considerable disposal problem. In view of the fact that there are many research teams working on transfer printing without waste support, it can bt; expectedfhat new and possibly complimentary processes may eventually be developed. As seen in Figure 18, transfer printing is still a young technology and as it matures, there will undoubtedly be significant improvemen ts. Acknowledgement: A number of the ideas presented in this paper were taken from a lecture by R. Peter of CIBA-GEIGY, Ltd., Basle, given at the Transfer Printing Symposium in Manchester, England on September 12, I A Technical paper Correlation of Properties and Chemical Structure of Disperse Dyes for Polyester was given by the author at the Southern Textile Research Conference in May,

Dyeing of Cotton Fabric with Basic Dye in Conventional Method and Pretreated with Cationic Polyacrylamide

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