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Dyeing Printing & Finishing

Cutting-Edge Research

AATCC's International Conference and Exhibition presented the science behind the latest we-processing technology Part Two.

AATCCs International Conference and Exhibition presented the science behind the latest wet-processing technology Part Two.The 1998 AATCC International Conference and Exhibition in Philadelphia featured a technical program of more than 70 presentations on a myriad of subjects, ranging from history of textile chemistry to retailing technology to cutting edge research and development.An article in the December issue of ATI discussed technological developments in textile chemistry. Part Two summarizes the content of selected papers presented at the conference concerning the dyeing of textile materials.R.F. Hyde and G. Thompson of BASF discussed how market changes affected requirements and practices in continuous dyeing of polyester/cotton blend fabrics. Demand figures showed that cotton remains the most consumed fiber at 40.3 percent of world consumption while polyester is second at 27.2 percent.The authors contended that demand for polyester/cotton blends will increase and that continuous dyeing of these blends will grow in importance in the future. Although the Thermosol-based continuous dyeing process for polyester/cotton blend fabrics has been practiced since its introduction in the 1950s, market conditions for dyed fabrics have changed dramatically.Greater emphasis on fashion, the resulting demand for smaller lot sizes, and more stringent demands from consumers with regard to quality and fastness all placed greater pressure on continuous dyers in recent years. These changes call for better control of reproducibility and Right-First-Time (RFT) dyeing. The paper advocated the use of Controlled Coloration in continuous dyeing. The term Controlled Coloration refers to the fusion of three principal factors, Management Technology, Application Technology, and Dyestuff Technology, to achieve reproducibility and RFT objectives. In order to achieve RFT consistently, management activity must be proactive and focused on managing production rather than reactive and focused on repairing what has already gone wrong. Since all continuous dyeing processes use the Thermosol process to fix dyes to polyester, selection of the particular disperse dyes is the major variable.The choice of dyeing method depends mainly on the selection of dye class to dye the cotton portion of the blend. The most common method of continuous dyeing is the one-pass method, where the disperse dye is thermofixed and then the cotton is dyed in subsequent steps in the dye range.This process is used with disperse dyes in combination with either reactive, vat or sulfur dyes. High-speed dye ranges, such as those commonly used in the United States, hold as much as 1,000 yards of fabric in process from entry to exit points in the line. Machine stoppages or control failures may result in large amounts of defective fabric.If reactive dyes are used to dye the cellulose portion of the blend, an alternative to the full Thermosol-pad-steam process that may be suitable for dyeing short lots is a pad-Thermosol process followed by a semi-continuous pad-batch method. Another alternative to the full pad-Thermosol/pad-steam method is to thermofix both the disperse and reactive dyes in the same stage, thereby completely eliminating a pad-steam stage.The requirements of reactive dyes for this process are that they fix at temperatures used to thermofix disperse dyes to polyester and that they fix under mildly alkaline conditions. High alkalinity negatively affects fixation of some disperse dyes and may cause yellowing of cellulose at high temperature. One-step dyeing also requires selection of disperse dyes that do not require reduction clearing since treatment with hydrosulfite is not possible with reactive dyes in the fabric.The authors point out that these alternative methods are not acceptable for all fabrics or depths of shade, so the conventional two-stage method for continuous dyeing of polyester-cotton fabrics remains important.Major developments in disperse dyes in recent years have been in dyes with low thermal migration and high wet-fastness properties. Disperse dyes based on thiophene chemistry and benzodifuranone chemistry were touted as examples of these developments. The chromophore in thiophene dyes is cleaved by hot alkali so these dyes are cleared using an alkali treatment in continuous processing.Dyes based on benzodifuranone were said to give high wet-fastness without reduction clearing and to fulfill all of the requirements of continuous dyeing. The main impetus for development of reactive dyes has been the desire to achieve better fixation so less dye enters the wastewater. Since bifunctional dyes can give dramatic improvements in fixation levels over monofunctional dyes, their use may help lower color pollution in wastewater.  Batch DyeingRepresentatives of Sumitoma Chemical America Inc. discussed the use of Sumafix Supra dyes in a low-salt dyeing process. According to the authors, the key to low-salt dyeing with reactive dyes is dye selection. For low-salt dyeing, the substantivity of the dye must be relatively insensitive to concentration of salt in the dyebath.Reduction of the amount of inorganic salt by one-third to one-half, compared to conventional dyeing, was said to be possible by using judicious dye selection and the low-salt dyeing method. Other advantages claimed for the low-salt dyeing method included: favorable economics because of value of the salt saved; environmental friendliness due to less salt in the aqueous effluent; excellent leveling because dyes migrate better in the presence of lower salt concentration; less inside-outside differences in package dyeing; more flexibility to the dyer in preparation and addition method for the salt; and better wash-off properties because there is less salt to be washed out of the fabric at the end of the dyeing process. Even though the low-salt dyeing process is based on selection of dyes with low sensitivity to concentration of salt in the dyebath, somewhat lower color yield is achieved with use of lower salt concentration. The paper did not discuss the effect of lower dye yield on the economics of the low-salt dyeing method or fastness properties achieved with the low-salt method.A paper by Y. Yang and S. Li of the Institute of Textile Technology discussed the single-bath, one-step dyeing of polyester/cotton with disperse/reactive dye combinations. Typically, polyester/cotton blends are dyed using a two-bath, two-step process for the following reasons: reactive dyes fix to cellulose under alkaline conditions, while disperse dyes require acidic dyeing conditions; reductive scouring and the high dyeing temperature typically used in disperse dyeing of polyester may hydrolyze fixed reactive dye on cotton or reactive dye in the dyebath, causing loss of color yield; and the high level of salt required for application of reactive dyes may adversely affect dispersion stability and yield of disperse dyes on polyester.The study demonstrated the feasibility of a single-bath, one-step process based on dye selection and process temperature, pH and alkalinity optimization. Appropriate disperse dyes for the process were those with good stability in the weak alkaline conditions used in the process.Reactive dyes suitable for the process were those having low reactivity reactive groups and parent dye structures stable under high-temperature alkaline conditions. Pyrimidine and monochlorotriazine dyes were said to work well in the process because of their inherently low reactivity.Although only a limited number of parent dye structural types was used in the study, the authors said that anthraquinone and phthalocyanine structures should perform better in the process than most azo dye structures. Optimum conditions for the process were said to have a dyeing time of 30 to 60 minutes, a temperature of 120 to 130C and a pH of 9 to 10.Advantages of the proposed one-bath, one-step dyeing process were said to be savings in time, water, energy and labor; an increase in productivity; and a decrease in problems with trimer in package and beam dyeings. EnvironmentalLynn Dewees of Waterford Compliance Group Ltd. presented a study concerning the fate of volatile organic compounds (VOC) present in liquid dyes. The impetus for the study was the concern that use of liquid dyes in dyeing of carpet would trigger air permit requirements under Title 5 of the Clean Air Act. Commonly, the assumption is made that all VOCs in liquid dye are emitted to the atmosphere.However, since the VOCs in liquid dyes have low vapor pressure and are extremely water soluble, the study hypothesized that part of the VOCs would be absorbed by water as the carpet passed through the steamer in the dyeing process. The results confirmed that the VOCs from liquid dyes were spilt between water and air emissions.At one of the two facilities studied, the mass of VOCs in the wastewater was 15 times that in the air. At the second facility, the ratio was 7-to-1. The paper concluded that the use of liquid dyes will not necessarily trigger Title 5-permitting requirements, and that each mill must review its own situation to determine applicability of Clean Air Act regulations.George Baughman of the University of Georgia presented the results of studies on presence of ionic copper in copperized dyes. The study included 16 commercial dyes having azo, formazan, or phthalocyanine structures. Twelve of these contained 0.007- to 0.2-percent cupric ion, while the other four contained more than 0.2-percent cupric ion. According to Baughman, dyers should not assume that all of the copper in copperized dyes is complexed by the dye and non-toxic.

February 1999



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