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‘Waterless’ polyester dyeing – a reality?

Background

Until the 1940s, the textile industry was dominated by natural fibres, the principal fibres being wool and cotton. There were no truly ‘synthetic’ fibres, only the regenerated cellulosic fibres, the viscose and the acetate rayons, while worldwide total fibre consumption by 1951 was a mere 11.5 megatonnes (million tonnes). Wallace Carothers of DuPont’s pioneering research laboratory led a team which had developed aliphatic polyester polymers in the period 1929 to 1931. These polymers are formed by the interaction of bi-functional molecules containing hydroxyl and carboxyl groups, the most common method being the esterification of a dibasic organic acid with a diol. The low melting point of the polymers discovered by Carothers’ team prevented fibre spinning, so that they side-stepped polyester, abandoning the project in favour of polyamides. Carothers and his team developed the polyamides in 1938, producing nylon 6.6 by the polymerisation of hexamethylene diamine and adipic acid with nylon textile materials having been produced in Wilmington and commercialised in the USA by 1941.

A review of the published work of Carothers and his co-workers led to parallel investigations. During 1939 to 1941, J.R. Whitfield and J.T. Dickson at the Calico Printer’s Association (CPA), Broad Oak Works at Accrington in the UK made significant, additional discoveries which led to commercial fibres. These included new aromatic polyesters, with high melting points, that were capable of yielding fibres by melt extrusion. Drawing during take-up partially oriented the long-chain polymers. The simplest and most important of these polymers was poly (ethylene terephthalate) (PET), formed from terephthalic acid and ethylene glycol. The inventors named this product Terylene and filed for patent protection [1].

The Second World War delayed the commercial development of this polyester fibre but, in 1946, DuPont purchased the patent rights from the CPA for the USA. Imperial Chemical Industries (ICI) received the rights to development for the rest of the world in 1947, including the scale-up of terephthalic acid production. The first sale of Terylene occurred in October 1948; DuPont developed Dacron whilst Hoechst developed Trevira with other producers entering the market under licences from ICI.

Worldwide fibre consumption

In 2012, worldwide fibre demand was 85.8 megatonnes and of this total, the production of synthetic fibres was 56 megatonnes (65.3% of the total). Polyester production was 41.4 megatonnes, so that polyester represented 73.9% of synthetic fibre production or 48.3% of total fibre demand. Although nylon has stood the test of time as the first commercially developed synthetic fibre, the figures above indicate the significance and importance of polyester both singly and in blends in the textile industry. It could perhaps be concluded that polyester is the only synthetic fibre that in real terms has fulfilled the early financial and technical aspirations of polymer and fibre research.

Development of dyeing methods for polyester

Disperse dyes were originally developed for use on cellulose acetates but were later found to be useful for polyester. Low molecular mass disperse dyes could be applied to polyester at atmospheric dyeing temperatures in the presence of the so-called ‘carriers’. Disperse dyes of higher molecular mass and more complex structures were developed later specifically for polyester. The development of the chemistry and application of disperse dyes has been extensively documented [2]. Carriers were essentially fibre plasticisers, typically phenols and chlorinated aromatics, which were used at relatively high concentrations of 2 to 5 g/l and were major pollutants in the effluent which would now be unacceptable. Until the introduction of polyester in the 1950s, there had been little requirement for dyeing machines to be pressurised to achieve temperatures above 100°C, but the diffusion of dyes into polyester is slow at the boiling point of water. In addition, the newer disperse dyes being developed for polyester required high temperature conditions for satisfactory dyeing to achieve the desired fastness and levelling properties whilst high temperature conditions resulted in shorter dyeing times. With adequate pump capacities and careful dye selection, high-temperature dyeing techniques were developed.

Following successful laboratory investigations [3], the first successful commercial high-temperature package-dyeing machine for yarn was the Belgium Steverlynck Static process [4], with other machinery makers following with their own designs. The developments in package-dyeing machinery have been reviewed [5]. Modern package-dyeing machines operate at temperatures up to 140°C, with flow rates of 60 litres/kg/min and liquor ratios as low as 3:1.

Early high-temperature machines for fabric depended on pressurising shallow-draft winches (for fabric in rope form) and jig-dyeing machines (for open-width fabric) which were not always successful approaches. Beam dyeing, a package-dyeing method for fabric, was more readily developed as a successful high-temperature method. A major development for H-T dyeing of fabric was the jet dyeing machine, the principle of which depends on injecting the dye liquor at high speed from an annular orifice around a rope of fabric as it passes through a venturi. The first commercial jet-dyeing machine, based on a 1958 patent by Burlington Industries in the USA, was launched by the Gaston County Company in 1961. By 1995, nightmare proportions had been reached regarding the selection of the optimum jet-dyeing machine for a particular fabric type, since there were 43 manufacturers of jet machines worldwide with over 100 design variants, ignoring those of machine size. Jet-dyeing machine developments were reviewed [6]. Although design and machine features have continued to develop and improve, some rationalisation and categorisation of machines have occurred into two principal machine configurations. These are machines based on a long shallow bath and machines based on compact ‘J-box’ related designs which have been termed ‘banana’ and ‘apple’ configurations respectively. Guidance has been given on the selection of the appropriate machine according to fabric type [7].

The low aqueous solubility of disperse dyes, particularly when dyeing dark colours, can lead to the presence of surface dye and a reduction in fastness properties, particularly to rubbing. This requires a ‘reduction clearing’ process, carried out with alkaline reducing agents. This raises a further environmental problem, although dyes that can be cleared with alkali have been developed. In high-temperature dyeing machines with H-T drains, dyeing liquors can be released at high temperature through heat exchangers to recover the sensible heat whilst this operation removes the oligomer released by the substrate. Although conventional polyester dyeing processes are carried out in the pH range of 5 to 6, alkaline application processes have been developed. In addition to the batchwise/exhaust dyeing processes discussed above, considerable volumes of polyester and blended-fibre fabrics, particularly with cellulosic fibres, are dyed or printed by continuous processes.

Supercritical fluids

Supercritical fluids are defined as gases that exist above the critical value for both temperature and pressure and although they do not contain two phases, they exhibit the properties of both a liquid and a gas. Supercritical fluids may act as both a solvent and a solute, have higher diffusion coefficients and lower viscosities than liquids whilst with the absence of surface tension, they exhibit better penetration into materials.

11Carbon dioxide has long been recognised as a gas with excellent dissolving properties for any hydrophobic substance under either liquid or supercritical conditions. As a result of its low critical point at a pressure of 73.858 bar and a temperature of 31.05°C, it has found many applications in industry, including classical extraction methods for coffee and tobacco. While for many years, extraction techniques were the main application, later patents covered the impregnation of polymers, plastics and rubber with various materials.

Supercritical fluid dyeing technology

Supercritical carbon dioxide (sCO2) dyeing has been comprehensively reviewed [8-10].The first patent to focus on the dyeing of textile substrates was submitted in 1988 and described a process in which dye-containing supercritical fluid flowed through or streamed across the substrate. In addition, the application of polar co-solvents such as water, alcohol and/or salts by changing the polarity of the supercritical fluid was described [11]. This patent was later supplemented by a number of more far-reaching patents [12]. After the first successful laboratory-scale dyeing of PET with this new technology in 1989 at the Ruhr-University of Bochum in Germany [13], the work was continued at Deutsches Textilforschungszentrum Nord-West e.V (DTNW) at Krefeld (Germany). Initial work was carried out in a static dyeing apparatus consisting of a 400 ml autoclave with a stirrable dyeing beam [8, 14]. Ciba-Geigy joined this consortium; developing and patenting many disperse dyes during 1992-93 suitable for carbon dioxide dyeing.

Based on optimum laboratory-scale dyeing conditions, the first semi-technical scale equipment was constructed in 1991 by Josef Jasper GmbH & Co of Velen in Germany in conjunction with DTNW. The autoclave had a capacity of 67 litres, designed to dye a maximum of four packages, each weighing 2 kg, several patents being filed by Jasper. In 1994, this pilot equipment from Jasper was installed in industry to assess the transfer of the technology to the dyeing of PET sewing threads. Unfortunately, many technical problems arose. In 1995, UHDE Hochdrucktechnik GmbH of Hagen in Germany, in conjunction with DTNW, constructed a new CO2-dyeing pilot-plant with an autoclave of 30 l with a capacity for two packages of yarn or fabric wound on a beam. In addition to the dyeing process, the UHDE equipment was extended to include an extraction cycle for removal and separation of excess dyes and spinning oils during the dyeing process, for cleaning the plant at colour changes and for re-cycling CO2.  An additional dye storage vessel and a high flow-rate pump were included. This plant was exhibited in Italy in 1995 and Japan in 1996 but was subsequently withdrawn.

In 1999, a German domestic-textiles producer and finisher (Ado Gardinenwerke GmbH & Co of Aschendorf) joined UHDE and DTNW in a three year project to evaluate results of research work and to scale-up a suitable plant [15]. Since 1995, growing interest was observed worldwide in the USA, Asia and Europe. Bach et al [15] summarised laboratory-scale work up to 2002, together with some of the attempts at scaling-up. Work in the USA, which included a joint project with Unifil Inc and Ciba-Geigy, appears to have terminated in 1999 [16]. A package-dyeing machine for PET yarn appears to have been constructed [17] but neither the results nor experience have been published. Similarly, no results were published concerning equipment constructed in 2004 by a consortium consisting of Hisaka, Mitsubishi Rayon and Teijin in Japan [18].

Some limitations

As indicated above, sCO2 dyeing is ideally suited for dyeing polyester with disperse dyes, dyeing equipment generally operating at temperatures up to 140°C under a considerable pressure of 250 to 300 bar. The high quality of the engineering required to withstand such conditions increases the cost of equipment in comparison with water-based dyeing machines.

Whereas disperse dyes are soluble in sCO2, reactive, direct and acid dyes that are used to dye natural fibres are almost insoluble in sCO2 which cannot break the hydrogen bonds in polar fibres such as cellulose, wool and silk to the same extent that occurs in conventional aqueous dyeing processes. This prevents dye diffusion into the fibres and, in addition, conventional disperse dyes only exhibit slight interaction with polar fibres. Various approaches have been attempted to overcome the problem of dyeing natural fibres in sCO2, including the use of swelling and cross-linking agents, co-solvents and fibre modification, which tend to detract from the major advantages of sCO2 methods when compared with aqueous procedures. A further alternative approach has been to modify the disperse dyes which are soluble in sCO2 with fibre-reactive functional groups for cellulose or protein fibres.

Advantages of sCO2 polyester dyeing

The advantages of this process for dyeing polyester with disperse dyes compared with conventional aqueous-based systems were discussed [8] and these are claimed to include the following:

  • – carbon dioxide is not toxic, can be obtained from natural sources and is readily reclaimed
  • – water is completely eliminated, including pre-treatment and effluent
  • – saving in energy since no drying is required
  • – dyebath auxiliary products are not required
  • – level dyeing and high degrees of dyebath exhaustion are obtained
  • – short dyeing cycles giving quick response (QR) and just-in-time (JIT) delivery
  • – no after-treatments are required

Latest developments

In 2010, the latest development from the Netherlands was the launch of the first sCO2 dyeing equipment by DyeCOO Textile Systems BV, following eleven years of development. This was initially carried out by DyeCOO’s parent company, FeyeCon Development & Implementation BV, with additional technical and engineering experience being provided by partners Stork Prints and the University of Delft. Specially developed disperse dyes were developed on an exclusivity basis by Chemische Fabriek Triade BV and a control system was supplied by Setex Schermuly Textile Computer GmbH of Germany.

The first production equipment with three dyeing vessels, the DyeOX 2250 series 1, was installed in Thailand during 2012. This machine will dye fabric batches of 175 to 200 kg at open width of 60 to 80 inches. A laboratory machine is available for colour matching. CO2 recovery is claimed to be 95%. This equipment is illustrated in Figure 1. Hunstman, Ikea and Nike have entered into a strategic partnership with the consortium to assist in promoting the new technology whilst Ikea have opened a ‘waterless dyeing’ facility at its Taiwanese contract manufacturers, Far Eastern New Century Corp. ((FENC).

2Figure 1 – DyeCOO Waterless dyeing equipment

Conclusion

Whilst other techniques have been evaluated, including processes such as solvent dyeing, to achieve so-called ‘waterless’ dyeing operations, this review indicates that the most promising technique currently available is that based on supercritical carbon dioxide. This technique is applicable to polyester using disperse dyes but further work will be required if this is to be extended to other fibre types. It perhaps remains to be seen whether high levels of RFT can be obtained by this technology and since equipment is expensive, perhaps dyeing costs need to be analysed before producers invest in this new technology, despite its environmentally-friendly credentials. Finally, current upward movement in dye prices may be a significant factor.

References

 [1] GBP 578 079 (29 July 1941)

 [2] Colorants and Auxiliaries, Volume 1 – Colorants, 2nd Edition. Ed. John Shore (Bradford:

       Soc. Dyers & Col., 2002)

 [3] G. L. Royer et al, Text. Res. J., 18 (1948), 598 and Amer. Dyestuff Rep., 41 (1952) 533

 [5] L. Drijvers, Teintex. 17 (1952) 294; BP 678 952; 697 687 (1951)

 [5] J. F. Gaunt, J. Soc. Dyers & Col., 109 (1993) 175 and 233

 [6] M. White, Rev. Prog, Col., 28 (1998) 80

 [7] J. Park TextileToday, 6, April and May 2013

 [8] D. Knittel, W. Saus and E. Schollmeyer, J. Text. Inst. 84 (4) (1993) 534

 [9] E. Bach, E. Cleve and E. Schollmeyer, Rev. Ptog. Coloration, 32 (2002) 88

[10] E.Bach and E. Schollmeyer in Environmental aspects of textile dyeing.

       Ed. R.M. Christie (Cambridge: Woodhouse Publishing, 2007)

[11] E. Schollmeyer, D. Knittel, H-J Buschmann, G.M. Schneider and K. Poulakis,

       DE3906724 (Deutsches Textilforschungszentrum, Germany, 1990)

[12] DE4200332 (1993); DE4202320 (1993); DE4344021 (1995)

[13] K. Poulakis, M. Spee, G.M. Schneider, D. Knittel, H-J Buschmann and E. Schollmeyer,

       Chemiefasern Textilind., 41/93 (1991) 142

[14] D. Knittel and E, Schollmeyer, Inst. J. Clothing Sci. Technol., 7 (1995) 36

[15] E. Bach, P. Nünnerich and A. Schüler, Final report BMCF Project no 0339775/8

        (DTNW), 0339778/2 (Ado) (2002), 0339777/4 (UHDE)

[16] C.L. Seastrunk and G.A. Montero, N.C. State Univ, (1999)

[17] G.A. Montero, C.B. Smith, W.A. Hendrix and B. Butcher, Ind. Eng. Chem. Rev.,

       39 (2000) 4806

[18] S. Aoyma, Teijin Ltd., 2005

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