Keywords: Disperse dyes, Dyeing, Supercritical carbon dioxide

Transcription

1 Indian Journal of Fibre & Textile Research Vol. 22, September 1997, pp Water-free dyeing of textile accessories using supercritical carbon dioxide" Dierk Knittel, Wolfgang Saus & Eckhard Schollmeyer German Textile Research Centre North-West e.v., Krefeld, Germany Received I November 1996; accepted 9 December 1996 Results are given about a new dyeing process for dyeing synthetic fibre material with disperse dyes. The use of supercritical carbon dioxide as dyeing medium completely avoids water pollution and the need of drying. Laboratory results show excellent levelness and fastnesses on the dyeing of poly(ethylene terephthalate) and polyamides. Especially, the good results are achievable for small lots like textile accessories. The state of technical development is briefly described. Keywords: Disperse dyes, Dyeing, Supercritical carbon dioxide 1 Introduction The costs of using water or of treating waste in industrial processes like conventional dyeing of textiles or accessories will increase significantly in future. Therefore, new concepts have to be evaluated. For dyeing of unmodified poly( ethylene terephthalate) (PET) or polyamide (PA) fabrics and for some other synthetic material, disperse dyes can be employed. Because of the hydrophobicity of such dyes and of the fibre a conventional aqueous dyeing liquor has to contain large amounts of dispersing agents and surfactants to obtain reasonable dyeing rates and useful shades. Considering the dissolving power of supercritical systems (SC-systems) research has been done at German Textile Research Centre North-West e.v., Krefeld (DTNW), FRG, for the evaluation of those systems as a dyeing medium using disperse dyestuffs'". Textile accessories constitute an important area of clothing manufacture. In this area, like one may imagine for zippers", often smaller lots of specially dyed material but with quick response to the market are required. Therefore, laboratory experiments of dyeing using the new process for treating preformed textile materials like accessories are presented. For those articles even a 'This paper is part of the thesis of W. Saus, University of Duisburg (1997). small pilot plant may be satisfactory for production. The process has been termed as SFD (Supercritical Fluid Dyeing)'. 2 Fundamental A description of the new dyeing medium is based on Fig. 1, where the pressure (P), temperature (1) and volume (V) of carbon dioxide (C0 2 ) are given". Especially the PoT projection illustrates the critical point (CP). When surpassing the conditions of CP the fluid gets solvent properties due to changes in its dielectric constant. Thus, the solvent power, being P- and T- dependent, of pure supercritical CO 2 will reach that of a liquid hydrocarbon like octane, sufficient for dissolving disperse dyes. The transport of dyestuff p /' " " " " "" Fig. I-Phase diagram of CO 2 (ref. 9) [CP-<:ritical point; Tcrt' -31 C; Pcrit -73 bar; g-gaseous; I-liquid; and s-solid]

2 KNITfEL et al. : WATER-FREE DYEING OF TEXTILE ACCESSORIES 185 dissolving disperse dyes. The transport of dyestuff and heat to the material to be dyed is thereby achieved by the supercritical fluid'. Table I shows the typical properties of supercritical fluids like CO 2 which lead to performance properties well suited for dyeing at low viscosity and high diffusion rates. Because of this, a quick transport and quick penetration, even into small pores,will be favoured. All dyeing systems using disperse dyes and exhaustion techniques may be changed to this new technology in future (i.e. even aramids and polyolefins are dyeable). The new process promises to be very ecological and to be highly flexible for quick response to the market. Other physicochemical aspects of supercritical fluid application and their effect may be found in the literature":". So, mainly the applications are described. 3 Materials and Methods 3.1 Materials Laces, knobs (PET, PA), velcro tapes, zippers, belting material, and PET standard fabric (for resist patterning) were used. Dispersant-free disperse dyes (Fa. Novartis AG, Basel) with code nos, and the protonated form of indicator dye Ethylred (Fa. Merck) were used. 3.2 Methods For a systematic investigation of the dyeing capabilities, a high-pressure laboratory apparatus has been built'". It consists of a heatable autoclave of 300 ern' capacity fitted with a pressure sealed stirrer. The system is safe to 500 bar and 350 C. Pressure is applied from a CO 2 gas cylinder via a membrane compressor. The sample to be dyed was wrapped around a perforated stainless steel tube or sewed onto the tube (knobs) and mounted inside the autoclave. Dyestuff without auxiliary chemicals was placed on the bottom of the vessel and the apparatus was closed and preheated. On reaching the working temperature, CO 2 was compressed to the working pressure under constant stirring. Pressure was maintained for the dyeing period of 0-60 min and released afterwards. Standard conditions for 'PET dyeing are C and bar for about min. Table I-Typical properties of supercritical systems Gas Liquid Supercritical fluid phase Density, g ern? Diffusion coefficient, 2 -I cm s 10-1 Viscosity, g em" S-I Resist Dyeing (Patterning) PET standard fabric was resist printed with a 8% paste of alginate thickener (Diagum A8 Diamalt) and dried at 120 C. Afterwards, it was treated like other samples in CO 2, Finally, the thickener and dyestuff, deposited within, were removed by rinsing with water. 4 Results and Discussion Figs 2-8 show supercritical fluid dyeing results on preformed materials having different make-up. The examples are mainly from PET and PA but some mixed materials were included too. Especially, in the case of zippers one has to state that simultaneously to the dyeing of the basic fabric, the teeth of the zipper are evenly dyeable. Similar results are obtainable for the hooks of velcro tapes. The laces shown in Fig. 4 consist of elasthane reinforced PET, which require special dyestuffs for even dyeing. But as shown interesting patterning effects are yet obtainable. In most cases the dyeings show excellent rubbing fastness even without any aftertreatment (depending on dyestuff and amount of dye)3.'2, whereas conventionally dyed samples often require subsequent reductive cleaning because of adherent dye aggregates. In case of surface contamination by residual dyestuff this can be removed by lowering the working temperature (below glass temperature of the sample) after the dyeing period and flushing the material with supercritical CO 2, Wash and light fastnesses usually orient themselves on the properties of the dyestuff used. Fig. 5 shows results on compact accessories, when dyeing PA and PET knobs simultaneously. The PET knob is almost completely dyed all over its cross-section, whereas the PAused reveals a ring dyeing after the usual treating time. The red

3 186 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 1997 Fig. 2-PA zipper dyed with DTNWI9 at 120 C and 260 bar Fig. 3-PA vclcro tape dyed with DTNW 12 at 120 C and 280 bar Fig. 4-Laees dyed with various disperse dyes ill supercritical CO! at o e and 300 bar for 1 min

4 KNITTEL et at. : WATER-FREE DYEING OF TEXTILE ACCESSORIES 187 Fig. 5-Dyeing of PET-,PA- and artificial horn knobs: blue-black PET with DTNWI9; red-violet PA with DTNW 19; and red artificial horn with protonated form of cthylrcd Fig. () -O.S PET wire dyed with Farbstoff DTNW 12 under standard conditions Fig. 7--PA belt dyed with DTNW 17 Fib. 8-PET standard web dyed with DTNW 12 in supereritical CO 2 after resist printing with alginate thickener

5 188 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 1997 knob (artificial horn) (Fig. 5) needs attention insofar as up to now such a material is not dyeable with disperse dyes in supercritical fluid dyeing (SFD). In this case, the use of a derivative of an acid like dye (protonated ethylred), which has proved suitable for polyolefinics too, despite of low solubility in CO 2 gives good results'", Fig. 6 shows the capability of SFD for dyeing compact materials like a PET wire of 0.5 mm diam. Using standard SFD conditions a ring dyeing is obtained, whereas on prolonged treatment an even dyed cross-section can be obtained. Fig. 7 shows dyeing on accessories of mixed constitution (i.e. belting with PA content). Thus, in special cases, depending on the blended sample future dyeing recipes have to be evaluated f9r even dyeing or otherwise one has a possibility of patterning. Differing from the otherwise water-free SFD procedure, another way of patterning is described in Fig. 8. For this sample, a pattern of resist (alginate thickener) has been printed onto the fabric prior to SFD. Since at present, alginate like other carbohydrate compounds are not dyeable in SFD (using disperse dyestuffs), the resist pattern introduces patterning of the PET -base fabric. Even if the example only shows weak contrast, this way of patterning may be improved further. Removing thickener after dyeing causes water consumption. For the examples presented, shade and dyestuff exhaustion may be adjusted by pressure and temperature variations. Since pressure is easier to vary, its regulation will be preferred. For the SFD process, even the dyestuffs which are normally not classified as disperse dyes like some pigments (for spinning mass dyeing), and some small molecular ionic dyestuffs may be used". 5 Conclusions As an illustration for the potential of using supercritical fluids for textile applications, some calculations on the water saving in supercritical fluid dyeing, if widely implemented, for German situation can be done. Dyeing of pure PET fibre materials in FRG (in different kind of preparation e.g. flocks, knitwear, yarns, fabrics, zippers, etc. amounts to about tonnes/annum. If all this could be done according to the SFD process, a saving in water consumption of about 1.1xl07 m' could be gained (using a mean value of 80 L waterlkg). This saving corresponds to the water consumption of the households of a town of about inhabitants". For the whole process of SFD for dyeing synthetic material like PET or PA (and some other) using carbon dioxide as fluid medium the benefits can be summarized as follows: - Complete elimination of water pretreatment and of water pollution, - saving of energy costs for drying textiles, - no need of auxiliary agents, - dyeing occurs with a high degree of levelness and dye exhaustion, - dyeing in a supercritical system requires little time, thus giving high flexibility and promoting 'just-in-time' delivery, - in the case of PET and PA, no aftertreatment like reductive washing is required if dyeing is done properly, -.carbon dioxide is non-toxic, can be gained from natural sources and can easily be recycled in a dyeing process. Especially, the short dyeing time needed as compared to that in conventional dyeing procedures offers advantages regarding flexibility of production. High-pressure apparatus are state of technical art. Development using SFD has only to deal with textile specific requirements (i.e. flow in upscaled apparatus, package of the goods, fluid ratio, dosage of dyestuff, etc.). An optimized gamme of dyestuffs for use in SFD yet exists II At present, the SFD technology is used for the dyeing of velcro tapes and pilot plants of L capacity are being evaluated for their performance at a yarn dyeing factory and at DTNW in order to obtain data for further scale-up":". But even this existing working volume may be now attractive for dyeing small lots of textile accessories. A study of RIZA on potential of large scale SFD of polyester (yarn or fabric) (based on technical information of 1993), which arrives at lower energy and operational costs, states that the process will need, depending on research activities, an implementation time of about 5-10 years for broad acceptance IS. If research promoted by

6 KNITTEL et al.: WATER-FREE DYEING OF TEXTILE ACCESSORIES 189 dyestuff manufacturers will provide suitable dyestuffs for SFD of polyester/cotton blends, this technology will reach general application. Acknowledgement The authors are grateful to Novartis AG (Ciba- Geigy), Switzerland, and to Carbo (FRG) and Diamalt (FRG) for supporting part of this research and to the country of Nordrhein-Westfalen (FRG) for institutional support. References I Saus W, Knittel D & Schollmeyer E, Text Prax Int, 27 (1992) Saus W, Knittel D & Schollmeyer E, Text Res 1,63 (1993) Knittel D, Saus W & Schollmeyer E, 1 Text Inst, 84 (1993) Knittel D, Saus W, Hoger S & Schollmeyer E"Melliand Textilber, 75 (1994) 388, Knittel D, Saus W, Hoger S & Schollmeyer E, Angew Makromol Chem, 21S (J 994) Knittel D, Saus W & Schollmeyer E, Techn Textilien, 38 (1995) Knittel D & Schollmeyer E, Melliand Int, 3 (1995) 20 I, 202, Reiflverschlusse., Du Pont Mag, 67 (1973) 12-15;.Ritschratsch, der Rei13verschlu13wurde 100, DNZ Int, 114 (1993) Chemical Engineering at Supercritical Fluid Conditions, edited by M E Paulaitis, J M L Penninger, R D Gray (Jr) & P Davidson (Ann Arbor Science Publishers, Ann Arbor Michigan) (\983). 10 Supercritical Fluid Science and Technology,edited by K P Johnston & J M L Penninger, ACS Sympos Ser, 406 (1989). 11 Novartis AG (Ciba-Geigy), Basel, CH. 12 Knittel D & Schollmeyer E,.Prevention of water pollution in dyeing processes of synthetic textiles, Eur Wat Pollut Control, 6(6) (1996) Exponat at ACHEMA 1997, Frankfurt, FRG (Fa. Uhde GmbH, D Hagen). 14 Exponat at ITMA 1995, Milano (Fa Uhde, D Hagen and DTNW Krefeld, D Krefeld). 15 Van Asselt W A & Wolterink J W K, Superkritisch C0 1 - verven, inventarisatie en mogelijkheden voor het verven van textiel. Report of Tebodin B, V. NL-7550 Hengelo (for RIZA-institute, NL-S200 Lelystad).