| The textile dyeing industry is believed to be one of the biggest consumers of water. In addition, the dyeing of fibers generates large amount of waste water which is difficult to be treated because the water is contaminated with dispersing agent, surfactants and residual dye. The value of water as a natural resource, as well as the cost of waste water treatment, are both becoming more serious concern to the dyeing industry. As a result, the textile industry has accelerated efforts to reduce or eliminate water consumption in the dyeing and coloration of textiles. One such approach is the use of supercritical carbon dioxide (SC-CO2) instead of water as the dyeing medium.Dyeing with supercritical carbon dioxide has advantages over conventional aqueous dying. Since no chemicals were used for SC-CO2 dyeing,. the CO2 could be easily recycled and the dye could be recovered eliminating virtually all waste. Energy savings could be realized by eliminating the need to dry the substrate at the end of the process. Supercritical CO2 dyeing processing has been investigated in many countries including Germany, USA, U.K., France, Italy, Netherlands, Japan, Korea and some others in the past few years. The solubilities of disperse dyes in SC-CO2, the dyeing process and mechanism of PET in SC-CO2, the development of disperse dyes suitable for CO2 dyeing, the dyeing of natural fibers in SC-CO2 have been focused to be investigated. Despite outstanding success in these areas, there is a lack of information on how to commercialization this technology.Our group began to investigate the SC-CO2 dyeing technology in 2001. Two apparatus had been built. One of them was used to study the dyeing process of PET fiber in SC-CO2. The other was used to determine the solubilities of disperse dyes in SC-CO2. Based on the experiences with these two apparatus, a pilot CO2 dyeing plant consisted of a dyeing autoclave with a 24 L capacity was built. By utilizing this plant, the key technologies of dyeing of textiles in SC-CO2 on industrial scale were studied including the levelness of package dyeing, the reproducibility of the dyeings and the compatibility of mixed disperse dyes. Extensive and systematic dyeing tests of this work revealed valuable information for scale-up and transfer of this technology into industry.1. At the beginning of the scaling-up of the SC-CO2 dyeing process, one of the technological problems usually encountered is the levelness of the dyed PET bobbins. The influence factors on the levelness were investigated. a) The result showed that the CO2 flow rate has the highest influence on the levelness. The higher the CO2 flow rate, the more uniform is the dyeing of the package. b) The bobbin features also influence the evenness of the dyeings. Differences in yarn package density result in permeability variations, which directly affect flow behavior within the package. Therefore, a precision bobbin winder was needed to ensure the uniformity of the yarn package density. The yarn package density also changed during the dyeing process due to the shrinkage of the non-heat-set yarn. This problem can partly be overcome by using elastic stainless steal tube instead of porous stainless cylinder. c) Our plant is able to reverse the direction of the CO2 fluid so that flow is either from inside-to-outside or outside-to-inside of the yarn package. When the reversal interval is set to 5 min, an even dyed yarn package can be obtained.Three batches of yarn package were dyed under the same conditions to test the reproducibility of the dyeings. The dye uptake of the three bathes were approximately equal and the color yield of the three batches were nearly identical, indicating that the parameters of the plant are reliable and the plant can be fully fill the demand of industrial practice.2. The uptake of disperse dyes in SC-CO2 dyeing was reported to be lower than that in aqueous dyeing. The dye uptake is influenced by the process conditions, the behavior of dyes and the properties of fibers. However, the design of the dyeing plant also plays an important role. a) As observed during the first test runs of the pilot plant, the dye uptake was low, resulting from the non-optimum design of the dyestuff autoclave in which melting of disperse dyes during CO2 dyeing occurred. After the structure of dyestuff autoclave was improved, the dyes can be dispersed evenly in the autoclave reducing agglomeration of dyes and deep dyeings can be obtained. b) The coloration of PET in SC-CO2 increases with CO2 flow rate, temperature and pressure. The dye uptake and fastness properties of PET dyed at 1100 kg/h,120℃,25MPa for 60 min in the pilot CO2 dyeing plant is similar to those of the water dyed sample.3. A fundamental understanding of chemicophysical principles in the supercritical CO2 dyeing will be valuable in designing machines, dyes and processes. The kinetic and thermodynamic of dyeing of polyester in SC-CO2 were studied. The diffusion coefficient of C.I. Disperse Red 60 in PET in supercritical CO2 increases with the increase of temperature; the activation energy calculated by the Arrhenius equation is 22.22 kJ/mol, which is lower than the activation energy in traditional water dyeing process of 163.84 kJ/mol. The SC-CO2 dyeing is actually a distribution process of the dye between the fiber and the fluid; and the uptake of the dye in PET fiber is linear to the amount of dye applied. The thermodynamic affinity of the dye decreases with the increase of temperature; The enthalpy of the dyeing of -23.63 kJ/mol and the entropy of the dyeing of -26.18 J/(mol K) were derived from a Van’t Hoff plot, which are similar to those in aqueous dyeing.4. Up to now, the behavior of single dyes in SC-CO2 was mostly reported. However, mixtures of dyes are always required to obtain a variety of shades in commercial dyeing. To avoid uneven dyeing, the dyes applied in a mixture should be compatible. Since the dyeing behaviors of disperse dyes in SC-CO2 dyeing are similar to those in aqueous dyeing, it was expected that the trichromatic combination for aqueous dyeing might be compatible for SC-CO2 dyeing. C.I. Disperse Orange 30 (O-30), Red 167 (R-167) and Blue 79 (B-79) have been known as a trichromatic combination for polyester dyeing in water. Compatibility of the afore mentioned disperse dyes in SC-CO2 dyeing were assessed through sorption curves and metric hue angles in this work. a) Both the dyeing-rate and build-up of the selected single dyes suggest that they have good compatibility in SC-CO2 dyeing. The uptake and exhaustion trend of the singles dyes in SC-CO2 dyeing are similar to those in aqueous dyeing. b) The compatibility of mixture of the selected dyes in SC-CO2 dyeing is confirmed by the on-tone uptake of dyes in a binary combination (Orange 30 and Blue 79 mass ratio 1:1) and a ternary combination (Orange 30, Red 167 and Blue 79 mass ratio 1:1:1) at different depth applied. c) Furthermore, the compatibility is supported by the constancy of metric hue angles of the two combinations at different depth applied. It concluded that the selected dyes could form a useful trichromatic combination in SC-CO2 dyeing.5. The dye uptake in supercritical CO2 is comparable to conventional water dyeing, but the hues, and in some cases the color yield vary significantly. Therefore, existing water dyeing color matching system are not suitable for CO2 dyeing. Database of color matching system for supercritical CO2 dyeing was established and its accuracy were verified. The K/S curves of each of the trichromatic disperse dyes(C.I. Disperse Orange 30, C.I. Disperse Red 167 and C.I. Disperse Blue 79) were parallel, which indicated that the database was reasonable. The color matching system was emended according to data of the samples dyed with mixtures of the disperse dyes, and by which the recipes for the standard samples could be found. |
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