| The dyeing industry adds values to the textiles,but the conventional dyeing process has the potential to cause serious environmental problems,owing to the huge amounts of water and energy consumptions,as well as a great deal of highly-colored effluent discharge.As one of the most commonly used types of dyes for pad dyeing of cotton,reactive dyes,apart from the aforementioned shortcomings,also face many other troubles that have to be settled urgently,such as serious dye hydrolysis and heavy discharges of inorganic salts and alkalis,etc.On the issues of high energy consumption,heavy discharges of wastewater and low dye utilization efficiency in the conventional pad dyeing of cotton fabric with reactive dyes,as well as the high cost of equipment investment,poor dyeing reproducibility,the serious wastes of fabrics,dyes and chemicals in the available shortened-wet steaming dyeing technology,which uses the high temperature unsaturated steam as the fixation medium,we proposed a new shortened-wet steaming dyeing process accomplishing dye-fiber reaction in the saturated atmospheric pressure steam.In order to overcome the disadvantages of slow heating of the wet fabric and serious reactive dye hydrolysis in the traditional pad-steam(P-S)dyeing process,a technical route of decreasing the water content of padded cotton fabric rapidly in low temperature prior to fixation in the saturated atmospheric pressure steam.Thus,a vacuum-dehydration procedure was incorporated in the P-S process,and consequently,a pad vacuum-dehydration steam(P-Vac-S)process for dyeing of cotton fabric using reactive dyes was established,with the expectation of increasing heating rate of the wet fabric and reducing dye hydrolysis during steaming,as well as enhancing utilization efficiency of dyes.In this paper,the main research contents and the principal conclusions are as follows:(1)Effect of the water contents of padded cotton fabrics on the steaming fixation of reactive dyes in the saturated atmospheric pressure steamVacuum-dehydrating water contents of the cotton fabrics padded with dye solutions containing 25 g·L-1 of C.I.reactive black 5 and 30 g·L-1 of Na2CO3 down to around 25.0%firstly,then being fixed in the saturated atmospheric pressure steam at 100 ± 1 ?,a dyed fabric with relatively higher K/S value and dye fixation rate was obtained.However,the color saturation of the dyeing was a bit poor as compared with the dyed fabrics having water contents much higher or lower than 25.0%.(2)Effect of various types of reactive dyes on the P-Vac-S dyeing of cotton fabricThe cotton fabrics padded with solutions containing 25 g·L-1 of reactive dyes and 30 g·L-1 of Na2CO3,being vacuum-dehydrated first to reduce the fabric water contents to(25.0 ± 2.0)%,then fixed in the saturated atmospheric pressure steam at 100 ± 1 ?,the optimum steaming time for C.I.reactive red 24(RR24,MCT),C.I.reactive blue 19(RB19,VS),C.I.reactive black 5(RB5,Bis-VS),C.I.reactive yellow 176(RY176,MCT/VS)were 6 min,2 min,3 min and 5 min,respectively.Which means that the higher the reactivity(VS>MCT),the shorter the optimum steaming time for the corresponding reactive dyes when applied in dyeing cotton fabric using the P-Vac-S process.The cotton fabrics padded with dye solutions containing 25 g·L-1 of reactive dyes,being vacuum-dehydrated first also to obtain fabrics with the water contents of(25.0 ± 2.0)%,and then fixed in the saturated atmospheric pressure steam at 100 ± 1? for the optimum steaming time of the corresponding respective dyes,the appropriate Na2CO3 concentration for RR24,RB19,RB5,RY176 were 30,20,30 and 25 g·L-1,respectively.In summary,for the reactive dyes with only one functional group,the lower the reactivity,the stronger the alkalinity demand of the medium,and the larger the amount of alkali required to realize the covalent bond formation between reactive dyes and cotton fibers.(3)Comparison of the P-Vac-S dyeing to conventional P-S dyeing of cotton fabric with reactive dyesThe(K/S)sum values of the dyed cotton fabrics increased with increasing dye concentrations of RY176 and RB5 both in the P-Vac-S dyeing and conventional P-S dyeing processes.The(K/S)sum values of the cotton fabrics dyed with P-Vac-S process were 18%?28%and 5%?20%higher than that with P-S process for RY176 and RB5,respectively.The color levelness of the cotton fabrics dyed with RY176 and RB5 using the P-Vac-S process and the P-S process both became poor with increasing dye concentration,and the color levelness of the fabrics dyed using the former process was poorer than that using the later process.The rubbing fastness of the cotton fabrics dyed with RY176 and RB5 using the two different processes rated equal to or higher than 4 and 3 respectively,and the wet rubbing fastness ratings of the cotton fabrics dyed with RB5 using the P-Vac-S process were a bit lower than that using the P-S process.(4)Effect of vacuum-dehydration temperature on the steaming fixation of cotton fabrics padded with reactive dye liquorsUnder the temperatures of 20?45? and the vacuum level of-0.096 MPa,if the water contents of all cotton fabrics padded with dye liquors containing 25 g·L-1 of RB5 and 30 g·L-1 of Na2CO3 were vacuum-dehydrated to(25.0 ± 2.0)%prior to steaming fixation in the saturated atmospheric pressure steam at 100 ± 1? for the same time,the vacuum-dehydration temperature variation would have no obvious influence on the color strength of the final dyeings.(5)The mechanism of vacuum-dehydration on the steaming fixation of the padded cotton fabricsThe water content of padded cotton fabric after vacuum-dehydration had a great effect on the dye distribution in the dyed fibers.For the fabric with extremely low water content(e.g.,1.4%),dye penetration in the dyed cotton fibers was poor,and'white core' phenomenon was quite obvious.Dye distribution in the dyed fibers of the padded cotton fabrics with the water contents of and above 25.0%was uniform.During vacuum-dehydration,accompanied with the rapid evaporation of water on the fabric and/or yarn surface,most of the dyes moved towards the yarn periphery,or even accumulated on the fabric surface.Dye diffusion into the fiber inner also occurred simultaneously with water evaporation on the yarn and/or fabric surface during vacuum-dehydration.When the water content of padded cotton fabric was vacuum-dehydrated to ca.25.0%,gradient distribution of dyes existed from periphery to the lumen of dyed cotton fiber in the cross-section.Vacuum-dehydrating the water content of the padded cotton fabric down to around 25.0%before steaming,dye penetration into the interior of the cotton fiber was accelerated with assistance of swelling of the cotton fibers and increasing of apparent dye concentration on the fiber surface,and fabrics with acceptable dye permeability in the dyed fibers were obtained.Research on the effect of water contents of the padded cotton fabric on steaming fixation of reactive dyes may provide a good reference for low add-on dyeing technology.Steaming fixation of padded cotton fabrics with low water content under the saturated atmospheric pressure steam might benefit the innovation of the current shortened-wet steaming fixation technology based on the unsaturated steam of high temperature.A vacuum-dehydration aided pad-steam process of dyeing cotton fabric with reactive dyes under the saturated atmospheric pressure steam could accelerate heating up of the wet fabric during steaming and increase the dye fixation rate,which may offer a great potential to save energy,as well as reduce the consumptions of dyes and chemicals. |
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