Coloration Technology

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1 doi: /cote Deep-colour vat dyeing of cotton knit fabric on a modified jet dyeing machine Hiroshi Wakoh, a, * Mina Furuie, a Daisuke Inaba, a Nobuto Azuma, a Koji Nakane, a Nobuo gata, a Toru Shimizu b and samu Ishimaru b a Graduate School of Engineering, University of Fukui, 3-9-1, Bunkyo, Fukui, , Japan wakohknowledge@me.com b Hisaka Works, Ltd, , Higashi-Konoikecho, Higashi-saka, saka, , Japan Received: 5 January 2014; Accepted: 18 December 2014 Coloration Technology Society of Dyers and Colourists To improve the vat dyeing of cotton knit fabric with Indanthren Black RB Coll. (CI Vat Black 9), the basic parameters of dyeing, including the concentrations of chemicals, the dyeing temperature and duration, and the apparent diffusion coefficient of the dye, were obtained by test dyeing with a stoppered Erlenmeyer flask and the cellophane film roll method. A stable vat dyeing process has been developed on a modified jet dyeing machine for the first time without using nitrogen to purge oxygen. Modifications were made to improve the airtightness of the machine and the equipment in the liquor circulating system, with a water inlet and outlet for the gentle oxidation of dyed fabric, and with monitoring by means of a sensor inserted in the liquor circulating system for in situ measurements of the redox potential and the ph of the dyeing liquors. These measurements made it possible to follow to their completion the process of dyeing and the process of gentle oxidation by overflow washing with water and final oxidation. ptimal conditions with regard to the amount of reducing agent, the dyeing temperature (80 C), and oxidising processes were established with this machine. It was found that, by using the modified machine and process conditions, dyeing proceeded stably and reproducibly (at 80 C) to yield grade A dyed fabric. Visual inspection confirmed that faultless deep-colour dyeing of the fabric was attained. Production has been proceeding successfully for the past 2 years. wing to its insolubility, complete removal of the dye from the wastewater has been possible. Today, cotton clothes are dyed with vat dyes in a continuous system, with stepwise operations of reducing the dye to leuco form, soaking the clothes in the bath under a nitrogen atmosphere, and then holding the clothes in an air atmosphere for oxidation [1 3]. Soaping, washing, and application of various auxiliary agents follow continuously to finish the processing [4]. However, cotton knit fabric, which is not put into the continuous system because of its overstretching on the conveyor, must be vat dyed in a batch system, where inhomogeneous reduction and oxidation of the dyed fabric often lead to some defects in the product. This report describes a new method for stable vat dyeing of cotton knit fabric in a batch, especially deep-colour dyeing, using a modified jet dyeing machine operating without the use of nitrogen gas to purge oxygen. Before designing the mechanical system for deep-colour dyeing with vat dyes, which has not been practised commercially until now, preliminary information on dye diffusion in cellulosic substrates was obtained using the cellophane film roll method, by which dye uptake at the surface and the diffusion behaviour of the dye within the roll were measured quantitatively [5 8]. As with other dyeing processes, it is desirable in vat dyeing that the temperature be as high as possible in order to reduce dyeing and processing time. However, increasing the temperature tends to result in unevenness in the dyed fabric, resulting in various defects in the product. The dyeing conditions, including the concentrations of reducing agent, sodium hydroxide, and the weight ratio of used dye to fabric, were examined by test dyeing in an Erlenmeyer flask under a nitrogen atmosphere. The possibilities of lowering the liquor ratio, the dyeing temperature, and the duration of each process were also examined in the flask experiment. The core technology for successful batch dyeing on a production scale consists in completely reducing the dye to leuco form for molecular dispersion in the dyeing liquor, preventing air penetration from outside into the reducing liquor, making the contact of the knit fabric with the liquor as even as possible, exposing it gradually to overflowing water for gentle and homogeneous oxidation initially, and then completing the oxidation with the replaced oxidising liquor. Any inhomogeneous contact of the fabric with the liquor leads to faults such as spot stains due to contact with undispersed leuco dyes, streaking due to insufficient evenness of contact of the knit with the dyeing liquor and unlacing of the rope during circulation, and spread stains due to uneven oxidation. These problems are crucially serious in vat dyeing on a jet dyeing machine. Rigorous airtightness is necessary. The speeds of the knit fabric and liquor circulation should be meticulously regulated, and the introduction of solutions of reducing agent and dye or oxidising agent into the dyeing liquor should be controlled so as to make the contact of the fabric with them as even as possible. To overcome these restrictions, necessary modifications to the dyeing machine are proposed in this study. In addition, monitoring of the redox potential and ph of the liquor in each process helps in following the progress of dyeing in situ and in making each operation controllable. These measurements are also useful in confirming the completion of processes of dyeing with leuco dyes and oxidation of the dyed fabric. The present study first reports preliminary investigations on dyeing in a Erlenmeyer flask and with the film roll method as a model for cotton knit dyeing. The measurements were performed for Indanthren Direct Black RB Coll. at 60, 70, and 80 C. Feasible conditions for stable production were established by referring to pre-examined data for The Authors. Coloration Technology 2015 Society of Dyers and Colourists, Color. Technol., 131,

2 each parameter, e.g. the concentration of reducing agent, sodium hydrosulfite and sodium hydroxide, the liquor ratio of dyeing, and the oxidation process. Based on the data obtained by the Erlenmeyer test, the film roll method, and dyeing on the modified test jet machine for 12 kg of cotton knit fabric, reasonable operating conditions were established for vat dyeing on a 80 kg scale. Modifications were made to the conventional jet dyeing machine to realise stepwise operations with the dyeing liquor, oxidation with water, and final oxidation with oxidising agent. After dyeing, soaping was carried out twice with foamless soap. We report that high quality of dyed fabric and reproducibility have been attained consistently on the modified machine under production conditions for the past 2 years. Experimental Materials Cotton knit fabric (30/1 cotton, 100% knit fabric, 24G-26 u is the spec of a circular knittng mabhine. If the units are changed into cm unit, the number of needles, 9.49/cm (24G) and the diameter is 66.04cm (26 u) ) was purchased from Verite Co. Ltd, Japan. The fabric was scoured and bleached with hydrogen peroxide under standard conditions. Cellophane film of mm thickness, supplied by Futamura Chemical Co. Ltd, Japan, was used after soaking in deionised water for 1 h at ambient temperature. Dyes and chemicals Indanthren Direct Black RB Coll. (CI Vat Black 9) was obtained from DyStar (Japan) and used as received. The structural formula is shown in Figure 1. Sodium hydrosulfite and sodium hydroxide were supplied by Wako Pure Chemical Industries, Ltd (Japan). Sodium meta-benzene sulfonate (Ludigol) was purchased from BASF (Japan). A scouring agent (Neonol HT: ethylene glycol mono-n-butyl ether) and a softener (S-M1: alkylether X and potassium phosphate mixture) were supplied by Fushimi Chemicals Co. Ltd (Japan). Soaping, which was required to be foamless in the jet dyeing machine, was carried out 2 times with a 1:1 mixture (by weight) of sodium tripolyphosphate and sodium meta-silicate. All the reagents were for production use. For the Erlenmeyer and the film roll expriments, deionised water was used, and for the production experiments, distilled water was used. Film roll method Sekido and Matsui were the first to report and give a full account of the cellophane film roll method to study the diffusion coefficients of some vat dyes [5 7]. Figure 2 shows the assembly of the roll of cellophane film. The roll was put in a 50 ml solution containing g of the dye, 0.14 g of sodium hydroxide, and 0.1 g of sodium hydrosulfite in a test tube, and stoppered tightly under a nitrogen atmosphere. The tube was placed in a vibrator at 60, 70, and 80 C and shaken for 72 h. After dyeing, the rolled film was unrolled quickly, washed lightly with water, kept exposed to air for 1 h, and used for dye concentration measurement. Figure 3 shows the unrolled films. The absorbance at the maximum absorption wavelength (520 nm) for each film layer was measured on a UV-vis spectrophotometer (UV-2450; Shimadzu, Japan) for the dye concentrations of the respective layers. Note that the dye concentration in each layer shown in Figure 4a was as dyed and oxidised, but not as soaped. The dye concentrations on the outer surface and inner surface of the first layer of the film roll, which were measured on a Shimadzu ATR-FTIR instrument by taking 100 spectra, are shown in Figure 4b, where a large excess of dye adsorption is shown at the outer surface of the layer. HN HN NH NH Figure 1 The chemical structure of Indanthren Direct Black RB Coll. (CI Vat Black 9) 2015 The Authors. Coloration Technology 2015 Society of Dyers and Colourists, Color. Technol., 131,

3 Glass rod (10 mm diameter) (a) Abs C 70 C 80 C Rubber ring Film roll Depth, µm (b) 100 Glass rod Transmission (%T) Wave number, cm 1 Figure 2 Schematic drawing of a cellophane film roll Figure 4 (a) Concentration vs layer number plot for dyeing at 60, 70, and 80 C. (b) ATR-FTIR spectra of dyed cellophane. The thick line shows the outer layer and the thin line shows the inner surface of the first layer of the roll dyed at 80 C. The layer was soaped after unrolling [Colour figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] Figure 3 Photos of unrolled films dyed at 60, 70, and 80 C (from top to bottom). The layers are shown from left to right [Colour figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] Determination of the dyeing conditions in the jet dyeing machine The concentrations of dye, sodium hydrosulfite, sodium hydroxide, and sodium meta-nitrobenzene sulfate, the liquor ratios, and the diagrams of temperatures and times for respective operations were pre-examined in the Erlenmeyer flask experiments. With a liquor ratio of 1:15, a dyeing temperature of 80 C, and respective operating times, the reproducibility of dyeing was confirmed on the test jet machine, and the conditions were transferred to production-scale dyeing on the modified jet machine. Modification of the conventional jet dyeing machine For our purposes, to use the machine for vat dyeing without using nitrogen gas, five principal modifications to the jet dyeing machine were necessary: (1) provision of controllable airtightness against oxygen penetration into the dyeing apparatus from outside air in respective operations; (2) provision of a probe and RP meter (Yokogawa, Japan) for measurements of the redox potential and ph in the circulating liquids; (3) provision of an inlet for feeding of concentrated solutions of reducing and oxidising agents; (4) provision of an inlet and outlet for overflowing water; (5) attachment of a small inspection window for occasional monitoring of the inside when necessary, aside from the round window for fabric set-in. The Hisaka (Japan) circular rapid model CUT-MR-M for the test dyeing of 25 kg of fabric and CUT-MR-1L for 100 kg dyeing were modified along these lines. Figure 5 presents a drawing of the modified Hisaka CUT-MR-1L jet dyeing machine. The sealing of the two windows was made airtight by using high-performance silicone rubber and Teflon â. The absence of penetration of oxygen from outside air was confirmed under operating conditions by measuring the stable redox potential of the liquid containing sodium hydrosulfite. The overflow water was pressurised to 1.3 kgf m 2 to pour water into the slightly pressurised can body. Results and Discussion Calculation of the approximate diffusion coefficient (D a ) The distribution of dye in layers of film roll dyed at 60, 70, and 80 C was measured to seek the most effective dyeing temperature. Figure 3 presents photos of unrolled films dyed at these temperatures. The distributions of vat dye in respective layers of the film roll with a solution containing g l 1 of dye, 8.4 g l 1 of sodium hydroxide, and The Authors. Coloration Technology 2015 Society of Dyers and Colourists, Color. Technol., 131,

4 Nozzle value Nozzle Drive reel Water level Cloth Dyeing autoclave Flow of water Flow of cloth Service tank B A Flow tube Heat exchanger RP sensor Suction value verflow valve Filter Main pump Control valve Feed pump Feed valve Water feed pump Figure 5 Schematic drawing of the modified Hisaka Circular Rapid Model Cut MR-1L jet dyeing machine 6.0 g l 1 of sodium hydrosulfite are shown. After unrolling, the films were washed lightly with water. It can seen in Figure 4a that the dye concentrations in the first layers dyed at the three temperatures are excessively high compared with those of the second and subsequent layers. Generally, vat dyes are known to have a dyeing property known as strike dyeing, i.e. a very rapid degree of dye exhaustion and a very low rate of diffusion within the fibre [2]. To find the cause of the high concentrations in the first layers, the dye concentrations on both surfaces of the first layers dyed at 80 C were measured on an ATR-IR instrument with 100- fold repetition. Figure 4b shows the spectra of both sides at around 1600 cm 1, corresponding to the overlapped absorptions by the stretching of C= and C=C of the aromatic rings [9]. It can be seen from Figure 4b that the absorptions are remarkably uneven at the two surfaces of the first layer, indicating excess deposition of the dye on the outer surface. Apparently this is the origin of strike dyeing, and it has been pointed out that diffusion of the dye inside the fibre is important for the level dyeing of cotton [2]. The values of the diffusion coefficient, D a, inside the film roll were estimated using the concentration data for the second and subsequent layers. For vat dyeing of cellophane, Sekido and Matsui [5 7] reported a method for estimating the approximate diffusion coefficient. They showed that the concentration ratios C i + 1/C i are expressed as C i + 1/ C i = (r i r i +1 )/(r i 1 r i ), where r i = ierfc if. Parameter f is related to D a by the equation D a ¼ 2 =4tf 2 where D a is the approximate diffusion coefficient, cm 2 min 1, e is the film thickness, cm, and t is the time, min. They gave a group of curves for C i + 1/C i, where i = 1, 2, 3,...,vsf, on which D a values were estimated from two successive C i values from the second layer inward. Table 1 shows D a values observed with the three concentrations of reducing agent at the three temperatures. The D a values increase greatly with temperature, about 4.5-fold from 60 to 80 C. As anticipated, dyeing at a higher temperature is advantageous for reduced striking and also for saving dyeing time. ð1þ Model dyeing by the film roll method for setting production conditions The dye concentration was adjusted so as to obtain a satisfactory deep black colour. The concentration of reducing agent was set at 1.2 times the dye in molar ratio to consume oxygen in the air space of the machine. As found in Table 1, the ratios of D a values for the dye at 60, 70, and 80 C were found to be 1:2.2:4.5. Equilibrium sorptions, however, contain excess concentrations on the outer surfaces of the film and hence, in productive dyeing, probably on the surfaces of cotton filaments. As this phenomenon cannot be avoided in dyeing and is undoubtedly associated with various fastnesses of the dyed knit fabric, a practical solution to diminishing the effect was adopted by applying effective soaping 2 times. At 60, 70, and 80 C, the apparent concentrations on the first layer become ca. 2:1.3:1, suggesting that the higher temperature is also advantageous for reducing the surface deposition. With these considerations, the dyeing temperature was set at 80 C for 40 min. Measurements of the redox potential and ph under dyeing conditions in the modified jet machine To monitor the progress of dyeing in situ, the redox potential and ph were measured for the dyeing liquid, overflowing water, and oxidising liquid. Figures 6 8 show Table 1 Values of the approximate diffusion coefficient D a at 60, 70, and 80 C Reducing agent NaH 2.8 g l 1, Na 2 S 2 4 2gl 1 NaH 5.6 g l 1, Na 2 S 2 4 4gl 1 NaH 8.4 g l 1, Na 2 S 2 4 6gl 1 D a,cm 2 min 1 60 C 70 C 80 C The Authors. Coloration Technology 2015 Society of Dyers and Colourists, Color. Technol., 131,

5 Reduction potential, mv Heating time, min Figure 6 Changes in the redox potential ( ) and ph (M) of the dyeing liquid with time ph Reduction potential, mv Auxiliary agent 40 C C 60 C 60 C min 60 C 10 min 20 min Time, min Figure 8 Changes in the redox potential ( ) and ph (M) with time after the addition of the oxidising liquor ph Reduction potential, mv Additional water, L ph Temperature, C Room temp. 40 min Chemical + dye verflow washing Ludigol 20 min 20 min 2 C min 1 2 C min 1 Figure 7 Changes in the redox potential ( ) and ph (M) with the quantity of overflow water in the production machine Time, min 200 the changes in the two parameters in respective processes with time. Figure 6 shows that the values for reducing liquid containing the dye, 8 g l 1 of sodium hydroxide, and 8gl 1 of sodium hydrosulfite decrease to stable values after 40 min of dyeing. Figure 7 shows that stable values are reached after the addition of 4800 l of overflowing water, indicating that the reducing liquid is washed out and the oxidation potential takes the value of that of the overflowing water. Figure 8 shows the decreases in the two parameters on the addition of oxidising liquor. It can seen that after 30 min both parameters decrease to constant values, indicating that oxidation has been completed. These measurements confirmed in situ that the respective steps had been completed, and that the set time courses of the successive steps were adequate. peration of modified jet dyeing machines Dyeing operations were performed with two modified machines to test and establish production conditions. Figure 9 shows a temperature control diagram for the successive processes. Liquor containing reducing agent in 20% excess is introduced for reduction of the dye from the service tank, and circulated for 5 min. Then 15 kg of prescoured fabric is introduced, dye solution is fed from the inlet, and the fabric and liquor are circulated. The temperature is increased to 80 C at a rate of 2 C min 1. The temperature is kept at 80 C for 40 min and then lowered to 60 C, and overflowing water is introduced in Fibonacci sequence 1, 2, 3, 5, 8, 13, 21, 34, and 55 l min 1 from the Figure 9 Temperature control diagram for dyeing in production inlet to the outlet valve. Then the oxidising liquor is introduced from the service tank and is circulated for 20 min at 60 C. Subsequent washing and soaping of the dyed fabric are performed as described in the experimental section. Dyeing of the knit fabric on the modified dyeing machine Note that the rate of oxidation becomes faster with temperature. This can lead to the formation of blotched or speckled defects in colour on oxidation. Therefore, the oxidation of the dyed fabric with overflowing water was regulated so as to make gradual contact with dissolved oxygen by adjusting the rate of the water introduced in Fibonacci sequence. In fact, such regulation was vital for faultless dyeing at this stage. The contact of the fabric with the oxidising liquor was postponed until the reducing liquor was washed out and the fabric was gently oxidised by the flowing water. The oxidation temperature with the agent was set at 25 C. Dyeing of 80 kg of knit fabric with Indanthren Direct Black RB Coll. at 12% owf was performed using the modified jet machine Hisaka Circular Rapid Model CUT MR-1L with 1200 l of reducing liquid in 1800 l capacity. Water supply for the overflow was set to a total of 4800 l. The dyeing and oxidising processes were recorded, and the achievement of stable states was confirmed by measurements of the redox potential and ph in respective steps, as shown in Figures The Authors. Coloration Technology 2015 Society of Dyers and Colourists, Color. Technol., 131,

6 Table 2 Parameters of the colour of the dyed knit fabric Temperature, C L* a* b* K/S Table 3 Colour fastnesses of the dyed knit fabric Test Standard Black Remark Light IS 105-B02 4 Grade 4 irradiation: irradiation until grade 4 of blue scale faded normally (irradiation time h) Perspiration (alkali), light (20 h) IS 105-B07 5 Method of colour fastness testing for light and sweat, method B Washing IS 105-C06 Pollution 5 Colour change 5 Rubbing IS 105-X12 Dry 4 5 Wet 2 3 Chlorinated water IS 105-E03 5 The degree of discolouration after washing for 30 min in chlorinated water of 20 ppm As regards ecological aspects of the given dyeing method, the vat dyes are insoluble in water, which is extremely advantageous for recovery from wastewater. Flocculation of the dye and removal were easily achieved by the conventional method of wastewater treatment. Inspection of the colours of obtained knit fabrics After finishing the 29 soaping, washing, and drying, the fabric was inspected for dyeing defects such as spots, streaking unevenness, blotches, and mottles. The results showed grade A excellence of the dyed fabric. The L, a, b values and K/S values are shown in Table 2. These values show that a deep colour of the knit fabric is obtained by the batch dyeing described above. Moreover, the black colour shows no discontunity. The dyed fabric was tested for light, washing, and rubbing fastness under dry and wet conditions according to the IS standards [10 14]. In addition to these fastnesses, perspiration light fastness was measured according to the Japanese standard. Table 3 shows the results, indicating the superior quality of the IS standards for dyed fabrics. Conclusions For the first time without using nitrogen to purge oxygen, deep-colour dyeing of cotton knit fabric with Indanthren Direct black RB Coll. has been successfully performed with a modified jet dyeing machine. The dyeing conditions, including the concentrations of the reducing agent, sodium hydroxide, and the dye, the dyeing temperature, and the processing times, were determined in test dyeing using an Erlenmeyer flask and on a test-size jet dyeing machine. The dyeing temperature was set at 80 C on the basis of measuring D a at C by the cellophane film roll method. It was found that on the outermost surface of the film roll the dye deposited in excess by so-called strike dyeing. The effect of this unwanted excess dyeing was reduced by effective 29 soaping with foamless soap. A conventional jet dyeing machine for 100 kg fabric was modified, with prevention of the penetration of oxygen from the outside, the provision of an inlet and outlet for regulated introduction of oxidising water, and in situ monitoring of the redox potential and ph to follow the progress of dyeing and oxidation. ver the past 2 years, deep-colour dyeing of knit fabric has run stably on the modified machine. wing to its insolubility, complete removal of the dye from the wastewater has been possible. References 1. J R Aspland, Text. Chem. Color., 24(1) (1992) J R Aspland, Text. Chem. Color., 24(2) (1992) C M Horne, Text. Chem. Color., 27 (1995) T Vickerstaff, Physical Chemistry of Dyeing, 2nd Edn (London: liver & Boyd, 1954) M Sekido and K Matsui, Sen i Gakkaishi, 20 (1964) M Sekido and K Matsui, Sen i Gakkaishi, 20 (1964) M Sekido and K Matsui, Sen i Gakkaishi, 20 (1964) R H Peters, Diffusion in Polymers, Eds J Crank and G S Park (New York: Academic Press, 1968) R M Silverstein, F X Webster and D J Kiemle, Spectrometric Identification of rganic Compounds, 7th Edn (Hoboken: John Wiley & Sons, 2005). 10. IS 105-B02:2014 Textiles: Tests for colour fastness. Part B02: Colour fastness to artificial light: Xenon arc fading lamp test (Basel: IS, 2014). 11. IS 105-B07:2009 Textiles: Tests for colour fastness. Part B07: Colour fastness to light of textiles wetted with artificial perspiration (Basel: IS, 2009). 12. IS 105-C06:2010 Textiles: Tests for colour fastness. Part C06: Colour fastness to domestic and commercial laundering (Basel: IS, 2010). 13. IS 105-X12:2001 Textiles: Tests for colour fastness. Part X12: Colour fastness to rubbing (Basel: IS, 2001). 14. IS 105-E03:2010 Textiles. Tests for colour fastness. Part E03: Colour fastness to chlorinated water (swimming-pool water) (Basel: IS, 2010) The Authors. Coloration Technology 2015 Society of Dyers and Colourists, Color. Technol., 131,