Each pigment dyeing trial takes approximately 6 hours to complete. Hence only one trial per day could be conducted; depending on machine availability, as the dyeing machine was also used for daily lab dyeings for the company. In the light of this, a full factorial experimental design, to determine the effects of the 6 varied parameters on K/S, wash fastness and wet and dry fastness to rubbing, could not be conducted in the time frame required to finish this MSc as it would have taken over 5 years for the dyeings to be completed, with additional time needed for testing and analysing. Sourcing of sufficient fabric for a full factorial experimental design would also have been quite daunting as each trial requires ± 1 kg of garments. A total of ± 730 kg of fabric would have been required, not taking into account additional fabric required for repeat dyeings, should there be human error. Therefore, in the light of the foregoing, only a
partial factorial experimental design, in terms of the effects of the varied parameters on K/S,
and wet fastness, could be conducted.
3.6. LOCATION OF TRIALS
All pigment dyeing trials were conducted at Spectrum Textiles (Pty) Ltd, which is a medium sized garment dyeing company, based in Durban. The company does not have its own laboratory, and the dyehouse personnel rely on customer feedback, in terms of pass or fail ratings of colour fastness and colour strength. For this research, the wash fastness and rub fastness tests for the pigment dyed garments were performed at the Textile Technology Laboratory at the Durban University of Technology, using a James H. Heal Washwheel and an AATCC Crockmeter. The Spectrophotometry tests, to determine colour strength, was conducted in Hammarsdale at the Advanced Textile Services Laboratory (ATS) at Gelvenor Consolidated Fabrics (Pty) Ltd using a DataColor SF600+ spectrophotometer.
3.7. PROCESS OPTIMIZATION
3.7.1. Cationisation Concentration
Various studies have been carried out since 2005 on the cationisation of cotton fabrics as well as garments, in relation to pigment dyeing (Fang et al: 2005, Patra et al: 2006, Ristić and Ristić: 2012, Kumar et al: 2013, Wu and Zhang: 2013, Choudhury: 2014, Saha et al: 2015). These studies have used cationising concentrations from 2% to 12% (owf), with good results on colour strength being reported for cationising concentrations between about 6 and 8%, when employing differing levels of pH, pigment concentrations, time, temperature and MLR. The results of these studies have informed the selection of variables and their levels in the present
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study. In particular, the concentration of cationising agent was fixed at 4, 6 and 8 % (owf), respectively while the pH, temperature and time used for the cationisation process were according to the recommendations of the dyestuff suppliers.
3.7.2. pH Level
In their research, Kumar et al (2013), used pH of 2, 4, 5 and 6 on cationised cotton fabric, and concluded that a better pigment uptake occurred at pH of 5. These results were, however, based on beaker dyeings, and it was therefore decided that, in the present study, pH levels of 3, 5 and 6 would be used for both the pigment and binder.
3.7.3. Pigment Concentration
Broadbent (2001), stated that “with increasing amounts of absorbed dyes, the colour of the goods become deeper, but duller with a slight change in hue. Deep shades have lower fastness to wet treatments and rubbing than pale shades”. This statement also holds true for pigments as reported by other researchers (Kumar et al: 2013, Wu and Zhang: 2013, Saha et al: 2015). Research work carried out on cotton fabrics by Patra et al (2006) and Kumar et al (2013), as well as Saha et al (2015) on cotton garments, used pigment concentrations of 1-3%, 4% and 5%, respectively, with the 5% results still showing unevenness. Cotton Incorporated (2000) stated that a Red pigment concentration of 9%, resulted in a high degree of staining of the equipment. Taking these studies into account, it was decided to cover Blue pigment concentrations (owf) of 5, 7 and 9 % for this research.
3.7.4. Binder / Fixer Concentration
In research on the pigment dyeing of cotton fabrics, Patra et al (2006) used binder concentrations (owf) at 3, 5, 7 and 10%, with 5% providing the best results. Saha et al (2015) used 2g/l of fixer on cotton garments, at a pigment concentration of 5%, with the results still showing unevenness. In the light of the above, concentration of binder/fixer at levels of 3, 5 and 7 % (owf) were selected. Higher concentrations would not be cost effective and would render the handle of the garment unacceptably harsh.
3.7.5. Temperature Level
Broadbent (2001) stated that “longer dyeing times at elevated temperatures usually result in better colour levelness”. Research on pigment dyeing of cotton fabrics, by both Fang et al (2005) and Patra et al (2006), involved temperatures ranging from room temperature to 80°C, and 50° - 100°C (with 10° increments), respectively. Fang et al (2005) studied the effects of
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temperature on cationisation, and concluded that temperatures higher than 80°C yielded a lower pigment uptake. Patra et al (2006) concluded that 60°C for the Blue pigment, produced the best dye uptake. Wu and Zhang (2013) also conducted research on the effects of temperature, ranging from 50° - 100°C. These trials were, however, on the pigment dyeing of Cotton/Polyester fabrics. They concluded that K/S increased with an increase in temperature from 50° to 70°C, after which it decreased with further increase in temperature (above 70°C). Based on the foregoing studies, temperatures of 60, 70 and 80°C were selected for this study.
3.7.6. Material-to-Liquor Ratio (MLR)
Broadbent (2001) stated that “lower liquor ratios minimise the consumption of energy for heating the water and that an increase in the liquor ratio causes a decrease in the degree of exhaustion and therefore a decrease in the depth of colour of the dyeing”. Kumar et al (2013) agreed with this statement, mentioning that “lower liquor ratio shifts the equilibrium in favour of the pigment on the fibre, with the equilibrium being reached more rapidly”.
Material-to-liquor ratios, for rotating drum machines, of 1:10 or lower was suggested by Park and Shore (2004), with additional ratios of 1:3 and 1:8 for 50 – 150 kg fabric. Ingamells (1993) also suggested material-to-liquor ratios ranging from 1:5 up to 1:30 or higher, for batchwise dyeing machines. Fang et al (2005) and Patra et al (2006) also conducted their research, using a short material-to-liquor ratio of 1:20. In light of the above, it was decided that the material- to-liquor ratio for this research would be 1:8, 1:15 and 1:20.