Effect of chemical dosing amount in conventional cotton fabric pretreatment process

Objectives: The objective of the study is to analysis the effect of chemicals on different physical properties and color fastness of treated cotton samples with standard rating values. Methodology: Five single baths are used for the pretreatment process with five different dosages such as 1g/L, 1.5 g/L, 2 g/L, 2.5 g/L, 3 g/L of NaOH and H2O2. After that, reactive dyeing was performed for these five samples with a standard recipe. Findings: With the gradual increment of NaOH and H2O2 dosages, the absorbency and whiteness index of pretreated samples were improved and for that particular reason bursting strength was degraded. On the other hand, for dyed samples, a little variation was observed in color fastness to wash and rubbing but the changes in the color shade of every sample were observable.


Introduction
Pretreatment is considered to be the heart of the wet processing of cotton. As an integral part of cotton wet processing, the scouring & bleaching process is usually named as pretreatment. Because of inherited characteristics of comfortability, absorbency, breathability, etc., cotton fabrics are still on the top in the list of people's demands. As it is a natural cellulosic fiber, it contains about 6% impurities of its total ingredients. Cotton fabric pretreatment process not only deals with the removal of protein, pectin, oil, wax, fat, mineral substances, etc. from the fabric but also need to achieve an acceptable absorbance value and whiteness index value with the least deterioration of fabric strength by applying minimum use of chemical & auxiliaries, power, time and water (1,2) .
As an important part of textile processing, usually, pretreatment contains two processes, such as the scouring process & bleaching process. Scouring deals with the underlying impurities trapped inside fibers. Scouring the fabric is considered to be one of the significant stages in pretreatment, in which the total or partial removal of non-cellulosic composites as well as lubricants of machinery and size chemicals are trapped inside fibres (3,4) . Specifically, after the reaction in the scouring process of cellulosic fibers, weight loss percentage of cotton fabric ranges to 5-10% and for wettability and absorbency properties, an observable improvement is achieved (5) . Besides, bleaching removes the chromophore group of natural coloring elements from the fibers and whitens the fabric according to the given dosages. Following three major functions, such as (a) soaking the cotton fabric with a bleaching agent and other chemicals, (b) increasing the temperature to the optimum level & for a certain period have to retain the temperature, (c) finally completely rinsing, washing and drying the cotton fabric; bleaching process has completed (6) . In some conditions, hydrogen peroxide bleaching sometimes may get combined with the cotton scouring process, where peroxide bleaching is accomplished under alkaline medium (7) . In the bleaching process with hydrogen peroxide which is not obnoxious for the environment, pH is maintained at 10.5-11 and 80-85 • C temperature is required with 4 hours of treatment, and also sodium silicate is usually used as a stabilizer in the bath (8,9) .
Reactive dyes, because of their vast range of color, intense brightness of light, and extremely good fastness properties, it becomes much admired in the dyeing of cotton fabrics (10) . In exhaust dyeing method, for dye exhaustion, 30-150 g/L of salts (usually sodium chloride or sodium sulfate is used) is needed to be added because of having a slightly negative charge in cotton fibers and reactive dyes show its anionic characteristics in the dyeing bath (11)(12)(13) .
Through this project work, some findings have been analyzed to observe how physical properties of single jersey cotton knitted fabric altered when H 2 O 2 & NaOH had been dosed at a different percentage. Similarly, after dyeing those cotton knitted samples the dyeing shades of each sample were also varied from each other. All those data & information have been thoroughly deconstructed in this study.

Materials
The experiment was done with the following materials.

Fabric
Single jersey 100% cotton knitted fabric and the GSM of the fabric was 160.

Pretreatment
The grey fabric was collected and from which we cut off 5 pieces of fabric for our experiment and the weight of every single piece was 10 grams. Pretreatment chemicals, dyes, auxiliaries, and fabric samples were weighted by using the digital electronic weight balance. For the proper clarification of this experiment 5 samples had been taken into 5 batches (Batch -A, Batch -B, Batch -C, Batch -D & Batch -E) where the liquor ratio (L:R) for all batches were 1:30

Dyeing
After completing the batch-wise pretreatment, 5 samples (A, B, C, D, & E) were dyed with the same reactive dyeing recipe mentioned in Table 7. For dyeing, the weight of every sample piece was 10 grams and the liquor ratio (L:R) was 1:30. After dyeing, all samples were washed with water then treated with acetic acid followed by soaping as mentioned in Table 7. At last, the samples were squeezed and then dried in the oven dryer machine at 100 0 C for 5 minutes.  Figure 1 shows that whiteness of the fabric is gradually improving when the dosages of NaOH & H 2 O 2 are getting increased in the pretreatment bath. Along with increasing H 2 O 2 dosages, the discoloration of the samples takes place by breaking the chromophore of natural color which is organic compounds with conjugated double bonds. And samples become whiter successively. On the contrary, the bursting strength of the samples is decreased successively after increasing the NaOH & H 2 O 2 dosages. That means due to applying high dosages of NaOH & H 2 O 2 , deterioration of cotton gets started due to oxidation, and that's why less pneumatic pressure is required to burst out the fabric samples. In Figure 3, the vertical & horizontal wicking height of the samples is going upwards in the bar chart by increasing the dosages of NaOH & H 2 O 2 . Due to successive increasing of NaOH dosages, the samples are started to losing more impurities (oils, wax, gums, soluble impurities and sold dirt) generally found in grey fabric and for that reason; the samples are being better absorbent to water.  Figure 5 shows that there is a stable as well as an acceptable grade of all the samples for change in color due to washing. But in the case of color staining, cotton fibre exhibits poor ratings. Due to undergoing the pretreatment process with more NaOH & H 2 O 2 gradually, the dye uptake % is increased than the dye fixation limit of the samples. As a result, color bleeding in the cotton fibre of multifibre fabric has appeared which is in the tolerance limit also.  As the same reason for colorfastness to wash, in Figure 6, there is also a slight decrease in colorfastness due to rubbing. But in comparison with the result of colorfastness to wash, the result of colorfastness to rubbing is better & more acceptable.    fabrics favor for the dyes uptake%.

Conclusion
From this study, it can be concluded that if the dosages of pretreatment chemicals are getting higher successively, then the absorbency and whiteness index of grey samples are improved and because of that reason bursting strength get decreased gradually. But while for dyeing of these pretreated fabric, color fastness to wash and rubbing showed a downward movement in their performance. As bursting strength, colorfastness to wash and rubbing influences the quality of the finished fabric, then a suitable pretreatment recipe should be maintained.

Conflicts of Interest
Authors have no conflicts of interest.

Funding
No Funding.