Key words:
Bleaching, cotton knit fabric, light shade, shade matching, color fastness, energy saving, cost effective
1. Introduction:
Energy is one of the most important ingredients in any industrial activity [1]. However, its availability is not infinite. Global energy crisis, as well as high cost of fuels resulted in more initiatives to conserve energy to best possible extent. Knit dyeing process is one of the most crucial and dominating sector in Bangladesh, as Bangladesh earns around 40% of its foreign currency through this sector [2]. Undoubtedly this is the sector where we can dig attention for energy saving purpose and hence, steps should be taken to make this sector stronger by reducing processing time, production cost and by saving environment. Process time is a very important factor for a dyer, because a dyer delivers his product within lead time that is given by the buyers. To deliver product, dyer must reduce his dying time that also ensure more production means more money. In this competitive world reduction of processing cost is also a major factor. Moreover, it is of utmost necessity to keep our environment free from pollution of different chemicals used in dyeing factories. So to save environment a dyer should use least number of chemicals and preferably no hazardous chemicals for performing dyeing. In cotton knit dyeing process, reduction of process time and reduction of chemicals in the preparatory stage can play a significant role in the above aspect. However without using peroxide in the preparatory process, it could be possible to obtain desired shade after dyeing. But actually up to the scope of searching through internet or books, no relevant research works were found in a justified way.
To run with the processing heads, focusing on scouring, bleaching and so on can be initiated here. Natural fibers (cotton) are yellowish or off-white in color due to color bodies present in the fiber and cotton also contain oils, fats, waxes, minerals, leafy matter and motes as impurities that interfere with dyeing and finishing [3]. The process of destruction of yellowish or off-white color bodies is called Bleaching and the process of removing impurities and making good absorbency is called Scouring
2. Materials and Methods
2.1 Raw materials
Same fabric of Table 1 has been used in all trials done both in lab and bulk.
2.2 Methods:
The fabric was dyed in exhaust method in the laboratory and in bulk scale at Padma Poly Cotton and Knit Fabrics Ltd.
2.2.1 Processes followed in laboratory trial:
A. Pretreatment:
The samples were categorized as dyed samples with bleaching and dyed samples without bleaching at same dye bath and same recipe.
2.2.2 Processes followed in bulk trial:
2.2.3 Process followed in testing
For colour fastness to wash ISO 105 C03 was method followed; for colour fastness to rubbing /crocking EN ISO 105×12 was method followed and for color fastness to perspiration ISO 105 E02 method was followedIn respect to colour fastness properties, the unbleached samples revealed the same better results as it was in case of bleached samples.
3. Results and discussions
3.1 Color Measurement Committee (CMC) DE pass/fail values:
The above result shows that dyeing without bleaching is possible up to 0.5% red, 0.5% blue and 0.1% yellow shades and also for specific combination shades (0.5%)
4. Cost analysis
In the bulk trial bleaching process needed 9 chemicals and without bleaching process only 6 chemicals were used. So process without bleaching saves extra 3 chemicals.
4.1 Cost of chemicals (Source: Purchasing department of Padma Poly Cotton)
From the table it is clear that process without Bleaching saves 757Tk/1000kg (approx.) fabrics.
4.2 Time saved
As per table 3, the process with bleaching needs 243 min and process without bleaching needs 185 min. So process without bleaching saves 58 min. As a result process without bleaching provides following benefits because of less time consumption:
1. Production will increase
2. Labor cost will decrease
3. Saves machine running cost. i.e. electricity, gas, and power cost.
4.3 Energy cost calculation for MCS dyeing machine [4]
1kw hour = 1 unit of electricity
1 unit cost = 5.5Tk (approx)
Dyeing machine is of 60 KW capacity
o it consume 60×1 = 60 unit per hour
Cost =60×5.5 = 330Tk/hr
Since process without bleaching save 58 minutes, So it saves 58×5.5 = 319Tk (approx.) energy cost.
Source: Maintenance department of Padma Poly Cotton and Knit Fabrics Ltd.
4.4 Water saved due to elimination of per-oxide
The trial was done in Material: Liquor = 1:6. For conventional pretreatment process 30000lt water was required, whereas without bleaching pretreatment process required 24000lt. So proposed process saves extra 6000lt of water.
A. Cost of treated water after Water Treatment Plant (WTP) :
So for 6000 lt of water process without bleaching saves 6x5.11 = 30.66 Tk(approx.)
B. Cost in ETP :
So, for 6000Lt of water process without bleaching saves 6x11.97 = 71.82 Tk (approx.)
Total cost for 6000lt water = 30.66+71.82 = 102.48Tk = 103 Tk. (approx.)
4.6 Savings from cost of heating
Here:
Heat Needed= Specific Heat of Water(4.186 kj/kg/°C)*mass of water*change in temperature.
Heating Cost Savings=(Heat Needed/Specific Enthalpy of steam)*cost of steam
Cost of steam: Considering the steam cost of PPC
4.7 Total saving in cost
So proposed process saves 1466Tk/1000kg (approx.) and 58 minute duration.
5. Conclusion:
Detailed experimental data were manipulated through the research work. As the elimination of bleaching results the grayness retaining in cotton fabric, CMC DE pass/fail comparison with conventionally treated fabric were investigated. Consequently color fastness to wash, rubbing and perspiration have been tested. During this experiment dyeing is performed for shade percentages of 2, 1.75, 1.5, 1.25, 1.00, 0,75, 0.50, 0.25, 0.10 with bleaching and without bleaching for Red, Yellow, Blue and combination shades. For Red, Blue and combination color up to 0.5% shade has been passed in data color result but below 0.5% the result in data color did not pass. It was passed up to 0.1% shade in case of yellow color. Besides, samples passed in data color results have been shown good color fastness (washing, rubbing and perspiration) values. Some bulk dyeing has been performed in factory after successful completion of laboratory trials. Experiments were done on two different combination shades % i.e. 2% and 1% and the results have been derived successfully. It can be concluded that dyeing with red (0.5%), blue (0.5%) and yellow (0.1%, 0.25%, 0.5%) colors, excluding bleaching process passed at data color 650 (CMC pass/fail) with conventional (including bleaching) dyeing process. In case of specific combination shade it was found that 0.5% shade is possible without bleaching operation. Cost analysis displays that 1220Tk (approx.) was saved per 1000 kg fabric dyeing through the proposed way.
References
1. http://www.fibre2fashion.com/industry-article/4/316/cost-effectiveness-in-textile-processing1.asp
2. Hasan, Md. Rakibul, "Scopes of Improvisation in Knit-Dyeing process of Cotton in Bangladesh to optimize the process-time", Bangladesh Textile Today, March-April 2010.
3. Tomasino, Dr. Charles, "Chemistry & Technology of Fabric Preparation & Finishing" North Carolina, USA.
4. Interview from Mr. Sehk Asadullah Tareq, Head of the department of Maintenance. Padma Poly Cotton & Knit Fabrics Ltd.
5. Interview from Sahida Sultana, Manager of the department of ETP and WTP. Padma Poly Cotton & Knit Fabrics Ltd.
