Performance Optimization of Super White Washed Stretch Denim Fabric by Deviating Washing Process Time and Machine RPM

: Over the last few decades, a lot of innovations and development works had already made denim “classic” in the fashion cycle. The modern consumers of today’s world are not only devoting themselves on clothing comfort but also emphasizing on economic and environmental sustainability. As a result, there has been increasing interests in terms of using environment friendly, non-toxic and biodegradable wet process such as super white wash. Though it enlarged higher consumer aspects and market opportunities in the denim washing technology, no research work had done yet regarding to super white wash. It can easily be applied to solid fabrics and finished garments. This research work was intended to analyze the physical and chemical changes of denim fabric while incorporating into super white wash at variable process parameters such as washing process time and machine rpm. Though its prime objective is to improve the whitening effect of textiles and apparels, it also can improve color strength (K/S), color fastness properties (color fastness to washing, color fastness to light, color fastness to perspiration), pilling resistance and fabric handle. In this article, it was demonstrated that all the fabric samples achieved gradual deterioration of tensile strength, tear strength and dimensional stability with the increase of washing process time and machine rpm up to a certain stage. Furthermore, the fabric samples showed better results regarding whitening index (WI), color strength (K/S), pilling resistance, fabric handle and color fastness properties with the gradual rise of washing process time and machine rpm. There were some notable changes in fabric GSM at the following washing conditions.


Introduction
Denim, one of the ancient clothing items in today's fashion industry, upholding its dignity and public acceptance even today within people around the world; beyond age, gender, climatic conditions and social status [1][2][3]. A lot of innovations and development works in the last few decades had already made denim "classic" in the fashion cycle till now [4]. The consumer expectations from denim fabric had changed over the years with technological advancements [5][6][7][8]. Nowadays, the modern consumers are not only devoted on infant designs and new appearances [9]; but also want to belong clothing comfort, user-friendly performance and consumer sustainability [10]. To meet the compatibility with rapidly changing and strategic market conditions at growing customers' demands; apparel technologists are now trying to introduce different infant variants of denim garments through physical and chemical modification of fabric's morphology [11][12][13][14][15]. For instances, there has been increasing eagerness at environment friendly, nontoxic, fully biodegradable products from modern textile dyeing, apparel washing and finishing operations [16][17][18][19][20]. Wet processes like enzyme wash, super white washing treatment can replace a number of mechanical and chemical operations, whose were basically imposed to improve both the comfortability and quality characteristics of fabrics [20][21][22][23]. Besides, super white wash is one of the garments wet processes; that is generally performed by using a strong oxidizing agent such as Hydrogen Peroxide (H 2 O 2) [24][25][26]. In addition, H 2 O 2 had played its role by breaking down chemical bond that builds up chromophore (color building group) and vanished away the ability of chromophore to absorb light [27][28][29][30]. It is a popular washing technique that was mainly introduced to denim fabric, with a view to increasing the whitening index and color strength (K/S) of the garments by deviating washing process time and machine rpm [30][31][32][33][34]. Thus, it also improved color fastness properties and hand feel rating of garments according to buyer proposed washing standards [35][36]. Beyond some very good advantages, there are few narrow drawbacks such that, this technical wet process sometimes damages celluloses very roughly [37], causes strength loss, pin holing at the side seam and pocket areas. Thus, it also causes corrosion to metallic machine parts of garments washing machines [38]. Apart from those, it has ecofriendly influences over modern fashion industry, as it doesn't cooperate with any aggressive and hazardous chemicals [39]. In a comparison to other wet processes, it encourages reduced consumption of utilities i.e. water, chemicals and power supply [40]. In this research work, the denim fabrics of same construction were washed at different ways by deviating process variables i.e. washing time and machine rpm. After that, the following samples are being tested at the laboratory for evaluating whitening index, color strength (K/S) value, colorfastness properties (color fastness to washing, color fastness to light, color fastness to perspiration), tensile properties (tensile strength, tear strength, abrasion resistance, pilling strength, hand feel).

Fabric
For conducting the following research work, off-white 3/1 "Z" twill fabric of cotton-spandex blend was used, where composition of cotton included 97.80% and spandex of 2.2%. The GSM of unwashed fabric was 345 grams per square meter and maximum fabric width was 52″.

Necessary Apparatus
For performing super white washing process and necessary lab tests, there were in need of garments washing machine, hydro extractor, garments dryer, datacolor spectrophotometer 650, tensile and tear strength tester, GSM cutter, pH meter, electrical balance, laundering machine, tumble dryer etc.

Determination of Tensile Strength
This tensile strength test was conducted according to ISO 13934. This method specifies about the determination of the maximum force of textile fabrics known as the grab test. For experimentation, test specimen gripped in its center part by jaws of specified dimensions, extended at constant rate until it ruptures. Then the maximum force was recorded.

Determination of Tear Strength
The determination of tear strength of different fabric samples treated with various washing processes were measured according to ISO 13937-1 [41]. Tear strength is the resistance of the fabric against tearing or force required to propagate the tear once it is initiated. The tear strength is required in high performance applications and in conventional textiles like bulletproof jackets, tents, aesthetic apparel. This is also important in the industrial textiles where heavy duty work is performed.

Determination of Pilling Resistance
The determination of pilling resistance of different fabric samples treated with super white washing process measured according to ISO 12945-2 [42]. Pilling test is carried out to determine the resistance to pilling and change in appearance of apparel fabrics. Generally, pills are formed during wear and washing by entanglement of loose fibers which obtrude from the fabric surface.

Determination of Dimensional Stability
The dimensional stability of the sample was measured according to ISO 23231 [43] which postulates about an accelerated procedure for the understanding of dimensional changes of fabrics. This standard signifies a procedure which implies an apparatus with programmable settings simulate numerous domestic and industrial laundering actions and wet processing operations in fabric manufacturing. In this research, this standard method was used for the determination of dimensional change. After washing, drying, conditioning and measuring the specimen, the change in dimension was calculated.
Dimensional Change (%) = {(Dimension after wash -Original dimension) × 100} / Original dimension Sample Conditioning: Conditioning specimen at least 4 hours as per ISO 139 prior to preparing and measuring.

Measurement of Color Fastness Properties
The following color fastness tests were done for this research work [

Effect of Washing Time and Machine RPM Variation on Whitening Index (WI) and Color Strength (K/S) of Super White Washed Denim
The following tables reveal that there was reduced whitening index and color strength, at the initial samples of washing process time and machine rpm. In case of washing time variation, minimum whitening index and color strength was depicted respectively 69. 21

Effect of Washing Time and Machine RPM Variation on Tensile Strength of Super White Washed Denim
The following diagrams reveal that there was a gradual falling of tensile strength in both the warp and weft direction with the increase of washing process time and machine rpm up to a certain stage. In terms of washing time variation, maximum tensile strength value of warp and weft was depicted respectively 207 & 160 lbs. at 35 min. After a gradual increase of process time, it was settling down in 190 and 151 lbs. at 80 min. For machine rpm variation, maximum tensile strength value of warp and weft was depicted respectively 204 & 164 lbs. at 16 rpm of washing machine. After a gradual increase of process machine rpm, it was settling down in 184 & 154 at 25 rpm of washing machine. From the above mentioned details, it can be assumed that the gradual increase of washing process time and machine rpm had established more frictional force among the garments, washing machine surface and chemical contaminated water. Hence, it leads to continuous drop of tensile strength up to a certain level. This is because, the -OH groups of polymer structure and many other strong covalent bonds in fiber had broken down with the following circumstances.     From the above mentioned details, it can be assumed that gradual enhance of washing time and machine rpm had established more abrasion and discontinuity between fiber & fiber matrix surface, thus leads to continuous drop of tear strength up to a certain level The following tables reveal that there was a gradual increase of negative shrinkage% in both the warp and weft direction with the increase of process time and machine rpm. For washing time variation, minimum shrinkage% in case of warp and weft were depicted respectively -2.1% and -3.8% at 35 minute. After a frequent increase of process time, it was moving up -3.1% and -5.3% respectively at 80 min. In terms of machine rpm variation, minimum negative shrinkage% of warp and weft were depicted respectively -1.3% and -3.2% at 16 rpm of washing machine. After a gradual rise of machine rpm, it was moving up at -4.8% and -5.2% at 25 rpm. Besides, it can easily be evaluated that weft shrinkage% for all the eight samples had quite large value than those of warp shrinkage%. This is because, the fabric blend contained 97.80% cotton and 2.20% spandex, which allowed the fabric to absorb more moisture both in hot air and cold water medium. And, it is to be noted that cotton fiber has higher tendency to shrink, when it comes in contact to water; whereas spandex fiber shrinks more where there is hot water or air contamination both in the garments washing and drying machine.

Effect of Washing Time and Machine RPM Variation on Pilling Resistance & Hand Feel Rating of Super White Washed Denim
The following diagrams reveal that there was a gradual improvement of both the pilling resistance and hand feel ratings with the continuous enhancement of washing process time and machine rpm up to a certain stage. In case of washing time variation, minimum pilling resistance and hand feel rating were depicted respectively 2.0 and 2.5 at 35 min process time. After a gradual rise of process time, it was moving up within 3-4 and 4 respectively at 80 min process time. For machine rpm deviation, minimum pilling resistance and hand feel rating was depicted respectively 2.0 and 2.5 at machine rpm of 16. After a gradual rise of machine rpm, it was ranged 3.0 and 4 respectively at machine rpm of 25. From the above mentioned details, it can be assumed that pilling not only creates unsightly and fuzzing appearance but also drops the hand feel rating and serviceability of textiles.   The following diagrams reveal that there was a gradual increase of GSM with respect to gradual enhance of process time and machine rpm. In case of washing time variation, maximum change of GSM% was depicted 4.3% at 35 min. After a frequent enhance of process time, it was moving up 7.9% at 80 min. For machine rpm variation, minimum change of GSM% was depicted 2.3% at machine rpm of 16. After a gradual increase of process temperature, it was moving up 4.3% at machine rpm of 25. During washing and drying of super white washed denim, the cotton-spandex fabric composition had shown a tendency to shrink in both the warp and weft direction, which leads to the rise of GSM%. And, EPI & PPI of denim garments had increased after washing; as a result, it can also be clearly mentioned that the enhancement of EPI and PPI also played a key role onto the enhancement of GSM%.  The above presented tables reveal the effect of washing process time and machine rpm on color fastness to washing properties of super white washed stretch denim fabric. It was investigated that at the initial washing time (35 minutes) and machine rpm (16 rpm), the samples demonstrated fair grading in case of color change and color staining. With the gradual rise of washing time and machine rpm, grading for both the color change and color staining moved towards moderate to good for all the samples.  The above mentioned tables reveal the effect of super white washing process time and machine rpm on color fastness to perspiration properties in both the acidic and alkaline medium. It was found that, in the very beginning of the increase of both the washing time and machine rpm, the super white washed sample experienced moderate rating in case of color change and color staining for acidic and alkaline medium. After the frequent rise of washing process time and machine rpm, all the samples had experienced steady improvement in terms of color change and color staining for both the acidic and alkaline medium. The following tables reveal the effect of super white washing process time and machine rpm on color fastness to light property. It was found that, in the very beginning of both the washing time and machine rpm enhancement, the super white washed samples had experienced moderate rating in blue wool standard grading, which indicates the samples had very low resistance against UV light. After the gradual rise of washing process time and machine rpm, the following samples had experienced moderate to good grading.

Conclusions
The prime aspect of this research paper is to evaluate, how washing process time and machine rpm affect the collective functions of stretch denim fabric. It was found that there was a gradual falling of tensile and tear strength in both the warp and weft direction, with the rise of washing process time and machine rpm. Furthermore, the negative weft shrinkage% for all the samples are higher than those of negative warp shrinkage%, as the fabric blend was composed of cottonspandex. Hence, it also leads to the rise of GSM change%. Beyond those narrow drawbacks, there were some improvement of color strength (K/S), whitening index (WI), pilling resistance and fabric handle on stretch denim under following washing and testing conditions. In addition, it was demonstrated that there was not significant amount of change in color fastness properties of denim fabric, while deviating washing time and machine rpm. Indeed, those analytical and experimental estimations may provide very few technical recommendations for the manufactures and washing technologists about the appropriate selection of washing process time and machine rpm, while undergoing super white washing process.

Compliance with Ethics Requirements
This article does not contain any studies with human or animal subjects performed by any of the authors.