Effect of Padder Roller Speed and Pressure During Pre-Treatment on Color Characteristics of Digitally Printed Cotton Knit Fabric Grace Wasike Namwamba and Devona L

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Effect of Padder Roller Speed and Pressure during Pre-treatment on Color Characteristics of Digitally Printed Cotton Knit Fabric Grace Wasike Namwamba and Devona L. Dixon, Textile Technology Southern University Agricultural Research and Extension Center, Baton Rouge, LA 70813

ABSTRACT

The purpose of this study was to determine the effect of padder roller speed (RPM) and pressure (PSI) on the color of digitally printed cotton fabrics. Single layers of double-knit cotton fabric were padded at speeds of 20, 40, and 60 rpm and pressure levels of 5, 15, 40, 60, and 70 psi using a 36” wide padder. The padding solution consisted of alginate, soda ash, urea, and distilled water. Color readings were taken using a spectrophotometer. Results indicated that roller speed and roller pressure both had an effect on the shade depth of the fabric. The deepest color depth was obtained at a roller speed of 40 rpm and roller pressure of 20 psi.

INTRODUCTION Printing directly onto a substrate requires the fabric be chemically pre-treated to ensure optimum dye penetration and color vibrancy. The padding solution or chemical pre-treatment may vary according to fiber chemistry and end-use requirements. Common pre-treatment solutions for digital textile printing of cotton (cellulosic) fibers include a mixture of alginate, soda ash, urea, and distilled water where alginate and urea are the thickeners for the pre- treatment solution. In this formulation, alginate is mixed with urea to control the temporary containment of droplets spreading toward the fabric (Baffoun, Viallier, Dupuis, & Haidara, 2005). The chemical pre-treatment is best applied using a padder (Namwamba, 2005) consisting of rollers. In the textile industry, the final result of roller printing depends on the efficiency of machine operation and control and the accuracy of the roll preparation (Hudson, Clapp, & Kness, 1993). Those practices also are relevant when padding fabric for digital textile printing. The proper combination of roller speed and padder pressure must be achieved to effectively apply padding solution for even consistent printing and optimal color characteristics. The proper padding solution applied to clean scoured fabric must be applied evenly and be completely absorbed by the substrate. A poorly applied chemical pre-treatment is not sufficiently capable of producing color consistency throughout the fabric and produces poor quality digitally printed fabric. Color inconsistency is attributed to matters related to certain printing parameters. Occurrences of poor quality or color inconsistency in printing have been linked to printing at different times, on different machines, and printing on large dimensions of the fabric (Tse,

1 Briggs, Kim, and Lewis, 1998). Changing printer technology, fabric pre-treatment, steaming, and other finishing processes are some to the variables that present challenges in color prediction and control (King, 2002). Color accuracy and appearance are major concerns in digital printing of fabrics. Accurate color that is consistent throughout the printed design is related to the overall appearance of the final digitally printed fabric. The appearance of color is highly critical in producing a quality digitally printed product. Color shade depth is an important factor in determining acceptability of textile fabrics. The purpose of this study was to determine the effect of padder roller speed (RPM) and pressure (PSI) on the color of digitally printed cotton fabrics. The padder for applying pre- treatment and fabric preparation is critical to the overall appearance of color of the final product. The padder parameters, which include roller speed and pressure, contribute to the final overall print quality and appearance. Padding parameters research is warranted for digital textile printing.

METHOD

Procedure: One hundred percent double–knit cotton fabric was padded with a pre- treatment fabric solution for digital textile printing. The pre-treatment formula consisted of alginate (.008%), soda ash (.04%), urea (.1%), and distilled water (.852%). A smooth, consistent, lump-free padding pre-treatment mixture was poured into the padder reservoir for fabric padding pre-treatment. Padding: Slightly over one yard of scoured double-knit tubular fabric was cut from the boat. The tubular fabric was then cut into two separate pieces of fabric. One layer of a single yard of fabric was loaded onto the padder. The lead of the fabric was lightly dipped into the padding pre-treatment mixture to make loading the fabric easier. Once loaded, the entire testing area of the fabric would pass through the padding pre-treatment solution in the padder reservoir into the rollers which removed excess solution. The padding speeds were 20, 40, and 60 rpm. The pressures of the rollers were 5, 15, 40, 60, and 70 psi. After pre-treatment padding, the fabric was hung and allowed to air dry over night.

2 Backing: It was necessary to back the dry pre-treated fabric in an effort to eliminate the curling edges of the knitted fabric. Freezer paper was applied to one side of the treated fabric. The freezer paper was applied using an iron warm enough to bond the paper to the fabric but not burn or singe the fabric. The fabric was allowed to completely cool over night before printing. Printing: The backed fabric was loaded into the Mimaki TX2-1600 Digital Textile Printer for printing. This printer uses 8-color piezo electric printheads in an 8x2 staggered layout. Reactive inks were used for printing. A rainbow print containing individual one inch stripes of cyan, yellow, magenta and black (CYMK). The print also contained measurable areas of the colors red, pink, violet and green. The printing was completed over the course of a few days. Critical to the printing of the pre-treated substrate is the method of digital printing and the dyes used. Piezo printing method allows for more control over the shape and size of ink droplet release (Namwamba, 2005). Furthermore, this printing method allows for higher printer resolution because the piezo process can deliver small and perfectly formed dots with great accuracy (Namwamba, 2005). Reactive dye ink is commonly used in printing on cotton (cellulosic) fibers. These dyes react with cellulose in the presence of alkali and electrolyte to form cross-linked compounds that are insoluble in water (Namwamba, 2005). Exceptionally good colorfastness characteristics are achieved with cotton (cellulosic) fibers as a result of the cross-linking.

Steaming: The reactive ink dyes were fixed with steam. The 100% double knit 1-yd. fabric samples were steamed in maximum group size of three. This was done in an effort to maintain consistency in the number of layers for each during steaming. Each group steamed for approximately 30 minutes in a low pressure bullet steamer at 245°F (120°C). In rolling the fabric to be steamed, a half yard of woven cotton fabric was used as the lead fabric for steaming. Another half yard of woven cotton fabric was used as the edge or outer most part of the rolled fabric. The lead and edge fabric technique was utilized so that the inter-most and outer-most layers of the fabric would not receive more steam than the center layers once rolled around the core. After steaming, the fabric was allowed to air dry overnight to remove all moisture before color readings were taken. Post treatment (Wash/rinse): In groups of three, the cotton knit digitally printed and steamed fabric samples were washed and rinsed in a commercial washing machine. No chemical

3 or detergent was added to the wash water. The wash/rinse post-treatment cycles consisted of two cold washes (20-30C° or 68-86F°) for 6 minutes and two cold rinse cycles, immediately followed by two hot washes (60C° or 140 F°) for 6 minutes and two cold rinse cycles. One additional cold rinse was added after the final wash/rinse cycle. The washer was set on small load and pre-set cold wash and pre-set hot wash guaranteeing the temperature in the designated range. The samples were immediately removed from the washer and allowed to air-dry and then placed into a commercial drier on high heat for 10 minutes to dry completely. Color Readings: Instrumental assessment of color change of the digitally printed fabric was done according to AATCC Evaluation Procedure 6:2001 (Instrumental Color Measurement) using the Colorguide 45/0 spectrophotometer. Color was evaluated at a 45° angle, underneath a north sky light source in the VarilouxTM light chamber. Ten (10) readings were randomly taken from each solid color represented in the print (cyan, yellow, magenta, black, pink, violet, green and red) in each condition (post-print, steamed, post wash/rinse). A total of 30 readings were taken for each color at each rpm/psi. For this study, the CIELAB color system was used based on AATCC standard procedures. The results of the color readings from the spectrophotometer were reported as L*, a*, b* values where L indicates lightness or darkness with values ranging from zero (black) to 100 (white). The a indicates the amount of red (positive value) and green (negative value). The b indicates the amount of yellow (positive value) and blue (negative value). Overall color change is indicated by ∆E. For this purposes of this paper, the focus is on L* of the post wash/rinse samples.

Statistical Analysis: Data obtained by instrumental color measurement were analyzed using

SPSS version 12.0 to determine if there was significant differences in L*and ∆E after the treatments. Descriptive statistics were also computed.

The General Linear Model (GLM) procedure was used to analyze the data. A significance level of p<0.05 was used. The statistical model used was as follows:

Yi,j = µi+ τi + ej(i)

4 where

yij is the value of the dependent variable for sample j receiving treatment i,

µ is the overall mean for the dependent variable,

τi is the effect of treatment i on the dependent variable, and

th ej(i) is the effect of j sample receiving treatment i.

Results and Discussion

Effect of Roller Speed on Color Depth: The general trend of L* was consistent among the

eight colors represented. Fabrics padded at 40rpm produced the darkest shades (see Table 1).

(L* indicates lightness or darkness with values ranging from zero (black) to 100 (white).)

Multiple comparisons using LSD indicated that there was no significant difference in L* values

for fabrics padded at 20 rpm and those padded at 60 rpm. Padding at a lower speed of 20 rpm

did not produce the darkest fabrics as expected. The explanation of the lightness of fabrics

padded at 20 rpm lies in the dye chemistry of the fiber reactive dyes used to print the fabric.

Fiber reactive dyes react with cellulose in the presence of alkali (NaOH) and electrolyte (NaCl)

to form cross-linked compounds that are insoluble in water. The slow padding speed means

greater uptake of the padding liquid into the fiber. This in turn would result in deeper dye

penetration into the fiber and less dye on the surface of the fabric. Less dye on the surface of the

fiber results in lighter colors.

Effect of Roller Pressure on Color Depth: Roller pressure had a significant (p<.05) effect on

L*. Increasing roller pressure produced darker colors up to PSI of 20. Higher pressure produced slightly lighter colors. (see Table 2). Higher pressures generally means that the pre-treatment liquor penetrates deeper into the fiber, whereas lower pressures result in more of a coating effect.

5 In the case where the fibers are coated, the fiber reactive dye would react on the fiber surface,

hence producing darker colors. For fibers padded at higher pressure, the dyes penetrate deeper

into the fiber, thus producing lighter colors.

Effect of Color on L* a* b* values: Results indicated that there were significant

differences in L* a* b* values for each color. Furthermore, each color represented had

significant interaction with roller pressure and roller speed. This further emphasizes the

complexity of color parameters in digital textile printing.

Conclusions

There are many complex parameters that affect color characteristics of digitally printed fabrics. The printing of the fabrics studied used a four-color process (CMYK). Findings indicated that each color had a unique reaction to the various processing parameters. The sum total of these colors gives the overall color shade depth. Results indicated that roller speed had a significant effect on the shade depth of the fabric. Fabrics padded at 40 rpm produced the darkest shades.

It is generally expected that lower padding speed imply deeper penetration of the pre- treatment liquid and hence enhanced colorfastness. The result obtained in this study did not follow this trend because the low padding speed translated to high liquid/solution uptake. This directly translated into higher viscosity of padding liquid because of the presence of alginate.

The increased viscosity hindered effective dye penetration, hence the lighter color.

Roller pressure has a significant effect on the color of digitally printed fabrics. Pressure of 20 psi produced the darkest colors. At lower pressure, increasing the RPM increased color depth. At high PSI, increasing RPM made colors lighter. The deepest colors were obtained at

RPM of 40 and PSI of 20.

REFERENCES

Baffoun, A., Viallier, P., Dupuis, D., and Haidara, H. (2005). Drying morphologies and related wetting and impregnation behaviours of ‘sodium-alginate/urea’ inkjet printing thickeners. Carbhydrate Polymers, 61. pp.103-110.

Haidara, H., Ayda, B., and Viallier, P. (November 2004). Morphology-dependent properties and swelling-induced transition in ‘sodium-alginate/urea’ thin films. Polymer, 45 (25), pp. 8333-8338.

Hudson, P., Clapp, A. and Kness, D. (Eds.) (1993). Joseph’s Introductory Textile Science, 6th Edition. USA: Harcourt Brace Jovanovich College Publishers.

King, K.M. (2000). Digital textile printing and mass customization. AATCC Rev. 2(6), pp. 9-12.

Namwamba, G. W. Digital Textile Printing. (New York: BookSurge Publications, 2005).

Schneider, R. and Sostar-Turk, S. (April 2003). Good quality printing with reactive dyes using guar gum and biodegradable additives. Dyes and Pigments, 57 (1), pp. 7-14.

Tse, M. K., Briggs, J., Kim, Y. and Lewis, A. (October 1998) Measuring print quality of digitally printed textiles. International Conference on Digital Printing Technologies.

Yang, Y. and Naarani,V. (2004). Effect of steaming conditions on colour and consistency of inkjet printed cotton using reactive dyes. Color Technology, 120.

Dependent RPM Color Mean N. Sig. Variable L 20 Black 25.106 10 .00 Cyan 39.327 10 .00 Green 48.938 10 .00 Magenta 36.485 10 .00 Pink 41.852 10 .00 Red 41.586 10 .00 Violet 25.280 10 .00 Yellow 81.664 10 .00 40 Black 24.828 10 .00 Cyan 35.914 10 .00 Green 47.411 10 .00 Magenta 35.576 10 .00 Pink 41.050 10 .00 Red 40.981 10 .00 Violet 23.904 10 .00 Yellow 81.104 10 .00 60 Black 26.514 10 .00 Cyan 36.509 10 .00 Green 48.563 10 .00 Magenta 36.426 10 .00 Pink 41.141 10 .00 Red 42.018 10 .00 Violet 25.802 10 .00 Yellow 81.174 10 .00 p<0.05

Table 1. Descriptive Statistics for Roller Speed

Dependent RPM Color Mean N. Variable L 5 Black 27.047 10 Cyan 36.408 10 Green 48.270 10 Magenta 36.345 10 Pink 41.191 10 Red 41.715 10 Violet 26.301 10 Yellow 81.061 10 15 Black 26.460 10 Cyan 39.632 10 Green 48.389 10 Magenta 36.391 10 Pink 41.347 10 Red 41.885 10 Violet 25.205 10 Yellow 81.323 10 20 Black 24.749 10 Cyan 36.058 10 Green 47.730 10 Magenta 35.691 10 Pink 41.330 10 Red 41.282 10 Violet 24.286 10 Yellow 81.278 10 40 Black 25.388 10 Cyan 37.135 10 Green 48.801 10 Magenta 36.574 10 Pink 40.810 10 Red 41.694 10 Violet 24.983 10 Yellow 81.575 10 60 Black 24.865 10 Cyan 36.359 10 Green 47.963 10 Magenta 35.849 10 Pink 41.418 10 Red 41.292 10 Violet 24.588 10 Yellow 81.273 10 70 Black 24.482 10 Cyan 37.177 10

9 Green 48.624 10 Magenta 36.153 10 Pink 41.642 10 Red 41.337 10 Violet 24.511 10 Yellow 81.344 10 p<0.05

Table 2. Descriptive Statistics for Roller Pressure.