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.