Colorfastness of Direct-to-Garment Inkjet-Printed T-Shirts
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Colorfastness of Direct-to-Garment Inkjet-Printed T-Shirts

The purpose of this research project was to compare the colorfastness of printed targets from five direct-to-garment inkjet printer manufacturers.

By Philip D. Age and Jean K. Dilworth, Eastern Illinois University

A target design was printed on 100% cotton and 50% cotton/50% polyester (50/50) blend T-shirts. The shirts were washed five times and compared to the unwashed original. The extent of color loss was analyzed using instrumentation and visual analysis.

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  • The purpose of this research project was to compare the colorfastness of printed targets from five direct-to-garment inkjet printer manufacturers. The data from this research was provided to the printer manufacturers to enable them to evaluate the quality of their printing process to that of other manufacturers in the marketplace. The colorfastness to home laundering of direct-to-garment printing has not been researched to validate product performance.

    A target design was printed on 100% cotton and 50% cotton/50% polyester (50/50) blend T-shirts. The shirts were washed five times and compared to the unwashed original. The extent of color loss was analyzed using instrumentation and visual analysis.

    Direct-to-Garment Printing
    Direct-to-garment inkjet printing on T-shirts was first introduced in 1993 by Patrice Giraud in a French government-sponsored project. He developed a piezo inkjet system that used water-based, UV-cured ink printed through industrial inkjet heads. Giraud’s printer could also print on fabrics, towels, and mouse pads, but was not a commercial success.

    Inkjet printer manufacturers struggled for commercial success with direct-to-garment inkjet printers from 1996 to 2004. Several issues contributed to a lack of success: inkjet inks were not opaque enough, inkjet heads clogged, colors lacked brightness, and color-banding and washability issues were noticeable. At the December 2005 Specialty Graphic Imaging Association Digital Expo, nine commercial companies offered direct-to-garment inkjet printers.1 At that time, ink manufacturers had reformulated their inks to resolve some of the earlier issues of ink flow, but had not addressed the ink’s colorfastness to laundering.

    Direct-to-fabric inkjet printing has had commercial success for short runs and color sampling, however, it has not been determined whether direct-to-garment printing will be commercially viable for high-volume production runs. Consumers are seldom aware of the complex processes used to print a colored image on a T-shirt, however, many consumers are conscious of product performance related to laundering. If a product’s color and image quality do not meet the consumer’s visual expectations, the product is blamed for the failure. Therefore, to be commercially viable, direct-to-garment inkjet printing must create colored images that can sustain numerous home launderings.

    Materials

    Original Color Target
    A color target (Fig. 1) was created in Adobe Photoshop CS2, including black (k), cyan (c), magenta (m), yellow (y), red (r), green (g), and blue (b) solid color values, and three continuous-tone color images. This color target was formatted as a portable document format (PDF PostScript v3.0), which is an industry standard in the graphic communications industry. The CIELAB color values associated with the target are listed in Table I. A PDF file was used to minimize adjustment of the color values in the original data. Participating companies were asked to print the color target without any alterations to the file.

    T-shirts
    Each company received 12, 100% cotton T-shirts and 12, 50/50 T-shirts. All T-shirts were single jersey knit made from 18/1 Ne yarn. The yarn for the 100% cotton T-shirt was open-end-spun and the 50/50 yarn was air-jet-spun. The 100% cotton T-shirts were provided by Delta Apparel and according to Technical Manager Mike Weaver, “achieve a CIE WI [whiteness index] value in the low 140s, calculated on the basis of AATCC Test Method 110, which corresponds to ASTM E313.”

    Inkjet Printers
    The five international companies working on this project used their proprietary direct-to-garment inkjet printers. All five companies used a formulated, pigmented ink. Four companies used water-based pigment ink and one company used a solvent-based pigment ink. Printing resolutions ranged from 360 to 720 dpi. All companies used the CMYK (cyan, magenta, yellow, and black) ink combinations. Company 2 used a solvent-based ink. The remaining four companies used water-based pigment inks. The companies used a variety of raster image processors (RIP) and used either Microsoft Windows 2000 or Windows XP to process the PDF file.

    Inkjet direct-to-garment and fabric printing requires additional post-processing steps. These steps may include steaming, washing, drying, and heat application with either a heat-transfer press or a gas or electric dryer. All participating companies used heat for post-processing which is essential to set the inks to the T-shirt. Four participating companies used a heat-transfer press and one used a gas dryer with a conveyor. The temperatures ranged from a minimum of 170F (76C) to a maximum of 350F (176C) with a dwell time of 30­90 s.

    Home Laundering Equipment
    A high-efficiency (HE) washing machine, HE dryer, and HE detergent were used for this research. High-efficiency washers are known to use 20%­66% less water than traditional agitator washers. The Soap and Detergent Association confirmed that the future of home laundering will include HE washers, dryers, and detergents because of the long-term cost savings and environmental benefits.3 The same equipment (one HE washer, one HE dryer, and one commercial HE detergent) was used for all wash cycles. This research was not a comparison of HE home laundering equipment or detergents. The washer used in this research was a front-loading Kenmore HE3 washer manufactured by Whirlpool. The HE dryer was a Whirlpool with a sensory drying system, wrinkle guard, and cool-down sensors.

    Wash Water
    Water was supplied to the washer from the municipal water department after softening. Water quality was consistently “zero” hardness, with a pH from 7.6 to 8.2, and chlorine content of 0.1 ppm. The pH of water sampled from the wash tub with detergent ranged from 9.2 to 9.6 at an average temperature of 90F (32C). The sample was taken 15 min into the 45-min wash cycle. The softened water samples were tested by a certified water testing professional throughout the process. Researchers verified the pH and water temperature of the wash cycle using a Myron L. Company Ultrameter II meter.

    Detergent
    HE detergents are formulated to be low-sudsing and quick-dispersing to get the best cleaning performance with HE washers. A commercial detergent was selected because no AATCC HE Reference detergent existed at the time of this study. All but one of the readily-available commercial HE liquid detergents contained optical brighteners. The detergent used in this study included optical brighteners. A liquid detergent was selected for two reasons: liquid detergents have a higher probability of completely dissolving and can be measured with greater accuracy and consistency. Since the wash water was of “zero” hardness, the amount of detergent was reduced to one-third of a cup after noting excessive sudsing using the recommended one-half cup.

    Color Measurement Instruments
    The color measurements of all printed samples were performed using an X-Rite SP64 spectrophotometer coupled to a PC with X-Rite Color Master software and an X-Rite 500 Series Spectrodensitometer. Both instruments used illuminant D65 and the 10° standard observer with the specular component excluded and the UV component included. The spectrophotometer was calibrated before each session, and every hour during sessions using the reference tile provided by the manufacturer.

    Method
    Printing
    Five companies participated in this study. Each company printed the provided color target (Fig. 1) on 12, 100% cotton T-shirts and 12, 50/50 T-shirts, and then returned them to the researchers for washing and analysis.

    Home Laundering
    Eight shirts from each set of 12 were randomly selected for home laundering (four of each were retained for backup). All shirts were turned inside-out to reduce washer and dryer tub abrasion. The T-shirts were washed according to the “Procedures” section of AATCC Test Method 172-2003 (TM172).2 TM172 was used (with HE detergent and no bleach) since a specific method for HE detergents and washing machines was not available.

    Garments were washed using the Warm/Cool setting for all loads. The water temperature range was 94F-117F (34C-47C) during each 45-min wash cycle. Each T-shirt specimen was washed with enough ballast to create a 4 ±0.25-lb load as described in TM172. After each wash load, the shirts were removed immediately and placed in the HE dryer.

    With the dryer temperature set to Energy Preferred, Medium/Casual and the cool-down sensory system in place, the drying temperature varied continuously throughout the drying process. The measured temperature ranged from 80F (26C) to 174F (79C). A Velleman digital thermometer with a flexible probe was used to record all temperatures. The time required to dry the shirts varied from 24 to 32 min, depending on the fiber content of the shirts. Shirts were removed immediately at the end of the drying cycle and conditioned to room temperature before color measurements were taken.

    Color Measurement
    All instruments were calibrated at the start of each series of measurements. The color data for five unprinted T-shirts of each fiber type were measured and averaged before and after laundering. The averages for the five shirts are listed in Table II.

    Three printed T-shirts of each type (company and fiber content) were selected and measured after each wash cycle. The colors of the T-shirt were measured using two thicknesses. On each shirt, five readings were taken for each color target and the white, non-printed area. The average of the five readings was recorded. Tables III and IV contain the color data for shirts printed by Company 1 for seven inkjet-printed colors before laundering, after the first wash cycle, and after the fifth wash cycle. Data for the remaining companies may be obtained directly from the author. In CIELAB color space, color differences are expressed as a single numerical value, , which indicates the size of the color difference but not the way in which the colors differ.

    Visual Color Assessment
    The printed targets were visually analyzed using a floor-model Just Normlicht viewing booth with a D65 illuminant to compare the unwashed printed T-shirts from each company to the shirts after five washes. The researchers performing visual evaluations scored “superior” on the Farnsworth-Munsell 100 Hue Test, considered to be the most comprehensive color-vision test of hue discrimination.

    According to Mike Bradbury of Color Solutions International, “All humans will perceive color differently, and the spread of these perceptions, from person to person, is quite wide. If you set up spectrophotmeters and their settings so that systematic and random errors are minimized, the instruments will also return varied results, but these will have a much narrower spread than with visual color assessment.”3 The loss of colored pigment was further verified by examining the color targets with a National digital microscope at 4× power.

    Results and Discussion
    CIELAB color space was used for instrumental color evaluations. Total color difference (*) was calculated and recorded using the CIE 1994 and CMC (2:1) color difference equations. For unprinted 100% cotton shirts, the between readings at zero and five washes was calculated to be 0.940 and was 1.264. For the unprinted 50/50 shirts, the was 0.567 and was 0.753 after five washes (Table II). Data collected from printed shirts after the first and fifth wash were compared to the data for the unwashed T-shirts and is reported in Tables III and IV. The results indicate that there are wide variations in colorfastness among direct-to-garment pigment inks after five cycles of home laundering.

    Home laundering was found to significantly affect the printed color targets on both 100% cotton and 50/50 T-shirts. The loss of colored pigment was clearly observed through the digital microscope. A digitally-captured photomicrograph of the black print on a 100% cotton T-shirt fabric is shown in Fig. 2. Fig. 3 shows the blue ink on a 50/50 shirt. Color loss and deterioration of color in the three continuous-tone images was also evident in the viewing booth.

    Color loss for 100% cotton samples was most evident on the shirts printed by Company 4 and Company 5 even after the first wash. The trend of color loss on these shirts was visually evident after the first two or three washes. Company 2 had the least overall visual color loss from the first through fifth washes. There was visual evidence of color loss for 100% cotton T-shirts on which the was above 2.0 and the was above 2.5.

    The visual evaluations for 50/50 T-shirts correlated to values similarly. Companies 3, 4, and 5 had visible color loss on 50/50 T-shirts after the first wash with certain colorants. The same companies had significant color loss after the fifth wash.

    The color changes for all garments with calculated values above 2.00 and values above 2.50 were readily visible when viewed in controlled conditions (D65 and D50 illumination in the viewing booth). After the fifth wash cycle, the highest for any sample was 4.95 (green print on 100% cotton). The highest was 5.537 (black ink on 100% cotton). The lowest was found on the blue area of a 50/50 T-shirt ( = 0.163, = 0.190).

    Conclusion
    To what extent consumers expect T-shirt colors to last depends on numerous factors. “Our customers expect our products to last. If it has the REI brand on it, it needs to last,” says Dana Parnello of REI.4 The final decision about “quality” will always be made by the consumer.

    To be commercially successful, direct-to-garment printers and ink manufacturers should continue to research pigmented ink formulations and the post-print processing.

    The researchers recommend that a below 2.0 and a below 2.5 after five washings be established as a realistic colorfastness benchmark for direct-to-garment printing on 100% cotton and 50/50 blends. The benchmark was determined by comparing the color values for the original target (Fig. 1) to values after five washes. Visually, the three continuous-tone images on 100% cotton and 50/50 shirts washed five times lacked detail in the highlights, midtones, and shadows. Based on MacAdam’s4 and Roberston’s5 research, a CMC of 1.0 represents a consistent color difference that humans can perceive, regardless of the area in color space in which the target and sample are located. The researchers’ proposed benchmarks are realistic for use by printer manufacturers to make decisions about the commercial viability of their printing processes.

    Future Research
    Research on the five companies’ printed samples will be continued to 10 or more washings, determining the color change at each stage. Researchers also plan to investigate the washfastness and lightfastness of various white pigment inks for dark garments using direct-to-garment printers.

    Acknowledgements
    This research was supported by X-Rite Corp., Myron L. Co., Delta Apparel Co., and the five companies that responded to the research request.

    References

    1. Sexton, D., SGIA Journal, 2006, Vol. 10, Second Quarter, pp7-9.
    2. AATCC Technical Manual, Vol. 81, 2006, pp308-310.
    3. Thiry, M. C., AATCC Review, Vol. 6, No. 5, May 2006, pp20-24.
    4. MacAdams, D. L., Journal of the Optical Society of America, Vol. 32, No. 5, May 1942, pp247-274.
    5. Roberston, D. A., Color Research and Application, Vol. 2, 1977, pp7-11.
    Jean K. Dilworth, School of Family & Consumer Sciences, Eastern Illinois University, Charleston, Illinois 61920, USA; telephone +1 217 581 6695; fax +1 217 581 6090; e-mail jkdilworth@eiu.edu.

    Originally published in AATCC Review, Vol. 7, No. 9, 2007, pp29-32; reprinted with permission from AATCC (AATCC Review), www.aatcc.org, copyright holder.

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