DIGITAL TEXTILE PRINTING USING SOLID INK TECHNOLOGY

Michael Korger1, Christine Steinem1, Joanna Kowol1, Maike Rabe1, Mark Leenen2, Jelmen Meijerink2, Tiny Ritzen2

1Hochschule Niederrhein – University of Applied Sciences, Research Institute for Textile and Clothing (FTB), Rheydter Str. 291, Mönchengladbach, Germany 2Océ Technologies B.V., St. Urbanusweg 43, Venlo, The Netherlands [email protected]

Abstract As well as conventional printing processes for textiles like flatbed, rotary screen or transfer printing, nowadays digital printing covers a wide range of textile printing applications which derive benefit from advantages such as a shorter turnaround, cheaper low volume and variable data printing. Although inkjet technology provides a great potential to produce high- quality prints with good color fastness on different textile materials, presently the large variety of textile materials requires different kinds of inks (containing reactive, acid or disperse dyes or pigments) and printers. At the same time a specific pre- and after-treatment of the textiles involving energy-consuming washing and drying steps are required. In this project an environment-friendly and flexible-purpose printing technology with hot-melt inks usable for different textiles is being elaborated. In this printing process neither water nor solvents are used and therefore, no fixation or drying process is needed, resulting in less energy consumption and virtually no emissions. Here the capabilities of this solid ink technology applied to textile media are presented. This includes investigations to improve ink fixation and color fastness against external stress like rubbing, washing or light with reference to the standards for suitable application fields like signage or interior decoration.

Introduction Digital textile printing is one of the fastest growing sectors within what is called “Industrial Printing” and therefore it has become more and more important also in the textile and clothing industry. In fact, in the worldwide market the segment “Soft Signage Printing” has grown 45 percent in its adaption of digital inkjet printing technology [1]. The breakthrough enabled by advanced new and eco-friendly technologies is expected to contribute to the European success in this fast growing industry. Digital printing is an on-demand process and therefore production of printed textiles can be done in short runs (low volumes) and on site. Table 1: Overview of common inkjet fabric requirements [2] Textile Inkjet Printing Fixation and Textile Preparation (Inks) Washing-off Finishing CEL Yes Reactive Yes like traditional Silk, PA, WO Yes Acid Yes like traditional PES Yes Disperse Yes like traditional No / Yes (for CO, CO/PES Pigment No like traditional high quality)

According to state of the art digital textile printing technology, usually water-based inks are applied to textiles through nozzles in an inkjet printing process with subsequent drying and fixation, e.g. using saturated steam. Then the coloring is based on chemical and physical bonding between textile and colorant which results in good color fastness. However, the handling is quite inflexible, because different textile materials require selected types of inks to obtain an excellent color performance (see Table 1). Additionally, specific textile pre- treatments and post-processing steps like washing-off contribute negatively to the eco- balance. Certainly, with the use of pigment inks (UV-curable or IR-drying latex inks), which contain binders to form a polymer matrix or film to fix the pigments onto the textile, the printing process becomes quite independent of the textile substrate. Even so, the printing quality including the range of color gamut and textile feel or grip is negatively affected [3]. Additionally, since textiles are highly porous materials, the UV-curable inks are absorbed deep into the textile and it is impossible to fully cure the ink. Harmful non-cured monomers will remain in the textile and can come into contact with human skin. Next to that, such textiles printed with current state of the art UV-curable inks will still smell bad lasting for weeks after printing. Furthermore, latex inks, like other water-based inkjet techniques, require energy-intensive drying or fixation processes. In the German-Dutch Interreg IV A project “SITex-Print – Solid Inks for Textile Printing” it is intended that those disadvantages should be removed by one sustainable and flexible- purpose printing technology using waterless hot-melt inks.

Solid Ink Technology The applied solid ink (CrystalPointTM) technology is already being used commercially for color printing on plain paper, where it results in prints with strong brilliant colors and sharp defined printed lines, text and other features. In the printing process the solid ink (in the form of TonerPearls™, Figure 1) is melted in the print heads and jetted onto the paper by piezo technology in pL-sized droplets which immediately solidify again on the surface. No drying or fixation step is required.

Figure 1: Cyan, magenta, yellow and black TonerPearlsTM

The fact that the ink solidifies fast on the medium makes the technology especially suitable for printing on plain paper. Textiles are even more porous media than plain paper. The strengths of CrystalPointTM technology on plain paper could also be applicable on textiles. At the same time, if CrystalPointTM does not require media pre- or post-treatment, it would require a much less complex workflow compared to traditional liquid textile inkjet inks.

Experimental and Results Printing experiments were performed on a PiXDRO LP50 desktop inkjet printer from Roth and Rau equipped with one print head (from Océ Technologies) and a heatable substrate table. TonerPearls™ from Océ Technologies were used as hot-melt inks. For first investigations different textile materials like polyester, cotton, polyamide, PVC coated glass fiber or polypropylene were printed with a simple grid pattern using black TonerPearls™. After that, washing fastness (DIN EN ISO 105-C6), sublimation fastness (DIN 54056), rubbing/crock fastness (DIN EN ISO 105-X12) and light fastness (DIN EN ISO 105-B02) of the printed fabrics were tested and evaluated. While on average the color fastness tests to washing and sublimation were quite unsatisfactory for all samples, the light fastness was sufficient (> level 5 blue scale) independent of the textile substrate. Looking at the Crockmeter tests for testing rubbing fastness (Figure 2), there are bigger differences in the results, not only related to chemical composition of the textile but rather to the textile structure (kind of texture). Moreover, the wet rubbing test results are usually better than the dry ones due to the water-repellent character of the hot-melt ink, whereby a one-layer print was always better than a three-layer print.

- Dry - - Wet - 5 5 4 4 3 3 2 1 layer 2 1 3 layers 1 1 2 3 d F 2 3 F CS CO ze PP G S 1 S CS CO PP ES ES a C-G P P PES ira PE PE PES v rceri re e PVC- revir PV T m T

CO, CO, mercerized

Figure 2: Color fastness to rubbing for different printed textile materials (black TonerPearls™, grid pattern, 1 layer and 3 layers). Crockmeter test results (DIN EN ISO 105-X12, dry and wet): grey scale for assessing staining (5 = no staining; 1 = strong staining)

Crock fastness tests (dry and wet) on additionally six different knitwear samples of polyamide, polyester or cotton material, which were printed with a filled black area at 750 dpi, did not result in a higher grade than three (not shown here). The same results were obtained when the samples were pre-treated as in usual inkjet printing processes. This also indicates that there is no direct chemical bonding between hot-melt ink and textile surface. In order to observe ink penetration into the textiles high-resolution SEM pictures of selected textile samples and their cross sections were taken (Figure 3). PES-1 PES-2

CO-1 CO-2

ink

Figure 3: HR SEM pictures of printed polyester (PES-1) and cotton (CO-1) fabric (black TonerPearls™, 3 layers) and their cross sections (PES-2, CO-2) prepared by Ion beam Cross section Polisher (ICP)

Comparing printed polyester (PES) with cotton sample (CO), for both, the major part of the ink is situated on or just between the upper fibers. Fiber wetting appears to be slightly improved for the cotton fabric due to the looser bundled structure of cotton staple fibers. By heating up the textile substrate, the wettability and the penetration of the ink into the fabric was improved (Figure 4) resulting in a slightly better rubbing fastness.

Figure 4: Microscope pictures (250x) showing influence of heated substrate table (left: T = 20 °C, right: T = 50 °C) on printed CO/silk satin (black TonerPearls™, 750 dpi, 2 layers)

The influence of temperature in the printing process can be used to regulate ink spreading and penetration, in order to achieve an optimum ink coverage of the textile surface. As resulted in different testings generally a large substrate surface area is required to obtain a good adhesion or fixation for the hot-melt ink. Summary and Outlook In digital textile printing solid ink technology offers an environmentally sound possibility to produce prints on textiles in one or more layers with high acuity. Color fastness of the printed textiles mainly depends on physical i.e. adhesive interaction between ink and textile. For a better ink fixation the contact between ink and textile fibers has to be maximized by e.g. choosing a woven structure as “open” as possible and temperature adjustment to fit the wetting behavior such as ink spreading and penetration. Further investigations to improve ink fixation are in progress. According to improved wet rubbing fastness results, first focus of hot-melt ink printing applications is soft signage and interior decoration for printing textile displays, banners, flags or wallpapers (wall fleece). Indoor applications additionally profit from the solvent- and emission-free technology. The usability of further application fields like reserve printing, where the ink has to protect the textile surface in one working step and to be removed afterwards, will be evaluated.

Acknowledgement: The project “Technologie-Kompetenz-Verbund Funktionale Oberflächen (TKVFO)” enclosing “SITex-Print” is supported by the European Regional Development Fund (ERDF) of the European Union, the Ministerium für Wirtschaft, Energie, Industrie, Mittelstand und Handwerk (MWEIMH) of Nordrhein-Westfalen, the Ministerie van Economische Zaken, Landbouw en Innovatie (EL&I), the province Limburg, the province Noord-Brabant, the province Gelderland, the province Drenthe and the province Overijssel within the framework of the INTERREG IV A programme Deutschland-Nederland.

References [1] H. Ujiie, State of Art of Inkjet Textile Printing: Status Report 2012, 28th International Conference on Digital Printing Technologies, Quebec, Canada (2012). [2] M. van Parys, Digital Technologies – State of the art, 2nd International Digital Textile Congress, Ghent, Belgium (2012). [3] R. Appelbaum, Pigmenttinten als interessante Alternative im Digitaldruck, Textilveredlung, 5/6, 5 (2012).