Making the chemistry work: Choose the right ink for your particular application.
By Samit Chevli
Digital textile printing is fast gaining ground and seeing market acceptance in printing on a variety of fabrics. Diverse markets such as apparel, home furnishings, tradeshow graphics, flag and banner, swimwear, and point of purchase displays are all showing rapid growth.
Compared to analog techniques, the cycle time from design conception to actual production is significantly reduced by using digital printing. Creating the best image quality for digital textile printing is obtained with proper consideration of several key areas:
- Fabric and ink selection
- Preprocessing and postprocessing of textiles
- Color software
- Printing equipment
This article will review the selection of ink types, fabric types, pretreatment/coating of the fabrics, and postprocessing in digital textile printing and how to optimize these elements for the best results.
Generally speaking there are four basic types of inks available to digitally print on textile substrates. Table 1 shows the ink and fabric selection matrix. Table 2 shows an example of a colorant for each ink type and brief description of its mode of attachment to the fabric.
It’s clear from the table that there are specific inks for each specific type of textile fiber/fabric. It is important to select inks that are compatible with the fabric and printer system since not all ink chemistries are available for a given printer system. A system that offers all the different ink chemistries will facilitate the widest selection of fabrics for printing.
Digital textile printing process
Digital printing of textiles with dye based inks require pretreatment and posttreatment of the printed fabric for full color and durability. The chemicals in the fabric coating permit reaction of the dye with the fiber during postprocessing.
After final drying additional treatments can be given to the printed fabric like waterproofing, flame retardant, and so forth.
Note: Pigment ink printing of textiles is an exception to the above flowchart. Pigment printing does not require any coating treatment of the fabric and the fixation treatment usually is a dry heat fusing requiring no washing and drying steps. However, color gamut obtained using pigment inks on textiles is lower than that obtained with dye inks and pigment inks also can make the fabrics feel slightly stiffer to touch.
Textile fabrics can be broken down into three basic types.
Woven fabrics. These have two sets of yarns running parallel and perpendicular to the selvage called warp and weft. These fabrics are dimensionally very stable.
Knitted fabrics. Formed by interlooping of yarns to form loops. Knitted loops are referred to as stitches. Vertical stitches are called wales and horizontal stitches are called courses. Knitted fabrics are dimensionally less stable than the woven fabrics.
Non-woven fabrics. These fabrics have intermingled fibers that are held together by adhesive, thermal bonding, spun bonding, and so forth.
Once the inkset for printing is selected, the selection of fabric needs careful attention to obtain the best image quality.
Surface defects on the fabric can lead to fabric striking the printhead resulting in printhead or image quality damage. Check for these factors:
- Fabric surface should be free of any lint.
- Fabric surface should be free of any broken fibers or filaments.
- Fabric roll should be wound with uniform tension across the width and length of the roll.
- There should be no wrinkles or creases on the fabric.
Dimension defect is another consideration. Most textile fabrics have a tendency to shrink. Care should be taken to preshrink the fabrics and make them dimensionally stable before starting to print, otherwise the printed fabric can change in dimension during postprocessing and lead to image distortion. While processing knitted fabrics, care should be taken to prevent excessive stretch during the coating step and also during printing step to prevent image defects due to dimensional stability.
In the simplest terms, pretreatment is an application of a solution to coat a fabric to achieve the following main objectives:
- Better image quality in digital printing by preventing lateral bleed. This is achieved by the use of natural or synthetic thickeners commonly used in the conventional textile printing paste.
- Application of necessary chemicals that are needed for the dye to exhaust onto the fabric and react with the fibers on a molecular level during the heat fixation process. Such chemicals need to be applied to the fabric prior to printing. They cannot be incorporated in the digital printing ink due to the need for stringent purity and low viscosity required of an ink to sustain good quality printing.
Since a universal pretreatment for all the ink types does not exist, each ink type requires its own unique pretreatment solution.
However, irrespective of what chemistry of pretreatment is used, the method of application is the same. Fabric is saturated in the pretreatment solution by dipping the fabric in an open width form, squeezing the fabric out by application of pressure, and drying the fabric in hot air. Most commonly pretreatments are done using textile equipment called a stenter/tenter frame.
It is not essential to invest in pretreatment equipment or possess detailed knowledge of the know-how of the pretreatment chemistry to print digitally. Commercially available pretreated fabrics are becoming more widely available as the digital textile market expands.
Once the pretreated/coated fabric is printed with digital inks it needs to go through a fixation process. This process enables exhaustion of dye from the fiber surface in the printed area into the molecular structure of the fiber. It also enables absorption of colorant on the fiber:
- Ionic forces of attraction in the case of acid inks on polyamide fibers.
- Dissolved in the fiber as in the case of disperse inks for polyester fibers.
- Reacts with the fiber and forms a covalent bond as in the case of reactive inks on cellulosic fibers.
- Binder self crosslinks forming a network in the case of pigment inks.
Table 3 shows typical fixation conditions for each type of textile ink. The fixation process ensures entrapment of dye colorants into the fiber structure or pigment and binder fusion to the fabric. This ensures:
- Bright, chromatic and deep colors are obtained that do not wash off.
- Printed fabric that can withstand rigors of daily use, such as fading due laundering, perspiration, exposure to light, and rubbing.
When investing in fixation equipment it is important to keep in mind that most of the equipment available is geared towards serving the conventional textile printing needs where throughput as high as 50 linear meters per minute is not uncommon. Digital textile printing runs at 30-100 meters per hour. The fixation equipment should complement the production capacity of the printer. Optionally, the postprocessing job can be outsourced to textile mills, which are already equipped with sophisticated machinery to do the job.
This final step in the digital printing process is essential for direct printing with all the dye based inks. Wash-off serves the following important functions for the dye based inks.
Removal of excess unreacted dye. In textile coloration with dyes, the adsorption of dyes onto textile fiber is never 100 percent complete and there is always some loose dye left on the fiber/fabric surface. This dye must be removed to realize the full end use potential of the printed product.
Removal of pretreatment or coating solution. Coating solution applied on the fabric during the pretreatment step imparts stiff handle/feel to the fabric. Wash-off process removes the coating solution and makes the fabric feel soft.
Wash-off also makes the white unprinted area come out looking bright and clean. Wash-off is not needed if fabrics are printed with pigment inks. This makes pigment inks particularly attractive since they do not need wet processing.
So many inks, so many fabrics: conclusions
With a variety of different textile substrates and ink chemistry available for printing, it is important to select the right substrate and ink chemistry for digitally printing onto textiles. Attention should be paid to proper preparation of the substrate to avoid print image quality defects. All the necessary steps of pretreatment/coating, fixation, and wash-off should be followed in order to obtain optimum results in digital printing. Equipment selected to perform pretreatment or posttreatment steps should complement the speed of the digital printer.