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Fabric Graphics, March April 2010, Digital Edition

Fabric Graphics, March April 2010, Digital Edition

Dyes are soluble and

Dyes are soluble and have an affinity to the substrate they are applied to. Pigments on the other hand are nonsoluble and have no affinity to the material they are applied to. A third category has some characteristics of each of the primary two and is called dispersed dyes. Keep in mind that in specific cases, pigments can become dyes and vice versa. What really matters to us as inkjet printers is that different fibers require the use of different types of colorants in order to deliver rich colors that last. For this purpose, five types of inks are generally used in inkjet printers to print on fabric, and all use either pigments, dyes or dispersed dyes as colorants. Standard dyes have an affinity with the substrate they are used on. This means that they are able to be easily absorbed by the compatible fiber. The simplest form of dyeing involves dissolving the dye in water and immersing the fibers in the solution. Fiber porosity is important in the dyeing process, as dye molecules need pores that are large enough to accommodate them. Dye retention is also dependent on certain chemical attractions that facilitate the penetration and make it permanent. Different types of fibers have different porosities, and their molecules can be either positively or negatively charged. A variety of dyes and treatments have therefore been developed to optimize penetration and dye retention based on the specific properties of each fiber. Using the right combination of dyes and ancillary treatments for each fiber will result in better color reproduction and better color fastness properties. Fibers that are printed with dyes include protein-based fibers, such as silk, and cellulose-based fibers, such as cotton and polyamides (nylon). However, not all of them are printed with the same type of dyes or are processed under the same conditions. The two most common types of dyes used for inkjet textile printing are acid and reactive dyes. Acid dyes Acid dyes are best used to print on protein-based fibers, such as wool, angora, cashmere and silk. Polyamides, such as nylon, can also be printed using this type of dye. Acid dyes are not indicated for the printing of cellulose-based materials, such as cotton and rayon, nor are they effective on other synthetic fibers, such as polyester. This type of dye is thought to fix to fibers by hydrogen bonding and ionic bonding. Compatible fibers contain many cationic sites (groups of ions with a positive charge) that are naturally attracted to the anionic (negatively charged) acid dye molecule. Think of the dyes and the fibers as being magnets with opposing charges that naturally attract each other. In simple, practical terms, the acid-dye-based ink is printed directly to the textile, then heat is applied, generally through a steamer, in order to “supercharge the magnets” and form a strong bond between the two. At the end of the steaming process, excess dyes that have not been absorbed by the textile need to be removed by washing the fabric. 2 8 Fabric Graphics Mar|Apr 2010

Fiber reactive dyes Fiber reactive dyes are mainly used to print cellulose (plant-based) fibers, such as cotton and rayon. They are also used to dye nylon, although the fixation process is slightly different. Reactive dyes have the ability to bond to the compatible fiber through a chemical reaction that requires a mildly alkaline environment. In chemical terms: the dye and the fiber form a covalent bond. The main advantage of this type of colorant over standard or acid dyes is that it is wash fast and durable when used on cotton, and it produces vibrant colors. The first reactive dyes where introduced commercially in 1956. Reactive-dye printing is possibly the most complicated of the available inkjet textile printing processes as the multiple factors and variables that influence the outcome and the chemical reactions between the dyes and the textile have to be controlled precisely in order to obtain a durable, vibrant print. If you just apply a reactive dye to cotton, nothing will happen and the dye will wash off as easily as it went on. The key in reactive dyeing is in initiating a chemical reaction that will cause the dye molecule to lose the chlorine atom and the fiber to lose the hydrogen atom in order to allow the two molecules to “stick” to each other. This chemical reaction is initiated by creating an alkaline environment (the opposite of an acid environment) with the introduction of a base, such as sodium carbonate (a stronger version of baking soda). The base can be introduced after the printing process, however, in inkjet printing, it is usually applied to the fabric as a coating (*) before printing. Heat is then applied to the printed textile in order to facilitate the chemical reaction. Excess dyes that did not bond are removed from the fabric through a washing process. Prepare for printing *For optimal results and image quality, textiles should be properly prepared for printing. Proper textile preparation should include the removal of loose fibers that can get stuck in the nozzles of the print heads and coating of the material. Coating the textile for inkjet printing keeps the ink droplets that are deposited by the printer in place to prevent the ink from wicking. The coating facilitates or prevents the penetration of the inks in the deep layers of the fabric. Just think of the difference between a photo printed on photo paper and one printed on copy paper. Photo paper is coated for inkjet printing and designed to keep the ink droplet in place on the surface with minimal dot gain (expansion of the dot), while on regular paper the ink tends to get absorbed by the paper and expand. It is important not to confuse the coating used to prepare a textile for inkjet printing with the coatings used for reactive dyeing. The two can be combined, but serve different purposes. 29

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