How to match inks to fabrics to ensure that your textile prints look beautiful and your colors won’t wash off.
By Michael Labella
Ever since the dawn of time, humans have been searching for colorants that would allow them to apply images to any type of surface, that would last forever and that would reproduce a wide range of vibrant colors. Unfortunately, we are not there yet.
Until the late 1800s, all dyes were obtained from natural sources, such as plants and insects. The synthetic organic dye industry began in 1856 when English chemist William Henry Perkin prepared the dye mauve from coal-tar chemicals. Today, virtually all colorants are produced synthetically from either organic sources, such as coal-tar, or they are synthesized in a chemical lab.
The third edition of the Color Index, edited by The Society of Dyers and Colourists, lists over 8,000 colorants. These colorants can be categorized as being either dyes or pigments, depending on their method of application, chemical structure and specific color within the Color Index. In general terms, 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 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.
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 (*see sidebar) 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.
Pigments are a class of colorants that are used in a non-soluble form and cannot be absorbed by any fiber. Where a chemical reaction allows the dye molecules to be absorbed within the structure of the fiber, binders are used to “glue” pigments to the surface of the fiber’s structure. Pigments are ground into extremely fine powders and suspended in a liquid carrier in order to be used in an inkjet printer. Although water is a solvent in itself in the ink world, we usually use the term solvent to refer to distillates derived from petroleum byproducts (think paint thinners).
While dyes are usually delivered in water-based formulations, pigmented inkjet inks are formulated using either water or solvents. Solvent-based inkjet inks have limited use in the world of textile printing as they often produce poor results. Water-based pigmented inks are widely used in inkjet printers for desktop applications. In the world of textile printing, they are quite popular on rotary screen systems to print on a wide range of fabrics, but mainly cotton and poly/cotton blends.
In inkjet textile printing, the use of pigments is still in its infancy as the inks are still unable to deliver the color density and durability obtained with traditional printing methods. Ink manufacturers are currently limited in the amount of pigments and binders they can incorporate within the ink formula without clogging the printer. Due to these limitations, current inkjet pigmented textile inks tend to produce less vibrant and saturated colors and have poor wash fastness properties. The most common use for these inks today is for direct-to-garment printing and sampling.
Dispersed dyes are the colorant of choice to print on hydrophobic fibers, such as polyester, acetate rayon, poly Lycra and acrylics. Dispersed dyes are dyes used in a non-soluble form and dispersed (like pigments) in the ink. The dyes have the ability to form a solid solution with the fiber or dissolve within the fiber and become part of it. Dispersed dyes are typically direct printed on to the textile and then processed through a heat press to fix and develop the color. Excess dyes need to be washed off.
A specific subset of dispersed dyes has the ability to sublimate (go from a solid state to a gaseous state without going through the intermediate liquid state). These dyes are used to manufacture dye sublimation inks, which have become quite popular in recent years. With dye sublimation, the inks are printed on a carrier media, such as paper, and then transferred onto the fabric by applying heat and pressure using a heat press.
In the heat press, the dyes take a gaseous form and the synthetic fibers “open” to receive the gas. Once the heat is removed, the fibers close, encapsulating the dye that returns to a solid state. Since the inks are not printed directly to the fabric in a dye sublimation transfer process, the fabric does not need any pretreatment and any excess dyes remain on the carrier so no washing is required. This makes dye sublimation transfer a versatile process that can be applied not only to polyester textiles, but virtually to any surface made of or coated with PES-based plastics.
In recent years, the cost of equipment and supplies for digital printing has decreased substantially while printer speeds have increased significantly making digital printing a viable option for short- and medium-run productions. The technology is flexible, the end product is fully customizable, the graphics are of photographic quality and a virtually unlimited number of colors can be applied to the textile without additional cost and with relative ease. However, the technology is no more than an evolution of the method used to apply the colorants rather than an evolution of the colorants themselves. No matter how good, cheap and flexible the technology is, the basic principles of fabric printing and dyeing remain unchanged.