TEXTILES.ORG
Knowing how and why inks and fabrics go together will help improve your printed products.
By Michael Labella
At a recent apparel show in Guatemala, I attended an interesting series of presentations organized by the Americas Apparel Producers Network. The theme I picked up on as I listened to these sourcing and manufacturing professionals was that buyers have learned to look at the big picture when deciding how and where to produce their goods.
In addition to the actual cost per item, other cost factors include the financing of long lead times, markdowns to eliminate excess inventory, intellectual property theft by manufacturers located in countries with little or no protection for the intellectual property of brands, and designers. As a result, major buyers are looking for ways to buy smarter and be more flexible in their purchasing in order to quickly respond to market demand and supply the market with new products more often while keeping inventories low. Additionally, an emerging middle class in Asia, especially China, is driving demand for foreign-made designed goods and brands expanding the market for high-end goods beyond North America and Europe.
While the specifics of this shift are outside the scope of this article, suffice to say that buyers and retailers are trying to avoid getting stuck with large quantities of products that might not be well received by customers, while being able to respond to market demands by being able to quickly order and replenish inventories for those products that sell. Increased margins of high-end goods certainly make this easy. Furthermore, bringing production closer to the market the goods are destined for reduces cost and lead times, increasing the efficiency of the supply chain while reducing the need for large inventories.
This is a trend that has been slowly, yet steadily developing with American and European buyers, and manufacturers have been looking at digital technology to help them deliver. Digital inkjet printing has already transformed the world of signs and promotional graphics into an on-demand, fast-turnaround industry. But due to limited speeds and the relatively high cost of consumables, it never took off in truly industrial, high volume, production environments such as those of the traditional textile industry.
In 2009, 27 billion square meters of fabric was printed using traditional printing methods (Source: John Provost presentation FESPA 2011); of that, 88 percent was printed using rotary or flat screen systems. This method for printing textiles is fast (over 40m per minute) and cost effective.
This shift in the market dynamics has not gone unnoticed by inkjet suppliers who, in collaboration with major textile manufacturers, have been investing significant R&D resources to develop solutions that could compete with traditional printing methods in terms of speed and cost, while providing the incredible flexibility that makes digital inkjet so attractive.
Technology improves equipment
Very recently, Italian printer manufacturer Reggiani Macchine introduced the Renoir digital inkjet printer, which uses Kyocera print heads and Sensient’s ElvaJet ink technology to print at speeds of up to 300 yards per hour, with minimal fabric pre-treatment, while reducing the cost of consumables by as much as 30 percent or more compared to older inkjet technologies. Garment manufacturers, such as Fabra of Ormond Beach, Fla., are replacing their automatic screen printing lines with the Reggiani Renoir, allowing them to go from concept to delivery in a matter of weeks rather than months and change design, colors and quantities on the fly without interrupting production: a process that on their older equipment could take a few days or more during which production would be halted.
In Italy, Miroglio Textiles, a world leader of textile manufacturing and design using Sensient’s ElvaJet Alpha inkjet technology in conjunction with MS’s industrial inkjet textile printers, can image upwards of 40 linear yards of fabric per minute while drastically reducing costly water usage and urea waste (a byproduct of some traditional printing methods).
Another development is in the wider variety of fabrics that can now be printed digitally, from silk to polyester, thick or thin, woven, non woven or knit, and even four-way stretch. Actually, today almost any kind of textile can be printed says Ken Bach of Aberdeen Fabrics. And in the new world of digital printing, dye sublimation and direct-dispersed processes seem to be the winners thanks to the incredible versatility of polyester-based technical fabrics.
But each of these materials has its own ink and color-fixation requirements says Dr. Martial Blanc of Sensient Technologies. There are also specific performance requirements based on the type of product made, from silk scarves to car seats, flags, garments, soft signage, shoes and upholstery. Indeed, inks play an important role in this new frontier. While the technology that delivers the ink to the textile is rapidly evolving and the challenges ink manufacturers face to formulate inks that are reliable and outperform those from the previous generation are significant, one thing that is not changing is the need to select the right colorant for the right type of fiber.
Inks and the fabrics they love
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 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. In specific cases, pigments can behave like dyes and vice versa. What really matters to us as 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 to print on fabric, and all use either pigments, dyes or dispersed dyes as colorants. And except for differences in the way they are formulated, all five types are available for traditional as well as digital inkjet printing platforms.
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 and the required end result. 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 digital 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.
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 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. The fabric then needs to be dried.
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. 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.
Reactive-dye printing is possibly the most complicated of the available textile printing processes because 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. 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 with the introduction of a base, such as sodium carbonate. 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 and then the fabric needs to be dried.
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.
Water-based pigmented inks are widely used in inkjet printers for desktop applications, and, 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 digital inkjet textile printing, the use of pigments is still in its infancy as formulating a pigment-based ink that meets the stringent standards of the textile industry is challenging, however a select few manufacturers are making great strides in this area and you should expect great advancements in the next few years. For now, the most common use for these inks today is direct-to-garment printing, sampling and custom-printed fabrics for interior décor, fashion and other specialty markets.
When printed using inkjet systems, the fabrics typically require minimal pre-treatment (primarily to reduce dot gain) while with traditional printing the fabric needs to be washed to remove excess color that did not bind to the textile. Whether textiles need to be washed when digitally printed with pigments greatly depends on the pre-treatment used.
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. Like with other processes, excess dyes need to be washed off.
Dye sublimation
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 and are the most widely used ink for digital textiles (52 percent of digitally printed fabrics are printed with dye sublimation inks). 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 or drying is required. (With the other methods, up to 50 liters of water per yard are used to wash the textile after printing.) This makes dye sublimation transfer a versatile and efficient process that can be applied not only to polyester textiles, but virtually to any surface made of or coated with PES-based materials.
The case for digital
In recent years, the cost of equipment and supplies for digital printing has decreased substantially and advancements in ink technology have drastically increased dye loads, further increasing the efficiency and speed of the process, while printer speeds have increased significantly, making digital printing a viable option not only for short- and medium-run productions but for larger industrial productions.
The technology is flexible, the end product is fully customizable down to the individual unit, the graphics are of photographic quality and a virtually unlimited number of colors and pattern combinations can be applied to the textile on the fly, in mid production and without additional cost. Digital printing requires less manpower to operate, allowing manufacturers in high-labor cost markets to once again be competitive.
If initial sales of this new generation of inkjet systems are any indication of what’s to come, brace yourself for a sweeping change in how and where textiles are going to be sourced, printed and delivered to our markets.