By Lisa Guerriero
Pre-coated fabrics are ideal for certain print scenarios. Commonly, a coating is used to ensure proper ink adhesion and dot gain. It’s the printer and ink set—not the textile alone—that determine whether a coating is needed for ink adhesion to optimize the fabric for digital print.
When discussing fabric printing, there are three main print processes—direct print, printing directly to a fabric using ink such as latex or solvent; direct dye-sublimation (dye-sub), where the sublimation process occurs directly on the fabric; and transfer dye-sub, when a graphic is first printed onto transfer paper and then passed on to the fabric with an external heat press. Most vendors recommend using coated fabric for direct printing and direct dye-sub, but not for transfer dye-sub.
Despite whether coatings are enacted to ensure quality printability, they are also used for enhancing image quality and providing practical features such as flame or mold resistance.
Coatings applied prior to purchase are typically proprietary to the manufacturer. Application is performed through one of several methods—knifing on or spread coating, bath coating or pad on, or hot-melt calendaring.
Hold On to Your Ink
The print process plays a large role in determining when a pre-coated fabric is required to ensure proper ink adhesion. “The coating is what holds the ink in its proper place until it is dried, cured, or heat set and offers the intended printability,” shares Eric Tischer, president, Verseidag seemee US, Inc.
The three main technologies used in printing to textiles—transfer dye-sub, direct dye-sub, and direct print—vary in terms of how the final image is sealed into the fabric. “The need for a coating is most often determined by the intended print method rather than the fabric itself. For instance, the same fabric that would require no coating if printed with paper-backed dye-sub will receive a disperse coating for direct printing,” explains Jeff Sanders, digital print fabrics sales manager, Pacific Coast Fabrics.
Transfer dye-sub is performed with uncoated fabrics, it typically does not require any treatment to aid in the sublimation process.
“While the dye-sub transfer process does not require that coated fabrics are used, the other printing processes require some sort of coating or finishing,” says Mike Compton, business development manager, Top Value Fabrics.
Direct dye-sub requires an ink receptive coating, otherwise the inks will bleed. “Without a proper coating, inks can absorb or wick into fabrics, causing poor, unacceptable print quality,” continues Tischer.
Direct printing to fabric using methods such as UV, solvent, latex, or aqueous yields varied response on when and how coatings are used. Steve Weiss, direct sales/print, national accounts, Dazian, LLC, comments that coatings for direct print are critical. “We apply specific coatings to allow ink to hold for direct solvent, direct UV, and direct latex. This is where the print vehicle will print directly onto the fabric and it is important to understand the machine and inks used in this print method.”
Jim Tufts, business unit manager, Perception Wide Format Media, agrees, saying that those textiles printed on latex, direct dye, or UV-curable platforms without a treatment or coating have the potential in some cases to be less robust in image quality and at times lack durability.
“If the print method is UV-curable, these printers can print on virtually any fabric, coated or uncoated. Latex printers do not need coated material, although often a coating will produce more vivid prints. Direct print solvent requires a coating to hold the inks in place,” counters Sharon Roland, advertising and PR manager, Fisher Textiles.
Practicality and Prettiness
There are additional benefits to treated textiles. A pre-coating is often used to help the inks adhere properly. If a coating isn’t required to hold those inks, sometimes it is used for other means, such as preserving appearance or to provide a desired effect.
Durability comes in many forms, from scratch resistance to preventive measures against fire, water, and mildew. “There are various other advantages that can be attained such as durability and dimension stability,” observes Scott Kacmarek, sales, Seattle Textile Company.
Dione Metnick, global product line manager, Brand Management Group, agrees that durability is a benefit, noting that scratch resistance and better adhesion lead to a longer shelf life for the product.
Flame resistance is a priority for many textile makers and customers, regardless of printing method. It’s now common to have flame-resistant chemicals added at the yarn stage, before the fabric is even created. If handled properly, image quality doesn’t suffer at all. “If it’s done right, you get a clean image that maintains its flame retardant capabilities as well,” notes Weiss.
The flag industry utilizes pre-coatings to promote fire and weather resistance, but most true coatings aren’t a fit since the dyes need to penetrate and be visible on both sides. “They inhibit penetration and do not allow for the creation of single images,” explains Mike Glaser, president, Glaser Mills.
In some cases, coatings help enhance fabric properties such as light diffusion or blockout capability, adds Blaise Humphries, business unit manager – Decoprint, DHJ International.
For appearance, coatings can prevent the dull look that sometimes results from direct printing with solvent inks. “With solvent inks, the ink will penetrate, which most often will lead to a more washed out print than you would otherwise achieve,” says Mike VonWachenfeldt, technical service manager, Glen Raven, Inc. The reduced absorption also results in a faster drying/curing time, he notes.
Preventing dullness is just one of the quality advantages that coated fabric holds for direct printing. “Those using solvent printers will also benefit greatly from either an acrylic or polyurethane coating to help achieve the proper color densities and finished look,” notes Joseph M. Rooney, sales manager North America, Heytex Corporation.
Several vendors point out that some finishes contain optical brighteners, which ensure a better white point for canvas as well as other fabrics. “The quality of the coating is just as important as the quality of the fabric. Our coatings are optimized for sharpness and clarity of the print as well as lightfastness—how a print fades, and hand—how a print feels,” explains Sanders.
Composition and Application
The chemical composition and application methods of pre-coatings vary among manufacturer, and are adjusted to meet the needs of the printing process—direct print, direct dye-sub, or transfer dye-sub—and the fabric. “The art and science of coating is every bit as important as the fabric itself. The way that a coating is formulated and the manner in which it is applied is a closely held craft secret that each fabric supplier uses to differentiate themselves from the competition,” observes Sanders.
Acrylics and polymers are prevalent chemicals, but most coatings contain a guarded blend of chemicals, which suppliers say is critical to the success of their textiles. “There are common coatings, but the exact blends are proprietary and are based on how they work with independent inks,” says Compton.
“Our coating combinations feature a variety of chemistries including urethane and acrylic polymers in order to maximize image quality, UV resistance, durability, and archival qualities,” shares Tufts.
At Qué Media Inc. one of the key agents in its chemical formulation is a surface tension relaxer. “It allows our coatings to flow smoothly over the surface of a textile substrate and saturate the base,” explains E. Tyler Reich, director of product development, Qué Media Inc.
While composition is closely guarded, the method of applying a coating to a pre-printed fabric is accessible. One popular and long-used application method is knifing on, also called knifing over or spread coating. In this approach, the coating is poured over the roll of fabric and a blade mechanism disperses a layer of it across the substrate.
Verseidag seemee utilizes a number of different coating applications, including knife and dip coating. According to Tischer, the process utilized depends on the product and the market it is used in.
Another common application method is pad-on/bath coating, of which there are several variations. Top Value Fabrics is one supplier using this approach. “In this process, the fabrics are placed through a dip bath, after which the excess treatment is removed via squeegee rollers on both sides. The thickness of the fabric dictates the pressure the squeegee rollers need to use in order to achieve the ultimate consistency,” explains Compton.
Dazian also utilizes this method. “We mostly use what we call bath coat, which coats both sides of the fabric, though we also apply coating to one side and this is rare, because we realize that printers actually print on either side of fabric—often by mistake,” notes Weiss.
In addition to these approaches, vendors, like Heytex, sometimes utilize hot-melt calendaring, in which the material is applied to the fabric through a heated roll system.
When determining which coating and application method to use, manufacturers need to consider what fibers and construction create the textile. “Each coating is made specifically for each fabric and the method of applying them varies upon a number of different factors,” says Sanders. “These include the category of fabric such as woven, knit, and warp knit. It is also based upon the construction of the fabric, taking into account its weight and density.”
The type of fabric also determines how many times a textile coated. Reich shares that fabrics are coated one to four times depending on the construction of the material. “Many different techniques are used to achieve the desired effect,” he concludes.
Maximizing the Potential
Pre-coated fabrics are used to provide proper ink adhesion for some printing processes, and to ensure practical qualities such as flame retardancy and benefits such as appearance. The best coated fabric is optimized to ensure outstanding graphic quality, while also meeting these demands.
Since there are many factors to consider when selecting and applying a coating, it takes a knowledgeable manufacturer to optimize the textile for the printer and application. Print professionals who are choosing fabric should communicate with their manufacturer or distributor to make sure the fabric and its coating are a match for their printer, ink, and other needs.
Jan2015, Digital Output