Understanding the Relationship Between Digital Textile Printing and Coatings
Significant gains are being made by digital printing in the overall textile printing market. And though traditional dye-sublimation (transfer process with paper) still has the lion’s share in this market, strides are being made with direct-to-textile printers. One area often overlooked is coatings and their impact on printing with textiles, so that will be a focus for this article.
When digital printing directly to a textile, whether using direct disperse, acid inks, pigment inks, or reactive inks, a pre-treat or topcoat must first be applied to the fabric. The chemical composition of this treatment will vary depending on the inks used and the coated textile. Each manufacturer uses a closely guarded secret to differentiate their product from the competition. In most cases, this is done during the textile manufacturing process — there is an exception that I will touch on later in this article. The end-user should understand how this process works and how it can impact their final output quality and color.
Methods for Applying Pre-Treat
There are three different generally accepted methods for applying pre-treat to textiles. A pad emulsion is the most widely used; this is where the fabric passes through a chemical bath and uses rollers to wipe away the excess. Next is a process like the pad emulsion process, but instead of rollers, it uses a knife/doctor blade to wipe away the excess coating. And last is spray coating. These processes aim to manage the chemical pick-up to maintain the coating’s consistency. The composition of the textile and the type of coating typically will dictate the method of application that is used. Remember that all pre-treats are different; some can discolor the fabric, some can affect light-fastness and wash-fastness, and some can make for sharper images.
Properties of Pre-Treat
A primary purpose of pre-treat is to prevent the ink from wicking or to migrate as it is printed. The other purpose of the pre-treat is to ensure that the dye or ink bonds to the fabric well. Another component of the pre-treat process is the addition of chemicals to help the fabric pass flame testing. The chemistry that prevents the ink from migrating also impacts the chemistry of the material that gives the textile its fire rating and can make it more difficult for the manufacturer to achieve the fire rating. Most manufacturers will focus on meeting the National Fire Protection Association (NFPA) 1989 standards and the State of California Fire Code (CAL 19) tests. Some printed items needing this fire rating are trade show displays, draperies/curtains and window treatments, upholstered items, tablecloths, wall coverings, carpets, tents, and costumes.
It’s good to note, at this point, that all coatings are not the same. An inadequate or inconsistent coating can cause problems with light fastness, wash fastness, crocking, and the fabric “hand” itself. And before I go too far, it’s good to define the terms that may not be known. Light and wash fastness is self-evident; crocking, on the other hand, is a term used to refer to the rubbing off color from a textile when it is subjected to abrasion. The American Association of Textile Chemists and Colorists (AATCC) has defined a test that can be used to determine the “crock” of a fabric. And when talking about fabric hand, this refers to feeling a cloth between your fingers and thumb – some materials have a natural stiff “hand,” while others (think of fine silk) are very soft to the touch. The goal of the pre-treat is to avoid altering the “hand” of the fabric drastically.
Other harmful or incorrectly applied coating examples are streaks, blotches, and inconsistent color – this is more prevalent when the pre-treat is sprayed on, though it can also show up in other methods. Asking for a fabric sample so you can print and then sublimate it is always a good practice. Some issues may not show up until you apply heat, such as yellowing of the coating. This also allows you to check the quality of the pre-treat coating by printing and transferring the image.
Advantages / Disadvantages of Direct-to-Textile Printing
A primary benefit of direct-to-textile printing is eliminating the cost of using transfer paper. Typically, this is not a significant difference as the direct-to-textile print process can use more ink; however, the cost savings can add up over the long run. For specific applications, particularly soft signage, flags, and banners, and backlit (light boxes), a significant advantage is the penetration of ink into the textile. When printing flags, the ink will bleed through so that you have the same color on the front and back sides of the flag or banner. Some sharpness of the image can be sacrificed, but as flags and banners are typically viewed from a distance, this is not an issue.
For applications such as upholstery, backdrops, or gaming tables where color fastness and high UV resistance are needed, the deep saturation of the ink that can be achieved by direct printing is a significant advantage.
The last advantage to consider is when printing to fabric that stretches. When starting with a white textile or fabric and using a dye sublimation transfer process, the ink stays on the surface, and when the fabric stretches, you can see a white line — or “smile.” By printing with a direct process, the ink will saturate the material more, and the likelihood of a “smile” occurring when the fabric stretches is reduced significantly.
There are a couple of disadvantages to consider when looking at direct printing. Some coatings can add stiffness — almost like a starched feel — to the fabric “hand,” which would not be appealing for wearable fabrics. And there is the issue of excess ink. If you don’t correctly heat and dry the material after printing, some excess ink can pool on the surface and run if it gets wet or smear and stain when folded/rolled for shipping. With a wearable fabric, sweat may also make the excess ink run.
Advantages with Dye-Sublimation Using a Paper Transfer Process
Dye-sublimation with paper transfer usually results in finer detail with graphics. The hand or feel of the fabric will also not be affected as much as with direct printing, so this is an excellent process for apparel printing.
Tips for Better Results
Storage and handling are essential when using pre-treated textiles. Once you open a roll, it is a good idea to use it as soon as possible. Always store them in the plastic wrapping that they came in (sealed) and acclimatize the roll for a few hours before loading them on the printer. Watch for streaks and compression marks near the end of the rolls. Most manufacturers will flag inconsistencies (quality issues) in the rolls — including where the roll is spliced if that occurs.
Exceptions to the Rule:
Pigments: Kornit and Mimaki
A coating is used for pigments to get more of the binder onto the printable fabric. The pigment is color plus a binder that holds the color onto the textile. Not having enough binder in the material happens because the binder molecule size is large and creates problems going through inkjet print heads. Pre-treat for pigments is a work in progress, and improvements are always happening. The industry is moving towards finding the right chemistry that will be able to get through print heads which will allow pigment printing to explode.
At the beginning of the article, I mentioned that direct-to-textile printing requires that a textile be pre-treated or top-coated before it can be used for direct printing. There are a couple of exceptions, and one is the Kornit Presto. With the Kornit process, the pre-treat is applied as part of the printing process, which means you can load virtually any textile into the printer (including polyester), use the pre-treat, and then print and “cure” the fabric with one process — the printer has an attached heater which sets the ink to the material. The other is a new collaboration announced during the Mimaki press conference at PRINTING United. This collaboration is with Neenah Coldenhove, and they are testing a new Texcol pigment transfer paper to add full-color graphics to a wide variety of natural-fiber textiles without a water-consuming process. Keep an eye on this process, as it has enormous potential for the home décor market.
Digital direct-to-textile printing and dye-sublimation are everywhere, from trade show graphics to retail applications and home décor. The market continues to grow as printing technologies improve to allow more opportunities. To learn more about textile printing, check out the PRINTING United Alliance and AATCC Digital Textile Printing Conference, Dec. 7-8, 2022, in Raleigh, Durham, North Carolina. Agenda and registration can be found here: printing.org/events/conferences/digital-textile-printing. Hope to see you there.
Ray assists association members with information on digital printing as well as digital equipment, materials, and vendor referrals. He oversees training and certification workshops at PRINTING United Alliance. Ray is project manager for both the PDAA Certification program and the PRINTING United Alliance Digital Color Professional Certification program and is an instructor for the Color Management Boot Camps as well as a G7 expert. Ray regularly contributes to the Association's Journal and won the 2016 Swormstedt Award for Best in Class writing in the Digital Printing category. Ray was inducted into the Academy of Screen and Digital Printing Technologies (ASDPT) in 2020. He also works with SkillsUSA to conduct the National Competition for Graphics Imaging Sublimation. Outside of work, Ray enjoys biking, international cuisine and spending time with his three fantastic grandkids.