Being solvent savvy
Solvent inks offer durability and performance for a wide range of applications.
By Vince Cahill
After the Turkish army ended its siege of Vienna in 1529, it left behind its many magnificently decorated tents with walls depicting gardens and fantastic designs. With that in mind, one can imagine that the promise of inkjet printing can provide environments and decoration to match the taste and purposes of its end users. Solvent-based inks offer the outdoor durability and performance for your longer sieges.
Solvent-based ink is used for printing fabric billboards, banners, flags, building wraps, canopies, tents, and architectural fabrics. Solvent inks offer longer resistance to weathering and sun exposure fading than aqueous inkjet inks—about three-year outdoor durability uncoated.
Solvent inkjet printing of textiles owns a substantial share of the outdoor advertising market. The Outdoor Advertising Association of America reports that in 2006, advertisers spent $6.8 billion on outdoor advertising in the United States, which increased 8 percent over 2005. About 7 percent, or $476 million,of U.S. outdoor advertising 2006 revenue was for graphics printed on textiles with solvent-based digital printing systems.
Aggressive solvent-based ink, however, is questionable for apparel applications since fabric can retain some solvent odor and skin irritants. Garment print providers are printing thermal adhesive-backed urethane films on polyester and coated paper carriers with aggressive and mild solvent inks and then contour cutting and heat transferring these films to garments. Aggressive solvent inkjet ink-printed films can withstand repeated laundering with almost no loss of color for more than 50 washings. Most brand-named eco-solvent inks also offer reasonable wash resistance. Print providers are using mild-solvent and so-called “eco”-solvent ink for even apparel applications. Solvent-based inkjet ink printed transfer films have also found applications in outdoor and stretchable fabric displays and signage. Many printable urethane films stretch while retaining their memory. Pigmented solvent ink that bites into the urethane polymer surface provides both the flexibility and durability that these applications demand. Some of these films also contain sublimation blockers that prevent film discoloration from migrating low-energy fabric dye.
Each process has a price
Sublimation transfer printing and disperse dye direct inkjet printing have increased their role for advertising and locale promotion. Solvent-based sublimation and disperse dye inks, along with aqueous- and oil-based inkjet inks are providing solutions. Every process, however, has its price. The strengths of a system will usually involve the loss of some advantage or the increaseof expense.
Inkjet printing sublimation transfer and direct dye inks can produce vibrant banners for in-store point-of-purchase and brand recognition advertising, exterior banners for short-term outdoor use, apparel and accessory decoration, and printing for awards, plaques, and novelties. The inks for sublimation transfer and direct disperse dye printing are chemically similar and often the same ink can be used as either for indirect sublimation application or direct disperse dye printing. Sublimation printing differs from direct disperse dye printing in that it is a transfer method that first prints an image in reverse onto a transfer medium, such as coated paper, which the user can then transfer with heat, pressure, and dwell time to the final print substrate. The dye changes from a solid on the transfer substrate into a gas that penetrates the receptive substrate with which it is in contact when placed under pressure with a conductive heat source. Direct disperse dye printing, in contrast, deposits its ink onto the substrate as a right reading image without the use of an intermediary transfer. Its dye disperses into the receptive substrate when printed and further develops its color and penetration of its substrate with the application of radiant or convection heat.
Each type of sublimation ink has advantages and disadvantages. Aqueous sublimation inks generally do not include hazardous petrochemical solvents. Print operations can typically use them with ventilation appropriate for an office environment. Unlike acid and reactive dyes, sublimation dye and its close relative disperse dye do not readily dissolve in water. They perform more like pigment than dye in inkjet ink formulations. Their water base tends to deform and cockle transfer papers. This tendency can puddle ink and distort printed transfer images and papers, resulting in media striking printhead nozzle plates. Printers using aqueous inks for printing transfers will typically use heavier and more costly transfer papers to compensate for cockling, adding about 30 to 40 percent to the cost of transfer paper. Water-based ink does not dry as fast as solvent-based ink, requires a greater amount of heat to dry, and can limit print production.
Solvent-based sublimation ink does not produce significant cockling, can perform on thinner and less expensive transfer papers than aqueous inks, and dries faster than its water-based cousin. It does not absorb into its transfer paper as much as aqueous-based ink and transfers more of its colorant during the heat transfer process. Printers use less ink than aqueous sublimation inks for comparable results. Solvent-based ink strengths can also produce disadvantages. It will not only rapidly evaporate and dry on transfer paper, but will also dry in print nozzles more rapidly, requiring more frequent head firing to rewet nozzles and head maintenance. Solvent inks require more frequent changes of air than water-based systems or the use of expensive activated charcoal solvent capture systems.
Oil-based sublimation ink does not dry by evaporation like solvent- and water-based inks. Instead in absorbs into the transfer paper. It does not dry in printhead nozzles and requires minimal maintenance. Xerox offered oil-based ink for sublimation with its X2 printer. Seiko Instruments initially offered its ColorPainter 64 with oil-based ink. However, while oil-based sublimation ink can produce saturated colors, it lacks the sharp vibrancy of aqueous- and solvent-based sublimation inks.
The downside for solvent dye-sublimation is exposure to more hazardous solvents than with aqueous- and oil-based inks, and the expense of venting or filtering solvents from the work environment. The use of solvent inks may also require Air Quality Management District permitting, a process that can cost a few thousand dollars for a small system. Some jurisdictions are more restrictive than others, including districts in California and Europe. However, solvent-based dye-sublimation can save print operations about 35 percent of their current print materials cost for sublimation transfer printing and about 50 percent for direct print over indirect dye-sublimation. The solvent-based dye-sublimation solution can afford industrial print operations a distinct cost advantage over competitors using aqueous- or oil-based inkjet printing systems. It is less applicable for storefront or office settings where inhabitants may object to solvent exposure.
Most printers using continuous ink (CIJ), piezo inkjet (PIJ), and valve-jet print heads can dispense solvent-based pigmented or dye-sublimation inks. Since most solvents evaporate more rapidly than water, systems using solvents require capping and purging stations and regimens to keep solvent-based ink from clogging in print head nozzles. While the purging procedures will typically produce a greater level of waste ink than aqueous-, oil-, and UV-cure systems, solvent ink can usually rewet ink in clogged nozzles and restore a device to its specified performance level. [Thermal inkjet (TIJ) print heads do not use solvent-based inks for commercial applications. Developers have demonstrated the use of some solvent-based inks and fluid with TIJ print heads, but their use has not gained acceptance as commercially viable.]
The future’s so bright
Vinyl, urethane, and other polymer-coated fabrics offer some of the best opportunities for solvent ink textile applications. These substrates provide the ideal receiver for solvent-based ink that can effectively bite into their surface for maximum adhesion and abrasion resistance. Polyester and related sublime-able fabrics offer the best fabric receivers for indirect solvent dye-sublimation and direct disperse dye inkjet printing. Polymer-coated fabric finds application with solvent-pigmented inks for outdoor advertising, marine and commercial upholstery, architectural, and some transportation graphics, while polyester fabrics with solvent-sublimation ink match the requirements of interior brand promotion and point-of-sale banners and displays, and short-term outdoor banners where vibrant dye-sublimation color is required. As architecture increases its use of coated and uncoated fabrics, the demandfor multiple-year outdoor durability will favor solvent-based solutions.
