The recent introduction of lower-cost solvent, wide-format printers has opened up the market for inkjet printing in outdoor applications. These systems are selling in increasing numbers and many new machine and ink vendors are appearing on the scene. However, popularity doesn’t equate to understanding the features and benefits of the many different inks available.
Webster’s New World College Dictionary defines solvent as "a substance, usually a liquid, which dissolves another substance." This can describe any liquid – including water. But in the inkjet world, the term is used to describe any ink that is not water-based. On top of this, the terms the industry uses to describe inks include soft, mild, safe and green, as well as hard, real, true and strong. No wonder people are confused.
One of the more intriguing terms used is eco-solvent ink. To most people, "eco" means ecological. But these inks generally contain glycol esters or glycol ether esters – both derived from mineral oil – hardly a renewable resource or an ecologically sound process. Perhaps the terms mild and aggressive might best be suited to describe the two groups that include solvent inks.
To Work, It Has To Stick
The vast majority of images for outdoor display are printed on nonabsorbent, uncoated materials, such as self-adhesive vinyl and scrim banner. The resulting print has to be weatherproof, fade-proof and largely scratch-resistant. In order to meet these performance criteria, the inks use a colorant in the form of a pigment – a very fine powder – and as the media is nonabsorbent, a resin or glue literally is used to stick the pigment onto the surface.
The solvent is really a carrier fluid to keep the ink in liquid form for jetting, and once the ink has been applied, the solvent evaporates. Most printers use forced drying in the form of heat to speed up the process. Non-aqueous liquids are used for the carrier fluid or solvent, as it is easier to produce a waterproof image using resins that are not water-soluble.
Two Choices: Mild or Aggressive
A mild-solvent ink – also known as soft or eco-solvent – generally uses very slow-drying liquids as the carrier fluid. Therefore, printers that use these inks have several heaters fitted to aid with ink drying. The faster the machine prints, the more heat is needed. One of the main benefits of slow-drying carrier fluids is that they make it easier to design and manufacture a reliable printer.
On the other hand, aggressive solvent inks – hard or true solvent – generally dry faster and need fewer heaters in the system. Because of this lower heat requirement, media that is prone to pucker under heat can be used more successfully than in a mild-solvent ink system. Aggressive solvents also have the benefit of softening up the media surface, which helps pigments to bond.
Hence, aggressive solvent inks tend to be more scratch- and weather-resistant, and work on a wider range of materials. The types of resins and additives that can be used with aggressive solvents also make it easier to produce an ink that is glossy when dry, so colors appear to have more "pop."
The running cost for both ink types are remarkably similar when purchased in cartridge form, although aggressive-solvent machines tend to use about 10 percent more ink due to the increased head-maintenance cycles required. However, most aggressive solvent printers now come with bulk ink-delivery systems as standard or an option. This reduces running costs considerably over cartridge-fed solvent machines. It remains to be seen if the mild-solvent machine vendors introduce bulk ink at a competitive cost.
A factor that sometimes is overlooked when selecting an ink is its resistance to other solvents. Finished prints often come into contact with harsh chemicals, particularly in cleaning agents. Glass cleaner or specialty vinyl-cleaning products can remove mild and less expensive aggressive-solvent-based inks.
Solvents have received a lot of bad press recently. However, the fact remains that there are equally as many "safe" aggressive solvents as there are toxic mild solvents. In addition, many commonly held myths about solvents confuse the issue: "If it don’t smell, then it is safe," and "The more aggressive a solvent, the more poisonous it is." For instance, dipropylene glycol monomethyl ether is a commonly used material in many low-odor mild-solvent inks with an exposure limit set by the U.S. Occupational Safety and Health agency at 100 parts per million in the work place. Compare this to some of the lactates used by aggressive-ink manufacturers that actually are food additives, are easily metabolized and have no set exposure limits.
Before using any ink, request a Material Safety Data Sheet from the supplier. By law, a supplier must provide this. A well-written MSDS provides specific information about exposure limits and ventilation-extraction requirements. It also should list the components in the ink, along with the Chemical Abstracts Service number of each item. A quick Web search using the CAS numbers will provide you with a wealth of health and safety information. Good resources also include OSHA and the Environmental Protection Agency sites at www.osha.gov and www.epa.org.