- June 01, 2002, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
Crosslinkers are an important class of additives ink and coating formulators use to enhance properties. These materials must be selected carefully to provide unique properties to the ink or coating film.
Oxidative systems: Perhaps the oldest crosslinking system is based on unsaturated oils, which can be crosslinked by oxygen. Although oxidative systems commonly are used in sheet-fed, oil-based inks, they are less common in fluid inks. However, oxidative systems can be used in fluid inks and provide unique properties that make them useful in areas such as high alkaline resistance. Oxidizable alkyds as well as epoxy esters can provide unique products for select applications. These systems can be formulated with or without oxidation catalysts and can provide excellent adhesion and product resistance.
Zinc ammonium carbonate and zirconium ammonium carbonate: These inorganic additives react with resin and substrate carboxylate groups to provide zinc or zirconium ion links, which result in higher heat and alcohol resistance. Such additives commonly are used in water-based systems. The pH of the formulations is often 9 or higher; otherwise the systems can become unstable.
Polyfunctional aziridines: These types of crosslinkers are based on a strained, three-membered ring that contains a nitrogen atom. This amine reacts readily with carboxylate function groups on resins and substrates, forming a link. These polyfunctional aziridines provide excellent adhesion and product resistance.
Silanes: Another class of adhesive promoters are silanes, or more specifically, alkoxy silanes. These materials can be added to solvent- or water-based systems and chemically react with the resin carboxy functionality. Their performance is somewhat similar to that of polyfunctional aziridines but may require more time and/or higher drying temperatures. Excellent adhesion and product resistance can be built into an ink system.
Polyethylenimine (PEI): Polyethylenimine adhesion promoters are used extensively in packaging. As primers, they provide improved adhesion to packaging films. These adhesion promoters can be sensitive to moisture. As a result, their effectiveness and, therefore, their formulations have a finite shelf life. They work well in combination with corona treatment. The formation of carboxylic functionality on the substrate will enhance bonding to the PEI adhesion promoter.
Organic titanates and zirconates: Organic titanates and zirconates represent another class of crosslinking agents. These adhesion promoters are used with many common ink resins, such as nitrocellulose and polyvinylbutyral. Organic titanates and ziconates are highly reactive, and their formulations have to be constructed carefully to assure stability. Although used for many years in solvent-based systems, some titanates have found use in water-based acrylic systems.
Photoinitiators: Photo-induced UV-curable inks and coatings fall into a special category of crosslinking additives. Photoinitiators absorb UV light and produce reactive chemicals that can polymerize and crosslink UV monomers to form high-molecular-weight films that provide these inks with high gloss and good product resistance. Both free radical and cationic photoinitiators are used in specific UV-curable systems and have been around for many years. Since the choice of photoinitiator controls the drying speed of the UV ink or coating, its selection is a key ingredient of a successful formula.
All of these different crosslinking systems have advantages and disadvantages. No system is perfect, and each requires various other additives to make a successful product. As many of these crosslinking systems are not FDA-compliant, careful selection is necessary to meet all end-use requirements of a given application.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; firstname.lastname@example.org.