- October 01, 2001, Richard M. Podhajny, Ph.D., Contributing Editor
One of the key areas of ink/coating formulation is the use of crosslinking additives to improve physical resistance properties, such as ink adhesion and resistance to abrasion, chemicals, heat, water, and block.
Crosslinking implies the linking of different molecules. In reality, it usually means increasing the molecular weight of the ink/coating film layer through a chemical reaction between the crosslinking additive and the resin binder. The crosslinking occurs within the ink/coating layer as well as between binder and substrate.
This short summary will cover the common additives used in packaging applications only.
UV Crosslinking: In the simplest example of crosslinking, unsaturated acrylic esters can be crosslinked with the aid of free radicals generated by a crosslinking additive called a photoinitiator. In this case the photoinitiator creates free radicals through absorption of UV energy, and these radicals then can react with monomers, multi-functional monomers, as well as oligomers, which will crosslink to form very high-molecular-weight films.
Besides crosslinking promoted via free radicals, cationic UV systems can crosslink via a “cation” phototinitiator that will react with the epoxide ring. The epoxide ring is somewhat strained in its three-atom ring and the oxygen. Presence of a cation will open the ring through the interaction of the cation with the oxygen.
Zinc oxide: Use of inorganic materials, such as zinc oxide, is common to improve heat and product resistance. Usually this is done in water-based inks where the high pH limits the zinc oxide activity until it is printed and the pH becomes acidic. The mechanism generally is given as a chemical reaction between the zinc/ammonium divalent complex reacting with two carboxylic groups, usually found on the binder and/or the substrate.
Titanium chelates: Chelates of titanium are complexes formed between titanium and electron donors. The mechanism of the crosslinking is not well defined, but it is thought the divalent titanium forms reaction products with carboxylic acid groups. Unlike other crosslinking agents, titanium complexes can cause discoloration in the presence of some chemicals due to its transition metal characteristics.
Multifunctional carbodiimides: These crosslinking agents are very effective and react with the carboxylic acid function groups on the resin binders, but the presence of water reduces the effective concentration of the carbodiimide due to formation of urea derivatives with water.
Polyfunctional aziridenes: Aziridine is a three-ring system composed of two carbons and one nitrogen atom. Like the epoxide ring (two carbon atoms and one oxygen atom), the ring is strained and can react with carboxylic acid function groups found on the resin binders.
Polyethylene amines: These chemicals contain amines on the backbone of PE that can react with carboxylic acids and crosslink through what appears to be acid-base neutralization. As such, crosslinked products formed from PEI tend to be water-sensitive.
Polysiloxanes: Silicon chemistry is the backbone of these crosslinking agents, which are used in many applications requiring improved ink adhesion and also water-resistance. Like the PEI crosslinking agents, some of the silanes have limited stability and can hydrolyze with water, reducing their reactivity. This chemistry provides high adhesion to glass surfaces. Use of siloxane crosslinkers in solvent-based systems must be formulated carefully to prevent contamination of catalytic incineration systems.
Use of crosslinking agents in inks and coatings is common. Although typically they are used at levels of about 1%, they can have a major effect on ink adhesion and ink product resistance. But undesirable side effects can occur, including increased viscosity, discoloration, and odor. This area is strictly the domain of the formulating chemist, who must carefully balance the physical chemical resistance desired against undesirable effects.
(Ed. Note: The Xerox web site that appeared at the end of the September column was incorrect. The correct site is: www.parc.xerox.com.)
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314, e-mail: email@example.com.