There are many chemically different types of insolubilizers or crosslinkers, but they all have the same function – to add water resistance to the coated paper surface. Water resistance is particularly important in offset printing, but also for wallpaper, label paper, poster paper and in the storage of board packages. In dou-ble-coated boards, crosslinkers are used in the precoating to impart water resis¬tance against the topcoat. The water resistance can be measured as wet rub and wet pick or seen as less pick, print mottle, or binder migration. The water sensitiv¬ity of paper and board coating originates from the fact that water-soluble binders tend to lose their binding power in contact with water and dissolve. This water sensitivity of binders can be described as the amount of O-atoms in the molecule (in hydroxy and carboxyl groups). The water sensitivity can be decreased by cross-linking the soluble binders with insolubilizers or by building an insoluble net around the binders.
Traditional insolubilizers in paper and board coating are based on formaldehyde and its amino compounds (melamine, urea) or on glyoxal. Imidazoline derivatives are also used as crosslinkers. Next-generation insolubilizers are based on zirco¬nium; the product most widely used is ammonium zirconium carbonate (AZC).
Glyoxal is the simplest aliphatic dialdehyde. It is an effective crosslinker of starch, and gives the coating an immediate curing. The crosslinking mechanism is a reaction with the hydroxy groups. Glyoxal first reacts with one starch molecule and then with another one, leading to crosslinking of two starch molecules. Glyoxal is a less effective crosslinking agent for synthetic water-soluble binders. Some of its derivatives, such as condensate products with urea or ethylene urea can be used here. Glyoxal is ineffective at pH >8.5. Difficulties have resulted with viscosity build-ups due to crosslinking in the wet form.
Melamine-formaldehyde (MF) and urea-formaldehyde (UF) resins have been used in paper coating since 1940. In the 1970s, methylated MF and UF resins for the most part replaced these resins. Melamine-formaldehyde (MF) resins and urea-formaldehyde (UF) resins have reactive imino- (>NH) and methylol (>N–CH2–OH) groups. Methylated MF and UF resins have also some functional methoxymethyl groups (>N–CH2–O–CH3). These groups undergo reactions with paper coating binders: the hydroxy group of starch and polyvinyl alcohol, and the carboxyl groups of latexes. Both UF and MF resins can also self-condensate. The reaction of MF/ UF and their derivatives is an acid catalyzed condensation reaction, which requires a certain temperature and pH. The curing reaction takes up to two weeks to complete.
Environmental pressures against formaldehyde-based resins, higher pH in coat¬ing formulation, and the need for faster curing are the reasons why ammonium zirconium carbonate (AZC) is more and more used in the paper industry. AZC reacts with carboxyl and hydroxy groups in the coating, hydroxy groups in starch and PVA, carboxyl groups in latexes, and oxidized starch. The reaction takes place when ammonia is evaporated and water is removed on drying. AZC forms cova¬lent bonds with carboxyl groups and weaker hydrogen bonds with hydroxy groups. High pH does not affect the reaction. Other zirconium-based crosslinkers are po¬tassium zirconium carbonate and zirconium acetate. The reaction mechanism is sim¬ilar for all these products.
In coatings the amount and type of binder present determines the addition level of crosslinkers. It is recommended to use 5 %-10 % crosslinker based on total dry binder. In general, starch-based coatings require a higher amount than coatings based on synthetic binders. It is important to optimize the dosage of crosslinker. An excessive amount can result in cracking problems or even in increased, instead of decreased, solubility of water-soluble coating components. A crosslinker should be added to the coating color as the last ingredient.