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Thursday, August 29, 2013

Inkjet Receptive Coatings

            What impacts one’s decision is ultimately durability and color quality when purchasing a fine art printing substrate. A large portion of these two factors is determined by the inkjet receptive coating that sits on top of the substrate and absorbs the ink. The ink must be able to penetrate and bond with the coating in order to have high quality image quality and long lasting durability. There are three major parts of an inkjet coating: the pigment, binder and additives.
            The pigment is usually a porous, viscous composition of silica pigments. Silica pigments have silanol groups that work to absorb and separate the solvent from the ink. The amount of silanol groups is determined by the pH levels of the silica, higher pH equals less silanol groups. It is also a highly viscous substance that is apt to crack if not combined with a binder.
            Binders function in a coating is improve the strength, hold the colorant at the surface and improve the smoothness of the coating. As silica is a highly viscous substance and combining it with a polymer makes it more stable. One of the most commonly used polymers as a binder in the industry is PVOH or PVA, which is a hydrolyzed version of polyvinyl acetate. This polymer works the best with silica in its partially hydrolyzed state, the OH molecules present in the polymer do not shrink as much when it dries which creates more stable color saturation and prevents cracking.
            Prevention of cracking is another reason binders are used, while pigments are great at absorbing the solvent and being porous, they are not the most stable compounds. Combining them with polymers enhances the overall durability of the combination and color accuracy. By itself silica would be able to absorb the ink but would not be stable enough. However, it is important to consider the molecular weight of the particular PVOH polymer used in the combination. If the molecular weight is too low, it will fill the holes in the silica and then the ink will not have a place to bond. On the other hands too high of a molecular weight lowers the viscosity to the point that it cannot work as intended. It is best to use as low of a molecular weight as possible, where the PVOH does not fill the gaps in the silica. Binders reduce the porosity and absorbance of a pigment, but increase the strength and durability.
            Additives help enhance the chemical attraction between the inks and coating. Many of the dyes used in inks have ionized compounds and adding a catatonic additive helps this bond with the coating. The coating would be functional without additives, however, they enhance the final properties of the coating. They can also play a role in altering color composition by changing the wavelength of reflected light.
            Together these components help filter the dye or pigments from the solvent and adhere these colorants to the binder. The porosity of the pigment separates the solvent from the colorant and isolates the colorant. Utilizing a coating provides superior ink adherence over uncoated paper or canvas. Manufacturer’s can create a product which has significantly improved qualities over what was previously available. A previous problem with coatings for canvas was that they cracked if the canvas was ever bent. This was solved with using the correct binders and additives, improved bonds between the pigments and other particle has largely eliminated this problem from printing. 
            Chemically evolved inkjet receptive coatings are used on many common substrates such as paper and canvas and understanding the composition can help you make an educated decision about what is the right product for your