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