POLYMER COATINGS for SUBSTRATES and PRINTED SURFACES

Polymer coatings are needed due to print head formulation limitations

The use of coating for substrates is increasing since dye based inks need ink receptive sites, and for this reason many paper companies are selling special paper (coated paper) for ink jet printing. Additionally, plastic substrates must be coated with ink receptive materials in order to print on nonporous substrate (plastic) using dye based waterborne ink. The ink receptive coating consists of a polymer binder and extender components such as clays that have been especially treated with wetting agents such that they are ink receptive. When dry these coating are flat (with out gloss) indicating a high Pigment Volume Concentration (PVC). The PVC is calculated by:

Dry Pigment Volume

PVC= --------------------------------------------------------------

Dry Binder Volume + Dry Pigment Volume

These dye based waterborne inks dry by two mechanisms absorption into the paper and also evaporation. These are the solvent transport mechanisms out of the fluid ink. Further, it should be noted that when the solvents evaporate they leave behind the soluble dye color deposited at the ink receptive site (clay extender pigment that was treated with wetting agents). In addition to the above, the coating must have adhesion to a substrate that is classified as either porous or nonporous. The adhesion to substrate problem becomes more critical when the surface is nonporous (plastic) since there are no fibers for the coating to cling to as with paper. Further, if the ink receptive coating is of a significant film thickness the possibility of cracking and delaminating increase since the coating must also have properties of flexibility similar to the substrate material. It is now easy to understand what at first appeared to be a simple formulation of vinyl emulsion, extender pigments, and wetting agents is again a complex balancing of components to match the required properties for the products intended applications.

UV PROTECTION for DYE BASED INKS

Dye based inks have more brightness and color saturation than pigment based inks however, once exposed to ultra violet radiation the intense colors show sever fading and sometimes within only a few days of exposure. There fore there is a definite need to protect documents that use dye based inks. It has long been known that UV radiation has much less effect on color dye fade if the dye colored material is kept in a vacuum. Then oxidation is a probable cause of color fading. There fore antioxidants and free radical scavengers have been added to ink formulations. Additionally, new materials such as hindered amine light stabilizers (HALS) have also been added to interfere with the degradation process. Further, other materials called UV absorbers that absorb UV radiation and then emit the radiation as harmless heat (IR) radiation have also been added to ink formulations in an attempt to increase the light fastness of the color but with no significant results.

However, while the above ink formulation techniques do not work, acrylic polymer coatings that have the above ingredients dissolved in the coating system have proven 100% effective and show major, major improvements in the light fastness of the color. Some common sense in the use of these protective polymer coatings is needed however:

DO NOT APPLY WATER BASED POLYMER COATINGS OVER WATER BASE INK!

The reason being water base inks will in all most all cases become soluble resulting in a destroyed document! Remember to apply solvent born coatings over water base inks. Again, be aware there are polymer coatings that could have strong organic polar solvents that will cause problems similar to the above where upon the dye becomes soluble in the solvent. Caution should be used by first testing a small portion of the document to see if there is any damage caused by the coating. In general, use solvent borne polymer coatings over water borne ink systems and use water borne polymer coatings over dye based inks that are in organic solvent systems.

Acrylic Polymer Film containing HALS and UV absorbers

Another technique is to laminate acrylic polymer film containing HALS and UV absorbers to the document thus avoiding the solvent problems. However, the film is expensive and only sold in certain thickness. In conclusion, both HALS polymer coating and film systems are effective.

RESEARCH WORK INDUSTRIES has experience in the complexities of balancing formulations for the desired properties along with capabilities of finding vendors of specialized raw materials, analysis and processing equipment needed for a successful development project.

RESEARCH WORK INDUSTRIES also develops trial samples for specific industrial applications.

RESEARCH WORK INDUSTRIES also has experience in Technical Writing along with building web sites for engineering products and services.

.