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Testing inks for product resistance.

Oct 1, 1995 12:00 AM


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While printing inks generally are known for providing printed products with an image, they frequently must be more than just decorative. One important property of ink, especially in package printing, is the ability to withstand the effects of coming in contact with the product inside the package. While this concern primarily is directed toward package printing, often brochures or instruction sheets inserted in packages also may contact products. Therefore, it's important to test inks and coatings for this property.

A good example is a soap or laundry detergent carton. While doing laundry a person may grasp the carton with wet, soapy hands. Under these circumstances, the ink film should not break down, nor should the pigment bleed in the soapy water. To test for this, immerse a printed strip of the ink in a concentrated solution of the product. Then establish a time factor, such as 24 hours, after which you can examine the print and solution for signs of bleed into the paper or solution.

To test soap wrappers, leave the ink print in direct contact with a moistened bar of soap and, after a time period, make a similar observation. Labels for liquor bottles may be tested for resistance to alcohol, which can spill down the bottle. Inks for bleach bottle labels should be tested for contact with chlorine solutions. Grease can be a powerful solvent and cause certain pigments to bleed, so packages that will contain greasy products should use inks with the appropriate properties.

You may ask what ingredients in inks create these problems. The answer lies in the pigment's chemical makeup. The difference between pigment and dye is that dyes dissolve in the solution or vehicle in which they are dispersed, similar to the way sugar dissolves in hot water.

Pigments, on the other hand, do not dissolve. They can be ground to a fine particle size, but always remain just that, a tiny particle. Some pigments, however, are dyes that have been 'laked' (coated) onto a clay base to take advantage of the bright, clean colors that only can be obtained from a dye-based product. These pigment dyestuffs are most vulnerable to bleeding or migrating into other products.

Examples of colors that need to be watched carefully are rhodamine red, victoria blue and the so-called "Fanal" or PMTA-based pigments such as purple, green and reflex blue. The latter frequently is used as a toner in black inks to promote a rich blue undertone, so in this case even black inks should be checked carefully.

I already have mentioned some products that will promote pigment bleed, such as alcohol, soap, detergents and grease. To this list add waxes (frozen food wrappers), glycols (antifreeze) as well as strong acids and alkalis. Coatings also can affect inks. While not too much spirit or shellac varnishing is performed these days, the alcohol in these coatings can cause bleeding with some of the pigments mentioned previously.

Ultraviolet-cured (UV) coatings, while in the wet state, act similar to an alcohol and will bleed sensitive pigments. UV coatings have an added danger - the high-intensity light used to cure them can cause rapid fading of the pigment, particularly if some bleeding also occurs. In addition, while not a product problem, note that some light tints containing opaque white (titanium dioxide) also can fade rapidly on exposure to UV light. Combinations containing iron blue, in particular, can be a real problem.

When printed material is destined to be a package or will otherwise come in contact with a product, printers should inform inkmakers so the ink will be formulated using the correct pigments not only for the shade of the ink, but also for the desired product resistance.

TERRY SCARLETT Contributing editor and president of Burntwood Industries, a consulting company specializing in inks and coatings