Redefining Plasticizers in Inks and Coatings

Published: July 31, 2002, By Dr. Richard M. Podhajny, Ph.D. Contributing Editor

The dictionary defines plasticizers as “any of various substances added to plastics or other materials to keep them soft or pliable.”

For the purpose of this discussion on ink and coatings, I will modify this definition as follows: “Plasticizers are chemicals that can soften binders used in ink and coatings, improving their flexibility.”

Most plasticizers are high boiling liquids that partially “dissolve” the polymer framework of select binders. These plasticizers effectively reduce the softening point (Tg) of the ink or coating binder.

Most plasticizer additives used in inks and coatings fall within the chemical class of esters. Esters are made from the reaction of carboxylic acids and select alcohols. These include phthalates, stearates, citrates, and a large variety of other esters. The most common are Di-isooctyl phthalate (DOP), Di-butyl phthalate, and Tri-ethyl citrate (TEC). Although esters have excellent solvency properties, other chemical classes can be used. Other plasticizer classes include ethers, oils, and soft polymers.

Careful selection is necessary to provide the best plasticizer for a particular ink or coating formulation. Due to incompatibility with some binders, not all plasticizer additives can be used in a given formula.

Besides low-molecular monomeric additives, polymeric plasticizers can be used. These can be co-binders, offering advantages over low-molecular plasticizers in some applications. For example, a soft PU resin can be used with nitrocellulose, providing “plasticizing” of the nitrocellulose and low or no plasticizer migration.

Nitrocellulose is a common solvent-based binder used in many flexographic and rotogravure inks. Without added plasticizer, nitrocellulose does not adhere to most packaging films since the binder forms a very rigid 3-D structure that cannot bend without cracking, and the internal stress is so strong that adhesion is not easily attainable.

In addition to plasticizers partially dissolving the binder, they can improve the adhesion of binders by dissolving slip additives and other surface contaminants, allowing the resin to bind with the desired film surface.

One of the undesirable effects of using plasticizers is they can raise the COF by making the ink or coating formulation “softer.” Not only is this effect evident in the ink and coating surface, but some “plasticizers” can migrate from front to back of the film within the roll.

Higher temperature and pressure will increase the rate of migration. As a rule of thumb, if the COF of a printed or coated film rises with time, plasticizer migration would be suspected. It is possible for the printed film to raise its COF from 1.5 to 3.5 or higher. The rate of migration is in the order of 8-24 hr depending on the film and plasticizer.

It is important to maintain low solvent retention levels in printed, coated, and laminated films since the presence of residual solvents enhances the migratory rates of plasticizers. A high residual solvent within the film will cause greater plasticizer migration within the roll under pressure.

Since most plasticizers are low-molecular liquid additives, they can migrate within the package construction. These plasticizers will divide themselves between different film layers, depending on their solubility in the films.

As a result, plasticizers used in food packaging applications need to be FDA-compliant since they can be expected to migrate through the film and be in contact with the food.