New Inks Have a High IQ

Published: August 31, 2004, By Dr. Richard M. Podhajny, Ph.D. Contributing Editor

Well, we've heard a lot about “smart packaging,” so it was inevitable such packaging would require “intelligent inks.”

With current packaging development focused on smart and active packaging, some critical R&D is being devoted to inks that can provide real-time information as to the package condition.

Conventional inks are used to provide the customer with brand identification, information about the packaging contents, UPC symbols, and advertising. Although this is vital to selling the package, the packaging designers of tomorrow are putting greater emphasis on providing more information about the package condition.

So, it's no surprise inks are being developed that can be used to provide a visual inspection as to the package contents and the package freshness. This can include information about temperature, level of oxygen, ammonia, and/or carbon dioxide. These sensor-type inks can be referred to as “intelligent inks.”

Many materials change color when subjected to heat, light, pressure, or electrical current. Photochromics change color when exposed to a specific wavelength of light. They are useful in security applications as well as monitors for UV exposure. Thermochromics change color as temperature changes and are used in medical and packaging applications. Electrochromics can change color when exposed to electrical currents or high pressure.

In the mid 1970s, Pilot Ink Co. of Japan introduced Metamo Inks, which utilized thermochromic polymers. Although the products worked, low market demand and product limitations dampened their commercial application in the US.

More recently, polymeric thermochromic materials have been developed at the Univ. of Rhode Island. Such materials have advantages in food packaging applications since they are not migratory. These thermochromic polymers could be incorporated into inks as well as into package substrates.

Conductive inks that can provide product identity using radio frequency identification (RFID) technology are examples of a development that focuses on product integrity and freshness. These inks can be formulated based on conductive silver or silver materials as well as highly conductive carbon blacks.

So, what's new in “intelligent inks”? For many years packaging research engineers have been trying to develop a colormetric indicator that could provide specific information about food packaging and its environment. Recent advances suggest inks that can sense (change color in) the presence of select gases may be available in the near term. Ideally, these would be able to detect the presence or level of oxygen, ammonia, and carbon dioxide. Such inks would be incorporated into the interior of the package and would require FDA status if used for food packaging. Color-changing reactions can take place with the ink providing an indication of the presence of the gas in question in the package headspace.

Inks that are sensitive to oxygen can take different forms. One such approach recently was described by the Univ. of Strathclyde, Glasgow, Scotland. A photoconductor (titanium dioxide) is used in the ink, which when activated by UV light, interacts with oxygen. The presence of an appropriate electron donor, redox-indicator dye, and encapsulated polymer can produce color indicative of oxygen's presence.

Also being investigated are inks that will detect ammonia and carbon dioxide. This R&D has spun off a commercial venture, Gas Sensor Solutions, Dublin, Ireland, which is making these inks available in the form of jet inks and conventional screen printing inks, and claims to be able to monitor oxygen as well as carbon dioxide. Applications include food, pharmaceutical, and medical packaging. Ink formulations allow chemical reactions to take place on the interior of the package, providing a chromatic show of the presence of one or more of these gases in the headspace of the package interior.

Are we there yet? If not, we are getting close. These developments are mapping new chemical methods to formulate “intelligent inks” that can provide a chromatic indicator to sense when the package interior provides — or doesn't provide — a suitable freshness. The above R&D examples are clearly the future of “intelligent inks.”