Hot or Not?

Although it is difficult to define exactly what smart packaging is, one significant component of any smart package is its ability to communicate with the person interacting with it. In food packaging, some packages can communicate freshness, while others can communicate a product’s history or expiration. In terms of the pharmaceutical market, packages integrating RFID tags to verify authenticity is another example. Other smart packages combine communication with functionality, like self-cooling beer kegs or self-heating soups and coffees.

According to a report from NanoMarkets, LC, titled, “Smart Packaging Markets; 2006-2013,” the global smart packaging market will grow to $4.8 billion in 2011 and reach $14.1 billion in 2013. The report lists various “smart materials” that enhance smart packages. These include shape memory alloys to control the opening and closing of packages depending on environmental conditions, piezoelectric materials to provide power for lighting and audio features on packaging, smart adhesives that can be used in conjunction with smart labels, and thermochromic inks to show when optimal or dangerous temperatures have been reached. NanoMarkets believes that smart materials will have a significant impact on smart packaging technology.

Thermochromic inks, in particular, have found their way into myriad packaging applications.

The heat is on. Or is it off?

Thermochromic inks change color with exposure to heat. They can go from colorful to colorless, colorless to colorful, or change from one color to another, according to Don Duncan, director of research, Wikoff Color Corporation. “Some cause a permanent, irreversible color change, and some give a temporary reversible color change,” he says. “Each themochromic colorant has a fixed temperature range over which its color change takes place.

Deanna Whelan, marketing manager, XSYS Print Solutions, adds, “Reaction temperatures vary depending on the end use. Inks for use in refrigerators are set to activate at low temperatures. Inks for touch are set to activate at typical body temperature. For a microwave oven, they are set at a high temperature.”

Duncan states that there are two types of thermochromic colorants: liquid crystals and leuco dyes. Liquid crystals show color change in a narrow temperature interval, making them suitable for displaying incremental changes in temperature. Their physical form makes incorporating them into packaging inks less common.

Leuco dyes change color over a broader temperature range and are available in a wider range of colors. “A gamut of colorants is available that will make a transition through a broad set of temperature ranges from –25ºC to +65ºC. Leuco dyes are the primary colorants used in packaging inks,” he says.

Chuck Boyce, managing director, Matsui International Co., Inc., explains that Matsui’s Chromicolor® product consists of three major components enclosed in a microcapsule: leuco dye, color developer, and a solvent. “When the solvent is at a high temperature, it is a liquid, which causes the dye and the color developer to come apart,” he says. “When the solvent solidifies, the dye and the developer combine to form a color. Different color change points are achieved by the use of different solvents. These microcapsules can then be used to create inks, paints, and plastics that can react at different temperatures to change color.”

What’ll they think of next?

Applications for thermochromic inks have expanded. John Signet, marketing manager, Water Ink Technologies, says thermochromic inks are used for security, novelty, and temperature-change indicators in packaging applications.

“Thermochromic inks are used in packaging and labels for refrigerated items such as beverages,” says Whelan. They are also used in specialized packaging for CDs and DVDs, “typically associated with a theme pertaining to the music or movie promoted, providing a package that will change color with the heat from the shopper’s touch,” she adds.

Duncan concurs. “Most of the new applications are novelty effects in packaging: areas of heat-sensitive, color-changing ink on ice cream cartons, beer labels, cups/mugs, lottery tickets, and CD/DVD packaging,” he says. “On the food containers, there is a minor functional purpose in letting the user know if the food is roughly in the optimal temperature range. On the other products, it’s strictly a ‘What’ll they think of next?’ purpose.”

Russ LaCoste, sales director for SICPA, says he’d like to see the number of applications increase. “We’d love to look into new areas frankly and try to get other people interested. I think you can use them in a lot more areas than they’re being used.” He adds that as ink companies expand into other areas of different temperature ranges, “I think it will open up more possibilities for some packaging providers to introduce this technology more across the board.”

Handling requirements

There aren’t many special handling requirements for thermochromic inks, but Whelan says converters should be educated before using them. “Shelf life is three months to one year in the container, depending on formulation,” she states. “Ink life in the printed form can be many years, but under normal use. These inks are sensitive to UV light and will have less life in the printed form if exposed to high temperatures and extensive light over long periods of time.” Otherwise, she says, the printed label or package should last for many years, under normal use.

“Most of our thermochromic packaging inks are water-based flexographic or gravure inks, and they have similar handling and stability issues as other water-based inks, plus a few others,” says Duncan. He adds that inks as supplied must be stored in a cool place; the cooler the better, without freezing. “Inks stored at +75ºF will have a shelf life of around three months, but inks stored at lower temperatures can last as long as three years,” he says.

Boyce adds that thermochromic inks are weak in light fastness resistance, so converters should avoid continuous exposure to ultraviolet as they tend to degrade the colors.

Signet states that thermochromic inks are very similar to other water-based inks in their shelf life and transparent environment. “From a waste standpoint, the pigments are inert and so the environmental issues will stem from the resins and other additives in the ink formulation.”

Applying the inks

For flexo and gravure applications, Duncan recommends that an anilox or cylinder of 175 linescreen (maxium) or lower should be used with very good airflow out of dryer ovens. This is because color strength of thermochromic pigments/dyes is low, and because leuco dye particle sizes can run from around 2-10 microns, with liquid crystal particles averaging 10-15 microns. Therefore, application of a higher ink film thickness is required.

Whelan concurs. “The thicker you can put down the ink, the better response you get,” she says. “Thinner ink film does not give the dramatic effects typically desired. And screen applications tend to provide the best results. Converters should understand that double bumping might be necessary.”

Shearing is also an issue. Duncan explains that many thermochromic pigments/dyes are microencapsulated, and will not stand up to very high shear conditions. Printing conditions where doctor blades are used can, over time, crush encapsulated particles creating a color change. “Therefore, it is recommended that inks get to the substrate with a minimum of recycling through pumps, mixers, and under doctor blades,” he says.

Additionally, Duncan says, these inks can separate. “Low-shear mixing is recommended before use to re-suspend particles.”

Holding their own

There are many different materials that can help packaging communicate with the consumer. Given the potential lifespan of thermochromic inks, these materials stack up well against other smart materials. “These inks hold their own,” says Whelan. “They are simply another option among other types of specialty inks designed for varying functions and end uses.” Boyce adds, “The life cycles possible using thermochromic inks are infinite.”

“I think it’s an interesting area and I think it has potential,” says LaCoste. “I think that it struggles with the same problems that any specialty printing products have. People are looking at the cost of packaging and it’s substantially more than what they’re used to paying. It may be different but they have to weigh the increased cost over what they’re going to get. So it has to be a pretty interesting application for anyone to consider it.” pP