Signaling a shift in temperature via a change in color can provide both functional and esthetic advantages to parts fabricated in plastics. But while the use of such thermochromic effects has been limited, developments suggest that the situation may be poised to change.
The pigments can be used to indicate that a part has become hotter or colder. Matsui International, Gardena, CA, a subsidiary of Matsui Shikiso Chemical, Kyoto, Japan, which dominates the global supply of such pigments, offers thermochromics that change color or become colorless at triggering temperatures from -25°C to 65°C, notes Chuck Boyce, managing director and vp. Current applications include bodies of appliances like tea kettles, baby spoons, toys, and novelty items like souvenir cups. But demand is small. Thermochromics account for far less than 1% of the special-effects volume at lnp, says Josh Blackmore, product marketing manager in Exton, PA, with use confined mainly to high-density polyethylene and polypropylene as well as some thermoplastic elastomers.
The increasing interest in visual effects to differentiate or personalize products (Dec 02 MP, 42; MPI, 50) has not yet translated into a boom for thermo-chromics. Compounders and pigment suppliers indicate no broad groundswell in demand, but there are some positive signs. Eckart America, Louisville, KY, added thermochromics in 2000, not because of market demand, but in line with its strategy of being a single source for pigments. It saw interest pick up in the last quarter of 2002, particularly for promotional and medical items, says David Boone, account manager. Pigment developments should spur greater use, believes Boyce, who hopes that Matsui’s thermochromics’ volume will rise 50% by 2004.
Some compounders are sensing an uptick. Roger Avakian, chief technology officer of PolyOne in Avon Lake, OH, has seen more interest in the last year or so for toys and applications related to workplace and food safety as well as graphics. The company does not have a thermochromic product line, but it provides the effects on a custom basis. In contrast, Ferro, Cleveland, OH, launched a new line of thermochromic masterbatches in Europe in 2002, says Mir Ali, technical manager of the plastic colorants division, and it hopes to offer around 10 standard color concentrates in North America in six months. Making customers more aware of thermochromic materials is a big issue, he adds.
The pigments have suffered from several drawbacks, admits Boyce. Lightfastness has been their “Achilles’ heel,” says Blackmore, with U.S. FDA compliance also an issue. Resistance to uv light has been a problem and lack of FDA certification has hindered growth, agrees Boyce. Plus, the materials are expensive.
The pigments will change color with the same intensity only a certain number of times, says Avakian. Also, the range of color choices now is limited, which restricts flexibility in tailoring colors for applications, he adds.
Processing at too high a temperature can damage the pigments, and thus limits the slate of suitable resins, says Boone. The highest-processing-temperature resin lnp uses is ABS, notes Blackmore. The stiffness of the polymer and shear during processing can affect the pigments’ performance, adds Ali. He believes that higher triggering temperatures would open up some applications.
Progress being made in addressing many of these drawbacks promises to prompt increasing interest, however. Avakian, for one, senses a greater understanding of the need to develop colors specifically tailored to end-use requirements. But more than a stronger market focus is at work.
Within the last two years, lightfastness of its pigments has been boosted by up to two times, notes Matsui’s Boyce. The company now is working on pigments that can handle higher processing temperatures (over 260°C), enabling them to be used for the first time with PET and nylon 6 and 66. He expects the materials to be available around June. About the same time, tests for U.S. FDA compliance should be completed. Matsui also is developing effects that trigger at higher temperatures, up to 90°C, says Boyce, with introduction likely during 2004.
The University of Rhode Island, Kingston, RI, is talking to established pigment makers about commercializing pigments it has developed, says Brett Lucht, project leader. He expects an agreement to be reached by the end of the second quarter.
Their chemical composition is completely different than current commercial reversible thermochromics, he says. The new materials are organic polymers and are not microencapsulated. He claims the pigments boast at least 10 times better uv stability, maintain their color-changing effect longer, and can be processed at temperatures up to 300°C, in addition to being FDA compliant. Triggering temperatures range from 40 to 150°C. Lucht says usage cost should be comparable to current products.
Another hopeful is Skyrad, Haifa, Israel. It has developed organic pigments with triggering temperatures from 40 to over 120°C, that can be processed at up to 270°C, says Ori Faran, CEO. Manufacturing is much simpler than for current pigments, he contends, which should enable prices to be three to five times lower. Laboratory-scale quantities became available in 2002, notes Faran, who is hoping for commercial launch this year.
Meanwhile, processing options are evolving. Avakian is noticing in-creasing interest in overmolding of thermochromic TPEs. Boyce notes that such TPEs already are featured in items like pens and hair-setters. Stuart Swain, product manager for color at RTP, Winona, MN, sees potential for housewares with handles that change color.
Injection molding now dominates processing of thermochromics, and will continue to, according to many compounders. However, thermoforming is garnering more interest, says Boyce, with products now commercial in Australia. RTP has been showing thermochromic sheet to thermoformers for about a year, notes Swain. Boyce also is seeing more interest in coextrusions for bottles, but adds that applications remain a couple of years from the market.
Thermochromic materials that change color irreversibly may provide warning that perishables like food and pharmaceuticals have been exposed to undesirable temperatures. Matsui, for one, is working on irreversible thermo-chromics, and hopes to have pigments on the market in 2005 or 2006, says Boyce. PolyOne’s Avakian sees particular promise for thermochromics that shift in response to a set time at a given temperature, not just when reaching that temperature.
Post-processing activation probed
One problem posed by irreversible thermochromics is how to process plastics containing them without triggering the color change. This may give an edge to use of the pigments in labels and coatings instead of masterbatches. However, Avakian notes that new technologies aim to activate the one-way thermochromic effect after plastics processing. Products might be available within three years, he says, and would be particularly good for film extrusion. He envisions broad platforms capable of being tailored for time/temperature response and color.
Compounded-in pigments would avoid the need for secondary operations, and the adhesion, durability, and waste issues with labels and coatings. Masterbatches also are more convenient for small runs, Avakian adds.