As summer comes to a close, the nights draw in and we start dusting off our winter coats in anticipation of the months ahead, this week’s Material Monday examines the use of thermochromic inks – reactive substances that, just like us, respond to changes in temperature.
Since making a name for themselves, back in the 1970s, as the key component in the production of mood rings – jewellery that claimed to double as an indicator of the wearer’s emotional state – thermochromic inks have existed, by and large, as a novelty tool in the armoury of marketers and printers. From colour-changing toys for children, to colour-changing beer cans and wine labels for adults, a variety of brands have found a use for this eye-catching technology, which has often been employed as a means of injecting intrigue into otherwise quotidian objects.
As the technology underpinning thermographics has developed, so two primary formulations of ink have emerged; thermochromatic liquid crystals and leuco dyes. Owing to their more stable chemical make-up, leuco dyes are more commonly used in the printing of mass-market consumer products, such as those beer cans or wine labels, which require a temperature-sensitive element. At lower temperatures, the dyes retain their colour, but become translucent when exposed to heat – a product of the separation of colorant and acid in the ink mixture. As a result, leuco dyes are often employed as a means of concealing (and then revealing) an underlying message or image.
Thermochromatic liquid crystals, by contrast, have a set of properties that render them ideal for more specialised applications. Rather than a binary distinction between coloured and translucent, TLCs exhibit different colours dependent on the surrounding temperature. A microencapsulation process is required before TLCs can be applied to substrates or mixed into solutions, but once this has taken place, the ink provides a remarkably accurate temperature analysis – making it ideal for incorporation into products such as dynamic thermometers.
The comparative fragility of thermochromatic crystals means that, despite its greater temperature range and accuracy, this type of ink is rarely used with traditional printing methods. Leuco dyes, by contrast, are compatible with offset and screen-printing, among other techniques, and can be found on a variety of substrates, from paper to metals and plastics. Consequently, they have found their way into baby bottles, as a safety precaution that allows parents to monitor the temperature of the liquid inside; and into packaging that allows brands to disguise seals of authenticity, and thus guard against counterfeiting.