From mood rings in the 1970s to the temperature-indicating labels of drinks today, colour change has been involved as a product gimmick for years. But what type of ink is used to perform this colour change, and what is the full range of its use?
Thermochromic ink is a specialised ink that changes colour with exposure to different temperatures. This change is classified as a part of thermochromism, which is any change in colour due to heat. In turn, both fall under the category of chromotropism, which is a change in colour due to any outside influences.
The basic two types of thermochromic inks are liquid crystals and leuco dyes. Liquid crystals are used less frequently than leuco dyes because of the difficulty in their production and the special training needed to use them. They are more sensitive to temperature change than leuco dyes, meaning that they are used in delicate experiments where small temperature change has to be noted. Leuco dyes are in greater use than liquid crystals in a variety of different products that require less-accurate temperature readings.
Thermochromic inks have several uses, ranging from frivolous to serious. Some of the more novelty uses are in mood rings, measuring the temperature of a liquid in a coffee cup and knowing when to take a bottle of syrup out of the microwave by the label. Other, more serious uses include taking someone's temperature with a piece of paper on the forehead, indicating the status of an experiment involving temperature change and using in a home-security system.
Basically, thermochromic ink works by transmitting a colour when cool and becoming clear when hot. For manufacturers, this means that the ink should be combined with other, non-thermochromic inks so that a message can appear when something is heated or cooled. The formation of the ink involves several microencapsulated chemicals carefully added together.
Thermochromic ink should not be confused with photochromic ink, another type that changes colour with exposure to UV rays, or sunlight. In addition, thermochromic ink can be manufactured to be reversible or irreversible; the preference comes if someone wants to monitor repeated temperature change or one change only.