3.6 Applications

Thermosensitive recording paper was launched onto the market by the National Cash Register Company in 1968. It consists of a paper sheet coated with a single thermosensitive layer comprising leuco dye and developer which are the essential colour-forming components as well as several further additives. By calendaring under high pressure a smooth surface is obtained. Local heating on contact with a thermal pen or thermal head brings the colour- forming components into reactive contact resulting in a distinct image of the heat pattern. The developing temperature of the thermosensitive recording paper can be lowered by the addition of a sensitiser. Then the melting temperature of the sensitiser determines the developing temperature. To prevent adhesion of the coloured leuco dye-developer complexes on the thermal head fillers are added which absorb the coloured melt. Moreover, stabilisers are used to increase the light stability and the long-term storage stability of the recorded images.

Thermochromic inks containing microencapsulated leuco dye" developer are available for all major ink types: water-based and photocuring inks for paper and plastics, epoxy-based inks for glass and wood as well as plastisol inks for textiles. Starting with novelty items such as mood rings in the 1970s, more and more applications are entering the market in which the thermochromic ink becomes a functional part of the product. Battery testers are an example of such a product. Battery testers consist of a printed layer structure. One layer is based on an electrically conductive ink and a second layer on a...