Liquid crystals take the cold

4 min read

Liquid crystals have a host of new commercial uses and potential uses other than in displays. Tom Shelley reports

Passive liquid crystal indicators are about to save many lives and much stomach upset by becoming a compulsory part of refrigerators. Added to that, closely related technologies have potential in truly hard-to-fake designer goods labels, and novel signs based on temperature responding four-colour printing. Applications abound in both consumer and industrial products, especially for those concerned to protect themselves against litigious attacks. Liquid crystals were discovered in the middle of the nineteenth century, and first described by Austrian Botanist Friedrich Reinitzer in 1888. The term 'liquid crystal' was coined in 1889 by his friend Otto Lehmann, a German professor of physics. However, it was not until the 1950s and 60s that scientists in the US, USSR and UK realised their true commercial potential. Apart from the LCD displays, B&H Liquid Crystal Devices in Wimbledon has built a very successful business on liquid crystal thermometers and other devices, mainly for product promotions, and has won no less than three DTI Smart Awards for some of its developments. The most recent, and the one likely to yield the biggest boost to the company's business, has been for superficially simple, but quite difficult to develop, fridge temperature indicators. Successful products already in the market from B&H include small square indicators mounted on the shelf edges of freezer cabinets in Tesco stores. Normally, these remain black, but if the temperature rises unacceptably, they turn green – alerting staff to a temperature anomaly. The latest development stems from a new law about to come into force in France, and eventually the rest of Europe, that all new refrigerators there will have to include some indication if they are not as cold as they should be. The need arises from the general decline in use of preservatives, dictated by consumer fears about their adverse effects, and a consequent increase in food poisoning. B&H managing director Stephen Barker explained to Eureka that there were a number of problems that had had to be overcome in order to turn aquarium type liquid crystal temperature indication into a suitable product for a refrigerator. Firstly, his team had to come up with liquid crystal colour indication that would remain positive from 5°C to ambient instead of the 1, 2 or 3 degree bandwidth obtained in conventional products. Second, the indicator had to be attached to something with enough thermal mass that it would not falsely indicate a over-high temperature, simply as a result of opening the door. Added to that, it had to be very cheap, be easy to fix in refrigerators of many different designs and capable of being produced in vast numbers. The temperature bandwidth was sorted out by experimental variation of the five ingredients which B&H uses to make up the contents of its 5 micron sized micro capsules, and making it indicate positively when temperature is correct, which is a narrower temperature range than all possible 'too warms'. Metal and plastic backings were found to provide insufficient thermal mass unless excessively thick, so the backing chosen is a 10mm thick flat plastic containment of either liquid or gel. OEM versions are designed to meet individual manufacturer requirements but a version designed for retrofit has to have a stick on pad from which it can be hung as an alternative to suspending it from the shelf bars. It can then cope with refrigerators with glass and plastic shelves, as well as shelf bar thicknesses that vary substantially from country to country. The final version is black when 'too warm', but shows 'OK' on a green background when cold. Initial development took nine months, and is still continuing in order to find ways to increase production output and further reduce costs. A seal to certify Still in development is the identification seal concept that won the company its 1998 SMART Award. Since hologram seals can now be readily counterfeited in the Far East and elsewhere, the idea is to make something more difficult to fake. A temperature changing liquid crystal seal would require a large amount of specialist expertise and experimentation to reproduce. Such seals could be made hard to spot by the uninitiated, but could be activated by either human touch or application of a cold spray. Samples have been made and the project is still very much on-going. It has even been considered as a means of authenticating banknotes, but in its present form, apparently lacks sufficient resistance to ultraviolet light. Colour the key Many 'too hot', 'too cold', 'much too hot', 'much too cold', 'OK' or 'danger', heat or cold triggered signs, indicators or colour changes can be devised, and are likely to find increasing favour in our increasingly litigious society. In 1996, B&H won its first SMART Award to study the feasibility or four colour printing thermally activated images and signs. These could either appear or disappear according to temperature or change in some way. This, like the anti-counterfeit seals, is still on-going. We do not know whether Eureka in conjunction with B&H is yet in a position to offer thermally activated four colour advertisements in the magazine. But we have seen a printed aid for children to learn their tables, in which pressing a hot thumb on the appropriate place reveals the answer. Interestingly, the liquid crystals areas required no perceptible lamination on the sheet, and had been directly printed onto the paper. The company has already successfully developed its Millennium Award winning colour changing bay feeding spoons. They incorporate micro-encapsulated liquid crystals, which warn mothers that their child's food is too hot. It was the subject of a Coffee Time Challenge some years back. The company has also developed baby bath thermometers using similar functionality. The company also produces liquid revealing 'Wash 'n' Win' and 'Dip 'n' inks, pressure sensitive inks, high temperature irreversible inks, microwave inks and a wide range of security inks. Almost anything in the ink or thermal colour changing business that anyone cares to imagine B&H can do, if the market is large and lucrative enough to warrant the necessary development effort. Now who wants a thermally triggered four-coloured advert? B&H Liquid Crystal Devices Email B&H Liquid Crystal Devices Pointers * Low cost, temperature responsive devices can indicate if a refrigerator has been running at an insufficiently low temperature, or issue other thermally induced warnings * Liquid crystal based seals look to be much more difficult to counterfeit than hologram seals yet cost about the same * Four colour printing of thermally activated signs and messages has also been shown to be possible Eureka says: The full potential of liquid crystals is still being discovered. The biggest lesson, however, is that it is the simplest applications which are often the most profitable. Never forget KISS - Keep It Simple Stupid!