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Temperature indication based on thermochromic effect

IP.com Disclosure Number: IPCOM000012547D
Publication Date: 2003-May-14
Document File: 3 page(s) / 38K

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The IP.com Prior Art Database



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Temperature indication based on thermochromic effect

Many domestic appliances are equipped with hot plates to supply heat, e.g. coffeemakers and irons. These hot plates are generally open and exposed to human touch (i.e. the consumer's touch), which leads to obvious problems if the plate is still hot even when the main power supply is switched off. It is desirable to have an indicator to tell a consumer when such a hot plate is too hot to touch, mainly for safety purposes. To this end, it is desirable to provide the hot plate with a thermochromic coating (i.e. a coating that changes colour reversibly when at a higher temperature).

        � � � � � � � � � � � � � � � Sol-gel coatings are used in steadily more appliances, for functional (e.g. abrasion resistance or gliding in irons) or decorative effects (colours in irons or shavers). Hence sol-gel technology/chemistry is a well-developed field. In many cases, fillers are used to tune the functionality (scratch-resistance) or decorative value (colour) of such coatings. One way to combine a decorative effect in sol-gel coatings, along with the thermochromic functionality desired above, would be to incorporate fillers into the sol gel coating which are thermochromic. However, it is not a trivial matter to find suitable materials that can withstand high temperatures (thermally stable), and are stable to repeated colour change.

1. Indirect effect

        � � � � � � � � � � � � � � � This option involves an indirect method to obtain a thermochromic effect in hot plates. Thermochromic materials change their colour, due to a change in its reflective, scattering or transmission properties, as a function of temperature. The preferred colour change in our case would be, for example, from blue to red when the temperature exceeds a tolerable (to touch) temperature. The effect should be reversible and should be stable as a function of time (switching frequency). For use on an ironing soleplate, such materials should also possess thermal stability to withstand high temperatures, which are often in excess of 200°C in an ironing soleplate. This can be done as follows (using the example of an ironing soleplate):

Particles of materials containing blue dye are dispersed in the transparent sol-gel coating based on MTMS, rather than any other pigment. Between this coating and the die-casted soleplate, an anodised coating is used to improve its adhesion. This coating can be coloured bright red with the help of a pigment (e.g. iron oxide). Another option is to print a pattern on the coating which is red (e.g. a pattern like dots or lines) with screen-printing.� The light scattering particles containing blue dye are (liquid) crystalline and scatter light strongly at lower temperatures (e.g. room temperature). Due to multiple scattering the red colour is disguised and the sole plate appear to be blue. When the temperature of the soleplate and the filler particles increases (say above Tcrit°C), the crystalline material melts and b...