A variety of devices in a wide range of industries include one or more component parts prepared from a material that undergoes a phase change at a temperature of interest. As the material undergoes the phase change, the component part typically expands or contracts, inducing a reaction, such as movement of an adjacent component part. For example, mechanical actuators are known where a material, typically a metal alloy, a polymer, or a wax, in the actuator expands or contracts to effect movement of a part in the actuator (U.S. Pat. Nos. 5,025,627; 5,177,969). Temperature relief valves having a thermal trigger composed of a eutectic material are known (U.S. Pat. No. 5,495,865). Fire sprinklers and fire extinguishes having a component part that responds to temperature increases are known (U.S. Pat. Nos. 4,896,728; 4,006,780). Temperature indicating devices for use in the medical industry and in the food industry are also known, where a component in the temperature-indicating device is composed of a material that undergoes a phase change at a selected temperature (U.S. Pat. Nos. 4,289,088; 4,170,956; 5,537,950; 5,988,102; 6,403,131). In these devices, a spring loaded indicator is held in place by a small quantity of meltable material, generally a eutectic metal alloy or an organic compound.
These and other devices require materials, preferably organic compounds, that exhibit very sharp melting points in the range of 50 to 100° C. There are few eutectic metal alloys that have a melting temperature in this temperature range of interest. Of the eutectic metal alloys available, for example lead/cadmium mixtures, toxicity of the material can be an issue for actuators used the medical or food industries. Moreover, low melting eutectic alloys are costly. Similarly, there are few organic compounds that have melting points in this temperature range of interest, and of those that do have the requisite melting point, often a physical or chemical property renders the compound undesirable for use in an actuator device.
Another problem with existing metallic and organic compounds is that it is not possible to vary the melting point and maintain abrupt melting behavior. For example, if the composition of an eutectic alloy is changed slightly, the melting point will either not change, broaden unacceptably, or give multiple melting points. Likewise, if a pure organic compound having a melting point at for example 85° C. is mixed with another compound the melting point of the mixture will invariably be lower and occur over a broader range. Moreover, while melting is a thermodynamic property, the behavior of a mechanical device, such as a food temperature indicator or a fire sprinkler, will be sensitive to the mechanical properties of the materials used. Many organic compounds having what appear to be sharp melting points, in fact do not provide the necessary mechanical properties for use in a thermo-mechanical device.
It would be very desirable to have a series of organic materials where the melting properties could be smoothly varied up or down while maintaining the necessary mechanical properties for use in a thermo-mechanical device. There remains a need in the art for such materials, and more specifically materials that have the following general properties: (i) a melting point between about 50 and about 100° C.; (ii) non toxic to humans; (iii) a low vapor pressure at 120° C.; (iv) stability at elevated temperature and moisture; (v) no odor; (vi) rapid rate of crystallization; (vii) low cost; (viii) readily produced in high purity; and (ix) sharp melting point and abrupt change in viscosity with temperature change.