Thermochromic compositions

DETAILED DESCRIPTION OF THE INVENTION Basic copper carbonate is a known compound and is generally designated by the formula CuCO.
sub.
3.
Cu(OH).
sub.
2.
The compound is green in color and, as reported by Day in the article mentioned above, is known to change in color from green to black at 320.
degree.
C.
This change is at much too high a temperature for basic copper carbonate to be used by itself in a thermochromic paint for heat-recoverable memory metal devices.
The sulfur compound used in the novel thermochromic compositions of this invention can be any sulfur-containing compound which is capable of effecting a distinct change of color of the copper compound at temperatures between about 165.
degree.
C.
to about 240.
degree.
C.
Among sulfur compounds which can be used are inorganic sulfur compounds, elemental sulfur, and sulfur containing organic compounds such as thiols, thioethers, sulfur-containing organic acids, sulfoxides, sulfones, sulfonamides, and the like.
Illustrative compounds which can be used include sodium sulfate, copper sulfate, free sulfur, dodecanethiol, sulfur-containing organic acids such as 3,3'-thiodipropionic acid, dimethyl sulfoxide, N-methyl-N-nitroso-p-toluene sulfonamide and the like.
The proportions of basic copper carbonate and the sulfur compound used in the composition are not critical.
Sufficient sulfur compound should be present to produce a distinct color change in the desired range.
Generally, this is achieved if the mole ratio of basic copper carbonate to sulfur compound is in the range of from about 0.
5:1 to about 2:1, preferably from about 0.
8:1 to 1.
5:1.
A mole ratio of 1:1 is particularly preferred.
Complexes of copper and sulfur-containing organic acids are discussed in the literature.
For example, in Chemical Abstracts Volume 56:13779h and 56:13780b, studies in which the dissociation constants and stability constants of such complexes in aqueous solution are reported.
It now has been found that complexes of copper and sulfur-containing organic acids undergo a thermochromic transition at temperatures in the range of from about 165