Heat-sensitive recording composition

A heat reactive recording composition capable of acquiring a stable color in selected regions contrasting with the background color of the sheet upon exposing these regions to an elevated temperature and comprising a paper or polymeric film substrate and a coating comprising (1) an N-substituted condensation product of a heterocyclic diimino compound and an aromatic aldehyde and (2) a cyclic polyketo compound reactive with amines and amides at elevated temperatures to form a color.

BACKGROUND OF THE INVENTION 
This invention relates to recording members containing heat reactive 
components and, more particularly, to recording members capable of use in 
thermographic copying, thermal printing, event recording, and as 
transparencies for overhead projection. 
Heat sensitive sheets containing the cyclic polyketo compounds of this 
invention, useful for copying and recording and characterized by the 
ability to form a mark of contrasting color when heated to an activation 
temperature of 50.degree. C are known in the art. They are used in 
thermographic processes wherein a recording member is positioned on a 
graphic original and exposed to infrared radiation to cause selective 
heating of the dark areas of the original sufficient to form a copy 
thereof on the heat sensitive member. The thermally responsive members 
have also been used to record the heated portions of a thermal print-head 
and also to record a colored trace when contacted by the hot stylus of a 
thermal recorder. The ninhydrinamine reaction wherein ninhydrin reacts 
with amino acids, primary amines, and certain derivatives of morpholine, 
piperidine, and pyrrolidine to give the dye commonly referred to as 
Ruhrman's purple, is well known. Isatin reacts with these same amines to 
give isatin blue. Alloxan reacts with the amines to give a red dye. These 
reactions have been used in numerous inventions for thermally responsive 
copy and recording papers and films. Lawton, U.S. Pat. No. 3,736,166 used 
ninhydrin with various morpholine and piperidine derivatives to prepare 
transparencies for overhead projection. Lawton, U.S. Pat. No. 3,293,061 
combined ninhydrin or hydridantin with isatin-amine condensates to provide 
thermographic copy sheets. Bauman and Lawton reacted ninhydrin or 
hydrindantin with complexes of amines and flavans or phenolic compounds to 
make thermographic copy sheets, see U.S. Pat. No. 3,149,991 and U.S. Pat. 
No. 3,149,992. Huffman, U.S. Pat. No. 3,664,858 combined ninhydrin with 
the adducts of morpholine or piperidine and organic acids in thermal 
recording members. Sus, U.S. Pat. No. 3,024,362 combined hydrindantin with 
amino acids or salts of primary amines with organic carboxylic and 
sulfonic acids to make a thermocopy paper. Allen, U.S. Pat. No. 2,967,785 
used the adducts of morpholine or piperidine with isatin or ninhydrin as 
the color forming material in thermocopy papers. Small, U.S. Pat. No. 
3,573,958 combined an amine with a halide or organometallic halide of 
germanium, silicone, lead, and tin with hydrindantin to provide a heat 
sensitinve recording sheet. In each case the normal dye formation of 
Ruhrman's purple with ninhydrin, red dye with alloxan, and isatin blue 
were obtained. 
It is often desirable and an objective of this invention to obtain colored 
records which have colors differing from those normally obtained with the 
above cyclic polyketo compounds. 
Undesirable odors and fumes are obtained by the heat dissociation or 
decomposition of the complexes, salts, or adducts during the imaging 
process. There is a problem of image bleaching during excessive heating, 
or aging, or prolonged exposure to ultraviolet containing light. 
Attempts to use heterocyclic diimino compounds such as the piperazine 
derivatives results in colorless or very pale images, general 
deterioration of coatings, and a coating layer unacceptable due to the 
very hygroscopic nature of the piperazines and their unpleasant odor. 
SUMMARY OF INVENTION 
I have found that the above objectionable problems can be minimized or 
eliminated by the combination of an N-substituted piperazine or other 
N-substituted heterocyclic diimino compound obtained by condensing the 
piperazine with an aromatic aldehyde, and a cyclic polyketo color 
precursor. The heterocyclic diiminoaldehyde condensate and the color 
precursor react at temperatures between 50.degree. C and 200.degree. C to 
form a number of different highly colored dyes. The volatility, odor, and 
hygroscopic characteristics of the original piperazine derivatives are 
eliminated. The materials for preparing the piperazine condensates are 
generally less expensive than those used for the adducts and complexes 
described in the prior art and the product yield is high which leads to a 
less costly record member.