Heat developable paper

Claim 1

A HEAT SENSITIVE SHEET ADAPTED FOR DRY HEAT MARKING PROCESSES, STABLE AND LIGHTLY COLORED UNDER NORMAL CONDITIONS AND RESPONSIVE TO HEATING TO AN ACTIVATION TEMPERATURE IN THE RANGE OF 50*C.-150*C. BY CHANGING VISIBLY AND IRREVERSIBLY TO A CONTRASTING COLOR THROUGH THE FORMATION OF A DYE, SAID SHEET INCLUDING A SOLID STABLE LIGHTLY COLORED NON-HYDROSCOPIC KETO COMPOUND SELECTED FROM THE GROUP CONSISTING OF KETONES AND ALDEHYDES OF MONO- AND POLYNUCLEAR AROMATIC COMPOUNDS AND QUINOID COMPOUNDS, AND A SOLID STABLE CRYSTALLINE CO-CRYSTAL ADDUCT OF A BISPHENOL AND AN AMINE SELECTED FROM THE GROUP CONSISTING OF PRIMARY AND AN AMINE SELECTED FROM THE GROUP CONSISTING OF PRIMARY AND SECONDARY AMINES AND HYDRAZINE, SAID AMINE AND KETO COMPOUND BEING DYE PRECURSORS WHICH WHEN COMBINED ARE REACTIVE AT A TEMPERATURE OF LESS THAN 150*C. TO FORM SAID DYE, SAID ADDUCT HAVING A DISSOCIATION TEMPERATURES BETWEEN 50*C. AND 150*C. AT WHICH SAID AMINE IS RELEASED FOR REACTION WITH SAID KETO COMPOUND.