Abstract: Premature color formation and fogging in composites used in two sheet thermographic imaging systems are prevented by providing a polymeric barrier coating between the two interactive layers. The barrier coat serves to keep the volatilizable organic acid in the donor sheet from diffusing into the dye precursor receptor sheet during storage or transport, but permits the desired diffusion at thermographic imaging temperatures. Suitable barrier coat layers, which must be acid-resistant, include chlorinated rubber, chlorinated polypropylene, styrene-acrylonitrile copolymer, chlorinated paraffins, polystyrene and vinyl chloride-vinyl acetate copolymer.

Abstract: A method for the amplification or intensification of dye images formed in a negative-working diazo system which comprises the steps of: (1) initially image-wise exposing a negative-working diazo layer to actinic radiation sufficient to convert a portion of the diazo compound contained therein to an active coupling constituent in the light-struck areas to form a first latent image, (2) developing the exposed diazo layer to effect coupling so as to form colored incipient images in the light-struck areas, (3) exposing the entire diazo layer from the same side as the initial image-wise exposure to actinic radiation of a greater amount than that used in the initial exposure, said amount of actinic radiation being sufficient to substantially photolyze the non-image areas while simultaneously creating second latent images beneath the already-developed areas, and (4) developing the exposed diazo layer again to effect coupling in said second latent image areas to form color therein and to amplify the initial colored i