Thermally developable materials with improved speed and contrast and methods of use

This invention relates to thermally developable imaging materials having improved speed and contrast. In particular it relates to thermographic and photothermographic materials containing specific co-developers combined with hindered phenol reducing agents, aromatic di- or tri-hydroxy compounds, or mixtures thereof. The invention also relates to methods of imaging these materials. Thermally developable materials include a reducing agent composition that can reduce silver ions and provide high contrast images. The reducing agent composition includes a) a hindered phenol reducing agent, an aromatic di- or tri-hydroxy compound, or mixtures thereof, and b) a thermally-releasable p-phenylenediamine co-developer that is represented by the following Structure I:wherein R-NH- is a p-phenylenediamine group and BLOCK is a blocking group that is cleaved upon exposure to at least 120° C. for at least 5 seconds and comprises one or more of the following thermally cleavable groups: -CONH-, -COO-, -CONHSO-, -PO-, -SO-, -COCHNHCO-, or -COS-, and BLOCK is directly linked to R-NH- through one of the thermally cleavable groups.