Diazotype imaging materials contain at least two active agents and usually require a third active agent to perform well. The first two agents are an acid-stabilized diazonium salt and an azo-coupler compound. The diazonium salt is stabilized against spontaneous reaction with itself and the azo-coupler by the presence of an acid environment. The image produced by diazotype materials is formed from the resulting dye of a reaction between a diazonium salt and the azo-coupler compound. A diazotype sheet containing only these two agents would not be of significant use in competition with modern imaging techniques because a protracted development time would be necessary to overcome the stabilization of the diazonium salt, or reduced amounts of acid would have to be used which would render the sheet less storage stable and of little practical use. It has therefore been the practice of the art to add active or latent acid-neutralizing compounds (basic materials) into the sheet in order to enable the diazonium salt and the azo-coupler to couple and form a dye more easily. The acid-neutralizing compounds have commonly been made available for neutralization in various manners including the direct application of bases in the gaseous state (e.g. ammonia) or in solution. In ammonia processed diazo systems, French Pat. No. 2,059,125 discloses that certain plasticizers are useful as development accelerators. It is shown that development times range from 10 to 40 seconds.
U.S. Pat. No. 3,754,916 discloses that benzimidazole compounds can be used as an acid-neutralizer and provides a storage stable material when the benzimidazole compound is included within a heat-softenable binder and also provides excellent imaging characteristics to diazotype sheets. Typical reaction times for the diazotype sheets shown were at least 40 seconds at 260.degree. F. (126.degree. C.) to react all of the dye forming components in the sheet. U.S. Pat. No. 4,168,171 discloses the use of imidazoles, and U.S. Pat. No. 4,230,789 teaches use of certain imidazolines as neutralizing agents for thermally developable diazo sheets. Comparable development times are disclosed.
Sheets of higher speed can be made by reducing the amount of diazonium salt (for quicker exposure) and increasing the amount of acid-neutralizer and/or decreasing the amount of acid (for quicker development). However, when such optimization has been made, it is found that in rapid processing with development following quickly upon exposure, there is a problem with the nitrogen from the diazo decomposition. It forms blisters or, in extreme cases, a very large number of small occlusions more usually known as vesicules. Such defects are unacceptable.