Drying of Overprinted Ink Films.

A recently dried first down (liD) ink film exerts a powerful catalytic effect on the drying of a second-down or overprinting ink film. As the liD ink film ages before being overprinted, its catalytic effect becomes less. Finally it produces an inhibiting action on the drying of the second-down ink. By means of a sandwich technique it was possible to prove that the accelerating material from the liD dried ink is a vapor. It was found that this accelerating material was not a drier per se so it has been termed a volatile drying accelerator. By a similar method it was shown that the retarding materials in aged liD ink films are not volatile at room temperature but that they can be removed from such films by prolonged heating at elevated temperatures. The effect of changes in pigment, type of varnish, per cent of drier in the liD ink, and percent of drier in the overprinting ink were studied in connection with the formulation and amount of the volatile drying accelerator and the retarding materials.