In 1970, it was learned that a polytetrafluoroethylene (PTFE) micropowder, in combination with wax, greatly increased the rub resistance of printing ink. An early investigation of the mechanism was done by printing on a transparent substrate and rubbing with paper. The microscope was then used to study the appearance.
Some of the rub streaks caused by the paper fibers had the appearance of a flowing liquid, which was interpreted as the wax melting under pressure (Figure 1). Other streaks had a zig-zag appearance, suggesting that the PTFE might shear along crystal planes (Figure 2). It was thought that some of the properties were contributed by each mechanism.