Glow-in-the-dark plastic contains a dispersed substance that absorbs light, and then slowly re-emits it. The correct scientific name for this process is phosphorescence. The phosphorescent substance used for Maozhang is most likely copper activated zinc sulphide (in use since the 1930s), although they could contain strontium aluminate (generally used only for industrial signage). When "charged" by expose to sunlight or electric light, the plastic glows in darkness for periods from a few minutes to a few hours.
Glow-in-the-dark plastic was used in two distinct ways on Maozhang. Either the Chairman's profile portrait glowed in the dark; or it was the background that glowed, with the Chairman remaining a dark silhouette.