This plastic was chosen because its atoms hold on to their electrons more weakly than the rubber atoms of the belt.
Because of this, the friction of the pulley and belt rubbing together can provide enough energy for electrons on the yellow atoms of the pulley to jump to the inner surface of the rubber belt. This leaves those plastic atoms with a net positive charge (i.e. they are then "ionized"):
Neutral pulley atoms (in yellow) =>
Positive ions left behind (in red)
+ Electrons transferred to belt (in blue)