A mechanism is proposed for the transfer of charge from a metal to a polymer which leads to a natural explanation of the (previously unexplained) linear relationship between the charge density and the metal work-function. In the simplest case of a single non-sliding contact it is found that the charge density does not depend on the metal. But if the contact is repeated many times, or if the metal slides on the plastic, the final charge density varies linearly with the metal work-function. In a single (non-sliding) contact electrons are believed to tunnel from the metal into trapping levels a short distance from the interface; thermodynamic equilibrium is not achieved because the difference in electrostatic potential across this thin layer of charge is not sufficient to raise the energy of the trapping levels up to the Fermi energy of the metal. But repeated contacts (or sliding contacts) transfer charge to greater depths and make equilibrium possible. This hypothesis predicts an equilibrium charge density linearly dependent on the metal work-function.