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| - For the purpose of enhancing wear resistance of the protective layer 21, fluorine resins such as polytetrafluoroethylene, silicone resins, or such inorganic materials as titanium oxide, aluminum oxide, tin oxide, zinc oxide, zirconium oxide, magnesium oxide, silica, and surface treated inorganic materials may be added, and it is further possible to add an electric charge transporting substance.
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