NOVELTY - The electret (E) film comprises a coextrusion composite material comprising a middle layer such as water proof antitensile layer, an upper layer such as adhesive barrier layer and a lower layer such as antiaging layer. The coextrusion composite material comprises medium density polyethylene, linear low density polyethylene and high density polyethylene. The upper and lower layers are bonded to a resistance layer and comprise linear low density polyethylene and low density polyethylene.    USE - The E film is useful for solar back plate.    ADVANTAGE - The E film exhibits excellent chemical resistance and weather resistance, high mechanical strength, good thermal stability, high bonding property and improved compatibility, and effectively blocks UV rays thus preventing the solar back plate from solar degradation.    DETAILED DESCRIPTION - The electret (E) film comprises a coextrusion composite material comprising a middle layer such as water proof antitensile layer (1), an upper layer such as adhesive barrier layer (2) and a lower layer such as antiaging layer (3). The coextrusion composite material comprises medium density polyethylene, linear low density polyethylene and high density polyethylene. The upper and lower layers are bonded to a resistance layer and comprise linear low density polyethylene and low density polyethylene. The middle layer further comprises an additive comprising a titanium dioxide (5 wt.%), fluorine additives (3 wt.%) and antiaging agent (5 wt.%). The upper and lower layer further comprises a source of fluorine additive and an antiaging agent. The mass ratio of medium density polyethylene, linear low density polyethylene and high density polyethylene is 1:1:1. The mass ratio of linear low density polyethylene and low density polyethylene is 1:1. The thickness of the upper layer, the middle layer and the lower layer is 1:2:10. The solar back plate E film is formed by an extrusion casting.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an electret film.    Water proof antitensile layer (1)    Adhesive barrier layer (2)    Antiaging layer. (3)