| 摘要 |
PURPOSE:To obtain a large piezoelectric factor by a method wherein a film of the copolymer of a monomer selected out of vinyl fluoride, ethylene trifluoride, and ethylene tetrafluoride with vinylidene fluoride is so formed that the birefringence degree becomes 10X10<-3> or more and thereafter changed into an electret. CONSTITUTION:The copolymer of at least a kind of monomer selected out of vinyl fluoride, ethylene trifluoride, and ethylene tetrafluoride with vinylidene fluoride is supplied to an extrusion molding machine 1, and then the film 8 extruded out of a die 2 for sheets is drawn off by means of a draw-off roll 5 at a high speed and cooled on cooling rolls 3 and 4. At this time, conditions such as the amount of resin extruded out of the die 2, temperature for extrusion molding, cooling temperature, and draw-off speed are selected, and then the film or the sheet is so molded that the birefringence degree becomes 10X10<-3> or more. The film or the sheet formed in such a manner is changed into the electret e.g. by corona discharging method. Thus, piezoelectric factor can be increased.
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