Study on Crystal Structure of Dielectric Polypropylene Films
The dielectric polypropylene film is not only the core of dry DC capacitor,but also the"key"of new energy power generation and flexible DC transmission,which directly influences both quality and safety of electric energy conversion.The crystal structure of polypropylene film plays decisive role in the perfor-mance of capacitor.In this paper,the crystal structure of the dielectric film of capacitor(BOPP),which is made of three kinds of polypropylene pellets by way of casting bi-axially drawing,is studied by such charac-terization method as wide-angle X-ray diffraction,Fourier transform infrared spectroscopy and differential scanning calorimetry.It is found by the characterization result that three kinds of BOPP dielectric films are predominantly composed of α-crystal structures.However,there are notable differences among the films in terms of their crystallinity,melting temperature and the crystallite size in various orientations.The melting point of dielectric film based on Borealis grains is minimum(169℃),the degree of crystallinity is highest(59.8%)and the crystals size along the[060]direction is maximum.The result of infrared spectrum shows that in BOPP species based on Borealis grains the 31 helical chain segments with sequence lengths n=14 is maximum.It is found by the study of specimens after melting recrystallization of BOPP film that the crysta-linity of three kinds of grains is almost identical(about 53%).However,the non-isothermal crystallization temperature of Borealis PP grains is highest(119℃)and obvious a β-phase melting peak is occurred dur-ing the melting period.In comparison,non-isothermal crystallization temperature of Sinopec PP grains is the lowest,which is 3℃less than that of the Borealis grains.
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