biaxial stretching of particle filled films
Manufacture of biaxially stretched films
Due to the biaxial stretching, polymer films in the lower micrometer range can be produced on a large industrial scale and with high cost-effectiveness. The mechanical, dielectric, optical properties and gas permeation behaviour can be decisively influenced by the stretching process. The generated microstructure of the polymer film is decisive for this. Especially in semi-crystalline polyolefins such as LDPE and PP, the typical spherolite structure is converted into a fibrist-like morphology. By adding suitable fillers, these properties can be further improved. In addition to the possibility of producing blown film, the complete process chain for the production of bixially stretched flat films is available at the LSP Erlangen.
Starting from additives and compounding, processing to cast film via twin-screw extruders and wide-slit nozzles to stretching on a Karo IV stretch frame from Brückner Maschinenbau GmbH & Co. KG is also possible. Extensive analysis methods are available at the LSP for each of these process steps. For example, in addition to film thickness, film roughness can also be determined by laser scanning microscopy or laser profilometry. The mechanical properties are accessible by tensile test, the dielectric properties by means of dielectric strength measurement and impedance spectroscopy. In addition, the morphology can be analyzed by means of differential calorimetry and scanning electron microscopy as well as the process-related degradation by means of (high-temperature) gel permeation chromatography.
Nanocomposite as an innovative dielectric for film capacitors
Biaxially stretched polypropylene is currently one of the most widely used polymeric materials for dielectrics in large-scale film capacitors due to its low dielectlosses and high dielectric strength. In recent years, the penetration rate has been increased to over 700 kV/mm by the development of highly isotactical polypropylene grades. Polymer nanocomposites have the potential to further increase these losses at low losses. Due to the interaction of the polymer chains with the nanoparticle surface or an applied functionalization, the polymer matrix is disturbed in an area around the nanoparticles, a so-called “interphase” is generated. Defects in the electronic structure of the polymer at the transition between the interphase and the matrix can capture and fix free charge carriers locally. As a result, they can no longer obtain energy from the electric field of the capacitor. A failure of the capacitor due to elective penetration is thus triggered at the same film thickness first at higher voltages.
However, industrial application is complicated by the strong agglomeration tendency and the difficult dispersion of nanoparticles, as well as their nucleusing effect and a correspondingly changed microstructure. Since the entire process chain is present at the LSP from compounding to biaxial stretching, suitable polymer nanofill combinations are currently being determined in a conveyor project with industrial participation and the influences of the individual process step for the final film properties.