The Ultrasonic Atomization Spray Technology Of Porous Polymer Membranes

Porous membrane is widely used in nano-materials, bio-medical, new energy materials and other fields. In this paper, the preparation method of the polymer porous membrane （thermally induced phase separation method,immersion precipitation phase conversion method, solvent evaporation method and so on） is briefly introduced. In order to apply the ultrasonic atomization spraying technology to the field of porous membrane preparation, the phase separation mechanism of the porous structure and the effect of ultrasonic atomization on the enhanced phase conversion were analyzed. We select the appropriate ternary polymer solution system, polymer / solvent / non-solvent （cellulose acetate （CA） / acetone / water）,the CA porous membrane was successfully prepared by adjusting the ratio of polymer and non-solvent. When the ratio of non-solvent to polymer reaches 4: 1 and above, CA film becomes porous. In addition, we found that we can adjust the final film diameter by adjusting the nozzle height in a certain range, membrane aperture and nozzle height is inversely proportional. The results of scanning electron microscopy （SEM） show that the surface and cross section of CA film prepared by ultrasonic spraying method is uniform in pore size, and the pore size is in the submicron scale, which overcomes the shortcomings of the film prepared by casting method and immersion precipitation method. The film was tested for thermal stability, liquid absorption and porosity, and both showed excellent results. The ion conductivity of the CA membrane gel electrolyte was about 3.78 x 10^-3S/cm, which was higher than that of the PP membrane gel electrolyte (2.2 x 10^-4S/cm).In the process of the preparation of the porous membrane, the droplet size is directly related to the pore size. Based on the CA / acetone / water ternary system, the qualitative relationship between process parameters, solution physical parameters and atomized droplet size was established. The droplet size increases with the increase of ultrasonic power, feed rate, solution surface tension and density, and decreases with the increase of frequency and solution viscosity. As the power increases, the droplet size distribution becomes wider and the peak size of the droplet size becomes larger, and the excessively high atomization power increases the droplet distribution range and the droplet size becomes more uniform. While the viscosity and surface tension as two different solution physical parameters, they effects the droplet size independently. And we finally get the droplet size prediction formula:d = 1.149f^-2/3Q^0.535σ^-1.456ρ^5.598η^0.462P^1.715In order to apply the ultrasonic spraying technology to the field of lithium ion battery separator, I fabricate PVDF-HFP porous membrane based on PVDF-HFP / acetone and tetrahydrofuran / n-butanol system. Its porosity is above 50%, the impedance is small, the ionic conductivity in the 10^-3 S/cm level, the battery cycle performance is good. Therefore,the ultrasonic spray method is suitable for the promotion of lithium ion battery separator preparation.