Preparation Of PVDF Microporous Membrane By Green Thermal Phase Separation

Poly(vinylidene fluoride) (PVDF) has been widely used in microfiltration, ultrafiltration, membrane bioreactors, membrane distillation, gas separation, pollutant removal from water, recovery of biofuels, separation for lithium ion batteries and ion exchange processes due to its outstanding properties, such as high thermal stability, good chemical resistance and membrane forming properties. The PVDF membranes including sheets, hollow fibers and tubes, can be fabricated by immersion precipitation and thermally induced phase separation (TIPS). Whereas, large amounts of organic solvents have been used to prepare PVDF membranes via NIPS or TIPS. Unfortunately, environmental pollution and raw-material wastes are obvious and urgent issues due to the difficultly in recycling organic solvents.Recently, the use of ionic liquids (ILs) as a class of green solvents has become widely popular as a replacement for the conventional volatile solvents, which are environmental pollutants or even toxic. ILs has plenty of unique and interesting properties, such as their negligible vapor pressure. They are also non-flammable, non-explosive, have excellent chemical and thermal stability, excellent solvent properties and high ionic conductivity. Moreover, they can be recycled and reduce environmental pollution in this respect.PVDF membranes were prepared by TIPS technology. In this study, PVDF was used as membrane material and ILs was used as diluents to replace organic solvent. The membrane formation mechanism of thermal induced phase melt process for preparation of PVDF membrane was researched, and the membrane microporous structure design, construction, reorganization and optimization mechanism were explored in the membrane making process. The membrane formation mechanism of green thermally induced phase melt membrane technology with ionic liquids as diluents for preparation of PVDF membrane was studied in order to provide theoretical basis and technical support for preparation of high performance polymer membrane.PVDF hollow fiber membrane and flat membrane were prepared by TIPS with four kinds of ionic liquids as the diluents respectively. Hollow fiber membranes and flat membrane were characterization thermogravimetry, scanning electron microscope (SEM) observation, crystallization, mechanical properties and water flux test and analysis. It is concluded that the 1 - butyl 3 - four fluoboric acid methyl imidazole salt ([BMIm]BF4) is the ideal diluents. And then focus on [BMIm]BF4 is used as a green environmental diluent to prepare PVDF hollow fiber membranes and flat membrane via TIPS melt technology. A phase diagram of PVDF/[BMIm]BF4 mixtures was presented, and the morphologies as well as properties of the PVDF membranes were investigated. The study results showed that the membrane porosity, pure water flux and elongation at break decreased as the increase of polymer concentration, while the membrane strength increased correspondingly. The pure water flux of the flat membrane is higher than the hollow fiber membrane. PVDF crystalline behavior in different polymer concentration was investigated by differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR). Experiments found that in the system, ionic liquid has no effect on it. [BMIm]BF4 as diluent for the preparation of PVDF microporous membrane can improve the hydrophilicity of membrane. Minimum of PVDF microporous membrane contact angle can be to 70.2°.