Hydrophilic PVDF membrane:

Polyvinylidene fluoride is a kind of crystalline polymer. It has good chemical stability, heat resistance and mechanical stability, and can be dissolved in some strong polar organic solvents at relatively low temperature. This property makes it easy to phase conversion membrane, and this new polymer membrane material has excellent performance.

PVDF is a kind of polymer with excellent strength and chemical stability, and it is an excellent treatment material for printing and dyeing wastewater. However, because PVDF membrane is insoluble in water, there are problems in the treatment of printing and dyeing water. Therefore, it is necessary to reform its performance and improve its practicability in printing and dyeing wastewater treatment. PVDF separation membrane has been used in chemical industry, food, medicine and so on. However, the surface energy of PVDF is extremely low, and this material is so hydrophobic that the water flux after film formation is relatively low. Therefore, when applied in the separation of oil and water system (especially the solution containing protein), the adsorption pollution will be more serious, and its flux will soon decay. This reduces the service life of the membrane and increases the cost of operation, and the application of membrane separation field is therefore restricted.

Material properties:

Polyvinylidene fluoride film materials are widely used. Compared with polyethylene, polypropylene, polyvinyl chloride, etc., it has many advantages, such as acid and alkali resistance and other harsh environmental conditions, high mechanical strength, good chemical stability, high separation accuracy, high efficiency. These advantages are attributed to polyvinylidene fluoride is a kind of excellent crystalline polymer.

The molecular weight of PVDF polymer is generally 4×10^5~8×10^5, the density is about 1.78g/cm³, the glass transition temperature is -39℃, the embrittlement temperature is -62 ℃ below, the crystallization melting point is 180℃, the thermal decomposition temperature is above 316℃, the long-term use temperature range is -50~150℃, At a certain temperature and pressure can still maintain good strength (such as 0.45MPa load pressure, thermal deformation temperature of 150℃). PVDF also has excellent UV protection and climate aging resistance, and its film will not be brittle and cracking when placed outdoors for 10 to 20 years. In addition, its chemical stability is good, only soluble in dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, N-methyl pirolidone and other strong polar solvents, at room temperature is not corroded by acids, bases, strong oxidants and halogens, aliphatic hydrocarbons, alcohols, aldehydes and other organic solvents have no effect on it. Because of these outstanding characteristics, PVDF becomes an ideal separation membrane material.

Membrane forming mechanism:

The preparation method of PVDF membrane is the phase transformation method, which is simple and mainly adopts a membrane preparation method. Therefore, the membrane forming mechanism of PVDF membrane is introduced based on the phase transformation method. The preparation of PVDF membrane by phase conversion method is described below. The film-forming process of PVDF membrane is generally divided into three main stages.

Stage 1: Reverse dissolution process. During this period, the casting liquid maintained a homogeneous state, but the concentration gradient began to form in the direction of the cross-section of the film liquid. The main reasons for the formation of the general gradient are solvent evaporation, absorption of non – solvent atmosphere, solvent and non – solvent double diffusion.

Stage 2: phase separation process. As the dissolution capacity of the system continues to decline, thermodynamic equilibrium begins to lose and phase separation occurs at the same time. There are two main phase separation processes. The liquid-liquid phase separation occurs for the amorphous polymer, and the liquid-liquid phase separation and solid-liquid phase separation occur for the crystalline polymer such as PVDF.

Stage 3: The fixed-line process. It mainly includes coagulation of membrane pore, interphase flow and solidification of polymer rich phase. This stage has a great influence on the final morphology and structure of the polymer membrane, but is not the main factor affecting the pore-forming. The decisive factor is the phase separation process of the membrane liquid.