Composite membranes and pure membranes belong to the same category of membranes, but they also have essential differences, composite membranes are a kind of membranes composed of multiple layers or materials with different functions, these layers are combined to form characteristics with reinforcing properties, or with special requirements, such as hydrophilic, hydrophobic, anti-corrosion, etc., with a porous support layer that provides mechanical strength and a selective layer with specific separation, filtration or permeation properties. The structural advantage of the composite is that it allows the respective properties to be optimized by using different materials in different layers. Pure membrane, also known as homogeneous membrane, is a membrane made of a single material, it has no additional layers or additives, its manufacturing method is also formed by a single-step process, has the ability to form a porous structure, pure membranes have homogeneity of the material composition in the entire membrane structure, they exhibit specific properties inherent in the materials used, such as chemical resistance, selectivity, or permeability, etc.
The manufacturing methods of composite film and pure film are different due to different properties.
Methods for the production of composite microporous membranes include:
1. Interfacial polymerization, which is to dissolve two or more monomers or polymers in separate solvents, and the solution is in contact at the interface, and a chemical reaction occurs, thus forming a thin selective film layer on the support layer.
2. The method of coating or solution casting is mainly to coat or impregnate on a preformed porous support layer with a solution containing the required selective material, and then dry or solidify the solution, which will form a thin selective film layer on the carrier.
3. The method of layer-by-layer assembly, it is to deposit alternate layers of different materials on the support layer, as the material of the support layer can be made by methods such as dip coating, spraying or spin coating, and the advantage of the layer-by-layer assembly method is that the structure and performance of the film can be precisely controlled.
4. Electrospinning method, which is a polymer solution or melt electrostatic pull into a fine fiber shape, so as to form a film layer of non-woven pad, the way of electrospinning fiber can be layered or combined with other materials to form a composite film with specific properties.
Methods for producing pure membranes include:
1. The method of phase conversion, phase conversion is a common method for the production of pure membranes, that is, the polymer is dissolved in a solvent, and then the polymer is formed through controlled cooling, immersion in a non-solvent or evaporation, resulting in the phase separation of the solvent, thus forming a porous membrane.
2. Mechanical stretching or orientation, the method of production by stretching or orientation polymer film or sheet, the process aligns the polymer chain to form a porous structure with specific properties, of course, the stretching method can be carried out according to the needs of uniaxial or biaxial stretching, etc., according to the actual needs, the pore size and structure can be effectively controlled.
3. The method of track etching, track etching is a technology that uses high-energy particles to impact the surface of polymer film to form a track or channel, this method can selectively etch tracks to form pores, this production method can accurately control the pore size, and the pore size is uniform, the shape is optional, and it is a very accurate film making technology.
4. The method of solvent casting, solvent casting is to dissolve the polymer in the solvent to form a solution or dispersion, and then the solution is poured onto the substrate or support and dried to form a solid film with a porous structure.
In production applications, composite and virgin membranes are often used in applications that require different combinations of properties. They can be used in water treatment, gas separation, biotechnology and pharmaceutical, for example, in nanofiltration or reverse osmosis processes to achieve high selectivity and retention of specific solutes; While pure membranes are often used in applications that require specific properties, such as PVDF or PES (polyethersulfone) membranes, pure polymer membranes are used in microfiltration or ultrafiltration processes for efficient separation of particles or biomolecules without the need for additional functionality.
The specific choice between composite microporous membranes and pure membranes depends on the desired performance and application requirements, as both types of membranes have their advantages and both can be customized with different production methods or processes to meet specific filtration and separation needs.