The characteristic of the composite membrane is that it is mainly made of the above two materials, which is a composite of a very thin dense layer and a porous support layer. The porous support layer is also called the base film, which enhances the mechanical strength; the dense layer is also called the skin layer, which plays a role of desalination, so it is also called the desalination layer. The thickness of the desalination layer is generally 50 nm, and the thinnest is 30 nm.
The asymmetric membrane made of a single material has the following shortcomings:
1. There is a compacted transition layer between the dense layer and the support layer.
2. The thinnest limit of the skin layer thickness is 100nm, it is difficult to reduce the pushing pressure by reducing the film thickness.
3. The desalination rate and the water permeation rate are mutually restricted, because the same material is difficult to have both desalination and support.
The composite membrane solves the above problems well. It can select materials with good desalination performance and materials with high mechanical strength according to the requirements of the dense layer and the support layer. Therefore, the dense layer of the composite membrane can be made very thin, which is beneficial to reduce the drag pressure; at the same time, the transition zone is eliminated, and the compression resistance performance is good.
The material of the base membrane is polysulfone, followed by polypropylene and polyacrylonitrile. Because polysulfone is cheap and easy to obtain, the membrane is simple, the mechanical strength is good, the compression resistance is good, the chemical performance is stable, non-toxic, and biodegradable.
To further enhance the strength of the porous support layer, polyester non-woven fabrics are commonly used.
