Introduction of reverse osmosis membrane materials

Membrane element

According to the needs of desalination, after a large number of research experiments, two types of membrane materials, cellulose acetate (CA) and aromatic polyamide, were screened out from a large number of polymer materials.
In addition, the skin layer of the composite membrane also uses some other special materials.

Cellulose acetate
Cellulose acetate is also called acetyl cellulose or cellulose acetate. Often use cellulose-containing cotton, wood, etc. as raw materials, undergo esterification and hydrolysis to produce cellulose acetate, and then process it into a reverse osmosis membrane.

Polyamides include aliphatic polyamides and aromatic polyamides. In the 1970s, aliphatic polyamides were mainly used, such as nylon-4, nylon-6 and nylon-66 membranes;
At present, the most used is the aromatic polyamide film. Membrane materials are aromatic polyamide, aromatic polyamide-hydrazide and some nitrogen-containing aromatic polymers.
The pH range of aromatic polyamide membrane can be as wide as 2~11, but it is very sensitive to free chlorine in water.

Composite film

The feature of the composite membrane is that it is mainly made of the above two materials, and it is composed of a very thin dense layer and a porous support layer. The porous support layer, also known as the basement membrane, plays the role of enhancing the mechanical strength; the dense layer is also called the epidermis, which plays the role of desalination, so it is also called the desalination layer. The thickness of the desalination layer is generally 50nm, and the thinnest is 30nm.

Asymmetric membranes made of a single material have the following disadvantages:

  1.  There is a compacted transition layer between the dense layer and the support layer.
  2.  The thinnest limit of 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 permeability rate are mutually restricted, because it is difficult for the same material to be excellent in both desalination and support.

The composite membrane solves the above problems well, and it can select materials with good desalination performance and high mechanical strength according to the requirements of the dense layer and the support layer, respectively. Therefore, the dense layer of the composite membrane can be made very thin, which is beneficial to reduce the dragging pressure; at the same time, the transition zone is eliminated, and the compression resistance is good. Polysulfone is the most common material of the base membrane, followed by polypropylene and polyacrylonitrile. Because polysulfone is cheap and easy to obtain, easy to make membrane, good mechanical strength, good compression resistance, stable chemical performance, non-toxic, and resistant to biodegradation.
In order to further enhance the strength of the porous support layer, polyester non-woven fabrics are commonly used.
The material of the desalination layer is mainly aromatic polyamide. In addition, there are piperazine amide, propylene-alkyl polyamide and condensed urea, furfuryl alcohol and trihydroxyethyl isocyanate, m-phenylenediamine and trimesoyl chloride, etc.

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Further reading

Filmtec RO membrane bw30365

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