Thin-film composite membrane
Thin-film composite membranes (TFC or TFM) are semipermeable membranes manufactured principally for use in water purification or water desalination systems. They also have use in chemical applications such as batteries and fuel cells. A TFC membrane can be considered as a molecular sieve constructed in the form of a film from two or more layered materials.
TFC membranes are commonly classified as nanofiltration (NF) and reverse osmosis (RO) membranes. Both types are typically made out of a thin polyamide layer (<200 nm) deposited on top of a polyethersulfone or polysulfone porous layer (about 50 microns) on top of a non-woven fabric support sheet. The three layer configuration gives the desired properties of high rejection of undesired materials (like salts), high filtration rate, and good mechanical strength. The polyamide top layer is responsible for the high rejection and is chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small, unfilterable molecules.
The first viable reverse osmosis membrane was made from cellulose acetate as an integrally skinned asymmetric semi-permeable membrane. This membrane was made by Loeb and Sourirajan at UCLA in 1959 and patented in 1960. The current generation of reverse osmosis (RO) membrane materials are based on a composite material patented by FilmTec Corporation in 1970 (now part of Dow Chemical Company).
Structure and materials
As is suggested by the name, TFC membranes are composed of multiple layers. Membranes designed for desalination use an active thin-film layer of polyamide layered with polysulfone as a porous support layer.
Other materials, usually zeolites, are also used in the manufacture of TFC membranes.
Thin film composite membranes are used in
- Water purification;
- as a chemical reaction buffer (batteries and fuel cells);
- in industrial gas separations.
Thin film composites membranes typically suffer from compaction effects under pressure. As the water pressure increases, the polymers are slightly reorganized into a tighter fitting structure that results in a lower porosity, ultimately limiting the efficiency of the system designed to use them. In general, the higher the pressure, the greater the compaction.
Surface fouling: Colloidal particulates, bacteria infestation (biofouling).
Chemical decomposition and oxidation.
A filtration membrane's performance is rated by selectivity, chemical resistance, operational pressure differential and the pure water flow rate per unit area.
Due to the emphasis on flow rate, a membrane is manufactured as thinly as possible. These thin layers introduce defects that may affect selectivity, so system design usually trades off the desired flow rate against both selectivity and operational pressure.
In applications other than filtration, parameters such as mechanical strength, temperature stability, and electrical conductivity may dominate.
Active research areas
Nano-composite membranes (TFN). Key points: multiple layers, multiple materials.
New materials, synthetic zeolites, etc. to obtain higher performance.