Membranes are used everyday in a variety of critical applications including water treatment, natural gas separation, beverage clarification, and HVAC. The composition of membranes vary based on application but in general they can be categorized as organic or inorganic based on the material that the membrane is composed of. Organic membranes are typically made from natural and synthetic polymers like cellulose acetate, polysulfones, PVDF, and polyamides. Inorganic membranes typically consist of metal, ceramics, and zirconates. Selection of an appropriate membrane material depends on a variety of factors including operating conditions, feed solution, and application. The advantages and disadvantages of each membrane type and the materials they are composed of should be carefully considered.
InorganicInorganic membranes consist of metal, ceramic, glass, and other inorganic material. They are traditionally used for applications with the most challenging conditions where high chemical and thermal stability are of utmost importance. While inorganic membranes excel in durability they usually are cost prohibitive for many standard filtration applications.
Organic membranes are composed of one or more synthetic or naturally occurring polymers. These polymers include polyamides, polypropylene, PVDS, polyether sulfones, cellulose esters, and many others. The properties of organic membranes depend heavily on the polymers they are made of. Selection of polymers for organic membranes should be based on application and manufacturing methods. Organic membranes are cost efficient and make up a majority of industrial membranes. The remainder of this article focuses on selection of polymers for organic membranes.
Choosing Membrane Materials
There are a variety of different polymers that can be used for the manufacture of organic membranes including polyamides, fluorinated polymers like PVDF, polysulfones, and cellulose based polymers like cellulose esters. The properties of these polymers vary and the intended use of the membrane should be carefully considered when choosing a membrane material. The table below lists polymers used to make membranes and compares important properties for membrane performance, efficiency, and durability.
|Material||pH Range||Tensile Strength, mPa||Oxidant Resistance||Cost|
|Polysulfone & Polyester Sulfone||1-13||70-90||Moderate||Moderate|
Why Use Eastman™ Cellulose Acetate
In comparing the materials above we can see that membranes produced with cellulose acetate have an advantage over the other types in terms of cost and oxidant resistance. Another important attribute to note is that cellulose acetate is hydrophilic. The result of this is better surface contact with aqueous feed streams and increased resistance to fouling. In addition, cellulose acetate membranes can withstand harsh cleaning environments that polymers like PVDF cannot. It is for these reasons that cellulose acetate is used for organic membranes in protein separations and other applications where fouling is a concern. Membranes produced with cellulose acetate also have an inherent selectivity for CO2 and CH4. This is why it is still the leading polymer for spiral wound and hollow-fiber membrane modules used in the separation of natural gas. In addition, all Eastman™ membrane materials are available with FDA approval for food contact in food and beverage processing operations making them an ideal choice for RO and FO membranes used to clarify or concentrate beverages like juice, milk, beer, and wine. See the table below for available Eastman™ membrane material grades of cellulose acetate.
|Grade||Polymer||Molecular Weight||Tg (°C)|
|Eastman™ M203||Cellulose Diacetate||30,000||182|
|Eastman™ M210||Cellulose Diacetate||40,000||185|
|Eastman™ M230||Cellulose Diacetate||60,000||189|
|Eastman™ M300||Cellulose Triacetate||75,000||172|