Manufacturing Membranes with Eastman™ Cellulose Acetate
Membrane Filter Materials
Membranes are used every day to provide filtration in a variety of applications critical to industry and infrastructure, including water treatment, natural gas separation, beverage clarification, and HVAC. The composition of filtration membranes varies based on application, but generally, they can be categorized as organic or inorganic based on the material the membrane filter is composed of. Organic filtration membranes are typically made from natural and synthetic polymers, such as cellulose acetate, polysulfones, PVDF, and polyamides. Inorganic filtration membranes are usually made of metal, ceramics, and zirconates. Selecting an appropriate membrane filter material depends on various factors, including operating conditions, feed solution, desired pore size, and application. The advantages and disadvantages of each filter type and the materials they are composed of should be carefully considered.
Membrane Filter Types
Inorganic
Inorganic filters are made of metal, ceramic, glass, or other inorganic materials that possess characteristics of high durability in challenging conditions. These filters are traditionally used for applications where high chemical and thermal stability are of utmost importance. While inorganic membrane filters excel in durability, they are usually cost prohibitive for many standard filtration applications. Furthermore, inorganic filters can only be manufactured in relatively large pore sizes and are typically only used for microfiltration or ultrafiltration applications.
Organic
Organic membrane filters are composed of one or more synthetic or naturally occurring polymers. These polymers include polyamides, polypropylene, PVDF, polyether sulfones, and cellulose esters. The characteristics of organic membranes depend greatly on the polymers they are made of. For example, the surface energy and hydrophobicity of the polymer should be considered to determine whether water or an organic solvent will be filtered. Polymer selection for organic membrane filters takes into account the application and the available filter manufacturing methods. Organic membranes are highly cost-efficient and make up the majority of industrial membranes used today. They are also made with various pore sizes that facilitate filtration processes all the way from microfiltration to nanofiltration to reverse osmosis. The remainder of this article focuses on polymer selection for fabricating organic membrane filters.
Choosing Membrane Materials
As mentioned, a variety of polymers can be used to manufacture organic membranes, such as polyamides, fluorinated polymers like PVDF, polysulfones, and cellulose-based polymers like cellulose acetate. The physical characteristics of these polymers vary, and the intended use of the membrane should be considered when choosing a membrane material. The table below lists polymers used to make organic membranes and compares important properties for filter durability, performance, and efficiency.
Material
pH Range
Tensile Strength (mPa)
Oxidant Resistance
Cost
Cellulose Acetate
4-9
30-60
High
Low
Polyamides
2-11
130-150
Low
High
Fluorinated Polymers
2-10
30-60
High
High
Polysulfone & Polyester Sulfone
1-13
70-90
Moderate
Moderate
Why Use Eastman™ Cellulose Acetate
When considering material selection for water-based solution filtration, cellulose acetate is a top option. An advantage can be observed in comparing the materials above, where membranes produced with cellulose acetate improve cost and oxidant resistance. Cellulose acetate and cellulose acetate membrane filters are also very hydrophilic, achieving better surface contact with aqueous feed streams and increased resistance to fouling by proteins and other organic material. This equates to obtaining better flow rates and service life from cellulose acetate membranes.
In addition, cellulose acetate membranes can withstand harsh cleaning protocols using oxidizers like chlorine that polymers like polyamides cannot withstand. Thus, cellulose acetate membranes can be used for conducting protein separations and other applications where the material must be disinfected. Membranes produced with cellulose acetate also have an inherent selectivity for CO2 and CH4. This is why cellulose acetate is still one of the leading polymers for spiral-wound and hollow-fiber membrane modules used in separating natural gas.
Eastman Cellulose Acetate Membrane Materials
Membrane manufacturers have counted on cellulose acetate from Eastman Chemical Company for decades to make high-quality filters for liquid and gas separation. Eastman's cellulose acetate, like other cellulose esters that Eastman manufactures, is derived from cellulose that originates from sustainably managed forests.
All Eastman membrane materials are available with FDA approval for food contact in food and beverage processing operations, making them an ideal choice for microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and forward osmosis membranes used to clarify or concentrate beverages such as juice, milk, beer, and wine.
Eastman manufactures several forms of cellulose acetate with varying degrees of substitution and molecular weight to fit requirements for different manufacturing methods and membrane applications. See the table below for Eastman membrane material grades of cellulose acetate.
