Cellulose acetate membranes characteristics

Our Investigation on durability and membrane characteristics changes under adverse conditions have much contributed to development of RO applications. Among these applications are those for ultra-pure water in electronic and pharmaceutical industries. Even under the circumstance of pH 7 and with 2 to 4 times per year of sterilization by H2O2 of as high as 1 %, the cellulose acetate membrane proved to show membrane life of more than 3 years. 

In other respects, it also shared the favorable characteristics of the NS-100 membrane. That is, it was resistant to pH 3 to 12, and showed far better compaction resistance than cellulose acetate. Also, it possessed the capability to operate at elevated temperatures, though some irreversible flux decline could still occur.34 Rejections of various organics were also good, as shown in Table 5.1 These were in sharp contrast to organic rejection data on cellulose acetate membranes. Initially, PA-300 was also postulated to possess good chlorine resistance.31 Subsequent experience showed it to be equally sensitive to chlorine as NS-100. 

These organic rejections, while greatly exceeding the capabilities of cellulose acetate membranes, are not appreciably different than rejection levels exhibited by aromatic polyamide membranes (FT-30, Permasep B-9). Nor do they match the organic rejection characteristics of the PEC-1000 membrane. The Solrox membrane is not resistant to chlorine, and its water flux is somewhat low (about 25 gfd at 650 psi net driving pressure). Consequently, it has not become a significant competitive membrane in the world marketplace. 

Sterile filtration of liquids and gases is now virtually always done using mem brane filters. The first U.S. patent for membrane filters was filed in 1922 and pertained to cellulose acetate membranes. A wide range of membrane filter media are now commercially available to suit various applications cellulose esters, polyvinylidinefluoride, polytetrafluoroethylene (PTFE), and polyhexam-ethyleneadipamide (nylon 66), separately or as laminates with polyethylene, polypropylene, and polyester for more robust phy.sicai characteristics. 

Semipermeable membranes and hollow fibers are produced from cellulose acetate. Dry-jet wet-spinning techniques are described to provide asymmetric and homogeneous hollow fiber membranes. Manipulation of spinning conditions leads to morphologies that permit higher rejection and higher fluxes. The excellent balance of the hydrophobic-hydrophilic characteristics for cellulose acetate makes this polymer useful for reverse osmosis [89-93]. Cellulose acetate membranes and hollow fiber membranes are commercially available for hemopurification. [94], for ultrafiltration [95], and for other commercial separation processes. 

This is widely used today. The characteristics of the membrane will vary with the extent of acetylation, the prewashing procedure employed by the manufacturer, and the additives used, as well as the pore size and the thickness of the membrane. Cellulose acetate membranes may be made transparent (cleared) for densitometry by treatment with a solvent mixture that partially dissolves the cellulose acetate fibers and eliminates the original air spaces. Advantages are the speed of separation (20 min-1 h) and the ability to store the transparent membranes for long periods. 

The mechanisms of formation and the structural characteristics of cellulose acetate membranes for reverse osmosis have been investigated. Scanning electron microscopy studies showed the membranes to be high molecular weight condensation structures of the cellular type. 

A rapid method for determining the amylase isoenzymes in human sera and urine is based on separation of the isoenzymes by electrophoresis on a cellulose acetate membrane and visualization using a Blue Starch-agar plate. " The two isoenzymes which are separated possess the same characteristics as salivary and pancreatic amylases. The relative proportions of the isoenzymes are useful in distinguishing hyperamylasemias arising from disorders of the pancreas and parotid gland, etc. 

Polybenzimidazolone membrane 21 developed by Teijin Ltd. had the following permeative characteristics Water permeation, 840 1/m2 - day salt rejection, 99.5% (1% NaCl aqueous solution, 80 kg/cm2)69). The membrane was less sensitive to plasticization with water than cellulose acetate and aromatic polyamide membranes... 

This so-called "active" layer has characteristics similar to those of cellulose acetate films but with a thickness of the order of 0.1 micrometer (jjm) or less, whereas the total membrane thickness may range from approximately 75 to 125 ym (see Figure 1). The major portion of the membrane is an open-pore sponge-like support structure through which the gases flow without restriction. The permeability and selectivity characteristics of these asymmetric membranes are functions of casting solution composition, film casting conditions and post-treatment, and are relatively independent of total membrane thickness. 

Membranes for Reverse Osmosis. The first commercially successful membrane was the anisotropic or asymmetric structure invented by Loeb and Sourirajan (1960 cited by Sourirajan, 1970). It is made of cellulose acetate and consists of a dense layer 0.2-0.5 jjim diameter. The thin film has the desired solute retention property while offering little resistance to flow, and the porous substructure offers little resistance to flow but provides support for the skin. The characteristics of available membranes for reverse osmosis and ultrafiltration are listed in Tables 19.2 through 19.4. 

The hollow fibre is the most crucial part of the microdialysis probe. It acts as a membrane, and its characteristics affect performance in the sampling step as well as the probe s suitability for the selected application. Hollow fibres are commercially available in different materials, the most common being polycarbonate (PC), regenerated cellulose (Cuprophan, CU), cellulose acetate (CA), polyacrylonitrile (PAN), polyethersulphone (PES), polysulphone (PE), and polyamide (PA). Generally, the fibres have an outer diameter between 200... 

Membranes used for NF are made of cellulose acetate and aromatic polyamide with characteristics such as salt rejections from 95% for divalent salts to 40% for monovalent salts and an approximate MWCO of 300 for organics. An advantage of NF over RO is that NF can typically operate at higher recoveries, thereby conserving total water usage due to a lower concentrate stream flow rate. NF is not effective on small-molecular-weight organics, such as methanol. 

Membrane material. During the early days, membranes were usually made of cellulose acetate. At present, membranes can also be made from aromatic polyamide and thin-filmed polymer composites. Different membrane materials have their own distinctive characteristics, such as hydraulic resistance, pH range, temperature range, chlorine tolerance, and biodegradation tolerance. 

Cellulose acetate is a thermoplastic resin made by treating cellulose with acetic anhydride to acetylate the hydroxyl groups. The membranes contain about 80% air space within the interlocking cellulose acetate fibers and come as dry, opaque, brittle films that crack easily. When the film is soaked in buffer, the air spaces fill with liquid and the film becomes pliable. Membrane characteristics depend upon the extent of acetylation, the prewashing procedure used by the manufacturer, the additives used, the pore size, and the thickness of the membrane. 

---