IE47585B1 - Flexible microporus cellulosic membranes and methods of formation and use thereof - Google Patents

Abstract

Microfiltration membranes comprise 10% to 50% cyanoethylated polysaccharide ether polymers blended with 50% to 90% polymers of cellulose nitrate, cellulose acetate or mixtures of the two. These membranes are flexible and strong and can be readily pleated for use in cartridge filters as well as used flat in conventional single- or multi-plate holders. They are also more thermally stable than conventional "mixed ester" (cellulose nitrate/cellulose acetate) membranes. Membranes are formed by wet or dry phase inversion casting.

Description

The invention herein relates to cellulosic membranes. More particularly, it relates to those membranes formed from cellulose derivatives and having pore sizes and configurations suitable for use in microfiltration processes.

Cellulose and its derivatives are well known and have been described 1n numerous books and publications. See, e.g., Ott et al, eds., Cellulose and Cellulose Derivatives, High Polymers, Vol. V, especially Section IX {Interscience, 1954) and Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 4, pages 593-683 (2d edn., 1967). Microporous membranes composed of cellulose derivatives, primarily cellulose nitrate and/or cellulose acetate are also well known and have been extensively described and studied; see Resting, Synthetic Polymeric Membranes (McGraw-Hill, 1971), especially Chapters 1, 2 and 5. Such membranes find utility in a wide variety of laboratory and commercial processes involving microfiltration.

Microfiltration is a membrane filtration process intended to effect filtration of particles having a size in the range of about 0.1 to about 10 micrometers. Microfiltration is to be distinguished from two other types of processes which also utilize semipermeable membranes for separation: ultrafiltration and reverse osmosis. Ultrafiltration operates at a significantly lower particle size range than microfiltration and is intended for filtration of individual polymer molecules. Reverse osmosis operates at yet lower particle size ranges and is used to filter individual Ions. Membranes intended for use in microfiltration have pores which are visible under normal light magnification, while those intended for use in reverse osmosis and ultrafiltration do not show visible pores. (In the past ultrafiltration and microfiltration have sometimes been considered to be synonymous, but current filtration practice and theory now clearly distinguish between the two, such that the membrane materials utilized for microfiltration are not considered to be equivalent to those used for ultrafiltration. See Resting, op. cit., Chapter 1.

Although the cellulose derivative membranes such as the cellulose nitrate and cellulose acetate mixed ester membranes have been used widely for a number of years, they suffer from two serious deficiencies: they are very brittle and they have little thermal stability. They are therefore very susceptible to breaking during handling and must be handled with great caution. Sterilization through autoclaving is obtained only with some difficulty because of the brittleness and low thermal stability. Their brittleness also of course precludes their being folded and therefore they cannot be pleated to make them suitable for use 1n cartidge filtration units. There have been attempts to alleviate the brittleness problem by Incorporating plasticizers into the compositions but this technique has not met with marked success.

Ethylcellulose has been found to confer some increase in strength to cellulose nitrate membranes but such membranes are susceptible to dissolution in alcohol which causes blockage of the pores.

It would therefore be highly desirable to have a membrane which would have pores of a size appropriate for use in microfiltration processes and yet be strong, flexible, unaffected by the chemical nature of common filtrates, and capable of being autoclaved for· sterilization.

The invention herein is a novel membrane for microfiltration which comprises a combination of cyanoethylated polysaccharide ether polymers and polymers of cellulose nitrate, cellulose acetate, or mixtures of cellulose acetate and cellulose nitrate polymers. In the present membrane the cyanoethylated polysaccharide ether polymers will be present as from 10% to 50% by weight of the composition with the balance of the composition being the cellulose nitrate and/or cellulose acetate polymers.

The ethers useful in the present Invention are the cyanoethylated ethers of homopolysaccharides, notably cellulose and chitosan. Preferred among these is cyanoethylcellulose. Cellulose, chitln and the related homopolysaccharides having reactive hydroxyl groups are known to react in the presence of a base with acrylonitrile to form cyanoethylated polysaccharide ethers. Reactions of this type have been described in the Ott et al reference mentioned above, particularly 1n Section IX-E-9. Consequently, cyanoethylcellulose may be formed by the reaction of acrylonitrile with cellulose in the presence of a base such as dilute sodium hydroxide, while cyanoethylcbitosan may be similarly formed.

The novel membranes of the present invention are formed by blending together the cyanoethylated polysaccharide ether polymers and the polymers of cellulose nitrate, cellulose acetate, or a mixture of cellulose acetate and cellulose nitrate. In the blend of this composition the cyanoethylated polysaccharide ether polymers will be present as from about 10% to 50% by weight of the composition, while the cellulose nitrate and/or cellulose acetate polymers will be present 10 as the balance (90% to 50% by weight) of the composition. Whether cellulose nitrate, cellulose acetate, or a mixture of the two types of polymers will be preferred for blending with the ether polymers will be determined by the particular application for which the finished membrane is ultimately to be used, and will be readily determined by those skilled in the art. For most purposes, however, it will be found that the cellulose nitrate polymers will be preferred.

The membranes of the present invention are formed by phase inversion casting in either wet or dry systems in accordance with conventional techniques of the prior art. In such techniques the 2θ membranes are cast from a slurry of the polymers and pore forming agents (nonsolvents) dispersed 1n suitable solvents. The phase inversion process is described 1n detail in Resting, op. cit., Section 5.1 and those details need not be repeated here. The phase inversion casting process may be carried out under wet or dry conditions.

These designations refer to the medium 1n which the final solvent removal after phase inversion is carried out. In the commonly used' dry process the final solvent removal is carried out in the presence of air or other gaseous medium which is chemically inert to the membrane. In a wet process the final solvent removal is carried out in a liquid solution which is also inert chemically to the membrane.

The phase inversion casting process may be carried out under any convenient conditions of the type described in the aforementioned Resting reference. The membrane Is cast onto an appropriate substrate commonly in ambient air and at a temperature of approximately 20°C to °C. Membrane thickness is controlled by use of a doctor blade. The particular pore size desired will be determined by control of the reaction conditions in a manner understood by those skilled in the art. The testing reference (especially Section 5.2) sets forth tha effects of variations 1n the solubility, concentration, structural regularity, miscibility, volatility and other physical properties of the polymers, solvents and nonsolvents of the types useful herein.

For Instance, it is known that the higher the solids concentration of the polymer, the smaller will be the pore size of the finished membrane. Solids contents will normally be 1n the range of about 4.25% to 8.0 % by weight, commonly 4.25% to 6.5% and preferably 5.0% to 5.5%.

Similarly, the higher the concentration of non-solvents the larger will be the pore size, while the higher the boiling point of the nonsolvent the larger will be the final pore size for a given concentration of nonsolvent. The effects of other variables are also well known. For Instance, the higher the processing temperature the smaller will be the final pore size. In a dry process the flow of air or other inert gas should be low enough to prevent skinning of the surface of the membrane (i.e. formation of a surface layer with pores of a size less than the pore size required for microfiltration) while yet retaining high enough flow velocity to allow removal of solvent volatiles.

The microfiltration membranes of the invention will have pore size distributions such that average pore sizes are 1n the range of 0.05 to 10.0 nticOTEtres, normally 0.1 to 5.0 micrcmstres. Mills the pore sizes of eadi membrane are not all uniform, the pore size distribution, is sufficiently small that one can readily differentiate between membranes having average pore sizes of ‘0.22 pm, 0.45 pm, 0.65 pm, and so forth.

The preferred solvent or swelling agents for use 1n the present Invention are acetone and acetonitrile, although other materials having similar properties under a given set of reaction conditions 1n conjunction with the materials of the present compositions can also be used. Similarly, a variety of nonsolvents (pore producing agents) may be used, preferred among which are ethanol and the isomeric propyl and butyl alcohols. The proportions of solvents and nonsolvents may be varied over a wide range as described 1n Section 5.1 of the testing reference. Typically there will be on the order of 50% to 55% solvent dnd about 40% nonsolvent in the solution or suspension containing the polymers. These proportions may be varied considerably, however, to regulate the rapidity of phase Inversion, to compensate for the 47S85 various types of materials which may be present, and to effect the formation of various sizes of pores.

The following examples will illustrate the novel membranes of the present invention and also their method of manufacture.

Example 1 Cellulose from cotton linters or wood pulp is reacted with an equal weight of 2% aqueous sodium hydroxide and acrylonitrile in the ratio of 15 parts by weight of acrylonitrile per part by weight of cellulose to produce cyanoethylcellulose having an 11.9 percent nitrogen content and a degree of substitution (D.S.) of approximately 2.5. The resulting cyanoethylcellulose may be used as produced. Example 2 Chitosan (hydrolyzed chitin) is cyanoethylated as in Example 1 to produce a cyanoethylchitosan which when incorporated into a cellulose nitrate membrane behaves as does the product of Example 1. Example 3 A solution containing 4% cellulose nitrate, li cyanoethylcellulose, 54.2 i acetone, 23.7% ethyl alcohol, 12.3% n-butyl alcohol, 3.3% water and 1.5% glycerol is allowed to desolvate completely 1n a dry phase inversion casting process to produce flexible heat resistant membranes having pore sizes suitable for microfiltration. In various experiments using this solution reaction conditions were varied according to the guidelines discussed above to produce membranes having substantially uniform pore sizes in the range of from 0.05 to 5.0 um.

Example 4 , A solution similar to that described in Example 3 but containing 1% cyanoethylchitosan instead of cyanoethyl-cellulose 1s allowed to desolvate partially and then is Immersed in water (wet phase inversion casting process) to produce microporous membranes equivalent to those formulated 1n accordance with Example 3.

Example 5 A solution similar to that described in Example 3 but containing 4% cellulose nitrate, 0.5% cellulose acetate, and 0.5% cyanoethylcellulose is utilized in a dry phase inversion casting process to form membranes equivalent to those of Example 3.

Example 6 A solution similar to that described 1n Example 3 but containing 4.5% cellulose acetate and 0.5% cyanoethyl-cellulose is used In a dry phase inversion casting process to form membranes equivalent to those of Example 3.

For comparison purposes, the properties of membranes formed in accordance with Example 3 above and having average pore sizes of O.22jim were oarpared with canrnercial 0.22 micrcrretres aallulase nitratecellulose acetate membranes of the prior art. The commercial materials . were all found to be quite brittle such that just ordinary handling and flexing caused rupture of many samples. On the other hand, the membranes of the present Invention were quite flexible, to the point where they could be readily bent into tight U-shapes and even creased without rupture of the membrane. In autoclave sterilization tests 1n which the membranes were placed unrestrained in an autoclave at 261 °F (127®C) for one hour, the membranes of the prior art not containing any cyanoethylated polysaccharide others were found to shrink by approximately 12% to 14%, while the membranes of the present invention shrank only approximately 0.08%. It is thus evident from these tests that the membrane compositions of the present invention are markedly superior to the membranes of the prior art.

The invention herein provides membrane filters for use in a wide variety of industrial and related microfiltration applications, including (but not limited to) filtration of pharmaceuticals, bio25 logical materials, foods, aerospace fuels and distilled water used in electronics, pharmaceutical, and aerospace processes. Generally, the filters are used in industrial applications where it is vital that all particulate matter and/or biological material greater than the pore size of the filter must be removed from the filtrate, nhe membranes provided by the present invention are especially useful for the filtering of liquid suspensions of particles having sizes in the range of 0.05 to 10.0 micrometres.

Claims (10)

Claims

1. A membrane for microfiltration which comprises a blend of 10% to 50% by weight of at least one cyanoethylated polysaccharide ether polymer and 50% to 90% by weight of at least one polymer selected from the group consisting of cellulose nitrate polymers, cellulose acetate polymers or mixtures thereof.

2. A membrane as in Claim 1 wherein said cyanoethylated polysaccharide ether polymer Is a polymer of cyanoethylcellulose or cyanoethylchitosan.

3. A membrane as in Claim 2 wherein said cyanoethylated polysaccharide ether polymer is a polymer of cyanoethylcellulose.

4. A membrane as in Claims 1 to 3 having a pore size distribution with an average pore size in the range of 0.05 to 10.0 micrometres.

5. A membrane as in Claim 4 wherein the average pore size is 1n the range of 0.1 to 5.0 ndcrenetres.

6. A method of formation of a membrane as in Claims 1 or 3 comprising casting said blend of polymers in a wet phase Inversion casting process.

7. A method of formation of a membrane as in Claims 1 or 3 comprising casting said blend of polymers in a dry phase inversion casting process.

8. A microfiltration process comprising filtering a liquid suspension of particles having sizes in the range of 0.05 to 10.0 miercmstres through a membrane as in Claims 1 cr 3.

9. A membrane for microfiltration substantially as hereinbefore described and exemplified.

10. A method of formation of a membrane as defined in claim 1 substantially as hereinbefore described and exemplified.