Note: Descriptions are shown in the official language in which they were submitted.
Technical Field
The invention herein relates to cellulosic membranes. More
particularly, it relates to those membranes formed from cellulose
derivatives and having pore sizes and configuratiorls suitable for use
in microfiltration processes.
Background of Prior Art
Cellulose and its derivatives are well known and have been
described in numerous books and publications. See, e.g., Ott et al,
eds., "Cellulose and Cellulose Derivatives", Hi~h Polymers, Vol. V,
especially Section IX (Interscience, 1954) and Kirk-Othmer, Encyclo-
pedia of Chemical Technolo~y, ~ol. 4, pages 593-683 (2d edn., 19~7).
Microporous membranes composed of cellulose derivatives, primarily
cellulose nitrate and/or cellulose acetate are also well known and
have been extensively described ancl studied; see Kesting, Synthetic
Polymer c Membranes (McGraw-~lill, 1971), especially Chapters 1, 2 and
5. Such membranes find utility in a wide variety of laboratory and
commercial processes involving microfiltration.
Micro-Filtration is a membrane filtration process intended to
effect filtration of particles having a size in the range o-F 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: ultrafiltrat;on and reverse osmosis. Ultra-filtration
operates at a significantly lower particle size range than micro-
f;ltration and is intended for Filtration of individual polyrner
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 magni-
fication, while those intended for use in reverse osmosis and ultra-
filtration do not show visible pores. (In the past ultrafiltration
and microf;ltration have sometimes been considered to be synonymous,
but curren-t filtration practice and theory now clearly distinguish
between the two, such that the membrane materials utilized for micro-
filtra'cion are not considered to be equivalent to those used for
ultrafiltration. See Kesting, 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
' ~
~Q~, Ei5S~i
-2-
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 caut;on. 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 in 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 ;ncrease in
strength to cellulose nitrate membranes but such membranes are sus-
ceptible 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 microfiltraiton
processes and yet be strong, flexible, unaffected by the chemical
nature of common filtrates, and capable of being autoclaved for
sterilization.
Brief Summary of Invention
The invention herein is a novel membrane for microfiltration
which comprises a combination of cyanoethylated polysaccharide ether
polymers and polymers of cellulose nitrate, ce11ulose acetate, or
mixtures of cellulose ace~a~e and cellulose nitrate polym~rs. In the
present membrane the cyanoethylated polysaccharide ether polymers will
25 be present as from 10% to 50% by weight of the composition with the
balance of the compos~tion being the cellulose nitrate and/or cellulose
acetate polymers.
Detailed Description of Invention
The ethers useful in the present invention are the cyano~
30 ethylated ethers of homopolysaccharides, notably cellulose and chitosan.
Preferred among these is cyanoethylcellulose. Cellulose, chi~in and
the related homopolysaccharides having reactive hydroxyl groups are
known to react in the presence of a base with acrylonitr;le to form
cyanoethylated polysaccharide ethers. Reactions of this type have
35 been described in the Ott et al reference mentioned above, particularly r
in Section IX-E-9. Consequently, cyanoethylcellulose may be formed by
the reaction of acrylonitrile with cellulose in the presence of a base
~65~;6
such as dilute sodium hydroxide, while cyanoethylchitosan may be
similarly formed.
The novel membranes of the present invention are formed by
blending together the eyanoethylated 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 com-
position 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
0 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 ~he 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 wit'n
conventional techniques of the prior art. In such techniques the
membranes are cast from a slurry of the polymers and pore forming
agents (nonsolvents) dispersed in suitable solvents. The phase
inversion process is described in detai1 in Kesting, 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 in which the fina1 sol~ent
removal after phase inversion is carried out. In the com~only used
"dry" process the ~lnal 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 carriPd 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 afsrementioned
Kesting reference. The membrane is cast onto an appropriate substrate
commonly in ambient air and at a temperature of approximately 20C to
25C. 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
~LQc~
art. The Kesting reference (especially Sect;on 5.2) sets forth the
effects of variations in the solubility, concentration, structural
regularity, miscibility, volatility and other physical properties of
the polymers, solvents and nonsolvents of the types useful herein.
For instance, ;t 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 in 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 con-
centration o~ 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 remGval of solvent
volatiles.
The microfiltration membranes of the invention will have
pore size distributions such that average pore sizes are in the range
of 9.05 to 10.0 um, normally 0.1 to 5.0 um. While the pore sizes of
each membrane are not all uniform, the pore sizeOdistribution is
sufficiently small tha~ one can readily differentiate between membranes
having average pore sizes of 0.22 um, 0.45 um7 0~65 um9 and so forth.
The preferred solvent or swelling agents for use in the
pr~sent invention are acetone and acetonitrile~ although other materials
having similar properties under a given set of reaction conditions in
conjunotion 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 in Section 5.1 of the Kesting
reference. Typically there will be on the order of 50% to 55% solvent
and about 40% nonsolvent in the solution or suspension containing the
polymers. These proportions may be Yaried considerably, however, to
regulate the rapidity of phase inversion, to compensate for the
5S6
various types of materials which may be present, and to effect the
~ormation 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 ch;tin) 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, 1% cyanoethyl-
cellulose, 54.2 % acetone, 23.7~ ethyl alcohol, 12.3% n-butyl alcohol,
3.3% water and 1.5% glycerol is allowed to desolvate completely in 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
5O0 um.
Example 4
A solution similar to that described in Example 3 but con-
taining 1% cyanoethylchitosan instead of cyanoethyl-cellulose ls
allowed to desolvate partially and then is immersed in water (wet
phase inversion casting process) to produce microporous membranes
equivalent to those formulated in accordance with Example 3.
Example 5
A solution similar to that described in Example 3 but con-
taining 4% cellulose nitrate, 0.5% cellulose acetate, and 0.5% cyano-
ethylcellulose is utilized in a dry phase inversion casting process toform membranes equivalent to those of Example 3.
~L~ i5
Example 6
A solution similar to that described in Example 3 but
containing 4.5~ cellulose acetate and 0.~% cyanoethyl-cellulose is
used in a dry phase inversion casting process to form membranes
S 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
0 22 um were compared with commercial 0.22 um cellulose nitrate-
cellulose 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 in
which the membranes were placed unrestrained in an autoclave at 261F
(127C~ for one hour, the membranes of the prior art not containing
any cyanoethylated polysaccharide ethers 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 ~rom these tests
that the membrane compositions of the present invention are markedly
superior to the membranes o~ the prior art. ~~
Statement of Industrial Apelication
The invention herein provides membrane filters for use in a
wide variety of industrial and related microfiltr~tion applications,
including (but not limited to~ filtration of pharmaceuticals9 bio-
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
partlculate matter and/or biological material greater than the pore
size of the filter must be removed from the filtrate.
~,
