Aminated polysulfone membrane and process for its preparation

A new process for preparing a separation membrane from aminated polysulfone resin or blends of aminated polysulfone-blended materials which utilizes an organic additive having a low or negligible water-solubility (instead of conventional water-soluble organics, polymers, or inorganic salts) in the casting solution. The resulting membrane exhibits high water-permeability and is capable of being formed at a high production rate when using a wet casting machine.

BACKGROUND OF THE INVENTION 
Polysulfone resin has been widely used to prepare separation membranes. 
However, since it is hydrophobic, its membranes tend to precipitate 
impurities, resulting in dramatically decreased water flux. It has been 
suggested to convert the polysulfone resin into a sulfonated polysulfone 
resin (C. Friedrich et al., "Asymmetric Reverse Osmosis and 
Ultrafiltration Membranes Prepared from Sulfonated Polysulfone," 
Desalination, 36 (1981), 39-62), or into aminated polysulfone (Japanese 
Provisional Application (A) No. 62-42704). The sulfonated or aminated 
polysulfone membranes have increased hydrophilicity and a wider range of 
uses. 
In conventional processes for converting aminated polysulfone resin to the 
desired membrane, a solution of the polymer is formed containing a 
water-soluble additive, e.g., water-soluble polymers, organic compounds, 
or inorganic salts. Examples of water-soluble polymer additives include 
polyethylene glycol, polypropylene glycol, and polyvinylpyrrolidone. 
Examples of water-soluble organic compounds include ethylene glycol, 
glycerol, and propylene glycol. Examples of water-soluble inorganic salts 
include lithium nitrate and lithium chloride. 
The solution having the water-soluble additives denoted above are 
conventionally cast onto a woven or non-woven reinforced cloth and the 
solvent thereafter evaporated. The cast reinforced cloth is immersed in a 
gelation medium, such as ice water, to form the membrane product. 
In Japanese Provisional Application (A) No. 62-42704, water-soluble lithium 
nitrate is used in a casting solution of aminated polysulfone. The 
solution must be evaporated at a temperature of over 50.degree. C, and, to 
prepare a suitable membrane, the processing times used are 40, 50, 60, and 
70 minutes. For this reason, the process is unsuitable for use with a 
conventional wet casting machine and high industrial production rate 
cannot be achieved. 
In sum, such conventional processes and the membrane thereby obtained have 
significant disadvantages. They cannot effectively be employed for mass 
production when using a wet casting machine. High production rate cannot 
be realized because of the long period for evaporation of solvent. 
Moreover there is the desire to produce an aminated separation membrane of 
high permeability to water, as well as a membrane having improved ability 
to separate and recover materials having cationic functional groups, such 
as dyestuffs, amino acids, and cationic electro-deposition paint. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention provides an improved separation membrane and a 
process for its preparation. The separation membrane is made from a 
casting solution composed of an aminated polysulfone resin or blends of 
aminated polysulfone and suitable blending materials and an organic 
additive having low or negligible water-solubility in an organic solvent. 
This contrasts with the use of water-soluble additives such as shown in 
the Japanese application. The process is capable of being performed at a 
high production rate in a commercial wet casting machine. The resulting 
aminated polysulfone membrane has superior properties, such as high water 
flux and the ability to separate materials having cationic functional 
groups. 
The basic process employed involves the following combination of steps: 
(1) Preparation of the casting solution by mixing an aminated polysulfone 
resin or blended aminated polysulfone with conventional solvents and an 
organic additive having low or negligible water-solubility (generally less 
than 50 g/l, preferably less than 10 g/l in water at 20.degree. C.). The 
mixture is stirred until the resin is completely dissolved. The organic 
additive may be added before, after, or simultaneously with the aminated 
polysulfone resin or the blend material. 
(2) Application of the resinous casting solution to a cloth substrate 
(woven or unwoven, but normally reinforced) by a conventional casting 
process. Typically the resin thickness is 3 to 25 mils, preferably 6 to 12 
mils, and the application is at room temperature. The present process is 
particularly suitable for being performed by a conventional wet casting 
process. 
(3) Gelation of the casting solution on the substrate to form the membrane 
by immersing the coated cloth into a suitable medium, such as ice water or 
an organic non-solvent at a temperature of 5.degree. to 15.degree. C. 
Normally solvent is first evaporated in a rapid manner, e.g., an 
evaporation period up to 10 seconds. 
DETAILED DESCRIPTION OF THE INVENTION 
The aminated polysulfones of the present invention are: 
1. Aminated polysulfone, per se. Such polymers are known in the art and can 
be made by treating polysulfone resin with chloromethylether and 
triethylamine, such as described in Japanese Professional Application (A) 
No 62-42704 cited previously. A preferred process is to react commercial 
polysulfone with trioxane and hydrogen chloride gas under the catalytic 
effect of acetic anhydride and zinc chloride. The amination reaction is 
carried out by reacting the chloromethylated polysulfone with 
triethylamine. 
2. Aminated polyethersulfone, aminated polyarylsulfone, especially aminated 
polyphenylsulfone, and aminated polyphenylsulfone. 
3. Blends of aminated polysulfone, such as aminated polysulfone/polysulfone 
(in a weight percentage of 5 to 70% aminated polysulfone of total blend); 
aminated polysulfone/polydifluoroethylidene (in a weight percentage of 5 
to 50% aminated polysulfone to total blend), and aminated 
polysulfone/polyolefin such as polyvinyl found (in the weight percentage 
of 5 to 50% of aminated polysulfone to total blend). 
Conventional solvents for dissolving aminated polysulfone are used. 
Illustrative solvents, include N-methyl-2-pyrrolidone, 
N,N-dimethylformamide, N,N-dimethylacetamide and dimethylsulfoxide. 
The organic additive having low or negligible water-solubility is selected 
from the groups dialkyl C.sub.1 -C.sub.10 phthalates, such as 
dimethylphthalate, dibutylphthalate, and dioctylphthalate 
(di(2-ethylhexyl)phthalate); trialkyl C.sub.1 -C.sub.6 phosphates, such as 
trimethylphosphate and triethylphosphate; ketones, such as cyclohexanone, 
and diethylketone; unsaturated carboxylic acids, such as maleic acid and 
fumaric acid. By the phrase "low or negligible water-solubility" is meant 
a solubility of less than 50 g/liter of water at ambient conditions 
(20.degree. C.). 
The casting solution will typically contain (weight percent): 
______________________________________ 
Broad Range 
Preferred Range 
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Aminated Polysulfone or Blend 
5 to 35 10 to 30 
Organic Additive Compound 
1 to 30 5 to 20 
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The balance of the solution is solvent. 
The resultant aminated polysulfone membrane produced by the present 
invention has various advantageous characteristics: 
(a) High water flux or permeability. 
(b) Good ion exchange capacity, such as 0.05 to 1.5, preferably 0.2 to 0.5, 
milliequivalent/g. 
(c) High solute rejection due to the improved hydrophilicity of the 
aminated polysulfone membrane surface. This decreases fouling of the 
membrane. 
While not fully understood, studies under the electron microscope and test 
data suggest that the sponge structure normally associated with the use of 
aminated polysulfone membranes made by conventional methods are converted 
into a finger-like structure when the present organic additive of low or 
negligible solubility is employed. The additive accelerates the gelation 
rate to form a membrane with a fingerlike structure which is more 
hydrophilic. This fingerlike structure across the cross-sectional view of 
the separation membrane provides high water flux. Thus a significant 
improvement in water flux is realized. 
This same structural characteristic makes the present membranes especially 
useful for separating and recovering valuable materials having cationic 
functional groups, such as dye-stuffs and amino acids, and treatment of 
cationic electro-deposition paint.

The following examples will serve to illustrate the invention and preferred 
embodiments thereof. All parts and percentages in the specification, 
examples, and claims are by weight unless otherwise indicated. 
EXAMPLES 
Example 1 
4.5 g of polysulfone, 4.5 g of aminated polysulfone resin (ion exchange 
capacity is 0.7 milliequivalent/g), and 3 g of cyclohexanone (as additive) 
were added to 38 g of N-menhyl-2-pyrrolidone (as solvent) to prepare a 
casting solution. The casting solution was then spread on a non-woven 
cloth with a glass rod after the mixture was dissolved, filtered, and 
defoamed. After the solvent of the casting solution was allowed to 
evaporate for a period of 10-15 seconds, the non-woven cloth with the 
casting solution spread thereon was immersed in 10.degree. C. ice water to 
gel and form a membrane. 
The performances of the resulting membrane were determined by thin channel 
flow ultrafiltration testing apparatus. The results were reported in pure 
water permeability (PWP)in terms of GFD (gal/ft.sup.2 /day) and rejection 
ratio (%) of solute for the polyethylene glycol (MW=100,000) aqueous 
solution. These test procedures are described in the Journal of Applied 
Polymer Science, Vol. 14 (1970), pp 1197-1214. 
Operation conditions were 30psi and 25.degree. C. The tested flux and 
solute rejection performance of the membrane were 330 GFD of pure water 
permeability and 70% solute rejection. 
Example 2 
The same process was employed as in Example 1, except 6 g of dimethyl 
phthalate was used as the additive instead of cyclohexanone. The test 
results for the resultant membrane were 260 GFD of pure water permeability 
and 71% solute rejection. 
Example 3 
The same process was employed as in Example 1, except 6 g of trimethyl 
phosphate was used as the additive instead of cyclohexanone. The test 
results for the resultant membrane were 280 GFD of pure water permeability 
and 64% solute rejection. 
Examples 1-3 thus illustrate that the aminated polysulfone membranes of the 
present invention have high water flux or permeability as well as high 
solute rejection. 
Example 4 
20% by weight of cationic electro-deposition paint was used as the test 
solution for long-term antifouling test of the following membranes. 
Polysulfone membrane was dissolved in 41 g of N-methyl-2-pyrrolidone, 3 g 
of glycerin as additive. The procedure for preparing the membrane was the 
same as described in Example 1. The membrane was denoted "PSM-01". 
For comparison, the long-term antifouling tests were conducted for both 
polysulfone and aminated polysulfone membranes made according to the 
present invention. 
The four membranes denoted in Table 1 were connected in series in four test 
cells. Cationic electro-deposition paint (sold by PPG Industries, Inc.) 
was employed as feed material and 2 kg/m.sup.2 operating pressure was 
maintained. The water flux through the membrane was measured. The results 
are shown in Table 1 below. 
TABLE 1 
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Water Flux (GFD) 
Duration of 
Polysulfone 
Operation 
Membrane Aminated Polysulfone Membrane 
(Hour) (PSM-01) Example 1 Example 2 
Example 3 
______________________________________ 
0 35 35 32 33 
50 12 34 31 32 
300 10 34 30 32 
500 8 34 30 32 
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As shown in Table 1, the aminated polysulfone membranes of the present 
invention exhibited excellent antifouling characteristics to cationic 
electro-deposition paint. Water flux remained essentially constant at 30 
GFD or better over 500 hours of operation, as compared to a 77% reduction 
when using a conventional polysulfone membrane. 
Example 5 
Table 2 below compares the properties of a membrane composed of 4.5 g 
polysulfone and 4.5 g of aminated polysulfone resin with a water-soluble 
additive (glycerin) and with organic additives of low solubility (Examples 
1-3). Tests were conducted as described in Examples 1-3. 
TABLE 2 
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COMATIVE 
EXAMPLE EXAMPLE EXAMPLE 
MEMBRANE 1 2 3 
__________________________________________________________________________ 
ADDITIVE 
Glycerin Cyclohexanone 
Dimethylphthalate 
Trimethylphosphate 
(3 g) (3 g) (6 g) (6 g) 
Pure Water 
71 330 260 280 
Permeability 
(GFD) 
Solution 
76 70 71 
Rejection % 
Structure 
Spongelike 
Fingerlike 
Fingerlike 
Fingerlike 
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As shown, water-soluble additives, such as glycerine, induce a spongelike 
structure in the membrane and result in lower water flux. With respect to 
Examples 1 to 3, the embodiments of the instant invention, the organic 
additives of low or negligible solubility induce the hydrophilic aminated 
polysulfone to form a fingerlike structure and result in higher water 
flux. 
As will be understood by those skilled in the art, various modifications 
may be made to the present invention without departing from the spirit 
thereof.