Background Art Small membrane areas possess the intrinsic properties of having the good selectivities, fluxes and lifetime, however the process of upscaling such membranes to commercial quantities introduces imperfections, reducing selectivity and membrane performance. These imperfections may be found on the membrane, at the interface between the membrane and other components within a membrane module and the term membrane as used herein is intended to include flat membranes as well as membranes in any configuration, including flat, spiral wound, tubular, etc.. Thus, the present invention also provides a process for blocking and/or plugging imperfections found in partially or completely torn or split capillaries, hollow fibers and tubelets. The size of the imperfection may vary from nano-meters to millimeters, centimeters and greater.

In water or solvent applications for RO, MF, UF and NF, membrane there is also a need to improve performance by blocking imperfections because the resultant increase in selectivity improves the economics of many applications, for example sea and brackish water desalination, the growing areas of MF, UF and NF in water, waste water treatment or pretreatment prior to RO.

There is extensive state of art describing materials and procedures for plugging or blocking imperfections. This is especially true in membranes for gas separation, where small imperfections have large effects on selectivity.

In all cases however each method has one or more of the following problems: 1. Complex procedure; 2. Insufficient life time of imperfection repair that requires repeat application; 3. Does not significantly improve selectivity; 4. Significantly reduces flux; and 5. Cannot fix a wide range of imperfections, and is not a general approach.

In addition to increasing dissolved solute rejections, plugging imperfections reduces the passage of bacteria and viruses which is of growing concern for MF, UF, NF and RO modules used in drinking water applications and in the reclamation of industrial and municipal waste water applications.

The procedures and materials used for blocking or filling imperfections in gas separation membranes (US 4,214,020; US 4,230,463; EP 0631 806A2; US 5,032,149; and US 5,034,024) cannot in general be used in membranes for water and solvent based applications. The differences arise from the physico- chemical properties of gases vs. liquid states. For example, contrary to the case of water, where hydrogen bonding and water of hydration must be considered, in gas separations said factors are not a consideration. Apart from the above differences the approaches presented herein may also be used to fill imperfections in gas separation membranes, though the primary examples are for membranes used in water and solvent applications.

The RO membranes are primarily asymmetric structures based on cellulosics, polyamides and composites. For example asymmetric polysulfone supports coated by a thin selective layer of a polyamide, polyureas, polyalcohol, sulfonated polyaromatics, polyfurfural aldehydes, etc.

The processes of the state of the art for improving membrane rejection to solutes is often carried out by forming a thin coating on the surface of the selective side of the membrane. This coating may be stabilized by some complexation, precipitation or crosslinking by covalent bond formation, however does not penetrate into the pores or imperfections The coating may be formed from polymers, oligomers and particles (ex. latex or colloids). Examples of these approaches may be found in the following US patents 3,373,056; 3,877,978; 4,634,531; 2,886,066; 4,828,700 ; 4,812,238; 4,239,714; and 4,704,325. These state of art methods have the following shortcomings. They are general coating methods which do not discriminate between coating the entire external membrane surface uniformly and the plugging or filling primarily the imperfections in a selective manner. In addition while some of these methods will fill imperfections to different degrees the plugging is of a relatively low density material. If the plugging was of high density it would necessarily block the intact surface and significantly reduce flux of the entire membrane. These low density plugs of material which fill the imperfections are unstable with the applications of pH extremes, pressure, flow; and/or temperature and are not stable upon exposure to organic solvents. Because of this instability the coating must be reapplied periodically. Many of the coating or plugging procedures do not include crosslinking or stabilization steps. The coatings are bound to the membrane surfaces by electrostatic or hydrophobic forces. When there is a crosslinking step it also crosslinks the material coating the membrane surface and results in reduced flux.

In the field of gas separation membranes the general procedure for plugging imperfections by coatings which have partial penetration into pores and imperfections (US 4,214,020). These approaches use hydrophobic polymers, highly permeable to gas, that coat the entire surface of the membrane. They cannot be used for water applications because of their hydrophobic nature. In an attempt to overcome the problem of coating the entire surface of the intact membrane, surfactant molecules with swelling agents have been used to fill submicroscopic pores (US 5,032,149 and US 5,034,024). These surfactant molecules are not stabilized, cannot be used to fill relatively large imperfections, and will not work well in membranes used for water applications.

Similarly, for ceramic membranes, pore modification is a general method for modifying all membrane pores and not selectively filling the imperfections (Koros- Desalination, US 4704 324).

The state of art does not teach how to plug a range of imperfections (from the microscopic to the macroscopic) without coating the intact membrane area.

It has now been surprisingly found according to the present invention that by taking into consideration the higher flow rate (in effect high linear velocity) through membrane imperfections as compared to the flux (or linear velocity) of the intact membrane areas, and considering the enhanced reaction rates of chemical reactions inside the imperfections caused by the fact that these reactants are retained by the imperfection substructure and are being concentrated inside the imperfection to a very substantial degree, a high amount of plugging materials can be localized and concentrated from a very dilute solution which plugging material can then be reacted, e. g., cross-linked, within the imperfection as compared to a much smaller, or even negligible amount of deposit that would accumulate on the surface of the intact membrane. The relative extent of material accumulation inside the imperfection can be controlled by adjusting concentration of a plugging material in the solution, the flow rate over the membrane surface, the utilization of pressure, and the duration of application.

Disclosure of the Invention Thus according to the present invention there is now provided a process for plugging at least one imperfection found in a membrane comprising applying a solution containing a reactive component in dilute concentration to the surface of said membrane under pressure, said components having low reaction rates at said dilute concentration, wherein at least one reactive component is at least partially entrapped within said imperfection for sufficient time so that a reaction with an other reactive component is induced as a result of a higher level of local accumulation and concentration of said components within said imperfection, whereby there are formed covalently linked reaction products within said imperfection thus permanently plugging the same In the case of torn or damaged or split capillaries, hollow fibers and tubelets (referred to hereinafter as"capillaries") it is also possible, by the method of the present invention, to selectively block such damaged fibers by selectively introducing reactants only into such damaged areas and/or fibers so that after completing the reaction the above damaged fibers and/or areas are blocked.

The selective mechanisms for introducing reactants only into the damaged fibers and not into the intact fibers can be achieved, according to the present invention, by introducing reactive particles (optionally expandable) and other non particulate reactants from the external walls of the fiber only into the broken ends of the fiber under the effect of pressure or vacuum in such a way that the interior of the fiber becomes blocked. Such particles are chosen to be of an appropriate size so that they will not permeate across the intact membrane and they will not block the pores or lumens of the intact membrane.

Thus according to the present invention there is now provided a process for plugging imperfections found in membranes comprising concentrating reactants inside said imperfections and causing reaction of said reactants to form reaction products within said imperfections thus plugging said imperfections. Included in this invention is also a process for selective plugging of imperfections and defective or torn capillaries through the cross section of the lumen with particles of an appropriate size and with particles that the size of which can be further expanded inside the lumen. These particles can contain other reactants as described.

Thus, as stated, the present invention provides a process for plugging at least one imperfection found in a membrane comprising applying a solution containing different reactive components or identical, self condensing or self polymerizing components of the same chemical structure in dilute concentration to the surface of said membrane under pressure, said components having low reaction rates at said dilute concentration, wherein at least one of said components is at least partially entrapped within said imperfection for sufficient time so that a reaction with said other component is induced as a result of a higher level of local accumulation and concentration of said mixture of components within said imperfection, whereby there are formed covalently linked reaction products within said imperfection thus permanently plugging the same.

In another embodiment of the present invention there is provided is a process for plugging the lumen of a broken or damaged hollow fiber, capillary or tubelet comprising first applying pressure or vacuum that causes the dispersion of solid or liquid particles to flow into the lumen of the capillary and reacted therein to form a stable plug. Such dispersions are chosen so that they will neither permeate across the membrane nor will they plug the pores of the intact membrane and will selectively pass into the lumen only through the damaged parts of the membrane.

It has now been further found that reactive reagents may be used in many cases at very low concentration in the solution, without a significant reaction taking place in the bulk of a solution facing the membrane being repaired and on the surface of this membrane. The reaction between the reactive components of the plugging solution/preparate will significantly occur only after they have been accumulated and concentrated within the imperfection. After reacting the plugging material inside the imperfection, the permeability of the solutes and/or of the microorganisms/viruses will be reduced and the rejection of the membrane towards these species will be improved.

The material deposited and reacted or precipitated within the imperfection or within the lumen of the capillary may be further densified and stabilized by additional precipitation and/or further chemical reactions such as crosslinking or by modifying the dimensions of the plug by agglomeration and/or expansion through a subsequent chemical reactions or thermal treatments. It has also been found that in many cases the reactants in solution begin to react slowly with each other if left for sufficiently long periods of reaction time so that they can interact and form larger molecules, oligomers, macromolecules, colloids, particles or precipitates which can plug progressively larger imperfections. Furthermore individual particles can grow in size with time by expansion due to evolution of gas inside the particle. Thus in the initial stages of the repairing process, small imperfections are filled and plugged efficiently. As reaction time continues the species in solution grow in size and efficiently plug larger imperfections. Thus this method is a general method for fixing a wide range of imperfections. It has also been further found that using particles and/or emulsions or liquid suspensions in addition to polymers and low molecular weight multi-functional reagents, large imperfections are more effectively plugged in comparison to the repairing process that is performed without such particles.

The materials that may be used in the present invention are low molecular weight multi-functional reagents for precipitation, complexation and crosslinking, monomers and initiators for polymerization, oligomers, particles (latex, colloids, emulsions, etc.) and liquids which do not dissolve but may form emulsions or liquid suspensions which react with other components to form a solid plug.

To selectively fill imperfection without depositing large amounts of plugging material on the intact surface, dilute solutions at low pressures are applied for short periods of time. If the solution concentration is increased, than either the pressure and/or the time of application should be adjusted. Several embodiments of the present invention are based on applying at least one solution under pressure for a given period of time. The solution may contain one or more of the following components depending on the type of membrane for repair (RO, NF, UF or MF): 1. Reactive particles or colloids of the same type containing the same functional groups which can self condense and/or react within the membrane imperfections and fuse together.

2. Reactive particles colloids or mixtures thereof, with different functional groups which may fuse together, react together within the membrane imperfections.

3. Reactive particles or colloids in a solution of reactive polymers, oligomers or low molecular weight reagents containing more than one reactive group where the particles, oligomers and low molecular reagent contain more than one reactive group which may react to form a crosslinked structure.

4. A mixture of two different polymers or oligomers, a polymer (oligomers) with different functional groups as in copolymers, terpolymers, block or graft polymers, which are soluble and remain in solution in certain pH ranges but precipitate in other pH ranges. For example a mixture of polyamines and polyacid under base or acid condition which precipitate at neutral conditions.

5. Reactive particles (liquid or solid) containing blowing agents and conditions that can increase their size either in solution and/or inside the imperfection and/or inside the lumen of the damaged hollow fiber, capillary or tubelet membrane.

6. Any combination of solutions described in sections 1 to 5 above.

7. Any of the above which contains a crosslinker or multi-functional molecules or particles in solution under conditions where the extent of reaction is slow in the solution but may be accelerated in the imperfections (as for example by concentration) and may optionally be further densified and/or crosslinked in subsequent steps.

8. A combination of monomers oligomers and polymers either capable of self condensation or with the addition of a crosslinker, slowly crosslinks or increases the molecular weight of the species in solutions such that over the time of application, increasingly larger molecules particles, colloids or precipitates are continuously formed in solution and can plug more efficiently progressively larger imperfections. These larger species also condense and are crosslinked within the imperfection at a higher rate than in solution, because of their greater concentration in the imperfection.

9. A plugging solution containing a liquid emulsion or suspension which can condense, fuse, expand or polymerize by itself or in combination with other components described in 1 to 8 above, in a relatively rapid rate within the imperfection, to form a plug and in a relatively slow rate in the solution or on the surface of the intact membrane so that no significant flux decline is caused to the membrane.

All the above solutions may be applied and subsequently may be subjected to additional steps such as curing, fixation, precipitation, temperature change; and, further crosslinking by immersion for example in solution of additional crosslinkers.

If in the first application step only precipitation or gelation occur without a chemical crosslinking then chemical crosslinking may be applied in a second step. The correct combination of solution concentration and the time of pressure application is easily determined by trial and error. The objective being to increase rejection to the desirable level, without a large decrease in flux ; and, further to decrease the passage of microorganisms and viruses. The application of pressure is the preferred driving force which concentrates the reactants and fillers within the imperfection. Other driving forces may also be used as for example an electric field, diffusion via a concentration gradient, centrifugal force, gravity, capillary forces, vacuum etc.

For example spiral wound RO membrane elements that exhibits only 99.0% salt rejection show that in a smaller isolated membrane samples taken from the same element, 99.5% to 99.9% rejection can be found. By the application of the present invention the rejection of the entire spiral wound element can be increased to around 99.5% rejection or more with less than a 10% loss of flux.

An RO membrane that is damaged will show before the repair according to the present invention a rejection of only 50% and after repair the original performance of more than 95% rejection will be achieved.

In another application, tublets of Ultra-filtration membranes in a modular membrane element have a retention to viruses of three orders of magnitudes (3 logs). Upon application of the repair process of the present invention the retention of viruses is improved to 4,5 or even 6 logs. This is also true for bacteria.

Thus, the present invention also provides, in a preferred embodiment thereof, a process for plugging an imperfection in a membrane selected from the group consisting of hollow fiber membranes, capillary membranes and tubelet membranes and for plugging the whole lumen such damaged membranes wherein the flux of the plugging substances is directed into the lumen of said membranes by a pressure or vacuum and wherein the reaction between the different components of the plugging solution occurs inside the lumen thus permanently plugging the same.

In an embodiment of the present invention a polymeric or oligomeric material is dissolved in a solvent, preferably water (for polymeric membranes) and a soluble crosslinker, containing at least two reactive groups, which may react with the polymer or oligomer. This mixture is applied under pressure to the membrane. The concentrations of reactive components and/or pH and/or temperature is in range so the reaction is sufficiently slow that at least in the initial stages mostly uncrosslinked and unreacted polymer or oligomer and crosslinker are pressed into the imperfection and undergo a more rapid reaction within the imperfections because of their higher concentration inside these imperfections. At the same time, but in the bulk solution above the membrane surface, the polymer or oligomer may react with the crosslinker at a lower rate and a population of larger molecules builds up in the bulk solution and these larger molecules are also pressed into the imperfection. These larger molecules can more efficiently fill the larger imperfections where they are also crosslinked. Therefore as a result of this procedure, a large portion of the molecules are crosslinked and bound inside the imperfection during the stage of the application of pressure. This step may be repeated if needed. Optionally if needed additional crosslinking and additional curing may take place, by application of more or different crosslinkers, with or without pressure, change of pH, heating, or drying.

In another embodiment of the present invention there is provided a polymer, copolymer, tripolymer or mixture thereof, and a crosslinker, under conditions where the materials are all initially dissolved or suspended at a given pH. As the reaction is allowed to continue within the imperfection pores and solution, the pH may then be changed to precipitate the materials within the pores. Additional crosslinking may occur in this condensed state at the same pH-Temperature or a different pH- Temperature combinations. In some cases the crosslinker may be left out until the material is pressed into the imperfection and precipitated. In another variation of this embodiment the polymers are applied with a crosslinker and the reaction occurs during application and then further precipitation can be carried out in an additional stage.

In another embodiment the solutions mentioned above under numbers in 1 and 2 contain particles, colloids emulsions and/or latex particles ; these particles may or may not be reactive with each other or with other components in the solution. These particles enhance the efficiency of plugging large imperfections.

Any component which becomes entrapped within the plug without reacting is a filler. As a variation of the above particles, colloids or latex may be used alone for certain applications where only large imperfections are expected (ex. in UF or MF membranes) the material will condense or react preferably within the imperfection as it is being concentrated.

In another embodiment monomers and initiators are dissolved in the solution in relatively dilute form, and when pressed into the membrane are concentrated and react within the imperfections at a greater rate than in the solution. As mentioned above, the polymer or oligomer may also form in solution and these will also be pressed into the membrane. A preferred monomer initiator combination is a water soluble monomer (ex acrylic acid) and free radical initiators (azo initiators, peroxides, hydro peroxides, and redox salt combination which form radical initiators) which undergo addition polymerization. Multi functional vinyl compounds which form crosslinked networks are also preferably included. Another preferred embodiment is a combination of water soluble monomers (ex. methyimethacrylate), which become insoluble upon polymerization. When further polymerization occurs, within the imperfection a water insoluble plug will form in the aqueous phase of the imperfection. Cationic and anionic polymerization and condensation polymerization may also be used.

In another preferred embodiment a liquid monomer suspension will interact with the initiator which will polymerize within the imperfection. A preferred case is the use of a multi-functional vinyl compound within the solution. Another case further provides the addition of a is liquid epoxy containing molecules, silicones, adhesives and curing agents in the liquid form. in another preferred embodiment, according to the present invention, a selective plugging of torn or damaged hollow fibers, capillaries or tubelets is achieved with the use of reactive particles alone or in combination with polymers and low molecular weight reactants which form an insoluble lumen plugs.

The benefits of the above approach over the state of art are: a. Efficient plugging from the same solution of different size and types of imperfections, ranging from molecular to microns and centimeters, without forming a heavy coating on the surface that will significantly reduce flux.. b. Stability. c. Fast steps. d. Used membranes may be repaired as well as new membranes because the components of the present invention will cure imperfections which already contain particles, colloids, precipitates that are already located inside the imperfection. The use of pressure during the reactions will force the reactive components into the spaces within the imperfections and form an insoluble plug.

The material already accumulated inside the imperfection will act as a filler and will help to create a stable crosslinked plug.

The imperfections may vary in size from angstroms to microns to a fraction of a millimeters. If the imperfection are a crack or slit the length may be from 1 nano-meter to 10 centimeters, or even larger. In case of damaged hollow fibers, capillaries or tubelets the lumen which have to be plugged may have inner diameter of 20 microns and up to 5 millimeters.

The object of the present invention is the use of reactive particles or polymers and/or oligomers with multi functional reagents which form crosslinked or condensed particles or plugs, in situ, within the imperfection, or low molecular weight monomers or multi functional compounds which can condense to form polymers or crosslinked materials inside such imperfections.

The particles may be 1 nano-meter to 50 microns and up to few millimeters, depending on the size of the imperfection to be and the type of membrane being repaired. At least the surface of the particles contains reactive groups (the interior of the particles may or may not contain reactive groups). These reactive groups may self condense with groups on other particles (with or without the same groups) or monomers, or oligomers or polymers present in a solution when pressed into an imperfection. Once within the imperfection, the particles fuse and cure to one another or to the membrane walls and or some or all of the additional components of the plugging solution. Alternatively the particles do not react, but become embedded in a crosslinked matrix formed from the polymer, oligomer, additional crosslinking agents, a polymerization process.

Alternatively a polymer or oligomer or monomer, soluble in a solution and pressed into the imperfection, precipitates by for example by the change of concentration or pH or increase in concentration of crosslinkers or increase of molecular weight, which causes the polymer or oligomer to become insoluble. This precipitate may, if necessary, be further cured to form a stable plug within the pore.

The curing may occur via a crosslinking or self condensation between the mixed components during the time they are being concentrated within the imperfection, or may be added at a latter stage of the process after the imperfection has been filled.

The invention includes forcing under pressure (hydrostatic, capillary, vacuum or osmotic) reactive particles, polymers or monomers which then cure within the imperfection. The reactive polymers, particles or monomers pass preferentially into the imperfection and do not accumulate substantially on and inside of the intact membrane pores, because of the higher flow rate of liquid into the damaged areas versus the flow on the intact membrane areas. Thus with the proper adjustment of pressure and time the imperfections are filled, with only a thin coating covering the area of the intact membrane. Because of the higher flux through the imperfection there is a greater accumulation of material within the imperfection than on the area containing intact pores, and the reaction rates are higher in the imperfections because of the higher concentration. It is of importance that at least one component of the plugging solution is sufficiently retained such that it and other components (of smaller size) are also retained, because of the first components' retention, and concentration.

Reactive epoxy particles which may be either latex, colloidal particles and may also extent to the micron range of 10 nm up to 100 microns and up to millimeter size. These epoxy particles may be made solely of one or more epoxy materials or epoxy particles with a surface coating of another material or a particle of another material coated with epoxy, wherein the epoxy is either chemically bonded or physically absorbed. The particles can self cure with heat, a catalyst, or by compression into the imperfection with low molecular weight reactive compounds (e. g. amines), oligomers and polymers containing active hydrogen atoms on 1st, 2nd, tertiary aliphatic and/or aromatic amines, hydroxyl, aliphatic or aromatic groups, carboxyl and sulfide groups, with catalysts or without catalysts.

The particles may be pressed into the imperfections and then cured with the application of an external crosslinker. Or preferably the crosslinker or hardener may be included in the same solution containing the reactive epoxy particles.

Other particles may contain reactive triazines and diazines and alkyl/halogens or benzyl halogen groups. The triazine and diazine are preferably (but not exclusively) reactive because of the presence of reactive halogen groups.

These particles may be applied in the same way as epoxy particles.

Another preferred process is the utilization of a solution containing a polymer or a mixture of different polymers or a mixture of a polymer and low molecular weight, multi-functional reactive compounds or polymers and oligomers. These mixtures are applied in a clear solution or in a mixture of a solution where some of the components have precipitated to form a colloidal, latex or particle suspension that may still contain soluble material which can be further precipitated. The above solution or suspension is applied under pressure to the membrane. After having been forced into the imperfection it is further precipitated and cured within the imperfection. This precipitation may occur due to the increased concentration within the imperfection, changing pH, adding salt, or by a crosslinking agent already in solution which through crosslinking, precipitates densifies the polymers or polymers and particles together. These solutions of polymers or polymers and a suspension of these particles may contain reactive particles described in the approach above which contain, epoxy, triazine, diazines and alkyl or benzyl halogen compounds.

In another embodiment of the present invention the two previous approaches may contain non reactive colloids, latex, particles, polymers or oligomers which act as fillers in the cured plug.

The curing of the precipitated plugs or coalescing particles within the imperfections may be carried out by a reaction between the groups already on the molecules within the precipitate or mixture, otherwise the reaction may occur between the groups and molecules applied to the plugs or precipitate after the latter has formed.

Any of the above reactions may occur, for example, between low molecular weight species, oligomers, polymers or particles (including colloidal and latex particles) containing an active hydrogen on a primary, secondary, aliphatic and aromatic amines (including tertiary amines without hydrogens but with reactive electron pair), hydroxyl, sulfide, carboxyl, multi-functional low molecular weight molecules, oligomers, polymers or particles (including colloidal particles) containing epoxy, or halo triazine and aliphatic and benzylic halogen containing compounds.

Examples of the above and other crosslinkers are disclosed in US 4,767,645; US 4,778,596; and, US 4,659,474.

The epoxy particles may be made in numerous ways. Some of the procedures are covered in the following patents and are included in the present invention. These patents are, US 5176959 German Offenlegungsshrift 3,644,37 and 3,726,497.

Epoxy particles may also be made by dissolving a multi functional epoxy compound (examples of such compounds are given in US 4,265,745) in a water miscible solvent and adding water to the solution with stirring. Either or both solutions way contain surfactants, emulsifiers and stabilizers to achieve a predetermined particle size or particle size range, and to stabilize the particles from aggregating or coalescing.

Other particles may be made to contain halogen groups on triazine and diazines and alkyl compounds. The materials for making the particles should contain reactive groups for curing or crosslinking reactions or binding reactions between particles or polymers and particles.

Particles on the nano, sub-micron or micron level may be polymers with reactive pendants. Such polymers are readily available with the following reactive pendants-amino groups with primary, secondary or tertiary aliphatic or aromatic amines, hydroxy groups and sulfides. These groups are readily used to add molecules which contain epoxy, alkyl halides, benzyi halides and halides bound to diazines and triazines. By reacting an excess of molecules containing two or more epoxy, or reactive halogens with the polymers containing amino, OH or SH groups, polymers and particles can be created with reactive epoxy and halogen groups. The excess should be sufficient so that the polymers do not crosslink or form a precipitating mass. The new epoxy or halogen containing materials will now be able to condense with one of their free epoxy or halide groups.

In another preferred embodiment, homogeneous polymer solutions are pressed into the imperfections and then precipitated and cured to a crosslinked mass within these imperfections. The precipitation may be carried out by a change in pH or by a crosslinking reagent which crosslink the polymers at a very low rate, so that a homogeneous solution will be maintained above the membrane, but once pressurized into the imperfections these substances will concentrate and undergo fast crosslinking and precipitation reactions will occur within the imperfections.

A mechanism which allows the mixing of polymers solutions with crosslinkers without precipitation or gel formation in the bulk solution is based on: 1. Maintaining low concentration of reacting components in the bulk solution above the membrane, 2. Maintaining pH at which the reagents do not precipitate, 3. Introducing into the reactive molecules at least some reactive groups that are characterized with slow reactivities, which in turn control overall reactivity.

Once placed in the imperfection, precipitation and/or crosslinking will occur as a result of higher concentration due to accumulation of material within the imperfection with or without: 1. Change of pH and or temperature.

2. Sufficient time of a reaction.

3. Combinations of 1 and 2 above.

In the case of polymers which have precipitated within the imperfection they may be crosslinked by in situ crosslinking or the application of an outside curing agent. For precipitates with amine, OH and SH group the crosslinker may be multi- functional epoxy, alkyl halides, benzyl halides triazine and diazine derivatives, aldehydes, activated double bonds. If the precipitates contain epoxy alkyl halides, benzyl halides triazine, diazine aldehydes and activated double bonds then the crosslinking molecules may contain amine, alkyl halides, benzyl halides, diazines and triazines.

A preferred embodiment of the present invention is the utilization of a mixture of polyamine and a polyacid. Normally upon mixing solutions of such polymers a precipitate forms. Outside this range, in the basic or acidic pH extremes the solutions are soluble. A mixture of these polymers is applied in the soluble state to fill the imperfections of a membrane, then the pH is a adjusted to precipitate the polymer mixture. The precipitated polymer may then be crosslinked by applying a multi-functional epoxy, multi halo alkyl, benzyl halogens, aldehyde compounds, reactive triazine or diazine derivatives. Alternatively these crosslinkers may be added to the polyamine/poly acid solutions. This can be done if the pH of the solutions does not hydrolyze the functional groups of the crosslinker. In this way the polymers are crosslinked as they concentrate within the imperfection and/or after they precipitate.

To get a clear solution of the polyamine and polyacid, the respective ratio of the two may be adjusted so that a pH can be found where the solution is clear and does not hydrolyze (to a large extent) any reactive component.

In the above case the relatively low concentration of polyamine, polyacid and a crosslinker, for example, are such that the reaction does not occur rapidly and the solution may be applied without precipitation or gel formation occurring in the solution. As a result of the application of the solution the polymers will concentrate in the imperfection, thus, the reaction rates will increase and the reaction will happen selectively inside the imperfection. In addition the increased concentration within the imperfection may cause a precipitation of the components.

In another mode a polyamine may be applied to the imperfections and then a polyacid may be applied to precipitate as an acid base complex. The polyacid may be citric and/or polyacrylic acid (molecular weights from 500 to 5x106 Daltons). The acid-base precipitate may be further crosslinked by chemically crosslinking either the polyamine, polyacid or both, as follows : 1. Crosslinker already in the polyamine in dilute form.

2. Application of the crosslinker with polyacid.

3. Application of a crosslinker after the precipitation of a crosslinker.

The polyamine in all the embodiments may be taken from the category of polyvinylamines and their co and tri polymers, polyaromatic compounds such as polyamino-styrene, amine containing engineering plastics of the aromatic polysulfones (ex polysulfone, polyether sulfone, polyphenylene sulfones, PEK, PEEK) polyethyleneimines and derivatives of polyethyleneimine. The different types of polyamines and amines which may be used may be chosen from the lists found in US 4,265,745; US 4,659,474 and US 4,039,440.

The polyacids may be low molecular weight polyacids of 2 or more carboxylic, sulfonic groups or phosphonic (such is citric and malic acid), succinic acid, polyacrylic acid (in molecular weight ranges of from 200 to 5 million), polyvinyl sulfonic acid and polystyrene sulfonic acid. The polyacids may be from naturally occurring polymers such as alginic acid homopolymers, random co, tri and ternary polymers or block and graft polymers may be used. The functional groups on all the polymers other than the homogeneous polymers may contain both basic amines and anionic functions (carboxylic acids). These polymers may also be chosen from the sulfonic, carboxylic, phosphonic derivatives of polysulfone, polyether sulfones, polyether ketones and other engineering plastics.

In another approach, useful for relatively large imperfections a hydrophilic reactive polymer, a crosslinker and particles may be used. The polymer should be able to wet the surface of the particles, with or without additional agents such as surfactants. In this case the particles may or may not be reactive and may be chosen from a wide range of candidates.

The solvent of the plugging solutions may be primary water. Organic solvents and/or salts may be added to the water to change the flow and plugging rates by changing for example surface tension, solution viscosity, and/or permeability.

In US 3,373,056 particles and polymers are used to plug membrane imperfections. The claimed polymer is polyvinyl methylether and the particles may include colloids and gels. Since there are no chemical reactions for curing the components the improvement is not permanent and must be reapplied periodically, for improving RO membranes.

In US 3,877,978 (Kremen Ap 15 1975) Semi permeable RO membranes (cellulosic and polyamide membranes) are improved by applying under pressure a copolymer of vinyl acetate and acrylic or methacrylic copolymers above a pH of 7.0. The polymers are insolubilized or precipitated by reducing the pH below 7.0 or using multivalent salts. As there are no chemical crosslinking, both changes in salt concentration or of pH can destroy the treatments, requiring periodic reapplication of the treatment. In addition the limitation of pH below 7.0 during operation is a practical short coming.

In EP 0432 358, Cadotte, which refers to US 4,964,998; US 4,960,518; and US 4,960,517 there is described the modification of RO membranes (polyamide membranes) with a relatively low molecular weight reagent that is reactive to amines or oxidizes amines. The claimed process is for improving the intact properties and not for fixing imperfections. The relatively low molecular weight of the reagents cannot improve the rejections when large size imperfections are present.

In US 4,634,531 there is described a process for improving stability and rejections (not for fixing imperfections) using two water soluble materials applied in sequence from an aqueous solution. The second water soluble compounds reacts with the first soluble compound to form an insoluble material on the membrane. The above patent states that particles do not work to achieve stability or high rejections.

From the examples which describe a first coating with polyamines and then a second coating with a water soluble crosslinker such as a multi functional aldehyde, it can be concluded that a coating is formed. This approach cannot be used to plug large imperfections. the present invention utilizes particles, polymers and particles, or monomers to form a polymer, under conditions for forming a plug, by precipitation and crosslinking inside the imperfection. Wherein, crosslinking may occur by an in situ crosslinker or by the application of an outside crosslinker while, the above process is used to modify the entire membrane surface; the present invention is concerned with selectively fixing or plugging holes and imperfections. In the present invention crosslinkers may be applied in a second stage to the precipitated plug, otherwise the crosslinkers are present within the solution of monomers, polymer, or polymers and particles, used to plug the imperfection and are thus intimately mixed.

In US 2,886,066, tannins are used to improve RO salt rejection of asymmetric or composite membranes. There is no chemical crosslinking, and the process is for improvement and not fixing imperfections. The Tannins adhere to the selective layer. The tannins change the surface properties of the contacted membranes. Since they are not covalently crosslinked, and/or bound to the membrane, the Tannins can be washed off and must be reapplied.

In US 4,828,700 water soluble coatings based on polycarboxylic acid copolymers also containing hydroxyl and amide pendants. There is no post crosslinking and thus the coating is not stable and cannot plug major defects or discontinuities. It is said to be good for plugging discontinuities too small for tannic acid. Their process cannot be applied to repair membranes in which the rejection is less than 5% of a defect free membrane.

In US 4,214,020 there is described a method for coating the exterior of hollow fiber assembled in a bundle, which are suitable for fluid separations. The coating on the exterior surface of the hollow fiber can include coatings which enter the pores. The process is for coating the entire surface of the exterior or filling all pores of the hollow fiber without sticking of the hollow fibers to each other, utilizing the application of pressure without flow. This invention is for uniform coating and not the filling of imperfection. The use of the term pores does not describe imperfections and the coating materials are sufficiently large molecular weight molecules or sufficiently large particles (for example colloidal dispersions) that the deposited material does not readily pass through the walls of the hollow fibers.

Small molecules may be used penetrating into the pores when the objective is for the material of the hollow fiber to substantially effect the fluid separation. After depositing the coating material the deposit may be further crosslinked. All the examples are for gas separation using silicone coatings, and fluid separation within the context of the patent is gas separation. The above patent only applies to a bundle of hollow fibers.

In US 4,812,238 a water soluble polymer containing amines and carboxylic acids can be crosslinked and insolubilized with nitrous acid when a concentrated NaN02 salt solution is used. This crosslinked layer can be a discriminating layer in a composite membrane. This formulation and process cannot be used to give a selective filling and forming of dense plugs within imperfections, as it is claimed to form uniform selective barriers, on the surface. The crosslinking requires a diazonium step after coating. The diazonium reaction is hard to control and in the examples is carried out on the dry film.

In a method of developed by Koros ceramic membrane with a given pore size distribution are narrowed by plugging 2000 A° pores with 100 to 200 A° particles placed in by diffusion. There is no reactive bonding between the particles or membrane and particles. This approach is not used for selective filling of imperfections in finished membrane.

In US 4,239,714 There is described a method for modifying the pore size distribution of a microporous separation medium by selectively blocking large pores but not small ones. The process is complex and involves an evaporation step of a volatile liquid to expose the surface of large pores into which the blocking molecule can fit in. The pore blocking molecule stays at the entrance of the large pores. It does not describe an in-depth formation of a dense plug within imperfection. It describes a method of obstructing the entrance to all pores larger than a preselected size. These pore blocking materials are applied in a solvent (for example proteins in water) and then chemically crosslinked. The pore blockers located at the entrance of the pores may easily come off.

In EP 0 395 184. There is described a method for reducing the pore size of ceramic Ultra-filtration. This approach reduces all pores and does not selectively plug large imperfections. In fact the larger imperfections will not be effectively filled as the basis for pore filling is capillary action.

In US 4,704,324 describes selective layers prepared by a reaction of a cationic compound and a nucleophilic compound or group on the same compound (for example a copolymer containing both the cationic and nucleophile) such that the product contains covalent bonds formed via cation charge elimination reactions.

In a preferred case the nucleophile is anionic, for example carboxyl group on a polymer or oligomer and forms a coacervate with the reactive cationic compound before the charge elimination reaction occurs. In one aspect membrane improvement is claimed but this is due to coating followed by a charge elimination reaction and not pore plugging. The coating materials may be polymers, monomers and particles (latex, colloids and dispersions). Even with particles the claimed objective is thin film formation.

In the above patent the main objective is the formation of a selective barrier, even in the case of membrane improvement. There is no description or claim for selective blocking of imperfections.

In US 4,927,540 selective RO membranes are improved by forming an additional selective layer comprising an ionic complex of one compound containing quaternary ammonium, imidazolium or pyridinium groups and a second compound bearing at least one carboxylate, phosphonate or sulfonate groups where at least one of the first and second compounds is a polymer or prepolymer bearing more than one ionic group per polymer. The objective of the above is a selective homogenous layer and not selective blockage of membrane imperfections.

Optionally the layers may be crosslinked by crosslinking agents, or groups on one of the above compounds. The formation of the ionic crosslinking results from a sequential coating of the cationic compound and anionic compound accomplished by the following options, by application of the solutions from opposite sides of the membrane, or by application of a solution of one of the components to a membrane containing functional groups of the opposite charge.

In US 4,230,463 Monsanto claims the improvement of gas separation membranes by sealing imperfections and using a process that plugs pores by a coating of a thin layer of a highly permeable polymer such as silicone rubber which is deposited on the surface of a porous asymmetric membrane. The silicone coating has good intrinsic selectivity for one component of the gas mixture to be separated.

In this approach surface pores were sealed without loss of selectivity of the porous polymer material. The present invention is different because RO, UF, NF, MF membranes are being repaired with materials of low permeability and the objective is to selectively plug imperfections with a densely packed crosslinked material that is relatively impermeable to water and solutes with minimal coating on the membrane's surface. The coating of membranes via organo pclysiloxane- polycarbonates as described in the above patent cannot selectively plug imperfections. The above process coats the entire membrane surface and if used with water permeable membranes a reduction in the transport of liquid across the membrane will occur.

In US 5,032,149 and US 5,034,024 disclose that gas selectivity, of polyamide and polyimide membranes, is improved by surfactants. A surfactant repairs submicroscopic cracks in the selective layer. A swelling agents is also used with the surfactants, and there are no reactions for stabilization. The use of surfactants for water applications as taught in the above patents cannot be used since the surfactants are removable. The surfactant penetrates into the pores of submicroscopic cracks and seals and is stabilized by physical adhesion. The above approach can only seal very small imperfections on gas applications membranes.

In EP 0,631,806 a coating is used as an external defect sealing layer. There is no disclosure of pore plugging rather a coating for covering a defect. This approach cannot seal large defects and results in a significant reduction in flux.

In NTIS report PB 82-157975"Post-Treatment Process for Reverse Osmosis Membranes" (film TecCorp) colloidal particles are made and used to plug pin holes made by a No. 19 needle. These pin holes reduce the rejection from 98%-99% for 3.2% NaCI in RO membranes to 20%-50%. When tested in RO modules, the procedure brings the membranes back to a 98% range. Large holes are not plugged and according to US 4,634,531 small imperfection cannot be fix by this approach.

The particles are pre-made, and there is no step to crosslink them for stabilization.

Thus, effect of the above process limited, and the particles must be reapplied. The chemistry and material for making these particles may however be used in the present invention in the following way: 1. In situ formation of colloidal particles in the pores under pressure; and 2. Pre-made colloidal particles mixed with reactive polymers, and/or multi functional reagent may be forced together into the imperfection to form a dense plug.

Materials and processes are incorporated by reference to US 4,659,474 and US 4,039,440. In these patents examples are given of crosslinking agents and hydrophilic polymers for the making of nano-filtration and reverse osmosis membranes. Process from both patents deposit a thin film on the surface, which becomes the selective barrier. In both cases examples are given of mixtures of polymers and crosslinkers deposited on the surface of the membrane with minimal pore penetration by various coating procedures such as pressure, immersion, dipping and casting. This surface film is further crosslinked by additional steps. The above patents do not teach the selective blockage of imperfection, by in situ formation of plugs. The hydrophilic polymers and crosslinkers in these two patents utilized in the present invention for the selective plugging of imperfections, by reference.

Particles, dispersions, latexs, colloids (for example synthetic latex particles) described in US 4,802,984 and US 4,497,917 and produced by emulsion polymerization having for example particle sizes of 0.05 to 0.25 microns; material examples include anionic acrylic, anionic nitrite-styrene-butadiene, styrene- butadiene-vinyl pyridine, terpolymer, vinyl and vinylidene chloride copolymer, resorcinol-formaldehyde, polyvinyl chloride-acrylic copolymer, and vinyl fluoride latexs. Commercially available latex particles may be purchased from Dow Chemical Co., B. F. Goodrich Co. And many others.

Multiple layer latexs or core shell latexs or particles produced by sequential emulsion polymerization of the same or different polymers.

Particles produced by suspension polymerization such as that from polystyrene and polymethylmethacrylate. <BR> <BR> <BR> <BR> <P> Dispersion of pigment.<BR> <BR> <BR> <BR> <BR> <BR> <P> Inorganic particles and dispersions.<BR> <BR> <BR> <BR> <BR> <BR> <P> Polymerized micelles.<BR> <BR> <BR> <BR> <BR> <BR> <P> Powdered coatings used on epoxy particles, or blocked polyurethane powders (ex Vestagon EPBF 1300).

Crosslinked microparticles (0.01 to 10u) based on urea, urethane or amide groups (US 5159,017) or based microparticles containing polymeric amide-acid and a neutralizing base as an emulsifier dispersant (US 5,176,958) or an epoxy (US 5,135,970).

Aqueous dispersions of epoxy containing compounds which are stabilized against flocculation by external stabilizers (US 4,122,067 and EP-OS 81,163 and others) or by internal emulsifiers (German offenkgungsschrift 3,643,751 and 3,820,301 and in EP-OS 51,483). <BR> <BR> <BR> <BR> <P> Carbon black.<BR> <BR> <BR> <BR> <BR> <BR> <BR> <P> Anodized particles based on organic, inorganic or combinations of both.<BR> <BR> <BR> <BR> <BR> <P> Colloidal dispesions based on organic or inorganic material or combinations of both.

"Dendrites.

Particles containing blowing agents such as : volatile liquids. chemical blowing agents (sodium bicarbonate, sulfonyl hydrazides, dinitrozopentamethylenetetraamine, azidocarbonamides.

The solutions containing the particles, dispersions, latexs, colloids, may contain additional agents that act to stabilize the solutions against precipitation and flocculation while in solution. These agents may be surfactants, emulsifiers, protective polymeric or particles which act to prevent aggregation.

While the invention will now be described in connection with certain preferred embodiments in the following examples so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention.

Description of Preferred Embodiments Example 1: A sea water RO membrane area 20 cm'is placed in a 10 cm long and 2cm wide flow cell and tested at 55 bars 20 ° C, flow rate 5 liters/min over the surface, and gave a rejection of 97.5% to 3.2% solution of NaCI with a flux of 650 liters/m'day. The cell is opened and a 10 cm crack is made on the membranes length by bending it back and forth for several times. Reinserting the membrane under the above conditions gives a rejection of only 90% to 3.2% solution of NaCI with a flux of 720 liters/m2 day.

The membrane is then processed as follows : 1.1,2 ethyleneglycol diglycidylether at a concentration of 0.5% maintaining a solution pH of 9.5, applying a pressure of 10 atm., for a period of 15 min.

2. Discard the above solution from the cell.

3. Introducing into a cell a mixed solution containing 0.2% of Polyethyleneimine and 0.1% polyacrylic acid, with a pH of 10.5, maintaining a pressure of 10 bars for a period of 15 minutes.

4. Discarding the second solution from the cell.

5. Introducing into the cell a solution of citric acid buffer at pH 4.0 and applying this solution to a membrane for a period of 15 minutes at a pressure of 10 atm.

6. Discarding the buffer solution.

7. Introducing into a cell a solution of Epoxy as in step 1 for a period of 30 minutes.

The membrane after this process had a rejection to 3.2% NaCI at 55 bars of 98% and a flux of 480 liters/m2 day.

Example 2: Example 1 is repeated using a 0.1% solution of polyethyleneimine and 0.05% solution of polyacrylic acid. The initial rejection and flux before making a crack was 98.2% and 620 liters/m2 day at 55 bar. After making the crack the salt rejection drops to 89% and the flux increases to 740 liters/m2 day.

After the repairing sequence of steps the salt rejection increases to 99.0% and the flux is 610 liters/m2 day.

NOTE: there was only a very small flux decline and all the rejections was recovered.

After standing for 72 hours, the membrane was re-tested and gave 99.2% rejection and 615 liters/m2 day.

Example 3: Example 2 was repeated using a brackish water membrane instead of a sea water membrane. The membranes was tested at 15 atm. and gave 96.5% rejection and 1030 liters/m2 day using a 1500 ppm solution of NaCI. After making a crack the rejection dropped to 93.0% and the flux increased to 1500 liters/m2 day. After repairing as in example 2 the salt rejection increased to 98% and the flux was 965 liters/m2 day at @15 atm.

Example 4: Example 1 is repeated without the first epoxy step and gave similar results.

Example 5: Example 4 is repeated without the first epoxy step and a polyethyleneimine and polyacrylic acid concentration of 0.1 and 0.35% respectively.

The membrane performance starting, after crack, and after the repair is 98.2%/720 liters/m2 day, 91%/816 and 98.7%/690 liters/m2 day respectively.

Example 6: A brackish water membrane as used in example 3 had a starting rejection of 97.2% 1420 liters/m2 day. After making 10 pin holes down to the non woven (but not through the non woven), the membrane was pressurized at 10 atm. for 40 minutes with a pH 9.5 solution containing 500 ppm solution of water insoluble epoxy particles (1 micron average diameter), 1000 ppm polyethyleneimine and 1000 ppm of the ethyleneglycol diglycidyl ether. The membrane after the pin holes, and then after repairing had rejection & fluxes of 57% & 1720 liters/m2 day and 98.1% & 1400 liters/m2 day to 1500 ppm NaCI solution. The particles were made by taking 2 grams of a novolak epoxy dissolving in 100 ml/acetone and adding this rapidly with stirring to 900 ml H20 containing 50 mg of SDS. This solution was diluted by four folds before application in the plugging experiment.

Example 7: Example 5 is repeated with the difference of adjusting the pH of mixed solution containing 0.1% polyethyleneimine and 0.35% polyacrylic acid from pH 10.5 to pH 9.2 and adding enough water soluble epoxy (ethylene glycol diglycidyl ether) to make a 0.1% solution. This solution was pressed through the membrane for 40 minutes at a pressure of 10 atm. The solution was discarded after this step and the membrane left to stand in. citric acid buffer at pH 6.5 for 20 minutes. The epoxy crosslinked and precipitated the polymer inside the crack. The starting membrane had a salt rejection of 95% and a flux of 700 liters/m2 day. The rejection after the crack was 29% and a flux of 20,000 liters/m2 day, after plugging the rejection was 96% and the flux 620 liters/m2 day.

Example 8: Example 7 is repeated with the difference that only polyethyleneimine without the polyacrylic acid is used. The rejections in 1500 ppm NaCI solution at 15 bars drops from 98.1% in the original membrane to 85% after the crack formation and is repaired after the plugging experiment to 98.5%.

Example 9: Example 8 is repeated by damaging the top selective layer over a 0.5 cm2 area and exposing the underlying supporting UF membrane.

Following the procedure of the example 8 the membrane was repaired and brought back to its original rejection of 98.5% plus an addition 0.6% to 99.1%. The rejection of the damaged membrane before the repair is only 82%.

Example 10: Example 6 is repeated with a membrane having a 3.5 mm diameter hole protruding through the RO selective layer and through the supporting UF layer down to the non-woven. The rejection and flux is 0% and 25,000 liters/m2 day to 1500 ppm NaCI at 12 atm. After repairing according to the procedure in example 6, the rejection was 95% and the flux was 650 iiters/m2 day respectively.

Example 11: A brackish water membrane as in example 8 was scratched with a sharp needle to make thin long lines down to the non-woven. The membrane was repaired according to example 8. The rejection/flux originally, after the crack, and after the repair are 97% & 826 liters/m2 day, 35% & 1020 liters/m2 day and 98% & 750 liters/m2 day respectively.

Example 12: An RO membrane as in example 8 is scratched with a sharp needle to make a 8 cm long scratch which penetrates to the non-woven. The rejection/flux at 15 atm., 1500 ppm NaCI goes from 97.8% & 650 iiters/m2 day to 75% & 1120 liters/m2 day. Instead of the epoxy a solution 0.3 millimoles/liter of cyanuric chloride is added to the polyethyleneimine solution at 10°C at pH 9.5 and pressed at 15 atm. through the membrane. The resultant membrane had a rejection of 98.2% & 616 liters/m2 day.

Example 13: Example 11 is repeated but instead of cyanuric chloride, a 0.2% solution of glutaraldehyde was added to the solution and the membrane's salt rejection is brought back to its original value with only a 10% loss of flux.

Example 14: Example 11 is repeated but instead of cyanuric chloride, using this time a solution of 0.15% of reactive Blue 4 (Aldrich catalog 24,481-3) dye with similar results as an example 11 with only a 5% loss in flux.

Example 15: Example 11 is repeated but instead of polyethyleneimine, polyvinyl alcohol having a molecular weight of 90,000 is used. The results are similar as in example 11.

Example 16: Example 15 is repeated with a copolymer of polyvinyl/vinylacrylic acid. The results are similar as in example 15.

Example 17: Example 8 is repeated using a copolymer of polyvinyl aniline/styrene sulfonate. The results are similar as in example 8.

Example 18: Example 8 is repeated using a nano-filtration membrane from DDS. A crack is made in the membrane by multiple bends to reduce the rejection of glucose from 96% to 86%. After correcting the membrane according to the procedure of example 8 the rejection was 98% to glucose.

Example 19: A UF membrane from Kalle has a 95% rejection to a 1% BSA solution. A 7 mm diameter hole is made, by removing the polysulfone membrane and exposing the non-woven. A solution containing 500 ppm of polystyrene colloidal particle (0.5 micron diameter) with carboxylic surface groups, 200 ppm polyethyleneimine and 200 ppm of the water soluble epoxy at pH 9.5 flowed along the membrane surface at 2 atm. for 30 minutes. After this treatment the rejection to BSA was 96% and the flux decreased by only 5%.

Example 20: A supported microporous membrane, Gelman AP-200W (acrylic copolymer on polyester support) with a nominal Cutoff of 0.2 microns is used. A 50 micron sized needle is used to make 50 micron sized holes. The intact membrane shows a clear permeate with a feed of 5 micron diameter polystyrene particles under pressure of 1 atm. The membrane with 50 micron sized holes gives a cloudy permeate under the same conditions. After repairing the punctured membrane with a 2000 ppm solution of polystyrene particles with sulfonic groups on the surface of 20 micron average diameter, 500 ppm polyethyleneimine and 500 ppm glutaraldehyde for 30 minutes gives a membrane that gives a clear permeate when tested again with 1000 ppm solution of 5 micron polystyrene particles.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative examples and that the present invention may be embodied in other specific forms without departing from the essential attributes thereof, and it is therefore desired that the present embodiments and examples be considered in all respects as illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.