FR2466265A1 - - Google Patents

Abstract

at. New membrane for filtration devices. b. Modified membrane for filtration. A modified membrane for filtration is obtained by contacting one of the surfaces of a membrane which has not undergone thermal curing or baking treatment with a solution containing a water-insoluble polymer or copolymer and a polar organic solvent. Preferably, the polar organic solvent used is methanol and the polymer and polyvinyl acetate. vs. Applies to filtration membranes, in particular, for the desalination of seawater by a reverse osmosis process.

Description

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  The present invention relates to a new and improved membrane

  for filtering apparatus, membrane obtained by treating its surface

  with a hydrophilic polymeric compound insoluble in water.

  Membranes of this kind are known, and in particular cellulose acetate membranes for reverse osmosis, used to prepare water.

  drinking water from seawater, these membranes being treated, after a long time

  this period of use, with an aqueous ammonia solution of a copolymer of vinyl acetate and an ethyl ester of maleic acid, the membrane after being immersed in the polymeric solution then being treated with a solution diluted hydrochloric acid, having a fold of 4 and containing 1% by weight

  of zinc chloride, which has the effect of precipitating the polymer on the

  brane. This process has the disadvantage that the ammoniacal solution affects the filtration capacity of the membranes, in particular the membranes of

  cellulose, which, on the other hand, have undergone initial curing treatment.

  or cooking, to give them the desired porosity.

  In the state of the art, it is known to treat cellulose acetate membranes with a solution composed of a copolymer or a polymer of vinyl acetate containing 5% of crotonic acid, which

  has the effect of dissolving said copolymer in an aqueous ammonia solution.

  that said copolymer then being rendered insoluble by acidification. We apply

  that also this copolymer on a cellulose acetate membrane that has undergone

  initially a heat treatment.

  It has been found that although it is possible to cover the

  cellulose acetate branes which have undergone heat treatment at

  relatively low erasures, with a copolymeric coating, this coating

  can be easily removed during the subsequent use of these.

  Another disadvantage of the two known methods for covering a mem-

  Cellulose acetate brane with a polymeric coating, as described above, is that only water soluble polymers can be used, so that the polyvinyl acetate must be previously modified and converted to a soluble copolymer. in water. It is clear from the foregoing

  that the use of polyvinyl acetate to form the coating of mem-

  cellulose acetate branes is impossible due to insolubility in

the water of this substance.

  To summarize the above, it is clear that the processes mentioned

  above require: firstly, the use of membranes that have been treated

  first, the preparation of copolymers which are soluble in water and the application of these aqueous copolymers by means of an ammoniacal solution, followed by a precipitation of the copolymer with a

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  acid and metal ions. Despite these preparations, the membranes

  have a relatively short practical life which could be explained by insufficient adhesion of the polymeric coating on said membranes,

  which are most often cellulose acetate.

  The present invention proposes to provide an improved membrane

  intended for filtration, a membrane obtained by treating its surface with a

  lution of a hydrophilic polymeric compound insoluble in water, which provides a long-lasting membrane. By "polymeric compound insoluble in

  water "means a compound which is not soluble in pure water.

  According to the present invention, this result is achieved in that the mem-

  modified brane, for use in filtration, is obtained by treating the surface of a membrane with a solution consisting of a hydrophilic polymeric compound insoluble in water, containing an organic solvent

  polar. This treatment is preferably carried out with a solution of a poly-

  mother or a vinyl acetate copolymer containing a polar organic solvent. A membrane of this type, which has been modified according to the invention, is very advantageous because the vinyl acetate polymer or copolymer

  the coating adheres perfectly to the surface of the membrane

  especially if it is a cellulose acetate membrane,

  thus substantially prolonging the duration of the membrane thus treated.

  Another important advantage of the present invention is that the treatment

  temperature of the membranes and, in particular, cellulose acetate membranes, may be suppressed when applied to them

  a polymeric coating according to the invention.

  Another great advantage of the invention lies in the fact that the membrane no longer needs to be covered with a coating constituted solely by a vinyl acetate copolymer, but that it is equally possible to use polyvinyl acetate, the latter polymer, if insoluble in water, is soluble in certain polar solvents such as methanol or mixtures thereof.

methanol and water.

  To obtain the desired porosity, the membranes no longer need to

  undergo an initial thermal baking treatment with the process according to the invention.

  tion. As a result, the manufacture of membranes thus treated allows considerable labor savings. The polymers or copolymers that can be used to treat the surface of the membranes in accordance with the invention are more particularly homopolymers of vinyl acetate having a molecular weight of between 2,000 and 100,000, but a copolymer containing at least 40% of vinyl acetate. vinyl acetate and, for the remainder, a monomer, the copolymer thus formed having to be naturally soluble in polar organic solvents,

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  especially in an alcohol or an aqueous solution of alcohol, for example

  in methanol or in a mixture of water and methanol, can just as easily be used. Among the monomers suitable for the desired modification, there may be mentioned unsaturated carboxylic acids or their esters corresponding to the following general formula

X -CH = CY-COOZ

  wherein X represents a hydrogen atom, or an alkyl or carbon group

  lower xyl, where Y is hydrogen, lower alkyl or CH2COH and Z is hydrogen or alkyl

inferior.

  By lower alkyl group is meant in this case> a group

  containing from 1 to 3 carbon atoms.

  It is advantageous to use dicarboxylic acids such as

  maleic acid, fumaric acid and itaconic acid, for example.

  Copolymerization copolymer formation is a single process

  versely known, that it would be superfluous to describe. It is obvious that, with respect to the above-mentioned carboxylic acids, they include not only the carboxylic acids themselves, but also their anhydrides as, for example, maleic anhydride is very suitable and easily applied. Unsaturated tricarboxylic acids, for example

  aconitic acid, can also be used and the same is true for monohydric acids.

  unsaturated carboxylic acids, such as, for example, crronal acids, acrylic acid and methacrylates.

  The concentration of the polymeric compound in the solution ranges,

  by weight, between 0.01 and 5% and is preferably between 0.5 and 1.5%.

  When a solution similar to the substrate is added, the concentration of the compound

  polymer in the latter is between 0.1 and 100 ppm.

  The solution containing the vinyl acetate polymer or copolymer contains at least 5% methanol and, although pure methanol (100% methanol) can also be used as the solvent, this is less advantageous because of the influence of restrictive of it on membranes, especially when it comes

  of cellulose acetate membranes.

  With regard to the concentration of the alcohol, the upper limit is preferably 75% and more preferably 45 to 55%, the organic solvent

  polar being preferably an alcohol and in particular a lower alcohol.

  The preferred alcohol is methanol. However, it is obvious that instead of

  methanol, one could also use ethanol, butanol and propanol.

  The following examples, which of course have no limiting character,

  will better understand the particularities of the invention.

EXAMPLE 1

  A tubular cellulose acetate membrane is made which is not subjected to the usual thermal firing treatment to give it the desired porosity. A 1% solution of a vinyl acetate copolymer, modified with 5% of crotonic acid, is poured into this tubular membrane, this copolymer having a molecular weight of approximately 50,000. The pH of the polymer solution is The solution consists of equal parts of water and methanol as polar organic solvent. This solution is removed after 10 seconds and the membrane is rinsed with water (instead of water, an aqueous solution of methanol could be used). After rinsing the membrane with water or with a solution

  aqueous methanol is tested with a saline solution containing 5000 ppm.

  sodium chloride at a pressure of 40 atm (4 mPa).

  The flow rate, that is to say the amount of water passed, amounts to 3.6 cm 3 / cm 2

  per hour, while salt retention is 96%.

  An identical experiment performed with membranes that did not undergo heat curing treatment, but were rinsed with water, resulted in a flow rate of 30 cm / cm / hr and saline retention.

of 3%.

  Thus, the obvious advantages of the invention are obvious.

EXAMPLE II

  We proceed as in Example I, except that we cover the inner face.

  cellulose acetate membrane with a homopolymer with a molecular weight of approximately 30 000. In this case also a solution

  composed of 50% water and 50% methanol.

  The results of a reverse osmosis performed at a pressure of 40 atm

  give a flow rate of 5.9 cm 3 / cm 2 / h and a salt retention of 85%.

  Long-term experiments have proven that the coating adheres

  to the membrane for about a year and a half. Throughout this period, we do not

  serve no reduction in coating properties.

EXAMPLE III

  A coating of cellulose acetate is applied to a cellulose acetate membrane

  as described in Example I. After the usual treatments, the

  brane for filtration at a pressure of 4-atm by adding 75 ml of a solution

  1% of the polymer according to Example I to the feed solution.

  After a period of three hours, the flow rate is 1.5 cm / cm / h and the retention is 85%. We continued the experiment for three days

  and a flow rate of 0.5 and retention of 95% was obtained. This last retention

has remained constant.

  Uncoated membranes that have undergone pre-heat treatment

  have a flow rate of 0.42 and a retention of 85% at a pressure of 4 atm. This unfavorable result clearly shows the advantages of the coating according to the

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present invention.

  Membranes without. heat-cured or baked coating have a flow rate of about 4 and a retention of 96% at a

40 atm pressure.

  It is clear from the above example that the present invention achieves a considerable flow rate and a high retention percentage at very low pressures. This results in considerable energy savings

  during filtration and therefore lower overhead costs.

EXAMPLE IV

  We proceed again as in example I but instead of using

  membranes made of cellulose acetate, a polyacrylic membrane is used.

  nitrile (Orlon). Treatment with a 1% solution of polyvinyl acetate, modified with 5% crotonic acid (polyvinyl acetate having a molecular weight of 50,000) in a solvent composed of 50% water and 50% of

  methanol, also results in a membrane having a retention percentage of

  very high output and high flow rate for a long period of time even at relatively low pressures of the order of 4 atm. The obtained results

  at this pressure are approximately the same as those indicated in the

ple III.

  For the sake of completeness, it should be noted that

  The bottom of the preformed membrane, which is in contact with the liquid being filtered and which is covered with a polymeric coating, is the only surface that comes into contact with the polar solvent. The other side of said

  cellulose acetate membrane does not come into contact with said organic solvent.

  polar, for example, with methanol.

EXAMPLE V

  The surface of a cellulose acetate tubular membrane, which is intended to come into contact with the medium to be filtered, is treated with a solution of 1% polyvinyl acetate in a methanol / water mixture (50/50 ) as described above in Example I. The membrane is also used for treatment with a solution

  0.1% sodium chloride-in water.

  At an operating pressure of 20 atm for a period of 1000 hours,

  a constant flow rate of 3.2 and a salt retention of 95% are obtained.

  The above results are also obtained with polysulfone membranes.

Claims (10)

  1. Modified membrane for filtration, obtained by treating one   of its surfaces with a solution of a hydrophilic polymeric compound   water-soluble, characterized in that said membrane surface is treated with a solution of a hydrophilic polymeric compound insoluble in   water containing a polar organic solvent.   2. Modified membrane according to claim 1, characterized in that the hydrophilic polymeric compound insoluble in water is a polymer   or a vinyl acetate copolymer.   io 3. Membrane according to claim 1 or 2, characterized in that   the membrane consists of a cellulose ester or polyacrylonitrile Trile.   4. Modified membrane according to any one of claims 1   at 3, characterized in that the polar solvent is an alcohol.   5. Membrane according to any one of claims 1 to 4, characterized   in that the polar organic solvent is composed of methanol, ethanol,   of propanol and / or butanol, the most preferred being methanol.   6. Modified membrane according to any one of claims 1   at 5, characterized in that the solution contains from 5 to 75% organic solvent.   than the polar, the remainder being water, the solution preferably containing from L5 to 55% of polar organic solvent and better still of 50% of solvent polar organic.   7. Modified membrane according to any one of the claims   previous, characterized in that it is formed of a membrane which has not undergone any hardening or curing heat treatment, directly after its manufacture, which has been treated with the solution containing the compound   polymer, the polymeric coating being preferably washed with a   aqueous solution after application.   8. Modified membrane according to any one of the preceding claims   33 cdentes, characterized in that only the surface of the membrane which comes into contact with the liquid to be filtered has been brought into contact   with the solution containing the polymeric compound.   9. Method for performing filtration using a membrane   modified, said membrane having been modified as specified in one of   conque of the preceding claims.   10. Process for performing filtration using a membrane according to claim 9, characterized in that a saline solution is desalted at a pressure below 20 atm and preferably at a pressure of between 3 and 7 atm.