WO1996006671A1

WO1996006671A1 - A membrane having osmotic properties and a method of producing it

Info

Publication number: WO1996006671A1
Application number: PCT/SE1995/000668
Authority: WO
Inventors: Peter Henning Hagqvist; Osborn HÄRMESTAD
Original assignee: Ab Electrolux
Priority date: 1994-08-31
Filing date: 1995-06-07
Publication date: 1996-03-07
Also published as: SE9402892L; SE502963C2; SE9402892D0

Abstract

A semipermeable membrane having osmotic properties is comprised of a microcrackled polymer film as a base membrane having a semipermeable surface layer applied thereto. The surface layer is comprised of a polymer of one or more polyethylene glycol-monoacrylates and/or -diacrylates of the general formula: CH2 = CH - CO - O - [-CH2 - CH2 - 0 -]n - R, in which R is hydrogen or -CO - CH = CH2, and n has a value such that the acrylates will obtain a mean molecular weight within the range of 100-1000. The surface layer polymer shall include at least one polyethelene glycol diacrylate, wherein R is -CO - CH = CH2. The membrane is used in a reverse osmosis water purifying process.

Description

A MEMBRANE HAVING OSMOTIC PROPERTIES AND A METHOD OF PRODUCING IT.

The present invention relates to a membrane having osmotic properties and also to the manufacture of such a membrane. More specifically, the invention relates to a membrane of this kind which is comprised of a specific type of base membrane on which a semipermeable layer having osmotic properties has been formed by polymerizing specific monomers. The invention also relates to the use of such a membrane in reverse osmosis water purifying processes.

The purification of water with the aid of reverse osmosis is a simple method which does not require the use of complicat¬ ed, space-consuming and expensive apparatus, as in the case of conventional water purification processes, and the reverse osmosis process has therefore been used widely in recent years. The reverse osmosis method has already obtained wide use in individual dwellings that are located remote from conventional water purification plants, and the use of such methods and processes can be expected to find wide use in the developing countries in particular.

In an osmotic process, two solutions containing different concentrations of a dissolved substance are separated from each other by a semipermeable membrane whose pores have a size such as to allow the molecules of the solvent to pass through but not the molecules of the dissolved substance or substances. The solvent molecules endeavour to penetrate the membrane to dilute the more concentrated solution, so as to finally equalize the concentrations. The driving force of this process can be illustrated in terms of a pressure which can be measured or determined in appropriate apparatus and which is dependent on such parameters as the types of substances dissolved and the types of solvent used, and also the difference in concentration.

When a pressure which exceeds the osmotic pressure is applied to the more concentrated solution, the osmotic process will instead proceed in the opposite direction and the solvent will be pressed from the more concentrated solution through the semipermeable membrane and out into the more diluted solution. It is this process that forms the basis of water purification by reverse osmosis. In this water purification process, the contaminated water that contains a high concentration of dissolved substances is thus present on one side of a semipermeable membrane and the purified or cleansed water is located on the other side of the membrane. By applying a sufficiently high pressure to that side of the membrane on which the contaminated water is located, the pure water can be caused to be pressed through the membrane pores and out into the solution with a lower concentration of dissolved substances, while the dissolved substances remain on the contaminated side of the membrane and can be removed therefrom. Reverse osmosis can also be referred to as a type of filtering process on a molecular scale.

The properties of the semipermeable membrane used are of a decisive importance to the practical purification of water with the aid of reverse osmosis. It is necessary for the membrane to exhibit a sufficient chemical resistance to the substances that may be present in the water (e.g. chlorine) , and also to have a sufficient mechanical strength to withstand the pressures applied, which are often relatively high and may reach about 10 MPa. However, the requirements of high separation factors, high rates of flow and high mechanical strength are often contradictory. In order to achieve a high rate of flow, the membrane must be as thin as possible, resulting in a greater risk that pin holes will form and therewith greatly reduce the separation factor, besides impairing the mechanical strength of the membrane.

In order to avoid these drawbacks, a thin surface layer having semipermeable properties is of en mounted on a thicker plastic membrane having coarser pores, this membrane func- tioning as a carrier. The composite membrane may possibly be mounted on a further carrier, such as a woven carrier or a wire net, in order to further enhance the mechanical strength.

EP-B1-0 015 149 describes the manufacture of a microporous membrane on which a semipermeable surface layer is applied. This patent specification also contains a large number of additional references to the state of the art.

There are several different methods by means of which the thin surface layer is formed on the surface of the membrane. One method involves casting a layer of the polymer used for the surface layer on the surface of the membrane from a solution of the polymer. However, this results solely in a purely mechanical connection which has been found unsatisfac¬ tory with regard to adhesion strength.

Another method of forming the surface layer is to polymerize suitable monomers of the surface layer on the membrane so as to form a polymer layer, and then primarily a polyamide layer. The primary achievement of this method is a polyamide which adheres to the membrane surface solely through addition forces, in other words a mechanical bond. The method is performed in a liquid and it is necessary to subsequently leach out unreacted chemicals. This is unsuitable from an environmental aspect.

Another method of producing the surface layer involves supplying the monomers in a solvent together with a photo- initiator which is decomposed by UV-light and/or heat. The polymerization reaction is initiated by exposing the mixture to UV-light or to heat, so that the initiator decomposes and forms free radicals which, in turn, initiate the polymeriza- tion process. This method also requires non-reacted material to be subsequently washed out in a leaching bath. In order for monomers to be suitable for the manufacture of a semipermeable layer having osmotic properties, it is necessary for the monomer when polymerized to produce a layer which has mechanical stability, chemical stability, a suitable pore size, and osmotic properties. The present invention removes the aforesaid drawbacks and provides a membrane having osmotic properties which fulfil the aforesaid requirements to a great degree.

In accordance with the invention a semipermeable membrane is provided, said membrane having osmotic properties which is characterized in that it is comprised of a microcrackled polymer film having applied thereon an outer layer comprised of a polymer of one or more polyethylene glycol monoacrylates and/or diacrylates of the general formula

CH₂ = CH - CO - O - [-CH₂ - CH₂ - O -]_n - R

in which R is hydrogen or -CO - CH = CH₂, and n is a value such that the acrylates will obtain a mean molecular weight within the range of 100-1000, wherein the surface layer polymer includes at least one polyethylene glycol diacrylate, where R is -CO - CH = CH₂. n will preferably have a value at which the mean molecular weight will lie within the range of 100-700.

Particularly preferred acrylates are those which have the aforesaid formulae, in which n has a value of 4-5 and/or 8-9. These acrylates then have the following formulae and data:

CH₂ = CH - CO - O -[-CH₂ - CH₂ - 0 -]_4-5 - CO - CH = CH₂ Polyethylene glycol diacrylate 200; Mw = 208 g/mole

CH₂ = CH - CO - 0 -[-CH₂ - CH₂ - O -]_«.₅ - H

Polyethylene glycol acrylate 200; Mw = 254 g/mole CH₂ = CH - CO - O -[-CH₂ - CH₂ - O -]8_β-9 - CO - CH = CH₂ Polyethylene glycol diacrylate 400; Mw = 508 g/mole

CH₂ = CH - CO - 0 -[-CH₂ - CH₂ - O -]_B_₉ - H Polyethylene glycol acrylate 400; Mw = 454 g/mole

It will be seen that the preferred acrylates are not normally specific, pure compounds but mixtures of acrylates having varying values with regard to the number of ethylene glycol groups in the polyethylene glycol part. The mean value, however, will lie within the recited ranges of the n-value.

The polyethylene glycol acrylate compounds used in accordance with the invention have not earlier been commercially available or described in the available literature. The compounds are therefore considered to be novel products.

The polyethylene glycol acrylate compounds according to the invention can be prepared in known manners. In this regard, a polyethylene glycol of desired molecular weight and degree of polymerization is reacted suitably with an acrylic acid halide, for instance the chloride, in the presence of an acid-binding substance, for instance a tertiary amine, such as triethylamine. A monoacrylate is produced by reacting one equivalent of polyethylene glycol with one equivalent of the acrylic acid halide, while a diacrylate is produced by reacting one equivalent of the polyethylene glycol with two equivalents of the acrylic acid halide. The reaction is suitably performed in a solvent that is able to dissolve the reactants and which is inert under the reaction conditions, for instance an ether, such as tetrahydrofuran. The reaction mixture is then worked up in a conventional manner, by removing the formed amine salt and removing the solvent.

According to the invention, a microcrackled polymer film is used as a base membrane or support material . Such a polymer film is characterized by containing a large number of microscopic, through-penetrating slits or cracks which impart the requisite porosity to the film. However, when the above illustrate acrylate monomers are polymerized thereon, the film will be covered entirely with a semipermeable surface layer withosmotic properties. The resultant osmotic membrane is comprised of a porous carrier film which imparts the requisite mechanical strength to the membrane and which carries a surface layer polymerized thereon and having semipermeable properties.

The microcrackled base membrane preferably consists of a polyolefin, such as polyethylene or polypropylene or copoly- mers thereof. An appropriate example of such a base membrane is a microcrackled polyethylene film sold by PPG Industries in the U.S.A. under the trade name TES IN^®. Such films have earlier found use in the clothing and printing fields, but have not been used for any purpose related to the^' present invention. The polyethylene film will suitably have a thickness of 175 μm, although other thicknesses are conceiv- able, these thicknesses being readily determined by the person skilled in this art having knowledge of the require¬ ments placed on the use intended.

A semipermeable layer is produced on a membrane surface of a microcrackled polymer film by applying one or more of the inventive acrylate compounds to the surface together with one or more polymerization initiators. The initiator/initiators is/are activated by applying UV-light, heat or some other energetic radiation form, and functions/function to initiate polymerization of the monomer or monomers. In this regard, it is important that at least one of the component acrylate compounds is a diacrylate. Monoacrylates may be included as components in a polymerization mixture but cannot be used alone because no cross-linking will take place due to their monofunctionality.

The polymerization initiator may comprise any initiator known for use in polymerizing acrylic compounds. One such initiator is comprised of a compound which decomposes under the influence of UV-light, heat or some other energetic radiation to form an activated radical which then initiates the monomer polymerization process. Different types of initiators can be used in the polymerization process, such as radical, anionic, cationic and coordination initiators. Such compounds may consist of benzophenone derivatives, organic peroxides or azo compounds, and a large number of such initiators are known to the person skilled in this art and are commercially avail¬ able. The following list recites a number of polymerization initiators, by way of example only:

IRGACURE^® (benzophenone derivative, retailed by Ciba-Geigy AG; activated by UV-radiation) .

Benzophenone (activated by UV-radiation) . Benzoyl peroxide (heat-activated) . Azobisisobutyronitrile (heat-activated) .

The polymerization reaction itself consists of a typical radical polymerization of a well known kind. The reaction can be performed in solvent or as a bulk polymerization process. There is often used a system in which the monomers are dissolved in a solvent, it lying within the competence of the skilled person to choose an appropriate solvent which will not disturb the polymerization reaction and which can also be used for the polymerization initiator. Alcohols, such as methanol and ethanol, are examples of suitable solvents in this regard. The monomer concentration of the solution may be between 1 and 15%, and is normally between 5 and 10% (w/v) .

In forming a semipermeable surface layer on a plastic foil membrane, a layer of a solution containing a monomer or monomers and a polymerization initiator is applied appropri¬ ately over the surface of the base membrane. The layer can be applied to said surface in a number of different ways known to the skilled person, and graphic processes, such as offset printing, are also possible methods of application. The layer of solution may be applied to a thickness of between 1 and 500 μm, and then preferably to a thickness of between 10 and 100 μm. The layer of solution is then exposed to either UV-light or heat, depending on the type of polymer¬ ization initiator used. When UV-light is used, the light will preferably have a wavelength between 170 and 450 nm, and the radiation dosage applied will preferably be between 50 and 200 mJ/cm². The layer may be irradiated for a period of between 5 and 60 seconds, and then typically between 10 and 20 seconds.

When the polymerization is complete, monomer and solvent residues are removed, for instance by washing with water. The membrane carrying the semipermeable layer is then ready for use.

The properties strived for with the semipermeable layer on the base membrane are mechanical stability, chemical stabili¬ ty, a desired pore size and osmotic properties. These properties are controlled primarily by the degree of cross- linking achieved and by the chemical structure of the surface layer. In this regard, the degree of cross-linking is determined by the proportion of diacrylate in the polymeriza¬ tion mixture, such that greater proportions of diacrylate will result in higher degrees of cross-linking. As mentioned in the aforegoing, a certain amount of diacrylate must always be present, since it is only these difunctional monomers that can be cross-linked. The diacrylates will normally comprise the major part of the polymerization mixture, while monoacry- lates are included to modify the properties of the polymer, for instance with regard to softness and pore size. The chemical structure of the monomers, primarily the length of the ethylene oxide segment between the acrylate groups, is also significant to the degree of cross-linking achieved and influences the chemical stability, pore size and the osmotic activity of the layer. Appropriate properties can be imparted to the semipermeable layer by a suitable choice of the polymerization components, if required after simple routine tests, as the person skilled in this art will know.

The invention will now be described in more detail with reference to the following examples, these examples having no limiting significance to the scope of the invention.

Example 1: Preparation of polyethylene glycol diacrylate 200

20 g polyethylene glycol PEG 200 (0.1 mole, 1 equiv.), 150 ml tetrahydrofuran and 20.2 g triethylamine (0.2 mole, 2 equiv.) were introduced into a three-neck round flask provided with a stirrer and drip funnel, and the mixture was cooled with an ice-water mixture. 19 g acrylic acid chloride

(0.2 mole, 2 equiv.) were then added very slowly while vigorously stirring the system. It is important to ensure that this addition is made very slowly.

During the reaction, the triethylamine immediately begins to bind the hydrogenchloride formed, and the amine salt precipi¬ tates out and causes the solution to become turbid. The salt settles readily. After all of the acid chloride had been added, the mixture was left to stand for 12 hours for further reaction, while stirring the mixture. The reaction process was then stopped by adding 1 ml ethanol, which esterifies non-reacted acid chloride.

The reaction mixture was worked-up by filtering off the amine salt, and all of the tetrahydrofuran and ethanol was removed in a rotatory evaporator. To the residue, acetone was added two times, in an amount of 5-10 ml in each portion, to remove amine salt residues. The product is soluble in acetone, but not the amine salt. The mixture was chilled in a freezer and then filtered. If so desired, the acetone can then be evaporated so as to obtain the product in a pure form. The pure product is a slightly yellowish liquid product.

Example 2: Preparation of polyethylene glycol diacrylate 400

The method was carried out in the same way as that described above, with the exception that the starting material used comprised 20 g polyethylene glycol PEG 400 (0.05 mole) , 10.1 g triethylamine (0.1 mole) and 9.5 g acrylic acid chloride (0.1 mole) . When all of the acid chloride had been added, the mixture was left to finally react for three hours, whereafter the reaction was stopped and the mixture worked up in the aforesaid manner. The pure product is a slightly yellowish liquid product.

Example 3: Manufacture of an osmotic membrane

A microcrackled polyethylene film having a thickness of 175 μm and comprising the material TESLIN^® (from PPG Industries, U.S.A.) was coated by means of an offset printing method with a 7 μm thick layer of polyethylene glycol diacrylate 400, containing 5 percent by weight of the UV-sensitive initiator IRGACURE^® 500 (from Ciba-Geigy AG, Basel, Switzerland) . The surface was heated with warm air, to reduce the viscosity of the mixture so as to obtain a homogenous monomer film. The surface was then irradiatedvith UV-light having a wavelength of between 170 and 450 nm at a dosage of 150 mJ/cm². When the surface had cured, any remaining, non-reacted residual monomers were leached out with water.

There was obtained a membrane comprising a base film of microcrackled polyethylene having polymerized thereto a semipermeable surface layer with osmotic properties. The membrane was well suited for use in reverse osmosis water purifying processes. Example 4: Manufacture of an osmotic membrane

A microcrackled polyethylene film having a thickness of 175 μm and comprised of the material TESLIN^® (from PPG Industries, U.S.A.) was coated by means of an offset printing method with a 50 μm thick layer of a solution containing 10 percent by weight polyethylene glycol diacrylate 400, and 1 percent by weight IRGACURE^® 500 (from Ciba-Geigy AG, Basel, Switzerland) in ethanol. The surface was heated with warm air to drive off the ethanol and to form a homogenous polymer film. The surface was then irradiated with UV-light having a wavelength of between 170 and 450 nm at a dosage of 200 mJ/cm². When the surface had cured, any remaining non-reacted monomers were leached out with water.

There was obtained a membrane which comprised a microcrackled base film having a semipermeable surface layer of osmotic properties polymerized thereto. The membrane was well suited for use in reverse osmosis water purifying processes.

The inventive membrane having osmotic properties can be used in a purely conventional manner in typical water purification apparatus by means of reverse osmosis. Such apparatus, their construction and their modus operandi are well known to the person skilled in this art-. It will be observed, however, that although the purification of water by means of reverse osmosis is a preferred area of use of the inventive membrane, the inventive membrane is not limited solely to this use. The inventive membrane can be used within any area in which a semipermeable membrane is required in osmotic processes, and those modifications to the properties of the membrane which are required for a specific application can be readily determined by the person skilled in this art who has knowledge of the invention, and on the basis of simple routine experiments if so required.

The present invention enables the provision of a special combination of a microcrackled plastic base film and a semipermeable layer of novel, specific acrylate polymers polymerized thereon. In this way, there is obtained an osmotic membrane which exhibits a successful combination of properties .concerning separation ability and mechanical and chemical stability.

Although the invention has been described and illustrated with reference to specific exemplifying embodiments thereof, it will be understood that the embodiments and examples described have no limiting significance and that variations and modifications are possible within the scope of the following Claims.

Claims

1. A semipermeable membrane having osmotic properties, characterized in that the membrane is comprised of a microcrackled polymer film having applied thereon a surface layer consisting of a polymer of one or more polyethylene glycol monoacrylates and/or diacrylates of the general formula

CH₂ - CH - CO - 0 - [-CH₂ - CH₂ - O -]_n - R in which R is hydrogen or -CO - CH = CH₂, and n has a value such that the acrylates will obtain a mean molecular weight within the range of 100-1000, wherein the surface layer polymer includes at least one polyethylene glycol diacrylate, wherein R is -CO - CH = CH₂.

2. A membrane according to Claim 1, characterized in that the microcrackled polymer film is comprised of polyethylene.

3. A membrane according to Claim 1 or Claim 2, characterized in that the polyethylene glycol monoacrylates and/or diacry¬ lates have a mean molecular weight in the range of 100-700.

4. A method of producing a membrane according to any one of Claims 1-3, characterized by bringing a microcrackled polymer film into contact with at least one polyethylene glycol monoacrylate or diacrylate of the formula

CH₂ = CH - CO - 0 - [-CH₂ - CH₂ - O -]_n - R in which R is hydrogen or -CO - CH = CH₂, and n has a value such that the acrylate will obtain a mean molecular weight within the range of 100-1000, wherein there is included at least one diacrylate, in mixture with an initiator, whereafter the initiator is activated to initiate polymerization of the polyethylene glycol acrylate compounds on the surface of the polymer film.

5. A method according to Claim 4, characterized in that the polyethylene glycol monoacrylates and/or diacrylates have a mean molecular weight in the range of 100-700.

6. A method according to Claim 4 or Claim 5, characterized by activating the initiator with the aid of UV-light, heat and/or some other type of radiation.

7. A method according to any one of Claims 4-6, character¬ ized in that the microcrackled polymer film is comprised of polyethylene.

8. The use of a membrane according to any one of Claims 1-3 in a reverse osmosis water purifying process.