The invention relates to a hydrophilic porous polymer membrane. Particularly, the invention relates to a polyolefin membrane, which is obtainable by blending a polyolefin polymer and a hydrophilic component with a solvent followed by extrusion of the blend. Furthermore, the invention relates to methods for making and to uses of such membranes.

Polymer membranes are commonly used for separation, filtration and concentration of solutions, suspensions and gasses. They have a broad application range and can be used in several applications like gas filtration and liquid filtration such as micro filtration, ultra filtration, nano filtration, as well as in reverse osmosis, electro- dialysis, electro-deionization, pertraction, and pervaporation. Examples of applications include waste water purification, fuel cells, controlled release of pharmaceutical components, batteries, filters and humidifiers. Most of the polymer membranes are made from hydrophobic materials like polyethylene (PE), polypropylene (PP), poly(vinylidene-difluoride) (PVDF), and polytetrafluoroethylene (PTFE). These membranes are not suitable as such for water filtration as the polymers are hydrophobic and hence require a relatively high pressure gradient to force water through the membrane because of high capillary force in hydrophobic membrane pores. In addition, hydrophobic surfaces are more prone to fouling than hydrophilic surfaces. Some membranes are inherently hydrophilic, such as membranes of cellulose-acetate and nylon based materials. The cellulose- acetate ester membranes, however, are prone to degradation by enzymes, and nylon has inherent disadvantages as it is difficult to prepare highly porous membranes from it. In contrast, many of the hydrophobic polymers are inherently stable. Hence, for a long time methods have been developed, to make hydrophobic membranes more hydrophilic, thus maintaining stability and improved flux.

A number of methods are currently used to make hydrophobic membranes hydrophilic by surface modification. Examples being surface grafting by redox initiators, chemical treatment, chemical grafting, corona treatment, photochemical treatment, halogenation, plasma treatment, and copolymers.

In one of these, plasma treatment (i.e., gas plasma treatment) is used to modify the surface of the membrane. Plasma treatment can damage or changing the original structure. In another method, a coating is applied or grafted on the surface, based on hydrophilic acrylate monomers. Furthermore, polymer blends are used, in which hydrophilic and hydrophobic polymers are mixed and processed into membranes.

Blending of hydrophilic and hydrophobic polymers has also been suggested. However, the intrinsic porosity and the structure of the resulting hydrophobic membrane are completely altered as compared to a membrane based on only hydrophobic polymer. Additionally, the natural incompatibility of hydrophobic (like highly crystalline UHMWPE) and hydrophilic polymers might lead to a phase separation, and it is hard to get desirable water transport performance.

In WO2005/069927, an electrically conductive microporous and macroporous film is suggested. The film comprises ultrahigh molecular weight polyolefin, a carbonaceous electrochemically active powder, a conductive agent and optionally a hydrophilic additive. A wide range of hydrophilic additives are suggested including inorganic powders and secondary polymers. The film is prepared by preparing a mixture of ultrahigh molecular weight polyethylene, carbonaceous electrochemically active powder, conductive agent with plasticizer and extruding the mixture before reducing the thickness of the film by calendaring, blown film or cast film methods. The disclosure of WO2005/069927 is not supported by experimental work.

JP 2002-194133 (pub. no.) discloses a porous film for a separator in a lithium battery or a capacitor. The film consists of polyolefin resin and a second polymer selected from a range of hydrophilic polymers and macromolecules. Examples presented describe ultra high molecular weight polyethylene with polyethyleneglycoldimethacrylate. The film is prepared by hotpressing a mixture of polyethylene, polyethyleneglycoldimethacrylate and paraffin followed by rolling, stretching and annealing before removal of paraffin. No examples concerning other second polymers are disclosed.

It is known in the art that small molecules like alcohol and/or surfactants can temporary hydrophilize otherwise hydrophobic PE membranes to obtain a waterflux. However, the hydrophilization typically only lasts for a few hours and is clearly not suited for long-term (1-4 years) use. Another way to get water through a hydrophobic membrane is using high pressure, i.e. a pressure above the water breakthrough pressure. A typical commercial polyethylene membrane (Solupor® grade 7P03A exhibits a typical water breakthrough pressure of about 2.2 bars and has no waterflux at pressures below 2 bars). It can be wetted by an alcohol or surfactant to obtain an initial water flux of about 3300 l/m ² h bar, but this is a only a short temporarily effect. Hence, both the membranes as well as the processes still await further improvement.

It is the object of the invention to provide an improved hydrophilic membrane. It is another object of the invention to provide a method of manufacturing the improved hydrophilic membrane.

It is yet another object of the invention to provide advantageous uses for such a membrane.

The improvement may for example be one or more of making the requirement of pre-treatment of the membrane less important or prevent need for redundant pre-treatment, to increase longtime hydrophilicity of the membrane, reduce cost of the membrane and/or manufacturing of the membrane, safe production process, increase strength or reproducibility of the membrane and increase the water flux obtainable for the hydrophilic membrane. One or more of the above objects was realized by a porous polymer membrane comprising (a) a polyolefin polymer, and (b) a hydrophilic component, where the hydrophilic component comprising (b1) a hydrophilic polymer and optionally (b2) a surfactant. The content of the polyolefin polymer (a) less than or equal to 98 wt-% based on total dry weight of the membrane (see below for definition) and the content of the hydrophilic component(s) (b) is more than or equal to 2 wt-% based on total dry weight of the membrane, and the porous polymer membrane is hydrophilic. Surprisingly, the membrane could be obtained by a blending technique, where the polyolefin polymer (a), the hydrophilic component (b) and optional additives are mixed together with a solvent to form a blend in the form of a gel, a solution or homogenous mixture followed by extruding the blend.

By the membrane being hydrophilic is herein meant a membrane that at 2O ⁰ C provides a water flux for demineralised water through the membrane of at least 0.5 l/(m ² h bar), measured at 0.5 bar (see experimental section).

By being porous is herein meant that the membrane has a plurality of open micro pores. It is preferred that the average pore size is at least 0.05μm. If the average pore size is much lower than 0.05μm then the water flux through the membrane becomes too low for micro filtration applications. By pore size and average pore size is herein meant (unless otherwise stated) the mean flow pore size measured directly or indirectly with air flow techniques, as shown in the experimental section below. It appeared to be advantageous to have an average pore size of at least 0.1 μm and even more advantageous when the average pore size is at least 0.4μm as this tends to lead to a higher water flux of the porous hydrophilic membrane. The average pore size should preferably be less than about 5μm to prevent impurities in the water from flowing through the membrane. Particularly, an average pore size of less than about 2μm or even an average pore size is less than 1 μm is preferred, as smaller pores tend to be less prone to fouling or irreversible clogging of the pores. For larger pores, reversible permeation is more likely to be successful when using back flush. It should be observed that binding or storing of particles inside the membrane during use may in some applications be desirable and in such applications the structure and the initial porosity is highly important and the pore size should not be too low.

The optimum pore size depends to a large extend on the specific application of the membrane. Hence, the surprising fact that the pore size of the membrane to some extent was tunable by adjusting the processing parameters and/or the composition of the membrane without deteriorating the water permeability of the membrane allows for a number of advantageous uses for individual ranges of pore sizes. For example, the hydrophilic porous polymer blend membrane may have an average pore size of about 0.5 nm or higher for reverse osmosis or nanofiltration applications. In a preferred embodiment, the pore size is about 10 nm or higher, allowing for ultrafiltration. In another preferred embodiment, the pore size is about 100 nm or higher, as to allow for optimal microfiltration. The preferred pore size will be about 10 μm or lower as to achieve high water flux and particle filtration with good separation. In a particularly preferred embodiment - allowing good filtration in the micro and ultrafiltration range - the pore size will be about 1 μm or lower.

The pores of the membrane should preferably be arranged so that the gas permeation as indicated by the Gurley number is below 50 s/50 ml. The Gurley number is the time it takes for a volume of air to pass through an area of the membrane and it is measured as described in the experimental section. The most desirable range of Gurley number (i.e. the optimum combination of highest and lowest Gurley numbers) depends on the actual application. In general it was found to be advantageous if the Gurley number is below 20 s/50 ml and preferably below 10 s/50ml, which tends to be very useful for some filtration applications. On the other hand, it was found that a too open structure could lead to uncontrolled transport of fluid through the membrane, and hence it is preferred that the Gurley number is at least 0.3 s/50 ml and more preferred more than 0.5 s/50 ml, and more preferred that the Gurley number is more than about 1 s/50 ml. Most preferred is a Gurley number is more than 2 s/50 ml. By total dry weight is herein meant the weight of the membrane except from water and solvent. The total weight therefore includes the polymer, the hydrophilic component, the surfactant as well as optional additives described elsewhere. The weight percentages (wt-%) as referred to herein are based on total dry weight of the membrane unless stated otherwise.

The polyolefin polymer can be any polymer known in the polymer membranes field as well as newly developed membrane. The polyolefin polymer is preferably selected from the group of polyethylene, polypropylene, higher polyalkenes or combinations comprising at least one of these polymers as well as physically or chemically treated polymers of these, such as for example plasma treated or co- polymerised polymers.

In one embodiment of the invention, the main polyolefin polymer comprises at least 90 wt-% polyethylene and/or polypropylene, as these polymers are readily available in suitable grades and provide suitable properties, particularly for high molecular weight grades. It is highly preferred that the polyolefin polymer comprises ultra-high molecular weight polyethylene (hereinafter UHMWPE), as UHMWPE allows for very high strengths through stretching of the membrane. UHMWPE is polyethylene with a weight average molecular weight of more than about 500,000 g/mole, such as 500,000 - 20,000,000 g/mole. The lower limit corresponds to the required (lower) tensile strength of the membrane whereas the upper limit corresponds to an approximate limit where the material becomes too rigid to process easily. The UHMWPE may be bi-modular or a multimodular mixture, as that increases processability. A membrane based on UHMWPE has as advantage that it is highly dimensional stable, also under stress, and that thin micro-porous membranes with high porosity can be made. Particularly, it was found that a high content of UHMWPE is advantageous as UHMWPE may be processed by extrusion and afterwards being stretched to form a very strong and affordable membrane as well as a membrane that is both chemically and mechanically stable (e.g. with regard to thermal cycling and swelling behaviour) even when blended with a fraction of another polyolefin or a hydrophilic component. Examples of very useful membranes include those wherein at least 40 wt-% of the polyolefin polymer (a) is UHMWPE. If high temperature resistant membranes are required, it may be advantageous to use membranes with at least 70 wt-% of the polyolefin polymer (a) being UHMWPE ₁ or even with at least 80 wt-% of the polyolefin being UHMWPE. In one embodiment, the polyolefin polymer is substantially UHMWPE. The remainder of the material preferably being another polyolefin, such as high molecular weight polyethylene (HMWPE) ₁ LLDPE ₁ LDPE, PP and the like. Preferably, mixtures of HMWPE and UHMWPE are used. A preferred polyolefin polymer comprises 40-80 wt% of UHMWPE and 60-20 wt% HMWPE. By HMWPE is herein considered polyethylene with a molecular weight of 100,000 to 500,000 g/mole.

In another embodiment according to the invention, at least 5 wt-% of the polyolefin polymer is HMWPE. Preferably at least 20 wt-% of the polyolefin polymer (a) is HMWPE, such as at least 30 wt-% of the polyolefin HMWPE, as a substantial amount of HMWPE (particularly in combination with UHMWPE) tends to improve processability of the membrane. Surprisingly it was found that introducing HMWPE allowed for tune the pore size of the membrane, and adjusting the amount of UHMWPE and HMWPE hence provides a means to control the average pore size of the membrane, which is highly desirable. It was further found that for most applications, the polyolefin polymer preferably comprises less than 60 wt-% HMWPE, and more preferably the polyolefin polymer comprises less than 25 wt-% HMWPE to prevent deterioration of membrane properties such as mechanical strength and micro structure of the membrane.

In a preferred embodiment of the invention, the main polyolefin polymer is a hydrophobic polymer comprising UHMWPE useful as self-supporting membrane. A hydrophilic polymer blend membrane comprising UHMWPE has as additional advantage that the membrane exhibits high strength and high porosity.

The solvent is at least partially an organic solvent as this facilitates the formation of the blend. A number of solvents are possible, but it was found that the use of decaline facilitated reaching a suitable combination of membrane properties, such as processability, pore size, homogeneity of blend and extractability of the solvent after extrusion. Other examples of usable solvents are a-polar or low-polar solvents or mixtures of solvents comprising decaline and/or other aliphatic or aromatic solvents, parafine (oil) and/or other oils or long chained alcohols or ethers.

The water permeability of the membrane (also referred to as the water flux through the membrane) is a very important property of the membranes according to the invention, as many separation applications (such as humidifiers and water filters) require a significant water flux. It was found that for most applications the water flux should typically be at least 100 l/(m ² h bar). Preferably the water flux is at least 400 l/(m ² h bar), and a water flux of at least 800 l/(m ² h bar) would be preferred, when measured at 0.5 bar. In most cases a water flux of at least 1000 l/(m ² h bar) is highly advantageous, as this is sufficient for typical low cost commercial hydrophilic membranes. By selecting structure, composition and/or composition adequately the water flux can be considerably higher such as at least 1200 l/(m ² h bar) and in some cases the flux may be more than 10000 l/(m ² h bar) or even 50000 l/(m ² h bar). Individual membranes may have a very high flux but typically the flux should be kept lower than 200000 l/(m ² h bar), such as less than 20000 l/(m ² h bar) to realize a reasonable filtering effect of the membrane. Furthermore, a very high water flux tend to lead to severe clogging or fouling the membranes, resulting in unwanted flow reduction and filtration capacity. However, this could be prevented by using a reversed flow system like backflush or backpuls.

In one embodiment of the invention, the present invention relates to an hydrophilic porous polymer membrane having a pore size of about 500 nm or less while showing a flux of 500 l/(m ² h bar) , if measured at 0.5 bar pressure. Preferably, the flux is about 1000 l/(m ² h bar) or more. Preferably, such flux is achieved with a membrane with a pore size of about 100 nm or less, as with a lower pore size, biofouling is further precluded.

In another preferred embodiment of the invention, the main blend polymer comprises UHMWPE and the hydrophilic porous polymer blend membrane having a pore size of about 200 nm or less exhibits a flux of 250 l/(m ² h bar), if measured at 0.5 bar pressure, preferably about 500 l/(m ² h bar) or more, and even more preferably, the hydrophilic porous polymer blend membrane having a pore size of about 200 nm or less while exhibits a flux of 800 l/(m ² h bar), if measured at 0.5 bar pressure.

It was found that the porosity of the membrane should be relatively high. The porosity for is defined as (1-BW/(rho x d))*100%, wherein BW is the baseweight of the membrane [in g/m ² ], rho the density of the membrane [in g/m ³¹ and d the thickness of the membrane [in m]. For some applications, where the requirement to the water flow is limited, a porosity of about at least 40 vol-% is advantageous. In most cases having a porosity of at least 80 vol-% or even at least 90 vol-% would be very useful, as this provides a very open structure, with high overall porosity.

It is highly preferred that the porosity is not evenly distributed. The most favorable structure is for the membrane to have a layered structure of fibril webs arranged substantially parallel to an outer main surface of the membrane, herein referred to as lasagna-like structure due to the visual similarity of the arrangement of the fibril webs with the pasta sheets in lasagna, where the fibril webs touch adjacent fibril webs in some areas and are separated by another matter (for example air, solvent or water phase) in other areas. The structure of the membrane may be examined by scanning electron microscope of cross sections prepared by freezing the membrane in liquid nitrogen followed by breaking by impact of a knife. The crack extending from the tip of the blade (not touched by the knife) forms a suitable sample for the investigation.

The fibril webs are formed by non-woven polymer fibrils of the polyolefin polymer and the hydrophilic component. The webs are themselves porous but with a porosity much lower than the overall porosity of the membrane. The polyolefin polymer and the hydrophilic component are typically intermingled in the individual fibril webs. The lasagna-like structure arises due to the preparation method of the membrane and hence connected to the fact that the membrane is prepared by blending followed by extrusion and stretching. The lasagna-like structure is highly advantageous for a range of separation applications, and it is highly surprising that the lasagna-like structure could be obtained for membranes prepared from a polymer blend with up to at least 35 wt% of hydrophilic polymer. For example, it has been found that membranes with the lasagna-like structure as described herein, provides superior filtration properties. It could be theorized without being limited thereto, that this is due to the highly tortuous pore structure, which forces fluid (gas, water or another liquid) to follow a relatively long path through such membranes. It is hence so much more surprising that despite the highly layered structure and hence highly tortuous pore structure, very high water fluxes could be realized as described elsewhere, which leads to a unique combination of high flow rates and high quality of filtration.

The density of the fibril webs of the lasagna-like structure may vary and depend on the thickness of the individual webs and the overall porosity of the membrane. In one embodiment the membrane has 3 to 15 fibril webs / 30μm of membrane cross section, the fibril webs being arranged substantially parallel to an outer surface of the membrane. However it is preferred that a cross section of the membrane has 4 to 12 fibril webs / 30μm, and it was found to lead to the most desired combination of properties when the cross-section of the membrane has 6 to 10 fibril webs / 30μm of membrane cross section.

The thickness of the individual fibril webs of the lasagna-like structure may also vary and depend on the density of webs of the webs and the overall porosity of the membrane. In one embodiment of the membrane according to the invention at least 70% of the fibril webs have a thickness of 0.02 to 2.5μm parallel to an outer surface of the membrane. It was found to lead to the most desired combination of properties when at least 90% of the fibril webs have a thickness of 0.02 to 2.5μm.

Even though the presence of a surfactant (b2) is optional, it is typically preferred to have the surfactant present except for applications where food contact, medical issues or similar restricts the acceptable amount of rest extractables, as the surfactant in some cases may leak partially out over time. In cases where a presence of a surfactant is acceptable, the amount of surfactant in the blend prior to extrusion is typically at least 1 wt-% based on total dry weight of the membrane. It is preferred to have at least 3 wt-% of the surfactant and most preferably, at least 4 wt-% of the blend based on total dry weight of the membrane is a surfactant. On the other hand, too high amount of surfactant tend to deteriorate the mechanical properties of the membrane and may lead to leaching of surfactant during use, which is typically not acceptable from a performance and/or a contamination point of view. Hence, it was found that the content of the surfactant (b2) in most cases should be less than 15 wt-% based on total dry weight of the membrane and preferably less than 10 wt-%. Experimental work has shown that it is most preferred that the content of the surfactant in the blend is less than 5 wt-%.

It should be observed that, some of the surfactant may be lost from the membrane during processing of the blend into the final membrane for example during the quenching (typically performed in a solvent bath) or even during use of the membrane. The extent of the loss of surfactant depends on the properties of the surfactant, such as the molecular weight, the melting/boiling point and/or the glass transition temperature of the surfactant as well as polarity and choice of solvent. However, the loss of surfactant is typically limited, and measuring the content of surfactant in a final membrane hence provides an approximation of the initial content of surfactant in the blend prior to extrusion. In other words, final membranes with compositions (including the (optional) content of surfactant) within the claimed range of the blend will be based on the same inventive idea as the rest of the membranes according to the invention and hence is within the scope of the claims.

A wide range of surfactants may be used in the membrane according to the invention, but it was found to be highly advantageous to use a non-ionic surfactant, which have a hydrophilic end group, and have a hydrophilic-lipophilic balance number (HLB number) about 2 to 10, as such surfactants tend to stabilize the hydrophilic polymer in the polyolefin membrane without leading to precipitation in the most preferred solvents, such as decaline. The term non-ionic surfactant is used herein in accordance with its classical definition as a molecule containing two structurally dissimilar groups having different solubilities in an aqueous solution. See Kirk-Othmer Enclycopedia of Chemical Technology, Online version Fifth Edition, Volume 10, Emulsions, p. 126, John Wiley Sons, New York, N.Y. 2001. Edward Kostanek, Published online 18 July 2003, (incorporated herein by reference). The traditional HLB scale ranges from 0 to 20 with lipophilic surfactants having HLB lower than about 9 and hydrophilic surfactants having HLB higher than about 11. Surfactants with HLB about 4-6 are typically w/o emulsifiers , and surfactants in the 8-18 range are typically o/w surfactants. Preferably, the surfactant is a non-ionic hydrophilic surfactant with a HLB number of 3-6. Most preferably the surfactant is selected from the group consisting of sorbitan mono-oleate, sorbitan monolaurate, sorbitan tristarate, sorbitan palmitate, sorbitan triolate, sorbitan monosterate, polyoxyethylene(2)stearyla ether, polyoxyethylene(2)oleyl ether, polyoxyethylene/polyoxypropylene block copolymer, mixtures thereof and mixtures comprising at least one of these. The most preferred surfactant is sorbitan mono-oleate or the surfactant comprises sorbitan mono-oleate as this surfactant provides a superior behavior when used in combination with decaline as the (main) solvent.

In a highly preferred embodiment, the content of the hydrophilic component (b) is at least 10 wt-% based on total dry weight of the membrane as this allows for a membrane with a more hydrophilic nature. The content of the hydrophilic component may be one compound (in which case it should be a hydrophilic polymer such as oxidized polyethylene) or the content of the hydrophilic component may be made up by a combination of two or more hydrophilic polymers and optionally further at least partially hydrophilic component(s), such as surfactant(s). It was also observed that the total content of the hydrophilic component (b) preferably should be below 40 wt-% based on total dry weight of the membrane, such as below 30 wt-%, as too high content of the hydrophilic component will lead to severe reduction of the mechanical properties of the membrane as well as processability of the membrane.

The hydrophilic polymer is typically selected from the group consisting of oxidized polyethylene (ox-PE) (including derivatives thereof, like ox-UHMWPE, ox-HMWPE), polyethylene oxide (PEO) or polyethylene glycol derivatives, PE wax, polyacrylic acid, polymethacrylic acid, polypropylene oxide, polyvinyl alcohol, ethylene vinyl acetate, cellulose and derivatives thereof, polyimide, polyetherimide, polyvinyl pyrolidone, polyethylenimine, polyamide, copolymers thereof, combinations thereof and combinations comprising at least one of these. Most preferably the hydrophilic polymer (b1 ) is ox-PE and/or PEO. For membranes comprising ox-PE it was surprisingly found that the average acid number (expressed in mg KOH / g ox-PE) tended to be more significant than the overall content of the ox-PE. In the experimental work, it was found to be advantageous that the ox-PE has an average acid number of at least 10 mg/g ox-PE, but higher values, such as at least 30 mg KOH / g ox-PE and most particularly at least 40 mg KOH / g ox-PE were very advantageous as higher acid numbers tend to lead to membranes with a more pronounced hydrophilicity and higher water flows than lower acid numbers for much lower contents of ox-PE.

Surprisingly, membranes with a combination of ox-PE and PEO performed very well. Particularly, these membranes tended to both have a very high initial water flow rate and the water flow rate tended to be very durable, i.e. staying high for a prolonged period of time. It could be theorized (without being limited thereto) the high initial water flow rate may be attributed to the PEO content whereas the long term effect may to a larger extend be attributed to the ox-PE content. Besides, the PEO also introduces a well know and highly desired antifouling effect. Likewise, the use of surfactant also tended to mainly increase the initial water flow rate. In a preferred embodiment, the membrane comprises at least 2 wt-% ox-PE (b1a) and at least 2 wt-% PEO (b1 b). A more preferred embodiment of the membrane under this aspect of the invention concerns a membrane wherein the content of ox-PE and/or PEO is at least 10 wt-% and most preferably the content of ox-PE and/or PEO is at least 15 wt-%. This allows for a membrane with both a very high initial water flow rate, and superior durability and extremely antifouling properties.

The required molecular weight of the hydrophilic polymer depends to a very large extent on the type of hydrophilic polymer. For PEO it was found that the weight average molecular weight should be relatively high such as at least

100,000 g/mole and preferably at least 200,000 g/mole. It should be observed that addition of PEO is also advantageous in enhancing the antifouling properties of the final membrane. For the ox-PE a wider range of molecular weights seemed to be acceptable. Best results for membranes comprising ox-PE were obtained when the weight average molecular weight of the ox-PE was about 1000 - 100,000 g/mole.

A particularly advantageous group of membranes is membranes comprising only oxidized PE as the hydrophilic polymer. In this case, ox-PE preferably has an average acid number of at least 10 mg/g ox-PE, but higher values, such as at least 30 mg/g oxidized PE and most particularly at least 40 mg/g ox-PE were very advantageous as higher acid numbers tend to lead to membranes with a more pronounced hydrophilicity and higher water flows than lower acid numbers for much lower contents of ox-PE. The combination of ox-PE and UHMWPE allowed for membranes with very high strengths and high hydrophilicity, which were very suitable as self-supporting membranes. As surfactants are typically not suitable for applications where rest extractables are not acceptable such as food contact or other applications that allows the surfactant to migrate into the mammal body there is a need for membranes which are hydrophilic but do not contain surfactants. Surprisingly, this was realized for membranes according to the invention where the hydrophilic polymer is ox-PE requiring only limited amounts of surfactant. Particularly it was found that the content of surfactant may be less than 1 wt-% surfactant, such as less than 0.5 wt-% surfactant. Most preferred was however, membranes that did not contain any surfactant, i.e. without any surfactant.

Adding additives, next to hydrophilic components, in the polyolefin porous membrane can be advantageous. For example, for water purification odors or impurities can be removed simultaneously while filtering the water. Electrically conducting materials in the hydrophilic membrane may for example enhance separation of charged molecules or ions, in an aqueous environment. Next to the antifouling effect of the hydrophilic component, other antifouling additive(s) may extend service life of the membranes and minimizing the need for chemical cleaning. Examples for additives for increasing the wettability of the hydrophilic membrane include organic powders as oxides, silicate, clay, diatomaceous earth and aluminum. Also additional carbon fibers, nanotubes, glass fibers or other fibers can be beneficial for the reinforcement of a hydrophilic porous polymer membrane, allow for higher freedom of design and/or increasing the lifetime of such materials and therefore minimizing the environmental impact of finally disposing it.

The final membrane may take any known membrane shape. Particularly, preferred shapes are sheet-like members. Such members may for example be used in a substantially flat shape or be folded into tubes having one or more layers of membranes or into members having a pleated (harmonica-like) surface. In another embodiment, the membrane is a hollow member, i.e. a shape that may be obtained by extrusion through a die having an insert, such as a hollow tube, a hollow box or a hollow fiber. These preferred shapes allows for a very versatile design of the final member comprising the membrane. It is preferred that the member is freestanding also referred to as self- supporting, i.e. capable of itself providing sufficient support strength to carry the weight of the membrane as well as the force exerted on the membrane during use. For membranes comprising UHMWPE this embodiment is the most preferred embodiment, as UHMWPE due to the high strength and stiffness allows for design of relatively thin membranes despite the more severe mechanical requirements of freestanding membranes. In another preferred embodiment, the membrane is arranged at least partially on a support member so that the membrane forms a flat main surface, a tubular main surface and/or pleated main surface. A tubular main surface may for example be obtained by spiral winding of one or more layers of a membrane according to the invention or by extruding a tube optionally followed by stretching for example by pressurized air or liquid.

In many applications, the membrane according to the invention is arranged in a module, such as a filtration module comprising the membrane itself (often shaped two- or three-dimensionally according to the specific needs of the applications by means well known in the art). Well know examples of filtration modules are spiral wound membrane module or a pleated membrane module. Preferred arrangements of the membrane are discussed elsewhere in the present description. The module further comprises a support and/or frame to protect the filter or to enhance handling of the filter. The support is typically of the type described for supported membranes. Such support may also be used for otherwise free standing membranes as a further precaution against mechanical damage during use. The frame typically has an outer shape that facilitates handling as well as fitting with the system in which the module is utilized. In such a module, the membrane is the essential element by providing the means for the separation process in which the membrane is to be utilized. In most applications, the membrane forms part of a larger system, such as a system for osmotic cleaning of water, a clean room (or a ventilation/air filtering system for a clean room), a fuel cell, a medical device, such as in implants or pills for example optimized for controlled release etc. In some cases the membrane is arranged in a module as described elsewhere but in all cases, the membrane is an essential feature of the whole system by providing properties or functions of the system that are essential for the function of the whole system. For example a clean room without proper air filtration would be completely useless due to contamination, and an osmotic water production plant containing reverse osmosis membranes is useless without prefilters, if the water is obtained from see or surface water. A preferred membrane has a thickness of about 0.5 mm or less, preferably about 0.2 mm or less, such as 10-100μm. A thinner membrane has the advantage of potentially higher water flux.

In one embodiment of the invention (regarding self-bearing membranes), the thickness of the membrane will be about 10μm or more, preferably about 20μm or more, to achieve a higher strength. The thickness generally will be about 500μm or less, preferably about 200μm or less. Suitable membranes for example may have a thickness of about 50μm, about 100μm or about 120μm. Although the membrane may be self-supporting, several types of membranes according to the invention are used on a support to ensure sufficient strength. In another embodiment of the invention, the final membrane will be put on a support layer either during manufacturing or after preparation of the membrane, and the thickness of the membrane may about 5μm or more, preferably about 10μm or more such as about 20μm or more. Often, such membranes have a thickness of about 10Oμm or less, preferably 50μm or less. Suitable examples include a laminate comprising one or more support layers from for example nonwoven, wovens, spunweb, web and/or grid from e.g. PET, PP, PTFE, UHMWPE, Nylon, an inorganic material such as a metal (such as aluminum or steel) or a ceramic or a glass. Examples of suitable ceramic and glass support layers are AI ₂ O ₃ and silicates, preferably sintered, porous AI ₂ O ₃ and silicates. The layers in the laminate can for example be bonded to each other by ultrasonic welding, gluing, thermo bonding or by laser welding.

In a preferred embodiment the hydrophilic porous polymer membrane according to the invention may be manufactured by mixing less than 90 wt-% polyolefin polymer (a) and at least 10 wt-% hydrophilic component based on total dry weight of the membrane with a solvent (and optionally additives) to form a blend. It should be observed that the total dry weight of membrane corresponds to the weight of the non- solvent components of the blend. Then the blend is extruded and the solvent is removed. Preferably, the solvent is removed by evaporation before stretching of the base member. In this way a base member is created with a unique porous structure, which enhances the formation of the highly advantageous layered structure of fibril webs in the final stretching operation. The preferred embodiments with regard to for example components and ranges are the same for this embodiment of the invention as the first aspect of the invention and are hence described elsewhere in the present description. Further aspects regarding the manufacturing are known in the art and described for example in US 5,376,445, US 5,370,889 and US 5,507,993 (incorporated herein by reference). It is noticed that premature phase separation, i.e. prior to extrusion, did not take place in the blend.

By blend is herein meant a mixture of components for the membrane and solvent. The mixture may be a highly viscous liquid typically in the form of a gel or an emulsion. The term extrusion as used herein encompasses the extrusion techniques known in the art, such as gel techniques, solvent extrusion, etc. In one embodiment, the blend is formed inside an extruder, such as an extruder with one or more screws, to process the blend into a highly viscous mass, such as a gel or an emulsion, which mass is drawn through a die, resulting in a thick member, such as for example a thick flat tape or thick tubular tape. Solvent is removed from the tape to form a base member.

The base member prepared as described above may be used directly as a membrane according to the invention, and is hence itself a membrane according to the invention. However, to increase specific strength, porosity, pore size and to reduce the cost per area of the membrane, the base member is preferably stretched by a factor of at least 1Ox by area to form the membrane. The stretching may be conducted batch wise or continuously. It was found to be advantageous to stretch biaxially by a factor of 2-10 in the machine direction and a factor of 3-10 in the transverse direction as this tended to lead to a suitable combination of membrane properties. Typically, a biaxially stretched a membrane according to the invention comprising UHMWPE exhibits a tensile strength in the machine direction of about 7 MPa or higher, preferably about 10 MPa or higher. In case a very high strength is required, the membrane can have a tensile strength of about 40 MPa or higher typically realized by calendaring of the membrane or the base material. The high strength allows for much thinner membranes and / or membranes that do not require rigid support during use. Furthermore, the elongation at break for such polyethylene membranes is typically in the order of 10-30% in the machine direction. This allows for a substantial (elastic) deformation during use without deteriorating the performance of the membrane. The solid content of the blend prior to extrusion is important for the processability of the membrane as well as the properties of the final membrane. A good combination of features was obtained when the dry content in the blend (i.e. the sum of polyolefin polymer, hydrophilic component and optional additives) was about 5 to 30 wt-% of the total weight of the dry content and the solvent. However the best combination of features was realized when the dry content of the blend was about 10 to 25 wt-% of the total weight of the dry content and the solvent. Additives are functional compounds such as for example rheology modifiers (such as oils), colorants and fillers (i.e. passive elements added for example to reduce weight or cost of the membrane). Additives may for example be added in the blend to increase processability or to affect the properties of the final membrane.

The way of processing (extrusion/stretching) is much more favorable than the traditional solvent casting method to produce hydrophilic membranes. Solvent cast membranes require a high cost and very well defined flat support for making the casting equally over the surface, in order to obtain a consistent film thickness. The method described in the present embodiment does not need a support for making a hydrophilic membrane, or if required can use a low cost support, like a non-woven support.

The hydrophilic porous polymer blend membranes of the present invention can be used in a large number of applications where filtration of water or water based media is required.

In a preferred embodiment of the invention, the hydrophilic porous polymer membrane is used in molecular separations, like particle filtration (in liquid or gas), micro filtration, ultra filtration, nano filtration, reverse osmosis. In one embodiment of the invention, the hydrophilic porous polymer membrane is used in a membrane bioreactor (MBR) and/or in a process for water purification. The membrane of the invention is in particular suitable in such process, because of the relatively high water flow rate at low pressure, and a low tendency for fouling.

In another embodiment of the invention, the hydrophilic porous polymer membrane is used in electrochemical applications, including electro-dialysis, electro-deionization and fuel cells.

In yet another embodiment of the invention, the hydrophilic porous polymer blend membrane is used in controlled release applications including pharmaceutical and nutraceutical components. Such applications may be for internal use, such as in an implant; or external use, such as in a bandage or another member arranged on the surface of the body such as on the skin or in an orifice of the body e.g. in the ear, eye or nose. The contact with the body may be directly or via a carrier material (solid, liquid or gas). The hydrophilic porous polymer blend membrane can also be used as a scaffold for functional groups.

In a further embodiment of the invention, the hydrophilic porous polymer blend membrane is used in pertraction, pervaporation and contactor applications.

As will be shown in the examples, it appeared possible to adjust the pore size of the hydrophilic porous polymer membrane by varying the blend formulation. It was very unexpected, that for the hydrophilic porous polymer membrane prepared by blending technique, it is possible to have a tuneable average pore size from micrometer to nanometer range while maintaining a relatively high water flux under low pressure gradient.

Test Methods: Water permeability:

The water permeability was measured at room temperature (20 ⁰ C) at a pressure gradient of 500 mbar across a disk with a 4 cm membrane diameter. 250 ml water is passed through the membrane under this pressure. The time elapsed for each 50 ml in the permeate side is recorded. Thereafter, the water flux is calculated according to the equation 1 :

J = Q / AtP (eq. 1 ) in which J is the flux [ l/(m ² h bar) ], Q is the amount of water (in liter) flowing through the membrane in the time (t) period (h), A is the effective area of the membrane (m ² ), and P is the pressure difference through the membrane. The five measurements are averaged, and the average value is reported.

Pore size

The mean flow pore size, expressed in micro meter, was obtained by using an empirical relation between the mean flow pore size, measured with the PMI and the Gurley number. The relation between the Gurley number and air permeability is described in ISO 5636-5 section 10.1. Calibrations between the Gurley number and mean flow pore size determined with the PMI apparatus (see below) resulted in the empiric correlation that the mean flow pore size [in μm] could be obtained by dividing 1.77 by the Gurley number (in seconds per 50 ml) measured according to ISO 5635-5. All values indicated in the example tables below are based on the Gurley number measurements.

The mean flow pore size, determined with a PMI apparatus, is also based on air permeability, but uses a wetting fluid, type Galwick. The common mean flow pore size method with the PMI apparatus is based on ASTM F316-03. Samples of 25 mm in diameter were wetted with a low surface tension fluid, type Galwick, and placed in a holder. A differential pressure of air removes the wetting fluid from the sample. After this wet run, a dry run (dry flow) is applied. PMI software calculates the mean flow pore size by using the differential pressure at which wet flow is equal to one- half of the dry flow.

Air permeability:

The Gurley test method (according to ISO 5636-5) covers the determination of the resistance of membranes to the passage of air. The method is applicable to membranes that permit the passage of air up to 50 ml in one second or more. In this test, a Gurley Densometer from Toyoseiki, type B was used, with a recording the time in 0.1 seconds; with a cylinder capacity of 50 milliliters, a cylinder weight of 567 gram and a measuring surface of 6.45 square centimeter (1 square inch). After calibration, a strip of a membrane is cut across the width of the roll. And a smooth, undamaged test specimen is placed over the clamping plate orifice and clamped. In this air permeability test method, no wetting liquid was used. The measurement is started, and the time is counted in units of 0.1 seconds, which is required for 50 milliliters of air to pass through the test specimen.

Thickness The thickness was measured with a Mahr Millitron, with a 12 mm in diameter footh using 0.5 N tension.

Examples

The invention will be elucidated with the following, non-limiting examples.

Example 1 :

The samples were prepared according to the following general method. A 16 to 24 wt-% solution of UHMWPE and hydrophilic components

(corresponding to component a and b in the claims) in decaline was extruded at a temperature of about 180 ⁰ C. The extruder head was fitted with a die with a 1 mm opening. The extruded film was cooled in a quench bath. The solvent was removed from the gel film by evaporation in an oven. The film from which solvent had been removed was biaxially stretched continuously or batchwise as indicated below, at a temperature of about 120 ⁰ C.

Materials used in the examples for preparation of membranes:

• UH 210, UHMWPE [MW 4,600,000] was obtained from DSM Stamylan

• PEO 200K, Poly(ethylene oxide) [MW 200,000] powder, was from Scientific Polymer Products catalog #136B

• PEO 300K, Poly(ethylene oxide) [MW 300,000] powder, was Polyox WSR N-750 from Dow Chemical Company

• PEO 400K, Poly(ethylene oxide) [MW 400,000] powder, was from Scientific Polymer Products catalog #136E • PEO 4000K,Poly(ethylene oxide) [MW 4,000,000] powder, was from Scientific

Polymer Products catalog #344

• SMO: The liquid surfactant Sorbitan mono oleate was Alkamuls S/80 from Rhodia

• Ox-PE (7 KOH), A-C 307A, Oxidized polyethylene with an acid number 5-9 mg KOH/g, was obtained from Honeywell

• Ox-PE (16 KOH), A-C 316A, Oxidized polyethylene with an acid number 15- 18 mg KOH/g, was obtained from Honeywell

• Ox-PE (28 KOH) ₁ Oxidized polyethylene, with an acid number 28 mg KOH/g, was obtained from Scientific Polymer Products catalog #406 • Ox-PE (43 KOH), A-C 395A, Oxidized polyethylene with an acid number 40-

45 mg KOH/g, was obtained from Honeywell

• PE-AA (40 KOH), A-C 540A, Ethylene Acrylic Acid Copolymer with an acid number 37-44 mg KOH/g, was obtained from Honeywell.

Table: Recipe per sample, always containing UHMWPE

O

¹ AN = Acid no.mg KOH / g Ox-PE

NJ

Ethylene-acrylic acid copolymer (i.e. not PEO)

The thicknesses of the samples were 30±10μm for continuously stretched samples, and 90±20μm for batch stretched samples.

Experimental results based on these membranes are shown in the tables below.

Table 1 : Addition of PEO and/or SMO

In Table 1 it is observed that the pure PE membrane is not hydrophilic as well as the PE membrane with only surfactant. Hydrophilic membranes may be realized by addition of about PEO, but to realize a high water flux, such as >500 l/(m ² h bar), incorporation of surfactant was needed. However, when PEO and SMO was combined, a surprisingly high and stable water flux was realized for a wide range of SMO and PEO concentrations, as well as for a wide range of PEO grades.

¹ Based on wt-% dry weight. The composition was topped by UHMWPE (UH210) to 100 wt-%. Average pore size calculated from Gurley number. Table 2: Addition of ox-PE and/or SMO

In Table 2 it is highly unexpectedly observed that for blends comprising UHMWPE and ox-PE, the acid number of the ox-PE is much more significant for the resulting hydrophilicity than the content of ox-PE. For samples with ox-PE having low acid numbers (even in combination with surfactant), the water flow rate was very limited if observed at all. On the other hand, for samples comprising ox-PE with acid numbers higher than 30 mg KOH per g ox-PE even 5 wt-% ox-PE lead to a very high and durable water flux. Surfactant is not required to realize a hydrophilic membrane with a high water flux when the membrane comprises ox-PE with a high acid number, but surfactant seems reduce the pore size, which in many applications are desirable for example due to reduced fouling for membranes with lower pore size.

' Based on wt-% dry weight. The composition was topped by UHMWPE (UH210) to 100 wt-%. ' Average pore size calculated from Gurley number. Table 3: Blending UH with ox-PE&SMO and PE-AA&SMO

Sample Composition ⁷ Mfp ⁸ Water flux Gurley

[μm] [l/(m ² h bar)] [s/50ml]

2 5% SMO 0.5 O 3.4

18 14% ox-PE (43 KOH) + 6% SMO 3.3 18635 0.5

19 14% PE-AA (40 KOH) + 4% , SMO 0.05 <100 37.2

Based on the observation in Table 2 that a high acid number of ox-PE tended to lead to high water flux it was investigated if the mere presence of a hydrophilic component with a high acid number was sufficient to achieve a high water flux. In Table 3, a sample without a hydrophilic polymer is compared with a sample with ox-PE or ethylene-acrylic acid copolymer (PE-AA); where both the ox-PE or ethylene- acrylic acid copolymer has a high acid number. It is observed that the mere presence of a high acid number component is not sufficient to realize a high water flux. In other words, the effect of the presence of ox-PE with a high acid number seems to lead to a membrane with truly unique properties, particularly when taken into account that the membranes are obtained by the very simple and low cost blending technique.

⁷ Based on wt-% dry weight. The composition was topped by UHMWPE (UH210) to 100 wt-%.

⁸ Average pore size calculated from Gurley number. Table 4: Blending UH with PEO and/or ox-PE

In Table 4 it is observed that SMO can, in combination with PEO, increases the water permeability by a factor 5 or more. Furthermore, the combination of PEO and ox-PE led to membranes with extremely high and stable water fluxes for relatively low pore sizes.

⁹ Based on wt-% dry weight. The composition was topped by UHMWPE (UH210) to 100 wt-%.

¹⁰ Average pore size calculated from Gurley number.