WO2014072404A1

WO2014072404A1 - Method for fabricating a membrane

Info

Publication number: WO2014072404A1
Application number: PCT/EP2013/073265
Authority: WO
Inventors: Arnaud David Henri Chiche; Petrus Johannes Lambertus Boonen; Marko Dorschu; Konraad Albert Louise Hector Dullaert; Roman STEPANYAN
Original assignee: Dsm Ip Assets B.V.
Priority date: 2012-11-07
Filing date: 2013-11-07
Publication date: 2014-05-15

Abstract

The invention relates to a method for fabricating a membrane, the method comprising the following steps: a) providing at least a first porous carrier substrate, b) providing at least a first layer of nanofibers on one side of the first porous carrier substrate, c) providing at least one adhesive after step (b), by electro-spinning or centrifugal-spinning to form a structure, wherein the adhesive has a softening temperature lower than the softening temperature and the degradation temperature of the first porous carrier substrate and the first layer of nanofibers, d) consolidating the structure obtained in step (c) by means of a temperature cycle or a pressure cycle thereby obtaining the membrane. The invention further relates to a membrane and a filter comprising the membrane.

Description

METHOD FOR FABRICATING A MEMBRANE

The invention relates to a method for fabricating a membrane.

Further, the present invention relates to a membrane. Furthermore, the present invention relates to a filter comprising the membrane.

A method for fabricating a membrane is known from WO12072725A. WO12072725A describes a method comprising the following steps: (a) obtaining at least a first porous carrier substrate, (b) providing a first layer of nanofibers on one side of the first porous carrier substrate, (c) providing an adhesive to form a further structure and (d) bringing the further structure up to or above the melting temperature of the adhesive and below the melting temperature of the at least first porous carrier substrate and of the first layer of nanofibers and reducing the temperature below the melting temperature of the adhesive thus obtaining the membrane. The adhesive in W012072725 is provided e.g. as a porous adhesive layer, a porous adhesive membrane or a non-woven layer of adhesive.

A disadvantage of the method described in WO12072725A is, that the adhesive that is applied to adhere the porous carrier substrate to the layer of nanofibers reduces a flux of air or liquid through the membrane.

A goal of the present invention, amongst other goals, is to provide a membrane with an increased flux of air or liquid. Further, another goal of the present invention is to provide a method for fabricating a membrane comprising diverse components, said method achieving the fabrication of a membrane presenting a good adhesion between said diverse components. Furthermore, it is a need in the prior art to provide membranes that do not show clogging after a prolonged use.

These goals are achieved by a method for fabricating a membrane, the method comprising the following steps of (a) providing at least a first porous carrier substrate; (b) providing at least a first layer of nanofibers on one side of the first porous carrier substrate; (c) providing a suitable amount of at least one adhesive after step (b) by electro-spinning or centrifugal-spinning in order to form a structure, wherein the adhesive has a softening temperature lower than the softening temperature and the degradation temperature of the first porous carrier substrate and the first layer of nanofibers; and (d) consolidating the structure obtained in step (c) by means of a temperature cycle or a pressure cycle or a combination thereof thus obtaining the membrane.

The method according to the present invention provides the fabrication of a membrane which is suitable for long-term use, because the components of the membrane adhere adequately together and the membrane presents less clogging. Advantageously, the method according to the present invention consists of carrying out the steps of providing one porous carrier substrate, one or more layer of nanofibers on one side of the first porous carrier substrate, providing a suitable amount of at least one adhesive after providing said layer of nanofibers in order to obtain a structure, optionally providing a second porous carrier substrate and consolidating the obtained structure according to the present invention. According to the present invention, the adhesive can be provided by electro-spinning or centrifugal-spinning after providing at least a first layer of nanofibers on one side of the first porous carrier substrate in step (b), or during and after step (b), or before and after step (b), or before, during and after step (b) to form the structure obtained in step (c). If more than one layer of nanofibers is provided in the method according to the present invention, such as two, three, four, five, six layers of nanofibers, the adhesive can be provided for the additional layer(s) before, during and/or after providing any of the second, third, fourth, fifth, sixth layer of nanofibers.

According to the present invention the above-mentioned goals are reached by providing a suitable amount of at least one adhesive after step (b) by electro-spinning or centrifugal-spinning to form a structure, wherein the adhesive has a softening temperature lower than the softening temperature and the degradation temperature of both the first porous carrier substrate and the first layer of nanofibers. Additionally, by providing a suitable amount of at least one adhesive after step (b), the method according to the present invention achieves the fabrication of a membrane which properties are better. Similarly obtainable good properties of the membrane can be achieved when a suitable amount of at least one adhesive is provided during and after step (b), or before and after step (b). The better properties are, amongst others, better adhesion of a porous carrier substrate and a first layer of nanofibers, or a better adhesion between the different components of the membrane. In other words, the diverse components of the membrane adhere better together and therefore the method according to the present invention provides an efficient way of fabricating long-lasting membrane. In the context of the present invention, the components of the membrane are intended to be any porous carrier substrate, any layer of nanofibers, any adhesive present in said membrane according to the present invention.

As a result of providing the adhesive by electro-spinning or centrifugal-spinning after step (b), or during and after step (b), or before and after step (b), or before, during and after step (b) to form the structure obtained in step (c), an increased flux through the membrane is obtained, the adhesion between the components of the membrane is improved and the obtained membrane presents less clogging after a prolonged use (because the membrane does not deteriorates in a short period of time, thereby preventing the clogging). Further, providing the adhesive "before and after" step (b) by electro-spinning or centrifugal-spinning is advantageous for obtaining an adhesion between the first porous carrier substrate and the first layer of nanofibers. Carrying out step (c) before step (b) accordingly provides an increased adhesion between the first porous carrier substrate and the first carrier layer of nanofibers. Providing the adhesive "during and after" step (b) by electro-spinning or centrifugal-spinning is advantageous for obtaining adhesion between the nanofibers of the first layer thus e.g. avoiding delamination. Generally, by providing the adhesive "after" step (b) by electro-spinning or centrifugal-spinning is advantageous for obtaining an adhesion between the first layer of nanofibers and a further layer of either nanofibers, or a further porous carrier substrate layer. Providing the adhesive "after" step (b) can also be used to improve or modify surface properties of the nanofiber membrane, like e.g. wear resistance.

In the context of the present invention, a suitable amount of adhesive is to be understood as an amount of adhesive between 0.002 and 0.5 g/m². According to the method of the invention, the structure obtained in step (c) is consolidated by means of a temperature cycle or a pressure cycle thus obtaining the membrane. The temperature cycle generally includes, bringing the structure up to or above the softening temperature of the adhesive and below the softening and degradation temperature of the at least first porous carrier substrate and of the first layer of nanofibers and reducing the temperature below the softening temperature of the adhesive thus obtaining the membrane. In the context of the present invention, in step (d) a temperature cycle which is carried out is to be understood as a repetitive exposure of the structure obtained in step (c) to low (such as below the softening temperature) and high (such as above the softening temperature) temperatures. For example, by carrying out a cyclical thermomechanical loading, the structure is consolidated (preventing fatigue failure). Accordingly, the temperature cycle consists of subjecting the structure obtained in step (c) to the specified low (or high) temperature then subjecting it to the specified high (or low) temperature for a specified number of cycles using an equipment known as a temperature cycle chamber. Similarly, a pressure cycle is the operation of exposing the structure of obtained in step (c) to a cyclical succession of exposure to low pressures (such as below 100Pa/below

0.001 bar) and high pressures (such as above 1000 Pa/above 0.01 bar), in order to consolidate the structure.Where a thermosetting adhesive is applied, the structure obtained in step (c) is consolidated in step (d) by a temperature cycle to cure the adhesive. Where a pressure sensitive adhesive is used, the structure obtained in step (c) is consolidated in step (d) by an appropriate pressure cycle, well known to a person skilled in the art. It is understood that consolidation by a temperature cycle can be combined with a pressure cycle, either in a combined heat and pressure cycle or successively a heat cycle preceded or followed by a pressure cycle. Whichever consolidation method is used, it may result in a change of morphology of the spun adhesive, resulting in small adhesive domains. Typically, the adhesive nanofibers will melt and flow, forming nanoparticles, microparticles, and droplets in the micron range, micro or nano webs, or any combination of these structures.

As used herein, the term "porous carrier substrate" refers to a substrate that allows normal manual manipulation without damaging or breaking. The porous carrier substrate, generally made of microfibers, may be adapted for carrying a layer that is as such not strong enough to remain undamaged during manipulation, or use. The surface weight of a porous carrier substrate is generally above 10 g/ m², generally below 300 g/ m² The surface weight of the porous carrier substrate is generally between 10 and 300 g/m², preferably between 20 and 200 g/m² and more preferably between 30 and 100 g/m².

The porous carrier substrate is not limited to fiber-type substrates (i.e. non-woven). It can be any textile, woven, knitted or in any other form. It can also be any porous membranes including ceramics, foams and films like precipitated, quenched or stretched films. In case of ceramics, the surface weight of the porous carrier substrate can be above 5000 g/m².

As used herein, the term "microfibers" refers to small diameter fibers generally having an average diameter between about 0.5 μηη and about 100 μηη, with an exemplary range of about 4 to about 50 μηη. Examples of microfibers include, but are not limited to, melt-blown fibers, spun-bonded fibers, paper-making fibers, pulp fibers, fluff, cellulose fibers, nylon staple fibers, although such materials can also be made larger in size than microfiber-sized. Microfibers can further include ultra microfibers, i.e., synthetic fibers having a denier per filament (dpf) of between about 0.5 and about 1 .5, provided that the fiber diameter is at least about 0.5 μηη.

Microfibers may be made of glass, carbon, ceramics, metals, and synthetic polymers, such as e.g. polyamides, polyesters, polyolefins, or natural polymers such as cellulose and silk.

As used herein, the term "nanofibers" refers to fibers having a number average diameter generally not greater than about 1000 nanometers (nm), but not greater than 800 nm or even not greater 600 nm. Nanofibers are generally understood to have a number average diameter range of about 40 to about 600 nm, more specifically from about 40 to about 300 nm, more specifically still from about 60 to about 100 nm. Other exemplary ranges include from about 300 to about 600 nm, from about 100 to 500 nm, or about 40 to about 200 nm. In the context of the present invention, to determine the number average diameter of the fibers, ten scanning electron microscopy (SEM) images at 5,000x magnification were taken of each nanofiber sample or web layer thereof. The diameter of ten clearly distinguishable nanofibers was measured from each photograph and recorded, resulting in a total of one hundred (100) individual measurements. Defects were not included (i.e. lumps of nanofibers, polymer drops, intersections of nanofibers). The number average diameter of the fibers can be calculated from one hundred (100) individual measurements.

The number average diameter of the nanofiber can be varied e.g. by varying the solution concentration of the polymer solution and thus the viscosity of the polymer solution used to make the nanofibers. A generally suitable viscosity is between 200 and 1000 mPa.s. The polymer solution can contain one or more suitable solvents.

Reducing the solution concentration can for example reduce the nanofiber diameter. Another possibility to vary the diameter is to modify the process conditions such as for example the applied electrical voltage, the flow rate of the polymer solution, the choice of polymer and/ or the spinning distance. The skilled person in the art can easily, without undue experimentation or burden, determine the best set of process variables to reach the desired properties of the nanofiber.

In the context of the present invention, the method for fabricating the membrane may involve at least one further layer of nanofibers, such as one further layer of nanofibers, two further layers of nanofibers, three further layers of nanofibers, four further layers of nanofibers. In those cases, the same or a different at least one adhesive can be provided before, during or after providing the second or further layer of nanofibers. According to the present invention, a further adhesive that may be added before, during or after providing at least one further layer of nanofibers, has a softening temperature lower than the softening temperature and degradation temperature of any porous carrier substrate or any layer of nanofibers in the membrane. The step of consolidating the structure (step (d) of the method according to the present invention) can be applied after every layer is deposited (multiple treatments), after several of the layers have been deposited (also several treatments) or at the very end as one unique treatment on the membrane. A layer of nanofibers may be made of any suitable material, such as for example a polymer such as polyamide, polyurethane, polyethersulfon,

polyvinylidenefluoride, celluloseacetate, polyesteramide, polyethyleneoxide, polyethyleneimine, polysulfon, polytetrafluoroethylene, polyolefins etc. Nanofibers have the advantage of having a large specific surface area, resulting in good absorbance properties for particles to be filtered from the flux of material. A layer of nanofibers can be provided by any web forming technology known in the art, like electro-spinning, melt-blowing, spun bonded extrusion and centrifugal-spinning.

In the embodiment wherein the adhesive is provided "during and after " the step of providing of a layer of nanofibers, it is an advantage to carry out step (c) according to the method of the present invention by electro-spinning in order to provide the adhesive as well as for providing a layer of nanofibers. The advantage is that the obtained membrane has an extended life-time, due to higher resistance to

delamination of the components of the membrane.

As used herein, the term "adhesive", is to be understood as a material adapted for mechanically fixing a first material with itself (more than one item made of the same material), or with a second material. The adhesive material is generally a thermoplastic polymer with a softening temperature lower than the softening temperature and degradation temperature of both the first porous carrier substrate and the first layer of nanofibers. Examples include - but are not limited to - polyolefins, polyamides and polyesters.

The softening temperature of a material herein is defined as the highest temperature of the melt temperature and/or the glass transition temperature of said material. Crystalline polymers have a melt temperature (T_m) and do not have a glass transition temperature (T_g). Semi-crystalline polymers have both a melt temperature (T_m) and a glass transition temperature (T_g) whereas amorphous polymers only have a glass transition temperature (T_g) and do not have a melt temperature (T_m). Glass transition temperature (T_g) measurements (inflection point) and melting temperature (T_m) measurements can be carried out via differential scanning calorimetry (DSC) on a Mettler Toledo, TA DSC821 , under N₂ atmosphere and at a heating rate of 5°C/min. Melting temperature (T_m) and glass transition temperature (T_g) can be determined using the second heating curve.

As used herein the degradation temperature of a material is defined as the temperature above which the material undergoes an irreversible loss of mechanical strength and/or chemical modification. The adhesive could also be a thermoset material provided that its curing temperature is below the softening temperature and degradation temperature of both the first porous carrier substrate and the first layer of nanofibers. Examples include, but are not limited to siloxanes and epoxies. Blends of the above mentioned materials can also be successfully used e.g. thermoplastics and thermosets, or thermosets and Pressure Sensitive Adhesives (PSA), or thermoplastics and PSA.

In the method of the invention, the first layer of nanofibers and the adhesive may be provided with a second porous carrier substrate at a side of the first layer of nanofibers opposite to the first porous carrier substrate prior to step (d). An advantage of a second or even further porous carrier substrate is that it protects the first layer of nanofibers during the membrane fabrication process, especially during the consolidation step and in particular where a pressure cycle is used for consolidating the further structure. A further advantage of a first layer of nanofibers between two porous carrier substrate layers is to prevent the first layer of nanofibers from surface induced damages (wear) and to reduce the stress applied by a liquid flow on the nanofiber membrane.

In the context of the present invention the term "at least a first layer of nanofibers on one side of the first carrier" is to be understood as one first layer on one side of the first carrier, or one first and one second layers on one side of the first carrier (i.e. two layers in total).

According to the present invention, a second layer of nanofibers is provided after step (b), in other words, a second layer of nanofibers is provided after providing the first layer of nanofibers. According to the present invention, a suitable amount of at least one adhesive is provided before, during or after, or any combination thereof, the step of providing the second layer of nanofibers, by electro-spinning or centrifugal-spinning to form a structure, wherein the adhesive has a softening temperature lower than the softening temperature and the degradation temperature of the first porous carrier substrate and the first layer of nanofibers, In the context of this embodiment of the present invention, if the method comprises providing a second carrier, said second carrier is provided after providing the second layer of nanofibers, (and the second layer of porous carrier substrate).

As used herein, the term "electro-spinning" refers to a technology that produces nano-sized fibers referred to as electro-spun fibers from a solution using interactions between fluid dynamics and charged surfaces. In electro-spinning, a polymer solution or melt provided from one or more needles, slots or other orifices is charged to a high voltage relative to a collection grid. Electrical forces overcome surface tension and cause a fine jet of the polymer solution or melt to move towards the grounded or oppositely charged collection grid. The jet can splay into even finer fiber streams before reaching the target and is collected as an interconnected web of small fibers. The dried or solidified fibers can have number average diameters of about 10 to 1000 nm, or from about 70 to about 200 nm, although 100 to 600 nm fibers are commonly observed. Various forms of electro-spun nanofibers include branched nanofibers, split nanofibers, nanofiber yarns, surface-coated nanofibers, nanofibers produced in a vacuum, and so forth. The production of electro-spun fibers is illustrated in many publication and patents, including, for example, P. W. Gibson et al, "Electro- spun Fiber Mats: Transport Properties," AIChE Journal, 45(1 ): 190-195 (January 1999). It is understood that electro-spinning might be discontinuous, thus resulting in electro- spraying, with the adhesive in the form of small droplets with a size of below 4 μηη, typically below 1 μηη.

In case electro-spinning is used for making the first layer of nanofibers, providing the adhesive "during" step (b) can be done either by feeding a part of the spinning nozzles/units for the first layer of nanofibers with the adhesive instead of the feedstock for the first layer of nanofibers, or by using some spinning blocks in a spinning line for the adhesive.

An alternative way of providing the adhesive "during" step (b) is introducing adhesive layers interspersed with nanofibers layers, thus building the first layer of nanofibers.

The term "centrifugal spinning" in this application refers to a method which uses a centrifuge encased in a jacket. One or more spinning orifices can be distributed more or less evenly over the outer circumference of the centrifuge. Rotation of the centrifuge causes the solution, which is fed to the centrifuge (under pressure) via a feed line, to be extruded through the spinning orifices in the direction of a jacket, through the diameter of the spinning orifices, the rotational speed of the centrifuge, and the distance between the outer circumference of the centrifuge and the inside of the jacket. Centrifugal spinning can also be carried out without orifice, the liquid (melt or solution) being deposited on a rotating surface or "cup".

A further advantage of the method according to the present invention is the formation of a better bond between layers in a multilayer membrane, e.g.

between a first layer of nanofibers and a first substrate layer, or between two or more consecutive layers of nanofiber, made of similar materials and fibers dimensions, or made of different materials and/or fiber dimensions. The better bond between layers of a multilayer membrane allows a long term use of the membrane preventing degradations, or splitting apart of the diverse components or layers. In other words, the method according to the present invention provides a membrane having an improved life-time.

Another advantage of the method according to the present invention is that with very low amounts of adhesive, a transition layer obtained achieves a good adhesion between the different components of the membrane (the porous carrier substrate and the layer of nanofibers). Also within a layer of nanofibers or between different or similar layers of nanofibers a good adhesion is obtained.

The amount of adhesive and/or the at least first layer of nanofibers with a thickness of 2000 nm is between 0.002 and 0.5 g/m². An additional advantage of these small amounts of adhesive is an increased flexibility in the material to be used as adhesive, without disturbing e.g. the hydrophobic or hydrophilic properties of the membrane. A further advantage of these small amounts of adhesive is that possible negative effects like toxicity, unwanted adsorption or leaching are reduced.

Another advantage of the method according to the present invention is that providing the adhesive by spinning allows a very homogeneous deposition of the adhesive.

A further advantage of the method according to the present invention is that the method achieves homogeneity across the whole membrane thickness, which reduces even further the amount of adhesive required and thus to increase even further a flux through the membrane. Such an advantageously homogeneous membrane thickness can be achieved by providing the adhesive after the spinning of the layer of nanofibers. This procedure is specific to the fabrication of particular membranes wherein the layer of nanofibers has a surface weight above 2 g/m² and/or wherein the layer of nanofibers is consisting of nanofibers having a thickness below 400 nm.

A further advantage of the present invention is that there is no need to use bi-component or coated fibers for the substrate like e.g. in US2008/01 10342 A1 , which gives more flexibility in the choice of materials, texture and morphology of the first porous carrier substrate and also reduces the need for substrate pre-treatment.

For the preparation of membranes wherein a thicker layer of nanofibers (such as when the layer of nanofiber consists of an amount of layers of nanofibers above 5-10 layers, and/or when the layer of nanofibers has a surface weight above 2 g/m² and/or for nanofibers having a number average size below 400 nm), the adhesive penetration into the layer of nanofibers is limited, and a deposition of the adhesive during and after spinning of the layer of nanofibers is recommended to insure a good cohesion with minor amount of adhesive.

The invention further relates to a membrane comprising (a) at least a first porous carrier substrate, (b) at least a first layer of nanofibers, wherein the first layer of nanofibers comprises an adhesive that has a softening temperature below the lowest softening temperature or degradation temperature of both the first porous carrier substrate and the first layer of nanofibers, characterized in that the amount of adhesive in the transition layer and/or the at least first layer of nanofibers with a thickness of 2000 nm, is between 0.002 and 0.5 g/m². The definitions, preferences and advantages recited in the context of the method according to the present invention are applicable to the membrane according to the present invention, as well as to any article comprising said membrane.

It is an advantage of the present invention that the component of the membrane according to the present invention consists of one, two, or more porous carrier substrates, at least one layer of nanofibers (such as one, two, three, four, five, six, seven, eight, nine, ten layers) and at least one adhesive (forming a transition layer) after a layer of nanofiber and/or between different layers of nanofibers, wherein the adhesive has a softening temperature below the lowest softening temperature or degradation temperature of both the one or two porous carrier substrates and the at least one layer of nanofibers, characterized in that the amount of adhesive and/or the at least one layer of nanofibers with a thickness of 2000 nm, is between 0.002 and 0.5 g/m².

According to the present invention, the membrane may comprise a second layer or more layers of nanofibers or a second porous carrier substrate, or more porous carrier substrate. In the context of the present invention, the porous carrier substrate can also be considered as a layer of material, to be designated as a substrate layer. Where these further layers of nanofibers, transition layers can be provided which comprises an adhesive, the amount of adhesive in the transition layer and/or deposited after the at least first layer of nanofibers with a thickness of 2000 nm, is between 0.002 and 0.5 g/m².

The invention further relates to a fluid filter comprising the membrane of the invention. An advantage of a fluid filter according to the present invention for the filtration of water or blood is an improved wettability, which gives more freedom in choosing an adhesive. Another advantage of a filter according to the present invention is a reduced adsorption of protein or other molecules in the fluid. Generally hydrophobic adhesives are easier to find than hydrophilic adhesives. However, large amounts of hydrophobic adhesives used in the state of the art decrease the flux of water based liquids through a filter. Due to the small amount of adhesive, hydrophobic adhesives can be used with only a minor effect on the flux. Preferably, the fluid filter of the invention comprises a hydrophobic adhesive. A hydrophobic adhesive has a limited swelling only and no dissolution in water based liquids, which is a key property for medical applications.

Brief description of the drawings

Fig. 1 through 4 Illustrate membranes. Fig. 1 and 2 are comparative membranes and Fig. 3 and 4 are membranes according to the present invention.

Fig. 1 shows a membrane, comprising a porous carrier substrate

(10), a layer of nanofibers (30), and a spun adhesive (20) that forms a transition layer between the porous carrier substrate (10) and the layer of nanofibers (30). This membrane is fabricated by providing the porous carrier substrate (10) followed by providing the adhesive (20) by electro-spinning subsequently followed by providing a layer of nanofibers (30).

Fig 2 shows a membrane comprising a porous carrier substrate (10), a layer of nanofibers (30), wherein the layer of nanofibers (30) comprises an adhesive (20). This membrane is fabricated by providing the porous carrier substrate (10) followed by electro-spinning of the layer of nanofibers during which an adhesive (20) is provided by electro-spinning.

Fig. 3 shows a membrane comprising a first porous carrier substrate

(10), a layer of nanofibers (30), and an adhesive (20) that forms a transition layer between the layer of nanofibers (30) and a second porous carrier substrate (1 1 ).

This membrane can be fabricated by providing a first porous carrier substrate (10), subsequently followed by providing a layer of nanofibers (2), providing an adhesive

(20) by electro-spinning and providing a second porous carrier substrate (1 1 ).

Fig. 4 shows a membrane comprising a porous carrier substrate (10), a spun adhesive (20), a layer of nanofibers (30) comprising an adhesive (21 ), again a spun adhesive (22) and a porous carrier substrate (10). A membrane according to Fig.

4 can be fabricated by providing the porous carrier substrate (10) followed by providing the adhesive (20) by electro-spinning subsequently followed by electro-spinning a layer of nanofibers (30) during which electro-spinning an adhesive (21 ) is electro-spun as well, subsequently followed by providing an adhesive (22) by electro-spinning and finally providing a porous carrier substrate (10). The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements and in particular the thickness of some layers may be exaggerated and not drawn on scale for illustrative purposes. Example

In Fig. 3 and 4 the porous carrier substrate is a non-woven polyamide with a mean flow pore size of about 39 μηι (Novatex 2579 from Freudenberg Filtration Technologies KG, Germany) and a basis weight of 70 g/m². The adhesive is an electro- spun of an amorphous electro-spun polyamide, with a softening temperature of 81 C. The preparation of the amorphous polyamide used, is described in WO2007006425 Experiment 1. The basis weight of this layer is about 0.03 g/m².

The layer of nanofibers is a layer of electro-spun nylon 4,6 (Stanyl® of DSM) with a basis weight of about 1 g/m².

Delamination of porous carrier substrate and the layer of nanofibers required a high force and occurred within the layer of nanofibers. A part of the layer of nanofibers remained on the substrate.

Despite the hydrophobicity of the resin used as an adhesive layer, this membrane is hydrophilic and can be easily wetted by water. This made it possible to use any polarity for the adhesive in the method of the present invention, even for applications in water or blood filtration.

The mean flow pore size is determined with a method using ASTM F 316. A capillary flow porometer test was performed on a Porolux 1000 system. A capillary flow porometer measures the pore sizes and distributions of through pores in filters. In the overall methodology, a filter is wetted with a liquid. This liquid has preferably a contact angle of zero with the filter material and a known surface tension with gas at the measurement temperature. If this is the case, the pore size can be calculated using the Washburn equation: pressure (mbar) = 4 x surface tension (dyn/cm) / pore size diameter (μηι). This is done by gradually increasing the pressure of gas over the sample in a closed container. The pressure at which an increase in gas flow is observed is then recalculated towards pore size. Typical parameters like bubble point, mean flow pore size, smallest pores and the pore size distributions are automatically calculated. The methods used for this purpose are described in ASTM F 316.

As opposed to other systems, the Porolux 1000 uses a pressure equilibrium routine. This states that between chosen boundaries the pressure and gas flow towards or through a sample have to be fully stabilized before a data point is taken as a true value. This results in very accurate measurement of the pore size diameters and very narrow but correct pore size distributions. Typically for non-woven materials, this will result in a one or two-point distribution, as all openings towards these structures are interconnected throughout the complete filter. With more discrete pores like filters prepared through emulsion polymerization, through laser shooting and other methods, more broad distributions can be found.

In the context of the present invention, the layer of nanofibers can also be designated by 'nanoweb'. Calendering the nanoweb and/or the nanoweb in combination with the - at least a first porous carrier substrate may reduce the mean flow pore size of the nanoweb. This may increase the strength of the nanoweb and/or the nanoweb/ porous carrier substrate combination. Calendering is the process of passing sheet material (in this case the nanoweb) through a nip between rolls or plates. The mean flow pore size (of the nanoweb) is influenced by a combination of the thickness of the nanoweb and the number average diameter of the nanofibers. For example, by increasing the thickness, the mean flow pore size may be reduced. By reducing the number average diameter of the nanofibers, the mean flow pore size can also be reduced.

With 'basis weight of the nanoweb' is meant the weight per square meter. Preferably, the basis weight of the nanoweb is in the range from 1 to 20 g/m², preferably 2-15 g/m². The basis weight is measured using ASTM D-3776, which is hereby incorporated by reference. The basis weight of the membrane construction can be determined in the same way. Preferably the basis weight of the membrane construction is in the range from 60 to 90 g/m², more preferably the basis weight is higher than 70 g/m².

The desired basis weight of the nanoweb, can be achieved by adjusting the flow rate of an electrospinning process using to spin the nanofiber and/or by adjusting the speed of the porous carrier substrate onto which the at least a first layer of nanofibers is spun.

When all four membranes of Fig. 1 to 4 are included in a filter and submitted to the same conditions (same solution to be filtered), the membrane according to the present invention (Fig. 3 and 4) last longer than the comparative membranes: the membrane of Fig. 1 is deteriorated the first, due to delamination of the diverse components, and the membrane of Fig. 2 presents significant clogging of material to be filtered. The membranes according to the present invention present good wear resistance, good adhesion of the diverse components and little clogging.

Claims

A method for fabricating a membrane, the method comprising the following steps:

a) Providing at least a first porous carrier substrate,

b) providing at least a first layer of nanofibers on one side of the first porous carrier substrate,

c) providing a suitable amount of at least one adhesive after step (b), by electro-spinning or centrifugal-spinning to form a structure, wherein the adhesive has a softening temperature lower than the softening temperature and the degradation temperature of the first porous carrier substrate and the first layer of nanofibers,

d) consolidating the structure obtained in step (c) by means of a temperature cycle or a pressure cycle thereby obtaining the membrane.

Method according to claim 1 , wherein the temperature cycle includes bringing the structure up to or above the softening temperature of the adhesive and below the softening and degradation temperature of the at least first porous carrier substrate and of the first layer of nanofibers and reducing the temperature below the softening temperature of the adhesive thus obtaining the membrane.

Method according to claim 1 or claim 2, comprising providing a suitable amount of at least one adhesive during and after step (b) by electro-spinning or centrifugal-spinning.

Method according to claim 1 or claim 2, comprising providing a suitable amount of at least one adhesive before and after step (b) by electro-spinning or centrifugal-spinning.

A method according to any one of claims 1 to 4, wherein the first layer of nanofibers and the adhesive is provided with a second porous carrier substrate at one side of the first layer of nanofibers opposite to the first porous carrier substrate prior to step (d).

A method according to any one of claims 1 to 5, wherein a second layer of nanofibers is provided after step (b).

A method according to claim 6, wherein a suitable amount of at least one adhesive is provided before, during or after, or any combination thereof, the step of providing the second layer of nanofibers, by electro-spinning or centrifugal-spinning to form a structure, wherein the adhesive has a softening temperature lower than the softening temperature and the degradation temperature of the first porous carrier substrate and the first layer of nanofibers,

A membrane comprising

a) at least a first porous carrier substrate,

b) at least a first layer of nanofibers, and porous carrier substrate wherein the first layer of nanofibers comprises an adhesive that has a softening temperature below the lowest softening temperature or degradation temperature of both the first porous carrier substrate and the first layer of nanofibers, characterized in that the amount of adhesive and/or the at least first layer of nanofibers with a thickness of 2000 nanometers, is in the range 0.002 and 0.5 g/m².

Membrane according to claim 6, wherein the basis weight of the at least a first layer of nanofibers is in the range from 1 to 20 g/m².

Fluid filter comprising the membrane of claim 6.

Fluid filter of claim 7, comprising a hydrophobic adhesive.