WO2023148102A1

WO2023148102A1 - Method for production of open-celled polymer foams for ultrafiltration applications

Info

Publication number: WO2023148102A1
Application number: PCT/EP2023/052042
Authority: WO
Inventors: Aniket Atul RAJE; Ulrich Alexander HANDGE; Joachim Koll; Oliver Gronwald; Martin Weber; Prokopios Georgopanos
Original assignee: Basf Se; Helmholtz-Zentrum Hereon Gmbh
Priority date: 2022-02-04
Filing date: 2023-01-27
Publication date: 2023-08-10

Abstract

A continuous foam extrusion process for producing an open-celled foam comprising the steps of a) providing a polymer melt comprising at least one water-insoluble polymer and at least one water-soluble polymer in an extruder, b) introducing at least one blowing agent into the polymer melt, c) introducing water as co-blowing agent into the polymer melt, and d) extruding the blowing-agent containing polymer melt through a die into a region of lower pressure.

Description

Method for production of open-celled polymer foams for ultrafiltration applications

Description

The present invention relates to a continuous foam extrusion process for producing an open- celled foam comprising the steps of a) providing a polymer melt comprising at least one water-insoluble polymer and at least one water-soluble polymer in an extruder, b) introducing at least one blowing agent into the polymer melt, c) introducing water as co-blowing agent into the polymer melt, and d) extruding the blowing-agent containing polymer melt through a die into a region of lower pressure.

Relevant Prior Art

Porous membranes with a pore diameter in the nanometer range can be used for ultrafiltration purposes. Achieving open pores with a diameter in the nanometer range in polymers without the use of organic solvents is very difficult. For the manufacturing of ultrafiltration membranes, these organic solvents such as tetrahydrofuran, /V-methyl-2-pyrrolidone, dimethylformamide are extensively used. Such organic solvents are harmful for the human body and the environment. To avoid organic solvent wastage, it is an industrial practice to recirculate the organic solvents by distillation. This process however consumes large amount of energy thus causing massive carbon emissions. Polymer foams present an alternative to generate pores in polymers without the use of organic solvents. It was however not yet possible to manufacture nano-scalar open porous polymer membranes on a large scale without the use of organic solvents.

This foam can be already realized by discontinuous processes such as solid-state foaming or batch foaming, with and without organic solvents by using a variety of methods. However, solid state foaming or batch foaming require total time ranging up to two days for a small quantity of samples. Thus, it is inefficient to produce such polymer foams on a large scale using these processes.

The methods that deliver such foam, or similar foams using batch foaming I solid-state foaming are as follows. Krause et al. (Journal of Membrane Science 187 (2001) 181-192, DOI: https://doi.org/10.1016/S0376-7388(01)00329-5) produced micro-scalar closed-cellular polymer foam from polysulfone. After preparation of the said foam, they used organic solvent tetrahydrofuran (THF) to form nano-cellular open pores within the walls of the micro-sized cells. This method uses organic solvents.

Krause et al. (Desalination 144 (2002) 5-7, DOI: https://doi.org/10.1016/S0011-9164(02)00280- 1) used discontinuous solid state foaming of polysulfone/polyimide blends with CO2 as physical blowing agent to develop nano-porous foams. Micro-scalar foams (- 1 - 10 pm) characterized as open-cellular as they were connected to each other, were achieved by the use of organic solvents such as THF. Nano-scalar open porous foams (2 - 50 nm) were achieved by increasing the CO2 saturation levels so that CO2 stays in a continuous phase, which lead to an open porous structure.

Gong et al. (Journal of Materials Science (2014) 49:2605-2617, DOI: https://doi.org/10.1007/s10853-013-7959-4) produced nano-porous cell walls in micro-scalar polycarbonate foams by using acetone with CO2 during foaming. The use of the said organic solvent induced crazing process within the polymer thus resulting in a nano-porous structure on the micro-porous cell walls. Similar to Krause et al., the said structure is only available by the use of an organic solvent.

Sorrentino et al. (Advances in Polymer Technology, Vol. 30, No. 3, 234-243 (2011), DOI: https://doi.org/10.1002/adv.20219) applied solid state foaming process on a variety of polymers that delivered micro-cellular morphologies. They indicated a tendency of formation of a ‘sub- micro-cellular morphology with nano-porous walls’ within the micro-cellular foams of poly(ethersulfone) and poly(etherimide). The words pore, porous and porosity are widely used in the polymer foam research to represent both open cells and closed cells in polymer foams. In this work, it is not denoted whether the ‘sub-micro-cellular morphology with nano-porous walls’ was closed or open. The images provided in this publication show a closed nano-cellular structure. As these ‘sub-micro-cellular structures with nano-porous walls’ are not open cellular, they cannot be permeable.

Guo et al. (Journal of Polymer Science, Part B: Polymer Physics 2015, 53, 975-985, DOI: https://doi.org/10.1002/polb.23719) & (DOI: https://doi.org/10.1177/026248931603500302) applied solid state CO2 foaming process on polysulfone (PSU) and poly(phenylsulfone) (PPSLI) where they used low temperatures between -10 °C to 60 °C for loading the samples with CO2. They obtained a nano-structure at the cell walls of PSU foam, but which was not open cellular and therefore not permeable. They however obtained an open nano-cellular structure on the cell walls of the micro-porous foam of PPSLI. They confirmed the permeance of these foams using a dye test. The manufacturing of these foams required a loading time of 48 hours. For applications on a large scale, it is not efficient to produce the said foams at this rate. Our invention thus introduces a method to produce these foams on a large-scale using foam extrusion, which is continuous process.

US 2004/0212119 A1 discloses a method for the production of foam sheet by extrusion of a mixture of a polysulfone or polyethersulfone and 1 to 10% by weight, based on the thermoplastic, of a volatile blowing agent under pressure at temperatures above 300 °C and extruding from the external atmosphere, where the blowing agent is water or a mixture of water with up to 200 weight parts, based on 100 weight parts of water, of an inert gas or of an organic liquid as a co-blowing agent.

US5866053A and US6383424B1 both use an extruder to melt and plasticize polymer material and/or mixture and introduces it to gas which results in continuous foam that can be molded into desired form and size using a die. The former method is only able to produce closed cell foams. The latter uses a more complex process implementing static mixer and melt pump and is therefore able to produce open celled foams. These methods were able to achieve only pores that are micro-cellular in nature. Although micro-pores are useful in certain membrane applications, they are too large for ultrafiltration membranes which require a pore size less than 100 nm.

US7838108B2 invented nano-cellular polymer foam by using a sophisticated combination of extruders where a blowing agent is mixed and dissolved into a molten polymer to extrude foams. Several other methods were introduced to achieve nano-cellular foam using foam extrusion.

In W02013048760A1 , carbon dioxide is mixed with polymer melt in an extruder where the amount of carbon dioxide exceeds the amount of carbon dioxide that is soluble in the polymer at the initial temperature and pressure, as the mixture passes through the extrusion die, a pressure drop of 50 bar resulted in expanded nano foams.

In US20190153181A1 , a polymer mixture containing a matrix polymer and a domain polymer is foamed in an extruder using carbon dioxide which results in nano-scalar foam in discrete regions where domain polymer exists. US10358537B2 mentions that the mixture of superheated fluid and supercritical gas alters properties for a variety of polymer materials and their mixtures that are useful in manufacture of polymer foams. Lee et al. (DOI: https://doi.org/10.1177/0021955X09343632) used n-butane and water together as blowing agent in an extruder with a mixture of polystyrene and silica which delivered bi-cellular foams. This bi-cellular foam is a collection of two set of closed cells, one larger (200 - 600 pm) and other smaller (50 - 100 pm). The smaller set exists within the cell walls between the larger cells separately. As they are separated from each other by cell walls that are non-open porous, it can be classified as a micro-scalar closed-cell foam as no interference is possible between these cells.

This principle stated in US10358537B2 was applied by Owusu-Nwantabisah et al. (Polymer Engineering and Science — 2017, DOI: https://doi.org/10.1002/pen.24673) using batch foaming where the combination of supercritical carbon dioxide and superheated water in foaming poly(ethersulfone) resulted in increased porosity and tendency towards open cellular structure. They reported only micro-cellular open-celled foams. The possibility of achieving a micro- cellular closed-cell-appearing structure with nano-cellular open cell structure throughout the polymer foam as described in this invention or in prior art, cannot be achieved by solely using this technique.

EP 1 424 124 A1 discloses a membrane producible by shaping a homogeneous polymer blend comprising at least one hydrophilic and at least one hydrophobic polymer having a solubility relating to the used foaming gas above the critical concentration. Described are four methods in general without details, a pressure cell process, autoclave process, extrusion process and solidspinning process.

EP 2 731 775 A1 (published as WO2013/048760) discloses the preparation of a polymeric nanofoam using a continuous extrusion process by providing a polymer melt of a polymer composition in an extruder, introducing carbon dioxide to a concentration above the solubility in the polymer melt, cooling the polymer melt without increasing the pressure to achieve conditions where all of the carbon dioxide is soluble in the polymer composition and then extruding the polymer composition and carbon dioxide mixture through an extrusion die so as to experience a pressure drop of at least five MegaPascals at a rate of at least ten MegaPascals per second and allowing the polymer composition to expand into a polymeric nanofoam. In the Examples PMMA compounded with silica nanoparticles are used.

Nano foams manufactured by any of the previously known foam-extrusion methods may be used for ultra-filtration applications but require a low thickness for a reasonable throughput. Object of the present invention was to provide a continuous process for producing open-celled polymer foams with open pores with an average pore diameter in the nanometer range without using organic solvents and which can be used as ultrafiltration membranes on a large scale.

This object was achieved by a continuous foam extrusion process for producing an open-celled foam comprising the steps of a) providing a polymer melt comprising at least one water-insoluble polymer and at least one water-soluble polymer in an extruder, b) introducing at least one blowing agent into the polymer melt, c) introducing water as co-blowing agent into the polymer melt, and d) extruding the blowing-agent containing polymer melt through a die into a region of lower pressure.

The cells with a diameter in the micrometer range (micro-scalar cells) in the foam realized by this invention allow the overall foam size to be large enough for structural stability of the foam while at the same time providing the required ultrafiltration capabilities presented by the open pores with a diameter in the nanometer range (nano-scalar open pores) on the walls of the micro-scalar cells.

Preferably the blowing agent is a physical blowing agent selected from carbon dioxide, argon, nitrogen, halogens, natural air, synthetic air, gaseous hydrocarbons or mixtures therefrom. The blowing agent preferably does not comprise an organic component, such as aliphatic alcohols, such as methanol, ethanol, propanol, isopropyl alcohol or butanol, aliphatic ketones, such as acetone or methyl ethyl ketone, or aliphatic esters, such as methyl or ethyl acetate. Most preferably the blowing agent is used together with water as co-blowing agent. Steps b) and c) may be combined to one step by introducing blowing agent and water through a single injection device in step b).

In a preferred process in step a) the temperature of the polymer melt is heated to a temperature in the range from 250 to 400°C, and in step b) the blowing agent, preferably carbon dioxide, is introduced at a pressure in the range from 5 to 15 MPa, and in step c) water is introduced at a pressure in the range from range from 5 to 15 MPa, and in step d) the blowing-agent containing polymer melt is extruded to obtain open-celled polymer foam strands; and optionally in step e) treatment of the foam strands, i.e. trough extraction with water. Preferably in step d) an annular slit die is used to produce hollow fibers.

Preferably in the process of the present invention a combination of water in the superheated state and a physical blowing agent such as CO2 in the supercritical state is used. In addition to their use as blowing agents, a polymer mixture that contains at least one component that is a water-insoluble polymer and at least one other component that is a water-soluble polymer is used. The individual domains of the water-soluble polymer in the polymer matrix should not exceed 400 nm in length and should preferably be interconnected. The two polymers in the polymer mixture, at microscopic scale should resemble figure 1. The interaction between water in superheated state and the water-soluble polymer is essential for the generation of nano-scalar open pores. Water in superheated state has a polarity high enough to dissolve the water-soluble polymer at a faster rate while at the same time, causing softening of the polymer matrix. This dissolution of the water-soluble polymer from the polymer mixture would result in the creation of the nano-scalar pores. The nano-scalar pore size would correspond to the size of the water- soluble polymer domains and thus a domain size smaller than 400 nm is recommended. In addition, the use of both superheated water and pressurized foaming agent is essential to the formation of the larger pores that are a direct result of the foaming that takes places when the mixture exits the extruder nozzle.

In addition, the present invention uses the continuous process of foam extrusion for mass production of the said foam. Contrary to solid state foaming, where physical foaming agents diffuse into the polymers at an elevated temperature well below the glass transition temperature so as to maintain the polymer in solid state, extrusion foaming implements the technique of diffusing foaming agent into polymer during the melt state where the polymer temperature is much higher than the glass transition temperature. This diffusion occurs in the mixing phase where the extruder components impose shear stresses at shear rates between 100 to 1000 s^-1. Superheated water undergoes drastic reduction in the polarity with increase in temperature and pressure, which transforms solubility properties of water near to those of organic solvents. Therefore, it is preferred that the polymer domains in the mixture are smaller than 400 nm in size and the percentage by weight content of the water-soluble polymer are enough to cause interconnection of these domains within the water-insoluble polymer matrix. Thus, the water-soluble parts within the polymer matrix dissolve in the foaming agent thus creating the nano-scalar open-pores on the cell walls of the micro-scalar polymer foam.

A preferred embodiment of this invention involves a polymer foam, wherein the foam provides appearance of closed cells at a micro scale that are made of nano-scalar open pores and a sol- vent free method to produce the same. This type of foam has uniformly distributed cells or bubbles that are separated from each other by cell walls that portray an appearance of closed cell foam when observed at a micro-scalar magnification as shown in figure 2. However, the microscalar cell walls comprise of open porous structures that are nano-scalar as shown in figure 3. These nano-scalar porous structures facilitate cell-to-cell connectivity of the larger micro-scalar cells. The polymer foam is therefore open-celled foam in a true sense. This nano-scalar connectivity further facilitates overall permittivity of the foam that is restricted to nano-scalar particles, colloids or fluids. Such a foam can take advantage of the microstructure for variety of applications that take advantage of properties (but not only restricted to) such as thermal conductivity, stiffness, strength, high porosity, lightweight and damping. This foam may also take advantage of the nano-open structure in filtration applications such as ultrafiltration and nanofiltration as the foam is only permeable at the nanoscale. A synergistic advantage of this type of foam is that when used as ultrafiltration or nanofiltration membranes, each micro cell acts as an ultrafiltration unit, causing the permeate to undergo multiple number of filtrations that are equal to or more than the minimum number of micro cells that exist linearly from the inlet side to the exit side of the membrane. Thus, in case of development of any defect in one of the cells causing large pores, larger particles passing through one unit would be restricted in the next unit.

On the other hand, when used as ultrafiltration membranes, the microstructure formed due to the consolidation of the nano-scalar open pores increases the porosity, thus allowing use of thicker membranes. This increases the structural stability of the overall membrane while maintaining the permittivity.

Preferably the open-celled polymer foam is characterized by an average pore diameter below 400 nm. More preferably the foam has open pores with an average pore diameter in the range from 20 nm to 400 nm. The average pore size can be measured by gas permeation based on the gas flow in the porous sample or by automatic or visual evaluation of scanning electron microscopy images. The pore sizes of every cell are measured using scanning electron micrographs taken at three locations of a freeze-fractured sample. For non-round pores, the pore diameter is calculated by averaging the largest and the smallest diameter. The diameters for all pores are averaged to obtain the average pore diameter of the particular foam.

The foam according to the invention is permeable to gases and liquids.

The said foam is manufactured without the use of any organic solvents. Generally, open celled foams are worse thermal insulators than closed cell foams as they allow a free flow of gas through them. In closed cell foams, as gas is trapped within the individual closed cells, thermal conduction is drastically reduced which gives closed cell foams their thermal insulation properties. Although the mentioned foam is open porous, it can be used as highly efficient thermal insulator for gases. The thermal conduction of a gas when passed through a nano-foam would be greatly reduced due to the nano-pores. Because of the Knudsen effect, when a gas passes through a pore size less than the mean free path of the gas molecules at the particular temperature, the thermal conduction is reduced to a near zero value. The open nano-pores in our invention would facilitate this effect, thus achieving very low to almost zero gas conducting capability while ensuring gas transfer from one end to another.

The micro scalar closed cells cause empty space in the overall foam to be much higher than that would be with only nano-scalar open celled foam. Therefore, the foam described in this invention would have a low bulk density thus facilitating low-weight thermal insulation products to be manufactured using it.

This is an organic solvent-free continuous process of producing ultrafiltration membranes that would be permeable only at nano-scale. In addition, this particular foam provides multiple layers of filtration due to the micro-porous structure. This foam also has applications in thermal insulation and lightweight polymer applications.

Figures 2 and 3 show an illustration of the foam according to the invention at different magnification. At lower magnification, the foam appears closed cell with cell diameters in the range from 1 - 10 pm as shown in figure 2. At higher magnifying (Figure 3), it is revealed that the cell walls are made of open-porous foam with pore diameters in the range from 10 - 100 nm.

Figure 4 is a schematic drawing of the equipment and process for producing the nano-scalar open cell foam according to the present invention.

To manufacture this foam, a polymer mixture, with at least one component as a water-insoluble polymer and at least one another component as a water-soluble polymer is chosen. Polymers are considered as "water-insoluble" when their solubility is less than 0.1 g per 100 mL of water. Water-soluble polymers that have a higher water solubility would require lower amounts of water to dissolve completely from the matrix to deliver the required nano-scalar pores. Preferably the water solubility of the water-soluble polymer should not be less than 50 mg/mL to maintain efficiency and effectiveness of the process. Most preferably the water-soluble polymer has a water-solubility of 80 - 120 mg/ml. The mixture however may not be restricted to two polymers and may also have more than two polymers. This mixture may be in the form of a single-phase polymer blend, a multi-phase polymer blend, alternating block copolymer, alternating copolymer, block copolymer, copolymer, dendrimer, graft copolymer, ionomer, random block copolymer, random copolymer, star block copolymer or any combination of these. This mixture however requires individual polymer domains to be smaller than 400 nm in order to facilitate the formation of the pores required for the said applications.

The polymer is a mixture between at least one water-insoluble polymer and at least one water- soluble polymer. The non-water-soluble polymer is the major component of the mixture comprising more than 50 wt% of the mixture. Preferable a polymer mixture of from 50 to 95 wt.-% of the at least one water-insoluble polymer and from 5 to 50 wt.-% of the at least one water-soluble polymer is used. Most preferably a blend of poly(ether sulfone) as water-insoluble polymer and poly(N-vinyl pyrrolidone) and/or poly(ethylene glycol) as water soluble polymer is used.

The non-water-soluble polymer or polymers preferably is selected from polyacetals, polyacrylics, polyamideimides, polyamides, polyanhydrides, polyarylates, polyarylsulfones, polybenzimidazoles, polybenzothiazinophenothiazines, polybenzothiazoles, polybenzoxazoles, polycarbonates, polycarboranes, polydibenzofurans, polydioxoisoindolines, polyesters, poly(ether etherketone)s, poly(ether ketone ketone)s, poly(etherimide)s, poly(ethersulfone)s, polyimides, poly(oxabicyclononane)s, poly(oxadiazole)s, polyoxindoles, poly(oxoisoindoline)s, poly(phenylsulfone)s, poly(phenylene sulfide)s, polyphthalides, polypiperazines, polypiperidines, polypyrazinoquinoxalines, polypyrazoles, polypyridazines, polypyridines, polypyromellitimides, polyquinoxalines, polysilazanes, polystyrenes, polysulfides, poly(sulfonamide)s, polysulfonates, polysulfones, poly(tetrafluoroethylene)s, poly(etherketone)s, polythioesters, polytriazines, polytriazoles, polyureas, poly(vinyl ester)s, poly(vinyl ether)s, poly(vinyl halide)s, poly(vinyl ketone)s, poly(vinyl nitrile)s, poly(vinyl thioether)s or any combination of these.

The water soluble polymer or polymers may be albumin, carrageenan, cellulose ethers, chitosan derivatives, dextran, divinyl ether-maleic anhydride, guar gum, hyaluronic acid, N-(2- hydroxypropyl) methacrylamide, pectins, poly(alkylene oxides) such as poly(ethylene oxide), polypropylene oxide), poly(ethylene oxide)-poly(propylene oxides) poly(acrylamide)s, polyacrylic acid, polyoxazoline, polyphosphates, polyphosphazenes, poly(vinyl alcohol), poly(pyrrolidone)s, poly(vinyl pyrrolidone), starch or starch based derivatives, xanthan gum or any combination of these. The water soluble polymer or polymers preferably is selected from albumin, carrageenan, cellulose ethers, chitosan derivatives, dextran, divinyl ether-maleic anhydride, guar gum, hyaluronic acid, N-(2-hydroxypropyl) methacrylamide, pectins, poly(ethylene glycol), poly(acrylamide)s, polyacrylic acid, polyoxazoline, polyphosphates, polyphosphazenes, poly(vinyl alcohol), poly(pyrrolidone)s, poly(vinyl pyrrolidone), poly(alkylene oxides) such as poly(ethylene oxide), polypropylene oxide), poly(ethylene oxide)-poly(propylene oxides) starch or starch based derivatives, xanthan gum or any combination of these.

Additives such as anti-corrosion additives, anti-drip agents, antioxidants, anti-ozonants, antistatic agents, carbon fibers, dyes, fillers, fire retardant, flow promoters, glass fibers, impact modifiers, mold release agents, pigments, thermal stabilizers, ultraviolet (UV) absorbers, or any combination of these may be added to the polymer mixture.

This polymer mixture may be pre-prepared into granules, flakes, powder, pellets or any combination of these. The process of foam extrusion is followed. This mixture is fed into an extruder hopper or inlet. This extruder can be one extruder or multiple extruders connected in either series or parallel where the extruder or extruders house a single screw, multiple screws, or a combination of both. The components of the required polymer mixture may also be directly mixed in the mentioned extruder

Preferably the polymer mixture is molten within the extruder due to heating and shear effect and transported in the feed direction. Primary blowing agent and water and optionally further organic liquids are introduced to the polymer melt either separately or together into the extruder at relatively higher pressure than the polymer melt. The introduction of the primary blowing agent and water is done by an inlet that is located at the compression zone of the extruder. This composition of physical blowing agent water and polymer melt is mixed together and transported towards the nozzle by means of extruder screw, kneading elements, static mixer, melt pump, or a combination of any of these. The gas and the water together diffuse within the polymer mixture during this process. Due to the temperature being higher than the boiling point of water, i.e. 100 °C (CAS No.: 7732-18-5), and overall pressure ranging from 100 bar to 400 bar, water stays in a superheated state. In superheated state, the dielectric constant of water reduces substantially with increase in temperature, thus allowing the polarity of water to reduce to a value that is similar to that of solvents such as methanol or THF. This causes the water-soluble polymer content in the polymer mixture to react rapidly with superheated water, thus aiding in the causation of a nano-scalar open-porous structure. The extruder nozzle is set at a pre-determined temperature that is equal to or lower than the previously exposed temperature in the extrusion components. As the mixture leaves the nozzle, polymer foam is obtained that appears as a uniform closed cell foam at a micro-scale but is open porous throughout in the nano-scale.

Foam extrusion may involve adding the polymer mixture in an extrusion assembly, providing gas and water to the mixture during processing and extruding the said foam.

The extrusion assembly may contain one or different combinations of the components such as extruder, melt pump, static mixer, nozzle, pressure sensors, temperature sensors, flow rate sensors, heaters and coolers. The extruder may contain an extruder barrel, a hopper, heaters, a single screw extruder, a multi screw extruder, kneaders, or a combination of these. The extruder contains at least one inlet for gas and at least one inlet for water. The respective inlets for gas and water may be present at the same location or separate locations at the extruder. The respective inlets for gas and water may be present at any stage in the extruder including the compression phase, mixing phase, metering phase, transition phase or any combination of these. The screws may be single-stage screw, multi-stage screw, kneading screw, mixing screw, conical screw. The nozzle may be a cylindrical nozzle, annular slit nozzle, rectangular nozzle, triangular nozzle, flat sheet nozzle, blow film nozzle, mesh nozzle, or any combination of these.

The foam may be mechanically cut or shaped into any shape or form if desired.

To produce this foam for the application of flat ultrafiltration membranes, the nozzle can be chosen from either a flat line nozzle or a blow film nozzle setup. For hollow fiber ultrafiltration membranes, an annular slit nozzle of the desired dimensions is necessary. It is also noted, that there exists extrudate swell in the extruded polymer foam due to extrusion shear and tensile stresses and foaming. The nozzle dimensions are required to be chosen with such that an offset due to extrudate swell is taken into consideration. An external drawing conveyor system may be used to compensate for the extrudate swell, by drawing the extrudate at a higher speed as compared to the speed of the extrudate at the nozzle.

The foaming agent used in this technique preferably is a combination of a gas and water. The gas can be selected from a variety of gases such as carbon dioxide, argon, nitrogen, air, gaseous hydrocarbons, or any combination of these. The water may be selected from distilled water, mineral water, de-calcified water, tap water, de-ionized water or a combination of these. The feed rate and pressure of water and carbon dioxide depends upon the extruder size, throughput, screw speed and pressure. The foaming agents may be supplied at different inlets in the extruder-line either at the same location or at separate locations. Preferably the blowing and co-blowing agents are injected at two separate inlets at the extruder. This ensures that their respective pressures are maintained independently, and the mixing occurs inside the extruder when interacting with the polymer melt within the extruder. Both foaming agents can be injected at the same location on the extruder axis at different inlets or can be injected at different locations along the extruder axis.

Flat sheet membranes or hollow fiber membranes produced by this technique are only permeable for fluids and particles smaller than the average pore size of the nano-scalar pores.

For its application in ultrafiltration, the foam, in the absence of unfoamed skin layer, can be used as it is after it exits the extruder though the nozzle. In case of a solid skin layer on the foam, the foam may require to be mechanically shaped in the desired form by physically removing the outer non-foamed skin layer if any, by either peeling or cutting using a sharp knife or cutting object. This procedure only may distort the nano-pores on the top and bottom side of the foam that come in contact with the cutting object, and does not affect those that are present in the concave micro structures at the top and bottom side. It also does not affect the internal structure of the foam that remains untouched and nor does affect the filtration performance.

The foam according to the invention may be used in various applications, such as thermal insulation applications, impact damping applications, acoustic applications. The foam according to the invention are preferably used in filtration applications such as ultrafiltration membranes in water treatment applications. The ultrafiltration membranes may be either flat sheet membranes or hollow fiber membranes. An ultrafiltration module made by consolidation of hollow fiber membranes or flat sheet membranes.

Examples:

Raw Materials;

PESLI E 2010: Polyethersulfone Ultrason® E 2020 P (BASF SE) with a viscosity number (measured based on ISO 1628-5 (1998) in a 1wt. % polymer solution in N- methylpyrrolidone) of 56 ml/g; a glass transition temperature (DSC, 10 K/min; according to ISO 11357 1 (2017) and 11357 2 (2020)) of 225 °C; a molecular weight Mw (GPC in THF, PS standard) of 48000 g/mol, and Mw/Mn = 2.7, which is abbrevi-ated as “PESLI E 2010” PESLI E 3010 Polyethersulfone Ultrason® E 3010 (BASF SE) with a viscosity number (measured based on ISO 1628-5 (1998) in a 1wt. % polymer solution in N- methylpyrrolidone) of 66 ml/g; a glass transition temperature (DSC, 10 K/min; according to ISO 11357 1 (2017) and 11357 2 (2020)) of 225 °C; a molecular weight Mw (GPC in THF, PS standard) of 58000 g/mol, and Mw/Mn = 3.3, which is abbrevi-ated as “PESLI E 3010

PVP K30 Poly(N-vinyl pyrrolidone) Luvitec® K30 (BASF SE), with a molecular weight Mw of 44000 to 540000 g/mol and a solution viscosity characterized by the K- value of 30, determined according to the method of Fikentscher (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as “PVP K30”

PEG Poly(ethylene glycol) (Sigma Aldrich® Polyethylene Glycol 200)

Pore analysis method:

For pore size analysis, scanning electron microscope (SEM) images of freeze-fractured samples were recorded at a magnification of 7000x. The SEM images were acquired with a Merlin SEM (Zeiss, Oberkochen, Germany) at an accelerating voltage of 1keV using a high efficiency secondary electron (HE-SE2) detector. Before measurement, the freeze-fractured specimens were sputter-coated with 1.5 nm platinum using a CCU-010 coating device (Safematic, Switzerland). Pore size analysis was performed with the software IMS (Imagic Bildverarbeitung AG, Switzerland). Pores with an area smaller than 2000 nm2 were excluded from the analysis.

Preparation of PESU/PVP-Blends

In our trials, polymer blends were manufactured using a poly(ethersulfone) (PESLI) and a poly(N-vinyl pyrrolidone) (PVP), where poly(ethersulfone) is a non-water-soluble polymer and poly(N-vinyl pyrrolidone) is a water-soluble polymer. Polymer combinations of PESU/PVP that were prepared were 92/08 wt%, 84/16 wt%, 76/24 wt%, 68/32 wt%. Two types of PESLI were chosen, which differed in molecular weights, viz. BASF Ultrason® E 2010 and BASF Ultrason® E 301 OP. The blends were prepared using a twin-screw mixing extruder with kneading elements, connected to a granulator that delivered pellets with approximately 2 mm diameter and 5 mm height. DSC measurement according to DIN-ISO 11357-2 at a heating rate of 10 K/min determined that the blend was mixed in a single phase. Table 1 : Composition of PESU/PVP-Blends

Example 1 :

Foam extrusion of PESU/PVP-blend and hollow fiber extrusion with pressurized CO2

A single screw extruder was used which had an inlet for CO2. CO2 was pressurized and injected into the extruder at the compression zone of the extruder screw. Polymer blend was added in the form of pellets into the extruder hopper. The extrusion temperatures used were between 250 °C to 350 °C. Pressurized CO2 was injected in the extruder as the melt proceeded forward and then mixed well together due to the extruder screw, a static mixer. Constant output feed rate was ensured by using a melt pump. The mixture then exited the extruder through an annular slit nozzle which was heated to a slightly lower temperature (T_extruder - 10 °C to T_extruder - 70 °C) than the extrusion temperature. Thus, the said mentioned foam was produced in the form of hollow fibers. Foams are open-celled with cavity size between 5 - 20 pm.

Pre produced polymer blends B5 - B8, comprising of the poly(ethersulfone) (BASF Ultrason® E 3020 P) and poly(N-vinyl pyrrolidone) (BASF Luvitec® K 30) were used. A single screw extruder that included a static mixer and a melt pump was used. A circular nozzle (nozzle diameter 1 mm) or an annular slit die was attached at the end of this extruder. The temperature of the extruder at all the zones including the static mixer and melt pump was set to 350 °C. The nozzle was set to a temperature of 280 °C. CO2 was supplied through an inlet in the extruder at 100 bar above ambient pressure.

An ink dip method using a solution of 1-1 methylene in ethanol was applied to verify that the extruded foams were open porous and permeable Example 2:

Foam extrusion of PESU/PVP-Bend with pressurized CO2 and water

Preparation of PESU/PVP-Blends

Polymer blends aremanufactured using poly(ethersulfone) (PESLI) and poly(/V-vinyl pyrrolidone) (PVP), where poly(ethersulfone) is a non-water-soluble polymer and poly(/V-vinyl pyrrolidone) is a water-soluble polymer. Pre-produced polymer blend B3, comprising of the poly(ethersulfone) (BASF Ultrason® E 2010) and poly(/V-vinyl pyrrolidone) (BASF Kollidon® K 30 of the composition 76 wt% and 24 wt% respectively are used. DSC measurement according to DIN-ISO 11357-2 at a heating rate of 10 K/min can confirm the phase separation of the said blend, single phase in this case.

Foam extrusion

A single screw extruder is used having an inlet for CO2 and an inlet for water as illustrated in figure 4. Both the fluids should be pressurized and injected into the extruder at the same location at the compression zone of the extruder screw. Polymer blend are added in the form of pellets into the extruder hopper. The extrusion temperatures are between 250 °C to 350 °C. Pressurized CO2 and water is injected in the extruder as the melt proceeds forward and then mixes well together due to the extruder screw and a static mixer. The pressures of CO2 and water is equal to or higher than 100 bar each. Constant output feed rate is ensured by using a melt pump. The mixture then exits the extruder through an annular slit nozzle which is heated to a slightly lower temperature (T_extruder - 70 °C to T_extruder - 10 °C) than the extrusion temperature but should be higher than the glass transition temperature of the mixture. Thus, the said mentioned foam is obtained in the form of hollow fibers.

The water after getting introduced into the extruder stays in the superheated state due to the high pressure and temperature. The water-soluble polymer dissolves easily in water and is removed when the polymer melt leaves the extruder nozzle. This dissolution creates nano-scalar pores in the polymer matrix which appear on the walls of the micro-scalar foamed cells created due to foaming of the water-insoluble polymer when it exits the extruder through the nozzle. Scanning electron microscopy results would confirm that the foams generated from these experiments contains micro scalar closed cells having nano-scalar open pores on their cell walls. This foam would be then permeable due to being overall open cellular in nature.

These hollow fibers when subjected to water flux measurements would bear a water permeance though them. Retention tests can be carried out to ensure retention of partials larger than the nano scalar pore size.

Example 3:

Foam extrusion of PESU/PEG Blend with pressurized CO2 and water

Preparation of PESU/PEG Blends

Polymer blends are manufactured using poly(ethersulfone) (PESU) and poly(ethylene Glycol) (PEG), where PESU is a non-water-soluble polymer and PEG is a water-soluble polymer. Preproduced polymer blend, comprising of the PESU (BASF Ultrason® E 2010) and PEG (Sigma Aldrich® Polyethylene Glycol 200) of the composition 80 wt% and 20 wt%, respectively, are used. DSC measurement according to DIN-ISO 11357-2 at a heating rate of 10 K/min can confirm the partial miscibility of this blend.

Foam extrusion

A single screw extruder is used. This extruder contains an inlet for CO2 and an inlet for water, as illustrated in figure 4. Both fluids should be pressurized and injected into the extruder at the same location at the compression zone of the extruder screw. Polymer blends are added in the form of pellets into the extruder hopper. The extrusion temperatures are between 200 and 220 °C. Pressurized CO2 and water are injected into the extruder as the melt proceeds forward and mixed well together due to the extruder screw and a static mixer. The pressures of CO2 and water are equal to or higher than 100 bar each. Constant output feed rate is ensured by using a melt pump. The mixture then exits the extruder through an annular slit nozzle with an outer diameter 2 mm and inner diameter 1 mm, which is heated to a slightly lower temperature (T_extruder - 55 °C to Textruder - 20 °C) than the extrusion temperature but should be higher than the glass transition temperature of the mixture. Thus, the said mentioned foam is obtained in the form of hollow fibers. Scanning electron microscopy results would confirm that the foams generated from these experiments contain open-celled micro-cellular closed cells having nano-cellular pores on their cell walls.

Further optimizations of process settings and blend compositions can improve the porosity of the pores and influence pore sizes.

These hollow fibers would bear water permeance when subjected to water flux measurements. Figure 5 and figure 6 show the microcellular foam and the nanopores on the cell walls, respectively.

Example 4

The experimental protocol of Example 3 is repeated.

For this example, a round nozzle with a cross-sectional diameter between 0.5 and 1 mm is used.

This smaller cross-sectional area than example 3 allows the nozzle pressures to grow 2-3 times higher and produce higher nucleation. This higher nucleation yields smaller cell sizes and high porosity.

Scanning electron microscopy results, as seen in figure 7, would confirm that the foams generated from these experiments contain open-celled nano-cellular pores with an average cell size of approximately 200 nm.

An ink dip method using a solution of 1-1 methylene in ethanol was applied to verify that the extruded foams were open porous and permeable.

Claims

1 . A continuous foam extrusion process for producing an open-celled foam comprising the steps of a) providing a polymer melt comprising at least one water-insoluble polymer and at least one water-soluble polymer in an extruder, b) introducing at least one blowing agent into the polymer melt, c) introducing water as co-blowing agent into the polymer melt, and d) extruding the blowing-agent containing polymer melt through a die into a region of lower pressure.

2. A process according to claim 1 , wherein the average pore diameter, determined by scanning electron microscopy, of the open-celled foam is in the range between 20 to 400 nm.

3. A process according to claim 1 or 2, wherein the at least one water-insoluble polymer is selected from polyacetals, polyacrylics, polyamideimides, polyamides, polyanhydrides, polyarylates, poly-arylsulfones, polybenzimidazoles, polybenzothiazinophenothiazines, polybenzothiazoles, polybenzoxazoles, polycarbonates, polycarboranes, polydibenzofurans, polydioxoisoin-dolines, polyesters, poly(ether etherketone)s, poly(ether ketone ke- tone)s, poly(etherimide)s, poly(ethersulfone)s, polyimides, poly(oxabicyclononane)s, poly(oxadiazole)s, polyoxindoles, poly(oxoisoindoline)s, poly(phenylsulfone)s, poly(phenylene sulfide)s, polyphthalides, poly-piperazines, polypiperidines, polypyrazino- quinoxalines, polypyrazoles, polypyridazines, polypyridines, polypyromellitimides, polyquinoxalines, polysilazanes, polystyrenes, polysul-fides, poly(sulfonamide)s, polysulfonates, polysulfones, poly(tetrafluoroethylene)s, poly(etherketone)s, polythioesters, polytriazines, polytriazoles, polyureas, poly(vinyl ester)s, poly(vinyl ether)s, poly(vinyl halide)s, poly(vinyl ketone)s, poly(vinyl nitrile)s, poly(vinyl thi-oether)s or any combination of these

4. A process according to any of claims 1 to 3, wherein the at least one water-soluble polymer is selected from albumin, carrageenan, cellulose ethers, chitosan derivatives, dextran, divinyl ether-maleic anhydride, guar gum, hyaluronic acid, N-(2-hydroxypropyl) methacrylamide, pectins, poly(alkylene oxides) such as poly(ethylene oxide), polypropylene oxide), poly(ethylene oxide)-poly(propylene oxides), poly(acrylamide)s, polyacrylic acid, polyoxazoline, polyphosphates, polyphosphazenes, poly(vinyl alcohol), poly(pyrrolidone)s, poly(vinyl pyrrolidone), starch or starch based deriva-tives, xanthan gum or any combination of these.

5. A process according to any of claims 1 to 4, wherein the polymer melt comprises

50 to 95 wt.-% of the at least one water-insoluble polymer and

5 to 50 wt.-% of the at least one water-soluble polymer.

6. A process according to any of claims 1 to 5, wherein the polymer melt consists of a blend of poly(ethersulfone) as water-insoluble polymer and poly(N-vinyl pyrrolidone) or poly(ethylene glycol) as water soluble polymer.

7. A process according to any o claims 1 to 6, wherein the blowing agent is a physical blowing agent selected from carbon dioxide, argon, nitrogen, halogens, natural air, synthetic air or mixtures therefrom.

8. A process according to any of claims 1 to 7, wherein in step a) the temperature of the polymer melt is heated to a temperature in the range from 250 to 400°C, and in step b) the blowing agent is introduced at a pressure in the range from 5 to 15 MPa, and in step c) water is introduced at a pressure in the range from range from 5 to 15 MPa, and in step d) the blowing-agent containing polymer melt is extruded to obtain an open-celled polymer foam strand.

9. A process according to any of claims 1 to 8, wherein in step d) an annular slit die is used to produce hollow fibers.

10. An open-celled foam with bi- or multimodal pore-size distribution obtainable with the process according to any of claims 1 - 9.

11. Use of the open-celled foam according to claim 10 in form of a flat sheet or hollow fiber membrane in an ultrafiltration module.

12. Use of the open-celled foam according to claim 11 in thermal insulation, impact dampening and acoustic applications.