Process for forming microporous membranes

(54) Title: MICROPOROUS MEMBER TRAINING PROCESS (51) Int. Cl .: B01D 69/00 (30) Unionist Priority: 28/03/2007 US 11 / 692,257 (73) Holder (s): 3M INNOVATIVE PROPERTIES COMPANY .

(72) Inventor (s): ILYESS H. ROMDHANE; SCOTT L. CILISKE

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(74) Attorney (s): NELLIE ANNE DANIELSHORES (86) International Application: PCT US2008054638 of 02/22/2008 (87) International Publication: WO

2008/118580 of 02/10/2008 “PROCESS OF FORMATION OF MICROPOROUS MEMBRANES”

Field

The present invention relates to a method of forming microporous membranes.

Background

Microporous membranes with diverse properties are used in many modern products, including filters, breathable articles, absorbent articles and medical articles. There are many known ways of making microporous membranes, including inducing a phase inversion in an absorbent layer. By manipulating the conditions that promote phase inversion, different morphologies can be generated in the resulting microporous membrane, adapting it to the specific needs of the end user.

One way to promote phase inversion is to put an absorbent formulation in contact with a coagulant. Methods of preparing microporous membranes are further described in US Patent Nos. 6,736,971 (Sale et al.), 5,869,174 (Wang), 6,632,850 (Hughes et al.), 4,992,221 (Malon et al.), 6,596,167 (Ji et al.), 5,510,421 (Dennis et al.), 5,476,665 (Dennison et al.) And in US patent application publications No. 2003/0209485 and 2004/0084364 (Kools).

A known way to introduce a coagulant into an absorbent layer is in the form of steam. Also known is the coagulation of the absorbent layer which consists of immersing the absorbent layer in a coagulation bath. Normally, the concentration and purity of the bath constantly change as the material diffuses into and out of the absorbent layer.

summary

Microporous membranes with a microstructure are described. The present description describes a method of forming sheets of microporous membranes. The microporous membrane is formed from an absorbent formulation and a first coagulant formulation. The absorbent formulation comprises a polymeric material and a solvent. The first coagulant formulation comprises a coagulant and a coating aid. The absorbent formulation and the first coagulant formulation are simultaneously molded by melting through a multilayer extrusion matrix. The absorbent formulation layer is melt-molded into the coagulant formulation forming an interface. A first surface of the absorbent formulation layer is located opposite the interface. At the interface, a portion of the first coagulant formulation diffuses into the absorbent formulation layer, making a first phase inversion. The first phase inversion results from the contact of the first coagulant of the first coagulant formulation with the polymeric material of the absorbent formulation reducing the thermodynamic stability of the polymeric material in the solvent. The first phase inversion forms a first microstructure of a microporous membrane.

In one embodiment, a first surface of a microporous membrane is placed in contact with a second coagulant, where the second coagulant is a vapor. A portion of the second coagulant diffuses through the first surface of the absorbent formulation layer, making a second phase inversion. The second phase inversion forms a second microstructure of the microporous membrane.

In one aspect, a method of forming a microporous membrane sheet that has at least two microstructures is described. The microporous membrane is formed from the simultaneous melting molding of an absorbent formulation with the first and second coagulant formulations. The second coagulant formulation comprises a third coagulant and a second coating aid. The absorbent formulation can be simultaneously molded by melting between the first and the second coagulant formulations with a multilayer extrusion matrix. A first interface is formed between the absorbent formulation and the first coagulant formulation, and a second interface is formed between the absorbent formulation and the second coagulant formulation. The first interface is opposite to the second interface. A portion of the first coagulant formulation diffuses into the absorbent formulation through the first interface resulting in a first phase inversion, and a portion of the second coagulant formulation diffuses into the absorbent formulation through the second interface resulting in a second phase inversion . The first and second phase inversions form the first and second microstructures of a microporous membrane, respectively. The first and second microstructures can be different or the same.

In one embodiment, a second coagulant formulation comprising a third coagulant and a second coating aid is molded by melting sequentially on the first surface of an absorbent formulation layer after simultaneous melting molding of the absorbent formulation and the first coagulant formulation.

Microporous membranes are commonly formed by exposing the absorbent layers to a coagulation bath that causes a phase inversion. In this description, the formulations of absorbent and coagulants and / or coagulants come into contact with each other as layers to perform a phase inversion resulting in the formation of microstructures. The simultaneous fusion molding of the formulations in a multilayer extrusion matrix forming a multilayer sheet removes the step of submitting the absorbent layers to coagulation baths. A multilayer extrusion die removes the cost and complexities of multi-stage coating processes, where the individual layers are coated. Additionally, the multilayer extrusion die allows thin multilayer coating which can be difficult with wet to wet coating methods. A multilayer sheet having thin layers can be formed with layers of specific thicknesses to satisfy particular specifications like a multilayer membrane. A coagulant formulation which has a coating aid of this description provides for the selection of a phase inversion rate, the control of the phase inversion depth in the membrane thickness, and the selection of a membrane microstructure.

Brief Description of Drawings

FIGURE 1 is a schematic representation of a multilayer extrusion die.

FIGURE 2 is a schematic representation of an expanded view of detail 2 in FIGURE 1.

FIGURE 3 is a schematic representation of the multilayer sheet having an absorbent formulation layer in a coagulant formulation layer.

FIGURE 4 is a schematic representation of a multilayer sheet having an absorbent layer between the first and second layers of coagulant formulations.

FIGURE 5 is a micrograph, obtained by scanning electron microscopy (SEM), of a microporous membrane that has a microstructure from example 1,

FIGURE 6 is a micrograph (SEM) of the cross section of a microporous membrane that has the first and second microstructures of example 2.

FIGURE 7 is a planar view of a micrograph (SEM) of a microporous membrane of FIGURE 6.

FIGURE 8 is a micrograph (SEM) of a cross section of a microporous membrane that has the first and second microstructures of example 3.

Detailed Description

These definitions should apply to the following terms, unless a different definition is given in the claims or elsewhere in the specification.

The term "absorbent formulation" refers to a polymeric material dissolved in a solvent at a selected concentration to form a thermodynamically stable absorbent formulation. The solvent of the polymeric material is called a "good" solvent or compatible solvent. The absorbent formulation has a suitable viscosity if it is melt-molded simultaneously with at least one coagulant formulation. The absorbent formulation can optionally further comprise a porogen (pore-forming) and / or a non-solvent.

The term "coagulant formulation" refers to a coagulant and coating aid useful in liquid polymer-induced phase separation. The coagulant (i.e., liquid) of the polymeric material of the absorbent formulation can be a "non-solvent". The coagulant can generally be considered incompatible with the polymeric material. The coagulant reduces the thermodynamic stability of a polymeric material in a solvent in the absorbent formulation. The coagulant formulation has a viscosity suitable for melt molding simultaneously with an absorbent formulation. The coagulant formulation can optionally additionally comprise more than one coating aid and / or more than one coagulant.

The term "coating aid" refers to additive (s) dissolved or dispersed (s) in a coagulant formulation. The coating aid (s) can be added to adjust the viscosity of the coagulant formulation sufficient for melt molding in an absorbent formulation. The concentration of the coating aid (s) can be selected to control the phase inversion rate, to control the depth of the phase inversion, or to form a specific membrane microstructure.

The term "simultaneous melt molding" refers to the formation of at least two or more layers on top of each other to form a multilayered sheet. The layers of absorbent formulation and coagulant formulation can be fused and molded through a multilayer extrusion die, where they come into contact with each other immediately at the die outlet. The matrix can be used to measure the absorbent formulation and coagulant formulations that have sufficient viscosity, so that the thickness of the layers is substantially controlled by controlling the formulation feed rate.

The term "phase inversion" refers to the transformation of a homogeneous system (for example, absorber formulation) into two or more phases. For example, a homogeneous polymer solution can be separated into two phases: a solid phase rich in polymers and a liquid phase low in polymers. In this description, for example, the phase inversion results from a process of exposing the absorbent formulation to a coagulant formulation, in which each formulation has a sufficient viscosity to be molded by melting simultaneously as layers through, for example, a matrix of multilayer extrusion to form sheets. Some other phase inversion mechanisms include thermally induced phase inversion and liquid-liquid phase inversion.

The term "microstructure" refers to a porous structure. The porous structure can comprise openings that can be asymmetrical or symmetrical. For example, a membrane can have more than one porous structure. The porous structure can be the result of a phase inversion of the polymeric material, where the regions rich in polymers can form the structure, and the regions low in polymers can form the openings within the structure. In membranes with more than one porous structure, the porous structures can be different or the same. The average pore diameter of a porous structure can be 0.05 to 25 microns.

The mention of numerical ranges with the ends includes all numbers comprised by that range (for example, 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.8, 4 and 5).

As stated in this specification and in the appended claims, the forms "one", "one", "o" and "a", in the singular, include references in the plural unless the content clearly indicates otherwise. Thus, for example, reference to a composition containing "a compound" includes a mixture of two or more compounds. As used in this specification and in the appended claims, the term "or" is generally used in a sense that includes "and / or" unless the content clearly indicates otherwise.

Except where otherwise noted, all numbers expressing quantities or ingredients, measurement of properties and so on used in this specification and claims are to be understood as being modified, in all instances, by the term "about." Consequently, unless otherwise stated, the numerical parameters established in the aforementioned specification and in the appended claims are approximations that may vary depending on the desired properties sought by those skilled in the art, using the guidelines of the present description. Undoubtedly and without trying to limit the application of the equivalence doctrine to the scope of the claims, every numerical parameter must be at least interpreted in the light of the number of significant digits presented and by the application of common rounding techniques. Although the numerical ranges and parameters that establish the broad scope of this description are approximations, the numerical values ​​established in the specific examples are reported as accurately as possible. Any numerical value, however, inherently contains errors necessarily resulting from the standard deviations found in their respective test measurements.

The present description describes a method of forming a microporous membrane sheet.

An absorbent formulation can be melt-molded as a layer to form a microporous membrane sheet. The absorbent formulation comprises a polymeric material that can be dissolved in a solvent with optional additives (e.g., porogens) and / or non-solvents to form a substantially homogeneous stable formulation. The absorbent formulation can be thermodynamically stable at room temperature without visible phase separation.

Absorbent formulations can be formed from one or more formulations that have adequate concentrations of solvents and / or non-solvents. The concentration of the absorbent formulation can be changed to accommodate a particular microstructure and also to maintain the integrity of the membrane. If a concentration of a polymeric material in an absorbent formulation is too low, a membrane may not be formed. If the concentration is too high, an unwanted irregular microstructure may result.

The concentration of the polymeric material in the absorbent formulation can vary based on the solvents, additives and non-solvents used to build the membrane. The concentration or percentage of solids of a polymeric material dissolved in a solvent in an absorbent formulation can be selected to achieve sufficient viscosity and / or surface tension for melting and molding as a layer on a multilayer sheet. The concentration is selected to be sufficient to allow diffusion of a coagulant formulation through an interface of the coextruded layers. The concentration of the polymeric material in a solvent of the absorbent formulation can be in the range of 10 to 25 weight percent. Preferably, the concentration of the polymeric material is in the range of 10 to 20 weight percent.

The viscosity of an absorbent formulation can be selected for melting and molding as an absorbent formulation layer in a coagulant formulation layer in a multilayer sheet. Viscosity is selected to provide the formation, at line speed, of a stable layer of, for example, a moving substrate. Likewise, surface tension, general microsphere stability and other fluid properties of the formulations can be selected to ensure a uniform coating. The formation of a stable layer promotes predictable diffusion of a portion of the coagulant formulation on the first surface of the layers of the coagulant and absorbent formulations. An appropriate choice of viscosity of the absorbent formulation also promotes the development of a uniform thickness of the absorbent formulation layer and, eventually, a uniform thickness of the microporous sheet that will be formed. The viscosity of the absorbent formulation can be in the range of 0.02 Pa.s to 4 Pa.s (20 to 4,000 centipoise). Preferably, the viscosity of the absorbent formulation is in the range of 0.025 Pa.sa 2 Pa.s (25 to 2,000 centipoise) and more preferably, in the range of 0.025 Pa.sa 1 Pa.s (25 to 1,000 centipoise).

Numerous polymeric materials are available and can be used to form the membranes of this description. Polymeric materials or a mixture of polymeric materials can be used for an absorbent formulation. Examples of polymeric materials may include polyether sulfones, polyetherimides, polyimides, polyamides, polysulfones, polyaryl sulfones, polyvinyl chloride, polyethylene terephthalate, polycarbonates, polyolefins such as polyethylene or polypropylene, cellulose ester such as cellulose acetate or cellulose nitrate, polystyrene, polystyrene acrylic polymer, methacrylic polymers, acrylic copolymers or methacrylic polymers and combinations thereof. The absorbent formulation can be a solution of polymers, which can be amorphous, crystalline, non-crystalline or partially crystalline, in the solvent. In one embodiment, the polymeric material of the absorbent formulation is polyetherimide (I).

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polyether sulfone (II).

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provide a homogeneous solution. The solvent of the absorbent solution can be a solvent for an optional non-solvent, that is, of the polymeric material, and optional additives present in the absorbent formulation. When present, the non-solvent can be added to the formulation at a concentration in the range of 0.05 to 5 weight percent. The selection of a solvent for the membrane can influence properties, such as the phase inversion rate or the type of microstructure, when the absorbent formulation comes into contact with a coagulant formulation. Some examples of solvents for the absorbent formulation may include water, dimethyl formamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidinone, tetramethylurea, acetone, methyl ethyl ketone, methyl acetate, ethyl acetate and other alkyl acetates , dimethyl sulfoxide and combinations thereof. The solvent can be oligomeric or polymeric in nature forming a polymer blend with the polymeric material. For phase inversion, the solvent of the absorbent formulation may comprise more than one solvent, a blend of solvents or a non-solvent. A non-solvent is a material that is miscible in the solvent of the absorbent formulation, but that, in itself, is insoluble in the polymer or can cause the polymer to coagulate. The non-solvent can be added to a solvent to influence the rate of a phase inversion or assist in the development of a microstructure.

The selection of a solvent for an absorbent formulation to provide a stable homogeneous solution for fusion molding in membrane formation involves basic principles of polymer solubility. Polymeric solvents can be categorized as good solvents, non-solvents and poor solvents. Good solvents are those in which the interactions (forces) between the polymer molecules and the solvent molecules are greater than the forces of attraction between a polymer molecule and another polymer molecule. The reverse is true for non-solvents. Poor solvents are those in which the interactions between the polymer and the solvent are equal to the forces of attraction between one molecule of the polymer and another molecule of the polymer.

Good solvents dissolve substantial amounts of polymer and can be miscible in the polymer in concentrations of at least 5 weight percent, while weak solvents may or may not be miscible, depending on the molecular weight of the polymer and the type of solvent.

In one embodiment, a stable homogeneous absorbent formulation can be obtained by first dissolving a polymer in a good solvent (for example, when polyether sulfone is the polymer, N-methyl-2-pyrrolidinone is used as the solvent). Other examples of good solvents for polyether sulfone include dimethylacetamide, dioxane, dimethyl sulfoxide, chloroform, tetramethylurea and tetrachloroethane.

Another method of evaluating solvents for polymer solubility includes determining Hildebrand's solubility parameters. These parameters refer to a solubility parameter represented by the square root of a material's cohesive energy density, which has units of (pressure)^1/2 and is equal to (AH-RT)^1/2V^1/2where ΔΗ is the enthalpy of vaporization of the material, R is the universal gas constant, T is the absolute temperature and V is the molar volume of the solvent. Hildebrand's solubility parameters are tabulated for solvents in: Barton, AFM, “Handbook of Solubility and Other Cohesion Parameters”, 2^naEd., CRC Press, Boca Raton, Fia., USA, (1991); for representative monomers and polymers in “Polymer Handbook”, 4^th Ed., J. Brandrup &amp; EH Immergut, Eds. John Wiley, NY, pages VII 675 to 714 (1999); and for many commercially available polymers in Barton, AFM, “Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameteçs”, CRC Press, Boca Raton, Fia, USA. (nineteen ninety).

A coagulant formulation can be melted and shaped as a layer of a multilayer sheet to form microporous membranes. The coagulant formulation can comprise a coagulant and a coating aid. In some embodiments, the coagulant does not comprise a coating aid. The coagulant can be a non-solvent from the polymeric material of the absorbent formulation.

Membranes can be formed by the simultaneous melting molding of a coagulant formulation with an absorbent formulation, resulting in a phase inversion and the formation of a microstructure. The coagulant of the coagulant formulation has a selected concentration to reduce the thermodynamic stability of the absorbent formulation when the formulations are melted and shaped as layers.

The coagulant is considered a non-solvent of the polymeric material of the absorbent formulation, where the polymeric material is insoluble in the coagulant. As a non-solvent, the attraction between the coagulant molecules and the polymeric chains of the polymeric material is less than the attraction between a polymeric chain of the material and a second polymeric chain of the same polymeric material. The coagulant may have limited solubility in the solvent of the absorbent formulation. The solvent of the absorbent formulation is preferably miscible in the coagulant during membrane formation.

Examples of coagulants in a coagulant formulation may include water, alcohols, ethers, oligoalkylene oxides and ethers of oligoalkylene oxides, polyalkylene oxide and their ethers. A particularly useful group of compounds suitable as a coagulant of the coagulant formulation are the oligoalkylene oxides (III) of the following general formula:

R¹ - (OR²)_x-OR³(III) where x is from 1 to 25; R¹, R³ are H or CnH2n + i, where n = 1 to 5; R² is CnH_2n, where n = 1 to 8. R¹ and R³ can be the same or different.

In addition, examples of coagulants as non-solvents for a polymeric material (eg, polyether sulfone) of an absorbent formulation can include 2-methoxy ethanol, propionic acid, t-amyl alcohol, methanol, ethanol, isopropanol, hexanol, heptanol, octanol, acetone, methyl ethyl ketone, methyl isobutyl ketone, butyl ether, ethyl acetate, amyl acetate, glycerol, diethylene glycol dibutyl ether and water. The concentration of coagulant can be determined by the polymeric material and solvent of the absorbent formulation and by the processing conditions of preparing the membrane. In one embodiment, a coagulant of a coagulant formulation for the polyether sulfone is water.

A coagulant formulation can have sufficient viscosity to allow a portion of the coagulant formulation to diffuse through a first surface formed with a layer of the absorbent formulation. The viscosity of the coagulant formulation can be adjusted by adding a coating aid to provide uniform thickness as a layer of coagulant formulation when melted and molded into the absorbent formulation layer. Other properties of the coating formulation to be considered include surface tension, general microsphere stability, in addition to the properties that can be selected to influence the coating uniformity of the layers. In addition to coating aids, other materials, such as surfactants, can be added to influence surface tension, interfacial stability, wettability and dimensional stability. The viscosity of the coagulant formulation containing a coating aid can be in the range of 0.02 Pa.s to 4 Pa.s (20 to 4,000 centipoise). Preferably, the viscosity of the coagulant formulation is in the range of 0.025 to 2 Pa.s (25 to 2,000 centipoise) and more preferably, in the range of 0.025 Pa.s to 1 Pa.s (25 to 1,000 centipoise). Some examples of coating aids include poly (ethylene glycols), polyhydric compounds, polyethers and combinations thereof. The coating aid concentration of a coagulant formulation is in the range of 20 to 95 weight percent. Preferably, the coating aid concentration is in the range of 25 to 90 weight percent. In one embodiment, the coating aid for a coagulant formulation is poly (ethylene glycol).

The coating aid can be compatible with the polymeric material and / or the solvent of the absorbent formulation layer. The coating aid can be added to a coagulant formulation to adjust the viscosity of the coagulant for melt molding as a layer. The coating aid of the coagulant formulation can be added at a concentration sufficient to allow a portion of the coagulant formulation to diffuse through the molded and molten absorbent and the layers of the coagulant formulation. In one embodiment, the coagulant formulation comprises water and poly (ethylene glycol).

In one embodiment, the coagulant can form a homogeneous solution with a coating aid. In contact with the auxiliary formulation, the coagulant formulation can diffuse into the absorbent formulation layer and coagulate the polymeric material causing a phase inversion, thus changing the thermodynamic stability of the polymeric material in the solvent of the absorbent formulation. Regions rich in polymers and regions low in polymers can form during the thermodynamic change of the polymeric material in the solvent. The regions rich in polymers form a continuous structure and the regions poor in polymeric material form the pores of the resulting microstructure.

In one embodiment, the coating aid can be added to a coagulant formulation at a selected concentration to control the rate (kinetics) of the phase inversion forming a membrane. The kinetics of the phase inversion can be generally dictated by the process conditions and the components of the coagulant formulation and the absorbent formulation.

In one embodiment, the coating aid can be added to the coagulant formulation at a selected concentration to produce a phase inversion that occurs when the coagulant formulation comes in contact with the absorbent formulation layer at a predetermined depth of a membrane thickness. The thickness of the membrane extends in the z dimension. The depth of the phase inversion can be dictated by the processing conditions and the type of absorbent and coagulant formulations. The depth of the phase inversion resulting in a microstructure can be in the range of 5 to 100 percent of the membrane thickness. More preferably, the depth of the phase inversion can be in the range of 5 to 75 percent and most preferably in the range of 10 to 60 percent.

In one embodiment, the concentration of a coating aid for a coagulant formulation is selected to effect a phase inversion resulting in a defined microstructure after contact of the coagulant formulation with the absorbent formulation layer. Proper selection of coagulant formulation and absorbent formulation layers can produce certain pore size and porosity distributions. For example, a membrane formed in the phase inversion of a polyether sulfone absorber formulation that comes in contact with two different coagulant formulations on opposite sides can result in a first microstructure that has a spongy open cell microstructure on the first surface and a second microstructure that has elongated, interconnected parallel pores that extend in a z dimension on a second surface. Multiple microstructures within a membrane, called multizone membranes, can provide high performance and high flow in filtration applications.

In another embodiment, a porogen can be an additive to an absorbent formulation. Porogen can be added to provide a selected microstructure within the membrane or to control the rate of phase inversion within the membrane. Some examples of porogens include poly (ethylene glycols), polyhydric compounds, polyethers and combinations thereof.

A selection of components of absorbent and coagulant formulations for simultaneous melting molding for the formation of microporous membranes has been described previously. In this description, the absence of a coagulation bath to induce a phase inversion eliminates a costly downstream process of membrane formation by traditional processes. The concentrations of solvents, non-solvents, polymeric materials and additives can be adapted in the absorbent and coagulant formulations to control the phase inversion rate and the depth of the phase inversion of a multilayered sheet or to obtain a membrane with a specific microstructure.

The measurement of the flow rate of the absorbent and coagulant formulations, which can correspond to the thickness of the coating, for example, of a multilayer extrusion matrix, can control the depth of a phase inversion in the layer (s) of the formulation of absorbent. The selection of a specific surface of the absorbent formulation layer to melt and shape the coagulant formulation can be determined prior to deposition of the absorbent and coagulant formulations as a multilayered sheet on a supporting cylinder or temporary support. In other processes described in the art, a coagulation bath used for the coagulation of the absorbent layer would have to be continuously filtered or the contents of the bath would have to be removed and refilled with new coagulants in a continuous solvent exchange process.

The concentration of the polymeric material of the absorbent formulation and the concentration of the coating aid of the coagulant formulation can be selected to provide formulations with viscosities suitable for melt molding, with or without a support cylinder or temporary support. The concentrations or solids levels of the absorbent and coagulation formulations can be selected to maintain sufficient viscosity for the melt molding of the absorbent and coagulant formulations and the subsequent phase inversion at an interface between the layers. Viscosities greater than the appropriate range required for simultaneous melting molding and phase inversion can form a membrane with little porosity, where the amount of solids is very low, resulting in a low viscosity formulation, and a membrane structure can not be attainable.

FIGURE 1 illustrates the schematic representation of a cross section of a multilayer extrusion die 200 that is capable of forming multilayer sheets. The multilayer extrusion die 200 includes an extrusion head 220 disposed adjacent to a support cylinder 240. The support cylinder 240 is a rotating cylinder that receives bed12 from the extruders of the extrusion head 220. In some embodiments, the support cylinder 240 can optionally support a liner 260 to support the coextruded layers (i.e., sheet) as they begin to form a microporous membrane.

The extrusion head 220 of FIGURE 1 includes a cavity for the absorbent formulation 320 and cavities for the coagulant formulations 300, 340 ending in extrusion slits that serve as routes for the coextrusion of the absorbent and coagulant formulations. The dimensions of the extrusion slits can be conveniently established by placing precision metal wedges, with a necessary thickness, between the bars of the die. By carrying out the method of the present description, an absorbent formulation is introduced into cavity 320 to subsequently form an absorbent formulation layer. Similarly, a coagulant formulation can be introduced into cavities 300 and / or 340 to subsequently form layers of coagulant formulation on one or more surfaces of the absorbent formulation layer.

FIGURE 2 illustrates an enlarged view of detail 2 of FIGURE 1. In detail 2, the absorbent / or coagulant formulations are extruded from the extrusion head 220 of FIGURE 1, specifically of the extrusion slits extending from cavities 300, 320, 340 creating absorbent formulations 321, the first 301 and the second coagulant formulations 341. In FIGURE 2, the absorbent formulations 321, the first 301 and the second 341 coagulant formulations are extracted in contact with the rotating support cylinder 240.

The multi-layer extrusion die 200 controls the shape of the article by molding a flow of cylindrical tubing onto a sheet. The measurement of the formulations can be self-measured, for example, a solution treatment system at constant pressure that feeds a slotted matrix with pressure cookers, inlet boxes or centrifugal pumps; or pre-measured, for example, a positive displacement syringe piston that measures the flow of formulations into the die cavity in relation to the width and speed of the molten multilayer sheet, thereby stabilizing the base weight of the incompressible liquid sheet at a value desired predetermined.

A method of forming a two-layer sheet 5 comprises simultaneously melting and molding an absorbent formulation and a coagulant formulation like the layers illustrated in FIGURE 3. An interface 20 is formed where the layers of the absorbent formulation 30 and the formulation coagulant 40 come into contact with each other. A first surface 10 of the absorbent formulation is located opposite the interface 20 formed between the molten absorbent 30 and the coagulant formulation layers 40 of the two-layer sheet 5. The simultaneous melting molding of the absorbent and coagulant formulations additionally provides the diffusing a portion of the formulation into the absorbent formulation through interface 20, performing a phase inversion. The phase inversion of the polymeric material of the absorbent formulation results in a first microstructure.

The thickness of the layers depends not only on the geometry of the multilayer extrusion die, but also on the flow and viscosity of the absorbent and coagulant formulations. The subsequent phase inversion on the first surface 10 of the two-layered sheet 5 can occur from the introduction of a vapor as a coagulant.

The phase inversion induced by the vapor phase (i.e., melting and air molding) generally includes a coagulant (e.g., water vapor) to induce a phase inversion. The coagulant can be introduced into a polymeric material of an absorbent formulation such as a vapor. A high vapor concentration can condense and reduce the thermodynamic stability of the polymeric material dissolved in a solvent. Similar to liquid-induced phase separation, polymer-rich regions and polymer-poor regions are formed from vapor-induced phase inversion, resulting in the formation of a microstructure. Examples of coagulants for vapor-induced phase separation include water, alcohols, amides and combinations thereof.

In one embodiment, multiple layers of the absorbent and coagulant formulations can be fused and shaped like a sheet to form a three-layer sheet 100 as illustrated in FIGURE 4. FIGURE 4 illustrates a three-layer sheet 100 that has a formulation layer of absorbent 30 and a first 40 and second 50 layers of coagulant formulation. The coagulant of the first and second coagulant formulations can be a liquid. The thickness of the absorbent and coagulant layers may depend on the viscosity and flow of the absorbent and coagulant formulations and the configuration of the multilayer extrusion die. An interface 70 can be formed where the absorbent formulation layer 30 is fused and molded to a first coagulant formulation layer 40. A first surface 60 of the three-layer sheet 100 can be formed where the absorbent formulation layer 30 is fused and molded into a second layer of coagulant formulation 50. The layer of the first coagulant formulation 40 and the absorbent formulation 30 at the interface 70 they provide the flow and diffusion of a portion of the layer of the first coagulant formulation 40 in the absorbent formulation layer 30, making a first phase inversion. The second coagulant formulation 50 and the absorbent formulation 30 on the first surface 60 provide the flow and diffusion of a portion of the layer of the second coagulant formulation 50 on the absorbent formulation layer 30, making a second phase inversion.

Phase inversions on the first and second surfaces of a layer of the absorbent formulation can occur when the first and second layers of the coagulant formulation are each located on opposite surfaces of the absorbent formulation layer. The resulting phase inversions on the first and second surfaces form the first and second microstructures. The morphology and depth of the microstructure can be different or the same in membranes that have more than one microstructure. The selection of the microstructures of the membranes can be additionally selected by the concentration and selection of the components of the absorbent and coagulant formulations and the processing conditions. In one embodiment, the absorbent and coagulant formulations can be simultaneously melted and molded at room temperature.

In one embodiment, the absorbent and coagulation formulations of FIGURE 1 can be simultaneously melted and molded using a multilayer extrusion matrix 200. The multilayer extrusion matrix 200 can melt and shape the absorbent and coagulant formulations as layers to form a sheet, where the layers are substantially linear in an x ​​or transverse dimension.

A phase inversion process can be initiated by diffusing a portion of a coagulant formulation layer into an absorbent formulation layer after the extrusion head 220 and coagulant formulation exit, as illustrated in FIGURE 1 and FIGURE 2 . The process can include measuring absorbent and coagulant formulations that have a selected viscosity, using a multilayer extrusion matrix 200 of FIGURE 1 to form a multilayer sheet, where the layers of absorbent and coagulant formulation coagulate, partially or totally, to form a microporous membrane.

Contact of the absorbent and coagulant formulations on a surface and the diffusion of a portion of the coagulant formulation into the absorbent formulation can cause the polymeric material of the absorbent formulation to become thermodynamically unstable. The polymeric material can separate from the solvent of the absorbent formulation forming a microstructure. During phase inversion, regions of the absorbent formulation layer are rich in polymeric material forming a structure, and some regions are poor in polymeric material forming pores. The membranes can additionally be subjected to solvent removal and subsequent drying after the development of the microstructures. Effective pore sizes of a membrane can be in the range of 0.05 to 50 microns. The pore size refers to the diameter of an opening within a microstructure formed during a phase inversion. Pore ​​sizes can be measured using the bubble point pressure method described in the Examples section. Other methods of measuring pore size and pore size distribution may include solute retention and flow / pressure techniques. Solvent phase inversion techniques are presented in Keating, “Synthetic Polymeric Membranes, A Structural Perspective”, 2^The Ed., John Wiley and Sons, 1995.

A phase inversion of this description may include diffusion of the absorbent and coagulant formulation layers across a liquid-liquid polymer surface. A portion of a layer of coagulant formulation diffuses into a layer of the absorbent formulation. The coagulant of the coagulant formulation can be thermodynamically compatible with the solvent of the absorbent formulation and / or other additives to facilitate the movement and diffusion of the coagulant within the absorbent formulation layer. The coagulant can generally be incompatible with the polymeric material of the absorbent layer, reducing the solubility or compatibility of the polymeric material in the solvent. The instability creates a phase inversion of the polymeric material resulting in the formation of a microstructure.

The microstructures of the membranes can have pores that are asymmetrical or symmetrical along the z dimension, or dimension of the thickness, of the membrane. Numerous microstructures can be formed that may depend on the absorbent formulation and the coagulant formulation used and the processing parameters. A first microstructure can be the same or different from the second microstructure. The first and second microstructures can provide a continuous or discontinuous path through the membrane. The formation of microstructures may depend on the concentration of some of the components (for example, polymeric material, coagulant, coating aid) of the absorbent and coagulant formulations. The microstructure morphology can also depend on the measurement (for example, layer thickness) of the absorbent and coagulant formulations and / or the rate of phase inversion. The microstructure morphology may also depend on the phase inversion mechanism and be related to the pressure and temperature conditions of the processing.

In general, the pores of a microstructure can be open or closed cells. A first face of the membrane can have pores with relatively small diameters, while the second face, or opposite, can have pores with relatively large diameters. The ratio between the pore sizes on the first face and the pore sizes on the second face of the membrane can be in the range of 10: 1 to 100: 1.

A convenient method to assess symmetry and pore diameter of membranes is through the use of scanning electron microscopy (SEM). FIGURE 5 of example 1 is a SEM micrograph showing the generally symmetrical porous structure. The pores can have an average diameter of 0.05 to 25 microns. Preferably, the pore diameter is in the range of 0.5 to 10 microns. In one embodiment, the pores of a microstructure have a generally symmetrical morphology.

FIGURE 6 of example 2 is a SEM micrograph showing a membrane having first and second microstructures. The membrane has a porous structure comprising the first and second faces, extending in an x ​​dimension. A first face has a first microstructure that has parallel elongated cylindrical pores that extend in a z dimension formed from a multilayer liquid-induced phase separation. The second face has a second microstructure that has ellipsoidal pores formed from a vapor-induced phase separation. FIGURE 7 of example 2 illustrates the planar view of a surface scan of the first face of the membrane. The absorbent formulation can have different coagulation formulations on different faces of the absorbent formulation by performing phase inversions, which can result in the formation of similar or different microstructures on the faces.

In one embodiment, an article has a porous structure that comprises the first and second faces extending in an x ​​dimension. The first face has a first microstructure and the second face has a second microstructure. The first and second faces are interconnected by the first and second microstructures.

FIGURE 8 of example 3 is a SEM micrograph showing a membrane having the first and second microstructures. The absorbent formulation layer of example 3 contained porogen and the first coagulant formulation layer comprised a coating aid. The depth of the first microstructure in a z dimension formed by a phase separation induced by multilayer liquid is in the range of 5 to 95 percent of the thickness of a membrane thickness. The depth of a second microstructure in a z dimension formed by a vapor-induced phase separation is in the range of 5 to 95 percent of the membrane thickness.

The thickness of the membranes may depend on the thickness of the absorbent and coagulant formulation layers when fused and molded and the subsequent removal of solvents followed by drying. The thickness of the membrane can be in the range of 20 to 200 microns and more preferably in the range of 40 to 150 microns. Most preferably, the thickness of the membrane is in the range of 50 to 100 microns.

In one embodiment, a microporous membrane can be formed by the simultaneous melting molding of an absorbent formulation between the first and the second coagulant formulations. The depth of a first microstructure formed by a multilayer phase separation is in the range of 5 to 95 percent of the membrane thickness. The depth of a second microstructure formed by a multilayer-induced phase separation is in the range of 5 to 95 percent of the membrane thickness.

In one embodiment, a microporous membrane having first and second microstructures formed by multilayer-induced phase separation and vapor-induced phase separation, respectively, the first and second microstructures are different.

In one embodiment, a microporous membrane that has first and second microstructures formed by multilayer-induced phase separation, the first and second microstructures are different.

In one embodiment, at least two absorbent formulations and at least two coagulant formulations are simultaneously melted and molded and additionally optionally comprise a steam coagulant to form a multizone microporous membrane.

It will be appreciated that a multizone microporous membrane can be formed which has multiple layers of absorbent and coagulant comprising multiple zones or microstructures.

Applications using filtration, reinforced adhesives and drug release properties can be envisaged. The utility of the process lies in the ability to form porous membranes useful in microfiltration and ultrafiltration applications.

The invention will receive further clarification by the following examples, which are exemplary and are not intended to limit the scope of the invention.

Examples

The present invention is, more particularly, described in the following examples, which are for illustrative purposes only, since numerous modifications and variations within the scope of the present invention will be apparent to those skilled in the art. Unless otherwise specified, all parts, percentages and ratios reported in the examples below are on a weight basis, and all reagents used in the examples have been obtained, or are available, from the general chemical suppliers described above or may synthesized by conventional techniques.

Bubble point pressure (PPB) - Standard ASTM E-128-99 (2005). PPB measurements were recorded on pre-moistened microporous membranes of 47 mm in diameter.

Water flow rate (TFA) - TFA measurements were recorded on microporous membranes. The membranes were pre-moistened with isopropanol and deionized water. The amount of time required to pass deionized water through the microporous membrane under reduced pressure (59 cm (Hg)) was recorded. The TFA method is further described in US patents No. 7,125,603 and 6,878,419 (Mekala et al.).

Surface treatment - Plasma treatment of membranes included chemical attack of the surface by exposing the surface to a mixture of O₂ and C₃F₈ (perfluoropropane) in plasma for 5 minutes. Known techniques for the chemical attack of polymeric surfaces have been described in Khulbe, KC et al .; J. Membrane Science: 171, 2000, pages 273 to 284 and Bauer, CJM et al .; J. Membrane Science, 57,1991, pages 307 to 320.

Example 1

The absorbent and coagulant formulations were fed into a multilayer extrusion matrix of FIGURE 1 (US patent published No. 2007/0128425) by syringe pistons (Model PHD 2000; Harvard Apparatus; Holliston, MA, USA) that were used to measure the absorbent and coagulant formulations in the multilayer extrusion die wells at volumetric flow rates in the range of 5 to 40 ml_ / minute.

A multilayer sheet was coextruded from a multilayer extrusion die and transferred to a support cylinder around which the corona treated polyester film support (20 to 50 microns thick) was loaded. The support was moved forward at a line speed of 150 cm / minute.

The central slot of the multilayer extrusion die was fed with an absorbent formulation that has 15% by weight of polyetherimide (PEI) (melt index = 9g / 10 min .; Sigma-Aldrich; St. Louis, MO, USA) dissolved in 1-methyl-2pyrrolidinone (NMP) (Sigma-Aldrich; St. Louis, MO, USA) (viscosity = 0.26 Pa.s (260 centipoise)) at a rate of 16 mUmin; one of the adjacent slits of the multilayer extrusion die was fed with a coagulant formulation which has 90 weight percent polyethylene glycol as a coating aid (PEG-400) (molecular weight = 400 g / mol; Sigma-Aldrich; St Louis, MO, USA) and 10 weight percent water as a coagulant. The viscosity of the coagulant formulation was 0.07 Pa.s (70 centipoise). The absorbent and coagulant formulations were melted and shaped as layers on top of each other through the multilayer extrusion matrix forming an interface, and the two-layer coating was started with a span of approximately 500 microns between the multilayer extrusion matrix and the cylinder of support.

The results of the multilayer-induced phase inversion are illustrated in the cross section MEV shown in FIGURE 5. Specifically, the resulting microporous membrane can be characterized by a mixture of closed cells and pore sizes with diameters of about 1 to 2 microns. The morphology of the membrane is generally symmetrical extending from one face of the membrane to the other.

Example 2

The absorbent formulation and the first coagulant formulation were fed to a multilayer extrusion matrix of FIGURE 1 (US patent published No. 2007/0128425) for syringe pistons (Model PHD 2000; Harvard apparatus; Holliston, MA, USA) that was used to measure the absorbent formulation and the first coagulant formulation in the cavities of the multilayer extrusion matrix at volumetric flow rates in the range of 5 to 40 mL / minute.

A multilayer sheet was extruded from the multilayer extrusion die and transferred to a support cylinder around which a corona treated polyester film support (20 to 50 microns in thickness) was loaded. The support was moved forward at a line speed of 150 cm / minute.

The central slot of the coating matrix was fed with a 13% absorbent formulation by weight of polyetherimide (PEI) (melt index = 9 g / 10 min .; Sigma-Aldrich; St. Louis, MO, USA) dissolved in 1-methyl-2-pyrrolidinone (NMP) (SigmaAldrich; St. Louis, MO, USA) (viscosity = 0.26 Pa (260 centipoise)) at a rate of mL / min; one of the adjacent slits of the multilayer extrusion die was fed with a first coagulant formulation with 80 weight percent polyethylene glycol as a coating aid (PEG-400) (molecular weight = 400 g / mol; Sigma-Aldrich; St Louis, MO, USA) and 20 weight percent water as a coagulant. The viscosity of the coagulant formulation was 0.054 Pa.s (54 centipoise). The absorbent formulation and the first coagulant formulation were melted and molded as layers on top of each other through the multilayer extrusion matrix forming an interface. A first surface of the absorbent formulation layer was opposite the interface formed of the absorbent formulation and coagulant formulation layers. The two-layer coating was started with a span of approximately 500 microns between the multilayer extrusion die and the support cylinder.

A second coagulant having a coagulant as a vapor was introduced on the first surface of a multilayer membrane having a first microstructure, which resulted in a vapor-induced phase separation. The absorbent formulation layers and the coagulant formulation were fused and molded as described above and subjected to a phase separation induced by liquid polymer followed by subsequent exposure of the first membrane surface to water vapor created by the injection of steam into a drying oven 8 meters long. Through the injection of steam, a relative humidity of between 50 and 60% was maintained. The parameters of the used oven are mentioned in table 1.

Table 1. Oven configuration

<td></td><td>Zone 1</td><td>Zone 2</td><td>Zone 3</td>

<td>Air temperature (° F)</td><td>49 ° C (120)</td><td>49 ° C (120)</td><td>49 ° C (120)</td>

<td>Nozzle type (* T / B)</td><td>- / Airfoils</td><td>- / Airfoils</td><td>TEC / TEC</td>

<td>Air pressure (* T / B)</td><td>0.5 water</td><td>0.5 water</td><td>1/1 water</td>

* T / B: Top / Bottom (airflow configuration of the air flotation nozzles or air bars - TEC (Grace TEC Systems; DePere, Wisconsin, USA) and Airfoils (Grace TEC Systems; DePere, Wisconsin, USA) ).

The results of the multi-zone microporous membrane are illustrated in the SEM cross section shown in FIGURE 6 and in the planar view of FIGURE 7. Specifically, the resulting microporous membrane is characterized by the mixture of closed cells and pore sizes of about 1 to 2 microns in diameter. The first zone of the liquid-induced phase inversion has a representative morphology of elongated porous spaces, having a thickness of about 32 microns (about 50 percent of the membrane thickness in a z dimension). The second vapor-induced phase inversion zone has a spongy microstructure that is about 35 microns thick (about 50 percent of the membrane thickness in a z dimension). The pores of the first and second zones appear to be interconnected between zones with different pore structures.

The membrane had filtration properties (PPB) of about 20.8 psi. The result of the TFA test of the untreated surface was no flow, and the TFA of the membrane treated with the chemical mixture on both sides was minutes.

Example 3

An absorbent formulation and a first coagulant formulation were fed into a multilayer extrusion die using an equipment arrangement similar to example 2.

The central slot of the coating matrix was fed with a 13% formulation by weight of polyether sulfone (PES) (molecular weight &lt;60,000 g / mol; Solvay; Alpharetta, GA, USA) dissolved in a mixture of 1 methyl-2 -pyrrolidinone (30 weight percent) (NMP) (Sigma-Aldrich; St. Louis, MO, USA) / polyethylene glycol (70 weight percent) (PEG-400) (molecular weight = 400 g / mol ; Sigma-Aldrich; St. Louis, MO, USA) as a porogen (pore-forming). The viscosity of the absorbent formulation was 3.4 Pa.s (3,400 centipoise). The flow rate of the absorbent formulation was 20 ml / minute. One of the adjacent slits of the multilayer extrusion die was fed with a coagulant formulation at a flow rate of 10 mL / minute. The two-layer coating was started with a span of approximately 500 microns between the multilayer extrusion die in the support cylinder.

A second coagulant was used similarly to example 2.

The results of the formation of a multi-zone microporous membrane are illustrated in FIGURE 8. Specifically, the resulting microporous membrane had an open cell morphology that has a pore size distribution of about 0.6 to 1 micron. The first zone of the liquid-induced phase inversion had elongated porous spaces and a thickness of about 75 microns, and a second porous zone resulting from the vapor-induced phase separation had a spongy microstructure with a thickness of about 25 microns.

The TFA was 2 minutes without the surface treatment, and the TFA was 18 seconds after the chemical attack on both sides of the membrane.

Various modifications and alterations of this invention will be evident to those skilled in the art without departing from the scope and spirit of this invention and it should be understood that this invention is not limited to the illustrative elements presented herein.