GB2232674A

GB2232674A - Microporous organic foam products

Info

Publication number: GB2232674A
Application number: GB9010639A
Authority: GB
Inventors: Michael John Baker
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1989-05-12
Filing date: 1990-05-11
Publication date: 1990-12-19
Anticipated expiration: 2010-05-11
Also published as: GB9010639D0; GB8910959D0; GB2232674B

Abstract

A method of forming elongate microporous foam bodies of controlled porosity profile and void content comprising forming a volume of a solution of an organic polymer in a solvent by extrusion through a single orifice and contacting the surface of said volume with a phase-separating liquid that is miscible with the solution but which is a non-solvent for the polymer. By control of temperatures and compositions of the polymer solution and the liquid the polymer is converted to a foam of desired pore profile and void content. The liquid and solvent are then removed by drying. The foam bodies can be used as filters for gas/liquid separation, or artificial veins.

Description

MICROPOROUS ORGANIC FOAM PRODUCTS.

The present invention relates to a method of forming elongate microporous organic foam bodies, to microporous foam bodies formed by such method and to uses for said foam bodies.

Micron cell size or microporous foams are known and have been produced by a phase-separation process wherein an organic polymer solution is precipitated or gelled to form a continuous matrix and then dried to produce the product foam eg: Coudeville et al; J. Vac. Sci. Technol. 18(3), April 1981, pp 1227-30, describe the production of a dextran foam from a dioxan/water solution thereof by rapidly reducing the temperature of the solution followed by freeze drying.

The present invention provides an alternative temperature and solvent driven process which allows the formation of elongate bodies of controlled porosity profile whereby the outer part of said body has different porosity characteristics to that of the interior, said interior having a void or being of a higher pore volume than said outer part. By use of the present method choice of solvents, phase-separating liquids and their respective temperatures can provide solid or hollow porous foam bodies having either pored or skin covered surfaees. Particularly advantageously production of solid or hollow foam polymer rods is provided from the same apparatus that may be used in processes for forming foam spheres.

Thus both solid spheres and tubes may be provided by use of the same single simple orifice by simply varying the temperatures, rate of supply and compositions of the liquids used.

According to the present invention there is provided a method of forming elongate microporous organic foam bodies of controlled porosity profile comprising: (a) forming a volume of a solution of an organic polymer in a solvent by extruding said solution through a single simple orifice of desired shape and (b) contacting the surface of the volume with a phase -separating liquid which is miscible with said solution but which is a non-solvent for said poLymer; the temperatures and compositions of said solution and said liquid being selected to eEfect phase-separation of the polymer from said solvent at a desired rate and thereby to convert said polymer to a foam of a desired pore profile and void content.

The present inventors have found that by decreasing the rate at which the polymer solution and the phase-separating liquid mix that the pore size toward the centre of the body can be increased up to and including the production of a central void, the size of said void being similarly dependent on this rate of mixing.

Conversely increasing the rate of mixingresults in a more uniform pore size distribution but with smaller pores being produced toward the exterior surface up to and including the formation of a skin of polymer of the order of several pore size thickness.

The parameters which can be used to control the porosity and thus central void size are set out in Table I below; each being alterable with the composition or temperature of the relevant liquid.

TABLE I

FUNCTION PORE INTERNAL DIAMETER THICKNESS OF INCREASING SIZE OF VOID OUTER SKIN I Molecular weight 1 Decreases Decreases Tncreases of ofpolymer.

Viscosity Decreases Decreases Increases of ofpolymer.

Solubility of I Increases Increases Decreases I polymer/solvent. I I Solubilty of I Increases Increases Decreases I polymer/phase-sep liquid.

I Miscibility of I Decreases Decreases Increases polymer solution I 1 /phase-sep.liquid I Speed of 1 No change No change No change extrusion.

Temp. of polymer I Increases Increases Decreases I solution.

I I I Temp. of phase I Increases Increases Decreases I -sep. liquid. I I Temp. difference I Increases Increases Increases I solution/phase I I -sep.liquid.

Thus, for example, by use of a solution and a phase-separating liquid both of room temperature (180C) a porous body having a central void might be created depending on the polymer and liquids selected. Alternatively by increasing the miscibility of the solution with the phase-separating liquid by choosing more miscible solvents or by increasing their temperatures a skin might be formed on the body surface and the void diameter thus increased relative to the diameter of the body. By decreasing the temperature of the phase-separating liquid and allowing it to warm up slowly after the solution is contacted with it a foam body having no void and no skin may be produced. A further option is provided by keeping the phase-separating liquid at room temperature and by contacting the solution with it in frozen form whereby a skinned body without a central void may be produced.

It will be appreciated that the above conditions are given as general, illustrative, guidelines only and that various ranches of temperatures will apply to different polymer, solvent and phase-separating liquid combinations. Further illustration of the control possible is given in the Examples accompanying this description. It should also be noted that increasing the diameter of the extrusion orifice results in increased pore size and a tendancy to form a void in the inner portion of the foam body.

The present method is particularly applicable to the production of hollow and high pore volume bodies since, without wishing to be bound by any particular theory, it appears that the phaseseparation of the polymer from the solvent by the non-solvent progresses inwards from their initial contact surface and that as phase separation proceeds the dissolved polymer migrates outwards towards this surface, producing a void or a region of lower density/higher pore volume content within the original volume.

It is this effect which also makes possible the production of bodies having either porous or fine skinned surfaces depending on the conditions selected.

The above mentioned effect can be used to form foam bodies of various cross-sectional shapes , in particular hollow tubes having foam walls, by extruding the polymer/solvent solution through a single simple orifice of desired cross-sectional shape into the non-solvent phase-separating liquid, the void formation effect resulting in the hollow centre of the body. By varying the temperature and composition of the solvent and/or the phase-separating liquid with time as the polymer/solvent solution is extruded it is possible to produce bodies, eg; tubes, in a continuous fashion but which have a varying inner diameter void or a blind end with an variable diameter of void developing adjacent it. Such structure would allow the tube to be pressurised with a gas or liquid thus facilitating its use in filtration apparatus.

A particular advantage of the method of the present invention is that it dispenses with the requirement to use multiple concentrically arranged orifice nozzles when extruding foam tubes as use of a single simple orifice will suffice. It should be noted that the outer diameter of the tube will be influenced by the orifice size and the pressure of the extrusion as well as the temperature and composition variables.

As illustrated the bore or void size may be controlled by selection of the temperatures of the liquids used; thus, for example, the method may employ control of the orifice temperature, use of a solution freezing liquid or may heat or cool the phase-separating liquid to control void formation and dimension.

The rate of migration of the dissolved polymer can be controlled by initially maintaining the non-solvent solution at a temperature below the freezing-point of the polymer/solvent solution so that the latter solidifies rapidly, for example in the above described extrusion process the solution is frozen by extrusion into sub-zero methanol non-solvent which is then caused to warm-up and precipitate the polymer as the frozen solid gradually melts. The faster the warm-up process the less time for migration and in this way the extrusion process yields a range of bores from zero (ie:a solid foam rod) t comparatively large ones (ie:a thin walled tube). In such freezing process care should be taken to ensure that no crystallisation of polymer occurs due to the cooling process being too slow.

Alternatively extrusion of the polymer solution into a freezing liquid may be employed to produce a solid shaped body prior to immersion in the phase-separating liquid; eg; liquid nitrogen, a liquid imiscible with most polymer solvents, is employed in Example 5 below.

The drying of the formed foam product may be continuous, eg; air-drying for robust foams, or batch-wise, eg; critical point-drying (CPD) for less dense foams. Alternatives such as freeze-drying can also be used.

The foams can be formed from a single monomer polymer, eg; polystyrene, polystyrene sulphonate or polypheny lsulphone, or copolymers, eg; styrene/acrylonitrile copolymer or styrene /acrylonitrile/butadiene copolymer or polymer mixtures such as polystyrene mixed with acrylonitrile/butadiene/styrene copolymer.

Suitable solvents and non-solvents will be apparent to those skilled in the art for each polymer system concerned. Generally usable solvents include aniline, chloroform, ketones such as acetone methylethylketone or methylisobutylketone or water and mixtures of these. Generally usable non-solvents include alcohols such as methanol or ethanol. Thus it will be realised that the range of usable polymers, solvents and miscible phase-separating non-solvents is not limited to those given above or those given below in the examples.

One application of the foams produced by the method of the present invention is in filtration systems for use in industry or for such applications as dialysis. For example a filter can be formed from a plurality of the aforesaid hollow foam tubes arranged in parallel and extending between and through a pair of spaced transverse plates sealed into an outer solid tube; the foam being sealed to the plates. The liquid or gas to be filtered is admitted to the outer tube at a position between the plates and the filtrate is obtained from the foam tube lumens at a position beyond one or both transverse plates.

It is further possible to fabricate the foam bodies from a polymer having pendant groups, such as sulphonic or quaternary -amino groups, which may be used for ion-exchange purposes, thus a hollow tube filter might be provided which has mechanical and chemical filtration capabilities. Thus polystyrene sulphonate is one of a number of polymers which is suitable for this task, but other such polymers will occur to a man skilled in the art.

Another application is in the production of porous artificial veins for surgical implantation. For this purpose a polymer is selected which is biologically compatible with human tissue and which is sufficiently flexible. The inclusion of a suitable plasticiser in the polymer can confer such flexibility; eg, a copolymer of polystyrene and butadiene would be suitable for such application.

A further application of the foam bodies of the present invention is in the provision of surfaces which are permeable to gases but not to liquids such as those which may be used in gas separation and degassing techniques. Such application is facilitated by controlling the temperature and compositions used in order to form a thin skin on the body's outer surface. To effect such skin covering in prior art extrusion techniques a separate covering step has been used. In the present method selection of parameters to ensure rapid ingress of phase separating liquid will provide a skin coating as desired as will selection of low polymer solubility in the solvent thus ensuring rapid separation.

Thus, for the purposes outlined above and other applications that will occur to a man skilled in the art, the present invention also Drovides foam bodies made by the aforesaid method.

The invention will now be illustrated by way of example only with reference to the following non-1imiting Examples.

Example 1.

A 5% by volume solution of polystyrene (PS) (MW. 250000-300000 in each example) in aniline was extruded through a silica tube of 320 microns internal diameter at a pressure of approximately 110 kPa (16 psi) into methanol at room temperature (180C) and the resultant foam tubing was dried by CPD. Ahe dry tubing had an inside diameter (ID) of approximately 220 microns, an outside diameter (on) of approximately 710 microns, a gravimetric density of approximately 125 mg/cc and an open pore size of approximately one micron at the outer periphery.

Example 2.

A 10% by volume solution of a polystyrene//acrylonitrile ibutadiene/styrene copolymer (PS/ABS) mixture (ra#tio 9:1 by weight) in a methylethylketonelmethylisobutyl ketone (MEK/MIBK) mixture (ratio 1:1 by volume) was extruded through a silica tube of 320 microns (ID) at a pressure of approximately 14 kPa (2 psi) into methanol at room temperature (180C) and the resultant foam tubing air-dried. The dried tubing had an ID of approximately 20n microns, an OD of approximately 380 microns and a gravimetric density of approximately 200 mg/cc and possessed a thin surface skin of approximately 1 micron thickness.

Example 3.

A 10% by volume solution of polyphenylsulphone (obtained from Polysciences, Northampton, Cat. No 9735, Lot No 3-0562) in chloroform was extruded through a silica tube of 250 microns ID at a pressure of apDroximately 28 kPa (4 psi) into acetone at room temperature (180C) and the resultant foam tubing dried by CPD. The dried tubing had an 1D of approximately 90 microns, an OD of approximately 250 microns, a gravimetric density of approximately 104 mg/cc and an open pore size of approximately n.5 microns at the outer periphery.

Example 4.

A 5% by volume solution of PS in aniline was extruded through a silica tube of 320 microns internal diameter at a pressure of approximately 35 kPa (5 psi) into methanol at -250C causing the extruded solution to freeze solid. The methanol solution was then allowed to warm-up to room temperature, resulting in the formation of a solid foam rod of OD approxiamtely 400 microns, gravimetric density of approximately 150 mg/cc and open pore size of approximately 0.5 microns at the outer periphery.

In order to demonstrate the effects of pre-phase-separation freezing on the polymer solution the following test examples were carried out on spheroid form extrusions; examples 5 and 6.

Comparative Example 5.

A 5% by volume solution of a PS/ABS (9:1 by weight) mixture in MEK/#tBK (1:1 by volume) was sprayed into liquid nitrogen and the resulting frozen spheres transferred to methanol at room temperature (18do). After drying by CPD, the product comprised foam spheres of approximately 0.5-1.0 mm diameter, which were shown by X-ray radiography to be without a central void and had a surface skin of approximately 1 micron thickness.

Comparative Example 6.

A 5% by volume solution of PS/ABS (9:1 by weight) mixture in MEK/MIBK (1:1 by volume) was sprayed into#methanol at room temperature. After air drying or CPD product foam balls of irregular shape were obtained. These balls also had a surface skin of approximately 1 micron thickness but in this case a central void was obtained. It can be seen that the nitrogen freezing not only imposes uniform sphericity upon the bodies but prevents formation of the inner void due to inhibition of the polymer migration.

Example 7.

Experiments were undertaken to determine the parameters required to form tubes by the present process using polymer having pendant groups suitable for ion-exchange purposes. Initial attempts to phase-separate a 5% solution of poystyrene sulphonate sodium salt into methanol or 0.25 molar sulphuric acid failed but it was found that by increasing the viscosity of the polymer solution a satisfactory result could be obtained, eg; by increasing the concentration or the molecular weight or both.

Thus: A 30% by volume solution of polystyrene sulphonate sodium salt (tis. 500000) in water was extruded through a silica tube of 320 microns internal diameter into methanol at room temperature (18 C) whereupon a centrally voided foam tube was obtained.

Claims

Claims.

1. A method of forming an elongate microporous organic foam body of controlled porosity profile comprising: (a) forming a volume of a solution of an organic polymer in a solvent by extruding said solution through a simple single orifice of desired shape and (b) contacting the surface of the volume with a phase-separating liquid which is miscible with said solution but which is a non-solvent for said polymer; the temperature and compositions of said solution and said liquid being selected to effect phase separation of the polymer from said solvent at a desired rate and thereby to convert said polymer to a foam of desired pore size profile and void content.

2. A method of forming a microporous organic foam body according to Claim l wherein the polymer solution is continuously extruded into a volume of phase separating liquid to produce a continuous rod of foam.

3. A method according to Claim 1 or Claim 2 wherein an elongate microporous foam body having a relatively low internal density and high pore volume is provided by selecting solution and phase-separating liquid compositions and temperatures such that one or more of: (a) polymer solution viscosity (b) polymer solubility in the solvent (c) polymer solubility in the phase-separating liquid (d) miscibility of the polymer solution with the phase -separating liquid (e) polymer solution temperature (f) phase-separating-liquid temperature and (g) the temperature differential between the polymer solution and the phase-separating liquid are selected in accordance with the principles provided in Table I as compared with those values which would provide a uniform solid foam rod.

4. A method according to Claim 3 wherein an elongate microporous foam body having a hollow inner core is provided by selecting solution and phase-separating liquid compositions and temperatures such that the values (a) to (g) selected in accordance with the principles provided in Table I are of increased effect.

5. A method according to any one of Claims 2 or 4 wherein the pores of the microporous foam body become smaller nearer its outer periphery.

6. A method according to any one of Claims 3 to 5 wherein the parameters (a) to (g) are selected such that the outer periphery of the body comprises a skin of the polymer of 0.1 to 100 microns thick.

7. A method according to Claim 5 wherein the outer periphery of the body comprises a skin of said polymer of 0.5 to 10 microns thick.

8. A method according to Claim 5 wherein the outer periphery of said body comprises a skin of said polymer of I to 5 microns thick.

9. A method according to any one of the above claims wherein the phase-separating liquid is initially at a temperature below the freezing point of the polymer solution whereby to solidify the latter and is thereafter caused to warm-up above the freezing-point whereupon a body having no central void and no surface skin is formed.

10. A method according to any of the above claims wherein the polymer solution is frozen before being contacted with the phase-separating liquid, the phase-separating liquid being at a temperature above the freezing point of the solution wherein a body having no central void but possessing a surface skin is provided.

11. A method according to any one of Claims 1 to 3 wherein the polymer solution and the phase-separating liquid are contacted at temperatures above either of their freezing points and a body having a central void and no skin is produced.

12. A method according to any of the above claims wherein the polymer solution is extruded into the phase-separating liquid under pressure.

13. A method according to Claim 10 wherein the polymer solution is extruded at a pressure of from 5 to 150 kPa.

14. A method according to Claim 11 wherein the polymer solution is extruded at a pressure of from 10 to 110 kPa.

15. A method according to any of the above claims wherein the body comprises a tube.

16. A method according to Claim 15 wherein the tube is cut to provide access to the tube lumen at both ends.

17. A method according to Claim 15 or 16 wherein the parameters of solution and phase-separating liquid composition and temperature are maintained constant as the tube is extruded such that it has an inner hollow diameter of substantially constant dimension.

18. A method according to Claim 15 or 16 wherein the parameters of solution and phase-separating liquid composition and temperature are altered while the tube is extruded such that its inner hollow diameter is increased or decreased in selected portions.

19. A method according to any of the above claims wherein the product foam is dried after formation.

20. A method according to Claim 19 wherein the product foam is dried continuously or batchwise.

21. A method according to Claim 19 or 20 wherein the product foam is dried by air drying.

22. A method according to Claim 19 or 20 wherein the product foam is dried by critical point drying.

23. A method according to Claim 19 or 20 wherein the product foam is dried by freeze drying.

24. A method according to any one of the above claims wherein the polymer is selected from homopolymers, copolymers or mixtures of homopolymers and/or copolymers.

25. A method according to Claim 24 wherein the polymer is a homopolymer and is selected from the polystyrenes.

26. A method according to Claim 25 wherein the polymer is polystyrene or polystyrene sulphonate.

27. A method according to Claim 24 wherein the polymer is polyphenylsulphone.

28. A method according to Claim 24 wherein the polymer comprises a copolymer and is selected from styrene/acrylonitrile or acrylonitrile/butadiene/styrene copolymers.

29. A method according to Claim 24 wherein the polymer comprises a polymer mixture of polystyrene with acrylonitrile/butadiene/ styrene copolymer.

30. A method according to Claim 30 wherein the polymer mixture has a ratio of 9:1 polystyrene to acrylonitrile/butadiene/styrene copolymer.

31. A method according to any one of the above claims wherein the solvent comprises one or more of aniline, chloroform, acetone, methylethylketone, methylisobutylketone or water.

32. A method according to any one of the above claims wherein the phase-separating liquid comprises methanol and/or ethanol.

33. A filter apparatus comprising foam tubes produced by a method according to any of the above claims.

34. A filter apparatus according to Claim 37 wherein the foam tubes are arranged in parallel and extend between and through a pair of spaced transverse plates wherein liquid or gas to be filtered is admitted to the interior of an outer tube between the plates and filtered liquid or gas exits from the ends of the foam tube lumens.

35. A filter apparatus according to Claim 33 or 34 comprising foam tubes of a polymer having pendant groups capable of ion-exchange.

36. A foam body according to any of the above claims wherein the polymer has pendant groups capable of ion-exchange.

37. A fiLter or foam body according to any one of Claims 33 to 36 wherein the pendant groups are sulphonic, carboxylic or amino groups.

38. An artificial vein comprising a tube as made by a method according to any of the above Claims 1 to 32.

3Q. An artificial vein according to Claim 38 wherein the polymer includes a plasticiser.

40. A gas11iquid separation apparatus comprising an elongate foam body as made by a method of any one of Claims I to 32.

41. A method of forming an elongate microporous foam body of controlled porosity profile and void content according to Claim 1 as described in any of the Examples 1 to 4 and 7.

42. An elongate microporous foam body of-controlled porosity profile and void content as provided by any of the Examples I to 4 and 7.

43. An elongate microporous foam body as provided by a method of according to any of the Claims 1 to 32.