GB2285261A - Increasing the crystallinity of aromatic ether ketone polymers with solvents - Google Patents

Abstract

A method of enhancing the crystallinity of a porous membrane (particularly an asymmetric membrane) formed of an aromatic ether ketone polymer comprises treating the membrane with a mixture of (i) a volatile plasticising compound having a molecular weight of 30 to 150 daltons, and (ii) a water-soluble compound having a molecular weight of 30 to 150 daltons and a lower volatility than (i).

Description

ENHANCING MEMBRANE CRYSTALLINITY The present invention relates to a process for enhancing the crystallinity of a porous membrane (particularly but not exclusively an asymmetric membrane) formed of an aromatic ether ketone polymer.

Aromatic ether ketone polymers (also referred to herein as an aromatic polyetherketones) are polymers in which inter-ring ether linkages and inter-ring ketone linkings together provide at least a major proportion of the linkages between aromatic units in the polymer backbone. We do not exclude the possibility that a portion of the aromatic rings may be replaced by a heterocyclic ring, eg pyridine, or a fused-ring system, e.g.

naphthalene.

Semi-crystalline aromatic polyetherketones are a family of high-performance engineering thermoplastics of exceptional stability, retaining their excellent mechanical properties of strength, stiffness and toughness at temperatures up to 3O00C.

They are moreover resistant to all conventional solvents at ambient temperatures and are highly resistant to the action of oxidising or hydrolysing agents.

Porous membranes (particularly asymmetric membranes) fabricated from aromatic polyetherketones are useful in a range of filtration applications, eg ultrafiltration, microfiltration, gas separation, pervaporation and reverse osmosis.

Processes for the production of asymmetric aromatic polyether ketone membranes are disclosed in EP-A-O 382 356 (ICI) and EP-A- 368 003 (Dow). As disclosed in these prior specifications, the process comprises forming a solution of the polymer into a desired shape and then treating the solution with a non-solvent for the polymer so as to cause gelation thereof and thus precipitation of the membrane.

Because of the insolubility of crystalline aromatic polyetherketones in conventional organic solvents (noted above), solutions of such polymers suitable for forming asymmetric membranes are obtainable only by use of strong acid solvents such as anhydrous methanesulphonic acid, trifluoromethanesulphonic acid, 98% sulphuric acid, or liquid hydrogen fluoride. Of these, 98% sulphuric acid is to be preferred on grounds of its low cost, low toxicity, and ease of handling.

The gelation is normally effected in an aqueous medium.

However, gelation in an aqueous medium of an aromatic polyetherketone solution in a strong acid such as sulphuric acid generally gives a polyetherketone membrane in which the polymer has only reached a fraction (typically one fifth to one half) of its normal level of crystallinity. The membrane therefore does not reach its full potential in terms of mechanical strength, stiffness, temperature resistance, and ability to withstand chemical and solvent attack.

This problem is recognised in EP-A-O 382 356 which proposes that the membrane be treated to enhance the crystallinity thereof. Such treatment includes heating the membrane dry above the Tg of the polymer, or treatment with a polar aprotic solvent, eg acetone, dimethylformamide, or dimethylacetamide. However, the membrane cannot be dried after such treatments without irreversible loss of permeability. Storing membrane wet is inconvenient and makes transportation difficult. It also promotes growth of micro-organisms in the membrane.

It is known (see Blundell and Osborn, Polymer, 1983, 24, 953) that crystallisable polyetherketones which have been very rapidly cooled from the melt can be obtained in an essentially non-crystalline form, and that, on heating such materials above their glass transition temperatures, a rapid and exothermic "cold-crystallisation" process is observed, leading to material containing typically 40-50% by weight of crystalline polymer.

Such a process is clearly unsuitable for crystallisation of an asymmetric ultrafiltration membrane as the temperatures required (typically 160-2000C) would dry out and possibly collapse the surface pores of the membrane, destroying its permeability.

An alternative crystallisation process has been disclosed in U.S. Patent No. 4,897,307 and involves solvent-induced crystallisation of polyetherketone articles including membranes, though crystallisation of a membrane was not in fact described.

This process involves (i) heating the article specifically at about 85-1450C in an organic liquid specifically of MW about 160320 daltons, (ii) removing the organic compound, and (iii) recovering the crystallised article. Such a process is likewise unsuitable for crystallisation of an asymmetric ultrafiltration membrane as removal of the liquid would again result in drying out and possible collapse of the surface pores of the membrane, destroying its permeability.

It is therefore an object of the present invention to provide a process for enhancing the crystallinity of an aromatic polyetherketone membrane which obviates or mitigates the abovementioned disadvantages.

According to the present invention there is provided a method of enhancing the crystallinity of a porous membrane formed of an aromatic ether ketone polymer comprising treating the membrane with a mixture of (i) a volatile plasticising compound having a molecular weight of 30 to 150 daltons, and (ii) a watersoluble compound having a molecular weight of 30 to 150 daltons and a lower volatility than (i).

The invention is particularly applicable to the treatment of a asymmetric membranes formed of an aromatic ether ketone polymer to enhance the crystallinity thereof.

By "asymmetric membrane" we mean a membrane (a) which comprises, on the side adjacent the solution to be filtered, a skin of thickness 0.02-2.0 microns supported on a substrate of thickness 25-250 microns; (b) in which both the skin and the substrate comprise the same polymer; and (c) in which the pore sizes of the substrate are greater than the pore sizes of the skin.

Preferably the (i) volatile plasticising compound and (ii) water-soluble compound each have a molecular weight of 40 to 140 daltons. Typically, the ratio by volume of (i):(ii) is 5-10:1.

If necessary or desired, the treatment mixture may also include (iii) a volatile third compound to ensure miscibility of compounds (i) and (ii). Preferably the ratio by volume of (iii):(ii) is 1-5:1.

The volatility of components (i) and (iii) is preferably such that they have a boiling point (at 1 atmosphere pressure) of less than 1800C, more preferably less than 1000C.

The method of the invention may be effected at relatively low temperatures (eg below 750 C) by immersion of the membrane in an admixture of (i), (ii) and possibly (iii). Most typically the treatment is effected at a temperature in the range 200 C750 C. Treatment times are typically in the range 5 min-5 hours.

The degree of crystallinity obtained by the treatment will generally be up to 50%, more typically up to 45%.

After crystallisation, the volatile plasticising compound and optional third compound are removed from the membrane by evaporation, leaving the low-volatility water-soluble compound in the pores of the membrane, preventing them from drying out and/or collapsing. The membrane will then re-wet spontaneously or at low pressure on contact with an aqueous feed.

The volatile plasticising compound (i) is typically a polar solvent such as chloroform (MW 120), dichloromethane (MW 85), dimethylformamide (MW 73), acetone (MW 58), or methyl ethyl ketone (MW 72). The water-soluble, low-volatility compound (ii) is typically a polyol such as diethylene glycol (MW 106), 1,4butanediol (MW 90), or glycerol (MW 92). The volatile third compound (iii) (if used) may for example be methanol (MW 32), ethanol (MW 46), or propan-1-ol (MW 60).

The process of the invention does not destroy the permeability or selectivity of the membrane. It is usually found that the process reduces the permeability of the membrane to some extent, but the selectivity of the membrane (as measured by its ability to reject solutes of a given molecular weight) is correspondingly increased. The selectivity of an ultrafiltration membrane is normally defined by a molecular weight cut-off (MWCO) which corresponds to the lowest molecular weight for which the membrane shows greater than 90% rejection. The process of the invention thus gives rise to a reduction in MWCO for a given membrane.

For an unsupported hollow-fibre polyetherketone membrane, the process of the invention produces enhanced mechanical properties; for example the pressure required to burst the membrane is significantly increased.

The treatment process of the invention is applicable to membranes formed from a wide range of aromatic polyetherketones using the procedures disclosed in EP-A-0 323 076. Examples of polymers from which the membranes may be fabricated are shown in the accompanying drawings, in which: Figure 1 illustrates polymers chains in which the aromatic rings are joined by ether or ketone bonds ((I-VI); Figure 2 illustrates polymer chains in which a portion of the aromatic rings are joined by direct links (VII-IX)) or are bicyclic rings (X); Figure 3 illustrates optional copolymer units bearing intercyclic -SO2-bonds (XI-XII); and Figure 4 illustrates certain copolymers containing ketone and ether links (XV and XVII) or in addition a mixture of biphenyl and sulphone linkages (XIV and XVI).

It will be appreciated that in figures 1-4, E represents an ether linkage; K K represents a ketone linkage; D represents a direct linkage; m represents a meta substituted aromatic ring; N represents a naphthalene ring; S represents a sulphone linkage except where it is used as a prefix to the polymer trivial name where it represents "sulphonated".

We do not exclude the possibility that at least a portion of the ether linkages in the polymers illustrated in Figures 14 may be replaced by thioether linkages.

The preparation of polymers illustrated in certain Figures of the drawings are described inter alia in Journal Macromolecular Science Review of Macromolecular Chem. Phys., (27 (2), 313-341, 1987 (General Formulae I-VIII); European Patent Specification No. 0,323,076 (General Formula IX); Polymer 1984, Vol 25 (August, 1151 (General Formula X); EPA 0,194,062 (General Formulae XIV) and British Patent Application No. 8910549 (General Formulae XVI)).

The membrane is preferably formed of a homopolymer, most preferably PEK, PEEK, PEEKK, or PEKEKK. PEK and PEEK are available from Imperial Chemical Industries plc, PEEKK from Hoechst, and PEKEKK from BASF.

We do not however exclude the possibility that the membrane may be fabricated from a copolymer eg. PEEK/PEK, PEEK/PES, PEK/PES, PEEK/PEES (wherein the PES and PEES copolymer units are represented by the General Formulae XI and XII respectively in Fig. 3 of the drawings). Further examples of copolymers are as depicted in Fig. 4 of the drawings and include PEKK/PE-m-KK (Formula XVII).

Furthermore we do not exclude the possibility that the membrane may be formed from a mixture of polymers, eg. PEEK/PEK or PEK/PES.

The invention is illustrated by the following non-limiting example.

EXAMPLE A hollow-fibre ultrafiltration membrane (internal diameter 0.6 mm; external diameter 0.8 mm) was prepared by spinning a solution of PEK in 98% sulphuric acid, as described in EP-A-0 382 356. The crystallinity level (as measured by differential scanning calorimetry) of the polyetherketone comprising the membrane was 16%. The membrane had a pure water permeability of 48 l/m2/hr/bar, a MWCO (for dextrans) of 19,000 daltons, and a burst pressure of 7.2 bar.

This membrane was treated at 560C for 30 minutes with a mixture of acetone (volatile plasticising compound, 70% by volume), glycerol (water-soluble, low volatility compound, 10% by volume), and methanol (miscibility-enhancing compound, 20% by volume). The membrane was then removed from the liquid and allowed to dry in air for 24 hours.

The crystallinity level of the treated membrane (measured by differential scanning calorimetry) of the polyetherketone comprising the membrane was now 40%. The treated membrane had a pure water permeability of 32 l/m2/hr/bar, a MWCO (for dextrans) of 12,000 daltons, and a burst pressure of 10.6 bar.

Claims (17)

1. A method of enhancing the crystallinity of a porous membrane formed of an aromatic ether ketone polymer comprising treating the membrane with a mixture of (i) a volatile plasticising compound having a molecular weight of 30 to 150 daltons, and (ii) a water soluble compound having a molecular weight of 30 to 150 daltons and a lower volatility than (i).

2. A method as claimed in claim 1 wherein the volatile plasticising compound (i) has a molecular weight of 40 to 140 daltons.

3. A method as claimed in claim 1 or 2 wherein the volatile plasticising compound (i) is a polar organic solvent.

4. A method as claimed in any one of claims 1 to 3 wherein the volatile plasticising compound has a boiling point of less than 1800C.

5. A method as claimed in claim 3 wherein the polar organic solvent is selected from chloroform, dichloromethane, dimethylformamide, acetone and methyl ethyl ketone.

6. A method as claimed in any one of claims 1 to 5 wherein the water soluble compound (ii) has a molecular weight of 40 to 140 daltons.

7. A method as claimed in any one of claims 1 to 6 wherein the ratio by volume of (i) to (ii) is 5-10:1.

8. A method as claimed in any one of claims 1 to 7 wherein the water soluble compound (ii) is a polyol.

9. A method as claimed in claim 8 wherein the polyol is selected from diethylene glycol, 1, 4-butane diol and glycerol.

10. A method as claimed in any one if claims 1 to 9 wherein a volatile third compound (iii) is included in the mixture to ensure miscibility of components (i) and (ii).

11. A method as claimed in claim 10 wherein compound (iii) has a boiling point less than 1800C.

12. A method as claimed in claim 10 or 11 wherein the ratio by volum of (iii) to (ii) is 1-5:1.

13. A method as claimed in any one of claims 1 to 12 wherein the treatment is effected at a temperature of less than 750C.

14. A method as claimed in claim 13 wherein the treatment is effected at a temperature of 200C to 750C.

15. A method as claimed in any one of claims 1 to 14 wherein the aromatic ether ketone polymer is PEK, PEEK, PEEKK, PEKEKK or PEKK/PE-m-KK.

16. A method as claimed in any one of claims 1 to 15 wherein the membrane is an asymmetric membrane.

17. A method of enhancing the crystallinity of a porous membrane substantially as hereinbefore described in the foregoing example.