WO2015015009A1

WO2015015009A1 - Microporous polyolefinsulfonic acids and methods for their preparation

Info

Publication number: WO2015015009A1
Application number: PCT/EP2014/066668
Authority: WO
Inventors: Daryl Joseph BRIGGS
Original assignee: Hydroxsys (Malta) Limited
Priority date: 2013-08-02
Filing date: 2014-08-01
Publication date: 2015-02-05

Abstract

The post-treatment of microporous polyolefin by sulfonation is disclosed The method provides sulfonated microporous polyolefins suitable for use reverse osmosis.

Description

MICROPOROUS POLYOLEFINSULFONIC ACIDS AND METHODS FOR THEIR PREPARATION

FIELD OF INVENTION

The invention relates to a method of sulfonating preformed microporous polyolefin sheets. In particular, the invention relates to sheets of sulfonated microporous polyethylene produced by the method and suitable for use in the fabrication of reverse osmosis membranes.

BACKGROUND ART

The publication of Anon (1966) describes a process for the preparation of cross-linked, microporous polyethylene, e.g. in the form of sheets or films, containing a plurality of pores of diameter 0.1 to 10 microns. The cross-linked, microporous polyethylene is useful in that it possesses pores of such a size as to permit the passage of gasses, e.g. oxygen, but prevent the passage of water. The cross-linked, microporous polyethylene is stated to have many uses, including use in the manufacture of battery separators. The publication of Fortuin and Simmelink (1994) describes a process for the production of a microporous film of linear polyethylene by extruding a solution of the polyolefin in a first solvent, i.e. an organic solvent, followed by cooling, removal of the solvent and stretching of the film. In the process both sides of the film are brought into close contact with a second solvent for the polyolefin before the film is contacted with the cooling agent, e.g. water, in which it is not soluble. The process produces a microporous film with a very high permeability to air and a high moisture vapour transmission rate.

It is known that the sulfonic acid group may be introduced into

polyethylene by treating the polyethylene with fuming sulfuric acid or chlorosulfonic acid, sulfur trioxide in chlorinated hydrocarbons, or gaseous sulfur trioxide.

The publication of Olsen and Osteraas (1969) describes a method of obtaining surface sulfonic acid groups by exposing the polyethylene surface directly to fuming sulfuric acid or chlorosulfonic acid. The polyethylene film was immersed in the acids for a time and at a temperature sufficient to achieve the optimum degree of surface sulfonation. This optimum degree of sulfonation was taken to be the point at which the maximum critical surface tension value was obtained without visible signs of charring. The publication of Walles (1973) describes methods of rendering resinous enclosures impermeable by sulfonation. One of the methods disclosed uses 10% by weight of a solution of sulfur trioxide in an inert solvent, such as a liquid polychlorinated aliphatic hydrocarbon. Examples of such liquid polychlorinated aliphatic hydrocarbons include methylene chloride, carbon tetrachloride, perchloroethylene, syntetrachloroethane and ethylene dichloride. A darkening of the substrate to a dark brown or a very black colour was observed.

The publication of Sano et al (1980) noted that processes where the substrate is sulfonated by immersion in a liquid sulfonating agent have disadvantages where the substrate is a fibrous material. The removal of excess sulfonating agent from the fibrous material is very difficult, giving rise to a large amount of waste liquor which is difficult to treat and causes an increase in the material cost of sulfonation. These disadvantages were circumvented by the use of gaseous sulfur trioxide.

The publication of Sano et al (1980) discloses treating fibrous

polyethylene at 10°C to 90°C with a gas comprising 10 to 80% by volume of gaseous sulfur trioxide and 90 to 20% by volume of an inert gas. The inert gas for use in diluting the sulfur trioxide may be air, nitrogen, helium, neon, argon, or other gasses having no adverse effect on sulfonation. The publication of Walles (1980) also describes a method where organic materials including the polyolefins polyethylene and polypropylene are sulfonated by contacting them with a gaseous mixture of sulfur trioxide and chlorine. In the methods disclosed in the publications of Sano et al (1980) and Walles (1980) the gaseous volume is required to be dry to avoid formation of sulfuric acid by reaction of water vapour with the sulfur trioxide.

The publication of Ihata (1988) describes the characterisation of

polyenesulfonic acid formed by the reaction of polyethylene films with sulfur trioxide.

The publication of Bonorand (2007) describes a method for making a sulfonated polymer electrolyte membrane (PEM) for use in an electrochemical fuel cell. In examples of the method a microporous polyethylene film containing ultra-high molecular weight polyethylene (UHMWPE) is immersed in a sulfonation mixture at room temperature and the reaction then conducted at either room temperature or 50°C for 15, 30 or 60 minutes. The

sulfonation mixture was a 30% by volume solution of sulfur trioxide in 1,2- dichloroethane . Equivalent weight analyses of up to 587 g/mol,

corresponding to an ion exchange capacity of 1.70 mmol/g were reported to be obtained by the exemplified methods. In contrast with the sulfonation of microporous polyethylene for use as a PEM in electrochemical fuel cells, the sulfonation of microporous

polyethylene for use in the fabrication of reverse osmosis membranes requires the product to be sulfonated throughout the body of the substrate, to permit high water flux rates, yet retain sufficient mechanical strength to maintain its integrity when exposed to high pressures.

It is an object of the present invention to provide a method of sulfonating one or more microporous polyolefin substrates that provides a product suitable for use in the fabrication of reverse osmosis membranes or at least to provide a useful choice in the selection of such methods.

STATEMENT OF INVENTION

In a first aspect the invention provides a method of sulfonating a preformed microporous polyolefin substrate comprising the steps of:

• wetting the substrate with a mixture of an organic solvent and a

solvent miscible polar aprotic co-solvent and cooling to a

temperature less than 20 °C to provide a permeated substrate; and then

• contacting a sulfonating agent with the permeated substrate at a

temperature and for a time sufficient to achieve the desired degree of sulfonation.

The desired degree of sulfonation is one that provides a sulfonated microporous polyolefin sufficiently hydrophilic to maintain a water flux rate of greater than 100 L m^"2 h^"1 at a pressure of 10 bar and temperature of 4 to 8°C Preferably, the polyolefin of the preformed microporous polyolefin substrate is selected from the group consisting of: polyethylene,

polypropylene, polybutylene and polymethylpentene . More preferably, the preformed microporous polyolefin substrate is a sheet of microporous polyethylene. Yet more preferably, the preformed microporous polyolefin substrate is a sheet of preformed microporous polyethylene of 12 to 20 μπι thickness. Most preferably, the preformed microporous polyolefin substrate is a sheet of preformed microporous polyethylene of 16 μπι thickness.

Preferably, the organic solvent is a chlorinated organic solvent. More preferably, the organic solvent is a chlorinated alkane. Yet more preferably, the organic solvent is selected from a group consisting of: methylene chloride and trichloromethane . Most preferably, the organic solvent is trichloromethane.

The solvent miscible polar aprotic co-solvent is miscible with the organic solvent and has a melting point above -80 °C. Preferably, the solvent miscible polar aprotic co-solvent is selected from the group consisting of: dimethylsulfoxide (DMSO) and dimethyl formamide (DMF) . More preferably, the solvent miscible polar aprotic co-solvent has a melting point above 0 °C. Most preferably, the solvent miscible polar aprotic co-solvent is

dimethylsulfoxide (DMSO) . Preferably, the ratio (v/v) of solvent miscible polar aprotic co-solvent to organic solvent in the mixture is in the range 3:1 to 11:1. More

preferably, the ratio (v/v) of solvent miscible polar aprotic co-solvent to organic solvent in the mixture is in the range 4:1 to 9:1. Most

preferably, the ratio (v/v) of solvent miscible polar aprotic co-solvent to organic solvent in the mixture is 4:1.

Preferably, the temperature is 70 to 90 °C and the time is 60 to 120 minutes. More preferably, the temperature is 85 °C and the time is 90 minutes .

Preferably, the organic solvent has a boiling point below 85 °C. Preferably, the sulfonating agent is a mixture of concentrated sulphuric acid and phosphorous pentoxide. In a first alternative, the sulfonating agent is a mixture of concentrated sulphuric acid and phosphorous pentoxide prepared by adding an amount of phosphorous pentoxide to a first volume of sulphuric acid to provide an intermediate mixture and refluxing the intermediate mixture for a period of time before cooling and adding a second volume of sulphuric acid to provide the sulfonating agent. In a second alternative, the sulfonating agent is a mixture of concentrated sulphuric acid and phosphorous pentoxide prepared by adding an amount of phosphorous pentoxide to a volume of sulphuric acid to provide a mixture and heating to a temperature below 100 °C for a period of time sufficient to dissolve the phosphorous pentoxide and provide the sulfonating agent.

In a second aspect the invention provides a sheet of a sulfonated

microporous polyolefin that has a tensile strength no less than 90% of that of the preformed microporous substrate from which it is prepared. Preferably, the preformed microporous polyolefin substrate is a sheet of preformed microporous polyethylene of 12 to 20 μπι thickness. More preferably, the preformed microporous polyolefin substrate is a sheet of preformed microporous polyethylene of 16 μπι thickness

Preferably, the sulfonated microporous polyolefin has a tensile strength no less than 95% of that of the preformed microporous substrate from which it is prepared.

Preferably, the sulfonated microporous polyolefin is capable of maintaining a water flux rate of greater than 100 L m^"2 h^"1 at a pressure of 10 bar and temperature of 4 to 8°C.

Preferably, the sulfonated microporous polyolefin is prepared according to the method of the first aspect of the invention.

Preferably, the sulfonated microporous polyolefin is a sulfonated

microporous polyethylene prepared according to the method of the first aspect of the invention.

In a third aspect the invention provides a method of preparing a

sulfonating agent comprising the steps of:

• adding an amount of phosphorous pentoxide to a first volume of

sulphuric acid to provide an intermediate mixture;

• refluxing the intermediate mixture for a period of time before

cooling; and then

· adding a second volume of sulphuric acid to provide the sulfonating agent .

Preferably, the intermediate mixture 1:5 (mol/mol) phosphorus pentoxide- sulphuric acid.

Preferably, the intermediate mixture is refluxed at greater than 185 °C. In the description and claims of this specification the following acronyms, terms and phrases have the meaning provided: "comprising" means

"including", "containing" or "characterized by" and does not exclude any additional element, ingredient or step; "consisting of" means excluding any element, ingredient or step not specified except for impurities and other incidentals; "DMSO" means dimethylsulfoxide; "DMF" means dimethyl

formamide; "homopolymer" means a polymer formed by the polymerization of a single monomer; "hydrophilic" means having a tendency to mix with, dissolve in, or be wetted by water; "hydrophobic" means having a tendency to repel or fail to mix with water; "internal sulfonation" means sulfonation of the surface of pores or channels within a the body of a microporous substrate; "microporous" means consisting of an essentially continuous matrix structure containing small pores or channels throughout the body of the substrate; "oleophobic" means having a tendency to repel or fail to mix with oil; "polyelectrolyte" means a polymer that comprises structurally repeating units bearing an electrolyte group that will dissociate in aqueous solutions making the polymer charged; "post-treated polymer" means a polymer that is modified, either partially or completely, after the basic polymer backbone has been formed; "preformed" means formed beforehand, i.e. prior to treatment; and "structural repeating unit" means a smallest structural unit that repeats in the polymer backbone. Any reference to a "preformed microporous substrate" specifically excludes a preformed sulfonated microporous substrate.

The terms "first", "second", "third", etc. used with reference to elements, features or integers of the subject matter defined in the Statement of Invention and Claims, or when used with reference to alternative

embodiments of the invention are not intended to imply an order of preference. The numbering of the Examples and the Comparative Examples is not intended to mean any pair of Example and Comparative Example is directly comparable. Where values are expressed to one or more decimal places standard rounding applies. For example, 1.7 encompasses the range 1.650 recurring to 1.749 recurring. In the absence of further limitation the use of plain bonds in the representations of the structures of compounds encompasses the diastereoisomers , enantiomers and mixtures thereof of the compounds. The invention will now be described with reference to embodiments or examples and the figures of the accompanying drawings pages.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 . Photographs of sheets of preformed microporous polyethylene prior to treatment (A) , following treatment according to the method described in Example 3 (B) and following treatment according to the method described in Comparative Example 1 (C) .

Figure 2 . Comparison on the FTIR spectra (525 to 4000 wavenumber (cm^-1)) obtained for sulfonated microporous polyethylene prepared according to the method described in Example 1 (upper trace) and the microporous

polyethylene prior to treatment (lower trace) .

Figure 3A. Comparison on the FTIR spectra (3000 to 3700 wavenumber (cm^-1)) obtained for sulfonated microporous polyethylene prepared according to the method described in Example 1 (upper trace) and the microporous polyethylene prior to treatment (lower trace) .

Figure 3B. Comparison on the FTIR spectra (525 to 1500 wavenumber (cm^-1)) obtained for sulfonated microporous polyethylene prepared according to the method described in Example 1 (upper trace) and the microporous

polyethylene prior to treatment (lower trace) .

Figure 4A. EDS spectrum of microporous polyethylene treated according to the method described in Comparative Example 2.

Figure 4B. EDS spectrum of microporous polyethylene treated according to the method described in Comparative Example 3.

Figure 4C. EDS spectrum of microporous polyethylene treated according to the method described in Example 4.

Figure 4D. EDS spectrum of microporous polyethylene treated according to the method described in Example 5. Figure 4E. EDS spectrum of microporous polyethylene treated according to the method described in Comparative Example 4.

Figure 5 . Flux rates observed over time for sheets of microporous polyethylene prior to treatment (open circles), following treatment according to the method described in Comparative Example 1 (solid

triangles) and following treatment according to the method described in Example 3 (solid circles) .

Figure 6. Comparison of replicated determinations of water flux rates observed over time for a sheet of sulfonated microporous polyethylene prepared according to the method described in Example 3. Determinations were performed on successive days (Day 1 (solid circles) ; Day 2 (small cross) ; Day 3 (inverted triangles) ; Day 4 (open squares) and Day 5 (solid triangles ) ) .

Figure 7 . Electronmicrographs obtained by SEM at increasing magnifications (x 4,000 (A, D, G, J and M) ; x 20,000 (B, E, H, K and N) and x 50,000 (C, F, I, L and O) ) of the surfaces of sheets of sulfonated microporous polyethylene prior to treatment (A, B and C) , treated according to the method described in Example 3 (D, E and F) , treated according to the method described in Comparative Example 1 (G, H and I), treated according to the method described in Example 4 (J, K and L) and treated according to the method described in Example 5 (M, N and O) . DETAILED DESCRIPTION

The disadvantages encountered when sulfonating fibrous polyethylene using liquid systems (Sano et al (1980)) are exacerbated when the substrate is microporous. An additional technical problem arises in respect of permeating a preformed microporous substrate with a liquid sulfonating agent. The method of the invention utilises a miscible combination of a polar aprotic co-solvent, such as DMSO, and a volatile organic solvent that serves as a wetting agent to pre-treat the substrate. In this context it will be understood that the term "wetting" refers to the ability to coat the initially hydrophobic surface of the substrate. The volatility of the organic solvent ensures that the surface of the substrate is wetted by the mixture while not preventing contact between the substrate and the sulfonating agent. By contrast, the publication of Bonorand (2007) discloses the sulfonation of non-aromatic polymer membrane materials such as microporous polyethylene with or without the use of a solvent or co- solvent. The publication discloses a method for sulfonating a hydrocarbon- based non-aromatic polymer membrane material directly, without a pre- irradiation step.

In the method described here the presence of co-solvent, e.g. DMSO, may result in a strongly exothermic reaction. The heating associated with this strongly exothermic reaction could significantly (greater than 10 %) degrade the tensile strength of the preformed microporous substrate. The melting point of DMSO is around 20 °C. Hence, cooling to a temperature less than 20 °C allows the latent heat of fusion to be exploited in moderating the reaction. Cooling using dry ice will reduce the temperature to around minus 80 °C. It is anticipated the same principle could therefore be exploited where other polar aprotic solvents are used with melting points above this temperature. Where reference is made to a polar aprotic co- solvent it will be understood that this includes mixtures of polar aprotic co-solvents that undergo a phase transition on cooling.

The moderated sulfonation reaction obtained according to the method described in the Examples is believed to promote sulfonation of the internal pores and channels within the body of the substrate whilst maintaining the structural integrity of the homopolymer. It has also been observed that superior sulfonation of microporous polyethylene sheets occurs when the sulfonating agent is prepared as described in Examples 1 and 3. Without wishing to be bound by theory it is believed this superior sulfonation may be attributed at least in part to a higher proportion of the sulfur trioxide being in a complexed form in the sulfonating agent due to the presence polyphosphate. The concentration of sulfur trioxide available for immediate participation in the sulfonating reaction may therefore be reduced, due to the oleophobicity, further promoting

moderation of the rate of reaction. In a preferred method of the invention, microporous polyethylene sheet is pre-treated by immersion in 4:1 (v/v) DMSO-trichloromethane and the temperature reduced to less than 20°C. The reduction in temperature is to promote both permeation of the sheet and crystallisation of the

dimethylsulfoxide (DMSO) within the pores and channels in the body of the substrate. When the permeated sheet is subsequently immersed in a sulfonating agent, such as a mixture of concentrated sulfuric acid and phosphorous pentoxide, the sulfonating agent is believed to more readily permeate the pores and channels of the body of the substrate for the reasons discussed above. As noted, such permeation of the body of the substrate by the sulfonating agent is desirable to promote sulfonation of the walls of the pores and channels within the body of the substrate and the degree of hydrophilicity required to permit high water flux rates. Following immersion, the microporous polyethylene sheet is incubated in the sulfonating agent at circa 85 °C for 90 minutes before rinsing to remove residual sulfonating agent.

MATERIALS

Flat sheets (16 μπι thickness) of porous (50% porosity, 0.08 μπι average pore diameter) polyethylene (CELGARD™ K2045, Celgard LLC) were used as the preformed microporous substrate for sulfonation. Samples of the

microporous polyethylene as supplied and without any treatement were designated ^ΛΡΕ' . Other reagents were used as supplied: phosphorous pentoxide, sulphuric acid (Merck), methanol ( Sigma-Aldrich) , chloroform ( Sigma-Aldrich) and dimethyl sulfoxide (Sigma-Aldrich) . Dry ice was obtained from BOC (NZ) Limited. EXAMPLE 1

Preparation of sulfonating agent

An amount of 250 g of phosphorous pentoxide was added to a volume of 469 mL of sulfuric acid to provide a mixture of 1:5 (mol/mol) phosphorous pentoxide-sulfuric acid. The mixture was heated to 190 °C and left to reflux for 17 hours. Once cooled a further volume of 500 mL of sulfuric acid was added and the solution was mixed thoroughly. Sulfonation of preformed microporous polyethylene sheet

A microporous polyethylene sheet was cut into 15 x 20 cm pieces. These were wetted with a mixture of DMSO-trichloromethane (9:1 (v/v)) . Excess of the mixture was removed from the sheets before the sheets were cooled to a temperature below 20°C by immersing in dry ice for 1 hour. Immediately after freezing the sheets were added to the sulfonating agent at room temperature. The system was then incubated at 80°C for 90 minutes. The samples were removed from the sulfonating agent and allowed to sit for 3 hours in order to dilute the acid. The sheets were then rinsed twice with methanol to remove residual sulfonating agent.

EXAMPLE 2

Preparation of sulfonating agent

An amount of 250 g of phosphorous pentoxide was added to a volume of 469 mL of sulfuric acid to provide a mixture of 1:5 (mol/mol) phosphorous pentoxide-sulfuric acid. The mixture was heated to 90°C to dissolve the phosphorous pentoxide.

Sulfonation of preformed microporous polyethylene sheet

A microporous polyethylene sheet was cut into 15 x 20 cm pieces. These were wetted with a mixture of DMSO-trichloromethane (9:1 (v/v)) . Excess of the mixture was removed from the sheets before they were cooled to a temperature below 20°C by covering them in liquid nitrogen. Immediately after freezing the sheets were added to the sulfonating agent at room temperature. The system was then incubated at 80°C for 90 minutes. The samples were then removed from the sulfonating agent and allowed to sit for 3 hours in order to dilute the acid. The sheets were rinsed twice with methanol to remove all residual sulfonating agent.

EXAMPLE 3

Preparation of sulfonating agent

An amount of 250 g of phosphorous pentoxide was added to a volume of 469 mL of sulfuric acid to provide a mixture of 1:5 (mol/mol) phosphorous pentoxide-sulfuric acid. The mixture was heated to at least 185 °C and left to refluxed. Once cooled a further volume of 500 mL of sulfuric acid was added and the solution was mixed thoroughly. Sulfonation of preformed microporous polyethylene sheet

A microporous polyethylene sheet was cut into 14 x 28 cm pieces. These were wetted with a mixture of DMSO-trichloromethane (4:1 (v/v)) . Excess of the mixture was removed from the sheets with paper towels and immediately placed between glass fibre sheets and kept in dry ice for at least an hour. The hardened sheets were then soaked in the sulfonating agent, covered with cling wrap and cured at 85 °C for 90 minutes. The glass fibre sheets were then removed and the pieces of sulfonated polyethylene sheets left exposed to humidity overnight. The sulfonated polyethylene sheets were then washed with methanol, to remove any residual sulfonating agent, dried and stored prior to evaluation. Samples of the sulfonated microporous polyethylene sheet prepared according to this method was designated ^BF' .

EXAMPLE 4

The sulfonating agent and samples for sulfonation were prepared as described in Example 3. The samples were wetted with a mixture of DMF- trichloromethane (4:1 (v/v)) . Excess of the mixture was removed from the sheets with paper towels and immediately placed between glass fibre sheets and kept in dry ice for at least an hour. The hardened sheets were then soaked in the sulfonating agent, covered with cling wrap and cured at 85 °C for 90 minutes. The glass fibre sheets were then removed and the pieces of sulfonated polyethylene sheets left exposed to humidity overnight. The sulfonated polyethylene sheets were then washed with methanol, to remove any residual sulfonating agent, dried and stored prior to evaluation.

Samples of the sulfonated microporous polyethylene sheet prepared according to this method was designated ^BF-DMF' .

EXAMPLE 5

The sulfonating agent and samples for sulfonation were prepared as described in Example 3. The samples were wetted with a mixture of DMSO- methylene chloride (4:1 (v/v)) . Excess of the mixture was removed from the sheets with paper towels and immediately placed between glass fibre sheets and kept in dry ice for at least an hour. The hardened sheets were then soaked in the sulfonating agent, covered with cling wrap and cured at 85 °C for 90 minutes. The glass fibre sheets were then removed and the pieces of sulfonated polyethylene sheets left exposed to humidity overnight. The sulfonated polyethylene sheets were then washed with methanol, to remove any residual sulfonating agent, dried and stored prior to evaluation.

Samples of the sulfonated microporous polyethylene sheet prepared according to this method was designated ^BF-MC . COMPARATIVE EXAMPLE 1

Samples of microporous polyethylene sheet were treated as described in Example 3, but without wetting with a mixture of DMSO-trichloromethane. The pieces of microporous polyethylene sheet were placed between glass fibre sheets and kept in dry ice for at least an hour. The hardened sheets were then soaked in the sulfonating agent, covered with cling wrap and cured at 85 °C for 90 minutes. The glass fibre sheets were then removed and the pieces of polyethylene sheets left exposed to humidity overnight. The polyethylene sheets were then washed with methanol, to remove any residual sulfonating agent, dried and stored prior to evaluation. Samples of the microporous polyethylene sheet prepared according to this method were designated 'BF-DMSO' .

COMPARATIVE EXAMPLE 2

Samples of microporous polyethylene sheet were treated as described in Example 3, but without exposure to the sulfonating agent or curing at 85 °C for 90 minutes. The pieces of microporous polyethylene sheet were wetted with the mixture of DMSO-trichloromethane before being placed between glass fibre sheets and kept in dry ice for at least an hour. The glass fibre sheets were removed and the pieces of polyethylene sheets left exposed to humidity overnight. The polyethylene sheets were then washed with methanol, dried and stored prior to evaluation. Samples of the microporous polyethylene sheet prepared according to this method were designated ^¾D' .

COMPARATIVE EXAMPLE 3

Samples of microporous polyethylene sheet were treated as described in Example 3, but without exposure to the sulfonating agent. The pieces of microporous polyethylene sheet were wetted with the mixture of DMSO- trichloromethane before being placed between glass fibre sheets and kept in dry ice for at least an hour. The hardened sheets were then covered with cling wrap and kept at 85 °C for 90 minutes. The glass fibre sheets were then removed and the pieces of polyethylene sheets left exposed to humidity overnight. The polyethylene sheets were then washed with methanol, dried and stored prior to evaluation. Samples of the microporous polyethylene sheet prepared according to this method were designated ^ΌΕ' .

COMPARATIVE EXAMPLE 4

Samples of microporous polyethylene sheet were treated as described in Example 3, but with the substitution of acetic acid for DMSO. The samples were wetted with a mixture of acetic acid-trichloromethane (4:1 (v/v) ) . Excess of the mixture was removed from the sheets with paper towels and immediately placed between glass fibre sheets and kept in dry ice for at least an hour. The hardened sheets were then soaked in the sulfonating agent, covered with cling wrap and cured at 85 °C for 90 minutes. The glass fibre sheets were then removed and the pieces of sulfonated

polyethylene sheets left exposed to humidity overnight. The sulfonated polyethylene sheets were then washed with methanol, to remove any residual sulfonating agent, dried and stored prior to evaluation. Samples of the sulfonated microporous polyethylene sheet prepared according to this method was designated ^BF-AA' .

A brief summary of the origin of the samples evaluated is provided in Table 1. The appearance of the samples prior to evaluation is shown in Figure 1.

Designation Description Appearance

PE Microporous PE sheet (CELGARD™ K2045, Celgard LLC) White

BF Microporous PE sheet (CELGARD™ K2 045, Celgard LLC) Blue/light sulfonated according to the method described in purple Example 3

BF-DMF Microporous PE sheet (CELGARD™ K2 045, Celgard LLC) Blue/light sulfonated according to the method described in purple Example 4

BF-MC Microporous PE sheet (CELGARD™ K2 045, Celgard LLC) Blue/light sulfonated according to the method described in purple Example 5

BF-DMSO Microporous PE sheet (CELGARD™ K2 045, Celgard LLC) Light brown sulfonated according to the method described in

Comparative Example 1

BF-AA Microporous PE sheet (CELGARD™ K2 045, Celgard LLC) Light brown, sulfonated according to the method described in blue patches Comparative Example

Table 1. Brief description of the origins of the samples evaluated. CHARACTERISATION OF SAMPLES

Fourier transform infrared (FTIR)

Spectra of the samples were recorded using a Thermo Electron Nicolet 8700 FTIR spectrometer equipped with a single bounce ATR and diamond crystal. An average of 32 scans with a 4 cm^"1 resolution was taken for all samples.

Elemental analysis

Analysis of the samples was performed using a Carlo Erba EA 1108 Elemental Analyser. The analytical method was based on the complete and instantaneous oxidation of the sample by flash combustion which converted all organic and inorganic substances into combustion products. The sample was held in a tin capsule and dropped into a vertical quartz tube, containing catalyst (tungstic oxide) and copper, which was maintained at a temperature of 1020 °C. Quantitative combustion was then achieved by passing the mixture of gases over a catalyst layer, then through copper to remove excess oxygen and reduce nitrogen oxides to nitrogen. The resulting mixture was directed to the chromatographic column where the combustion products were separated and detected by a thermal conductivity detector. All samples were run in duplicates .

PE BF BF-DMSO

C (i *) 84. .98 ± 0 , .09 82 .26 ± 0 .01 84.36 ± 0.04

H (i *) 14. .08 ± 0 , .25 13 .56 ± 0 .18 13.95

S (i *) - 0. 45 ± 0. 01 < 0.3

Atomic ratio (S/C x 10²) - 0. 21 ± 0. 01 *

Table 2. Elemental analysis results (* indicates the sulfur content was too low fo accurate analysis) .

Surface analysis

The contact angle of the samples were determined using the captive bubble method. The samples were immersed in deionized water with the surface to b analysed facing downwards. An air bubble was trapped on the lower surface of the sample using a syringe. An image of the bubble was captured and the contact angle was calculated from its geometrical parameters. At least fiv measurements of each sample were performed.

PE BF BF-DMF BF-MC BF-AA BF-DMSO

Contact angle (°) 82 ± 2 46 ± 6 42 ± 2 44 ± 5 47 ± 8 76 ± 5

Table 3. Contact angles of the samples.

The sample surfaces were visualized using a Hitachi S-4700 field emission scanning electron microscope (SEM) at 3 kV. Samples were coated with platinum, using Hitachi E-1030 ion sputter coater, for 80 seconds prior to imaging. Energy dispersive X-ray spectroscopy (EDS) of the coated samples was done at 20 kV. Electron micrographs of the samples at increasing magnification are presented in Figure 7.

Differential scanning calorimetry (DSC) were performed on a selection of the samples, contained in aluminium pans, using a TA Instruments Q1000 DSC from 25 to 200 °C at a 10 °C min^"1 heating rate in nitrogen atmosphere. All samples were run in duplicates. PE BF BF-DMSO

Melting point (°C) 139.6 ± 0.2 140.2 ± 0.9 139.4 ± 0.4

Enthalpy of melting, AHm (Jg^-1)* 219.8 ± 15.6 206.3 ± 7.5 216.7 ± 1.7

Degree of crystallization (%) 71.3 ± 5.3 70.4 ± 2.6 74.0 ± 0.6

Table 4. Calorimetry results of PE, BF and BF-DMSO (* indicates enthalpy of melting was taken from 100 to 160 °C with a sigmoidal baseline) .

Mechanical properties

The static mechanical properties of the samples were determined at ambient temperature, using an Instron 5567 universal testing machine. The

measurements were undertaken in accordance with ASTM D822-02. The stress- strain curves of dog bone shaped test specimens were obtained at a constant crosshead speed of 50 mm min^"1 and a 1000 N load cell. Distance between the sample grips was 50 mm. The Young's modulus, tensile strength and

elongation at break were determined from at least five measurements.

The burst strengths of the samples were determined using a Lorentzen and Wettre Burst-o-matic bursting strength tester. Test specimens were clamped between two concentric plates, each having circular openings in the centre. Pressure was applied to the underside of the test specimen by a rubber diaphragm and the pressure at specimen failure was recorded. At least 10 measurements of each sample were performed.

PE BF BF-DMSO

Burst strength (kPa) 345.5 ± 12.9 343.8 ± 10.0 344.4 ± 8.6 Tensile strength (MPa) 172.0 ± 4.3 160.0 ± 7.3 167.0 ± 6.6

Modulus (GPa) 2.15 ± 0.20 2.13 ± 0.30 2.36 ± 0.40 Elongation at break (%) 57.0 ± 2.4 55.4 ± 1.4 57.6 ± 0.8

Table 5. Mechanical properties of PE, BF and BF-DMSO.

Flux testing

Flux tests were performed on the samples using a Sterlitech flux rig equipped with a PolyScience cooling unit. The samples were mounted in the flux cell and clamped at 55 bar. Deionized water was fed into the rig at 2.5 L min^"1 and 4 to 8 °C. The time to collect a predetermined volume of permeate was noted. Tests were done at a feed pressure of either 10 or 20 bars. The flux rate (J) was calculated according to the following equation:

V

1 = txA where V is the permeate volume (L), t is the time (h) for the collection V and A is area of the sample (m²) which was determined to be 0.014 m².

DISCUSSION

The publications of Gilbert (1961) and Kucera and Jancar (1998) disclose the reactions of sulfur trioxide and review the homogenous and

heterogeneous sulfonation of polymers. The use of sulfonating agents, including sulfur trioxide (SO₃) are described.

PE BF BF- -DMF BF -MC BF-AA BF- DMSO

Contact angle (°) 82 ± 2 46 ± 6 42 ± 2 44 ± 5 47 ± 8 76 ± 5

Flux rate (Lnr²!!^"1)* 3.67 ± 2.11 748 ± 170 538 ± 46 556 ± 50 54 ± 14 5.26 ± 0.03

Table 6. Correspondence between contact angles (Table 3) and initial flux rates of the samples (* indicates average flux rate of the first 30 minutes of the test at 20 bar for PE and BF-DMSO samples and 1 minute for all other samples) .

The sulfonating agent used in the method described in this specification is believed to produce sulfur trioxide according to the following scheme:

H₂S0₄ + P2O5 → S0₃ + 2HP0₃

A possible mechanism for the sulfonation of the microporous polyethylene substrate is by abstraction of a hydrogen atom to provide a polyethylene radical that is reactive towards sulfur trioxide to yield the sulfonic acid group according to the following scheme:

It is anticipated that such abstraction may occur in a range of polyolefin substrates including, in addition to polyethylene, polypropylene,

polybutylene and polymethylpentene . Sulfonation of the microporous polyethylene substrate according to the method described in this

specification has been confirmed by FTIR. The spectrum obtained for PE (Figure 2, lower trace) is similar to that previously described in the publications of Socrates (1994), Nand et al (2012), Cameron and Main (1985) and Raghavan and Torma (1992) . The bands at about 2914 cm^"1 and 2847 cm^"1 can be assigned, respectively, to the C¾ asymmetric and symmetric stretching. The bands at 1471 cm^"1 and 1462 cm^"1 result from C¾ bending deformations. The bands at 730 cm^"1 and 718 cm^"1 can be attributed to the C¾ rocking deformations. The spectrum obtained for BF (Figure 2, upper trace) exhibited additional peeks at about 3400 cm^"1 and in the region 550 to 1250 cm^"1. The band observed at about 1200 cm^"1 can be attributed to the 0=S=0 stretching of the sulfonic groups as disclosed in the publication of Cameron and Main (1985) . This band could also have contributions from the P=0 groups of any residual phosphorous pentoxide as disclosed in the publication of Socrates (1994) . The other well resolved band at 1040 cm^"1 is assigned to the S=0 stretching as disclosed in the publications of Socrates (1994) and Cameron and Main (1985) . The bands at about 590 cm^"1 and 906 cm^"1 can be assigned,

respectively, to the S-0 and C-S stretching as disclosed in the

publications of Socrates (1994) and Cameron and Main (1985) . Sulfonic acid groups covalently bonded to the polyethylene backbone is indicated by the band at 590 cm^"1 and this sample also exhibited a broad peek at about 3400 cm^"1 (Figure 3A) attributed to the O-H stretching of the sulfonic group as disclosed in the publication of Cameron and Main (1985) . The absence of this peek in the spectrum of PE is consistent with the absence of O-H groups .

The results of elemental analysis (Table 2) were consistent with

sulfonation of the polyethylene substrate when the method described in Example 3 was employed. The amount of sulfur detected in the sample prepared according to the method described in Comparative Example 1 is consistent with the wetting of the substrate with a mixture of a solvent miscible polar aprotic co-solvent (e.g. DMSO) and an organic solvent (e.g. trichloromethane ) is essential for efficient sulfonation. In the absence of this wetting step only the outer surface of the substrate may be sulfonated resulting in a very low amount of sulfur being detected by elemental analysis. The non-quantification of sulfur in samples of sulfonated polyethylene with low degrees of sulfonation is consistent with the results disclosed in the publication of Tricoli and Carretta (2002) . The results obtained using the method described in Example 3 are consistent with a higher degree of sulfonation being achieved. The S/C ratio indicates that an average 0.21% of all carbon atoms are sulfonated. Without wishing to be bound by theory it is envisaged that this high degree of sulfonation is promoted by the organic solvent wetting the inner walls of the pores of the microporous polyethylene substrate while the solvent miscible polar aprotic co-solvent fills up the pores. The solvent miscible polar aprotic co- solvent is then displaced by the sulfonating agent. Employing a high ratio of solvent miscible polar aprotic solvent to organic solvent ensures only the surface of the microporous substrate is wetted. A high ratio of solvent miscible polar aprotic solvent to organic solvent is also required for the principle of exploiting the latent heat of fusion.

The high degree of sulfonating being artifactual due to the retention of the solvent miscible polar aprotic co-solvent (i.e. DMSO) was discounted by determining the EDS spectra for samples of microporous polyethylene treated with only DMSO as described in Comparative Example 2. Followed by heating in a water bath as described in Comparative Example 3. The spectra obtained for the sample designated D and the sample designated DH are presented, respectively, in Figures 4A and 4B. No sulfur was quantified on the surface of either sample. These results are consistent with DMSO as a polar aprotic solvent not wetting the hydrophobic surface of the microporous polyethylene sheet .

Consistent with an increasing degree of sulfonation the contact angles determined for samples of microporous polyethylene treated according to the methods described in Example 3 and Comparative Example 1 decreased (Table 3) . The lowering of a contact angle indicates an increase in the

hydrophilic character of a sample as disclosed in the publication of Bayrammoglu et al (2011) . The initial water flux rates determined for these samples also correlated with the degree of sulfonation and associated hydrophilicity (Table 6) . The low flux rate of the untreated microporous polyethylene (PE) is attributed to its high hydrophobicity . By contrast, the sulfonated microporous polyethylene substrate prepared according to the method described in Example 3 demonstrated a more than 200-fold increase in flux rate. This large increase is attributed to the walls of the pores within the body of the substrate being sulfonated, thereby increasing the hydrophilicity and allowing water to pass through easily. The low flux rate of the samples prepared according to Comparative Examples 1 and 4 supports the importance of the inclusion of an aprotic polar co-solvent (DMSO or DMF) in the mixture used to wet the preformed microporous substrate prior to contacting with the sulfonating agent. It is anticipated that acetone and acetonitrile could also be used as polar aprotic co-solvents.

The flux rates over prolonged periods of time were determined (Figure 5) . The flux rate observed for the sample of sulfonated microporous

polyethylene prepared according to the method described in Example 3 demonstrated an initial decrease stabilising after three to four hours.

This initial, moderate decrease in flux rate is attributed to compaction of the sheet under pressure consistent with observations made elsewhere, such as those reported in the publication of Dimov and Islam (1991) . The observed decrease in initial water flux rate was a consistent property of the sample when tests were repeated over a five day period (Figure 6) .

Electron micrographs obtained by scanning electron microscopy of the samples in increasing magnifications are presented in Figure 7. All three samples were observed to have a fibrous morphology with pores evident within the network of fibres. Notably, sulfonation of the microporous ethylene substrate according to the method described in Example 3 had no observed effect on the size or distribution of the pores or the network of fibres. This observation is consistent with the increased water flux rate being attributable to an increase in the hydrophilicity of the substrate and not a degradation of its structure.

Sulfonation of the microporous polyethylene substrate according to the method described in Example 3 did not appear to significantly affect the physical characteristics of the substrate measured in terms of melting point, enthalpy of melting and corresponding degree of crystallinity (Table 4) . The degree of crystallinity was calculated according to the following equation :

ΔΗπι

Crystallinity (%) xlOO

ΔΗ'πι where ΔΗτη is the sample enthalpy of melting and ΔΗ°τη is the enthalpy of melting of 100 % crystalline polyethylene, taken to be 293 J g^"1 as disclosed in the publication of Nand et al (2012) . These observations are consistent with the modification of the properties of the substrate being a surface phenomenon, although sulfonation of amorphous regions of

polyethylene cannot be excluded. The suitability of the sulfonated microporous polyethylene for use in the fabrication of membranes for use in high pressure applications, such as reverse osmosis, was assessed by determination of the burst strength, tensile strength, modulus and elongation of samples prepared according to the methods described in Example 3 and Comparative Example 1 (Table 5) . The method described in Example 3 did not significantly affect the burst strength, modulus and elongation at break of the microporous polyethylene substrate. A minor decrease in the tensile strength (no greater that 10%) was observed, but was not statistically significant. Again, the

preservation of these mechanical properties indicates that the changes in the characteristics of the substrate are a surface phenomenon.

Although the invention has been described with reference to embodiments or examples it should be appreciated that variations and modifications may be made to these embodiments or examples without departing from the scope of the invention. Where known equivalents exist to specific elements, features or integers, such equivalents are incorporated as if specifically referred to in this specification. In particular, variations and

modifications to the embodiments or examples that include elements, features or integers disclosed in and selected from the referenced publications are within the scope of the invention unless specifically disclaimed. The advantages provided by the invention and discussed in the description may be provided in the alternative or in combination in these different embodiments of the invention.

INDUSTRIAL APPLICABILITY

The invention provides an improved method of sulfonating preformed microporous polyolefin sheets for use in the fabrication of membranes.

REFERENCED PUBLICATIONS Anon (1966) Micro-porous polyethylene and a process for the manufacture thereof United Kingdom Patent No. 1,051,320.

Bayrammoglu et al (2011) Immobilization of chloroperoxidase onto highly hydrophilic polyethylene chains via bio-conjugation : Catalytic properties and stabilities Bioresource Technology, 102, 475-482 Bonorand (2007) Methods for making sulfonated non-aromatic polymer electrolyte membranes United States Patent Application Publication No. US 2007/0218334 Al .

Cameron and Main (1985) The action of concentrated sulphuric acid on polyethylene and polypropylene: Part 2 - Effect on the polymer surface Polymer Degradation and Stability, 11, 9-25.

Dimov and Islam (1991) Sulfonation of polyethylene membranes Journal of Applied Polymer Science, 42, 1285-1287.

Fortuin and Simmelink (1994) Microporous film of polyethylene and process for the production thereof United States Patent No. 5,376,445. Gilbert (1961) The reactions of sulfur trioxide, and of its adducts with organic compounds .

Ihata (1988) Formation and reaction of polyenesulfonic acid. I. Reaction of polyethylene films with S0₃ Journal of Polymer Science: Part A, Polymer Chemistry, Vol. 26, 167-176. Kucera and Jancar (1998) Homogenous and heterogeneous sulfonation of polymers: A review Polymer Engineering and Science, 38(5), 783-792.

Nand et al (2012) Characterization of antioxidant low density polyethylene/ polyaniline blends prepared via extrusion Materials Chemistry and Physics, 135, 903-911.

Olsen and Osteraas (1969) Sulfur modification of polyethylene surfaces. II. Modification of polyethylene surfaces with fuming sulfuric acid Journal of Polymer Science, Part Al, Vol. 7, 1921-1926.

Raghavan and Torma (1992) DSC and FTIR characterization of biodegradation of polyethylene Polymer Engineering and Science, 32, 348-442.

Sano et al (1980) Method for producing strong-acid cation exchange fibre Canadian Patent No. 1085089.

Socrates (1994) Infrared Characteristic Group Frequencies (Second Edition) John Wylie and Sons, West Sussex, England. Tricoli and Carretta (2002) Polymer electrolyte membranes formed of sulfonated polyethylene Electrochemistry Communications, 4, 272-276.

Walles (1973) Resinous enclosure members rendered impermeable by

sulfonation United States Patent No. 3,740,258.

Claims

A method of sulfonating a preformed microporous polyolefin substrate comprising the steps of:

• wetting the substrate with a mixture of an organic solvent and a solvent miscible polar aprotic co-solvent to provide a wetted substrate ;

• cooling the wetted substrate to a temperature less than 20 °C to provide a permeated substrate; and then

• contacting a sulfonating agent with the permeated substrate at a temperature and for a time sufficient to achieve the desired degree of sulfonation.

The method of claim 1 where the polyolefin of the preformed

microporous polyolefin substrate is selected from the group

consisting of: polyethylene, polypropylene, polybutylene and polymethylpentene .

The method of claim 1 where the preformed microporous polyolefin substrate is a sheet of microporous polyethylene.

The method of any one of claims 1 to 3 where the organic solvent is a chlorinated organic solvent.

The method of claim 4 where the organic solvent is a chlorinated alkane .

The method of claim 5 where the organic solvent is selected from a group consisting of: methylene chloride and trichloromethane.

7) The method of claim 6 where the organic solvent is trichloromethane.

The method of any one of claims 1 to 7 where the solvent miscible polar aprotic co-solvent is selected from the group consisting of: dimethylsulfoxide (DMSO) and dimethyl formamide (DMF) .

The method of claim 8 where the solvent miscible polar aprotic co- solvent is dimethylsulfoxide (DMSO) .

The method of any one of claims 1 to 9 where the ratio (v/v) of solvent miscible polar aprotic co-solvent to organic solvent in the mixture is in the range 3:1 to 11:1. 11) The method of claim 10 where the ratio (v/v) of solvent miscible polar aprotic co-solvent to organic solvent in the mixture is in the range 4:1 to 9:1.

12) The method of claim 11 where the ratio (v/v) of solvent miscible

polar aprotic co-solvent to organic solvent in the mixture is 4:1.

13) The method of any one of claims 1 to 12 where the temperature is 70 to 90 °C and the time sufficient is 60 to 120 minutes.

14) The method of claim 13 where the temperature is 85 °C and the time sufficient is 90 minutes. 15) The method of any one of claims 1 to 14 where the organic solvent has a boiling point below 85 °C.

16) The method of any one of claims 1 to 15 where the sulfonating agent is a mixture of concentrated sulphuric acid and phosphorous

pentoxide.

17) A sheet of sulfonated microporous polyolefin having an average S/C ratio greater than 0.20% and a tensile strength no less than 90% of that of the preformed microporous substrate from which it is prepared.

18) The sheet of sulfonated microporous polyolefin of claim 17 where the sheet has a tensile strength no less than 95% of that of the preformed microporous substrate from which it is prepared.

19) A sheet of sulfonated microporous polyolefin where the sheet is

capable of maintaining a water flux rate of greater than 100 L m^"2 h^"1 at a pressure of 10 bar and temperature of 4 to 8°C. 20) The sheet of sulfonated microporous polyolefin of any one of claims 17 to 19 prepared according to the method of any one of claims 1 to 16.

21) The sheet of sulfonated microporous polyolefin of any one of claims 17 to 20 where the polyolefin is polyethylene.