WO2012112118A1

WO2012112118A1 - Expanded polymeric membranes

Info

Publication number: WO2012112118A1
Application number: PCT/SE2012/050175
Authority: WO
Inventors: Franciscus Hubertus Jacobus Maurer; Valerie Claes
Original assignee: Franciscus Hubertus Jacobus Maurer
Priority date: 2011-02-20
Filing date: 2012-02-17
Publication date: 2012-08-23

Abstract

A method of manufacturing an expanded polymeric membrane comprising a glassy polymer having a glass transition temperature of at least 160°C and possessing two ortho-positronium (o-Ps) lifetimes is described. The polymeric membrane is treated by CO₂ at temperatures between 25°C and 200°C and pressures between 2 and 40 MPa. Uses of the expanded polymeric membrane include pervaporation and nanofiltration.

Description

TITLE:

Expanded Polymeric Membranes

Field of the invention

The present invention is related to expanded polymeric membranes and methods of their manufacture and uses. Expanded polymeric membranes find applications in processes for separating a mixture of (fluid) components. Examples of the latter are gas and vapor separation.

Background

This application claims priority to SE 1130006-8 filed 2011-02-20 and to US application 61/445,576, filed 2011- 12-23, both of which are incorporated by reference for all purposes.

De Sitter et al. in WO 2009027376 and in Journal Membrane Science vol. 278 (2006), pp. 83- 91 disclose a method for preparing filled polymeric membranes. The polymer is poly (1- trimethylsilyl-l-propyne), (PTMSP) and nanoparticles of silica are used to increase the gas permeability of the polymeric membranes. Silica particle aggregates comprising interstitial nanometric- sized cavities were observed in the polymeric matrix membrane and are described by Winberg et al. in Macromolecules, vol.38 (2005), pp.3776-3782. PTMSP is one of the most permeable polymers known. The route of transportation of the penetrants in a polymeric membrane and its separation performance is strongly determined by the intrinsic free volume cavity sizes of the polymer; in other words by the non-occupied space of the macromolecules in the polymer matrix. Free volume cavity sizes can be measured quantitatively by Positron Annihilation Lifetime Spectroscopy (PALS) and using the Tao-Eldrup equation (1) which relates ortho-Positronium lifetimes x₀-p_s (o-Ps) and radii of free volume cavities R. x₀__Ps = 0.5 [ 1- R/Ro + (1/(2π)) sin (2nR/R₀) ]^_1 (1) with RQ = R + 0.166 nm and x₀_p_s in ns. The volume of the free volume cavity size is: v = (4π/3) R³. (2) A detailed description of the PALS measurement technique can be found in Winberg et al. Macromolecules, vol. 38 (2005), pp. 3776-3782. Permeabilities of penetrants through polymeric membranes increase with increasing free volume cavity sizes and thus with increasing ortho-Positronium lifetimes.

The interstitial cavities in aggregates of the fillers in filled polymer membranes may contribute positively to the permeability of penetrants. To increase the permeability of PTMSP, De Sitter et al. in WO 2009027376 propose to increase the filler amount and to control the aggregate size.

A drawback of using filled polymers, specifically the inclusion of filler particles and its varying content in polymeric membranes and the presence of aggregates is often a decrease in selectivity of the membrane in separating penetrants. Particle fillers and aggregates of particles form a non-selective path for the penetrants passing the membrane.

Another drawback of using filled polymeric membranes is that particle size and particle aggregate size limits the minimum thickness of the membrane. A membrane is most efficient when it is as thin as possible while completely and homogeneously covering a porous substrate used as a support.

A further drawback of using filled polymeric membranes is the possible occurrence of filler pore blockage by moieties of a fermentation broth, due to the presence of the filler particles.

The present invention provides a method of manufacturing expanded polymeric membranes with expanded free volume cavity sizes which overcome shortcomings of known

manufacturing methods.

Summary of the Invention

The expanded membranes, made by the method of the invention, possess at least equal or improved properties compared to presently available commercial polymeric membranes and which allow improved separation processes. Recently a comparison of the performance of commercial poly dimethyl siloxane (PDMS) pervaporation membranes and expanded silica filled and expanded unfilled PTMSP pervaporation membranes have been described in literature by Claes et al. J. Membrane Science vol.382 (2011), pp. 17-185. The expanded membranes outclass the commercial PDMS membrane in flux and separation factor for ethanol-water mixtures. Particularly, said membranes provide an improved pervaporation process, in particular for concentrating ethanol out of ethanol/water mixtures. The membranes also provide an improved nanofiltration process.

The invention also provides expanded polymeric membranes with long term stability. The free volume cavities in polymers below the glass transition temperature tend to reduce their size as a function of elapsed time after processing. This process is called physical aging and is a consequence of the non equilibrium state of the free volume in glassy polymers. The decrease of the lifetimes in said expanded glassy polymer membranes is extremely slow due to the high glass transition temperature of the starting glassy polymer used. Recently the influence of treatment conditions on the free volume cavity sizes of expanded PTMSP membranes and their behavior as a function of time have been published by Claes et al. in Macromolecules, vol. 44 (2011) 2766-2772.

Therefore, according to the first aspect of the invention, there is provided a method of manufacturing an expanded polymeric membrane. The method comprises a first step of preparing a polymeric membrane comprising a glassy unfilled polymer having a glass transition temperature of at least 160 C. The glassy polymer may comprise one or more additives commonly used in the manufacturing of polymers and known to the skilled man, such as processing aids or thermal and UV stabilizers. The glassy polymer may also be blends or mixtures comprising one or more further polymers to a maximum of 50 wt%.

The polymeric membrane may be in the shape or form of film, fiber, hollow fiber, disk or any other suitable form, and may be manufactured by precipitation, spin coating, gel spinning, or electro- spinning from solution or any other method of manufacturing of polymeric membranes, known in the art. The polymeric membrane may include a porous substrate used as a support to increase mechanical stability.

An important feature of the invention includes a step of treating the formed polymeric membrane with pressurized C0₂ or supercritical C0₂ (T_c = 31.1°C, p_c = 7.37 MPa) at temperatures between 25 °C and 200°C and pressures between 2 and 40 MPa. According to a second aspect of the invention, expanded polymeric membranes are provided having improved separation processes.

The expanded polymeric membranes obtained possess two expanded main free volume cavity sizes which are characterized by their ortho-Positronium lifetimes (o-Ps), which may be measured with positron annihilation lifetime spectroscopy (PALS). The expanded glassy polymer possesses a shorter o-Ps lifetime of about 2-5 ns and a larger o-Ps lifetime about 6.5- 15 ns. Both lifetimes of the expanded polymeric membranes show an increase of more than 7.5 % over the values of the lifetimes of the untreated polymeric membranes.

According to a third aspect of the present invention, there is provided an apparatus for separating a mixture of components by pervaporation comprising the expanded polymeric membrane of the invention. The process of separating a mixture of components is preferably a pervaporation process. More preferably, said mixture of components consists (essentially) of a mixture of water and ethanol. In said process the mixture of components is separated in an ethanol- rich fraction and an ethanol -poor fraction. This does not exclude other mixtures of components such as water and butanol. In some embodiments, the apparatus for separating a mixture of components may employ a nanofiltration process.

According to a fourth aspect of the invention there is provided a use or application of said expanded polymeric membrane in a process of separating a mixture of components by a pervaporation or nanofiltration process.

The invention describes herein additionally provides for improving the performance of polymeric membranes, enabling a balance of high permeabilities and high selectivities.

Brief description of the drawings

Figure 1 relates to the disclosure in Example 2. Figure 1 represents Poly(l-trimethylsilyl-l- propyne) (PTMSP) free volume cavity sizes determined from o-Ps lifetimes with equation 1 and equation 2, as a function of elapsed time at 23 °C after preparation and treatment. PTMSP expanded membranes have been treated in C0₂ at 16 MPa and 150°C for 6 h. Experimental data of the free volume cavity sizes as a function of elapsed time of untreated PTMSP membranes are also disclosed. The enlarged free volume cavity sizes of the expanded polymeric membrane remained appreciable higher than the untreated membrane even after 3 years. The relaxation time for the expanded free volume cavity sizes is estimated to be about 30 years.

Detailed description of the invention

Specific embodiments of the invention will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention.

A glassy polymer refers to a polymer having a glass transition temperature above the temperature at which the polymer will be used. The glassy polymers used for the present invention have a glass transition temperature of at least 160°C, measured by Differential Scanning Calorimetry with a heating rate of 20°C/min. The glassy polymer may also be a microporous polymer.

The glassy polymers preferable possess two large main free volume cavities, where at least one is larger than 0.4 nm in diameter. Possible glassy polymers include, but are not limited to, substituted polyacetylene polymers, such as PTMSP and PMP and amorphous perfluoro polymers, such as Teflon(R) (copolymer of tetrafluoroethylene and 2,2-bis(trifluoromethyl)- 4,5difluoro-l,3-dioxole) and Hyflon (R) (copolymer of tetrafluoroethylene and 2,2,4- trifluoro-5-trifluoromethoxy-l,3-dioxole) and their mixtures or blends.

The glassy polymer may comprise additives commonly used in polymers such as thermal and UV stabilizers, and other polymeric materials mixed or blended with the glassy polymer, to a maximum content of 50 wt%.

The polymeric membrane and the expanded polymeric membrane may comprise a porous substrate as a support, allowing mechanical stability. Porous supports may be manufactured, for example, from polyvinyliden (PDVF), polyacrylonitrile (PAN), ceramics, or aerogels. A porous support may be in the form of an ultrafiltration membrane. Description of the treatment process

A high-pressure reactor may be used in combination with a diaphragm pump for the treatment of the membranes. The reactor, in which the polymeric membrane is placed, is filled with liquid C0₂ prior to heating. The liquid C0₂ may contain co-solvents to control the solubility parameter 5co₂ of the liquid and the interaction between the liquid and the glassy polymer.

The solubility parameter of C0₂ is dependent on temperature and pressure. In the temperature range between 30 C and 190 C and pressures between 5 and 30 MPa, the solubility parameter of C0₂ varies between 0 - 15 MPa 1/2. The use of a co-solvent, as for example methanol, can increase the solubility parameter of C0₂ and may decrease the glass transition temperature of the polymeric membrane during treatment. An advantage is that the interaction between C0₂ and the polymeric membrane can be adjusted, dependent on the specific configuration of the polymer used.

As the pressure increases with increasing temperature to the set point temperature, a regulation valve may be used to prevent the pressure from crossing the set point pressure. Once the desired temperature and pressure is reached, the reactor may be kept for several hours under stable conditions. Preferentially the time period under these conditions is between 0.25 hours and 10 hours.

After this treatment time the C0₂ pressure is slowly released at a rate of 0.15MPa^_1 or slower. Higher rates of depressurization may lead to undesirable foaming with pores larger than 0.1 μιη. To prevent C0₂ from liquefying during depressurization, the reactor temperature may be kept above 35 C.

Positron annihilation lifetime spectroscopy (PALS)

The expanded membranes of the present invention possess two main o-Ps lifetimes, the shorter lifetime being 2-5 ns and the larger lifetime being 6.5-15 ns as measured by PALS at

23 C in air. Positron annihilation lifetime spectroscopy is a suitable method that directly probes the free volume cavity sizes in polymers.

A positron emitted from a radioactive source, sandwiched between two polymer slabs or stacked polymer films, may form ortho-positronium (o-Ps) with an electron with parallel spin in the polymer. o-Ps is formed and annihilates in regions of low electron density. In a polymer, the low electron density regions are the free volume sites of the polymer, and the lifetime of o-Ps is therefore directly related to the size of the free volume cavities. Assuming that the ortho-positronium is localized in a spherical potential well having an infinite potential barrier, the free volume cavity size can be estimated by equation 1 and the volume of the equivalent sphere by equation 2. O-Ps lifetimes of thin (10 nm-100 μιη) polymeric membranes comprising a substrate may be characterized by PALS with a low energy positron beam instead of a radioactive source.

Many glassy polymers such as polystyrene, poly methyl methacrylate, polyvinylchloride, styrene acrilonitrile copolymer, polycarbonate, and others posses only one O-Ps lifetime of a value of about 2 ns at room temperature and therefore posses only one main free volume cavity size. Polymers such as PTMSP and PMP however posses two o-Ps lifetimes with values of about 1.9 ns and 6.2 ns (example 1) for PTMSP and 2.9 ns and 6.4 ns for PMP (De Sitter, PhD thesis (2008) Leuven University, Belgium). The corresponding main large free volume cavity sizes are larger than any free volume cavity sizes known in amorphous polymers including poly dimethyl siloxane (PDMS). Consequently PTMSP and PMP have large gas and vapor permeabilities. The oxygen permeability of PTMSP is about ten times larger than the oxygen permeability of PDMS.

Separation processes

The expanded polymeric membranes of the invention can find application in apparatus for separating a mixture of components by pervaporation. They can find application in nanofiltration apparatuses as well. Expanded glassy polymeric membranes can

advantageously be used in processes for separating a mixture of (fluid) components.

Examples of the latter are gas and vapour separation.

Pervaporation is a fractionation process, in which a liquid mixture is obtained at atmospheric pressure on the feed side of the membrane and the permeate is removed as a vapour.

Transport through the membrane is induced by the vapour pressure difference between the feed and the permeate vapour. The pressure difference can be achieved by using a vacuum pump at the permeate side, or by cooling the permeate to create a partial vacuum.

Pervaporation is used on an industrial scale to separate ethanol from its dilute aqueous solutions. One of the applications wherein ethanol/water separation is the key factor is the production of bio-ethanol to be extracted from an ethanol/water mixture. This can be performed by conventional techniques, such as distillation and solvent extraction, but these processes are very energy consuming. Pervaporation with ethanol- selective membranes can be used to concentrate low-concentration bio-ethanol from fermentation broths in an

economically effective way. It is known to use polydimethylsiloxane (PDMS) membranes for the ethanol recovery from fermentation broths. PDMS possesses only one o-Ps lifetime (P. Winberg et al. Polymer 45 (2004) 8253-8264). The O-Ps lifetime of PDMS at 23 ^°C is about 4 ns. The expanded polymeric membranes of the present invention can be used instead of PDMS for the separation of ethanol from its dilute aqueous solution by pervaporation. Filled silicone rubber membranes in contact with fermentation broth, used in a pervaporation process, are reported to deteriorate fast in pervaporation performance due to filler pore blockage (L.M.Vane et al. Journal of Membrane Science, 364 (2010) 102-110. An advantage of the present invention is that the treatment does not change the interaction between membrane and fermentation broth in contrast to the incorporation of filler particles.

Nanofiltration comprises the separation of industrial gases, filtration of particulate matter from liquid suspensions, air, industrial flue-gas and the separation of liquid mixtures. Other applications may include ion separation in electrochemical processes, membrane dialysis of blood and urine, artificial lungs and kin, controlled release of therapeutic drugs, controlled release of fertilizers, affinity separation of biological molecules, membrane -based sensors for gas and ion detection, and membrane reactors.

Example 1: Preparation and properties of expanded polymeric membranes.

Poly(l-trimethylsilyl-l-propyne) (PTMSP, molecular mass M_w ~ 4.5xl0⁵ g mol^"1) was purchased from Gelest, Inc (USA) and dissolved in an analytical graded toluene, obtained from Merck ( Belgium). Technical grade liquid carbon dioxide was purchased from Air Products (Belgium).

PTMSP membranes were prepared from toluene solutions containing 3 wt% of polymer. The polymer solutions were magnetically stirred for 4 days until complete dissolution, after which they were cast in a Petri dish. After casting, the membranes were dried 10 days at ambient conditions and subsequently annealed for 2 h at 80 C in a nitrogen atmosphere. A high- pressure reactor from Premex Reactor AG (Switzerland) with an internal volume of 1 L was used in combination with a ProMinent Orlita MhS 30/10 diaphragm pump (Belgium) for the treatment of membranes. Circular disks with a thickness of approximately 100 μιη and a diameter of about 9 cm were cut from the PTMSP membranes and placed in the reactor, which was then sealed. First, the reactor vessel, pump and piping were flushed with C0₂ until liquid C0₂ was obtained at the outlet. Subsequently, the reactor was filled with liquid C0₂ prior to heating. As the pressure increases upon heating due to phase transition and expansion of the liquid C0₂, a regulation valve was used to prevent the pressure from crossing the set point pressure. PTMSP membranes were treated at 8 different conditions (Table 1). Once the desired temperature and pressure were reached, the reactor was kept for 6 h under stable conditions. After this treatment the C0₂ pressure was slowly released at 0.15 MPas^"1, as higher rates of depressurization can lead to foaming of the polymer. To prevent C0₂ from liquefying during depressurization, the reactor temperature was kept above 35°C.

Subsequently, PALS measurements were performed in air at 23 °C on stacked membranes at a total thickness of 1 mm on either side of the radioactive source. Each sample was measured five times with a total number of counts of about 2.3 milj. The spectra were analyzed with PALSFIT software from RISOE (Denmark) with four lifetimes and a fixed first lifetime of para-Positronium annihilation at 260 ps. The longest two lifetimes x₃ and x₄ in the spectrum represent the two main ortho-Positronium lifetimes. Ortho-Positronium lifetimes were determined for untreated and C0₂ treated membranes. Results are presented in Table 2.

Example 2: Long term properties of polymeric membranes

The preparation of two PTMSP membranes is described in example 1. The expanded polymeric membrane was treated in C0₂ at 16 MPa and 150 C for 6h.

In Figure 1 a comparison between the time dependency of free volume cavity sizes of treated and untreated PTMSP membranes, calculated from o-Ps lifetimes with equation 1 and equation 2, is presented. Free volume cavity sizes of untreated (■,□) and C0₂-treated (·, ο) PTMSP membranes treated at 16 MPa and 150°C for 6 h and stored at 23 °C as a function of the elapsed time t in seconds are shown. Free volume cavity size in nm (filled symbols) derived from x₃ and free volume cavity size in nm (unfilled symbols) derived from x₄. As can be seen, the unexpanded membrane possesses free volume cavity sizes of about 0.15 nm and about 0.6 nm while the expanded membrane possesses free volume cavity sizes of about

0.35 nm 3 and about 1.15 nm 3. In this example, the treatment approximately doubled the calculated free volume cavity sizes. The enlarged free volume cavity sizes of the expanded polymeric membrane were calculated to remain appreciably higher than the untreated membrane even after 3 years. The relaxation time for the expanded free volume cavity estimated to be about 30 years.

Table 1. Treatment conditions of PTMSP membranes.

Temperature (°C)

Pressure (MPa)

40 70 110 150

12 X X

16 X X

20 X X

24 X X

Table 2. Ortho-Positronium lifetimes x₃ [ns] and x₄ [ns] of the untreated and C0₂ treated PTMSP membranes, and the standard deviations on the measurements (five measurements per sample).

Claims

1. A method of manufacturing an expanded polymeric membrane, the method comprising:

placing a polymeric membrane comprising a glassy polymer into a reactor;

treating the polymeric membrane with pressurized C0₂ at a temperature of between 25°C and 200°C and a pressure of between 2 and 40 MPa; and

reducing the pressure inside the reactor at a controlled rate to form an expanded polymeric membrane

wherein the glassy polymer has a glass transition temperature of at least 160°C.

2. A method according to claim 1, wherein the polymeric membrane comprises a porous substrate support.

3. The method according to claim 1 or 2, wherein treating the polymeric membrane with pressurized C0₂ is performed at a temperature of between 30°C and 190°C and a pressure of between 5 and 30 MPa.

4. The method according to any preceding claim, wherein:

the glassy polymer possesses a first ortho-Positronium (o-Ps) lifetime and a second o-Ps lifetime longer than the first o-Ps lifetime,

the first o-Ps lifetime and the second o-Ps lifetime of the glassy polymer in the expanded polymeric membrane is at least 7.5% greater than in the polymeric membrane, and/or

the expanded polymeric membrane has a first o-Ps lifetime value of 2-5 ns and a second o-Ps lifetime value of 6.5-15 ns, as measured by PALS at 23°C.

5. The method according to claim 4, wherein the glassy polymer is poly[l- (trimethylsilyl)-l-propyne] (PTMSP); poly [4-(methyl)-2-pentyne] (PMP); or a combination thereof.

6. The method according to any preceding claim, wherein the controlled rate at which the pressure inside the reactor is reduced is 0.15MPa^_1 or slower.

7. An expanded polymeric membrane produced according to any of claims 1-6.

8. The expanded polymeric membrane according to claim 7, wherein the expanded polymeric membrane possesses enlarged free volume cavity sizes relative to the unexpanded polymeric membrane for at least 3 years.

9. An apparatus for separating a mixture of components by pervaporation comprising the expanded polymeric membrane according to either of claims 7 and 8.

10. An apparatus for separating a mixture of components by nanofiltration comprising the expanded polymeric membrane according to either of claims 7 and 8.

11. Use of the polymeric membrane according to either of the claims 7 and 8 for separating a mixture of components by pervaporation.

12. The use according to claim 11, wherein said mixture of components is a mixture of water and ethanol and wherein, in said processs, the mixture of components is separated to form an ethanol-rich fraction and an ethanol-poor fraction.

13. Use of the polymeric membrane according to either of the claims 7 and 8 for separating a mixture of components by nanofiltration.

14. A method for separating ethanol and water in mixture comprising water and ethanol, said method comprising:

contacting the mixture with an expanded polymeric membrane according to either of claim 7 and 8 to produce an ethanol-rich fraction on a first side of the expanded polymeric membrane and an ethanol-poor fraction on a second side of the expanded polymeric membrane.