Co-extruded ultra thin films

Abstract

A process for producing an ion exchange precursor resin membrane involves co-extruding an ion exchange precursor resin with an incompatible polymer to form a multilayer film having a layer of the ion exchange precursor resin supported on a layer of the incompatible polymer. The layer of incompatible polymer is then removed from the layer of ion exchange precursor resin to provide the ion exchange precursor resin membrane. The ion exchange precursor resin membrane may be converted to an ion exchange resin membrane by hydrolysis, and subsequent acidification if desired. Ion exchange resin membranes and ion exchange precursor resin membranes having a uniform thickness of 25 microns or less may be formed by the process.

Claims

1. A process for producing an ion exchange precursor resin membrane comprising: co-extruding an ion exchange precursor resin with an incompatible polymer to form a multilayer film having a layer of the ion exchange precursor resin supported on a layer of the incompatible polymer; and, removing the layer of incompatible polymer from the layer of ion exchange precursor resin to provide the ion exchange precursor resin membrane.

2. The process according to claim 1, wherein the co-extruding is accomplished by blown film extruding.

3. The process according to claim 1, wherein the co-extruding is accomplished by melt-blowing.

4. The process according to claim 1, wherein the ion exchange precursor resin comprises a non-ionic form of a perfluorosulfonic acid (PFSA) resin.

5. The process according to claim 4, wherein the PFSA resin comprises a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer.

6. The process according to claim 4, wherein the PFSA resin comprises a perfluorosulfonic acid-polytetrafluoroethylene copolymer of Formula (I), ##STR00002## where x, y and z represent fractional weight composition of particular moieties and are numbers between 0 and 1, where x+y+z=1.

7. The process according to claim 4, wherein the non-ionic form of the PFSA resin comprises sulfonyl halide protected acid groups.

8. The process according to claim 1, wherein the ion exchange precursor resin comprises a polymer composite comprising a polymer and filler.

9. The process according to claim 8, wherein the filler comprises a metal oxide, a manganese-based compound, a cerium-based compound, an inorganic filler grafted with one or more functional moieties, or a mixture thereof.

10. The process according to claim 1, wherein the incompatible polymer comprises a polyolefin.

11. The process according to claim 1, wherein the multilayer film comprises two layers, a first layer of the ion exchange precursor resin and a second layer of the incompatible polymer.

12. The process according to claim 1, wherein the multilayer film comprises three layers, a first layer of the ion exchange precursor resin sandwiched between second and third layers of the incompatible polymer.

13. The process according to claim 12, wherein the second and third layers comprise the same incompatible polymer.

14. The process according to claim 1, wherein removing the layer of incompatible polymer from the layer of ion exchange precursor resin is accomplished by peeling the layer of incompatible polymer from the layer of ion exchange precursor resin.

15. A process for producing an ion exchange resin membrane comprising converting the ion exchange precursor resin membrane produced in accordance with claim 1 into the ion exchange resin membrane.

16. The process according to claim 15, wherein converting comprises hydrolyzing the ion exchange precursor resin membrane.

17. The process according to claim 16, wherein hydrolyzing is accomplished with a base, and the ion exchange resin membrane produced therefrom is further converted to a protonated form using an acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order that the invention may be more clearly understood, embodiments thereof will now be described in detail by way of examples, with reference to the accompanying drawings, in which:

(2) FIG. 1 depicts a schematic representation of a multilayer die used in a co-extrusion process of the present invention;

(3) FIG. 2 is a graph depicting rheological data measured on PFSA precursor (Nafion™ R-1000) using a rotational rheometer in dynamic mode with a frequency sweep at 240° C. under dry nitrogen atmosphere;

(4) FIG. 3 is a graph depicting rheological data measured on a nanocomposite blend of PFSA precursor (Nafion™ R-1000) with 6 wt % of hydrophilic silica using a rotational rheometer in dynamic mode with a frequency sweep at 240° C. under dry nitrogen atmosphere;

(5) FIG. 4 depicts schematic representations of easily delaminated multilayer films obtained in accordance with a process of the present invention;

(6) FIG. 5 depicts TEM images on lead acetate stained membranes, where FIGS. 5A and 5B are for a solution-cast NRE-211 membrane, FIGS. 5C and 5D are for a melt extruded Nafion™ R-1000 membrane and FIGS. 5E and 5F are for a melt blown Nafion™ R-1000 membrane;

(7) FIG. 6 depicts transmission XRD patterns obtained for A) highly oriented R-1000-CS-E, B) R-1000-CS-MB, and C) NRE-211;

(8) FIG. 7 depicts spectra obtained from the integration of the transmission X-ray scattering patterns obtained at an incidence angle of 90° at room temperature for solution-cast NRE-211 membrane, and extruded and melt-blown Nafion™ R-1000 membranes;

(9) FIG. 8 is a graph depicting proton conductivity measured at room temperature in water for solution-cast NRE-211 membrane, and extruded and melt-blown Nafion™ R-1000 membranes;

(10) FIG. 9 is a graph depicting dry/wet water uptake (WU) and volume change (VC) measured for solution cast NRE-211 membrane, extruded and melt-blown Nafion™ R-1000 membranes, and the melt blown nanocomposite membrane of Example 2;

(11) FIG. 10 is a graph presenting polarization curves obtained in a hydrogen/air fuel cell operated at 80° C./100% relative humidity (RH) and 95° C./30% RH measured for solution cast NRE-211 membrane, 25 and 18 microns melt-blown Nafion™ R-1000 membranes (Films 2 and 3); and,

(12) FIG. 11 is a graph showing polarization curves obtained in a hydrogen/air fuel cell operated at 95° C. and 30% RH measured for solution cast NRE-211 membrane, melt-blown Nafion™ R-1000 membranes (Film 2), and the melt-blown nanocomposite membrane (Film 6).

DESCRIPTION OF PREFERRED EMBODIMENTS

Materials

(13) For the Examples described herein, the precursor form of Nafion™, a perfluorosulfonic acid (PFSA) copolymer in the sulfonyl fluoride form with an Ion Exchange capacity IEC=1 mmol/g was used as ionomeric material (grade R-1000 purchased from Ion Power). For nanocomposite systems, fumed silica nanoparticles with a surface area of 380 m.sup.2/g were used as inorganic filler (grade CAB-O-SIL EH-5 purchased from Cabot). A branched low density polyethylene LDPE (Novapol™ LF-Y918-A) with MFI=0.75 g/10 min at 190° C./2.16 kg was the incompatible polymer used as a support material for the external layers.

(14) Rheology

(15) To determine melt viscosities of polymeric materials, dynamic rheological measurements were performed in an ARES (Advance Rheometric Expansion System) rotational rheometer in dynamic mode, using 25-mm diameter parallel plates in oscillatory shear mode under dry nitrogen atmosphere at T=240° C. Prior to testing, samples were dried 24 h under active vacuum at 60° C. The test comprises a frequency sweep over a range spanning from 100 down to 0.1 rad/s. Small deformations (10% or 15%) oscillatory motions were imposed on the samples for all time and frequency sweeps to avoid any irreversible damage of the structure of the material. The measurements allow to evaluate the response of the materials tested in term of elastic or storage modulus (G′), viscous or loss modulus (G″), and the dynamic complex viscosity (η*).

(16) Rheological measurements were carried out to characterize the viscoelastic properties of ionomeric materials used in the Examples described herein. The measurements allow the evaluation of the response of the materials tested in terms of elastic or storage modulus (G′), viscous or loss modulus (G″), and the dynamic complex viscosity (Eta* or η*) as a function of the oscillation frequency. The results of the frequency sweep test at 240° C. are shown in FIG. 2 for PFSA precursor Nafion™ R-1000, and FIG. 3 for PFSA nanocomposite with 6 wt % EH-5 filler. For Nafion™ R-1000 the viscosity increases as the shear rate is decreased but at sufficiently low shear rates it becomes independent of shear rate and shows a linear plateau typical of Newtonian melts. For Nafion™ R-1000+6 wt % EH-5, the consequence of adding silica filler is an increase in melt viscosity and deviation from the linear viscoelastic range at low frequency. Also, the contribution of the viscous or loss modulus (G″) clearly governs the rheological behaviour of both ionomeric materials in the range of frequencies investigated.

(17) Melt-Processing

(18) PFSA precursor Nafion™ R-1000 pellets were directly melt-blown without previous preparation. Nanocomposites based on Nafion™ R-1000 and fumed silica EH-5 were previously compounded at 240° C. using a twin-screw extruder Leistritz Nano-16 mm.

(19) The melt-blowing experiments were performed on a LabTech multilayer co-extrusion blown film line equipped with a flat spiral die system. The multi-layer die has more than one spiral ‘layer’ and the die is fed from several extruders. For the examples described herein, a die fed by three melt streams was used, one melt stream of ionomeric materials coming from extruder (B) and the other two melt streams of support polymer coming from separate extruders (A and C) with a pancake die with four plates as illustrated in FIG. 1. The multilayer melt-blowing film line is equipped with three single-screw extruders LabTech 12.5 mm (type LBE 12.5/30).

(20) Hydrolysis

(21) PFSA precursor thin films obtained by melt blowing are converted to the acid form by using the following process. 1) hydrolysis in a solution of 15% KOH/35% DMSO/50% de-ionized (DI) water at 80° C., followed by washing with DI water to remove all traces of un-reacted KOH. 2) Acid conversion to the H.sup.+ form by exchanging the K.sup.+ for H.sup.+ ions using a 10 to 15% solution of nitric acid (HNO.sub.3), followed by DI water washing. 3) Activation: the H.sup.+ form of PFSA membranes are activated with 7.5% H.sub.2O.sub.2 at 80° C. for 1 h, soaked in DI water at 80° C. for 1 h, and finally treated with 15% H.sub.2SO.sub.4 at 80° C. for 1 h. The treated membranes are washed thoroughly with DI water.

(22) Proton Conductivity

(23) In-plane proton conductivities were measured using a Solartron™ 1260. A strip of membrane (in H.sup.+ form) was set between two Pt electrodes and an alternating current was passed through the plane of the sample. In the case of room temperature and liquid water conditions, the samples were immersed in Millipore water. Room temperature varied from 20° C. to 22° C. Nyquist plots between 5 MHz to 10 Hz were collected and membrane resistance was extrapolated by fitting the semi-circle part of the data to equivalent circuits. Proton conductivities were calculated from the equation below:
σ=d/RS
where σ is proton conductivity, d is the distance between the Pt electrodes, R is membrane resistance and S is the cross-sectional area of the sample.
Water Uptake (WU) and Volume Change (VC)

(24) Water Uptake (WU) and Volume Change (VC) were determined as follows. After measuring mass of wet and dried membranes (in H.sup.+ form), WU was calculated from the equation below:

(25) $WU=\frac{\mathrm{wet}\mathrm{mass}-\mathrm{dry}\mathrm{mass}}{\mathrm{dry}\mathrm{mass}}\times 100\%$
The mass of dried membranes were obtained after drying them in a vacuum oven at 80° C. overnight. For VC measurements, thickness, width and length of wet and dried membranes (in H.sup.+ form) were determined. Wet/dry volume change was calculated from the equation below:

(26) $VC=\frac{\mathrm{Volume}\mathrm{of}\mathrm{wet}\mathrm{membrane}-\mathrm{volume}\mathrm{of}\mathrm{dried}\mathrm{membrane}}{\mathrm{Volume}\mathrm{of}\mathrm{dried}\mathrm{membrane}}\times 100\%$
Dimensions of dried membranes were obtained after drying them in a vacuum oven at 80° C. overnight.
X-Ray Diffraction Spectroscopy (XRD)

(27) Crystalline orientation was determined from wide-angle transmission X-ray spectroscopy analysis. The equipment used was a Bruker D8 Discover X-rays goniometer equipped with a Hi-STAR™ two-dimensional area detector. The generator was set up at 45 kV and 0.65 mA.

(28) Transmission Electron Microscopy (TEM)

(29) Membranes in the acid form were previously immersed in a saturated lead acetate solution for 2 h at room temperature to stain the ionic domains. The samples were encapsulated in epoxy resin. The cured epoxies containing the membranes were then microtomed at room temperature into thin slices of 50 nm using a diamond knife. TEM of ultrathin sections of the samples were obtained with a Philips CM 200 instrument with an acceleration voltage of 200 kV.

Preparative Example 1

Nafion™ R-1000

(30) Five different multilayer films were prepared by melt-blowing co-extrusion process using Nafion™ R-1000 as ionomeric material, and LDPE as a support material. The structures obtained in this example are tri-layer films, where Nafion™ R-1000 is the mid-layer, and LDPE the external layers (FIG. 4B).

(31) The extruder (B in FIG. 1) used for the Nafion™ R-1000 mid-layer has the following profile temperature:

(32) Zone 1: 220° C., Zone 2: 230° C., Zone 3: 240° C., Die temperature 240° C.

(33) The two extruders (A and C in FIG. 1) used for LDPE external layers have the following profile temperature:

(34) Zone 1: 185° C., Zone 2: 190° C., Zone 3: 200° C., Die temperature 200° C.

(35) Control parameters for the five different multilayer films prepared are listed in Table 1. LDPE is incompatible with the PFSA precursor, which prevents any interfacial bonding between the layers during melt processing. The external layers are peeled easily to obtain a thin membrane of PFSA precursor. Thin membranes with different thicknesses ranging from 30 to 6 microns were obtained. Blow-up ratios (BUR) used were between 4.5 and 5.5, and draw-down ratios were between 1 and 2.

(36) TABLE-US-00001 TABLE 1 Melt-Blowing Parameters for Ionomer Precursor Films Parameter Film 1 Film 2 Film 3 Film 4 Film 5 RPM extruder A (LDPE) 106.4 99.5 106.5 86.9 86.9 RPM extruder B (PFSA) 28.4 20.3 16.8 9.7 7.1 RPM extruder C (LDPE) 130 100.1 130.1 104.4 104.4 Blower speed (rpm) 810 628 810 769 625 Nip roll speed (m/min) 1 1 1 1 1 Wind-up speed (m/min) 1.2 1.2 1.2 1.2 1.2 Lay-Flat (mm) 143 133 147 128 170 Average total film 147 180 138 111 83 thickness (μm) BUR 4.552 4.234 4.679 4.074 5.411 DDR 1.212 1.096 1.259 1.794 1.811 Thickness PFSA 30 25 18 10 6 precursor layer (μm)

Preparative Example 2

Nafion™ R-1000+6 wt % Silica

(37) A multilayer film was prepared by a melt-blowing process using a composite of Nafion™ R-1000+6 wt % silica as ionomeric material, and LDPE as a support material. The structure obtained in this case is a tri-layer film, where nanocomposite PFSA is the mid-layer, and LDPE the external layers (FIG. 4B).

(38) The extruder (B in FIG. 1) used for Nafion™ R-1000+6 wt % EH-5 mid-layer has the following profile temperature:

(39) Zone 1: 220° C., Zone 2: 230° C., Zone 3: 240° C., Die temperature 240° C.

(40) The two extruders (A and C in FIG. 1) used for LDPE external layers have the following profile temperature:

(41) Zone 1: 185° C., Zone 2: 190° C., Zone 3: 200° C., Die temperature 200° C.

(42) Control parameters used for the multilayer film based on nanocomposite PFSA ionomer prepared are listed in Table 2. Even though the rheological measurements show an increase in melt viscosity and a deviation from the linear viscoelastic range at low frequency after nanoparticles incorporation, the parameters were very similar since extrusion is a high frequency process.

(43) TABLE-US-00002 TABLE 2 Melt-Blowing Parameters for Nanocomposite Ionomer Precursor Film Parameter Film 6 RPM extruder A (LDPE) 99.5 RPM extruder B (PFSA) 20.3 RPM extruder C (LDPE) 100.1 Blower speed (rpm) 614 Nip roll speed (m/min) 1 Wind-up speed (m/min) 1.2 Lay-Flat (mm) 144 Average total film thickness (μm) 176 BUR 4.584 DDR 1.035 Thickness PFSA precursor layer (μm) 25

Example 3

Post-Preparation Processing of Multilayer Films

(44) The processes described in Examples 1 and 2 permit melt-blowing of a continuous multilayered cylinder. Once the cylinder is obtained, it is flattened and drawn through nip rolls to a winder. A cutting device can be used before winding to split the cylinder into two sheets that can be then wound to produce the finished rolls of films or go through chemical treatment baths in case of a continuous process.

(45) Melt blown films were hydrolyzed by peeling one LDPE layer away from the PFSA layer and following the process outlined above. The second LDPE layer was used as a support during manipulation and chemical treatment to avoid deformation and wrinkling of the very thin films. Tests were previously done to ensure that LDPE is not affected by the chemical reagents used for the hydrolysis and activation.

Example 4

Comparisons

(46) Examples 1 and 2 show that it is possible to produce thin uniform films from ion exchange resin precursor and composites of ion exchange resin precursor. Properties of melt-blown ion exchange resin membranes produced from the films of Example 1 (Film 2) and Example 2 (Film 6) were compared with Nafion™ NRE-211 (a solution-cast commercial ion exchange resin membrane) and with an extruded membrane prepared from Nafion™ R-1000-CS in a bench-top 5 cc microextruder (DSM) equipped with 5 cm width flat dies. The extruded Nafion™ R-1000-CS membranes were melt-cast directly from the flat die and rapidly quenched on a chill roll, producing a monolayer membrane. For the comparative study, the thickness of the membranes selected is about 25±5 microns.

(47) Transmission Electron Microscopy (TEM) was used to examine the morphology and arrangement of the hydrophobic/hydrophilic phase separation within the ionomeric materials. High resolution TEM images on lead acetate stained membranes prepared by solution-casting (Nafion™ NRE-211), melt-extrusion (Nafion™ R-1000) and melt-blowing (Film 2) are presented in FIG. 5. Two magnifications are shown for each membrane. The fine phase separation of hydrophilic and hydrophobic domains characteristic of PFSA ionomers is visible in all cases. For solution-cast film of NRE-211 (FIG. 5A and FIG. 5B), ordered and aligned ionic domains agglomerated in dark spheres ranging from 3 to 10 nm in diameter embedded in a pale background of hydrophobic fluoropolymers can be observed. The very clear regions could suggest free volume left by solvent evaporation. In the case of the melt-extruded membrane (FIG. 5C and FIG. 5D), ionic domains are smaller and uniform (4 to 6 nm), less ordered, and more interconnected. Melt-blown membrane (FIG. 5E and FIG. 5F) shows ionic domains having even smaller size as illustrated in the high magnification image.

(48) 2D XRD patterns obtained in transmission mode are shown in FIG. 6, where solution cast NRE-211 and melt-blown R-1000 show an isotropic halo, while extruded R-1000 shows an anisotropic halo. The spectra in FIG. 7 were obtained by integration of the XRD patterns at an incidence angle of 90°, where spectra of extruded and melt-blown membranes are compared with the NRE-211 solution-cast membrane. All the samples show two diffraction peeks at 2 theta of 17.2° (100) and 39.6° (101) corresponding to d-spacing of 5.5 Å and 2.4 Å respectively, attributed to PTFE backbone of PFSA. A more pronounced intensity can be observed for the extruded membrane compared to the melt-blown and solution-cast NRE-211 membrane, which indicates that NRE-211 and melt-blown R-1000 (Film 2) present less anisotropy compared to the extruded-R-1000 membrane as shown from the higher intensity of the amorphous halo horizontally, where the machine direction axis is vertical. The anisotropic pattern is due to higher orientation generated from stretching in the machine direction during the cast-extrusion process to achieve the required thickness. Integrating the halo at 0° and 90° incidence angle reflects the two normals to the diffraction plane relative to the MD/TD plane. To quantify the isotropy/anisotropy of the samples, we determined an orientation ratio (OR) defined as the ratio of the intensity of the peek at 2 theta=17.2° (100) integrated at 0 and 90°. OR calculated for NRE-211, melt-blown and extruded R-1000 were 1, 1.1 and >1.5, respectively.

(49) After hydrolysis according to the protocol previously described, the proton conductivity of hydrated membranes were measured by impedance spectroscopy at room temperature in water. The results in FIG. 8 show that room temperature (RT) conductivity was not affected by the process used for membrane manufacturing, and was around 7.10.sup.−2 S/cm. However, the incorporation of silica seems to slightly reduce the conductivity in water to 3.10.sup.−2 S/cm, which is typical in nanocomposite membranes. The composite membranes are expected to excel at low relative humidity (RH), where the fillers have the ability to retain more water than the ionomeric polymer at higher temperature.

(50) FIG. 9 shows water uptake (WU) and dry/wet volume change (VC) measured for solution-cast NRE-211 membrane, extruded membrane, melt-blown Nafion™ R-1000 membrane (Film 2), and melt blown nanocomposite membrane (Film 6). Membranes prepared by melt blowing show reduced water uptake and volume change compared to the NRE-211 solution-cast membrane and the extruded membrane. Thus, the process of the present invention produces ion exchange membranes with increased dimensional stability.

Example 5

Membrane-Electrode Assemblies

(51) Membrane-electrode assemblies (MEA) were prepared by placing the membranes between two electrodes (0.4 mg Pt/cm.sup.2 at the cathode and 0.1 mg Pt/cm.sup.2 at the anode) and tested in fuel cell hardware with an active area of 25 cm.sup.2. Testing was done at the facilities of the NRC-IFCI using a Scribner Associates test stand model: 850 C. Hydrogen crossover was measured by cyclic voltammetry for melt-blown Nafion™ R-1000 membranes (Film 2) and NRE-211 solution-cast membranes. The limiting current densities for hydrogen oxidation at the cathode determined were 1.89 and 2.07 mA/cm.sup.2, respectively, demonstrating that the membrane of the present invention has lower hydrogen permeability than the standard solution-cast membrane.

(52) FIGS. 10 and 11 show I-V preliminary polarization curves measured for solution-cast NRE-211 membrane, melt-blown Nafion™ R-1000 membrane (Films 2 and 3), and melt blown nanocomposite membrane (Film 6). The results show that melt blown membranes have similar beginning of life (BOL) performance compared to NRE-211 reference, with a slightly improved performance for thin melt-blown membranes (18 microns) and the composite membrane, especially at high temperature and low RH conditions (95° C., 30% RH).

REFERENCES

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(54) Other advantages that are inherent to the structure are obvious to one skilled in the art. The embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed. Variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims.