WO2014042593A1

WO2014042593A1 - Thin film composite membranes

Info

Publication number: WO2014042593A1
Application number: PCT/SG2013/000396
Authority: WO
Inventors: Gang Han; Tai-Shung Chung
Original assignee: National University Of Singapore
Priority date: 2012-09-11
Filing date: 2013-09-11
Publication date: 2014-03-20

Abstract

Disclosed is a polymeric support layer that is free of macrovoids and has a thickness of 10 to 500 µm and a mean pore size of 5 to 50 nm. The polymeric support, preferably a polyimide support, can be coated with a polyamide thin film layer having a thickness of 50 to 300 nm to form a composite membrane. Also disclosed are methods of preparing the above-described polymeric support and composite membrane.

Description

THIN FILM COMPOSITE MEMBRANES

BACKGROUND

Pressure retarded osmosis (PRO) is a promising technology for harvesting osmotic power, a potential source of renewable energy without greenhouse gas emissions.

In a typical PRO process, river water is separated from pressurized seawater by a semi-permeable membrane. Osmotic power is generated as a result of a transmembrane pressure. The power can be collected by feeding a pressurized stream to a hydro-turbine to generate electricity.

The membrane used in a PRO process must withstand high pressures and also have a high water permeability and an acceptable salt rejection rate. Few commercially available membranes have met that all these requirements.

There is a need to develop a durable and high-performance PRO membrane.

SUMMARY

This invention provides, among others, a thin film composite membrane that is unexpectedly durable and has an unexpectedly high water flux rate and an acceptable salt rejection rate, thus suitable for osmotic power generation via a PRO process.

One aspect of this invention relates to a polymeric support that is free of macrovoids and has a thickness of 10 to 500 μπι (e.g., 50-300 μιτι) and a mean pore size of 5 to 50 nm (e.g., 10-35 nm). Examples of the polymeric support include a polyimide support, a polyamide-imide support, a polysulfone support, and a polyethersulfone support.

In one embodiment, the polymeric support has a toughness rate of higher than 1.5 l0⁶ Jm^"3.

Also within the scope of this invention is a composite membrane that contains the just-mentioned polymeric support, as a support layer and a polyamide thin film layer, having a thickness of 50 to 300 nm (e.g., 100-200 nm), adhered to the polymeric support layer. Preferably, the composite membrane has a water permeability rate of higher than 2.0 LnvVbar^"1 and a salt permeability rate of lower than 3.0 Lm^"2h^"'. Yet another aspect of this invention relates to a method of preparing a polymeric support. The method includes the following steps: (i) mixing 10 to 25 wt% of a polymer and 5 to 25 wt% of a pore former in a solvent to form a polymer solution, (ii) casting the polymer solution on a plate, e.g., a glass plate, and (iii) immersing the plate in water to coagulate the polymer and deplete the pore former and the solvent from the polymer solution, thus forming a polymeric support.

The above-described method can include a subsequent step of casting a 0.5 to 10 wt% (e.g., 1 to 5 wt%) m-phenylenediamine (MPD) aqueous solution on a surface of the polymeric support and then casting a 0.01 to 10 wt% (e.g., 0.1 to 5 wt%) trimesoyl chloride (TMC) hexane solution on top of the MPD aqueous solution. A composite membrane, i.e., a polymeric support coated with a polyamide thin film layer, is thus formed.

If desired, one can further (i) immerse the composite membrane in an active chlorine solution for 1 to 60 minutes, (ii) immerse the active chlorine solution-treated composite membrane in a washing solution for 2 to 180 minutes, and finally (iii) immerse the washed composite membrane in a rinsing solution for at least 10 hours. The composite membrane thus treated has an improved water permeability.

The details of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

DETAILED DESCRIPTION

The polymeric support of this invention has a mean pore size of 5 to 50 nm. "Mean pore size" is the average size of the pores in the polymeric support. It is measured by solute rejection assays using a neutral solute such as polyethylene glycol or polyethylene oxide. See, e.g., Han et al. J. Membr. Sci. 440 (2013) 108-121.

Further, the polymeric support is free of macro voids, namely, it contains no more than 3 macro voids per μιτι³. A macro void is an undesirable open space in the support that

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has a volume of at least 10 nm . It is significantly larger in volumn than the pores in the polymeric support. A macro void can be a finger-like big pore formed during preparation of the support. Typically, the polymeric support has a surface water contact angle of 40° to 80°, a toughness rate of higher than 1.5 χ 106 Jm^'3, a pure water permeability rate of higher than 500 Lm^rf'bar^"1, and a surface roughness rate of lower than 60 nm.

Methods for measuring a surface water contact angle, a toughness rate, a pure water permeability rate, and a surface roughness rate can be found in Han et al., Chem. Eng. Sci. 80 (2012) 219-231.

The polymeric support can be coated with a polyamide thin film layer to form a composite membrane. More specifically, the composite membrane thus formed contains both a polymeric support layer having a thickness of 10 to 500 μιτι and a polyamide thin film layer having a thickness of 50 to 300 nm.

Preferably, the composite membrane contains a support layer having a toughness rate of higher than 1.5 χ 10⁶ Jm^"3.

In one embodiment, the composite membrane has a polyimide support layer and a polyamide thin film layer.

The composite membrane of this invention can be a flat sheet, a hollow fiber, or of any other desired shape. In addition to the PRO process, it can also be used in other applications, such as nanofiltration, reverse osmosis (RO), and forward osmosis (FO).

Described below are exemplary procedures of preparing a polymeric support and a composite membrane of this invention.

As stated above, the polymeric support can be prepared in three steps: First, a polymer and a pore former are mixed in a solvent to form a polymer solution. Second, the polymer solution is cast on a plate. Third, the plate having the polymer solution cast thereon is immersed in water. Water is miscible with the polymer solvent, but does not dissolve the polymer. As such, the polymer starts to coagulate to form the polymeric support as a result of the solvent being gradually mixed with the water and depleted from the polymer solution. The pore former, while dissolvable in water, acts at this stage to generate pores in the polymeric support to be formed and is washed away with the water afterwards.

Examples of the polymer include a polyimide, a polyamide-imide, a

polysulfone, and a polyethersulfone. Examples of the pore former includes alkylene glycols, such as diethylene glycol and polyethylene glycol. Examples of the solvent include dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethylsulfoxide, and l,3-dimethyl-2-imidazolidinone.

The polymeric support thus prepared can be coated on its surface with a polyamide thin film layer via interfacial polymerization between MPD and TMC. To achieve this, the polymeric support can be fixed in a frame so that only one surface of the support is exposed to the reactants. First, a 0.5 to 10 wt% MPD aqueous solution is cast on the exposed surface of the support. Then, excess water droplets on the surface are removed by a filter paper. Subsequently, a 0.01 to 10 wt% TMC hexane solution is cast on top of the MPD aqueous solution to initiate polymerization. A composite membrane, i.e., a polymeric support coated with a polyamide thin film layer, is thus formed.

The composite membrane can be post-treated as follows. It is first immersed in an active chlorine solution for 1 to 60 minutes, then immersed in a washing solution for 2 to 180 minutes, and finally immersed in a rinsing solution for at least 10 hours.

Examples of the active chlorine solution include NaOCl, HOC1, Ca(OCl)₂, LiOCl, and other alkaline hypochlorite solution; examples of the washing solution include

NaHS0₃, NaHC0₃, and Na₂S₂0₃; and examples of the rinsing solution include methanol, ethanol, acetone, tetrahydrofuran, and a mixture thereof.

The specific examples below are to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present invention to its fullest extent. All publications cited herein are incorporated by reference in their entirety. Example 1 -

Preparation of a polymeric support.

A polymeric support was prepared by the conventional non-solvent induced

Loeb-Sourirajan wet phase inversion method using a polyimide polymer following the detailed procedure below.

A polymer solution was prepared by mixing 18 wt%,polyimide polymer

(Matrimid® 5218, Vantico Inc.) and 17 wt% diethylene glycol in N-methyl-2- pyrrolidone at room temperature. The polymer solution was then cast on a clean glass plate. The entire assembly was immersed into a water bath at room temperature and the polyimide polymer started to coagulate resulting in a polyimide support with a thickness of about 50 μιη. The polyimide support was later removed from the water bath and washed thoroughly with deionized water.

Scanning electron microscope (SEM) images of the prepared polyimide support showed unexpectedly that it was free of macro voids, and no trace of big pores on the surface, with a fully sponge-like cross-section structure. This

morphology helped subsequent formation on a surface of the support of a polyamide thin film layer with an unexpectedly high water permeability rate and other superior properties. See, e.g., Han et al, J. Membr. Sci. 440 (2013) 108-121 ; Widjojo et al, J. Membr. Sci. 383 (201 1) 214-223; Han et al, Chem. Eng. Sci. 80 (2012) 219-231 ; and Li et al, Ind. Eng. Chem. Res. (2012), DOI: 10.1021/ie2027052.

Several properties of a prepared polyimide support were determined: pure water

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permeation: 545.7 Lm^" h^" bar^" molecular weight cut-off: 414.6 kDa, mean pore size: 22 nm, surface water contact angle: 61.5+3.5, and porosity: 75.2%.

Example 2

Preparation and post-treatment of thin film composite membranes.

A polyamide thin film layer was coated on top of the polymeric support prepared in Example lvia an interfacial polymerization reaction between MPD and TMC. To achieve this, the polymeric support was fixed in a frame so that only one surface of the support was exposed to the reactants. A 2 wt% MPD aqueous solution was cast on the surface of the support. Excess water droplets on the surface were removed by a filter paper. Subsequently, a 0.2 wt% TMC hexane solution was cast on top of the MPD aqueous solution to initiate polymerization. A thin film composite membrane, i.e., a polymeric support coated with a polyamide thin film layer, was thus obtained.

A post-treatment procedure was used to modify the water and salt

permeability of the composite membranes. Three batches of thin film composite (TFC) membranes, i.e., TFC, TFC200, and TFC600, with different water and salt permeability rates were prepared. The TFC membranes were not subjected to any post-treatment. The TFC200 and TFC600 membranes were first immersed for 7 min in fresh 200 ppm and 600 ppm NaOCl aqueous solutions, respectively. The membranes were then placed in a 1000 ppm NaHS03 aqueous solution and soaked for 5 min. Finally, the membranes were immersed in methanol overnight and the post-treated membranes were rinsed thoroughly and stored in deionized water at

room temperature.

Table 1 below summarizes four mechanical properties of a polyimide support and a TFC200 membrane: Young's modulus (for rigidity), tensile strength (for

stretch resistance), elongation at break (for ductile capability), and toughness (for the overall strength and robustness). Both the polyimide support and the TFC200

membrane exhibited excellent results in all four categories.

Table 1. Mechanical properties of the polymeric support and TFC200 membrane

Membrane ID Young's modulus Tensile strength Elongation at Toughness

(MPa) (MPa) break (%) ( ioe j m³)

Polymeric support 290.6±21.4 8.3±0.3 27.1±3.0 1.82±0.11

TFC200 membrane 290±62.8 7.9±0 7 37.1-.KU 2.72±0.56

'Toughness was calculated bv takina the inteeral underneath the stress-strain curve Both SEM and Atomic force microscopy (AFM) images of the prepared

composite membranes showed that the polyamide thin film layer had a special "flake- like" surface morphology, which is different from the typical "ridge-and-valley"

morphology. The thin film layer had a rather flat structure without small upward

leaf-like "ridges." The layer was very thin with an average thickness of 155 nm,

which helped to achieve an unexpectedly high water flux rate for the composite

membrane.

The durability of the composite membrane under a high-pressure PRO process was also tested. A TFC200 membrane was pressurized at 15 bar in a PRO process.

Images from both SEM and AFM showed little change in the surface morphology of the membrane before and after the test.

Table 2 below shows that post-treating composite membranes improved their water permeability rates. A TFC membrane, which was not post-treated, had a water permeability rate of 1.47 Lm^h 'bar^"1 (LMH/bar) and a salt permeability rate of 0.23 Lm^"2h^_I (LMH). A post-treated TFC200 membrane unexpectedly exhibited an about 4-fold increase in the water permeability rate (5.34 LMH/bar) and a post-treated TFC600 membrane exhibited an ever higher water permeability rate (10.03

LMH bar).

As shown in Table 2, post-treatment also increased salt permeability.

Table 2. Performance comparison of post-treated composite membranes

FO performance⁶

Membrane Water permeability Salt permeability

(LMH/bar) (L H)

TFC 1.47 0.23

TFC200 5.34 2.00

TFC600 10.03 5.40

' Determined by permeate flux measurements in RO tests at 1-4 bar with a deiormed water feed solution at 25 "C.

* Water flux and reverse salt flux were measured in FO tests under the PRO mode with a 1.0 M NaCl draw solution and a deioni2ed water feed solution at 25 °C.

Note the post-treatment also improved FO water flux rates of the composite membranes.

OTHER EMBODIMENTS

All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.

Claims

What is claimed is:

1. A polymeric support, wherein the support is free of macrovoids and has a thickness of 10 to 500 μη and a mean pore size of 5 to 50 nm.

2. The polymeric support of claim 1 , wherein the support has a thickness of 50 to 300 μηι and a mean pore size of 10 to 35 nm.

3. The polymeric support of claim 1 , wherein the support is a polyimide support.

4. The polymeric support of claim 1 , wherein the support is hydrophilic with a surface water contact angle of 40° to 80° and has a toughness rate of higher than 1.5 x 10⁶ Jm^"3, a pure water permeability rate of higher than 500 Lm^' 'bar^"1, and a surface roughness rate of lower than 60 nm.

5. A composite membrane comprising:

a polymeric support layer having a thickness of 10 to 500 μπι, and a polyamide thin film layer having a thickness of 50 to 300 nm, wherein the thin film layer is adhered to the support layer, and the support layer has a mean pore size of 5 to 50 nm and is free of macrovoids.

6. The composite membrane of claim 5, wherein the support layer has a toughness rate of higher than 1.5 ^χ 10⁶ Jm^"3.

7. The composite membrane of claim 5, wherein the composite membrane has a water permeability rate of higher than 2.0 Lm^"2h^"'ba ¹ and a salt permeability rate of lower than 3.0 Lm^"2h^"'.

8. The composite membrane of claim 5, wherein the support layer is a polyimide support layer, a polyamide-imide support layer, a polysulfone support layer, or a polyethersulfone support layer.

9. The composite membrane of claim 8, wherein the support layer is a polyimide support layer.

10. The composite membrane of claim 8, wherein the composite membrane has a water permeability rate of higher than 2.0 Lnrti 'bar^"1 and a salt permeability rate of lower than 3.0 Lm^"2h^"'.

1 1. The composite membrane of claim 10, wherein the support layer is a polyimide support layer.

12. The composite membrane of claim 10, wherein the thin film layer has a thickness of 100 to 200 nm, and the support layer has a thickness of 50 to 300 μιη and a mean pore size of 10 to 35 nm.

13. The composite membrane of claim 12, wherein the support layer is a polyimide support layer.

14. The composite membrane of claim 5, wherein the thin film layer has a thickness of 100 to 200 nm, and the support layer has a thickness of 50 to 300 μιη and a mean pore size of 10 to 35 nm.

15. A method of preparing a membrane structure, the method comprising: mixing 10 to 25 wt% of a polymer and 5 to 25 wt% of a pore former in a solvent to form a polymer solution,

casting the polymer solution on a plate, and

immersing the plate in water to coagulate the polymer and deplete the pore former and the solvent from the polymer solution, whereby a support layer is formed.

16. The method of claim 15, wherein the pore former is an alkylene glycol.

17. The method of claim 16, wherein the pore former is diethylene, glycol.

18. The method of claim 17, wherein the solvent is N-methyl-2-pyrrolidone and the polymer is a polyimide.

19. The method of claim 15, wherein the solvent is dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethylsulfoxide, or 1,3 -dimethyl -2- imidazolidinone.

20. The method of claim 19, wherein the polymer is a polyimide, a polyamide-imide, a polysulfone, or a polyethersulfone.

21. The method of claim 15, wherein the polymer is a polyimide, a polyamide-imide, a polysulfone, or a polyethersulfone.

22. The method of claim 21 , wherein the polymer is a polyimide.

23. The method of claim 15, further comprising casting a 0.5 to 10 wt% m- phenylenediamine (MPD) aqueous solution on a surface of the support layer and then casting a 0.01 to 10 wt% trimesoyl chloride hexane solution on top of the MPD aqueous solution, whereby a membrane structure having a support layer coated with a polyamide thin film layer is formed.

24. The method of claim 23, wherein the MPD aqueous solution is 1 to 5 wt% and trimesoyl chloride hexane solution is 0.1 to 5 wt%.

25. The method of claim 23, further comprising:

immersing the membrane structure in an active chlorine solution for 1 to

60 minutes,

immersing the active chlorine solution-treated membrane structure in a washing solution for 2 to 180 minutes, and

immersing the washed membrane structure in a rinsing solution for at least 10 hours.