Technical Field

The present invention relates to a smooth thin film composite membrane and a method of forming the same.

Background

Thin film composite (TFC) polyamide membranes have long been used for reverse osmosis and nanofiltration. The membranes are typically prepared by immersing the support in an aqueous diamine solution, removing excess water and then applying hexane solution containing trimesoyl chloride. The resultant membranes are characterized with ridge and valley structure on the surface, and have a high surface roughness, typically in the range of 50 to 150 nm. High surface roughness is disadvantageous for such membranes as they lead to higher fouling. There has therefore been a shift towards the use of smooth TFC membranes. The problem with smooth TFC membranes is that there is lack of a reliable and scalable process for fabricating TFC membranes. Most of the existing methods of preparing smooth TFC membranes are complicated, are difficult to scale up and/or do not result in good adhesion between the support and selective layers of the TFC membrane. Improving the adhesion between the support layer and the selective layer requires complicated and tedious pre- or post-treatment steps.

There is therefore a need for an improved smooth TFC membrane and an improved method of forming the smooth TFC membrane which is low-cost, simple and easily scalable.

Summary of the invention The present invention seeks to address these problems, and/or to provide an improved smooth thin film composite (TFC) membrane and an improved method of preparing a smooth TFC membrane.

According to a first aspect, the present invention provides a thin film composite (TFC) membrane comprising: a support layer; and a selective layer on a surface of the support layer, the selective layer formed of a cross-linked polyamide, wherein the TFC membrane has a surface roughness of £ 20 nm.

According to a particular aspect, the support layer may be a porous support layer.

According to a particular aspect, the support layer may comprise a polymer. For example, the polymer may comprise, but is not limited to, polyethersulfone (PES), polysulfone (PSF), polyacrylonitrile (PAN), polypropylene (PP), polyethylene (PE), polystyrene (PS), polycarbonate (PC), polyvinylidene difluoride (PVDF), polyimide (PI), polyamide-imide (PAI), or blended polymers thereof.

The support layer may have a suitable thickness. For example, the support layer may have a thickness of 1-10,000 mhi.

The selective layer may have a suitable thickness. For example, the selective layer may have a thickness of 1-1000 nm.

The TFC membrane may have suitable properties. For example, the TFC membrane may have a pure water permeability rate of > 2 LMH/bar. The TFC membrane may have a salt rejection rate of > 90%.

The TFC membrane may be used for any suitable application. In particular, the TFC membrane may be for use in organic solvent nanofiltration, reverse osmosis, or nanofiltration.

According to a second aspect, the present invention provides a method of forming the TFC membrane according to the first aspect, the method comprising: providing a support layer in a first solution comprising a polyfunctional amine; adding a second solution comprising a polyfunctional acyl halide and an organic solvent to the first solution to enable formation of a selective layer on a surface of the support layer through interfacial polymerization; removing the TFC membrane after a pre-determined period of time; and drying the TFC membrane.

In particular, the support layer may be as described above in relation to the first aspect. According to a particular aspect, the polyfunctional amine comprised in the first solution may be, but is not limited to: m-phenylenediamine (MPD), p-phenylenediamine, p- xylylenediamine, cyclohexanediamine, piperazine, branched or dendrimeric polyethylenimine, or a combination thereof. The first solution may comprise a suitable amount of polyfunctional amine. In particular, the first solution may comprise 0.01-50 wt% polyfunctional amine.

According to a particular aspect, the polyfunctional acyl halide comprised in the second solution may be, but is not limited to: trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, 1,3,5-cyclohexane tricarbonyl chloride, 1,2,3,4-cyclohexane tetracarbonyl chloride, or a combination thereof.

The second solution may comprise a suitable amount of polyfunctional acyl halide. In particular, the second solution may comprise 0.01-50% (wt/volume ratio) polyfunctional acyl halide.

The organic solvent comprised in the second solution may be any suitable organic solvent. According to a particular aspect, the organic solvent may be, but is not limited to: hexane, heptane, cyclohexane, isoparaffinic hydrocarbon, or a mixture thereof.

The removing may be after a suitable pre-determined period of time. For example, the pre-determined period of time may be 0.1-1000 minutes.

The drying may be by any suitable means. According to a particular aspect, the drying may comprise air drying. The drying may comprise drying the TFC membrane for a suitable period of time. For example, the drying may be for 0.1-1000 hours.

The method may further comprise treating the TFC membrane. The treating may comprise any suitable post-treatment of the TFC membrane. According to a particular aspect, the treating may comprise washing the TFC membrane in a solution, wherein the solution may be an alkaline-containing, alcohol-containing, oxidising agent-containing solution, or a mixture thereof. In particular, the solution may be an alkaline-containing solution. Even more in particular, the solution may comprise, but is not limited to: NaOH, glycerol, sodium hypochlorite, or a mixture thereof. Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

Figure 1 shows a schematic representation of the synthesis of the TFC membrane according to one embodiment of the present invention;

Figure 2(a) shows FESEM image of a TFC polyamide membrane fabricated using a polysulfone support via conventional interfacial polymerization at a supported interface using 2 wt% MPD and 0.15 % TMC (w/v ratio), Figure 2(b) shows the FESEM image of a front side (facing TMC solution) of a TFC membrane prepared according to the method of the present invention, Figure 2(c) shows FESEM image of the back side (facing MPD solution) of the TFC membrane prepared according to the method of the present invention, Figure 2(d) shows the AFM image of the membrane fabricated via conventional interfacial polymerization, Figure 2(e) shows AFM image of the front side (facing TMC solution) of a TFC membrane prepared according to the method of the present invention and Figure 2(f) shows the AFM image of the back side (facing MPD solution) of the TFC membrane prepared according to the method of the present invention; Figure 3(A) shows the reverse osmosis desalination performance of TFC membrane according to one embodiment of the present invention with different annealing time at 25°C. The annealing duration is 3 hours, 20 min, 10 min, 5 min, separately (from right to left). Figure 3(B) shows the performance of the membranes at different annealing temperatures; Figure 4(a) shows a plot of the water permeance and NaCI rejection of TFC composite membranes versus the pure water permeance of support membranes and Figure 4(b) shows the contact angle data of the support membranes; and

Figure 5(a) shows the reverse osmosis desalination performance of TFC composite membranes using a polysulfone support containing different SDS concentrations. Figure 5(b) shows the variation of the TFC membrane thickness versus the SDS concentrations included in MPD solution and Figure 5(c) shows the variation of the TFC membrane roughness versus the SDS concentration included in MPD solution.

Detailed Description

As explained above, there is a need for an improved TFC membrane which has a lower surface roughness so that there is lower fouling of the membrane.

In general terms, the present invention provides a smooth thin film composite (TFC) membrane with a lower surface roughness, thereby being more fouling resistant. The present invention also provides an improved method of forming the TFC membrane which is less complicated and easy to scale up, and wherein no pre-treatment steps are required to prime the support layer. Further, the method of the present invention does not require addition of toxic or expensive chemicals. The method is also a fast method, thereby shortening the time for fabricating the membrane as compared to convention TFC membrane fabrication processes.

According to a first aspect, the present invention provides a thin film composite (TFC) membrane comprising: a support layer; and a selective layer on a surface of the support layer, the selective layer formed of a cross-linked polyamide, wherein the TFC membrane has a surface roughness of £ 20 nm.

The TFC membrane may have a smooth surface. In particular, the TFC membrane may have a surface roughness 0.1-20 nm. For example, the surface roughness of the TFC membrane may be 0.5-18 nm, 0.6-15 nm, 0.8-12 nm, 1-10 nm, 1.5-9.5 nm, 2-9 nm, 2.5- 8.5 nm, 3-8 nm, 3.5-7.5 nm, 4-7 nm, 4.5-6.5 nm, 5-6 nm. Even more in particular, the surface roughness may be £ 10 nm, particularly about 7-9 nm.

The TFC membrane may be any suitable TFC membrane. For example, the TFC membrane may be, but not limited to, a flat sheet membrane, a tubular membrane, or a hollow fibre membrane, such as an outer selective hollow fibre membrane.

According to a particular aspect, the support layer may be a porous support layer. The support layer may comprise any suitable material. According to a particular aspect, the support layer may comprise a polymeric material. The polymeric material may comprise any suitable polymer. For example, the polymer may comprise, but is not limited to, polyethersulfone (PES), polysulfone (PSF), polyacrylonitrile (PAN), polypropylene (PP), polyethylene (PE), polystyrene (PS), polycarbonate (PC), polyvinylidene difluoride (PVDF), polyimide (PI), polyamide-imide (PAI), or blended polymers thereof. In particular, the support layer may comprise polypropylene (PP). The advantage of using PP is that it is a cheap and readily available material. PP is also solvent-resistant which does not require additional cross-linking. The support layer may be of a suitable thickness. According to a particular aspect, the thickness of the support layer may be 1-10,000 mhi. For example, the thickness of the support layer may be 10-10,000 mhi, 20-9000 mhi, 30-8000 mhi, 50-7000 mhi, 75-5000 mGh, 100-4500 mhΐ, 150-4000 mhΐ, 200-3500 mGP. 250-3000 mhΐ, 300-1500 mhΐ, 500-1000 mGh, 700-800 mGP. In particular, the thickness may be 10-10,000 mhi. Even more in particular, the thickness may be 10-100 mhi.

The selective layer may be of a suitable thickness. In particular, the thickness of the selective layer may be 1-1000 nm. For example, the thickness may be 5-800 nm, 10-700 nm, 20-600 nm, 50-500 nm, 75-400 nm, 100-300 nm, 150-250 nm, 175-200 nm. In particular, the thickness may be about 2-50 nm. The membrane according to the first aspect also has a suitably high pure water permeability rate and high salt rejection rate. The calculations of the water permeability rate and salt rejection rate are described in detail with specific reference to a particular TFC membrane in the Example section. However, it would be understood by a person skilled in the art that the calculations may apply to other TFC membranes within the scope of the present invention, and not necessarily restricted to the specific support layer and selective layer described in the Example.

According to a particular aspect, the TFC membrane may have a high pure water permeability rate of > 2 LMH/bar. In particular, the pure water permeability rate may be 2-100 LMH/bar, 2.2-75 LMH/bar, 2.3-50 LMH/bar, 2.5-25 LMH/bar, 2.8-20 LMH/bar, 3.0- 10 LMH/bar, 3.5-8 LMH/bar, 4.0-7 LMH/bar, 4.5-5 LMH/bar. Even more in particular, when the support layer comprises PP, the pure water permeability rate may be about 2 LMH/bar.

The TFC membrane may have a favourably high salt rejection rate of ³ 90%. In particular, the salt rejection rate may be 90.0-99.9%, 90.5-99.0%, 91.0-98.5%, 91.5-98.0%, 92.0- 97.5%, 92.5-97.0%, 93.0-96.5%, 93.5-96.0%, 94.0-95.5%, 94.5-95.0%. Even more in particular, when the support layer comprises PP, the salt rejection rate may be about 92%.

The TFC membrane may be used for any suitable application. In particular, the TFC membrane may be for use in, but not limited to, organic solvent nanofiltration, reverse osmosis, or nanofiltration. Even more in particular, the TFC membrane may be for use in reverse osmosis desalination.

According to a second aspect, the present invention provides a method of forming the TFC membrane according to the first aspect, the method comprising: providing a support layer in a first solution comprising a polyfunctional amine; adding a second solution comprising a polyfunctional acyl halide and an organic solvent to the first solution to enable formation of a selective layer on a surface of the support layer through interfacial polymerization; removing the TFC membrane after a pre-determined period of time; and drying the TFC membrane.

The support layer may be as described above. In particular, the support layer may be a polymeric support layer. Even more in particular, the support layer may comprise polypropylene (PP).

The first solution may comprise any suitable polyfunctional amine. For the purposes of the present invention, a polyfunctional amine may be defined as an organic compound comprising one or more amine groups. For example, the polyfunctional amine may be, but not limited to, m-phenylenediamine (MPD), p-phenylenediamine, p-xylylenediamine, cyclohexanediamine, piperazine, branched or dendrimeric polyethylenimine, or a combination thereof. In particular, the polyfunctional amine comprised in the first solution may be MPD. The first solution may comprise a suitable amount of polyfunctional amine. According to a particular aspect, the first solution may comprise 0.01-50.0 wt % polyfunctional amine based on the total weight of the first solution. For example, the first solution may comprise 0.05-45.0 wt %, 0.1-40.0 wt %, 0.5-30.0 wt %, 1.0-25.0 wt %, 1.5-20.0 wt %, 2.0-15.0 wt %, 2.5-10.0 wt %, 3.0-7.0 wt %, 3.5-5.0 wt % polyfunctional amine based on the total weight of the first solution. In particular, the first solution may comprise 0.01-10.0 wt % polyfunctional amine based on the total weight of the first solution. Even more in particular, the first solution may comprise 0.1 -3.0 wt % polyfunctional amine based on the total weight of the first solution. The second solution may comprise any suitable polyfunctional acyl halide. For the purposes of the present invention, a polyfunctional acyl halide may be defined as an organic compound consisting of one or more acid halide groups. For example, the polyfunctional acyl halide may be, but is not limited to, trimesoyl chloride (TMC), isophthaloyl chloride, terephthaloyl chloride, 1,3,5-cyclohexane tricarbonyl chloride, 1,2,3,4-cyclohexane tetracarbonyl chloride, or a combination thereof. In particular, the polyfunctional acyl halide may be TMC.

The second solution may comprise a suitable amount of polyfunctional acyl halide. According to a particular aspect, the second solution may comprise 0.01-50.0 wt/volume (w/v) ratio polyfunctional acyl halide based on the total weight of the second solution. For example, the second solution may comprise 0.05-45.0 w/v ratio, 0.1-40.0 w/v ratio, 0.5-30.0 w/v ratio, 1.0-25.0 w/v ratio, 1.5-20.0 w/v ratio, 2.0-15.0 w/v ratio, 2.5-10.0 w/v ratio, 3.0-7.0 w/v ratio, 3.5-5.0 w/v ratio polyfunctional acyl halide based on the total weight of the second solution. In particular, the second solution may comprise 0.01-10.0 w/v ratio polyfunctional acyl halide based on the total weight of the second solution. Even more in particular, the second solution may comprise 0.15 w/v ratio polyfunctional acyl halide based on the total weight of the second solution.

The organic solvent comprised in the second solution may be any suitable organic solvent. For example, the organic solvent may be, but not limited to, hexane, heptane, cyclohexane, isoparaffinic hydrocarbon, or a mixture thereof. In particular, the organic solvent may be hexane.

According to a particular embodiment, the first solution may comprise 0.01-10.0 wt % MPD. According to a particular embodiment, the second solution may comprise 0.01-10 w/v ratio of TMC. In particular, the second solution may comprise 0.01-10 w/v ratio TMC and hexane. Even more in particular, the second solution may comprise 0.15 w/v ratio TMC in hexane. According to a particular embodiment, the first solution may comprise MPD and the second solution may comprise TMC and hexane. For example, the first solution may comprise 0.1-3.0 wt % MPD aqueous solution and the second solution may comprise 0.15 w/v ratio TMC in hexane.

The adding a second solution may comprise introducing the second solution to the surface of the support layer saturated with the first solution for a pre-determined period of time to form the selective layer on the support layer. Following the pre-determined period of time, the TFC membrane comprising the support layer and the selective layer formed on the support layer may be removed from the mixture of the first solution and the second solution. The pre-determined period of time may be any suitable period of time. The pre determined period of time may be 0.1-1000 minutes. For example, the pre-determined period of time may be 0.2-750 minutes, 0.3-500 minutes, 0.4-250 minutes, 0.5-100 minutes, 1-90 minutes, 2-75 minutes, 3-60 minutes, 5-45 minutes, 6-30 minutes, 8-25 minutes, 10-20 minutes, 12-15 minutes. In particular, the pre-determined period of time may be 0.1-100 minutes. Even more in particular, the pre-determined period of time may be 1-10 minutes.

The drying may be by any suitable means. According to a particular aspect, the drying may comprise air drying. The drying may comprise drying the TFC membrane for a suitable period of time. The drying may be for 0.1-1000 hours. For example, the drying may be for 0.2-750 hours, 0.3-500 hours, 0.4-250 hours, 0.5-100 hours, 1-90 hours, 2- 75 hours, 3-60 hours, 4-45 hours, 5-40 hours, 10-30 hours, 15-25 hours, 17-20 hours. In particular, the drying may be for 0.1-100 hours. Even more in particular, the drying may be for about 3 hours.

The method may further comprise treating the TFC membrane. The treating may comprise any suitable post-treatment of the TFC membrane. According to a particular aspect, the treating may comprise washing the TFC membrane in a solution, wherein the solution may be an alkaline-containing, alcohol-containing, oxidising agent-containing solution, or a mixture thereof. In particular, the solution may be an alkaline-containing solution. Even more in particular, the solution may comprise, but is not limited to: NaOH, glycerol, sodium hypochlorite, or a mixture thereof. The TFC membrane of the present invention and formed from the method of the present invention may exhibit a lower surface roughness with comparable or improved water permeability as compared to conventional TFC membranes without losing salt rejection.

In particular, the method of the present invention enables the use of hydrophobic PP as a support layer. The advantage of using PP as a support layer is that it is cheap and solvent resistant, thereby making the TFC membrane formed more suitable for a wide range of applications. Further, in view of the low cost of PP, the overall cost of producing the TFC membrane is also reduced, thereby providing a low-cost solvent resistant membrane.

Another advantage of the present invention is that it provides a direct interfacial polymerization method which leads to the formation of TFC membranes. In particular, the direct interfacial polymerization between the polyfunctional amine containing first solution and the polyfunctional acyl halide containing second solution enables formation of a thin polyamide film at the interface of the support layer and the subsequent attachment of the thin film on the support layer. In this way, support-independent growth of thin films is achieved which results in the TFC membrane having a smoother surface, thereby having a lower fouling rate.

Having now generally described the invention, the same will be more readily understood through reference to the following embodiment which is provided by way of illustration, and is not intended to be limiting. Examples

Example 1

Preparation of TFC membrane

Thin film composite (TFC) membranes were prepared by placing a polypropylene support membrane in m-phenylenediamine (MPD) aqueous solution first and then gently pouring hexane solution containing trimesoyl chloride (TMC) on top. The MPD concentration was 0.1 wt %, and the TMC concentration was 0.15 % (weight/volume ratio). After a reaction time of 1 to 10 min, the support was lifted up and left to air dry for 3 hours. The membrane was then treated in a 1000 ppm NaOH solution for 10 min. A schematic representation of the preparation method is shown in Figure 1.

Organic solvent nanofiltration (OSN) and reverse osmosis (RO) tests

The solvent permeability (A, L. nr ² . IT ¹ . bar ¹ ) and solute rejection (R, %) in RO were tested with 1000 ppm NaCI solution at 15 bar with a cross-flow filtration system, and the relevant values in OSN were tested with 50 ppm dye solutions at 5 bar with a dead-end filtration cell.

The permeability of each membrane was determined by dividing the permeate volume ( V) with the membrane area (S, 13.2 cm ² ), filtration time (t) and transmembrane pressure (D P) according to the following equation:

The rejection rate (R) of salts was calculated from the following equation, where C _p and C _f correspond to the solute concentrations in the permeate and feed solutions, respectively:

Solute concentration was determined by conductivity measurement for NaCI solutions and by UV-Vis absorption for dye solutions.

Surface characterizations of the TFC membrane

The membrane surface morphology was examined with field emission scanning electron microscopy (FESEM, JEOL JSM-6700). The membrane was first dried with a freeze- dryer and then coated with platinum by Cressington sputter coater ion 208 HR. The membrane surface roughness was measured with atomic force microscope (AFM, Agilent Technologies, Santa Clara, CA) under tapping mode. The AFM images were processed with NanoScope Analysis 1.50. The contact angle as well as the surface energy of the membranes was calculated with a goniometer (VCA Optima, AST Products Inc.) at room temperature. The element compositions of membrane surfaces were analyzed using X-ray photoelectron spectroscopy (XPS, Kratos AXIS UltraDLD, Kratos Analytical Ltd., England) equipped with a monochromatized AIK a X-ray source (1486.71 eV, 5 mA, 15 kV). A Vision Procession software was used to determine the atomic concentrations on the membrane surface. Figures 2(a) to (d) compare the scanning electron microscope (SEM) and atomic force microscope (AFM) images of TFC membranes with TFC membrane prepared via conventional and free-standing interfacial polymerization methods (as explained in J Duan et al, Journal of Membrane Science, 2015, 473:157-164). It can be seen from Figure 2 that while the conventional membrane showed typical ridge and valley structure with high surface roughness of about 200 nm, the TFC membrane prepared from the free-standing approach according to the present invention was much smoother with a surface roughness of £ 10 nm.

Impact of post-annealing on membrane performance

A drying process was required after the attachment of free-standing thin film onto the support layer to ensure good adhesion between the two layers. Figure 3 demonstrates that the drying time and temperature are critical for membrane performance in RO. When the membrane was left to air dry, a drying duration of 3 hours was needed to prevent the polyamide film from detachment from the support. At increasing drying temperatures (where the drying duration for each temperature was optimized), the membrane permeability decreased accompanied by slight reduction in NaCI rejection as well. This may be due to the different rates of water evaporation on the surface and beneath. Due to the low transport resistance on the surface, water evaporated fast into the air. With increasing transport resistance along the depth of membrane cross-section, the evaporation rate decreased. At higher temperature, the difference became more significant, leading to dramatically different shrinking rate of the film and support. As a result, defects were formed that reduced the rejection. In addition, the fast water evaporation on the surface at higher temperature led to closure of surface pores and reduced the water permeability. At 120°C, surface pores collapsed completely, thereby blocking water from passing through the membrane. Air drying provides the best match between the two layers and reserves most surface pores, providing the highest water permeability and salt rejection. TFC membrane on different support

TFC membranes were fabricated from a first solution comprising 0.1% MPD and a second solution comprising 0.15% TMC under identical conditions and subsequently attached onto different polymeric supports (i.e. PES, PSF, PAN, PP). The RO performance of the TFC membrane prepared on different supports was measured. The supporting membranes were used directly without any post-modification. All membranes showed water permeability > 2 LMH/bar and salt rejection > 90% (except for the PAN membrane whose rejection was lower, but still much higher than prior art TFC membranes).

It is well known that due to the hydrophobicity of PP, conventional interfacial polymerization approach fails to produce usable TFC membranes on PP support. Tedious pre-treatments such as plasma bombardment are required to increase surface hydrophilicity prior to interfacial polymerization, making PP-based TFC membranes commercially non-viable, despite of the low price of PP membranes. However, as can be seen from Figure 4(a), TFC membrane prepared according to the method of the present invention on a PP support shows reasonable water permeability of about 2 LMH/bar and NaCI rejection of about 92%. Since PP is resistant to many organic solvents such as dimethylformamide, it may be used in organic solvent nanofiltration.

Effect of surfactant on TFC membrane formation

The effect of surfactant on TFC membrane formation was also investigated on the method of the present invention. Similar to conventional interfacial polymerization, introduction of sodium dodesyl sulfate (SDS) increased the surface roughness, and influenced the permeability and rejection as well. The results are shown in Figure 5. In addition, when the SDS concentration was above the critical concentration, the salt rejection decreased sharply. This may be due to the excess SDS that aggregates at the interface, which causes interferences to the interfacial polymerization processes and reduces the performance. Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.