The Investigation of Mechanical Characteristics of PVDF Membrane for Membrane Distillation
Zahra Mirzaei, Sina Rezvani, Ahmad Riahi, Jasmin Dadras
Department of Chemistry, Payam Noor University, Tehran, Iran.
*Corresponding Author E-mail: dadras@chemhitech.ir
ABSTRACT:
Membrane distillation (MD) is one of the most applicable processes for purifying water by the hydrophobic membrane. Nanocomposite membranes have good potential in membrane process due to their extraordinary properties. In this study, PVDF flat-sheet membranes were produced by dry-wet phase inversion (DIPS) technique via dual coagulation bath. Also, the nanocomposite membranes were modified by hydrophobic SiO2 nanoparticles. The pore size and percentages of macro-voids of fabricated membranes were recognized by secondary electron microscope (SEM) and mechanical properties were elicited via tensile test. As a result, the pore size and percentages of macro-voids increased for 3wt% SiO2 and then decreased when 6wt% SiO2 were embedded. The percentages of macro-voids for nanocomposite membranes were higher than the percentages of macro-voids for the neat membrane. The decrease in pore size should be related to agglomeration. Furthermore, the tensile strength increased by adding the SiO2 nanoparticles and as a consequence, the elongation at break descended.
KEYWORDS: PVDF nanocomposite membrane, Hydrophilic SiO2 nanoparticles, Membrane distillation, Membrane structure, Mechanical properties.
1. INTRODUCTION:
Membrane distillation (MD) was known as a suitable process for water desalination. MD uses the vapor pressure differences to desalination the saline water. The proper structures for membranes should result high flux and salt rejection with high wetting resistance and fouling resistance. Increasing in pore size and decreasing in tortuosity factor caused to increase in the flux of water. A wetting resistance improved whenever an average pore size tends to bubble point (BP). Also, the water contact angle (CA) and the bubble point test show the hydrophobicity of the membrane surface (1-6).
Polyvinylidene fluoride (PVDF) has high thermal stability, excellent mechanical properties and good chemical resistance. PVDF was used as suitable polymer for fabrication of diverse membranes. Nowadays, PVDF membranes have been used for MD. Most researchers utilize a various type of nanoparticles to improve the membrane properties like membrane performance, thermal, mechanical and chemical stability. Nevertheless, there are a few researches on incorporating nanoparticles in MD membranes. Hou et al. prepared PVDF membranes by hydrophilic CaCO3 for direct contact membrane distillation (DCMD) configuration (7, 8). They showed the water flux 14.9% increased when hollow fiber membranes were used, while 30.8% for the flat sheet membranes. Baghbanzdeh et al. used hydrophilic CuO in PVDF membranes for vacuum membrane distillation (VMD) configuration. The flux increased by 153.4% without decreasing in selectivity (9, 10). Also, Baghbanzdeh et al. investigated the effect of employing the hydrophilic SiO2 and non-woven fabric backing material on PVDF membranes. The results showed the water flux increased significantly. Yatim et al. incorporated the fluorosilaned- TiO2 in PVDF for MD. The contact angle increased and the membrane has good anti-fouling properties. Besides that, Tian et al. studied the effect of adding multi-walled carbon nanotubes (MWCNT) with different carboxyl content into PVDF membranes (6,11-16). They found a composite membrane containing carboxyl content of 0.49wt% MWCNT had better mechanical properties and higher hydrophobicity. In this study, the effects of hydrophilic SiO2 on mechanical properties have been studied for the first time. Furthermore, the change in the structure of membranes due to adding SiO2 in dual coagulation bath process was observed (17-22).
Compared to other hydrophobic materials such as polytetrafluoroethylene (PTFE) and polypropylene (PP) membranes, PVDF membranes can be fabricated without going through stretching and sintering processes and can be easily dissolved in many organic solvents, for example, N-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide, dimethylsulfoxide. In 2011, Liu et al. has comprehensively reviewed the current progress on the production and modification of PVDF membranes for liquid–liquid or liquid–solid separation. This article summarized various PVDF modification methods which are useful for different industrial applications.
Typically, PVDF membrane is transformed into hydrophilic membrane for pressure-driven membrane processes by introducing non-solvent additives or inorganic fillers into the membrane matrix. However, in MD process, PVDF membrane is modified to enhance its hydrophobicity so that a lower value of surface energy can be obtained to prevent liquid solution penetration at the membrane pores. Conventionally, porous hydrophobic MD membranes are produced mainly in two ways. One is using hydrophobic materials; the other is transforming hydrophilic membrane into a membrane having hydrophobic properties. Various methods, such as polymer nanofibers, thermally induced phase separation, and non-solvent-induced phase separation have been reported in the literature for the preparation of hydrophobic MD membranes. Of these, many researchers preferred to prepare membrane using a simple blending process. Normally, the degree of hydrophobicity is measured based on water contact angle in which the higher the water contact angle value the greater the hydrophobicity of the membrane and vice versa. However, it is found that hydrophobic MD membranes are always associated with pore-wetting and fouling problems, especially when operated at higher feed temperature. Previous studies show that PTFE and PP membranes experienced structural changes in their microporous structure upon significant temperature changes which futher led to pore-wetting problem. Ge et al. on the other hand found that the water contact angle of PVDF membrane decreased with increasing water (feed) temperature and explained that the membrane wetting was caused by the temperature dependence of the membrane properties. With respect to membrane fouling, Gryta stated that feed temperature is the most influential factor for the membrane fouling because high feed temperature tends to cause more volatile compounds to evaporate and diffuse through the membrane as well as increase the solute concentration on the interface between the feed and membrane phase. It is also elucidated that foulants could be attracted to the surface layer of the membrane due to the hydrophobic and electrostatic interaction between organic materials in the feed water and membrane surface.
PVDF Kynar 740 as the main polymer (pellet, melt viscosity: 1850±250 Pa. s, melting temperature: 165- 175˚C) were purchased from Arkema. NMP as the solvent was bought from Daejung co. Hydrophilic SiO2 was purchased from Notrinoavar co. The isopropanol and deionized water were used as the first and second coagulation bath, respectively.
PVDF membranes were fabricated by dry-wet phase inversion (DIPS)(23-26). The casting solution as produced as follow:
To prepare the casting solution the 15% PVDF and 85% NMP were heated for 48h at 50 ˚C. Then the solution was mixed for 24h at 50 ˚C to achieve a homogeneous solution. After that, it was left for 24h for degassing(27-30).
To prepare the nanocomposite membranes based on Table1, the required amount of SiO2 were added to the dope solution. These solutions were stirred for 5h.
The casting solution was cast on clean glass by automatic thin film (MTI Corporation) and 120 μm in thickness. The casting film was immediately dipped in isopropanol bath as first coagulation bath for 5s and then transferred to deionized water bath (at 25 ˚C) as second bath for 48h to completely solidified and removed the residual solvent. Next, the membrane was left at the air to dry (31-34).
Table 1: Composition of dope solution.
|
Membrane |
|
Dope solution |
|
First coagulation bath |
Second coagulation bath |
|
|
PVDF (wt%) |
NMP (wt%) |
SiO2 (wt%) |
|
|
|
MW-0.0 |
15 |
85 |
0.0 |
Deionized water |
Deionized water |
|
MA-0.0 |
15 |
85 |
0.0 |
Isopropanol |
Deionized water |
|
MA-3.0 |
15 |
85 |
3.0 |
Isopropanol |
Deionized water |
|
MA-6.0 |
15 |
85 |
6.0 |
Isopropanol |
Deionized water |
Figure 1: Schematic diagram of flat sheet membrane production process by DIPS.
For morphological studying of cross section of membranes, the secondary electron microscope (SEM- TESCAN- Vega3) were used. The samples were cut via liquid nitrogen to avoid any distortion and coated by sputtering machine(35-39).
To characterize the mechanical properties, the single fiber tester (LLY- 06ED) was used. According to standard, the samples with 30 mm in length, 5 mm in width gauge length of 10 mm with the rate of 5 mm.min -1 were tested. Each recorded value was an average of 5 measurements. Also, the thickness of each membrane was measured 3 times (40-45).
The cross-sectional SEM image of neat and nanocomposite membranes were presented in Figure 1. Images processing are done by ImageJ software. This software has been used by other researchers for these purposes. SEM images show that by using isopropanol as first coagulation bath in dual bath process, the finger-like macrovoids disappeared and sponge-like structure appeared more. It referred to nature of isopropanol alcohol. Isopropanol is less harsh than water to disturb the solution system equilibrium (Figure2). In the other words, the water is so stronger non-solvent than isopropanol for PVDF/NMP system. So, the symmetric morphology was seen in MA-0.0 sample. These observations about the effect of different coagulation bath on PVDF/NMP were mentioned before.
Also, the mean pore size of each membrane was calculated. Figure 4 shows the mean pore size versus the concentration of SiO2 nanoparticles. By increasing in the weight percentage of the hydrophilic SiO2 nanoparticles, the size of polymer lean phase grows faster. But, at the higher amount of SiO2 nanoparticles, the pore size decreases due to agglomeration. The average pore size increase from 186nm for sample MA-0.0 to 380nm for sample MA-3.0. This phenomenon could be related to the nature of hydrophilic SiO2. The hydrophilic SiO2 will be keen to attract a non-solvent. So, it means the isopropanol and water as the non-solvent can diffuse more and the larger pores produce.
The percentage of macro-voids versus nanoparticles concentration was plotted in Figure 5. The fraction of macro-voids increases up to 3wt% of SiO2, though, decrease by increasing the nanoparticles concentration. This observation confirmed the effect of agglomeration which was told before.
Figure 2: Cross- sectional morphologies of memranes with magnefication of 6.0k. (a) MA-0.0, (b) MA-3.0, (c) MA-6.0.
Figure 3: Phase diagrame of ternary PVDF/NMP/non-solvent include water and isopropanol [10].
Figure 4: Surface pore size vs. nanoparticles concentration.
Figure 5: Fraction of membrane macro-voids vs. nanoparticles concentration in nanocomposite membranes.
The tensile strength and elongation at break were shown in Table 2. The tensile strength increases significantly than neat PVDF membrane because of existence of SiO2. It might be revealed the physical adsorption and intercalation of polymer chains and nanoparticles. These intercalations limit the movement and mobility of polymer chains. So, they cause the increase in tensile strength. Notwithstanding the agglomeration, the MA-6.0 sample has higher tensile strength than other PVDF membranes. It reveals the advantage of using silica nanoparticles on mechanical performances of membranes. On the other hand, the elongation at break decreases from 57% for MA-0.0 sample to 53% and 53% for MA-3.0 and MA-6.0. The reduction in elongation at break was seen because the longitudinal stress transfer to nanoparticles in nanocomposite membranes. The improvement in tensile strength by using nanoparticles was mentioned before for aromatic polyamide (PA) membranes [11].
Table 2: The tensile strength and elongation at break of composite membranes.
|
Membrane |
Tensile strength (MPa) |
Elongation at break (%) |
|
MA-0.0 |
2.1 |
57±7 |
|
MA-3.0 |
2.7 |
53 ±4 |
|
MA-6.0 |
3 |
53 ±5 |
In this work, the effect of adding hydrophilic SiO2 nanoparticles by using dual coagulation bath on cross-section sight and mechanical properties of PVDF flat sheet membranes have been studied. The SEM analysis revealed that at first the average pore size and the fraction of macro-voids increased. In the following, the maximum pore size and fraction of macrovoids decreased due to agglomeration. Also, the nanocomposite membranes were strengthened rather than neat PVDF membranes.
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Received on 04.05.2020 Modified on 29.05.2020
Accepted on 16.06.2020 ©Asian Pharma Press All Right Reserved
Asian J. Res. Pharm. Sci. 2020; 10(3):133-137.
DOI: 10.5958/2231-5659.2020.00024.7