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Preparation and properties of homogeneous-reinforced polyvinylidene fluoride hollow fiber membrane
Applied Surface Science 264 (2013) 801–810
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and properties of homogeneous-reinforced polyvinylidene fluoride hollow fiber membrane Xuliang Zhang, Changfa Xiao ∗ , Xiaoyu Hu, Qianqian Bai State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300387, China
a r t i c l e
i n f o
Article history: Received 7 June 2012 Received in revised form 23 October 2012 Accepted 23 October 2012 Available online 30 October 2012 Keywords: PVDF Homogeneous-reinforced Hollow fiber membrane Interface Tensile strength
a b s t r a c t Homogeneous-reinforced (HR) polyvinylidene fluoride (PVDF) hollow fiber membranes include PVDF polymer solutions (coating layer) and the matrix membrane prepared through the dry-wet spinning process. The performance of HR membranes varies with the polymer concentration in the polymer solutions and is characterized in terms of pure water flux, rejection of protein, porosity, infiltration property, a mechanical strength test, and morphology observations by a field emission scanning electron microscope (FESEM). The results of this study indicate that the tensile strength of the HR PVDF membranes decreases slights compared with that of the matrix membrane, but the elongation at break increases much more and the hollow fiber membranes are endowed with better flexibility performance. The HR PVDF hollow fiber membranes have a favorable interfacial bonding between the coating layer and the matrix membrane, as shown by FESEM. The infiltration property is characterized by the contact angle experiments. Pure water flux decreases while the rejection ratio with an increase in polymer concentration increasing. The protein solution flux of the HR PVDF membranes is higher than that of the matrix membrane after 100 min of infiltration. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Membrane technologies have enjoyed great popularity over the last 30 years. They have been extensively used in separation facilities to separate mixtures because of their flexibility and capacity to remove contaminants from wastewater to low levels [1]. Compared with flat membranes, a hollow-fiber-shaped membrane has a high permeability per installation area and is suitable for water treatment. Furthermore, hollow fiber membranes are mechanically self-supporting and are easy to assemble in modules for different membrane applications [2]. These characteristics make hollow fiber membranes attractive for industrial use and such membranes have been widely applied in Membrane Bioreactor (MBR) systems. The MBR has emerged as an attractive technology for advanced municipal and industrial wastewater treatment, as it combines the biological degradation process of activated sludge with a direct solid–liquid separation through membrane filtration [3]. In this way, both high mixed liquor suspended solids (MLSS) and a very good quality effluent, suitable for processing applications, or maybe even drinking water can be achieved in the MBR process [4]. In addition, the MBR offers many advantages over conventional wastewater treatment processes, including low sludge
∗ Corresponding author. Tel.: +86 022 83956138. E-mail address: [email protected] (C. Xiao).
production and reduced footprint [5]. In the MBR filtration process, hollow fiber membranes are liable to be damaged and broken by the high-pressure water cleaning process or disturbance of the aerated airflow. In the use of such water treatment, the filtration membrane is required, not only for its being superior in separation performance and permeation performance, but also for its high mechanical property, which has not been seen till now [6]. However, the most popularly used Ultrafiltration (UF) and Microfiltration (MF) hollow fiber membranes nowadays, which are composed of skin layer and support layer, are usually prepared through the immersion-precipitation method. Normally, these hollow fiber membranes have high permeability but low mechanical endurance [7]. Several researches have been carried out to improve the mechanical properties of hollow fiber membranes. First, according to the membrane formation mechanism, as a production method of producing a filtration membrane having fairly high strength, there is a method of thermally induced phase separation (TIPS). However, since the structure of the membrane produced by such a method is a homogeneous structure, which has no distribution of pore diameters in a sectional direction of the membrane and has uniform pore diameters, it is difficult to obtain sufficient permeation performance for industrial water treatment [8,9]. Second, a hollow fiber membrane reinforced with a fabric or tubular braid has an excellent mechanical strength as a support for the separation membrane. Zenon Environmental Inc. [10] has produced a reinforced microporous membrane comprising a tubular
0169-4332/$ – see front matter Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.135
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Fig. 1. Schematic diagram describing the HR PVDF membrane induced by the dry–wet spinning process.
braid microporous support coated on its outer surface with a thin tubular asymmetric semipermeable film of polymer. Mitsubishi Rayon Co., Ltd. [6] has invented a porous membrane comprising a porous body and a reinforcement fiber. The reinforcement fiber exists linearly and continuously between both ends, facing orthogonal to the porous body permeation direction. Kolon Industries, Inc. [11] has also produced a braid-reinforced hollow fiber membrane that includes a reinforcing material of tubular braids and a polymer resinous thin film coated onto the surface of the reinforcing material. PET threads- reinforced PVDF hollow fiber membranes were prepared by Li et al. [7], and the tensile/rupture strength of the reinforced membranes were significantly improved to 10 MPa. However, such a hollow fiber membrane has a problem in the porous membrane and is easily peeled from the braid because the porous membrane and reinforced fiber are thermodynamically incompatible. The requirement of the interfacial bonding strength between the porous membrane and the reinforced fiber is higher than the tensile strength due to the techniques of aeration and antiwash in the filtration process. Such problems limit the application of these heterogeneous-reinforced membranes in engineering practice. As a semi-crystalline polymer, PVDF has widespread applications because of its excellent properties, such as good mechanical strength, stability against vigorous chemicals, and good thermal stability [12]. In the past, membranes formed of PVDF using a wet or dry–wet spinning method were known as filtration membranes that were excellent in water permeability. Further, since these membranes are produced by being separated in microphase, an adequate molecular orientation is not obtained, and the tensile/rupture strength of the filtration membranes is as little as about several MPas [13,14]. Distinctive from these previous reinforced methods, a new idea is introduced in this study. According to the theory of
thermodynamic compatibility and the principle of skin/core composite spinning of chemical fiber, hollow fiber membranes that included a PVDF casting solution (coating layer) and the matrix membrane (preparation of porous PVDF hollow fiber membrane through the melt spinning and stretching process) were prepared through the dry-wet spinning process. The process is called the homogenous-reinforced (HR) method. The mechanical properties of the HR membranes and the effects of the PVDF concentration in the polymer solutions on the membrane structure and membrane filtration properties are studied. 2. Experimental 2.1. Materials PVDF (W no. 1300 powders, Tm = 170 ◦ C) was purchased from Kureha Chemical Industrial Co., Ltd. (Tokyo, Japan). N,Ndimethylacetimide (DMAc, > 99%) and Polyvinylpyrrolidone (PVP, K30, Mw = 30,000) were obtained from Tiantai Fine Chemical Co., Ltd. (Tianjin, China). Tween 80 (Tw-80) was purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co., Ltd. (Tianjin, China). The PVDF matrix membrane (the PVDF membrane prepared by melting spinning) was supplied by Tianjin Motian Membrane Engineering & Technology Co., Ltd (Tianjin, China). 2.2. Preparation of HR PVDF hollow fiber membranes The HR hollow fiber membranes were prepared using the dry–wet spinning method. Fig. 1 shows the spinning apparatus. According to the method described, the PVDF matrix membrane was coated with the polymer solutions and guided through a precipitation bath, where the polymer solutions were converted into
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Table 1 Spinning parameters of HR PVDF membranes with different dope composition. Prepared membrane M0 M6 M10 M14 M18
PVDF (wt%)
PVP (wt%)
Tw-80 (wt%)
DMAc (wt%)
Matrix membrane
Dope composition
0 6 10 14 18
0 7 7 7 7
0 3 3 3 3
0 84 80 76 72
PVDF hollow fiber membrane
Dope solution temperature Extra coagulation Air gap Take up speed
60 ◦ C Water, 20 ◦ C 10 cm 15 cm/min
a microporous layer. The polymer solutions were prepared by the blending of different compositions consisting of PVDF, PVP K30, Tw-80, and DMAc (Table 1), under constant mechanical stirring in a three-necked round-bottom flask for 4 h at 70 ◦ C. All the membranes were prepared under an environmental humidity of 60% and at a temperature of 20 ◦ C. The HR membranes with the different PVDF concentrations, i.e., 6, 10, 14 and 18 weight % (wt%) were labeled M6, M10, M14, and M18, respectively. The PVDF matrix membrane was designated as M0. Before the field emission scanning electron microscope (FESEM) tests, we put the resulting membranes in glycerol water solutions (three parts glycerol to two parts water) and then dried them in air, so as to retain the porous structure. Before the measurements of the contact angle, we put the resulting membranes in a vacuum for 12 h at 50 ◦ C to get dry membranes.
Each membrane was initially pressurized for 20 min at 0.15 MPa. The pressure was then reduced to the operating pressure of 0.1 MPa, and the PWF was measured. Before the protein filtration, the membranes were compacted at 0.1 MPa for 60 min, then the PSF was measured at 0.1 MPa. The rejection was calculated using Eq. (2): R (%) =
J=
V , S×t
(1)
where J is the PWF (L m−2 h−1 ), V is the quantity of the permeate (L), S is the membrane area (m2 ), and t is the testing time (h). The filtration experiments were then carried out using the protein solution. The protein solution flux (PSF) of the membranes was measured with 2 g/L egg albumen solution and calculated using Eq. (1).
Manometer Vaue
Membrane model Pump
× 100%,
(W1 − W2 )/w × 100%, (W1 − W2 )/w + W2 /p
(2)
(3)
where W1 and W2 are the weights of the wet and the dry membranes, respectively, w is the water density, andp is the polymer density (1.78 g/cm3 ) that comes from the PVDF material density. 2.3.4. Tensile break strength measurements The tensile strength and elongation at break of the hollow fiber membranes were determined at room temperature by using a YG061F electronic single yarn tensile tester (Shandong, China). The tensile rate was 10 mm min−1 . Five runs were performed for each specimen. The change ratios of elongation at break were defined as Eq. (4): Ratio (%) =
Ea − E0 × 100%, E0
(4)
where Ea is the elongation at break of the HR membrane with different PVDF concentrations, and E0 is the elongation at break of the matrix membrane. 2.3.5. Infiltration property of polymer solution The infiltration property of the membrane was investigated by the contact angle () between the polymer solution and the membrane surface. The contact angle was measured using a JY-820 Contact Angle Meter produced by Chengde Testing Machine Co., Ltd. (China). To minimize the experimental error, the contact angles were measured five times for each sample with the average value reported. The hollow fiber membranes were cut lengthwise and the polymer solution droplets were deposited on the outer surface of the PVDF matrix membranes. The change ratios of the contact angle at 5 s and 30 s when compared with that at 0 s can be defined as Eq. (5):
Permeate Fig. 2. Schematic diagram for filtration experiments.
Cp Cf
2.3.3. Porosity determination The membrane porosity (ε) was defined as the pores’ volume divided by the total volume of the porous membrane. It can be determined by Eq. (3): ε=
2.3.2. Membrane permeability The pure water flux (PWF) of the membranes was determined using Eq. (1). The pressure difference across the membrane was 0.1 MPa under the condition of outside feeding. Fig. 2 shows the PWF measurement apparatus. The membrane module contained two hollow fiber membranes with an effective length of 19–20 cm.
1−
where Cp and Cf are the concentration at permeate and feed, respectively.
2.3. Membrane characterizations 2.3.1. Morphology examination The morphology of the membranes was observed using FESEM (X4800, Hitachi, Japan). The samples were frozen in liquid N2 , followed by fracturing to expose their cross-sectional areas. Thereafter, they were sputtered with gold and recorded through FESEM.
Ratio (t) =
Ct − C0 × 100%, C0
(5)
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Fig. 3. Cross-sectional FESEM morphology of PVDF hollow fiber membranes: (a) M0, (b) M6, (c) M10, (d) M14, (e) M18.
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Fig. 3. (Continued)
where Ct is the contact angle at time t (t = 5, 30 s), and C0 is the contact angle at time 0 s.
3. Results and discussion 3.1. Morphologies of PVDF hollow fiber membranes
2.3.6. Apparent viscosity of the polymer solutions The viscosities of the casting solutions with different PVDF dosage were determined at 60 ◦ C by a rotational viscometer (NDJ-7, Shanghai, 750 r/min). 2.3.7. Maximum pore size of the membrane The maximum pore size of the membranes was obtained using the bubble point method with alcohol as wetting liquid and calculated using Eq. (6) rp =
2 , P
(6)
where rp is the maximum pore size of the membrane (m), is the surface tension at the alcohol/air interface (N m−1 ), and P is the pressure. At room temperature (20 ◦ C), is equal to 22.32 × 10−3 N m−1 . 2.3.8. Differential scanning calorimetry (DSC) The membrane crystallization was characterized through differential scanning calorimetry (PerkinElmer, DSC-7) at 10 ◦ C/min increasing temperature rate. Melting heat was determined from the melting peak area. Crystallinity was evaluated using Eq. (7) Xc (%) =
Hf × 100%, Hf∗
(7)
where Hf is the melting enthalpy for a 100% crystalline PVDF and the value is 104.75 J/g [15]. Hf∗ is the melting enthalpy of the PVDF membranes.
The FESEM micrographs in Fig. 3 show the structure of the hollow fiber membranes prepared by the HR method for the different concentrations (6, 10, 14, and 18 wt%) of PVDF in the polymer solutions. Fig. 3(a1, b1, c1, d1, e1) and (a2, b2, c2, d2, and e2) show the whole cross-sectional structure of the membrane and the enlarged structure, respectively. The low magnification image of the crosssection in Fig. 3(a1) shows that the PVDF matrix membrane is a kind of homogeneous membrane. Moreover, the interconnected sponge-like microstructure can obviously be observed as the magnification is increased. There is no skin layer at the cross-section near the outer surface as shown in Fig. 3(a2). Although a larger pore size and a higher porosity can improve the permeability of the membrane, it can also easily cause the embedded pollution in the filtration process. Obviously, a thin skin layer (coating layer) with small pores is formed at the outer surface of the PVDF matrix membrane due to the HR method with the PVDF casting solution. The thickness of the coating layer increases with the increase in PVDF concentration in the polymer solutions, as shown in Table 2. The coating layer thickness accounts for one-seventh and a quarter of the wall thickness when the PVDF concentration increases from 6 to 18 wt%. The enhancement of the wall thickness increases the resistance of the membranes in water filtration and decreases the water flux. With the minimal concentration (6 wt%) of the PVDF polymer solution, the thickness of the coating layer is thin, and there is no apparent finger-like pore structure within the membrane. In this case, the performance of the coating layer induced by the phase separation is poor, and sometimes the membrane structure is not easy to form. With the increment in the PVDF concentration (10 and 14 wt%) in the polymer solutions, the thickness of the coating layer increases and leads to the typical finger-like structure. The addition of PVDF may delay the coagulation process, and a
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Table 2 Characterization of PVDF hollow fiber membranes. Membrane ID
M0
M6
M10
M14
M18
ODa ,IDb /mm Wall thickness (mm) Coating layer thickness (mm)
1.214,0.729 0.235 ± 0.009 0
1.410,0.839 0.283 ± 0.007 0.035 ± 0.003
1.200,0.721 0.258 ± 0.024 0.040 ± 0.002
1.237,0.707 0.281 ± 0.017 0.051 ± 0.005
1.234,0.716 0.277 ± 0.023 0.068 ± 0.005
a b
Outer diameter. Inner diameter.
long finger-like structure will become into a short finger-like structure and thereafter a sponge-like structure, as shown in Fig. 3(c2, d2). Generally, an instantaneous demixing leads to a finger-like structure, whereas a delayed demixing leads to a sponge-like structure of the as-spun membrane [16]. The viscosity of the solutions increases with increase in PVDF concentration in the polymer solutions; consequently, the kinetic exchange mechanism between the solvent in the polymer solutions and the nonsolvent in the coagulation bath becomes slow, and then a delayed demixing behavior becomes more distinct, inhibiting the growth of the finger-like pore structure. The micrographs in Fig. 3(b2, c2, d2, e2) show three parts near the outer surface of the HR membranes. The outermost layer is the PVDF coating layer, and the innermost layer is the PVDF matrix membrane. The middle layer, between the PVDF coating layer and the PVDF matrix membrane, is the interface layer, which endows the HR membranes with high interfacial bonding. Fig. 4 shows the FESEM results of the outer surfaces of the hollow fiber membranes. The matrix membrane (M0) has a rougher outer surface within obvious big pores; this surface was prepared by the melt spinning and stretching process. The outer surface of the HR PVDF hollow fiber membrane (M10) appears denser than M0. Fig. 1 shows a schematic diagram describing the HR PVDF membrane induced by the dry–wet spinning process, based on the
results of FESEM as seen in Fig. 3. The PVDF matrix membrane is first goes through the co-extrusion device directly. The polymer solutions have some effects on the matrix membrane, such as infiltration and diffusion, as shown in the process (I). Infiltration occurs at the time the polymer solutions contacted the surface of the matrix membrane, and then the matrix membrane slightly dissolves during the infiltration stage owing to the codissolved solvent DMAc. The entanglement of PVDF chains in the polymer solutions and the matrix membrane is controlled by the interface diffusion between the polymer solutions and the matrix membrane [17]. The degree of infiltration and diffusion effects significantly affect on the interfacial bonding strength. The HR PVDF hollow fiber membranes solidify in the coagulation bath, as shown in the process (II), and then they are taken up by the wire-wrapped rolls, as shown in the process (III). As it can be seen from Fig. 3, the cross-section model of the HR PVDF membrane has been built up for the results of the FESEM. The PVDF matrix membrane prepared by the meltspinning method has high porosity and a big pore size. The coating layer possesses a compact surface and a small pore size, which were prepared via the phase inversion method induced by immersion precipitation. The properties of the interface layer are between that of the coating layer and the matrix membrane.
3.2. Infiltration properties of the polymer solutions
Fig. 4. Outer surface FESEM morphology of PVDF hollow fiber membranes: (a1) M0, (a2) M10.
Generally, one phase contacts with the other phase of the solution or melt states in the two phases of the composite materials, and then combines with the curing reaction. The adhesion action refers to the state in which the two surfaces are combined with the chemical or physical interaction [18]. An interesting phenomenon occurs in Fig. 3(b2, c2, d2): a significant transition layer structure appears on the HR membranes. This indicates that the interfacial bonding effect contributes to the infiltration and diffusion effects from the polymer solutions to the matrix membrane. Poor infiltration between the polymer solutions and the matrix membrane will lead to defects or stress concentration near the interface. Consequently, the interfacial bonding strength decreases; on the contrary, favorable infiltration can improve greatly on the interfacial bonding strength. The contact angle measurement was used to characterize the infiltration ability [7]. The contact angle between the matrix membrane and the polymer solutions must be small enough to achieve a sufficient infiltration and contacting between them. As shown in Fig. 5, with the increase in PVDF concentration, the contact angles dramatically increase at 0 s, and then the interfacial bonding strengths decrease. The changes in the apparent viscosity and surface tension have a similar trend with the contact angle. The apparent viscosity of the polymer solutions containing 18 wt% PVDF is up to 2400 mPa s, which is obviously greater than those of the low PVDF concentration. The contact angle is greater than 90◦ at 0 s with the 18% PVDF concentration. This negative infiltration pattern will affect the interfacial bonding strength. The swelling states appear on the matrix surface when it contacts with DMAc in the polymer solutions as a favorable solvent. The interface layer is on the condition of the dispersion induced by the interdiffusion of the PVDF molecules between the polymer solutions and the matrix membrane. The deeper the degree of the interdiffusion is, the higher
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Fig. 5. Effects of the PVDF concentration on the time dependence of contact angle and the viscosity of the polymer solutions.
the interfacial bonding strength. The degree of the interdiffusion in this study is defined as the time dependence of the contact angle on the membranes. As can be seen, the change rates of the contact angle (5 s, 30 s) when compared with 0 s are significantly reduced as the PVDF concentration reaches 18 wt% in the polymer solution, and then the degree of the interdiffusion decreases. The degree of the interdiffusion with 18% PVDF concentration is worse than 10%, which is well coincident with the value of the negative infiltration pattern. The change rates of the contact angle at 5 s are nearly almost the same as at 30 s. This indicates that the effect on the diffusion occurred mainly at the first few seconds. Thus, the appropriate time of the polymer solutions exposed in the air environment significantly affects the diffusion time. Therefore, the appropriate diffusion time of the polymer solutions not only obtains the best infiltration and the diffusion effect on the matrix membrane, but also results in a slight reduction in the properties of the matrix membrane, which provides the major tensile strength of the HR membranes. 3.3. Mechanical properties of the HR PVDF membrane 3.3.1. Morphologies of membrane after tensile strength test Fig. 6 shows the significant difference in morphology between the inner and the outer surfaces of the HR PVDF membrane (M14) before and after the tensile strength measurements. Samples were collected approximately at 3 cm of the membrane fracture point. The outer surface image after the test exhibits larger pores, while no obvious pores can be identified on the outer surface before the test. Fig. 6(b) compares the cross-section morphology of the HR PVDF membrane before and after tensile strength measurements. Lots of microvoids are created on the outer surface during the stretching process, whereas there is no observable change in the cross-section. The fine interfacial bonding is formed between the coating layer and the matrix membrane. There is no significant
Fig. 6. Outer surface and cross-sectional FESEM morphology of HR PVDF membranes (M14) before and after tensile strength measurement: (a1, b1) before measurement, (a2, b2) after measurement.
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Fig. 8. Effects of PVDF concentration on the porosity and the maximum pore size.
Fig. 7. Effects of PVDF concentration on the mechanical properties of membranes.
flake phenomenon in the interface after stretching. Meanwhile, the thickness of the coating layer and the pore structure of the HR PVDF membrane changed to some extent. This indicates that the interface has high bonding strength after the tensile test due to the interdiffusion of the PVDF molecules. It is important for the resistance of antiwash performance and service life of the hollow fiber membranes. 3.3.2. Tensile strength of the hollow fiber membranes Fig. 7(a) shows the results of the tensile strength and elongation at break of the HR PVDF membranes. The tensile strength of the HR membranes, except for that of the M10 membrane, decreases slightly compared with the matrix membrane. The tensile strength of the HR PVDF membranes depends mainly on the matrix membrane. In the HR process, the erosion effect of the solvent DMAc on the matrix membrane may cause some unfavorable influence on the tensile strength of HR PVDF membranes. We know that the tensile strength of PVDF membranes prepared by the wet spinning method is almost 1–3 MPa [19,20]. A low tensile strength in a PVDF membrane may easily cause fiber breakage in the state of serviceability. The HR PVDF hollow fiber membranes prepared by the HR method rather than by the wet phase inversion method have higher mechanical properties. The tensile strength of the HR PVDF membranes is nearly 10 MPa, which is adequate for MBR application. The elongation at break increased much more, from 50% (M0) to 99% (M14). The crystallinity of PVDF in the matrix is higher than that in the coating layer, as shown in Table 3. It is beneficial to the tensile strength and elastic modulus with the increase in crystallinity of the polymer. There have been more amorphous chains in the coating layer than in the matrix because of the lower
crystallinity. Therefore, the coating layer and the matrix membrane undergo various changes during deformation at the different strain rates. The orientation of the amorphous chains first occurs at the coating layer; the matrix at the stage of the strain increases and the stress remains. The strain increases with the continued transmission of external force that passes through the interface layer, and then the supermolecule structure of PVDF goes through some complex changes. These phenomena cause significant changes to the elongation at break of the HR membranes in macroscale. The change ratios of elongation at break increase from 5.79% for the M6 membrane to 85.44% for the M14 membrane. A higher interfacial stress transfer coefficient is achieved due to the better bonding strength of the HR membranes, which is proportional to the change ratios of elongation at break. It also agrees well with the infiltration properties results. The better the infiltration properties of the polymer solutions to the matrix membrane surface, the higher the interfacial stress transfer coefficient. The elongation at break then decreases as the PVDF concentration increases at 18%. When the PVDF concentration increases to a certain level, the polymer gradually shows that the unique elastic nature and the rigidity of the molecular chains has an increase to some extent. The membrane structures change much, as shown in Fig. 3, resulting in the weakening of the elasticity and in getting a peak of the elongation. 3.4. Permeation properties of the hollow fiber membranes Fig. 8 shows the maximum pore size and porosity of the hollow fiber membranes. The maximum pore size and porosity have the same decreasing tendency as the PVDF concentration increase in the polymer solutions. The HR PVDF membranes have a denser outer surface, as shown in Fig. 4. This means that the HR PVDF membranes will possess a smaller maximum pore size than the matrix membrane, which could improve the rejection of the membrane. The porosity of HR PVDF membranes also decreases with the increase in PVDF concentration.
Table 3 Crystallinity of the PVDF membrane. Membrane
Coating layer (14%PVDF)
Matrix-PVDF(M0)
Hf (J/g) Xc (%)
40.8 38.9
50.06 47.8
Fig. 9. Effect of the PVDF concentration on pure water flux and protein rejection of the membranes.
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the PSF of the matrix membrane is lower than that of the HR membranes. The high permeability and rejection of the HR PVDF membranes in the protein solution effectively reduce the frequency of the backwashing and aeration, and this can indirectly improve the service life of the membrane. In addition, the HR membranes have nearly the same tensile strength as the matrix membrane. The PSF increases when the PVDF concentration in the polymer solution changes from 6 to 10 wt% and then decreases when it changes to 14 wt%. The M18 membrane has the lowest PSF compared with other HR membranes, but this PSF is still greater than that of the M0 membrane. The better performance of the HR membranes of the protein solution has great value in MBR system applications. 4. Conclusion Fig. 10. Effects of PVDF concentration on the protein solution flux changes with time.
Fig. 9 shows the PWF and separation of the protein solution of the prepared HR PVDF hollow fiber membranes. The PWF of the HR PVDF membranes decreases with the increase in PVDF concentration in the polymer solutions. According to Fig. 9, PWF decreases significantly from 1230.4 L m−2 h−1 ·0.1 MPa for the M0 membrane to 442.1 L m−2 h−1 ·0.1 MPa for the M10 membrane, and then decreases slightly with the increase in PVDF concentration from 14 to 18 wt%. The PWF decreases while the rejection ratio increases with the increase in PVDF concentration. The wall thickness of the HR PVDF membranes is thicker than the matrix membrane, which extends the membrane filtration path and increases the intrinsic resistance of the membrane. Therefore, the PWF of the HR PVDF membranes is lower than that of the matrix membrane. As shown in Fig. 9, the PWF decreases with the increase of PVDF concentration in polymer solution. This is because the highly polymer concentration results to lower membrane porosity as shown in Fig. 8 and the porosity is a key factor impacting the membrane permeability. The rejection depends more on the denseness of the skin layer than on the structure of the cross-section [21]. The skin layers of the HR PVDF membrane are denser than those of the PVDF matrix membrane (Fig. 4). Thus, the maximum pore size of the HR PVDF membranes is smaller than that of the matrix membrane. With the application of the HR method, the increasing rate of the protein separation is significant for some special water treatment. Fig. 10 presents the PSF of the PVDF hollow fiber membranes. Under constant trans-membrane pressure (TMP), the effects of membrane fouling or concentration polarization are usually distinguished by a more or less considerable permeation flux decline [22]. As observed, the reduction of PSF for the HR PVDF membranes (M6, M10, M14, and M18) is lower than that of the matrix membrane (M0). For example, the PSF decreases from 136.6 to 51.8 L m−2 h−1 ·0.1 MPa for the M0 membrane, whereas it decreases from 126.1 to 124.8 L m−2 h−1 ·0.1 MPa for the M18 membrane. After filtration of the protein solution for 100 min, the PSF of the M0 membrane is considerably lower than that of the HR membranes. At the early stage of the protein solution filtration, the matrix membrane has larger pore entrances than the HR membranes, which results in a series pore-blockage and irreversible membrane fouling. The HR membranes with smaller pore size could reduce the pore-blockage phenomenon at the beginning of the filtration. The mechanism of filtration is then transferred to cake filtration more rapidly than the matrix membrane under the same filtration condition, which reduces the irreversible membrane fouling and slows down the membrane flux decline [23]. Fig. 4 also shows that the matrix membrane has a rougher outer surface than the HR membranes have, and this more easily causes membrane fouling. So
The HR PVDF hollow fiber membranes were prepared through the dry–wet spinning method of coating PVDF polymer solutions on the PVDF matrix hollow fiber membrane surface. The HR PVDF hollow fiber membranes have a favorable interfacial bonding between the coating layer and the matrix membrane due to the existence of the interface layer, as seen from the FESEM. The outer surface of the HR PVDF membrane is denser than that of the matrix membrane. With an increasing PVDF concentration in the polymer solutions, the contact angle significantly increases at 0 s, which results from the degree of the interdiffusion with 18% PVDF concentration being worse than 10%, and then the interfacial bonding strength decreases. The tensile strength of the HR PVDF membranes shows a slight reduction when compared with the matrix membrane, but the elongation at break increases much more, from 50% (M0) to 99% (M14). The porosity and maximum pore size of the HR PVDF membranes decrease with an increase in PVDF concentration, which can result in lower PWF and higher rejection of protein. The HR PVDF membranes also have higher PSF than the matrix membrane. Acknowledgements The authors thank the financial support of the National Basic Research Development Program of China (973 Program, 2012CB722706), the National Natural Science Foundation of China (20874073 and 21274109), the Science and Technology Plans of Tianjin (10SYSYJC27900), and the Basic Research Program of China National Textile and Apparel Council. References [1] E. Yuliwati, A.F. Ismail, T. Matsuura, M.A. Kassim, M.S. Abdullah, Effect of modified PVDF hollow fiber submerged ultrafiltration membrane for refinery wastewater treatment, Desalination 283 (2011) 214–220. [2] X.Y. Hu, Y.B. Chen, H.X. Liang, C.F. Xiao, Preparation of polyurethane/poly (vinylidenefluoride) blend hollow fiber membrane using melt spinning and stretching, Materials Science and Technology 27 (2011) 661–665. [3] S.I. Patsios, A.J. Karabelas, An investigation of the long-term filtration performance of a membrane bioreactor (MBR): the role of specific organic fractions, Journal of Membrane Science 372 (2011) 102–115. [4] N. Fallah, B. Bonakdarpour, B. Nasernejad, M.R. Alavi Moghadam, Long-term operation of submerged membrane bioreactor (MBR) for the treatment of synthetic wastewater containing styrene as volatile organic compound (VOC): effect of hydraulic retention time (HRT), J. Hazard. Mater. 178 (2010) 718–724. [5] M.J. Kim, B. Sankararao, C.K. Yoo, Determination of MBR fouling and chemical cleaning interval using statistical methods applied on dynamic index data, Journal of Membrane Science 375 (2011) 345–353. [6] K. Murase, H. Habara, H. Fujiki, T. Hirane, M. Mizuta, Porous membrane, US 2002/0046970 A1, 2002-04-25. [7] J. Liu, P. Li, Y. Li, L. Xie, S. Wang, Z. Wang, Preparation of PET threads reinforced PVDF hollow fiber membrane, Desalination 249 (2009) 453–457. [8] A. Cui, Z. Liu, C. Xiao, Y. Zhang, Effect of micro-sized SiO2-particle on the performance of PVDF blend membranes via TIPS, Journal of Membrane Science 360 (2010) 259–264. [9] X. Lu, X. Li, Preparation of polyvinylidene fluoride membrane via a thermally induced phase separation using a mixed diluent, Journal of Applied Polymer Science 114 (2009) 1213–1219.
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[10] M. Mailvaganam, L. Fabbricino, C.F.F. Rodrigues, A.R. Donnelly, Hollow fiber semipermeable membrane of tubular braid, US 5472607, 1995-12-05. [11] M.S. Lee, S.-H. Choi, Y.-C. Shin, Braid-reinforced hollow fiber membrane, US 7267872 B2, 2007-09-11. [12] X.C. Cao, J. Ma, X.H. Shi, Z.J. Ren, Effect of TiO2 nanoparticle size on the performance of PVDF membrane, Applied Surface Science 253 (2006) 2003–2010. [13] Q. Li, Z.-L. Xu, L.-Y. Yu, Effects of mixed solvents and PVDF types on performances of PVDF microporous membranes, Journal of Applied Polymer Science 115 (2010) 2277–2287. [14] P. Sukitpaneenit, T.-S. Chung, Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and theology, Journal of Membrane Science 340 (2009) 192–205. [15] C.-H. Du, Y.-Y. Xu, B.-K. Zhu, Structure formation and characterization of PVDF hollow fiber membranes by melt-spinning and stretching method, Journal of Applied Polymer Science 106 (2007) 1793–1799. [16] W.-Z. Lang, Y.-J. Guo, L.-F. Chu, Evolution of the precipitation kinetics, morphologies, permeation performances, and crystallization behaviors of polyvinylidenefluoride (PVDF) hollow fiber membrane by adding different molecular weight polyvinylpyrrolidone (PVP), Polymers for Advanced Technologies 22 (2011) 1720–1730.
[17] National Research Council (U.S.), High-Performance Structural Fibers for Advanced Polymer Matrix Composites, The National Academies Press, Washington DC, USA, 2005. [18] D.E. Packham, Preparation of metallic surfaces with microfibrous topography and their effect on adhesion of hot-melt and thermoset adhesives, International Journal of Adhesion and Adhesives 6 (1986) 225–228. [19] M.-C. Yang, T.-Y. Liu, The permeation performance of polyacrylonitrile/polyvinylidine fluoride blend membranes, Journal of Membrane Science 226 (2003) 119–130. [20] N. Li, C. Xiao, S. An, X. Hu, Preparation and properties of PVDF/PVA hollow fiber membranes, Desalination 250 (2010) 530–537. [21] M. Amirilargani, T. Mohammadi, Preparation and characterization of asymmetric polyethersulfone (PES) membranes, Polymers for Advanced Technologies 20 (2009) 993–998. [22] M. Amirilargani, A. Sabetghadam, T. Mohammadi, Polyethersulfone/ polyacrylonitrile blend ultrafiltration membranes with different molecular weight of polyethylene glycol: preparation, morphology and antifouling properties. Polymers for Advanced Technologies 2011, in press. [23] K.J. Hwang, T.T. Lin, Effect of morphology of polymeric membrane on the performance of cross-flow microfiltration, Journal of Membrane Science 199 (2002) 41–52.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and properties of homogeneous-reinforced polyvinylidene fluoride hollow fiber membrane Xuliang Zhang, Changfa Xiao ∗ , Xiaoyu Hu, Qianqian Bai State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300387, China
a r t i c l e
i n f o
Article history: Received 7 June 2012 Received in revised form 23 October 2012 Accepted 23 October 2012 Available online 30 October 2012 Keywords: PVDF Homogeneous-reinforced Hollow fiber membrane Interface Tensile strength
a b s t r a c t Homogeneous-reinforced (HR) polyvinylidene fluoride (PVDF) hollow fiber membranes include PVDF polymer solutions (coating layer) and the matrix membrane prepared through the dry-wet spinning process. The performance of HR membranes varies with the polymer concentration in the polymer solutions and is characterized in terms of pure water flux, rejection of protein, porosity, infiltration property, a mechanical strength test, and morphology observations by a field emission scanning electron microscope (FESEM). The results of this study indicate that the tensile strength of the HR PVDF membranes decreases slights compared with that of the matrix membrane, but the elongation at break increases much more and the hollow fiber membranes are endowed with better flexibility performance. The HR PVDF hollow fiber membranes have a favorable interfacial bonding between the coating layer and the matrix membrane, as shown by FESEM. The infiltration property is characterized by the contact angle experiments. Pure water flux decreases while the rejection ratio with an increase in polymer concentration increasing. The protein solution flux of the HR PVDF membranes is higher than that of the matrix membrane after 100 min of infiltration. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Membrane technologies have enjoyed great popularity over the last 30 years. They have been extensively used in separation facilities to separate mixtures because of their flexibility and capacity to remove contaminants from wastewater to low levels [1]. Compared with flat membranes, a hollow-fiber-shaped membrane has a high permeability per installation area and is suitable for water treatment. Furthermore, hollow fiber membranes are mechanically self-supporting and are easy to assemble in modules for different membrane applications [2]. These characteristics make hollow fiber membranes attractive for industrial use and such membranes have been widely applied in Membrane Bioreactor (MBR) systems. The MBR has emerged as an attractive technology for advanced municipal and industrial wastewater treatment, as it combines the biological degradation process of activated sludge with a direct solid–liquid separation through membrane filtration [3]. In this way, both high mixed liquor suspended solids (MLSS) and a very good quality effluent, suitable for processing applications, or maybe even drinking water can be achieved in the MBR process [4]. In addition, the MBR offers many advantages over conventional wastewater treatment processes, including low sludge
∗ Corresponding author. Tel.: +86 022 83956138. E-mail address: [email protected] (C. Xiao).
production and reduced footprint [5]. In the MBR filtration process, hollow fiber membranes are liable to be damaged and broken by the high-pressure water cleaning process or disturbance of the aerated airflow. In the use of such water treatment, the filtration membrane is required, not only for its being superior in separation performance and permeation performance, but also for its high mechanical property, which has not been seen till now [6]. However, the most popularly used Ultrafiltration (UF) and Microfiltration (MF) hollow fiber membranes nowadays, which are composed of skin layer and support layer, are usually prepared through the immersion-precipitation method. Normally, these hollow fiber membranes have high permeability but low mechanical endurance [7]. Several researches have been carried out to improve the mechanical properties of hollow fiber membranes. First, according to the membrane formation mechanism, as a production method of producing a filtration membrane having fairly high strength, there is a method of thermally induced phase separation (TIPS). However, since the structure of the membrane produced by such a method is a homogeneous structure, which has no distribution of pore diameters in a sectional direction of the membrane and has uniform pore diameters, it is difficult to obtain sufficient permeation performance for industrial water treatment [8,9]. Second, a hollow fiber membrane reinforced with a fabric or tubular braid has an excellent mechanical strength as a support for the separation membrane. Zenon Environmental Inc. [10] has produced a reinforced microporous membrane comprising a tubular
0169-4332/$ – see front matter Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.135
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Fig. 1. Schematic diagram describing the HR PVDF membrane induced by the dry–wet spinning process.
braid microporous support coated on its outer surface with a thin tubular asymmetric semipermeable film of polymer. Mitsubishi Rayon Co., Ltd. [6] has invented a porous membrane comprising a porous body and a reinforcement fiber. The reinforcement fiber exists linearly and continuously between both ends, facing orthogonal to the porous body permeation direction. Kolon Industries, Inc. [11] has also produced a braid-reinforced hollow fiber membrane that includes a reinforcing material of tubular braids and a polymer resinous thin film coated onto the surface of the reinforcing material. PET threads- reinforced PVDF hollow fiber membranes were prepared by Li et al. [7], and the tensile/rupture strength of the reinforced membranes were significantly improved to 10 MPa. However, such a hollow fiber membrane has a problem in the porous membrane and is easily peeled from the braid because the porous membrane and reinforced fiber are thermodynamically incompatible. The requirement of the interfacial bonding strength between the porous membrane and the reinforced fiber is higher than the tensile strength due to the techniques of aeration and antiwash in the filtration process. Such problems limit the application of these heterogeneous-reinforced membranes in engineering practice. As a semi-crystalline polymer, PVDF has widespread applications because of its excellent properties, such as good mechanical strength, stability against vigorous chemicals, and good thermal stability [12]. In the past, membranes formed of PVDF using a wet or dry–wet spinning method were known as filtration membranes that were excellent in water permeability. Further, since these membranes are produced by being separated in microphase, an adequate molecular orientation is not obtained, and the tensile/rupture strength of the filtration membranes is as little as about several MPas [13,14]. Distinctive from these previous reinforced methods, a new idea is introduced in this study. According to the theory of
thermodynamic compatibility and the principle of skin/core composite spinning of chemical fiber, hollow fiber membranes that included a PVDF casting solution (coating layer) and the matrix membrane (preparation of porous PVDF hollow fiber membrane through the melt spinning and stretching process) were prepared through the dry-wet spinning process. The process is called the homogenous-reinforced (HR) method. The mechanical properties of the HR membranes and the effects of the PVDF concentration in the polymer solutions on the membrane structure and membrane filtration properties are studied. 2. Experimental 2.1. Materials PVDF (W no. 1300 powders, Tm = 170 ◦ C) was purchased from Kureha Chemical Industrial Co., Ltd. (Tokyo, Japan). N,Ndimethylacetimide (DMAc, > 99%) and Polyvinylpyrrolidone (PVP, K30, Mw = 30,000) were obtained from Tiantai Fine Chemical Co., Ltd. (Tianjin, China). Tween 80 (Tw-80) was purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co., Ltd. (Tianjin, China). The PVDF matrix membrane (the PVDF membrane prepared by melting spinning) was supplied by Tianjin Motian Membrane Engineering & Technology Co., Ltd (Tianjin, China). 2.2. Preparation of HR PVDF hollow fiber membranes The HR hollow fiber membranes were prepared using the dry–wet spinning method. Fig. 1 shows the spinning apparatus. According to the method described, the PVDF matrix membrane was coated with the polymer solutions and guided through a precipitation bath, where the polymer solutions were converted into
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Table 1 Spinning parameters of HR PVDF membranes with different dope composition. Prepared membrane M0 M6 M10 M14 M18
PVDF (wt%)
PVP (wt%)
Tw-80 (wt%)
DMAc (wt%)
Matrix membrane
Dope composition
0 6 10 14 18
0 7 7 7 7
0 3 3 3 3
0 84 80 76 72
PVDF hollow fiber membrane
Dope solution temperature Extra coagulation Air gap Take up speed
60 ◦ C Water, 20 ◦ C 10 cm 15 cm/min
a microporous layer. The polymer solutions were prepared by the blending of different compositions consisting of PVDF, PVP K30, Tw-80, and DMAc (Table 1), under constant mechanical stirring in a three-necked round-bottom flask for 4 h at 70 ◦ C. All the membranes were prepared under an environmental humidity of 60% and at a temperature of 20 ◦ C. The HR membranes with the different PVDF concentrations, i.e., 6, 10, 14 and 18 weight % (wt%) were labeled M6, M10, M14, and M18, respectively. The PVDF matrix membrane was designated as M0. Before the field emission scanning electron microscope (FESEM) tests, we put the resulting membranes in glycerol water solutions (three parts glycerol to two parts water) and then dried them in air, so as to retain the porous structure. Before the measurements of the contact angle, we put the resulting membranes in a vacuum for 12 h at 50 ◦ C to get dry membranes.
Each membrane was initially pressurized for 20 min at 0.15 MPa. The pressure was then reduced to the operating pressure of 0.1 MPa, and the PWF was measured. Before the protein filtration, the membranes were compacted at 0.1 MPa for 60 min, then the PSF was measured at 0.1 MPa. The rejection was calculated using Eq. (2): R (%) =
J=
V , S×t
(1)
where J is the PWF (L m−2 h−1 ), V is the quantity of the permeate (L), S is the membrane area (m2 ), and t is the testing time (h). The filtration experiments were then carried out using the protein solution. The protein solution flux (PSF) of the membranes was measured with 2 g/L egg albumen solution and calculated using Eq. (1).
Manometer Vaue
Membrane model Pump
× 100%,
(W1 − W2 )/w × 100%, (W1 − W2 )/w + W2 /p
(2)
(3)
where W1 and W2 are the weights of the wet and the dry membranes, respectively, w is the water density, andp is the polymer density (1.78 g/cm3 ) that comes from the PVDF material density. 2.3.4. Tensile break strength measurements The tensile strength and elongation at break of the hollow fiber membranes were determined at room temperature by using a YG061F electronic single yarn tensile tester (Shandong, China). The tensile rate was 10 mm min−1 . Five runs were performed for each specimen. The change ratios of elongation at break were defined as Eq. (4): Ratio (%) =
Ea − E0 × 100%, E0
(4)
where Ea is the elongation at break of the HR membrane with different PVDF concentrations, and E0 is the elongation at break of the matrix membrane. 2.3.5. Infiltration property of polymer solution The infiltration property of the membrane was investigated by the contact angle () between the polymer solution and the membrane surface. The contact angle was measured using a JY-820 Contact Angle Meter produced by Chengde Testing Machine Co., Ltd. (China). To minimize the experimental error, the contact angles were measured five times for each sample with the average value reported. The hollow fiber membranes were cut lengthwise and the polymer solution droplets were deposited on the outer surface of the PVDF matrix membranes. The change ratios of the contact angle at 5 s and 30 s when compared with that at 0 s can be defined as Eq. (5):
Permeate Fig. 2. Schematic diagram for filtration experiments.
Cp Cf
2.3.3. Porosity determination The membrane porosity (ε) was defined as the pores’ volume divided by the total volume of the porous membrane. It can be determined by Eq. (3): ε=
2.3.2. Membrane permeability The pure water flux (PWF) of the membranes was determined using Eq. (1). The pressure difference across the membrane was 0.1 MPa under the condition of outside feeding. Fig. 2 shows the PWF measurement apparatus. The membrane module contained two hollow fiber membranes with an effective length of 19–20 cm.
1−
where Cp and Cf are the concentration at permeate and feed, respectively.
2.3. Membrane characterizations 2.3.1. Morphology examination The morphology of the membranes was observed using FESEM (X4800, Hitachi, Japan). The samples were frozen in liquid N2 , followed by fracturing to expose their cross-sectional areas. Thereafter, they were sputtered with gold and recorded through FESEM.
Ratio (t) =
Ct − C0 × 100%, C0
(5)
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Fig. 3. Cross-sectional FESEM morphology of PVDF hollow fiber membranes: (a) M0, (b) M6, (c) M10, (d) M14, (e) M18.
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Fig. 3. (Continued)
where Ct is the contact angle at time t (t = 5, 30 s), and C0 is the contact angle at time 0 s.
3. Results and discussion 3.1. Morphologies of PVDF hollow fiber membranes
2.3.6. Apparent viscosity of the polymer solutions The viscosities of the casting solutions with different PVDF dosage were determined at 60 ◦ C by a rotational viscometer (NDJ-7, Shanghai, 750 r/min). 2.3.7. Maximum pore size of the membrane The maximum pore size of the membranes was obtained using the bubble point method with alcohol as wetting liquid and calculated using Eq. (6) rp =
2 , P
(6)
where rp is the maximum pore size of the membrane (m), is the surface tension at the alcohol/air interface (N m−1 ), and P is the pressure. At room temperature (20 ◦ C), is equal to 22.32 × 10−3 N m−1 . 2.3.8. Differential scanning calorimetry (DSC) The membrane crystallization was characterized through differential scanning calorimetry (PerkinElmer, DSC-7) at 10 ◦ C/min increasing temperature rate. Melting heat was determined from the melting peak area. Crystallinity was evaluated using Eq. (7) Xc (%) =
Hf × 100%, Hf∗
(7)
where Hf is the melting enthalpy for a 100% crystalline PVDF and the value is 104.75 J/g [15]. Hf∗ is the melting enthalpy of the PVDF membranes.
The FESEM micrographs in Fig. 3 show the structure of the hollow fiber membranes prepared by the HR method for the different concentrations (6, 10, 14, and 18 wt%) of PVDF in the polymer solutions. Fig. 3(a1, b1, c1, d1, e1) and (a2, b2, c2, d2, and e2) show the whole cross-sectional structure of the membrane and the enlarged structure, respectively. The low magnification image of the crosssection in Fig. 3(a1) shows that the PVDF matrix membrane is a kind of homogeneous membrane. Moreover, the interconnected sponge-like microstructure can obviously be observed as the magnification is increased. There is no skin layer at the cross-section near the outer surface as shown in Fig. 3(a2). Although a larger pore size and a higher porosity can improve the permeability of the membrane, it can also easily cause the embedded pollution in the filtration process. Obviously, a thin skin layer (coating layer) with small pores is formed at the outer surface of the PVDF matrix membrane due to the HR method with the PVDF casting solution. The thickness of the coating layer increases with the increase in PVDF concentration in the polymer solutions, as shown in Table 2. The coating layer thickness accounts for one-seventh and a quarter of the wall thickness when the PVDF concentration increases from 6 to 18 wt%. The enhancement of the wall thickness increases the resistance of the membranes in water filtration and decreases the water flux. With the minimal concentration (6 wt%) of the PVDF polymer solution, the thickness of the coating layer is thin, and there is no apparent finger-like pore structure within the membrane. In this case, the performance of the coating layer induced by the phase separation is poor, and sometimes the membrane structure is not easy to form. With the increment in the PVDF concentration (10 and 14 wt%) in the polymer solutions, the thickness of the coating layer increases and leads to the typical finger-like structure. The addition of PVDF may delay the coagulation process, and a
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Table 2 Characterization of PVDF hollow fiber membranes. Membrane ID
M0
M6
M10
M14
M18
ODa ,IDb /mm Wall thickness (mm) Coating layer thickness (mm)
1.214,0.729 0.235 ± 0.009 0
1.410,0.839 0.283 ± 0.007 0.035 ± 0.003
1.200,0.721 0.258 ± 0.024 0.040 ± 0.002
1.237,0.707 0.281 ± 0.017 0.051 ± 0.005
1.234,0.716 0.277 ± 0.023 0.068 ± 0.005
a b
Outer diameter. Inner diameter.
long finger-like structure will become into a short finger-like structure and thereafter a sponge-like structure, as shown in Fig. 3(c2, d2). Generally, an instantaneous demixing leads to a finger-like structure, whereas a delayed demixing leads to a sponge-like structure of the as-spun membrane [16]. The viscosity of the solutions increases with increase in PVDF concentration in the polymer solutions; consequently, the kinetic exchange mechanism between the solvent in the polymer solutions and the nonsolvent in the coagulation bath becomes slow, and then a delayed demixing behavior becomes more distinct, inhibiting the growth of the finger-like pore structure. The micrographs in Fig. 3(b2, c2, d2, e2) show three parts near the outer surface of the HR membranes. The outermost layer is the PVDF coating layer, and the innermost layer is the PVDF matrix membrane. The middle layer, between the PVDF coating layer and the PVDF matrix membrane, is the interface layer, which endows the HR membranes with high interfacial bonding. Fig. 4 shows the FESEM results of the outer surfaces of the hollow fiber membranes. The matrix membrane (M0) has a rougher outer surface within obvious big pores; this surface was prepared by the melt spinning and stretching process. The outer surface of the HR PVDF hollow fiber membrane (M10) appears denser than M0. Fig. 1 shows a schematic diagram describing the HR PVDF membrane induced by the dry–wet spinning process, based on the
results of FESEM as seen in Fig. 3. The PVDF matrix membrane is first goes through the co-extrusion device directly. The polymer solutions have some effects on the matrix membrane, such as infiltration and diffusion, as shown in the process (I). Infiltration occurs at the time the polymer solutions contacted the surface of the matrix membrane, and then the matrix membrane slightly dissolves during the infiltration stage owing to the codissolved solvent DMAc. The entanglement of PVDF chains in the polymer solutions and the matrix membrane is controlled by the interface diffusion between the polymer solutions and the matrix membrane [17]. The degree of infiltration and diffusion effects significantly affect on the interfacial bonding strength. The HR PVDF hollow fiber membranes solidify in the coagulation bath, as shown in the process (II), and then they are taken up by the wire-wrapped rolls, as shown in the process (III). As it can be seen from Fig. 3, the cross-section model of the HR PVDF membrane has been built up for the results of the FESEM. The PVDF matrix membrane prepared by the meltspinning method has high porosity and a big pore size. The coating layer possesses a compact surface and a small pore size, which were prepared via the phase inversion method induced by immersion precipitation. The properties of the interface layer are between that of the coating layer and the matrix membrane.
3.2. Infiltration properties of the polymer solutions
Fig. 4. Outer surface FESEM morphology of PVDF hollow fiber membranes: (a1) M0, (a2) M10.
Generally, one phase contacts with the other phase of the solution or melt states in the two phases of the composite materials, and then combines with the curing reaction. The adhesion action refers to the state in which the two surfaces are combined with the chemical or physical interaction [18]. An interesting phenomenon occurs in Fig. 3(b2, c2, d2): a significant transition layer structure appears on the HR membranes. This indicates that the interfacial bonding effect contributes to the infiltration and diffusion effects from the polymer solutions to the matrix membrane. Poor infiltration between the polymer solutions and the matrix membrane will lead to defects or stress concentration near the interface. Consequently, the interfacial bonding strength decreases; on the contrary, favorable infiltration can improve greatly on the interfacial bonding strength. The contact angle measurement was used to characterize the infiltration ability [7]. The contact angle between the matrix membrane and the polymer solutions must be small enough to achieve a sufficient infiltration and contacting between them. As shown in Fig. 5, with the increase in PVDF concentration, the contact angles dramatically increase at 0 s, and then the interfacial bonding strengths decrease. The changes in the apparent viscosity and surface tension have a similar trend with the contact angle. The apparent viscosity of the polymer solutions containing 18 wt% PVDF is up to 2400 mPa s, which is obviously greater than those of the low PVDF concentration. The contact angle is greater than 90◦ at 0 s with the 18% PVDF concentration. This negative infiltration pattern will affect the interfacial bonding strength. The swelling states appear on the matrix surface when it contacts with DMAc in the polymer solutions as a favorable solvent. The interface layer is on the condition of the dispersion induced by the interdiffusion of the PVDF molecules between the polymer solutions and the matrix membrane. The deeper the degree of the interdiffusion is, the higher
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Fig. 5. Effects of the PVDF concentration on the time dependence of contact angle and the viscosity of the polymer solutions.
the interfacial bonding strength. The degree of the interdiffusion in this study is defined as the time dependence of the contact angle on the membranes. As can be seen, the change rates of the contact angle (5 s, 30 s) when compared with 0 s are significantly reduced as the PVDF concentration reaches 18 wt% in the polymer solution, and then the degree of the interdiffusion decreases. The degree of the interdiffusion with 18% PVDF concentration is worse than 10%, which is well coincident with the value of the negative infiltration pattern. The change rates of the contact angle at 5 s are nearly almost the same as at 30 s. This indicates that the effect on the diffusion occurred mainly at the first few seconds. Thus, the appropriate time of the polymer solutions exposed in the air environment significantly affects the diffusion time. Therefore, the appropriate diffusion time of the polymer solutions not only obtains the best infiltration and the diffusion effect on the matrix membrane, but also results in a slight reduction in the properties of the matrix membrane, which provides the major tensile strength of the HR membranes. 3.3. Mechanical properties of the HR PVDF membrane 3.3.1. Morphologies of membrane after tensile strength test Fig. 6 shows the significant difference in morphology between the inner and the outer surfaces of the HR PVDF membrane (M14) before and after the tensile strength measurements. Samples were collected approximately at 3 cm of the membrane fracture point. The outer surface image after the test exhibits larger pores, while no obvious pores can be identified on the outer surface before the test. Fig. 6(b) compares the cross-section morphology of the HR PVDF membrane before and after tensile strength measurements. Lots of microvoids are created on the outer surface during the stretching process, whereas there is no observable change in the cross-section. The fine interfacial bonding is formed between the coating layer and the matrix membrane. There is no significant
Fig. 6. Outer surface and cross-sectional FESEM morphology of HR PVDF membranes (M14) before and after tensile strength measurement: (a1, b1) before measurement, (a2, b2) after measurement.
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Fig. 8. Effects of PVDF concentration on the porosity and the maximum pore size.
Fig. 7. Effects of PVDF concentration on the mechanical properties of membranes.
flake phenomenon in the interface after stretching. Meanwhile, the thickness of the coating layer and the pore structure of the HR PVDF membrane changed to some extent. This indicates that the interface has high bonding strength after the tensile test due to the interdiffusion of the PVDF molecules. It is important for the resistance of antiwash performance and service life of the hollow fiber membranes. 3.3.2. Tensile strength of the hollow fiber membranes Fig. 7(a) shows the results of the tensile strength and elongation at break of the HR PVDF membranes. The tensile strength of the HR membranes, except for that of the M10 membrane, decreases slightly compared with the matrix membrane. The tensile strength of the HR PVDF membranes depends mainly on the matrix membrane. In the HR process, the erosion effect of the solvent DMAc on the matrix membrane may cause some unfavorable influence on the tensile strength of HR PVDF membranes. We know that the tensile strength of PVDF membranes prepared by the wet spinning method is almost 1–3 MPa [19,20]. A low tensile strength in a PVDF membrane may easily cause fiber breakage in the state of serviceability. The HR PVDF hollow fiber membranes prepared by the HR method rather than by the wet phase inversion method have higher mechanical properties. The tensile strength of the HR PVDF membranes is nearly 10 MPa, which is adequate for MBR application. The elongation at break increased much more, from 50% (M0) to 99% (M14). The crystallinity of PVDF in the matrix is higher than that in the coating layer, as shown in Table 3. It is beneficial to the tensile strength and elastic modulus with the increase in crystallinity of the polymer. There have been more amorphous chains in the coating layer than in the matrix because of the lower
crystallinity. Therefore, the coating layer and the matrix membrane undergo various changes during deformation at the different strain rates. The orientation of the amorphous chains first occurs at the coating layer; the matrix at the stage of the strain increases and the stress remains. The strain increases with the continued transmission of external force that passes through the interface layer, and then the supermolecule structure of PVDF goes through some complex changes. These phenomena cause significant changes to the elongation at break of the HR membranes in macroscale. The change ratios of elongation at break increase from 5.79% for the M6 membrane to 85.44% for the M14 membrane. A higher interfacial stress transfer coefficient is achieved due to the better bonding strength of the HR membranes, which is proportional to the change ratios of elongation at break. It also agrees well with the infiltration properties results. The better the infiltration properties of the polymer solutions to the matrix membrane surface, the higher the interfacial stress transfer coefficient. The elongation at break then decreases as the PVDF concentration increases at 18%. When the PVDF concentration increases to a certain level, the polymer gradually shows that the unique elastic nature and the rigidity of the molecular chains has an increase to some extent. The membrane structures change much, as shown in Fig. 3, resulting in the weakening of the elasticity and in getting a peak of the elongation. 3.4. Permeation properties of the hollow fiber membranes Fig. 8 shows the maximum pore size and porosity of the hollow fiber membranes. The maximum pore size and porosity have the same decreasing tendency as the PVDF concentration increase in the polymer solutions. The HR PVDF membranes have a denser outer surface, as shown in Fig. 4. This means that the HR PVDF membranes will possess a smaller maximum pore size than the matrix membrane, which could improve the rejection of the membrane. The porosity of HR PVDF membranes also decreases with the increase in PVDF concentration.
Table 3 Crystallinity of the PVDF membrane. Membrane
Coating layer (14%PVDF)
Matrix-PVDF(M0)
Hf (J/g) Xc (%)
40.8 38.9
50.06 47.8
Fig. 9. Effect of the PVDF concentration on pure water flux and protein rejection of the membranes.
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the PSF of the matrix membrane is lower than that of the HR membranes. The high permeability and rejection of the HR PVDF membranes in the protein solution effectively reduce the frequency of the backwashing and aeration, and this can indirectly improve the service life of the membrane. In addition, the HR membranes have nearly the same tensile strength as the matrix membrane. The PSF increases when the PVDF concentration in the polymer solution changes from 6 to 10 wt% and then decreases when it changes to 14 wt%. The M18 membrane has the lowest PSF compared with other HR membranes, but this PSF is still greater than that of the M0 membrane. The better performance of the HR membranes of the protein solution has great value in MBR system applications. 4. Conclusion Fig. 10. Effects of PVDF concentration on the protein solution flux changes with time.
Fig. 9 shows the PWF and separation of the protein solution of the prepared HR PVDF hollow fiber membranes. The PWF of the HR PVDF membranes decreases with the increase in PVDF concentration in the polymer solutions. According to Fig. 9, PWF decreases significantly from 1230.4 L m−2 h−1 ·0.1 MPa for the M0 membrane to 442.1 L m−2 h−1 ·0.1 MPa for the M10 membrane, and then decreases slightly with the increase in PVDF concentration from 14 to 18 wt%. The PWF decreases while the rejection ratio increases with the increase in PVDF concentration. The wall thickness of the HR PVDF membranes is thicker than the matrix membrane, which extends the membrane filtration path and increases the intrinsic resistance of the membrane. Therefore, the PWF of the HR PVDF membranes is lower than that of the matrix membrane. As shown in Fig. 9, the PWF decreases with the increase of PVDF concentration in polymer solution. This is because the highly polymer concentration results to lower membrane porosity as shown in Fig. 8 and the porosity is a key factor impacting the membrane permeability. The rejection depends more on the denseness of the skin layer than on the structure of the cross-section [21]. The skin layers of the HR PVDF membrane are denser than those of the PVDF matrix membrane (Fig. 4). Thus, the maximum pore size of the HR PVDF membranes is smaller than that of the matrix membrane. With the application of the HR method, the increasing rate of the protein separation is significant for some special water treatment. Fig. 10 presents the PSF of the PVDF hollow fiber membranes. Under constant trans-membrane pressure (TMP), the effects of membrane fouling or concentration polarization are usually distinguished by a more or less considerable permeation flux decline [22]. As observed, the reduction of PSF for the HR PVDF membranes (M6, M10, M14, and M18) is lower than that of the matrix membrane (M0). For example, the PSF decreases from 136.6 to 51.8 L m−2 h−1 ·0.1 MPa for the M0 membrane, whereas it decreases from 126.1 to 124.8 L m−2 h−1 ·0.1 MPa for the M18 membrane. After filtration of the protein solution for 100 min, the PSF of the M0 membrane is considerably lower than that of the HR membranes. At the early stage of the protein solution filtration, the matrix membrane has larger pore entrances than the HR membranes, which results in a series pore-blockage and irreversible membrane fouling. The HR membranes with smaller pore size could reduce the pore-blockage phenomenon at the beginning of the filtration. The mechanism of filtration is then transferred to cake filtration more rapidly than the matrix membrane under the same filtration condition, which reduces the irreversible membrane fouling and slows down the membrane flux decline [23]. Fig. 4 also shows that the matrix membrane has a rougher outer surface than the HR membranes have, and this more easily causes membrane fouling. So
The HR PVDF hollow fiber membranes were prepared through the dry–wet spinning method of coating PVDF polymer solutions on the PVDF matrix hollow fiber membrane surface. The HR PVDF hollow fiber membranes have a favorable interfacial bonding between the coating layer and the matrix membrane due to the existence of the interface layer, as seen from the FESEM. The outer surface of the HR PVDF membrane is denser than that of the matrix membrane. With an increasing PVDF concentration in the polymer solutions, the contact angle significantly increases at 0 s, which results from the degree of the interdiffusion with 18% PVDF concentration being worse than 10%, and then the interfacial bonding strength decreases. The tensile strength of the HR PVDF membranes shows a slight reduction when compared with the matrix membrane, but the elongation at break increases much more, from 50% (M0) to 99% (M14). The porosity and maximum pore size of the HR PVDF membranes decrease with an increase in PVDF concentration, which can result in lower PWF and higher rejection of protein. The HR PVDF membranes also have higher PSF than the matrix membrane. Acknowledgements The authors thank the financial support of the National Basic Research Development Program of China (973 Program, 2012CB722706), the National Natural Science Foundation of China (20874073 and 21274109), the Science and Technology Plans of Tianjin (10SYSYJC27900), and the Basic Research Program of China National Textile and Apparel Council. References [1] E. Yuliwati, A.F. Ismail, T. Matsuura, M.A. Kassim, M.S. Abdullah, Effect of modified PVDF hollow fiber submerged ultrafiltration membrane for refinery wastewater treatment, Desalination 283 (2011) 214–220. [2] X.Y. Hu, Y.B. Chen, H.X. Liang, C.F. Xiao, Preparation of polyurethane/poly (vinylidenefluoride) blend hollow fiber membrane using melt spinning and stretching, Materials Science and Technology 27 (2011) 661–665. [3] S.I. Patsios, A.J. Karabelas, An investigation of the long-term filtration performance of a membrane bioreactor (MBR): the role of specific organic fractions, Journal of Membrane Science 372 (2011) 102–115. [4] N. Fallah, B. Bonakdarpour, B. Nasernejad, M.R. Alavi Moghadam, Long-term operation of submerged membrane bioreactor (MBR) for the treatment of synthetic wastewater containing styrene as volatile organic compound (VOC): effect of hydraulic retention time (HRT), J. Hazard. Mater. 178 (2010) 718–724. [5] M.J. Kim, B. Sankararao, C.K. Yoo, Determination of MBR fouling and chemical cleaning interval using statistical methods applied on dynamic index data, Journal of Membrane Science 375 (2011) 345–353. [6] K. Murase, H. Habara, H. Fujiki, T. Hirane, M. Mizuta, Porous membrane, US 2002/0046970 A1, 2002-04-25. [7] J. Liu, P. Li, Y. Li, L. Xie, S. Wang, Z. Wang, Preparation of PET threads reinforced PVDF hollow fiber membrane, Desalination 249 (2009) 453–457. [8] A. Cui, Z. Liu, C. Xiao, Y. Zhang, Effect of micro-sized SiO2-particle on the performance of PVDF blend membranes via TIPS, Journal of Membrane Science 360 (2010) 259–264. [9] X. Lu, X. Li, Preparation of polyvinylidene fluoride membrane via a thermally induced phase separation using a mixed diluent, Journal of Applied Polymer Science 114 (2009) 1213–1219.
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