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Microstructure construction of polypropylene-based hollow fiber membranes with bimodal microporous structure for water flux enhancement and rejection performance retention
Separation and Purification Technology 213 (2019) 328–338
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Microstructure construction of polypropylene-based hollow fiber membranes with bimodal microporous structure for water flux enhancement and rejection performance retention ⁎
Dajun Luoa,b, Fujian Weib, Huiju Shaoa,b, Lan Xiea, Zhenyu Cuib, Shuhao Qina,b, , Jie Yua,b, a b
T
⁎
College of Materials and Metallurgy, Guizhou University, Guiyang 550025, People's Republic of China National Engineering Research Center for Compounding and Modification of Polymeric Materials, Guiyang 550014, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bimodal microporous structure Hydrophilicity Polypropylene hollow fiber membrane
The low porosity, low pore connectivity and poor hydrophilicity of polypropylene hollow fiber membranes (PPHFMs) reduced membrane performance and limited its application in water treatment. Hence, the bimodal microporous structure including two different grades and independent pore size distributions was constructed in hydrophilic polypropylene (PP)/poly(ethylene-co-vinyl alcohol) (EVOH)/maleic anhydride grafted polypropylene (PP-g-MAH) hollow fiber membrane (PP/EVOH/MAH-HFM) via melt blending combining with meltspinning and stretching (MS-S). The addition of PP-g-MAH formed compatible interfaces to optimize the bimodal microporous structure in PP/EVOH/MAH-HFM. The porosity and pure water flux of PP/EVOH/MAH-HFM with stretching ratio of 200% respectively increased to 81.5% and 322.7 L/(m2·h), which respectively increased by 27.3% and 118.0% compared to PPHFM with stretching ratio of 200% due to the large micropores in bimodal microporous structure. The rejection performance of PP/EVOH/MAH-HFM was also retained due to the small micropores in bimodal microporous structure. The reactive groups in EVOH and PP-g-MAH significantly improved the hydrophilicity of PP/EVOH/MAH-HFM, while also improving the antifouling property of membrane. Moreover, the formation mechanism of bimodal microporous structure was systematically investigated in combination with physical modeling and FESEM method. This work supplied a new membrane structure and provides a new perspective for the fabrication of high performance polypropylene-based hollow fiber membranes by MS-S.
1. Introduction Polypropylene hollow fiber membrane (PPHFM) is important in membrane separation technology and widely used in many fields, e.g., desalination of sea water, wastewater treatment, gas separation, petroleum chemical industry and energy storage [1–3]. There are mainly two fabrication techniques for manufacturing PPHFM. One is thermally induced phase separation (TIPS) method [4,5]. Another one is melt spinning and stretching (MS-S), which is mainly applicable to semicrystalline polymers, such as polypropylene (PP), polyethylene, and polyurethane [1,6,7]. The PPHFM prepared by MS-S exhibit higher mechanical properties, uniform pore distribution and uniform pore size. And MS-S is a clean and economical process. Hence, MS-S shows more advantages over TIPS. Although the PPHFM prepared by MS-S exhibit many advantages, it still has several disadvantages, such as low porosity, poor connectivity
⁎
and poor hydrophilicity. The disadvantages reduce the water flux of the PPHFM and limit applications of PPHFM in wastewater treatment, desalination of sea water and biomedical applications [8–10]. The porosity and connectivity of the membrane could be increased by increasing stretching ratio [11]. However, the pore size was also proportional to the stretching ratio, which would decrease the rejection performance of the membrane [12]. Therefore, in general, the high water flux and high rejection performance can not be simultaneously achieved. In the field of inorganic porous materials, there was an inorganic material with a special bimodal pore structure including two different grades and independent pore size distributions [13–15]. The large pores show a smaller diffusion resistance and the small pores serve as sites for adsorption and reaction of material. Imagining if the PP-based hollow fiber membrane (PPBHFM) prepared by MS-S has the bimodal microporous structure, the large micropores can improve the porosity and
Corresponding authors at: College of Materials and Metallurgy, Guizhou University, Guiyang 550025, People's Republic of China. E-mail addresses: [email protected] (S. Qin), [email protected] (J. Yu).
https://doi.org/10.1016/j.seppur.2018.12.052 Received 5 September 2018; Received in revised form 12 December 2018; Accepted 21 December 2018 Available online 21 December 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental
connectivity of membrane, and the small micropores can retain the rejection performance of membrane. The high water flux can be achieved while retaining rejection performance. To our knowledge, work on the bimodal microporous PPBHFM prepared by MS-S has not been published. PPHFM formation mechanism of MS-S was based on the separation of oriented lamellae during stretching [16–18]. The uniform microporous structure (pore diameter less than 0.3 μm) could be obtained by the separation of oriented lamellae [19]. It has been reported that microporous membranes could also be obtained by separating the twophase interfaces of immiscible blends [20–22]. Mei et al. [21]. prepared the PP/easy hydrolytic degradation polyester microporous hollow fibers by blend spinning and cold stretch process. The large-sized pore structure (pore diameter greater than 3 μm) could be obtained by the separation of two-phase interfaces. Unfortunately, the PP phase did not form an oriented lamellar structure in this work, because the large amount of easy hydrolytic degradation polyester (30 wt%) was added and no stress was applied during melt spinning. The micropores formed by lamellae separation were not formed in a large amount. Therefore, it is possible to realize the construction of bimodal microporous structure in PPBHFM by adding a suitable amount of incompatible second phase and adjusting the manufacturing process. Hydrophilic PPHFM is usually made by modifying the base membrane surface. Such as, radical grafting reactions, plasma treatment, ultraviolet grafting reactions, coating and so on [8–10,23,24]. However, all the above methods aim at the surface modification of PPHFM. Moreover, weak interaction between PPHFM and hydrophilic groups resulted in the release and elution of hydrophilic compound from the surface in the practical application [25]. It has been reported that PPHFM can obtain permanent hydrophilicity by blending with hydrophilic polymer [1,21]. Therefore, the hydrophilic polymer as the incompatible second phase mentioned above can not only construct bimodal microporous structure, but also improve the hydrophilicity of PPBHFM. The assumption structure of PPHFM and hydrophilic PPBHFM with bimodal microporous structure are shown in Fig. 1. In this paper, the construction of bimodal microporous structure in PPBHFM and the improvement of hydrophilicity of PPBHFM were achieved by melt blending and MS-S with poly(ethylene-co-vinyl alcohol) (EVOH) as hydrophilic incompatible second phase. The formation mechanism of bimodal microporous structure was systematically investigated in combination with physical modeling and FESEM method. The results showed that the bimodal microporous structure was well constructed in PP/EVOH/MAH hollow fiber membrane (PP/ EVOH/MAH-HFM) because the compatible phase interfaces were formed by adding PP-g-MAH. The small micropores formed by separation of PP lamellae. The compatible phase interfaces separate to form large micropores while also forming a small amount of microfibrils. Both porosity and hydrophilicity of PP/EVOH/MAH-HFM were improved. The high water flux could be achieved while retaining rejection performance. The introduction of bimodal microporous structure is referential for the development of high performance PPBHFM.
2.1. Materials PP (T30S) was supplied by Sinopec Dushanzi Petrochemical Company (Xinjiang, China). The isotacticity is 98% and the powder ash is 0.01%. The melting point, Tm, of the resins obtained by differential scanning calorimetry was 166 °C. The melt flow index of PP was 3.35 g/ 10 min. EVOH (L171B) was purchased from Kuraray International Trading (Shanghai) Co., Ltd. (Shanghai, China). The content of vinyl was 27 mol%. The melt index was 4.0 g/10 min, and the melting point was 191 °C. As compatibilizer, PP-g-MAH (CA100) was purchased from Arkema (China) Investment Co., Ltd. (Beijing, China). The melt index was 8.0 g/10 min, and the melting point was 143 °C. 2.2. Sample preparation PPBHFM were prepared by three steps. Firstly, the extrusion of blends. For PP/EVOH blend, the dried mixture (90 wt% PP and 10 wt% EVOH) was extruded on a twin-screw extruder (L/D = 48, D = 42 mm) at 215 °C. For PP/EVOH/MAH blend, the dried mixture of 10 wt% EVOH and 5 wt% PP-g-MAH was first extruded and then the blend was extruded with 90 wt% PP. All samples from extruder were dried for 12 h at 80 °C. Secondly, the precursor hollow fibers were prepared via melt spinning. All pellet samples were extruded and spun to form hollow fibers on a single-screw extruder with a single-hole tube-in orifice spinneret (inner and outer diameters of 0.50 and 2.50 mm) at 190 °C. Melt-draw ratio was 56. Finally, the annealing and stretching of precursor hollow fibers. The precursor hollow fibers were annealed under relaxation conditions at 150 °C for 1 h. The annealed hollow fibers were firstly cold-stretched by 20% at 20 °C. After cold-stretching, the temperature inside the stretcher was raised to 140 °C and hot-stretching was started. The hot-stretching ratio were 80, 130, 180, 230 and 280%, respectively. Finally, PPBHFM were re-annealed in the stretcher at 140 °C for 1 h to prevent shrinkage. 2.3. Characterizations Differential scanning calorimetry. TA-Q10 from TA Instruments (America) was employed to investigate the crystallization behavior of samples. All tests were carried out under a nitrogen atmosphere. For pure PP and blends, the thermal history of the samples was firstly eliminated by heating the samples at 220 °C for a period of 5 min. Subsequently, the samples were cooled to 50 °C at 10 °C/min rate and held at 50 °C for 5 min. Then, the samples were heated to 220 °C at 10 °C/min rate again. For hollow fibers, the samples were heated to 220 at 10 °C/min rate. Field emission scanning electron microscope. Quanta FEG250 of FEI company (America) was used to observe the micromorphology of samples and measure the oxygen content of membranes.
Fig. 1. The assumption model of (a) PPHFM and (b) hydrophilic PPBHFM with bimodal microporous structure. 329
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Fig. 2. (a) Crystallizing curves and (b) secondary melting curves of pure PP and blends with rate of 10 °C/min.
Contact angle measurements. The water contact angle (WCA) was measured using a 25S optical contact angle tester from Germany DSA company (Germany) to evaluate the hydrophilicity of the samples. Pore size distribution. The pore size distribution of samples was measured using an automatic mercury porosimeter (AutoPore IV 9510, Micromeritics, America) . Porosity. Weighing method was selected to measure the porosity of samples [11]. The porosity was calculated as:
ρ −ρ ·100% P= 0 ρ0
FRR =
V AΔt
Jp ⎞ ·100% DRt = ⎛1 − J w1 ⎠ ⎝
(5)
Jw2 − Jp ⎞ ·100% DRr = ⎛ ⎝ Jw1 ⎠
(6)
J − Jw2 ⎞ DRir = ⎛ w1 ·100% ⎝ Jw1 ⎠
(7)
⎜
(1)
⎜
⎜
⎟
⎟
⎟
Particle size distribution. The particle size distribution of residual carbon particles in the permeate solution was measured by laser particle sizer (ZEN3600, Malvern, UK) 3. Results and discussion
(2) 3.1. Crystallization behavior and morphology of pure PP and blends
where J is the pure water flux at the pressure of 1 bar, L/(m2·h); V is the volume of permeate water, L; Δt is the testing time, h; A is the effective area of each membrane module, m2. Rejection. The rejection performance of the membranes was characterized by rejecting carbon ink with a concentration of 0.1%. The average particle size of the carbon particles obtained by laser particle sizer (ZEN3600, Malvern, UK) was 221.9 nm. Rejection (R%) was calculated as follows[11]:
Cp ⎤ ·100% R% = ⎡1 − ⎢ Cr ⎥ ⎣ ⎦
(4)
Furthermore, the antifouling property of membranes was also evaluated by the total flux decrease ratio (DRt), reversible flux decrease ratio (DRr) as well as irreversible flux decrease ratio (DRir).
where P is the porosity of membranes; ρ0 is the density of pure PP or blends; ρ is the density of membranes. Pure water flux. The internal pressure mode was used to measure the pure water flux. The test temperature was approximately 20 °C. All the modules were pressurized under 1.5 bar for 0.5 h. Next, the pure water test was carried out under 1 bar. The pure water flux was calculated as follows [11]:
J=
Jw2 ·100% Jw1
3.1.1. Non-isothermal crystallization behavior of pure PP and blends The DSC curves in Fig. 2 show a single exothermic and endothermic peak in the crystallizing and secondary melting curves of pure PP. The crystallizing and melting peaks temperature values are 110.6 °C and 165.6 °C, respectively. Two crystallizing peaks and melting peaks respectively are observed in the PP/EVOH blend (Fig. 2a,b), which shows the PP/EVOH blend is thermally immiscible system. The PP phase begins to crystallize after the EVOH phase is completely crystallized. The crystallizing and melting peaks temperatures of EVOH phase in the PP/ EVOH/MAH blend shifts to low temperature. The results show that the compatibility of blend system is improved by adding PP-g-MAH, but most EVOH are still crystallized independently. In addition, the crystallizing peak temperatures of PP in PP/EVOH blend and PP/EVOH/ MAH blend shift to high temperature. Moreover, the time need to reach the maximum of exothermic peak during non-isothermal crystallization decrease remarkably after the addition of EVOH. As we know, the crystallizing peak corresponds with both rate of nucleation and growth of polymer during non-isothermal crystallization. PP/EVOH blend and PP/EVOH/MAH blend show a heterogeneous nucleation process implying that the EVOH essentially acts as an effective nucleating agent for PP phase and promotes the crystallization rate of PP during nonisothermal crystallization. The result is consistent with the reports in the literature [26]. The melting enthalpy (ΔH) of pure PP, PP/EVOH blend and PP/EVOH/MAH blend are 80.3, 73.1 and 76.8 J/g,
(3)
where Cr is the concentrations in original solution; Cp is the concentrations in permeate solution. The turbidimeter (2100Q, HACH, America) was used to measure turbidity to determine the concentration of original solution and permeate solution. Antifouling property. The pure water flux test was first carried out by external pressure method to obtain pure water flux Jw1, L/(m2·h), and then bovine serum albumin (BSA, 1000 mg/L in phosphate buffered solution, PH = 7.0) solution was used instead of the pure water for testing. After BSA solution filtration for 1 h, the flux of BSA solution Jp, L/(m2·h), was measured. Then, after backwashing at 0.1 MPa for 10 min, a second pure water flux test was performed, and the secondary pure water flux was recorded as Jw2, L/(m2·h). The flux recovery ratio (FRR) used to characterize the antifouling property of membranes can be calculated by the following equation [8]: 330
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Fig. 3. FESEM images of (a) pure PP and (b) PP/EVOH blend; the diameter distribution of EVOH particles in (c) PP/EVOH/MAH blend and (d) PP/EVOH blend; and (e, e1) FESEM images of PP/EVOH/MAH blend.
Meanwhile, the diameter of spherical particles formed by EVOH is significantly reduced. The diameter distribution of EVOH particles is in the range of 0.2–1.8 μm, and the average diameter is 0.8 μm (Fig. 3c). The main reason may be the fact that the addition of PP-g-MAH improves the compatibility of the blend system and forms a compatible interface between the PP phase and the EVOH phase.
respectively. Since the PP/EVOH blend contains only 90 wt% of PP, the ΔH value is decreased. However, the total PP content increased after the addition of 5 wt% PP-g-MAH, so the ΔH value of PP/EVOH/MAH blend increased again. 3.1.2. Morphology of pure PP and blends The phase morphology in pure PP and blends are displayed in Fig. 3. The PP/EVOH blend exhibits typical two-phase structure, in which EVOH forms the “island” structure as large spherical particles are evenly distributed in the “sea”. The diameter distribution of EVOH particles is in the range of 0–9 μm, and the average diameter is 4.0 μm (Fig. 3d). The phase separation and a clear interface between PP phase and EVOH phase are also observed in Fig. 3b. However, there is no obvious phase separation after adding PP-g-MAH, but the interfaces between PP phase and EVOH phase are still observed in the Fig. 3(e1).
3.2. Characterization of bimodal microporous structure 3.2.1. DSC analysis of hollow fibers As shown in Fig. 4, two melting peaks are observed in the precursor and annealed hollow fibers of PP/EVOH and PP/EVOH/MAH, which show the PP phase and EVOH phase are still independently crystallized during the crystallization process. EVOH phase is still independently exists in the PP phase. After the annealing, the lamellar thickness of the 331
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Fig. 4. First melting curves of (a) precursor hollow fibers and (b) annealed hollow fibers with rate of 10 °C/min.
in the uniform small micropores. A small amount of microfibrils can be observed inside the large micropores. The shape of the EVOH particles remains irregularly spherical. The longitudinal cross-section images of hollow fiber membranes with stretching ratio of 200% are given to further prove that the bimodal microporous structure is well constructed in the PP/EVOH/ MAH-HFM (Fig. 6). A longitudinal cross-section image reveal that PPHFM has a nearly isotropic structure (Fig. 6a). A poor connectivity can be observed in the PPHFM due to superposition of lamellae (Fig. 6a1). As shown in Fig. 6b, a bimodal microporous structure is formed in the membrane wall of PP/EVOH-HFM. The pore size of the large micropores is large due to the incompatibility of the interfaces. Moreover, A poor connectivity is also observed in the PP/EVOH-HFM. However, Fig. 6(c) shows that a perfect bimodal microporous structure is constructed in the membrane wall of PP/EVOH/MAH-HFM. The large micropores are evenly distributed in the uniform small micropores. Since the large micropores reduce the proportion of lamellae superposition, a perfect connectivity is also observed in the PP/EVOH/MAHHFM. Importantly, PP/EVOH/MAH-HFM is not the case of gradient structure membrane, where smaller pores are in the top dense layer for selectivity and greater pores are in the support porous layer for flux. The top layer and support layer are not in the same level of the gradient
polymer increases and the defects of crystal phase are also reduced. Hence, the melting peaks temperature of annealed hollow fibers move to high temperature. The enthalpies of hollow fibers also increase significantly after annealing. Moreover, a small peak is observed at 154.2 °C after annealing, which is due to the formation of a certain number of metastable folding chain conformations during annealing [27].
3.2.2. Morphology of bimodal microporous structure The inner surface images of all membranes with stretching ratio of 200% are shown in Fig. 5. The uniform micropores and microfibrils can be clearly observed in PPHFM (Fig. 5a, a1). However, there are two types of pore structures (bimodal microporous structure: the small micropores and the large micropores) in the PP/EVOH hollow fiber membrane (PP/EVOH-HFM) (Fig. 5b, b1). There is clear separation between the two-phase interfaces, and the shape of the EVOH particles become oval. Due to the presence of the large sized EVOH particles and incompatible interfaces, the outer diameter of the membrane and the stress distribution become non-uniform during stretching, the distribution of the small micropores becomes more confusing. A better bimodal microporous structure can be clearly observed in PP/EVOH/ MAH-HFM (Fig. 5c, c1). The large micropores are uniformly distributed
Fig. 5. The inner surface of hollow fiber membranes with stretching ratio of 200%. (a, a1) PPHFM; (b, b1) PP/EVOH-HFM; and (c, c1) PP/EVOH/MAH-HFM. 332
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Fig. 6. The longitudinal cross-section images of hollow fiber membranes with stretching ratio of 200%. (a, a1) PPHFM; (b) PP/EVOH-HFM; and (c) PP/EVOH/MAHHFM.
EVOH precursor hollow fiber are observed in Fig. 7B. The interface between PP phase and EVOH phase is clear because PP phase and EVOH phase are incompatible. The spherical particles of EVOH will become elliptical after melt spinning because there is stress perpendicular to the traction direction during spinning (Fig. 7b2). However, the diameter of spherical particles formed by EVOH is significantly reduced and the number of particles significantly increased after adding PP-g-MAH. Meanwhile, the compatible interfaces between the PP phase and the EVOH phase will be formed (Fig. 7c1). The compatible interface is more easily deformed under the stress perpendicular to the traction direction during spinning because it is amorphous. Therefore, the central EVOH particles may still maintain a spherical structure after melt spinning (Fig. 7c2).
structure membrane. But PP/EVOH/MAH-HFM with bimodal microporous structure has both greater and smaller pores in the same level. 3.3. The formation mechanism of bimodal microporous structure 3.3.1. The phase morphology in blends and precursor hollow fibers Based on the analysis of the above DSC and FESEM, the model of the phase morphology in blends and precursor hollow fibers are built (Fig. 7). The phase morphologies of pure PP and PP precursor hollow fiber are represented in Fig. 7A. The oriented lamellae in PP precursor hollow fiber will formed via the mechanisms of stress-induced crystallization (Fig. 7a2). The phase morphology of PP/EVOH blend and PP/
3.3.2. The formation of bimodal microporors structure Based on the model of the phase morphology in precursor hollow fibers, the model about the formation of bimodal microporors structure in hollow fiber membranes are built (Fig. 8). The precursor hollow fibers are annealed at appropriate temperatures to perfect the lamellar structure, but the morphology of each phase does not change. For pure PP samples, the micropores and microfibrils will be formed by the separation of the lamellae during the stretching of annealed hollow fiber (Fig. 8a3) [18]. For blends samples, there are two types of pore structures (bimodal microporous structure) in the PP/EVOH-HFM (Fig. 8b3). One is the small micropores formed by separation of PP lamellae. Another one is a large micropores formed by the separation of interfaces between the PP phase and the EVOH phase. The interface separation at the vertices in the stretching direction of EVOH particles develops into the larger interface micropores. Since the stretching process is performed near the melting point temperature of PP phase and the strength of EVOH phase is slightly higher than that of PP phase, EVOH still maintains the ellipsoidal granular structure during stretching. For PP/ EVOH/MAH sample, there is a perfect bimodal microporous structure in the hollow fiber membrane (Fig. 8c3). The compatible phase interfaces separate to form large micropores while also forming a small amount of microfibrils. The presence of microfibers limit the further
Fig. 7. The model of the phase morphology in blends and hollow fibers. (A) Pure PP; (B) PP/EVOH; and (C) PP/EVOH/MAH. 333
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Fig. 8. The model of the formation of bimodal microporous structure in (A) PPHFM; (B) PP/EVOH-HFM; and (C) PP/EVOH/MAH-HFM.
Fig. 9, it can be seen that both types of micropores are formed simultaneously during the stretching process. For the small micropores formed by separation of PP lamellae, the pore size and number of the micropores increase as the stretching ratio increases. The thickness of the lamellar stacks also becomes more uniform. However, when the stretching ratio exceeds 200% and continues to increase, the dislocations and slips occur in the lamellae. Meanwhile, the microfibrils break and the pore size decreases. The formation and enlargement of small micropores are present throughout the stretching process. Therefore, unseparated lamellae stacks can still be observed at low stretching ratios (Fig. 9b, c). For the large micropores formed by the separation of interfaces, the pore size and number of the micropores also increase as the stretching ratio increases. However, when the stretching ratio exceeds 200% and continues to increase, the microfibrils break and the micropore shape becomes elongated. The corresponding pore size is also decreased.
separation of the phase interface. Meanwhile, the diameter of spherical particles formed by EVOH is significantly reduced after adding PP-gMAH. Therefore, the size of large micropores formed by phase interfaces separation in PP/EVOH/MAH-HFM is smaller than that of PP/ EVOH-HFM. Theoretically, the large micropores in the bimodal microporous structure can improve the porosity and pure water flux of the membrane, and the small micropores can maintain the rejection performance of the membrane. The hydrophilicity of the PP/EVOH/MAHHFM will also be improved because EVOH is a polyhydroxy polymer.
3.4. Development of bimodal microporous structure during stretching 3.4.1. Morphology of PP/EVOH/MAH-HFMs According to the above discussion, a perfect bimodal microporous structure was well constructed in the PP/EVOH/MAH-HFM. Hence, the PP/EVOH/MAH sample was selected to study the formation process of bimodal microporous structure during the stretching process. From
Fig. 9. The inner surface images of PP/EVOH/MAH-HFMs with different stretching ratio. (a) 0%; (b) 100%; (c) 150%; (d) 200%; (e) 250%; and (f) 300%. 334
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Fig. 10. The pore size distributions of (a) PPHFM-200 and PP/EVOH/MAH-HFM-200; and (b) PP/EVOH/MAH-HFMs with different stretching ratio.
3.4.2. Pore size distributions and porosity of PP/EVOH/MAH-HFMs Fig. 10 presents the pore size distributions of PPHFM and PP/ EVOH/MAH-HFMs with different stretching ratio, and the relevant data of pore structure are listed in Table 1. There is a peak in the pore size distribution curve of the PPHFM-200 (the PPHFM with stretching ratio of 200%), and the most probable pore size is 226.6 nm (Fig. 10a). However, there are two peaks in the pore size distribution curve of PP/ EVOH/MAH-HFM-200 (the PP/EVOH/MAH-HFM with stretching ratio of 200%), and the most probable pore sizes are 183.2 nm and 3209 nm, respectively. It shows that the bimodal microporous structure is well constructed in the PP/EVOH/MAH-HFM-200. The PP/EVOH/MAHHFM-100 (PP/EVOH/MAH-HFM with stretching ratio of 100%) also shows a single peak in the pore size distribution curve (Fig. 10b). The large micropores formed by phase separation have a small pore size and can not be formed in large amounts at low stretching ratio. Moreover, with the increase of the stretching ratio, the pore sizes of two types of micropores show a trend from increase to decrease, which is consistent with the conclusion of FESEM. All results demonstrate that the bimodal microporous structure is formed in the PP/EVOH/MAH-HFMs.(See Table 2.) The porosity of PPHFM-200 is 64%, while the porosity of PP/ EVOH/MAH-HFM-200 reaches 81.5%. The porosity comparison between PPHFM and PP/EVOH/MAH-HFM-200 prepared in this work and the previously reported similar PPBHFM with different filler content is displayed in Fig. 11a [1,19,22,28–30]. It can be remarkably noted that the PP/EVOH/MAH-HFM-200 is higher than that of other PPBHFMs. The large micropores formed by interfaces separation significantly increase the porosity of the PP/EVOH/MAH-HFM-200. In addition, with the increase of the stretching ratio, the porosity of PP/EVOH/MAHHFMs with different stretching ratio show a trend from increase to decrease (Fig. 11b). The results are consistent with the results of the FESEM and pore size distributions.
Table 2 The list of abbreviations. Abbreviation
Description
PP EVOH PP-g-MAH MS-S TIPS PPHFM PPHFM-200
Polypropylene Poly(ethylene-co-vinyl alcohol) Maleic anhydride grafted polypropylene Melt spinning and stretching method Thermally induced phase separation method Polypropylene hollow fiber membrane Polypropylene hollow fiber membrane with stretching ratio of 200% Polypropylene-based hollow fiber membrane PP/EVOH hollow fiber membrane PP/EVOH/MAH hollow fiber membrane PP/EVOH/MAH hollow fiber membrane with stretching ratio of 100% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 150% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 200% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 250% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 300%
PPBHFM PP/EVOH-HFM PP/EVOH/MAH-HFM PP/EVOH/MAH-HFM-100 PP/EVOH/MAH-HFM-150 PP/EVOH/MAH-HFM-200 PP/EVOH/MAH-HFM-250 PP/EVOH/MAH-HFM-300
3.5. Properties of PP/EVOH/MAH-HFMs 3.5.1. Hydrophilicity of PP/EVOH/MAH-HFMs The water contact angle (WCA) of pure PP is 103.8° (Fig. 12a). The WCA of PP/EVOH/MAH blend dropped to 83.2°, indicating that the hydrophilicity of PP/EVOH/MAH blend is improved. EVOH and PP-gMAH contain a large number of hydrophilic groups, which is responsible for the decrease in WCA of PP/EVOH/MAH blend. Normally, PP/EVOH/MAH-HFMs will inherit the hydrophilicity of PP/EVOH/ MAH blend. A full-spectrum scanning of the EDS energy spectrum on
Table 1 The parameters of membrane structure and propertiesa. Sample
Dmin (nm)
Dmax (nm)
Oxygen Content (%)
Pure Water Flux (L/(m2·h))
Rejection (%)
PPHFM-200 PP/EVOH/MAH-HFM-100 PP/EVOH/MAH-HFM-150 PP/EVOH/MAH-HFM-200 PP/EVOH/MAH-HFM-250 PP/EVOH/MAH-HFM-300
226.6 98.1 120.3 183.2 118.8 96.6
– – 2102 3209 2543 1613.6
1.84 6.22 6.08 5.93 4.82 4.53
148.0 109.1 167.5 322.7 257.1 157.6
99.84 99.94 99.91 99.92 99.97 99.92
a
± ± ± ± ± ±
Dmin is the most probable pore size of small micropores; Dmax is the most probable pore size of large micropores. 335
6.1 5.2 4.0 5.4 5.9 6.8
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Fig. 11. (a) The porosity comparison between PPHFM-200 and PP/EVOH/MAH-HFM-200 prepared in this work and the previously reported similar PPBHFM with different filler content (Filler: PP-g-MA, PP-g-AA, nylon 6, silicon dioxide and atactic polypropylene, etc); (b) the porosity of PP/EVOH/MAH-HFMs with different stretching ratio.
formed by the separation of the lamellae. The phenomenon can be explained by the longitudinal cross-section structure of the membrane. As shown in Fig. 6c, PP/EVOH/MAH-HFM with bimodal microporous structure has both greater and smaller pores in the same level. However, even if the carbon particles pass through the large interface micropores on the surface of the PP/EVOH/MAH-HFM, the small micropores formed by the separation of the lamellae need to be further faced later. To verify the correctness of rejection, the laser particle size analyzer was employed to measure the particle size of residual carbon particles in the permeate solution. As shown in Fig. 13, the size of residual carbon particles in the permeate solution of PPHFM and the PP/ EVOH/MAH-HFMs are significantly reduced. The result confirms the correctness of the rejection. Moreover, the size of the carbon particles in the permeate solution of PP/EVOH/MAH-HFMs with different stretching ratio are small than that of PPHFM. Meanwhile, with the increase of the stretching ratio, the carbon particles in the permeate solution of PP/EVOH/MAH-HFMs with different stretching ratio show a trend from increase to decrease. The result shows that the diameter of small micropores formed by the separation of the lamellae in the PP/ EVOH/MAH-HFM is lower than that of the PPHFM, which is consistent with the result of the pore size distributions.
Fig. 12. The water contact angles of (a) pure PP and (b) PP/EVOH/MAH blend.
the membranes were performed to further explain the changes in the hydrophilicity of PP/EVOH/MAH-HFMs. The oxygen content data are listed in Table 1. The oxygen content of the surface of PPHFM-200 is 1.84%, while the oxygen content of the surface of PP/EVOH/MAHHFM-200 increased to 5.93%. The result of EDS energy spectrum also confirmed that the hydrophilicity of PP/EVOH/MAH-HFMs is improved. However, as the stretching ratio increases, the surface oxygen content decreases, which may be due to an increase in the pore size of the large micropores.
3.5.2. Pure water flux and rejection of PP/EVOH/MAH-HFMs Pure water flux of the membranes is affected by various factors such as inner and outer surfaces, porosity, pore connectivity, and hydrophilicity. Pure water flux of the hollow fiber membranes is summarized in Table 1. Pure water flux of PPHFM-200 is 148 ± 5 L/(m2·h), while pure water flux of PP/EVOH/MAH-HFM-200 is 322.7 ± 4 L/(m2·h). The higher pure water flux of PP/EVOH/MAH-HFM-200 can be attributed to the large micropores in bimodal microporous structure and the improvement of hydrophilicity. Meanwhile, with the increase of the stretching ratio, the pore sizes and porosity of PP/EVOH/MAH-HFMs show a trend from increase to decrease. Therefore, when the stretching ratio of PP/EVOH/MAH-HFMs exceeds 200%, the pure water flux of PP/EVOH/MAH-HFMs is decreasing. The rejection property of the membranes was characterized by rejecting carbon ink with a concentration of 0.1% (Table 1). The rejection of PPHFM-200 is 99.84%. The rejection of PP/EVOH/MAH-HFMs with different stretching ratio does not decrease but increase slightly, which is not affected by the large interface micropores. The results show that the rejection of membranes is determined by the small micropores
Fig. 13. The particle size distributions of residual carbon particles in the permeate solution. 336
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Fig. 14. (a) Time-dependent water permeation fluxes and (b) a summary of antifouling index of all membranes.
micropores in bimodal microporous structure. The reactive groups in EVOH and PP-g-MAH significantly improved the hydrophilicity of PP/ EVOH/MAH-HFM, while also improving the antifouling property of membrane. This work supplied a new membrane structure and provides a new perspective for the development of high performance polypropylene-based hollow fiber membranes by MS-S.
3.5.3. The antifouling property of PP/EVOH/MAH-HFMs The antifouling property of membranes was tested using BSA aqueous solution as a foulant. The time-dependent water permeation fluxes and antifouling indexes of the hollow fiber membranes are shown in Fig. 14. The membrane had a higher FRR value but a lower DRt and DRir, indicating that the membrane had excellent antifouling properties. In other words, it was not only indicated that the surface of the membrane was difficult to be contaminated by adsorbates, but also that the adsorbates could be easily cleaned [8]. The PPHFM-200 had poor antifouling properties of 53.15%,54.24%,7.39% and 46.85% for FRR, DRt, DRr, and DRir values, respectively. Interestingly, PP/EVOH/MAHHFM-200 showed elevated properties of 74.72%,52.89%,27.61% and 25.28% for FRR, DRt, DRr, and DRir values, respectively. And the antifouling property of PP/EVOH/MAH-HFMs was inversely proportional to the stretching ratio. In general, the large micropores on the surface of the membrane were more likely to be attached by adsorbates, which reduced the antifouling property of the membrane. However, the results showed the BSA was difficult to adhere onto the surface of PP/EVOH/ MAH-HFMs and could be removed during the process of cleaning. The enhancement of antifouling properties of PP/EVOH/MAH-HFMs was attributed to the hydrophilic EVOH and PP-g-MAH which were mainly concentrated in the large micropores. Since the hydrophilic EVOH and PP-g-MAH in the large micropores prevent the adhesion of BSA, the existence of a large microporous in bimodal microporous structure did not reduce the antifouling properties of the PP/EVOH/MAH-HFMs.
Acknowledgements The national natural science foundation of China (51763003 and 21604016), Outstanding youth program of Guizhou province (20170430178) and Joint research program of Guizhou province (20177251) are acknowledged for the financial support Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2018.12.052. References [1] A. Saffar, P.J. Carreau, A. Ajji, M.R. Kamal, Development of polypropylene microporous hydrophilic membranes by blending with PP-g-MA and PP-g-AA, J. Membr. Sci. 462 (2014) 50–61. [2] M. Henares, P. Ferrero, P. San-Valero, V. Martínez-Soria, M. Izquierdo, Performance of a polypropylene membrane contactor for the recovery of dissolved methane from anaerobic effluents: mass transfer evaluation, long-term operation and cleaning strategies, J. Membr. Sci. 563 (2018) 926–937. [3] D. Luo, F. Wei, H. Shao, L. Xiang, J. Yang, Z. Cui, S. Qin, J. Yu, Shape stabilization, thermal energy storage behavior and thermal conductivity enhancement of flexible paraffin/MWCNTs/PP hollow fiber membrane composite phase change materials, J. Mater. Sci. 53 (2018) 15500–15513. [4] N. Tang, Q. Jia, H. Zhang, J. Li, S. Cao, Preparation and morphological characterization of narrow pore size distributed polypropylene hydrophobic membranes for vacuum membrane distillation via thermally induced phase separation, Desalination. 256 (2010) 27–36. [5] M.C. Yang, J.S. Perng, Microporous polypropylene tubular membranes via thermally induced phase separation using a novel solvent-camphene, J. Membr. Sci. 187 (2001) 13–22. [6] J. Kim, S.S. Kim, M. Park, M. Jang, Effects of hollow properties on the preparation of polyethylene hollow fiber membranes by stretching, J. Membr. Sci. 318 (2008) 201–209. [7] H. Liu, C. Xiao, X. Hu, M. Liu, Post-treatment effect on morphology and performance of polyurethane-based hollow fiber membranes through melt-spinning method, J. Membr. Sci. 427 (2013) 326–335. [8] H. Shao, Y. Qi, D. Luo, S. Liang, S. Qin, J. Yu, Fabrication of antifouling polypropylene hollow fiber membrane breaking through the selectivity-permeability trade-off, Eur. Polym. J. 105 (2018) 469–477. [9] S.C. Yu, Y.P. Zheng, Q. Zhou, S. Shuai, Z.H. Lü, C. Gao, Facile modification of polypropylene hollow fiber microfiltration membranes for nanofiltration, Desalination 298 (2012) 49–58. [10] Y.H. Zhao, K.H. Wee, R. Bai, Highly hydrophilic and low-protein-fouling polypropylene membrane prepared by surface modification with sulfobetaine-based
4. Conclusions It was confirmed by FESEM and pore size distribution tests that the bimodal microporous structure was well constructed by melt blending combining with MS-S in PP/EVOH/MAH-HFM. The presence of compatible interface enabled the EVOH particles to be less susceptible to deformation during membrane formation and optimized the bimodal microporous structure in PP/EVOH/MAH-HFM. During the stretching process of PP/EVOH/MAH-HFM, the small micropores in bimodal microporous structure were formed by the separation of PP lamellae, and the large micropores in bimodal microporous structure were formed by the separation of compatible interfaces between the PP phase and the EVOH phase. Moreover, two types of micropores were formed simultaneously. With the increase of the stretching ratio, the pore sizes of the two types of micropores showed a trend from increase to decrease. The porosity and pure water flux of PP/EVOH/MAH-HFM-200 respectively increased to 81.5% and 322.7 L/(m2·h), which respectively increased by 27.3% and 118.0% compared to PPHFM-200 due to the large micropores in bimodal microporous structure. The rejection performance of PP/EVOH/MAH-HFMs was also retained due to the small 337
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Microstructure construction of polypropylene-based hollow fiber membranes with bimodal microporous structure for water flux enhancement and rejection performance retention ⁎
Dajun Luoa,b, Fujian Weib, Huiju Shaoa,b, Lan Xiea, Zhenyu Cuib, Shuhao Qina,b, , Jie Yua,b, a b
T
⁎
College of Materials and Metallurgy, Guizhou University, Guiyang 550025, People's Republic of China National Engineering Research Center for Compounding and Modification of Polymeric Materials, Guiyang 550014, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bimodal microporous structure Hydrophilicity Polypropylene hollow fiber membrane
The low porosity, low pore connectivity and poor hydrophilicity of polypropylene hollow fiber membranes (PPHFMs) reduced membrane performance and limited its application in water treatment. Hence, the bimodal microporous structure including two different grades and independent pore size distributions was constructed in hydrophilic polypropylene (PP)/poly(ethylene-co-vinyl alcohol) (EVOH)/maleic anhydride grafted polypropylene (PP-g-MAH) hollow fiber membrane (PP/EVOH/MAH-HFM) via melt blending combining with meltspinning and stretching (MS-S). The addition of PP-g-MAH formed compatible interfaces to optimize the bimodal microporous structure in PP/EVOH/MAH-HFM. The porosity and pure water flux of PP/EVOH/MAH-HFM with stretching ratio of 200% respectively increased to 81.5% and 322.7 L/(m2·h), which respectively increased by 27.3% and 118.0% compared to PPHFM with stretching ratio of 200% due to the large micropores in bimodal microporous structure. The rejection performance of PP/EVOH/MAH-HFM was also retained due to the small micropores in bimodal microporous structure. The reactive groups in EVOH and PP-g-MAH significantly improved the hydrophilicity of PP/EVOH/MAH-HFM, while also improving the antifouling property of membrane. Moreover, the formation mechanism of bimodal microporous structure was systematically investigated in combination with physical modeling and FESEM method. This work supplied a new membrane structure and provides a new perspective for the fabrication of high performance polypropylene-based hollow fiber membranes by MS-S.
1. Introduction Polypropylene hollow fiber membrane (PPHFM) is important in membrane separation technology and widely used in many fields, e.g., desalination of sea water, wastewater treatment, gas separation, petroleum chemical industry and energy storage [1–3]. There are mainly two fabrication techniques for manufacturing PPHFM. One is thermally induced phase separation (TIPS) method [4,5]. Another one is melt spinning and stretching (MS-S), which is mainly applicable to semicrystalline polymers, such as polypropylene (PP), polyethylene, and polyurethane [1,6,7]. The PPHFM prepared by MS-S exhibit higher mechanical properties, uniform pore distribution and uniform pore size. And MS-S is a clean and economical process. Hence, MS-S shows more advantages over TIPS. Although the PPHFM prepared by MS-S exhibit many advantages, it still has several disadvantages, such as low porosity, poor connectivity
⁎
and poor hydrophilicity. The disadvantages reduce the water flux of the PPHFM and limit applications of PPHFM in wastewater treatment, desalination of sea water and biomedical applications [8–10]. The porosity and connectivity of the membrane could be increased by increasing stretching ratio [11]. However, the pore size was also proportional to the stretching ratio, which would decrease the rejection performance of the membrane [12]. Therefore, in general, the high water flux and high rejection performance can not be simultaneously achieved. In the field of inorganic porous materials, there was an inorganic material with a special bimodal pore structure including two different grades and independent pore size distributions [13–15]. The large pores show a smaller diffusion resistance and the small pores serve as sites for adsorption and reaction of material. Imagining if the PP-based hollow fiber membrane (PPBHFM) prepared by MS-S has the bimodal microporous structure, the large micropores can improve the porosity and
Corresponding authors at: College of Materials and Metallurgy, Guizhou University, Guiyang 550025, People's Republic of China. E-mail addresses: [email protected] (S. Qin), [email protected] (J. Yu).
https://doi.org/10.1016/j.seppur.2018.12.052 Received 5 September 2018; Received in revised form 12 December 2018; Accepted 21 December 2018 Available online 21 December 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental
connectivity of membrane, and the small micropores can retain the rejection performance of membrane. The high water flux can be achieved while retaining rejection performance. To our knowledge, work on the bimodal microporous PPBHFM prepared by MS-S has not been published. PPHFM formation mechanism of MS-S was based on the separation of oriented lamellae during stretching [16–18]. The uniform microporous structure (pore diameter less than 0.3 μm) could be obtained by the separation of oriented lamellae [19]. It has been reported that microporous membranes could also be obtained by separating the twophase interfaces of immiscible blends [20–22]. Mei et al. [21]. prepared the PP/easy hydrolytic degradation polyester microporous hollow fibers by blend spinning and cold stretch process. The large-sized pore structure (pore diameter greater than 3 μm) could be obtained by the separation of two-phase interfaces. Unfortunately, the PP phase did not form an oriented lamellar structure in this work, because the large amount of easy hydrolytic degradation polyester (30 wt%) was added and no stress was applied during melt spinning. The micropores formed by lamellae separation were not formed in a large amount. Therefore, it is possible to realize the construction of bimodal microporous structure in PPBHFM by adding a suitable amount of incompatible second phase and adjusting the manufacturing process. Hydrophilic PPHFM is usually made by modifying the base membrane surface. Such as, radical grafting reactions, plasma treatment, ultraviolet grafting reactions, coating and so on [8–10,23,24]. However, all the above methods aim at the surface modification of PPHFM. Moreover, weak interaction between PPHFM and hydrophilic groups resulted in the release and elution of hydrophilic compound from the surface in the practical application [25]. It has been reported that PPHFM can obtain permanent hydrophilicity by blending with hydrophilic polymer [1,21]. Therefore, the hydrophilic polymer as the incompatible second phase mentioned above can not only construct bimodal microporous structure, but also improve the hydrophilicity of PPBHFM. The assumption structure of PPHFM and hydrophilic PPBHFM with bimodal microporous structure are shown in Fig. 1. In this paper, the construction of bimodal microporous structure in PPBHFM and the improvement of hydrophilicity of PPBHFM were achieved by melt blending and MS-S with poly(ethylene-co-vinyl alcohol) (EVOH) as hydrophilic incompatible second phase. The formation mechanism of bimodal microporous structure was systematically investigated in combination with physical modeling and FESEM method. The results showed that the bimodal microporous structure was well constructed in PP/EVOH/MAH hollow fiber membrane (PP/ EVOH/MAH-HFM) because the compatible phase interfaces were formed by adding PP-g-MAH. The small micropores formed by separation of PP lamellae. The compatible phase interfaces separate to form large micropores while also forming a small amount of microfibrils. Both porosity and hydrophilicity of PP/EVOH/MAH-HFM were improved. The high water flux could be achieved while retaining rejection performance. The introduction of bimodal microporous structure is referential for the development of high performance PPBHFM.
2.1. Materials PP (T30S) was supplied by Sinopec Dushanzi Petrochemical Company (Xinjiang, China). The isotacticity is 98% and the powder ash is 0.01%. The melting point, Tm, of the resins obtained by differential scanning calorimetry was 166 °C. The melt flow index of PP was 3.35 g/ 10 min. EVOH (L171B) was purchased from Kuraray International Trading (Shanghai) Co., Ltd. (Shanghai, China). The content of vinyl was 27 mol%. The melt index was 4.0 g/10 min, and the melting point was 191 °C. As compatibilizer, PP-g-MAH (CA100) was purchased from Arkema (China) Investment Co., Ltd. (Beijing, China). The melt index was 8.0 g/10 min, and the melting point was 143 °C. 2.2. Sample preparation PPBHFM were prepared by three steps. Firstly, the extrusion of blends. For PP/EVOH blend, the dried mixture (90 wt% PP and 10 wt% EVOH) was extruded on a twin-screw extruder (L/D = 48, D = 42 mm) at 215 °C. For PP/EVOH/MAH blend, the dried mixture of 10 wt% EVOH and 5 wt% PP-g-MAH was first extruded and then the blend was extruded with 90 wt% PP. All samples from extruder were dried for 12 h at 80 °C. Secondly, the precursor hollow fibers were prepared via melt spinning. All pellet samples were extruded and spun to form hollow fibers on a single-screw extruder with a single-hole tube-in orifice spinneret (inner and outer diameters of 0.50 and 2.50 mm) at 190 °C. Melt-draw ratio was 56. Finally, the annealing and stretching of precursor hollow fibers. The precursor hollow fibers were annealed under relaxation conditions at 150 °C for 1 h. The annealed hollow fibers were firstly cold-stretched by 20% at 20 °C. After cold-stretching, the temperature inside the stretcher was raised to 140 °C and hot-stretching was started. The hot-stretching ratio were 80, 130, 180, 230 and 280%, respectively. Finally, PPBHFM were re-annealed in the stretcher at 140 °C for 1 h to prevent shrinkage. 2.3. Characterizations Differential scanning calorimetry. TA-Q10 from TA Instruments (America) was employed to investigate the crystallization behavior of samples. All tests were carried out under a nitrogen atmosphere. For pure PP and blends, the thermal history of the samples was firstly eliminated by heating the samples at 220 °C for a period of 5 min. Subsequently, the samples were cooled to 50 °C at 10 °C/min rate and held at 50 °C for 5 min. Then, the samples were heated to 220 °C at 10 °C/min rate again. For hollow fibers, the samples were heated to 220 at 10 °C/min rate. Field emission scanning electron microscope. Quanta FEG250 of FEI company (America) was used to observe the micromorphology of samples and measure the oxygen content of membranes.
Fig. 1. The assumption model of (a) PPHFM and (b) hydrophilic PPBHFM with bimodal microporous structure. 329
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Fig. 2. (a) Crystallizing curves and (b) secondary melting curves of pure PP and blends with rate of 10 °C/min.
Contact angle measurements. The water contact angle (WCA) was measured using a 25S optical contact angle tester from Germany DSA company (Germany) to evaluate the hydrophilicity of the samples. Pore size distribution. The pore size distribution of samples was measured using an automatic mercury porosimeter (AutoPore IV 9510, Micromeritics, America) . Porosity. Weighing method was selected to measure the porosity of samples [11]. The porosity was calculated as:
ρ −ρ ·100% P= 0 ρ0
FRR =
V AΔt
Jp ⎞ ·100% DRt = ⎛1 − J w1 ⎠ ⎝
(5)
Jw2 − Jp ⎞ ·100% DRr = ⎛ ⎝ Jw1 ⎠
(6)
J − Jw2 ⎞ DRir = ⎛ w1 ·100% ⎝ Jw1 ⎠
(7)
⎜
(1)
⎜
⎜
⎟
⎟
⎟
Particle size distribution. The particle size distribution of residual carbon particles in the permeate solution was measured by laser particle sizer (ZEN3600, Malvern, UK) 3. Results and discussion
(2) 3.1. Crystallization behavior and morphology of pure PP and blends
where J is the pure water flux at the pressure of 1 bar, L/(m2·h); V is the volume of permeate water, L; Δt is the testing time, h; A is the effective area of each membrane module, m2. Rejection. The rejection performance of the membranes was characterized by rejecting carbon ink with a concentration of 0.1%. The average particle size of the carbon particles obtained by laser particle sizer (ZEN3600, Malvern, UK) was 221.9 nm. Rejection (R%) was calculated as follows[11]:
Cp ⎤ ·100% R% = ⎡1 − ⎢ Cr ⎥ ⎣ ⎦
(4)
Furthermore, the antifouling property of membranes was also evaluated by the total flux decrease ratio (DRt), reversible flux decrease ratio (DRr) as well as irreversible flux decrease ratio (DRir).
where P is the porosity of membranes; ρ0 is the density of pure PP or blends; ρ is the density of membranes. Pure water flux. The internal pressure mode was used to measure the pure water flux. The test temperature was approximately 20 °C. All the modules were pressurized under 1.5 bar for 0.5 h. Next, the pure water test was carried out under 1 bar. The pure water flux was calculated as follows [11]:
J=
Jw2 ·100% Jw1
3.1.1. Non-isothermal crystallization behavior of pure PP and blends The DSC curves in Fig. 2 show a single exothermic and endothermic peak in the crystallizing and secondary melting curves of pure PP. The crystallizing and melting peaks temperature values are 110.6 °C and 165.6 °C, respectively. Two crystallizing peaks and melting peaks respectively are observed in the PP/EVOH blend (Fig. 2a,b), which shows the PP/EVOH blend is thermally immiscible system. The PP phase begins to crystallize after the EVOH phase is completely crystallized. The crystallizing and melting peaks temperatures of EVOH phase in the PP/ EVOH/MAH blend shifts to low temperature. The results show that the compatibility of blend system is improved by adding PP-g-MAH, but most EVOH are still crystallized independently. In addition, the crystallizing peak temperatures of PP in PP/EVOH blend and PP/EVOH/ MAH blend shift to high temperature. Moreover, the time need to reach the maximum of exothermic peak during non-isothermal crystallization decrease remarkably after the addition of EVOH. As we know, the crystallizing peak corresponds with both rate of nucleation and growth of polymer during non-isothermal crystallization. PP/EVOH blend and PP/EVOH/MAH blend show a heterogeneous nucleation process implying that the EVOH essentially acts as an effective nucleating agent for PP phase and promotes the crystallization rate of PP during nonisothermal crystallization. The result is consistent with the reports in the literature [26]. The melting enthalpy (ΔH) of pure PP, PP/EVOH blend and PP/EVOH/MAH blend are 80.3, 73.1 and 76.8 J/g,
(3)
where Cr is the concentrations in original solution; Cp is the concentrations in permeate solution. The turbidimeter (2100Q, HACH, America) was used to measure turbidity to determine the concentration of original solution and permeate solution. Antifouling property. The pure water flux test was first carried out by external pressure method to obtain pure water flux Jw1, L/(m2·h), and then bovine serum albumin (BSA, 1000 mg/L in phosphate buffered solution, PH = 7.0) solution was used instead of the pure water for testing. After BSA solution filtration for 1 h, the flux of BSA solution Jp, L/(m2·h), was measured. Then, after backwashing at 0.1 MPa for 10 min, a second pure water flux test was performed, and the secondary pure water flux was recorded as Jw2, L/(m2·h). The flux recovery ratio (FRR) used to characterize the antifouling property of membranes can be calculated by the following equation [8]: 330
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Fig. 3. FESEM images of (a) pure PP and (b) PP/EVOH blend; the diameter distribution of EVOH particles in (c) PP/EVOH/MAH blend and (d) PP/EVOH blend; and (e, e1) FESEM images of PP/EVOH/MAH blend.
Meanwhile, the diameter of spherical particles formed by EVOH is significantly reduced. The diameter distribution of EVOH particles is in the range of 0.2–1.8 μm, and the average diameter is 0.8 μm (Fig. 3c). The main reason may be the fact that the addition of PP-g-MAH improves the compatibility of the blend system and forms a compatible interface between the PP phase and the EVOH phase.
respectively. Since the PP/EVOH blend contains only 90 wt% of PP, the ΔH value is decreased. However, the total PP content increased after the addition of 5 wt% PP-g-MAH, so the ΔH value of PP/EVOH/MAH blend increased again. 3.1.2. Morphology of pure PP and blends The phase morphology in pure PP and blends are displayed in Fig. 3. The PP/EVOH blend exhibits typical two-phase structure, in which EVOH forms the “island” structure as large spherical particles are evenly distributed in the “sea”. The diameter distribution of EVOH particles is in the range of 0–9 μm, and the average diameter is 4.0 μm (Fig. 3d). The phase separation and a clear interface between PP phase and EVOH phase are also observed in Fig. 3b. However, there is no obvious phase separation after adding PP-g-MAH, but the interfaces between PP phase and EVOH phase are still observed in the Fig. 3(e1).
3.2. Characterization of bimodal microporous structure 3.2.1. DSC analysis of hollow fibers As shown in Fig. 4, two melting peaks are observed in the precursor and annealed hollow fibers of PP/EVOH and PP/EVOH/MAH, which show the PP phase and EVOH phase are still independently crystallized during the crystallization process. EVOH phase is still independently exists in the PP phase. After the annealing, the lamellar thickness of the 331
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Fig. 4. First melting curves of (a) precursor hollow fibers and (b) annealed hollow fibers with rate of 10 °C/min.
in the uniform small micropores. A small amount of microfibrils can be observed inside the large micropores. The shape of the EVOH particles remains irregularly spherical. The longitudinal cross-section images of hollow fiber membranes with stretching ratio of 200% are given to further prove that the bimodal microporous structure is well constructed in the PP/EVOH/ MAH-HFM (Fig. 6). A longitudinal cross-section image reveal that PPHFM has a nearly isotropic structure (Fig. 6a). A poor connectivity can be observed in the PPHFM due to superposition of lamellae (Fig. 6a1). As shown in Fig. 6b, a bimodal microporous structure is formed in the membrane wall of PP/EVOH-HFM. The pore size of the large micropores is large due to the incompatibility of the interfaces. Moreover, A poor connectivity is also observed in the PP/EVOH-HFM. However, Fig. 6(c) shows that a perfect bimodal microporous structure is constructed in the membrane wall of PP/EVOH/MAH-HFM. The large micropores are evenly distributed in the uniform small micropores. Since the large micropores reduce the proportion of lamellae superposition, a perfect connectivity is also observed in the PP/EVOH/MAHHFM. Importantly, PP/EVOH/MAH-HFM is not the case of gradient structure membrane, where smaller pores are in the top dense layer for selectivity and greater pores are in the support porous layer for flux. The top layer and support layer are not in the same level of the gradient
polymer increases and the defects of crystal phase are also reduced. Hence, the melting peaks temperature of annealed hollow fibers move to high temperature. The enthalpies of hollow fibers also increase significantly after annealing. Moreover, a small peak is observed at 154.2 °C after annealing, which is due to the formation of a certain number of metastable folding chain conformations during annealing [27].
3.2.2. Morphology of bimodal microporous structure The inner surface images of all membranes with stretching ratio of 200% are shown in Fig. 5. The uniform micropores and microfibrils can be clearly observed in PPHFM (Fig. 5a, a1). However, there are two types of pore structures (bimodal microporous structure: the small micropores and the large micropores) in the PP/EVOH hollow fiber membrane (PP/EVOH-HFM) (Fig. 5b, b1). There is clear separation between the two-phase interfaces, and the shape of the EVOH particles become oval. Due to the presence of the large sized EVOH particles and incompatible interfaces, the outer diameter of the membrane and the stress distribution become non-uniform during stretching, the distribution of the small micropores becomes more confusing. A better bimodal microporous structure can be clearly observed in PP/EVOH/ MAH-HFM (Fig. 5c, c1). The large micropores are uniformly distributed
Fig. 5. The inner surface of hollow fiber membranes with stretching ratio of 200%. (a, a1) PPHFM; (b, b1) PP/EVOH-HFM; and (c, c1) PP/EVOH/MAH-HFM. 332
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Fig. 6. The longitudinal cross-section images of hollow fiber membranes with stretching ratio of 200%. (a, a1) PPHFM; (b) PP/EVOH-HFM; and (c) PP/EVOH/MAHHFM.
EVOH precursor hollow fiber are observed in Fig. 7B. The interface between PP phase and EVOH phase is clear because PP phase and EVOH phase are incompatible. The spherical particles of EVOH will become elliptical after melt spinning because there is stress perpendicular to the traction direction during spinning (Fig. 7b2). However, the diameter of spherical particles formed by EVOH is significantly reduced and the number of particles significantly increased after adding PP-g-MAH. Meanwhile, the compatible interfaces between the PP phase and the EVOH phase will be formed (Fig. 7c1). The compatible interface is more easily deformed under the stress perpendicular to the traction direction during spinning because it is amorphous. Therefore, the central EVOH particles may still maintain a spherical structure after melt spinning (Fig. 7c2).
structure membrane. But PP/EVOH/MAH-HFM with bimodal microporous structure has both greater and smaller pores in the same level. 3.3. The formation mechanism of bimodal microporous structure 3.3.1. The phase morphology in blends and precursor hollow fibers Based on the analysis of the above DSC and FESEM, the model of the phase morphology in blends and precursor hollow fibers are built (Fig. 7). The phase morphologies of pure PP and PP precursor hollow fiber are represented in Fig. 7A. The oriented lamellae in PP precursor hollow fiber will formed via the mechanisms of stress-induced crystallization (Fig. 7a2). The phase morphology of PP/EVOH blend and PP/
3.3.2. The formation of bimodal microporors structure Based on the model of the phase morphology in precursor hollow fibers, the model about the formation of bimodal microporors structure in hollow fiber membranes are built (Fig. 8). The precursor hollow fibers are annealed at appropriate temperatures to perfect the lamellar structure, but the morphology of each phase does not change. For pure PP samples, the micropores and microfibrils will be formed by the separation of the lamellae during the stretching of annealed hollow fiber (Fig. 8a3) [18]. For blends samples, there are two types of pore structures (bimodal microporous structure) in the PP/EVOH-HFM (Fig. 8b3). One is the small micropores formed by separation of PP lamellae. Another one is a large micropores formed by the separation of interfaces between the PP phase and the EVOH phase. The interface separation at the vertices in the stretching direction of EVOH particles develops into the larger interface micropores. Since the stretching process is performed near the melting point temperature of PP phase and the strength of EVOH phase is slightly higher than that of PP phase, EVOH still maintains the ellipsoidal granular structure during stretching. For PP/ EVOH/MAH sample, there is a perfect bimodal microporous structure in the hollow fiber membrane (Fig. 8c3). The compatible phase interfaces separate to form large micropores while also forming a small amount of microfibrils. The presence of microfibers limit the further
Fig. 7. The model of the phase morphology in blends and hollow fibers. (A) Pure PP; (B) PP/EVOH; and (C) PP/EVOH/MAH. 333
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Fig. 8. The model of the formation of bimodal microporous structure in (A) PPHFM; (B) PP/EVOH-HFM; and (C) PP/EVOH/MAH-HFM.
Fig. 9, it can be seen that both types of micropores are formed simultaneously during the stretching process. For the small micropores formed by separation of PP lamellae, the pore size and number of the micropores increase as the stretching ratio increases. The thickness of the lamellar stacks also becomes more uniform. However, when the stretching ratio exceeds 200% and continues to increase, the dislocations and slips occur in the lamellae. Meanwhile, the microfibrils break and the pore size decreases. The formation and enlargement of small micropores are present throughout the stretching process. Therefore, unseparated lamellae stacks can still be observed at low stretching ratios (Fig. 9b, c). For the large micropores formed by the separation of interfaces, the pore size and number of the micropores also increase as the stretching ratio increases. However, when the stretching ratio exceeds 200% and continues to increase, the microfibrils break and the micropore shape becomes elongated. The corresponding pore size is also decreased.
separation of the phase interface. Meanwhile, the diameter of spherical particles formed by EVOH is significantly reduced after adding PP-gMAH. Therefore, the size of large micropores formed by phase interfaces separation in PP/EVOH/MAH-HFM is smaller than that of PP/ EVOH-HFM. Theoretically, the large micropores in the bimodal microporous structure can improve the porosity and pure water flux of the membrane, and the small micropores can maintain the rejection performance of the membrane. The hydrophilicity of the PP/EVOH/MAHHFM will also be improved because EVOH is a polyhydroxy polymer.
3.4. Development of bimodal microporous structure during stretching 3.4.1. Morphology of PP/EVOH/MAH-HFMs According to the above discussion, a perfect bimodal microporous structure was well constructed in the PP/EVOH/MAH-HFM. Hence, the PP/EVOH/MAH sample was selected to study the formation process of bimodal microporous structure during the stretching process. From
Fig. 9. The inner surface images of PP/EVOH/MAH-HFMs with different stretching ratio. (a) 0%; (b) 100%; (c) 150%; (d) 200%; (e) 250%; and (f) 300%. 334
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Fig. 10. The pore size distributions of (a) PPHFM-200 and PP/EVOH/MAH-HFM-200; and (b) PP/EVOH/MAH-HFMs with different stretching ratio.
3.4.2. Pore size distributions and porosity of PP/EVOH/MAH-HFMs Fig. 10 presents the pore size distributions of PPHFM and PP/ EVOH/MAH-HFMs with different stretching ratio, and the relevant data of pore structure are listed in Table 1. There is a peak in the pore size distribution curve of the PPHFM-200 (the PPHFM with stretching ratio of 200%), and the most probable pore size is 226.6 nm (Fig. 10a). However, there are two peaks in the pore size distribution curve of PP/ EVOH/MAH-HFM-200 (the PP/EVOH/MAH-HFM with stretching ratio of 200%), and the most probable pore sizes are 183.2 nm and 3209 nm, respectively. It shows that the bimodal microporous structure is well constructed in the PP/EVOH/MAH-HFM-200. The PP/EVOH/MAHHFM-100 (PP/EVOH/MAH-HFM with stretching ratio of 100%) also shows a single peak in the pore size distribution curve (Fig. 10b). The large micropores formed by phase separation have a small pore size and can not be formed in large amounts at low stretching ratio. Moreover, with the increase of the stretching ratio, the pore sizes of two types of micropores show a trend from increase to decrease, which is consistent with the conclusion of FESEM. All results demonstrate that the bimodal microporous structure is formed in the PP/EVOH/MAH-HFMs.(See Table 2.) The porosity of PPHFM-200 is 64%, while the porosity of PP/ EVOH/MAH-HFM-200 reaches 81.5%. The porosity comparison between PPHFM and PP/EVOH/MAH-HFM-200 prepared in this work and the previously reported similar PPBHFM with different filler content is displayed in Fig. 11a [1,19,22,28–30]. It can be remarkably noted that the PP/EVOH/MAH-HFM-200 is higher than that of other PPBHFMs. The large micropores formed by interfaces separation significantly increase the porosity of the PP/EVOH/MAH-HFM-200. In addition, with the increase of the stretching ratio, the porosity of PP/EVOH/MAHHFMs with different stretching ratio show a trend from increase to decrease (Fig. 11b). The results are consistent with the results of the FESEM and pore size distributions.
Table 2 The list of abbreviations. Abbreviation
Description
PP EVOH PP-g-MAH MS-S TIPS PPHFM PPHFM-200
Polypropylene Poly(ethylene-co-vinyl alcohol) Maleic anhydride grafted polypropylene Melt spinning and stretching method Thermally induced phase separation method Polypropylene hollow fiber membrane Polypropylene hollow fiber membrane with stretching ratio of 200% Polypropylene-based hollow fiber membrane PP/EVOH hollow fiber membrane PP/EVOH/MAH hollow fiber membrane PP/EVOH/MAH hollow fiber membrane with stretching ratio of 100% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 150% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 200% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 250% PP/EVOH/MAH hollow fiber membrane with stretching ratio of 300%
PPBHFM PP/EVOH-HFM PP/EVOH/MAH-HFM PP/EVOH/MAH-HFM-100 PP/EVOH/MAH-HFM-150 PP/EVOH/MAH-HFM-200 PP/EVOH/MAH-HFM-250 PP/EVOH/MAH-HFM-300
3.5. Properties of PP/EVOH/MAH-HFMs 3.5.1. Hydrophilicity of PP/EVOH/MAH-HFMs The water contact angle (WCA) of pure PP is 103.8° (Fig. 12a). The WCA of PP/EVOH/MAH blend dropped to 83.2°, indicating that the hydrophilicity of PP/EVOH/MAH blend is improved. EVOH and PP-gMAH contain a large number of hydrophilic groups, which is responsible for the decrease in WCA of PP/EVOH/MAH blend. Normally, PP/EVOH/MAH-HFMs will inherit the hydrophilicity of PP/EVOH/ MAH blend. A full-spectrum scanning of the EDS energy spectrum on
Table 1 The parameters of membrane structure and propertiesa. Sample
Dmin (nm)
Dmax (nm)
Oxygen Content (%)
Pure Water Flux (L/(m2·h))
Rejection (%)
PPHFM-200 PP/EVOH/MAH-HFM-100 PP/EVOH/MAH-HFM-150 PP/EVOH/MAH-HFM-200 PP/EVOH/MAH-HFM-250 PP/EVOH/MAH-HFM-300
226.6 98.1 120.3 183.2 118.8 96.6
– – 2102 3209 2543 1613.6
1.84 6.22 6.08 5.93 4.82 4.53
148.0 109.1 167.5 322.7 257.1 157.6
99.84 99.94 99.91 99.92 99.97 99.92
a
± ± ± ± ± ±
Dmin is the most probable pore size of small micropores; Dmax is the most probable pore size of large micropores. 335
6.1 5.2 4.0 5.4 5.9 6.8
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Fig. 11. (a) The porosity comparison between PPHFM-200 and PP/EVOH/MAH-HFM-200 prepared in this work and the previously reported similar PPBHFM with different filler content (Filler: PP-g-MA, PP-g-AA, nylon 6, silicon dioxide and atactic polypropylene, etc); (b) the porosity of PP/EVOH/MAH-HFMs with different stretching ratio.
formed by the separation of the lamellae. The phenomenon can be explained by the longitudinal cross-section structure of the membrane. As shown in Fig. 6c, PP/EVOH/MAH-HFM with bimodal microporous structure has both greater and smaller pores in the same level. However, even if the carbon particles pass through the large interface micropores on the surface of the PP/EVOH/MAH-HFM, the small micropores formed by the separation of the lamellae need to be further faced later. To verify the correctness of rejection, the laser particle size analyzer was employed to measure the particle size of residual carbon particles in the permeate solution. As shown in Fig. 13, the size of residual carbon particles in the permeate solution of PPHFM and the PP/ EVOH/MAH-HFMs are significantly reduced. The result confirms the correctness of the rejection. Moreover, the size of the carbon particles in the permeate solution of PP/EVOH/MAH-HFMs with different stretching ratio are small than that of PPHFM. Meanwhile, with the increase of the stretching ratio, the carbon particles in the permeate solution of PP/EVOH/MAH-HFMs with different stretching ratio show a trend from increase to decrease. The result shows that the diameter of small micropores formed by the separation of the lamellae in the PP/ EVOH/MAH-HFM is lower than that of the PPHFM, which is consistent with the result of the pore size distributions.
Fig. 12. The water contact angles of (a) pure PP and (b) PP/EVOH/MAH blend.
the membranes were performed to further explain the changes in the hydrophilicity of PP/EVOH/MAH-HFMs. The oxygen content data are listed in Table 1. The oxygen content of the surface of PPHFM-200 is 1.84%, while the oxygen content of the surface of PP/EVOH/MAHHFM-200 increased to 5.93%. The result of EDS energy spectrum also confirmed that the hydrophilicity of PP/EVOH/MAH-HFMs is improved. However, as the stretching ratio increases, the surface oxygen content decreases, which may be due to an increase in the pore size of the large micropores.
3.5.2. Pure water flux and rejection of PP/EVOH/MAH-HFMs Pure water flux of the membranes is affected by various factors such as inner and outer surfaces, porosity, pore connectivity, and hydrophilicity. Pure water flux of the hollow fiber membranes is summarized in Table 1. Pure water flux of PPHFM-200 is 148 ± 5 L/(m2·h), while pure water flux of PP/EVOH/MAH-HFM-200 is 322.7 ± 4 L/(m2·h). The higher pure water flux of PP/EVOH/MAH-HFM-200 can be attributed to the large micropores in bimodal microporous structure and the improvement of hydrophilicity. Meanwhile, with the increase of the stretching ratio, the pore sizes and porosity of PP/EVOH/MAH-HFMs show a trend from increase to decrease. Therefore, when the stretching ratio of PP/EVOH/MAH-HFMs exceeds 200%, the pure water flux of PP/EVOH/MAH-HFMs is decreasing. The rejection property of the membranes was characterized by rejecting carbon ink with a concentration of 0.1% (Table 1). The rejection of PPHFM-200 is 99.84%. The rejection of PP/EVOH/MAH-HFMs with different stretching ratio does not decrease but increase slightly, which is not affected by the large interface micropores. The results show that the rejection of membranes is determined by the small micropores
Fig. 13. The particle size distributions of residual carbon particles in the permeate solution. 336
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Fig. 14. (a) Time-dependent water permeation fluxes and (b) a summary of antifouling index of all membranes.
micropores in bimodal microporous structure. The reactive groups in EVOH and PP-g-MAH significantly improved the hydrophilicity of PP/ EVOH/MAH-HFM, while also improving the antifouling property of membrane. This work supplied a new membrane structure and provides a new perspective for the development of high performance polypropylene-based hollow fiber membranes by MS-S.
3.5.3. The antifouling property of PP/EVOH/MAH-HFMs The antifouling property of membranes was tested using BSA aqueous solution as a foulant. The time-dependent water permeation fluxes and antifouling indexes of the hollow fiber membranes are shown in Fig. 14. The membrane had a higher FRR value but a lower DRt and DRir, indicating that the membrane had excellent antifouling properties. In other words, it was not only indicated that the surface of the membrane was difficult to be contaminated by adsorbates, but also that the adsorbates could be easily cleaned [8]. The PPHFM-200 had poor antifouling properties of 53.15%,54.24%,7.39% and 46.85% for FRR, DRt, DRr, and DRir values, respectively. Interestingly, PP/EVOH/MAHHFM-200 showed elevated properties of 74.72%,52.89%,27.61% and 25.28% for FRR, DRt, DRr, and DRir values, respectively. And the antifouling property of PP/EVOH/MAH-HFMs was inversely proportional to the stretching ratio. In general, the large micropores on the surface of the membrane were more likely to be attached by adsorbates, which reduced the antifouling property of the membrane. However, the results showed the BSA was difficult to adhere onto the surface of PP/EVOH/ MAH-HFMs and could be removed during the process of cleaning. The enhancement of antifouling properties of PP/EVOH/MAH-HFMs was attributed to the hydrophilic EVOH and PP-g-MAH which were mainly concentrated in the large micropores. Since the hydrophilic EVOH and PP-g-MAH in the large micropores prevent the adhesion of BSA, the existence of a large microporous in bimodal microporous structure did not reduce the antifouling properties of the PP/EVOH/MAH-HFMs.
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4. Conclusions It was confirmed by FESEM and pore size distribution tests that the bimodal microporous structure was well constructed by melt blending combining with MS-S in PP/EVOH/MAH-HFM. The presence of compatible interface enabled the EVOH particles to be less susceptible to deformation during membrane formation and optimized the bimodal microporous structure in PP/EVOH/MAH-HFM. During the stretching process of PP/EVOH/MAH-HFM, the small micropores in bimodal microporous structure were formed by the separation of PP lamellae, and the large micropores in bimodal microporous structure were formed by the separation of compatible interfaces between the PP phase and the EVOH phase. Moreover, two types of micropores were formed simultaneously. With the increase of the stretching ratio, the pore sizes of the two types of micropores showed a trend from increase to decrease. The porosity and pure water flux of PP/EVOH/MAH-HFM-200 respectively increased to 81.5% and 322.7 L/(m2·h), which respectively increased by 27.3% and 118.0% compared to PPHFM-200 due to the large micropores in bimodal microporous structure. The rejection performance of PP/EVOH/MAH-HFMs was also retained due to the small 337
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