Effect of Annealing on the Morphology of Porous ... - Springer Link

Macromolecular Research, Vol. 22, No. 6, pp 618-623 (2014) DOI 10.1007/s13233-014-2087-9

www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673

Effect of Annealing on the Morphology of Porous Polypropylene Hollow Fiber Membranes Sung Wook Han1, Seung Moon Woo2, Deuk Ju Kim2, O Ok Park1, and Sang Yong Nam*,2 1

Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea 2 Department of Materials Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Gyeongnam 660-701, Korea Received November 18, 2013; Revised February 26, 2014; Accepted March 2, 2014 Abstract: Three different polypropylene resins were selected to develop microporous membranes through melt spinning, annealing and stretching. The effects of take-up speed, cooling rate, stretching, and annealing temperature on the crystalline structure and orientation of the membrane were investigated using field emission scanning electron microscopy. An annealing process was employed to generate and enlarge the pores and lamellar structure, and improved the crystallinity of hollow fiber precursors, before the stretching process. The annealed hollow fiber precursor was stretched by cold, hot, and cold/hot complex stretching. We observed that porous structure on the outer and inner surfaces of the stretched precursor fibers were produced by cold and hot complex stretching. Keywords: melt spinning, stretching method, hollow fiber membrane, porous structure, annealing.

Introduction

structure of the membrane formed have to be considered. Some of these processing steps are difficult to quantify experimentally. After several trials, three key parameters were chosen to for study: initial composition of melt solution, spinning temperature, and melt-draw ratio. The dry process studied consists of three steps: extrusion, followed by annealing, then stretching.8 The first step was to prepare the fiber precursor through melt spinning. Prepared hollow fiber precursors were composed of a crystalline part and an amorphous part. When high shear stress is applied, row-nucleated lamellar structures can be created by stress-induced crystallization effects during melt spinning. Spun fiber precursors are annealed to enhance the crystalline structures of their crystalline regions. Porous structures are created by deforming and separating these stacked row lamellae (Figure 1). Stretched membranes have small, slit-shaped pores with narrow size distributions. As mentioned above, the stretching method is a valuable and simple way to produce porous membranes. This is because the process does not require any solvents, diluents or additives. Therefore, the stretching process is clean and economical. Membranes prepared by this method have high mechanical strengths due to their highly ordered structures. Kim et al. studied the effects of stretching processes on the morphology of high-density polyethylene (HDPE).3 They found that by using a melt spinning cold and hot stretching (MSCHS) process, the membranes produced were highly porous and had slit-shaped pores. As a result, membrane pore size and water flux varied in accordance to the variation of crystallite size

Because the productivity and efficiency of separation processes can be significantly improved by large surface area per unit volume of the fiber module, hollow fiber membranes have shown substantial potential for commercial application in reverse osmosis (RO), ultrafiltration (UF), and microfiltration (MF) systems. Microporous polyolefin hollow fiber membrane preparation can be achieved by three types of process: nonsolvent-induced phase separations (NIPS), thermally induced phase separations (TIPS), and stretching processes.1,2 Because of its facility, most membranes are prepared by NIPS methods. This method of hollow fiber membrane production is well-known for producing porous membranes. And use polymer precipitation by solvent and non-solvent exchange. TIPS begins by dissolving a polymer in a diluent at an elevated temperature. The solution is then cast or extruded into a desired shape, such as a flat sheet or hollow fiber. Next, the formed product is cooled to induce phase separation and crystallization. The TIPS process has been utilized to make microporous membranes from partially crystalline polymeric materials (polypropylene (PP), polytetrafluoroethylene (PTFE), polyethylene (PE)).3-6 Melt spinning is a more complex TIPS type process, with more post-cooling processing steps than TIPS, such as stretching, diluent extraction, etc.7 When melt cooling, many parameters that have significant influence on the *Corresponding Author. E-mail: [email protected] The Polymer Society of Korea

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Effect of Annealing on the Morphology of Porous Polypropylene Hollow Fiber Membranes

Figure 2. Apparatus of melt spinning and take-up system for preparation of PP hollow fiber precursor. Figure 1. Formation of pores between lamellae.

and crystallinity of the precursor fibers. K. Y. Lin et al. prepared microporous PP films using a dry process.9 Their membranes, produced under the same conditions, showed smaller spherulite structures. The aim of this study was to perform a membrane morphology analysis of TIPS by varying fundamental parameters, such as annealing temperature, cooling rate, take-up speed, and stretching process. In addition, we investigated the effects of precursor crystallinity on membrane morphology.

Experimental Materials. Three different grades of polypropylene were used to prepare precursors for hollow fiber membranes in this study. PPs (melt flow index: 4-8 g/10 min at 230 oC, density: 0.9 g/cm3) were supplied by Lotte chemical Corp. (Korea) and used without further purification. Basic properties, such as melting temperature and crystallization temperature of PPs are listed in Table I. Preparation of PP Hollow Fibers by Melt Spinning. The dry process consisted of three steps: extrusion, followed by annealing, then stretching. PP pellets were dried in a vacuum oven for 24 h at 60 oC prior to melt spinning. The melt spinning system is illustrated in Figure 2. Hollow fiber precursors were introduced into the single screw extruder equipped with a tube-in-orifice spinneret. The applied barrel temperature profile, from feeding to metering zone, had four zones set to 180, 210, and 230 oC, respectively. Nitrogen gas was introduced into the spinneret for the formation of the hollow fibers. The temperature between the extruder die zone and the Table I. Properties of Sample for Melt Spinning Sample

Tm (oC)

Tc (oC)

Melt Flow Index (g/10 min)

Density (g/cm3)

PP_1

164

108

8

0.9

PP_2

163

109

8

0.9

PP_3

163

106

4

0.9

Macromol. Res., Vol. 22, No. 6, 2014

spinneret zone was fixed at 180 oC. The spun fiber precursors were completely cooled to room temperature then wound on a take-up winder. In this process, exposure time for cooling was controlled by different take-up speed of winder to investigate the effect of cooling on crystallinity. The exposure time with different take-up speed is calculated to 2.1 s (128 m/min), 1.8 s (145 m/min) and 1.6 s (161 m/min) respectively. Takeup speeds ranged from 80 m/min to 161 m/min. At speed less than 80 m/min, I could not guarantee the constant fiber dimensions due to unstable fiber tension. Take-up speed over than 160 m/min caused the breaking of fiber. The spinning process variables investigated were the type of polymer, take-up speed, and stretching method. Next, fiber precursors were annealed in an air-circulating oven for 2 h. Annealing temperatures investigated were 100, 120, and 140 oC. Sample designations, (An_100), (An_ 120), and (An_140), indicate their annealing temperatures. Stretching of the annealed fiber precursors was conducted using three different processes: “cold stretching”, “hot stretching”, and “cold/hot complex stretching”. The cold stretching was measured to be around room temperature. And hot stretching carry out at 125 oC condition. The stretching direction was parallel to the extrusion direction (ED). 3-cm-long sections of fiber precursors were stretched by approximately 30% of their length using two air-pressurized clamps. For cold/hot complex stretching, samples prepared by cold stretching were directly stretched again by up to 100% of their length, at 125 oC. Finally, the hollow fiber membranes were annealed at 125 oC for 10 min to prevent shrinkage of the stretched hollow fibers. Crystallinity of PP hollow fibers. Crystallinity was measured by Differential Scanning Calorimetry (DSC, Q-20, TA instrument, USA). The samples were sealed in aluminum DSC pans, heated to 200 oC at a rate of 30 oC/min under a nitrogen atmosphere, and kept there for 5 min. Then, samples were cooled to 30 oC at various cooling rates. Precursor crystallinity (Xc) was calculated by the following equation: H Xc  %  = ---------  100 H 0

(1)

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where H is the heat of fusion of the precursor and H0 is 209 J/g, the heat of fusion of 100% crystalline PP. Morphology of PP Hollow Fibers. Images of hollow fiber precursor and stretched membrane morphologies were obtained using a field emission scanning electron microscope (FESEM, XL30 S FEG, Philips, Netherlands) and an accelerating voltage of 15 kV. Dry membranes were fractured in liquid nitrogen and coated with gold for 500 s.

Results and Discussion Crystallinity of Unprocessed PPs. Cooling process is one of the factors in determining the structure of membranes. If the cooling rate is too fast, an interconnected sponge structure is obtained because phase separation directly enters in the unstable region.10 On the other hand, when the cooling rate is too slow, the membrane shows a spherulitic structure.11 In the case of solid-liquid (SL) phase separations, only spherulitic structures are observed and they are controlled by varying cooling rate, polymer concentration, and polymer-diluent interactions.12 Efficient cooling conditions are required to prevent chain relaxation at the die exit and rapid cooling can affect lamellae growth.13 Therefore, cooling condition optimization is required in order to obtain a desired lamellae structure. Because of this, we studied the crystallinity of PPs obtained using various cooling rates in order to optimize melt spinning conditions. Figure 3 shows PP crystallinity measured by DSC as a function of cooling rate for the three different PP granules. The crystallinities of the PPs changed with the type of base mate-

rials and with cooling rate. The cooling curve peak was observed to narrow and shift to higher temperatures with decreasing cooling rate. This indicated that the PPs crystallized completely when slower cooling rates were used. According to the theory of nucleation growth (NG),14 as cooling rate decreases, more nuclei are created, and the number of spherulites increases. Therefore, higher crystallinity is obtained by slower cooling.15 On the other hand, decreased PP crystallinity, observed using a faster cooling rate, was due to the crystallization rate being lower than the cooling rate. The obvious decrease in crystallization temperature (Tc) with increasing cooling rate indicates that cooling rate indeed influences PP crystal nucleation and growth. This phenomenon is often observed in non-isothermal polymer crystallizations. Morphological Analysis of PP Hollow Fiber Precursors; Effect of Take-Up Speed. Take-up speed affects the crystal structure of the microporous membrane precursors. First, we investigated the effect of take-up speed without annealing. The cross sectional morphologies of PP hollow fiber precursors, prepared by melt spinning with various take-up speeds, are shown in Figure 4. All membranes show dense, non-porous structures. Hollow fiber membrane diameter varies with the type of base polymer and decreases with increasing take-up speed. Figures 5 and 6 show SEM images of the inner and outer surfaces of unannealed hollow fiber precursors. Row nucleated lamellar structures developed on all surfaces. Stacks of lamellae were also observed. In all cases, lamellar structures developed perpendicular to the drawing direction. Precursors prepared at lower take-up speeds had larger lamellae thicknesses compared to those prepared at higher speeds.

Figure 3. Crystallinity and DSC curves of PP granules with various cooling rate. 620



Figure 4. Cross section morphologies of PPs hollow fiber precursor with take-up speeds.

Figure 5. Inner surface morphologies of PPs hollow fiber precursor with take-up speeds.

Figure 7. Effect of annealing on crystallinity of PP hollow fiber precursors: (a) PP_1, (b) PP_2, and (c) PP_3. Figure 6. Outer surface morphologies of PPs hollow fiber precursor with take-up speeds.

Effect of Annealing Temperature on Crystallinity. Spun fibers are annealed to enhance their crystallinity. Therefore, we studied how the crystallinity of the prepared hollow fiber precursors varied with the annealing process. Figure 7 shows Macromol. Res., Vol. 22, No. 6, 2014

crystallinities, calculated by equation 1, for annealed PP hollow fiber precursors. The crystallinities of the hollow fiber precursors were quantified based on enthalpy change data (Hm), determined by DSC. The crystallinities of unannealed precursors varied with the type of base polymer. Most of crystallinity is almost like initial value or showed decreased with increasing take-up speed due to the decrease of crystallization 621

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time by the finishing air-cooling unit, as shown in Figure 7. The annealing temperature has a pronounced influence on the structure and performance of the resulting hollow fibers, as shown in Figure 7(a)-(c). Annealing at high temperature resulted in a small increase in the crystallinity of the PPs. The crystallinity of annealed precursors also increased with increasing of take-up speed. Figure 7 showed that take-up speed had little effect on crystallinity in this work.16 This might be because higher annealing temperatures favor the rearrangement of crystal imperfections which form as a result of stress-induced crystallization during the melt spinning process. As a result of annealing, micropores form more easily in fiber walls when hollow fibers are strained. The porosity of the hollow fiber membranes increased with increasing annealing temperature. Effects of Stretching Method and Take-Up Speed. Annealed hollow fiber precursors were stretched by cold, hot, and coldhot complex stretching methods to investigate the effects of stretching method on membrane morphology. The coldstretching process was developed to make PP and PE hollow fiber membranes with porous structures.17 This process has been applied to dense and nonporous films and hollow fibers to make lamellae and porous structures. In this study, the cold-stretching process was applied to porous hollow fiber membranes via TIPS. Figure 8 shows morphologies formed by cold stretching the annealed PP_1 hollow fiber precursor using various annealing temperatures. Increases in porosity were clearly observed at the surface of membranes as a result of cold-stretching. Lamella structures on the outer surfaces of hollow fiber membranes were enhanced by increasing takeup speed and tiny fibrils along the spinning direction were clearly seen in the 100% cold-stretched sample. J. Kim et al. also reported that annealed PE hollow fiber precursors produce lamella structures by cold stretching.3 From these result, we can confirm that the cold-stretching method destroys the crystal structure and it is a useful method for preparing porous membranes. The effects of take-up speed on the mor-

Figure 8. Effect of take-up speed on the morphologies of annealed hollow fiber precursors (PP_1) by cold stretching. 622

Figure 9. Effect of take-up speed on the morphologies of annealed hollow fiber precursors (PP_1) by hot stretching.

phologies of annealed PP_1 hollow fiber precursors prepared by hot stretching are shown in Figure 9. Slit-shaped pores were observed in all annealed samples when a high take-up speed was used. In the case of An_140, stretching resulted in larger skin pores and decreased membrane thickness with increasing take-up speed. Fibril structures were observed in An_140 membranes when take-up speeds under 161 m/min were used. However, the shapes and sizes of the pores were irregular. S.H. Tabatabaei et al. reported that annealing at 140 oC significantly increased crystallinity.18 The stretching process resulted in fibril structures. Figure 10 shows surface images of hollow fiber precursors prepared by the cold/hot stretching method. After cold stretching, fibril-type membrane pores were grown by hot stretching. Pore diameters frequently changed after hot stretching and formed lamella structures. Some pores that were invisible after cold stretching, initiated pores, were enlarged during the hot stretching process. These phenomena could explain the increase in the number of pores observed during the hot stretching stage. Cold/hot stretching was developed, and has been reported in many studies, for the

Figure 10. Morphologies of PPs hollow fiber membrane prepared by cold and hot complex stretching method. Macromol. Res., Vol. 22, No. 6, 2014


fabrication of microporous structures in hollow fiber membranes.13 Sadeghi et al. also reported that a porous PP membrane with lamella structures was made by the cold/hot stretching process.19 Also, increasing take-up speed increased the interlamellar microfibril length and the distance between each lamellar stack. A certain orientation is needed to develop lamellar structure for the precursor film. From these results, we can confirm that the stretching method is useful for producing lamellar structures and for optimizing porosity.

Conclusions In this work, we have investigated the structure of PP hollow fiber membranes produced using various cooling rates, annealing temperatures, and stretching techniques. Our findings can be summarized as follows: · Microporous PP hollow fiber membranes with slit-shaped pores were successfully fabricated using a post-annealing cold/ hot complex stretching method. · We investigated the effects of annealing temperature on the crystallinity of PP hollow fiber precursors. Using a sequential annealing process improved crystallinity. · The annealing temperature highly improve crystal phase orientation. · DSC results showed changes in lamellae size distribution at each step, from hollow fiber precursors to final products. · We found that while the annealing process had a profound effect on membrane morphology, PP hollow fiber membrane pore distributions varied in accordance with variations in precursor fiber crystallite size and crystallinity.

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