Preparation and Characterization of MicroporousPolyethylene Hollow Fiber Membranes

LI-QIANG SHEN, ZHI-KANG XU, YOU-YI XU

Institute of Polymer Science, Zhejiang University, Hangzhou 310027, China

Received 25 October 2000; accepted 26 July 2001Published online 25 January 2002

ABSTRACT: Microporous polyethylene (PE) hollow fiber membrane with a porosity of43% and N2 permeation of 4.96 cm3 (STP)/cm2 s cmHg was prepared by melt-spinningand cold-stretching method. It was found that PE with a density higher than 0.96 g/cm3

should be used for the preparation of microporous PE hollow fiber membranes. Byincreasing the spin–draw ratio, both the porosity and the N2 permeation of the hollowfiber membranes increased. Annealing the nascent hollow fiber at 115°C for 2 h wassuitable for attaining membranes with good performance. By straining the hollow fiberto higher extensions, the amount and size of the micropores in the hollow fiber wallincreased, and the N2 permeation of the membranes increased accordingly. © 2002 JohnWiley & Sons, Inc. J Appl Polym Sci 84: 203–210, 2002; DOI 10.1002/app.10305

Key words: polyethylene; membrane; morphology

INTRODUCTION

Microporous membranes fabricated from hydro-phobic polymers such as polyethylene (PE) andpolypropylene (PP) have great advantages overhydrophilic membranes made from polyacryloni-trile, cellulose, and others when used in fieldssuch as membrane distillation and membrane ex-traction because of their strong hydrophobicity.1

Generally speaking, there are two ways to pre-pare polyolefin membranes: thermally inducedphase separation (TIPS)2,3 and melt-spinning andcold-stretching (MSCS).4–7

For the TIPS method, the polymer and diluentare mixed to form homogenous solution at ele-vated temperatures. Upon cooling, the system un-dergoes liquid–liquid or solid–liquid phase sepa-ration, which results in the formation of polymer-

rich and diluent-rich phases. After solidificationof the polymer-rich phase, the diluent is extractedand the sites occupied by the diluent are left toform the microporous membrane. Therefore, byselecting appropriate diluents and cooling condi-tions, is relatively convenient to the control thepore size. However, it is obvious that this processresults in a waste solvent problem. On the otherhand, the MSCS method does not need any sol-vent, which is more environment-beneficial thanthe TIPS method. The micropores of the mem-branes in the MSCS process are formed bystretching the pure polymer hollow fiber or film.This means that the MSCS process does not in-volve any phase separation and its handling isrelatively easy.

As we know, the preparation of microporouspolyolefin hollow fiber membranes through MSCSmethod is based on the hard elastic property ofthe materials. That is, under proper conditions,the melt-spinning hollow fibers fabricated fromhighly crystalline polymers such as PE and PPmay exhibit high-elastic recovery such as elas-

Correspondence to: Z.-K. Xu (xuzk@ipsm.zju.edu.cn).Contract grant sponsor: National Natural Science Founda-

tion of China; contract grant number: 59673030.Journal of Applied Polymer Science, Vol. 84, 203–210 (2002)© 2002 John Wiley & Sons, Inc.

203

tomers and high-yield stress such as normal plas-tics.8 The lamellae in hard elastic fibers aligned inrows normal to the fiber direction. Upon strain-ing, they will separate and micropores bridged byfibrils will be formed. Then, if the fibers are heat-set, macromolecular chains in the fibrils will re-order or recrystallize to form stable structure andthe micropores are fixed. During the formation ofthe micropores, there are many factors whichwould influence the micropore structure, for ex-ample, the spinneret temperature, the melt-drawratio, and the annealing condition.4

There has been much literature, mainly pat-ents, that described the preparation of micro-porous PP or PE hollow fiber membranes throughthe MSCS method.4–6 However, there were fewdetailed investigations about the factors that in-fluence the membrane structures.7 In this article,factors such as spin–draw ratio, annealing tem-perature and time, and fiber extension on thepreparation of microporous PE hollow fiber mem-branes were discussed.

EXPERIMENTAL

Materials

Two types of high-density PEs were used.Yangtze 5000S with a melt index of 1.08 g/10 minwas purchased from Yangtze Petrochemical Co.(China) and Hi-Zex 2200J with a melt index of 5.4g/10 min was purchased from Mitsui Petrochem-ical Co., Ltd. (Japan). High-purity nitrogen(99.99%) was used for the gas permeation mea-surement of the membranes.

Preparation of Microporous PE Hollow FiberMembranes

The melt-spinning system was the same as wasused in Hangzhou Hualu Membrane EngineeringCo., Ltd. A tube-in orifice spinneret with innerand outer diameters of 9 and 13 mm was used.First, the PE melts were extruded out of the spin-neret at 190°C by using nitrogen as bore gas.Then, the hollow fibers were taken up at a spin–draw ratio of 680 and annealed for 2 h at 115°C.Subsequently, the hollow fibers were strained tolow extensions at temperatures lower than 40°Cand then to 130% at temperatures higher than80°C. At last, the hollow fibers were heat-set at110°C for 30 min. All the preparation parameters

are same as described above unless specified oth-erwise.

Characterization

Crystallinity

A Perkin–Elmer DSC 7 was used to measure thecrystallinity (Xc) of the sample at a heating rate of10°C/min. The crystallinity was defined as

Xc �%� � �H/�H0 � 100

where �H is the measured heat of fusion for thesample, and �H0 is the theoretical heat of fusionfor the pure PE crystal (taken as 293 J/g).9

Morphologies of the Samples

The morphologies of the samples were examinedby using a Hitachi S-570 scanning electron mi-croscopy (SEM) at an accelerating voltage of 20kV. The surfaces of the samples were coated witha thin film of gold to prevent charging prior to theSEM examinations.

Mechanical Property

The elastic recoveries of the hollow fibers weretested by using an XL-250A tensile testing ma-chine (Guangzhou Test Instrument Factory). Theelastic recovery (Er) was defined as

Er �%� � �l1 � l2�/�l1 � l0� � 100

where l0 is the fiber length before straining, l1 isthe fiber length when strained to 100%, and l2 isthe fiber length when the load was returned tozero after strained to 100%.

Porosity

The porosity (P) of the hollow fiber membrane wasdetermined with a density method. A certainlength of hollow fiber was weighed. The outer andinner diameters of the hollow fiber were mea-sured as an average of 10 samples by using amicroscope. Then the density (�) of the hollowfiber was defined as

� �W

��R2 � r2�l

204 SHEN, XU, AND XU

where W is the weight of the hollow fiber, R and rare the outer and inner diameters, respectively,and l is the length of the hollow fiber.

Thus, the porosity of the hollow fiber mem-brane was defined as

P �%� � ��0 � �1�/�0 � 100

where �0 is the density of the hollow fiber beforestraining and �1 is the density of the hollow fiberafter straining.

N2 Permeation

About 10 hollow fiber membranes were bundledin a U-shape and their open ends were fastenedwith epoxy resin to fabricate a module. The lengthof every hollow fiber membrane was about 10 cm.Then, a nitrogen pressure of 15 cmHg was appliedto the inside of the hollow fiber membrane at25°C, and the amount of nitrogen that permeatedthrough the membrane was determined. The N2permeation (F) was calculated according to thefollowing equation

F �VN2

A � P � t

where VN2 is the nitrogen volume that permeatesthrough the membrane, A is the membrane areathat is calculated on the basis of the inner diam-eter of the hollow fiber membrane, P is the nitro-gen pressure between the upside and downside ofthe membrane, and t is the time when nitrogenpermeates through the membrane.

RESULTS AND DISCUSSION

Hollow Fibers Spun from Different Types of PEs

It was reported that PEs with density higher than0.96 g/cm3 should be used to gain PE hollow fiberswith good hard elasticity.10–12 Because hard elas-ticity is the basis of microporosity of PE hollowfiber membranes, it is important to select properPE materials for the spinning of microporous PEhollow fiber membranes. In this study, two typesof PEs, Yangtze 5000S and Hi-Zex 2200J, wereused.

Figure 1 SEM micrographs of the outer surfaces ofstrained hollow fibers spun from (a) Yangtze 5000S (a)and (b) Hi-Zex 2200J. Magnification: �10,000 (a) and�3000 (b).

Figure 2 SEM micrographs of the outer surfaces ofannealed hollow fibers spun from (a) Hi-Zex 2200J and(b) Yangtze 5000S. Magnification: �15,000 (a) and�10,000 (b).

Table I Hollow Fibers Spun from Different PEs

Type of PEMelt Index(g/10 min)

Density(g/cm3)

Crystallinity(%)

Er of Hollow Fiberbefore Straining (%)

Porosity(%)

N2

Permeationa

Hi-Zex 2200 J 5.40 0.968 93.1 82 34 4.35 � 10�2

Yangtze 5000 S 1.08 0.955 82.8 45 0 0

a Unit of N2 permeation is cm3 (STP)/cm2 s cmHg.

MICROPOROUS PE HOLLOW FIBER MEMBRANES 205

As shown in Table I, before straining, the re-sulting hollow fibers exhibit very different elasticrecovery, which indicates different hard elastic-ity. It is well known that higher elastic recoverymeans better hard elasticity. Data shown in TableI reveal that the hollow fiber spun from Hi-Zex2200J exhibits higher elastic recovery just likeother kinds of hard elastic fibers; however, thehollow fiber spun from Yangtze 5000S performsmore like normal PE fibers. After these two kindsof hollow fibers were strained to the same exten-sion of 130% and heat-set, the hollow fiber spunfrom Yangtze 5000S shows a N2 permeation of 0and a porosity of 0, whereas the hollow fiber spunfrom Hi-Zex 2200J exhibits just the opposite prop-erties. The results were confirmed by the SEMmicrographs of the hollow fibers. In Figure 1(a)

for Yangtze 5000S, no micropore can be seen inthe fiber wall, whereas there are many intercon-nected micropores in Figure 1(b) for Hi-Zex2200J.

The reason for this result may be complicatedbecause there are many factors that influence thepreparation of microporous PE hollow fiber mem-branes. It was pointed out that two importantfactors which are in favor of hard elasticity areperfect crystal structure and amorphous regionwith good mobility.13 It can be seen from Table Ithat Hi-Zex 2200J shows higher crystallinity andhigher density. Thus, it may be easier for Hi-Zex2200J to form relatively perfect crystal structurewhen extruded from spinneret than Yangtze5000S. Moreover, the higher melt index of Hi-Zex2200J, which indicates lower molecular weight,

Figure 3 Relationship between spin–draw ratio and elastic recovery of unstrainedhollow fiber.

Figure 4 Relationship between spin–draw ratio and properties of hollow fiber mem-brane.

206 SHEN, XU, AND XU

may contribute to the higher mobility of the amor-phous regions, which is in favor of the formationand closing of microvoids under extension. There-fore, with more perfect crystal structures andmore mobile amorphous regions, the hollow fibersspun from Hi-Zex 2200J exhibit better hard elas-ticity than the hollow fibers spun from Yangtze5000S. This is consistent with the literature.10–12

Consequently, after straining and heat-setting,the hollow fibers spun from Hi-Zex 2200J showmicropores in its wall, whereas hollow fibers spunfrom Yangtze 5000S do not, as illustrated in Fig-ure 1(a,b).

Figure 2(a) shows that the hollow fiber spunfrom Hi-Zex 2200J possesses well-formed thickcrystalline lamellae perpendicular to the fiber di-rection, which is a main feature of hard elasticfiber. However, it can be seen from Figure 2(b)that the wall of the hollow fiber spun fromYangtze 5000S is similar to that of ordinary ori-ented hollow fiber. These fibers indicate that thehollow fiber from Hi-Zex 2200J has good hard

elasticity, whereas that from Yangtze 5000S doesnot.

As a result of the above discussion, Hi-Zex2200J was used only in the following investiga-tion.

Effect of Spin–Draw Ratio

The effect of spin–draw ratio on the resultinghollow fiber properties was examined and the re-sults are illustrated in Figures 3 and 4. Figure 3shows that the elastic recoveries of the un-strained hollow fibers increased slightly with in-creasing spin–draw ratios. In addition, the poros-ities and N2 permeations of the microporoushollow fiber membranes both increase with in-creasing spin–draw ratios, as illustrated in Fig-ure 4.

One possible explanation for the phenomenamay be that the higher spin–draw ratio can stim-ulate the formation of more perfect row-nucleatedcrystals, which increases hard elasticity of theunstrained hollow fibers. Besides, more perfectoriented crystalline lamellae can contribute to theformation of more and larger micropores in thefiber wall, which increases the membrane poros-ity and gas permeation. Figure 5(a,b) illustratesthe morphologies of the resulting hollow fibersspun from spin–draw ratios of 680 and 950. TheSEM micrographs show clearly that the micro-pores of the hollow fiber spun with spin–drawratio of 950 are more and larger than those of thehollow fiber spun with spin–draw ratio of 680.

Effect of Annealing

After the hollow fiber is spun from the spinneretand taken up, it should be annealed below themelting point of PE to increase elasticity of thehollow fiber; then the micropores can be morereadily formed in the fiber wall during the strain-

Figure 5 SEM micrographs of the outer surfaces ofhollow fiber membranes spun with spin–draw ratio of(a) 680 and (b) 950. Magnification: �10,000.

Table II Hollow Fibers Annealed at Different Temperatures

AnnealingTemperature (°C)

Hollow Fiber beforeStraining Hollow Fiber after Straining

Er (%) Crystallinity (%) N2 Permeationa Porosity (%)

No annealing 51 86 — —90 70 88 3.70 � 10�2 29

110 74 89 4.08 � 10�2 32115 82 90 4.35 � 10�2 34

a Unit of N2 permeation is cm3 (STP)/cm2 s cmHg.

MICROPOROUS PE HOLLOW FIBER MEMBRANES 207

ing process. In the annealing process, annealingtemperature and time are the two key factorsinfluencing structure and properties of the hollowfiber membranes.

After the hollow fibers are extruded out of thespinneret, they rapidly undergo stress crystalli-zation and hence the row-nucleated crystals form.However, the process is so short that there aremany molecular chains that align irregularly inlamellae; thus, many defects exist after the hol-low fibers are taken up. Upon annealing underproper conditions, these unstable defects will re-order to form more perfect structures. The resultis that the defects will reduce and more stablecrystals will form. Moreover, density of the hollowfiber will show some increase because of highercrystallinity, as has been mentioned by Kim et

al.7 Thus, the elasticity of the hollow fiber im-proves after annealing. This phenomenon was ob-served in this study also. As shown in Table II,the elastic recoveries of the annealed hollow fi-bers enhance to a great extent in contrast to theunannealed hollow fiber. Besides, it can be seenthat the elastic recoveries of the unstrained hol-low fibers increased with increasing annealingtemperatures in Table II. This may be that higherannealing temperature is in favor of the rearrang-ing of imperfect regions in the fiber wall. As aresult of improved elasticity, the micropores inthe fiber wall formed more easily when the hollowfiber was strained; the porosity and N2 perme-ation of the hollow fiber membrane both in-creased. Within the temperature scope examinedin this study, hollow fiber membrane with higher

Figure 6 Relationship between annealing time and properties of annealed hollowfiber.

Figure 7 Relationship between annealing time and properties of strained hollowfiber.

208 SHEN, XU, AND XU

porosity and N2 permeation can be obtained athigher annealing temperatures.

In addition, the effect of annealing time wasexamined in this study. Figure 6 shows that theelastic recovery and crystallinity of the hollowfiber increase with increasing annealing time inthe initial stage of about 2 h. However, moreannealing time has almost no influence on theelastic recovery and crystallinity of the hollowfiber. The explanation may be that the reorderingof the molecular chains in the fiber wall is adynamic process, which needs some time to fulfill.Once the process is mostly completed, increasingthe annealing time further has no effect. Thus,the properties of the hollow fiber did not changemuch after about 2 h of annealing. Because of thechanges of the hollow fiber in elasticity and crys-tallinity, the porosity and N2 permeation of thestrained hollow fiber change accordingly. Asshown in Figure 7, similar to the curves shown inFigure 6, the curves level off also above annealingtime of 2 h. All these results indicate that about2 h of annealing is proper for obtaining hollowfibers with reasonable performances in this study.

Effect of Extension

The micropores of PE hollow fibers are formed bystraining. For a typical process of straining, thehollow fibers were first strained to lower exten-sions under somewhat low temperatures to in-duce microvoids in the fiber wall. Then, the hol-low fibers were strained to higher extensions un-der higher temperatures to form more and largermicropores without the breaking of the hollowfibers.

In this study, the PE hollow fibers werestrained to different extensions and heat-set at110°C for 30 min. The results are presented inTable III. As can be seen, the porosity and N2permeation of the hollow fibers increased with

increasing extensions. Figure 8 shows the SEMmicrographs of the hollow fibers with differentextensions. It is obvious that the amount and sizeof the micropores increased with increasing ex-tensions.

When a hard elastic fiber is strained,14 first,the lamellae of the fiber rotate, that is, the c-axisof the lamellae originally perpendicular to thefiber direction rotates to a position parallel to thefiber direction. Then, the lamellae are separatedand the molecular chains of amorphous regionsare oriented; thus, the microvoids bridged byfibrils are created. If the fiber is extended further,molecular chains in the lamellae will be pulledout at higher extensions, and larger and moremicrovoids can be formed, but the cross-sectionalarea of the fiber will reduce markedly. The resultof increased porosity and gas permeation withincreasing extensions in this study confirms theabovementioned theory. Moreover, in this study,the inner and outer diameters of the unstrainedhollow fiber are 395 and 440 �m, respectively. Incomparison with the data listed in Table III, it

Figure 8 SEM micrographs of the outer surface ofhollow fiber membranes with different extensions: (a)extension of 100%, (b) extension of 130%, and (c) exten-sion of 170%. Magnification: �3000.

Table III Microporous Hollow Fibers Strainedto Different Extensions

Extension(%)

N2

PermeationaPorosity

(%)OD/ID

(�m/�m)

100 3.78 � 10�2 30 415/375130 4.35 � 10�2 34 409/369170 4.96 � 10�2 43 394/357

a Unit of N2 permeation is cm3 (STP)/cm2 s cmHg.OD, outer diameter of hollow fiber; ID, inner diameter of

hollow fiber.

MICROPOROUS PE HOLLOW FIBER MEMBRANES 209

can be concluded that the cross-sectional areas ofthe fibers reduce a great deal, which is anotherproof for the theory.

The financial support of the National Natural ScienceFoundation of China (Grant No. 59673030) is gratefullyacknowledged.

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