TOKYO METROPOLITAN UN工VERSITY

(1)

、環境調和・材料化学 745

0ya Hiroyoshi

(2)

Fabrication of Gradient−Structure Hollow−Fiber Microfiltration Membranes

Using Non−Solvent lnduced Phase Separatio皿

ADissertation Presented to

TOKYO METROPOLITAN UN工VERSITY

Hiroyoshi o】日lYA

2009／3

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Promoter： Prof． Dr． Hiroyoshi KAWAKAMil Prof． DL Satoru KATO

Prof． Dr． M紐s紐fumi YAMATO

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Fabrication of Gradient−Structure Hollow−Fiber

Microfdtration Membranes

Using Non−Solvent lnduced Phase Separation

Contents

CHAPTER 1 Introduction ¹

1．1Background and obj ective s 2

1．20rganization of this thesis 10

1。3 References 14

CHAPTER 2 Theory of membrane formation 17

2．1General understanding of］¶PS process 18

2．20riginal control method of phase separation by MPS 20

2．3 References 26

CHA．PTER 3 Optimal dope viscosity fbr nucleation and grow重h 29

3．11ntroduction 30

3．2Experimental 36

3．2．lMaterials 36

3．2．2Fabrication of PSF丘bers 36

3．2．3Characterization ofPSF fibers 40

3．2．4SEM analysis 40 3．3．Results 41 3．3．lPolymer solution properties 41

3．3．2Physical characteri stics 42

3．3．3 Pure water flux 43

3．3．4SEM analysis 44

3．4Discussion 46

3．4．1Membrane鉛㎜ing mechanism of outer sur鉛ce 46 3．4．2Membrane鉤㎜ing mechanism of㎞er sur飴ce 52

3．4．3 Macrovoid formation 54

3．5Conclusion 55

3．6 References 59

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CIIAPTER 4 Optimal spinning condition jbr gmdient structure 65

4．2．1Materials 70

4．2．2Fabrication of PSF fibers 71

4．2．3Characterization of P SF fibers 73

4．3Results 74

4．3．1 Spinning 74

4．3．2Pure water flux 76

4．3。3SEM analysis 78

4．4Discussion 84

4．4．1Membrane fbr血ng mechanism of cross section 84

4．4．2Membrane fbrming mechanism of outer surface 88 4．4．3Membrane鉛rming mechanism of㎞er sur飴ce 89

4．5Conclusion 90

4．6 References 94

CHAPTER 5 Optimal spinning condition br micr・filtration membrane 97

5．2．1Materials 98

5．2．2Fabrication of PSF fibers 99

5．2．3Characterization of P SF fibers 101

5．3．Results 103

5．3．1 Spirming 103

5．3．2Characterization of P SF fiber 104

5．4Discussion 114

5．5Conclusion 120

5．6 References 124

CHAPTER 6 Conclusion and future scope 125

6．1Conclusion 126

6．2Future scope 136

List of Publications

Ais：1kg！g）1：gggg1ng！uldement

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CHAPTER l

Introduction

This chapter describes the background and obj ectives of this thesis． Polymeric

membrane classifications and applications are described first。 Next is described why

h・ll・w fiber micr・filtrati・n（MF）membrane pr・duced by n・n−s・lvent induced phase

separation（NIPS）was important． In the latter part of this chapter， the organization of

this thesis is explained．

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CHAPTER l

Introduction

1．1Background and Objectives

The fbur developed industrial membrane separation processes are MF，

ultrafiltration（UF）， reverse o smo sis（RO）， and electrodialysis（ED）． There are two

maj or configurations， hollow fiber type and flat sheet type． These membrane processes

are all well established， and the market is served by a number of experienced companies．

The range of application ofthe three pressure−driven membrane water separation

processes−RO， UF and MF−is illustrated in Figure 1．1． Photos of hollow fiber

membrane and flat sheet membrane are shown in Figure 1．2。

RO membranes enable the separation ofwater from ions and low−molecular

weight organic constituents． These membranes have been used for the desalination of

seawater for some time． UF membranes have pores血the range of 1㎜一〇．Ol pm．

This enables precise separation， concentration， and purification of dissolved and

suspended constituents according to their relative molecule size（molecular weight）．

MF membranes have a pore sizes range of O．08−10 pm depe尊ding on the selected

membrane． These membranes enable efficient and precise separation and concentration

of suspended and colloidal particles．・

ln recent years， membrane separation teclmology has gained attention in process

water treatment， potable water production， wastewater treatment， and other water

treatment applications． Hollow−fiber MF membranes have become prevalent in the field

of water treatment． Advantages over flat−sheet membrane s include high mechanicaI

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CHAPTER l

Introduction

RO

UF

MF

O．1nm lnm 10・nm O．1脚 1pm

Pore diameter

to vm 100 pm

Figure 1．1 Reverse osmosis， ultra血ltration， and・microfiltration are related process differing principally in the average pore diameter of the membrane filter．

Hollow fiber membrane

Source：Asahi Kasei Chemicals

Flat sheet membrane

Source：Membrane Solutions

Figure 1．2 Ho且且ow fiber membranes and fiat sheet membrane．

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CHAPTER 1

1ntroduction

strength and a self−supporting structure to enable back−flushing and greater membrane

surface area in a given space． Advantages over ultrafiltration include higher water

permeate flux with a given membrane surface area．

There are inside−out and outside−in filtration mode for hollow fiber membrane

filtration． Figure 1．3 shows schematic drawing ofthese filtration mode． Inside−out

filtration has a high cro ss−flow velocity over the membrane surface and prevents

membrane fbuling． This makes inside−out filtration suitable fbr concentration and

purification ofhighly concentrated solutions． On the other hand， outside−in filtration

utilizes the lager area of the outer surface of the meml）rane fiber；the filtration load per

unit area may be reduced． Additionally， a physical cleaning technique such as air−

scrubbing， may be utilized． These feature s make this mode of operation well suited for

high volume water clarification． Therefbre， outside−in filtration by hollow−fiber

membranes is generally used fbr water clarification．

lnside−out filtration Outside瞳in filtration

Figure 1．3 lnside−out and outside−in mtra重ion mode．

Typical fabrication methods fbr hollow fiber membrane are dry−wet spinning through

NIP S and melt sptming through thermally induced phase separation（TPS）． Figure 1．4

shows schematic drawings ofthe difference between Nll｝S and TIP S processes．

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CHAPTER l

Jnt7「oduction

TIP S， a method whereby phase separation is induced by lowering the

temperature of a molten polymer solution， has long been used to produce MF

membranes［1−4］・Hollow−fiber MF membrane produced by melt spinning through

TP S has sponge−like structure， and thi s structure provides higher mechanical strength．

At the same time， with mek spinning through TIIP S， there i s the tendency for formation

of uniform membrane wall morphology， which limits the degree of permeate flux which

can be ol）tahled． −

NIPS is a method whereby non−solvent is absorbed in the air gap（AG）or

polymer solution is immersed血non−solvent to effect phase separation． Membrane

produced by NIPS generally has an anisotropic membrane structure to engender high

water permeate flux． However， it is very difficult to fbrm large pores by NIPS while

maintaining mechanical streng‡h． MF membrane produced through NrPS has been

limited t6 fiat sheet membrane which is cast・n a n・nw・ven supP・rt． Table1．l

summarize maj or differences between hollow fiber by NIPS and by TIP S． Figure 15

shows s chematic drawings o f hollow fiber spinning and casting of flat sheet membrane．

ln other words， if the fabrication method of hollow−fiber MF membrane that has

afilter layer on the outer sufface to reduce the filtration resistance and pore sizes which

gradually increase toward the inner surface structures can be established， this method

would lead to a new development in MF membrane technology which outstrip s the

water permeability of conventional hollow−fiber MF membrane．

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CHAPTER！

Introduction

TS隣＄

｛ThereVtsa麗y iRCieeceti gehase ＄eparation）

細》崩鰐鶴甕穂畷篇獣撫1繍癖騰

麟鞭鎌

Figure 1．4 Membrane formation by phase separation．

Table 1．1 Comparison betWeen hollow fiber by NIPS and by Tll｝S．

NIPS TIPS

Materials Soluble polymer ThermOP雇astic

Pore range RO−UF ^MIF

Advantage High permeate flux High mechanica畳strength Simple spinning proceSS

Disadvantage Complicated spinning process

Low・mechanica且strength Low perme飢e伽x

Structure Anisotropic Isotropic

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CH 4PTER 1 1ntroduction

Hollow fiber spinning by NIPS

Non−solvent

Polymer Solutjon

er Membrane

owashing

nd drying

プ

Non−solvent bath

Flat sheet casting by NlPS

Polymer solution

Casting box

Support layer

Flat sheet Membrane

To drying

Non■solvent bath

Figllre 1．5 Hollow fiber spinnimg an 血at sheet casting．

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CHAPTER l

Int「oduction

In this study， fabrication of practical hollow−fiber MF membranes， suitable fbr

outside−in filtration， through non−solvent induced phase separation（NP S）by dry／wet

spinning， was studied． Polysulfbne（P SF）hollow−fiber MF membranes having gradient

structure， more specifically， having a filter layer in the region of the outer surface and

gradually increasing pore size across the thickness of the membrane wall toward the

inner surface were designed and their spiming conditions were examined． Figure 1．6

shows the chemical structure of P SF． Figure 1．7 shows the images of gradient structure．

Featuring excellent chemical resistance， heat resistance， and mechanical strength

in addition to high permeate flux， PSF hollow丘ber membranes have long been the

subj ect of study for application as separation membranes for RO［5］， gas separation

［6，7］，UF［8］， and dialysis［9］． In contrast， despite an extensive search ofpublished

literature， we were unable to find reports related to hollow−fiber MF membranes by dry−

wet spilming． If P SF hollow−fiber MIT membranes can be achieved by dry−wet spinning，

this teclmology would not only have high academic value but would also be extremely

usefU．l fbr practical industry．

o

CH3 Cl

I

CH3

o

OHSロO

Figure 1．6 Structure of polysulfone．

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CUA1）TER l Intp「oduction

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C『◎ss§ec豊1◎鶴◎f 睡◎1国◎W簸e『

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縄1響齢響1醜x

畷羅1琿覇↓

ぼ

睡譲1蛙鋼肇紬△x

8・η・τ

」：職馨ぎ馨『搬e醜馨聲1鱒《ll 『鶴2タ驚⇔

△酔：継e§§w蓼嫡欝ξ蓼騰醜◎ee r睡 M21

△x ：鵬e齢蝕『翻騰豊隔《細錨（『醜》

：・pare麟鵬㈹

：1瞬彌dvl蜘量y（穐・s響《；）

：§騨㎡窺騰騨◎『◎§鰍y《嫡1麟縷憾◎麟総§趨：豊◎Wt屡◎§1琶y葛蕊鷹oず《c選1魏㊧麟◎醜睡§§夢

Figure 1。7 Hollow−fiber MZF membranes having gradient structure．

(15)

CUAPTER 1

1ntroduction

1．20rganization of this thesis

Chapter l is the introduction．

Chapter 2 describes general understanding of the membrane fbrmation

mechanism through N】P S and an original ．control method of phase separation by N正PS

in order to make MF−class pores． How to promote nucleation and growth in a practical

PSF hollow−fiber spi皿ing process is considered． PSF solution separates into a

polymer−rich phase and polymer−lean phase． ih order to obtain MF−class pores， it is

necessary to promote nucleation and growth ofpolymer lean phase． Therefbre the time

in which nucleation and groWth of the polymer lean phase is allowed before

solidification must be important， and dope viscosity， bore liquid concentration， and AG

conditions were fbcused on in order to control nucleation and growth time．

Chapter 3 focuses on dope viscosity in determining the optimum dope

composition fbr promotion of nucleation and growth． All sample preparations were

made with N−methylpyrrolidone（NMP）as solvent， polyethylene glycol（PEG）as

additive， aqueous NMP solution as bore liquid， and water as coagulation bath．

Particular focus was placed on the influence of PEG molecular weight（Mw）on

membrane structure． Characterization of the obtained membrane s was perfbrmed l）y

measuring pure water permeate flux， tensile strength at break， and tensile elongation at

break， and by analyzing scanning electron microscope（SEM）images ofhollow−fiber

cross sections， outer surfaces， and inner surfaces．

This chapter reveals that dope viscosity increased sharPly as PEG Mw was

raised to 20 kDa or higher， as did water permeate且ux． SEM image analysis also

(16)

CH4PIER l

Introduction

revealed that outer surface pore size increased with PEG Mw． Moq）hology of the inner

surface transformed markedly as PEG Mw was raised from 6，000 to 20，000．

Ih Chapter 4， fabrication ofpractica1 hollow−fiber microfiltration membranes，

suitable fbr outside−in filtration， through non−solvent hlduced phase separation（NIP S）

by dry／wet spirming， was studied． Polysulfbne hollow−fiber MF membrane s having

gradient stnlcture， more specificall》山aving a filter layer in the region ofthe outer

surface and gradually increasing pore size across the thickness ofthe membrane wall

toward the㎞er surface were designed， and their spinning conditions were examined，

For all sample preparations， we used N−methylpyrrolidone as solvent，35 kD molecular

weight polyethylene glycol（PEG）as additive， aqueous NMP solution as l）ore liquid，

and water as coagulation bath． Particular focus was placed on the effect on membrane

structure ofNMP concentration of bore liquid and air AG conditions in order to obtain

gradient structure。 Characterization ofthe obtained membranes was performed by

measuring pure water permeate flux， tensile strength at break， and tensile elongation at

break， and by analyzing scanning electron microscope（SEM）images ofhollow−fiber

cross sections， outer surfaces， and i皿er surfaces． It was necessary to raise the

proportion of membrane formation from the outer surface with respect to membrane

f（）rmation from the inner surface in order to fabricate a P SF hollow−fiber MF membrane

having a filter layer in the region ofthe outer surface and gradually increasing Pore size

across the thickness ofthe membrane wall toward the inner． It was possible to fabricate

the above P SF hollow−fiber MF having this gradient structure by selecting more than 70

Wt％NMP concentration ofbore liquid and setting AG distance below 50 mm．

(17)

CH4PIER l

Introduction

hChapter 5， particular focus was placed on the effect ofNMI concentration of

bore liquid and air gap conditions on membrane structure in order to control inner and

outer surface pore size as MF membranes． Characterization ofthe obtained membranes

was performed by measuring pure water permeate flux， tensile strength at break， and

tensile elongation at break， and by analyzing scanning electron microscope images of

hollow−fiber cross sections， outer surfaces， and inner surfaces．

It was fbund that a solvent neutral state was attainable by selecting 95 wt％

NMP solution as bore liquid with our temary dope system， and that it was possible to

obtain the desired gradient structure with membrane fbrmation only from the outer

surface． Pore size of the filter later in the region of outer su士face could be controlled by

a（漸usting AG temperature and／or AG humidit）1． hl particular， MF−1evel pores were

obtained by setting the AG temperature to 60°C or more．

Chapter 6 describes the conclusion of this thesis and fUture scope． The hollow一

丘ber MF membrane obtained in this study had a gradient structure with an average pore

size of O．23 ptm on the outer surface． Pure water permeate flux was about 4，900

1・m−2・h−1at 100 kPa． Retention of T500 was O％and that of O．137μm latex beads was

almost 100％． Tensile strength was over 4．O MPa， and tensile elongation at break was

over 80％． These characteristics and perfbrmance figures are suf且cient fbr use in MF at

the practical level， and such membran6s would be well suited fbr application in water

treatment and other fields．且owever， in this study the temperature of the coagulation

bath was a（加sted to the same temperature as the AG Therefbre， no distinction was

made between the in且uence on membrane fbrmation of coagulation bath temperature，

(18)

CHAPTER l

lntroduction

AG temperature， and AG humidity． More precise understanding of membrane

fomiation mechanism in the A q p articularly the p ore formation mechanism on the outer

surface， requires more precise control of the temperature and the humidity of the AG

independently under a certain constant coagulation temperature． Moreover， a

quantitative experiment concerning the amount of water vapor absorbed by the nascent

hollow fiber in the AG is necessary．

(19)

CUAPTER l

Introduction

1．3 References

［1］D．R Lloyd， J．W． Barlow， Microporous Membrane Formation via Thermally−induced

Phase S eparation， AIChE． Symp． S er．，84（1988）28．

［2］BJ・Cha， K・、Char， J．−J． Kim， S・S・Kim and C・K・Kim， The effects of diluents

molecular weight on the structure ofthermally−induced phase separation membrane． J．

Membrane． Sci．，108（1995）219．

［3］且．Matsuyama， H． OkafUj i， T． Maki， M． Teremoto and N． Kubota， Preparation of

polyethylene hollow fiber membrane via thermally induced phase separation， J．

Membrane． S ci．223（2003）119．

［4］B．J． Cha and J． M． Yang， Preparation ofpoly（vinylidene且uoride）hollow fiber

membranes fbr microfiltration using modified TIP S process， J． Membrane． Sci．

291（2007）191．

［5］1．Cabasso， E． Klein and∫． K． Smith， Research and development ofNS−1 and related

polysulfone hollow fibers for reverse osmosis desalination of seawater， U． S． NTIS，

PB Rep．（1975）， PB−248666，150．

［6］Z．Borneman， J． A． Van t Hof and C．A． Smolders and H．M． Van Veen， Hollow丘ber

gas separation membranes：structure and properties， Special Publication−Royal S ociety

of Chemistry（1986），62（Membrane． Gas S ep． Enrich．），145．

［7］s．c． Pesek and w． J． Koros， Aqueous Quenched Asymmetric Polysulfone Hollow

Fiber prepared by dry／wet phase separation， J． Membrane． S ci．88（1994）1．

［8］P．Aptel， F． Ivaldi and J． P． Lafaille， Development ofpolysulfbne hollow fiber， Proc

2nd World Congr． Chem． Eng．，4（1981）191．

(20)

CHAPTER l

Introduction

［9］J．Fowler and J． Hagewood， Preparation of Polysulfbne Dopes fbr Production of

且ollow Fiber Membranes， ht． Fiber J．，9（1994）28．

(21)

CHAPTER l

Introduction

、

(22)

CHAPTER 2

Theory of membrane血）rmation

This chapter describes general understanding of membrane fbrmation

mechanism through NPS and an original control method of phase separation by NIPS

in order to make MF−class pores． How to promote nucleation and growth in a practical

PSF hollow−fiber spinning process is considered． PSF solution separates into a

polymer−rich phase and polymer−lean phase． In order to obtain MF−class pores， it is

necessary to promote nucleation and growth of polymer lean phase． Therefbre the time

in which nucleation and growth of the polymer lean phase is aUowed befbre

solidification must be important， and dope viscosity， bore liquid concentration， and AG

conditions were fbcused on in order to control nucleation and growth time．

(23)

Cll］caPTER 2

z吻那ゾme〃7branefor〃2at加

2．1General understanding of membrane formation mechanism through NIPS

Hollow−fiber membranes are generally produced by dry／wet spi㎜ing， or by melt

spinning， which utilizes phase separation characteristics of polymer solutions． Phase

separation methods have been classified according to the mechanism of phase

separation［1，2］． Three『broad categories are evaporation induced phase separation

（EIP S）， non−solvent induced phase separation（NPS）， and thermally induced phase

separation（TP S）． With dry−wet spiming， membrane formation is generally through

EIPS， NIPS， or a combination of the two；with melt spillning， it is generally through

T工PS．

EIP S， a method whereby solvent evaporates in an air gap（AG）until polymer

concentration rises above the dissolution limit， thereby effecting phase separation， is

used to produce RO and gas separation membranes as it is well suited to the formation

of a very fine porous skin on the membrane surface to function as a separation layer［3，

4］．This techniques is schematically represented in Figure 2．1

Figure 2．1 Phase separation by solvent evaporation．

(24)

CHA1）TER 2

TheOjつノqゾ〃le〃7b7anθプ〜）7〃lation

N正PS， a method whereby non−solvent is absorbed in the AG（also called vapor

induced phase separation（VIP S）［5】）or polymer solution is immersed in non−solvent to

effect phase separation， i s generally used to produce UF membranes as it is well suited

to the formation of an anisotropic membrane structure to engender high water permeate

flux． These technique is schematically represented in Figure 2．2．

Polymer 1 solvent

Vapor phase

or

Liquid phase

Figure 2．2 Non−solvent imduced phase separation．

TIPS， a method whereby phase separation is induced by lowe血g the

temperature of a molten polymer solution， has long been used to produce MF

membranes［6−9］． One drawback of hollow−fiber MI？ membranes produced by melt

spinning th「・ugh TIPS is the tendency f・r f・rmati・n gf unif・rm membrane wall

morphology， which limits the degree ofpermeate flux which can be obtained． This

technique is schematically represented in Figure 2．3． Fabrication ofho llow−fiber MF

membranes with anisotropic wall morphology by dry−wet spinning through NIP S has

therefore become a focus of research．

(25)

CH41）TE1〜2

TheOT y of〃le〃1ゐranefor〃2α o刀

Polymer solvent

Liquid phase

Figure 2．3 Thermally induce phase separation．

Dry−wet spinning through NIP S is commonly used to produce hollow一丘ber

membranes of polysulfbne（PSF）and polyether sulfbne（PES）． Featuring excellent

chemical resistance， heat resistance， and mechanical strength in addition to high

permeate flux， these have long been the su切ect of study fbr application as separation

membranes fbr RO［10］， gas separation［11，12］， UF［13］， and dialysis［14］． In contrast，

despite an extensive search of pubIished literature， I w．as unable to find reports related to

hollow−fiber MF membrane s by dry−wet spinning．

2．2 Original control method of phase separation by NIPS

The dry−wet spiming process involves the phase separation of a polymer

solution into polymer−rich and polymer−lean phases， which can be induced by non−

solvent， vapor， or solution． The simplest system comprises three components，

polymer／solvent／non−solvent， as described by the ternary phase diagram shown in

Figure 2．4．

(26)

CH41）TER 2

TheOT y of〃le〃zbraneプわr〃lation

ln the metastable regions A or C betWeen the binodal and spinodal curves， the

polymer solution separates into a polymer−rich phase and a polymer−lean phase． In

these regions， nucleation and groWth occur over time． h region A， where the volume of

the polymer−rich phase is greater than that of the polymer−lean phase， nucleation and

growth ofthe polymer−lean phase may initiate the phase separation process． In region C，

where the volume of the polymer−lean phase is greater than that ofthe polymer−rich

phase， nucleation ofpolymer−rich phase may occur． If the polymer composition passes

through the critical point where the binodal and spinodal curves meet， a bicontinuous

structure fbrms［1］． Finally， when the concentration of non−solvent exceeds a certai l

threshold， complete solidification occurs and the structure is fixed， with the polymer−

lean phase washing out to leave pores． As shown in Figure 1，the critical point is

generally located at a very low polymer concentration［1］． Therefore， actual membrane

fbrmation may occur in regions A or B．

in practical membrane forming methods， an additive is generally used． If the

additive is used in the polymer solution， the phase diagram can be represented by a

tetrahedron． Due to the complexity ofthree−dimensional repre sentation it is usual to

analyze such a quatemary system in a pseudo−temary diagram［15］， where the additive

is considered together with the polymer as one single component． Boom et al．［16］

explained the membrane formation mechanism in a quaternary system with polymeric

additive by introducing two time scales． The shorter time scale is valid fbr the exchange

(27)

CIL4PTER 2

z碗θoワ（珈7e励7α刀θカ7〃lat加

polymer

solvent

■：dope solution

binodal spinodal

non■solvent

●：critical point

Figure 2．4 T量le ternary p血ase diagram．

of solvent and non−solvent． On this time scale， the two polymers effectively behave as

one component and the system −can be treated as pseudo−temary． The binodal curve of

the pseudo−ternary system is denominated by the virtual binodal． Transport of low

molecular weight components through the polymeric network is possible；transport of

the polymers with respect to each other is not possible． The longer time scale is the

time scale at which the two polymers can move relative to each other：The polymeric

additive moves into the polymer−lean phase． When the poIymeric additive is able to

diffUse into the polymer−lean phase， the virtual binodal shifts toward real binodal， and

the thermodynamic situation changes dramatically． We believe thi s insight into the

(28)

CH41）TER 2

蹄εo型q加θ〃ibranefor〃7αtion

mechanism provides an accurate description of phase separation behavior in membrane

f（）rmation．

Kim and Lee［17］investigated the thermodynamic and kinetic effects of Mw of

PEG additive and additive loading in a PSF／NMP∠PEG dope system on the structure

fbrmation ofPSF flat sheet membrane． They studied precipitation rate by observing

turbidity formation after immers血g the transp arent film in a non−solvent bath in order to

discriminate between the tWo different precipitation types of instantaneous demixing

and delayed demixing de scribed by Mulder［1］and by Reuvers and Smolders［18】．

According to the results of Kim and Lee， both an increase in PEG Mw with a given

loading and an increase in PEG loading with a given Mw result in a diminished

precipitation rate， although the precipitate type of illstantaneous demixing remained

unchanged． They attributed these results to dope viscosity．

hl conside血g an actual fabrication method ofhollow−fiber MF membrane using

polymeric additive， Kim and Lee s precipitation rate can be considered to govern the

amount of time available for the nucleation and groWth of masses ofpolymer−lean phase

within a matrix ofpolymer−rich phase after phase separation was induced． In this case，

the precipitation rate is analogous to the rate at which the polymer−rich phase loses

liquidity as the concentration of non−solvent exceeds a certain threshold． Coagulation

composition is de丘ned as that in which the concentration ofnon−solvent is high enough

to cause coagulation of the polymer−rich phase． Simply put， a lower precipitation rate

provides more time befbre coagulation composition is reached， enabling nucleation and

groWth of the polymer−lean phase to progress fUrther， so that lager pores can fbrm・

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CHAPTER 2

TheOTJ／of〃2e〃ib7aneプbr〃lalion

Next I consider how to promote nucleation and growth in a practical PSF

hollow−fiber spinning process． In industrial spinning processes， water generally is used

as coagulation bath． Water is extremely non−solvent fbr P SE When the composition of

nascent P SF hollow fiber discharged from spinneret enters the bath， coagulation

composition is reached in an instant and the structure is fixed．

Phase separation is therefbre induced in the Aq and ifAG conditions are such

that coagulation composition is avoided， and if the nascent hollow fiber has a long

residence time in the AG nucleation and groWth of the polymer−lean phase will progress

fUrther before entry into the coagulation bath and larger pores will form． With sufficient

residence time in the Aq the diffUsion rate ofnon−solvent becomes slower when dope

viscosity is higher， as Kim and Lee point out， which increases the time to reach

coagulation composition． Thus nucleation and growth ofthe polymer−lean phase is

promoted， and lager pores are fbrmed．

In this stud）i， therefore the time in which nucleation and groWth of the polymer

lean phase is allowed befbre solidification must be important， and dope viscosity， bore

liquid concentration， and AG conditions were fbcused on in order to control nucleation

and growth time． Original control methods of phase separation by NIPS are

schematically represented in Figure 2．5．

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CE4P7ER 2

Theory ofme〃ibraneformation

ControI of outer layer by air gap distance and humidi量y

Bore

liquid

Polymer solution

Dope Broad contro■of pore size by polymer solution viscosity

Hollow

fiber

Low

Vlscosity

⇔

Hlgh Vlscosity

Control of inner Iayer by solvent concentration of bore−liquid

鱗㈱灘纏灘鰻難鑑撒灘鱒該灘騰

Bore

Iiquid

Polymer solution

Figure 2．5 0riginal contro1 method of phase separation by NI］？S．

(31)

CHAP皿iR 2

Theory of〃2e〃7braneプb7〃lation

2．3 References

［1］MW．V Mulder， Basic Principle ofMembrane Technology， Klu甲er Academic

Publishers，1996．

［2］S．Jain， J． B ellare， A Review of TIP S and DIP S Technique s for Membrane

Manufacture， Indian Chem．， Engr．，46（2004）155．

［3］R．E Kesting， Synthetic Polymeric Membranes， John Wiley＆Sons，1985， pp．238−

251．

［4］1．Pimau and W．J． Koros， Structures and gas separation properties of asymmetric

polysulfone membranes made by dry， wet， and dry／wet phase inversion， J． Appl． polym

Sci．43（1991）1491．

［5］H．Caquineau， P． Menut and C． Dupuy， lhfluence ofthe Relative Humidity on Film

Formation by Vapor Induced Phase Separation， Polym． Eng． Sci．，43（2003）798．

［6］D．R． Lloyd， J．W． B arlow， Microporous Membrane Formation via Thermally−induced

Phase S eparation， AIChE． Symp． S er．，84（1988）28．．

［7］B．J． Cha， K． Char， J．−J． Kim， S．S． Kim and C．K． Kim， The effects of diluents

molecular weight on the structure of thermally−induced phase separation membrane． J．

Membrane． Sci．，108（1995）219．

［8］H．Matsuyama， H． OkafUji， T． Maki， M． Teremoto and N． Kubota， Preparation of

polyethylene hollow fiber membrane via thermally induced phase separation， J．

Membrane． S ci．223（2003）119．

(32)

CHAPTER 2

7乃θ01四加e〃ibrane／b7〃lation

［9］B．J． Cha and工M， Yang， Preparation ofpoly（vinylidene fluoride）hollow fiber

membranes for microfiltration using modified TIP S process， J． Membrane． Sci．

291（2007）191．

［10］1．Cabasso， E． Klein and J． K． Smith， Re search and development ofNS−1 and

related polysulfone hollow fibers for reverse osmosis desalination of seawater， U． S．

NTIS， PB Rep．（1975）， PB−248666，150．

［11］Z．Borneman， J． A． Van t Hof and C．A． Smolders and H．M． Van Veen， Hollow

fiber gas separation membranes：structure and properties， Special Publication−Royal

Society of Chemistry（1986），62（Membrane． Gas Sep． Enrich．），145．

［12】S．C． Pesek and w． J． Koros，Aqueous Quenched Asymmetric Polysulfone且0110w

Fiber prepared by dry／wet phas e separation， J． Membrane． S ci．88（1994）1．

［13］P．Aptel， F． Ivaldi and J． P． Lafaille， Development of polysulfone hollow fiber， Proc

2nd World Congr． Chem． Eng．，4（1981）191．

［14］J．Fowler and J． Hagewood， Preparation of Polysulfone Dopes for Production of

Hollow Fiber Membranes， Int． Fiber J．，9（1994）28．

［15】P．S．T． Machado， A，C．且abert and C． P． Borge s， Membrane formation mechanism

based on precipitation kinetics and meml）rane morphology：f【at and hollow fiber

polysulfone membranes， J． Membrane． S ci．，155（1999）171．

［161 R．M． Boom，1． M， Wienk， Th． van den Boomgaard and C．A． Smolders，

Microstructure in phase inversion membranes Part 2． The role of a polymeric additive． J．

Membrane． S ci．，73（1992）277．

(33)

CH∠P1：ER 2

Theo7 y of〃le〃かαη⑳ア〃lat加

［17］J−H．Kim and K−H． Lee， Effect of PEG additive on membrane formation by phase

inversion， J． Membrane． S ci．138（1998）153．

［18］A．J． Reuvers and C．A． Smolders， Formation of membranes by means of immersion

precipitation． Part H． The mechanism of fbrmation of membranes prepared丘om the

system cellulose aceteate−aceteon−water， J． Membrane． S ci．34（1987）67．

(34)

CHAPTER 3

0ptimal dope viscosity for nucleation and growth

In this chapter， fabrication of polysulfone hollow−fiber membranes through non−

solvent induced phase separation（NIP S）by dry／wet spinning was studied． For all sample

preparations， I used N−methylpyrrolidone（NMP）as solvent， polyethylene glycol（PEG）as

additive， aqueous NMP solution as bore liquid， and water as coagulation bath． Particular

f（）cus was placed on the influence of PEG molecular weight（Mw）on membrane structure．

Characterization of the obtained membranes was perfbrmed by measuring pure water

permeate flux， tensile strength， and tensile elongation at break， and by analyzing scaming

electron microscope（SEM）images of hollow−fiber cross sections， outer surfaces， and inner

surfaces．

Dope viscosity increased sharply as PEG Mw was raised to 20 kDa or higher， as did

water permeate flux． SEM image analysis revealed that outer sur魚ce pore size increased with

PEG Mw． Mo仙ology of the inner surface transformed markedly as PEG Mw was raised

丘om 6 kDa to 20 kDa．

(35)

CHAPTER 3

0pti〃ia1 dope viscosiり7プ〜）アnucleation and growth

3．11ntroduction

Following the development of a practical cellulose acetate reverse osmosis（RO）

membrane by Loeb and S ouriraj an of UCLA in the early 1960s［1］， membrane separation

technology has come to 1）e used in a many fields of industry． hl the 1990s， membrane

separation teclmology gained attention in process water treatment， potable water production，

wastewater treatment， and other water treatment applications． Hollow−fiber microfiltration

（MF）membranes have become prevalent in the field ofwater treatment． Advantages over

flat−sheet membranes include high mechanical strength and a self−supporting structure to

enable back−flushing and greater membrane surface area in a given space． Advantages over

ultrafiltration（UF）include higher water permeate flux with a given membrane surface area．

Hollow−fiber membranes are generally produced by dry／wet spinning， or by melt

spinning， which utilizes phase separation characteristics ofpolymer solutions． Phase

separation methods have been classified according to the mechanism of phase separation［2，

3］．Three broad categories are evaporation induced phase separation（EPS）， non−solvent

induced phase separation（N工P S）， and thermally induced phase separation（TP S）． With dry−

wet sphming， membrane fbrmation is generally through EIP S， N［PS， or a combination of the

two；with melt spiming， it is generally through TIP S．

EPS， a method whereby solvent evaporates in an air gap（AG）until polymer

concentration rises above the dissolution limit， thereby effecting phase separation， is used to

produce RO and gas separation membranes as it is well suited to the fbrmation of a very fine

porous skin on the membrane surface to fUnction as a separation layer［4，5］． N正P S， a method

whereby non−solvent is absorbed in the AG（also called vapor induced phase separation

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CHAPTEI〜3

（ipti〃zal dope vis cos iリノプ〜）アnu（rZeoオionα〃d growth

（VPS）［6］）or polymer solution is immersed in non−solvent to effect phase separation， is

generally used to produce UIF membrane s as it i s well suited to the formation of an anisotropic

membrane structure to engender high water p ermeate flux． TIP S， a method whereby phase

separation is induced by lowering the temperature of a molten polymer solution， has long

been used to produce MF membranes［7−10］． One drawback ofhollow−fiber MF membranes

produced by melt spinning through TIP S is the tendency for formation of uniform membrane

wall morphology， which limits the degree ofpermeate flux which can be obtained．

Fabrication ofhollow−fiber MIF membranes with anisotropic wall morphology by dry−wet

spinning through Nll｝S has therefore become a focus of research．

Dry−wet spinning through Nll｝S is commonly us ed to produce hollow−fiber

membranes ofpolysulfbne（PSF）and polyether sulfbne（PES）． Featu血g excellent chemical

resistance， heat resistance， and mechanical strength in addition to high permeate flux， the se

have long been the subj ect of study for application as separation membranes for RO［11］， gas

separation［12，13］， UF［14］， and dialysis［15］． In contrast， despite an extensive search of

published literature， we were unable to find reports related to hollow−fiber MF membrane s by

dry−wet spinning．

There have been a variety of reports related to control of morPhology and performance

of hollow−fiber membranes of PSF and PES by dry−wet spi皿ing． In the fbrmation ofholl6w−

fiber membrane s， there are a greater number offactors in complex interaction than there are in

the fbrmation of flat−sheet membrane s． These include spinneret size and shape， flow rate of

polymer solution and bore Iiquid， distance and hu皿idity ofthe AG and take−up speed・

(37)

CHAPTER 3

（）pti〃ial dope viscosiり7プb7 nzacleation and growth

As with flat sheet manufacturing， though， dope composition is generally extremely

important in hollow−fiber spinning． Due to the inherent chemical resistance of PSF and PES，

the choice of solvent is for practical purposes limited to dimethylacetamide（DMAc）or N−

methylpyrrolidone（NMP）． Experiments to control P SF and PE S membrane mony）hology and

perfbrmance through modification of dope composition have therefbre come to fbcus on

additive species and additive loading．

Many researchers have therefore investigated the effect of additive species and

additive loading on PSF or PES hollow−fiber membranes． From the thermodynamic point of

view， additive s work to b血ig the dope composition toward the binodal state． From the kinetic

point ofview， additives act to modulate the interdiffUsion rate between solvent and non−

solvent by altering dope viscosity． It has long been known that a more porous membrane

structure is obtainable with a thermodynamically less stable dope， i．e．， one nearer a binodal

composition［2，16−18］．

To improve gas pemieability of gas separation membranes or to increase water

permeate flux of UF membranes， low molecular weight（Mw）non−solvents such as water or

various alcohols have been used［19−26］． In each case this is because the additive brings the

dope composition nearer to the point ofphase separation， thereby modulating the speed of

demixture ofthe dope composition in the process of coagulation［2］， generally influencing the

morphology ofthe skin layer．

On the other hand， to obtain a nearly sponge−like structure in membrane morphology

by suppressing macrovoid formation， to promote pore interconnectivity， and to increase the

porosity of a flat−sheet membrane s top surface and support layers， high Mw additives such as

(38)

CE4P7ER 3

0ρtimα1 dope viscosityfor nucleation and growth

polyvinylpyrrolidone（PVP）and polyethylene glycol（PEG）have long been used， In addition

to bringing the dope composition nearer to the point ofphase separation， high Mw additives

modulate the interdifUsion of solvent and non−solvent by raising dope viscosity， which

influences not only surface morphology but also the broad macrostructure ofthe membrane，

Cabasso et al． report that it is possible to obtain MF−class pores on the outer surface of

aPSF hollow−fiber membrane when using PVP as additive［27］， and B oom et al， report that it

is possible to eliminate macrovoids through the addition of PVP to the dope［28］． Tbrre stiana−

Sanchez et aL studied PVP， PEG and water as additive， reporting that increas ing the loading

of any ofthem resulted in greater water permeate flux in PE S hollow−fiber UF membranes

［29］．

Liu et a1． reported that it is possible to suppress macrovoids and to improve water

permeability of a PES hollow−fiber membrane by raising dope viscosity using PEG with Mw

of 400 Da as additive and bringing the dope composition nearer to the binodal state by adding

non−solvent water［30］． They also claimed that it is possil）le to open MF class pores on the

outer surface by setting the air gap distance to over 10 cm．

There have also been皿any studies on the effect of PVP or PEG additive Mw on the

morphology and performance of PSF and PES membrane s． Kim and Lee［18］studied the

effect ofboth Mw and additive loading of PEG on P SF flat sheet membrane formation， and

discussed their results丘om both thermodynamic and khletic points of view， They indicated

that there is a trend toward formation of larger p ores when using additive PEG with higher

Mw and higher loading．

(39)

CHAPTER 3

0ptimal dope viscos ityノわr nzacleation and growth

Xu et al． report increasing water permeate flux in PES hollow−fiber UF membranes

using the two additives PEG and PVP simultaneously， particularly when PEG Mw was raised

to l O，000［31］． Chakarbarty et al．［32］studied the effect of Mw of PEG additive on PE S flat

sheet membrane morphology and performance， and concIuded that higher PEG Mw raises

water permeability and lowers s olute retention．

In addition to dope composition， many other spinning parameters have been studied to

ef〔 ect control ofmorphology and performance of PSF and PES hollow−fiber membranes． AG

conditions in particular have a strong influence on membrane morphology， especially outer−

surface structure［33，34］． Tsai et al． report eliminating voids in P SF hollow−fiber UF

membranes by a（恥sting AG distance and humidity［35］． S imilarly， Chung et al． report that it

is possible to eliminate voids in PES hollow−fiber UF membranes by increasing AG distance

［36］．

Liu et al． report outer−surface pore size increasing with greater AG distance in PE S

hollow−fiber UF membranes using PEG and water as dope additive［37］． Control of inner−

surface morphology is generally effected by a（巧usting the mixture ratio of bore liquids

comprising water， a strong non−solvent of PSF and PES， and a solvent such as NMP［26，34］．

Yan et al． report altering NMP and PEG concentrations in bore liquids comprising water and

NMP or water， NMP， and PEG as a powerfUl means of a（ljusting imer・・surface pore size［38］．

While some ofthese studies demonstrate the fbrmation ofMF−level pores血PSF or

PE S hollow−fiber membranes， on the outer surface or within the membrane wall by altering

dope composition［27，29］， on the outer surface by controlling AG distance and humidity［37］，