Preparation and structure of synthetic membranes

Desalination, 46 (1983) 303-312

303

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PREPARATION

A.

AND

STRUCTURE

OF

SYNTHETIC

MEMBRANES

Walch

Kalle

Niederlassung

O-6200

der

Wiesbaden

1

Hoechst

AG,

P.O.B.

3540,

(Germany)

ABSTRACT Processing

via

differing

areas

phase.

The

tion

of

fied

into

synthetic of

mechanism

the

structure

of

various

a few

formation

are

has

in

the

separation

membrane

basic

on

membranes

some

accepted or

and

in

particular however,

largely

the

liquid

the may

forma-

be

classi-

are

exemplified

adequate

for

microfiltration,

gas

future

in

in

Details

hyperfiltrat,ion,

set

been gaseous

structures,

principles. of

ultrafiltration, Accents

membranes

application

in

the

separation.

research

and

development

activi-

ties.

INTRODUCTION Membranes the and to

continue

fields

of

physiology. the

fical

the

kidney)

and

application

means cially

nology.

water

the

milk on

The

metal

improved

and

finishing production

electroplating OOll-9164/83/$03.00

and

comes

of

In the

to

Membrane

separation and

and

for

impetus

for

and

similar

processes

recovery

0 1983 Elsevier Science Publishers B.V.

of

espewere

separation

encouraged

the

the by

processes

membrane

the

for

where

purified

industry,

exploit and

in (arti-

thin-film

segments, of

applied

therapeutic

biotechnolog

installations

efficiencies

being

Further

food

processing

industries

effluents

from

in

medicine,

problems

(multilayer,

concentrated

advantage

such

are

for

analysis).

are

research

biology,

separation

purposes

whey

take

intensive

membranes

medicine

clinical

membranes

to

success

problems.

in

membranes.

early

of

chemistry,

engineering

products

synthetic

in

modified

of

object

synthetic

process

for

and/or

of

the

polymer

diagnostic

assay

substrates

and

of

industry,

calorimetric

be

and

Moreover,

solution

pharmaceutical

to

physical

the

for

textile

technology

solution

rinse

tech-

for

waste

recycling waters

of con-

taining

water-soluble

lfsh.ed.

Additional

offered

to

latices,

the

by

from

the

for

via

the

preparation

and

tion

which

cess

of

The from

energy membrane

The

and

variety

1.

a.

are

with

of

of

combustion desalinato

practical

the

development for

the

pro-

experience of

example,

ecolocon-

1 summarizes

typi-

volume

future.

membrane

.differing

of

the

years.

market

following

selection

0,

and

for

widely

for

seawater

Table

in

the

industry,

are an

processes

calls

properties the

and

principal

appropriate

membrane

process:

separation

based the

and

The

the

essentially

Separation sizes

b.

past

applications

separation

mechanisms

There

the

of

guiding

a particular

The

in

of

Additional

twenty

estimated

the

and

include

past

which

of

separation)

contributed

to

were

polymer

concentration

of and

materials.

their

characteristics.

considerations

led

estab-

means

electronics

the

processes

and

membranes

the

research

raw

by

membranes

enrichment

has

protect

processes

by

(gas

decisively

applied

membrane

and

brackish

during

installations

synthetic

performance

for

of

increasing wide

and

technology

NaOH

of

(pervaporation).

for

least

fully

improvement

and

synthetic

gas,

not

now

cleaning

phase

phase

water

initially

Cl,

gas

of

natural but

beneficial

steadily

for

has

membrane

existing

serve cal

of last

combination

gically

vapor

ultrapure

deacidification process,

the

the

are

process the

process

from

application

of

by of

electrolysis

oils

for

industry

electrowinning

components

solution

fields

lubricating

possibilities

chemical

chloride-aTkali specified

and

three

on

permeants

mechanisms

large

Separation

based

on

to

separated

be

differences

(sieve

substances

of

differences

separation:

in

the

molecular

effect).

in

the

charges

(electrochemical

of

the

effect).

305 c.

Separation

depending

materials

in

chemical

The

to

dispersed

3.

Mechanical

4.

Compatibility

of

The

properties

ly,

1.

dissolved

substances).

chemical

of

the

of

of

by

dry

stability

membrane

deposits

be

the

of

physico-

separated

(e.g.

the

rejection

the

of

membrane.

materials

being

processed

predisposition

membrane

subgaseous,

fouling

to

and

the

scaling)).

MEMBRANES

by

a

and

towards

and

of

with

(e.g.

synthetic

characterized

rial,

solubilities

of

to

membrane

flux

adsorption,

SYNTHETIC

Formation

the

or

formation

be

of (volume

of

OF

in

because

substances

separated

(biocompatibility,

FORMATION

phase

the

be

and

formation

differences

H-bonding).

permeation

stance

the

membrane

features

polarity,

2.

the

on

the

polymer

following

film

spinning

polycondensation,

membranes

by

by

somewhat

three

by

of

in-situ

posttreatment

arbitrari-

mechanisms:

extrusion

process,

or

can,

the

polymer

mate-

polymerization

of

the

and

corresponding

films.

2.

Formation

3.

Production (glass

In

the

of

of or

excluded

area

which

gel

surface

should

are

rally

this

shown

by

tool thetic

for

leads

examining

membranes.

sol-gel

modified

porous

be

more

electron the

inorganic

cover

treated

or

for

ion-exchange they

preferably to

transition.

capillaries),

today

not

a

membranes

instance.

membranes

such

less

gel

formation

asymmetric

micrographs

microstructure

and

and

for

gas

(method

membranes

which

provide

morphology

1)

increasing

Membranes

hyperfiltration by

(method

a wide,

superficially.

ultrafiltration,

prepared

scanning

by

contribution

because

microfiltration, ration

layer

ZrO,/carbon

following

are

a

sepa-

2). as

a of

Gene-

will

reliable syn-

be

306 MEMBRANE

FORMATION

For were

many

years,

commercially

nitrate,

hols

and

water.

humidity,

since

phase

asymmetry have

of

their

a

polyamide,

for

Porous

2).

diameter porosity.

active

materials

formation

pore

been a

1 yields membranes

by

of

heterogeneous

fillers;

@Accurel,

pores

an

as-cast

Fig.

4).

polymer

stretching

the

by

very

flow

exact

velocity

addition

of

surface also

"dead

end",

polysulfone (Fig. polymer

dense

using

membranes by

(e.g.

addition it

film

by

af.ter

film

(@Celgard,

this

is

the

open

3) solutions

produced

Moreover,

pore

recently

binary

systems

the

increasing

Most

element

be

with

respectively),

from

homogeneous can

by

solution.

composite

prepared

Porous

also,

chemi-

diameters

since

air

sup-

membranes

pore

achieved and

reas

to

composite

minimal

were

and,

from

chemical utilized

increases

humidity,

films

its

In addition of

with

films

prefiltration of

chloride, fluoride)

self-contradictory

casting

(or

of

layer

generally

the

method

biaxially

is

period

pores.

or

pm

extensively

initially

membranes.

porosity

adequate

to

had

in

with

micro-

0,4

polyvinyl

because

supporting

surface

asymmetric

as

use

weak

disintegrate

about

polymer

were

composite

evaporation

Porous

to

sol-

skin

electron

polyvinylidene

membranes

temperature,

microfilters

ing

(e.g.

additional

requirement

air

layer

only

Any

will

having

the

commercialized.

a gel-membrane

the

The

membrane

polymers

one

the

high

during

extremely

observed

exhibit

scanning

alco-

tempera-

of

membrane.

by

application PS

However, of

is

for

This

of

control

other

membrane

combine

of

carefully

surface

shown

for

esters,

evaporation

the

membrane

as

nitrate

(PS)

matrices

(Fig.

the

been

compatibility,

must

on

from

recently

sistivity.

cal

by

across

of

conditions be

cellulose

blends,

a mixture

must

esters

from

corresponding in

initiated

opened

cellulose

1).x

Polysulfone

porting

the

polytetrafluoroethylene,

just

produced

characteristically

cellulose

(Fig.

on

be

graduated

porosity

are

based can

circulation is

formed

Microfilters

studied

and

air

microfilters

pores

diameter

have

polymers

Definite

been

the

graphs

or

the

and

in

They

acetate

separation

Such

vents.

when

microfilters

dissolving

ture,

may

MICROFILTRATION

only

available.

cellulose

instance,

FOR

accord-

without method

of

only

extractable

possible

treatment,

to

create

i.e.

mono-

@'Goretex,

'Figures 1 - 20 are referred to in the text but not reproduced be shown as slides during the presentation of the Paper.

here. They will

307 @'Poreflon,

5)

Fig.

nuclear

fragments

nuclear

tracks

MEMBRANE The

of

They

are

for

means

of

Today

these

increasing

higher

by

In

properties

the

cellulose

xanthogenate

have

in

medical

In

may 8).

(with

technical

areas

as

no

guaranteed

from

hemo-

(mechanical be

obtained

However,

and

need

in for

by case

of

dif-

sufficient

(low

sufficient

an

applications

rates

(Fig.

got

membranes

specific

transport

be

tetra-

regeneration

membranes

transport

to

and

preferably

cut-off,

dialysis

as

asymmetric

membranes

have

gel-like

particular

too.

structures

MW

more

7).

of

case

convective

distinct

etching

coagulation

in

which

ultrafiltration

to

as

heteropous

convective

asymmetric

mechanical

neutron-induced

dissolved

symmetric

competitors

favored

permeability)

with

the

(Fig.

competition,

are

bar,

by

acid

polymers.

permeability)

or

followed

of

diafiltration

10

are

cellulose

homogeneous

a certain

"technical"

subsequently

6).

('s'Cuprophan)

more

synthetic

fusive

and

Fig.

from

represented

heteroporous

ions,

sulfuric

number

is

film

ultrafilters

instance,

dilute

applications

the

ULTRAFILTRATION

cast

complex

(@Nadir),

various

heavy

FOR

precursors

amminecopper

there

or

bombarding

("Nuclepore,

FORMATlON

membranes.

by

or

compaction

permeate

up

drainage

etc.). For is

preparation

cast

into

period sed

(period

into

water) with

a

a

other

of

is

not

take

coagulation

sible

place,

formation to

of

numerous

a continuous critical

the

for

casting

but

immer-

(usually is

miscible

Diverging

phase

the

phase)

liquid

solution.

solution

evaporation

gas

a

polymer

the

bafore

polymer

short

another

the

structural

influences

need

of

or

a

from

separation

as

- cast

does

film

enters

extremely

sen-

bath.

casting

keep

the

the

after

containing

microfilters

significantly

polymer

to

air

bath

in of

or

with

a non-solvent

components

the

ultrafilters

directly

contact

preparations

The

common

and

"precipitation"

which

common

of

film

and

polymer (large

parameters

in

features particular

coagulation. scale) constant

is

during This

membrane (Fig.

the

results

production 9).

course in in

of

the order

308 The development more

hydrophilic

of asymmetric polymers

and progressively

bic polymers

for applications

aggressive

plants

must

agents

cellulose

of sufficient

in solutions bility

bridging

Today, offers

polysulfone

oxidizing

However,

requires

drophobic

again

ted by positive of which

to a blend

ethalpy

resemble

10).

used

pH range

since

it

and stability

or dispersed

hydrophilicity

were made

The most

followed

with

components

sweet

or sour

of "per se" hy-

to blend

ones,

of polysulfone

appropriate

however,

often

successful

limi-

examples

or polyvinylidene

by subsequent

hydrolysis

alcohol).

of membrane

for gas separation

asymmetric

esters

ciently

still

membranes

represent

working

resistivity ment

(Fig.

formation they

for hyperfiltration

formally

have to be in-

in the following.

Again mixed

of its varia-

FOR HYPERFILTRATION/GASSEPARATION

principles

those

prote-

and even to dense

electropaints,

of mixing.

of poly(viny1

polymer

with

12).

hydrophilic

acetate),

FORMATION

’ Because cluded

with

are the sulfonation

fluoride/poly(vinyl

MEMBRANE

Attempts

polymers

wide

macromolecular

a balanced

membranes.

hydrophobic

and because porous

is preferentially

or cataphoretic

and

membrane

of these 'membranes to processes

diversified

(e.g. anaphoretic

where

as a membrane

and hyperfiltration

11,

or celluhydropho-

and sanitized).

industry)

(Fig.

the

temperatures

(low adsorption)

resistivity,

agents

higher

remained

to finely

still

application

increasingly whey)

ultra-

high temperature

towards

employing

acetate

porous

complexes

in the food industry sterilized

(pharmaceutical

with

led to the use of more

compatibility

from coarsely

membranes

(e.g.

be periodically

Nevertheless because

began

as polyelectrolyte

lose acetate more

ultrafilters

the initially

membrane

against

based

species.

common

have protected

on cellulose applied

but still

Ease and costs

concentrations their

acetate

effi-

of manufacture,

of chlorine

application

or

within

pretreat-

certain

areas. However, because

CA membranes

of their

degradation.

are precluded

susceptability

The Du Pont

Company

from many

to hydrolysis

applications

and biological

and Monsanto-Chemstrand

309 first

prepared

amides. and of

The

polar

asymmetric

structural

heterocyclic

asymmetric are

rature

solution

same

prepared

that

are

by

the for

periods

(e.g.

asymmetric

for

the

in

exhibit

for

Figure

The

CA

is

Most

points

of

asymmetric

fundamentally

since

the

those

for

the

evapoused

dimethyl

than,

poly-

low-tempe-

However,

longer

polyamides

preparation

preparation

membranes. are

boiling

the 13.

N-methylpyrrolidone,

higher

polyof

(preferably

polyamides

solvents

aromatic units

used

compiled

corresponding

on

basic

polycondensation

dimethylacetamide,

oxide)

the

polycondensation).

from

as

ration

based

of

polycondensates

membranes

amides

membranes

membranes formulas

sulf-

example,

acetone-

formamide. Polyacrylonitrile the

preparation

layer in

in PAN

copolymers of

membranes

desalination

and

Recently,

however,

obtaining

a

the

porous As

always step by

considered

or

the

too

dense

ultrafiltration Industries

layer

by

for

active for

use

purposes. succeeded

in

plasma

treatment

of

every the

can

cast

an

thickness

permeability

controlled.

to

polymer

prepare

appropiate

of

Hence,

membranes

of

the it

the

was

a

into

active

membrane

similar

supporting

be

logical

to

layer

cannot next

asymmetric

matrix

with

a thin

either

refractory

ones

homogene-

film. The

supporting

solvents

in

protected layer.

the

the

to

of

is

gel-layer

by

the

can

pass

Thus

by

by

usually

protects

the If

porous

extracted the

into

solution.

active

or

is

membrane.

spraying. pores

formed

by

porous

support

will

of

Then

the of

support-

the

porous

a water-soluble

polymer the

be

Penetration

surface

support,

must

generally

the

the

the

intermediate

the

during layer

or

supporting

coating

to

soluble PS

dipping

solution

the

thin-film

stability,

gel-layer

casting

the

a water

fine-porous

prepared

progressively

membrane.

for

chemical

gel-layer

be

solvents

prevented

a thin

the

must

solution

casting ist

This of

is

its

fabricate

with

solvents

site

these

thin-film

polymer.

layer

casting

layer

framework

support

the

the

Because

active

ing

framework

against

employed

of

for

Moreover,

optimally

an

not

water

attempt

coating

ous

been However,

porous

Electric surface

mentioned,

membrane.

be

too

suited

Sumitomo

resultant

to

only

either

permselective

already

the

is

also

membranes.

surface.

asymmetric and

have

asymmetric

from

molecules

the of

intermediate

operation finally

of lie

the only

compoon

310 the

porous

can

be

support

linking

the

high

possible

as

active

can

is

the

be

with

brought

by

between

or

water

case

way

layer

active

a

layer by

the

surporous

polymer

organic

and

solution

of

iso-

improvement

supporting be

a

its

diisocyanate,

might

direct

as The

at

significant

and

be

cross-linking

fabrication

an

It

cross-

should

multifunctional with

of

resistance.

covalently

toluylene

this

acids

In

content

During

(e.g.

active

the

amines

stability.

drop.

polyepichlorohydrin)

contact

In

mechanical

hydrodynamic

a dissolved

agent

Finally

functional

formed

into

dichloride).

achieved.

its

its

cross-linking).

soaked

adhesion

poor pressure

layer,

minimize

also

cross-linking

phthaloyl

have

a sudden

amine-modified

(interfacial

subsequently the

to

(e.g.

support

will

by

intermediate

layer

gel-layer face

and

delaminated

in

framework

formed

interfacial

from

is

multi-

polyconden-

sation. addition

In

to

CA,

several

mides,

poly(ether/amides),

zolone

(PBIL)

structural ration the

form

unit

of

gives some

thin-film

which

Improved streams

of

only

volume

installed

of

In for

tration

plants

refined

and

concentrate

this from

waste

Figure

accelerated

using

in

has

purpose

in

the

prepa-

demonstrates and

Figure

16

membranes,

only

aggressive

process

by

and

the

supplanting

hyperfiltration

Often,

of

water

processing,

than

1978-1981

purified

application

brackish

gradually

more

during

of

15

the

than

in whey is

USA

1972-1981.

more

other

example,

the

lists

commercialized.

purposes

permeate

were

final

products

hyperfiltration plants

were

ultrafil-

using

ultrafiltration

ultra-

plants

may

be

(e.g.

to chemi-

industry). or

membranes 17

composite

agents

For

14

employed

membranes

towards

streams

development

composite

and

polya-

polybenzimida-

Figure

Figure

composite

hyperfiltration

cal/pharmaceutical The

membranes.

been

for

fact,

the

films. polymers

resistance

desalination.

filtration.

including and

barrier

asymmetric

cleaning

polymers

typical

typical

have

membranes

seawater market

some

chemical

and

synthetic

of

composite

of

a overview of

suitable

formulas

morphology

other

poly(ether/urea),

evolution

during

the

hyperfiltration

of years

asymmetric,

homogeneous

1953-1979

membranes

as

is an

summarized

example.

and in

311 MEMBRANE

pathway

possible

shown

structure

molecular

we

be

more

defined a

low

packaging to

logy

ted

by

clearly

all

having

by

of

but

urgent

still

contradictory of

In

limits

membrane

sently

became

lead

of

in

the

come

by

tive

in

the

molecular

view

active

Difficulties

but

morphoand

supplemenIn

accents

in

(Figure

aqueous

be

solutions

proved

as

characterized, as

18).

an

old

by

accessibility

facilitating

selective

the

thickness

a few

significant.

polymers

of

hundred

composite

by

approaches

order

to

(ac-

Angstroms

Problems of

in

of

pre-

mechanical

to

increase

improve the

thick-

selectivity

have

been

gas

separation

approach'

an

additional

coating

by

partially to

being

over-

eliminate less

selec-

permeable.

properties,

roughly

structurized formation

also

expe-

layer.

attention

fouling

brane

are

but

the

new

in to

polymer

"renaissance"

of

"Monsanto

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tive)

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chlorine

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DEVELOPMENT

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R & 0 originate case

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polyethylene,

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IN RESEARCH

figure

principles

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last

Furthermore

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ACCENTS

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riments.

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- FUTURE

FORMATION

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focused

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active

(Figure

be

19).

layers

on

on

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membrane

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promote

dependance topography, secondary

of where mem-

312 The availibility

of improved

way

aided

for computer

use of heavy Rising

ions,

separation

problems

understanding

of biological membrane

"transport-active"

activities Finally, cations

distinct

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membranes

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research

by the government.

take a look beyond

structures

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even to non-separation

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processes may be

activity

or

20).

The author is very much obliged to Priv.-Doz. Dr. W. Pusch, Max-Plan&-Inetitut fiirBiophysik (D-6000 Frankfurt/Main), for stimulating discussions and significant contributions to the paper.