Note: Descriptions are shown in the official language in which they were submitted.
2 ~ 3 ~
1 --
COMPOSITIONS USEFUL FOR PREPARING CELLULOSE ESTER
MEMBRANES FOR LIQ~ID SEPARATIONS
This invention relates to a novel composltion
useful for preparing a semi-permeable cellulose ester
membrane useful for liquid separations. This invention
further relates to a process utilizing said composition
to prepare a semi-permeable cellulose ester membrane
useful for liquid separations. This invention still
further relates to a semi-permeable cellulose ester
membrane containing a certain organic compound or
compounds useful for liquid separations.
Cellulose ester membranes have long been used
in membrane liquid separation processes such as
microfiltration9 ultrafiltration, dialysis, and reverse
osmosis. Typically, cellulose ester membranes are
prepared by extruding, molding, or casting the membranes
from blends containing a polymer, a solvent, and an
optional non-solvent. Solvents are compounds in which
the polymer substantially dissolves at the membrane
fabrication temperature. Non-solvents are compounds in
which the polymer is substantially insoluble at the
membrane fabrication temperature. Solvents which have
been used to prepare cellulose ester membranes include
sul~olane, dimethylformamide, N-methylpyrrolidone, and
35,971-F _l_
: :
.
~3~7l~
acetone. Non-solvents which have been used for
cellulose esters include rnethanol, propanol, water, and
maleic acid. Polyethylene glycol has been used as a
non-solvent for cellulose triacetate. Even residual
amounts of such solvents and non-solvents generally
cannot be left in the membranes because they cause
unacceptable contamination of the fluids being treated.
Avoiding such contamination is particularly important in
the treatment of blood by dialysis or the desalination
of drinking water by reverse osmosis. The solvents and
non-solvents are kherefore typically completely removed
during membrane fabrication by extensive leaching. Once
the solvents and non-solvents are removed from the
membranes, they present problems of disposal or
extensive repurification before reuse.
Following formation of the membranes and
removal of the solvents and non-solvents, it is often
desirable to dry the water-wet cellulose ester membranes
prior to fabrication of devices, storage, or shipment.
However, cellulose ester membranes generally should not
be directly dried without pretreatment because direct
drying may cause adverse structural changes such as
crazing or pore collapse which adversely affect membrane
performance. Therefore, the pore structure of the
membranes preferably is protected during the drying
process. This is generally accomplished by
incorporating a non-volatile, water-soluble compound
such as glycerol into the pore structure of the
membranes prior to drying. The non-volatile, water-
soluble material also preferably serves as a surfactant
or wetting agent for the later rewetting of the
membranes. ~uch a process is commonly called
"replasticization."
35,971-F -2-
2~3.~7~
--3--
Such membrane preparation processes are
complex, time consuming, and expensive because they
require complete removal of the extruding, molding, or
casting solvents and non-solvents, Eollowed by
replastici~ation with a non-volatile compound i~ the
membranes are to be dried. The extrusion, molding, or
casting solvents must be able to dissolve the cellulose
ester to form an extrudable, moldable, or castable
homogeneous blend. However, iE such a compound is also
used for the replasticization agent, membrane integrity
could be adversely affected. Therefore, to date, the
solvents used for extrusion, molding, or casting
necessarily di~fer from the replasticization agents,
resulting in a multi-step membrane preparation process.
Such complex, multi-step membrane fabrication processes
may result in significant variations in membrane
performance.
What is needed are extrusion, molding, or
casting compositions for cellulose esters in which the
solvents and non-solvents are not harmful or deleterious
or which can be converted into substances which are not
harmful or deleterious in the end use of the membranes.
Accordingly, such solvents and non-solvents would not
have to be leached from the membranes prior to use.
Furthermore, solvents and non-solvents which function
both as plasticizers during extrusion, molding, or
casting and replasticization agents during drying are
highly desired.
This invention is a novel composition useful
for preparing a semi-permeable cellulose ester membrane
for liquid separations comprising a mixture of.
35,971-F -3-
'~ :
~3~fi ~
~ .
A. at least one cellulose ester~
B. at least one solvent of gl.ycerol monoacetate,
glycerol diacetate, glycerol triacetate, or
mixtures thereof, to .solubilize the cellulose
ester at the membrane formation temperature,
and
wherein the cellulose ester and solvent are present in a
ratio to form said semi-permeable membrane.
Optionally, a non-solvent glycerol may be
included in said mixture.
In another aspect, this invention is a process
for preparing a semi-permeable cel].ulose ester membrane
from said composition, comprising:
A. forming a mixture of:
i. at least one cellulose ester,
ii. at least one solYent of glycerol
monoacetate, glycerol diacetate, glycerol
triacetate, or mixtures thereof, to
solubilize the cellulose ester at the
membrane formation temperature;
B. heating the mixture to a temperature at which
the mixture becomes a homogeneous fluid;
C. extruding, molding, or casting the homogeneous
fluid into a semi-permeable membrane; and
D. passing the membrane through one or more quench
zones wherein the membrane gels and solidifies,
35,971-F -4-
.
2~3~P~
--5--
wherein the semi~permeable membrane so formed is useful
for a membrane liquid separation process.
Another aspect of the invention is a semi-
permeable cellulose ester membrane comprising a
cellulose ester membrane useful for liquid separations
containing at least one of glycerol monoacetate,
glycerol diacetate, glycerol triacetate, or mixtures
thereof, and optionally glycerol.
Because the solvents ~glycerol monoacetate,
glycerol diacetate, glycerol triacetate, and mixtures
thereof) and non-solvent (glycerol) from which the
membrane is prepared may be converted into subskances
the presence of which are compatible and/or accepted in
the end use of the membrane, the membrane does not
require extensive leaching prior to use. Furthermore,
in some cases the solvents and non-solvent may serve as
both the plasticizer and replasticization agent, thus
eliminating the need for a separate replasticization
step.
The Figure illustrates the miscibility of
various compositions of cellulose triacetate, a
monoacetate glycerol mixture (Hallco~ C-918 Monoacetin,
~trademark of the C. P. Hall Company), and glycerol at
ambient temperature and pressure.
This invention consists of compositions useful
for forming cellulose ester membranes comprising at
least one cellulose ester, at least one solvent of
glycerol monoacetate, glycerol diacetate, glycerol
triacet~te, or mixtures thereof. The solvent is present
in an amount sufficient to solubilize the cellulose
ester at the membrane formation temperature. An
35,971-F -5-
7 ~
--6~
optional non-solvent glycerol may be present in the
mixture. The cellulose ester, solvent, and optional
non-solvent are present in a ratio useful to form a
semi-permeable membrane suitable for a liquid separation
process.
Cellulose esters and their synthesis are well
known in the art. See "Cellulose Esters, Organic,"
Enc~clopedia of Polymer Science and Engineeri~~, 2nd
edition, Vol. 3, Wiley Interscience, New York, 1985, pp.
158-226. Preferred cellulose esters useful in this
invention include cellulose acetate, cellulose
propionate, cellulose butyrate, cellulose nitrate,
cellulose methacrylate, cellulose phthalate, and
mixtures thereof. Mixed cellulose esters such as
cellulose acetate propionate, cellulose acetate
butyrate, and cellulose acetate methacrylate are also
within the scope of the invention. The term "mixed
cellulose esters" refers to cellulose esters in which
the polymer backbone contains at least two different
cellulose ester moieties. Mixed cellulose esters are
thus distinct from physical mixtures or blends of two or
more different cellulose esters. Cellulose esters which
are more preferred for use in this invention are the
cellulose acetates, commonly referred to as cellulose
acetate, cellulose diacetate, cellulose triacetate, and
mixtures thereof. The cellulose acetates possess
different acetyl contents depending upon the degree of
3Q substitution. The acetyl content ranges from 11.7
weight percent for a degree of substitution of 0.5 to
44.8 weight percent for a degree of substitution of 3Ø
Cellulose diacetate, with an acetyl content of from
about 32.0 to about 41.0 weight percent, and cellulose
triacetate, with an acetyl content of from about 41.0 to
35,971-F -6-
2~3.1L674
--7--
about 44.8 weight percent, and mixtures thereof, are
especially preferred for use in this invention.
Preferred solvents useful in this invention are
glycerol monoacetate (acetin), glycerol diacetate
(diacetin), glycerol triacetate (triacetin), and
mixtures thereof. More preferred solvents are glycerol
monoacetate, glycerol diacetate, and mixtures thereof.
The solvents useful in this invention may optionally
contain small amounts of other compounds which are not
deleterious to the membrane or unacceptable in its
applications. Preferably the presence of these minor
impurities in the solvent is less than 15 weight
percent, more preferably less than 5 welght percent.
The optional non-solvent preferred in this
invention is glycerol.
The concentrations of the components in the
extrusion, molding, or casting composition may vary.
Miscibility of the composition at the extrusion,
molding, or casting temperature is one ~actor to be
considered in forming the extrusion, molding, or casting
composition. Miscibility of polymer solutions may be
readily determined empirically by methods known in the
art. The amount of glycerol acetate, glycerol
diacetate, or glycerol triacetate solvent used in the
composition is advantageously sufficient to solubilize
the cellulose ester polymer at the extrusion, molding,
or casting temperature. That is, no solvent other than
the glycerol acetates is necessary to solubilize the
cellulose ester. The end use of the membrane is another
factor in determining the appropriate blend composition,
because the preferred pore size and transport rate
35,971-F -7-
,
6 ~ ~
-8-
through the membrane vary, dependent upon the intended
membrane use.
In the case of membranes useful for
ultrafiltration or microfiltration, the concentration of
cellulose ester is preferably at least 10 but less than
80 weight percent, more preferably at least 15 but less
than 60 weight percent. The concentration of the
solvent (glycerol monoacetate/glycerol
diacetate/glycerol triacetate/mixtures thereof) is
0 preferably 20 to 90 weight percent, more preferably 40
to 85 weight percent. The concentration of the optional
non-solvent (glycerol) is preferably 0 to ~0 weight
percent, more preferably 5 to 60 weight percent.
In the case of membranes useful for dialysis,
the concentration of cellulose ester is preferably 10 to
60 weight percent, more preferably 15 to 55 weight
percent. The concentration of the solvent (glycerol
monoacetate/glycerol diacetate~glycerol
triacetate/mixtures thereof) is preferably 40 to 90
weight percent, more preferably 45 to 85 weight percent.
The concentration of the optional non-solvent (~lycerol)
is preferably 0 to 50 weight percent, more preferably 5
25 to 40 weight percent.
In the case of membranes useful for reverse
osmosis, the concentration of cellulose ester is
preferably 25 to 65 weight percent, more preferably 35
30 to 60 weight percent. The concentration of the solvent
(glycerol monoacetate/glycerol diacetate~glycerol
triacetate/mixtures thereof) is preferably 15 to 75
weight percent, more preferably 35 to 65 weight percent.
The concentration of the cptional non-solvent (glycerol)
3 5 , 97 1 -F -8 -
~3~67~
is preferably 0 to 30 weight percent, more preferably 0
to 20 weight percent.
The compositions of this invention may be used
to fabricate membranes useful for membrane liquid
separation processes such as microfiltration~
ultrafiltration, dialysis, and reverse osmosis. Such
membranes may be fabricated by several alternative
process schemes. In Gne preferred process9 the
cellulose ester composition is extruded, molded, or
cast, then air quenched. In another preferred process,
the cellulose ester composition is extruded, molded, or
cast, quenched, leached, and dried. In still another
preferred process, the cellulose ester composition is
extruded, molded, or cast, quenched, leached,
replasticized, and dried. The choice of a membrane
fabrication process is in part determined by the
membrane properties desired.
The membranes are first extruded, molded, or
cast from the cellulose ester compositions hereinbefore
described. In the case of casting, a homogeneous blend
is prepared which possesses a suitable viscosity for
casting at a given temperature. For casting, the
viscosity of the blend is preferably 2 to 25 poise (0.2
to 2.5 Pa-s). The casting b~end may be cast at room
temperature or at elevated temperatures depending upon
the viscosity of the blend. The blend is preferably
cast at a temperature of from about 25 to about 200C.
3 In the embodiment wherein the membrane is useful for
ultrafiltration or microfiltration or dialysis, the
casting blencl preferably contains 8 to 50 weight percent
cellulose ester, more preferably 12 to 40 weight percent
cellulose ester. In the embodiment wherein the membrane
is useful for reverse osmosis, the blend preferably
35,971-F _g_
-
2~3 ~rdJ~
- l o - ~
contains 10 to 60 weight percent cellulose ester, more
preferably 15 to 50 weight percent cellulose ester. The
blend may be cast by pouring the blend onto a smooth
support surface and drawing down the blend to an
appropriate thickness with a suitable tool such as a
doctor blade or casting bar. Alternately, the blend may
be cast in a continuous process by casting the blend
onto endless belts or rotating drums. The casting
surface is such that the finished membrane may
therea~ter be readily separated from the surface. For
example, the membrane may be cast onto a support having
a low surface energy, such as silicone, coated glass, or
metal, or a surface to which the membrane will not
adhere. The blend may also be cast onto the surface of
a liquid with which the polymer is immiscible, such as
water or mercury. Alternately, the blend may be cast
onto a support surface, such as a non-woven web, which
may thereafter be dissolved away from the finished
membrane. The membrane may also be cast onto a
permanent support surface which does not substantially
impede transport through the membrane. The membranes
are then processed in a manner similar to that described
hereinafter for extruded membranes.
In the case of extrusion, the components of the
extrusion composition may be combined prior to extrusion
by mixing in any convenient manner with conventional
mixing equipment 9 as for example in a Hobart mixer.
Alternatively, the extrusion composition may be
homogenized by extruding the mixture through a twin
screw extruder, cooling the extrudate, and grinding or
pelletizing the extrudate to a particle size readily fed
to a single or twin screw extruder. The components of
the extrusion composition may also be combined d~rectly
35,971-F -10-
c~ 7~
in a melt-pot or twin screw extruder and extruded into
membranes in a single step.
The viscosity of the mixture rnust not be so
high that the mixture is too viscous to extrude at
temperatures which do not deleteriously affect the
polymer. On the other hand, the viscosity must not be
so low that the mixture does not maintain its desired
shape upon exiting the extrusion die. The membrane may
retain its desired shape upon extrusion by cooling or by
coagulation. In the embodiment wherein the membrane is
useful for ultrafiltration or microfiltration or
dialysis7 the mixture preferably contains 8 to 80 weight
percent cellulose ester, more preferably 15 to 60 weight
percent cellulose ester. In the embodiment wherein the
membrane is useful for reverse osmosis, the mixture
preferably contains 10 to 80 weight percent cellulose
ester, more preferably 15 to 60 weight percent cellulose
ester.
The mixture is heated to a temperature which
results in a homogeneous fluid possessing a viscosity
suitable for extrusion. The temperature should not be
so high as to cause significant degradation of the
cellulose ester. The temperature should not be so low
as to render the fluid too viscous to extrude. The
extrusion temperature is preferably 20C to 250C, more
preferably 25C to 220C.
The mixture of polymer, solvent, and optional
non-solvent is extruded through a sheet, hollow tube, or
hollow fiber die (spinnerette). Hollow fiber
spinnerettes typically are multi-holed and thus produce
a tow of multiple hollow flbers. The hollow fiber
spinnerettes include a means for supplying fluid to the
35,971-F
.
. ~
: .
-12- ~3~7~
core of the extrudate. The core fluid is used to
prevent the collapsing of the hollow fibers as they exit
the spinnerette. The core fluid may be a gas such as
nitrogen, air, carbon dioxide, or other inert gas or a
liquid which is a non-solvent for the polymer such as
water or glycerol. The temperature and composition of
the core fluid can affect the properties of the
membrane.
The extrudate exitlng the die enters one or
more quench zones. The environment of the quench zone
may be gaseous or liquid. Within the quench zone, the
extrudate is subjected to cooling to cause gelation and
solidification of the membrane. In one preferred
embodlment, the membranes are quenched in air. Within
the quench zone, the mernbranes gel and solidify, The
temperature of the air zone is preferably 10C to 100C,
more preferably 20C to 80C. The residence time in the
air zone is preferably less than 180 seconds, more
preferably less than 30 seconds, even more preferably
less than 10 seconds. Shrouds may be used to help
control air flow rates and temperatures in the air
quench zone.
Following or instead of the air quench, the
membranes may optionally be quenched in a liquid which
is substantially a non-solvent for the polymer, such as
water or a mixture of water and the glycerol acetate
solvents and/or the non-solvent glycerol. Some removal
of the solvent and/or non solvent from the membrane may
occur in the liquid quench zone. The temperature of the
liquid quench zone is preferably at least about 0C to
60C, more pre~erably 2C to 30C. The residence time in
the liquid quench zone at the liquid quench temperature
should be sufficient to gel and solidify the membranes.
35,971-F -12-
2~3~
-13-
The residence time in the quench liquid is preferably
less than about 60 seconds, more preferably less than 30
seconds.
The fibers or films are optionally drawn down
using godet rollers or other conventional equipment to
the appropriate si~e. Line speeds are not generally
critical and may vary over a wide range. Minimum
preferred line speeds are at least 10 feet per minute (3
m/min) for reasons of economy in operation~ more
preferably at least 100 feet per minute (30O5 m/min).
Maximum preferred line speeds are less than 1000 feet
per minute (304.8 m/min) for ease in handling, more
preferably less than about 500 feet per minute
(152.~ m/min).
The desired thickness for the membrane will
depend upon its intended end use and other factors. In
the embodiment wherein the membrane is useful for
ultrafiltration or microfiltration, films are preferably
10 microns to ~00 microns in thickness; hollow fibers
for ultrafiltration or microfiltration preferably
possess an outside diameter of 100 microns to 5000
microns, more preferably 200 microns to 3000 microns,
with a wall thickness of preferably 10 microns to 500
microns, more preferably 15 microns to 200 microns. In
the embodiment wherein the membrane ls useful for
dialysis, films are preferably 10 microns to 75 microns
in thickness; hollow fibers for dialysis preferably
3 possess an outside diameter of 100 microns to 500
microns, more preferably 175 microns to 300 microns,
with a wall thickness of preferably 5 to 50 microns,
more preferably 10 to 30 microns. In the ernbodiment
wherein the membrane is useful for reverse osmosis,
films are preferably 10 microns to 500 microns in
35,971-F -13-
'~
.
2 ~3 3 ~L ~ rt
- 1 4 -
thickness; hollow fibers for reverse osmosis preferably
possess an outside diameter of 100 microns to 800
microns, more preferably 150 microns to 500 microns,
with an outer diameter to inner diameter ratio of
preferably 1.2 to 3.5, more pre~erably 1.8 to 2.5.
In another preferred embodiment, following
quenching the membranes are passed through at least one
leach zone containing a liquid which is substantially a
non-solvent for the polymer such as water or a mixture
of water and the glycerol acetate solvents and/or the
non-solvent glycerol to remove at least a portion of the
solvent and optional non-solvent. The leach bath need
not remove all of the solvent and optional non~solvent
from the membrane. The leach bath preferably contains
up to 75 weight percent glycerol in water. more
preferably up to 50 welght percent glycerol in water.
The minimum temperature of the leach bath is such that
removal of the solvent and optional non-solvent from the
membrane occurs at a reasonable rate. The minimum
temperature of the leach bath is preferably at least
-10C, more preferably at least 0C, even more preferably
at least about 10C. The maximum temperature of the
leach bath is below that temperature at which membrane
integrity is deleteriously affectedO The temperature of
the leach bath is preferably less than about 100C, more
preferably less than about 90C. The residence time in
the leach bath is preferably less than about 1000
seconds, more preferably less than about 400 seconds.
The membranes may be drawn down to the desired size
prior to entrance into the leach bath, during the
residence time in the leach bath, subsequent to exiting
the leach bath, or a combination thereof.
35,971-F -14-
2 ~ 7 ~
-15-
Following leaching, the membranes are
optionally dried. The membranes may be dried in air or
an inert gas such as nitrogen, The air or inert gas
used to dry the membrane should have a low enough
initial water content so that drying of the membrane
takes place at reasonable rates. Room air is a suitable
and convenient source for drying the membrane. The
membranes may be dried at temperatures at which drying
takes place at a reasonable rate and which do not
adversely affect the membrane. The drying temperature
is preferably at least about 10C, more preferably at
least 20C. The drying temperature is preferably less
than about 120C, more preferably less than about 90C.
The drving time is preferably at least about 30 seconds,
more preferably at least about 60 seconds. The
membranes may be dried under reduced pr~ssures.
In still another preferred embodiment, the
membranes may be leached in a liquid such as water, then
subjected to a replasticization step prior to drying.
The membranes are leached to remove at least a portion
of the solvent and optional non-solvent. The leach bath
need not remove all of the solvent and optional non-
solvent from the membrane. The minimum temperature of
the leach bath is such that removal of the solvent and
optional non-solvent occurs at a reasonable rate. The
minimum temperature of the leach bath is preferably at
least about -10C, more preferably at least about 0C,
even more preferably at least about 10C. The maximum
temperature of the leach bath is below that temperature
at which membrane integrity is adversely affected. The
maximum temperature of the leach bath is preferably less
than about 100C, more preferably less than about 90C.
The residence time in the leach bath is preferably less
35,971-F -15-
,,
.~
~ ~ 3 Q~
-16-
than about 1000 seconds, more preferably less than about
400 seconds.
The leached membranes may be replasticized by
passing them through at least one replasticization zone
such as a glycerol and water bath. In the embodiment
wherein the membrane is usefuL for microfiltration or
ultrafiltration, the replasticization bath preferably
contains up to about 80 weight percent glycerol, more
preferably up to about 60 weight percent glycerol. ~n
the embodiment wherein the membrane is useful for
dialysis, the replasticization bath preferably contains
up to about 70 weight percent glycerol, more preferably
up to about 60 weight percent glycerol. In the
embodiment wherein the membrane is useful ~or reverse
osmosis, the replasticization bath preferably contains
up to about 70 weight percent glycerol, more pre~erably
up to about 50 weight percent glycerol. The minimum
temperature of the replasticization bath is such than
replasticization of the membrane occurs at a reasonable
rate. The minimum temperature of the glycerol
replasticization bath is preferably at least about 10C,
more preferably at least about 20C. The maximum
temperature of the replasticization bath is below that
temperature at which membrane integrity is adversely
affected. The maximum temperature of the
replasticization bath is preferably less than about
80C, more preferably less than about 60C.
3 In one preferred embodiment of the invention,
as the concentration of the glycerol acetates in the
leach and/or replasticization bath rises, a portion of
the bath may be withdrawn, subjected to a gentle
hydrolysis to convert the glycerol acetates to glycerol
and sodium acetate, then the treated portion returned to
35,971-F -16-
:
3 7 ~
-17~
the leach and/or replasticization bath. This recycle
reduces or eliminates the need for disposal of sol~/ent
containing leach or replastici~ation water while
simultaneously converting the glycerol acetates to a
replasticizer. The membranes may be drawn down to the
appropriate size at any point before, during, or a~ter
the leach or replasticization bath.
The membranes formed by the hereinbefore
described processes may contain significant amoLtnts of
solvent and optional non-solvent, depending on the
quench, leach, and replasticization conditions used.
The membranes may contain up to about 75 percent solvent
and optional non-solvent following fabrication. For
some applications, such membranes containing significant
levels of solvent and optional non-solvent may be stored
for long periods of time without adverse impact on
membrane separation properties. Furthermore, since the
solvents and optional non-solvent used are or can be
converted into substances the presence of which are
compatible and/or accepted in the end use of said
membrane, membranes containing significant levels of
solvent and optional non-solvent may be used in liquid
separations without extensive leaching prior to use
depending upon the application.
The membranes formed by the described processes
may be used in membrane liquid separation processes such
as microfiltration, ultrafiltration, dialysis, and
3 reverse osmosis~ The membrane device fabrication
process is generally performed so as to tailor the
resulting membrane device for its specific end use.
Such adaptation is readily achieved by one skilled in
the art.
35,971-F -17-
~3~
~18-
Ultrafiltration and microfiltration are
pressure driven filtration processes using porous
membranes in which particles or solutes are separated
from solutions. Separation is achieved on the basis of
differences in particle size or molecular weight. Such
membranes may be characterized by the hydraulic
permeability and the sieving coefficient. The hydraulic
permeability is a measure of the volume of solvent
transported through the membrane under the influence of
a pressure gradient. The hydraulic permeability, Ph, is
expressed at a specified temperature.
(amount of solvent permeated)
ph = _________________ __________________________
(membrane area) (~P across membrane) (time)
where ~P is the pressure differential across the
membrane. The hydraulic permeability is commonly
expressed in units of
milliliters
_______________________________
(meter~2 (hour) (centimeters Hg)
The ultrafiltration and micro~iltration membranes of
this invention preferably have an hydraulic permeability
for water at 25C o~ at least about
milliliters
_______________________________ .
(meter)2 (hour) (centimeters Hg)
The sieving coefficient is designated ~.
~ = Cf/Cs
where C~ is the concentration of a small volume of
filtrate at a given moment and Cs is the simultaneous
35,971-F -18-
~3 ~
--19--
concentration of the filtering solution. In a closed
system, both C~ and Cs increase with time when
O <~< 1.
Ultrafiltration and microfil~ration membranes
may also be characterized by their porosity and pore
size. Porosity re~ers to the volumetric void volume of
the membrane. Membranes of this invention use~ul for
ultrafiltration and microfiltration preferably have a
porosity of between about 20 and about 80 percent. Pore
size may be estimated by several techniques, including
by scanning electron microscopy and/or measurements of
bubble point, solvent flux, and molecular weight cuto~f.
Such techniques are well known in the art for
characl;erizing the pore size of microporous membranes,
see Robert Kesting, S~nthetic Pol~meric Membranes, 2nd
edition, John Wiley & Sons, New York, 1985, pp. 46-56;
Channing R. Robertson (Stanford University), Molecular
and Macromolecular Sievin~ by Asymmetric Ultrafiltration
Membranes, OWRT Report, NTIS No. PB85-1577661EAR,
September 1984; and ASTM Test Method F316-86. The
average pore sizes of the membranes of this invention
useful for ultrafiltration are preferably from about 20
Angstroms to about 500 Angstroms. The average pore
sizes of the microfiltration membranes of this invention
are preferably ~rom about 0.05 micron to about 10
microns. The rejection of various solutes may be tested
by successively feeding a solution containing a solute
to one side of the ultrafiltration or microfiltration
membrane of a given temperature and pressure and
analyzing the permeate collected from the other side o~
the membrane to determine the extent o~ solute
rejection. The percent rejection is calculated using
the equation 100 X [1 - (Cp/CF)] where Cp is the
35,971-F -19-
2~3~7~
~20-
concentration of the solute in the permeate and CF is
the concentration o~ the solute in the feed. A series
of different solutes with different nominal molecular
weights may be used such as Blue Dextran 2~000,000, AP
Ferritin 490,000, Albumin 69,ooo, Cytochrome C 12,400,
Vitamin B-12 1335 and Methylene Blue 320.
Ultrafiltration membranes of this invention preferably
possess molecular cut-offs of bet~een about 500 and
about 300,000.
Dialysis is a process whereby solute molecules
are exchanged between two liquids by diffusion through a
semi permeable membrane under the influence of a
chemical potential gradient across the membrane
separating the two liquids. The ability of a membrane
to perform a dialysis separation may be characterized by
the overall diffusive mass transfer coefficient, Ko~,
for the solute of interest. In particular, membranes
useful for the dialysis of blood may be characterized by
the overall diffusive mass transfer coefficient for
urea, K0y(urea).
K0v(urea) is commonly expressed in units of
centimeters/minute. Membranes of thls invention useful
for blood dialysis preferably possess an overall
diffusive mass transfer coefficient for urea at 37C of
at least about 20 x 10-3 centimeters/rninute. Membranes
of this invention useful for dialysis preferably have an
hydraulic permeability for the removal of water from
3 blood at 37G of at least about 10 milliliters
hour meter2 cmHg
Reverse osmosis is used for the purification or
concentration of solutions containing salts or other low
35/971-F -20-
:, :
-21- 2~3~
molecular`~weight solutes, such as in the desalination of
brackish water or seawater. In reverse osmosis, a
pressure exceeding the osmotic pressure is applied to
the feed solution (high concentration side), causing the
solvent to permeate through the semi-permeable membrane
from the high concentration side to the low
concentration side of the membrane. The pressure in
excess of the osmotic pressure is the effective pressure
and constitutes the driving force for solvent transport
across the membrane. The salt or other solute is
"rejected" by the membrane, that is, the salt does not
readily pass through the membrane. The membrane's
separation performance may be characterized by the flux
of water through the membrane and the salt rejection.
The rate of water flux through the me~lbrane =
amount of water permeated
____________.._~_________
(membrane area) (time)
under given conditions of feed concentration,
temperature, and pressure. The rate of water flux is
commonly expressed as GFD, gallons per square foot of
membrane area per day (1 GFD = 40.7 L/m2day) .
Concentration of Permeate Wate~
Salt Rejection(%)= 1- -----~ X 100
Concentration of Feed Water
Membranes of this invention useful for home
reverse osmosis applications at a feed pressure of 50
pounds per square inch (345 kPa) and 0.05 weight percent
NaCl feed concentration preferably have a water flux of
at least about 0.4 GFD (16.3 L/m2day) and a salt
rejection of preferably at least 80 percent. Membranes
of this invention useful for low pressure reverse
35,971-F -21-
2~3~ ~7~
-22-
osmosis applications at a feed pressure of 250 pounds
per square inch (1724 kPa) and 0.15 percent NaCl feed
concentration preferably have a water flux of 2 G~D
(81.4 L/m2day) and a salt rejection o~ pre~erably 90
percent. Membranes of this invention use~ul for
standard pressure reverse osmosis applications at a feed
pressure of 400 pounds per square inch and 0.15 weight
percent NaCl feed concentration preferably have a water
flux of 2 GFD (81.4 L/m2day) and a salt rejection of
preferably at least about 90 percent. Membranes of this
invention useful for seawater reverse osmosis
applications at a feed pressure of 80o pounds per square
inch (5516 kPa) and 3.5 weight percent NaCl feed
concentration have a water flux of 0.5 GFD (20.4
L/m2-day) and a salt rejection of 97 percent.
The following examples are presented to
illustrate the invention only and are not intended to
limit the scope of the invention or claims.
Example 1 - Miscibility Dia~ram for Cellulose
Triacetate/Hallco~M C-918 Monoacetin!Glycerol Blends
A miscibility diagram for cellulose triacetate,
HallcoTM C-918 Monoacetin (~Mtrademark of C.P. Hall
Company), and glycerol blends is shown in the Figure.
The diagram is constructed by determining the
miscibility of various compositions of the three
components at room temperature and pressure.
3o
Cellulose triacetate with an acetyl content of
about 44 weight percent is obtained from Daicel
Chemical. Hallco~M C-918 ~onoacetin is obtained from
the C.P. Hal] Company. Hallco~' C-918 Monoacetin is not
a pure compound but is a mixture of glycerol acetates
35, 971-F -22-
2~3:~ 67~
-23-
and glycerol. Analysis by gas phase chromatography
gives the following lot composition ~or Hallco'M C-918
Monoacetin:
HallcoT`' C-918 Monoacetln Composition (Area Percent)
Glycerol Monoacetin 50.0
Glycerol Diacetin28.8
Glycerol Triacetin1.9
Glycerol 19.2
Glycerol is obtained from J. T. Baker Chemical Company.
Solutions of various compositions of the three
components are prepared in 2,2,2-trifluoroethanol.
Drops of each solution are placed on a microscope slide,
the 2,2,2-trifluoroethanol evaporated, and the resulting
; film microscopically examined with a polarizing
microscope equipped with phase contrast objectives and a
phase contrast condenser. Films from compositions
within the boundaries of the areas labeled "l" in the
Figure exhibited single phase characteristics. Films
from compositions within the boundaries of the area
labeled "M" exhibited multi-phase characteristics.
Example 2 - Ultrafiltration Applications
Example 2-1 - Cellulose Diacetate Hollow Fibers
A mixture of cellulose diacetate from Eastman
Chemical designated CA394-60, Hallco~M C-918 Monoacetin,
and glycerol is mixed in a Hobart mixer to give a blend
comprising by weight 34 percent cellulose diacetate, 47
35,971-F -23-
~1~ 3 ~1~ rl ~;
-24-
percent Hallco~M C~918 Monoacetin, and 19 percent
glycerol.
The mixed blend is extruded into hollow fibers
with a 3/4 inch (1.9 cm) single screw extruder equipped
with a 16 hole spinnerette at a temperature of` about 128
to 149C. The fibers are air quenched and drawn at a
rate of 100 feet/minute (30.5 m/min). One sample
received an additional cold draw of 10 percent. The
solvent and non-solvent are removed from the fibers by
flushing with water.
The fibers are formed into test cells, each
containing 160 fibers, to measure hydraulic permeability
and solute rejection rates. To measure the hydraulic
permeability of the membranes, water inside the bores o~
the hollow fibers at room temperature is subjected to a
pressure of about 9 cmHg. The water permeating outside
of the fibers is measured by observing the increase in
water volume outside the fibers. To determine the
solute rejection rates, solutions of the various solutes
are fed to the test cell at room temperature to the
bores of the hollow fibers by means of a pump. The
permeate is collected and analyzed to determine the
extent of solute rejection. The percent rejection is
calculated using the equation 100 x [1-(Cp~CF)] where Cp
is the concentration of the solute in the permeate and
CF is the concentration of the solute in the feed. The
different solutes used and their nominal molecular
3 weights are Blue Dextran 2,000,000, AP Ferritin 490,000,
and Albumin 69,000. Data are reported in Table IIA.
35,971-F -24-
~3~7~
-25~
TABLE IIA
PERFORMANCE CHARACTERISTICS OF HOLLOW FIBER
ULTRAFILTRATION MEMBRANES
._ ~
Hydraulic Solute Rejectiorl (%)
P~rmeabillty
Fiber Size __ __ __
Sample (Micrometers) ml . AP Blue
m2 hr cmHg Albumln Ferritin Dextran
~_ __. __ I__ __ __
1 237x275 468 847 937 98 0
__ , ~. . . ~_ ,__ ~__ .................. _
2 233x270 485 85~ 93 1 97 7
___ ~ ~ _ . _ . ___ __
3~ 22~x261 844 391 58 8 92 3
~cold ~rawn addit ional lO~
Example 2-2 -_Cellulose Triacetate Flat Sheets
A casting solution is prepared by dissolving
about 300 g of cellulose triacetate powder in about 1700
g f Hallco~ C-918 Monoacetin under heating at about
200C with stirring for about 1-1/2 hours. The blend is
cooled and stored.
About 50 grams of the blend are heated with
stirring to about 160C. The viscosity of the solution
is about 11.25 poise (1.1 Pa-s) at 160C as measured in
a Brookfield viscometer. The solution is hot cask onto
a Pyrex glass plate heated to about 140 to 150C with a
20 mil (0.5 mm) casting bar. Immediately after casting,
the membrane is quenched in room temperature water. The
membrane is leached in room temperature water overnight.
The final thickness of the wet membrane is about 5.7 mil
(0.14 mm).
A membrane is cut from the cast film and
mounted in an Amicon ultrafiltration cell with an
35,97l-F -25
':
~3~ 67~
-26-
effective surface area of 12 cm2. The measured flux of
distilled water through the membrane is 8.5 GFD (346
L/m2day) at 8.75 psi (60.3 kPa). A solution of Blue
Dextran 2,000,000 is made up in a concentration of about
2.8 grams/liter, buffered with 1.0 gram/liter of dibasic
sodium phosphate. The measured flux of the Blue Dextran
solution through the membrane is 5.4 GFD (219.7
L/m2day) at 8.85 psi (61 kPa), with a rejection rate
of Blue Dextran of greater than 99.7 percent.
Example 3 - Dialysis Applications
Hollow fiber membranes are spun from various
blend compositions containing cellulose diacetate,
glycerol monoacetate, glycerol diacetate, and glycerol
via the spinning proce~sses designated process I, process
II, and process III in~Figure~ ~ The cellulose
diacetate is obtained ~rom Eastman Chemical Products,
Inc. The product designation for the cellulose
diacetate is CA-394-60 in all cases except for Example
3-5 in which it is CA-398-30. The glycerol monoacetate
is obtained from C.P. Hall under the product designation
HallcoTU C~918 Monoacetin. HallcoTM C-918 Monoacetin is
not a pure compound, but a mixture of glycerol acetates
and glycerol, having the analysis as shown in Example 1.
The glycerol diacetate is obtained from C.P. Hall under
the product designation Hallcor" C-491 Diacetin.
HallcoTM C-491 Diacetin is not a pure compound 9 but a
mixture of glycerol acetates and glycerol. Analysis by
3 gas phase chromatography gives the following lot
composition ~or Hallco~U C-491 Diacetin:
35,971-F -26-
2 ~ 3 ~ 6 r~ ~
-27-
HallcoTM C-491 Diacetin Co~ osition (Area Percent)
Glycerol Diacetate 52.5
Glycerol Monoacetate 29.3
Glycerol Triacetate 14.9
Glycerol 3.3
The glycerol can be obtained ~rom The Dow Chemical
Company. The glycerol triacetate can be obtained from
Eastman Chemical.
The blend compositions may be homogeniæed by
perforrning an initial compounding extrusion, pelletizing
the exl;rudate, and re-extruding the pellets into hollow
fibers. The fibers from the dif~erent spin runs are
assembled into test cells and the hydraulic permeability
and diffusive mass transfer coef~icient for urea
determined. Data are shown in Table III.
Process I
Example 3-1
A blend composition is prepared consisting of
about 34 percent cellulose diacetate (CD~), 47 percent
Hallco~M C-918 Monoacetin, and 19 percent glycerol by
weight. (The resulting composition corresponds to about
34 percent cellulose diacetate 7 27.9 percent glycerol,
23.5 percent glycerol monoacetate, 13.5 percent glycerol
diacetate, and 0.9 percent glycerol triacetate by
weight.) The blend is extruded at about 162C and
quenched in air. The fiber size measured wet is about
35,971-F -27-
~316 '~
--28--
211 microns in internal di,ameter with a wall thickness
of about 12 microns.
Example 3-2
A blend composition is prepared consisting of
about 34 percent cellulose diacetate (CDA), 38 percent
HallcoTb' C-918 Monoacetin and 28 percent glycerol by
weight. (The resulting composition corresponds to about
34 percent cellulose diacetate, 35.3 percent glycerol,
10 19.0 percent glycerol monoacetate, 10.9 percent glycerol
diacetate, and 0.7 percent glycerol triacetate by
weight.) The blend is extruded at about 167C, quenched
in air, and cold drawn 10 percent. The resulting wet
fiber size is about 207 microns in internal diameter
with a wall thickness of about 17.5 micron~.
Proce~s II
Example 3-3
A blend composition is prepared consisting of
about 35 percent cellulose diacetate (CDA), 53 percent
Hallco~ C-918 Monoacetin, and 12 percent glycerol by
weight. (The resulting composition corresponds to about
35 percent cellulose diacetate, 22.2 percent glgcerol,
26.5 percent glycerol monoacetate, 15.3 percent glycerol
diacetate, and 1.0 percent glycerol triacetate by
weight. Following the initial compounding step, the
blend contains about 39.0 percent cellulose diacetate,
3 21.7 percent glycerol, 24.5 percent glycerol
monoacetate, 14.7 percent glycerol diacetate, and 1.0
percent glycerol triacetate by weight.) The blend is
extruded at about 152C, quenched in air, passed through
a bath at a temperature of about 75C containing 50
weight percent glycerol and 50 weight percent water,
35,971-F -28-
.
~.
~3~ 7~
-29-
drawn 13 percent, and dried in air at about 50C for
about 5 minutes. The final membrane composition is
about 57.0 percent cellulose diacetate, 36O0 percent
glycerol, 3.2 percent glycerol monoacetate, 3.1 percent
glycerol diacetate, and l.0 percent glycerol triacetate
by weight. The final wet fiber size is about 213
microns in internal diameter with a wall thickness of
about 15 microns.
Example 3-4
A blend composition is prepared consisting of
about 42 percent cellulose diacetate (CDA), 47.3 percent
HallcorM C-918 Monoacetin, and 11.7 percent glycerol by
weight. (The resulting composition corresponds to 42
percent cellulose diacetate, 20.8 percent glycerol, 23.6
percent glycerol monoacetate, 13.7 percent glycerol
diacetate, and 0.9 percent glycerol triacetate by
weight. Following the initial compounding step, the
blend contains about 42.0 percent cellulose diacetate,
20.8 percent glycerol, 23.2 percent glycerol
monoacetate, 14.0 percent glycerol diacetate, and 1.0
percent glycerol triacetate by weight.) The fiber is
processed in the manner described in Example 3-3. The
final membrane composition is about 57.0 percerlt
cellulose diacetate, 34.0 percent glycerol, 4.6 percent
glycerol monoacetate, 4.0 percent glycerol diacetate,
and 1.0 percent glycerol triacetate by weight. The
final wet fiber size is about 223 micron internal
3 diameter with a wall thickness of about 15 microns.
35,971-F -29-
2 ~ '7 ~
-3
Process III
Example 3-5
The initial blend composition is about ~2
percent cellulose diacetate (CDA), 47 percent Hallco~M
C 918 Monoacetin, and 11 percent glycerol by weight.
(The resulting composition corresponds to about 42.0
percent cellulose diacetate, 20.0 percent glycerol, 23.5
percent glycerol monoacetate, 13.5 percent glycerol
diacetate, and 0.9 percent glycerol triacetate by
weight.) The blend is extruded at about 166C, quenched
in air, passed through a water bath at a temperature of
about 75C to remove the plasticizer, passed through an
aqueous solution at ambient temperature containing about
3o to 60 weight percent glycerol, and dried in air at
755 for about 2 minutes. The final wet fiber size is
about 206 microns in internal diameter with a wall
thickness o~ about 22 microns.
Example 3-6
The initial blend composition is about 34
percent cellulose diacetate (CDA), 38 percent HallcoTM
C-918 Monoacetin, and 28 percent glycerol by weight.
(The resulting composition corresponds to about 34.0
percent cellulose diacetate, 35.2 percent glycerol, 19.0
percent glycerol monoacetate, 10.9 percent glycerol
diacetate, and 0.7 percent glycerol triacetate by
weight.) Fibers are extruded in the manner described in
Example 3-5. The fiber size measured wet is about 207
microns in internal diameter wîth a wall thickness of
about 12 microns.
The overall mass transport rate for urea,
Kov(urea)~ is determined by providing a water pool in a
35,971-F -30-
2~3.~9i7~
-31-
supply reservoir and pumping the same through the fiber
lumens9 the pool surrounding the fibers in the dialysis
test unit being initially a water-urea solution.
Measurements are made to determine the urea
concentration in the recirculating fluid at given time
intervals.
Tests are conducted at 37C and there is no
pressure differential across the fiber wall surface
during the tests.
The overall mass transfer rate for urea,
Kov(urea), is determined by taking into account the
differences in concentration of urea in the supply
reservoir and in the dialysis test unit on the outside
of the fibers as a function of time and the fiber area
in accordance with the equation:
N = K0v(urea) A(C1-C2)
wherein
N = flux across the membrane in moles/minute
Cl = concentration on one side of the ~embrane
in moles
; (centimeter)3
C2 = concentration on the other side of the
3o membrane in
moles
(centimeter)3
A = membrane area in (centimeters)2.
35,t~71-F -31-
2~3~
-32-
In a two-chamber system without a pressure
differential or resultant ultra~iltration the trans~er
of urea across the membrane wall may be integrated over
a time interval, t, to yield the further equation:
-(C~-C2)t=- rVl+v2
: (C1-C2)t L V1V2 A ~ KOv(urea) t
wherein V1 is the volume of supply reservoir solution,
and V2 is the volume of the solution in the dialysis
beaker.
In the tests, the volumes, V1 and V2 and the
area A are measured separately so that a plot of the
values on each side of the integrated equation produced
a straight line9 the slope of which allows KOv(urea) in
units of centimeter per minute to be calculated.
Hydr-aulic permeability and the mass transfer
coefficient for urea for membranes produced in Example 3
are summarized in Table III.
TABLE III
DIALYSIS APPLICATIONS
* milliliters
__________________________ _____
(meter)2 (hour) (millimeters Hg)
3o
** centimeters/minute
35,971-F -32-
2 83.~ 6 7~
~_ .___ _~_
Example Ph KOv(urea)**
__ ~ ,, ~
3_1 _ 0.034 +/- 0.004
3-2 ~ O ~L .~ :
3-3 17.9 ~/- __ 0.059 ~/- ___
_ ~ _
3-4 2.9 l/- 0.3 0.0235 +/~
. , ,,. . . . __
3-5 6.3 ~/- 0.6 0.033 +/- 0.004
_ __
3-6 210 ~/- 27 0.0267 +/- .0025
_ __
1 0
Example 4 - Reverse Osmosis Applications
Hollow fiber cellulose triacetate (GTA)
membranes are spun from various blend compositions
containing glycerol monoacetate, glycerol diacetate, and
glycerol via conventional spinning processes. The
blends are prepared by mixing the various components to
form a tacky powder of plasticized CTA which is then
extruded in a conventional single screw extruder. The
hollow fibers are extruded at about 180-190C, passed
into air, quenched in water for about 3 seconds at about
4-6C, then leached in water at about 30 degrees to
remove most of the plasticizer. The fibers are then
optionally annealed for about 100 seconds in water at
about 65C. The cellulose triacetate is obtained from
Daicel Chemical Industries Ltd. under the product
designation Cellulose Triacetate Flakes. The glycerol
monoacetate is obtained from C.P. Hall under the product
designation HallcorM C-918 MonoacetinO Hallco~M
C-918 Monoacetin i3 not a pure compound, but a ~ixture
of glycerol acetates and glycerol. Analysis by gas
phase chromatography gives the following lot composition
for HallcoTU C-918 Monoacetin:
35,971-F -33-
2~3~7~
_31~_
Hallco"' C-918 Monoacetin Composition (Area Percent)
Glycerol Monoacetate 46
Glycerol Diacetate 29
Glycerol Triacetate 5
Glycerol 20
The glycerol diacetate is obtained from C.P. ~all under
the product designation HallcorM C-491 Diacetin.
Hallco7U
C-491 Diacetin is not a pure compound, but a mixture of
glycerol acetates and glycerol. Analysis by gas phase
chromatography gives the following lot composition for
Hallco~ C-491 Diacetin:
HallcorU C-491 Diacetin Composition (Area Percent)
Glycerol Diacetate 49
Glycerol Monoacetate 24
Glycerol Triacetate 24
Glycerol 3
The glycerol can be obtained from The Dow Chemical
Company. The glycerol triacetate can be obtained ~rom
Eastman Chemical.
The wet hollow fibers obtained are formed into
test cells for performance evaluation. A test cell
contains a single hollow fiber which is passed through a
stainless steel tube equipped with "Septa" tubing plugs
at both ends. The "Septa" plugs serve as a barrier to
prevent feed solution from leaking out from the feed
35,971-F -34-
2~3~.~ 7~
-35-
compartment. A "Septa" plug is a silicon rubber disc
commonly used in gas or liquid chromatograph de~ices to
separate the sample injection port from the outside of
the systems.
The water flux and salt rejection of the hollow
fiber are measured by pumping a f'eed solution of 0.15
weight percent NaCl aqueous solution through the test
cell under an operational pressure of 250 psi (1724 kPa)
at a temperature of about 25C. The permeate or product
water which exits from the inside or lumen of the f`iber
is then collected. The concentration of NaCl ln the
product water is then measured.
Table IVA reports the results of tests
investigating the effect of different ratios of Hallco~M
C-918 Monoacetin and Hallco~M C-491 Diacetin in the
blend composition on membrane performance. Table IVB
lists the effect of additional glycerol in the blend
composition on water flux and salt rejection. Table IVC
shows the effect on membrane performance of additional
glycerol and glycerol triacetate in the blend
composition.
35,971-F -35-
~,
-36~
_ ~ ~ _ ~ __ ~_
~ o C`~ C-, ~ ~o
.o +l +l o o +l o
c~l c~ ~ ,_ a~ ~ ~ c,
a~ u~ t- __ _ a
¢ r- o ~, _ '`~ o~ _ ~r o '~ ,~
_ O r-~ O O U~ O O C~l 11~ ~ OD OD
~ 1 +1 +1 +1 +1 +1 +1 -~1 -1-1+1 +1 +1 +1 o~
.~ ~ ~ C~ CD~r C~C`l ~ V
~ê ~ _ _ ~ _____ O
_ ~D In 00 C~ ~
. C~ O ~ OD ,O
V l 00, C+- +~ l l ~ ~
Cr ~ ~ 00 t- 00 ~ ~
H ~ _ _ ~ . _ ___ ~'5 ~
m ~ c~ O O C~ ~ co ~ ~
~ ~ l ~ +l +l l l ~ ~o
_ ~_ ~ ~_ ~ ___
~ ~ ~ C~ O 00 ~ ~t~ O ~
U~ o C`l C~ C~ C~ C`l C`l
E c,~ ,. :~ c~ ~ o c
r ~ o ~D o ~ o _ a 6, .':
'O 0~ l l l C~ cr~ C~
R. __ ____ ___ C~ ~ 2 ~
~ C ~o ~ ~r ~ ~o ~ ~ ~ ~
_~ _ _ . ~ _ _ _ ---- '` ~:C bD
~ C`J ~ ~ ~ ~o * ~ *
--36--
- 3 7~
~1 ~
¢ ~4 C3 UE r~ ~ 8 l
_ _ O __ t
::~ t A' tl _~ ~
~ ~) ~ IQ~ c~
m _ ~ N ~ _ 8 ~ ._
_ 8 E D , ~_ 8
~t ~ O O C`l ~
~0 _ ~:1 ~O ~ ~1
L~ ~
'~
~3~ ~J~ 7~
_ ~ __ . __
O O O O N ~
~ ~ -1-1 +1 +1 +1 ~.~
eD ~ t- _ ~
u~ C~ _ a) C~. _ c~ ,,,'~'
¢ ;Ç C ~ t~lo N _ CC~ O O O
:~ r. l ~ +1 +1 -~ 1 +1 +1 +1 +1 4~
,_~ Ci~ ~D _ ~ N ~D _.
_ _ _ _ N
o, ~r ~ c~ ~
O C+~ C~ +1 ~1 ~O
Cl~ 00 00 _
_~ _ _ _ _ -o
~ U~ C~! _ ~ ) O ~ a)
,, ~1~ ,~, N a~ I~ _
;~ X Ç ~ C~O ~ O o N O
v ~ +1 +1 +1 +1 +1 +1 +1 +1
~ 1 ~ U ~3 ~ C`l ' c~ In C~l ~
H ~ C`~ C~iCl:) o ~g ~ C~ N
1~ ~ _ N --~ N r~
m _ u~ _, ~D ~Q ~ ~,
¢ L. C~ o X X X X O
k,V~ ~ oO Ci) C~ ~ ~u
_ _ __
t ~ ~o r- c.~ o
_ C~L. O ~ ~ .
~ --O .._ _ __
_ U 1:-- N ~ ~ O
'~a __ _. _ __ ,_
~3 o* o O ~ O i~
a~ .~ ~ ~ ~ ~ c)O
~ _ _ _-- ~0
. . E o ~ c~ ~ _
38--
~' ,' ~. . :
:' ' ::: ~: --
