Note: Descriptions are shown in the official language in which they were submitted.
~13'7~
Semipermeable membrane devices, and in particular
semi-permeable ultrafiltration membrane devices, have been
employed to concentrate or separate polymeric emulsions or
latices Such latices typically comprise solid polymeric
particles dispersed in water or a water-alcohol or other liquid
phase. Often such latices contain a surfactant material which
has been added during the manufacturing process to aisperse the
polymeric particles in the liquid phase. Typical latices would
include, but not be limited to: styrene-butadiene latices, poly-
vinyl-chloride latices and the like.
Past commercial attempts to concentrate such latices
through the removal of a portion of the liquid phase after manu-
facture from a permeate zone of a semipermeable membrane device
have not been successful. Such lack of success has been due in
part to the inability of the semipermeable membranes to maintain
the initially or originally high flux rates during the
separation or concentration process. Quite often the flux rates
rapidly diminish with time to an unsatisfactory or very low flux
value, and, therefore, require, such as described in U.S. Patent
2~ 3,956,114, Joseph Del Pico, issued May 11, 1976, the periodic
employment of a solvent in order to help maintain or restore
such original flux value. Thus, it is desirable to provide a
rapid, simple, and inexpensive process which will permit the
concentration of polymeric latices in a semipermeable membrane
process, such as a low-pressure, ultrafiltration process, and
for such process to operate in a commercially satisfactory
and continuous manner without severe flux degradation.
My invention relates to an improved process for the
concentration or separation of polymeric latices, and in parti-
cular, my invention concerns an improved process forstabilizing a polymeric
cl~. - 3 -
.J.
latex during a concentration process by ultrafiltration through
the addition of surfactant to the latex to stabilize the latex
and thereby maintain acceptable flux rates during the concen-
tration process~
I have found -that, by adding surfactants, particularly
anionic and nonionic surfactants, to polymer latices prior to or
during the concentration or separation process of the latices
with a semipermeable membrane, the latices are stabilized and
good flux rates are maintained. I believe that the addition of
the surfactants to the polymer latices provides for adsorption
.~ on the surface of the latex particles or electrical boundary
layers, which prevent or retard the polymer particles from
coalescing during the semipermeable membrane process.
~he invention relates to an improvement in a process
for the concentration or separation of an aqueous polymeric
~ latex, which comprises polymer particles dispersed in an aqueous-
i liquid phase, by a semipermeable membrane which permits the
passage of th~ liquid phase and r~tains the pol~mer particles.
The improvement comprises adding to the latex an amount of a
2Q co.mpatible surfactant effective to stabilize the latex contact-
ing the semi-permeable membrane to maintain the dispersion of
the polymer particles in the liquid phase during the con-
centration or separation, said surfactant forming polymer-
surfactant particles thereby preventing the formation of
coagulum from the latex and degradation of the flux rate of
the process, the polymer-surfactant particles having
dimensions such that a substantial portion thereof do not pass
through the membrane. In particular the amount of surfactant
may be in excess of the amount of surfactant which may be lost
through the semi-permeable membrane during concentration or
separation.
r~ - 4 - .
~3~7~
The concentration and/or separation of a polymer
latex is carried out by introducing the latex into the feed
zone of a semi-permeable membrane device, wherein a feed zone
is separated from a permeate zone through the employment of
a particular semi-permeable membrane. The semi-permeable
membrane device may comprise one or more reinforced tubes,
such as a braided tube having a semi-permeable membrane parti-
cularly of cellulose acetate or other membrane material, on
the inside or the outside of the tube, or may comprise a
spiral-type module device, such as described, for example,
in U.S. Patents 3,367,504, 3,386,5~3 3,397,790, and
3,417,870.
-- Typically, polymer latices are separated or
concentrated in an ultrafiltration, rather than a reverse
osmosis process, wherein the pressures employed are
about 10 to 200 psi, for e~ampl~, 20 to 100 psi. The
temperatures employed in such proce~;ses may vary,
depending upon the viscosity of the latex to be con-
centrated, the flux rate of the membrane and other
factors, but typically range
- 4a -
.~
~ 3~
from about 70 to 180F, for example, 90 to 140 F. In the process,
the latex is introduced into one end of the feed zone of the semi-
permeable membrane device, and a concentrated latex is removed
from the other end of the feed zone and a portion of the liquid
phase,typically water and low-molecular weight salts,are removed
from the permeate zone. The concentration process may be directed
to sending the latex through one or more semipermeable membrane
devices in a series, or more typically, the latex is introduced by
a pump ~nto the feedzone of the semipermeable membrane, and the
concentrated fraction recycled, employing the same pump or another
pump, back to the introductory feed zone portion of the device,
while the permeate fraction comprising the liquid phase, typically
water with low-molecular-weight materials~ and often containing
some of the surfactant in the latex is removed from the permeate
zone.
I have found that in the process of concentrating a latex
mechanical shear is placed on the latex, since the latex is
pumped about in a typical ultraf;ltrat:;on system and that this
mechanical shear force contribute~ to the destabilization of ~he
latex and the formation of coagulum which reduces flux rate.
Further, the ultrafiltration semipermeable membrane process re-
moves the liquid-water phase and some surfactant and their salts
in the polymer latex which contributes further to destabilization
of the latex. When the latex destabilizes, then coagulum; that
is, aggregates of latex polymeric particles, coagulate and dest-
abilization of the latex occurs, resulting in fouling of the mem-
brane surface and pores with reduced flux rates resulting.
Thus, many polymeric latices are unstable in the presence of
the high-mechanical shear required to pump the latex into the feed
zone, and to recyle the processed latex back into the feed zone of
the semipermeable membrane device. The mechanical shear that is
developed in the seals and impellers of the high-volume centri~uga
pumps; that ls, pumps that have relat~vely low shear and high
volume, are employed in pumping latices through ultrafiltration an
_ 5 -
.3~6~
reverse-osmosis systems. Such pumps are an opened-face impeller~
while pumps which have a closed-face impeller or gear pumps or
pumps that have close tolerances are not employed in pumping
latices9 since such pumps tend to destabilize rapidly the latices.
Furthermore, diaphragm pumps, although they produce a pulsating
f1OW, are not normally used, except with an accumulator which
evens out the flow rate.
Another pump recommended for use with an ultrafiltra~ion
process for the separation of a latex is a low-shear screw pump
which has large tolerances. Thus, for example, low-shear screw
pumps with large tolerances and centri~ugal pumps with an opened-
face impeller are used in ultra~iltration processes for the con-
centration of polymeric latices, while other pumps9 which place a
much higher mechanical shear on the late~, are not recommended,
since otherwise very large uneconomical amounts of surfactant may
be required to stabilize the latex.
However, regardless of what pumps are used, quite often the
latex becomes unstable, eYen though the latex may contain surfac-
tants added usually during the manufacturing o~ poly~erization
process of ~he latex. Addition of these surfac~ants added during
manufacturing often provides for only a low order of stability.
The use o~ additlonal surfactants, as required in my process,
often is not necessary under normal conditions, because the latex
is not subject to a high shear or other factors such as concentra-
tion polymerization layers employed or found in an ultrafiltrationprocess.
In addition, I have found that latices which are to be con-
centrated in a semipermeable separation process are often unstable
at the concentrations found in the polarization concentration laye
formed adjacent the semipermeable membranes employed in the ultra-
filtration and reverse-osmQsis devices. Since there is a hi~her
concentration of polymer particles in the polarization layer ad-
jacent the membrane skin, this concentration is often sufficient
, l
~37~7~
during the process to effect also the destabilization of the
l~tex, Thus, tha higher temperatures~ the higher concentration of
the polarîzation layer and the greater shear caused by the pumps
and the pumpi ng processes dur;ng the ultrafiltration process cause
a more frequent and energetic collision of the macro-mol ecules of
the polymer particles, and thus lead to a greater tendency of coa-
gulation of the particles and destabili~ation of the latex, which
coagulation results in fouling of the membrane and reduction in
flux rate.
I have found that destabilization of the latex during a mem-
brane separation process may be avoided, prevented or at least
considerably reduced along with the r~sulting coagulum from the
d~stab~lized latex, by employing additional and minor amounts of
a surfactant to the latices prior to or durlng the concentration
process. The amount of the surfactant to be added may vary,
dependiny upon the particular polymeric latices to be employed and
the conditions under which the process is to be operated, but
typically may comprise about 0.05 to 2.0% o~ the surfactant based
on the weight of the polymer in the latices, for example, from
2Q about 0.1 to about 1.0% such as 0.4 to 0.8%. The su~factant may
be added in a continuous manner into the latex prior to pumping
or during recyle, or where a batch process is used~ the surfactant
may be added to and mixed with the batch of the latex to be con-
centrated prior to separation and concentration. Where a portion
of the surfactant is removed with the liquid phase from the
permeate zone, it may be ~ound necessary to add additional sur-
factant during the recycling of the concentrated fraction back to
the feed zone, to maintain the desired concentration level of the
latex to prevent destabilization.
The amount of surfactant required to stabilize the latex
l¦during a particular process may be determined by carrying out
767~
I .
the particular process under similar temperature and pressure
conditions with the desired pump9 e~ther in a pil~ plant or in
la commercial unit and continually adding smaller incremental
amounts of surfactant to reach and determine the minimum con~
centration ~evel required for stabilization of the latex under
the commercial operating condi~ons to be employed. Another
method for determining-the amount of surfactant to be employed is
to test the latex by mixing the latex in a blender while adding
incremental amounts of surfactant, and observing for coagulum
under the high shear blending conditions. Such a test is a
typical test for mechanical stability of latices, as set forth
in ASTM D 1076-73 (Test No. 16).
My process will be described in reference to particular po1y-
meric latices; however, my process is useful with a wide variety
; 15 of polymeric latices, such as natural latex, butyl rubber, nitrile
rubber~ ethylene-propylene copolymers and terpolyme~s, homo
and copolymers of diene polymers like butadiene-styrene copoly-
mers~ as well as terpolymers with acrylonitrile, acrylate latex,
polyvinyl alcohol and polyvinyl-acetate emulsions, homo and
~0 copolymers of vinyl-halides like polyvinyl-chloride and vinyl
chloride-vinyl acetate copolymers and other polymeric emulsions
and latex compositions where it is desired to concentrate the
latex to a higher concentration value. My process is particularl
applicable to vinyl-chloride polymer latices~ such as polyvinyl-
chloride latex or a vinyl~chloride~vinyl-acetate Jatex and the
like and natural rubber latex since such latices tend generally
to be relatively unstable as compared to styrene-butadiene rubber
latices.
The polymeric latices may be concentra~ed typically up to as
hlgh as 70% conc~ntration. For example, with polyvinyl-chloride
and vinyl-halide/vinyl-acetate copolymer emulsions, the latex
L3~67~
is usually manufactured at about 25 to 35% polymer, and is con-
centrated up to 50 to 60%~ Styrene-butadiene rubber latices are
oFten concentrated from about 10 to 20%; for example, 15%, up to
45 to 60% concentration levels, o~ higher if desired. My process
may also be employed on waste streams which contain a polymeric
latex where it is desired to concentrate the latex from a ~ery low
value; for example, less than 1%~ up to 20%, and~ thereafter9 to
mix the concentrated fraction recovered with other latex concentra-
tion ~or further concentration to a higher level. ThPrPfore, in
the concentration processes for polymeric latices, the feed
stream may range from very low amounts (as low as 0.1 to about 1%)
to concentration levels of 55 to 75% or higher. Where very high
concentrations occur, the polymer often becomes viscous, so that a
hi~her temperature must be employed in the ultrafiltration process
and when such occurs, often additional amounts of surfactant are
required in order to prevent destabilization of the latices due to
the morP energetic polymer molecules a~; the higher temperature
process levels.
The surfactants useful in my process and to be added to the
polymeric lat;ces encompass a wide variety of surfactants and
surfactant-functioning materials. Any material may be used as a
urfactant in my process as I use the term which stabilizes the
polymeric latices under the membrane concentration layer condi-
tiuns and high-~shear pumping conditions of the process. Typically
the surfactant should be compatib~e with the polymer latices; that
is, not lead to an electrical imbalance, for example, adding an
anionic surfactant to a cationic stabilized latex, and preferably
the surfactant employed is the same surfactant or same type or
class as used by the manufacturer in the latex~ and more particu-
larly, the use of nonionic surfactants is preferred, It isrecognized that s~me latices are sold as unstable-type latices and
37679
are compounded in this manner so that they may be used for a
particular process. However, such latices are not of the type
useful in ultrafiltration processes and are not generally used
in such processes, due to such compounded and intentional destabi-
lization of the latices.
The polymeric latices are usually prepared by polymerization
of the monomer in an aqueous medium in the presence of a suit-
able polymerization catalyst to provide a latex of lO to 60%
total solids. The aqueous medium may be surfactant-free or it
¦may contain a surfactant or a surfactant may be added later in
the process.
Suitable surfactants used in latex manufacture and useful
in my process include organic su1fates and sulfonates, such as
sodium lauryl sulfate, but are not limited to: ammonium lauryl
sulfate, the alkali-metal and ammonium salts of sulfonated
petroleum or paraffin oils, the sodium salts of aromatic sulfonic
acidsS such as the sodium salt of naphthalene sulfonic acids,
the sodium salts of dodecane-l-sulfonic acid, octadiene-l-sulfonic
acid, etc.; aralkyl sulfonates, such as sodium isopropyl benzene
sulfonate, sodium dodecyl benzene sulfonate and sodium isobutyl
naphthalene sulfonatea alkali-metal and ammonium salts of sul-
fonated discarboxylic acid esters and amides, such as sodium dioct l
sulfosuccinate, sodium octadecyl sulfo succinamate and the like
and others.
Cationic surfactants, such as the salts of strong inorganic
acids and organic bases3 containin~ long carbon chains, for exampl ,
lauryl amine hydrochloride, the hydrochloride of diethylaminooctyl
decylamine, trimethyl cetyl ammonium bromide, dodecyl trimethyl
ammonium bromide, the diethyl cyclohexylamine salt of cetyl sulfon c
ester and others may be used. One preferred class, however, is
the anionic surfactants such as the alkali~metal and ammonium salt
L37~73~
of aromatic sulfonic acids, aralkyl sulfonates and long-chain
~kyl sulfates. Suitable anionic surfactants would comprise sodium
lauryl sulfate, ethoxylated sodium sulfo succinate~ and alkylaryl
polyether sulfates.
In addition to the above and other polar or ionic emulsifiers,
and surfactants, another most preferred class which may be used,
singly or in combination with one or more of the foregoing types
of surfactants, includesthe so-called "nonionic" surfactants, such
as the polyether alcohols prepared by condensing ethylene or pro-
pylene oxide with higher alcohols, the fatty alkylol-amine con-
densates, the digylcol esters of lauric~ oleic and stearic acids
and others. Specific nonionic surfactants include C8-Cg alkyl
phenoxy polyetho~y ethanols or propanols containing from about 20
to 100 ethoxy or pr~xy groups like tertiary octyl and nonylphenoxy
polyethoxy ethanols.
My invention will be described for the propose of illus~ration
only in connection with the concentration of certain polymeric
latices; however, it is recognized ancl within the sp~rit and scope
of my inventi~n that Yarious changes, modifications and alteration
may be made without departing from the spirit and scope of my in
vention.
BRIEF DESCRIPTION OF THE DRAWING
The drawing shows an illustrated schematic process of an ultra
f~ltration device employed for the concentration of a polymeric
lat~x employing my invention.
DESCRIPT~ON OF THE EMBODIMENTS
The drawing shows an ultrafiltration device and process in
which a polymeric latex 10 is placed in a batch container 12, and
a surfactant 16 added and mixed by a mixer 14 with the polymeric
latex. The polymeric latex 10 with the additional surfactant is
withdrawn from the container 12 through line 18 and through a
3'76 ;'~
centrifugal opened-face impeller highovolume pump 20 into an ultra
f~ltration membrane device 22 comprising for example a module with
a plurality of tubes having a semipermeable membrane coated on the
inside diameter of the reinforced tubes or a spiral module ultra-
filtration membrane device, for example, with a cellulose-acetate
semipermeable membrane.
A permeate fraction 34 is removed from line 24 from the permea e
zone, the permeate fraction comprising the liquid phase, prima~ly
water, plus also some low-molecular-weight salts if present in the
lU ori~nal polymeric latex 10, and also small amounts of surfactants
in some cases. The concentrated latex is removed from the other
end of the feed zone through line 28 and is recycled through line
30 to be reintroduced into the semipermeable membrane device 22
unt~l the desired level of concentration is obtained, and then
the concentrated latex 32 is removed continuously through line 26.
Add;tional surfactant 36 is shown introduced into the recycle line
0 to maintain the surfactant level~ The drawing illustrates a
typical batch process for the concentration of a manufactured
atex. Of course, where desired, rather than employing a single
emipermeable membrane unit 22~ a series of such units may be
mployed, with the latex progressively concentrated as it passes
hrough each membrane device.
Example 1.
A polyvinyl-chlorlde latex having a solids content of about
4.5% was introduced into an ultrafiltrat~on process as set forth
n the drawing, and it was found that the centrifugal pump could
nly run for approximately two hours at 2600 rpm before the latex
oagulated, The addition of an anionic or a nonionic surfactan~
o the polyvinyl-chloride latex, at approximately 0.4% of the
eight of the polymer, permitted the latex to be run in the
ltrafiltration process and to be concentrated to approximately
- 12 -
3~G79~
64% solids without difficulty. One surfactant employed was
Tergitol 7, an anionic surfactant similar to the surfac~ant em-
ployed by the manufacturer in stabilizing the polyvinyl-chloride
latex during manufacture. Tergitol 7 is a trademark of Union
Carbide Corp. to identify a sodium sulfonate derivative of 1,
9-dlethyltridecanol-6 A nonionic surfactant Triton X-100, an
alkylaryl polyether alcohol, which is a ~rademark of Rohm & Haas
C~o., was also added and found to be satisfactory.
~e~.
~ A poly~inyl-chloride emulsion of a different manufacturer, the
Example 1, having about 30% solids 9 when placed in an ultrafiltra-
tion system of the type described, and could not be pumped at all
without destabilization oF the latex and formation of coagulum.
The addition of between 5 and 50 m1 per gallon of an anionic sur-
lS factant of the same type as employed by the manufacturer to the
latex provided additional stability and permitted the latex
to be concentrated in the ultraf~ltrat;ion process.
Example 3
.
A 50%-solids styrene-butadiene rubber latex of about 50%-solid
2Q was diluted to 0.~% solids, and run w;th both tubular and spiral
ultrafiltration membrane devices. After several hours of running
at a steady state, as the temperature increased from 15 to 35C,
the process flux dropped from 60 to 10 gfd for the tubes (gallons
per square foot of membrane per day). The add;tion of about 5X
- 25 of a nonionic surfactant Triton X-100, based on the polymer
weight, at a rate of 14 ml to 15 gallons of a latex pre~ented the
process flux of the tubes and the spiral membrane from decreasing
with time. After addition of the surfactant the flux of the
membrane was then approximately 200 gfd at 50C.
Thus 9 the addition of surfactants to polymer latices prior to
¦or during the process of ultrafiltration stabilized the latices
3'767~
~and prevented coagulum from forming and decre~sing the flux rate.
¦The addition of surfactant also prevented pump fai7ure9 which
failure often occurs by virtue of the coagulant plu~ging up the
l seals in the internal portion of the pump. ~ly process provides
S la rapid, simple and an effective means to overcome the difficultie
¦of the prior art and to permit the commercial concentration and
¦seperation of polymeric latices.
- 14 -
