Ultrafiltration process for the concentration of polymeric latices

In a process for the concentration of an aqueous polymeric latex, which latex comprises polymeric particles dispersed in an aqueous phase by a semipermeable membrane process, such as an ultrafiltration process and which latex is subject to destabilization, the improvement which comprises: adding to the latex a sufficient amount of a compatible surfactant to stabilize the latex and to maintain the dispersion of the polymeric particles in the aqueous phase of the latex during the concentration of the latex in a semipermeable membrane process.

BACKGROUND OF THE INVENTION 
Semipermeable membrane devices, and in particular semipermeable 
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 disperse the polymeric 
particles in the liquid phase. Typical latices would include, but not be 
limited to: styrene-butadiene latices, polyvinyl-chloride latices and the 
like. 
Past commercial attempts to concentrate such latices through the removal of 
a portion of the liquid phase after manufacture 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 my prior application (now 
U.S. Pat. No. 3,956,114, 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. 
SUMMARY OF THE INVENTION 
My invention relates to an improved process for the concentration or 
separation of polymeric latices, and in particular, my invention concerns 
an improved process for stabilizing a polymeric 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 concentration 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. 
The concentration and/or separation of a polymer latex is carried out by 
introducing the latex into the feed zone of a semipermeable membrane 
device, wherein a feed zone is separated from a permeate zone through the 
employment of a particular semipermeable membrane. The semipermeable 
membrane device may comprise one or more reinforced tubes, such as a 
braided tube having a semipermeable membrane particularly 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. Pat. Nos. 3,367,504; 3,386,583; 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 example, 20 to 100 psi. 
The temperatures employed in such processes may vary, depending upon the 
viscosity of the latex to be concentrated, the flux rate of the membrane 
and other factors, but typically range from about 70.degree. to 
180.degree. F.; for example, 90.degree. to 140.degree. F. In the process, 
the latex is introduced into one end of the feed zone of the semipermeable 
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 into the feed zone of the semipermeable 
membrane, and then 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 
ultrafiltration system and that this mechanical shear force contributes to 
the destabilization of the latex and the formation of coagulum which 
reduces flux rate. Further, the ultrafiltration semipermeable membrane 
process removes the liquid-water phase and some surfactant 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 destabilization of the latex occurs, resulting in 
fouling of the membrane 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 recycle 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 centrifugal pumps; that is, 
pumps that have relatively low shear and high volume, are employed in 
pumping latices through ultrafiltration and 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 latices, since such pumps tend to destabilize rapidly 
the latices. Furthermore, diaphragm pumps, although they produce a 
pulsating flow, are not normally used, except with an accumulator which 
evens out the flow rate. 
Another pump recommended for use with an ultrafiltration process for the 
separation of a latex is a low-shear screw pump. Thus, for example, 
low-shear screw pumps and centrifugal pumps with an opened-face impeller 
are used in ultrafiltration processes for the concentration of polymeric 
latices, while other pumps, which place a much higher mechanical shear on 
the latex, 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, even though the latex may contain surfactants added usually 
during the manufacturing or polymerization process of the latex. Addition 
of these surfactants added during manufacturing often provides for only a 
low order of stability. The use of additional 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 
concentration polarization layers employed or found in an ultrafiltration 
process. 
In addition, I have found that latices which are to be concentrated in a 
semipermeable separation process are often unstable at the concentrations 
found in the concentration polarization layer formed adjacent the 
semipermeable membranes employed in the ultrafiltration and 
reverse-osmosis devices. Since there is a higher concentration of polymer 
particles in the concentration polarization layer adjacent the membrane 
skin, this concentration is often sufficient during the process to effect 
also the destabilization of the latex. Thus, the higher temperatures, the 
higher concentration of the polarization layer and the greater shear 
caused by the pumps and the pumping processes during the ultrafiltration 
process cause a more frequent and energetic collision of the 
macro-molecules of the polymer particles, and thus lead to a greater 
tendency of coagulation of the particles and destabilization 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 membrane separation 
process may be avoided, prevented or at least considerably reduced along 
with the resulting coagulum from the destabilized latex, by employing 
additional and minor amounts of a surfactant to the latices prior to or 
during the concentration process. The amount of the surfactant to be added 
may vary, depending 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% of the surfactant based on the 
weight of the polymer in the latices; for example, from about 0.1 to about 
1.0% such as 0.4 to 0.8%. The surfactant may be added in a continuous 
manner into the latex prior to pumping or during recycle, or where a batch 
process is used, the surfactant may be added to and mixed with the batch 
of the latex to be concentrated prior to separation and concentration. 
Where a portion of the surfactant is removed with the liquid phase from 
the permeate zone, it may be found necessary to add additional surfactant 
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 during any 
particular process may be determined by carrying out the particular 
process under similar temperature and pressure conditions with the desired 
pump, either in a pilot plant or in a commercial unit and continually 
adding smaller incremental amounts of surfactant to reach and determine 
the minimum concentration level required for stabilization of the latex 
under the commercial operating conditions 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 polymeric latices; 
however, my process is useful with a wide variety of polymeric latices, 
such as natural latex, butyl rubber, nitrile rubber, ethylene-propylene 
copolymers and terpolymers, homo and copolymers of diene polymers like 
butadiene-styrene copolymers, as well as terpolymers with acrylonitrile, 
acrylate latex, polyvinyl-alcohol and polyvinyl-acetate emulsions, homo 
and 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 particularly applicable to 
vinyl-chloride polymer latices, such as polyvinyl-chloride latex or a 
vinyl-chloride-vinyl-acetate latex 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 concentrated typically up to as high as 70% by 
weight concentration. For example, with polyvinyl-chloride and 
vinyl-halide/vinyl-acetate copolymer emulsions, the latex is usually 
manufactured at about 25 to 35% polymer, and is concentrated 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, or 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 very 
low value; for example, less than 1%, up to 20%, and, thereafter, to mix 
the concentrated fraction recovered with other latex concentration for 
further concentration to a higher level. Therefore, 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 higher 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 more energetic polymer molecules at the higher temperature 
process levels. 
The surfactants useful in my process and to be added to the polymeric 
latices encompass a wide variety of surfactants and surfactant-functioning 
materials. Any material may be used as a surfactant in my process as I use 
the term which stabilizes the polymeric latices under the membrane 
concentration layer conditions and high-shear pumping conditions of the 
process. Typically the surfactant should be compatible 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 particularly, the use of 
nonionic surfactants is preferred. It is recognized that some latices are 
sold as unstable-type latices and 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 destabilization 
of the latices. 
The polymeric latices are usually prepared by polymerization of the monomer 
in an aqueous medium in the presence of a suitable polymerization catalyst 
to provide a latex of 10 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 sulfates 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 acids, such as the sodium salt of naphthalene 
sulfonic acids, the sodium salts of dodecane-1-sulfonic acid, 
octadiene-1-sulfonic acid, etc.; aralkyl sulfonates, such as sodium 
isopropyl benzene sulfonate, sodium dodecyl benzene sulfonate and sodium 
isobutyl naphthalene sulfonate; alkali-metal and ammonium salts of 
sulfonated discarboxylic acid esters and amides, such as sodium dioctyl 
sulfosuccinate, sodium octadecyl sulfo succinamate and the like and 
others. 
Cationic surfactants, such as the salts of strong inorganic acids and 
organic bases, containing long carbon chains, for example, lauryl amine 
hydrochloride, the hydrochloride of diethylaminooctyl decylamine, 
trimethyl cetyl ammonium bromide, dodecyl trimethyl ammonium bromide, the 
diethyl cyclohexylamine salt of cetyl sulfonic ester and others may be 
used. One preferred class, however, is the anionic surfactants such as the 
alkali-metal and ammonium salts of aromatic sulfonic acids, aralkyl 
sulfonates and long-chain alkyl 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, 
includes the so-called "nonionic" surfactants, such as the polyether 
alcohols prepared by condensing ethylene or propylene oxide with higher 
alcohols, the fatty alkylol-amine condensates, the digylcol esters of 
lauric, oleic and stearic acids and others. Specific nonionic surfactants 
include C.sub.8 -C.sub.9 alkyl phenoxy polyethoxy ethanols or propanols 
containing from about 20 to 100 ethoxy or proxy groups like tertiary octyl 
and nonylphenoxypolyethoxy ethanols. 
My invention will be described for the purpose of illustration only in 
connection with the concentration of certain polymeric lactices; however, 
it is recognized and within the spirit and scope of my invention that 
various changes, modifications and alterations may be made without 
departing from the spirit and scope of my invention.

DESCRIPTION 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 centrifugal opened-face impeller high-volume 
pump 20 into an ultrafiltration 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 
ultrafiltration membrane device; for example, with a cellulose-acetate 
semipermeable membrane. 
A permeate fraction 34 is removed from line 24 from the permeate zone, the 
permeate fraction comprising the liquid phase, primarily water, plus also 
some low-molecular-weight salts if present in the original 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 until the desired level of concentration is obtained, 
and then the concentrated latex 32 is removed continuously through line 
26. Additional surfactant 36 is shown introduced into the recycle line 30 
to maintain the surfactant level. The drawing illustrates a typical batch 
process for the concentration of a manufactured latex. Of course, where 
desired, rather than employing a single semipermeable membrane unit 22, a 
series of such units may be employed, with the latex progressively 
concentrated as it passes through each membrane device. 
EXAMPLE 1 
A polyvinyl-chloride latex having a solids content of about 34.5% was 
introduced into an ultrafiltration process as set forth in the drawing, 
and it was found that the centrifugal pump could only run for 
approximately two hours at 2600 rpm before the latex coagulated. The 
addition of an anionic or a nonionic surfactant to the polyvinyl-chloride 
latex, at approximately 0.4% of the weight of the polymer, permitted the 
latex to be run in the ultrafiltration process and to be concentrated to 
approximately 64% solids without difficulty. One surfactant employed was 
Tergitol 7, an anionic surfactant similar to the surfactant employed by 
the manufacturer is 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-diethyltridecanol-6. A nonionic 
surfactant Triton X-100, an alkylaryl polyether alcohol, which is a 
trademark of Rohm & Haas Co., was also added and found to be satisfactory. 
EXAMPLE 2 
A polyvinyl-chloride emulsion of a different manufacturer, when Example 1, 
having about 30% solids, when placed in an ultrafiltration 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 ml 
per gallon of an anionic surfactant of the same type as employed by the 
manufacturer to the latex provided additional stability and permitted the 
latex to be concentrated in the ultrafiltration process. 
EXAMPLE 3 
A 50%-solids styrene-butadiene rubber latex of about 50%-solids was diluted 
to 0.5% solids, and run with both tubular and spiral ultrafiltration 
membrane devices. After several hours of running at a steady state, a the 
temperature increased from 15.degree. to 35.degree. C., the process flux 
dropped from 60 to 10 gfd for the tubes (gallons per square foot of 
membrane per day). The addition of about 5% of a nonionic surfactant 
Triton X-100, based on the polymer weight, at a rate of 14 ml to 15 
gallons of a latex prevented 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 50.degree. C. 
Thus, the addition of surfactants to polymer latices prior to or during the 
process of ultrafiltration stabilized the latices and prevented coagulum 
from forming and decreasing the flux rate. The addition of surfactant also 
prevented pump failure, which failure often occurs by virture of the 
coagulant plugging up the seals in the internal portion of the pump. My 
process provides a rapid, simple and an effective means to overcome the 
difficulties of the prior art and to permit the commercial concentration 
and seperation of polymeric lactices.