CA1051137A - Process for concentrating latices - Google Patents

Abstract

Abstract of the disclosure:
The present invention is related to the process of con-centrating K-polymer-latices by ultra filtration carried out by means of a semi-permeable membrane made of a synthetic polymer having a determined molecular weight, comprising that the membrane is pretreated with an emulsifier solution prior to ultra filtration. This operation substantially prevents the formation of solid matter deposits on the membrane.

Description

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The present invention is related to a process ~or the concentration of latices which are obtained by emulsion-poly.
mer~zing vinyl chloride or vinyl chloride and monomers copoly-merizable herewith, carried out by ultra filtration by means 5 . of a seml-permeable membrane made of a synthetic polymer.
Upon emulsion-polymerizing ~inyl chloride or vinyl chloride and monomers copolymerizable herewith a latex is formed which conta~ns in addition to the hydrosoluble polymerization agents such as emulsifiers an~ activators up to 50 weight % o~ polymer, calculated on the we~ght of the latexO In some cases it is necessary or at lea~t desirable to concentrate such latices.
For certain application purposes lt is useful to further oon-centrate the latlces, for example as bonding agent. ~hen working the latices up to yield a powdery polymer, usually by a drying process9 e.g~ by spray dryingj a process which re-. ~ quires the evaporation of the total quantity of wat~r, economical reasons recommend the further-concentration o~ the .original latices to a rate of up to 70 weight % of the polymer~
It is known to carry out the concentration by means of a vacuum evaporator or a film evaporator. Howe~er, the con-centration of latice~ based on polyvinyl chloride by means of a vaouum evaporator or a ~ilm evaporator is problematic 7 because such latices - as generally known - are more or less sensitive to tempera-ture and, besides, tend to foaming, a fact which hampers the evaporation. Furthermore, local overheating in the evaporator may encrust the evaporator, these crusts may peel of~ the evaporator walls and appear in the latex as un-desirable granulated deposit.
29 It has also been proposed to carry out the concentration

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of the emulsion~polymer latices by ultra filtration by means o~semi-permeable membranes made of synthetic polymer~ Chemical Engineering Progress", vol. 64, 1~ no. 129 pg. 31 - 4~ and "Chemie-Anlagen ~ Verfahren", 19719 no. 89 pg 529 57 and 58~.
Howe~er, the practical realization of this me~hod failed hitherto due to the fact that after a short operation period the above mentioned relatively high contents in polymer (up to about 50 weight %) of the latice~ obtained by emulsion-poly-meriæation show an ef~ect known as concentration pol~riza~ion o~ semi~permeable membran~sO The described e~fect is indicated by clogging o~ the membrane pores a~ter merely a short fil-tration period and by encrusting of the entire membrane ~ur~ace with a solid material so that the filtration ~ets gradually . more and more di~ficult and ~inally impos~ible~
Object o~ the present invention is there~ore to offer a process which avoids these disadYantages.
The present invention solved this problem by a process for the concentration of latices which are obtained by emulsion~
polymerizing vinyl chloride or vinyl chloride and monomers co~
polymerizable herewith9 by ultra filtration by means of a ~emi-permeable membrane made of a synthetic polymer, a process which comprises the use of a ~emi-permeable membrane with a partition cu* at a molecular weight of from 5 000 to 100 000, this membrane being tr0ated prior to ultra filtration wlth an aqueous solution of one or several emulsifiers which ar~
suitable t~ emulsion-polymerizing vinyl chloride or vinyl chloride and monomers copolymerizable herewith.
Actually, suitable membranes for the process according to 29 the in~rention are all membrane~ made of synthetic polymers, . .
i.' - . ' ' . ~ ' ~ '' ~5:~37 provided they have a suffi~ient chem~c~l stabil~ty in respect to the latex to be filtered~ The followlng well known membrane materials may be cited i.a.: polyacrylic acid9 polystyrene, styrene-copolymers, sulfonated polyphenylene oxide , cro~s-linked polyvinyl alcohols~ polyolefins such as polyethylene, polypropylene and ethylene-copolymers, vinylchloride - copoly-mersp polyacrylonltrile, polyvinylene-carbonate, polyvinylene glycol, polyacrylates such aæ polyethyl acrylate e.g~ cross-linked with trimethylolpropanetrimethacrylate and polymethyl-1Q methacrylate, e.g. poly-(galactosemethacry~ate)methylmethacrylate;
polyimides~ polyvi-nyl pyrrolidones, e~g. cross-linked wit~
methylene-bls-(4-phenylisocyanate)~ poly~midazopyrrolones, e~g. pyrromellithic acid dianhydride-3,~'-diaminobenzidine 9 polyamides, e,g. polycaprolactam cross-linked with toluene~
2,4-~iisocyanate, polyamide hydrazides as well as membranes based on cellulose, such as cellulose-acetate membranes, cellu-loseacetate-palmitate membranes and cellulose nitrate membranesO
Such membranes can be prepared for e~ample according to US
patents nos~ 3 13~ 132~ ~ 65~ 030, 3 710 945 and 3 737 042.
Especially suitable proved to be membranes of polyacrylonitrLle~
polyamides as well as of polymethyl-methacrylate. As far as the structure of the membranes 1s concerned, known asymmetric membranes a~e used pre~erably. 5uch asymmetric membranes may consist e.g. of a thin (about 001 to 2 ~ thick) partition layer and a highly porous substructure o~ the same material a~
æupport for the partition layer which can be achieved by known manufacturing prooesses through dif~erent.precipitatiorl con-ditions of the polymer. There may also be used asymmetric 29 membranes consisting of a dens~ but extremely thin (about 400 ~ ' ~
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to 2000 ~ thick).polyMer film which had been spread onto a mechanically stable micro-porous support.
m e ultra filtration of the latices i~ carried out in such a way that the membranes are positioned according to ~nown membrane sets (moduli) or assembled to an apparatus~ Plate moduli are used which normally group several flat membranes (platen membranes) to form a package. Also suitable moduli are tubular moduli composed of bundles of perforated metal tubes each o~ which being lined with a tubular membrane on a porous support, or spiral shaped moduli of wound spiral membranes.
When using plàten membranes and wound spiral membranes the distance separating two membranes should usefully vary from 2 to 20~ preferably ~rom 3 to 8 ~m. ~hen u~ing tubular membranes the most suitable tube diameter varies from 10 to 50, preferably from 15 to 25 mm.
The ultra filtration membranes are characterized by the definition o~ "partition cut" which indicates the minimum limit of the molecular weight o~ macromolecules being still retained by the membrane.
Membranes with a partition cut at a molecular weight ~umerical average) of from 5000 to 100 000, pre~erably with a partition cut at a molecular weight of from 10 000 to 50 000 are used for the concentration of latices, A~ per the invention the membrane m~erials are treated, prior to their use for ultra filtration, with the aqueous æolutio~ of one or several emulsifiers such as they are suitable for the emulsion polymerization of viny~chloride or o~ vinyl chloride and monomers eopolymerizable herewith~ Such emulsifiers 29 suitable for emulsion-p~lymeriz~ng ~inylchloride which may be . - ~ . ~ . , .. ~;

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1~5~7 considered for the pretr~atment are~ ~or example, alkyl sul fonates and alkyl sulfates having from 8 to 20 carbon atoms, prefarably ~rom 12 to 18 carbon atoms, suc~ a~ lauryl sul~ate;
alkylaryl sulfonates~ the alkyl radicals of which haYe totally from 8 to 18 carbon atoms, such as dodecylbenzene sulfonate, dibu~yl naphthalene sul~onate, octadecylbenzene sul~onate;
salt of higher ~atty acids having from 8 to 22 carbon atoms, such as stear$c acid, lauric acid~ pal~itic acid; salts of fatty acids cont~ning epoxy group~, such as epox~stearicacid;
sal~s of acid phosph~nic acic alkyl ester, aryl ester or alkyl-aryl ester, the alcohvlic or phenolic components of which have from 6 to 18 carbon atoms, such as diethylhexyl phosphoni.c acid; oxyalkyl sul~onic aclds having from 8 to 18 carbon -atoms in the alkyl chain and havin~ from 1 to 15 alkylene oxide radlcals~ each radical ha~ing ~rom 2 to ~ carbon atoms, or their ~alts; ~ul~o-phthalic acid esters having from 4 to 12 carbon atoms in the alcohol component or their salts; ~ulfo-succinic acid esters or their salts and p~lyalkylene oxide deriv~tives of phenols or amides. As salts may be considered ~o in general the alcali metal salts, alcali earth metal salts and ammonium salts, preferably alcali metal salts or ammonium ~altsO Respecting further emulsifiers reference is made to F.Kainer, 'IPoly~inylGhlorid and Vinylchloridmischpolymerisate"
965, p. 36 - 44. Especially pro~ed to be the ammonium salts or alcali me~al salts o~ the sul~o succinic acid diesters, the alcohol componsnt of which is an alkyl radical, pre~erably branched, having from 5 to 15 carbon atoms, pre~erably ~rom 8 to 12 carbon atoms. The same emulsi~ier can be used ~or the 29 pretreatment ~f the membrane and for the emulsion-polymerization - i , , 1all5~37 o~ the latex to be ~iltered, or it i~ al~o poss~ble to use a different emulsifier of the afore described groups, or their mixtures.
The pH of the treatment solution is not of critioal ~m-portance. It has to be ad~usted in such a way that the ~ -:
membrane ma~erial is spared, the operations are generally -carried out at pH values ranging from 4 to 10.
According to the in~ention a solution, pre~erably an a~ueous solution, is prepared of the emulsifier or o~ the e~ulsifier mixture containing from 0.5 weight % to 70 weight %
- of emulsifier, preferably ~rom 095 to 40 weight %, especially from 1 to 10 weight %0 . Prior to their use ~or ultra filtration the membranes are put in co~tact to these solutions for at least half an hour, preferably for a period of from half a~ hour to 48 hours, especially from 10 to 30 hours, for example by immersing the membranss in a vessel replete with the emulsifier solutio~.
It i~ useful to -treat the support m~terial o~ the membranes at the æame time. The temperature at which the emulsifier treat-ment is carried out has its lower limit set by the freezing point of the emulsifier solution and its upper limit by the heat resistance temperature of the chosen membrane material.
Generally, it is possible to operate at about from 0C ~o about . 80C, the treatment is preferably carried through at tempera-ture~ ~rom 20 to 50C.
So as to keep small the stagnant partition layer at the membrane, the flow speed of the latex along the msmbrane set should be at least 0.5 m pe~ second~ preferably a speed of from 2g 0.5 to 5 m per second should be ohosen9 especially from ~ to 3 m ' - , .

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per second. The upper limit of the speed is given by pressure lo~s ~nd shear gradient. The ~iltring pressure i5 ~rom 0.1 to 6 bar7 preferably from 0.~ to 2 bar, above the pressure on the filtration product face. In a pre~erred embodiment the latex to be concentrated is directed in a circulation movement.
The process according to the invention substantially preve~ts the formation of solid deposits on the membrane, but it does ~ot succeed in avoiding it entirely. However, the thin - covering which is forming on the membrane has a soft and fl~y 1~ co~sistency and can be rinsed o~ easily with water; this cover~ng does not clog the membranes in a~y way or encrust the ~urface of the membranes, such as it occurred in the past as a consequence o~ the concentration polarization. When it is desirable to carry out the process of the invention con-tinuously, the membrane shou~d be separat.ed ~rom the lakex in intervals of ~rom about 24.to 240 hours of ultra ~iltration period and ri~sing with water should take place in the opposite direction to the latex flOwg the flow speed of the water ~hould be at least 0.5 m per ~econd.` The thin a~d fluffy covering o~
the membrane is rinsed of~ in a few minu.tes and the ultra filtra~ion process can be continued. I~ the permeability of the membrane should abate after a prolonged ~iltering period, a repition of the pretreatment of the membrane as per the invention is recommended.
ThP process according to the invention offers a simple method for concentrating continuously aqueous la-~ices which are formed upon emulsion polymerization o~ vinylchloride or of vinylchloride and up to 40 weight %9 preferably from 1 to 20 29 weigh~ ~ calculated on the total monomers - of monomers ~ '~

.' copolymerlzable herewith, so that polymer contents o~ up to 80 weight % are attainedO By copolymeri~ation i~ to comprise also the graft polymerization - the graft polymers should contain at least 70 weight %, preferably at least 80 weight % -calculated on the graft polymer - of v~nylchloride unit~.
Suitable monomers are ole~inicall~ unsaturat.ed ~ompounds such as they are described by US patents nos. 3 663 520 and

3 691 080~ especially Yinyl esters o~ straight-chain or branched carboxylic acids of ~rom ~ to 209 preferably from 2 to 4 carbon atoms, e.g. vinyl acetate, vinyl propionate, Yin butyrate, vi~yl 2-ethylhexonate, vi.nyl fumarate, vinyl stearate;
vinyl ethers ~uch as ~inyl methyl ether, vinyl dodecyl ether;
furtherm~re unsaturated monocarboxylic acids such as crotonic acid, acrylic acid, methacrylic acid and the corresponding eæters with alcohols of from ~ to 10 carbon atomsS e.g methyl ester, butyl ester or octyl ester; moreover9 unsaturated di-carboxylic acids such as maleic acid, fumaric acid, itaconic acid, their anhydrides, imides and esters (the latter with alcohols having ~rom ~ to 10 carbon atoms). ~urther useful comonomers are acrylonitrile, aromatic vinyl monomers such as ætyrene; d~ole~ins such as ethylene, propylene or butylene;
vinylidene halides, e.g. vinylidene chloride. For carrying out the graft polymerization there may be used especially diolefins such as butadiene9 chloroprene and cyclopentadiene.
The monomers are used as such or as mixtures of each other.
Concent~ated latices are needed as such for many purposes, especially for coating~ If they are subject to a ~urther treatment such as a drying process, e.g. spray drylng~ much 29 less energy is needed, since a ~maller quantity of water has - _ g _ .

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The prooes~ according to the in~ention includes a further advantage. Namely, the speclfied latices usually contain also solutions of ~arious low-molecular auxiliary materials such as emulsifiers, acti~atorsg buffer substances which are used ~or the polymerization GenerallyJ the emulsi~iers are the same as those specified page 5, par. 4. When polymerizing the latices there may be used e.g. from 0.001 to 3 weight %, preferably from 0.1 to 0.3 weight %, calculated on monomers of radic~l-forming activators, for example persulfates such as potassium persulfate, sodium persulfate or ammonium persulfate~
hydro~en peroxide, tert.-butyl hydropsroxide, cyclohexa~one perox.lde or other hydrosoluble peroxidic compounds such as perborates, percarbonates, perphosphates as well as mix-tures f various acti~ators these activators may also be used in the presence of from 0.01 to 1 weight % ralculated on mono~
mers - of one or several reducing agents which are suitable for building up a redox-catalyst system, such ~ sulfite~, bisulfites, dithionites7 thiosul~ates, aldehyde sulfoxylates, e.g. formaldehyde-sulfoxylate, ascorbic acid. The poly-merization may possibly be carried through in the presence o~
from 0.05 to 10 ppm - calculated on metal per monomers - of ~oluble metal salts, e.g. of copper, of sil~er, of iron or of chromium. When using the membranes submitted to a pre-treatment according to the lnvention t it iS possible to - æeparate from the polymer a part of the low~molecular sub ~tancesO This may be important for the quality o the product.
For example, polymers for foodstu~fs wrapping material and for 29 more transparent sheets ~hould preferabl~ contain a reduced H

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rate of an emulsifier. Furthermore it is possible to recycle into the polymerization the auxiliary materials belng recoYered in the filtration process.
The process according to the invention is not strictly limited to the concentration of said latices 9 but it may also be applied for emulsion polymer latices made of other monomer~, e.g~ of Yinylidene chloride, ~inyl acetate and mix~ures of these monomers of these monomers o~ each other and with other copolymerizable monomers.
The following e~amples illustrate the i~e~tion:
E X A M P L E 1:
A vessel contains 40 m3 o~ a polyYinyl chlorlde latex .
which was prepared cont.inuously by emulsion polymerization5 contianing 45 weight ~ of solid matter and havi~g a K-value o~
th~ polymer of 54. 70 m~ of latex per ~our were circulation~
contacted by means of a pump with a membrane of polyme~hyl-methacryla~P having a si~e of 12m2 and being constructed asym- -me~rically such as it is commercially aYailable. The partition cut of this membrane is at a molecular weight of 24 000. The membrane is bonded on both sides on 30 pla-tens of a porous æupport fabric o~ 120 mrn heigh~ and 1700 mm length, The dista~ce from one platen to the next was 3 mm, the ~low speed of the latex between the platens was 1,80 m/sec~ Prior to its first use the membrane was immersed ~or 12 hours ln a æolution f 5 weight % of di-isodecyl-sulfosuccinic acid ester (Na salt) in water. The ~lltration pressure o~ the latex in the ultra ~iltration device was adjusted to 2.5 bar by means of a throttle, the filtrate could escape freely flowing wi-thout coun~er-29 pressure. The yield per hour wa~ 52 l of ~iltrate duri.ng the . . ..
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_~ 7~_91 ~OS~IL37 first 24 hours) a~ter ~urther 24 hours the filtration output fel~ to 45 l/h. The ~eeding of latex to the membrane was interrupted after 120 hours. Visual inspection of the membrane at that moment showed a soft polyvinyl chloride covering whic~
could be remo~ed ~asily by rinsing with water. The membrane was then rinsed ~or two minutes with l m3 of water in the opposite direction to the latex flow. Subsequently, the apparatus wasagain ~ed wlth latex as per the above mentioned . description. The ~iltration output was again 52 liters per hour. At the end of about 10 days the sol~d matter contents o~ the latex had increased from-45 weight % to.60 weight %.
The ~ ration product was clear a~d contained 0.5 weight % of dissolved auxiliary materials but iio polyvinyl chloride.
E~en after 10 ~ays the membrane was oo~ered with a thln, soft layer of poly~inyl chloride only wllich could be easily removed by rinsing with water. The latex concentra~ed to 60 weight %
of solid matter was dried in a spray drying device to yield polyvinyl chloride powder~ The discharge o~ solid matter was more than twlce the discharge obtained upo~ drying the non pre ooncentrated latex, a~ the same evaporation capacity of the drying device.
E X A M P L E 2~
The latex obta~ned according to example 1 containing 60 weight % of solid matter was subject to ~urther concen-tration in the apparatus described ~y example 1. A red~ced filtration output of 36 l/h was determined. After a treatment of about 120 hours the latex contained 70 % of solid matter~
This latex was still capable of being pumped and sprayed in 29 the spray drying device. At the same evaporation capacity o~

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IL3~7 the dryer the solid matter discharge was about three times higher than ~he rate obtained by spraying the initial latex obtainable by polymerization and containing about 45 ~ of solid matter. Despite the prolonged treatment period the polyvinyl chloride covering o~ the membrane was not thicker than the one described by example 1~
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A vinylohloride graft polymer latex prepared by continuous emulsion polymerization of 92 parts of ~inylchlorlde, 7 parts of butadiene and 1 part o~ acrylonitrile, containing 45 weight %
of solid matter and having a K-value o~ the polymer of 60, is concentrated. in a tubular membrane (so-called tubular module) at an inside diameter o~ 24 mm and a length of 3000mm. The membrane of the tubular-module consists of polyamide and has a partition cut at a molecular weight of 40 000 (numerical average).
Prior to be put to use the membrane i~ immersed in a 3 %
aqueous solution of a sodium alcane sulfonate for 10 hours at 20C. This alcane sulfonate has the ~ollowing chain length distribution: up to C14 : 9 weight %, C14 : 27 weight %~
C15 : 34 weight %, C16 : 22 weight ~0, above C1~ : 8 weight %0 Approximately one fifth of all 21kanes is bi-sul~onated. The latex pressure in the membrane i~ 3 bar, the flow speed o~ the latex is 2 meters per second. The measured ~i].tration output i~ 3 1 of ~iltrate per m of membrane sur.~ace and hour.
The latex is concentrated up to 50 ~eight ~ of solid matter.
After 100 days of contlnuous operation~ while the spraying recycle is carried out according to the description of example 1, 29 the tubular membrane is ~erely covered wlth a thin so~t layer of .

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polyvinyl chlor~de which can be easily removed by rinsing with water.
E X A M P L E 4:
A vinyl chloride copolymer latex co~taining 35 weight % o~
solid matter a~d having a K value of the polymer o~ 70 and which i8 prepared by continuous emulsion polymerization of 95 parts of vinyl chloride and 5 parts of vinyl acetate is concentrated in the same apparatus as per the descriptlon given by example 1 Prior to its use the membrane of the apparatus had been treated for 12 hours at about 20C with an aqueous 5 weight % solution f of the sodiu~ salt of diisodecyl sulfosuccinic acid ester~
The filtratioll pressure o~ the'latex is adjusted to 277 bar9 the filtrate can escape freely flowing without counter-pressure.
The filtration output is 95 liters per hour~ A desoreasing *iltration output could not be observed until the desired final rate of solid matter contents, namely 45 weight %, was achieved.
The apparatus is operated continuously for 100 days, while the same spraying recycle as described by example 1 i~ carried out.
At the end of this period the membrane i~ merely co~red with a thin soft polyvinyl chloride layer which can be easily re-moved by rin~ing with water.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the concentration of a latex obtained by emulsion polymerization of vinyl chloride or emulsion copoly-merization of vinyl chloride and at least one monomer copoly-merizable therewith the resulting polymer containing at least 60 weight percent vinyl chloride, in which a semi-permeable membrane formed from a synthetic polymer and having a partition cut at a molecular weight of from 5,000 to 100,000 is treated for at least half an hour with an aqueous solution containing from 0.5 weight percent to 70 weight percent of at least one emulsifier which is suitable for use in the emulsion polymerization or copolymerization of vinyl chloride, and the latex is passed through the treated semi-permeable membrane at a filtering pressure of from 0.1 to 6 bar and a flow speed of at least 0.5 m/sec.

2. A process as claimed in claim 1 in which the semi-permeable membrane has a partition cut at a molecular weight of from 10,000 to 50,000.

3. A process as claimed in claim 2 in which the membrane is formed from polyacrylonitrile, polymethylmethacrylate or a polyamide.

4. A process as claimed in claim 1, claim 2 or claim 3 in which the membrane is treated by immersion in an aqueous solution containing from 1 to 10 weight % of the emulsifier for a period of from half an hour to 48 hours.

5. A process as claimed in claim 1, claim 2 or claim 3 in which the emulsifier used to treat the membrane is an ammonium salt or an alkali metal salt of a sulfosuccinic acid diester, the alcohol component of the diester being a branched alkyl radical having from 8 to 12 carbon atoms.

6. A process as claimed in claim 1, claim 2 or claim 3 in which the latex is passed through the membrane at a flow speed of 0.5 to 5 m per second.