CA1059690A - Vinyl halide stripping process - Google Patents

Abstract

VINYL HALIDE STRIPPING PROCESS

Abstract of the Disclosure Vinyl halide polymers and copolymers are formed in the presence of a linear, branched chain or cyclic alkane, and the polymer is then heated to remove the alkane and unreacted vinyl halide therefrom. The polymers so formed have reduced amounts of vinyl halide therein and hence are less likely to contaminate the air with airborne vinyl halide.

Description

59 6~ 0 The present invention relates to the polymeri~ation of .- .,~
vinyl halides, particularly vinyl chloride, and to a method and : means for reducing the content of unreacted vinyl halide in the polymer.
The polymerization of vinyl halides, such as vinyl chloride, to form a homopolymer of the vinyl halide or a copolymer thereof ~ith a copolymerizable monomer,such as a vinyl ester, an ol~fin, etc., is well Icnown. TD prepare polymers that are readily recoverable in particle form, as contrasted to latex form, suspension, solution or bulk polymerization techniques employing a catalyst or initiator are used. The polymerization is effected under pressure and elevated temperatures, e.g.
120-140F., are developed. At the end of the reaction, the pressure vessel used as the reactor is vented and the pressure is rapidly lowered to sub-atmospheric pressure to recover `
unreacted vinyl halide, and then the slurry of polymer is ': ~ nl-ri f~ .or1 ~nr~ ~r; .o~ :
These techniques have been quite effective in the past, ` but as of Januar~ 1, 1975 very stringent Federal regulations will impose very low levels o~ airborne vinyl chloride monomer that are permissible. Polyvinylchloride (PVC) containing large amounts o~ unreacted vinyl chloride i~ thus undesirable, since it ~ay ;
act to increase the airborne vinyl chloride content in areas where the PVC is stored or processed, thus necessitating costly measures to make those areas conf~rm to the Federal standards.
It is thus an object of this invention to provide a methocl for reducing the content of unreacted vinyl halide monomer in a polymer of a vinyl halide.

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Thls ob~ect ~ accomplished by th~ presen~ inventlon by polymerizing the vinyl halide in the presence of an alkane D
and heating the polymer thus obtained to remove unr~acted vinyl -~
halide and copolymerizable monomer, It has been found that the pre~ence of the alkane during the polymerization allows removal of ~reater quantities of unreacted vinyl h~lide and othe~ monomer. It is believed that the alkane enters the oily phase containing the vinyl halide and ~hu~ is phy~ically trapped inside the vinyl halide polymer, Removal of the alkane from ~he polymer also resul~ in stripping additional amounts of unreacted vinyl halide therefr~m~ perhap~
due to an inereased porosity of the polymer ari~ing from ~he use of the alkane.
Quite surprisingly, two further i~po~tant effect~
have been noted. Firs~, the polymer re~ulting from the stripping has enhanced dry blending properties, i.e. it will blend more rapidly with monomeric and polymeric plasticizers, pig~ents and other additives in conventional dry blending equipment, such ~s Banb.ury mix~rs, etc. In addition, it is posæible to carry the .
~0 polymerization ~o h~gher yields while still retaining the dry blending properties. Thus; in prior art polymerizations of vinyl halide~, the reac~ion is terminated shortly ~fter the pressure in ~he reactor ~tarts to drop, and a pre~ure drop of S psi is conventionally employed to maximize the dry blendin~ properties o~ the polymer. I larger pressure drops are used, the try blending properties of the polymer ~re adversely affected. By ~he use of the alkane in the polymeriza~ion, prxssure drops a3 much as 40 p8i can be accepted whlle still obtaining a polymer with good dry b~nding properties, ~ 3 -: . . ' ; .. . . ; ' ' ~ ~ , !"

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The present invention i~ operable in any vinyl halidepolymerization technique that i8 employed to give recovery of polymer a~ particulate solids, namely su~pension, solution or bulk polymeri8ation~ In all of these teehniques, elevated pressure and an initiator are employed. The ~details of these polymerization techni~ues will not be set forth here~n as they are well known and form no part of the present invention. Furthermore, the vinyl halide may be polymer~zed alone to form a homopolymer, or may be polymerized with up to 100% of a copolymerizable monomer, ba~ed on the vinyl halide, containing an ethylenically unsaturated double bond, e g. vinyl e~tar~, vinylidene halides, acry~onitrile, or olefins.
U.S. Patent 3,324,097 d~scribes an emulsion polymer-ization of ~inyl chloride to obtain vinyl chloride latices, and propo~es the use of hydrocarbon~ of at least 8 carbon atoms to prevent wet polymer build-up in the reaction vessel. In such a process, the polymer is recovered as a latex, which is not amenable to heating under reduced pre~sure for removal of unreacted monomerO
The alkane is used in an amount s~fficient to reduce the conten~ of unreacted vinyl halide monomer in the polymer, e.g. rom 0.1 to 10% by ~eight, based on the weight of the ` :
vinyl halide an~ copolymerizable monomer, preferably from 0.5 to 3% by weight. Since the alkane i8 ~tripped off after ~he polymerization, there is actually no upper limit on the amount to be employed, except that quite large amounts, e.g. more than 15%, may give ri~e to odors in the resin. Since there is no point in using more of the alkane than i8 necessary, eeonomics will dictate the use of smaller amounts where possible. : :
The alkane is pr~ferably added along with the reactants charged ~o ~he pol~merization vessel. If desired, however, the alkane may be added in a manner known per ~ during the cour~e of the polymerizationO

. ' ' ' ~5~369() Suitable alkanes for u~e in the invention are straight, branched chain or cyclic alkane~, and ~he term "~lkane" as used herein and in the appended claims includes straight, branched and cyelic ~tructures. Any alkane may be u~ed, prefer~bly those that can be removed from the polymer at the stripping conditions, such as alkanes of 1 to 20 carbon a~oms, preferably 1 to 15 carbon atoms, and most preferably 5 to 10 carbon atoms. While straight, branched or cyclic alkanes may be employed, it i~ presently pre-ferred to use normal or secondary alkanes, such as n~pentane~
i-hexane, n-heptane and ~-octane.
The alkane is removed from the polymer at the end of ~he reaction by heating the polymer containing the alkane and unreacted monomer at a tempe~a ture of at least about 160F. at reduced pre~sure for a suitable period of time to ensure maximum removal of the alkane and unreacted monomer from the slurry obtained from the polymerization, e.g. 15 to 180 minutes. Here again, economic~ will dictate the ultimate selection of operating parameters.
For example, the polymer is usually at an elevated temperature a~ the end of the reaction~ say abou~ 110 to about 155F., and hence a s~ripping temperature of at least about 160~F~
will be used. Temperatures closely approaching 230F. tend to adversely affect the dry blending proper~ies of the polymer and would not normally be employed. However, if the advantages of using temperatures higher than 230F. were justified from the overall economics, then such temperatures could be used. Normally, ho~ever,a ~ripping temperature of at least about 160~.
preferably ~o provide an increase in temperature of at least about ~ 05~6~90 15 F., such as about 160F. to abou~ 200~F., e.g. 160F. to 180F., may be employed to advantage.
Generally~ the efficienc:y of the stripping is an r inverse function o the pressure, and hence it ls preferred to u6e reduced pressure. Here again,, the degree of vacuum used depends on a trade~off of increa~ed ~tripping efficiency versus the cost of vacuum equipment. A suitable range of pressure is from about 50 to about 200 ~n Hg, and most preferably 75 to 150 mm Hg.
After s~cripping the alkane and unreacted vinyl halide from the slurry, ~he pressure is brough~ back to atmo~pheric and the slurry i~ centrifuged to remove water from ~he slurry9 and then the polymer is dried in a conventional mannerg e.g. at a tempera~ure of 125F. to 350F. and atmo3pher~c pre~ure.
In ~he most preferred embodiment of the invention, the reaction maS8 obtained from the polymerlzation i~ sub~ected to reduced pre~sure to recover unreacted vinyl h~lide monome~ and to remove ~ater vapor (where present) and then the p~lymer i~ heated ~o a temperature of at least about 160F.~ preferably about 160 to about 200F., at a pre~sure o~ from 50 to 200 mm Hg, for s~ch time as ~o effect significant removal of unreacted vinyL halide from the polymer, such a~ from about 15 minutes to 180 minute~.
After the stripping, the pre~ure i~ brought back to a~mospher~c and the polymer i8 sent to c~ntrifugation and dryin~.
The present invention finds particular utility in a ~uspension polymerization, since this is normally used to prepare re~in~ that are suitable for dry blending. A150, su~pen-~ion polymerization accounts for the large~t p~sduction of vinyl res~ns. In a ~uepen~ion polym~rization, a pres~ure vessel 1~
charged with w~ter and the monomer or monomer mixture, c~talyst, protective colloid and other ingredlents are added to the water and the polymerization reaction is allowed to proceed ~t the desired tempera~u~e9 with cooling if necessary to main~ain the temperature. The degree of completion i~ determined by the amount of pre~ure drop béfore the reaction i~ terminated, as described above.

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Charged to the reactor is a mixture generally comprising:
Parts Vinyl halide and copolymerizable monomer 100 Water 100-400 Dispersing agent (protective colloid) 0.03-0.3 Catalyst 0.01-0.15 Other recipes may be used, as described in the art. Suitable dispersing agents include the water soluble pro~ective colloids such as gelatin~ partially or completely hydrolyzed polyvinyl acetate, polyalkylene oxides such as polyethylene oxide9 methyl cellulose or hydroxyethyl cellulose. Suitable catalysts include diacyl peroxides such as iauroyl and benzoyl peroxide; peroxy-dicarbonates such as diisopropyl peroxydicarbonate; acyl cyclo-alkyl sulphonyl peroxides such as acetyl cyclohexyl sulphonyl peroxide; and azo compounds such as a,a'-azodiisobutyronitrile.
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A~ter the reaction has proceeded to the desired degree, the reactor is vented to remove water vapor and unreacted vinyl halide and other monomer, and the pressure is reduced to the desired degree of vacuum. To heat the polymer, a heated jacket or introduc~ion of steam into the reactor or any other sui~able heating may be used. When the stripping of the alkane and unreacted monomer is completed, the vessel is brought ~ ;
back to atmospheric pressure and the stripped slurry is sent ~o 'r drying storage, blendin~, etc. just as in the case o~ convention~l vinyl polymers.
The Examples that ~ollow also illustrate the invention in terms o~ a suspension polymerization, bu~ the scope o~ ~he invention is not to be limited thereto.

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- 1~59690 In the specification and appended claims, all parts and proportions referred to are by weight, unless othe~ise noted. Further3 the percentage of alkane in Examples I V is based on the weight of vinyl halide and copolymerizable monomer~ -and the vinyl chloride content in ppm is based on dry solids.
Example I
Several production runs in a commercial PVC reactor were made by charging to the reactor for each run the following mixture:
Demineralized water 1100 gallons Vinyl chloride 760 gallons IPP * 1 pound `~
* Methocel 60 HG 50 * 3.5 pounds Sodium bicarbonate (buffer) 1.0 pound * IPP is diisopropvl peroxy dicarbonate * Methocel 60 ~G $0 is a me~hyl cellulose derivative manufactured by Dow Chemical Company These runs served as controls. Additional runs were made using the same mixture but including 57.5 pounds of Soltrol 10, a mixture of 90-95~/O isooctanes and the remainder other isoparaf~ins.
Tne runs were carried out by heating the reaction mass to 131F, and the pressure rose to about 115 psig. After 8-9 hiours, the pressure fell to l:L0 psig. and the reaction was terminated by venting the reac~or. After ventin~, the pressure was reduced to -10 inches Hg and s~eam was admitted to the reactor to heat the polymer mass to 180F. During steam addition, the vacuum was increased to -25 inches Hg, and the polymer mass held at 175-180F. at this pressure for 30 minutes, a~ter ~hich time the ,1 .

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vacuum was broken and the slurry ~ent to centrifuga~cion and drying. Samples of the wet cake atld fully dried resin for each run were obtained for each run, and the resul~ are rsported in Table I below. The results are re~ported as an av~rage of several runs o each ~ype.

Table I_ Control I v_ntion Isooctane (70 of vinyl chloride) O
Stripping Temp, (F~) 180 180 Stripping Pressure (in. Hg)-25 -25 St~ipping Time (rnin.) 30 30 Vinyl Chloride (ppm) wet cake 224 28 dry cake 50 2 . 7 DOP dry-up time (min . )4 . 0 3 .1 TOTM dry-up time (lliin.) 706 4.1 The DOP and T~TM dryoup tlmes are a measure of the dry blend~ng .
propertie~ of the polymer and are dete~ined as iEollow~. In both tests~ a charge compri~ing the resin, stabili~er and filler i~ mixed in a sigma head miKer of 650 ml eapaei~y t 87Co + 0.1C.
for 5 minutes, after which ~ime dioctylphthalate (DOP) or trioctyl-tr~mellltate (TOlM~ is added over a period of 1 minute.
The mixer is conr;ected ~o a Brsbender pla~tico~der and ~he transition point ~om a werc lumpy mixture to a dry-free flowing mixture i3 read from the plasticord~r chart. The DOP or T~TM
dry-up time is r~ported a~ the time in minutes from the addition of the DOP or TOT~I to the tran~itlon point.

~ ` . " ~ ' . ' ' ' The charge used for the DOP dry-up time i~
210 gr~m~ resin 21 grams basic whiLte lead carbonate 15 grams No. 33 109 ~rams DOP, The charge :for the TOTM dry-up time is:
200 gramæ re~in 8 gram~ epoxidized soybean oil 18 grams diba~ic lead ph~halate 1 gram~risto wax 165 20 gram~ clay No. 33 96 gram~ TOl'M
A~ can be seen from Example I, there is a remarkable reduction in t&e ~mount of unreacted vinyl ch1oride when the alkane is u~ed, a~ well a~ an in~prov~ment in dry~blending propertie~. Sa[~ple~ of the con'cro1 and lnveneion run were pl~ced in gla8~ stoppered Erlenmeyer 1a~k~ and heated at 65C. for one hour. On opening the flasks9 no odor was de~ected.
Ei~h~ceen tho~sand po~md~ of the polymer prepared using the alkane were dry-blended with a commercial blend of pigmenk~, etc. using production techn~ques, and a commerc~ally acc~ptable dry blend wa3 pr~pared, . . .
E~LE I~
Following the pro~edure of E~c~mple I, ~ix addi~lonal runs were made, Runs 1 and 2 emp~oying no isooctane and hen~
being con~rol run~" and Runs 3-6 using 1% i!~ooctane and hence * Trademark - 10 - , , r ~ 105~691~
i illustrating the invention. Runs 1 through 3 employed a 5 psi pressure dropg which is the pressurle drop most commonly used in the production o~ dry blending vinyl chloride homopolymers.
Runs 4, 5 and 6 employed pressure drops of 10, 15 and 20 psi, respectively, and thus resulted in subs~antially greater yield of polymer. The polymer obtained fxom the six runs ~as analyze~
~or unreacted ~inyl chloride in the wet calce and the dry blending properties. The results are reported in Table II below.
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- Table II
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Run 1 2 3 4 5 6 Pressure Drop - `
(psi) 5 5 5 10 15 20 Vacuum Strip Temp. (F.) 180 180 180 180 180 180 Vacuum Strip Time (min.) 30 30 30 30 30 30 Iso-octane (%) 0 0 Vinyl chloride (ppm) ~et cake 245 289 37 164 75 232 DOP dry-up time 1 1 2 2 2 2 (min.) 3.7 4.0 2.3 2.6 2.5 2.7 TOTM dry-up time (min.) - 6.7 4.2 5.8 5.5 4.5 1 - Standard _ 3.3 min.

2 - S~-ndard 2.6 min.

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It can be seen ~rom the data in Table II that the strlp~
ping tempera~u~e D~ 180 ad~ersely a~fected the dry blending properties of Runs 1 and 2, which represented the control runs, since these runs had a longer DOP dry-up time than the standard of 3.3 minu~es established for the polymer made under the ambient conditions of that day. All of Runs 3 through 6 resulted in PVC
that had substantially better dry blending properties than the standard used for the specific conditions of runs being made in the period of Runs 3 through 6. This is true even for Runs 4 ;
through 6, which carried the reaction toward the higher yields represented by the large pressure drops. This is contrary to expectations, since normally the combination of higher drying temperatures and higher yie7d is accompanied by a loss of dry blending proper~ies.
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Exam~le III -Following the procedure of Example I, a vinyl chloride vinyl acetate copolymer was prepared from the mixture below, using a polymerization temperature of 150F. and a vent pressure of '-~0 psig:
Vinyl chloride91 pounds Vinyl acetate 9 pounds IPP 4.5 grams ~auroyl peroxide68 grams Gelatin (100 Bloom)159 grams Sodium bicarbonate23 grams ~~-c~'u;ueL-,la,le272 grams Water 160 pounds `

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Three runs w~re made. In Runs 1 and 2 the above mixture ~as used, and irl Run 3 the mixture included l7~, of isoostane.
After the reac~ors were ventedJ the polymer mass wa~ kept a~c the polymerization temperature in the case of Run l~ or heated to 1~5F. in the ca3e of Run6 2 and 3, to ~rip 8a~e$ and unreac~ed vinyl chloride and vinyl acetate ~rom the polymer. A ~trip ~ime of 120 minutes wa~ used for all t~ree run~ ~t a pressure of -25 inehes ~18. The unreacted vin~l chloride ~a~ me~ured in ~he wet cake and dry eake as de~eribed above, and the re~ults are 10 reported in ~able III belowO

T~ble III

Run~ 2 3 Isooct~ne (~/0) 0 0 Stripping Temp. (F~ 150 165 165 Stripp1ng Pre~s, (inO Hg) -25 -25 -25 Stripping Time (min. ) 120 120 1 20 Vinyl Chloride (ppm) wet ealce 4204 1284 376 dry cake 330û 1~35 240 Following the procedure of ~xample I~ three b~tche~
of PVC w~re prepared at a polymerization temperature of 145 and a vent pr~ssure of 120 p~ig. Run l was a con~rol, and Runs 2 and

3 employed 1% n-hep~cane. The unreae'ced vinyl chloride content in the wet and dry c~ke were determined, and the resu1ts are repor~ed in Table IV belo~7.

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Table IV
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Run 1 2 3 - n-heptane (%) O
Stripping Temp. (F.) 145 145 170 S~ripping Press. (in. Hg) -25 -25 -25 Stripping Time (min.) 60 60 60 Vinyl Chloride (ppm) `
wet cake 13816 2651 486 dry cake 7496 946 322 ,' ExamPle V
Following the procedure of Example 1, two batches of vinyl chloride homopolymer were prepared using a polymerization temperature of 131F. and a pressure drop of 30 p5i. Run 1 contained no alkane and Run 2 contained 1% o~ iso-hexane. At the end of the reaction, the polymer was heated to a tempera~ure of 160F. pressure of -25 inches Hg for 60 minutes. The unreacted vinyl chloride content was determined in the wet and dry cake and the DOP dry-up time was also determined for both runs. The results are reported in TabLe ~ o^low.

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5~ 690 Table V

Run , 1 2 Iso-hexane ~%) O
Stripping Temp. (F) 160 160 Stripping Press. (in. Hg) -25 -25 Stripping Time (min.) 60 60 Vinyl Chloride (ppm) wet cake 1350 340 dry cake - 2~4 60 DOP dry-up time (min.) 5.8 4.5 , ~
As can be seen ~rom the above Examples, the use o~
the alkane in reducing the content of unreacted vinyl chloride in the polymer is not limited to vinyl halide polymers employed for dry blending only. Thus, the present invention is applicable to the production o~ vinyl halide homopolymers and copolymers that are I-ormed into thick sheets or thin ~ilms, that are injection molded or compression molded or eætruded into articles of desired shape9 and to such homopolymers and copolymers that are blended with the usual range of additives by dry blending or by any other technique. While the use o~ the alkane does improve the dry blending properties of such resins ~hat are ultimately dry blended, there is a significant reduction of unreacted vinyl halide in the vinyl halide homopolymers and copolymers, xegardless o~ whethex or not such polymers are used f~r dry bJ~ending.

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Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the suspension, solution or bulk polymerization of a monomer system selected from the group consisting of a vinyl halide and a mixture of a vinyl halide and up to 100%, by weight of the vinyl halide, of at least one copolymerizable monomer under elevated pressure in a pressure vessel to form a polymer thereof in solid form, the improvement which comprises effecting the polymerization in the presence of an alkane in an amount of from about 0.1 to about 10% based on the monomer system, venting said vessel to atmospheric pressure at the termination of the polymerization, and thereafter stripping unreacted vinyl halide from said polymer by heating the polymer to a temperature of at least about 160°F and at a pressure of about 50 mm Hg to about 200 mm Hg.

2. The process according to claim 1, wherein said polymerization is the suspension polymerization of said monomer system.

3. The process according to claim 1, wherein said alkane has 1 to 20 carbon atoms.

4. The process according to claim 3, wherein said alkane is a normal or secondary alkane.

5. The process according to claim 4, wherein said alkane has 5 to 10 carbon atoms.

6. The process according to claim 5, wherein said alkane is i-hexane, n-heptane or i-octane.

7. The process according to claim 1, wherein said stripping is at about 160°F to about 200°F.

8. The process according to claim 1, wherein said stripping is at about 160°F to about 180°F.

9. The process according to claim 1, wherein said heating is at a temperature at least 15°F higher than the temperature of the polymer after said venting.

10. The process according to claim 1, wherein said alkane is present from the start of the polymerization.

11. The process according to claim 1, wherein said polymer is a homopolymer of vinyl chloride or a copolymer of vinyl chloride and up to 100% by weight of the vinyl chloride of one or more copolymerizable monomers.

12. The process according to claim 11, wherein said polymer is a copolymer of vinyl chloride and a vinyl ester.

13. The process according to claim 1, wherein said polymerization is the suspension polymerization of vinyl chloride under elevated pressure in a pressure vessel to form a vinyl chloride homopolymer, the polymerization is terminated by venting the vessel to the atmosphere when the pressure in the vessel shows a pressure drop of between about 5 and about 30 psi, and the reaction product thus obtained comprising vinyl chloride homo-polymer is heated to a temperature of at least about 160°F at a pressure of from about 50 to about 200 mm Hg for stripping of said alkane and unreacted vinyl chloride.

14. The process according to claim 13, wherein said reaction product is subjected to said stripping in said pressure vessel.

15. The process according to claim 13, wherein said alkane has 1 to 20 carbon atoms.

16. The process according to claim 13, wherein said stripping is at about 160° to about 200°F.

17. The process according to claim 13, wherein said stripping is at about 160° to about 180°F.

18. The process according to claim 17, wherein said stripping is for about 10 to about 180 minutes.