CA2074995A1

CA2074995A1 - Method for the preparation of a vinyl chloride polymer

Info

Publication number: CA2074995A1
Application number: CA 2074995
Authority: CA
Inventors: Ichiro Kaneko; Tadashi Amano
Original assignee: Individual
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1991-08-02
Filing date: 1992-07-30
Publication date: 1993-02-03
Also published as: JPH0539309A

Abstract

ABSTRACT OF THE DISCLOSURE

An improvement is proposed for the suspension polymeriza-tion of vinyl chloride to give a resin powder having an increased bulk density and containing a decreased amount of fish eyes. The improvement can be obtained by using, as the suspending agent, three kinds of water-soluble polymers in combination, of which the first two, which are dissolved in the aqueous suspension medium already at the start of the polymerization eaction, are a specific partially saponified polyvinyl alcohol and a specific hydroxypropyl methyl cellulose and the third polymer, which is introduced into the polymerization mixture ata specified moment during proceed-ing of the polymerization reacion, is also a specific partially sapo-nified polyvinyl alcohol different from the first partially saponified polyvinyl alcohol, each polymer being added in a specified amount.

Description

2~7~

METHOD FOR T~IE PREPARATION OF A VINYL
CHLORIDE POLYP~ER

sAcKGRouND OF THE INVENIION
The present invention relates to a method for the preparation of a vinyl chloride polymer or, more particularly, to an i:mprove-ment in the me~od for the preparation of a vinyl c:lhloride polymer having a high bulk density and capable of giving a shaped article contai~ing little amount of fish eyes.
As is well k:alow:n, vinyl chloride-based polymeric resins are resins having excellent physical properties and widely used and consumed in large quantities as a material of either rigicl, i.e. un-plasticized~ or :flexible, i.e. plasticized, shaped a:rtic:les. Various methods are applicable to the preparation of shaped article~ of a vi-nyl chloride-based resin including calendering, extru~ion moldîng, injection molding and other~ depending on the type of the resin composition and the desired ~haped articles. In the molding pro-Ce95 of an un~lasticized vinyl chloride-based resin, in particular, it is desirable that the resin powder has a hulk density as large as possible in view of the increased efficiency thereby in the rate of extrusion out of an extruder machine.
Various proposals and atte:mpts have been made heretofore for the improvement in this regard in the preparation of a vinyl chloride based resin by the suspension polymerization of vi~yl chloride monomer in an aqueous suspensio:n medium containing a : ' ' ~ , ... . , .. , . . - . . . - . ~

. . . .

2~7~95 .~
water-soluble polymeric suspending agent including a method dis-closed in Japanese Patent Kokai No. 59 168008, in which a portio:n of the vinyl chloride monomer is additionally introduced into the polymerization mi~ture in the course of the proceeding polymeriz-ation reaction, a method di~closed in Japanese Patent Publication No. 1-27088, in wh;ch the suspending agent is a partially ~aponi~
ed polyvinyl alcohol of a high degree of saponification~ a method disclosed in Japanese Patent lPublication No. 1 2û7624, in which a portion of the suspending agent is additionally introduced into the polymeTization mixture in the course of the proceeding polymeri~-ation reactio~, and so on.
Although the above mentioned methods disclosed in Japa-nese Patent Kokai No. 59-168008 and Japanese Patent Publica tion No. ~L-27088 are iIldeed ef:fective in obtainirlg a res;n powder having a somewhat increased bulk density, these methods have a problem that the resin powder obtained thereby con$ains an in-creased amount of fish eyes or resin particles like glass beads to give defective shaped articles. The method disclosed in Japanese Patent Publication No. 1-207624 has no problems in respect of fish eyes but the method has a limitation when a further imcreased bulk density is desired of the resi~ powder.

SU~ARY OF THE INVENIION
The p~esent inve:rltion accordingly has an object to provide a novel and im~roved method for the preparation of a vinyl chloride-2 ~

based resin powder ha~ing an increased bulk dens;ty and capableof giving a shaped article containing little amount of ~lsh eyes.
Thus, the present invention provides an impro~ement, in a rnethod for the preparation of a vinyl chlo:ride polymer by the ~us-pensioIl polymerization reaction of vinyl chlor:ide monomer in an aqueous suspension m~dium containing a water soluble polymLeric suspending agent in the presence of a monomer-soluble polymer-ization initiator, which comprises using a combination of three kinds of water-soluble polymers as the sllspending agent includ-i:~g: .
(a) a ~lrst partially saponifiled polyvinyl alcohol having an averagedegree of polymerization in the range ~rQm 15û0 to 3000 and a de-gree of saponi~lcat;on in the range fro~ 75 to 85%;
(b) a hydroxypropyl methyl cellulose, of which the degree of substi-tution with metho~cy grollps is in the range from 26 to 30% by weight and degree of substitution with hydro~cypropoxy groups is in the range from 4 to 1~% by weight, giving a 2% by weight aqu-eous solution having a vi~cosity in the range from 5 to 4000 centi-poise at 20 C; and (c) a second partially saponi~led polyvinyl alcohol having an aver-age degree of polymerization of at least 1500 and a degree of sapo-ni~lcation of at least 86%, of which the total amount of the polymers (a) and (b) is in the ranges from 0.03 to 0.1 part by weight per 100 parts by weight of the ~Tinyl chloride monomer with a weight ratio of the polymer (a) to the polymer (b) in the range ~rom 2:8 to 8:2 and t:he amount of .;~,, . . - , . .
.
. .
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the polymer ~c) is in the ranges from 0AO1 tn 1 part by weight per 100 parts by weight of the vinyl chloride monomer, the water-solu-ble polymers (a) and (b) being contained in the aqueous suspension medium at the moment of starting of the polymerization reaction and the water-soluble polymer (c) bei:ng introduced into the aque-ous suspension medium at a moment when corlversion of the vinyl chloride monomer to polymer is in the range from 5% to 60%.

DETAILED DESCR~ ON OF TEE PREE13RRE:D EMBODIMENTS
A8 iS described above, the most characteristic ~eatuLre of the i~vention consists in the use of three kinds OI water-soluble poly-mers in combinatioll as a suspending agent while each o:f the wa-ter-soluble polymers is i~troduced into t:he aqueous susp~sion medium at a speci:fied moment. Namely, two of the three water-so-luble polymers, i.e~ (a) and (b), including the ~lrst partially saponi-fied polyvinyl alcohol and the hydroxypropyl methyl cellulose are contained in the aqueous suspension medium already at the mo-ment of starti~g of the polymerization reaction while the third wa ter-soluble polymer, i.e. (c), or the second partially saponi~led poly-vinyl alcohol is introduced into the suspension medium only at a moment when co:nversion of the morlomer mto polymer is from 5 to 60%. Preferably, the polymers (a) and (b) are used in a weight pro-portion i:n the range from 8:2 to 2:8 or, more preferably, fro~ 7:3 to 3:7. When the amount of the polymer (a) is too large relative to the polymer (b)s the resultant polyvinyl chloride resin would contain an increased amount of :~lSh eyes while, when the amount of the ~., .: , , ' 2~7~5 polymer (a) is too small relative to the polymer (b), the bulk den-sity of the resultant polyvinyl chloride resin powder cam~Lot be as large as desired.
The first partially saponi~led polyvinyl alcohol as the water~
soluble polymer (a) has an average degree of polymerization in the range ~rom 1500 to 3000 and a degree of saponification in the range ~om 75 to $5%. When the average dlegree of polymerizatio~
and/or degree of sapo~ cation thereof are too low~ fwll stabiliza-tion of the suspension as the pol~ymerization mixture cannot be ob-tained resulting in occurrence of coarse particles ;n the resin pow-der or a decrease in the bulk de~sity thereof while, when the aver-age degree of polymerization and/or degree of sapon;~cation there-of are too high, the content of ~lsh eyes in the resultant resin pow-der is somewhat increased.
The hydroxypropyl methyl cellulose as the water-soluble polymer (b) should have a degree OI substitutio~ with methoxy groups in the range from 26 to 30% by weight and degree of sub-stitution with hydroxypropoxy groups in the range from 4 to 15%
by weight and should give a 2% by weight aqueous solution having a viscosity in the range from 5 to 4000 centipoise or, preferably, from 5 to 1000 centipoise at 20 C. When the degree of substitution with methoxy groups and/or the degree of substitution with hydr-oxypropoxy groups are too low, full stabilization of the suspension as the polymerization mixture cannot be obtained resulting in oc-currence of coarse particles in the resin powder or a decrease in the bulk density thereof. When either or both of these parameters are .... ..

2 ~ 7'~

-~;
too high, on the other hand, the particle size distribution of the re-sultant resin powder would be somewhat broadened.
The total amount of the polymers (a) and (b) is in the range from 0O03 to 0.1 part by weight or, pre~erably, from 0.03 to 0.06 part by weight per 100 parts by weight of the vinyl chloride mo-nomer while ~e weight ratio of the polymer (a) to polymer (b) is in the range ~rom 2:8 to 8:2 or, pre~erably, from 3:7 to 7:3.
lE3esides the abo~Te described two water-soluble polymers ~a) and ~b), the improvement of the invention can be achieved by th use of a second partially saponi~led polyvi:nyl alcohol as the water-solulble polymer (c) which should have an average degree of poly-merization of at least 1500 and a degree of saponi~lcat;on of at least 86%. When the.average degree of polymerization a.nd/or the degree of saponification ar0 too low, the bull~ density of the resin powder obtained thereby caImot be increased as desired. The amount of this secolld partially saponi~led polyvinyl alcohol is in the range ~rom 0.01 to 1 part by weight or, preferably, from 0.02 to 0.5 part by weight per 100 parts by weight of the vinyl chloride mo-nomer.
It is essential that t:he above described water-soluble polymer (c)~ i.e. the second partially saponi~led polyvinyl alcohol, is intro-duced into the aqueous suspension medium only at a moment when conversion of the vin~l chloride mo~omer to polymer i~ in the range from 5% to 60% or, pre~erably, from 30% to 60%. When the moment of introduction of the polymer (c) is too early9 the par-ticle size distribution of the resultant resi:~ powder would be too ` ~ ~

~ ~ 7 '~

fime so as to decrease the bulk derlsity of the powder. When the mo-ment of introductio:ll thereof is too late, the bulk density of the res-in powder is also not so high as desired. The % conversion of the monomer into polymer can be determined with eonsiderable accu-racy by making reference to the pre-established relatio~ship be-tween the polymerization time, i.e~ the length of time elapsed from the start of the polymerization reactlon, and the monomer conver-sion. The water-soluble polymer (c) can be introduced inl;o the aqu-eous suspension medium continuously, intermittently or at one time during the above spec;~led period. It is a convenient way that this additional water~soluble polymer ~c) is introduced into the re-actor in the form of an queous solution prepared be~or~hand.
The improvement according to the in~rention can be obtained not only in the homopolymeri~ation of vi~yl chloride monomer alone but also in the copolymerization of a monomer mixture of vinyl chloride with one or more of comonomers copolymerizable with vinyl chloride provided that the mi:gture is mainly composed of vinyl chloride or, in particular, at least 50% by weight thereo:f iæ
~inyl chloride. The comonomers copolymeriable with vi~yl chlo-ride mclude a-ole~l~s such as ethylene, propylene, l-butene, l-pen-tene, l-hexene, l-heptene, l-octene, l-nonene? l-decene, l-unde-cene, l-dodecene, l-tridecene and l-tetradecene, acrylic acid and esters thereof such a~ methyl acrylate and ethyl acrylate, methac-rylic acid and ~sters thereof such as methyl methacrylate, maleic acid and esters thereof, vinyl esters such as vinyl acetate and vinyl propionate, vinyl ethers such as lauryl vinyl ether and isobutyl vi-2 ~ 7 ~

nyl ether, maleic anhydride, acrylonitri:le, styrene, vinylidenechloride and so on.
As in the suspension polymerization of vinyl chloride mono-mer in general, the polymerization reaction according to the in-vention proceeds in the presence o:f a monomer-soluble polymeriza-tion initiator exempli~led by percarbonate compounds such as di-isopropylperoxy dicarbonate, di-2-ethylhe~ylperoxy dicarbonate and diethoxyethylperoxy dicarbonate, perester cornpounds such a tert-butylperoxy neodecanoate, tert-butylperoxy pi~Talate, tert-hexylperoxy pi~alate, a-cumylperoxy neodecanoate and 2,4,4-tri methylpentyl-2-peroxy-2-neodecanoate, peroxides such as acetyl cyclohe~ylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-pero~cyphen-oxy acetate, 3,~,5-trimethylhexanoyl peroxide and lauroyl pero2~-ide, azo compounds such as azobis-2,4-dimethyl valeronitrile and azobi~(4-methoxy-2,4-dimethyl valeronitrile), and so on. These polymerization initiators can be used either s;ngly or as a combi-nation of two kirLds or more according to need. It is optional that the polymerizatio~ initiator is a combination of one or more of the mo~omer-soluble ones described above with a water-solub]le initia-tor such as potas~ium persulfate~ ammonium persulfate, hydrogen peroxide, cumene hydroperoxide and the like.
The amount of the polymerization initiator added to the poly-merization mixture can be conventional but it is usually in the range from 0.01 to 0.3 part by weight per 100 parts by weight of the vinyl chloride monomer or the monomer mixture. The poly-merization initiator can be inroduced into the polymerization :rnix-,. . . .
. - ~
' 2 ~

ture in any conventional maImer. Namely, it can be introduced thereinto either as such or in the ~rm of an organic solution pre-pared beforehand by using an organic solvent or in the form of an aqueous emulsion prepared by USiIlg a dispersing agent.
The procedure of suspension polymer:;zation of vinyl chlorid according to the invention can be performed in a converltional manner excepting the requirements that ths suspending agellt is a combination of the above sI)eci~led three different water-soluble polymers (a), (b) and (c) and that the third water-soluble polymer, i.e. (c), is introduced into the polymerization mixture at a specified moment during proceeding of the polymerizatioll reactlon while the other two water-soluble polymers, i.e. (a) and (b), are contained in the aqueous suspensio:n medilim already at the start of the polymerization reaction. The other conditions of the procedure can be conYentional including the way by which the polymerization re-actor is charged with the aqueous medium, vinyl chloride mono-m~r, polymeriza~ion initiator and other optional ingredients such as comonomers, dispersion aid and the like, amounts and propor-tions of these ingredients, polymerization temperature and so on.
It is of course optional that the polymerization mi~ture is further admixed with various kinds of known additives conventionally us-ed in the suspe~sion polymerization of vi~yl chloride including polymerizatio~ regulators, chain trans~er agents, pEI-controlling agents, gelation improvers, antista~ic agents, crosslinking agents, stabilizers, fillers, antioxidants, buffering agents, scale-deposition inhibitors and so on according to need.

, 2~7'~9~

~ 10--In the following, the improvement obtai~ed according to the invention is described in more detail by way of examlples which, however, never limit the scope of the invent;on in any way.
Example 1.
Into a stainless-steel jacketed polymerization reactor of 2000 liters capacity equipped with a stirrer we:re introduced 900 kg of deionized water, 180 g of (a~ a ~lrst partially saponified polyviIlyl alcohol having an average degree of polymerization of 260û and a degree oiE saponi~lcation of 80.2%, 120 g of (b) a hydroxypropyl methyl cellulose giving a 2% by weight aqueous solution having a viscosity of 49.5 cerltipoise at 20 (:, of which the degrees of substi-tution of metho~y and hydro~ypropoxy groups were 29.2% by weight and 8.9% by weight, respectively, and 240 g of di-2-ethyl-hexylperoxy dicarbonate. A~ter evacuation of the reactor down to a pressure of 50 r~mHg, 600 kg of vinyl chloride monolner were in-troduced thereinto to form a polymerization mi~ture under agita-~io~
The polymerization mixture was heated up to a temperature of 57 C by passing hot water through the jacket of the reactor and the polymerization reaction was effected by keeping the polymer-ization mixture at this temperature. At a moment when conver-sion of the monomer i~to polymer had reached 30% in the polymer-ization mixture, 600 g of (c) a second partially saponi:fied polyvinyl alcohol having an av~rage degree of polymerization of 2000 and a degree of saponificatio:n of 98% were introduced thereinto i:a the form of an aqueous solution without interruption of the polymeriz-... . ., , - ~ ...

,.: . - , ~ - -~: .

2~7~5 ation reaction. When the pressure inside of the reactor had drop-ped to 6.0 kgf/cm2, the unreacted monomer was purged out of the reactor to terminate the polymerization reaction. The polymeriza-tion mixture in the form of an aqueolls slurry was ~ischarged out of the reactor and dehydrated alld dried into a polyvinyl chloride resin powder in a conventional manner.
The amounts of the above mentioned three water-soluble polymers (a), (b~ and (c) correspond to 0.03%, 0.02% alld 0.1%, re-spectively, by weiht relative to the amount of the vinyl chloride monomer.
The resin powder product obtained above was evaluated by measuring the parameters shown below by the testing procedures respectively give~ there to give the results shown in Table 1.

Bulk density:
Measurement wa~ per~ormed according to the procedure spa-ci~led in JIS K 67a1.
Particle size distributio~:
The resi~ powder was screened by succsessively passing through a set of sieves of fill~e~ess of 60 meshes, 80 meshes, 100 meshes, 150 meshes and 200 meshes and the fractions of the pow-der passillg through the respective sieves were recorded in % by weight of the starting resin powder.
Number of ~lsh eyes:
A resin compound was prepared from 100 parts by weight of the resin powder, 50 parts by weight of dioctyl phthalate, 0.5 part -: . . -,. . ~, .
' 2~7'~5 ~ 12--by weight of tribasic lead sulfate~ 1.5 parts by weight of lead stea-rate, 0.1 part by weight of titanium dioxide and 0.05 part by weight of carbon black and a Z5 g portio:n of the compound was milled for 7 minutes on a 6-inch testing roller mill at 140 C follow-ed by sheeting into a sheet of 0.2 mm thickness, of which the num-ber of translucent spots was visually counted for a 100 cm2 widé
areaO

Example 2.
The experimental procedure was substantially the same as in E~{ample 1 except that the amount of the second partially sapo-nified polyvinyl alcohol added to the polymerization mixt~re was decreased to 240 g. The amounts of the three water-soluble poly-mers (a), (b) and ~c) correspond to 0.03%, 0.02~o and 0.04%, respec-tively, by weiht relative to the amount of the ~rinyl chloride mo-nomer. The results obtained by the evaluation tests ~f the product resin powder are shown in Table 1.
Example 3.
The experime~tal procedure was substantially the same as in Example 1 except that the second partially saponified polyvinyl alcohol, i.e. the water-soluble poly:mer (c), was introduced i~to the polymerization mixture at a ~noment when conversion of the mo-nomer into polymer was 10%. The results obtained by the evalua-tion tests of the product resin powder are shown in Table 1.
Example 4.
The experimental procedure was sllbstantially the same as in Example 1 except that the second partially saporlified polyvinyl . . . - .
. . .
.
~ . . -' ' ` :

, .

2~7~

-~3-alcohol, i.e. the water-soluble polymer (c), was introduced into the polymerization mixturP at a moment when conversion of the mo-nomer into polymer was 60%. The results obtained by the evalua-tion tests of the product resin powder are shown in Table 1.
Example 5~comparative~
The experimental procedure was substantially the same as in Example 1 except that the second partially saponified polyvinyl alcohol, i.e. the water-soluble polymer (c), was entirely omitted.
The results obgained by the evaluation tests of the product resil:~
powder are shown in Table 1.
Example 6~comparative).
The experimen$al procedure was substantially the same as in Example 1 except that the second partially sapo~ ed polyvinyl alcohol~ i.e. the water-soluble polymer (c), was introduced into the polymerization mi~ture at a moment when conversion of the mo-~omer i:rlto polymer was 3%. The results obtained by the evalua~
tion tests of the product resin powder are shown in Table 1~ A
small amount of glassy beads was found in the resin powder.
13xam~e 7 (comparative3.
The experimental procedure was substantially the same as in Example 1 except that the seco:nd partially saponified polyvinyl alcohol, i.e. the water-soluble polymer (c), was introduced into the polymerization mixture at a moment when conversion of the mo-nomer into polymer was 70%. The results obtained by the evalua-tion test~ of the product resin powder are shown in Table 1.

.

2~ c~5 ~ 4 Exarnple 8 (comparative).
The experimental procedure was substantially the same as in Example 1 except that the amount of the secoIld partially sapo-nified polyvinyl alcohol, i.e. the water-soluble polymer (c), was de-creased to 360 g, i.e. 0.06% by weight of the monomer, which was introduced ;nto the polymerization mixture ~lready before the start of the polymeri~ation reaction. The .results obtained by the e~raluation tests of the product resin powder are shown in Table 1.
A small amount of glassy beads was fou~d in the resin powder.
Example 9 (comparative).
The experimehtal procedure was substantially the same as in Example 1 except that the amount of the first partiall~y sapo~
ed polyvinyl alcohol, i.e. the water-soluhle polymer ~a), was in-creased to 300 g, i.e. 0.0~% by weight OI the monomer9 and the hy-droxypropyl methyl cellulose, i.e. the water-soluble polymer (b), was entirely omitted. The results obtained by the evaluatio~ tests of the product resin powder are shown in Table 1.
Example 10 (comparative).
The e}cperimental procedure wa~ substantially the same as in Example 1 except that the ~lrst partially saponified polyvinyl alcohol, i.e. the water-soluble polymer (a~, was entirely omitted and the amount of the hydroxypropyl methyl cellulose, i.e. the wa-ter-soluble polymer (b)~ was increased to 300 g, i.e. 0.05% by weight of the monomer. The result~ obtained by the evaluation tests of the product resin powder are ~hown in Table 1.

2 ~ 7 L~

Example 11 The experimental procedure was substantially the same as in E~ample 1 except that the filrst partially saponifïed polyvi:nyl al-co~ol, i.e. the water-soluble polymer (a), was replaced with the same amount of another partially saponi~led polyvinyl alcohol havillg an average degree of polymerization of 1700 and a degree of saponi~lcatioIl of 78.0%. The results obtained by the evaluation tests of the product resin pow~er are shown in Table 1.
Example 12.
The experimental procedure was substantially t:he same as i~ Example 1 except that the :first partially saponi~ed polyvinyl al-cohol, i.e. the water-soluble polymer (a~, was replaced with the same amount of another partially saponi:fied poly~inyl alcohol having an average degree o:f polymerization of 2000 and a degree of saponification of 80.0%. The results obtained by the evaluation tests of the product resin powder are shown in Table 1.
Example 13 (comparative).
The experimental procedure was substantially the same as in Example 1 except that the first partially saponif~led polyvinyl al-cohol, i.e. the water-soluble polymer (a), was replaced vvith the same amount of another partially saponi~led polyvinyl alcohol having an average degre~ of polymerization of 1000 and a degree of saponification of 88.5%. The results obtained by the evaluation tests of the product resin powder are shown in Table 1. A small amount of glassy beads was found in the resin powder.

2~ 7~

Example 14 (com~arative~.
The experime:tltal procedure was substa~tially the same as in Example 1 except that the ~lrst partially saponified polyvinyl al-cohol, i.e. the water-soluble polymer (a), was replaced with the same amolmt of another partially saponi~led polyvinyl alcohol having an average degree of polymerization of 780 and a degree of saponi~lcation of 72.9%. The results obtained by the evaluation tests of the product resin powder are shown in Table 1.
Example 15 (comparatlve).
The experimental procedure was substantially the same as in Example 1 e~cept that the first partially sapQnified polyvinyl al-cohol, i.e. the water-soluble polymer (a), was replLaced with the same amount of another partially saponified polyvi~yl alcohol :having an average degree of polymerization o:f 1000 and a degree of saponi~lcation oP 98.0%~ The result~ obtained by the evaluation tests of the product resin powder are shown in Table 1.
E~ample 16.
Th0 experimental procedure was substantially the same a~
in Example 1 except that the sec~d partially saponi~led polyvinyl alcohol, i.e. the water-soluble polymer (c), was replaced with the same amount of another partially saponi~led polyvinyl alcohol having an average degree of polymerization of 2000 and a degree of saponi~lcation of 88.0%. The results obtained by the evaluation tests of the product resin powder are shown in Table 1.

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2 ~ 7 '~

~ 7-Example 17 (compa ative~.
The experimental procedure was substantially the same as in Example 1 except that the hydroxypropyl methyl cellulose, i.e.
the water-soluble polymer (b), was replaced with the same amount of anQther hydro~yp:ropyl methyl cellulose giv;ng a 2% by weight aqueous solution having a viscosity of 100 centipoise at 20 C~ of which the degrees of substitution of methoxy and hydroxypropoxy groups were 21.5% by weight and 8.0% by weight, respectively.
The results obtained by the evaluation tests of the product resin powder are show:~ in Table 1.
Example 18.
The e~perimental procedure was substantially the ~ame as in Example 1 except that the hydroxypropyl methyl cellulose, i.e.
the water-soluble polymer (b), was replaced with the same amount of another hydroxypropyl methyl cellulo~e giving a 2% by weight aqueous solution having a viscosity of 400 centipoise at 20 C, OI
which the degrees of substitutioll of methoxy and hydroxypropoxy groups were 28.0% by weight arld 6.0% by weight, respectively.
The results obtained by ~he evaluation tests of the product resin powder are shown in Table 1.

2~7~

--lB--Table 1 Example Bulk Particle size distribution, ~o, passing Number of No. densi- through screen of fish eyes, ty, pieces/100 g/cm3 ~0 80 100 150 200 cm2 mesh mesh mesh mesh me~h 0,612 100 9~.1 61.2 10.9 0.4 10 2 0.607 100 89.~ 58.~ 9.1L ~ 0 3 0.600 1~0 9~.0 6~.1 14,1 1.1 30 0.609 100 ~2.9 60.8 10.2 0.3 10 0.~6 100 82.~L 48~6 8.~ 0.9 6 ~ 42 100 97.6 86.2 31.~ 16.2 200 7 û.529 ~00 81.9 4~.1 8.4 0.7 ~0 : ~ ~.572 1~0 95.1 74.~ ~4.3 3.4 1000 : 9 0.5~6 97.6 64.3 28.9 10.~ 0.3 :l~0 0.512 96.2 71.~ 46.1 17.4 ~.8 40 :~ 11 0.~99 100 86.2 ~0.1 9.2 0.3 10 12 0.602 100 90.3 68.~ 10.0 0.3 10 13 0.59~ 9g.~ 39.5 62.3 19,9 1.1 70 ; 14 0.~4l) 9~.2 61.8 2~.5 9.3 0.2 10 0.642 10~ gO.~ 59.3 10.0 0.3 10 16 0.~95 100 91.3 ~9.8 10.4 0.4 1~
17 0.562 98.6 84.3 6~.2 26.1 1.4 30 18 0.697 100 92.1 59.8 11.0 0.5 40 : ~ : :

.

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Claims

1. In a method for the preparation of a vinyl chloride polymer by the suspension polymerization reaction of vinyl chloride mono-mer in an aqueous suspension medium containing a water-soluble polymeric suspending agent in the presence of a monomer-soluble polymerization initiator, the improvement which comprises using, as the suspending agent, a combination of three kinds of water-soluble polymers including:
(a) a first partially saponified polyvinyl alcohol having an average degree of polymerization in the range form 1500 to 3000 and a de-gree of saponification in the range from 75 to 85%;
(b) a hydroxypropyl methyl cellulose, of which the degree of substi-tution with methoxy groups is in the range from 26 to 30% by weight and degree of substitution with hydroxypropoxy groups is in the range from 4 to 15% by weight, giving a 2% by weight aqu-eous solution having a viscosity in the range from 5 to 4000 centi-poise at 20°C; and (c) a second partially saponified polyvinyl alcohol having an aver-age degree of polymerization of at least 1500 and a degree of sapo-nification of at least 86%, of which the total amount of the polymers (a) and (b) is in the ranges from 0.03 to 0.1 part by weight per 100 parts by weight of the vinyl chloride monomer with a weight ratio of the polymer (a) to the polymer (b) in the range from 2:8 to 8:2 and the amount of the polymer (c) is in the ranges from 0.01 to 1 part by weight per 100 parts by weight of the vinyl chloride monomer, the water-solu-ble polymers (a) and (b) being contained in the aqueous suspension medium at the moment of starting of the polymerization reaction and the water-soluble polymer (c) being introduced into the aque-ous suspension medium at a moment when conversion of the vinyl chloride monomer to polymer is in the range from 5% to 60%.

2. The improvement as claimed in claim 1 in which the total amount of the polymers (a) and (b) is in the ranges from 0.03 to 0.06 part by weight per 100 parts by weight of the vinyl chloride monomer.

3. The improvement as claimed in claim 1 in which the hydr-oxypropyl methyl cellulose gives a 2% by weight aqueous solution having a viscosity in the range from 5 to 1000 centipoise at 20 °C.

4. The improvement as claimed in claim 1 in which the amount of the second partially saponified polyvinyl alcohol is in the ranges from 0.02 to 0.5 part by weight per 100 parts by weight of the vinyl chloride monomer.

5. The improvement as claimed in claim 1 in which the second partially saponified polyvinyl alcohol is introduced into the aque-ous suspension medium at a moment when conversion of the vinyl chloride monomer to polymer is in the range from 30% to 60%.