CA1110397A - Process for producing polyvinyl chloride - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
The invention is concerned with a process for the production of polyvinyl chloride by suspension polymerization of vinyl chloride monomer, alone or in mixture with other copolymerizable monomers, in a medium consisting essentially of water and containing a water-soluble suspending agent. The process of the invention is characterized in that the polymeriza-tion is conducted in the presence of polyvinyl acetate having an average molecular weight between 10,000 and 30,000, and being partially hydrolyzed so that its saponification number is between 300 and 600. The invention enable one to produce a product having high porosity and high uniformity in the ganulo-metric distribution of the particles, which permits almost complete removal of residual monomer in the final stripping stage.

Description

3~7 The present invention is concerned with a process for the production of polyvinyl chloride by water suspension polymerization of vinyl chloride monomer.
It is known, for example Erom Italian Patent Nos. 560,363, 525,427, 526,394 and 562,180, as well as U.S.
Patent No. 2,~76,47~ and German Patent No. 2,358,099, that poly-vinyl alcohols, water soluble modified celluloses, such as methyl cellulose and methyl hydroxypropyl cellulose, and maleic anhydride/vinyl acetate copolymers, are effective as suspending ~-agents, alone or in mixture, in polymerizing vinyl chloride monomer (VCM), alone or in mixture with other unsaturated monomers, such as viny].idene chloride, acrylic and ~.ethacrylic esters, vinyl esters, styrene! acrylonitrile, methacrylonitrile , ~-ethylene and other olefines.
The present invention provides an improved process for the production of polyvinyl chloride by suspension : ~
polymerization of vinyl chloride monomer, alone or in mixture -with other copolymerizable monomers, in a medium consisting ;~.
essentially of water and containing a water soluble suspending ~. . :
agent, the improvement comprising conducting the polymerization ln the presence of polyvinyl acetate having an average molecular ~-weight between 10,000 and 30,000, and being partially hydrolyzed so that its saponification number!~is between 300 and 500.
According to a preferred embodiment, the partially hydrolyzed polyvinyl acetate is utilized in an amount of 0.001% to 0.3% by weight, based on the weight of the monomer or monomers.
The partially hydrolyzed polyvinyl acetate preferably has a saponification number between 380 and 480.
The advantages provided by the use of partially `

hydrolyzed polyvinyl acetate, according to the invention, are as follows:

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3~7 (a) an improved porosity of the polymer being obtained, maintainable also by carrying out the reaction up to high conversion percentages of the monomer, thereby resulting in an increased system capacity; ~ .
(b) type of porosity being obtained, allowing - in the ~
final stripping step an almost complete removal from the polymer ~r of the residual monomer retained therein, which is of outstanding ~ ;
importance in view of the proved danger to man's health of vinyl !, chloride monomer;
(cj less forming of foam;
(d) an increased crumbleness in the polymer adhering ; to the reactor walls, which consequently makes it easier to clean the same by conventional mechanical means; and , (e) a high bulk density of the product obtained.
A modified polyvinyl acetate, suitable to the purpose of the present invention, can be obtained by any partial hydrolysis process, from a polyvinyl acetate having an average - molecular weight between 10,000 and 30,000, operating at both alkaline and acid pH, and using to this purpose, - by mere way of non-limiting example - hydroxides or alcoholates of alkaline ~metals, such as sodium or potassium, or acids, such as sulphuric acid, hydrochloric acid, phosphoric acid, perchloric acid and the like, the reaction being carried out in a solution of a ~-~
suitable solvent, such as for example metal alcohol, ethyl alcohol, acetone, methyl acetate, ethyl aceta-te or mixtures thereof and, at the end of the process, it can be separated from the solvent, in a per se known manner, by means of suitable coagulation or evaporation processes of the volatile constituents, or it can be used dissolved in said reaction solvent, or as emulsion or suspension in water.

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3~7 The following non-limiting examples illustrate the effects of using partially hydrolyzed polyvinyl acetate, having a saponification number between 300 and 500, according to the invention, in the water suspension polymerization of vinyl chloride monomer.

. Polyvinyl chloride was prepared as follows: :~
.~` a~ 2,400 kg water, 1,600 kg vinyl chloride monomer (VCM), 1.6 ~g water soluble polyvinyl alcohol, and 2.55 kg .
lauroyl peroxide, were charged into a 5 cu. m. reactor, provided with a Pfaudler stirrer, and the mixture was heated at 55C and ;. :;
. ~ .
^. stirred at 130 r.p~m.
~ The polymerization was carried out up to a decrease ;~
;~ in pressure o 0.3 atm., obtaining a conversion of about 80%. .
; ~ The porosity of the product obtained was 0.35 cc/g, expressed as inner pore volume.
b) A preparation was repeated as at point a)hereabove adding to the ~

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., - above mentioned ingredients 1.28 ~g polyvinyl acetate, having a number average molecular weight of 16 t 000 and partially hydrolized, so that its saponification number was equal to 450, according to the concept of the present invention.
The porosity of the product obtained was 0.40 c~c/g, - expressed as inner pore volume.

a) Example 1, a) was repeated, carrying out the ` reaction up to a decrease in pressure of 3.5 atm., corresponding to a conversion of about 93%.
The porosity fell to a rate of 0.26 cc/g.
- The suspension of the polymer obtained was then subjected to final stripping for 2 hours at 65C under vacuum.
The content of VCM residue in the suspension was equal to 200 ppm.
After drying and screening, the content of VCM
residue in the polymer granules was 75 ppm.
b) Example 1, b~ was repeated, carrying out the reaction up to a decrease in pressure of 3.5 atm., corres-ponding to a conversion of about 93%.
The porosity obtained kept at a rate of 0.38 cc/g.
The suspension of the polymer obtained was thensubjected to final stripping for 2 hours at 65C under vacuum. The content of VCM residue in the suspension was equal to 28 ppm.
After drying and screening, the conkent of VCM
residue in the polymer granules was 5 ppm.
The appended table shows the complete data as referred to the preceding examples, as well as to further examples of preparations. qn th~ table, all of the component partst shown in the first seven lines, are intended as parts by weight and are related to 100 parts VCM. Polymerizationswexe carried out .

3~-in 5 cu. m. autoclaves, provided with Pfaudler type stirrers and operating at 130 r.p.m.
The first two columns relate to examples l,a) and l,b), whereas the third and fourth column relate to examples

2,a) and 2,b), respectively; however, it should here be noted that the number or rates shown in the table indicate, as specified above, parts by weight for the various components, related to 100 parts by weight of VCM, instead of indicating the absolute values shown in example 1 and 2.
-Whereas, the subsequent columns of the table show ~`~ the data relating to some other examples, indicating the use of different suspending agents; particularly:
~` - the fifth and sixth column show examples 3, a) and ; 3, b), the preparation of which differs from that of examples 2, a) and 2, b~ in that, instead of 0.10 parts of water soluble polyvinyl alcohol, use was made of 0.09 parts of a product commercially known as METHOCEL 65 HC*;
- the seventh and eight column show examples 4, a) ~-and 4, h), the preparation of which differs from that of examples 2, a) and 2, b) in that, instead of 0.10 parts of water soluble polyvinyl alcoholl use was made of 0.08 parts of maleic anhydride/vinyl acetate copolymer;
- finally, the ninth and tenth column show examples ;
5, a) and 5, b), the preparation of which differs from that of examples 2, a) and 2, b?,inlthat the amount of water soluble polyvinyl alcohol was reduced from 0.10 to 0.06 parts by weight, and 0.04 parts of METHOCEL 65 HG* were added thereto.
In addition to the above mentioned data of the preparation components and to the test data mentioned in examples 1 and 2, also the following data are shownin the same table:
* trademark ~ .

3~37 .. , :
- VCM residue (line 10 of the table) in p.p.m.
- bulk density (line 11 of the table) in g/cc;
- formation of ~fish-eyes in the PVC (line 12 of the table), expressed as number of fish-eyes)> per s~. dm. of a film calendered at a temperature of 150-l60C, 0.2 mm thick, and after calendering for 5 minutes.

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The above table is a clear confirmation of the outstanding results in usiny partially hydrolized polyvinyl acetate, according to the invention, in the polymerization of vinyl chloride.
Similar values to those shown in the above table have also been obtained by using, instead of lauroyl peroxide (line 5 of the table), other oil soluble catalysts, such as for example peroxydicarbonates also generated in situ Erom alkyl-halo-formates, hydrogen peroxide and alkali metal bases;
perpivalates; azodiisobutyronitrile and other peroxides or hydroperoxides and the like, known to those skilled in the art.

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3~7 .

SUPPLEMENTARY DISCLOSURE
: ', his supplementary disclosure relates to a process for ;
producing polyvinyl chloride by-water suspension polymerization of vinyl chloride monomer (VCM~ with conversion up to 93~, which enables one to obtain a product having not only a high porosity, as described in the original disclosure, but also an excellent granulometric distribution, generally within the range of 75 to : -~ 160 microns.
The water suspension polymerization of VCM is one of ::
the most known and widely used methods for producing vinyl ~
10 chloride polymers (VCP). . :
The method usually consists in mixing liquid VCM
with water and with an initiator of radicalic polymerization, ~
while the monomer itself is kept in dispersion by stirring - ~ :
at a high temperature, until polymerization has been ~
obtained to the desired degree of conversion. ;~.
AS widely known, it is most important to obtain a VCP having~
- besides a high porosity of thegranules, .
. - a rather uniform granulometric distribution, tha-t 2Q is to say, a minimum~quantity of too small or too large particles, or none at all. :
Asalready indicated in the original disclosure, the importance of obtaining a h~igh.porosity of the granules :~
derives not only from technical reasons - such as the requirement for the granules to absorb the plasticizers .
rapidly and in great quantity for forming "dry blends" - but :
also from sanitary reasons, in that it has been noticed that only with highly porous granules it is possible to obtain a more ~-thorough removal of the VCM residues, which has resulted to be cancerous.

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The importance of obtaining a rather uniform granulo-me-tric distribution derives instead essentially from technical reasons, particularly in relation to the requirements of the subsequent working stages, for the production of finished or semifinished objects.
The combination of the two aEore-specified characteristics, though having been notoriously desirable for a long time, is not easy to obtain, even if the specialized literature describes various processes for carrying out polymerization in such a way as to achieve this object.
This forms a problem - so far unsolved on an industrial production level - partlcularly in the case of - carrying out polymerization in reactors equipped with turbine stirrers.
~s described in the U~S. Patent No. 3,772,226, the properties of the VCP obtained are influenced by different variables, depending on the technique used for suspension polymerization. As evidenced in this patent, the porosity of the polymer granules tends to clearlydecrease if polymerization ; 20 is carried on up to more than 90~ of the monomer conversion;
hence, the known processes o~ suspension polymeri~ation are interrupted at about 85~ of the monomer conversion~
As also described in the above U.S. Patent, it is known that the use of the suspending agents, in suspension polymerization, may notably influence the characteristics of the polymer obtained: whereby a great variety of substances, generally hydrosoluble, have been suggested for this purpose.
It has also been proposed to use a combination of "primary" and "secondary" or "auxiliary" suspending agents, in order to obtain an acceptable compromise between porosity and dimension of the particles. It is known that the"primary"
suspending agents act as regulators of the particles dimensions:

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they are usually hydrosoluble substances, such as cellulose or gelatin derivatives, polyvinyl alcohols, maleic anhydride/
; vinyl acetate copolymers, and others. The "secondary" suspending agents, which have been used with the intention of increasing the porosity o:E the granules, comprise partial esters of fatty ~ .
acids with polyalcohols, monoglycerides of unsaturated fatty acids, polyethylene glycol esters, polysaccharides, and others.
In order to reduce the negative effect - on the granulometry of the particles - produced by the secondary suspending agents, it has been proposed to add other ingredients, as for example aliphatic alcohols and acids, acting as surfactants able to control the monomer spreading coeficient on water ~see, for example, Italian Patent No. 674.764),or an exactly proportioned mixture of primary suspending agents, emulsifying agents (chiefly anionic) and inorganic coagulants (as described in Italian Patent No. 552.180). . :~
A substantial improvement on the known technique has ~.
been obtained by means of the process according to the original disclosure, in which it has been proposed to use the combination of a conventional suspending agent with a secondary suspending ~: agent consisting of partially hydrolyzed polyvinyl acetate an average molecular weight between 10,000 and 30,000 and a saponification number between 300 and 500.
The process according to the original disclosure allows to obtain the two fold advantage of being able to carry out polymerization with monomer conversion up to 93% and, at the same time, to keep to porosity of the granules at excellent values, hence with very scarce VCM res.idues in the final :
product.
Since this important result was the chief object of the researchers - in connection with the afore-mentioned sanitary 3~7 problems - less importance was given to the fact that the use of the new secondary suspendiny agent led to the production of a polymer with rather wide granulometric distribution and, ~ :
; in particular, with an undesired increase in the percentage of the finer particles.
It is an object of the invention to now provide a process for the production of VCP by water suspension polymerization of VCM, allowing to keep the excellent characteristics of porosity already described in -the original disclosure - particularly with monomer conversion up to 93~ - but accomplishing at the same time also a very uniform granulo-metric distribution, particularly within the desired range of 75 to 160 microns.
This object is achieved, according to the invention of the present supplementary disclosure, in a proc~ss for the production of polyvinyl chloride by suspension polymerization of vinyl chloride monomer, alone or in mixture with other copolymerizable monomers, in a medium consisting essentially of water and containing a water-soluble suspending agent, by conducting the polymerization in the presence of polyvinyl acetate having an average molecular weight between 10,000 and 30,000, and being partially hydrolyzed so that its saponification number is between 500 and 600. :
The partially hydrolyzed polyvinyl acetate utilized in the practice of the above process preferably has a saponification number between 520 and 540.
In actual fact, Applicant had already noticed - when carrying out researches in connection with partially hydrolyzed polyvinyl acetate - that the preferred range specified in the original disclosure - namely, a polyvinyl acetate being partially hydrolyzed so as to have a saponification number between 3gO and

4~0 - was not very cri-tical and that in practice, for a VCP with 3~7 : K ~ 70, there was no considerable worsening in the results even if reaching the specified limits of the saponi~ication number, of 300 and 500.
In carrying on such studies, Applican-t has now ..
discovered that, in reality, even if exceeding these limits -particularly up to a saponification number of 600 - the final - result, in respect to the porosity of the grains, remains practically constant, whlle it has surprisingly been possible to establish that there is considerable improvement in the results - especially in the preferred range of partial ~ hydrolyzation, wit~ saponification number between 520 and : 540 - concerning the uniformity in the granulometric distribution ~ -of the particles of the final product.
More specifically, it was found that by using a polyvinyl acetate with saponification number be-tween 500 and 600, the latter does not interfere and therefore does not alter the action of the primary or water-solu:ble suspending agent (for ~ -example, hydrosoluble polyvinyl alcohol and/or hydrosoluble modified celluIose), for the purpose of obtaining a proper and , uniform granulometric distribution of the particles of the final VCP. : ~
`- In actual fact, in working for a VCP with K _ 60-63, .
it was surprisingly found that there is even a general . ~:
improvement in the results, both in respect to the porosity : of the grains and to the granulometric distribution.
Though the reasons for this substantial improvement -~
are not fully comprehensible, it can be observed that the secondary suspending agent proposed in the present supplementary disclosure - namely a partial].y hydrolyzed polyvinyl acetate having a saponification number between 500 and 600 - is less soluble in water and more soluble in the VCM than the secondary .~: - 13 -~t , 3~

suspending agent proposed in the original disclosure, namely a polyvinyl acetate hydrolyzed to a saponification number between 300 and 500.
The partially hydrolyzed polyvinyl acetate utilized in the process according to both the original and supplementary disclosures, that is, having a saponification number between 300 and 600, preferably has a viscosity between 0.5 and 2 cps, measured in a 4~ methanol solution at 20C.
The excellent results obtained with the process `~
according to the present supplementary disclosure can be obtained with any type o~ stirrer, both of the Pfaudler and of the turbine type, with or without breakwater, and with any -type of catalyst commonly used in the polymerization of VCM. Hence, also in this respect, the process according to the present supplementary disclosure is particularly advantageous, in that it is not critical relatively to the working system applied on an industrial production level.
The partially hydrolyzed polyvinyl acetate, used in the process according to the invention, may be obtained with any system known to the skilled in the art, for example with the system described in the Encyclopaedia of Chemical Technology -(Kirk Othmer) Vol. 21, page 360, second edition.
; The partially hydrolyzed polyvinyl acetate is normally i obtained in solution with solvents, such as methyl alcohol, ethyl alcohol, acetone, methyl ace-tate, ethyl acetate, or mixtures thereof, and can be added in the polymerization reaction either dissolved in such solvents or as an emulsion or suspension in water, or even in a solid Eorm after separation from the solvents.
The amount o~ partially hydrolyzed polyvinyl acetate used in the process according to the supplementary disclosure is not critical and may vary from 0.01 to about 0.1~-, and preferably between 0.02 and 0.06% by weight, calculated on the R

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weight of -the monomer or monomers.
The amount of primary or water-soluble suspending agent may in turn vary wlthin the same limits, tha-t is, from about 0.01 to 0.1%, and preferably between 0.02 and 0.06% by weight, calculated on the weight of -the monomer or monomers.
The weight proportion between the primary suspending agent and the secondary suspending agent, namely the partially hydrolyzed polyvinyl acetate, may vary within a wide range from 1:10 to ~
10:1. ' . ~, The polymerization is generally conducted in the presence of the usual initiators of radicalic polymerization, ; which comprise for example benzoyl peroxide, lauroyl peroxide, ~;~
azodiisobutyronitrile, and the like. These catalysts can be used in a proportion of about 0.01 to 0.5% by weight calculated on the weight of the monomer or monomers. ~ ;
The temperature may vary, but it is generally between 50 and 70C, and preferably between 55 and 65C.
As already indicated above, the polymerization can be carried out up to a 93~ conversion of the monomer, without decreasing the porosity of the grains. Such porosity -expressed in cc/g and measured with the mercury porosity me-ter according to ~he method C. ERBA - keeps at the optimum level of 0.25-0.26 cc/g for a VCP with K = 60-63. Similar results are obtained with vinyl chloride polymers having a different K.
The afore-described process allows moreover to obtain, ;
as already indicated, a VCP in particles having a uniform ;~ granulometric distribution and, in particular, a product containing no more than 3% o granules having dimensions below 75 microns.
The annexed tables show in detail the characteristics of the VCP which can be obtained with the process according to 3~

the invention of the present supplementary disclosure. These tables report the da-ta of different tes-ts carried out on a VCP
with K = 63 (Table II) and, respectively, on a VCP with K = 70 able III), the different tests are divided into groups A, B, C and D, referring to the following further examples 3, 4, 5 and 6 respectively, the difference between the various tests lying only in the different choice of the suspending agents used.

~` The VCM is water suspension polymerized into a 5 cu. m.
reactor equipped with a Pfaudler stirrer. The reactor is charged with:
2,400 kg demineralized water ~; 1,600 kg vinyl chloride monomer (VCM) 2.55 kg lauroyl peroxide The quantities and the types of suspending agents used are indicated in Table II, group A.
The mixture is heated at 61C while being stirred at 115 r.p.m. into the closed reactor, up to obtaining a monomer conversion in the polymer not below 93%, which is evidenced through a decrease in pressure of 3.5 atm.

The VCM is water suspension polymerized into a 5 cu.
m. reactor equipped with a turbine stirrer. The reactor is charged with:
2,400 kg demineralized water 1,600 kg vinyl chloride monomer (VCM) 2.55 kg lauroyl peroxide The quantities and the types of suspending agents used are indicated in Table II, group B.
The mixture is heated at 61C while belng stirred at 150 r.p.m. in-to the closed reactor, up to obtaining a monomer conversion not below 93%, which is evidenced -through a decrease ~,f'~, ' .

3~7 in pressure of 3.5 atm.

The VCM is water suspension polymerized into a 5 cu. m.
reactor equipped with a Pfaudler stirrer. The reactor is charged with:
2,400 kg demineralized water 1,600 kg vinyl chloride monomer (VCM) 2.55 kg lauroyl peroxide The guantities and the types of suspending agents used are indicated in Table III, group C.
The mixture is heated a-t 55C while being stirred at 130 r.p.m. into the closed reactor, up to ob-taining a monomer ;
conversion not below 93~, which is evidenced through a decrease in pressure of about 3.5 atm.

The VCM is water suspension polymerized into a 5 cu. m.
reactor equipped with a turbine stirrer. The reactor is charged with:
2,400 kg demineralized water 1,600 kg vinyl chloride monomer (VCM) 2.55 kg lauroyl peroxide `
The quantities and the types of suspending agents used are indicated in Table III, group D.
The mixture is heated at 55C while being stirred at 150 r.p.m. into the closed reactor~ up to obtaining a monomer conversion not below 93%, which is evidenced through a decrease in pressure of about 3.5 atm.

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3~7 When lnterpre-ting the data reported in Tables II and III, one must bear in mind thatO -- the data contained in column 1 of each of the yroups ~' A, ~, C or D, relate to tests carried out on VCP obtained according to techniques known per se, particularly by using sorbitol monostearate as secondary suspending agent;
- the data contained in column 2 of each of the yroups ;~
A, B, C or D, relate to a VCP obtained by using, as secondary suspending agent, polyvinyl alcohol having a saponification number of 315, that is, close to the lower margin of the range specified in the parent patent. It can be observed that the results are already better than those ob-tained according to the known technique, but much inferior to those which can be obtained according to the present invention;
- the data contained in columns 3 to 5, refer instead to tests carried out on VCP obtained by using, as the secondary suspending agent, partially hydrolyzed polyvinyl acetate having a saponification number of 450, namely within the preferred range specified in the original disclosure;
: 20 - finally, the data contained in columns 6 to 9, refer to tests carried out on VCP obtained according to the teachings of the present supplementary disclosure, that is, by using - as :~ secondary suspending agent ~ partially hydrolyzed polyvinyl . acetate having a saponification number of 525.
The first remark relates to the comparison between the data contained in column 1 and the remaining data. It can be ~ ~:
seen quite evidently that the ~CP obtained according to the known technique has - compared to the VCP obtained according to the original disclosure or according -to the supplementary disclosure - :
a markedly lower porosity and, hence, a higher bulk density, but above all, a high amount of monomer residue both in the slurry and in the dry VCP. ~ :

~,..

3~s~97 On compariny subsequently the data contained in columns 3 to 5 with those contained in columns 6 to 9, two kinds of considerations can be made:
a) first of all it can be observed -that the data referrin~ to the porosity and to the presence of monomer residue are substantially equivalent; in particular, there is ~
even an improvement in the data if referring to a VCP with ~ :
K = 63 (see Table II), b) furthermore, one notes a clear improvement in the~
granulometric distribution, with a reduction below significant values of the amount of particles below 75 microns and of that ;
above 170 microns, and with a simul-taneous concentration of most of the granules around the preferred dimension oE 100 nicrons.

~ .

" ~ , ~ 7 --

Claims (30)

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

1. A process for the production of polyvinyl chloride by suspension polymerization of vinyl chloride monomer, alone or in mixture with other copolymerizable monomers, in a medium consisting essentially of water and containing a water-soluble suspending agent, which comprises conducting the polymerization reaction in the presence of polyvinyl acetate having an average molecular weight between 10,000 and 30,000, and being partially hydrolyzed so that its saponification number is between 300 and 500.

2. A process according to claim 1, wherein said poly-vinyl acetate is partially hydrolyzed so that its saponification number or value is between 380 and 480.

3. A process according to claim 1, wherein the amount of partially hydrolyzed polyvinyl acetate is between 0.001% and 0.3% by weight, calculated on the weight of vinyl chloride monomer used, or on the weight of the mixture of vinyl chloride monomer and other copolymerizable monomers used.

4. A process according to claim 1, wherein said suspending agent comprises completely water-soluble polyvinyl alcohol.

5. A process according to claim 1, wherein said suspending agent comprises a modified water-soluble cellulose.

6. A process according to claim S, wherein said suspending agent comprises methyl cellulose or methyl hydroxy propyl cellulose.

7. A process according to claim 1, wherein said suspending agent comprises a maleic anhydride/vinyl acetate copolymer.

8. A process according to claim 1, wherein said suspending agent comprises a mixture of water-soluble polyvinyl alcohol and a modified water-soluble cellulose.

9. A process according to claim 8, wherein said modified water-soluble cellulose is methyl hydroxypropyl cellulose.

10. A process according to claim l, wherein the polymerization is conducted in the presence of a catalyst selected from the group consisting of peroxides, hydroperoxides, peroxydicarbonates and oil soluble azoderivatives, and mixtures thereof.

11. A process according to claim 10, wherein said catalyst is lauroyl peroxide.

12. A process according to claim 10, wherein said catalyst is azodiisobutyronitrile.

13. A process according to claim 10, wherein said catalyst is peroxydicarbonate.

14. A process according to claim 13, wherein said peroxydicarbonate is generated in situ from an alkyl-halo-formate and hydrogen peroxide, in the presence of an alkaline substance.

15. A process according to claim l, wherein said partially hydrolyzed polyvinyl acetate is added to the medium as a solution in methanol.

16. A process according to claim 1, wherein said partially hydrolyzed polyvinyl acetate is added to the medium as a solution in ethanol.

17. A process according to claim 1, wherein said partially hydrolyzed polyvinyl acetate is added to the medium as a solution in methanol and methyl acetate.

18. A process according to claim 1, wherein said partially hydrolyzed polyvinyl acetate is added to the medium as a solution in methanol and water.

19. A process according to claim 1, wherein said partially polyvinyl acetate is added to the medium as a solution in methyl acetate.

20. A process according to claim 1, wherein said partially hydrolyzed polyvinyl acetate is added to the medium as a solution in ethanol and ethyl acetate.

21. A process according to claim 1, wherein said partially hydrolyzed polyvinyl acetate is added to the medium as a solution in ethyl acetate.

22. A process according to claim 1, wherein said partially hydrolyzed polyvinyl acetate is added to the medium as an aqueous suspension or emulsion.

CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE

23. A process for the production of polyvinyl chloride by suspension polymerization of vinyl chloride monomer, alone or in mixture with other copolymerizable monomers, in a medium consisting essentially of water and containing a water-soluble suspending agent, which comprises conducting the polymerization in the presence of polyvinyl acetate having an average molecular weight between 10.000 and 30,000, and being partially hydrolyzed so that its saponification number is between 300 and 600.

24. A process according to claim 23, wherein the partially hydrolyzed polyvinyl acetate has a viscosity between 0.5 and 2 cps, measured in a 4% methanol solution at 20 C.

25. A process for the production of polyvinyl chloride by suspension polymerization of vinyl chloride monomer, alone or in mixture with other copolymerizable monomers, in a medium consisting essentially of water and containing a water-soluble suspending agent, which comprises conducting the polymerization in the presence of polyvinyl acetate having an average molecular weight between 10,000 and 30,000, and being partially hydrolyzed so that its saponification number is between 500 and 600.

26. A process according to claim 25, wherein the partially hydrolyzed polyvinyl acetate has a saponification number between 520 and 540.

27. A process according to claims 25 or 26, wherein the amount of partially hydrolyzed polyvinyl acetate employed is from 0.01% to 0.1% by weight, calculated on the weight of the monomer or monomers.

28. A process according to claims 25 or 26, wherein the amount of partially hydrolyzed polyvinyl acetate employed is from 0.02% to 0.06% by weight, calculated on the weight of the monomer or monomers.

29. A process according to claims 25 or 26, wherein the weight proportion between the water-soluble suspending agent and the partially hydrolyzed polyvinyl acetate is between 1:10 and 10:1.

30. A process according to claim 25, wherein the polymerization is conducted at a temperature between 50 and 70°C.