CA1126449A

CA1126449A - Method for the preparation of polyvinyl chloride resins

Info

Publication number: CA1126449A
Application number: CA297,336A
Authority: CA
Inventors: Satoshi Kuwata; Kouji Azuma; Kazuhiko Kurimoto; Kinya Ogawa
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1977-02-24
Filing date: 1978-02-20
Publication date: 1982-06-22
Also published as: IT1094154B; HU178740B; DK80978A; PL107759B1; FR2381792B1; ES467284A1; NL181659B; PT67691A; ATA126678A; BE864149A; NO780590L; GB1596689A; JPS53104684A; NL181659C; CU34877A; SE440784B; DE2807180C2; SU841592A3; IT7820524A0; NL7802025A

Abstract

ABSTRACT OF THE DISCLOSURE
A method for preparing a low molecular weight polymer in an aqueous medium from a monomer mixture mainly composed of vinyl chloride monomer, comprising: adding an organic compound containing at least one mercapto group and at least one hydroxy or carboxyl group per molecule to the aqueous medium, and wherein at least 50 weight percent of the organic compound is added to the aqueous medium when the monomer conversion is between 1% and 30%. Polymers produced by the method disclosed have a relatively low degree of polymerization, good heat stability and desirable particle size distribution.

Description

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The present invention relates to a method for the preparation of vinyl chloride polymers and, in particular, to a method for the preparation of vinyl chloride polymers with a relatively low degree of polymerization.
It is well known in the art to introduce a chain transfer ayent into a polymerization mixture in order to obtain a vinyl chloride polymer with a relatively low degree of polymerization. Several classes of compounds are known as suitable chain transfer agents including, for example, (1) saturated hydrocarbons such as n-pentane and n-he~ane, (2) saturated and unsaturated chlorinated hydrocarbons such as carbon tetrachloride, trichloroethylene and perchloroethylene, and (3) aldehydes such as propionaldehyde and n-butyraldehyde.
These chain transfer agents, however, have their respective defectsO For e~ample, the saturated hydrocarbons, as a chain transfer agent, must be introduced into the polymerization mixture in an amount as large as 8 to 10% by weight based on the weight of the vinyl chloride monomer when vinyl chloride polymers with an average degree of polymerization of about 700 are desired by aqueous suspension polymerization at a temperature of about 60C. A considerable amount of the chain transfer agent is emitted to the atmosphere during the course of polymerization and saturated hydrocarbons absorbed in the polyvinyl chloride product gradually dissipate during storage and processlng of the polymer, thus polluting the environment as well as posing a serious health problem for the workers due to the toxicity of the vinyl chloride monomer.
The amount of the chain transfer agent can be reduced to about 0.7 to 1.0% by weight when the polymerization is :30 carried out under the same conditions as above but with a `

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saturated or unsaturated chlorinated hydrocarbon as the chain transfer agent. Chlorinated hydrocarbons are, however, inherently toxic. Alclehydes can be effective in a further reduced amount of 0.2% to 0.5% by weight but besides their toxicity they are disadvantageous as their thermal decomposition products, produced in the course of polymerization, retard the polymerization velocity.
It is also known in the prior art that various mercaptans are efficient chain transfer agents (see, for example, British Patent Specification No. 579,353). The inventors of the present invention have previously proposed a method for the suspension polymerization of vinyl chloride monomer to obtain vinyl chloride polymers with a relatively low degree of polymerization by adding a compound having at least one mercapto group and at least one hydroxy or carbo~yl group per molecule, such as 2-mercaptoethanol, thiopropylene glycol, 2-mercapto-propanol and thioglycolic acid, into an aqueous polymerization medium containing a specific comblnation of water-soluble - polymeric substances as a dispersiny agent.
The above method proposed by the inventors, has several advantages: (1) the acti.vity of the mercapto group-containing compounds, as the chain transfer agent, is very hiyh so that the amount of the chain transfer agent can be greatly reduced, (2) no retarding effect is induced in the velocity of polymerization, (3~ the mercapto group-containing compounds are not absorbed in the vinyl chloride polymers produced by the polymerization, and thus do not affect the quality of the polymer products, (4) the polymer products have well developed porosity resulting in good workability during processing and (5) the unreacted monomer is easily separated and removed from the polymer products after completion of the polymerization.

-. ., ,, , ~2~4~9 However, after further investigations it was found that the dispersion of the monomer in the aqueous medium was inadequate or unstable leading -to lower quality polymer products with coarser particle size distribu-tion.
rrhis problem is serious, especially, when the amount of the vinyl chloride monomer charged into the polymerization reac-tor is increased in order to obtain higher productivity.
An object of the invention is to obviate or mitigate the disadvantages of the prior art as described above.
According to an aspect of the in~ention there is provided a method for preparing a low molecular weight polymer in an aqueous medium from a monomer mixture mainly composed of vinyl chloride monomer comprising adding an organic compound con-taining at least one mercapto group and at least one functional group selected from the group consisting of hydroxy and carboxyl groups per molecule to the aqueous medium, and wherein at least 50 weight percent of said organic compound is added to the aqueous medium when the monomer conversion is between 1% and 30%.
According to a further aspect of the invention, there is provided a method for preparing a low molecular weight polymer in an aqueous medium from a monomer mixture mainly composed of vinyl chloride monomer comprising adding an organic compound containing at least one mercapto group and at least one functional group selecteA from the group consisting of hydroxy and carboxyl groups per molecule to said aqueous medium when the monomer conversion is between 1% and 30%.
The organic compound used as the chain transfer agent is, as defined above, a hydrophilic organic compound having at least one mercapto group (~SH) and at leas-t one hydroxy l~Z~9 group t-OI-!) or carboxyl group (-COOH) per molecule. Among the compounds falling within the above definition, those compounds having 2 to 7 carbon atoms, preferably 2 to ~ carbon atoms, in a molecule as exemplified by thioalcohols such as 2-mercapto-ethanol, 2-mercaptopropanol, 2-hydroxypropylmercap-tan and thio-glycerin and mercapto-containing carboxylic acids such as thio-glycolic acid, thiohydroacrylic acid, thiolactic acid and thiomalic acid are especially suitable. Compounds having more than 7 carbon atoms per molecule are less preferable because of their lower activity as a chain transfer agent.
Though dependent on the conditions of polymerization such as polymerization temperature and the desired properties of the polymer products such as the average degree of polymerization, the amount of the chain transfer agent to be added to the polymerization mixture is usually in the range from 0.001% to 0.5% by weight or, preferably, from 0.005% to 0.3% by weight based on the weight of the monomer or monomer mixture in the polymerization mixture. Naturally the above named compounds a~e not, necessarily, used singly but they may be used in combinations of two or more.
It is essential that the chain transfer agent be added to the aqueous polymerization medium at the time when the monomer conversion is in the range from 1% to 30% or, preferably from 2% to 20%. By this delayed introduction of the chain transfer agent, a vinyl chloride polymer product with desirable properties, especially desirable particle size distribution as well as superior heat stability is obtained.
With the same amount of the chain transfer agent, the average molecular weights of the resultant polymer products are about the same both in the prior art method, i.e. addition of ~6~4~

the chain transfer agent at the beginning of the polymerization, and in the present method, i.e. delayed addition of the chain transfer agent.
If -the total amount of the chain transfer agen-t is introduced in-to the aqueous medium at a tinle when the monomer conversion is lower than 1~, the beneficial effect on the particle size distribution cannot be obtained because the suspended monomer droplets have not been sufficiently stabilized.
On the other hand, if the chain transfer agent is introduced after the monomer conversion has exceeded 30%, it is difficult to obtain a vinyl chloride polymer product with as low an average degree of polymerization as desired because the chance for the chain transfer agent to exhibit its activity is largely lost.
The procedure for introducing the chain transfer agent into the polymerization mixture is not limited. For example, a predetermined amount of the chain transfer agent may be added at a time during the above defined period of time or it may be added in several portions over the period of time for monomer conversion of 1% to 30%. Alternatively, the chain transfer agent may be introduced continuously over the above defined period.
Although it is within the scope of the present inven-tion that part of the chain transfer agent be added to the polymerization mixture before the monomer conversion reaches 1%, for example, at the beginning of the polymerization reaction, it is essential that at least 50% o~ the total amount of the chain transfer agent be introduced into the polymerization mixture within the period when the monomer conversion is in the above specified range.

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This method of divided addition of the chain transfer agent, i.e. addition of 1% to 50% of the chain transfer agent before the monomer conversion reaches 1% or, preferably~ at or before the initiation of the polymerization and the remainder at the moment when the mqnomer conversion is in the range from 1 to 30% resulted, unexpectedly, in less residual monomer in the polymer products.
This divided addition of the chain transfer agent is especially advantageous in improving the particle size distri-bution when an increase is desired in the amount of vinylchloride monomer charged into the polymerization reactor. In this case/ however, it should be kept in mind that too small an amount of the chain transfer agent added at the earlier stage, i.e. before the 1% monomer conversion stage, will not produce a desirable ef~ect while too large an amount of the chain transfer agent added at the earlier stage could adversely effect the particle size distribution, and, therefore, it is recommended that the amount of the chain transfer agent added at the earlier stage be in the range from 0.0005% to 0.028% by weight based on the weight of the monomer or monomer mixture, the remainder of the chain transfer agent being added at the later stage, i.e.
when the monomer conversion is in the range of 1% to 30%.
Aside from the use of a mercapto-containing organic compound as the chain transfer agent the polymerization procedure is largely the same as in the conventional polymerization of vinyl chloride in an aqueous medium containing suspending agents selected from water-soluble polymeric substances such as partially saponified polyvinyl alcohol, methylcellulose, polyvinyl pyrrolidone, vinyl acetate-maleic anhydride copolymers, starch and gelatine, optionally in combination with anionic ~ 6 --,!
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or nonionic surface active agentsO The polymerization initiator is usually a monomer-soluble one exemplified by organic peroxides such as diisopropylperoxy dicarbonate, acetylcyclohexylsulfonyl peroxide, tert-butylperoxy pivalate and lauroyl peroxide, and azo compounds such as azobisisobutyronitrile, ~,~'-azobis-2,4-dimethylvaleronitrile and ~,a'-azobis-4-methoxy-2,4-dimethyl-valeronitrile.
The method described herein is advantageous especially, in the suspension polymerization of vinyl chloride or a mixture of vinyl monomers mainly composed of vinyl chloride.
The comonomers to be copolymerized with vinyl chloride to give a copolymer include vinyl esters such as vinyl acetate, vinyl ethers, acrylic acid, methacrylic acid or esters thexeof, maleic acid, fumaric acid and anhydrides or esters thereof, aromatic vinyl compounds such as styrene, unsaturated nitrile compounds such as acrylonitrile, vinylidene halides such as vinylidene fluoride and vinylidene chloride and olefins such as ethylene and propylene.
The delayed addition of the chain transfer agent to the polymerization mixture is also applicable to the emulsion polymerization of vinyl chloride with an emulsifying agent.
The examples illustrate the method of the present invention in further detail. All parts are parts by weight and the procedures for the determination of the average molecular weight, the amount of residual vinyl chloride monomer, the dioctyl phthalate absorption and the heat stability were as follows:

Average molecular weight: by the method as speci~ied in JIS K 6721;

~2~9 Residual vinyl chloride monomer: gas chromatographic analysis for a solution of 1 g of the polymer in 25 ml of tetrahydrofuran;
Dioctyl phthalate absorption: 20 g of dioctyl phthalate were added to 10 g of the vinyl chloride polymer obtained in an example to form a blend which was kept standing at room temperature for 1 hour followed by centrifugal separation to remove the excess of dioctyl phthalate remaining unabsorbed in the polymer. The dioctyl phthalate absorbed in the polymer thus determined was expressed in % by weight based on the weight of the polymer;
Heat stability: a blend of 100 g of the vinyl chloride polymer obtained in an example 7 0.5 g of an organotin mercaptide (TUS-8831, product of Nitto Chemical Co., Japan) and 0.5 g of stearic acid was milled for 5 minutes in a roller blender with the temperature of the rolls at 160C and shaped into a sheet 0.~ mm thick. The sheet thus prepared was kept in an oven at 170C for 40 minutes and the appearance of the sample taken out of the oven was examined visually, the results being recorded in 5 grades as follows.
A: the same as before heating (even after extension of the heating time to 60 minutes); B: slightly discolored;
C: discolored to brown; D: discolored to dark brown; E: completely blackened.
In the tables, the abbreviations DMVN, PV and LPO are ~or a ,a ' -azobis-2,~-dimethylvaleronitrile, tert-butyl-peroxy-pivalate and lauroyl peroxide, respectively.

EXAMPL~_1 Into a 50 liter capacity stainless steel polymerization *A Trade Mark ,, _ ~ _ :, ~

reactor were introduced 30 kg of deionized water, 20 g of partially saponified polyvinyl alcohol, 15 kg of vinyl chloride monomer and a polymeriza-tion initia-tor as indicated in Table 1 in an amount also as indicated in Table 1 and the polymerization reaction was initiated by elevating the temperature. At the time when the monomer conversion reached a percentage as indicated in Table 1, a mercapto-containing organic compound as indicated in Table 1 was introduced into the polymerization mixture and the polymerization was continued at the temperature given in Table 1. After a time ~or polymerization as indicated in Table 1, the polymerization reaction was stopped and, after removal of the unreacted vinyl chloride monomer, the resultant polymer in the aqueous slurry was dehydrated and dried into a finished polyvinyl chloride product.
The polyvinyl chloride product thus obtained was examined for average degree of polymerization, particle size distribution, amount of residual vinyl chloride monomer, dioctyl phthalate absorption and heat stability to give the results as set out in Table 1.
Experiments No. 9 and ~o. 10 in Table 1 were undertaken for comparative purposes. In each experiment, 10 g of 2-mercap-toethanol was introduced into the polymerization mixture either at the time when the monomer conversion was about 50%
; (Experiment No 9) or before the initiation of the polymeriza-tion reaction; i.e. together with the deionized water, partially saponified polyvinyl alcohol/ vinyl chloride monomer and polymerization initiator [Experiment No. 10). The other conditions for these experiments wexe as shown in Table 1 and the results of the tests undertaken on the polymer products obtained in these comparative experiments were as set out in Table 1.
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The chain transfer agents used were as follows:
(a) 2-mercaptoethanol;
(b) 3-mercaptopropanol;
(c) thio~lycolic acid;
(d) 2-mercaptopropanol.

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~o ,,, .,,;.., 64~9 Into the same stainless steel polymerization reactor as used in ~xample 1 were introduced 30 kg of deionized water, 20 g of partially saponified polyvinyl alcohol, lS kg of vinyl chloride monomer and ~ azobis-2,4-dimethylvaleronitrile in an amount as indicated in Table ~ below and the polymerization reaction was initiated by elevating the temperature. In the course of polymerization, 2-mercaptoethanol in an amount as indicated in Ta~le 2 was added to the polymeriæation mixture at a time when the monomer conversion was in a range as indicated in Table 2 and the polymerization reaction was continued at a temperature of 62C or 58C.
After a polymerization time of 8 hours or 9 hours the polymerization reaction was stopped and, after removal of the unreacted vinyl chloride monomer, the resultant polymer in the aqueous slurry was dehydrated and dried to give the ; finished polyvinyl chloride product. The properties of the polymer products thus obtained were as set out in Table 2.
; Experiments No. 13 and No. 14 in Table 2 were under-taken for comparative purposes. In each experiment 2-mercapto-ethanol (~xperiment 13) and trichloroethylene (Experiment 1~) was introduced into the polymerization reactor together with deionized water, partially saponified polyvinyl alcohol and vinyl chloride monomer~ The other conditions for these experiments and the properties of the thus obtained comparative polymer products are given in Table 2.

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-Into the same stainless steel polymerization reactor as used in Example 1 were introduced 30 kg of deionized water, 20 g of partially saponified polyvinyl alcohol, a monomer mixture composed of 12.7 kg of vinyl chloride monomer and 2.3 kg of vinyl acetate monomer and 7.5 g of ~,al-azobis-2,4-dimethyl-valeronitrile and the polymerization reaction was initiated. At the time when the monomer conversion reached 12%, 15.0 g of

2-mercaptoethanol was added to the polymerization mixture and the polymerization reaction was continued. After an overall polymerization time of 11 hours and 20 minutes the polymerization reaction was stopped and, after removal of the unreacted monomers, the resultant copolymer in the aqueous slurry was dehydrated and dried to give a finished product of vinyl chloride-vinyl acetate copolymer. The properties of the thus prepared copolymer product were as set out in Table 3 under the heading of Experiment No. lS.
Experiments No. 16 and No. 17 in Table 3 were under-taken for comparative purpose. In Experiment No. 16, 15 g of 2~mercaptoethanol was introduced into the polymerization reactor together with the deionized water, partially saponified polyvinyl alcohol, vinyl chloride monomer and other ingredients and the overall polymerization time was extended to 12 hours.
For Experiment No. 17, 200 g of trichloroethylene was used as the chain transfer agent in place of 15 g of 2-mercaptoethanol and the overall polymerization time was extended to 12.5 hours.
The properties of the vinyl chloride-vinyl acetate copolymers obtained were as set out in Table 3.

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TAsLE 3 _ -I
Examp].e Control _ _ Experiment No. 15 16 17 _ Chain transfer 2-Mercapto- 2-Mercapto-Trichloro-agent, (g) ethanol ethanol ethylene (15.0) (15.0) (200) _ _ _ DMVN used, (g) 7.5 7.5 7.5 ~ _ Polymerization temperature, (C) 56 56 56 _ _ % Monomer conversion : when chain transfer 12 0 0 agent was added, (hrs. from the begin-ning of polymerization) (1) (0) (0) __ _ _ ~ Overall polymeriza-:~ tion time,(hrs.) 11.33 12 12.5 : ._ . .
Average degree of polymerization 560 550 570 ~ , _ _ ~_ Particle size distribution/ 60 mesh 100.0 16.5 100.0 % passed _ through screen : of 200 mesh 2.8 1.5 3.5 ,.

Into a 1,000 liter-capacity stainless steel polymeriza-tion reactor were introd~ced S00 kg of deionized water, 120 g of partially saponified polyvinyl alcohol, 60 g of hydroxypropyl methylcellulose, 350 kg of vinyl chloride monomer and a chain transfer agent and a polymerization initiator as indicated in Table ~ in amounts also given in Table 4 and the polymerization reaction was initiated by elevating the temperature. When the monomer conversion reached a percentage as shown in Table 4, an additional amount of the same chain transfer agent as initially used was added to the polymerization mixture and the polymerization reaction was continued at the same polymerization temperature until the pressure inside the polymerization reactor dropped to 7 kg/cm2G when the polymerization reaction was stopped by purging the unreacted vinyl chloride monomer.
The resultant vinyl chloride polymerizate was dehydrated and dried in a fluidizing drier operated with hot air at 75C for 20 minutes after the temperature of the polymerizate reached 70C~
For comparison, the same experimental procedures as above were repeated except that all the 2-mercaptoethanol was added before starting the polymerization reaction (Experiment No. 25); or part of the chain transfer agent was added before starting the polymerization reaction and the remainder was added when the monomer conversion was 0.5% (Experiment No. 26); or trichloroethylene instead of the mercapto-containing chain transfer agent was added before starting the polymerization reaction (~xperiment No. 27).

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_ Into the same polymerization reactor as used in Example 4 were introduced 600 kg of deionized water, 120 g of partially saponified polyvinyl alcohol, 60 g of hydroxypropyl methylcellulose, 250 kg of vinyl chloride monomer and a chain transfer agent and a polymerization initiator as indicated in Table 5 and the polymerization reaction was initiated by elevating the temperature. When the monomer conversion reached a percentage as shown in Table 5, an addi-tional amount of the same chain transfer agent as initially used was added and the polymerization reaction was continued at the same temperature until the pressure inside the polymerization reactor dropped to 7 kg/cm2G when the polymerization reaction was stopped by purging the unreacked vinyl chloride monomer. The treatment of the polymerizate after completion of the polymerization reaction was the same as in Example 4 and the results of the experiments are summarized in Table 5.
For comparison, all the 2-mercaptoethanol was added before starting the polymerization reaction (Experiment No. 31), the other conditions being the same as above.

, Example Control _ Experirnent No. 28 29 30 31 Chain transEer agent (a) (a) (a) (a) _ _ _ _ _ First addition of chain transfer agent, weight % of monomer 0.010 0.005 0.015 0.030 _ Second addition of chain transfer agent, 0.0250.020 0.025 0.000 weight % of monomer (at % monomer co.nversion) (15)(10) (20) _ _ Polymerization initiator, DMVN DMVN DMVN DMVN
(weight % of monomer) (0O04)(0.04)(0.04) (0.04) _ _ _ .
Polymerization temperature, (oc) 6262.5 62 63 l __ __ Particle size distribution, ¦ 60 mesh 100 100 100 98 % passed through¦ 100 mesh 89 90 _ _ 78_ 45 screen of 1 200 mesh1.5 0.8 1.5 3.5 Heat stability A B C C
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Residual monomer in the polymer, p.p.m. 0.8 1.5 1.5 20 _ _. . _ ~verage degree of polymerization 660 680 650 660 _ ;

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Claims

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

1. A method for preparing a low molecular weight polymer in an aqueous medium from a monomer mixture mainly composed of vinyl chloride monomer, said method comprising:
adding an organic compound containing at least one mercapto group and at least one functional group selected from the group consisting of: hydroxy and carboxyl groups, per molecule, to said aqueous medium, and wherein at least 50 weight percent of said organic compound is added to said aqueous medium when the monomer conversion is between 1% and 30%.

2. A method for preparing a low molecular weight polymer in an aqueous medium from a monomer mixture mainly composed of vinyl chloride monomer, said method comprising:
adding an organic compound containing at least one mercapto group and at least one functional group selected from the group consisting of: hydroxy and carboxyl groups, per molecule, to said aqueous medium when the monomer conversion is between 1%
and 30%.

3. The method of claim 2, wherein said organic compound is added to said aqueous medium when the monomer conversion is between 2% and 20%.

4. The method of claim 1, 2 or 3, wherein said organic compound has 2 to 7 carbon atoms per molecule.

5. The method of claim 1, 2 or 3, wherein said organic compound has 2 to 4 carbon atoms per molecule.

6. The method of claim 1, 2 or 3, wherein said organic compound is selected from the group consisting of:
thioalcohols and mercapto containing carboxylic acids.

7. The method of claim 1, 2 or 3, wherein said organic compound is selected from the group consisting of: 2-mercapto-ethanol, 2-mercaptopropanol, 2-hydroxypropylmercaptan, thioglycerin, thioglycolic acid, thiohydroacrylic acid, thiolactic acid, thiomalic acid and mixtures thereof.

8. The method of claim 1, 2 or 3, wherein said organic compound is 2-mercaptoethanol.

9. The method of claim 1, 2 or 3, wherein the weight of said organic compound added to said aqueous medium is from 0.001% to 0.5% relative to the weight of said monomer mixture.

10. The method of claim 1, 2 or 3, wherein the weight of said organic compound added to said aqueous medium is from 0.005% to 0.3% relative to the weight of said monomer mixture.

11. The method of claim 1, wherein 1 to 50 weight percent of said organic compound is added to said aqueous medium before the monomer conversion reaches 1%.

12. The method of claim 1, wherein 1 to 50 weight percent of said organic compound is added to said aqueous medium at or before the initiation of polymerization.

13. The method of claim 1, 11 or 12, wherein the weight of said organic compound added to said aqueous medium prior to 1% conversion of said monomer is from 0.0005% to 0.028% relative to the weight of said monomer mixture.

14. The method of claim 1, wherein said aqueous medium additionally contains a water-soluble polymeric suspending agent selected from the group consisting of: partially saponified polyvinyl alcohol, methylcellulose, polyvinyl pyrrolidone, vinyl acetate-maleic anhydride copolymers, starch and gelatin.

15. The method of claim 14, wherein said aqueous medium additionally contains an anionic or nonionic surface active agent.

16. The method of claim 1, 2 or 3 wherein a monomer-soluble polymerization initiator selected from the group consisting of: organic peroxides and azo compounds is used.

17. The method of claim 1, 2 or 3, wherein a minor portion of said monomer mixture is selected from the group consisting of: vinyl esters, vinyl ethers, acrylic acid, methacrylic acid and esters thereof, maleic acid, fumaric acid, anhydrides and esters thereof, aromatic vinyl compounds, unsaturated nitrile compounds, vinylidene halides and olefins.