IL46787A - Process for the manufacture of polyvinyl chloride with reduced scale formation - Google Patents

Description

T ibD V*3 1»VtB -\Λ$.'>·>> 13 878 T' ttn oinp *>9 nopva ηηχη»π ηιη Process for the manufacture of polyvinyl chloride with reduced scale formation STMJT$?ER ΟΗΜΪΟΑΤ- COMPANY C. 44738 REDUCTION OF POLYMER SCALE FORMATION DURING PVC MANUFACTURE Abstract Use of a small amount of an ammonium or an alkali metal borate as a buffer in the suspension polymerization of PVC to thereby control the pH between about 7· 5 and 10 reduces the amount of polymer scale which forms on the inner walls of the reactor. Vihen initiators having a 10 hour half-life at a temperature below about 6o°C, such as diisopropyl peroxydi-carbonate (IPP) and t-butyl perpivalate (TBP) , are used further reactor cleanliness can be achieved by using them in a solvent comprising a C5-C12 straight or branched hydrocarbon, such as hexane or heptane.

Technical Description of the Invention The present invention is an improved process for. reducing the amount of PVC scale forming on the inner walls of the reactor during the suspension polymerization of vinyl chloride monomer (VCM).

A number of approaches have been suggested in an attempt to obtain a suspension polymerization of VCM which would give a reduced amount of polymer scale on the internal surfaces of the reactor. One general approach has been the use of various additives. For example, U.S. Patent No. 3,488, 28 to Koyanagi et al. has suggested the combined use of a non-ionic surfactant as a dispersing agent, an organic solvent which is inert to the polymerization reaction, and an alkaline reaction medium. This latter condition is obtained by the addition of alkali metal and alkaline earth metal the hydroxides, carbonates, acetates, phosphates and alcohol- ates of these metals. Hie use of a coating of a polar organic compound, dye, and/or pigment on the inner walls of the re¬ actor was suggested by U. S, Patent No. 3* 69,946 to Koyanagi et al. Addition of the oxide or hydroxide of an alkali metal to keep the pH above 8.0 was described in Japanese Patent No. 70/37,988 and Belgian Patent No. 780, 4l8.

The process of the present invention gives an unex¬ pectedly reduced formation of polymer scale due to the use of an ammonium or alkali metal borate.

The ammonium and alkali earth metal borates which are intended to be included within the scope of the term "borate" as used herein are: ammonium borate, NH4HB407 ·3Η20; anhydrous borax, Na2B407; borax pentahydrate, Na2B407-5H20; sodium borate decahydrate, Na2B407 · 10H20 and potassium tetraborate, Κ2Β407· 8H20. The borate can either be added to the polymerization medium or can be generated in situ by the addition of equimolar amounts of boric acid and an ammonium or alkali metal hydroxide or bicarbonate. The preferred borates are the sodium borates.

Despite the use of certain borates, e.g., borax, as a buffer for the production of various other polymers, Encycl.

Polymer Sci. Technol., 2:569-570 (1964), 71 Chem. Abstracts 22 58y (Italian Patent No. 809,717) and French Patent No.2, 163, 96J, it was not known that this particular type of substance would give a clean reactor when utilized in a suspension PVC process. The amount of borate that is used in the pol meriza^^* tion medium according to the present invention, gives a pH of between about 7.5 and 10 during the entire course of the reaction, preferably between about 8.0 and 9-0 and retards to a substantial degree the formation of polymer scale on the inside surfaces of the reaction vessel.

Suspension polymerization of PVC is well known in the art. Some recent examples of such processes are described in U.S. Patent Nos. 3, 488,328, to Koyanagi et al., 3,511,822 to Kraft et al. and 3,669,946 to Koyanagi et al.

Basically, such a type of process according to the present invention comprises the polymerization of an aqueous suspension polymerization medium of from about 10 to 0$ by weight of a vinyl chloride monomer charge, said vinyl chloride monomer optionally containing from 0$ - 0$ of the well-known monomers copolymerizable with said vinyl chloride as listed in U.S. Patent No. 3,488,328, from about 0.02$ to 2$ of suspending agent, optionally, from about 0.001$ to 5$ of a chain transfer agent, from about 0.01$ to 0.5$ by weight of borate as a buffering agent, from about 0.01$ to 0.3$ of initiator, with the remainder being water, said percentages being based on the total weight of the suspension. The suspension polymerization is preferably carried out at a temperature of between 4o°C. and 75 °C.

The use of the above mentioned borates is essential to the success of the present invention. It must be incorporated in the polymerization medium, either by addition of the borate or the in situ generation of the borate, in an amount which ^ will give a pH of between about 7.5 and 10 throughout the reaction, preferably between about 8.0 and 9-0. The amount of borate in the medium will generally vary between about O.OVo and about 0.5$> preferably between about 0.2 and about of the aqueous suspension containing the vinyl chloride monomer. If the borate is not used the pH will drop from about 7 to about 4 as the p^oymerization proceeds and wall fouling will occur. The above described borates give an un- expectedly increased degree of reactor cleanliness compared to use of other buffers or alkaline substances which produce a similar pH range in the aqueous suspension. The use of these borates does not affect the heat stability, relative viscosity or particle size of the resulting product.

The suspending agents which can be used in the present process are those synthetic or natural polymers having protective colloidal preperties such as partially saponified polyvinyl acetate, cellulose ether and gelatin. Methyl cellulose ether derivatives commercially available under the name "Methocel", a registered trademark of Dow Chemical Company, are preferred.

Initiators suitable for use in this process are also well known to persons in the art. Preferred compounds are 2, 2 ' -azobisisobutyronitrile, ^ye^1 -azobis (»<, ^-dimethylvalero- nitrile), lauroyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate (IPP), t-butyl peroxypivalate ( B ) , and β-^,β^' -azobis (e^,¾-dimethyl-^l-methoxyvaleronitrile) . When catalysts such as IPP and TBP, are used, either alone or combined with conventional free radical initiators in a weight ratio of 1 :3 ' f-to 1 : 6, further reactor cleanliness is achieved by utilizing them when they are present in a solvent comprising a straight or branched Cs-Ci2 saturated hydrocarbon, e.g. hexane. Although the use of a variety of solvents has been proposed as a means to utilize such initiators, U.S. Reissue Patent No. 25, 763 and PPG Technical Service Bulletin No.350 (Jan. 4, 196 ) , it has not been realized heretofore that use of the C5-C12 saturated or branched hydrocarbons, preferably hexane or heptane, further improves reactor cleanliness in a suspension polymerization of PVC. The amount of C5-C12 hydrocarbon solvent that is used will range from about 0. 1 wt$ to about 0. 4 wt$, based upon the weight of vinyl chloride monomer, preferably about 0. 2 to about 0.3 wt$.

It is preferable to add the initiator-solvent mixture to the reactor medium only after the reaction medium has reached the desired reaction temperature and after agitation has begun. This results in enhanced reactor cleanliness when these initiators are employed.

The optional chain transfer agents include halogenated aliphatic hydrocarbons containing from 1 to 20 carbon atoms, preferably 1-5 carbon atoms, and from about 1 to 30 halogen atoms. The preferred halogens are chlorine and bromine.

Exemplary of particularly desirable halogenated hydrocarbon chain transfer agents in the present invention are carbon tetrachloride, chloroform, bromoform, ethyltrichloride, per-chloroethylene, 1, -dichloroethylene, n-butylbromide, 2-bromo-propane, tetrabromoethane, 1, 2-dibromotetrachloroethane, trifluoro 1-chloroethane, tribromochloromethane, tetrabromoethane, dibromodichloromethane, bromotrichloromethane and the like. Other chain transfer agents that can be used include the well known alkyl mercaptan chain- transfer agents, e.g., n-decyl mercaptan, t-dodecyl mercaptan, and the like.

The aqueous suspension which is to be reacted comprises the catalyst, the vinyl chloride monomer charge, borate, buffer, suspending agent and, if desired, chain transfer agent, and the reaction is carried out in a closed reaction vessel or polymerizer under autogenous pressure. The vessel is equipped with an agitator, baffles, temperature controls and ordinarily is jacketed so as to better control the temperature. The reaction vessel must have enough cooling surface area to effectively remove the heat of polymerization and thereby enable an operator to keep the temperature and, consequently, the pressure under strict control. Agitation of the mixture can be accomplished by various means and may vary considerably. It should be sufficiently vigorous to disperse the liquid vinyl chloride monomer charge in the form of small globules distributed throughout the aqueous medium.

■The reaction is continued until a significant sure drop is noticed (i.e., about 70-95$ conversion) at which point the batch may be transferred to a stripper to remove unreacted monomer. The batches are then finished or dried to produce the granular polymer utilizing methods well known to persons of skill in the art. The following Examples further illustrate the invention: EXAMPLE 1 A stainless autoclave having a 2 gallon capacity and equipped with a paddle type stirrer was charged with , 500 g. of distilled deionized water, 2. 1 g. of methyl cellulose, 5. 0 g. of borax (Na2B407' 10 Η2θ) and 2. 1 g. of azobis (isobuty-ronitrile) at room temperature. The pH of this mixture was 8. 5. The autoclave was closed and evacuated by a vacuum pump. Then 3* 000 g. of vinyl chloride monomer was charged into the reactor. The reaction mixture was heated to 70°C. with good agitation (an r.p.m. of 4θθ ) . The vapor pressure of the reaction mixture (originally about 165 psi at 70°C. ) started to 'descend after 4 hours of reaction. When the pressure was 90 psi, the reaction mixture was cooled and remaining vinyl chloride monomer was recovered. About 90$ conversion was obtained. The pH of reaction slurry was about 8. 0, and the reactor was very clean as evidenced by substantially no deposition of polymer on the wall, baffle and agitator of the reactor.

EXAMPLE 2 The same polymerization described in Example 1 was repeated three additional times using the same reactor without solvent cleaning. This reactor was still very clean and could undoubtedly be used for more polymerizations before solvent cleaning was necessary.

EXAMPLES 3-8 The same reactor used in Examples 1 and 2 was charged with 4, 500 g. of distilled deionized water, 2.1 g. of methyl cellulose, and 5 - 0 g. of borax. The pH of the solution was 8. 5. The mixture was heated to 5 °C. and then 2.1g. of azobis ( EXAMPLES 9-11 The general procedure employed in Examples 1 and 2 was repeated with the absence of borax in the polymerization recipe. The inside wall of the reactor was completely coated with polymer even after the third batch and some peeling of the polymer was noted on the inside walls of the reactor when e fourth un was com eted EXAMPLES 12-14 'φ The general procedure reported in Examples 3-8 was repeated with the absence of borax in the polymerization recipe. The inside of the reactor again exhibited a high level of wall fouling.

EXAMPLE 15 A mixture of 4,500 g of distilled deionized water, 2.1 g of methyl cellulose and 10 g of sodium tripolyphosphate were charged into a 2 gallon stainless steel autoclave. To that mixture was added 4 g of diisopropyl peroxycarbonate (IPP) at 30°C. The pH of the mixture was 8.4. The autoclave was closed and was evacuated by a vacuum pump. Then 3000 g of vinyl chloride monomer was charged into the reactor. Polymerization was conducted at 50°C until a 35 pound pressure drop was attained. The reaction mixture was cooled and the remaining vinyl chloride monomer was recovered. About 90 conversion was obtained. The pH of the reaction slurry was 8.0 and the reactor showed a thin coating of polymer build-up on its inner surface. This example demonstrates that even though sodium tripolyphosphate maintains the pH in the range of 8-9* it does not pr£ ent wall fouling.

EXAMPLE l6 The same reactor used in Example 15 was charged with the same amount of ingredient as described in Example 15, and the polymerization was repeated. The reactor surface had a much thicker build-up of polymer scale.

EXAMPLE 17 ^ A solution of 2. 1 g. of methyl cellulose and 5 g- of sodium borate in 4, 500j^ g. of distilled deionized water was placed into a 2-gallon stainless steel reactor equipped with a paddle type stirrer. The mixture was heated to 4o°C. and the reactor was evacuated. Vinyl chloride monomer (J>,O0O g) was introduced and the mixture was heated to 50°C. with agitation. Initiator solution ( 10 g. of 25 weight percent IPP in hexane) was introduced into the reactor to give a 0. 25 addition of hexane based upon the weight of VCM, and the polymerization proceeded until a 30 pound pressure drop was observed The inner surface of the reactor was very clean with almost no polymer scale.

EXAMPLE l8 The reactor used in Example 17 was employed, without solvent cleaning, for 4 more consecutive polymerizations of vinyl chloride following the above procedure. The reactor was still clean enough for more runs.

EXAMPLE 19 Solid IPP was used as the initiator, in the absence of hexane, in the same process reported in Examples 17 and 18. The inner wall of the reactor showed significant amounts of polymer scale after only two runs.

Claims (15)

C- 232/ 2 What is Claimed:

1. In a suspension polymerization process for forming i^ polyvinyl chloride from a monomer charge containing a vinyl chloride monomer in a reactor in the presence of a catalytic amount of initiator wherein the improvement comprises incorporating in the polymerization medium an amount of an ammonium or alkali metal borate as a buffer which is sufficient to maintain the pH of the polymerization medium between about 7.5 and 10.0 and thereby reduce the amount of polyvinyl chlorid scale formed during the polymerization.

2. A process as claimed in Claim 1 wherein the amount of borate that is added maintains the pH at between about 8.0 and 9.0.

3. A process as claimed in Claim 1 wherein the amount of borate that is used varies between about 0.01 and about 0. by weight of the polymerizatio medium.

4. A process as claimed in Claim 3 wherein the amount of borate that is used varies between about 0.2$ and about 0.3$ by weight of the polymerization medium.

5. A process as claimed in Claim 1 wherein the borate is selected from the group consisting of the borates of ammonium, sodium and potassium.

6. A process as claimed in Claim 5 wherein the borate is a borate of sodium.

7. A process as claimed in Claim 1 wherein the borate is added to the polymerization medium.

8. A process as claimed in Claim 1 wherein the borate is generated in the polymerization medium by addition to said medium of boric acid and a compound selected from the group consisting of the hydroxides bicarbonates of ammonium, Λ sodium and potassium.

9. A process as claimed in Claim 8 where the compound is a hydroxide or bicarbonate of sodium.

10. A process as claimed in Claim 1 wherein the initiator that is used has a 10 hour half-life at temperatures below about 6o°C and is dissolved in a C5-Ci2 straight or branched hydrocarbon solvent.

11. A process as claimed in Claim 10 wherein the initiator is selected from the group consisting of diisopropyl peroxy-dicarbonate and t-butyl perpivalate.

12. . A process as claimed in Claim 10 wherein the amount of C5-C12 hydrocarbon solvent ranges between about 0. 1 and θΛ based o the weight of vinyl chloride monomer.

13. . A process as claimed in Claim 10 wherein the amount of C5-C12 hydrocarbon solvent ranges between about 0. 2 and 0. % based on the weight of vinyl chloride monomer.

14. A process as claimed in Claim 10 wherein the hydrocarbon solvent is selected from the group consisting of hexane and heptane.

15. . A process as claimed in Claim 10 wherein the initiator solvent mixture is added after the reaction medium has reached the desired reaction temperature and after agitation has begun.