High Impact Polystyrene by Prepolymerization in a Water-in-Oil

14 High Impact Polystyrene by Prepolymerization in a Water-in-Oil Emulsion Downloaded by UCSF LIB CKM RSCS MGMT on December 3, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0099.ch014

Followed by Suspension Polymerization R U D O L F B. D E JONG Chemische Werke Hüls A.G., 4370 Marl, Germany

During the suspension polymerization of solutions of rubber in styrene under the conditions of a "normal" suspension polymerization, stabilization of the droplets of the prepolymerization mixtures in water by the suspending agent impedes the inversion of the rubber phase by restricting agitation within that mixture. This difficulty may be overcome by performing the prepolymerization in a water-in-oil emulsion. W/o emulsions can be made with standard suspending agents as emulsifiers by adding the aqueous phase with stirring to the rubber solution. Phase ratio w:o, type and concentration of the emulsifier, rate of agitation and polymerization are important reaction parameters during emulsification and polymerization. The transition from the w/o emulsion polymerization to a standard suspension polymerization occurs automatically with increasing styrene conversion.

Ti/iost impact polystyrenes on the market today are made by polymerizing solutions of rubber in styrene. The styrene solution of rubber is prepolymerized in bulk with stirring to 12-40% conversion. Polymeri zation is then completed either in bulk in the absence of agitation or in aqueous suspension with agitation. The degree of agitation during the prepolymerization step has a profound effect upon the properties of the final product (5, 6). A high rate of agitation leads to a product with a low gel content and small rubber particles, whereas a product with a large proportion of gel and with large rubber particles is obtained if the agitation during the prepolymerization step is only moderate. The rub221

In Multicomponent Polymer Systems; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

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ber particle size and the gel content are two important structural param eters determining the physical properties of high impact polystyrene.

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Prepolymerization may be carried out in the presence of water (7), which aids in heat transfer. A t the same time, however, because of the low viscosity of water compared with the polymeric phase, agitation within the organic phase is less intensive and therefore must be accel erated or prolonged to arrive at a rubber particle size and gel content equivalent to that obtained during bulk agitated prepolymerization. The rubber particle size in the final product increases several fold if the prepolymerization is carried out in the presence of a dilute aqueous solution of an alkane sulfonate or polyvinyl alcohol in place of pure water. The addition of a surface-active agent converts the coarsely dis persed oil-water mixture—obtained as above in the presence of pure water—into an oil-in-water emulsion. In this case even prolonged stirring during prepolymerization does not decrease the rubber particle size ap preciably in the final product. The stabilization of the droplets of the organic phase in water by the emulsifier obviously impedes or prevents agitation within the polymeric phase. Figure 1 shows the influence of these three prepolymerization methods (under otherwise equal reaction conditions) on the dispersion of rubber particles in polystyrene. As explained above the rubber particle size in the final product is a measure for the rate of agitation—under otherwise equal reaction con ditions—within the rubber-polystyrene-styrene solution during prepoly merization. Figure 1 shows that agitation is least effective if the organic

Figure 1. Interference phase contrast micrographs of rubber particles in polystyrene. Prepared by prepolymerization (A) in bulk, (B) in the presence of water, and (C) in an o/w emulsion.


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phase is present as an oil-in-water emulsion ( o / w ). A n optimum in agi tation efficiency is obtained only in those cases where the organic phase is the continuous phase—i.e. i n the presence of water in a w / o emulsion. Heat transfer during prepolymerization in a w / o emulsion should be better, and agitation within the polymeric phase should be as good as during the prepolymerization in bulk.


Experimental The following investigations were performed with a solution of 6% polybutadiene in styrene. The polybutadiene used was Buna CB-10 (Stereokautschuk-Werke Hiils G m b H & Co. K G ) which has a cis content of about 9 5 % and a Mooney viscosity M L - 4 of 45-50. The screening procedure of the potential emulsifiers was as follows. The compound to be tested was either dissolved in water or in the rubber solution de pending on which was the better solvent. The aqueous phase was then added dropwise to the rubber solution with agitation. The stirrer speed chosen was as high as possible but below the speed at which air was beaten into the mixture. Agitation was stopped a short time after the aqueous phase was added. Immediately afterwards, the rate of droplet coalescence was determined. In those cases where the droplets failed to coalesce or d i d so only slowly—i.e. when an emulsion was obtained— the emulsion type was determined by the dye solubility method ( 1 ). A mixture of the oil-soluble red Sudan III and the water-soluble malachite green was added to the emulsion. A green color indicated an o / w emul sion and a red color a w / o emulsion. Prepolymerization was then performed with this w / ο emulsifier. The polymerization was carried out in a 2-liter cylindrical glass vessel fitted with a four-stage cross-blade agitator operating at 500 rpm. The stirring motor was connected to a watt meter. The watt meter indicated major changes in the viscosity of the prepolymerization mixture such as the demulsification of a w / o emulsion—i.e., the transition of the highly viscous organic phase to the low viscosity of the aqueous phase. Prepolymerizations were performed at 60°-100°C. The final polymerizations—i.e., the prepolymerization in a w / o emul sion followed by suspension polymerization were carried out in a 40-liter stainless steel reactor. The stirrer speed was varied between 200 and 420 rpm. Prepolymerizations were performed at 60°-100°C, the ensuing suspension polymerizations at up to 140°C. Emulsifiers for the w/o Emulsion Polymerization The compounds tested as potential w / o emulsifiers for the rubber solution during this investigation can be divided into three groups: (a) finely divided solids, (b) low molecular weight emulsifiers, and (c) high molecular weight compounds. Finely divided solids, such as calcium phosphate, are used in large quantities as dispersing agents during the suspension polymerization of styrene. Finely divided solids modified by the controlled adsorption of various amphiphilic compounds have been



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used as emulsifiers for the w / o emulsion polymerization of styrene (8). A l l attempts to polymerize the rubber solution in a w / o emulsion with the aid of these emulsifiers were unsuccessful. Emulsifiers suitable for preparing w / o emulsions have H L B numbers in the range of 2-7 ( H L B = hydrophile-lipophile balance). The com mercial emulsifiers offered in this range are fatty acid ( stearic acid, oleic acid, etc. ) esters of polyhydric alcohols ( glycol, glycerol, sorbitol, etc. ). Other typical w / o emulsifiers are the fatty acid salts of multivalent metals (calcium, magnesium, zinc, etc.). Some emulsifiers produced w / o emul sions with pure styrene. However, none of the emulsifiers, which belong to this group and which were examined by us under the conditions de scribed above, produced w / o emulsions with the rubber solution. After the essential part of this work had been completed, a patent (3) appeared in which the prepolymerization of a solution of rubber in styrene in a w / o emulsion was described. Zinc stéarate and other compounds with H L B numbers below 10 are claimed as emulsifiers. A number of water-soluble high molecular weight compounds are used as protective colloids or suspending agents during the suspension polymerization of styrene. Many of these dispersants depress surface and interfacial tensions to a moderate degree. The applicability of the following dispersants as emulsifiers for the w / o emulsification of the rubber solution was investigated: poly (vinyl alcohol) ( P V A ) (MW ca. 80,000, degree of hydrolysis 8 7 % ) , hydroxyethylcellulose ( H E C ) (ap parent viscosity of a 2 % aqueous solution: 300 cp at a shear rate of 1 sec" ), poly (vinyl pyrrolidone) ( P V P ) and the 1:1 copolymer of styrene and maleic anhydride hydrolized and neutralized with aqueous sodium hydroxide ( S M N a ) . The following additional compounds were exam ined: a high viscosity hydroxyethylcellulose (apparent viscosity of a 1% aqueous solution: 3,000 cp at a shear rate of 1 sec" ), a high viscosity S M N a and the sodium salt of the azeotropic styrene acrylic acid co polymer ( S A N a ) . The high viscosity S M N a copolymer was obtained by neutralizing a 1:1 styrene—maleic anhydride copolymer with a dilute aqueous solution of sodium hydroxide in the presence of a small quantity of polyhydric alcohol (as a crosslinking agent). Using a phase volume ratio w:o of 1:1 and the rubber solution as the oil phase, w / o emulsions were successfully made with P V A , H E C , S M N a , and SANa. No emulsions were obtained with P V P . It was pos sible to increase the stability of the w / o emulsions, at the same level of emulsifier concentration, by using the high viscosity H E C or the high viscosity S M N a in place of the corresponding low viscosity products. The S A N a copolymer gave the most stable emulsions. It seems worth mentioning that these emulsifiers are not composed of large blocks of hydrophilic and lipophilic groups like the styrene-poly(ethylene oxide) 1

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graft copolymers, for instance, which are reported to be good w / o emul sifiers (2). O n the contrary, the hydrophilic and lipophilic groups of the emulsifying suspending agents are quite small and are arranged in the polymer chain in an alternating or random manner. It is significant that the efficiency of the emulsifier increases with increasing emulsifier vis cosity. It is not yet clear whether the viscosity of the aqueous phase, or the viscosity at the water/oil interface, or both, are responsible for this effect. Formation and Stability of w/o Emulsions as a Function of the Process Conditions The formation of a w / o emulsion with the rubber solution as the oil phase depended, like the production of w / o emulsions with low molec ular weight oils, to a large extent on the process conditions. It cannot be ruled out, therefore, that a few of the products which were examined as emulsifying agents during these investigations and which were dis carded as ineffective, would have yielded positive results if they had been tested under different conditions. To produce w / o emulsions the aqueous phase had to be added to the rubber solution. The reverse order of addition yielded o / w emul sions; at a phase volume ratio w:o of 1:1 it was impossible to change an o / w emulsion into a w / o emulsion even when good w / o emulsifiers, which displayed only a moderate tendency to produce o / w emulsions, were used. The tendency to form w / o emulsions and the stability of these emulsions increased with the following factors: (the tendency to form o / w emulsions decreased): (a)

with decreasing water content and increasing emulsifier con centration ( S A N a was an apparent exception; emulsions made with 0.1% of this emulsifier appeared to be more stable than those with 1% ); (b) with decreasing viscosity of the oil phase (an increase in the rubber content of the oil phase favored the production of o / w emulsions ) ; (c) with an increasing rate of agitation (similar observations were made during the emulsification of low molecular weight oils (4).

Demulsification of w/o Emulsions Of great importance to the polymerization is the smooth resolution of the w / o emulsions at the end of the prepolymerization. The transition of w / ο emulsion polymerization to suspension polymerization ( type o / w ) must be complete and must occur without great effort. It has already been pointed out that the stability of w / o emulsions increased with de-



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creasing viscosity of the oil phase and that the tendency towards forma tion of o / w emulsions increased with increasing viscosity of the oil phase. Therefore, it was expected that the w / o emulsions would become un stable during the polymerization—i.e., with increasing styrene conver sion. Demulsification was not only caused by increasing viscosity of the oil phase, it also depended on the type and quantity of the emulsifier used to prepare the particular emulsion. B y increasing the emulsifier concentration, demulsification was delayed. Usually demulsification oc curred without outside inducement. However, only a few w / o emulsions prepared with S A N a resolved, either partially or not at all. These w / o emulsions separated only a small quantity of the aqueous phase; the entire organic phase remained as a w / o emulsion. There are probably two reasons why the remaining w / o emulsions did not disintegrate fur ther. First, by discarding water the w / o emulsion should have acquired an increased stability. Secondly, agitation—probably necessary for the demulsification of a highly viscous w / o emulsion—within the w / ο emul sion is reduced greatly owing to the fact that the remaining w / o emulsion merely slips through the separated aqueous phase when agitated. These w / o emulsions disintegrated readily when a typical o / w emulsifier (e.g., an alkane sulfonate ) was added. This result may indicate that only those w / o emulsions demulsify which are prepared with an emulsifier having a distinct tendency to produce o / w emulsions. Development of a Polymerization Recipe for Producing High Impact Polystyrene The production of a high impact polystyrene with optimum gel con tent and rubber particle size required the careful balancing of a number of factors during the w / o emulsion polymerization: phase volume ratio w:o, type and quantity of emulsifier, rate of polymerization, rate of agi tation, and styrene conversion during the w / o emulsion polymerization. The phase ratio w:o chosen for the prepolymerizations was the one nor mally used for a suspension polymerization—i.e., a phase volume ratio w:o of 0.8:1 to 1:1. Of the emulsifiers mentioned above (not counting mixtures of these emulsifiers ) the high viscosity H E C seemed to provide the best over-all combination of properties required for the w / o emul sion polymerization, demulsification, and the suspension polymerization. The required amount of this emulsifier ranged between 0.2 and 0.5% of the aqueous phase, depending on the phase ratio w:o, rate of agitation, and rate of polymerization used and on the desired styrene conversion during the w / o emulsion polymerization. Using an agitator speed of 300 rpm and a rate of polymerization of 5-10% per hour, the w / o emulsions demulsified at a styrene conversion of about 3 0 % . A n optimum rubber



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Figure 2. Interference phase contrast micrographs of the rubber dispersion in high impact polystyrene. Prepared by prepolymenzation (A) in a w/o emulsion and (B) in bulk. particle size and gel content in the final product was obtained by regu lating and controlling the polymerization rate during the w / o emulsion polymerization or the conversion during the w / o emulsion polymeriza tion, while keeping the other factors within the above limits. Suspension polymerization was started immediately after demulsification. To limit water occlusions in the final polymer beads to about 1 % , the rate of styrene conversion during the first hour after demulsification was not allowed to exceed 1 0 % per hour. This guaranteed a certain agitation within the globules of the organic phase, which apparently was necessary to bring about complete demulsification. The reaction conditions used during the remainder of the suspension polymerization were those of the combined bulk-suspension polymerization process during the suspension polymerization step. Figure 2 shows the rubber dispersion in two high impact polystyrenes, which were prepared on the one hand by prepoly merization in a w / o emulsion and on the other hand by prepolymerization in bulk under otherwise equal reaction conditions. The rubber particle size in both products is about equal. This shows that the agitation con ditions within the polymer mixture during the w / o emulsion polymeriza tion must have corresponded closely to those of the bulk agitated prepolymerization. Conclusions H i g h impact polystyrene can be made by prepolymerization in a w / o emulsion with ensuing suspension polymerization. The processes which



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take place within the polymer mixture during the w / o emulsion poly merization correspond to those of the bulk agitated polymerization. Out wardly the conditions used for the w / o emulsion polymerization corre spond to those of a "normal" suspension polymerization. A number of suspending agents are suitable emulsifying agents for w / o emulsions. The efficiency of a particular emulsifying—suspending agent increases with increasing viscosity of that emulsifier. A successful w / o emulsion polymerization requires the careful balancing of the following factors: phase ratio w:o, viscosity of the rubber solution, type and concentration of the emulsifier, rate of agitation, and rate of polymerization. Demulsi fication of the w / o emulsion and transition to the suspension polymeriza tion occurs automatically with increasing styrene conversion. The reaction conditions necessary during the suspension polymerization correspond to those of the combined bulk-suspension polymerization process during the suspension polymerization step. Literature (1) (2) (3) (4) (5) (6) (7) (8)

Cited

Becher, P., "Emulsions: Theory and Practice," Reinhold, New York, 1965. Bartl, H . , Bonin, von H . , Makromol Chem. 57, 74 (1962). Dainippon Celluloid Co. Ltd., Japanese Patent 18,710 (Aug. 14, 1968). Rodger, W. Α., Trice, V. G., Rushton, J . H . , Chem. Eng. Progr. 52, 515 (1956). Ruffing, N . R., Kozakiewicz, Β. Α., Cave, Β. B., Amos, J . L . , U . S. Patent 3,243,481 (March 29, 1966). Schroeder, C. W., Lunk, Η. E., Doyle, M . E., Belgian Patent 619,901 (Jan. 7, 1963). Stein, Α., Walter, R. L . , U . S. Patent 2,862,907 (Dec. 2, 1958). Wenning, H . , Makromol. Chem. 20, 196 (1956).

RECEIVED February 25, 1970.


High Impact Polystyrene by Prepolymerization in a Water-in-Oil

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