CA1078549A - Suspension polymerization - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Large particles are obtained in the suspension polymerization of styrene employing carboxymethyl methylcellulose as the suspending agent by employing a relatively small quantity of the suspending agent in the initial phase of polymerization and after 20 to 40 weight percent conversion, additional suspension agent is added. This process produces primarily large polymer particles having a narrow particle size distribution which are particularly useful for molding into relatively large blocks or articles.

Description

_~ ~;'8S~s't One particularly convenient means of obkaining styrene polymers is by suspension polymerization.
monomer and polymerization promoter such as a peroxide is dispersed in an aqueous suspending medium by means of agitation to form dropléts. The droplets subsequen~ly polymerize to form beads or pearls of polystyrene, Depending upon the end use of the particular polystyrené, it may be desirable that the pearls or beads be large, small or have a broad particle size distribution. For the preparation of expandable polystyrene particles which are to be molded into relatively large blocks or articles, it is desirable that large polymer particles and preferably of narrow particle size distribution be employed. A typical large particle size polystyrene particulate mass would pass a 20 mesh U.S. sieve size screen and be substantially retained on a 40 mesh U.S.
sieve size screen. In conventional suspension polymeri-zation procedures, usually a much broader particle size distribution is obtained. A variety of polymerization systems are known wherein the particle size distribution is varied by a variety of means, such as increasing or decreasing the quantity of suspending agent, employing two different suspending agents at the same time or employing a colloidal suspending agent toge-ther with an anionic detergent. British Patent 1,344,060 discloses the preparation of nonspherical particles in a suspension polymerization system by a two-stage addition of the sus-pending agent.

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~7~54L9 It would be desirable if there were available an improved suspension polymerization methoa for the preparation of polystyrene.
It would also be desirable if there were avail-able an improved suspension polymerization process ~or polystyrene which produced primarily larger beads.
It would be desirable if there were available an improved suspension polymerization method for the preparation o~ large particles of polystyrene employing a carboxymethyl methylcellulose as the suspending agent.
These benefits and other advantages in accor-dance with the present in~ention are achieved in a method for the preparation of polystyrene particles wherein styrene monomer containing a styrene soluble free radical i~itiator is dispersed in an aqueous suspending medium containing carboxymethyl methyl-Cellulose, heating the suspension to a temperature sufficient to initiate polymerization of the styrene monomer to form styrene polymer and subsequently recovering generally spherical polystyrene particles from the sus-pending medium, the improvement which comprises providing from 0.02 to 0.04 weight percent of carboxymethyl methyl-cellulose in and based on the weight of the aqueous medium, agitating the suspending medium until from 20 weight percent to 40 weight percent of the styrene monomer is converted to polymer and subsequently adding from 0.1 to 0.3 weight percent of carboxymethyl methylcellulose to the suspending medium and maintaining agitation while Completing the polymerization and cooling.

18,3~4-F -2-`` ~ 854~
Styrene of commercial polymerization purity is satisfactory for the practice of the present invention as is carboxymethyl methylcellu-lose which is described in United States Patent 2,476,331. Carboxymethyl methylcellulose is a mixed cellulose ether which is water soluble and con-tains an average of about 0.7 to 2.1 methoxyl groups and from 0.02 to 0.3 carboxymethyl groups per anhydro-glucose unit.
Free radical initiators suitable for the practice of the pre-sent invention include: lauroyl peroxide, octanoyl peroxide, p-chloro-benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, benzoyl peroxide, di-isopropyl peroxy dicarbonate, t-butyl perbenzoate, di-t-butyl peroxide, dicumyl peroxide, azobisisobutyronitrile and the like. Usually a mixture of initiators, preferably azobisisobutyronitrile, benzoyl peroxide and t-butyl perbenzoate are used.
Polymerization in accordance with the present invention is generally accomplished employing temperatures between about 60C and 140C;
beneficially, from about 70C to 120C. Usually depending upon the proper-ties desired and the resultant polymer, the temperature of the polymeri-zation reaction is raised either continuously or stepwise from a lower value to a higher value during the course of the reaction. Such tempera-ture programming is familiar to those skilled in the art of styrene poly-merization. Generally, in the suspension polymerization of polystyrene, the weight ratio of styrene to water is maintained within the range of 0.5:1 to 1.5:1 and beneficially from 1:1 to 1.2:1. Beneficially, in the practice of the present invention, the appropriate amount of water containing the carboxymethyl methylcellulose is added to the polymerization vessel or alternatively the water added and the carboxymethyl methylcellulose dispersed therein. The polymerization reactor is then freed of substantial amounts of oxygen by the application of vacuum or a nitrogen purge or both. The monomer con-taining the free radical inltiator is added to the reactor with agitation sufficient to disperse the monomer as a plurality of droplets. The agitation is maintained throughout the polymerization. The temperature of the reaction mixture is raised to poly-merization temperature and when 20 to ~0 weight percent of the styrene added is converted to polymer, the remaining quantity of carboxymethyl methylcellulose is added as an aqueous solution. Due to the individual nature of suspension polymerization reactors, optimum agitation speed must be determined for each vessel. Generally, the optimum agitation speed will be about 20 percent -- less than the speed of agitation found satisfactory for conventional small particle size polymerizations carried out in the same vessel. If the vessel is e~uipped with a view port, that amount of agitation just sufficient to maintain dispersion without visible pooling of the monomer on the upper surface of the reaction mixture is generally found to be satisfactory.
The invention is further illustrated but not limited by the followiny e~amples wherein the average 8,384-F -4-.. . . .
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bead size was determined in accordance with the American Society for Testing Material procedure D-1921-6~ Method A.
Example 1 A one-gallon Parr bornb having a double-looped agitator was charged with 1,043 parts of deionized water, 20.6 parts by weight of a 2 weight percent aqueous solution of carboxymethyl methylcellulose (CMMC) of commercial purity, and 12 8 parts by weight of sodium dichromate serving as a water-phase polymerization inhibitor. The agitator speed was adjusted as indicated in Table I and the reactor purged three times by pressurizing to 30 lbs./sq. inch gauge with nitrogen and venting of the nitrogen. The reactor was then charged with 1,563 parts by weight of styrene having dissolved therein 1.36 parts by weight of azobisiso-lS butyronitrile, 2.35 parts by weight of benzoyl peroxide and 1.07 parts by weight of tertiarybutyl perbenzoate.
The polymerization schedule was; the temperature of the reactor and contents were raised to 70C over a period of one hour, the reactor and contents were then heated from 70 to 120C at a uniform ra-te of temperature rise over a period of six hours. The reaction mixture was maintained at 120C with agitation for an additional three hours and subsequently cooled to room temperature.
Additional suspending agent as a 2 weight percent carboxymethyl methylcellulose solution in 204 parts of water were added at the times indicated in Table I.
Four polymerization runs were made; two with an agitator speed of 125 rpm, and two at the speed of 225 rpm. The comparative data indicates the difference in bead size 18,384-F -5-~07~
obtained when all of the suspending ~gent is initiall~ l added to the reactor and when sequential addition is made.

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18, 384-F --7-107854~ Il Example 2 A 30-gallon reactor was employed to make five polymerization runs employing the same proportions and ingredients utilized in Example 1. The same polymeri- . -zation schedule was employed. Two speeds of agitation were utilized and one run made with all of the car~
bXymethyl methylcellulose initially present. The results are set forth in Table II.

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The foregoing clearly illustrate the increase in particle size obtained by the delayed addition of the suspending agent.
As is apparent from the foregoing specifi-cation, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and descrip~
tion.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for the preparation of poly-styrene particles wherein the styrene monomer containing a styrene soluble free radical initiator is dispersed in an aqueous suspending medium containing carboxymethyl methylcellulose, heating the suspension to a temperature sufficient to initiate polymerization of the styrene monomer to form styrene polymer and subsequently recovering generally spherical polystyrene particles from the sus-pending medium, the improvement which comprises providing from 0.02 to 0.04 weight percent of carboxymethyl methyl-cellulose in and based on the weight of the aqueous medium, agitating the suspending medium until from 20 weight percent to 40 weight percent of the styrene monomer is converted to polymer and subsequently adding from 0.1 to 0.3 weight percent of carboxymethyl methylcellulose to the suspending medium and maintaining agitation while completing the polymerization.