Transparent Impact-Resistant Polystyrene Structure

Transparent impact-resistant polymers or resinous compositions are highly desirable for many packaging operations. Polystyrene and polymethylmethacrylate are both resins having a highly desirable degree of transparency for many packaging applications; however, both polystyrene and polymethylmethacrylate are often found to be too brittle to provide a reliable package, particularly when used as containers or as vacuum-formed oriented sheet, tubs, lids, and the like. In an effort to improve the impact resistance of resins such as polystyrene, rubber has been added to the polystyrene in one manner or the other to provide either a dispersion of solid rubber particles within a polystyrene matrix or a dispersion of rubber particles containing occluded polystyrene dispersed within a polystyrene matrix. Employing such techniques a substantial increase in the impact resistance of the polymer composition is obtained; however, the resultant product is usually opaque or translucent and is unsatisfactory for packaging applications which require a transparent material. Biaxial orientation of certain rubber containing impact-resistant styrene polymer films results in a product having improved transparency; however, when such articles are obtained in a heavier or thicker section the opacity is usually too great or the impact resistance is inadequate. In an attempt to produce transparent impact-resistant styrene polymers, considerable effort has been expended in employing as a polymer matrix a copolymer of methylmethacrylate and styrene, the methylmethacrylate and styrene being employed in such a proportion that the refractive index of the reinforcing rubber and the methylmethacrylate-styrene polymer differ by an insignificant value thereby providing a resinous body which does not scatter large amounts of light and, at least to the unaided eye, in thin sections, appears to be transparent. The resultant polymers, using refractive index matching, usually are polymers which have methylmethacrylate as a major component. Such impact-resistant polymers are set forth in the following patents: British Pat. No. 1,180,085; U.S. Pat. No. 3,261,887; U.S. Pat. No. 3,300,545; U.S. Pat. No. 3,400,175 and U.S. Pat. No. 3,427,274. Styrene polymers of improved transparency without the use of methylmethacrylate are described in U.S. Pat. No. 2,957,833 wherein styrene is polymerized in the presence of rubber with mixing and incremental addition of styrene to provide a polymer of improved transparency. Another technique of providing a rubber-reinforced styrene polymer of improved transparency is shown in U.S. Pat. No. 3,574,151 wherein styrene is polymerized in the presence of rubber and a small quantity of .beta.-bromostyrene.

It would be desirable if there were available an improved polymer structure which could be readily prepared employing a mass or bulk polymerization system, the polymer being impact resistant, transparent and composed primarily of styrene and a reinforcing diene rubber.

It would also be desirable if there were an improved impact-resistant transparent polymer structure having polymerized therein styrene as the monomeric component and a rubber dispersed therethrough.

These benefits and other advantages in accordance with the present invention are achieved in an improved stryene polymer structure, the structure comprising a continuous matrix of polystyrene, the structure having a dispersed phase of a reinforcing diene rubber, the diene rubber being present in a proportion of about five to about twelve parts by weight per one-hundred parts by weight of the combined weight of styrene and rubber, the rubber being in the form of a plurality of particles of generally cellular nature and containing occlusions of polystyrene, the particles being of both monocellular and multicellular configuration, the rubber particles having a weight average particle diameter not exceeding two microns and cell walls not greater than about 0.15 microns in thickness, the dimensions and configuration of the rubber particles being determined by means of an electron microscope on a sample of the polymer which has been treated with osmium tetroxide in accordance with the procedure set forth in Polymer Engineering and Science, by K. Kato, 7, 38(1967), with the further limitation that when the structure has a thickness of about 10 mils the light absorbance, measured using a wave length of 640 millimicrons, is not greater than about 0.10, and the structure has a notched izod impact value when measured in accordance with the American Society for Testing Materials, Specification D 256A of at least 0.65 foot pounds per inch of notch.

Also contemplated within the scope of the present invention is a method for the preparation of a generally transparent impact-resistant polymer structure, the steps of the method comprising providing a rubbery reinforcing diene polymer which is a polymer prepared from 1,3-butadiene, isoprene, or mixtures thereof, the diene rubber having an inherent viscosity in the range of 0.9 to 2.5 as determined at 25.degree. C employing 0.3 gram of rubber per deciliter of toluene, the diene rubber being employed in a proportion of from about five to twelve parts by weight, preparing a solution of the diene rubber in styrene and from about 0.01 percent to 0.3 percent by weight based on the combined weight of the rubber and styrene of a bromine-containing compound selected from the group consisting of: allyl bromide, 2-(bromomethyl)-3-bromopropene, 3,3-dibromo-1-propene, 3-bromo-1-pentene, 3,4-dibromo-1-butene, 2-(dibromomethyl)-1-butene, 2-(bromomethyl)-1-butene, 2-(bromomethyl)-1-pentene, 2-(dibromomethyl)-1-pentene, 2-(bromomethyl)-1-hexene, 3-bromo-1-hexene, 2-(dibromomethyl)-1-hexene, 3-bromocyclohexene, and 3-bromocyclopentene, and mixtures thereof, heating the mixture with agitation to a temperature to initiate free radical polymerization maintaining agitation until at least about 30 weight percent of styrene is converted to polymer and subsequently polymerizing the mixture until about 70 to 90 percent conversion of styrene to polymer to thereby provide an improved styrene polymer structure, the structure comprising a continuous matrix of polystyrene, the structure having a dispersed phase of a reinforcing diene rubber, the diene rubber being present in a proportion of about five to about twelve parts by weight per one-hundred parts by weight of the combined weight of styrene and rubber, the rubber being in the form of a a plurality of particles generally cellular nature and containing occlusions of polystyrene, the particles being of both monocellular and multicellular configuration, the rubber particles having a weight average particle diameter not exceeding two microns and cell walls not greater than about 0.15 microns in thickness, the dimensions and configuration of the rubber particles being determined by means of an electron microscope on a sample of the polymer which has been treated with osmium tetroxide in accordance with the procedure set forth in Polymer Engineering and Science, by K. Kato, 7, 38(1967), with the further limitation that when the structure has a thickness of about 10 mils, the light absorbance, being measured using a wave length of 640 millimicrons, is not greater than about 0.10, and the structure has a notched izod impact value when measured in accordance with the American Society for Testing Materials, Specification D 256A of at least 0.65 foot pounds per inch of notch.

In the preparation of polymers in accordance with the present invention styrene of commercial purity is satisfactory. Bromine-containing compounds hereinbefore set forth of commercial purity are satisfactory for use in the present invention.

If desired, in the polymerization of styrene polymers of the present invention a diluent may be employed. Usually it is desirable to employ a diluent or solvent which may be present in a quantity of up to 20 parts by weight per 100 parts styrene. The diluent generally aids in the polymerization by increasing heat transfer, by reducing the viscosity of the polymerizing mixture and in easing the problem of handling viscous syrups. Suitable diluents are hydrocarbons that are generally non-reactive under the polymerization conditions and are a solvent for the monomers and the polymer produced. Such diluents include substituted aromatic compounds such as ethylbenzene and the xylenes.

The bromine-containing compounds may be used individually or in admixture with each other. Similarly the free-radical initiating components may be used alone or in admixture with each other. If desired, a satisfactory product can be obtained using thermal initiation. The preferred free radical initiators are those which decompose to produce alkoxy radical fragments or aryloxyradical fragments. The following compounds are preferred initiators: tertiary butyl perbenzoate, tertiary butyl peracetate and 1,1-bis(t-butyl-peroxy-) cyclohexane.

Also useful in the preparation of polymers in accordance with the invention are: .alpha.,.alpha.'bis(t-butyl peroxy)diisopropylbenzene, di-t-amyl peroxide, t-butyl peroxy-3,3,5-trimethyl hexanoate, 2,5-dimethyl-2,5-bis(benzoyl peroxy)hexane, di-t-butyl peroxide, dicumyl peroxide, di-t-butyl diperoxy phthalate, t-butyl peroxy crotonate, t-butyl peroctoate, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-chlorobenzoyl peroxide, t-butylperoxy isopropyl carbonate, 1,1-bis-(t-butylperoxy)-3,3,5-trimethyl cyclohexane, t-butyl hydroperoxide, cumene hydroperoxide, 4-(t-butyl)-1,1-bis(t-butyl peroxy)cyclohexane, 1,1,3,3-tetramethyl butyl hydroperoxide, lauroyl peroxide, decanoyl peroxide, methyl ethyl ketone peroxides, cyclohexanone peroxides, .alpha.,.alpha.'-azo-diisobutyronitrile and the like. Usually such compounds are employed at a level of 0.01 to 0.5 weight percent based on the weight of the monomer. Rubbers useful for the preparation of polymers in accordance with the present invention are rubbers which are soluble in styrene in an amount of twelve parts by weight of rubber per one-hundred parts by weight of styrene at a temperature of 80.degree. C., the rubbers consisting essentially of the polymerization product of butadiene, isoprene, and copolymers thereof containing up to 30 weight percent styrene or mixtures thereof. Many such rubbers are well known and commercially available. Oftentimes, one or more such rubbery polymers may be employed in the polymerization, however, the total amount of the rubber should be in the proportion of from about five to twelve parts by weight of rubbery polymer per 100 parts by weight of styrene and rubber. Generally, in the preparation of polymers in accordance with the present invention employing the foregoing ingredients, it is desirable to combine the rubbery polymer and styrene with a diluent such as ethylbenzene. Any diluent may be employed which is generally non-reactive with the polymerizing components and the product of the polymerization under conditions of polymerization and devolatilization of the reaction mixture. The diluent must also be a solvent for the styrene polymer formed during the polymerization. In dissolving the rubber in styrene or a styrene mixture, generally it is desirable to include with the styrene a major portion of the solvent or diluent which is to be used in order to obtain the solution as rapidly as possible. Generally, a small portion of the diluent is employed to dissolve the initiator and the bromine-containing compound or grafting agent such as allyl bromide, 2-(bromomethyl)-3-bromopropene or mixtures thereof. The reaction mixture is then raised to appropriate polymerization initiating temperature such as a temperature between about 60.degree. C and 100.degree. C at which time the solution of the initiator and bromine-containing compound is added to the reaction mixture with agitation. The temperature of the reaction mixture is then raised to a temperature within the range of 80.degree. to 130.degree. C and maintained in this range for a period of about 3 to 10 hours with agitation, a nitrogen or other inert atmosphere being maintained within the reactor. When the solids content of the reaction mixture reaches from about 30 to 50 percent conversion, agitation may be discontinued if desired and the temperature of the reaction mixture raised over a period of 2 to 5 hours to a temperature within the range of 170.degree. to 190.degree. C. When polymerization, from a practical standpoint, is complete, generally in a range from about 70 to 95 percent conversion of the monomers to polymer solids, the reaction mixture is devolatilized usually at a temperature from about 200.degree. to 240.degree. C, beneficially under a vacuum of from about 0.1 to 50 millimeters of mercury. The polymer may be prepared by either batch or continuous-process polymerization. The following examples serve to illustrate the invention but should not be construed as limiting thereof.

A plurality of polymerization runs was carried out employing the following procedure. A two-liter jacketed reactor having an agitator was employed in all polymerizations. The agitator was a rotatable hollow shaft having two helical vanes mounted external to the hollow shaft. The vanes had a clearance of about 1 mil. from the inner wall of the reactor vessel. When the agitator was rotated, the helical vanes forced the reaction mixture downwardly along the shaft and adjacent walls of the reaction vessel. (More conventional agitators with horizontal crossbars can also be used satisfactorily). Styrene plus 95% of the diluent employed and rubber were added to the reactor. The reactor was nitrogen-purged and heated to a temperature of about 90.degree. C with stirring. The polymerization initiator and bromine-containing compound dissolved in the remaining 5% of diluent was then added. The agitator was set at a speed of about 20 rpm and the temperature of the reaction mixture raised to about 105.degree. C generally cycling from 100 to 110.degree. for a period of 4 to 5 hours. During this period, nitrogen was maintained over the reaction mixture. Periodically, samples of the reaction mixture were removed from a sampling connection on the bottom of the reaction vessel and when the reaction mixture contained between about 38 and 42 percent solids, the mixture was transferred to tubes and polymerization completed in a heating block programmed to raise the temperature of the reaction mixture from about 100.degree. to 175.degree. C over a period of 5 hours. At the end of that time, the solids content of the reaction mixture was from about 78 to 84 percent. The mixture was then forced from the tubes with nitrogen pressure and devolatilized in a vacuum oven maintained at a temperature of about 210.degree. C and under a pressure of about one millimeter of mercury for a period of about one and one-half hours. At the end of one and one-half hours of devolatilization, the product was removed from the oven and maintained in an inert atmosphere for a few minutes to minimize surface yellowing. The cooled slab was then granulated and portions molded into test plaques for light absorbance testing. Light absorbance was measured using a Beckman Model B spectrophotometer wherein the film sample was placed approximately 9 centimeters from the center of the photocell light detector. The wavelength employed was 640 millimicrons and the reading obtained was corrected to 10 mils thickness. Notched Izod impact tests were conducted employing procedure ASTM D-256A.

The rubbers employed were:

Diene 55: a polybutadiene rubber of 2.30 deciliter/gram inherent viscosity at a concentration of 0.3 gram per deciliter; and

Stereon S-700: approximately 80 parts by weight butadiene and 20 parts by weight styrene in which 18% is randomly present and 2% S is as block.

The results are set forth in the following Table.

Resins of the foregoing examples within the scope of the present invention had weight average particle diameter of less than two microns and cell walls not greater than about 0.15 microns in thickness. Majority of the particles showed occlusions of polystyrene. Some particles were monocellular while others were multicellular. Similar beneficial results are obtained when the hereinbefore delineated bromine-containing compounds are employed.

As is apparent from the foregoing specification, 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 description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto-appended claims.