Transparent impact resin and process for the preparation thereof

Abstract: Styrene is polymerized employing a free radical initiator in the presence of a styrene-diene block copolymer of the AB or AB(BA) n configuration to provide a polymer containing from about 15 to 30 weight percent diene units polymerized therein. The polymer contains polydiene domains in the form of rods and spheres having diameters of 70 nanometers or less.

Description: Transparent impact resistant styrene containing polymers are highly desirable for many applications, particularly in the field of packaging, and are suitable for such applications as tub and cup lids, blister packs and the like. Polystyrene is much too brittle for many applications and impact polystyrene does not have sufficient transparency for many of such packaging applications. Impact resistant transparent resins have been prepared by a variety of methods employing anionic polymerization with butyllithium as initiator. Other techniques have employed methylmethacrylate in the styrene polymer to prepare impact resistant resins wherein the rubber domains would not significantly interfere with the transmission of light. It is the technological problem of the invention to provide an improved impact resistant transparent styrenic resin which does not require anionic polymerization and its attendant purity problems. This problem is solved by providing an impact resistant alkenyl aromatic resinous composition in the form of a matrix of a polyalkenyl aromatic monomer having therein a block copolymer, said resinous composition being obtained by free radical polymerization of the alkenyl aromatic monomer in the presence of the block copolymer, the block copolymer having a weight average molecular weight of at least 40 000 and having the AB or AB(BA) n ―configuration, wherein A represents a block of polymerized alkenyl aromatic monomer and B represents a block of polymerized conjugated diene monomer and n is an integer from 1 to 5, characterized in that, a) the composition is transparent and contains from 70 to 85 weight percent of alkenyl aromatic units and from 30 to 15 weight percent of conjugated diene units polymerized therein, b) the weight average molecular weight of the polyalkenyl aromatic monomer matrix lies in the range of from 75 000 to 350 000, c) the block copolymer is contained in the matrix as a plurality of domains in the form of rods and spheres having diameters less than 70 nanometers and no occlusions being observable by electron microscopy, d1) the block copolymer contains from 39 to 70 weight percent of polymerized alkenyl aromatic monomer and from 61 to 30 weight percent of polymerized butadiene therein or d2) the block copolymer contains from 35 to 50 weight percent of polymerized alkenyl aromatic monomer and from 65 to 50 weight percent of polymerized isoprene, which optionally can be partially replaced by butadiene, and e) when the block copolymer used is a copolymer comprising butadiene, the block copolymer having a weight average molecular weight from 40,000 to 150,000, crosslinking between the polymerized diene blocks has been prevented by addition of an antioxidant at or near the end of the polymerisation. Also within the scope of the invention is a method for the preparation of an impact resistant alkenyl aromatic resinous composition by the free radical polymerisation of at least one alkenyl aromatic monomer in the presence of a block copolymer and forming a matrix of polyalkenyl aromatic monomer having therein the block copolymer having a weight average molecular weight of at least 40000 and having the AB or AB(BA) n configuration, wherein A represents a block of polymerized alkenyl aromatic monomer and B represents a block of polymerized conjugated diene monomer and n is an integer from 1 to 5, characterized by forming a transparent composition containing from 70 to 85 weight percent of alkenyl aromatic unit and from 30 to 15 weight percent of conjugated diene units polymerized therein, providing a weight average molecular weight of the polyalkenyl aromatic monomer matrix of from 75 000 to 350 000 having therein the block copolymer as a plurality of domains in the form of rods and spheres having diameters less than 70 nano-meters and no occlusions of polymerized alkenyl aromatic monomers being observable by electron microscopy and i) the block copolymer having a weight average molecular weight from 40,000 to 150,000, containing from 39 to 70 weight percent of polymerized alkenyl aromatic monomer and from 80 to 30 weight percent of polybutadiene therein or ii) the block copolymer contains from 35 to 50 weight percent of polymerized alkenyl aromatic monomer and from 65 to 50 weight percent of polymerized isoprene, which optionally can be partially replaced by butadiene, and when a block copolymer comprising butadiene is used, preventing crosslinking between the polymerized diene blocks by adding an antioxidant at or near the end of the polymerization. When a polybutadiene block copolymer is used, the molecular weight of the block copolymer is from 40,000 to 150,000. By an alkenyl aromatic monomer is meant an alkenyl aromatic compound having the general formula: wherein "Ar" represents an aromatic hydrocarbon radical, or an aromatic halohydrocarbon radical of the benzene series, and "R" is hydrogen or the methyl radical. Examples of such alkenyl aromatic monomers are styrene, ortho-methylstyrene, meta-methyl-styrene, para-methyl-styrene, ar-ethylstyrene, ar-vinylxylene or mixtures thereof. The preferred block copolymers to be used for the practice of the invention are block copolymers of alkenyl aromatic monomer and diene of the AB or AB(BA) n configuration wherein A represents a block of polymerized alkenyl aromatic monomer and B is a block of polymerized conjugated diene such as polybutadiene, polyisoprene or copolymers of butadiene and isoprene and n is an integer of from 1 to 5. Preferably n is one. The proportion of alkenyl aromatic monomer should be from 20 to 70, preferred from 39 to 70 weight percent and the proportion of diene from 61 to 30 weight percent with the further limitation that the weight average molecular weight is above 40,000 as determined by gel permeation chromatography and up to a molecular weight wherein the desired morphology is not obtained. Mixtures of butadiene and isoprene are also useful as the B block. Polymers in accordance with the present invention are believed to be a mixture of a variety of components. The matrix is principally polyalkenyl aromatic monomer having a weight average molecular weight as determined by gel permeation chromatography of from 75,000 to 350,000. It is believed some graft copolymer exists wherein the alkenyl aromatic monomer has grafted onto the blocked copolymer. The free-radical initiator may be any of those known in the art such as, for example, 4 - (t - Butyl) - 1,1 - bis(t - butylperoxy) cyclohexane, benzoyl peroxide, and lauroyl peroxide. Generally, the amount of initiator employed is in a range of 0.025 to 0.2 weight percent based on the combined weight of the monomer plus block copolymer. Oftentimes, and particularly when the diene polymer is polybutadiene, it is desirable to employ a chain transfer agent to help maintain the molecular weight of the polymer in a desirable range. Suitable chain transfer agents include organic thiols such as aliphatic mercaptans typically dodecyl mercaptan, halogenated hydrocarbons such as carbon tetrachloride or alphamethylstyrene dimer. Such chain transfer agents when added are generally in a concentration of from 0.001 to 0.5 weight percent based on the combined weight of monomer plus block copolymer. Similarly, when the diene is polybutadiene, antioxidant is added at or near the end of the polymerization to prevent crosslinking between the diene polymer blocks. Such antioxidants are well known to those skilled in the art and include hindered phenols such as 2,6 - di - (tertiarybutyl) - 4 - methylphenol and combinations of hindered phenols and alkylated aryl phosphites. Generally, polymers in accordance with the present invention are prepared by polymerization in mass, solution or suspension at temperatures ranging from 105° to 175°C. If desired, inert solvents may be incorporated prior to polymerization in order to reduce the viscosity of the reaction mixture. Typical solvents include benzene, toluene, xylene, ethylbenzene, cyclohexane and methylcyclohexane. On completion of the polymerization step, such solvents are generally removed from the polymer by exposing molten polymer to reduced pressure such as from 0.14 to 136 mbar (0.1 to 100 millimeters of mercury) until the solvents and residual monomer have been removed from the polymer. The resultant polymer compositions are moldable, i.e., they may be directly heat fabricated into a desired form such as sheet or film, or extruded as strands, the strands being cooled and cut into pellets. The invention is further illustrated by the following examples, wherein all parts and percentages are by weight unless otherwise specified. Example 1 An agitated pressure reactor was charged with 115 grams of a styrene-butadiene block copolymer dissolved in 334 grams of styrene, 121 grams of ethylbenzene. The block copolymer had a weight average molecular weight of 90,000 as determined by gel permeation chromatography and contained 45 weight percent styrene and 55 weight percent butadiene. One and one-tenth grams of alphamethylstyrene dimer was employed as a chain transfer agent together with 1.7 grams of mineral oil as a lubricant and 0.24 grams of 4 - (tertiaryburyl) - 1,1 - bis - (tertiarybutylperoxy)cyclohexane as free radical initiator. The resultant reaction mixture was heated to 105° and agitated, by an agitator rotating at about 30 revolutions per minute. Over a period of 5―1/2 hours the temperature of the reaction mixture was increased from 105 to 155°C. At this time, 2.8 grams of 2,6 - di - (tert - butyl) - 4 - methylphenol and 1.8 grams of trisnonylphenyl phosphite were added as antioxidants. When the antioxidants had been dispersed in the reaction mixture, the reaction mixture was placed in trays and heated in a vacuum oven at 175° for one and one-half hours under a pressure of 27 mbar (20 millimeters of mercury) to remove ethylbenzene and unreacted styrene. The resulting polymer product contained about 19 weight percent polymerized butadiene units therein. The resultant polymer was ground and test bars compression molded therefrom. The following results were obtained: Notched Izod impact (0.38 ft-lbs/inch notch) 2.02 kg/cm; tensile strength at rupture, (3375 pounds per square inch) 23,270 kPa; tensile modulus 248,000 pounds per square inch (17.09,91 × 10³ kPa); Vicat heat distortion temperature 97.2°C; and melt flow rate using condition G of 1.71 grams. A 10 mil (0.254 mm) thick sheet was molded from the polymer and examined for light absorbence in a Beckman Model B spectrophotometer. The wave length employed was 640 nanometers and the film was placed about 9 centimeters from the center of the photocell light detector. The light absorbency was 0.07. An electron micrograph of sections of the molded specimen showed polybutadiene domains having the shapes of rods and spheres of about 35 nonometers in diameter and no visible occluded polystyrene. Example 2 In a manner similar to Example 1, a plurality of polymerizations were conducted by utilizing a variety of styrene-butadiene block copolymers. The results are set forth in Table I wherein product composition S/B indicates the weight ratio of styrene to butadiene units in the final product. Under the heading of Elastomer S/B comp. is the weight proportion of styrene to butadiene in the block copolymer and M w is its weight average molecular weight in gram moles; n. Izod impact is the notched Izod impact; H. D. indicates heat distortion in degrees Centigrade; nm, nanometers; MFR Sch. G., melt flow rate, schedule G. All polymerizations were made using 500 ppm (based on styrene plus elastomer) of 4 - (t - butyl) - 1.1 - bis(t - butylperoxy)cyclohexane. Ethylbenzene diluent was used in all the polymerizations at the level of 15-22% of the total feed solution, 0.2-0.4 Percent alpha-methylstryene dimer (chain transfer agent) and 0.3% mineral oil were also included in the feed solutions. All product polymers contain 1% 2 - 6 - di - tert - butyl - 4 - methylphenol and 0.5% tris nonylphenyl phosphite antioxidants. For the purpose of comparison, the first two runs in Table I used block copolymers outside the claimed range of composition (1st run) and molecular weight (2nd run). Both runs were unsatisfactory as the products had high absorbence of light. Electronmicroscopy showed that the polybutadiene domains of both products contain the "core and shell" structure. This structure consists of a polybutadiene shell of diameter larger than 100 nanometers and an occluded polystyrene core. Example 3 In a manner generally similar to Example 1, a plurality of polymerizations were conducted utilizing a variety of block copolymers containing styrene and isoprene and some block copolymers of styrene with isoprene and butadiene copolymer blocks. The results are set forth in Table II, wherein the term "S/I" designates a styrene isoprene block copolymer; the heading "Comp" indicates the weight percent composition and under "M.W." the weight average molecular weight as determined by gel permeation chromatography. Under "Comments", the letters refer to footnotes indicating deviation from the procedure used in Example 1. All of the resultant polymers prepared indicated polydiene domains having the shapes of rods and spheres and less than about 50 nanometers in diameter with no visible occluded polystyrene. The stabilizers employed were either a mixture of 2,6 - di - (tert - butyl) - 4 - methylphenol with trisnonylphenyl phosphite as employed in Example 1; or (n - octadecyl - 3 - (4 - hydroxy - 3.5 - ditert - butylphenyl) - propionate), to provide a level of about 0.2 weight percent in the final product. In a manner similar to the foregoing example, other alkenyl aromatic monomers and mixtures thereof are readily employed to provide transparent impact resisting resins.

Claims: An impact resistant alkenyl aromatic resinous composition in the form of a matrix of a polyalkenyl aromatic monomer having therein a block copolymer, said resinous composition being obtained by free radical polymerization of the alkenyl aromatic monomer in the presence of the block copolymer, the block copolymer having a weight average molecular weight of at least 40 000 and having the AB or AB(BA) n configuration, wherein A represents a block of polymerized alkenyl aromatic monomer and B represents a block of polymerized conjugated diene monomer and n is an integer from 1 to 5, characterized in that, a) the composition is transparent and contains from 70 to 85 weight percent of alkenyl aromatic units and from 30 to 15 weight percent of conjugated diene units polymerized therein, b) the weight average molecular weight of the polyalkenyl aromatic monomer matrix lies in the range of from 75 000 to 350 000, c) the block copolymer is contained in the matrix as a plurality of domains in the form of rods and spheres having diameters less than 70 nanometers and no occlusions being observable by electron microscopy, d1) the block copolymer contains from 39 to 70 weight percent of polymerized alkenyl aromatic monomer and from 61 to 30 weight percent of polymerized butadiene therein or d2) the block copolymer contains from 35 to 50 weight percent of polymerized alkenyl aromatic monomer and from 65 to 50 weight percent of polymerized isoprene, which optionally can be partially replaced by butadiene, and e) when the block copolymer used is a copolymer comprising butadiene, the block copolymer having a weight average molecular weight from 40,000 to 150,000, crosslinking between the polymerized diene blocks has been prevented by addition of an antioxidant at or near the end of the polymerization. A method for the preparation of an impact resistant alkenyl aromatic resinous composition by the free radical polymerisation of at least one alkenyl aromatic monomer in the presence of a block copolymer and forming a matrix of polyalkenyl aromatic monomer having therein the block copolymer having a weight average molecular weight of at least 40 000 and having the AB or AB(BA) n configuration, wherein A represents a block of polymerized alkenyl aromatic monomer and B represents a block of polymerized conjugated diene monomer and n is an integer from 1 to 5, characterized by forming a transparent composition containing from 70 to 85 weight percent of alkenyl aromatic unit and from 30 to 15, weight percent of conjugated diene units polymerized therein, providing a weight average molecular weight of the polyalkenyl aromatic monomer matrix of from 75 000 to 350 000 having therein the block copolymer as a plurality of domains in the form of rods and spheres having diameters less than 70 nanometers and no occlusions of polymerized alkenyl aromatic monomers being observable by electron microscopy and i) the block copolymer having a weight average molecular weight from 40,000 to 150,000, containing from 39 to 70 weight percent of polymerized alkenyl aromatic monomer and from 61 to 30 weight percent of polybutadiene therein or ii) the block copolymer contains from 35 to 50 weight percent of polymerized alkenyl aromatic monomer and from 65 to 50 weight percent of polymerized isoprene, which optionally can partially be replaced by butadiene, and when the block copolymer used is a copolymer comprising butadiene, preventing crosslinking between the polymerized diene blocks by adding an antioxidant at or near the end of the polymerisation.

Inventors: LYONS CHARLES EDWARD, TUNG LU HO, LYONS, CHARLES EDWARD, TUNG, LU HO
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1981
Main classification: C08F287
IPC codes: C08F287