Impact modified poly(alkenyl aromatic) resin compostions

Abstract: Thermoplastic compositions comprising rubber modified polyalkenyl aromatic resin and a modifying agent having a core-shell structure comprised of an interpenetrating, crosslinked (meth)acrylate core and a crosslinked styrenic resin shell have significantly improved impact strength, thermal resistance and surface appearance following molding into shaped articles.

Description: The present invention relates to thermoplastic resin compositions having improved impact strength, thermal resistance and surface appearance after molding. More particularly, it pertains to combining a rubber modified poly(alkenyl aromatic) resin and an impact modifier having a core-shell structure comprised of a crosslinked (meth) acrylate core and a crosslinked styrene resin shell which forms an interpenetrating network between the core and the shell with minimal grafting between the structures. BACKGROUND OF THE INVENTION It is known that rubber modified poly(alkenyl aromatic) resins are useful thermoplastic materials having a wide range of applications. Such resins because of their relatively low cost have become widely used commodities in spite of some deficiencies in impact strength, thermal resistance and surface appearance, e.g., gloss in thermoformed articles. One of the more important modifiers for improving properties of rubber modified poly(alkenyl aromatic) resin compositions is a saturated rubber known as Kraton G, available from Shell Chemical Company. Kraton G is a saturated triblock copolymer styrene-butadiene-styrene or styrene-ethylenebutylene-styrene (SEBS rubber). These modifiers provide useful impact strength improvements as well as superior thermal aging performance. The latter property is especially important in rubber modified poly(alkenyl aromatic) resin compositions designed for medium to high heat requirement applications. Such superior performance, however, has been achieved at relatively great expense compared to conventionally available impact modification systems and many manufacturers have sought to improve the properties of rubber modified poly(alkenyl aromatic) resin compositions with other impact modifier candidates. Yu et al, U.S. 3,944,631, discloses the modifications of styrene-acrylonitrile copolymers by adding an effective amount of an additive comprising a cross-linked (meth)acrylate rubbery core and a cross-linked interpenetrating styrene-acrylonitrile resin shell. The modifier is said to improve both impact strength and weather resistance in articles molded therefrom. In the copending, commonly assigned application of Bates, Ting and Haaf, Serial No. 839,406, filed March 14, 1986, now allowed (US 4 684 696), there are disclosed compositions comprising the core-shell modifiers of Yu et al. in polyphenylene ether resins, alone, or in further combination with rubber modified poly(alkenyl aromatic) resins. It has now been discovered that useful rubber modified polyalkenyl aromatic compounds can be provided which exhibit vastly improved impact strength combined with thermal stability and high gloss while other physical properties such as Beat Distortion Temperature (HDT) are unchanged or only slightly affected, such as flow and tensile strength. These improvements are achieved by combining the base resin with a modifying compound comprising the aforementioned core-shell modifier of Yu et al. This crosslinked core-shell modifier appears to improve the properties of the rubber modified poly(alkenyl aromatic) resin without suffering from the thermal degradation often experienced following the use of conventional modifiers of the unsaturated, i.e., non-hydrogenated type. In accordance with the present invention, there is provided a thermoplastic resin composition consisting of, by weight (a) 100 parts of a rubber modified poly(alkenyl aromatic) resin; and (b) 1 to 40 parts of a core-shell interpolymer consisting of a crosslinked (meth)acrylate core and an interpenetrating crosslinked styrenic resin shell. The rubber modified poly(alkenyl aromatic) resin (a) preferably comprises units of the formula wherein R¹ and R² are selected from lower alkyl or alkenyl groups having from 1 to 6 carbon atoms or hydrogen; R³ and R⁴ are selected from chloro, bromo, hydrogen or lower alkyl of from 1 to 6 carbon atoms; R⁵ and R⁶ are selected from hydrogen, lower alkyl or alkenyl groups of from 1 to 6 carbon atoms, or R⁵ and R⁶ may be concatenated together with hydrocarbyl groups to form a naphthyl group. More preferably said rubber modified component (a) comprises units wherein said rubber modified component (a) comprises units in which (i) R¹, R², R³, R⁴, R⁵ and R⁶ are each hydrogen; (ii) R¹ is methyl and R², R³, R⁴, R⁵ and R⁶ are each hydrogen; (iii) R¹, R², R⁴, R⁵ and R⁶ are each hydrogen and R³ is methyl; and (iv) R¹ and R² are each hydrogen and at least two of R³, R⁴, R⁵ and R⁶ are bromines and the remainder are hydrogen. The invention also contemplates a process for improving the impact strength, thermal resistance and surface appearance of articles molded from a thermoplastic resin composition, said process comprising: (1) providing an intimate mixture consisting of, by weight, (a) 100 parts of a rubber modified poly (alkenyl aromatic) resin and (b) 1 to 40 parts of a core-shell inter polymer consisting of a crosslinked (meth) acrylate core and an intererpenetrating crosslinked styrenic resin shell; and (2) molding said mixture into an article. Also provided by the invention are articles made by the process as above defined. DETAILED DESCRIPTION OF THE INVENTION The improved thermoplastic compositions of the present invention are based upon a combination of a rubber modified polyalkenyl aromatic resin and a core-shell modifier having a cross linked acrylate core which is surrounded and interpenetrated by a crosslinked styrene shell. The polyalkenyl aromatic compound is preferably polystyrene or another polymer formed from the following: vinyl toluene, alpha methyl styrene, ethyl vinyl benzene, vinyl naphthalene, mono and polychlorostyrene, mono and poly-bromostyrene, and the like. The integrity of the core-shell structure is maintained by the interpenetration and entanglement of the crosslinked components rather than by grafting. The base resin can be improved with various amounts of the core-shell modifier depending upon the particular thermoplastic application. At least 1 part by weight of the core-shell modifier is required per 100 parts of the base resin. Less than one part will not ordinarily provide very much beneficial effect in the overall composition. Also, greater than 40 parts by weight of the core shell modifier is not utilized per 100 parts of the base resin since the beneficial properties of the rubber modified polyalkenyl aromatic resin can be overshadowed by the high rubber content of this loading of the core-shell modifier. In typical embodiments, 5 to 25 parts by weight of the core-shell modifier will be utilized per 100 parts of the base resin. Regarding the core-shell modifier, a preferred embodiment would be comprised of a crosslinked butyl acrylate core. This acrylate core will ordinarily comprise approximately 40 to 90 weight percent of the core-shell combination. As stated previously, the preferred shell component is comprised of crosslinked polystyrene. The polyalkenyl aromatic resins useful in the present invention are the well-known styrenic-based materials. The rubber modified styrenes can be provided by combining or grafting natural or synthetic rubber compounds (e.g., a diene rubber, an ethylene-propylene-diene terpolymer rubber (EPDM), a polybutadiene, or an acrylate rubber) with styrene polymers. Those skilled in the art will be able to provide impact improved compositions comprising various proportions of the rubber modified polyalkenyl aromatic resin and the core-shell impact modifier. Conventional thermoplastic compounding technology such as single or twin screw extrusion can be utilized to provide compositions of the present invention. The above-described weight ratios and percentages represent the the compositional formulations of the present invention. The order of combining the components to provide final products may be varied as described below. The preferable core-shell interpolymer modifiers are those having a crosslinked acrylate rubber core, such as butyl acrylate. Surrounding this crosslinked core is a shell-like structure of crosslinked styrenic resin, preferably styrene, which surrounds and interpenetrates the crosslinked core. Incorporation of small amounts of other monomers such as acrylonitrile and/or methyl methacrylate with styrene in the shell can also provide useful products if the resulting copolymer shell does not cause significant incompatibility with the rubber modified polyalkenyl aromatic matrix. The integrity of such preferable core-shell structures is maintained by the interpenetrating network of the several crosslinked moieties rather than by grafting the structures together. The interpenetrating network is provided when the monomers forming the shell structure are polymerized and crosslinked in the presence of the previously polymerized and crosslinked acrylate core. The core-shell interpolymer compositions may be formed by the following type of two-step, sequential polymerization process: 1. emulsion polymerizing an acrylate monomer charge (herein designated "acrylate", for purposes of the present invention), of at least one C₂-C₁₀ alkyl acrylate, C₈-C₂₂ alkyl(meth)acrylate, or compatible mixtures thereof, in an aqueous polymerization medium in the presence of an effective amount of a suitable di- or polyethyleneically unsaturated crosslinking agent for such type of monomer, with the C₄-C₈ alkyl acrylates being the preferred acrylate monomers for use in this step; 2. emulsion polymerizing a monomer charge of styrene in an aqueous polymerization medium, also in the presence of an effective amount of a suitable di- or polyethyleneically unsaturated crosslinking agent for such monomers, said polymerization being carried out in the presence of the product from Step 1 so that the cross linked (meth)acrylate and crosslinked styrene components form an interpolymer wherein the respective phases surround and penetrate one another. Such a two-stage polymerization process is analogous to the three step process for producing an acrylate-styrene-acrylonitrile (ASA) terpolymer, as taught by Yu et al. in U.S. 3,944,631. This core-shell product, which is used as the interpolymer impact modifier in the present invention generally comprises from 40% to 90%, by weight, of at least one of the above-identified crosslinked acrylates, and from 10% to 60%, by weight of the crosslinked styrene component. It contains little graft polymerization between the crosslinked styrenic copolymer components and the crosslinked acrylate polymeric component. In the preferred embodiments, the core will comprise 50 to 80 weight percent crosslinked acrylate, based on the weight of crosslinked core and crosslinked shell taken together. Further details regarding this type of polymer composition can be found in the aforementioned U.S. 3,944,631 of Yu et al. The core-shell interpolymer provided by the foregoing process can be isolated and dried by conventional means and can be provided in powder or pellet form. The compositions of the present invention will generally be comprised of approximately 1 to 30 parts by weight of the core-shell interpolymer modifier based upon 100 parts of the base resin. It is particularly preferred that about 5 to 25 parts by weight of the core-shell interpolymer additive will be used per 100 parts of the base resin. The foregoing constituent ingredients can be compounded and molded by conventional means. The order of mixing and degree of shear experienced during extrusion can be varied. It would be expected that the physical properties could vary as such processing conditions are varied. Those skilled in the art will be able to achieve optimum processing conditions which may vary for different thermoplastic applications. Thus in one instance, each of the ingredients could be blended and extruded at once, thereby providing thermoplastic resin having a particular property profile. Alternately it may be desirable to pre-blend or precompound some of the ingredients while the remaining ingredients are charged later in a compounding or extrusion process. Additionally, it is expected that conventional additives such as fillers, pigments, stabilizers, plasticizers and flame retarding compounds can be incorporated in the thermoplastic compositions of the present invention, thereby providing a variety of useful products. For fillers, both reinforcing and non-reinforcing fillers are contemplated, including glass, clay and other mineral fillers. Conventional heat and oxidative stabilizers may be used. Among the useful flame retarding compounds which may be utilized are organic and inorganic halogen and phosphorus compounds with or without synergists such as, for example, antimony trioxide. DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention are taught in the following examples. All parts are by weight unless otherwise indicated. Synthesis of Core-Shell Additive Preparation A: crosslinked core/crosslinked styrene shell A crosslinked polybutylacrylate core having an interpenetrating, crosslinked polystyrene shell was prepared in a five liter, three-necked flask equipped with a heating/cooling jacket, a Teflon blade agitator, a thermometer and a nitrogen purge. The following solutions were prepared: Solution A: n-butyl acrylate 751.4 g 1-3 butyleneglycol diacrylate 1.27 g Solution B: Sodium Metabisulfite 1.75 g deionized water 27.4 g Solution C: Ammonium Persulfate 4.84 g deionized water 76.1 g Solution D: styrene 250.6 g divinyl benzene (55% active solution from Dow Chemical) 2.65 g Into the reaction flask was charged: 3,460 g deionized water, the following emulsifying agents: 2.1 g Sipex UB sodium lauryl sulfate (from Alcolac, Inc.) and 4.2 g Aerosol A-268, a disodium sulfosuccinate (from American Cyanamid), and 14.6 g of Solution B. The flask was stirred with N₂ sparge at room temperature for 30 minutes, to reduce the O₂ content. Thereafter, 150.5 g of N₂ sparged Solution A was added. The flask contents were heated to 55°C and then 13.5 g of Solution C was added to initiate polymerization. After 1.5 hours of reaction, a sample showed 4.1% resin solids indicating approximately 96% conversion. The remainder of Solution A as well as 14.6 g Solution B and 40.4 g Solution C were added. After 2.5 hours of additional reaction time at 55°C a sample showed 17.2% resin solids, indicating greater than 97% conversion. The reaction mixture was cooled to 35°C and Solution D was added and mixed for 15 minutes at 35°C. The reaction mixture was then heated to 60°C and the remainder of Solution C was added. The mixture was reacted for 1.25 hours. The temperature was raised to 75°C and maintained for 45 minutes. A final sample showed 22.4% resin solids indicating a conversion greater than 98%. The product latex was coagulated in a solution of 0.25 weight percent CaCl₂ in methanol at a rate of 1600 ml methanol per 800 ml latex. The coagulum was filtered, rinsed with fresh methanol, and dried in a vacuum oven at 60°C. The product had a rubber content of 75% by weight, a number average latex particle diameter of 211 nanometers, a swell index in methyl ethyl ketone (MEK) of 8.1 and a percent Gel fraction from MEK extraction of 91.1%. Examples 1-3 Several blends of a high impact polystyrene (HIPS) and the impact modifier of Preparation A were blended by a Henschel mixer, then extruded by a 2.54cm (one inch) Wayne single-screw extruder at a temperature profile of 177-185-191-196°C (350-365-375-385°F). Extruded materials were injection molded into ASTM specimens by a 85 Toshiba injection molding machine at a temperature setting of 177°C (350°F) (melt) 540°C (130°F) (mold). The compositions used and the results obtained are set forth in the Table : Example Control 1 2 3 HIPS 100 100 100 100 Geloy IM-XS - 5 10 20 HDT°C (°F) 0.63cm (1/4") @ 1821 KPa (@264 psi) 76 (168) 77 (171) 77 (171) 75 (167) Izod J/m (ft-lb/in) 298 (5.6) 346 (6.5) 426 (8.0) 458 (8.6) Gardener Nm (in-lb) 5.5 (50) 8.25 (75) 9.35 (85) 13.75 (125) TYS MPa (psi) 31.88 (4620) 30.15 (4370) 29.39 (4260) 28.22 (4090) TBS MPa (psi) 25.67 (3720) 24.08 (3490) 23.46 (3400) 21.94 (3180) TE (%) 52 50 54 47 Flow Channel cm (in) 28.58 (11.25) 27.94 (11) 27.15 (10.69) 26.52 (10.44) Gloss (60 degree) 40 43 45 48 Kraton G, Sipex UB, Aerosol A-268, Huntsman 1897 and Geloy IM-XS are registered trade marks. The data in the Table demonstrate that the impact modifier of Preparation A is a very powerful impact modifier for HIPS. Not only is the impact strength of the composition significantly increased, but other important physical properties such as thermal resistance are either unchanged or slightly affected. Surface gloss is very high. Many variations of the invention will suggest themselves to those skilled in this art in light of the foregoing detailed description. For example, instead of a modifier having a cross-linked n-butyl acrylate core, one having a cross-linked 2-ethylhexyl methacrylate core can be used. Modifiers having an alpha-methylstyrene, a vinyl toluene, or a nuclearly substituted dibromostyrene resin phase can be used. Instead of a diene-rubber modified HIPS, there can be used HIPS in which the rubber is an acrylate rubber or an EPDM rubber. All such obvious modifications are within the full intended scope of the appended claims.

Claims: A thermoplastic resin composition consisting of, by weight (a) 100 parts of a rubber modified poly(alkenyl aromatic) resin; and (b) 1 to 40 parts of a core-shell interpolymer consisting of a crosslinked (meth)acrylate core and an interpenetrating crosslinked styrenic resin shell. A composition as defined in Claim 1 wherein component (a) comprises units of the formula wherein R¹ and R² are selected from lower alkyl or alkenyl groups having from 1 to 6 carbon atoms or hydrogen; R³ and R⁴ are selected from chloro, bromo, hydrogen or lower alkyl of from 1 to 6 carbon atoms; R⁵ and R⁶ are selected from hydrogen, lower alkyl or alkenyl groups of from 1 to 6 carbon atoms, or R⁵ and R⁶ may be concatenated together with hydrocarbyl groups to form a naphthyl group. A composition as defined in Claim 2 wherein said rubber modified component (a) comprises units in which (i) R¹, R², R³, R⁴, R⁵ and R⁶ are each hydrogen; (ii) R¹ is methyl and R², R³, R⁴, R⁵ and R⁶ are each hydrogen; (iii) R¹, R², R⁴, R⁵ and R⁶ are each hydrogen and R³ is methyl; and (iv) R¹ and R² are each hydrogen and at least two of R³, R⁴, R⁵ and R⁶ are bromines and the remainder are hydrogen. A composition as defined in Claim 1 wherein the rubber in component (a) comprises a natural or a synthetic rubber. A composition as defined in Claim 4 wherein the synthetic rubber is selected from a diene rubber, an EPDM rubber, an acrylate rubber, or a mixture of any of the foregoing. A composition as defined in Claim 5 wherein said rubber is a polybutadiene. A composition as defined in Claim 1 wherein modifying agent (b) is present in an amount of from 5 to 25 parts by weight per 100 parts by weight of the resin (a). A composition as defined in Claim 1 wherein said crosslinked (meth)acrylate core is a crosslinked acrylate core. A composition as defined in Claim 8 wherein said acrylate is butyl acrylate. A composition as defined in Claim 1 wherein the crosslinked (meth)acrylate core comprises from 40 to 90 parts by weight per 100 parts by weight of the crosslinked core and crosslinked shell, taken together. A process for improving the impact strength, thermal resistance and surface appearance of articles molded from a thermoplastic resin composition, said process comprising : (1) providing an intimate mixture consisting of, by weight, (a) 100 parts of a rubber modified poly(alkenyl aromatic) resin and (b) 1 to 40 parts of a core-shell inter polymer consisting of a crosslinked (meth)acrylate core and an interpenetrating crosslinked styrenic resin shell; and (2) molding said mixture into an article. A process as defined in Claim 11 wherein the intimate mixture is provided by melt mixing. A process as defined in Claim 12 wherein step 1(a) comprises units of the formula : wherein R¹ and R² are selected from lower alkyl or alkenyl groups having from 1 to 6 carbon atoms or hydrogen; R³ and R⁴ are selected from chloro, bromo, hydrogen or lower alkyl or from 1 to 6 carbon atoms; R⁵ and R⁶ are selected from hydrogen, lower alkyl or alkenyl groups of from 1 to 6 carbon atoms, or R⁵ and R⁶ may be concatenated together with hydrocarbyl groups to form a naphthyl group. A process as defined in Claim 13 wherein said rubber modified component of step 1(a) comprises units in which (i) R¹, R², R³, R⁴, R⁵ and R⁶ are each hydrogen; (ii) R¹ is methyl and R², R³, R⁴, R⁵ and R⁶ are each hydrogen; (iii) R¹, R², R⁴, R⁵ and R⁶ are each hydrogen and R³ is methyl; and (iv) R¹ and R² are each hydrogen and at least two of R³, R⁴, R⁵ and R⁶ are bromines and the remainder are hydrogen. A process as defined in Claim 12 wherein the rubber component of step 1(a) comprises a natural or a synthetic rubber. A process as defined in Claim 15 wherein the synthetic rubber is selected from a diene rubber, an EPDM rubber, an acrylate rubber, or a mixture of any of the foregoing. A process as defined in Claim 16 wherein said rubber is a polybutadiene. A process as defined in Claim 11 wherein the modifying agent of step 1(b) is present in an amount of from 5 to 25 parts by weight per 100 part by weight of the resin (a). A process as defined in Claim 11 wherein said crosslinked (meth)acrylate core is a crosslinked acrylate core. A process as defined in Claim 19 wherein said acrylate is butyl acrylate. A process as defined in Claim 11 wherein the crosslinked (meth)acrylate core comprises from 40 to 90 parts by weight per 100 parts by weight of the crosslinked core and crosslinked shell, taken together. An article made by the process of Claim 11.

Inventors: TING SAI-PEI, TING, SAI-PEI
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1988
Main classification: C08L51
IPC codes: C08L25, C08L51, C08L9, C08L21