IMPROVED FLOW CARBONATE POLYMER BLENDS

Abstract: Disclosed is a polymer blend composition containing an aromatic carbonate polymer and a rubber-modified monovinylidene aromatic copolymer comprising a star-branched rubber and optionally, a linear rubber. The polymer blend compositions according to the invention have a desirable balance of processability and when molded, impact strength.

Description: IMPROVED FLOW CARBONATE POLYMER BLENDS This invention relates to compositions containing a carbonate polymer and a rubber-modified monovinylidene aromatic copolymer. This invention relates particularly to a blend composition having improved processability and when molded, having improved impact strength. Carbonate polymers derived from reactions of dihydroxyorganic compounds, particularly the dihydric phenols, and carbonic acid derivatives such as phosgene have found extensive commercial application because of their excellent physical properties. These thermoplastic polymers appear suitable for the manufacture of molded parts wherein impact strength, rigidity, toughness, heat resistance, excellent electrical properties, glass-like transparency and good clarity are required. Unfortunately, however, these polymers are expensive in price and require a high amount of energy expenditure in extrusion and molding processes. In order to reduce the cost of processing carbonate polymers, said polymers may contain additives that reduce costs and lower the temperatures required for molding processes. The blends resulting from the processing of carbonate polymer and additive generally exhibit improved melt flow properties at the sacrifice of other desirable features such as heat resistance, and impact strength. In addition, blends of carbonate polymer and additive often do not exhibit a desirable glossy finish. In view of the deficiencies of the conventional carbonate polymers and blends thereof, it would be highly desirable to provide an economical carbonate polymer composition which exhibits improved processability while retaining, to some degree, the desirable properties characteristic of carbonate polymers such as impact strength and heat resistance, and exhibiting a glossy finish. The present invention is such a desirable carbonate polymer composition. The composition possesses a desirable balance of good processability, improved gloss, good thermal and physical properties, and especially, improved impact resistance. The composition is a heterogeneous blend comprising an aromatic carbonate polymer blended with an effective amount of a rubber-modified copolymer comprising a monovinylidene aromatic monomer, an ethylenically unsaturated nitrile monomer, and a rubber component comprising a star-branched rubber having three or more arms and optionally, a linear rubber. Preferably, the rubber-modified copolymer is a composition prepared using bulk, mass-solution or mass-suspension polymerization techniques. In another aspect, the present invention is a process for preparing a carbonate polymer composition which exhibits a desirable balance of good processability, improved gloss, good thermal and physical properties, and especially, improved impact resistance wherein a carbonate polymer is blended with an effective amount of a rubber-modified copolymer comprising a monovinylidene aromatic monomer, an ethylenically unsaturated nitrile monomer, and a rubber component comprising a star-branched rubber having three or more arms and optionally, a linear rubber. In a further aspect, the present invention involves a method of molding or extruding a polymer blend composition whereby a carbonate polymer is blended with an effective amount of a rubber-modified copolymer comprising a monovinylidene aromatic monomer, an ethylenically unsaturated nitrile monomer, and a rubber component comprising a star-branched rubber having three or more arms and optionally, a linear rubber. In yet a further aspect, the invention involves molded or extruded articles of a polymer blend composition comprising a carbonate polymer blended with an effective amount of a rubber-modified copolymer comprising a monovinylidene aromatic monomer, an ethylenically unsaturated nitrile monomer, and a rubber component comprising a star-branched rubber having three or more arms and optionally, a linear rubber. The carbonate polymer blend compositions of the present invention are especially useful in the preparation of molded objects notably parts having large surfaces prepared by injection molding techniques and having predictable finished dimensions, good heat resistance, and good room temperature and low temperature impact resistance. Such properties are particularly desired for exterior automotive body panel applications such as door panels and fascia, or other automotive applications such as instrument panels, fenders, hoods, trunk lids, side cladding parts, mirror housings, cowl vent grills, etc. These compositions can even find use in instrument housings such as for power tools or information technology equipment such as telephones, computers, copiers, etc. Suitable carbonate polymers employed in the present invention are well known in the literature and can be prepared by known techniques, for example several suitable methods are disclosed in US-A-3,028,365, US-A-4,529,791 , and US-A-4,677,162. In general, carbonate polymers, preferably aromatic carbonate polymers can be prepared from one or more multihydric compounds by reacting the multihydric compounds, preferably an aromatic dihydroxy compound such as a diphenol, with a carbonate precursor such as phosgene, a haloformate or a carbonate ester such as diphenyl or dimethyl carbonate. Preferred diphenols are 2,2-bis(4-hydroxyphenyl)-propane, 1 ,1- bis(4-hydroxyphenyl)-1 -phenylethane, 3,3-bis(para-hydroxyphenyl)-phthalide and bishydroxyphenylfluorene. The carbonate polymers can be prepared from these raw materials by any of several known processes such as the known interfacial, solution or melt processes. As is well known, suitable chain terminators and/or branching agents can be employed to obtain the desired molecular weights and branching degrees. It is understood, of course, that the carbonate polymer may be derived from (1 ) two or more different dihydric phenols or (2) a dihydric phenol and a glycol or a hydroxy- or acid-terminated polyester or a dibasic acid in the event a carbonate copolymer or heteropolymer rather than a homopolymer is desired. Thus, included in the term "carbonate polymer" are the poly(ester-carbonates) of the type described in US-A-3,169,121 , US-A-4,156,069, and US-A-4,260,731. Also suitable for the practice of this invention are blends of two or more of the above carbonate polymers. Of the aforementioned carbonate polymers, the polycarbonates of bisphenol-A are preferred. The aromatic carbonate polymer of the present invention preferably has a melt flow rate (MFR), determined under conditions of 300°C and an applied load of 1.2 kilogram (kg), equal to or greater than 2, more preferably equal to or greater than 5, more preferably equal to or greater than 7, and most preferably equal to or greater than 10 grams per 10 minutes (g/10 min.). Generally , the melt flow rate of the aromatic carbonate polymer is equal to or less than 80, preferably equal to or less than 40, more preferably less than or equal to 22, and most preferably equal to or less than 17 g/10 min. The aromatic carbonate polymer of the present invention is present in an amount equal to or greater than 10 weight percent, preferably equal to or greater than 20 weight percent, more preferably equal to or greater than 30 weight percent, even more preferably equal to or greater than 40 weight percent, and most preferably equal to or greater than 50 weight percent based on the weight of the polymer blend composition. The aromatic carbonate polymer of the present invention is present in an amount equal to or less than 90 weight percent, preferably equal to or less than 80 weight percent, more preferably equal to or less than 70 weight percent, even more preferably equal to or less than 60 weight percent, and most preferably equal to or less than 50 weight percent based on the weight of the polymer blend composition. Suitable rubber-modified copolymers employed in the present invention comprise a monovinylidene aromatic and ethylenically unsaturated nitrile copolymer in a matrix or continuous phase and rubber particles dispersed in the matrix. The matrix or continuous phase of the present invention is a copolymer comprising polymerized therein a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer or a copolymer comprising polymerized therein a monovinylidene aromatic monomer, an ethylenically unsaturated nitrile monomer and one or more vinyl monomer that can be copolymerized with them. Copolymer, as used herein, is defined as a polymer having two or more monomers interpolymerized. These compositions are generically known as SAN-type or SAN since poly(styrene-acrylonitrile) is the most common example. The weight average molecular weight (M w ) of the matrix copolymer is typically equal to or greater than 50,000, preferably equal to or greater than 80,000, and more preferably equal to or greater than 100,000. The weight average M w of the matrix copolymer is typically equal to or less than 300,000, preferably equal to or less than to 240,000 and most preferably equal to or less than 180,000. Molecular weight, unless otherwise specified is weight average molecular weight, is measured by gel permeation chromatography (GPC). Monovinylidene aromatic monomers include but are not limited to those described in US-A-4,666,987; US-A-4,572,819 and US-A-4,585,825. Preferably, the monomer is of the formula: R" I Ar-C=CH 2 wherein R is hydrogen or methyl, Ar is an aromatic ring structure having from 1 to 3 aromatic rings with or without alkyl, halo, or haloalkyl substitution, wherein any alkyl group contains 1 to 6 carbon atoms and haloalkyl refers to a halo substituted alkyl group. Preferably, Ar is phenyl or alkylphenyl, wherein alkylphenyl refers to an alkyl substituted phenyl group, with phenyl being most preferred. Preferred monovinylidene aromatic monomers include: styrene, alpha-methylstyrene, all isomers of vinyl toluene, especially paravinyltoluene, all isomers of ethyl styrene, propyl styrene, vinyl biphenyl, vinyl naphthalene, and vinyl anthracene, and mixtures thereof. Typically, such monovinylidene aromatic monomer will constitute from an amount equal to or greater than 50 weight percent, preferably from an amount equal to or greater than 60 weight percent, more preferably from an amount equal to or greater than 65 weight percent, and most preferably from an amount equal to or greater than 70 weight percent based on the total weight of the matrix copolymer. Typically, such monovinylidene aromatic monomer will constitute less than or equal to 95 weight percent, preferably less than or equal to 85 weight percent, more preferably less than or equal to 80 weight percent, and most preferably less than or equal to 75 weight percent based on the total weight of the matrix copolymer. Unsaturated nitriles include, but are not limited to, acrylonitrile, methacrylonitrile, ethacrylonitrile, fumaronitrile and mixtures thereof. The unsaturated nitrile is generally employed in the matrix copolymer in an amount equal to or greater than 5 weight percent, preferably in an amount equal to or greater than 10 weight percent, more preferably in an amount equal to or greater than 15 weight percent, and most preferably in an amount equal to or greater than 20 weight percent based on the total weight of the matrix copolymer. The unsaturated nitrile is generally employed in the matrix copolymer in an amount less than or equal to 50 weight percent, preferably equal to or less than 45 weight percent, more preferably less than or equal to 35 weight percent, and most preferably less than or equal to 25 weight percent based on the total weight of the matrix copolymer. Other vinyl monomers may also be included in polymerized form in the matrix copolymer, including conjugated 1 ,3 dienes (for example butadiene, isoprene, etc.); alpha- or beta-unsaturated monobasic acids and derivatives thereof (for example, acrylic acid, methacrylic acid, etc., and the corresponding esters thereof such as methylacrylate, ethylacrylate, butyl acrylate, methyl methacrylate, etc.); vinyl halides such as vinyl chloride, vinyl bromide, etc.; vinylidene chloride, vinylidene bromide, etc.; vinyl esters such as vinyl acetate, vinyl propionate, etc.; ethylenically unsaturated dicarboxylic acids and anhydrides and derivatives thereof , such as maleic acid, fumaric acid, maleic anhydride, dialkyl maleates or fumarates, such as dimethyl maleate, diethyl maleate, dibutyl maleate, the corresponding fumarates, n-phenyl maleimide, etc. These additional comonomers can be incorporated in to the composition in several ways including, interpolymerization with the monovinylidene aromatic and ethylenically unsaturated nitrile matrix copolymer and/or polymerization into polymeric components which can be combined, for example blended in to the matrix. If present, the amount of such comonomers will generally be equal to or less than 20 weight percent, more preferably equal to or less than 10 weight percent and most preferably equal to or less than 5 weight percent based on the total weight of the matrix copolymer. The matrix copolymer is present in an amount equal to or greater than 40 weight percent, preferably equal to or greater than 50 weight percent, more preferably equal to or greater than 60 weight percent, even more preferably equal to or greater than 70 weight percent, and most preferably equal to or greater than 75 weight percent based on the weight of the rubber-modified copolymer. The matrix copolymer is present in an amount equal to or less than 95 weight percent, preferably equal to or less than 90 weight percent, more preferably equal to or less than 85 weight percent, even more preferably equal to or less than 80 weight percent, and most preferably equal to or less than 75 weight percent based on the weight of the rubber-modified copolymer. The various techniques suitable for producing rubber-modified copolymer are well known in the art. Examples of these known polymerization processes include bulk, mass-solution, or mass-suspension polymerization, these are generally known as mass polymerization processes. See, for example, US-A-3,660,535; US-A-3,243,481 and US-A-4,239,863. In general, continuous mass polymerization techniques are advantageously employed in preparing the rubber-modified monovinylidene aromatic copolymer of the present invention. Preferably, the polymerization is conducted in one or more substantially linear, stratified flow or so-called "plug-flow" type reactor such as described in US-A-2,727,884, which may or may not comprise recirculation of a portion of the partially polymerized product or, alternatively, in a stirred tank reactor wherein the contents of the reactor are essentially uniform throughout, which stirred tank reactor is generally employed in combination with one or more "plug-flow" type reactors. The temperatures at which polymerization is most advantageously conducted are dependent on a variety of factors including the specific initiator and type and concentration of rubber, comonomers and reaction diluent, if any, employed. In general, polymerization temperatures from 60 to 160°C are employed prior to phase inversion with temperatures from 100 to 190°C being employed subsequent to phase inversion. Mass polymerization at such elevated temperatures is continued until the desired conversion of monomers to polymer is obtained. Generally, conversion of from 65 to 90, preferably 70 to 85, weight percent of the monomers added to the polymerization system (that is, monomer added in the feed and any additional stream, including any recycle stream) to polymer is desired. Following conversion of a desired amount of monomer to polymer, the polymerization mixture is then subjected to conditions sufficient to cross-link the rubber and remove any unreacted monomer. Such cross-linking and removal of unreacted monomer, as well as reaction of diluent, if employed, and other volatile materials is advantageously conducted employing conventional devolatilization techniques, such as introducing the polymerization mixture into a devolatilizing chamber, flashing off the monomer and other volatiles at elevated temperatures, for example, from 200 to 300C, under vacuum and removing them from the chamber. Alternatively, a combination of mass and suspension polymerization techniques are employed. Using said techniques, following phase inversion and subsequent size stabilization of the rubber particles, the partially polymerized product can be suspended with or without additional monomers in an aqueous medium which contains a polymerized initiator and polymerization subsequently completed. The rubber-modified monovinylidene aromatic copolymer is subsequently separated from the aqueous medium by acidification, centrifugation or filtration. The recovered product is then washed with water and dried. Various rubbers are suitable for use in the present invention. The rubbers include diene rubbers, ethylene propylene rubbers, ethylene propylene diene (EPDM) rubbers, acrylate rubbers, polyisoprene rubbers, halogen containing rubbers, and mixtures thereof. Also suitable are interpolymers of rubber-forming monomers with other copolymerizable monomers. Preferred rubbers are diene rubbers such as polybutadiene, polyisoprene, polypiperylene, and polychloroprene, or mixtures of diene rubbers, that is, any rubbery polymers of one or more conjugated 1 , 3-dienes, with 1 , 3-butadiene being especially preferred. Such rubbers include homopolymers and copolymers of 1 , 3-butadiene with one or more copolymerizable monomers, such as monovinylidene aromatic monomers as described hereinabove, styrene being preferred. Preferred copolymers of 1 , 3- butadiene are block or tapered block rubbers of at least 30 weight percent 1 ,3-butadiene rubber, more preferably from 50 weight percent, even more preferably from 70 weight percent and most preferably from 90 weight percent 1 , 3-butadiene rubber, and up to 70 weight percent monovinylidene aromatic monomer, more preferably up to 50 weight percent, even more preferably up to 30 weight percent, and most preferably up to 10 weight percent monovinylidene aromatic monomer, weights based on the weight of the 1 , 3-butadiene copolymer. Linear block copolymers can be represented by one of the following general formulas: S-B; Sι-B-S 2 ; B,-Sι-B 2 -S 2 ; In which S, Si, and S 2 are non-elastic polymer blocks of a monovinylidene aromatic monomer, with equal or different molecular weights and B, Bι, and B 2 are elastomeric polymer blocks based on a conjugated diene, with equal or different molecular weights. In these linear block copolymers, the non-elastic polymer blocks have a molecular weight of between 5,000 and 250,000 and the elastomeric polymer blocks have a molecular weight of between 2,000 and 250,000. Tapered portions can be present among the polymer blocks, S, Si, and S 2 and B, B^ and B 2 . In the tapered portion the passage between the blocks B, B 1 ( and B 2 and S, S 1 f and S 2 can be gradual in the sense that the proportion of monovinylidene aromatic monomer in the diene polymer increases progressively in the direction of the non-elastomeric polymer block, whereas the portion of conjugated diene progressively decreases. The molecular weight of the tapered portions is preferably between 500 and 30,000. These linear block copolymers are described for example in US-A-3,265,765 and can be prepared by methods well known in the art. Further details on the physical and structural characteristics of these copolymers are given in B.C. Allport et al. "Block Copolymers", Applied Science Publishers Ltd., 1973. The rubbers preferably employed in the practice of the present invention are those polymers and copolymers which exhibit a second order transition temperature, sometimes referred to as the glass transition temperature (T g ), for the diene fragment which is not higher than 0°C and preferably not higher than -20°C as determined using conventional techniques, for example ASTM Test Method D 746-52 T. T g is the temperature or temperature range at which a polymeric material shows an abrupt change in its physical properties, including, for example, mechanical strength. T g can be determined by differential scanning calorimetry (DSC). The rubber in the rubber-modified copolymer of the present invention is present in an amount equal to or greater than 5 weight percent, preferably equal to or greater than 10 weight percent, more preferably equal to or greater than 15 weight percent, even more preferably equal to or greater than 20 weight percent, and most preferably equal to or greater than 25 weight percent based on the weight of the rubber-modified copolymer. The rubber in the rubber-modified copolymer of the present invention is present in an amount equal to or less than 60 weight percent, preferably equal to or less than 50 weight percent, more preferably equal to or less than 40 weight percent, even more preferably equal to or less than 30 weight percent, and most preferably equal to or less than 25 weight percent based on the weight of the rubber-modified copolymer. The rubber dispersed in the matrix copolymer comprises (1) a branched rubber component and optionally, (2) a linear rubber component. Branched rubbers, as well as methods for their preparation, are known in the art. Representative branched rubbers and methods for their preparation are described in Great Britain Patent No. 1 ,130,485 and in Macromolecules, Vol. II, No. 5, pg. 8, by R. N. Young and C. J. Fetters. A preferred branch rubber is a radial or star-branched polymer, commonly referred to as polymers having designed branching. Star-branched rubbers are conventionally prepared using a polyfunctional coupling agent or a polyfunctional initiator and have three or more polymer segments sometimes referred to as arms, preferably between three to eight arms, bonded to a single polyfunctional element or compound, represented by the formula (rubber polymer segment^Q wherein preferably, k is an integer from 3 to 8, and Q is a moiety of a polyfunctional coupling agent. Organometalic anionic compounds are preferred polyfunctional initiators, particularly lithium compounds with C1- 6 alkyl, C 6 aryl, or C 7 . 20 alkylaryl groups. Tin-based and polyfunctional organic coupling agents are preferably employed; silicon-based polyfunctional coupling agents are most preferably employed. The arms of the star-branched rubber are preferably one or more 1 , 3-butadiene rubber, more preferably they are all the same type of 1 , 3-butadiene rubber, that is, 1 , 3- butadiene tapered block copolymer(s), 1 , 3-butadiene block copolymer(s) or 1 , 3- butadiene homopolymer(s) or a combination thereof. A star-branched rubber with such a structure may be represented by the formula XmYnZoQ (1 ) wherein X is one or more 1 , 3-butadiene tapered block copolymer, Y is one or more 1 , 3- butadiene block copolymer and Z is one or more 1 , 3-butadiene homopolymer, Q is a moiety of a polyfunctional coupling agent and m, n, and o are independently integers from 0 to 8 wherein the sum of m + n + o is equal to the number of groups of the polyfunctional coupling agent and is an integer from at least 3 to 8. Preferred star-branched rubbers are represented by formula (1) wherein m is equal to zero, for example, Y n Z 0 Q. More preferred are star-branched rubbers represented by formula (1) wherein m is equal to zero and n and o are integers equal to or greater than 1 and less than or equal to 3 and the sum of n + o is equal to 4, for example Y 2 Z 2 Q, Y 1 Z 3 Q, and Y 3 Zι;Q. Even more preferably, all of the arms of the star- branched rubber are the same type of rubber, that is, all 1 , 3-butadiene tapered block copolymers, for example, X m Y n Z o Q wherein n and o are equal to zero, more preferably all 1 , 3-butadiene block copolymers for example, X m Y n Z 0 Q wherein m and 0 are equal to zero and most preferably all 1 , 3-butadiene homopolymers, for example, X m Y n Z 0 Q wherein m and n are equal to zero. A more preferred star-rubber has four arms of 1 , 3-butadiene represented by the formula X m Y n Z 0 Q wherein Z is one or more 1 , 3-butadiene homopolymer, Q is a moiety of a tetrafunctional coupling agent, m and n are equal to zero, and o is equal to 4. Further, a more preferred star-rubber has four arms of 1 , 3-butadiene represented by the formula X m Y n Z 0 Q wherein Y is a 1 , 3-butadiene and styrene block copolymer, Z is one or more 1 , 3-butadiene homopolymer, Q is a moiety of a tetrafunctional coupling agent, m is equal to zero, n is equal to 1 , and o is equal to 3. Moreover, a most preferred star- rubber has six arms of 1 , 3-butadiene represented by the formula X m Y n ZoQ wherein Y is one or more 1 , 3-butadiene and styrene block copolymer, Z is one or more 1 , 3- butadiene homopolymer, Q is a moiety of a hexafunctional coupling agent, m is equal to zero, the sum of n and o is equal to 6. When m and/or n are not equal to zero, styrene and butadiene are the preferred comonomers comprising the tapered block copolymer and/or block copolymer arms of the star-branched rubber. Tapered block copolymer arms and/or block copolymer arms may be attached to the polyfunctional coupling agent through a styrene block. Alternatively, tapered block copolymer arms and/or block copolymer arms may be attached to the polyfunctional coupling agent through a butadiene block. Methods for preparing star-branched or radial polymers having designed branching are well known in the art. Methods for preparing a polymer of butadiene using a coupling agent are illustrated in US-A-4,183,877; US-A-4,340,690; US-A-4,340,691 and US-A-3,668,162, whereas methods for preparing a polymer of butadiene using a polyfunctional initiator are described in US-A-4,182,818; US-A-4,264,749; US-A-3,668,263 and US-A-3,787,510. Other star-branched rubbers useful in the composition of the present invention include those taught in US-A-3,280,084 and US-A-3,281 ,383. The branched rubber in the rubber-modified copolymer of the present invention is present in an amount equal to or greater than 10 weight percent, preferably equal to or greater than 20 weight percent, more preferably equal to or greater than 30 weight percent, even more preferably equal to or greater than 40 weight percent and most preferably equal to or greater than 50 weight percent based on the total weight of the rubber in the rubber-modified copolymer. The branched rubber of the rubber-modified copolymer of the present invention is present in an amount equal to or less than 100 weight percent, preferably equal to or less than 90 weight percent, more preferably equal to or less than 80 weight percent, more preferably equal to or less than 70 weight percent, even more preferably equal to or less than 60 weight percent, and most preferably equal to or less than 50 weight percent based on the total weight of the rubber in the rubber-modified copolymer. Linear rubbers, as well as methods for their preparation, are well known in the art. The term "linear rubber" refers to straight chains of polymerized monomer or comonomers which include uncoupled and dicoupled rubber wherein one or two polymeric segments or arms have been attached to a multifunctional coupling agent represented by the formula (rubber polymer segment^Q wherein k is an integer from 1 to 2. The rubber polymer segments in a dicoupled linear rubber having the formula (rubber polymer segment^Q, can be the same type, that is, both 1 , 3-butadiene homopolymers, more preferably 1 , 3-butadiene taper block copolymers, and most preferably 1 , 3- butadiene block copolymers, or they can be different, for example, one rubber polymer segment can be a 1 , 3-butadiene homopolymer and the other polymer segment a 1 , 3- butadiene block copolymer. Preferably, the linear rubber is one or more 1 , 3-butadiene homopolymer, more preferably one or more 1 , 3-butadiene tapered block copolymer, most preferably one or more 1 , 3-butadiene block copolymer or combinations thereof. The preferred comonomers comprising the tapered block copolymer and/or block copolymer linear rubber are styrene and butadiene. If present, the linear rubber of the rubber-modified copolymer of the present invention is present in an amount equal to or less than 90 weight percent, preferably equal to or less than 80 weight percent, more preferably equal to or less than 70 weight percent, even more preferably equal to or less than 60 weight percent, and most preferably equal to or less than 50 weight percent based on the total weight of the rubber in the rubber-modified copolymer. The linear rubber in the rubber-modified copolymer of the present invention is present in an amount equal to or greater than 1 weight percent, preferably equal to or greater than 10 weight percent, more preferably equal to or greater than 20 weight percent, more preferably equal to or greater than 30 weight percent, even more preferably equal to or greater than 40 weight percent and most preferably equal to or greater than 50 weight percent based on the total weight of the rubber in the rubber- modified copolymer. In addition, the rubber-modified copolymer may also optionally contain one or more additives that are commonly used in polymers of this type. Preferred additives of this type include, but are not limited to: stabilizers, antioxidants, impact modifiers, plasticizers, such as mineral oil, antistats, flow enhancers, mold releases, etc. If used, such additives may be present in an amount from at least 0.01 percent by weight, preferably at least 0.1 percent by weight, more preferably at least 1 percent by weight, more preferably at least 2 percent by weight, and most preferably at least 5 percent by weight based on the weight of the polymer blend composition. Generally, the additive is present in an amount less than or equal to 25 percent by weight, preferably less than or equal to 20 percent by weight, more preferably less than or equal to 15 percent by weight, more preferably less than or equal to 12 percent by weight, and most preferably less than or equal to 10 percent by weight based on the weight of the polymer blend composition. Preferably, a low molecular weight additive having a surface tension of less than 30 dynes/cm (ASTM D1331 , 25°C) is included in the rubber-modified copolymer. In particular, a low molecular weight silicone oil is used to improve impact properties as described in US-A-3,703,491. Preferably, the silicone oil is polydimethylsiloxane having a viscosity of from 5 to 1000 centipoise (cps), preferably from 25 to 500 cps. The composition typically contains the low molecular weight silicone oil from 0.01 to 5.0 weight percent, based on the total weight of the rubber-modified copolymer, preferably from 0.1 to 2.0 weight percent. The effect of such silicone oil is enhanced by the incorporation of other additives such as wax and tallow, wherein each is also incorporated at a level of from 0.5 to 1.5 weight percent, based on the total weight of the rubber-modified copolymer. Alternatively, fluorinated compounds such as a perfluoropolyether or a tetrafluoroethylene polymer can be used as the low molecular weight additive. Mixtures of such additives can also be used. Advantageously the cis content of the branched and linear rubbers will be independently equal to or less than 75 percent, preferably equal to or less than 55 percent, and most preferably equal to or less than 50 percent as determined by conventional IR. Preferably, branched rubbers according to the present invention have relatively high average molecular weights and have relatively low solution viscosities (less than 60 cps, 5 weight percent solution in styrene at 25°C) and high Mooney viscosities (greater than 35). Mooney viscosity is determined under standard conditions in accordance with ASTM D 1646 and is reported as X-ML 1+4 at 100°C where X is the measured number, M is the Mooney unit, L designates the roter size (large), 1 is the time in minutes the specimen is allowed to heat in the viscometer, 4 is the time in minutes at which reading X was taken and 100°C is the test temperature. Preferably, the molecular weight of the branched rubber, as determined by GPC using laser scattering techniques, is equal to or greater than 60,000; more preferably equal to or greater than 90,000, even more preferably equal to or greater than 120,000, and most preferably equal to or greater than 180,000. The molecular weight of the branched rubber is preferably less than or equal to 300,000, more preferably less than or equal to 260,000, even more preferably less than or equal to 230,000, and most preferably less than or equal to 200,000. Preferably, the molecular weight of the linear rubber, as determined by GPC with a refractive index detector and polystyrene standards, is equal to or greater than 140,000, more preferably equal to or greater than 160,000, more preferably equal to or greater than 180,000, and most preferably equal to or greater than 200,000. The molecular weight of the linear rubber is preferably less than or equal to 400,000, more preferably less than or equal to 300,000, more preferably less than or equal to 250,000, and most preferably less than or equal to 200,000. Generally, the solution viscosity of the branched and linear rubbers are independently at least 5 cps, preferably at least 10 cps, preferably at least 15 cps, and most preferably at least 20 cps. Preferably, the branched and linear rubbers independently have a solution viscosity of less than or equal to 200 cps, preferably less than or equal to 100 cps, preferably less than or equal to 50 cps, and most preferably less than or equal to 40 cps. Generally, the ML 1+4 at 100°C Mooney viscosities of the branched and linear rubbers are independently at least 15, preferably at least 25, preferably at least 35, and most preferably at least 45. Preferably, the branched and linear rubbers independently have a Mooney viscosity of less than or equal to 100, preferably less than or equal to 85, preferably less than or equal to 75, and most preferably less than or equal to 60. The rubber, with graft and/or occluded polymers if present, is dispersed in the continuous matrix phase as discrete particles. Preferably, the rubber particles comprise a range of sizes having a mono-modal distribution. The average particle size of a rubber particle, as used herein, will, refer to the volume average diameter. In most cases, the volume average diameter of a group of particles is the same as the weight average. The average particle diameter measurement generally includes the polymer grafted to the rubber particles and occlusions of polymer within the particles. The average particle size of the rubber particles is equal to or greater than 0.01 micrometer (μm), preferably equal to or greater than 0.15 μm, more preferably equal to or greater than 0.5 μm, and most preferably equal to or greater than 0.75 μm. The average particle size of the rubber particles is equal to or less than 5 μm, preferably equal to or less than 2.5 μm, more preferably equal to or less than 1.5 μm, and most preferably equal to or less than 1 μm. The volume average diameter can be determined by the analysis of transmission electron micrographs of the compositions containing the particles. Rubber cross-linking is quantified by the light absorbance ratio (LAR). In the rubber-modified copolymer of the present invention, it is preferred that the rubber particles have a light absorbance ratio preferably equal to or greater than 1 , more preferably equal to greater than 1.5, and most preferably equal to or greater than 2. The preferred light absorbance ratio of the rubber particles is less than or equal to 5, preferably less than or equal to 4, more preferably less than or equal to 3, even more preferably less than or equal to 2.5, and most preferably less than or equal to 2. Light absorbance ratio is the ratio of light absorbance for a suspension of the rubber particles in dimethylformamide to the light absorbance for a suspension of the rubber particles in dichloromethane, as described in the examples hereinbelow. The light absorbance ratio, which is a measure for degree of crosslinking, is dependent on the amount and kind of the polymerization initiator and the temperature and the residence time at the removal step for the volatile components. It also depends on the types and amounts of the matrix monomers, antioxidant, chain transfer agent, ect. A suitable light absorbance ratio can be set by a person skilled in the art by choosing the appropriate conditions for the production process in accordance with the trial and error method. The rubber-modified copolymer of the present invention preferably has a melt flow rate, determined under conditions of 220°C and an applied load of 10 kg, equal to or greater than 0.1 , more preferably equal to or greater than 1 , more preferably equal to or greater than 5, and most preferably equal to or greater than 10 g/10 min. Generally , the melt flow rate of the rubber-modified copolymer is equal to or less than 100, preferably equal to or less than 50, more preferably less than or equal to 20, and most preferably equal to or less than 10 g/10 min. The rubber-modified copolymer of the present invention is present in an amount equal to or less than 90 weight percent, preferably equal to or less than 80 weight percent, more preferably equal to or less than 70 weight percent, even more preferably equal to or less than 60 weight percent, and most preferably equal to or less than 50 weight percent based on the weight of the polymer blend composition. The rubber- modified copolymer of the present invention is present in an amount equal to or greater than 10 weight percent, preferably equal to or greater than 20 weight percent, more preferably equal to or greater than 30 weight percent, even more preferably equal to or greater than 40 weight percent, and most preferably equal to or greater than 50 weight percent based on the weight of the polymer blend composition. Optionally, the polymer blend composition comprises an impact modifier. Preferable impact modifiers are rubber materials having a T g equal to or less than 0°C, preferably equal to or less than -10°C, more preferably equal to or less than -20°C, and most preferably equal to or less than -30°C. Suitable rubbers include polymers such as acrylate rubbers, particularly homopolymers and copolymers of alkyl acrylates having from 4 to 6 carbons in the alkyl group; or polyolefin elastomers, particularly copolymers of ethylene, propylene and optionally a nonconjugated diene. In addition, mixtures of the foregoing rubbery polymers may be employed if desired. Preferably, the impact modifier is a grafted homopolymer or copolymer of butadiene that is grafted with a polymer of styrene and methyl methacrylate. Some of the preferred rubber-containing materials of this type are the known methyl methacrylate, butadiene, and styrene-type (MBS-type) core/shell grafted copolymers having a T g equal to or less than 0°C and a rubber content greater than 40 percent, typically greater than 50 percent. They are generally obtained by graft polymerizing styrene and methyl methacrylate and/or equivalent monomers in the presence of a conjugated diene polymer rubber core, preferably a butadiene homo- or co-polymer. The grafting monomers may be added to the reaction mixture simultaneously or in sequence, and, when added in sequence, layers, shells or wart-like appendages can be built up around the substrate latex, or core. The monomers can be added in various ratios to each other. Other impact modifiers useful in the compositions of this invention are those based generally on a long-chain, hydrocarbon backbone, which may be prepared predominantly from various mono- or dialkenyl monomers and may be grafted with one or more styrenic monomers. Representative examples of a few olefinic elastomers which illustrate the variation in the known substances which would suffice for such purpose are as follows: butyl rubber; chlorinated polyethylene rubber; chlorosulfonated polyethylene rubber; an olefin polymer or copolymer such as ethylene/propylene copolymer, ethylene/styrene copolymer or ethylene/propylene/ diene copolymer, which may be grafted with one or more styrenic monomers; neoprene rubber; nitrile rubber; polybutadiene and polyisoprene. If used, the impact modifier is preferably present in an amount of at least 1 percent by weight, preferably at least 2 percent by weight, more preferably at least 5 percent by weight, even more preferably at least 10 percent by weight, and most preferably at least 15 percent by weight based on the weight of the polymer blend composition. Generally, the impact modifier is present in an amount less than or equal to 50 percent by weight, preferably less than or equal to 40 percent by weight, more preferably less than or equal to 30 percent by weight, even more preferably less than or equal to 25 percent by weight, and most preferably less than or equal to 20 percent by weight based on the weight of the polymer blend composition. The polymer blend composition of the present invention can be employed in mixtures, alloys or blends with other polymer and/or copolymer resins, for example, mixtures with polysulfones, polyethers, polyether imide, polyphenylene oxides or polyesters. In addition, the claimed polymer blend compositions may also optionally contain one or more additives that are commonly used in polymer blend compositions of this type. Preferred additives of this type include, but are not limited to: fillers, reinforcements, ignition resistant additives, stabilizers, colorants, antioxidants, antistats, flow enhancers, mold releases, nucleating agents, etc. Preferred examples of additives are fillers, such as, but not limited to talc, clay, wollastonite, mica, glass or a mixture thereof. Additionally, ignition resistance additives, such as, but not limited to halogenated hydrocarbons, halogenated carbonate oiigomers, halogenated diglycidyl ethers, organophosphorous compounds, fluorinated olefins, antimony oxide and metal salts of aromatic sulfur, or a mixture thereof may be used. Further, compounds which stabilize polymer blend compositions against degradation caused by, but not limited to heat, light, and oxygen, or a mixture thereof may be used. If used, such additives may be present in an amount from at least 0.01 percent by weight, preferably at least 0.1 percent by weight, more preferably at least 1 percent by weight, even more preferably at least 2 percent by weight, and most preferably at least 5 percent by weight based on the weight of the polymer blend composition. Generally, the additive is present in an amount less than or equal to 25 percent by weight, preferably less than or equal to 20 percent by weight, more preferably less than or equal to 15 percent by weight, even more preferably less than or equal to 12 percent by weight, and most preferably less than or equal to 10 percent by weight based on the weight of the polymer blend composition. Preparation of the polymer blend compositions of this invention can be accomplished by any suitable mixing means known in the art, including dry blending the individual components and subsequently melt mixing, either directly in the extruder used to make the finished article (for example, the automotive part), or pre-mixing in a separate extruder (for example, a Banbury mixer). Dry blends of the compositions can also be directly injection molded without pre-melt mixing. The polymer blend compositions of this invention are thermoplastic. When softened or melted by the application of heat, the polymer blend compositions of this invention can be formed or molded using conventional techniques such as compression molding, injection molding, gas assisted injection molding, calendering, vacuum forming, thermoforming, extrusion and/or blow molding, alone or in combination. The polymer blend compositions can also be formed, spun, or drawn into films, fibers, multi-layer laminates or extruded sheets, or can be compounded with one or more organic or inorganic substances, on any machine suitable for such purpose. Some of the fabricated articles include exterior automotive body panel applications such as door panels and fascia, or other automotive applications such as instrument panels, fenders, hoods, trunk lids, side cladding parts, mirror housings, cowl vent grills, etc. These compositions can even find use in instrument housings such as for power tools or information technology equipment such as telephones, computers, copiers, etc. EXAMPLES To illustrate the practice of this invention, examples of preferred embodiments are set forth below. However, these examples do not in any manner restrict the scope of this invention. The compositions of Examples 1 to 13 were prepared by mixing polycarbonate resin pellets dried at 120°C for at least 4 hours, ABS pellets and other additives in a plastic bag. The dry blended mixture was feed to a 30 mm Werner and Pfleider fully intermeshing corotating twin screw extruder. The following were the compounding conditions on the Werner and Pfleider extruder: Barrel temperature profile: 210°C, 240°C, 255°C, 255°C, and 255°C and screw speed of 289 rotations per minute (RPM). The extrudate was cooled in the form of strands and comminuted as pellets. The pellets were dried in an air draft oven for 4 hours at 120°C and then were used to prepare test specimens on a 70 ton Arburg injection molding machine, having the following molding conditions: Barrel temperature of 250°C; Mold temperature of 79°C; Injection time: 3 seconds; Holding time 5 seconds; cooling time 20 seconds; Holding pressure: 230-40 bar; Back pressure: 2.5 turns; and Screw speed: 3.8. Example 1 An ABS resin (33.7 percent) which was mass produced and comprised 14.8 percent star-branched butadiene homopolymer and 24 percent acrylonitrile having a average rubber particle size of 1.0 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. Example 2 An ABS resin (33.7 percent) which was mass produced and comprised 15.9 percent star-branched butadiene homopolymer and 22.7 percent acrylonitrile having a average rubber particle size of 0.65 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. Example 3 An ABS resin (33.7 percent) which was mass produced and comprised 15.6 percent star-branched butadiene homopolymer and 22.7 percent acrylonitrile having a average rubber particle size of 0.83 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. Example 4 An ABS resin (33.7 percent) which was mass produced and comprised 15.6 percent star-branched butadiene homopolymer and 22.7 percent acrylonitrile having a average rubber particle size of 0.83 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 14 g/10 min. Example 5 An ABS resin (33.7 percent) which was mass produced and comprised 15.6 percent star-branched butadiene homopolymer and 22.7 percent acrylonitrile having a average rubber particle size of 0.83 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 22 g/10 min. Example 6 An ABS resin (33.7 percent) which was mass produced and comprised 17.1 percent star-branched butadiene homopolymer and 22.4 percent acrylonitrile having a average rubber particle size of 0.98 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. Example 7 An ABS resin (29.7 percent) which was mass produced and comprised 17.1 percent star-branched butadiene homopolymer and 22.4 percent acrylonitrile having a average rubber particle size of 0.98 micometers was melt blended into a homopolymer of bisphenol-A (70 percent) having a MFR of 10 g/10 min. Example 8 An ABS resin (44.7 percent) which was mass produced and comprised 17.1 percent star-branched butadiene homopolymer and 22.4 percent acrylonitrile having a average rubber particle size of 0.98 micometers was melt blended into a homopolymer of bisphenol-A (55 percent) having a MFR of 10 g/10 min. Example 9 An ABS resin (33.7 percent) which was mass produced and comprised 9.4 percent star-branched butadiene homopolymer and 9.4 percent linear styrene and butadiene block copolymer rubber and 20.5 percent acrylonitrile having a average rubber particle size of 1.2 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. Example 10 An ABS resin (33.7 percent) which was mass produced and comprised 7.6 percent star-branched butadiene homopolymer and 7.6 percent linear styrene and butadiene block copolymer rubber and 22.6 percent acrylonitrile having a average rubber particle size of 0.86 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. Example 11 An ABS resin (33.7 percent) which was mass produced and comprised 10.6 percent star-branched butadiene homopolymer and 4.6 percent linear styrene and butadiene block copolymer rubber and 24.4 percent acrylonitrile having a average rubber particle size of 0.81 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. Example 12 An ABS resin (33.7 percent) which was mass produced and comprised 10.6 percent star-branched butadiene homopolymer and 4.6 percent linear styrene and butadiene block copolymer rubber and 24.4 percent acrylonitrile having a average rubber particle size of 0.81 micometers was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 14 g/10 min. Example 13 An ABS resin (33.7 percent) which was mass produced and comprised 22.1 percent linear styrene and butadiene block copolymer rubber and 20.2 percent acrylonitrile having a average rubber particle size of 0.81 was melt blended into a homopolymer of bisphenol-A (66 percent) having a MFR of 10 g/10 min. The formulation content and properties of Examples 1 to 13 are given in Table 1 below in parts by weight of the total composition. In Table 1 : "PC" is a bisphenol-A polycarbonate homopolymer commercially available as CALIBRE™ 300 from Dow Chemical; "ABS" is a mass produced acrylonitrile butadiene styrene terpolymer wherein the rubber was dissolved in a feed stream of styrene, acrylonitrile, ethyl benzene, 1 weight percent mineral oil and 0.14 weight percent of di-(tertiarybutyl peroxy) cyclohexane initiator to form a mixture. The mixture was polymerized in a continuous process while agitating said mixture. The polymerization occurred in a multi staged reactor system over an increasing temperature profile. N-dodecylmercaptan was added for molecular weight control. During the polymerization process, some of the forming copolymer grafts to the rubber particles while some of it does not graft, but instead, forms the matrix copolymer. The resulting polymerization product was then devolotalized, extruded, and pelletized; "IRGANOX™ 1076" is a phenolic antioxidant available from Ciba Geigy; "LUBRIL™ JK" is octadecanoic acid, 12-(1 -oxooctadecyl)oxy-2-octyldodecyl ester a mold release commercially available from Rhodia; "Star-branched rubber" is a butadiene homopolymer commercially available as BUNA™ HX565 from Bayer having a 5 percent solution viscosity in styrene of 44 cps, a Mooney viscosity of 59, a M w of 200,000, and a Mn of 110,000; "Linear rubber" is a 70/30 butadiene/styrene block copolymer commercially available as STEREON™ 730 from Firestone having a 5 percent solution viscosity in styrene of 30 cps, a M w of less than 182,000, and a Mn of 140,000; "Percent (%) AN" is the percent acrylonitrile in the ABS; "LAR" is the light absorbance ratio determined using a Brinkmann model PC 800 probe colorimeter equipped with a 450 nm wavelength filter, from Brinkmann Instruments Inc., Westbury, New York, or equivalent, is used. In a first vial, a 0.4 gram (g) sample of rubber-modified copolymer is dissolved in 40 milliliters (ml) of dimethylformamide (DMF). From the first vial, 5 ml of the resulting DMF solution is added to a second vial containing 40 ml of DMF. From the first vial, 5 ml of the resulting DMF solution is added to a third vial containing 20 ml of dichloromethane (DCM). The probe is zeroed in neat DMF. The absorption of the DMF solution in the second vial and the absorption of the DCM solution in the third vial are determined. The light absorbance ratio is calculated by the following equation: (Absorbance of Sample in DMF) (Absorbance of Sample in DCM) "M w " is weight average molecular weight measured by gel permeation chromatography. Matrix polymer (for example, SAN) determinations were made using polystyrene standards with a UV detector set at 254 nanometers and polycarbonate determinations were made using polystyrene and polycarbonate standards with a UV detector set at 228 nonometers; "Particle size" is reported volume-weighted (D 43 ) mean particle diameter and was determined by the analysis of transmission electron microscope (TEM) images on samples of the ABS. Samples prepared from polymer pellets cut to fit a microtome chuck. The area for microtomy was trimmed to approximately 1 square millimeter (mm 2 ) and stained in OsO 4 vapor overnight at 24 °C. Ultrathin sections were prepared using standard microtomy techniques. 90 nanometer thin sections were collected on Cu grids and were studied in one of two transmission electron microscopes: either a Hitachi H- 600 TEM operating at 100 kilovolt (KV) or a Philips CM12 TEM operating at 120 KV. The resulting images were analyzed for rubber particle size distribution and rubber gel phase volume using a computer with the NIH Image image analysis software (NIH Image is a public domain computer program developed at the U.S. National Institutes of Health and available on the Internet at http://rsb.info.nih.gov/nih-image/). Images were analyzed at a resolution of 0.007 micometer/pixel with the features segregated so that the stained rubber was black and the unstained matrix was white. Image artifacts were removed with the editing tools built into the software. Rubber gel sizes were determined by measuring the area of the gel particle, then converting the area to a diameter which is the diameter of a circle with the same area as that observed for the gel particle. The nomenclature used in this work is based primarily on the work of T. Allen ("Particle Size Measurement" Vol. 1 , Fifth Edition, Terence Allen, 1998, Chapman & Hall) and is summarized by the following single general equation: where D πm is the weighted mean diameter for the distribution, N, is the number of particles of diameter d„ / runs over all k sizes in the distribution and the (n-m) th root function (that is, square-root for n-m=2, cube-root for n-m=3, identity function for n-m=1 , etc.) is used to assure that the dimensionality of the result is appropriate. Particle size measurement is sensitive to the TEM sectioning process since the sections do not always run through the center of the observed particles. Although this effect will result in a generally smaller estimate of average particle size than the true value, a consistent approach yields consistent relative results. The sizing method presented hereinabove is practical without the need for assumptions particle size distribution or accurate measurement of section thickness. Correction for sectioned viewing in either the phase volume determination or the particle size reporting was not made. The following tests were run on Examples 1 to 13 and the results of these tests are shown in Table 1 : Tensile property testing was done in accordance with ASTM D 638. Tensile Type 1 test specimens were conditioned at 23 °C and 50 percent relative humidity 24 hours prior to testing. Testing was performed using an INSTRON 1125 mechanical tester. Testing was performed at room temperature. Flexural properties were determined in accordance with ASTM D 790. Testing was performed using an INSTRON mechanical tester. Flexural property test specimens were conditioned at 23 °C and 50 percent relative humidity 24 hours prior to testing. Testing was performed at room temperature. Deflection temperature under load (DTUL) was determined on a Ceast HDT 300 Vicat machine in accordance with ASTM D 648-82 where test specimens were unannealed and tested under an applied pressure of 1.82 megapascals (MPa). Impact resistance as measured by the Notched Izod test (Izod) was determined according to ASTM D 256-90-B at 23°C, -29°C, -40°C, and -45°C. Ductile Brittle Transition Temperature (DBTT) was determined at the point where one out of four test specimens had a brittle failure. Specimens were cut from rectangular DTUL bars and measured 3.18 millimeter (mm) in thickness and 50.8 mm in length. The specimens were notched with a TMI 22-05 notcher to give a 0.254 mm radius notch. A 22 kilogram pendulum was used. Impact resistance as measured by instrumented impact (Dart Impact) was determined according to ASTM D 3763 using a General Research Corp. Dynatup 8250 instrumented impact tester with a 45.4 kg weight. Test results were determined at 23°C and -29°C on a 64 mm by 3.18 mm thick disk. MFR was determined according to ASTM D 1238 on a Tinius Olsen plastometer, for PC resins at 300°C and an applied load of 1.2 kg and for ABS resins and PC/ABS blends at 230°C and an applied load of 3.8 kg. Table 1

Claims: CLAIMS: 1. A polymer blend composition comprising, in admixture: (a) an aromatic carbonate polymer and (b) a rubber-modified copolymer comprising (i) a continuous matrix phase comprising a copolymer of a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer and (ii) a rubber comprising (1 ) a star-branched rubber having three or more arms and (2) optionally, a linear rubber, wherein the rubber is dispersed as discrete rubber particles in the matrix. 2. The polymer blend composition of Claim 1 wherein the ethylenically unsaturated nitrile is from 10 to 35 weight percent of the copolymer. 3. The polymer blend composition of Claim 1 wherein the monovinylidene aromatic monomer is styrene and the ethylenically unsaturated nitrile monomer is acrylonitrile. 4. The polymer blend composition of Claim 1 wherein the arms of the star- branched rubber are one or more 1 , 3-butadiene rubbers. 5. The polymer blend composition of Claim 1 wherein the structure of the star- branched rubber is represented by the formula XmYnZoQ (1 ) wherein X is one or more 1 , 3-butadiene tapered block copolymer, Y is one or more 1 , 3-butadiene block copolymer and Z is one or more 1 , 3-butadiene homopolymer, Q is a moiety of a polyfunctional coupling agent and m, n, and o are independently integers from 0 to 8 wherein the sum of m + n + o is equal to the number of groups of the polyfunctional coupling agent and is an integer from at least 3 to 8. 6. The polymer blend composition of Claim 5 wherein the block copolymer is a block copolymer of styrene and 1 , 3-butadiene. 7. The polymer blend composition of Claim 5 wherein the block copolymer comprises from 50 to 90 weight percent 1 , 3-butadiene based on the weight of the block copolymer. 8. The star-branched rubber of Claim 5 represented by formula (1 ) wherein m is equal to zero. 9. The star-branched rubber of Claim 5 represented by formula (1 ) wherein m and n are equal to zero and o is equal to 4. 10. The mass polymerized rubber-modified monovinylidene aromatic copolymer composition of Claim 1 wherein the rubber comprises: (i) from 10 to 90 weight percent of the star-branched rubber and (ii) from 90 to 10 weight percent of the linear rubber, weight percents are based on the total weight of the rubber in the rubber-modified copolymer. 11. The polymer blend composition of Claim 10 wherein the linear rubber is one or more 1 , 3-butadiene rubbers. 12. The polymer blend composition of Claim 10 wherein the linear rubber is a 1 , 3-butadiene homopolymer. 13. The polymer blend composition of Claim 10 wherein the linear rubber is a block copolymer of styrene and 1 , 3-butadiene. 14. The polymer blend composition of Claim 13 wherein the block copolymer comprises from 50 to 90 weight percent 1 , 3-butadiene based on the weight of the block copolymer. 15. The polymer blend composition of Claim 1 wherein the arms of the star- branched rubber comprise a 1 , 3-butadiene homopolymer and the linear rubber comprises the same or a different 1 , 3-butadiene homopolymer. 16. The polymer blend composition of Claim 1 wherein the arms of the star- branched rubber comprises a 1 , 3-butadiene homopolymer and the linear rubber comprises a 1 , 3-butadiene block copolymer. 17. The polymer blend composition of Claim 1 wherein the arms of the star- branched rubber comprises a 1 , 3-butadiene block copolymer and the linear rubber comprises a 1 , 3-butadiene homopolymer. 18. The polymer blend composition of Claim 1 wherein the arms of the star- branched rubber comprises a 1 , 3-butadiene block copolymer and the linear rubber comprises the same or a different 1 , 3-butadiene block copolymer. 19. The polymer blend composition of Claim 16 wherein the block copolymer is a block copolymer of styrene and 1 , 3-butadiene comprising from 50 to 90 weight percent butadiene based on the weight of the block copolymer. 20. The polymer blend composition of Claim 1 wherein the average rubber particle size is from 0.15 to 1.5 micometers. 21. The polymer blend composition of Claim 1 wherein the polymer blend comprises: (a) from 90 to 10 weight percent aromatic carbonate polymer and (b) from 10 to 90 weight percent rubber-modified copolymer, weight percents are based on the weight of the polymer blend composition. 22. The polymer blend composition of Claim 1 wherein the rubber-modified copolymer comprises: (ii) from 70 to 90 weight percent copolymer and (iii) from 30 to 10 weight percent rubber, weight percents are based on the weight of the rubber-modified copolymer. 23. The polymer blend composition of Claim 1 wherein the rubber comprises: (1) from 30 to 70 weight percent butadiene homopolymer star-branched rubber and (2) from 70 to 30 weight percent styrene and butadiene block copolymer linear rubber, weight percents are based on the total weight of the rubber. 24. The polymer blend composition of Claim 1 wherein the rubber comprises: (1 ) 50 weight percent butadiene homopolymer star-branched rubber and (2) 50 weight percent styrene and butadiene block copolymer linear rubber, weight percents are based on the total weight of the rubber. 25. The polymer blend composition of Claim 1 wherein the star-branched rubber is a 1 , 3-butadiene homopolymer having a solution viscosity, measured as a 5 weight percent solution in styrene at 25°C, of less than 60 centipoise. 26. The polymer blend composition of Claim 1 wherein the matrix copolymer has a molecular weight of 80,000 to 240,000. 27. The polymer blend composition of Claim 1 wherein the rubber-modified copolymer has a melt flow rate of 1 to 10 gram per 10 minutes at conditions of 220°C and an applied load of 10 kg. 28. The polymer blend composition of Claim 1 wherein the aromatic carbonate polymer has a melt flow rate of 10 to 17 gram per 10 minutes at conditions of 300°C and an applied load of 1.2 kg. 29. A method for preparing a polymer blend composition comprising the step of combining: (a) an aromatic carbonate polymer and (b) a rubber-modified copolymer comprising (i) a continuous matrix phase comprising a copolymer of a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer and (ii) a rubber comprising (1 ) a star-branched rubber having three or more arms and (2) optionally, a linear rubber, wherein the rubber is dispersed as discrete rubber particles in the matrix. 30. The method of Claim 29 wherein the monovinylidene aromatic monomer is styrene, the ethylenically unsaturated nitrile monomer is acrylonitrile, the star- branched rubber has four arms comprising a 1 , 3-buatadiene homopolymer and the linear rubber comprises a styrene and 1 , 3-butadiene block copolymer. 31. A method for producing a molded or extruded article of a polymer blend composition comprising the steps of: (A) preparing a carbonate polymer composition comprising (a) an aromatic carbonate polymer and (b) a rubber-modified copolymer comprising (i) a continuous matrix phase comprising a copolymer of a monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer and (ii) a rubber comprising (1) a star-branched rubber having three or more arms and (2) optionally, a linear rubber, wherein the rubber is dispersed as discrete rubber particles in the matrix, and (B) molding or extruding said carbonate polymer composition into a molded or extruded article. 32. The method of Claim 31 wherein the molded or extruded article is selected from the group consisting of automotive door panels, fascia, instrument panels, fenders, hoods, trunk lids, side cladding parts, mirror housings, cowl vent grills, power tool housings, telephone housings, computer housings and copier housings. 33. The composition of Claim 1 in the form of a molded or extruded article. 34. The molded or extruded article of Claim 33 is selected from the group consisting of automotive door panels, fascia, instrument panels, fenders, hoods, trunk lids, side cladding parts, mirror housings, cowl vent grills, power tool housings, telephone housings, computer housings and copier housings.

Inventors: CERASO JOSEPH M, PHAM HOANG T, STANLEY JAMIE J, WECKLE CHERYL L, CERASO, JOSEPH, M., PHAM, HOANG, T., STANLEY, JAMIE, J., WECKLE, CHERYL, L.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 2000
Main classification: C08L69
IPC codes: B60K37, B62D29, C08F279, C08F287, C08F291, C08F297, C08J5, C08L51, C08L53, C08L55, C08L69