WO2000024829A1

WO2000024829A1 - Polycarbonate resin/abs graft copolymer/san blends

Info

Publication number: WO2000024829A1
Application number: PCT/US1999/017810
Authority: WO
Inventors: John Robert Campbell; Pratima Rangarajan; Ronald James Wroczynski
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1998-10-23
Filing date: 1999-08-03
Publication date: 2000-05-04
Anticipated expiration: 2001-04-23
Also published as: KR20010075652A; EP1129138A1; TWI227723B; CN1331727A; CN1150275C; JP2002528589A

Abstract

Thermoplastic resin blends containing an aromatic polycarbonate resin, an acrylonitrile-butadiene-styrene graft copolymer and a styreneacrylonitrile having a reduced styrene-acrylonitrile oligomer content exhibit an improved balance of flow properties and ductility.

Description

POLYCARBONATE RESIN/ ABS GRAFT COPOLYMER/SAN BLENDS

FIELD OF THE INVENTION

The present invention relates to thermoplastic resin compositions, more specifically, to thermoplastic resin compositions containing a polycarbonate ("PC") resins, an acrylonitrile-butadiene-styrene ("ABS") graft copolymer and a styrene-acrylonitrile ("SAN") copolymer.

BRIEF DESCRIPTION OF THE RELATED ART

Polycarbonate resins are tough, rigid engineering thermoplastics having good impact strength. However, the flow characteristics of polycarbonate resins sometimes cause difficulties in processing. Various prior art attempts have been made to blend polycarbonate resins with other polymeric modifiers to increase the flow characteristics while still retaining the toughness and impact resistance of the polycarbonate resin. For example,

ABS graft copolymers have been blended with polycarbonate resins to yield a lower cost blend having improved processing characteristics, while retaining good impact resistance (see U.S. Patent No. 3,130,177, issued to Grabowski, and Plastics World, November 1977, pp. 5-58). However, various attempts to further improve flow characteristics of polycarbonate resin /ABS graft copolymer blends have resulted in a brittle material or undesirably low heat deflection temperature ("HDT"). To produce polycarbonate resin/ ABS graft copolymer blends which have good flow characteristics, while having good low temperature ductijity and high HDT would be very desirable and useful.

SUMMARY OF THE INVENTION

In a first aspect, the present invention is directed to a thermoplastic resin composition, comprising: (a) an aromatic polycarbonate resin;

(b) an acrylonitrile-butadiene-styrene graft copolymer; and

(c) a styrene-acrylonitrile copolymer having a reduced styrene-acrylonitrile oligomer content.

The resin composition of the present invention exhibits an improved balance of flow properties and ductility.

In a second aspect, the present invention relates to a process for making a thermoplastic resin composition, comprising blending together an aromatic polycarbonate resin, an acrylonitrile-butadiene-styrene graft copolymer and a styrene-acrylonitrile copolymer having a reduced styreneacrylonitrile oligomer content.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, the thermoplastic resin composition of the present invention comprises, based on 100 parts by weight ("pbw") of thermoplastic resin composition, from 40 to 95 pbw, more preferably from 50 to 90 pbw, even more preferably from 55 to 80 pbw, of the aromatic polycarbonate resin, from 3 to 58 pbw, more preferably from 7 to 47 pbw, even more preferably from 10 to 40 pbw, ABS graft copolymer and from 2 to 57 pbw, more preferably from 3 to 43 pbw, even more preferably from 5 to 35 pbw, SAN.

Aromatic Polycarbonate Resin

The aromatic polycarbonate resin component of the composition of the present invention comprises one or more aromatic polycarbonate resins. Aromatic polycarbonate resins suitable for use as the polycarbonate resin component of the thermoplastic resin composition of the present invention are known compounds whose preparation and properties have been described, see, generally, U.S. Patent Nos. 3,169,121; 4,487,896; and 5,411,999, the respective disclosures of which are each incorporated herein by reference.

In a preferred embodiment, the aromatic polycarbonate resin component of the present invention is the reaction product of a dihydric phenol according to the structural formula (I):

HO - A - OH (I)

wherein A is a divalent aromatic radical,

with a carbonate precursor which contains structural units according to the formula (II):

O

II

- (O - A -O -Q - _(π)

wherein A is defined as above.

As used herein, the term "divalent aromatic radical " includes those divalent radicals containing a single aromatic ring such as phenylene, those divalent radicals containing a condensed aromatic ring system such as, for example, naphthlene, those divalent radicals containing two or more aromatic rings joined by a non-aromatic linkage, such as for example, an alkylene, alkylidene or sulfonyl group, any of which may be substituted at one or more sites on the aromatic ring with, for example, a halo group or (Cl-C6)alkyl group.

In a preferred embodiment, A is a divalent aromatic radical according to the formula (III): CH₃

CH₃

(III).

Suitable dihydric phenols include, for example, one or more of 2, 2-bis- (4-hydroxyphenyl) propane ("bisphenol A"), 2,2-bis(3,5-dimethyl-4- hydroxyphenyl)propane, bis(4-hydroxyphenyl) methane, 4,4-bis(4- hydroxyphenyl)heptane, 3,5,3',5'-tetrachloro-4,4'-dihydroxyphenyl)propane,

2,6-dihydroxy naphthalene, hydroquinone, 2,4'-dihydroxyphenyl sulfone. In a highly preferred embodiment, the dihydric phenol is bisphenol A.

The carbonate precursor is one or more of a carbonyl halide, a carbonate ester or a haloformate. Suitable carbonyl halides include, for example, carbonyl bromide and carbonyl chloride. Suitable carbonate esters include, such as for example, diphenyl carbonate, dichlorophenyl carbonate, dinaphthyl carbonate, phenyl tolyl carbonate and ditolyl carbonate. Suitable haloformates include, for example, bishaloformates of a dihydric phenols, such as, for example, hydroquinone, or glycols, such as, for example, ethylene glycol, neopentyl glycol. In a highly preferred embodiment, the carbonate precursor is carbonyl chloride.

Suitable aromatic polycarbonate resins include linear aromatic polycarbonate resins and branched aromatic polycarbonate resins. Suitable linear aromatic polycarbonates resins include, e.g., bisphenol A polycarbonate resin. Suitable branched polycarbonates are known and are made by reacting a polyfunctional aromatic compound with a dihydric phenol and a carbonate precursor to form a branched polymer, see generally, U. S. Patent Nos. 3,544,514; 3,635,895; and 4,001,184, the respective disclosures of which are incorporated herein by reference. The polyfunctional compounds are generally aromatic and contain at least three functional groups which are carboxyl, carboxylic anhydrides, phenols, haloformates or mixtures thereof, such as. for example, l,l,l-tri(4-hydroxyphenyl)ethane, 1,3,5,-trihydroxy- benzene, trimellitic anhydride, trimellitic acid, trimellityl trichloride, 4- chloroformyl phthalic anhydride, pyromellitic acid, pyromellitic dianhydride, mellitic acid, mellitic anhydride, trimesic acid, benzophenonetetracarboxylic acid, benzophenone-tetracarboxylic dianhydride. The preferred polyfunctional aromatic compounds are l,l,l-tri(4-hydroxyphenyl)ethane, trimellitic anhydride or trimellitic acid or their haloformate derivatives.

In a preferred embodiment, the polycarbonate resin component of the present invention is a linear polycarbonate resin derived from bisphenol A and phosgene.

In a preferred embodiment, the weight average molecular weight of the polycarbonate resin is from about 10,000 to about 200,000 grams per mole ("g/mol"), and in another preferred embodiment, the weight average molecular weight of the polycarbonate resin is from about 10,000 to about 100,000 grams per mole ("g/mol"), as determined by gel permeation chromatography relative to polystyrene standards.- Such resins typically exhibit an intrinsic viscosity of about 0.3 to about 1.5 deciliters per gram in methyl ene chloride at 25°C.

Polycarbonate resins are made by known methods, such as, for example, interfacial polymerization, transesterification, solution polymerization or melt polymerization.

Copolyester-carbonate resins are also suitable for use as the aromatic polycarbonate resin component of the present invention. Copolyester- carbonate resins suitable for use as the aromatic polycarbonate resin component of the thermoplastic resin composition of the present invention are known compounds whose preparation and properties have been described, see, generally, U.S. Patent Nos. 3,169,121; 4,430,484; and 4,487,896, the respective disclosures of which are each incorporated herein by reference.

Copolyester-carbonate resins comprise linear or randomly branched polymers that contain recurring carbonate groups, carboxylate groups and aromatic carbocyclic groups in the polymer chain, in which at least some of the carbonate groups are bonded directly to the ring carbon atoms of the aromatic carbocyclic groups.

In a preferred embodiment, the copolyester-carbonate resin component of the present invention is derived from a carbonate precursor, at least one dihydric phenol and at least one dicarboxylic acid or dicarboxylic acid equivalent. In a preferred embodiment, the dicarboxylic acid is one according to the formula (IV):

II li

HO — C — A' — C — OH ,

wherein A' is alkylene, alkylidene, cycloaliphatic or aromatic and is preferably a non-substituted phenylene radical or a substituted phenylene radical that is substituted at one or more sites on the aromatic ring, wherein each of such substituent groups is independently (C1-C6) alkyl, and the copolyester carbonate resin comprises first structural units according to formula (II) above and second structural units according to formula (V):

wherein A' is defined as above.

Suitable carbonate precursors and dihydric phenols are those disclosed above. Suitable dicarboxylic acids, include, for example, phthalic acid, isophthalic acid, terephthalic acid, dimethyl terephthalic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dimethyl malonic acid, 1,12-dodecanoic acid, cis-l,4-cyclohexane dicarboxylic acid, trans-l,4-cyclohexane dicarboxylic acid,

4,4'- bisbenzoic acid, naphthalene-2,6-dicarboxylic acid. Suitable dicarboxylic acid equivalents include, for example, anhydride, ester or halide derivatives of the above disclosed dicarboxylic acids, such as, for example, phthalic anhydride, dimethyl terephthalate, succinyl chloride.

In a preferred embodiment, the dicarboxylic acid is an aromatic dicarboxylic acid, more preferably one or more of terephthalic acid and isophthalic acid.

In a preferred embodiment, the ratio of ester bonds to carbonate bonds present in the copolyester carbonate resin is from 0.25 to 0.9 ester bonds per carbonate bond.

In a preferred embodiment, the copolyester-carbonate copolymer has a weight average molecular weight of from about 10,000 to about 200,000 g/mol.

Copolyester-carbonate resins are made by known methods, such as, for example, interfacial polymerization, transesterification, solution polymerization or melt polymerization.

ABS Graft Copolymer

The ABS graft copolymer component of the composition of the present invention comprises one or more ABS graft copolymers. ABS graft copolymers suitable for use as the ABS graft copolymer component of the composition of the present invention are well known in the art. ABS graft copolymers are a two-phase systems based on a styrene-acrylonitrile (SAN) copolymer continuous phase and a dispersed elastomeric phase, typically based on butadiene rubber. Small amounts of styrene and acrylonitrile are grafted onto the rubber particles to compatibilize the two phases.

Three major processes which can be used to prepare ABS include emulsion, bulk /mass and suspension polymerization or combinations thereof. The emulsion polymerization of ABS is a two step process involving polymerization of butadiene to form a rubber latex, followed by addition and polymerization of acrylonitrile and styrene during which grafting to the rubber and production of the SAN continuous phase takes place. The rubber content of an ABS graft when made in emulsion may range from about 10 to 90 weight percent and the SAN will be grafted from about 10 to 90 weight percent of the ABS graft composition. The ratio of the styrene to acrylonitrile ranges from 50:50 to 85:15. When made in emulsion, the rubber latex will have a particle size ranging from about 0.15 to about 0.8 microns by weight, preferably 0.3 microns. Compositionally, the rubber phase may be comprised of polybutadiene, styrene-butadiene or butadiene-acrylonitrile copolymers, polyisoprene, EPM (ethylene/propylene rubbers), EPDM rubbers (ethylene/propylene/diene rubbers containing as diene, a nonconjugated diene such as hexadiene-(l,5) or norbornadiene in small quantities) and crosslinked alkylacrylate rubbers based on C1-C8 alkylacrylates, in particular ethyl, butyl and ethylhexylacrylate. One or more rubber grafted resins from about 10 to 90 and about 90 to 10 weight percent may also be used. The latex emulsion is broken and the ABS is recovered at the end of the polymerization. In the bulk process, the polymerization is carried out in styrene /acrylonitrile monomer rather than in water. Instead of making the rubber, a pre-produced rubber is dissolved in the monomer solution. The rubber-monomer solution is then fed into the reactors and graftin /polymerization is carried out. When produced via bulk or bulk-suspension process, the soluble rubber will range from about 5 to 25 weight percent and the dispersed rubbery phase will have a diameter ranging from about 0.5 microns to about 10 microns. A large weight percent of the free SAN phase is present depending upon the amount of rubber employed.

In place of styrene and acrylonitrile monomers used in the grafted or ungrafted resins, monomers such as, alpha methyl styrene, para-methyl styrene, mono, di or tri halo styrene, alkyl methacrylates, alkyl acrylates, maleic anhydride, methacrylonitrile, maleimide, N-alkyl maleimide, N-aryl maleimide or the alkyl or halo substituted N-aryl maleimides may be replaced for the styrene or acrylonitrile or added to. Like the bulk process, suspension polymerization uses rubber dissolved in the monomer solution, but after polymerizing SAN to low conversions, the rubber /SAN /monomer mixture is suspended in water and the polymerization is completed.

SAN Copolymer

The SAN copolymer component of the composition of the present invention comprises one or more SAN copolymers. Conventional SAN copolymers comprise from about 0.1 to about 10 weight percent oligomer content, wherein the oligomers can be generally defined as those components of the SAN having a molecular weight of about 15,000 grams per mole or less.

More typically the oligomers are defined as having a molecular weight of about 10,000 grams per mole or less.

Preferred SAN copolymers are SAN with relative weight average molecular weights of from about 40,000 g/mole to about 110,000 g/mole, more preferably 50,000 g/mole to about 90,000 g/mole, and even more preferably from about 60,000 g/mole to about 85,000 g/mole, wherein the molecular weights are measured by gel permeation chromatography relative to narrow dispersivity polystyrene standards. The SAN copolymer typically comprises from about 10 to 40 weight percent, preferably 15 to 35 weight percent, more preferably 20 to 30 weight percent acrylonitrile, with the balance styrene.

We have discovered that by removing at least some of the lower molecular weight end ( i.e., at least some of the oligomer content) of the distribution from the SAN as originally polymerized, a decreased notched Izod ductile-brittle transition temperature (which indicates improved ductility) is realized. Additionally, the removal of at least some of the low molecular weight end of the distribution results in only a slight increase in blend viscosity. Thus, the resulting viscosity-ductility balance is very attractive. Moreover, by selecting the amount of oligomers removed, it is possible to tailor the polycarbonate /ABS /SAN blend to have an acceptable blend viscosity, while resulting in a product with a wide range of properties. Removal of the low molecular weight end of the distribution also results in an increase in the Tg (and HDT) of the blends. Such an increase in Tg (and HDT) is not achievable by simply moving to higher molecular weight SANs.

Reduction of the content of low molecular weight materials, that is, styrene-acrylonitrile oligomers, in the styrene-acrylonitrile copolymer component of the composition of the present invention may be accomplished by any suitable manner.

In a preferred embodiment, the oligomer content of the SAN copolymer component of the present invention is reduced by chemical fractionation. Suitable chemical fractionation techniques are well known in the art.

In a preferred chemical fractionation technique, the SAN copolymer is dissolved in a first solvent, such as, for example, methyl ethyl ketone, in which high molecular weight SAN copolymer species and low molecular weight SAN oligomeric species are soluble and then a second solvent, such as, for example, isopropanol or methanol, in which the high molecular weight SAN copolymer species are relatively insoluble, is added to the solution with mixing and at a slow enough rate to prevent precipitation of the high molecular weight SAN copolymer species. The resultant mixture is then allowed to separate into two layers, that is, a layer of the first solvent and a layer of the second solvent, and the fractionated high molecular weight SAN copolymer species are isolated from the layer of first solvent by addition of more of the second solvent.

Although one preferred method of removing the oligomer content is by chemical fractionation, this should not be viewed as limiting the present invention. Other methods may be used to remove the oligomer content and the present invention encompasses such methods. Further, the oligomer content can be removed at any time. That is, oligomers can be removed from the SAN component before blending with the polycarbonate component, or the oligomers can be removed after blending, or a combination of removing the oligomers before blending and after blending can be used.

Other Components

In addition, certain additives can be included in the resin composition of the present invention, such as antistatic agents, fillers, pigments, dyes, antioxidants, heat stabilizers, ultraviolet light absorbers, lubricants, flame retardants and other additives commonly employed in polycarbonate/ABS/SAN blends.

Suitable antistatic agents which may optionally be incorporated into the resin blend of the present invention include, but are not limited to, the reaction products of polyethylene oxide block polymers with epichlorohydrin, polyurethanes, polyamides, polyesters or polyetheresteramides.

Suitable fillers which may optionally be incorporated into the resin blend of the present invention include, but are not limited to talc, glass fiber, calcium carbonate, carbon fiber, clay, silica, mica and conductive metals, and the like.

Suitable mold release agents may optionally be incorporated into the resin blend of the present invention. The products of the present invention can be made by combining and mixing the components of the composition of the present invention under conditions suitable for the formation of a blend of the components, such as for example, by melt mixing using, for example, a two-roll mill, a Banbury mixer or a single screw or twin- screw extruder, and, optionally, then reducing the composition so formed to particulate form, for example, by pelletizing or grinding the composition.

The blends of the present invention can be molded into useful shaped articles by a variety of means such as injection molding, extrusion, rotational molding, blow molding and thermoforming to form articles such as, for example, computer and business machine housings, home appliances.

Various demonstrations of the present invention are included in the Examples immediately following. However, the Examples should be considered as being illustrative and should not be construed as limiting the scope of the invention as defined in the appended claims.

EXAMPLES

The following abbreviations are used in the Examples:

PC-1: Linear polycarbonate resin having an absolute weight average molecular weight of about 29,000 g/mole; PC-2: Linear polycarbonate resin having an absolute weight average molecular weight of about 24,000 g/mole;

ABS: An acrylonitrile-butadiene-styrene graft copolymer containing about 50 wt% butadiene and 50 wt% of a styrene-acrylonitrile copolymer (75 wt% styrene and about 25 wt% acrylonitrile);

SAN-1: Copolymer of 75 wt% styrene and 25 wt% acrylonitrile having a relative weight average molecular weight of about 62,600 g/mol;

SAN-2: SAN-1 having about 3.5 wt% of the oligomer content removed by chemical fractionation and having a relative weight average molecular weight of about 66,600 g/mol;

SAN-3: SAN-1 having about 7.0 wt% of the oligomer content removed by chemical fractionation and having a relative weight average molecular weight of about 71,500 g/mol; and

SAN-4: Copolymer of about 75 wt% styrene and 25 wt% acrylonitrile and having a relative weight average molecular weight of about

87,600 g/mol.

Examples 1-2 and Comparative Examples C1-C2

The blends of Examples 1-2 of the present invention and of Comparative Examples C1-C2 were each made by combining the components described below in the relative amounts (each expressed in parts by weight) set forth in TABLE I. Blends containing the ingredients listed in TABLE I were prepared by Henshel blending the components for about one minute, and then the blend was. added into the hopper of the extruder. In a typical small scale lab experiment, a six barrel Welding Engineers 20mm extruder was used to compound these blends at 320-400 rpm with melt temperature of approximately 550°F. The compounded materials were injection molded on a 28 ton lϊngel molder at about 525° F. Test specimens were 3.2± 0.2 mm thick unless otherwise specified. Notched Izod impact was measured according to ASTM test procedure D256. Notched Izod data were collected at a range of temperatures. Ductile /Brittle temperatures were determined as the temperature at which the impact energy fell below 8 ft-lb/in.

Viscosity of the samples was measured using a Goettfert Capillary Rheometer. Viscosity was measured at about 550° F at frequencies ranging from about 100 to about 6300 Hz.

Absolute weight average molecular weights of the polycarbonate resins were determined by gel permeation chroma tography relative to absolute molecular weight polycarbonate standards.

SAN-1 and SAN-4 are conventional grades of SAN. SAN-2 was prepared by taking about 300 grams of an SAN-1 type SAN and dissolving it in about 1.5 liters of methyl ethyl ketone and about 2.1 liters of isopropanol was added drop wise while the solution was stirred. The addition of isopropanol was maintained at a slow enough rate to prevent precipitation of polymer. The mixture was left to stand for about 1 hour. The top layer was decanted and concentrated to a dryness to give about 10.5 grams of residue. The polymer dissolved in the bottom layer was precipitated upon slow addition to methanol in a blender. The precipitate was filtered and dried in a vacuum oven overnight at about 40° C and then at 80° C for several days.

SAN-3 was prepared by taking about 250 grams of an SAN-1 type SAN and dissolving it in about 1.25 liters of methyl ethyl ketone and about 1.3 liters of isopropanol added dropwise, as with formation of SAN-2. The same procedure as was used to formulate SAN-3 was used to formulate SAN-2. The blend viscosity, the notched Izod ductile /brittle transition temperature results, the polycarbonate phase Tg, and the HDT are set forth below in Table I , for each of the compositions.

TABLE I

Cl C2

PC-1 36.8 36.8 36.8 36.8 PC-2 27.7 27.7 27.7 27.7

SAN-1 22.0 — SAN-2 22.0 SAN-3 22.0 SAN-4 22.0

ABS 13 13 13 13

Antioxidants and flow improver 0.5 0.5 0.5 0.5

Properties

Notched Izod

Ductile/Brittle transition temperature (°C) -10 -38 -25 -35

Blend viscosity (Pa-sec, measured at 100 sec-1) 209 293.1 226.6 257.9

PC phase Tg (°C) 144.46 143.5 149.1 147.9

HDT (°C) 108 111 111

ΗDT not measured for C2 The above Examples 1 and 2 and Comparative examples Cl and C2 demonstrate that by removing at least some of the oligomer distribution from the SAN component of a polycarbonate /ABS /SAN blend, it is possible to obtain an attractive notched Izod ductile /brittle transition temperature while maintaining a relatively low blend viscosity.

While the preceding discussion includes very particular disclosure, various modifications to the disclosure may occur to an artisan of ordinary skill, and all such modifications should be considered to be within the scope of the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A thermoplastic resin composition, comprising:

(a) an aromatic polycarbonate resin;

(b) an acrylonitrile-butadiene-styrene graft copolymer; and

(c) a styrene-acrylonitrile copolymer having a reduced oligomer content.

2. The composition of claim 1, wherein the oligomer content of the styrene-acrylonitrile copolymer is reduced by chemical fractionation.

3. The composition of claim 1, wherein the styrene-acrylonitrile copolymer has a weight average molecular weight of from about 40,000 grams per mole to about 110,000 grams per mole.

4. The composition of claim 3, wherein the styrene-acrylonitrile copolymer has a weight average molecular weight of from about 50,000 grams per mole to about 90,000 grams per mole.

5. The composition of claim 4, wherein the styrene-acrylonitrile copolymer has a weight average molecular weight of from about 60,000 grams per mole to about 85,000 grams per mole.

6. The composition of claim 1, wherein the aromatic polycarbonate resin has a weight average molecular weight of from about 10,000 grams per mole to about 200,000 grams per mole.

7. The composition of claim 6, wherein the aromatic polycarbonate resin comprises two or more aromatic polycarbonate resins.

S. The composition of claim 6, wherein the aromatic polycarbonate resin comprises an aromatic polycarbonate resin having a weight average molecular weight of about 29,000 grams per mole.

9. The composition of claim 6, wherein the aromatic polycarbonate resin comprises an aromatic polycarbonate resin having a weight average molecular weight of about 24,000 grams per mole.

10. The composition of claim 1, wherein the thermoplastic resin composition comprises, based on 100 parts by weight of the composition, from about 4 parts by weight to about 59 parts by weight of the acrylonitrile- butadiene-styrene copolymer.

11. The composition of claim 1, wherein the thermoplastic resin composition comprises, based on 100 parts by weight of the composition, from about 5 parts by weight to about 46 parts by weight of the acrylonitrile- styrene copolymer.

12. The composition of claim 11, wherein the acrylonitrile-styrene copolymer comprises, based on the total weight of the copolymer, from about 10 weight percent to about 40 weight percent acrylonitrile and from about 60 weight percent to about 90 weight percent styrene.

13. A process for blending a thermoplastic resin composition, comprising blending together an aromatic polycarbonate resin, an acrylonitrile-butadiene-styrene graft copolymer and a styrene-acrylonitrile copolymer having a reduced oligomer content.

14. The process of claim 13, wherein said oligomer content of the styrene-acrylonitrile copolymer is reduced by chemical fractionation.

15. The process of claim 13, wherein the styrene-acrylonitrile copolymer has a weight average molecular weight of from about 40,000 grams per mole to about 110,000 grams per mole.

16. The process of claim 13, wherein the styrene-acrylonitrile copolymer has a weight average molecular weight of from about 50,000 grams per mole to about 90,000 grams per mole.

17. The process of claim 13, wherein the styrene-acrylonitrile copolymer has a weight average molecular weight of from about 60,000 grams per mole to about 85,000 grams per mole.

18. An article molded from the thermoplastic resin composition of claim 1.