STYRENIC COPOLYMER BLEND COMPOSITIONS HAVING IMPROVED COLOR STABILITY

Abstract: The present invention pertains to thermoplastic polymer blends which contain a monovinylidene aromatic copolymer and an acetal polymer and which may also optionally contain an elastomeric material such as an elastomeric thermoplastic polyurethane or an elastomeric copolyester and/or a non-elastomeric polycarbonate or polyester resin. The indicated polymer blends are characterized in that they also contain one or more oxirane-containing stabilizer ingredients and are thereby imparted with improved thermal stability and, surprisingly, with improved U.V. stability and improved impact strength as well. Said polymer blends have good processability and are suitable for use in the preparation of a variety of molded utilitarian articles having a beneficial combination of chemical and physical properties.

Description: - 1 - STYRENIC COPOLYMER BLEND COMPOSITIONS HAVING IMPROVED COLOR STABILITY The present invention pertains generally to thermoplastic polymer blends which contain a monovinylidene aromatic cox-tolymer in combination with an acetal polymer and which contain a minor proportion of an oxirane-containing ingredient. In certain preferred embodiments hereof, the indicated polymer blends also contain an elastomeric polymer such as elastomeric thermoplastic polyurethanes or copolyester elastomers and/or one or more non-elastomeric thermoplastic polycarbonate or polyester homopolymer or copolymer resin ingredients. In another preferred embodiment, the monovinylidene aromatic copolymer is a rubber-modified monovinylidene aromatic copolymer having from 1 to 40 weight percent of dispersed rubber particles contained therein. Blends of various types of polymeric materials have been suggested over the years in a variety of prior art references. For example, U.S. Patent 4.665,126 to Kusumgar et al. discloses t certain polymeric molding compositions containing a predominant amount (for example from 60 to 95- weight percent) of an acetal polymer ingredient in combination with relatively lesser amounts (for example from 4 to 30 and from 1 to 10 weight percent, respectively) of a thermoplastic polyurethane (TPU) and a multiphase composite interpolymer such as, for example, a butadiene-based, rubber-modified styrene/methyl- methacrylate polymer. Such Kusumgar et al. formulations are said to have improved impact strength relative to that of the acetal polymer per se and relative to that of comparable two component acetal/TPϋ or acetal/multiphase composite 10 interpolymer blends and to be useful in various molding applications. U.S. Patent 4,694,042 to McKee et al. pertains to thermoplastic polymer blends containing a minor A c proportion (that is from 5 to 50 parts) by volume of a partially or completely crystalline polymer such as nylon, polyacetal, etc. wherein said crystalline polymer, even though employed in minor volumetric proportion, is nevertheless considered to form a 20 coherent phase and wherein the second, major proportion component forms a dispersed phase therein. Within the indicated McKee et al. blends, said major proportion (that is from 50 to 95 parts by volume) component consists of one or more crosslin ed, emulsion- 25 polymerized elastomeric polymers such as, for example, butadiene or acrylate rubber-based graft copolymers containing either from 10 to 50 weight percent of a shell having a glass transition temperature of less than 30 -10°C or a substantially lesser amount of a hard polymer shell of styrene, methylmethacrylate or styrene acrylonitrile copolymer. Acetal resin-based compositions are not evident in the working examples. British Patent 1,311.305 discloses thermoplastic molding compositions composed of a mixture of from 50 to 99 weight percent of an acetal polymer and from 1 to 50 weight percent of a butadiene or acrylate rubber-modified, two-phase polymer mixture. Such thermoplastic molding compositions are described as having considerably improved impact strength relative to that of the acetal polymer per se. Preferred embodiments of this reference utilize 80 to 95 weight percent of the acetal polymer component. U.S. Patent 4,639,488 to Schuette et al. discloses impact resistant polyacetal-based molding materials containing from 30 to 95 weight percent of an acetal polymer and from 5 to 70 weight percent of an emulsion polymerized elastomeric graft copolymer composed, on a graft copolymer weight basis, of from 60 to 90 weight percent of a butadiene-based core (or "grafting base") and from 10 to 40 weight percent of a grafted shell of a styrene and/or methylmethacrylate- based polymer or copolymer. Such molding materials are said to have high impact strength at low temperatures, to exhibit good thermal stability and to resist discoloration in the presence of light. U.S. Patent 4,179,479 to Carter discloses thermoplastic polymer blend compositions containing from 40 to 100 weight percent of a thermoplastic polyurethane in combination with up to 60 weight percent of a thermoplastic polymer which can be an ABS resin, an acetal resin, a polycarbonate resin, a polyester resin or mixtures thereof. Such compositions are also required to contain 0.5 to 10 weight percent of an acrylic polymer processing aid to improve the processabuity and molding characteristics thereof. U.S. Patent 4,117,033 to Gale discloses polymer blends containing an acetal resin in combination with from 0.1 to 5 weight percent of a low molecular weight copolyether-ester resin. Said copolyether-ester resin is said to improve the melt processabuity of the indicated acetal resin. U.S. Patent 4,683.267 to Lindner et al. discloses molding compounds consisting of a mixture of from 60 to 99.00 parts by weight of an acetal resin, from 0 to 40 parts by weight of an elastomer softening below the melting point of said acetal resin and from 0.01 to 40 parts by weight of an aliphatic, rubber-like, high molecular weight adipate-carbonate mixed ester. Elastomers said to be useful in the Lindner et al. blends include homopolymers and copolymers of alpha- olefins, homopolymers and copolymers of 1,3-dienes, copolymers and homopolymers of vinyl esters and copolymers and homopolymers of acrylate and methacrylate esters. Another publication concerned with blends of polyacetal resins and polystyrene resins is Japanese Kokai No. 64-38463, published February 8, 1989. Such publication is essentially concerned with polyacetal/ polystyrene blends wherein the polyacetal constitutes the major portion by weight thereof and requires in all events that the ratio of the polyacetal melt flow rate (MFR, ASTM D-11238 at 190°C and 2l60g) to the polystyrene melt flow rate (ASTM D-16238 at 200°C and 5000g) be from 5:1 to 100:1. According to such publication, excellent surface appearance is obtained by operating within, and only by operating within, the indicated range of polyacetal: polystyrene melt flow rate ratios. Also according to such publication, the polymer blends thereof optionally may contain small amounts of additional polymer ingredients such as a polyurethane resin, an olefinic homopolymer or copolymer resin, acrylate resins, polyamide resin, ABS resins or polyester resins. Certain blends of rubber-modified styrenic copolymers such as ABS resins with polycarbonate resins are discussed and described in U.S. Patents 4,526,926 and 4,624,986 (Weber et al.) and in U.S. Patents 10 4,163,762 and 4,243,764 (Rudd). Not contemplated, however, by said patents are acetal resin-containing (or acetal resin and thermoplastic polyurethane or elastomeric copolyester-containing) blends as are - c provided in accordance with the present invention. Published German -application DE 3,802,753 A1 discloses polymeric molding materials containing homo- or copolyoxymethylene (POM), thermoplastic polyurethane 20 elastomers (TPU), obtained by the reaction of aromatic di-isocyanates with linear polyols, and polyalkylene terephthalate resin. Such mixtures are described as being useful for applications in vehicles and electrical appliances and to provide improved impact resistance. 25 Preferred compositions are from 40-95 percent POM and from 5-60 percent of a mixture consisting of 60-98 percent TPU and 2-40 percent polyalkylene terephthalate. In spite of the foregoing prior art activities, 30 there has remained a continuing need to provide improved engineering thermoplastic materials having a balance of processabuity, good aesthetics with no pearlescence and having alternative, advantageous property profiles such as mechanical strength, impact resistance, environmental stress crack resistance, creep and chemical resistance and practical toughness. Certain improved styrenic copolyme /acetal polymer-based blend compositions of the sort indicated are disclosed and claimed within U.S. patent applications 474,415; 474,416 and 474,171 all of which were filed on February 2, 1990. The present application is directed to a further improvement in such blend compositions which are characterized in that one or more oxirane-containing ingredients are incorporated therein and in that the resulting compositions have notably improved thermal and ultraviolet (U.V.) light stability and improved impact strength. In accordance with the foregoing, the present invention, in one of its aspects, is a polymer blend composition which contains: (A) a monovinylidene aromatic copolymer ingredient which is either (1) a non-rubber-modified monovinylidene aromatic copolymer containing, in polymerized form and on an aromatic copolymer ingredient weight basis, from 55 to 99 weight percent of one or more monovinylidene aromatic monomers and from 1 to 45 weight percent of one or more relatively polar comonomer ingredients; or (2) a rubber-modified monovinylidene aromatic copolymer containing, on a rubber-modified copolymer weight basis, from 30 to 99 weight percent of one or more monovinylidene aromatic copolymers as described in item (A) (1) above and from 1 to 70 weight percent of dispersed particles of a rubbery polymer having a glass transition temperature of 0°C or lower; and (B) an acetal homopolymer or copolymer ingredient which can be either linear or branched and which can be employed either singly or in combination; said composition being characterized in that it contains, on a total polymer blend composition weight basis, from 0.01 to 15 weight percent of an oxirane- containing stabilizer ingredient. In one of its preferred embodiments, the aforementioned polymer blend composition employs as its monovinylidene aromatic copolymer ingredient a rubber- modified monovinylidene aromatic copolymer containing, on a rubber-modified copolymer weight basis, from 2 to 35 weight percent of dispersed particles of a rubbery polymer such as a homopolymer of a 1 ,3-conjugated alkadiene monomer or a copolymer of from 60 to 99 weight percent of a 1 ,3-conjugated alkadiene monomer with from 1 to 40 weight percent of a monoethylenically unsaturated monomer. In another preferred embodiment, the indicated polymer blend composition further contains one or more elastomeric thermoplastic polyurethane ingredients and/or one or more copolyester elastomer ingredients. Particularly preferred elastomeric polymers for use within such embodiment are ester-containing or ester- based elastomeric materials (such as, for example, ester-based elastomeric thermoplastic polyurethanes and copolyester elastomers) used either alone or in combination with each other or in combination with up to 70 weight on a total elastomer weight basis of a non- ester-based elastomeric material such as, for example, an ether-based thermoplastic polyurethane. In another preferred embodiment hereof, the subject polymer blend composition further contains, in addition to the indicated elastomeric copolyester or thermoplastic polyurethane ingredient, one or more non- elastomeric thermoplastic polycarbonate or polyester resin ingredients. The indicated polymer blends can have a highly advantageous and controllable combination of physical, chemical and aesthetic properties and can be beneficially employed in the preparation of molded articles for use in a wide variety of applications including various interior and exterior automotive applications, household appliance applications, housings for electronic and/or business equipment and the like. While oxirane-containing materials have been known in the prior art as being thermal stabilizers for glass reinforced polyacetal compositions (see, for example, Published European Application Number 281,148) and as being thermolysis stabilizers for polyurethanes (see, for example, U.S. Patent 4,775,558), the use of such materials as additives or stabilizers within monovinylidene aromatic copolymer/acetal polymer blends of the type of concern herein is not thought to have been known heretofore. Moreover, the beneficial U.V. stabilization and impact strength improvements which are achieved by incorporating such ingredients within the subject polymer blends is believed to constitute a totally unexpected and surprising technical result. As has been noted above, the polymer blend compositions hereof contain a monovinylidene aromatic copolymer ingredient which can either be rubber-modified or non-rubber-modified. In either case, suitable monovinylidene aromatic monomer constituents include styrene, alkyl substituted styrenes.such as alpha-alkyl- styrene (for example alpha-methylstyrene, alpha- ethylstyrene etc.), various ring-substituted styrenes such as para-methylstyrene, ortho-ethylstyrene, 2,4- dimethylstyrene, etc., ring-substituted halo-styrenes such as chloro-styrene, 2,4-dichloro-styrene, etc. and the like. Such monovinylidene aromatic monomer (especially styrene) typically constitutes from 55 to 99 weight percent of said monovinylidene aromatic copolymer and preferably constitutes from 60 to 95 (more preferably from 65 to 90) weight percent thereof. Such monovinylidene aromatic copolymers are typically normally solid, hard (that is non-elastomeric) materials having a glass transition temperature in excess of 25°C. Suitable relatively polar comonomer ingredients for use as the minor constituent in (that is constituting from 1 to 45 weight percent of) the indicated monovinylidene aromatic copolymers include ethylenically unsaturated nitriles such as acrylonitrile, methacrylonitrile, ethacrylonitrile, etc.; ethylenically unsaturated anhydrides such as maleic anhydride; ethylenically unsaturated amides such as acryla ide, methacrylamide, etc.; esters (especially lower, for example C 1 -C 6 , alkyl esters) of ethylenically unsaturated carboxylic acids such as methyl methacrylate, ethylacrylate, hydroxyethylacrylate, n- butyl acrylate or methacrylate, 2-ethyl-hexylacrylate, etc.; ethylenically unsaturated dicarboxylic acid i ides such as N-alkyl or N-aryl malei ides such as N-phenyl maleimide, etc. Especially preferred for use as the relative polar comonomer ingredient herein are the aforementioned ethylenically unsaturated nitriles. Preferably said relatively polar comonomers or mixtures thereof constitute from 5 to 40 (more preferably from 10 to 35) weight percent of the indicated monovinylidene aromatic copolymer. Especially preferred polymer blend compositions hereof are those wherein the monovinylidene aromatic copolymer is rubber modified and comprises on a total rubber odified-copolymer weight basis from 1 to 70 (preferably from 1 to 40, more preferably from 2 to 35, and most preferably from 3 or 5 to 20, 25 or 30) weight percent of dispersed particles of a rubbery polymer having a glass transition temperature of 0°C or lower. Especially preferred rubbery polymers for use herein are those having a glass transition temperature of -20°C or lower. Examples of suitable such rubbery polymers include homopolymers of 1,3-conjugated alkadiene monomers; copolymers of from 60 to 99 weight percent of said 1,3-conjugated alkadienes with from 1 to 40 weight percent of a monoethylenically unsaturated monomer such as, for example, monovinylidene aromatic monomers (for example styrene, etc.) and ethylenically unsaturated nitriles such as acrylo-nitrile, methacrylonitrile etc.; ethylene/propylene copolymer rubbers and rubbery ethylene/propylene/non-conjugated diene copolymers. Especially preferred rubbery polymers for use herein include polymers composed of from 60 to 100 weight percent of 1,3-butadiene and from 0 to 40 weight percent of styrene or acrylonitrile. One particular class of rubber-modified monovinylidene aromatic copolymer ingredients of interest for use herein are graft copolymer compositions wherein the above-discussed rubbery polymer particles serve as substrates having grafted thereto a portion of -1 1- the above-described monovinylidene aromatic copolymer as a grafted superstrate and wherein the remainder of said monovinylidene aromatic copolymer constitutes a continuous matrix phase in which the indicated grafted rubbery particles are dispersed. In such instances, the matrix phase typically constitutes from 40 to 95 (preferably from 60 to 95) percent of the overall weight of the indicated rubber-modified compositions and the grafted copolymer constituents constitutes the remainder thereof. Typically the grafted copolymer constituent will have a grafted superstrate to graftable rubber substrate ratio (that is a graft to rubber or "G/R" ratio) of from 0.1:1 to 1:1 (preferably from 0.35:1 to 0.45:1). Typically, the above-described rubber-modified monovinylidene aromatic copolymer ingredient will have a melt flow rate (MFR) of from 0.5 to 12 (preferably from 1 to 10) grams per 10 minutes as determined pursuant to ASTM D-1238 at 230°C and 3.8 kg. In certain especially preferred embodiments hereof, the dispersed rubbery polymer particles are of a sort which have a bimodal particle size distribution. For example, it has been observed that substantially higher impact strength is obtained within the polymer blend compositions of interest when the indicated rubbery particles are predominantly composed (for examplefrom 50 to 90, preferably from 65 to 75, weight percent on a total rubbery particle weight basis) of particles having a volume average particle size of less than one micron (preferably from 0.05 to 0.8 micron) and wherein the remainder of said rubbery particles (for example from 10 to 50, preferably from 25 to 35, weight percent thereof) have a volume average particle size of one micron or greater (preferably from 1 to 3 micron) . The use of such bimodal rubber polymer particle has been found to give notably higher impact strength relative to comparable polymer blend compositions wherein the dispersed rubbery polymer particles are composed completely of rubber particles having sizes (that is diameters) of one micron or greater. The aforementioned rubber-modified monovinylidene aromatic graft copolymer hereof can suitably be prepared in any known manner by free radical polymerization of the selected comonomer materials in the presence of the modifying rubber material. Suitable techniques thus include conventional mass, solution, suspension or emulsion polymerization processes. If emulsion polymerized graft copolymers are to be employed, care should be taken to remove or neutralize residual acid moieties. Otherwise decomposition of the acetal polymer component can result. Especially preferred for use herein are rubber-modified monovinylidene aromatic graft copolymers prepared via mass, solution, mass/suspension or mass/solution polymerization techniques. In general, mass polymerization involves polymerizing a solution of the rubber and monomer(s) at conditions sufficient to form discrete rubber particles of the desired particle size dispersed throughout the polymerized monomer. The polymerization is advantageously conducted in one or more substantially linear stratified flow or so-called plug-flow reactors such as described in U.S. Patent No. 2,727,884 which may or may not comprise recirculation of a portion of the partially polymerized product or in a stirred tank reactor wherein the contents of the reactor are essentially uniform throughout. The polymerization is advantageously conducted in an organic liquid reaction diluent or solvent such as aromatic or inertly substituted aromatic hydrocarbons (for example benzene or toluene) and in the presence of a free-radical initiator such as the peroxide initiators, (for example dibenzoyl peroxide or 1,1- bistertiary butyϊperoxycyclohexane) . In general, the initiator will be employed in an amount from 100 to 5000 weight parts per million weight parts of the monomers employed. The organic liquid reaction diluent is generally employed to control , the viscosity of the polymerization mixture and is generally employed in an amount from 2 to 20 weight percent based on the total weight of the rubber, monomer and diluent. The polymerization mixture can further contain other adducts such as a plasticizer or lubricant (for example mineral oil); and antioxidant (for example an alkylated phenol such as di-tert-butyl-p-cresol); a polymerization aid (for example a chain transfer agent such as an alkyl • mercaptan) or a mold release agent, (for example zinc stearate). Temperatures at which polymerization is normally conducted are dependent on the 1 specific components employed but will generally vary from 60° to 190°C. In the preparation of the rubber-reinforced polymer resin, the mass polymerization can be continued to the desired completion and then treated to remove any unreacted monomer such as by flashing off the monomer and other volatiles at an elevated temperature under vacuum. Mass/suspension polymerization involves initially mass polymerizing the monomer/rubber mixture and, following phase inversion (that is the conversion of the polymer from a discontinuous phase dispersed in a continuous phase of the rubber solution through the point where there is no distinct continuous or discontinuous phase in the polymerization mixture and to the point where there is a continuous polymer phase having the rubber dispersed therethrough) and subsequent size stabilization of the rubber particles, suspending the partially polymerized product, with or without additional monomer(s), in an aqueous medium which generally contains a polymerization initiator. Subsequently, polymerization is completed using suspension polymerization techniques. In one preferred embodiment hereof, the above- described mass or mass/suspension-polymerized rubber- modified monovinylidene aromatic graft copolymer ingredient is employed in combination with a finely divided, emulsion polymerized particulate elastomeric material. Such particulate elastomeric materials typically have a volume average particle size in the range of from 0.05 to 0.5 (especially from 0.15 to 0.2) micron and, when employed, constitute from 1 to 15 percent by weight of the overall blend composition. Such emulsion polymerized particulate elastomeric materials may be suitably prepared by emulsion polymerizing suitable monomers such as butadiene, isoprene or higher alkyl esters of acrylic acid or methacrylic acid, optionally in the presence of not more than 30 percent by weight of monomers, such as styrene, acrylonitrile, methyl acrylate, methyl methacrylate or any other monomer and polar comonomer described above. Preferably such elastomeric materials contain adhesion promoting groups such as carboxyl, carboxamido, carboxylic anhydride or epoxide groups. This can be suitably achieved if acrylic or methacrylic acid, an amide of one of these, glycidyl acrylate or, instead of the free acid, tert.-butyl acrylate is used as a comonomer, in an amount of from 0.1 to 10 percent by weight. It is particularly advantageous if a shell which has a glass transition temperature of less than -10°C and which contains su h an adhesion promoting monomeric building block is grafted onto the indicated emulsion-polymerized elastomeric polymer. Graft monomers which have proven particularly useful are esters of acrylic acid, such as n-butyl acrylate, preferably in combination with multifunctional crosslinking agents and/or with comonomers containing the stated adhesion promoting groups. Advantageously, the shell amounts to 10-50 percent by weight of the total elastomeric polymer. While not being particularly critical for the purposes of the present invention, the above-described monovinylidene aromatic copolymer ingredient will generally constitute from 5 to 90 weight percent of the polymer blend compositions hereof. Preferably, said monovinylidene aromatic copolymer is employed in amounts corresponding to from 10 to 90 (more preferably from 15 to 85, especially from 20 to 65) parts by weight per 100 parts of the combined or total weight of the overall polymer blend composition. In those embodiments wherein the blends hereof are binary in character and are composed of the monovinylidene aromatic copolymer and the acetal polymer only, said blends will preferably contain from 50 to 80 (more preferably from 55 to 75) parts by weight of the monovinylidene aromatic copolymer per 100 parts by weight of the polymer blend composition. On the other hand, when three component monovinylidene aromatic copolymer/acetal polymer/-elastomeric polymer compositions are involved, then the aromatic copolymer will preferably be employed in amounts ranging from 10 to 65(more preferably from 15 to 60) parts by weight per 100 parts by weight of the polymer blend composition. Furthermore, when 4 component polycarbonate or polyester resin-containing systems are prepared, the monovinylidene aromatic copolymer will preferably be employed in amounts ranging from 5 to 65 (more preferably from 10 to 40 and most preferably from 15 to 30 or 35) parts by weight per 100 parts by weight of the overall polymer blend composition. The acetal (sometimes termed polyoxymethylene) resin can be any of those commonly known in the art or commerically available. Thus the acetal resin either can be linear or branched and can be a copolymer or a homopolymer or mixtures of these. Copolymers can contain one or more co onomers such as those generally used in preparing acetal resins. Comonomers more commonly used include alkylene oxides of 2 to 12 carbon atoms, in a less than 20 wt. percent amount. Polyoxymethylenes which contain from 0.5 to 10 percent, in particular from 1 to 5 percent of ethylene oxide are particularly important commercially and are especially preferred for use herein. As a general rule, the available acetal resins have thermally stable terminal groups, such as ester or ether groups, for example acetate or methoxy groups. The polyoxymethylenes have, in general, a molecular weight of from 10,000 to 100,000. As an alternative to molecular weight, melt flow rate (MFR) is commonly.used to characterize resins, and those with higher molecular weights have lower melt flow rates. Preferred acetal resins for use in the compositions of the present ' invention have MFRs of from 0.1 to 60 (preferably from 0.5 to 30 and most preferably from 0.5 to 5 or 10) grams/10 minutes, as measured pursuant to ASTM D-1238 at 190°C and 2.16Kg. If the MFR is too high, the melt viscosity of the acetal will be too low and it will be difficult to achieve sufficient intimate mixing of components at appropriate shear rates. If the MFR is too low, the temperature for the compounding operation may become too high and degradation can result. As will be e.vident in the examples, and assuming all other parameters are equal, the lower the MFR, the higher the toughness of the compositions of the present invention. While not critical, the acetal polymer ingredient of the subject polymer blend compositions can generally constitute from 5 to 90 weight percent) of said polymer blend compositions. Preferably, said acetal polymer is utilized in an amount corresponding to from 10 to 90 (more preferably from 15 to 85, and especially from 15 to 55) parts by weight per 100 parts by weight of the total or combined weight of the indicated polymer blend composition. In certain preferred embodiments hereof, it is desirable to employ the indicated acetal polymer ingredient in relatively larger proportions such as for example at levels ranging (on a per 100 parts by weight total polymer blend composition basis) from 40 to 90 (more preferably from 45 to 80 and most preferably from 50 to 75) parts by weight. These latter types of blend compositions are particularly beneficial in those instances wherein high heat distortion characteristics are desired. In other preferred embodiments, it may be desirable to employ said acetal polymer in smaller proportions such as, for example, at levels ranging from 20 to 50 (especially from 25 to 35 or 40) parts by weight per 100 parts by weight of the polymer blend composition in question. As has been noted above, the polymer blend compositions hereof also contain a minor proportion (for example, from 0.01 to 15 parts by weight per 100 parts by weight of the subject polymer blend compositions) of one or more oxirane-containing stabilizer ingredients. The inclusion of the indicated oxirane- containing ingredients within the polymer blend compositions hereof has been observed to substantially improve the thermal color stability of the subject polymer blends during the melt processing (for example, melt blending and/or injection molding) thereof and to thereby widen the processabuity window of such blends by allowing the use of increased processing temperatures without encountering severe thermally induced discoloration problems. Additionally, it has been surprisingly found that the indicated oxirane-containing additives or ingredients can also impart notably improved impact strength to the subject polymer blends - 1 9- as well as providing unexpectedly enhanced ultraviolet (U.V.) light stability characteristics. Oxirane-containing ingredients suitable for use herein include the various known epoxidized organic materials which have sufficiently high boiling points and decomposition temperatures (for example, preferably at least 180°C, more preferably at least about 200°C and most preferably at least about 220°C) so as to be melt processable within the subject polymer blend compositions without undergoing substantial decomposition or evaporative loss thereof. Examples of suitable oxirane-containing materials for use herein include epoxide derivatives of unsaturated triglycerides such as epoxidized soybean oil, epoxidized linseed oil, epoxidized palm oil, epoxidized tung oil, epoxidized coconut oil, epoxidized peanut oil, epoxidized olive oil, epoxidized rapeseed oil, etc.; epoxy phenol novolac resins; epoxy cresol novolac resins, diglycidyl ethers of bisphenol A; diglycidyl ethers of poly (oxypropylene) glycol; glycidyl ethers of polyethylene glycols; glycidyl ethers of polyhydroxy aliphatic alcohols such as 1,4-butanediol, 1,4-butenediol, glycerin, trimethylol propane, pentaerythritol, etc.; and glycidyl esters of polyvalent aromatic, aliphatic or cycloaliphatic carboxylic acids such as phthalic acid, terephthalic acid, tetrahydrophthalic acid, adipic acid, etc. Particularly suitable and preferred for use herein are the aforementioned epoxidized unsaturated triglycerides (especially epoxidized soybean oil and epoxidized linseed oil) and epoxy resins derived from the reaction of epichlorohydrin with aromatic or aliphatic polyols such as bisphenol A, polyethylene glycol, polypropylene glycol, etc. As a general proposition,the aforementioned oxirane-containing stabilizer ingredients can be employed within the polymer blend compositions hereof in an amount ranging from 0.01 to 15 weight percent based upon the total weight of said compositions. Preferably, however, said stabilizer ingredient will more typically be employed in an amount ranging from 0.1 to 10 (more preferably from 0.2 to 5 and most preferably from 0.5 to 2 or 3) parts by weight per 100 parts by weight of the subject polymer blend composition. Polycarbonate resins suitable for use herein include aromatic polycarbonates which contain the repetitive carbonate group, O II -o-c-o- and which have a divalent aromatic radical attached to said carbonate group. Preferably, the polycarbonate can be characterized as possessing recurring structural units of the following formula and structural isomers thereof: wherein A is a single bond or is a divalent aliphatic radical such as an alkylene or an alkylidene radical usually with 1-7 carbon atoms, or a cycloalkylene or cycloalkylidene radical usually with 5-15 carbon atoms, with all including their aromatically and aliphatically substituted derivatives. In other variations of the (R')n (R")n polycarbonate resin, A can also represent -0-, -S-, - CO-, -SO- or - SO 2 -. In the indicated structural formula, R' and R 1 ' are substituents other than hydrogen such as, for example, halogen or a saturated or unsaturated monovalent aliphatic radical having usually 1-7 carbon atoms, and n equals 0 to 4. Typical of the above-mentioned structural unit are those which result from the reaction of phosgene (or other carbonyl-providing species) with bis- (hydroxyphenyl) alkanes, bis (hydroxyphenyl) cycloalkanes, bis (hydroxyphenyl) sulphides, bis (hydroxyphenyl) ethers, bis (hydroxyphenyl) ketones, bis (hydroxyphenyl) sulphoxides, bis-(hydroxyphenyl) sulphones, α, α'-bis (hydroxyphenyl)-isopropylbenzene, bis (3,5-bromo-4-hydroxyphenyl) sulfone, bis (tetrabromo-4-hydroxyphenyl) propane, bis-(3,5,6-tri- chloro-2-hydroxyphenylJ methane, 2,2'-chloro-4,4'- cyclohexylidene phenol, tetrachlorohydroquinone and chloroethylene phenol. Further possible structural units are those which result from bis-(3,5-methyl-4- hydroxyphenyl) propane, 4,4'-bis (4-hydroxy-phenylthio) phenylsulfone and phenophthalein. It is understood, of course, that the carbonate polymer may be derived from two or more different hydric phenols or a copolymer of a hydric phenol with a glycol if a copolymer carbonate rather than a carbonate homopolymer is desired. Also suitable for the practice of this invention are blends of any of the above carbonate polymers. Also included in the term "polycarbonate polymer" are the ester carbonate copolymers of types described in U.S. Patent Numbers 3,169,121; 4,330,662 and 4,105,633. Typical comonomers are dicarboxylic acid, for example, terephthalic acid. Additionally included in the scope of this invention are so-called "branched polycarbonates" which are made by using the above-described polyhydric monomers in combination with a suitable branching agent, normally tri- or higher polyfunctional molecules. Suitable polyhydric reactants for use in preparing various polycarbonate resins are also described, in U.S. Patent Numbers 3,062,781; 2,970,131 and in German Offenlegungsschrift Nos. 1,570,703; 2,211,956 and 2,211,957. The polycarbonate resins employed herein preferably have a melt flow rate, measured according ASTM D-1238 (condition 0 : 300°C, 1.2 kg load), of from 0.5 to 200 g/10 min, preferably from 2.5 to 100 g/10 min, more preferably from 5 to 90 g/10 min, and especially preferred from 8 to 75 g/10 min. Thermoplastic polyester resin components suitable for use herein are those which are obtained by reaction of glycol and dicarboxylic acid such as, for example, as are described in U.S. Patent Number 2,465,319. The glycol preferably has the general formula: HO-(CH 2 ) n -OH in which n is an integer from 2 to 12, such as for example ethylene glycol, 1,2- or 1,3-propane diol, 1,2-, 1,3- or 1,4- butane diol, 1,5- or 1,4-pentane diol, 1,6- hexane diol, 1,7- heptane diol or 1,8-octane diol. In other preferred cases cycloaliphatic diols, typically containing up to 21 carbon atoms, are employed, such as, for example, cyclohexane-l,4-dimethanol, 2,2-bis-(4- hydroxycyclohexyl) propane, 2,4-dihydroxy-l,l,3,3- tetramethyl cyclobutane and 2,2-bis-(3-β-hydroxy- ethoxyphenyl)-propane. Dicarboxylic acid components suitably employed to prepare said polyester resins include those having the general formula O O II II HO—C-R"'--B—R""—C—OH with R" ' and R n " each representing the -(CH 2 ) πι -group, with m being zero or an integer from 1 to 4. B is a divalent aromatic radical represented by the following formulas or structural isomers thereof: or a cyclo aliphatic group. D may be: O II -(CH 2 ) P - . -(CH2)p-C-(CH2)p- ; -(CH2)p-0-(CH2) p - ; -O-(CH 2 ) q -O- . -(CH 2 )p-S-(CH 2 )p- ; -S * -(CH 2 ) q -S - . —SO 2 — ; and where p may be zero or an integer from 1 to 5 and q is an integer of from 1 to 5. D m a*y also be: and structural isomers thereof. Typical dicarboxylic acids include phthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, 4,4'-diphenyl dicarboxylic acid, succinic acid, adipic acid, sebacic acid, azelaic acid and cyclohexane diacetic acid. The polyester resin obtained from reaction of the indicated dicarboxylic acid and a glycol may be branched by incorporation of relatively small quantities of tri-or tetrahydric alcohols or tri- or tetrabasic polycarboxylic acids of the type described, for example, in German Offenlegungsschrift No. 1,900,270 and in U.S. Patent Number 3,692,744. In addition to the homopolymer derived from one type of dicarboxylic acid and one type glycol, copolymer resins are often preferred, polymerized from a combination of one or more dicarboxylic acids and a combination of one or more glycols. Such a product, made from terephthalic acid, and a combination of cyclohexane dimethanol and ethylene glycol is commerically available from Eastman Laboratories under the tradename KODAR PETG (TM) Copolyester. The homo- and copolyesters derived from dicarboxylic acid and glycol have preferably a molecular weight ranging from 5,000 to 200,000, more preferably from 10,000 to 60,000- Mixtures of various thermoplastic polyester and/or polycarbonate resins can suitably be employed if and as desired such as, for example, polycarbonate and polyethylene glycol terephthalate or polybutylene glycol terephthalate or any other combination of the various polyester and polycarbonate resins mentioned above. The indicated thermoplastic polyester or polycarbonate resin ingredient, when employed within the compositions hereof, can typically be employed in amounts ranging from 5 to 90 parts by weight thereof per 100 parts by weight of the subject polymer blend compositions. Preferably, said ingredient is employed in an amount corresponding to from 10 to 75 (more preferably from 15 to 55, especially from 20 to 45 or 50) parts of the combined weight of the polymers contained within the subject polymer blend composition. Elastomeric materials suitable for use herein include, as noted above, thermoplastic polyurethanes and elastomeric copolyester materials. Thermoplastic polyurethanes suitable for use herein include any of those generally known in the art and thus include those prepared from a diisocyanate, a polyester, poly- caprolactone or polyether and a chain extender. Such thermoplastic polyurethanes are substantially linear and maintain thermoplastic processing characteristics. A preferred group of polyether-based polyurethanes used in the polymer blend composition of the present invention are the reaction products of: (i) 4,4'-methylene bis(phenyl isocyanate), (ii) a polyether polyol (such as for example, a poly (oxy-1,2 propylene) glycol or a polyoxytetramethylene glycol) having a number average molecular weight within the range of 600 to 3000 (preferably from 1000 to 2500) and (iii) chain extending agent such as diol extenders selected from the group consisting of aliphatic straight chain diols having from 2 to 6 carbon atoms, bis(2-hydroxy-ethyl) ether of hydroquinone, bis(2-hydroxy-ethyl) ether of resorcinol, and mixtures of any two or more of such diol extenders and/or other difunctional chain extending agents containing 2 active hydrogen-containing groups which are reactive with isocyanate groups. Suitable chain extending agents for use herein may include any difunctional compounds containing two active hydrogen-containing groups which are reactive with isocyanate groups. Examples of such suitable chain extending agents thus include diols including ethylene glycol, propylene glycol, butylene glycol, 1,4- butanediol, butenediol, butynediol, xylylene glycols, amylene glycols, 1,4-phenylene-bis-β-hydroxyethyl ether, 1,3-phenylene-bis-β-hydroxy ethyl ether, bis-(hydroxy- methyl-cyclohexane) , hexanediol, thiodiglycol and the like; diamines including ethylene diamine, propylene diamine, butylene diamine, hexamethylene diamine, cyclohexalene diamine, phenylene diamine, toluylene diamine, xylylene diamine, 3,3'-dichlorobenzidine, 3,3'- dinitrobenzidine and the like; alkanol amines such as, for example, ethanol amine, aminopropyl alcohol, 2,2- dimethyl propanol amine, 3-aminocyclohexyl alcohol, p- aminobenzyl alcohol and the like. If desirable, a small amount of polyfunctional material may be utilized. This polyfunctional chain extender, however, should not be present in an amount greater than about 1 percent by weight. Any suitable polyfunctional compound may be used for such purpose such as, for example, glycerine, trimethylolpropane, hexanetriol, pentaerythritol and the like. As used herein, the term "aliphatic straight chain diols having from 2 to 6 carbon atoms" means diols of the formula HO(CH 2 )n OH wherein n is 2 to 6 and there is no branching in the aliphatic chain separating the OH groups. The term is inclusive or ethylene glycol, 1,3- propanediol, 1,4-butanediol, 1,5-pentanediol and 1,6- hexanediol. Preferred diol extenders for use herein include 1,4-butanediol, 1,6-hexanediol and the bis(2- hydroxy-ethyl) ether of hydroquinone; an especially preferred diol extender being 1,4-butanediol. Other diisocyanates which may be used in place of or in combination with the preferred species mentioned above [that is 4,4'-methylene bis (phenyl isocyanate)] include ethylene diisocyanate, ethylidene diisocyanate, propylene diisocyanate, butylene diisocyanate, cyclopentylene-1,3-diisocyanate , cyclohexylene-1,4-diisocyanate, 2,6-tolylene diisocyanate, 2,2-diphenylpropane-4,4'-diisocyanate, P- phenylene diisocyanate, m-phenylene diisocyanate, xylylene diisocyanate, 1,4-naphtylene diisocyanate, 1,5- naphthylene diisocyanate, diphenyl-4,4'-diisocyanate, azobenzene-4,4'diisocyanate, diphenyl sulfone- 4,4'diisocyanate, dichlorohexamethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, 1- ■ chlorobenzene-1,4-diisocyanate, furfurylidene diisocyanate and the like. The polyether polyol and chain extending agent are typically used in the polyurethane reaction medium in a ratio of 0.5 to 2.5 equivalents (for example mole equivalents) of the chain extender per equivalent of the polyol. Preferably, the equivalents ratio is from 1 to 2. Most preferably the ratio is from 1.2 to 1.6 equivalents of extender per equivalent of the polyol when said polyol has a molecular weight of 2000, and especially when the extender is an aliphatic straight chain diol. When the aforementioned hydroquinone or resorcinol extender are used, the equivalents ratio may be lower than the above-mentioned preferred ranges, for example, as low as 0.5 equivalents of the extender per equivalent of the polyol. In preparing the foregoing polyether-based polyurethanes, the polyether polyol and the chain extender and the diisocyanate are typically used in relative proportions to each other such that the overall ratio of isocyanate equivalents or groups to total hydroxyl equivalents or groups or other active hydrogen- containing groups (that is polyol plus extender) is within the range of 1:1 to 1.08:1.0 and preferably is within the range of 1.02:1.0 to 1.07:1.0. The most preferred ratio of isocyanate (NCO) groups to total hydroxyl (OH) groups (or combined hydroxyl plus other active hydrogen groups) is within the range of from 1.03:1.0 to 1.06:1.0. The term equivalent(s) as used with respect to the polyurethane pxeparation herein is based on the hydroxyl (or other active hydrogen) groups and the isocyanate groups within the reactants. Suitable techniques for the preparation of the aforementioned polyether-based thermoplastic polyurethanes are known in the art and are discussed, for example, within the teachings in Columns 4-6 of U.S. Patent 4,665,126 to Kusumgar et al. 0 The polyether-based thermoplastic polyurethanes employed in the practice of the present invention are typically characterized by a ClashBerg modulus (Tf) which is less than -10°C The T g (glass j - transition temperature) of the polyurethanes is essentially the same value. The polyether-based polyurethanes may suitably have, for example a Shore A Hardness of 95A or less, and a weight average molecular weight in excess of 100,000. 0 A preferred group of thermoplastic polyester- based polyurethanes for use in the present invention are the reaction products of: (i) 4,4'methylenebis(phenyl isocyanate; (ii) a polyester of adipic acid and a glycol having at least one primary hydroxyl group; and (iii) a difunctional chain extender of the sort described above having 2 active hydrogen-containing groups which are reactive with isocyanate groups. In preparing the polyester precursor of this group of polyurethanes the adipic acid is condensed with a suitable glycol or mixture of glycols which have at least one primary hydroxyl group. The condensation is stopped when an acid number of from 0.5 to 2.0 is reached. The water formed during the reaction is removed simultaneously therewith or subsequently thereto such that the final water content is from 0.01 to 0.02 percent preferably from 0.01 to 0.05 percent. Any suitable glycol may be used in reaction with the adipic acid such as, for example, ethylene glycol, propylene glycol, butylene glycol, hexanediol, bis-(hydroxymethylcyclohexane) , 1,4-butanediol, diethylene glycol, 2,2-dimethyl propylene glycol, 1,3- propylene glycol and the like. In addition to the glycols, a small amount of trihydric alcohol up to 1 percent may be used along with the glycols such as, for example, trimethylolpropane, glycerine, hexanetriol and the like. The resulting hydroxyl polyester has a molecular weight of at least 600, a hydroxyl number of 25 to 190 and preferably between 40 and 60, and an acid number of between 0.5 and 2 and a water content of 0.01 to 0.2 percent. Any suitable chain extending agent including those described above for the polyether-based thermoplastic polyurethanes) having active hydrogen containing groups reactive with isocyanate groups may be used in preparing the subject polyester-based materials. Examples of such extenders thus include diols such as ethylene glycol, propylene glycol, butylene glycol, 1,4- butanediol, butenediol, butynediol, xylylene glycols, amylene glycols, 1,4-phenylene-bis-β-hydroxyethyl ether, 1,3-phenylene-bis-β-hydroxy ethyl ether, bis-(hydroxy- methyl-cyclohexane) , hexanediol, thiodiglycol and the like. Moreover, polyether polyols may also be employed as the chain extending agent (or as a portion thereof) with the result being a copolyester/polyether based thermoplastic polyurethane which ,is also suitable for 10 use in the practice of the present invention. Although thermoplastic polyurethanes based upon adipate polyesters are generally preferred for use herein, other polyester-based thermoplastic - r polyurethanes can also be suitably employed within the present invention such as, for example, those in which there is employed (in place of the adipic acid) succinic acid, suberic acid, sebacL.c acid, oxalic acid, methyl adipic acid, glutaric aci ^ d, pimelic acid, azelaic acid, 20 phthalic acid, terephthalic acid, isophthalic acid and the like as well as those prepared using hydroxycarboxylic acids, lactones, and cyclic carbonates such as ε-caprolactone and 3-hydroxy-butyric acid in place of the adipic acid component. Similarly 25 polyester-based thermoplastic polyurethanes prepared using the above-described alternative diisocyanates in place of 4,4'-methylene bis (phenyl isocyanate) can also be suitably employed in the practice of. the present 30 invention. The aforementioned types of polyester-based thermoplastic polyurethanes are generally known materials. Suitable methodology for the preparation thereof is disclosed within Column 7 of U.S. Patent 4,665,126. Especially preferred thermoplastic polyurethanes for use herein include those having a Shore hardness (ASTM D2240) between 70 on the "A" scale and 60 on the "D" scale. If desired, the thermoplastic polyurethane employed in the practice of the present invention can have incorporated in it additives such as pigments, fillers, lubricants, stabilizers, antioxidants, coloring agents, fire retardants, and the like, which are commonly used in conjunction with polyurethane elastomers. Elastomeric polymer ingredients suitable for use herein also include polyester-based elastomers other than the ester-based polyurethane materials which have been discussed above. Examples of such other elastomers include elastomeric copolyether-ester resin material and elastomeric adipate-carbonate mixed ester resin materials. Suitable copolyether-ester elastomer ingredients can be generally described as comprising a multiplicity of recurring intralinear long-chain and short-chain ester units connected head-to-tail through ester linkages, said long chain ester units generally constituting from 25 to 85 weight percent of said copolyether-ester elastomer and corresponding to the formula: 00 -OGO— ■ CRC— wherein G is a divalent radical remaining after removal of terminal hydroxyl groups from a poly-(alkylene oxide) glycol having a carbon-to-oxygen mole ratio of 2-4.3 and a molecular weight of 400-6000; and R is a divalent radical remaining after removal of carboxyl groups from a dicarboxylic acid having a molecular weight less than 300; and said short chain ester units generally constituting from 15 to 75 weight percent of said elastomer and corresponding to the formula: O O II II -ODO—CRC- wherein D is a divalent radical remaining after removal of hydroxyl groups from a low molecular weight diol having a molecular weight less than 250; and R is as defined above. Preferably, the indicated polyether-ester elastomers have a relatively low molecular weight as evidenced by their exhibiting an inherent viscosity of 0.05-0.95 (preferably from 0.1 to o.8 and most preferably from 0.1 to 0.5) when measured in m-cresol at a O.lg/dl concentration at 30°C. A more detailed description of the aforementioned polyether-ester elastomers (including preferred embodiments thereof, preparation methodology, the use of small amounts of the low molecular weight materials as a processing aid for polyacetal resins and the use, as per German Patent 2,449,343, of higher molecular weight versions as impact modifiers for polyacetal resins) is presented within U.S. Patent 4,117,033 to Gale. Suitable elastomeric adipate-carbonate mixed ester materials for use herein include those described within U.S. Patent 4,683,267 to Lindner et al. for use as property modifiers for polyoxymethylene resin-based molding compositions. Such materials are rubber-like high molecular weight compounds corresponding to the formula wherein -35- X and X' represent residues of the reaction product of a polyhydric alcohol and adipic acid having a molecular weight of from 800 to 3,500; k represents an integer of from 0 to 10; and m represents an integer greater than 20, preferably from 22 to 100; which compounds have a limiting viscosity number [η] (Staύdinger Index) in tetrahydrofuran of from 0.8 to 2.5 dl/g. The following are examples of polyhydric alcohols which may be used, optionally as mixtures, for the polyesters from which the residues X and X' are derived: ethylene glycol, propylene glycol-(l,2) and - (1,3), butylene glycol-(1,4) and -(2,3), hexane diol- (1,6), octane diol-(1,8), neopentyl glycol, cyclohexane dimethanol, l,4-bis-(hydroxymethyl cyclohexane), 2- methyl-l,3-propane diol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol and dibutylene glycol. The reaction products obtained from adipic acid and the alcohols are polyesters having hydroxyl end groups. The molecular weights thereof range from 800 to 3,500. The adipate-carbonate mixed esters are prepared from these polyesters by a reaction with difunctional carbonic acid aryl esters. These correspond in particular to the following general formula: )) ■+- OAr •Jlr wherein Ar represents a substituted or unsubstituted aryl group having from 6 to 18 carbon atoms, preferably 6 carbon atoms; and k and X' are as defined above. Further details concerning preferred embodiments of the indicated adipate-carbonate mixed 10 esters and concerning suitable techniques for the preparation thereof are contained in the indicated Lindner et al. patent. When the above-described elastomeric polymer - c * ingredients are employed within the subject polymer blends hereof, they are typically utilized in an amount ranging from 1 to 80 (preferably from 3 to 70 and most preferably from 10 to 65) parts by weight per 100 parts of the combined weight of the stated polymer 20 ingredients. In certain preferred embodiments, said elastomeric ingredient is employed in amounts ranging from 3 to 40 (especially from 5 to 30 and most preferably from 10 to 25) parts by weight per 100 parts by weight of the total polymer ingredients. 25 In some instances, it is preferred that the elastomeric ingredient be employed at relatively lower levels such as, for example, in amounts of from 3 to 25 (especially from 5 or 10 to 20 or 25) parts by weight on 30 a 100 parts total polymer weight basis. In other cases, as for example when a more elastomeric character is desired in the subject blend composition, it is preferred that the indicated elastomeric ingredient be employed in amounts ranging -37- from 20 or 25 to 60, 70 or 80 (more preferably from 20 or 25 to 35 or 40) parts by weight .per 100 parts total weight of the specified polymer ingredients. In those instances wherein relatively low levels (for example from 3 to 20 or 25 parts by weight/100 parts total polymer) of the elastomeric ingredient is to be employed, it„has been found to be especially advantageous and preferred to either (a) employ one or more of the above-described ester- containing or ester-based elastomeric materials (especially the ester-based thermoplastic polyurethane) either alone or in combination with each other as the elastomeric ingredient or (b) to employ, on a total elastomeric ingredient weight basis, at least 30 weight percent (preferably 50 weight percent or more) of such an ester-based or ester-containing elastomer in combination with up to 70 weight percent (preferably 50 weight percent or less) 'of an ether-based thermoplastic polyurethane material. On the other hand, in those cases where a relatively larger amount(such as, for example, and on a 100 parts by weight total polymer basis, from 20 or 25 to 40 or 70 parts by weight) of the elastomeric material is to be employed, it has been found that elastomeric materials which are somewhat less effective and/or desirable for use as the sole elastomeric ingredient at low usage levels can in fact be more satisfactorily employed as the sole elastomeric ingredient at said higher usage levels. The polymer blend compositions hereof are conveniently prepared by dry blending the individual polymer ingredients to be employed in particulate (for example pelletized) form along with the oxirane- containing stabilizer ingredient in the quantitative proportions desired in a given instance and thereafter melt compounding the particulate polymer mixture in accordance with known extrusion compounding techniques. In connection with the indicated melt compounding operation, it is generally preferred to conduct such operation at a temperature of not exceeding 240°C, especially at a melt temperature in the range of from 180 to 230°C (more preferably from 180 to 210°C) . Various optional additives may also be included in the polymer blend compositions hereof for different purposes as well known in the art, including bisphenol- type, ester-type or hindered phenol-type additives and anti-oxidants as disclosed, for example, in U.S. Patent Nos. 3,103,499 and 3,240,753, amine and amidine as disclosed, for example, in U.S. Patent Nos. 3,313,767 and 3,314,918, nucleants, UV screens and absorbers, metal soaps, glass fibers, glass beads, talc, polymeric substances other than those critical to this invention such as additives commonly known as mold release agents, plasticizers, antistatic agents, etc. which are compatible with the blends and color pigments which are compatible with acetal polymers. However, the use of the mentioned additives is not considered to be necessary for the operability of present invention. As has been noted above, the inclusion of the aforementioned oxirane-containing ingredients has been found to substantially improve the thermal stability, U.V. light resistance and impact strength of the subjeci polymer blends and can thereby serve to enhance the recycling capability of such blends which may of necessity entail additional melt processing (that is more thermal exposure) and/or prolonged U.V. exposure. In addition to the foregoing, it has also been found that the inclusion of the indicated oxirane- containing ingredients can also serve to offset or minimize the adverse thermal and/or chemical destabilizing effect and/or reduced impact strength that can otherwise be imparted to the subject acetal- containing polymer blends by virtue of the inclusion therein of various additives such as mold release agents, plasitcizers, antistatic agents, colorants, and the like. The polymer blend compositions of the present - invention have good processabuity characteristics and are suitable for use in a wide variety of injection molding and extrusion applications. Such compositions can be formulated so as to be particularly useful in such applications wherein good thermal/dimensional stability, creep resistance and chemical resistance properties and/or paintability is required. Suitable exemplary end-use applications thus include automotive interior and exterior parts, tool casings, appliance housings and the like. The present invention is further understood and illustrated by reference to the following examples thereof. The various thermoplastic resins and oxirane- containing additives employed within such examples are identified and described in Tables A and B respectively. Table A -41- Table B Example 1 In this example, a 4 component POM/ABS/PC/TPU blend was prepared containing the indicated polymer components, respectively, in a 30/23/30/17 weight ratio and also containing 5 weight percent of ESO-1. In preparing the indicated 4 polymer component blends the individual thermoplastic polymer components, in pelletized form, were weighed out and combined in the desired proportions, tumble blended for 15 minutes, melt compounded with the epoxidized soybean oil ingredient using a BUSS Ko-Kneader operated at approximately 220- 240°C, 20 kg throughput and pelletized for subsequent drying and injection molding (at 180-240C) into appropriate testing specimens. For comparative purposes, the corresponding polymer blend was prepared without the epoxidized soybean oil component (that is, ESO-1) being included. Both of the resulting polymer blend compositions were injected molded under a variety of time and temperature conditions as set forth in Table I below. The degree of discoloration exhibited by the resulting molded samples was determined using a DATACOLOUR DC 3890 photospectrometer using a sample of the same resin composition molded at a temperature of 180°C and a 300 second residence time as the color standard. The delta E's reported in Table I below represents the difference in color (that is, degree of discoloration) as between the 180°C/300 second molded "standard" and the sample molded under the more severe molding conditions described in Table I. Table I As can be seen from the Table I results, the presence of the epoxidized soybean oil provided a substantial reduction in the degree of discoloration which was otherwise exhibited under a given set of molding conditions. Examples 2-6 In these examples, a series of 4 component 30/23/30/17 weight ratio POM/ABS/PC/TPU blends were prepared containing frbm 1 to 5 weight percent of the different oxirane-containing additives identified in Table II below. For comparative purposes, the same polymer blend was also prepared without including any oxirane-containing additive. Each of the resulting blends were then injection molded under two different sets of molding conditions, namely at 210°C for a medium residence time (that is, 210 seconds) and at 240°C for a long residence time (that is, 475 seconds). The color difference or discoloration encountered under the more severe molding conditions (that is, relative to that of the samples molded at 210°C and a medium residence time) is set forth in Table II below. Table II As can be seen from the Table II results, the various oxirane-containing additives tested provided substantial thermal stability benefits within the subject polymer blend compositions. Examples 7-10 In this series of examples, the procedure of Examples 2-6 was repeated except that a 3 component 33/52/15 weight ratio POM/ABS/TPU polymer blend was employed in place of the 4 component blend of Examples 2-6. The thermal stability evaluation results for these blends are set forth in Table III below along with that of a control composition containing no oxirane- containing stabilizer ingredient. Table III Examples 11-14 In these examples, several different 2 component ABS/P0M and 3 component ABS/P0M/TPU polymer blend compositions were prepared containing 3 weight percent of ES0-2 and were subjected to accelerated U.V. stability testing in a Heraeus S-150 xenon-arc apparatus with intermittent exposure for 24 hours at 30-40 percent relative humidity and 95°C. Comparative compositions were prepared and tested in each instance which did not contain the requisite oxirane containing additive. Delta E discoloration values for the various samples (that is, on the basis of before and after accelerated U.V. exposure photospectrometer readings) are set forth in Table IV below. Also set forth in Table IV are the U.V. stability results for a comparative 2 component P0M/TPU blend composition both with (C-16) and without (C-15) 3 weight percent of ES0-3 having been added thereto. As can be seen from the Table IV results, the 2 and 3 component ABS/P0M and ABS/P0M/TPU compositions containing the ES0-2 additive exhibit less discoloration upon accelerated U.V. exposure than do the corresponding ESO-2-free comparative compositions. As can also be seen, the U.V. stability of the 2 component POM/TPU comparative composition is actually worsened when the epoxidized soybean oil ingredient ESO- 3 is incorporated therein. Table IV *Comparative Experiments Example 15 In this experiment the 2 component ABS/POM compositions of Example 11 and comparative experiment C- 11 above were molded into test specimens and were subjected to room temperature (that is, 23°C) Notched Izod Impact Strength Testing pursuant to ISO 180. The C-11 composition had a room temperature Notched Izod impact strength of 15 kJ/π_2 whereas the ESO-2-containing Example 11 composition had a corresponding value of 20 kJ/π_2. Examples 16-26 In these examples a series of 3 component 33/52/15 weight ratio POM/ABS/TPU compositions were prepared containing different amounts of the various oxirane-containing ingredients identified in Table V below. The resulting compositions were subjected to room temperature Notched Izod and Notched Charpy Impact Strength Testing (pursuant to testing methods ISO 180 and ISO 179, respectively) along with a comparative control composition in which no oxirane-containing ingredient was included. As can be seen from the results in Table V, the compositions of the present invention (that is, containing the oxirane ingredient) have notably enhanced Charpy and/or Izod impact strength values relative to those of the comparative control composition. Table V 1. N.B . -Sample did not break under the test condi tions employed . Examples 27-40 In these examples, a series of 4 component 30/23/30/15 weight ratio POM/ABS/PC/TPU polymer blend compositions containing different amounts of various oxirane-containing additives were prepared and subjected to room temperature Izod " and Charpy impact strength testing pursuant to Examples 16-26 above. Also tested was a control composition having the types and proportions of the 4 polymer ingredients but not having any oxirane-containing additive incorporated therein. The results for this series of examples are set forth in Table VI below and illustrate that substantially improved Charpy impact strength can be obtained within the 4 component composition in question with several of the types and levels of the oxirane- containing additives that were tested. Table VI * 1. N.B. - Sample did not break under the test conditions employed. While the present invention has been described and illustrated with reference to particular embodiments and examples thereof, such is not be interpreted as in any way limiting the scope of the instantly claimed invention.

Claims: -51 - CLAIMS 1. A polymer blend composition which contains: (A) a monovinylidene aromatip copolymer which is either ( 1) a non-rubber-modified monovinylidene aromatic "copolymer containing, in polymerized form and on an aromatic copolymer weight basis, from 55 to 99 weight percent of one or more monovinylidene aromatic monomers and from 1 to 45 weight percent of one or more relatively polar comonomers; or (2) a rubber-modified monovinylidene aromatic copolymer containing, on a rubber-modified copolymer weight basis, from 30 to 99 weight percent of a^monovinylidene aromatic copolymer as described in item (A) (1) above and from 1 to 70 weight percent of dispersed particles of a rubbery polymer having a glass transition temperature of 0°C " or lower; and (B) one or more acetal homopolymer or copolymers; said composition being characterized in that it contains on a total polymer blend composition weight basis, from 0.01 to 15 weight percent of an oxirane-containing stabilizer ingredient. 2. The polymer blend composition of Claim 1 further characterized in that the monovinylidene aromatic copolymer is a rubber-modified monovinylidene aromatic copolymer containing, on a rubber-modified copolymer weight basis, from 2 to 35 weight percent of dispersed particles of a rubbery polymer which is a homopolymer of a 1,3-conjugated alkadiene monomer or is a copolymer of from 60 to 99 weight percent of a 1,3- conjugated alkadiene monomer with from 1 to 40 weight percent of a monoethylenically unsaturated monomer. 3. The polymer blend composition of Claim 2 which is further characterized in that the rubber modified monovinylidene aromatic copolymer ingredient is one which is prepared by mass or mass/suspension graft polymerization techniques. 4. The polymer blend composition of Claim 3 which is further characterized in that said composition also contains one or more elastomeric thermoplastic polyurethane or copolyester elastomer ingredients. 5. The polymer blend composition of Claim 4 which is further characterized in that said composition also contains one or more non-elastomeric thermoplastic polycarbonate or polyester resin ingredients. 6. The polymer blend composition of Claim 1 which is further characterized in that the relatively polar comonomer of the monovinylidene aromatic copolymer is an ethylenically unsaturated nitrile, an ethylenically unsaturated anhydride, an ethylenically unsaturated amide, an ester of an ethylenically unsaturated carboxylic acid or an.ethylenically unsaturated dicarboxylic acid imide. 7. The polymer blend composition of Claim 6 further characterized in that the monovinylidene aromatic copolymer is rubber-modified and in that the rubbery polymer of said rubber-modified monovinylidene aromatic copolymer is a homopolymer of a 1,3-conjugated alkadiene monomer; a copolymer of from 60 to 99 weight percent of a 1,3-conjugated alkadiene monomer with from 1 to 40 weight percent of a mono-ethylenically unsaturated monomer; an ethylene/propylene copolymer rubber; or a rubbery ethylene/propylene/non-conjugated diene copolymer. 8. The polymer blend composition of Claim 4 characterized in that the elastomeric thermoplastic polyurethane or copolyester elastomer ingredient contains one or more ester-containing elastomeric materials employed either ^ alone or in combination with each other or in combination with up to 70 weight percent, on a total elastomeric material weight basis, of an ether-based thermoplastic polyurethane ingredient 9. The polymer composition of Claim 2 characterized in that said composition contains, on a total composition weight basis, from 5 to 90 weight percent of the monovinylidene aromatic copolymer and 5 to 90 weight percent of the acetal homopolymer or copolymer ingredient. 10. The polymer blend composition of Claim 4 characterized in that said composition contains from 1 to 80 weight percent of the elastomeric copolyester or thermoplastic polyurethane ingredient on a total composition weight basis. 11. The polymer blend composition of Claim 5 characterized in that said composition contains from 5 to 90 weight percent of the non-elastomeric thermoplastic polycarbonate or polyester resin ingredient on a total composition weight basis. 12. The polymer blend composition of Claim 1 characterized in that said composition contains from 0.1 to 10 weight percent of the oxirane-containing stabilizer ingredient on a total composition weight basis. 13. The polymer blend composition of Claim 1 characterized in that said composition contains from 0.2 to 5 weight percent of the oxirane-containing stabilizer ingredient on a total composition weight basis. 14. The polymer blend composition of Claim 1 characterized in that the oxirane-containing stabilizer ingredient is an epoxidized unsaturated triglyceride or an epoxy resin derived from the reaction of epichlorohydrin with an aromatic or aliphatic polyol. 15. A method for preparing a polymer blend composition, said method being characterized in that it involves the steps of dry blending the following polymer ingredients together in particulate form and melt compounding the resulting mixture with from 0.01 to 15 weight percent on a total composition weight basis of an oxirane-containing stabilizer ingredient at a temperature of 240°C or less, said polymer ingredients being composed of: -55- (A) a monovinylidene aromatic copolymer which is either (1) a non-rubber-modified monovinylidene aromatic copolymer containing, in polymerized form and on an aromatic copolymer weight basis, from 55 to 99 weight percent of one or more monovinylidene aromatic monomers and from 1 to 45 weight percent of one or more relatively polar comonomers; or (2) a rubber-modified monovinylidene aromatic copolymer comprising, on a rubber-modified copolymer weight basis from 30 to 99 weight percent of a monovinylidene aromatic copolymer as described in item (A)'(1) above and from 1 to 70 weight percent of dispersed particles of a rubbery polymer having a glass transition temperature of 0°C or lower; and (B) one or more acetal homopolymer or copolymers.

Inventors: GKOGKIDIS ANTONIOS, GKOGKIDIS, ANTONIOS
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: C08K5
IPC codes: C08L25, C08L51, C08L55, C08L59, C08L67, C08L69, C08L73, C08L75