Solvent resistant polycarbonate blends
FIELD OF THE INVENTION 
     This invention relates generally to methods for increasing the solvent and flammability resistance of amorphous polycarbonates and to polymer blends which comprise a mixture of an amorphous polycarbonate resin with a polycarbonate oligomer. 
     BACKGROUND OF THE INVENTION 
     Polycarbonates are well-known resins which have good property profiles, particularly with respect to impact resistance, electrical properties, optical clarity, dimensional stability and the like. Polycarbonates are used in applications requiring extreme toughness, transparency, resistance to burning, and maintenance of useful engineering properties over a wide temperature range. Typical applications include: bubble helmets for astronauts, canopies for supersonic aircraft, furnishings for commercial aircraft, break-resistant windows, transparent bullet-resistant laminates, impact-resistant lenses, combination electrical insulation and mechanical housings for appliances, automotive instrument panels, and the like. 
     One polycarbonate resin that has been used successfully in such environments is based on bisphenol A (&#34;BPA&#34;) The relevant art prior to BPA-polycarbonates and the subsequent development of this material as an engineering resin is reviewed in H. Schnell, Chemistry and Physics of Polycarbonates, Wiley-Interscience, New York (1964) and K. Johnson, Polycarbonates Recent Developments, Noyes Beta Corp., Parkridge, N.J. (1970). 
     While BPA-polycarbonate provides significant benefits, this resin and related amorphous polycarbonates have certain disadvantages. Most notably, the resistance of amorphous polycarbonates to organic solvents is rather limited. The tendency of organic solvents to crystallize, craze, crack or mar the surface of objects made from such polycarbonates naturally limits their application. For example, tensile strains may readily be &#34;frozen in&#34; to polycarbonate parts during fabrication by injection molding and, when compounded with stresses encountered in service, result later in crazing on exposure to unfavorable environments. The presence of solvents or solvent vapors in such environments may alter threshold conditions for crazing to the point where parts under mechanical stress will fail. Therefore, environmental conditions as well as stresses to be encountered in fabrication and in service must be considered carefully in designing for polycarbonates. 
     There have been a number of attempts to improve the solvent resistance of BPA-polycarbonate resins. One effort involved the use of a cross-linking agent, as disclosed, for example, in U.S. Pat. Nos. 4,604,434, 4,636,559, 4,701,538, and 4,767,840. This technique is often effective, but difficulties are sometimes encountered by reason of swelling of the cross-linking agents polycarbonate in the presence of organic liquids, and loss of ductility with increasing levels of cross-linking agents. 
     Another limitation of certain BPA-polycarbonates is flame resistance. Although polycarbonate resins will normally merit the rating of &#34;self-extinguishing&#34; according to ASTM Method D 635, they will often receive poorer ratings under UL-94 testing procedures. 
     Thus, there continues to be a need for BPA-polycarbonates that have increased solvent and flame resistance in combination with conventional processability. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the foregoing and other limitations by providing blended polymer compositions which comprise: (A) at least one amorphous polycarbonate; and (B) at least one polycarbonate oligomer containing hydroquinone carbonate structural units. The present invention is based on the discovery that the solvent and flammability resistance of an amorphous polycarbonate are increased by blending therewith an effective amount of a linear polycarbonate oligomer containing hydroquinone structural units. 
     Accordingly, the present invention provides a method for increasing the solvent and flame resistance of amorphous polycarbonates. Also provided is a novel class of solvent and flame resistant polycarbonate polymer blends. The polymer blends have particular application in the preparation of shaped articles for use in environments characterized by the presence of solvents, solvent vapors, and/or potential for fire conditions. 
    
    
     DETAILED DESCRIPTION 
     In one of its aspects, the present invention is directed to blended polymer compositions which comprise: 
     (A) up to about 80 percent by weight of at least one amorphous polycarbonate; 
     (B) at least about 20 percent by weight of at least one linear polycarbonate oligomer comprising carbonate structural units of the following formulae (I) and (II): ##STR1##  wherein in formula (II) A 1  is a divalent group represented by the following formulae (III) or (IV): ##STR2## wherein in formula (III) each of R 1  and R 2  independently is a linear or branched alkyl group, and in formulae (III) and (IV) each of R 3 , R 4 , R 5  and R 6  independently is a linear or branched alkyl group, or is a halogen atom, m and n independently are 0-4 and o and p independently are 0-3; and wherein at least about 30 mole percent of the carbonate structural units in said oligomers have formula (I). 
     It is not certain whether any or all of the components (A), and (B) used to prepare the polymeric compositions of the present invention interact chemically upon blending. Therefore, the present invention contemplates compositions comprising said components and any reaction products thereof, as well as optional components as described hereinafter. 
     1. Amorphous Polycarbonate Resins 
     The term &#34;amorphous&#34; when used in conjunction with the polycarbonate resins of component (A) is intended to describe polycarbonates that are substantially devoid of crystallinity or stratification at processing temperatures up to about 180° C. Such materials may possess some order, but it is insufficiently developed to result in loss of transparency or to be observed in routine x-ray crystallography spectra. Such polycarbonates, however, may become crystallized to some degree by prolonged heating at elevated temperatures or by exposure to solvents such as acetone, as by immersion. 
     The amorphous polycarbonates (A) are available commercially and can also be prepared by several methods known in the art including those set forth in W. Christopher, et al., Polycarbonates, Reinhold Publishing Corporation, New York (1962). For example, they may be produced by the Schotten-Baumann reaction of phosgene with a diol in the presence of an appropriate hydrogen chloride acceptor or by a melt transesterification reaction between the diol and a carbonate ester. In the former, phosgene reacts directly with the diol to produce a polymer in a solution. The reaction may be run by employing an acid acceptor such as pyridine or triethylamine, either in solution in a solvent such as methylene chloride or interfacially in a mixture of such a solvent and water. The polymer may be recovered as a solution after aqueous washes to remove the acid acceptor which has been neutralized or is present in excess. In transesterification, a diphenyl carbonate reacts with the selected diol to give a phenol and a molten solvent-free polymer. 
     A number of diols including aliphatic, cycloaliphatic and aromatic dihydroxy compounds may be used in the above processes. Suitable aromatic dihydroxy compounds include, for example, the bis(monohydroxyaryl)alkanes and the bis(monohydroxyaryl) sulphones such as: 
     2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane 
     2,2-bis-(4-hydroxyphenyl)propane (bisphenol A) 
     2,2-bis-(4-hydroxyphenyl)pentane 
     2,4&#39;-dihydroxydiphenylmethane 
     bis-(2-hydroxyphenyl)methane 
     bis-(4-hydroxyphenyl)methane 
     bis-(4-hydroxy-5-nitrophenyl)methane 
     1,1-bis(4-hydroxyphenyl)ethane 
     3,3-bis(4-hydroxyphenyl)pentane 
     2,2-dihydroxybiphenyl 
     2,6-dihydroxynaphthalene 
     bis-(4-hydroxydiphenyl) sulfone 
     bis-(3,5-diethyl-4-hydroxyphenyl) sulfone 
     2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)propane 
     2,4&#39;-dihydroxydiphenyl sulfone 
     5&#39;-chloro-2,4&#39;-dihydroxydiphenyl sulfone 
     bis-(4-hydroxybiphenylyl)sulfone 
     4,4&#39;-dihydroxydiphenyl ether 
     4,4&#39;dihydroxy-3,3&#39;-dichlorodiphenyl ether 
     4,4&#39;dihydroxy-2,5-dihydroxydiphenyl ether 
     2,2-bis-(3,5-dibromo-4-hydroxyphenyl)propane. 
     Suitable aliphatic diols include, for example, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,3-propanediol, 1,5-pentanediol, 2-ethyl-1,3-propanediol, 1,6-hexanediol, 1,8-octanediol, 1-ethyl-1,3-hexanediol, 1,10-decanediol, and the like. 
     Suitable cycloaliphatic dihydroxy compounds include, for example, 1,4-cyclohexanediol, 1,2-cyclohexanediol, 2,2-(4,4&#39;-dihydroxy-dicyclohexylene)-propane, 2,6-dihydroxydecahydronaphthalene, the corresponding bis-alkoxylated aromatic dihydroxy compounds thereof, and the like. 
     For the purposes of the invention, it is also possible to utilize polycarbonates (A) prepared by copolymerization of two or more of the above diols or terpolymerization of the above diols. Amorphous copolyester carbonates such as those disclosed in U.S. Pat. Nos. 4,238,597 and 4,506,065, as well as amorphous branched polycarbonates such as those disclosed in U.S. Pat. No. 3,544,514 may also be used as component (A) of this invention. 
     In one embodiment the amorphous polycarbonate (A) consists essentially of carbonate structural units of the following formula (V): ##STR3## wherein each of R 7  and R 8  independently is a linear or branched alkyl group, each R 9  and R 10  independently is a linear or branched alkyl group, or is a halogen atom, and x and y independently are 0-4. 
     For example, such structural units may be derived from monomers including bisphenol A, 3,3&#39;,5,5&#39;-tetrabromobisphenol A, 3,3&#39;,5,5&#39;-tetrachlorobisphenol A and 3,3&#39;,5,5&#39;-tetramethylbisphenol A. 
     The weight average molecular weight of the amorphous polycarbonates (A) is generally within the range of about 20,000 to about 80,000. Usually, the polycarbonates (A) will have a weight average molecular weight within the range of about 40,000 to about 60,000 (as determined by gel permeation chromatography relative to polystyrene). In addition, it is preferred in one embodiment that the polycarbonates (A) are homopolymers. For example, homopolymeric BPA-polycarbonates such as GE&#39;s Lexan® polycarbonates having a number average molecular weight (relative to polystyrene) of about 20,000 to about 30,000 are particularly preferred. 
     2. Linear Polycarbonate Oligomers 
     The blended polymer compositions of the present invention also contain at least about 20 percent by weight of at least one linear polycarbonate oligomer (B) comprising structural units of the formulae (I) and (II). The oligomers (B) have an overall linear structure, apart from the carbocyclic rings present in formulae (I)-(IV). They include oligomers having degrees of polymerization (i.e., average number of monomer units per oligomer molecule) from 3 to about 30 and preferably to about 20, with a major proportion being up to about 8 and a still larger proportion up to about 10. Mixtures of oligomers having varying degrees of polymerization are preferred. 
     An essential feature of the oligomers (B) is the presence of hydroquinone (&#34;HQ&#34;) carbonate structural units (i.e., units of formula (I)) in amounts of at least about 30 mole percent. Hydroquinone carbonate levels of at least about 60 mole percent are preferred. Most often, the oligomers (B) contain hydroquinone units in amounts of about 65 mole percent. ##STR4## 
     Also present in the oligomers (B) of this invention are carbonate units of formula (II). In that formula, A 1  may be a divalent bisphenol group of formula (III), in which each of R 1  and R 2  is an alkyl group as defined. Most often, both R 1  and R 2  are methyl. ##STR5## 
     The R 3  and R 4  groups may be alkyl or halo as defined, and may be present in quantities up to 4 per aromatic ring. The values of m and n are usually each 0 or 2, and each R 3  or R 4  group (when present) is usually methyl or bromo. Thus, the divalent groups of formula (III) are derived from bisphenols known in the art, especially bisphenol A or 2,2-bis(4-hydroxyphenyl)propane. 
     The divalent A 1  groups may also have formula (IV); i.e., they may be derived from spirobiindane bisphenols which may contain R 5  and R 6  substituents as previously defined, up to 3 such substituents being present per aromatic ring. The preferred spirobiindane bisphenol is the unsubstituted 6,6&#39;-dihydroxy-3,3,3&#39;3&#39;-tetramethylspiro(bis)indane. ##STR6## 
     The oligomers (B) may be prepared by a variety of well-known processes including: 
     interfacial polymerization as set forth in F. Millich and C. Carraher, Jr., eds., Interfacial Synthesis, Vol. II, Marcel Dekker, Inc., New York (1977), Chapt. 13; and 
     melt transesterification as described in H. Schnell, Chemistry and Physics of Polycarbonates, Wiley-Interscience, New York (1964), (Section III, 3, A, 4). 
     In a preferred embodiment, the oligomers (B) are obtained by separation from a mixture of crude cyclic carbonates which contain both cyclic and linear oligomers (hereinafter &#34;crude cyclics&#34;). The crude cyclics may be prepared by contacting: (i) a mixture of hydroquinone bischloroformates and bischloroformates of a dihydroxyaromatic compound of the formula HO--A 1  --OH (wherein A 1  is as defined above) with, (ii) at least one oleophilic aliphatic or heterocyclic tertiary amine, and (iii) an aqueous base comprising an alkali or alkaline earth metal hydroxide or carbonate solution, in (iv) a substantially non-polar organic liquid which forms a two-phase system with water. The bischloroformates are preferably maintained in low concentration wherein the molar ratio of said amine to said bischloroformates is about 0.06-2.0:1 and the molar ratio of said base to said bischloroformates is at least about 1.4:1. The linear oligomers (B) may be obtained by purification of the crude cyclics such as rapid stirring in acetone followed by vacuum filtration. For a more detailed description of the preparation and isolation of cyclic polycarbonate oligomers, reference is made to U.S. Pat. No. 4,644,053 (Brunelle et al.) which is incorporated by reference herein. 
     (i) Bischloroformates 
     The bischloroformate mixture employed in the preparation of the crude cyclics may be a mixture of substantially pure monomeric hydroquinone and bisphenol bischloroformates, which may be prepared, for example, by the reaction of the corresponding dihydroxyaromatic compound with phosgene in the presence of a dialkylaniline, as disclosed in British Patent 613,280, the disclosure of which is incorporated by reference herein. 
     For larger scale reactions, it is usually preferred for the sake of economy to employ crude bischloroformate mixtures which may contain oligomeric carbonate bischloroformates, a majority of said oligomeric materials having degrees of polymerization up to about 5. Numerous methods for preparing such crude bischloroformates are known; suitable methods are disclosed, for example, in the following U.S. patents: 
     
         ______________________________________                                    
       3,255,230     3,974,126                                            
       3,312,661     4,638,077                                            
       3,966,785                                                          
______________________________________                                    
 
    
     The disclosures of these patents are also incorporated by reference herein. 
     A preferred method for preparing bischloroformate compositions useful in the preparation of crude cyclics is disclosed in copending, commonly owned application Ser. No. 07/299,572. It comprises passing phosgene into a mixture of water, a substantially inert, water-immiscible organic liquid, an alkali or alkaline earth metal hydroxide, and hydroquinone or a mixture of hydroquinone and bisphenol A containing at least about 40 mole percent hydroquinone; the ratio of moles of water to gram-atoms of alkali or alkaline earth metal hydroxide in said mixture being in the range of about 5.0-5.5:1. 
     (ii) Tertiary amines 
     The tertiary amines employed in the preparation of the crude cyclics (&#34;tertiary&#34; in this context denoting the absence of N--H bonds) generally comprise those which are oleophilic (i.e., which are soluble in and highly active in organic media, especially those used in the oligomer preparation method), and more particularly those which are useful for the formation of polycarbonates. Reference is made, for example, to the tertiary amines disclosed in U.S. Pat. Nos. 4,217,438 and 4,368,315, the disclosures of which are also incorporated by reference herein. They include aliphatic amines such as triethylamine, tri-n-propylamine, diethyl-n-propylamine and tri-n-butylamine and highly nucleophilic heterocyclic amines such as 4-dimethylaminopyridine (which, for the purposes of this invention, contains only one active amine group). The preferred amines are those which dissolve preferentially in the organic phase of the reaction system; that is, for which the organic-aqueous partition coefficient is greater than 1. This is true because intimate contact between the amine and the bischloroformates is essential for the formation of the crude cyclic mixture. For the most part, such amines contain at least about 6 and preferably about 6-14 carbon atoms. 
     The most useful amines are trialkylamines containing no branching on the carbon atoms in the 1- and 2-positions. Especially preferred are tri-n-alkylamines in which the alkyl groups contain up to about 4 carbon atoms. Triethylamine is often preferred by reason of its particular availability, low cost, and effectiveness in the preparation of products containing low percentages of high polymers. 
     (iii) Aqueous base 
     Also employed in the preparation of the crude cyclics mixture containing oligomers (B) is an aqueous alkali or alkaline earth metal hydroxide or carbonate solution (hereinafter sometimes &#34;aqueous base&#34;), such as lithium, sodium, potassium or calcium hydroxide or sodium or potassium carbonate. It is most often lithium, sodium or potassium hydroxide, with sodium hydroxide being preferred because of its availability and relatively low cost. The concentration of the solution is not critical and may be about 0.2-16 M. 
     (iv) Non-polar solvent 
     The fourth essential component in the preparation of crude cyclics is a substantially non-polar organic liquid which forms a two-phase system with water. The identity of the liquid is not critical, provided it possesses the stated properties. Illustrative liquids are aromatic hydrocarbons such as toluene and xylene; substituted aromatic hydrocarbons such as chlorobenzene, o-dichlorobenzene and nitrobenzene; chlorinated aliphatic hydrocarbons such as chloroform and methylene chloride; and mixtures of the foregoing with ethers such as tetrahydrofuran. Methylene chloride is generally preferred. 
     To prepare the crude cyclic oligomer mixture according to the above-described method, the reagents are brought into contact under conditions whereby bischloroformates are maintained at low concentration, generally up to about 0.5 M. Actual high dilution conditions, requiring a large proportion of organic liquid, may be employed but are usually not preferred for cost and convenience reasons. Instead, simulated high dilution conditions known to those skilled in the art may be employed. For example, in one embodiment of the method the bischloroformates, and optionally other reagents, are added gradually to a reaction vessel containing the organic liquid. 
     Although addition of bischloroformates neat (i.e., without solvents) is within the scope of this embodiment, it is frequently inconvenient because many bischloroformates are solids. Therefore, they are preferably added as a solution in a proportion of the organic liquid. The proportion of organic liquid used for this purpose is not critical; about 25-75% (by weight) of the total, and especially about 40-60%, is preferred. 
     The reaction temperature is generally in the range of about 0°-50° C. It is most often about 0°-40° C. and preferably 20°-40° C. 
     The molar proportions of the reagents constitute another factor affecting yield and purity. For example, when the molar ratio of amine to bischloroformates is about 0.06-2.0:1 to about 0.1-0.25:1 and that of base to bischloroformates is from about 1.4:1 to about 2.5-3.1:1, a higher proportion of cyclic oligomers is obtained. In general, lower proportions of base (typically a molar ratio of about 1.4-2.0:1) are employed with a crude bischloroformate composition than with substantially pure monomer bischloroformates (about 2.75-3.1:1). 
     A factor of some importance is the concentration of available amine, which should be maintained at a level as constant as possible during the entire bischloroformate addition period. If all amine is present in the reaction vessel into which bischloroformates are introduced, its concentration steadily decreases, principally by dilution. On the other hand, if amine is introduced continuously or in equal increments during bischloroformate introduction, its available concentration is initially low and increases more or less steadily during the addition period. These fluctuations can result in a high and constantly varying proportion of &#34;high polymer&#34; in the product. Such high polymers are oligomers (linear or cyclic) having degrees of polymerization greater than about 30, and are generally not preferred. 
     It has been found advantageous to introduce the amine in one initial large portion, usually about 40-95% and preferably about 40-75% by weight of the total amount, followed by incremental or continuous addition of the balance thereof. By this procedure, the concentration of available amine is maintained at a fairly constant level in the organic phase during the entire addition period, and it is possible to minimize the proportion of high polymer in the product. 
     Under these conditions, it is usually advantageous for the reaction vessel to initially contain about 1-40% and preferably about 1-20% of total aqueous base. The balance thereof is also introduced continuously or incrementally. 
     In one embodiment, the pH of the reaction mixture is typically in the range of about 9-14 and preferably about 10-11. When bischloroformates (and optionally amine) are added to all of the aqueous base, on the other hand, the initial pH remains on the order of 14 during essentially the entire reaction period. 
     The linear oligomers (B) are recovered by conventional operations such as combining the crude cyclic mixture, as a solid or in solution, with a diluent which is a non-solvent for said linear oligomers (B). Illustrative non-solvents include ketones such as acetone and methyl isobutyl ketone and esters such as methyl acetate and ethyl acetate. Acetone is a particularly preferred non-solvent. Recovery of the oligomers (B) is normally effected by simply separating the same from the diluent by known methods such as vacuum filtration. 
     3. Additives and/or Fillers 
     The polymeric compositions of the present invention may contain one or more additives and/or fillers of the types used in the polymer art. Such materials may be included in the polymer blends of the present invention to modify or to obtain desirable properties. Fillers such as glass powder, quartz products, graphite, metal powders, powders of high melting synthetic resins, natural fibers, glass fibers, metal fibers, and the like may generally be included in the polymer compositions in amounts up to about 25 percent by weight. 
     Typical additives may be employed for color, thermal, hydrolytic and ultraviolet stabilization of the polycarbonate compositions of the present invention in amounts up to about 2 percent by weight. Other additives such as mold release agents, pigments, flame retardants and the like may also be present. 
     4. Blending Techniques 
     The blended polymer compositions of the present invention including the amorphous polycarbonate (A), the linear polycarbonate oligomer (B), and optional additives and/or fillers can be prepared by melt blending techniques well-known to those skilled in the art. A blending method which results in the formation of an intimate blend of the components is required. Suitable procedures include solution blending and melt blending. A particularly useful procedure is to intimately mix the polymers using conventional mixing equipment such as a mill, a Banbury mixer, a Brabender Torque Rheometer, a single or twin screw extruder, continuous mixers, kneaders, etc. For example, the components may be intimately mixed in the form of granules and/or powder in a high shear mixer. One preferred process involves: (1) mixing powdered components (A) and (B) at ambient temperature in a mixer; (2) drying the mixed powder at about 115° C.; and (3) melt blending the mixed powder in an extruder, to obtain the blended polymers of the present invention. &#34;Intimate&#34; mixing means that the mixture is prepared with sufficient mechanical shear and thermal energy to produce a dispersed phase which is finely divided and homogeneously dispersed in the continuous or principal phase. 
     Thus, in another of its aspects the present invention relates to methods for increasing the solvent and flammability resistance of amorphous polycarbonates which comprises blending at least one amorphous polycarbonate (A), with an effective amount of at least one linear polycarbonate oligomer (B). An effective amount of oligomer (B) is usually at least about 20 percent by weight, and may be up to about 60 percent by weight or more. The polymer blends of the present invention are characterized as having improved flame and solvent resistance. 
     The blended polymer compositions of the present invention can be processed into shaped articles by extrusion, coextrusion, thermo-forming, blow-molding, injection-molding, compression-molding, calendaring, laminating, stamping, pultrusion, etc. In particular, shaped articles can be prepared by injection molding the blended polymer compositions of the present invention. 
     EXAMPLES 
     Various features and aspects of the present invention are illustrated further in the examples that follow. While these examples are presented to show one skilled in the art how to operate within the scope of this invention, the examples are not to serve as a limitation on the scope of the invention since such scope is only defined in the claims. 
     Unless otherwise indicated in the following examples and elsewhere in the specification and claims, all parts and percentages are by weight, all temperatures are in degrees Centigrade, and if not specified, are ambient temperature, and all pressures are at or near atmospheric. 
     1. PREPARATION OF BISCHLOROFORMATES 
     EXAMPLE 1 
     A 30 liter reactor was charged with 12.5 liters of CH 2  Cl 2 , 752.4 grams of hydroquinone (6.84 moles) and 1176.5 grams of bisphenol A (5.16 moles). Phosgene was added at the fastest possible rate such that 2500 grams were delivered in close to one hour. The reaction contained no initial water and the reactor was maintained with the best possible stirring. During the phosgenation reaction, the stoichiometric amount of sodium hydroxide (24 moles; 1920 grams of 50% NaOH) was added subsurface. Following completion of the NaOH and phosgene addition, the reaction was pH controlled at pH=3-5, for 1/2 hour. Following this half hour, four liters of water were added, the pH was raised to 8 and the temperature was maintained below 20° C., until no phosgene was detected. The organic layer was then washed with 1 N HCl, and was used directly in the preparation of crude cyclics. Derivatization and hplc analysis indicated that the yield of hydroquinone bischloroformate was 44%, with about 20% dimeric bischloroformate present. 
     EXAMPLE 2 
     A 30 liter reactor was charged with 660 grams of hydroquinone (6 moles), 136 grams of bisphenol A (6 moles), and 8.2 liters of CH 2  Cl 2 . The reactor was stirred while cooling to 10°-15° C. for about 10 minutes, then phosgene was admitted at a rate of 36 g/min for 67 minutes (24.0 moles). Concomitant to the phosgenation, a methylene chloride slurry consisting of 933 grams of calcium hydroxide (12.6 moles) and 1.3 liters of CH 2  Cl 2  was added. Water was also added continuously at a rate of 18 ml/min (1.2 liters total). The reaction temperature was maintained between 15°and 18° C. Following completion of the phosgenation, the reaction was stirred for 30 minutes, while purging with nitrogen. At that time, about 2.4 liters of water was added, and 50% NaOH was added, while maintaining the pH at 8, until phosgene could no longer be detected in the reactor. Finally, the phases were separated, and the organic phase was washed with 0.1 M HCl. Derivatization and analysis of the reaction product indicated 68% of theoretical hydroquinone bischloroformate and 79% of theoretical bisphenol A bischloroformate. The mixture was used in the cyclization reaction without further purification. 
     2. PREPARATION OF CRUDE CYCLIC CARBONATES 
     EXAMPLE 3 
     The bischloroformate solution (nominal concentration=0.6 M) prepared in Example 1 (20 liters) was placed in a pressure vessel and weighed, so that addition over 30 minutes with constant flow was achieved. The reactor was charged with 20 liters of CH 2  Cl 2 , 240 m of Et 3  N, and one liter of water. The reactor was brought to reflux, while stirring, and at that point addition with base was started. After approximately 2% of the base was added, bischloroformate addition was started, and addition continued over 30 minutes. Concomitant to bischloroformate addition, an additional portion of Et 3  N (240 ml) was added (over 30 minutes), and 30% NaOH was added to maintain the pH at 9.5-11 (normally 1.2 moles of base/liter of bischloroformate solution is necessary). After the addition was complete, 5 liters of water were added, followed by 5 liters of 1.0 M HCl. The phases were separated, and the organic phase was washed sequentially with 1 N HCl, 0.1 N HCl, and water (three water washes). The product (crude HQ-BPA cyclics containing linear oligomeric material) was isolated by pumping the CH 2  Cl 2  solution into boiling water, leaving an off-white powder, which was removed by filtration and was dried. 
     3. ISOLATION OF LINEAR OLIGOMERIC POLYCARBONATE 
     EXAMPLE 4 
     The linear oligomeric hydroquinone-BPA carbonate was obtained as follows. A total of 2,072 g. of crude HQ-BPA cyclic carbonate from Example 3 was divided into six lots, each containing about 350 g. Each of these lots was stirred rapidly with 3.5 L of acetone in a Waring blender for about 5 minutes; the mixture was then vacuum filtered. The residues were combined and treated similarly with acetone two more times. After drying in a vacuum oven, the final residue (i.e., the linear oligomeric hydroquinone-BPA carbonate) weighed 731 g., corresponding to 35% of the crude cyclics. (The yield of purified cyclics was 1,296 g., corresponding to 63% of the crude cyclics.) GPC analysis (in 1,2,4-trichlorobenzene at 150° C.) of the oligomeric material revealed the M w  to be 5394 and the M n  to be 2006 (relative to polystyrene). In all cases, the molar ratio of hydroquinone to BPA was 65:35. 
     4. PREPARATION OF POLYMER BLENDS 
     EXAMPLES 5-8 
     In a Henschel mixer were placed 750 g. of BPA polycarbonate (&#34;BPA PC&#34;) resin (M n  =28,000 relative to polystyrene) and 750 g. of oligomeric hydroquinone-BPA carbonate obtained from Example 4 were blended for 30 seconds. Also prepared were blends comprised of 25 and 10 weight percent HQ-BPA oligomeric carbonate. These mixtures, along with a control batch of 100% BPA polycarbonate resin (M n  =28,000 relative to polystyrene), were then dried in an air circulating oven at 115° C. for several hours prior to extrusion, which was achieved on a Werner and Pfleiderer, WP-28 twin screw extruder having six heating zones. Table 1 presents the details for the extrusions of each of these materials. 
     
                                           TABLE 1                                 
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EXTRUSION OF BLENDS OF BPA POLYCARBONATE                                  
AND OLIGOMERIC HYDROQUINONE-BPA CARBONATE                                 
EX.                                                                       
   WT % BPA POLYCARBONATE                                                 
                    TORQ (%)                                              
                          CURR(amps)*                                     
                                  T-1                                     
                                     T-2                                  
                                        T-3                               
                                           T-4                            
                                              T-5                         
                                                 T-Die                    
__________________________________________________________________________
5  50               93    16.5    319                                     
                                     554                                  
                                        636                               
                                           575                            
                                              597                         
                                                 586                      
6  75               84    14.5    316                                     
                                     575                                  
                                        646                               
                                           576                            
                                              595                         
                                                 586                      
7  90               72    12.5    309                                     
                                     580                                  
                                        639                               
                                           575                            
                                              589                         
                                                 584                      
8  100              69    11.8    327                                     
                                     593                                  
                                        625                               
                                           576                            
                                              583                         
                                                 584                      
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 T-1, T2, T3, etc. give the zone temperatures in degrees F.               
 *current required to maintain screw speed                                
 
    
     5. EVALUATION OF BLENDS 
     EXAMPLES 9-12 
     Prior to molding, the extrudate materials from Examples 5-8 were dried in an air circulating oven at 250° F. overnight. The extrudates were then injection molded into test bars on an 85 ton Van Dorn molding machine using the following conditions: the mold temperature was 180° F., the extruder barrel set temperature was 570° F., the injection pressure was 400 psi, and the back pressure was about 100 psi. The test bars obtained were 0.125 inch thick Izod bars, flame bars, and tensile bars. 
     Evaluation of the materials for resistance to acetone was accomplished by clamping a 21/2&#34; long Izod test bar in a curved stainless steel stress jig (61/4&#34; radius, which imposed a 1 percent applied strain on the part). This assembly was then immersed in a jar of acetone for ten minutes. The test was carried out three times for each extrudate material. The materials were also evaluated for notched Izod impact strength, and melt viscosity. Table 2 summarizes the pertinent results. 
     
                       TABLE 2                                                     
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EVALUATION OF BLENDS OF BPA                                               
POLYCARBONATE AND HYDROQUINONE-BPA                                        
OLIGOMERIC CARBONATES                                                     
     WT %                       BEHAVIOR                                  
EX.  BPA PC   MI.sup.1                                                    
                     KI.sup.2                                             
                           NI.sup.3                                       
                                IN ACETONE                                
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 9   50        0.7   7045   4.7 all bars remained intact                  
10   75        7.2   3055  16.2 only 1 of 3 bars cracked                  
11   90       12.3   1970  16.0 instantaneously cracked                   
12   100      11.6   2660  16.5 instantaneously cracked                   
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 .sup.1 melt flow index (g/10 min) ASTM D 1238                            
 .sup.2 Kasha index                                                       
 .sup.3 notched Izod (ftlbs/inch of notch) ASTM D 256                     
 
    
     The melt viscosity as measured by both melt index (MI) and Kasha index (KI) increased as more of the oligomeric hydroquinone-BPA carbonate was employed. This was not anticipated since the oligomeric material itself has a melt viscosity that is too low to measure by the MI and KI procedures. 
     Further inspection of the data assembled in Table 2 points out that except in the case of the 50:50 blend, the traditionally excellent notched Izod impact resistance of BPA-polycarbonate was maintained; in all cases (except for the 50:50 blend) the breaks were the result of ductile failure. 
     EXAMPLES 13-16 
     The test bars molded in Examples 9-12 were evaluated for flammability resistance. Table 3 summarizes the pertinent results. 
     
                       TABLE 3                                                     
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FLAME RESISTANCE EVALUATION                                               
OF VARIOUS MATERIALS                                                      
      WT % BPA                                                            
EX.   POLYCARBONATE  OI.sup.1                                             
                             UL-94  % CHAR.sup.2                          
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13    100            25      Failed.sup.3                                 
                                    21.3                                  
14    50             34      V-0    18.2                                  
15    75             33      V-0    18.8                                  
16    90             32      V-1    19.6                                  
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 .sup.1 oxygen index  ASTM D 2863                                         
 .sup.2 wt % relative to test bar before char                             
 .sup.3 flamming drips on first ignition (1 bar) or second ignition (4    
 bars)                                                                    
 
    
     Although the above examples are limited to only a few of the variables applicable to the compositions and methods within the scope of the present invention, it should be understood that the scope of the present invention can be further appreciated by the description preceding these examples. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.