Patent Publication Number: US-4544677-A

Title: Foamable polycarbonate compositions, articles and methods

Description:
This invention relates to foamable thermoplastic compositions comprising in admixture (a) a polycarbonate resin, alone, or in combination with (b) a polyester resin, (c) a minor amount of a chemical foaming agent and (d) a minor amount of a foam nucleating agent. Also provided are articles made by foaming such compositions. 
     BACKGROUND OF THE INVENTION 
     High molecular weight aromatic polycarbonates and blends thereof with linear polyesters are known, and these can include finely divided particulate fillers having a low aspect ratio, such as silica, and polymeric particles, see, e.g., commonly assigned U.S. patent applications Ser. Nos. 553,172, 553,173, and 553,175, filed Nov. 18, 1983, Richard B. Allen. 
     Also, it is known to add a foaming agent to thermoplastics as disclosed in U.S. Pat. Nos. 3,781,233, 3,799,954, 4,097,421, 4,097,425, 4,280,005, and 4,351,911. 
     In U.S. Pat. Nos. 4,280,005 and 4,351,911, it is reported that when a foaming agent is added directly to a polyester, highly irregular foaming occurs which results in an inferior product in that the product contains voids and sink marks. The product is also warped. However, when the foaming agent is added to a polyester containing a filler such as fibrous glass, minerals or mixtures thereof in an amount of at least 5 percent, a foamed product is produced having a rigid cellular core within a solid integral skin. Fibrous glass is a filler with a high aspect ratio, i.e., a length to diameter ratio of greater than 1.0, substantially so. 
     When a foaming agent is added directly to a polycarbonate, the material will not form a foam when injected into a cavity, the polymer collecting instead as solid masses in the mold. However, it is known to use glass fibers to aid in the formation of gas bubbles which ultimately form cells, and foams. Glass fibers do not, however, produce an entirely uniform, void-free structure with polycarbonates and the glass fibers produce severe and adverse reduction in impact strength, especially at low temperatures and an unsmooth surface appearance, especially if recent techniques, such as counterpressure foam molding, are used to produce the parts. High impact strengths are important because applications of polycarbonate structural foams have begun to expand beyond the traditional business equipment and computer housings into uses such as automobile load floors, seats, truck battery boxes, and the like, and low temperature impact strength, especially with the latter are critical. 
     It has now been discovered that structural foams comprising aromatic polycarbonate resins and related resins can be nucleated without the presence of glass fiber to yield foams that are superior in impact strength. Nucleating agents that can be used include but are not limited to amorphous silica, precipitated silica, titanium dioxide, particulate impact modifiers, foamed silica, dispersible polytetrafluoroethylene particles, and the like. The use of surfactants is also preferred to help bubble formation and to improve nucleating agent dispersion. Most preferably, the nucleating agent should be sub-micron in size and it should be coated with a hydrophobic coating. Surfactants which can be used include but are not limited to silicones, polyglycol ethers, fluorocarbons, fatty acid derivatives, and the like. 
     The compositions of the instant invention can be molded, for example, in standard injection molding machines, to produce a variety of foam molded products having good surface appearance, uniform cell structure, and exceptional impact strength. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there are provided foamable thermoplastic compositions, comprising 
     (a) an aromatic polycarbonate, an aromatic polyester carbonate, an aromatic dihydric phenol sulfone carbonate, or a mixture of any of the foregoing, alone, or in combination with 
     (b) a polyester resin comprising units of an aliphatic diol, a cycloaliphatic diol, or a mixture of said diols and an aromatic diacid or a cycloaliphatic diacid, 
     (c) a minor effective amount of a foaming agent; and 
     (d) a minor effective amount of a particulate foam nucleating agent substantially insoluble in the composition and having an aspect ratio of not greater than about 1.0, the amounts of (c) and (d) being sufficient to provide a rigid, substantially void-free, substantially uniform cellular core within a solid integral skin. 
     In preferred features of the invention the compositions will be substantially free, that is, will contain 0 or no more than about 2% by weight of fillers with an aspect ratio of greater than about 1.0, e.g., talc platelets or glass fibers, etc. Other preferred features are those in which component (a) comprises from about 99.97 to about 50.0 parts by weight; component (b) comprises from about 0 to about 50 parts by weight; component (c) comprises from about 0.02 to about 2.0 parts by weight; and component (d) comprises from about 0.01 to about 40 parts by weight based upon the total weight of components (a), (b), (c) and (d) combined. 
     The thermoplastic compositions of the present invention may also include pigments, such as carbon black, impact modifiers, flow promoters and the like, as well as flame retardant agents. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be better understood by reference to the drawing in which 
     FIG. 1 is a photomicrograph at 30X magnification of a cross section of glass fiber filled polycarbonate foamed according to the prior art, showing non uniform cells and numerous voids; and 
     FIG. 2 is a photomicrograph at 30X magnification of a cross section of a non-glass filled nucleated polycarbonate foamed according to the present invention, showing substantially uniform cells and substantially no voids. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Polycarbonate homopolymers useful in this invention are especially aromatic polycarbonates. These can be made by those skilled in the art or obtained from various commercial sources. They may be prepared by reacting dihydric phenol with a carbonate precursor, such as phosgene, a haloformate or a carbonate ester. Typically, they will have recurring structural units of the formula: ##STR1## wherein A is a divalent aromatic radical of the dihydric phenol employed in the polymer producing reaction. Preferably, the aromatic carbonate polymers have an intrinsic viscosity ranging from 0.30 to 1.0 dl./g. (measured in methylene chloride at 25° C.) By dihydric phenols is meant mononuclear or polynuclear aromatic compounds containing two hydroxy radicals, each of which is attached to a carbon atom of an aromatic nucleus. Typical dihydric phenols include 2,2-bis-(4-hydroxy-phenyl)propane; 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)propane, 4,4&#39;-dihydroxydiphenyl ether, bis(2-hydroxyphenyl)methane, mixtures thereof and the like. The preferred aromatic carbonate polymer is a homopolymer derived from 2,2-bis(4-hydroxyphenyl)propane(bisphenol-A). 
     Poly(ester-carbonates) for use in the invention are known and can be obtained commercially. Generally, they are copolyesters comprising recurring carbonate groups: ##STR2## carboxylate groups: ##STR3## aromatic carbocyclic groups in the linear polymer chain, in which at least some of the carboxylate groups and at least some of the carbonate groups are bonded directly to ring carbon atoms of the aromatic carbocyclic groups. These poly(ester-carbonate) copolymers, in general, are prepared by reacting a difunctional carboxylic acid, such as phthalic acid, isophthalic acid, terephthalic acid, homophthalic acid, o-, m-, and p-phenylenediacetic acid, the polynuclear aromatic acids, such as diphenic acid, 1,4-naphthalic acid, mixtures of any of the foregoing, and the like, with a dihydric phenol and a carbonate precursor, of the types described above. A particularly useful polyester carbonate is derived from bisphenol-A, isophthalic acid, terephthalic acid, or a mixture of isophthalic acid and terephthalic acid, or the reactive derivatives of these acids such as terephthaloyl dichloride, isophthaloyl dichloride, or a mixture thereof, and phosgene. The molar proportions of dihydroxy diaryl units to benzenedicarboxylate units to carbonate units can range from 1:0.2-1.00:0.80-0.00 and the molar range of terephthalate units to isophthalate units can range from 99:1 to 1:99 in this preferred family of resins. When the molar proportion of carbonate units is  0, the resin is a wholly aromatic polyester. See Robeson, U.S. Pat. No. 4,324,869. 
     The aromatic dihydric phenol sulfone resins used in this invention are a family of resins which can be made by those skilled in this art. For example, homopolymers of dihydric phenol, and a dihydroxydiphenyl sulfone and a carbonate precursor can be prepared, as well as copolymers of a dihydric phenol and a carbonate precursor can be made according to the description in Schnell, et al., U.S. Pat. No. 3,271,367. A preferred material is made by polymerizing bis-(3,5-dimethyl-4-hydroxy phenyl) sulfone, alone, or especially in combination with bisphenol-A, with phosgene or a phosgene precursor, in accordance with the description in Fox, U.S. Pat. No. 3,737,409. Especially preferred is a coplymer made by reacting 1-99, preferably 40-99 wt. percent of the sulfone, 99 to 1, preferably 60 to 1 wt. percent of the bisphenol with phosgene. 
     Polyesters suitable for use herein are derived from an aliphatic, aliphatic ether or cycloaliphatic diol, or mixtures thereof, preferably containing from about 2 to about 10 carbon atoms, and one or more aromatic or cycloaliphatic dicarboxylic acids. Preferred polyesters are derived from an aliphatic diol and an aromatic dicarboxylic acid having repeating units of the following general formula: ##STR4## wherein n is an integer of from 2 to 4. The most preferred polyester is poly(ethylene terephthalate). 
     Also contemplated herein are the above polyesters with additional amounts of polyols and/or acids in the amounts of from 0.5 to 50 wt. percent based on the total composition. The acids can be aliphatic or cycloaliphatic with the number of carbon atoms ranging from 2 to 20. Likewise, the glycols can be cyloaliphatic or aliphatic with the number of carbon atoms covering the same range. Polyalkylene ether glycols can also be used where the alkylene portion has from 2 to 10 carbon atoms and the entire glycol portion varies in molecular weight from 100 to 10,000. All such polyesters can be made following the teachings of, for example, U.S. Pat. Nos. 2,465,319 and 3,047,539. 
     The polyesters which are derived from a cycloaliphatic diol and an aromatic dicarboxylic acid are prepared, for example, by condensing either the cis- or trans-isomer (or mixtures thereof) of, for example, 1,4-cyclohexanedimethanol with an aromatic dicarboxylic acid so as to produce a polyester having recurring units of the following formula : ##STR5## wherein the cyclohexane ring is selected from the cis- and trans-isomers thereof and R represents an aryl or cycloaliphatic radical containing 6 to 20 carbon atoms and which is the decarboxylated residue derived from an aromatic dicarboxylic acid. 
     Examples of aromatic dicarboxylic acids represented by the decarboxylated residue R are isophthalic or terephthalic acid, 1,2-di(p-carboxyphenyl)ethane, 4,4&#39;-dicarboxydiphenyl ether, etc., and mixtures of these. Acids containing fused rings can also be present, such as in 1,4- or 1,5-napthalenedicarboxylic acids. Also contemplated are cycloaliphatic diacids, such as cyclohexane dicarboxylic acid. The preferred dicarboxylic acids are terephthalic acid or a mixture of terephthalic and isophthalic acids. 
     Another preferred polyester may be derived from the reaction of either the cis- or trans-isomer (or a mixture thereof) of 1,4-cyclohexanedimethanol with a mixture of isophthalic and terephthalic acids. Such a polyester would have repeating units of the formula: ##STR6## 
     Still another preferred polyester is a copolyester derived from a cyclohexane dimethanol, an alkylene glycol and an aromatic dicarboxylic acid. These copolyesters are prepared by condensing either the cis- or trans-isomer (or mixtures thereof) of, for example, 1,4-cyclohexane-dimethanol and an alkylene glycol with an aromatic dicarboxylic acid so as to produce a copolyester having units of the following formula: ##STR7## wherein the cyclohexane ring is selected from the cis- and trans-isomers thereof, R is as previously defined, n is an integer of 2 to 10, the x units comprise from about 1 to about 99 percent by weight, and the y units comprise from about 99 to about 1 percent by weight. 
     Such a preferred copolyester may be derived from the reaction of either the cis- or trans-isomer (or mixtures thereof) of 1,4-cyclohexanedimethanol and ethylene glycol with terephthalic acid in a molar ratio of 80:20:100. These copolyesters have repeating units of the following formula: ##STR8## wherein x and y are as previously defined. 
     The polyesters described herein are either commercially available or they can be produced by methods known in the art, including those set forth in U.S. Pat. No. 2,801,466. 
     The polyesters employed in the practice of this invention will usually have an intrinsic viscosity of from about 0.4 to about 2.0 dl./g., as measured in a 60:40 phenol:tetrachloroethane mixture, or similar solvent at 23°-30° C. 
     The relative amounts of the polymers can and usually do vary widely in the blend, with particular amounts depending on specific requirements and the nature of the polymers being employed. Proportions can range, for example, between 99:1 and 1:99, based on 100 parts by weight of the two polymers together. Best amounts in a given instance will be readily determinable by those skilled in the art. 
     For those embodiments of the invention having flame retardant properties, conventional additives can be used in conventional amounts. For example, sulfonate salts in small amounts can be used, such as a sulfonate salt, e.g., potassium benzenesulfonic acid at least 0.1, and preferably 0.4 parts per hundred by weight of polycarbonate, or aromatic halogen compounds, such as tetrabromobisphenol A homopolymers or copolymers or oligomers, alone, or with a suitable synergist, such as antimony oxide, can be used in effective amounts to provide flame resistance, e.g., about 2-12 parts by weight of bromine per 100 parts by weight of flammable resin content. 
     The foaming agents (c) of the present invention can be selected from chemicals containing decomposable groups such as azo, N-nitroso, carboxylate, carbonate, heterocyclic nitrogen containing and sulfonyl hydrazide groups. Generally, they are solid materials that liberate gas(es) when heated by means of a chemical reaction or decomposition. Representative compounds include azodicarbonamide, dinitrosopentamethylene tetramethylene tetramine, p,p&#39;-oxy-bis(benzenesulfonyl)-hydrazide, benzene-1,3-disulfonyl hydrazide, azo-bis-(-isobutyronitrile), biuret and urea. The dihydrooxadiazinones of U.S. Pat. No. 4,097,425 are members of an especially preferred family of foaming agents. The foaming agent may be added to the polymer in several different ways which are known to those skilled in the art, for example, by adding the solid powder, liquid or gaseous agents directly to the resin in the extruder while the resin is in the molten state to obtain uniform dispersal of the agent in the molten plastic. The temperature and pressures to which the foamable composition of the invention are subjected to provide a foamed polyester/polycarbonate composition will vary within a wide range, depending upon the amount and type of foaming agent that is used. The foaming agents may be used in amounts of from about 0.01 to about 2.5  parts by weight and preferably from about 0.02 to 2.0 parts by weight per 100 parts by weight of the total composition (resin). The preferred foaming agents are dinitrosopentamethylene tetramine, p-toluene sulfonyl semicarbazide, 5-phenyltetrazole, calcium oxalate, trihydrazino-s-triazine, 5-phenyl-3,6-dihydro-1,3,4-oxadiazin-2-one, 3,6-dihydro-5,6-diphenyl- 1,3,4-oxadiazin-2-one and mixtures of any of them. 
     The purpose of foam nucleating agent (d) is to provide a surface for bubble formation. This is analogous to the use of &#34;boiling chips&#34; to moderate bubble formation and to prevent &#34;bumping&#34; in boiling liquids. Numerous attempts to form foams from the same resins (a), alone, or with (b), without nucleating agent were unsuccessful. Although glass fibers are better than nothing, they ruin impact properties, as will be seen. The retention of impact properties requires judicious selection of particle size, shape, and surface polarity, but the most important characteristic appears to be aspect ratio, that is the dimension in one direction divided by another. When the length divided by diameter is equal to about 1.0 in aspect ratio, the impact strength is excellent. If the aspect ratio of the foam nucleating agent is greater than about 1.0, e.g., 1.5, the impact strength drops, and if it is significantly greater, impact strength as measured by standard test methods is very poor, especially at low temperatures, e.g., -40° C. As has been mentioned before, also, high aspect ratio fillers, e.g., glass fibers will show up as surface imperfections, especially if counterpressure molding is used, but low aspect foam nucleating agents, especially in sub-micron sizes, do not detract from surface smoothness. 
     Although numerous inorganic, non-fibrous particulate compounds can be employed for use as foam nucleating agents according to this invention, their selection by those skilled in this art is a simple matter. The compositions can be formulated and foam molded in standard injection molding machines into workpieces suitable for measuring cell structure and flow and ultimately unnotched Izod impact strength. The best formulations can then, if desired, be scaled up for molding into panels and then tested for drop ball impact strength, which provides useful information on resistance to ductile impact. The latter test is the most sensitive to fine formulation changes and has the added advantage of being closest to actual end use requirements. 
     As has been mentioned, it has been discovered that satisfactory results are achievable with siliceous or non-siliceous inorganic patriculate material, alone, or together with an organic polymeric particulate material. Illustratively, the inorganic material can be amorphous silica, precipitated silica, fumed silica, and titanium dioxide, and the organic polymers can be acrylate copolymers, such as those described in U.S. Pat. No. 4,320,212 (Liu); dispersible polytetrafluoroethylene particles; and polyphenylene ether(oxide) resin particles such as described in U.S. Pat. Nos. 3,306,874 and 3,306,875 (Hay), as well as in U.S. Pat. Nos. 3,257,357 and 3,257,358 (Stamatoff). 
     In the present invention, it is preferred to use silica as component (d). This is readily available in a variety of forms with suitable particle sizes, aspect ratios and surface polarities. Although a wide variety of silicas are used in plastics, for component (c) herein, only a synthetic type will be used. These differ from silicas of mineral type, which are mainly used in the form of quartz and glass as fillers. Amorphous, i.e., non-crystalline silicas on the other hand, are typically used in generally small amounts, to achieve special effects. They are made by well known processes, such as one of the pyrogenic processes, i.e., the AEROSIL process in which silicon tetrachloride is mixed with hydrogen and oxygen at 1000° C. or higher, or the electric arc process in which quartz and coke are heated at 1500° C. or above and the SiO and CO which are formed thereby are further reacted with atmospheric oxygen. Wet proccesses are also useful to prepare silicas suitable for use herein, i.e., a precipitation process in which sodium water glass and sulfuric acid are stirred and silica precipitates, or a hydrothermal process, in which quartz is heated with lime under pressure at 180° C., and the calcium silicate is mixed with hydrochloric acid. In all cases, the silica is obtained in a highly dispersed, i.e., very finely divided form. They are not crystalline, but are amorphous, as determined by x-rays. Silicas suitable for the present method are available from a number of commercial sources. The most important characteristics appear to be average primary particle size, which is measured in conventional ways, and expressed in microns (or nanometers, nm); compacted apparent density, measured according to Standard Test Method DIN 53 194 or lSO 787-11; and expressed in g./1., and pH-value, measured according to Standard Test Method DIN 53 200 or ASTM D 1208-65, and expressed in pH units measured on a 5% aqueous suspension. In accordance with conventional practice, the silicas can be surface treated with silanes, silicones, and the like. In Table 1 are listed a number of commercial sources for suitable silicas. The list is not exhaustive but is provided as an aid to understanding and practicing the invention: 
     
                       TABLE 1                                                     
______________________________________                                    
Suitable Commercial Sources of Silicas                                    
                              Average                                     
                              Agglom-                                     
                              erated Surface                              
Manufac-                                                                  
        Trade                 Size,  Area                                 
turer   Name       Type       Microns                                     
                                     m.sup.2 /g                           
______________________________________                                    
CABOT.sup.a                                                               
        CABOSIL    Hydrophilic                                            
                              2.5    250                                  
        M-5        Fumed                                                  
CABOT   CABOSIL    Hydrophobic                                            
                              0.014  70 + 15                              
        N70-TS     Fumed                                                  
CABOT   CABOSIL    Hydrophilic                                            
                              0.008  325 + 25                             
        HS-5       Fumed                                                  
DAVI-   SYLOID     Synthetic  2.5    250                                  
SON.sup.b                                                                 
        234        Amorphous                                              
DAVISON            Silica     140    --                                   
                   Gel                                                    
IMSIL.sup.c                                                               
        IMSIL      Amorphous  1.8     2                                   
        A108                                                              
IMSIL   A108H      Amorphous, 1.8     2                                   
                   surface                                                
                   treated                                                
TULCO.sup.d                                                               
        TULLANOX   Hydrophobic,                                           
                              0.018  140 + 15                             
        A-50       fumed                                                  
______________________________________                                    
 .sup.a Cabot Corp., Boston, Ma                                           
 .sup.b Davison Chemical Div., W.R. Grace &amp; Co., Baltimore, MD 21203      
 .sup.c Illinois Minerals Co., Cairo, IL 62914                            
 .sup.d Tulco Co., Boston, Ma                                             
 
    
     In preferred embodiments, the foam nucleating agent (d) will carry a surface coating sufficient to render it hydrophobic. Hydrophobic surface treated particulate agents suitable for use herein are commercially available, such as the material known as CabOSil N-70-TS listed in Table 1. 
     As has been mentioned, component (d) can be used in conjunction with an organic polymer or polymers, for example, acrylate copolymers, polyphenylene ethers or polytetrafluoroethylenes. 
     The preferred acrylate polymers are acrylate-based core-shell multi-phase composite interpolymer resins. More particularly, the acrylate-based core-shell multi-phase composite interpolymer resin is a core-shell interpolymer comprising about 25 to 95 percent by weight of a first elastomeric phase and about 75 to 5 percent by weight of a final rigid thermoplastic shell phase. One or more intermediate phases are optional, for example, a middle stage polymerized from about 75 to 100 percent by weight styrene. 
     The first stage or core of the multiphase composite interpolymer component is polymerized from about 75 to 99.8 weight percent C 1  to C 6  alkyl acrylate resulting in an acrylic rubber core having a T g  below about 10° C. and crosslinked with 0.1 to 5 weight percent crosslinking monomer and further containing 0.1 to 5 percent by weight graft-linking monomer. The preferred alkyl acrylate is butyl acrylate. The crosslinking monomer is a polyethylenically unsaturated monomer having a plurality of addition polymerizable reaction groups all of which polymerize at substantially the same rate of reaction. Suitable crosslinking monomers include polyacrylic and poly methacrylic esters of polyols such as butylene diacrylate and dimethacrylate, trimethylol propane trimethacrylate, and the like, di- and tri-vinyl benzene, vinyl acrylate and methacrylate, and the like. The preferred crosslinking monomer is butylene diacrylate. 
     The graftlinking monomer is a polyethylenically unsaturated monomer having a plurality of addition polymerizable reactive groups, at least one of which polymerizes at substantially different rate of polymerization from at least one other of said reactive groups. The function of the graftlinking monomer is to provide a residual level of unsaturation in the elastomeric phase, particularly in the latter stages of polymerization, and consequently, at or near the surface of the elastomer particles. When the rigid thermoplastic shell phase is subsequently polymerized at the surface of the elastomer, the residual unsaturated, addition-polymerizable reactive groups contributed by the graft-linking monomer participates in the subsequent reaction so that at least a portion of the rigid shell phase is chemically attached to the surface of the elastomer. 
     Among the effective graftlinking monomers are allyl group-containing monomers of allyl esters of ethylenically unsaturated acids, such as allyl acrylate, allyl methacrylate, diallyl maleate, diallyl fumarate, diallyl itaconate, allyl acid maleate, allyl acid fumarate, and allyl acid itaconate. Somewhat less preferred are the diallyl esters of polycarboxylic acids which do not contain polymerizable unsaturation. The preferred graftlinking monomers are allyl methacrylate and diallyl maleate. A most preferred interpolymer has only two stages, the first stage comprising about 60 to 95 percent by weight of the interpolylmer and being polymerized from a monomer system comprising 95 to 99.8 weight percent of n-butyl acrylate, 0-2.5 weight percent of butylene diacrylate as cross-linking agent, 0.1 to 2.5 percent by weight allyl methacrylate or diallyl maleate as graftlinking agent with a final stage polymerized from about 60 to 100 percent by weight methyl methacrylate. A preferred two stage interpolymer of this type is commercially available under the tradename, ACRYLOID KM 330, from Rohm &amp; Haas Chemical Company. 
     The final or shell stage monomer system can be comprised of C 1  to C 16  methacrylate, styrene, acrylonitrile, alkyl acrylates, allyl methacrylate, diallyl methacrylate, and the like, as long as the overall T g  system is at least 20° C. Preferably, the final stage monomer system is at least 50 weight percent C 1  to C 4  alkyl methacrylate. It is further preferred that the final stage polymer be free of units which tend to degrade poly(alkylene terephthalates), for example acid, hydroxyl, amino, and amide groups. 
     The multiphase composite interpolymers are prepared sequentially by emulsion polymerization techniques wherein each successive outer stage coats the previous stage polymer. By way of illustration, the monomeric C 1  -C 6  acrylate, the cross-linking monomer and the graft-linking monomer are copolymerized in water in the presence of a free-radical generating catalyst and a polymerization regulator which serves as a chain transfer agent, at a temperature on the order of from 15° C. to 80° C. The first elastomeric phase is formed in situ to provide a latex of the core copolymer. 
     Thereafter, the second rigid thermoplastic phase monomers are added and are emulsion polymerized with the core-copolymer latex to form the interpolymers. A more detailed description of the preparation of the acrylate-based interpolymers for use herein can be found in U.S. Pat. Nos. 4,034,013 and 4,096,202, both being specifically incorporated herein by reference. 
     Preferred polyphenylene ether resins are homopolymers or copolymers having units of the formula: ##STR9## in which Q, Q&#39;, Q&#34; and Q&#34;&#39; are, independently, selected from the group consisting of hydrogen, halogen, hydrocarbon radicals, halohydrocarbon radicals, hydrocarbonoxy radicals, and halohydrocarbonoxy radicals; and n represents the total number of monomer units and is an integer of at least about 20, and more usually at least 50. 
     The polyphenylene ether resins are, in general, self-condensation products of monohydric, monocyclic phenols produced by reacting the phenols with oxygen in the presence of complex metal catalysts, with the molecular weight being controlled by the reaction time, longer times providing a higher average number of repeating units. Particular procedures are known to those skilled in the art and are described in the patent literature, including the Hay and Stamatoff patents mentioned above. 
     Suitable phenolic monomers include but are not limited to: 2,6-dimethylphenol; 2,6-diethylphenol; 2,6-dibutylphenol; 2,6-dilaurylphenol; 2,6-dipropylphenol; 2,6-diphenylphenol; 2-methyl-6-ethylphenol; 2-methyl-6-cyclohexylphenol; 2-methyl-6-tolylphenol; 2-methyl-6-methoxyphenol; 2-methyl-6-butylphenol; 2,6-dimethoxyphenol; 2,3,6-trimethylphenol; 2,3,5,6-tetramethylphenol; and 2,6-diethoxyphenol. 
     Some of the polymers which can be produced and which are within the above formula are: poly(2,6-dilauryl-1,4-phenylene)ether; poly(2,6-diphenyl-1,4-phenylene)ether; poly(2,6-dimethoxy-1,4-phenylene)ether; poly(2,6-diethoxy-1,4-phenylene)ether; poly(2-methoxy-6-ethoxy-1,4-phenylene)ether; poly(2-ethyl-6-stearyloxy-1,4-phenylene)ether; poly(2,6-dichloro-1,4-phenylene) ether; poly(2-methyl-6-phenyl-1,4-phenylene)ether; poly(2,6-dibenzyl-1,4-phenylene)ether; poly(2-ethoxy-1, 4-phenylene)ether; poly(2-chloro-1,4-phenylene)ether; poly(2,6-dibromo-1,4-phenylene)ether; and the like. 
     Also included within the above formula are copolymers prepared from mixtures of phenolic monomers. Special mention is made of those based on the reaction of 2,6-dimethylphenol with other phenols, for example, with 2,3,6-trimethylphenol or 2-methyl-6-butylphenol, to produce the corresponding copolymer, for example, poly (2,6-dimethyl-co-2,3,6-trimethylphenol), poly(2,6-dimethyl-co-2-methyl-6-butylphenol), and so forth. 
     Especially preferred for use in this invention are homopolymers having alkyl substituents in the two positions ortho to the oxygen atom, that is, those of the above formula in which Q and Q&#39; are alkyl and particularly alkyl having from 1 to 4 carbon atoms. Most preferred is poly(2,6-dimethyl-1,4-phenylene ether). 
     In other embodiments, agents for helping particulate dispersion and to reduce surface tension will be included. These generally will be non-ionic surfactants, and will typically be, for example, silicones, polyglycol ethers, fluorocarbons, fatty acid derivatives, and many others. Amounts of from 0.01 to about 5.0 parts by weight per 100 parts by weight of composition will be used. 
     In some embodiments, the compositions of the present invention will include a modifier to increase the flow of resin during molding. Many materials are suitable for this purpose, but special mention is made of paraffin waxes, fatty acid esters, polyethylene waxes or mixtures. Suitable commercially available materials will be illustrated in the examples, and they can be used in a wide concentration range, preferably 0.1 to 5 parts by weight per 100 parts by weight of total composition. 
     In other embodiments, the compositions present invention can include an effective amount of (f) a modifier to increase the impact strength of the composition. These can comprise organic polymeric materials generally used to enhance the impact strength of thermoplastics. Typically, they can be a block copolymer of butadiene and styrene, a block copolymer of polycarbonate and polysiloxane, linear low density polyolefins and the like. They will be used in conventional amounts, e.g., from 0.5 to 20 parts by weight, preferably from 2 to 10 parts by weight, per 100 parts of total composition. 
     The relative amounts of components used in the method of the present invention can vary widely. For example, the aromatic carbonate polymer component (a) can comprise from 100 to 1 parts by weight and polyester resin component (b) can comprise correspondingly from 0 to 99 parts by weight. Effective amounts of the other additives have been discussed above. In any event, one will use the lowest amount to obtain the desired result, since there is the least tendency to adversely affect physical properties in so-doing. 
     The resins and other ingredients may be prepared by any of the well known methods outlined above. They may be blended with one another at room temperature. The blend consisting of the polycarbonate, any optional resin, the foam nucleating agent, any impact modifier, etc., are fed into an extruder at a temperature of 480° F. to about 550° F. The extrudate is then comminuted into pellets or other suitable shapes. The decomposable chemical foaming agent can be dry tumbled with the resin blend, or it can previously have been added, e.g., as a concentrate in polycarbonate resin. This mixture is then fed into a conventional molding machine. The molding temperature may be from about 500° F. to about 580° F. with the mold temperature being from about 100° F. to 250° F., preferably from about 140° F. to about 200° F. 
     The foamable composition may be handled in any conventional manner employed for the fabrication or manipulation of thermoplastics such as low and high pressure injection molding to provide thermoplastic products which have uniform cell structure, smooth surfaces, high impact, modulus and tensile strength, improved flow and cycle time. 
     The composition may contain additional materials such as pigments and dyes, stabilizers, antioxidants, mold-release agents, ultraviolet stabilizers, and the like. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following examples are set forth herein to illustrate in more detail the preferred embodiments and to illustrate more clearly the principle and practice of this invention to those skilled in the art. Unless otherwise specified, where parts or percents are mentioned, they are parts or percents by weight. 
     Formulations were compounded by dry blending the components followed by extrusion on a vented, one inch single screw Haake Buchler extruder with a length/ diameter ratio of 25. Generally, the polycarbonate powder was not dried prior to extrusion. The extrusion parameters were as follows: 
     
         ______________________________________                                    
Extrusion conditions                                                      
                   Actual     Set                                         
______________________________________                                    
Zone 1             285° C.                                         
                              285° C.                              
Zone 2             290° C.                                         
                              290° C.                              
Zone 3             295° C.                                         
                              295° C.                              
Die                295° C.                                         
                              295° C.                              
Melt               296° C.                                         
                              --                                          
Pressure           350 psi    --                                          
RPM                50-150     --                                          
Amps               20         --                                          
______________________________________                                    
 
    
     Prior to molding, the compounded resin was dry blended with the blowing agent at a 5/95 blowing agent resin concentrate/polycarbonate resin ratio. The blowing agent resin concentrate contains 5 percent by weight of the blowing agent in poly(bisphenol-A carbonate). The dry blend was further dried at 110° C. for 4 hours prior to molding. After drying, the material was foam molded to a density of 0.90 g./cc. into Izod impact bars (5&#34;×1/2&#34;×1/2&#34;) and drop ball impact plaques (51/2&#34;×11&#34;×1/4&#34;). Molding was done in a 101/2 oz. Reed molding press utilizing about 15% and about 80% of shot capacity for Izod bars and plaques respectively. Molding parameters were as follows: 
     
         ______________________________________                                    
MOLDING CONDITIONS                                                        
______________________________________                                    
Nozzle Temp., °F.                                                  
                      550                                                 
Front Temp., °F.                                                   
                      580                                                 
Center Temp., °F.                                                  
                      580                                                 
Rear Temp., °F.                                                    
                      480                                                 
Mold Temp., °F.                                                    
                      180                                                 
Injection: Hold       100     psi                                         
Std.                  0       psi                                         
Fast                  700     psi                                         
Cure Time             55      sec.                                        
Clamp Open            1.0     sec.                                        
Cycle Time            62      sec.                                        
Shot Time             0.5-1.0 sec.                                        
Amt. Blowing Agent Concentrate                                            
                      5%      by weight                                   
______________________________________                                    
 
    
     For testing, the procedure of American Society of Testing Materials (ASTM D-256) was used on unnotched Izod bars. Drop ball impact testing was done according to ASTM D-2444, slightly modified. Particle size and surface area measurements on the nucleating agents were obtained from the suppliers thereof, or by optical microscopy when not so available. Intrinsic viscosities were measured at 30° C. in CHCl 3 , unless otherwise noted. 
     EXAMPLES 1-8 
     Compositions comprising an aromatic polycarbonate resin (General Electric Company, LEXAN 100), a chemical blowing agent, and siliceous foam nucleating agents having an aspect ratio of about 1.0 were formulated, foam molded and tested. For comparison purposes, a composition having a siliceous nucleating agent with aspect ratio significantly greater than 1, fibrous glass, was molded and tested. The formulations used and the results obtained are set forth in Table 2. 
     
                                           TABLE 2                                 
__________________________________________________________________________
Foamed Compositions of Polycarbonate                                      
and Silica                                                                
               Example                                                    
               A*  1   2   3   4   5   6   7   8                          
__________________________________________________________________________
Composition (parts by weight)                                             
Poly(bisphenol-A carbonate)                                               
               100 100 100 100 100 100 100 100 100                        
5-phenyl-3,6-dihydro-1,3,4-                                               
               0.25                                                       
                   0.25                                                   
                       0.25                                               
                           0.25                                           
                               0.25                                       
                                   0.25                                   
                                       0.25                               
                                           0.25                           
                                               0.25                       
oxadiazin-2-one blowing agent                                             
silica, gel.sup.a                                                         
               --  0.5 --  --  --  --  --  --  --                         
silica, hydrophilic, fumed.sup.b                                          
               --  --  0.5 --  --  --  --  --  --                         
silica, granular gel.sup.c                                                
               --  --  --  0.5 --  --  --  --  --                         
silica, hydrophobic, fumed.sup.d                                          
               --  --  --  --  0.5 --  --  --  --                         
silica, amorphous.sup.e                                                   
               --  --  --  --  --  0.5 --  --  --                         
silica, amorphous, surface                                                
               --  --  --  --  --  --  0.5 --  --                         
treated.sup.f                                                             
silica, hydrophilic, fumed.sup.g                                          
               --  --  --  --  --  --  --  0.5 --                         
silica, hydrophobic, fumed.sup.h                                          
               --  --  --  --  --  --  --  --  0.5                        
Glass, fibrous, 1/8&#34; chopped                                              
               5   --  --  --  --  --  --  --  --                         
Properties                                                                
Unnotched Izod Impact, ft.-lbs.,                                          
               7.36                                                       
                   8.3 17.4P                                              
                           17.5P                                          
                               19.6                                       
                                   --  --  15.2P                          
                                               --                         
ft.-lbs., 25° C.                                                   
                   P   4NB 1NB 4NB 5NB 5NB --  5NB                        
Drop ball impact, ft. lbs.                                                
25° C.  35              88.0                                       
-40° C. 9               25.9                                       
__________________________________________________________________________
 *Control; P = partial break; NB = no break; S = specimens used           
 .sup.a DAVISON, SYLOID 234                                               
 .sup.b CABOT, CABOSIL M5                                                 
 .sup.c DAVISON, Silica gel                                               
 .sup.d CABOT, CABOSIL N70TS                                              
 .sup.e IMSIL, IMSIL A 108                                                
 .sup.f IMSIL, IMSIL A 108H                                               
 .sup.g TULLANOX A50                                                      
 .sup.h CABOT, CABOSIL HS5                                                
 
    
     The foregoing results demonstrate that highly impact resistant foams are prepared according to the present invention when a particulate foam nucleating agent comprising synthetic silica with an aspect ratio of about 1.0 is included. When, on the other hand, fibrous glass is present, impact strenth is much lower, and at -40° C. the ductile impact strength is vastly inferior to, for example, the composition of Example 4, foamed. The cells in the foams of Examples 1-8 are substantially uniform and void-free, as well, whereas the cells in Example 1A are non-uniform and the foamed article contains numerous voids. 
     EXAMPLES 9 
     The general procedure of Examples 1-8 is repeated, substituting for the silica, a non-siliceous inorganic particulate material in particulate form. The composition used and the results obtained are set forth in Table 3. 
     
                       TABLE 3                                                     
______________________________________                                    
Foamed Compositions of Polycarbonate and                                  
Particulate Non-Siliceous Materials                                       
Example                  9                                                
______________________________________                                    
Composition (parts by weight)                                             
Poly(bisphenol A carbonate)                                               
                         100                                              
5-phenyl-3,6-dihydro-1,3,4-                                               
                         0.25                                             
oxadiazin-2-one blowing agent                                             
Titanium dioxide, coated.sup.a                                            
                         0.5                                              
Properties                                                                
Unnotched Izod Impact,   17.5P                                            
ft.-lbs., 25° C.  2NB                                              
Drop ball impact, ft. lbs.                                                
 25° C.           80                                               
-40° C.           25                                               
______________________________________                                    
 .sup.a 0.15 micron particle size                                         
 
    
     Impact resistant foams according to this invention are obtained. Flow was difficult. 
     EXAMPLES 10-17 
     The general procedure of Examples 2 and 4 was repeated except that 0.2 parts of various surfactants were added to each. The formulations used and the results obtained are set forth in Table 4: 
     
                                           TABLE 4                                 
__________________________________________________________________________
Foamed Compositions of Polycarbonate,                                     
Particulate Silica and Surfactants                                        
                  Example                                                 
                  10  11  12  13  14  15  16  17                          
__________________________________________________________________________
Composition (parts by weight)                                             
Poly(bisphenol-A carbonate)                                               
                  100 100 100 100 100 100 100 100                         
5-phenyl-3,6-dihydro-1,3,4-                                               
                  0.25                                                    
                      0.25                                                
                          0.25                                            
                              0.25                                        
                                  0.25                                    
                                      0.25                                
                                          0.25                            
                                              0.25                        
oxadiazin-2-one blowing agent                                             
fluorocarbon surfactant.sup.c                                             
                  --  0.2 0   0   0   0.2 0   0                           
Hydrophobic fumed silica.sup.a                                            
                  0.5 0.5 0.5 0.5 --  --  --  --                          
Hydrophilic fumed silica.sup.b                                            
                  --  --  --  --  0.5 0.5 0.5 0.5                         
Silicone fluid surfactant.sup.d                                           
                  --  --  0.2 --  --  --  0.2 --                          
Anionic polyglycol ether surfactant.sup.e                                 
                  --  --  --  0.2 --  --  --  0.2                         
Properties                                                                
Unnotched Izod Impact,                                                    
                  17.4                                                    
                      19.6                                                
                          16  --  17.4                                    
                                      17.7                                
                                          --  19.6                        
ft. lbs., 25° C.                                                   
                  4NB 3NB     5NB 4NB 2NB 5NB 1NB                         
Drop ball impact, ft. lbs.                                                
25°  C.    73.8                                                    
                      ND  ND  82.6                                        
                                  ND  ND  ND  ND                          
-40° C.    25.9                                                    
                      ND  ND  8.2 ND  ND  ND  ND                          
__________________________________________________________________________
 .sup.a CABOT, CABOSIL N 70TS                                             
 .sup.b CABOT, CABOSIL M5                                                 
 .sup.c DuPont Co., ZONYL FSN                                             
 .sup.d Dow Corning, Silicone fluid 200                                   
 .sup.e Sandoz Co., SANDOZIN D100                                         
 
    
     The surfactant-containing compositions produced high impact strength foams with generally larger cell sizes. 
     EXAMPLES 18-22 
     The general procedure of Example 4 was repeated, except that several wax-like materials were added as flow modifiers. The formulations used and the results obtained are set forth in Table 5: 
     
                       TABLE 5                                                     
______________________________________                                    
Foamed Compositions of Polycarbonate,                                     
Particulate Silica and Flow Modifiers                                     
Example           18     19     20   21   22                              
______________________________________                                    
Composition (parts by weight)                                             
Poly(bisphenol-A carbonate)                                               
                  100    100    100  100  100                             
5-phenyl-3,6-dihydro-1,3,4-                                               
                   0.5    0.5    0.5  0.5  0.5                            
oxadiazin-2-one blowing agent                                             
hydrophobic fumed silica.sup.a                                            
liquid polyethylene.sup.b                                                 
                  --      0.3   --   --   --                              
solid polyethylene.sup.c                                                  
                  --     --      0.3 --   --                              
primary methyl behenate.sup.d                                             
                  --     --     --    0.3 --                              
linear low density polyethylene.sup.e                                     
                  --     --     --   --    2.0                            
Properties                                                                
Drop ball impact strength,                                                
ft.-lbs.  25° C.                                                   
                  88.0   68.3   67.8 86.9 85.0                            
-40° C.    25.9   39.8   28.0 24.8 31.8                            
______________________________________                                    
 .sup.a CABOT, CABOSIL N 70TS                                             
 .sup.b BARECO DIV., PETROLITE CO., VYBAR 825 P                           
 .sup.c BARECO DIV., PETROLITE CO., HYPAC Q 507                           
 .sup.d WITCO CHEM. CO., KEMESTER 902790                                  
 .sup.e UNION CARBIDE CO., BAKELITE 7047                                  
 ND = not determined                                                      
 
    
     Certain of the modifiers help impact strength. Ethylene bis stearamide and an ester wax degraded during processing and could not be used. 
     EXAMPLES 23-27 
     The general procedure of Example 4 was repeated, including in the compositions amounts of various impact modifiers. The formulations used and the results obtained are set forth in Table 6: 
     
                       TABLE 6                                                     
______________________________________                                    
Foamed Compositions of Polycarbonate,                                     
Particulate Silica and Impact Modifiers                                   
Example          23      24     25   26   27                              
______________________________________                                    
Composition (parts by weight)                                             
Poly(bisphenol A carbonate)                                               
                 100     100    100  100  100                             
5-phenyl-3,6-dihydro-1,3,4-                                               
                 0.25    0.25   0.25 0.25 0.25                            
oxadiazin-2-one blowing agent                                             
hydrophobic fumed silica.sup.a                                            
                 0.5     0.5    0.5  0.5  0.5                             
Butadiene/styrene block                                                   
                 --       10    --   --   --                              
copolymer.sup.b                                                           
Polycarbonate/polisiloxane                                                
                 --      --      10   5   --                              
block copolymer.sup.c                                                     
Acrylic rubber core shell                                                 
                 --      --     --   --    10                             
copolymer.sup.d                                                           
Properties                                                                
Drop ball impact strength,                                                
ft.-lbs.,  25° C.                                                  
                 88.0    86.3   74.7 88.8 59.6                            
-40° C.   25.9    23.8   53.8 44.8 25.9                            
______________________________________                                    
 .sup.a CABOT, CABOSIL N 70--T5                                           
 .sup.b SHELL, KRATON G 1651                                              
 .sup.c General Electric Co., LR resin 3220                               
 .sup.d Rohm &amp; Haas Co., ACRYLOID KM 330                                  
 ND = not determined                                                      
 
    
     Foamed compositions according to this invention are obtained. When a butadiene based core shell polymer was added, the processing temperatures caused it to degrade. The block polycarbonate/polysiloxane caused a very substantial improvement in low temperature impact strength. 
     EXAMPLES 28-29 
     The general procedure of Example 4 was repeated and pigments and flow modifiers were added. The formulations employed and the results obtained are set forth in Table 7: 
     
                       TABLE 7                                                     
______________________________________                                    
Pigmented Foamed Compositions of                                          
Polycarbonate and Particulate Silica                                      
Example           B*     10     20   28   29                              
______________________________________                                    
Composition (parts by weight)                                             
Poly (bisphenol A carbonate)                                              
                  100    100    100  100  100                             
5-phenyl-3,6-dihydro-1,3,4-                                               
                   0.25  0.25    0.25                                     
                                      0.25                                
                                           0.25                           
oxadiazin-2-one blowing agent                                             
hydrophobic fumed silica.sup.a                                            
                  --     0.5    0.5  0.5  0.5                             
carbon black pigment 0.2                                                  
                  --     --     0.2  0.2                                  
solid polyethylene flow                                                   
                  --     --     0.3  --   0.3                             
modifier.sup.b                                                            
Glass fibers      5.0    --     --   --   --                              
Titanium dioxide  0.5    --     --   --   --                              
Properties                                                                
Drop ball impact, ft.-lbs.                                                
 25° C.     35    88.0   68   85.4 74.2                            
-40° C.     9     25.9   28   26.8 29.6                            
______________________________________                                    
 *Control                                                                 
 .sup.a CABOT, CABOSIL N 70TS                                             
 .sup.b Bareco Co., HYPAC Q 507                                           
 
    
     The use of carbon black is not seen to adversely affect the impact strength of the present invention. The foamed product of Example 31 is shown in FIG. 2. The foamed product of Example 1A is shown in FIG. 1. In contrast to Example 1A, Example 31 produces uniform cells. 
     The foregoing patents, applications, and publications are incorporated herein by reference. Many variations will suggest themselves to those skilled in this art in light of the above, detailed description. For example, the polycarbonate can be replaced in part by poly(ethylene terephthalate), poly(1,4-butylene terephthalate) poly(1,4-cyclohexyldimethylene)terephthalate, and the like. The blowing agent 5-phenyl-3,6-dihydro-1,3,4-oxadiazinedione can be replaced with 5-phenyl tetrazole. All such obvious variations are within the full intended scope of the appended claims.