Process for the preparation of branched thermoplastic polycarbonates

Abstract: Thermoplastic branched polycarbonate resins are prepared by equilibration of linear aromatic polycarbonate with polyhydric phenol having more than two hydroxy groups in the presence of a catalyst.

Description: Polycarbonates are well known high performance engineering thermoplastics characterized by many advantageous physical properties, such as high optical clarity, toughness, dimensional stability and excellent impact strength over a wide temperature range. Polycarbonate resins suitable for use in blow molding applications such as the production of hollow articles of large volume and large panels are required to have additional advantageous physical properties of high melt strength (high shape retention) and high shear sensitivity characteristics such as a melt index ratio greater than about 2.0 and a complex viscosity ratio greater than about 3.0. In order to achieve such properties, polycarbonates with a critical degree of branching are required. It is known that branched polycarbonates, for example, bisphenol-A ("BPA") polycarbonates, which exhibit high melt strength properties and are thus suitable for blow-molding applications, can be prepared by a heterogeneous interfacial polymerization of the polycarbonates. The use of pclyhydric phenols having three or more hydroxy groups per molecule, for example, 1,1,1-tris-(4-hydroxyphenyl)-ethane (THPE), 1,3,5-tris-(4-hydroxyphenyl)-benzene, and 1,4-bis-(4',4"-dihydroxy-tri-phenylmethyl)-benzene, as branching agents for high melt strength blow-moldable polycarbonate resins prepared interfacially has been described in U.S. Patent Nos. Re. 27,682 and 3,799,953. Other methods known to prepare branched polycarbonates through heterogeneous interfacial polymerization methods include the use of cyanuric chloride as a branching agent (U.S. Patent No. 3,541,049); branched dihydric phenols as a branching agents (U.S. Patent No. 4,469,861); and 3,3-bis-(4-hydroxyaryl)-oxindoles as branching agents (U.S. Patent No. 4,185,009). Additionally, aromatic polycarbonates end-capped with branched alkyl acyl halides and/or acids and said to have improved properties are described in U.S. Patent No. 4,431,793. Such interfacial polymerization processes as described above are typically conducted in a mixed aqueous-organic system which results in recovery of the polycarbonate in the organic phase. Usually a dihydric phenol is reacted with a carbonate precursor in the presence of a chain terminating or molecular weight controlling agent wherein the branching agent is employed as a comonomer in the preparation of a branched polycarbonate. Blow-moldable grade, branched polycarbonates possessing properties of high melt strength prepared in such interfacial polymerization methods, however, are typically specialty products which require a plant to stop making conventional grades of linear polycarbonate, thus hindering plant flexibility and increasing production costs in their manufacture. It is thus an object of the present invention to provide an alternative method for the preparation of branched polycarbonates possessing properties of high melt strength and shear sensitivity which does not involve interfacial polycondensation methods. Another object of the invention is to provide a method for modifying the melt properties of linear polycarbonate resin in order to provide a resin suitable for use in blow molding applications. Another object of the invention is to provide a method for extruder processing of linear polycarbonate to produce blow molding grade polycarbonate resin from standard grade resin. SUMMARY OF THE INVENTION In accordance with the present invention a novel and improved method is provided wherein conventional grades of linear polycarbonates can be conveniently employed as starting materials in the preparation of blow-moldable branched polycarbonates possessing the requisite physical properties of high melt strength and shear sensitivity. The present invention thus provides a process for the preparation of a thermoplastic, branched-chain polycarbonate which comprises contacting a linear aromatic polycarbonate having a number average molecular weight of from 10,000 to 30,000 with a polyhydric phenol having more than two hydroxy groups per molecule in the presence of a catalytic amount of a carbonate equilibration catalyst selected from tetrabutylammonium tetraphenylborate, tetramethylammonium tetraphenylborate, lithium tetraphenylborate, sodium tetraphenylborate, sodium bis(2,2'-biphenylene) borate, potassium tetraphenyl borate, tetramethylammonium tetraphenylborate, tetramethylphosphonium tetraphenylborate, tetra-n-butylphosphonium tetraphenylborate and tetraphenylphosphonium tetraphenylborate. As will be appreciated by the persons skilled in the art, the subject invention thus provides a convenient process for the preparation of blow-moldable branched polycarbonates heretofore manufactured as special-order products. By the process of this invention, conventional linear non-branched polycarbonates can be converted to the branched polycarbonates. A wide variety of linear non-branched polycarbonate resins may be employed. Commercially available linear polycarbonates can be used to conveniently and effectively produce materials with a range of melt rheological characteristics. The present invention is more fully illustrated by the following detailed description. DETAILED DESCRIPTION OF THE INVENTION The thermoplastic branched polycarbonates according to the process of the present invention are obtained by contacting a preforined, non-branched, linear aromatic polycarbonate with a polyhydric branching agent having more than two hydroxy groups per molecule in the presence of a catalytic amount of a suitable carbonate equilibration catalyst. Linear polycarbonate useful in this invention include any of the several aryl polycarbonates which can be obtained commercially, or prepared by any of the usual procedures, for example, by reacting a dihydroxy aromatic compound with a phosgene in an interfacial polymerization process. Typical of some dihydroxy aromatic compounds suitable for the preparation of linear aryl polycarbonate useful as starting materials herein are described in U.S. Patent Nos. 4,727,134. Dihydroxy aromatic compounds of the bisphenol-A type, and especially, bisphenol-A, are often preferred for their particular suitability in preparing linear aryl polycarbonate resins useful for purposes of the present invention. In the event an aryl polycarbonate copolymer rather than a homopolymer is desired for use in the preparation of the branched aromatic carbonate polymers herein, two or more different dihydroxy aromatic compounds or a copolymer of a dihydroxy aromatic compound with a glycol or with hydroxy or acid terminated polyester, or with a dibasic acid may be employed in the preparation thereof. Further, blends of aryl polycarbonate homopolymer with any of the above-described materials to provide the aryl polycarbonate substrates may be employed herein. Carbonate precursors which can be used for production of the linear aryl polycarbonates useful as substrates herein are well known and described, for example, in U.S. Patent Nos. 4,469,861 and 4,431,793, illustrative examples of such precursors include carbonyl halides, diaryl carbonate esters or haloformates. The carbonyl halides can be carbonyl chloride, carbonyl bromide and mixtures thereof. Typical of the diaryl carbonate esters which can be employed herein are diphenyl carbonate, di-(halophenyl) carbonates such as di-(chlorophenyl) carbonate, di-(bromophenyl) carbonate, di-(trichlorophenyl) carbonate, di-(tribromophenyl) carbonate, di-(alkylphenyl) carbonate, di-(chloronaphthyl) carbonate, phenyl tolyl carbonate, chlorophenyl chloronaphthyl carbonate, or mixtures of any of the foregoing. The haloformates suitable for use herein include mono- or bis-haloformates of dihydric phenols, for example, bischloroformates of hydroquinone, monochloroformate of bisphenol-A, or bishaloformates of glycols, for example, bishaloformates of ethylene glycol, neopentyl glycol, polyethylene glycol. While other carbonate precursors will occur to those skilled in the art, carbonyl chloride, also known as phosgene, is generally preferred. In utilizing the interfacial polymerization process to prepare aryl polycarbonate resin, the dihydroxy aromatic compounds are dissolved in an alkali metal salt solution, for example, sodium hydroxide, and a water-immiscible solvent, such as, for example, methylene chloride, dichloroethane or chlorobenzene, is added. After introducing the carbonate precursor, for example, phosgene, at room temperature, the polycarbonate intermediates are then isolated directly from the organic phase by distilling off the solvent or by precipitation. The chain length of the particular polycarbonate product substrates and thus the desired molecular weight can be adjusted and regulated by adding a chain terminating agent or chain stopper to the reaction mixture to obtain a viscosity of a predetermined value. Such chain stoppers are in most cases a monofunctional phenol, such as, for example, phenol m- and p-methylphenol, m- and p-ethylphenol, m- and p-propylphenol, m- and p-isopropylphenol, m-bromophenol, p-butylphenol, para-tertiary-butylphenol and p-cumylphenol. The addition of the agent is usually made initially prior to the addition of the carbonate precursor, but can be made at any time up to the point of reaction where the degree of polymerization approaches that of a high polymer. The quantity of chain terminating agent which can be added to prepare the linear aryl polycarbonate resins particularly suited for the preparation of the blow-moldable grade of branched polycarbonates in accordance herewith is an amount effective to prepare substrates having a number average molecular weight of about 6,000 to about 50,000. This amount will vary as a function of the mole percent of aromatic dihydroxy compound employed in the reaction. As will be appreciated by those skilled in the art in the preparation of polycarbonate product substrates herein having the desired properties, the addition of such necessary chainterminating agents will not be influenced or hindered by the addition of branching agents. In accordance with the process of the present invention conversion of the aforesaid linear aromatic polycarbonate resin to branched chain polycarbonate is effected by contacting the resin with a polyhydric branching agent having more than two hydroxy group per molecule in the presence of a catalytic amount of a suitable carbonate equilibration catalyst. Without intending to limit the scope of the present invention to any theory or reaction mechanism, it is believed that the reaction is initiated by the formation in situ of a reactive phenoxide from reaction of the equilibration catalyst with the polyhydric branching agent which can thereafter readily undergo an addition reaction with electrophillic carbonate carbon atoms on the linear polycarbonate backbone causing chain scission to form a lower molecular weight fragment and a branched aromatic polycarbonate. It is further believed that the reaction continues until equilibration is attained and a product having a new molecular weight distribution and which has shorter branched chains than the linear aromatic polycarbonate substrate is formed. Polyhydric phenols suitable as branching agents in the present invention include any triol or tetrol or higher hydroxy substituted polyhydric phenol, for example,1,1,1-tris-(4-hydroxyphenyl)ethane (or 4,4′,4″-ethylidyne trisphenol or THPE); 1,3,5-tris-(2-hydroxyethyl) cyanuric acid ([1,3,5-tris-(2-hydroxyethyl)-1,3,5-triazine-2,4,6-(1H, 3H, 5H)-trione]; 4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-heptane-2; 2,2-bis-4,4-(4,4′-dihydroxyphenyl)-cyclohexyl propane; 1,3,5-trihydroxybenzene (phloroglucinol)); 1,2,3-trihydroxybenzene (pyrogallol); and 1,4-bis-(4′4″-dihydroxytriphenyl-methyl)-benzene. Such compounds and examples of additional polyhydric phenols suitable for use herein and their method of preparation are described, for example, in U.S. Patent Nos. 3,799,953 and Re. 27,682. Other commercially available polyhydric phenols useful herein include, for example, 2′,3′,4′-trihydroxyacetophenone; 2,3,4-trihydroxybenzoic acid; 2,3,4-trihydroxybenzophenone; 2,4,4′-trihydroxybenzophenone; 2′,4′,6′-trihydroxy-3-((4-hydroxyphenyl)propiophenone; (phloretin); pentahydroxyflavone; 3,4,5-trihydroxyphenylethylamine (5-hydroxydopanine); 3,4-trihydroxyphenethyl alcohol; 2,4,5-trihydroxypyrimidine (isobarbituric acid); tetrahydroxy-1, 4-quinone hydrate (tetrahydroxy-1,4′-benzoquinone); 2,2′,4,4′-tetrahydroxybenzophenone; and 1,2,5,8-tetrahydroxyanthraquinone (quinalizarin). Of course, a mixture of two or more of such polyhydric phenols may be employed to achieve particularly desired properties of branched polycarbonate. While other polyhydroxy phenols suitable for the practice of the present invention will occur to those skilled in the art, 1,1,1-tris-(4-hydroxyphenyl) ethane or THPE is preferred as the compound is readily available at competitive cost. The term "catalysts" as used herein includes compounds which function as conventional catalysts and compounds which undergo chemical changes and function as conventional initiators or promoters. The carbonate equilibration catalysts used in the present invention are selected from tetrabutylammonium tetraphenylborate, tetramethylammonium tetraphenylborate, lithium tetraphenylborate, sodium tetraphenylborate, sodium bis-2,2'-biphenyleneborate, potassium tetraphenylborate, tetramethylammonium tetraphenylborate, tetramethylphosphonium tetraphenylborate, tetra-n-butyl-phosphonium tetraphenylborate and tetraphenylphosphonium tetraphenylborate. The choice of any particular catalyst selected is not critical to the practice of this invention, thus the use herein of such catalysts described above or similar catalysts may depend upon such factors as their thermal stability, the desired rate of reaction and the chemical nature of the particular linear polycarbonate and branching agents employed. Tetrabutylammonium tetraphenylborate, however, is particularly preferred because of its high activity. Preparation of the branched aryl polycarbonates in accordance with this invention can be effected by dry mixing the above-described reagents prior to their reaction, for example, by melt condensation in a Helicone at temperatures ranging from about 250°C to about 350°C, for approximately 5 to 30 minutes, or by dry mixing the reagents and continuously feeding the mixture through an extrusion device at temperatures ranging from about 200°C to about 350°C. In general equilibration is permitted to proceed for a period of time sufficient to achieve the desired melt properties in the product branched resin. Generally, the level of polyhydric branching agent is not critical to the practice of the present invention as varying the level of branching agent will effect the number of branching sites and the average branched polycarbonate chain length. For example, low levels of branching agents will produce few branching points with relatively long chains, and higher levels will increase the number of branching points, but will decrease the average length of the chains. The amount of branching agent, therefore, will depend upon the various properties of particular branched polycarbonates desired and end uses contemplated. In the preparation of blow-moldable polycarbonate resins, however, it is preferred to employ levels of branching agent ranging from about 0.1 to about 2.0 mole percent based on molecular weight and the amount of linear polycarbonate resin in the reactor. In similar manner, the amount of carbonate equilibration catalyst employed herein is not critical to the practice of the invention, as such amount may depend upon the particular catalyst employed, the reaction rate desired, e.g. the rate at which the branching reaction reaches equilibrium and a stable branched polycarbonate product is obtained, the chemical nature of the particular substrate and branching agent employed, and the particular properties of various branched polycarbonates desired and end uses contemplated. Depending on such variables, an effective amount of catalyst can easily be determined for the preparation of a particular branched polycarbonate without undue experimentation. It is thus preferred that when borate-containing initiators are employed in the preparation of blow-moldable, high melt strength branched chain polycarbonates, the amounts thereof should range from about .01 to about.1 mole percent based upon the amount of linear polycarbonate substrate present. In the process according to the invention, it is also possible to include adjuvants of all kinds at any stage before, during or after the preparation of the branched aryl polycarbonates. Examples which may be mentioned in this context include fire retardants, dyestuffs, pigments, mold release agents, stabilizers against the action of moisture, heat and UV, lubricants and fillers, such as glass powders, quartz products, graphite, molybdenum sulphide, metal powders, powders of higher-melting plastics, for example, polytetrafluoroethylene powder, natural fibers, such as cotton, sisal and asbestos, and also glass fibers of very diverse kinds, metal filaments and fibers which are stable when present in the melt of the polycarbonates and do not significantly damage the polycarbonates. The thermoplastic, branched-chain polycarbonates obtained according to the invention preferably have melt rheological properties comprising a melt index ratio (MIR) of at least 2.0 and a complex viscosity ratio (R*) of at least 3.0 and a number average molecular weight of from about 5,000 to about 20,000. Further, the branched-chain polycarbonates obtained preferably have intrinsic viscosities of about 0.50 to 0.55 measured on solutions of 1.0, 0.5, and 0.33 percent in chloroform, 25°C and melt viscosities of 4,000 to 15,000 centi seconds by the test procedures of U.S. 4,465,820 at Col. 4, line 15-35. The branched polycarbonates according to the invention can be used for both processing by extrusion, and injection molding, and particularly for the manufacture of hollow articles and large panels by blow-molding processes which require materials possessing high melt strength and excellent shape retention of the extrudate. The following detailed examples will serve to more fully illustrate the practice of preferred embodiments of the present invention. EXAMPLES 1-8 To illustrate the effect of THPE branching agent level on the properties of linear polycarbonate substrate, a series of melt condensations are performed in a Helicone mixer. In Examples 1-6, 500 grams of a commercially available grade of linear BPA-polycarbonate was dry mixed with an amount of tetrabutylammonium tetraphenylborate catalyst sufficient to supply 0.025 mole percent borate, based upon the amount of substrate present, and indicated amounts of THPE branching agent. The mixtures are then placed in a Helicone at temperatures ranging from 270°C-300°C for 20 to 30 minutes, and the resulting branched polycarbonate thereafter extracted and evaluated for melt rheological properties of MIR and R*. The results are summarized in Table 1 below. Comparative Examples 7 and 8 are also provided to illustrate similar properties of commercially available branched and linear polycarbonate, respectively, such that the desirable properties of the branched polycarbonates prepared in accordance with the present invention are fully demonstrated and appreciated. The Effect of Varying THPE Levels on Melt Properties of Resulting Branched Aryl Polycarbonates Example No. THPE (mole%) MIR 2 kg wt. (grams) R* ( T ref) 1 0.6 2.37 0.260 4.3 (265°C) 2 0.8 3.05 0.217 5.0 (256°C) 3 1.0 2.67 0.263 5.0 (260°C) 4. 1.2 2.39 0.292 5.0 (255°C) 5 1.4 2.71 0.351 ----- 6. 1.6 2.51 0.468 5.2 (245°C) 7 Comm. 150 2.42 0.147 3.8 (278°C) 8 Comm 130 1.35 0.381 1.5 R is defined as the ratio of the complex melt viscosity of a material low shear (H/radian/sec) to that at high shear (as during extrusion, i.e. 100 radian/sec), typically 20,000 poise. R* is thus a measure of the shear thinning behavior of the polymer. Experience has taught that good flow molding performances is obtained when R* is equal to or greater than 3.5. R* values are obtained by determining the complex viscosity on a Rheometrics Dynamic Spectrometer at 3 different temperatures (typically 230, 250, and 270°C). Using this data fitted to the Arrhenius equation, the optimum processing extrusion temperature is calculated, i.e., that temperature at which the melt viscosity is 20,000 poise at 100 radian/sec. Then, the viscosity at low shear is calculated at this temperature. R* is then calculated by dividing this viscosity by 20,000 poise. The results of Examples 1-6 especially the results of R and 2 kg wt. and Comparative Examples 7-8 indicate that any of the levels of THPE examined produce a branched polycarbonate material displaying significantly higher properties than those of the non-branched resin Comm. 130, and properties somewhat better than those displayed by the branched polycarbonate Comm. 150 prepared by conventional interfacial technology. EXAMPLES 9-11 Additional dry mixtures such as described in Examples 1-6 above were prepared in Examples 9 and 10 and the melt equilibrium thereafter carried out in an extruder which has a very short residence time compared to a Helicone. In comparative Example 11, Lexan 130 is extruded. The melt rheological properties of all materials summarized in Table 2 below. Melt Properties of Extruded Material Example No THPE (mole %) MIR 2K6 Wt. R* (at temp.) IV 9 0.5 2.12 .423 3.04(257°C) .541 10. 1.0 2.50 .589 4.02(243°C) .495 11 .0 1.35 .381 .600 The results of Table 2 further demonstrate the desirable melt rheological properties of branched polycarbonates prepared by extrusion methods in accordance with the present invention. Further GPC analysis of the extruded pellets compared with GPC analysis of the same material after heating at 270°C for 10 minutes in a melt indexer indicates that the material completely equilibrated in the extruder and thus had completely reacted. Moreover, thermal aging tests performed on the extruded material on a melt rheometer for 60 minutes at 300°C indicate the extruded material prepared in accordance with present invention to be thermally stable.

Claims: A process for the preparation of a thermoplastic, branched-chain polycarbonate which comprises contacting a linear aromatic polycarbonate having a number average molecular weight of from 10,000 to 30,000 with a polyhydric phenol having more than two hydroxy groups per molecule in the presence of a catalytic amount of a carbonate equilibration catalyst selected from tetrabutylammonium tetraphenylborate, tetramethylammonium tetraphenylborate, lithium tetraphenylborate, sodium tetraphenylborate, sodium bis(2,2'-biphenylene) borate, potassium tetraphenyl borate, tetramethylammonium tetraphenylborate, tetramethylphosphonium tetraphenylborate, tetra-n-butylphosphonium tetraphenylborate and tetraphenylphosphonium tetraphenylborate. The process of Claim 1 wherein the polyhydric phenol is 1,1,1-tris-(4-hydroxyphenyl)ethane. The process of Claim 1 or Claim 2 wherein the polyhydric phenol is present in an amount of from 0.1 to 2.0 mole percent, and the catalyst is present in an amount of from 0.01 to 0.1 mole percent, based on the amount of moles of the linear aromatic polycarbonate. The process of Claim 1 wherein the thermoplastic branched-chain polycarbonate is prepared by melt condensation at a temperature of from 200°C to 350°C for 1 to 30 minutes. The process of Claim 1 wherein the thermoplastic branched-chain polycarbonate is prepared in an extrusion process at a temperature of from 200°C to 350°C. The process of any preceding claim wherein the branched-chain polycarbonate has a melt index ratio of at least 2.0 and a complex viscosity ratio of at least 3.9.

Inventors: BODEN EUGENE PAULING, KRABBENHOFT HERMAN OTTO, BODEN, EUGENE PAULING, KRABBENHOFT, HERMAN OTTO
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1989
Main classification: C08G64
IPC codes: C08G64