Moldable crystalline polyester composition

Abstract: Poly(ethylene terephthalate) modified with an impact modifier, a small amount of polycarbonate, and a nucleator is a rapidly-crystallizing tough molded or extruded material, whose surface and mold-release properties are enhanced by the presence of small amounts of ethylene/acrylate copolymers. Plate-out during processing also is reduced.

Description: This invention is concerned with compositions containing polyester, such as poly(ethylene terephthalate), impact modifier, mold release agent, and nucleating agent, which exhibit resistance to plate-out on injection molding or extrusion. Polymers and copolymers containing predominantly units derived from ethylene glycol and terephthalic acid, which we shall designate PET, when properly formulated, are attractive candidates for many packaging operations. One major aspect is in clear formulations where the polyester remains amorphous, such as in the packaging of water, carbonated beverages, and the like. A second aspect is when the polyester is crystallized, so as to attain a heat stability without distortion up to temperatures above 230 degrees C. The crystalline polymers in a practical aspect require one or more additives for improvement of impact strength, an additive to improve mold release or release from any contact with metal surfaces, and one or more additives known as "nucleating agents" which will control the rate and degree of crystallization during the molding process. One solution to this problem has been offered by Carson, U.S-A-4,713,268, where he teaches an acrylic core/shell impact modifier, suitably heat-stabilized, a small amount of polycarbonate, and a poly(alkylene terephthalate), preferably where the alkylene group is butylene, as a nucleator. Another formulation which offers lower cost involves the use of a polyolefin, such as linear low density polyethylene (LLDPE), as a nucleating component of polyester blends (as taught in Gartland et al, U.S. Patent 4,463,121). The polyolefin is also useful as a mold release agent; it is generally less effective as a nucleator than inorganic nucleators, such as talc. When a polyolefin, such as LLDPE, is employed in combination with a mixture of polycarbonate and an acrylic core/shell modifier, under certain conditions of molding, such as extended runs or attempts to increase production rates by use of higher temperatures and faster mold cycles, the LLDPE will "plate-out", necessitating stopping or slowing the molding process to remove the separated polyolefin. A similar result is seen when such a formulation is extruded into sheet and then thermoformed into appropriate packaging articles. Plate-out is manifested by unattractive, dull surfaces, appearance of a film of polyolefin at the thermoforming rolls, or a slippery feel to the surface of the formed article. We have now surprisingly found that replacement of LLDPE with a copolymer of ethylene and a vinyl or vinylidene monomer containing ester groups, such as a (meth)acrylate ester, such as an ethylene/methyl acrylate (E/MA) or ethylene/ ethyl acrylate (E/EA) copolymer, or a vinyl alkanoate, such as vinyl acetate, drastically reduces plate-out from the blend on molding, resulting in longer molding runs and fewer spoiled moldings. Similarly, replacement in an extrusion process results in less plate-out, as noted by better appearance of the sheet, longer runs without surface mar, and lack of buildup of polyolefin at the die lips. Polyesters modified with larger amounts of ethylene/ ester copolymers, additionally with nucleator, are known to the art, but are not taught in combination with the core/shell modifier and the polycarbonate of the present invention as being freer of plate-out and mold-release problems. In the broadest aspect therefore the invention provides the use, in a composition containing (a) poly(ethylene terephthalate) resin having an intrinsic viscosity of from 0.7 to 1.1 dl/g, (b) core/shell impact modifier, (c) polycarbonate and, optionally (d) nucleating agent, and/or filler, and/or thermal and/or oxidative stabiliser, of (e) 1 to 5%, by weight of the stated components (a) to (e), of copolymer containing less than 99% by weight ethylene units and units of vinyl and/or vinylidene monomer containing ester groups, the copolymer containing no acid or acid salt functionality, to enhance the surface and/or mould release properties of the composition. In another aspect of the invention, there is provided a rapid-crystallizing, modified polyester resin composition comprising: a rapid-crystallizing modified polyester resin composition comprising: 1. from 70 to 94 %, preferably about 85% to about 90%, of a poly(ethylene terephthalate) resin having an intrinsic viscosity of from about 0.7 to about 1.1 dl./g.; 2. from 4 to less than 29%, preferably from about 5 to about 10%, of impact modifier polymer having an elastomeric inner stage (core) and a rigid outer stage (shell), preferably a core/shell polymer having: from about 60 to about 90% of a first elastomeric stage polymerized from a monomer system comprising (i). at least about 75% of an alkyl acrylate preferably butyl acrylate, or of butadiene; (ii) optionally up to about 5% of a crosslinking monomer, preferably an alkylene glycol diacrylate or a polyol triacrylate; (iii) optionally up to about 5% of a graftlinking monomer, preferably allyl methacrylate or diallyl maleate; and from about 10 to about 40 parts of a second, rigid, stage polymerized from a monomer system comprising at least 55% of an alkylmethacrylate wherein the alkyl group has from 1 to 4 carbon atoms; 3. optionally up to 5 weight percent of an inorganic nucleating agent, such as talc; and 4. optionally up to 5% of one or more thermal and/or oxidative stabilizers; 5. from 1 to 8 % of an aromatic polycarbonate, preferably bisphenol-A polycarbonate; and 6. from 1 to 5 % of a copolymer of ethylene with a vinyl or vinylidene monomer containing ester groups, such as an alkyl acrylate such as methyl acrylate or ethyl acrylate, or a vinyl alkanoate, such as vinyl acetate, the ethylene content being less than about 99%, the copolymer containing no acid or acid salt functionality. Preferably the core/shell polymer contains from about 10 to about 40 parts of a final rigid stage polymerized from a monomer system consisting, or substantially consisting, of methyl methacrylate. The invention further relates to the above compositions in the form of a rapidly-crystallizing sheet or molding. The invention further provides to a process for producing a molding of a toughened poly(ethylene terephthalate) comprising a. forming a composition as described above, b. subjecting it to conditions of heat and shear in an extruder sufficient to form a uniform melt; c. passing the melt through an extruder die to form a strand; d. cooling and cutting the strand to form pellets; e. subjecting the pellets to extruder conditions of heat and shear sufficient to reform the uniform melt; f. passing the uniform melt into a mold; g. cooling and releasing the molded object; wherein the molded object exhibits less plate-out than an object molded similarly from the described formulation absent component 6, the ethylene copolymer. The invention further relates to a process wherein similar components are processed in a similar way to produce a molded object exhibiting better release from the mold than an object molded similarly from the described formulation absent the ethylene copolymer component 6. Further, the invention relates to a process for producing an extrudate of a toughened poly(ethylene terephthalate) comprising treating an admixture of similar materials in a similar manner in an extruder, but passing the melt through an extruder die or die-lips to form a sheet, then optionally passing the sheet through shaping equipment, thermoforming dies, or onto calender rolls to form a finished shaped object wherein the extruded shaped object exhibits less plate-out than an object molded similarly from the described formulation absent the ethylene copolymer component a(4). All percentages are by weight and based on 100 parts of the composition or of the core/shell polymer, respectively. By PET is meant condensation polymers formed by the reaction of ethylene glycol with terephthalic acid or a dialkyl terephthalate. Further are included copolymers containing small amounts of other glycols, such as propylene glycol, 1,4-butylene glycol, cyclohexane-1,4-dimethanol and the like, or of units derived from other related diacids, such as isophthalic acid, 2,6-napthalene dicarboxylic acid, and the like. One of ordinary skill in the art may readily select those copolymers which have a sufficiently high crystallization rate (related to PET "homopolymer") to be useful in forming crystallized molded or extruded articles. Core/shell impact modifiers as used in the invention are well-known polymers in the art. Normally they are formed in emulsion from a core polymer of an alkyl acrylate, such as butyl acrylate, 2-ethylhexyl acrylate, and the like, usually containing a crosslinking monomer, such as a glycol di(meth)acrylate, divinylbenzene, or the like to retain the dimensions of the core during processing and to increase the impact strength, and a graftlinking monomer, such as diallyl maleate, allyl (meth)acrylate, trisallyl cyanurate, and the like, to leave double bonds available for attachment of the later-polymerized shell to the core, thus improving the dispersion of the rubbery core in the matrix polymer. Typical of such core-shell polymers are those disclosed in U. S-A-4,264, 487 which should be consulted for details of such core/shell modifiers useful in this invention. It is a requirement of a crosslinking monomer that it be polyethylenically unsaturated monomer copolymerizable with the other monomers of the same stage, the reactivity of its unsaturated groups being such that substantially all of them react during the same polymerization so that substantially all of the crosslinker monomer units are chemically attached only to a single stage. It is a requirement of a graftlinking monomer that it be polyethylenically unsaturated monomer copolymerizable with the other monomers of the same stage but having sufficiently low reactivity of one or more of the unsaturated groups to allow significant residual unsaturation to remain in that stage polymer to that substantially all of the graftlinker monomer units are chemically attached to two or more polymer stages. The distinction between graftlinking monomers and crosslinking monomers may not be clean-cut, depending on relative reactivities of double bonds of these monomers with various core and shell monomers, and on the extent of conversion. Thus, in some instances, a crosslinking monomer may participate in grafting reactions, and a graftlinking monomer may cause the core to become crosslinked. In general therefore, in the final polymer, a crosslinking monomer is defined as polyethylenically unsaturated monomer which is chemically attached to only a single stage and graftlinking monomer is defined as polyethylenically unsaturated monomer which is chemically attached to two or more adjacent stages. However, whatever their attachment in the final polymer, the following are to be regarded as preferred crosslinking monomers: butylene diacrylate or dimethacrylate, divinyl benzene and trimethylolpropane trimethacrylate or triacrylate; and the following are to be regarded as preferred graftlinking monomers: allyl methacrylate and diallyl maleate. When polymers based on butadiene-rich cores are utilized, the presence of a graft-linking monomer is usually unnecessary. Cross-linking monomers may be utilized. Useful for the present purpose are homopolymers of butadiene, or copolymers of butadiene with up to 25% of co-monomers such as styrene, butyl acrylate, acrylonitrile, and the like. Typical of such core-shell polymers are those disclosed in U. S-A-4,180,494 which should be consulted for details of the butadiene-based core/shell modifiers useful in this invention. The shell of the core/shell modifier may be a rigid polymer, that is, one with a glass temperature (as an isolated polymer apart from the core) of above room temperature. The shell will contain at least 55% of at least one alkyl methacrylate wherein the alkyl group is from 1 to 4 carbon atoms, such as methyl, ethyl, propyl, or butyl. Other co-monomers may be present, such as styrene, an alkyl acrylate, an aryl (meth)acrylate, (meth)acrylic acid, and the like. Preferred for ease of isolation of the resulting core/ shell modifier, is an outer stage or shell polymer of methyl methacrylate homopolymer. The rubbery core should be present as from about 60% to about 90% of the total core/shell polymer. The outer shell should comprise from about 10 to about 40 parts of the core/shell modifier. Intermediate shells, either rubbery or hard, may also be present. A butadiene//styrene//methyl methacrylate "three-stage" polymer such as taught in Ferry et al., U. S. 3,985,704, may be utilized in the present invention. Since the core/shell polymer is separately isolated prior to forming the blend, such as by spray-drying, coagulation, and the like, it is convenient at that time to add the nucleating and thermal/oxidative stabilizers described below, especially as the thermal/oxidative stabilizer is generally required to protect the core/ shell polymer. Core-shell polymers based on butadiene especially require such stabilization. However, it may be more convenient to add these components at the time the blend is formed and processed. Nucleators may optionally be added, depending on the exact processing conditions to be used. Useful nucleators are inorganic compounds, such as talc, functionalized alumina, organic salts such as sodium salicylate, or certain organic polymers, such as poly(butylene terephthalate), certain polyolefins, polyamides, and the like. Thermal/oxidative stabilizers are of several types: (a) organic phosphites, such as triphenyl phosphite, tris(nonylphenyl) phosphite, and the like, or other phosphorous-acid based derivatives, such as those taught in U.S-A-3,953,539; (b) organic sulfur compounds, such as dilauryl thiodipropionate, pentaerythritol tetra(thiodipropionate), alkyl esters of thioglycolic acid, and the like; (c) phenolic antioxidants based on hindered phenols, such as 2,6-di-t-butyl-4-methylphenol, octadecyl (3',5'-di-t-butyl-4'-hydroxyhydrocinnamate, and the like; (d) combinations, such as molecules containing both a hindered phenol and an S-alkylated thioester group. The stabilizers may also include a component, such as a trisaryl phosphite, which will prevent transesterfication reactions between the polyester and the polycarbonate. The polycarbonate of the invention is normally bisphenol A polycarbonate, but other known polycarbonates, such as those based on bisphenol S, may be used. The copolymer of ethylene with a vinyl or vinylidene monomer containing ester groups refers to a group of commercial or experimental polymers wherein the mol percent of the ester is generally below about 30%. The "monomer containing ester groups" refers to two general types of monomers: (a) molecules where the copolymerizable double bond resides in the acid portion of the ester molecule, such as acrylate esters, methacrylate esters, maleic esters, itaconate esters, and the like. Preferred for their commercial availability are copolymers containing ethyl acrylate or methyl acrylate. (b) molecules where the copolymerizable double bond resides in the alcohol portion of the molecule, such as vinyl alkanoates, isopropenyl alkanoates, and the like. Preferred because of commercial availability is vinyl acetate. Experimentation has shown that these copolymers are more effective in preventing plate-out and encouraging mold release than are the prior art homopolymers of polyethylene, copolymers of ethylene with small amounts of polyolefin, or copolymers containing ethylene and an acid, such as (meth)acrylic acids, or salts thereof, the latter polymers being also known as ethylene ionomers. It should be noted that certain of these copolymers, in the absence of the core/shell polymer, are still somewhat effective impact modifiers at room temperature. However, they degrade noticeably on re-processing of scrap, and require additional amounts of appropriate thermal stabilizers. On balance of processing, retention of properties on re-molding, and low temperature toughness, the blend of the present invention is preferred. The blend of the present invention may further contain other impact modifiers, reinforcing fillers, lubricants, dyes, pigments, stabilizers, and the like. Such may be combined prior to melt-processing and/or during the melt-processing operation. The blend may be melt-processed in conventional blending or molding equipment. It may be pre-pelletized by conventional extrusion/pelletization techniques and then re-molded or re-extruded by the ultimate user to form the desired object. Alternatively, the pre-mixed blend of ingredients may be melt-mixed and molded or extruded in one operation. The blend may be molded into useful tough objects of good surface, for use in such applications as computer and business machine housings, automotive parts, and the like . It may also be extruded or molded into food containers which will exhibit sufficient toughness to be frozen and sufficient heat stability to be used in warming operations, such as in a microwave oven. Extruded sheets will exhibit good thermoformability, so they can be readily shaped by conventional thermoforming techniques into trays, containers, and the like. Quantification of plate-out is difficult, and it may be necessary to rely on comparative experiments and the judgment of an experienced operator. Plate-out is a visual observation, either seen as a film on the mold surfaces or as defects in the molded object, and one test requires establishment of equilibrium molding conditions with a particular formulation and then a measurement of the number of moldings which can be made from various formulations before the mold must be cleaned. For extrudates, plate-out is usually observed at a die or at a pinch point, such as at a thermoforming roll or rolls, or orifice contacted by the warm or molten polymer. Build-up at thermoforming rolls is the most common observation, and again can be observed but not easily quantified, except the time the formulation can be run before re-cleaning is required. Surface gloss after a number of cycles or a set time of extrusion may also be employed. EXPERIMENTAL Dry blends were made of the following compositions, wherein PET is a commercial poly(ethylene terephthalate), intrinsic viscosity = 0.95, sold for use in molded crystallizable objects; the acrylic additive is a blend comprising 74.5 parts of a spray-dried core/shell impact modifier of the composition 80 (butyl acrylate 99/diallyl maleate 0.5/ trimethylolpropane triacrylate 0.5// 20 MMA; 19 parts of bisphenol-A polycarbonate; 1.5 parts of talc and 3.5 parts of a blend of phenolic and sulfur-containing thermal stabilizers (containing dilauryl thiodipropionate, tris( 2,4-di-t-butylphenyl) phosphite and tetrakis(methylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane; the polyolefin is linear low density polyethylene, containing a hindered phenol antioxidant; and the E/EA is a poly(ethylene/ethyl acrylate) copolymer, melt index 6.0, containing 18% ethyl acrylate. The dry blends were dried at 70 degrees C. overnight, and extruded into pellets on a 25.4 mm. single screw extruder operating at 100 rpm, with barrel temperatures zone 1=243 degrees C., and zones 2-4=254 degrees C. The pellets were re-dried at 70 degrees C. overnight before molding. The blends studied contained the following parts by weight of the indicated components: Blend A* B* C* D* E PET 97 97 90 87 87 Polyolefin 3 3 E/EA 3 3 Acrylic Additive 10 10 10 Molding was performed in a Newbury molder and with a ASTM chromed, polished, mold for test parts for further physical testing by ASTM methods. Mold temperature was 140 degrees C.; the barrel temperatures were zone 1=266 degrees C.; zone 2= 260 degrees C.; zone 3= 254 degrees C.; zone 4= 260 degrees C. The injection pressure was 150 psi ( 1.03 mPa); the cycle time was 25 seconds hold, and 25 seconds cool. Mold release was judged by the operator on a scale of 1-5, 5 being best. Plate-out was visually judged and the number of molding shots required before plate-out appeared was noted. EXAMPLE 1 This experiment compares the performance of the polyolefin and the ethylene/acrylic copolymer in the absence of the acrylic additive. The blends are not tough enough for many packaging applications. The ethylene/ethyl acrylate copolymer additive is slightly better in resistance to plate-out, but neither additive exhibits a major plate-out problem. Blend Mold release Plate-out A* 4 Start at 20th shot B* 4 None after 25th shot EXAMPLE 2 In this experiment, the toughened blend (containing the acrylic additive) is shown to have poor mold release, but no plate-out problems. When the polyolefin, taught by Gartland et al., is added to the toughened blend, mold release is improved, but early plate-out is encountered. Upon replacement of the polyolefin, with the E/EA copolymer, greatly improved resistance to plate-out is seen. Blend Mold release Plate-out C 1 No plate-out after 25 shots D 3 Start at 3rd shot; very bad at 20th shot E 3 Start at 22-25th shot; in repeat runs, none at 25th shot EXAMPLE 3 A copolymer of ethylene/ methyl acrylate may be substituted for the ethylene/ ethyl acrylate copolymer of Example 1 with similar improved results. EXAMPLE 4 The butyl acrylate-based heteropolymer impact modifier of Example 1 may be replaced with an equal amount of a butadiene//styrene//methyl methacrylate copolymer as exemplified in U. S. Patent 3, 985,704 with similar improved results. The butadiene-based impact modifier is stabilized with about 3.5% by weight of a stabilizer combination claimed in U. S. Patent 5,164,434.

Claims: The use, in a composition containing (a) poly(ethylene terephthalate) resin having an intrinsic viscosity of from 0.7 to 1.1 dl/g, (b) core/shell impact modifier, (c) polycarbonate and, optionally (d) nucleating agent, and/or filler, and/or thermal and/or oxidative stabiliser, of (e) 1 to 5%, by weight of the stated components (a) to (e), of copolymer containing less than 99% by weight ethylene units and units of vinyl and/or vinylidene monomer containing ester groups, the copolymer containing no acid or acid salt functionality, to enhance the surface and/or mould release properties of the composition. A polyester resin composition comprising: a. from 70 to 94%, of poly(ethylene terephthalate) resin having an intrinsic viscosity of from 0.7 to 1.1 dl./g.; b. from 4 to less than 29% of a core/shell polymer having an elastomeric inner stage (core) and a rigid outer stage (shell), c. optionally up to 5% of inorganic nucleating agent; and d. optionally up to 5% of thermal and/or oxidative stabilizer; e. from 1 to 8 % of an aromatic polycarbonate; and, as surface and/or mould release enhancer: f. from 1 to 5 % of copolymer of ethylene with vinyl or vinylidene comonomer containing ester groups, the ethylene content being less than 99%, and the copolymer containing no acid or acid salt functionality; the percentages being by weight. A composition as claimed in Claim 2 wherein the core/shell polymer comprises from 60 to 90% by weight of a first elastomeric stage polymerized from a monomer system comprising a. at least 75% of alkyl acrylate, such as butyl acrylate or of butadiene; b. optionally up to 5% of crosslinking monomer; c. optionally up to 5% of graftlinking monomer; and from 10 to 40% of second rigid stage polymerized from a monomer system comprising at least 55% of alkyl methacrylate wherein the alkyl group has from 1 to 4 carbon atoms; the percentages being by weight. A composition of Claim 2 or 3 wherein the vinyl or vinylidene monomer containing ester groups is methyl acrylate, ethyl acrylate, or vinyl alkanoate such as vinyl acetate. A composition of any of Claims 2 to 4 wherein the core/shell polymer is from 5 to 10 % of the stated ingredients of the composition, wherein the first elastomeric stage is polymerized from a monomer system comprising at least 75% of butyl acrylate, the crosslinking monomer is alkylene glycol diacrylate, the graftlinking monomer is allyl methacrylate or diallyl maleate. A composition of any of Claims 2 to 5 wherein the amount of component (a) is 85 to 90% and/or the amount of component (b) is 5 to 10%, by weight of the stated components. A composition of any of Claims 2 to 6 wherein the polycarbonate is bisphenol-A polycarbonate. A composition of any of Claims 2 to 7 wherein the core/shell polymer contains a final shell stage substantially consisting of units of methyl methacrylate. A composition of any of Claims 2 to 8 in the form of a rapidly-crystallizing sheet or molding. Use according to Claim 1 of the ethylene copolymer in a composition of any of Claims 2 to 9. A process for producing a shaped article of toughened poly(ethylene terephthalate) embodying a use as claimed in Claim 1 or 10 comprising; a. forming a mixture as defined in any of Claims 1 to 9; b. subjecting it to conditions of heat and shear in an extruder so as to form a uniform melt; c. passing the melt through an extruder die or die lips to form a strand or a sheet; and either d.1. cooling and cutting the strand to form pellets; d.2. subjecting the pellets to extruder conditions of heat and shear so as to re-form the uniform melt; d.3. passing the uniform melt into a mold; and d.4. cooling and releasing the molded object; or e. passing the sheet through shaping equipment, thermoforming dies, or onto calender rolls to form a finished shaped object.

Inventors: BRADY JEAN MARIE, LAI CHOUNG-HOUNG, BRADY, JEAN MARIE, LAI, CHOUNG-HOUNG
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1993
Main classification: C08L67
IPC codes: C08L51, C08L69, C08L67, C08L23, C08J5