Background of the Invention Polyesters such as poly (ethylene terephthalate) (PET) are engineering thermoplastics used in a wide variety of end use applications such as fibers, films, automotive parts, food and beverage containers and the like. They can be processed by a variety of techniques including injection molding, compression molding, extrusion, thermoforming, blow molding, and combinations thereof. Sometimes, it is desirable to add impact modifiers to improve the toughness of polyesters used in the manufacture of molded parts such as those used in automotive, appliance, cookware or food storage applications. U. S. Patent 4,172,859 discloses that polymeric materials that serve well as impact modifiers should (i) possess a modulus 1/10 that of the polyester matrix material, (ii) be well dispersed within the matrix material in discrete phases of 0.01 micron to 3.0 micron in size, and (iii) be well bonded to the matrix.

Low modulus polymers commonly used as impact modifiers fall into several general classes. The first class comprises rubbers based on butadiene or isoprene, e. g., polybutadiene, polyisoprene, natural rubber, styrene-butadiene (SBR), acrylonytrile-butadiene (ABN or nitrile rubber),

styrene-butatene-styrene (SBS) or hydrogenated SBS (styrene-ethylene- butene-styrene) block copolymers (SEBS), or acrylonytrile-butadiene- styrene (ABS) polymers containing high levels of butadiene. Butadiene- based rubbers generally have lower glass transition temperatures (Tg's) which help to improve low temperature toughness, but they may not be stable under the high temperatures at which polyesters are processed. The second major class of impact modifiers comprise elastomers based on polyethylene, e. g., ethylene-propylene rubbers (EPR) or EPRs with a small amount of side chain diene moiety (EPDM), ethylene-acrylate copolymers such as ethylene/methyl acrylate, ethylene/ethyl acrylate, ethylene/butyl acrylate and ethylene/methylacrylate/glycidyl methacrylate, or ethylene- vinyl acetate copolymers (EVA). A third group of impact modifiers consists of core-shell impact modifiers such as those that contain a poly (methyl methacrylate) (PMMA) hard shell with either a butadiene methacrylate- butadiene-styrene (MBS) or butyl acrylate (acrylic) core, e. g., PARALOID manufactured by Rohm & Haas Company. Core-shell impact modifiers based on acrylonitrile-butadiene-styrene) (ABS) also are commercially available, e. g., BLENDEX manufactured by GE Specialty Chemicals).

Other elastomers that may serve as impact modifiers include polyesters, e. g., HYTREL manufactured by E. I. duPont de Nemours Company and ECDEL manufactured by Eastman Chemical Company, and polyurethanes, e. g., PELLETHANE manufactured by Dow Chemical Company, or silicone rubbers.

By properly matching the melt viscosities of the matrix and impact modifier at the temperatures of melt processing, a fine discrete impact modifier phase can be created by the shearing forces obtained during melt processing. A mixing screw must be designed properly to create the appropriate shear fields during a compounding/extrusion process.

However, impact modifiers dispersed by purely mechanical action may re- coalesce during a later stage of processing where shear may be reduced.

Alternatively, impact modifiers can be manufactured to an inherently small size using latex or other polymerization processes. Impact modifiers manufactured in this way often contain a stiff shell of harder polymer and are, therefore, often referred to as core-shell impact modifiers. These impact modifiers can be made in 0.2-0.5 micron sizes ideally suited for impact modification of nylons, polycarbonates and polyesters. Nonetheless, these core-shell impact modifiers also must be dispersed by shearing action during melt processing, and are prone to re-coalesce during later stages of molding or compounding.

One way to enhance dispersion and prevent coalescence is to introduce functional groups into the impact modifier that either are highly soluble in the matrix polymer or will react with the matrix polymer.

Interaction between these functional groups and the matrix during compounding creates a thin interlayer of material that make the impact modifier and matrix more energetically compatible. Compatibility related to these functional groups leads to good mixing and good dispersion of the impact modifier. The enhanced compatibility also will reduce the possibility that the impact modifier phases will re-coalesce later during processing.

Therefore, impact modifiers containing functional groups that react rapidly with the matrix-polymer produce well-dispersed impact modifier phases of small particle size [See"Rubber Toughened Engineering Plastics", A. A.

Coller, Chapman & Hall, London, 1994. The incorporation of functional groups into the impact modifier also will ensure a good bond between the impact modifier and matrix, i. e. interfacial adhesion between these immiscible phases.

Impact modifiers can be functionalized with a variety of reactive or non-reactive monomers. These functional monomers can be incorporated into the impact modifier directly during preparation of the impact modifier or subsequently by means of a grafting polymerization step. Non-reactive impact modifiers (for example an EPDM-grafted-SAN) compatibilize

themselves to the matrix through a closer matching of solubility parameters, without actually bonding to the matrix polymer. The reactive groups of reactive impact modifiers chemically bond to the matrix polymer but, to be effective, they must do so in the limited time available in the extruder during compounding. U. S. Patent 4,172,859 lists a variety of functional groups which can be grafted or copolymerized onto ethylene-based elastomers for use with polyesters and nylons. In practice, mateic anhydride (MAH) functionalized impact modifiers work well for nylons, and there are many commercial products available, e. g., EPR-MAH, EVA-MAH, and SEBS-MAH. However, the reaction between maleic anhydride and polyesters is not fast enough for significant compatibilization in the timescales encountered during normal compounding. A functional group that reacts particularly well with polyesters is the monosubstituted oxirane, or epoxy, group such as is present in glycidyl methacrylate (GMA), glycidyl acrylate, allyl glycidyl ether, and 3,4-epoxy-1- butene (EpB).

The following patent documents describe polyester compositions which contain epoxy-containing, ethylene-based polymeric materials. U. S.

Patent 4,172,859 describes thermoplastic compositions comprising blends of polyesters and epoxy-functionalized, random ethylene copolymers. This patent makes no reference to catalyst residues present in the polyesters.

U. S. Patent 4,284,540 describes the use of ethylene/GMA copolymers as a toughening agent for polyesters when combined with 0.1 to 5 weight percent of an added barium catalyst. This patent also notes that ethylene/GMA copolymers increases the crystallization rate of PET and that PET containing antimony catalyst residues are preferable for promoting a reaction with epoxy-containing olefinic materials. The patent does not, however, provide any data that show any toughness enhancement due to these catalyst residues.

U. S. Patent 4,753,980 discloses that polyester compositions containing 3-40 weight percent of either ethylene/ethyl acrylate/GMA

terpolymer or ethylene/butyl acrylate/GMA terpolymer possess superior low temperature toughness when compared to analogous polyester compositions which contain an ethylene/methyl acrylate/GMA terpolymer.

There is no reference to catalyst residues in this patent. U. S. Patents 5,098,953,5,086,119,5,086,118,5,086,116, and 5,068,283 disclose that the toughness of polyester compositions containing ethylene/GMA copolymers or ethylene/alkyl acrylate/GMA terpolymers can be improved by adding a functional crosslinking agent to the compositions. The functional crosslinking agent contains, in one molecule, at least two functional groups having reactivity with epoxy group, carboxyl group or hydroxyl group.

U. S. Patent 5,206,291 describes compositions comprising a polyester containing residues an ethylene/GMA copolymer. Stewart et al., Polymer Engineering and Science, 33 (11), 675 (1993), discloses that PET containing antimony catalyst residues reacts faster with an ethylene/GMA copolymer than does PET catalyzed by other metals. U. S. Patent 5,436,296 discloses that an ethylene/GMA copolymer may be used to compatibilize blends of polyethylene and polyester. European Patent Publication EP 481,471 B1 and Penco et al., Journal of Applied Polymer Science, 57,329 (1995) disclose compositions comprising a polyester, a linear low density polyethylene, an ethylene/ethyl acrylate/GMA terpolymer and 0.5% to 1 % of an amine for the opening of an epoxy ring.

U. S. Patents 5,483,001,5,407,999, and 5,208,292 and Die Angewandte Makromolekulare Chemie, p. 89, disclose polyester compositions having improved toughness which contain an ethylene/alkyl acrylate/GMA terpolymer, an ethylene/alkyl acrylate/maleic anhydride terpolymer, and a catalyst such as dimethylstearylamine which accelerates the reaction between the functional groups of the two terpolymers. U. S.

Patent 5,652,306 and European Patent Publication EP 737,715 A2 disclose polyester compositions containing MBS or acrylic-type core-shell impact

modifiers combined with small amounts of an ethylene/alkyl acrylate/GMA terpolymer.

While several of the preceding patents discuss the use of added catalysts to promote a reaction between epoxy-containing ethylene polymers and a polyester, in none of these patents is there any disclosure that the toughness of the blend is affected by the presence of residues of catalysts used in the preparation of the polyester. Polyesters typically are prepared using metal catalysts that remain in the polyester product.

Examples of these catalysts include organic and inorganic compounds of arsenic, cobalt, tin, antimony, zinc, titanium, magnesium, gallium, germanium, sodium, lithium and the like. Titanium and antimony compounds are frequently used in the preparation of PET.

There is reference in U. S. Patent 4,284,540 and in Stewart et al.

Poly. Eng. & Sci., 33 (11), 675 (1993), that certain residual catalysts present in PET can significantly affect the rate of reaction of epoxy functional polymers with PET. U. S. 4,284,540 notes that, among the above mentioned residual polyester polymerization catalysts, antimony catalyst residues are preferred for promoting the reaction between polyester and an epoxy group. The patent does not, however, provide any toughness data related to the presence of these catalyst residues. Stewart et al. quantified the rate of reaction between PET and a copolymer of ethylene and glycidyl methacrylate (E/GMA) by monitoring the rise in torque of a mixture of these two components in an instrumented mixing bowl (also known as a torque rheometer). According to Stewart et al., torque rheometry provides a simple and straightforward method for monitoring the viscosity of polymer blends as a function of blending time. The rheometer continuously measures the torque required to turn the rotor blades that shear and mix the sample within the mixing bowl. For a given material and set of processing conditions, the torque measured is approximately a linear function of the viscosity of the sample. Any change in viscosity with time is

in turn related to such effects as changes in molecular weight of the sample (for example, an increase due to a reaction or a decrease due to degradation) or the formation of grafts, branches or crosslinks in the sample. The work by Stewart et al. shows that mixtures of E/GMA with PET containing antimony catalyst residues gives rise to significant increases in torque with mixing time. This led to the conclusion that PET containing antimony catalyst residues accelerated the reaction between the PET and the E/GMA. PET containing residual antimony catalyst was found to produce a more rapid increase in torque than PET containing other residual catalysts. It is often implied by those knowledgeable in the art that a rapid reaction between the E/GMA and the PET containing residual antimony catalyst should lead to better dispersion of the E/GMA (i. e., a smaller particle size) and better bond to the PET. This superior dispersion should, in turn, lead to improved toughness in the resultant blend.

Furthermore, since the PET containing residual antimony catalyst reacts faster than the PET containing other catalysts, it is implied that the use of PET containing antimony catalyst residues should lead to tougher blends with epoxy containing ethylene copolymers.

Contrary to the teachings of the prior art discussed above, we have discovered that the toughness of blends of polyesters with epoxy-containing ethylene polymers is strongly affected by the presence of residues of catalysts used in the manufacture of the polyesters. Furthermore, when epoxy-containing ethylene polymers are blended with polyesters containing antimony catalyst residues, the resulting polyester composition exhibits surprisingly low toughness values. Indeed, it has been found that superior toughness values are obtained when making blends using polyesters that do not contain antimony catalyst residues. Although we do not wish to be bound by any technical theories, it is believed that antimony catalyst residues present in the polyester accelerates an epoxy-epoxy reaction within the epoxy-containing ethylene copolymer. For example, subsequent

work using the same mixing bowl experiment as performed by Stewart et al. has shown that a similar increase in torque with time can be obtained when antimony acetate is added directly into an epoxy-containing copolymer with no PET present. The resultant epoxy-containing ethylene copolymer is highly crosslinked, suggesting that the catalyst is highly active in promoting reactions within the impact modifier itself. Transmission electron microscope images of blends of antimony-catalyzed PET with epoxy- containing ethylene copolymers show that the impact modifier has formed into large phases, many greater than 1 micron in size. These phases are too large to produce maximum toughness in the PET.

Brief Summary of the Invention It now has been discovered that, contrary to the teachings of the prior art, compositions comprising certain polyesters and certain epoxy- containing impact modifiers exhibit improved toughness when the polyester utilized contains no, or essentially no, antimony residues but rather contains titanium and/or germanium metal residues, e. g., residues resulting from the use of a titanium catalyst in the manufacture of the polyester. We have found that polyesters containing titanium and/or germanium residues are preferred to those containing antimony residues. The polyester compositions provided by the present invention comprise: I. about 75 to 98 weight percent of a thermoplastic polyester comprised of: (A) diacid residues comprising at least 85 mole percent terephthalic acid residues, 2,6-naphthalenedicarboxylic acid residues or a mixture of terephthalic acid and 2,6-naphtha- lenedicarboxylic acid residues; (B) diol residues comprising at least 85 mole percent ethylene glycol residues; and

(C) at least 25 parts per million by weight (ppmw) of a metal component selected from Ti, Ge or a mixture thereof; and II. about 25 to 2 weight percent of an impact modifying polymer comprised of about 0.5 to 20 weight percent of epoxy-containing residues derived from monomers selected from glycidyl methacrylate, glycidyl acrylate, allyl gycidyl ether, 3,4-epoxy-1- butene, or a mixture of any two or more of such monomers, wherein the weight percentages are based on the total weight of components I and 11. Stewart et al. Poly. Eng. & Sci., 33 (11), 675 (1993), disclose polyester compositions comprising 50/50 blends of (i) PET containing titanium residues and (ii) an ethylene/glycidyl methacrylate copolymer. These compositions of Stewart et al. actually contain more E/GMA than PET on a volume basis whereby they are very low modulus materials, suitable for torque rheometry studies but unsuitable for the types of applications prepared for by the manufacture of molded articles or extruded sheet or film material.

Detailed Description of the Invention The polyester component of our novel compositions are commercially available and/or may be prepared by batch or continuous processes using conventional melt phase or solid state condensation procedures well known in the art. Also, the polyester component may be obtained from post consumer waste, e. g., recycled polyester. Polyesters useful in the present invention are comprised of diacid residues comprising at least 85 mole percent terephthalic acid residues, 2,6-naphthalenedicar- boxylic acid residues or a mixture of terephthalic acid and 2,6-naphthalene- dicarboxylic acid residues; (B) diol residues comprising at least 85 mole percent ethylene glycol residues; and (C) at least 25 ppmw Ti, Ge or a mixture thereof; wherein the polyester is made up of 100 mole percent diacid residues and 100 mole percent diol residues. Up to 15 mole percent

of the diacid component of the polyesters may be derived from diacids other than terephthalic and 2,6-naphthalenedicarboxylic acid residues. For example, up to 15 mole percent of the diacid residues may be residues derived from dicarboxylic acids containing about 4 to about 40 carbon atoms such as succinic, glutaric, adipic, pimelic, suberic, azaleic, sebacic, terephthalic, isophthalic, sulfodibenzoic, sulphoisophthalic, maleic, fumaric, 1,4-cyclohexanedicarboxylic (cis-, trans-, or cis/trans mixtures), and the like.

The diacid residues may be derived from the dicarboxylic acids, esters and acid chlorides thereof, and, in some cases, anhydrides thereof.

Similarly, up to 15 mole percent of the diol residues may be derived from diols other than ethylene glycol. Examples of other diols which may be used in the preparation of the polyester component include those containing 3 to about 10 carbon atoms such as propylene glycol, 1,3- propanediol, 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, neopentyl glycol, 2,2,4,4-tetramethyl-1,3-cyclobutanediol, diethylene glycol, 1,4- cyclohexanedimethanol (cis, trans, or cis/trans mixtures) and the like.

Small amounts, e. g., up to 2 mole percent, of a branching agent such as trimellitic anhydride, pyromellitic dianhydride, glycerol, pentaerythritol, polyvinyl alcool, styrene-maleic anhydride (SMA) and the like may be included in the polyester if desired.

The polyester component of the compositions of the present invention should have an inherent viscosity (IV) in the range of about 0.4 to about 1.4 dUg, preferably about 0.55 to 0.95 dL/g, measured at 25°C using 0.50 g of polyester per 100 mL of a solvent consisting of 60 weight percent phenol and 40 weight percent tetrachloroethane. The polyester component preferably is unmodified PET having an IV of about 0.55 to 0.95 dL/g.

The polyester component of our novel compositions contains at least 25 ppmw of a metal selected from Ti, Ge or a mixture thereof, typically about 25 to 200 ppmw and preferably about 50 to 100 ppmw Ti, Ge or a mixture thereof. The metal preferably is titanium. The titanium and

germanium may be introduced to the polyester by using a titanium and/or germanium compound as a catalyst in the preparation of the polyester. It is also possible to introduce the titanium and/or germanium into the polyester subsequent to the preparation of the polyester. The polyester compositions of the present invention are substantially and essentially free of antimony, e. g., the polyester compositions contain less than 20 ppmw antimony, preferably no detectable antimony.

The second component of the compositions of the present invention is an impact modifying polymer comprised of about 0.5 to 15 weight percent of epoxy-containing residues derived from monomers selected from glycidyl methacrylate, glycidyl acrylate, allyl gycidyl ether, 3,4-epoxy-1-butene, or a mixture of any two or more of such monomers. These epoxy-containing monomers may be introduced into the impact modifier during polymerization, or they may be subsequently grafted onto the impact modifier. Such epoxy-containing, impact modifier polymers are well known in the art and are available from a plurality of manufactures.

Impact modifiers that may be modified with a functional epoxy group include, but are not restricted to, polyethylene; polypropylene ; polybutene; ethylene based copolymers and terpolymers containing vinyl acetate, alkyl acrylate, alkyl methacrylate where the alkyl group could be methyl, ethyl, butyl or ethylhexyl; ethylene-propylene copolymers (EPR); ethylene- propylene-diene (EPDM); natural rubber; polybutadiene; polyisoprene; acrylonitrile-butadiene (nitrile rubber); styrene-butadiene (SBR); styrene- butadiene-styrene (SBS); styrene-ethylene-butene-styrene (SEBS); acrylonitrile-butadiene-styrene (ABS); methyl methacrylate-butyl acrylate (acrylic core-shell); methyl methacrylate-butadiene-styrene (MBS core- shell); organic silicone rubbers; elastomeric fluorohydrocarbons; elastomeric polyesters; polyurethanes; or combinations thereof. Of these materials, those based on polyethylene are preferred.

Preferred epoxy-containing impact modifiers include copolymers and terpolymers having the respective general formulas E/Y and E/X/Y wherein: X represents residues derived from wherein R'is alkyl of up to about 8 carbon atoms, preferably alkyl of 1 to 4 carbon atoms, and R2 is hydrogen, methyl or ethyl, preferably hydrogen or methyl, and X constitutes about 10 to 40 weight percent, preferably 15 to 35 weight percent, and most preferably 20 to 35 weight percent, of terpolymer E/X/Y; Y represents residues derived from glycidyl methacrylate, glycidyl acrylate, allyl glycidyl ether or 3,4-epoxy-1-butene which constitute about 0.5 to 20 weight percent, preferably about 2 to 10 weight percent, of copolymer E/Y or terpolymer E/X/Y; and E represents ethylene residues that constitute the remainder of the copolymer or terpolymer composition.

Of these, copolymers based on ethylene-GMA (E/GMA) containing about 2 to 10 weight percent GMA residues, and terpolymers based on ethylene-methyl acrylate-GMA, ethylene-ethyl acrylate-GMA and ethylene-butyl acrylate-GMA containing about 20 to 35 weight percent alkyl acrylate residues and about 2 to 10 weight percent GMA residues are particularly preferred. The concentration of the epoxy- containing impact modifiers in the compositions of the present invention preferably is about 10 to 25 weight percent, based on the total weight of components I and 11, for injection molding uses and about 2 to 15 weight percent, based on the total weight of components I and 11, for compositions intended for extrusion into sheet or film or for use in thermoforming applications.

The impact modifier and polyester may be pellet blended before extrusion or they may be fed from separate streams. These reactive impact

modifiers also may be combined with non-reactive impact modifiers of similar composition. The polyester compositions of this invention can be readily prepared by conventional compounding technology, such as the use of single or twin screw extruders. The resultant blends are readily extruded into film or sheeting and injection molded, compression molded or thermoformed into desired shapes or objects.

Other additives normally used in polyesters such as stabilizers, antioxidants, pigments, colorants, plasticizers, flame retardants, mold release agents, slip agents and the like may be used as desired. Glass fibers or other inorganic fillers can also be included. Although not required, small amounts of nucleating agents such as polyethylene, polypropylene, talc and the like may be used.

A particularly preferred embodiment of the invention consists of a polyester composition comprising: I. about 75 to 98 weight percent of a thermoplastic polyester comprising poly (ethylene terephthalate) having an inherent viscosity of about 0.55 to 0.95 dUg, measured at 25°C using 0.50 g of polyester per 100 mL of a solvent consisting of 60 weight percent phenol and 40 weight percent tetrachloroethane, containing about 50 to 100 ppmw Ti; and II. about 25 to 2 weight percent of an impact modifying polymer selected from ethylene/methyl acrylate/glycidyl methacrylate copolymers containing about 20 to 35 weight percent methyl acrylate residues and about 2 to 10 weight percent glycidyl methacrylate residues. Alternatively, up to 50% of the amount of component 11 may consist of an ethylenelmethyl acrylate copolymer containing about 20 to 35 weight percent methyl acrylate residues.

Examples The novel polyester compositions provided by the present invention are further illustrated by the following examples wherein all percentages

given are by weight unless otherwise specified. The following polyesters, prepared using conventional polycondensation procedures and the catalyst specified, were used in the examples: Polyester A : Poly (ethylene terephthalate) having an IV of 0.70 prepared from ethylene glycol and terephthalic acid using 100 ppmw titanium catalyst provided as titanium tetraisopropoxide.

Polyester B: Poly (ethylene terephthalate) having an IV of 0.72 prepared from ethylene glycol and terephthalic acid using 200 ppmw antimony catalyst provided as antimony triacetate.

Impact Modified 1: An ethylene/methyl acrylate/glycidyl methacrylate terpolymer containing 24% methyl acrylate residues and 8% glycidyl methacrylate residues, available from Elf Atochem under the name"Lotader AX8900"and having a melt flow of 6.5 g/10 minutes measured by ASTM D1238 at 190°C using a 2.16 kg weight.

Impact Modifier 2: An ethylene/glycidyl methacrylate copolymer containing 8% glycidyl methacrylate residues, available from Elf Atochem under the name"Lotader AX8840"and having a melt flow of 5 g/10 minutes measured by ASTM D1238 at 190°C using a 2.16 kg weight.

The polyester compositions of the examples were prepared by feeding the polyester and impact modifier through separate feeders into the main hopper of a 30 mm twin-screw, compounding extruder. The extruder barrel temperatures was set at 290°C (554°F), screw speed was 300 revolutions per minute and the throughput rate was 11.4 kg (25 pounds) per hour. Test bars 3.2 mm thick by 12.8 mm wide by 125 mm long were molded on a ToyoA molding machine using a chitled-4°C (25°F) mold. The test bars thus prepared contained essentially amorphous polyester. The test bars then were crystallized in an air oven at 120°C for 3 hours. Test bars were machined into notched Izod test coupons and tested in accordance with ASTM D256.

Morphology was examined using either TEM (transmission electron microscopy) or SEM (scanning electron microscopy). Specimens for TEM examination were cut using a cryomicrotome. Specimens for SEM examination were freeze fractured in liquid nitrogen. The particle sizes listed represent the diameters of the most common large particles of the impact modifier phases present in the SEM or TEM photo.

EXAMPLE 1 The polyester composition of Example 1 consisted of 80% Polyester A containing about 100 ppmw titanium and 20% Impact Modifier 1. This composition has a 23°C (72°F) Notched Izod value of 616 Joules per meter (11.52 foot-pounds per inch). The TEM particle size of the impact modifier was 0.2 microns.

EXAMPLE 2 The polyester composition of Example 2 consisted of 80% Polyester A containing about 100 ppmw titanium and 20% Impact Modifier 2. This composition has a 23°C (72°F) Notched Izod value of"No Break". A"No Break"result indicates that the material did not completely fracture under the indicated conditions with an Izod value of greater than 1000 Joules per meter (53.5 foot-pounds per inch). The TEM particle size of the impact modifier was 0.2 microns.

COMPARATIVE EXAMPLE 1 The polyester composition of Comparative Example 1 consisted of 80% Polyester C containing about 200 ppmw antimony and 20% Impact Modifier 1. This composition has a 23°C (72°F) Notched Izod value of 287 Joules per meter (5.36 foot-pounds per inch). The TEM particle size of the impact modifier was 2 microns. The size of the impact modifier phases in

this blends is larger than that believed to produce optimal toughness in PET.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.