BLENDS OF POLYESTER COPOLYMER AND ETHYLENE VINYL ACETATE WITH IMPROVED GAS BARRIER PROPERTIES

Abstract: The present invention relates to polymer blends having improved gas barrier properties. The blends are a mixture of polyester copolymers (PETCOPs) and ethylene vinyl acetate (EVA) copolymers that are particularly useful in films requiring gas barrier properties. Bottle liners and closures prepared from the blends described herein have improved gas barrier properties relative to EVA copolymers alone. Films of the present invention may also be useful in a multilayer laminate structure wherein at least one layer comprising a blend of EVA/PETCOP.

Description: TITLE BLENDS OF POLYESTER COPOLYMER AND ETHYLENE VINYL ACETATE WITH IMPROVED GAS BARRIER PROPERTIES This application claims the benefit of U.S. Provisional Application No. 60/138,099, filed June 8, 1999. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to polymer blends having improved gas barrier properties. The present invention particularly relates to blends obtained from a mixture of polyester copolymers and ethyl ene vinyl acetate copolymers that are particularly useful in films or articles requiring gas barrier properties. Description of Related Art Ethylene vinyl acetate ("EVA") copolymers are useful resins for a multitude of purposes. EVA resins are sold by E.I. DuPont de Nemours and Company of Wilmington, Delaware. EVA resins have been used extensively in the preparation of cap liners for beverage containers, for example. Despite commercial success in a variety of applications, it is known that films or products made from EVA copolymers can have poor gas barrier and organoleptic properties. Polyester copolymers can have exceptional gas barrier properties. Polyester copolymers (PETCOPs) can be useful in packaging and other applications where gas permeation can be undesirable. For example, bottles for carbonated beverages. However, use of PETCOPs in certain applications can be undesirable, because films and/or articles prepared from PETCOPs can be hard or brittle. Use of PETCOPs in applications where a "soft" or resilient polymer is desirable can prove to be unsatisfactory. For example, in bottle cap liner and/or bottle closures, a soft polymer is desirable in order to obtain an efficient seal when the liner is compressed against the perimeter of an opening. Use of a PETCOP liner in this application can be significantly less effective than using a soft polymeric liner such as an EVA copolymer. The present inventor has found that blends of EVA copolymers and polyester copolymers, and films and/or articles made therefrom, have significantly improved gas barrier properties compared with EVA copolymers alone. The blends described herein can be useful in applications where soft polymers having low gas permeability are desirable, such as, for example, bottle cap liners. Blends of EVA copolymers and PETCOPs are known. For example, U.S. Pat. No. 4,271,063 ('063) describes a blend obtained from a polyester copolymer, filler, and from 0.1 to 25 % by weight of a copolymer of ethylene and vinyl acetate. The blends described therein are obtained by melt-blending the copolymers, followed by post-reaction of the resultant blend by heating the solid blend in a vacuum or under a stream of inert gas. However, polymers of the type described in '063 are not sufficiently soft polymers to be useful in the applications described herein because the continuous phase is the harder material (PETCOP). SUMMARY OF THE INVENTION In one aspect, the present invention is a polymeric blend comprising an ethylene-vinyl acetate (EVA) copolymer and a polyester copolymer (PETCOP), wherein EVA is included in an amount of greater than 25% by weight of the total weight of the copolymer composition. In another aspect, the present invention is a polymeric blend comprising an ethylene-vinyl acetate (EVA) copolymer and a polyester copolymer (PETCOP), wherein EVA and PETCOP are included in the blend at a weight ratio of from about 0.33:1 to about 5.67:1 EVA:PETCOP. In another aspect, the present invention is a film obtained from a polymeric blend, the blend comprising an EVA copolymer and a polyester copolymer, wherein EVA is included in an amount of greater than 25% by weight of the total weight of the copolymer composition, and wherein the film has increased gas barrier properties relative to a film obtained from EVA copolymers alone. In another aspect the present invention is an article obtained from a polymeric blend comprising an EVA copolymer and a polyester copolymer, wherein EVA is included in an amount of greater than about 25% by weight of the total weight of the copolymer composition, and wherein the article has increased gas barrier properties relative to an article prepared from an EVA copolymer alone. In still another aspect, the present invention is a method for preparing a film comprising an EVA copolymer and a polyester copolymer, wherein EVA is included in an amount of greater than 25% by weight of the total weight of the copolymer composition. Caps and other closures prepared from EV A/polyester copolymer (EVA/PETCOP) as a liner have improved gas barrier properties compared to that of EVA alone. Films of the present invention may also be useful in a multilayer laminate structure wherein at least one layer comprises a blend of EVA/PETCOP. DETAILED DESCRIPTION The present invention relates to a polymeric composition comprising an EVA copolymer and a polyester copolymer. EVA copolymers used in the practice of the present invention are prepared from ethylene and vinyl acetate. Ethylene vinyl acetate copolymers and copolymeric blends are known. Conventional methods can be used to prepare EVA copolymers useful herein. For example, U.S. Pat. No. 4,477,325 and U.S. Pat. No. 4,338,227 each describes various EVA copolymers, the method of preparation, and uses thereof. EVA copolymers useful herein are commercially available from DuPont. For the purposes of the present invention, the term "copolymer" includes any random, block, alternating, or graft copolymers. EVA copolymers useful in the practice of the present invention can include from about 2 to about 40 weight percent vinyl acetate, relative to the weight of ethylene therein. Preferably the weight percent of vinyl acetate is from about 5 to about 30. More preferably the weight percent of vinyl acetate is from about 7 to about 25, and is most preferably from about 7 to about 18 weight percent. The melt index ("MI") of EVA copolymers suitable for use in the present invention ranges from about 0.1 to about 20. MI is measured in accord with ASTM D-1238 condition E. Preferably the MI is in a range from about 0.1 to about 15; more preferably in a range of from about 0.2 to about 12. Most preferably, the MI is within a range of from about 0.3 to about 10. PETCOPs particularly useful in the practice of the present invention can be selected from the group consisting of: poly(ethylene terephthalate) (PET); poly(- butylene terephthalate) (PBT); poly (-propylene terephthalate) (PPT); poly( ethylene isophthalate (PEI), block and random copolyesters obtainable from these, and blends of any of these. Suitable comonomers that may be included in the polymer chain include monobasic and polybasic carboxylic acid and derivatives thereof, hydroxy carboxylic acids and derivatives thereof, monohydric and poyhydric phenols and derivatives thereof, monohydric and polyhydric aliphatic (including alicyclic) alcohols and derivatives thereof. Preferably, PETCOPs useful in the practice of the present invention are selected from the group consisting of copolymers of PET, PBT, and PPT that have a melting point of 240°C or lower. As used herein, the term polyester copolymers includes polyester homopolymers, and the terms can be used interchangeably. Conventional processes can be used to prepare PETCOPs useful in the practice of the present invention. Polyester copolymers can be prepared, for example, according to the process described in U.S. Patent Nos. 2,465,319 and 3,047,539 and 4,578,437 incorporated herein by reference Alternatively, polyester copolymers useful herein can be purchased commercially from, for example, E. I. DuPont de Nemours & Co. PETCOPs that are prepared according to presently unknown, or non-conventional processes are also contemplated to be useful in the practice of the present invention. Polyester copolymers suitable for use in the present invention can have intrinsic viscosity in the range of from about 0.4 to about 1.8 as determined by ASTM test method D2857. Preferably the viscosity of the polyester copolymer is in the range of from about 0.45 to about 1.2. More preferably the viscosity is in the range of from about 0.5to about 1.0, most preferably from about 0.5 to about 0.9. Polyester copolymers suitable for use in the present invention, preferably, are processable at melt temperatures below 250°C to avoid excessively degrading the EVA copolymer during blending. PETCOP useful herein can have intrinsic viscosity in the range of from about 0.4 to about 1.0. EVA PETCOP blends of the present invention are blends comprising greater than about 25 to about 85 weight percent EVA, relative to the total weight of the blend. Preferably, blends of the present invention comprise from about 30 to about 85 weight percent EVA, more preferably from about 40 to about 80 weight percent, and most preferably from about 50 to about 70 weight percent EVA. EVA/PETCOP blends of the present invention comprise from about 15 to about 75 weight percent PETCOP, relative to the total weight of the blend. Preferably, blends of the present invention comprise from about 15 to about 70 weight percent PETCOP, more preferably from about 20 to about 60 weight percent, and most preferably from about 25 to about 50 weight percent PETCOP. The EVA:PETCOP weight ratios of blends of the present invention are within a range of from about 0.33:1 to about 5.67:1. The compositions of this invention are prepared by melt blending a polyester copolymer and a copolymer of ethylene and vinyl acetate in a twin-screw extruder such as a Werner Ffleiderer™ extruder or other conventional plasticating devices such as a Brabender, Banbury mill, or the like. Before the melt blending step, the polyester must be pre-dried to remove moisture and avoid polymer degradation. The melt temperature at the extruder die is approximately in the range of 220-245°C. The film of polymer blend formed from the die is cooled onto a water-cooled chill roll. Alternatively, instead of film, extruded strands of the blend composition are formed and cooled under water and then pelletized. In addition to the two polymeric components described above, polymeric blends of the present invention can further comprise a third component that can act as a compatibilizer. A compatibilizer, for the purposes of the present invention, is any material that increases the stability of the phase morphology between EVA and PETCOP components claimed herein. While a compatibilizer is not required in the practice of the present invention, its inclusion can be preferred. Suitable compatibilizers are selected from the group consisting of: copolymers of ethylene and vinyl acetate; ethylene copolymer obtained from a copolymer of ethylene and an alpha, beta-unsaturated carboxylic acid which is optionally neutralized with metal cations; ethylene copolymers modified with unsaturated carboxylic acids or derivatives thereof, such as ethylene/maleic anhydride copolymers. Modified ethylene copolymers include polyethylenes and ethylene copolymerized with olefins, vinyl acetate, or acrylates. Ethylene/maleic anhydride copolymers sold under the name of Fusabond® by E. I. DuPont de Nemours and Company are examples of compatibilizers useful in the practice of the present invention. Mixtures of any of these are also useful in the practice of the present invention. Compatibilizers useful herein can be present in an amount from about 0 to about 25% weight percent, based on the weight of the total polymer blend. Preferably, the compatibilizer is present in an amount of from about 0 to about 25 weight percent, more preferably in an amount of from about 0 to about 15 weight percent. Most preferably, the compatibilizer is present in an amount of from about 0 to about 10 weight percent. Other components can optionally be included in blends of the present invention. For example, pigments or colorants, processing aids, antioxidants can optionally be added to blends described herein. The advantages of adding a particular optional component can depend upon the targeted application. Pigments can include, for example, titanium dioxide, carbon black and/or any other conventional pigment or pigment known in the art, whether conventional or non-conventional. Processing aids can include polyethylene waxes, fatty esters, fatty amides, and colloidal silica for example. Antioxidants can include, for example, phenolic compounds, phosphites, and thioesters. In the blends of the present invention, oxygen permeability can be reduced by as much as a factor of 40 relative to the unblended EVA copolymer. Copolymer blends of the present invention are those blends described herein having oxygen permeability values (OPV) of less than about 650 cc mil/100 in 2 /day/atm, as measured at 30°C and 50% RH (relative humidity). Preferably, the OPV of the blends described herein are less than about 300, more preferably less than about 100, and most preferably less than about 50 cc mil/100 in 2 /day/atm. Carbon dioxide permeability values (measured at 23°C under dry conditions) for polymers of the present invention are preferably less than about 900, more preferably less than about 400, most preferably less than about 250 cc mil/100 in /day/atm. In another embodiment, the present invention is a polymeric article obtained from the blends of the present invention. Polymeric articles of the present invention have oxygen permeability values that are significantly reduced compared with an article obtained from EVA copolymers that have not been blended according to the process of the present invention. Polymeric articles, for the purposes of the present invention, include any thing that is prepared from the claimed blends. Articles obtained from the blends described herein can include, for example: films, sheets, molded articles, laminate articles, cap liners, closures, and bottles. In choosing the most advantageous EVA:PETCOP weight ratio, consideration should be given to the "softness" of polymers obtained from the blended copolymers. Blends suitable for the uses intended herein have a Shore D hardness, as determined by ASTM D2240 of less than about 75. Shore D hardness values of less than about 65 are desirable in some applications. Shore D hardness values of less than about 55, and preferably less than 45 can be desirable in some applications. The "softness" of polymers can be also characterized by the temperature at which the polymer begins to deform. The blends have been characterized by Vicat softening temperature, as determined by ASTM D1525. EXAMPLES The following examples and comparative examples are presented for illustrative purposes only, and are not intended to limit the scope of the present invention. General Example Polymer blends are prepared by dry mixing pellets of EVA and polyester copolymer with, optionally, a compatibilizer in pellet form and subsequently melt compounding in a 28mm Werner and Pfleiderer™ twin-screw extruder with high- shear mixing, if required. The set temperatures across the barrel are 130°C- 180°C-200 o C-200 o C-200°C. The melt temperature of the polymer blend is typically between 220 to 245°C. The melt blend is extruded through a die forming a film ranging 2 to 11 mil in thickness. The components comprise ethylene vinyl acetate having a vinyl acetate content of 2 to 40% with a melt index of 0.1 to 20 dg/min at 190°C, polyester copolymer having a melting point below 240°C and an intrinsic viscosity of from 0.4 to 1.8 and an ionomer or modified ethylene vinyl acetate ethylene copolymer as a compatibilizer. Oxygen Permeability Value Determination The oxygen permeability value (OPV) is calculated in volume of oxygen (cubic centimeters) permeating through a sample area, 100 square inches with a thickness of 0.001 inch (mil), over a 24-hour period under an O partial pressure difference of one atmosphere at either 23°C or at 30°C, at a relative humidity of 50%. A MoCon OxTRAN-1000 instrument is used to measure the permeability of the film, according to ASTM D3985 standard method. The actual film measured for permeability varied between 4-11 mils in thickness. The oxygen permeability values of films with varying blend compositions are reported in Table 1 in units cc mil/100 in 2 /atm/day at specified temperatures. Carbon dioxide permeability is measured similarly on a MoCon Permatran-C 4/40 instrument at 23°C under dry conditions. The film samples are placed under vacuum to remove moisture before measuring carbon dioxide transmission rates. Vicat Softening Temperature Determination Vicat Softening Temperature is determined according to ASTM D1525 standard method. Shore D Hardness Shore D hardness is determined according to ASTM D2240 standard method. Example 1 - 9 EVA and copolyester pellets are blended in a twin screw extruder according to the procedure described above in the General Example. The copolyester is dried overnight for approximately 16 hours before extruder blending. Typical drying temperature is about 50°C, however the precise temperature is not critical. The polymer is cast into a 4-mil film and evaluated for OPV and carbon dioxide permeability. The results are reported in Table 1. Examples 1 - 20 Ethylene vinyl acetate and copolyester polymers are blended in the proportions set out in Table 1, according to the procedure of the General Example. The O 2 and CO 2 permeability values are determined, and the results are reported in Table 1. Table 1 *100% Elvax®: OPV @ 23°C/50%RH = 870 ccmil lOOin-day; "Not an example of the present invention. EVA= Elvax® 313 5 χ, Copolyester=Crystar® 3991 , S=Surlyn® 1706, E=Elvax® 3165; Fl=Fusabond® 100D=HDPE grafted maleic anhydπde, F2=Fusabond® 2 5 0D=EVA grafted maleic anhydπde

Claims: WHAT IS CLAIMED IS: 1. A polymeric blend comprising an ethylene-vinyl acetate (EVA) copolymer and a polyester copolymer PETCOP, wherein EVA is included in an amount of greater than 25% to about 85% by weight of the total weight of the copolymer composition. 2. The blend of Claim 1 wherein EVA is included in an amount of from about 30 to about 85 percent by weight. 3. The blend of Claim 2 wherein EVA is included in an amount of from about 40 to about 80 percent by weight. 4. The blend of Claim 3 wherein EVA is included in an amount of from about 50 to about 70 percent by weight. 5. The blend of Claim 1 wherein the ratio of EVA:PETCOP is from about 0.5:1 to about 20:1. 6. The blend of Claim 1 wherein a compatibilizer is included in an amount of up to about 25 percent by weight of the total blend. 7. The blend of Claim 6 wherein the compatibilizer is included in an amount of from about 0.01 to about 20 percent by weight. 8. The blend of Claim 7 wherein the compatibilizer is included in an amount of from about 0.01 to about 15 percent by weight. 9. The blend of Claim 8 wherein the compatibilizer is included in an amount of from about 0.01 to about 10 percent by weight. 10. The blend of Claim 9 wherein the compatibilizer is selected from the group consisting of: copolymers of ethylene and modified vinyl acetate; ethylene copolymers obtained from a copolymer of ethylene and an alpha, beta-unsaturated carboxylic acid which is optionally neutralized with metal cations. 11. The blend of Claim 10 wherein the compatibilizer is selected from the group consisting of: ethylene copolymers obtained from a copolymer of ethylene and an alpha, beta-unsaturated carboxylic acid, or derivative thereof, which is optionally neutralized with metal cations . 12. The blend of Claim 11 wherein the compatibilizer is a copolymer of ethylene and vinyl acetate. 13. An article obtained from a polymeric blend of any one of the preceding Claims, wherein the article has increased gas barrier properties relative to an article obtained from EVA copolymers alone. 14. The article of Claim 13 having an OPV at 30°C/50%RH of less than about 650. 15. The article of Claim 14 having an OPV at 30°C/50%RH of less than about 300. 16. The article of Claim 15 having an OPV at 30°C/50%RH of less than about 100. 17. The article of Claim 16 having an OPV at 30C/50%RH of less than about 50. 18. The article of Claim 14 wherein the article has a Shore D hardness of less than about 75. 19. The article of Claim 18 wherein the article has a Shore DHardness of less than about 65. 20. The article of Claim 14 wherein the article has a Shore D Hardness of less than about 55. 21. The article of Claim 14 wherein the article has a Shore D Hardness of less than about 45. 22. The article of Claim 14 wherein the article is a cap liner, 23. A method for preparing a film comprising an EVA copolymer and a polyester copolymer, wherein EVA is included in an amount of greater than 25% to about 85% by weight of the total weight of the copolymer composition . 24. A polymeric blend comprising an ethylene-vinyl acetate (EVA) copolymer and a polyester copolymer (PETCOP), wherein EVA and PETCOP are included in the blend at a weight ratio of from about 0.33:1 to about 5.67:1 EVA:PETCOP. 25. The article of Claim 14 wherein the article has a Vicat Softening Temperature of at least about 50°C.

Inventors: KAPUR JANE, VISIOLI DONNA LYNN, KAPUR, JANE, VISIOLI, DONNA, LYNN
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 2000
Main classification: C08L23
IPC codes: B65D53, C08J5, C08L23, C08L51, C08L67