Polyester Containers Having Improved Gas Barrier Properties

The present invention relates to packages for protecting comestibles and to the polyesters from which such packages are made. More specifically, it relates to films and molded containers which are formed from such polyesters.

Presently there is a strong interest in the packaging industry for protecting comestibles, such as foodstuffs, medicines, and especially carbonated beverages, by enveloping the substances in packages which are formed from various polymers. One polymer in which there is an especially strong interest is polyethylene terephthalate. Containers formed from this material, which may be biaxially oriented, possess many desirable characteristics. Molded biaxially oriented containers which are formed from polyethylene terephthalate and certain copolyesters are disclosed in U.S. Pat. No. 3,733,309. While molded containers formed from polyethylene terephthalate have, as indicated, many desirable characteristics, there is a need in the art to provide improved polyester containers which have gas permeabilities which are lower than those of containers formed from polyethylene terephthalate. Such improved containers would be much more versatile in their utilization and allow the containers to be used to package substances for which the polyethylene terephthalate containers may not be suitable.

The present invention provides a novel polyester which is suitable for use in forming packages for protecting comestibles. The present invention also provides improved containers made of the polyester which exhibit improved resistance to gas permeability. The polyester of the present invention comprises a diacid component comprising [thiobis(p-phenyleneoxy)]diacetic acid, [sulfonylbis(p-phenyleneoxy)]diacetic acid, and mixtures thereof. Containers formed from the novel polyester exhibit a desirable balance of physical properties, including improved gas barrier properties, which render them advantageous for use in the packaging of comestibles.

U.S. Pat. No. 4,110,315 discloses a molded container which is formed from a linear copolyester comprising units derived from bis(4-.beta.-hydroxyethoxyphenyl) sulfone. The polyester further comprises units derived from isophthalic acid, optionally terephthalic acid, and ethylene glycol. It is disclosed that packages formed from these polyesters exhibit lower carbon dioxide and oxygen permeabilities than polyethylene terephthalate and a higher glass transition temperature than polyethylene terephthalate. The disclosure of this patent differs from the present invention in several important respects. Firstly, the composition of the reference requires the presence of isophthalic acid, while the polyester of the present invention is based upon terephthalic acid. Furthermore, the first diacid employed in the present invention contains the unique moiety represented by the formula ##STR1## the presence of which is critical to attaining the advantages provided by the present invention. The combination of polyester-forming moieties provided by the present invention is neither disclosed nor suggested by the prior art.

In one aspect of the present invention, there is provided a novel polyester which comprises the reaction product of a diol containing up to about 8 carbon atoms and a diacid component. The diacid component comprises about 5 to 60 mole percent of a first diacid selected from [thiobis(p-phenyleneoxy)]diacetic acid, [sulfonylbis(p-phenyleneoxy)]diacetic acid, and mixtures thereof, and further comprises about 40 to 95 mole percent of terephthalic acid. The polyester exhibits an inherent viscosity of about 0.5 to 1.5.

In another aspect, the present invention provides containers having improved resistance to gas permeability. The containers are formed from the novel polyester of the present invention.

The invention described herein relates to a novel polyester and to containers formed therefrom. As used herein, the term "container" is intended to mean shaped articles formed from the specified polyester which are suitable for use in packaging comestibles. Such shaped articles include sheets and films which are extruded from the polyester and which may be used as such or as barrier layers, container liners, as components of laminates or other composites, etc. Such shaped articles also include molded containers, such as bottles.

The polyester of the present invention comprises the reaction product of a diol component and a diacid component. The diol component comprises one or more diols containing up to about 8 carbon atoms. Examples of such diols include ethylene glycol; 1,4-butanediol; 1,4-cyclohexanedimethanol; propylene glycol; diethylene glycol; 2,2,4-trimethyl-1,3-pentanediol; 2,2-dimethyl-1,3-propanediol; 2,2,-diethyl-1,3-propanediol; 2-methyl-2-propyl-1,3-propanediol; 1,3-butanediol; 1,5-pentanediol; 1,6-hexanediol; 1,2-cyclohexanedimethanol; 1,3-cyclohexanedimethanol; 2,2,4,4-tetramethyl-1,3-cyclobutanediol; o-, m-, and p-xylylene diols; etc. The diol component is preferably selected from ethylene glycol, 1,4-butanediol, and 1,4-cyclohexanedimethanol, and is more preferably selected from ethylene glycol and 1,4-butanediol. Most preferably, the diol component comprises ethylene glycol. Additionally, minor amounts (i.e., less than about 50 mole percent, and, preferably, less than about 10 mole percent, based on the total amount of diol present in the polyester) of other known polyester-forming glycols may also be employed. Such diols may include, for example, 2,4-dimethyl-2-ethylhexane-1,3-diol; 2-ethyl-2-butyl-1,3-propanediol; 2-ethyl-2-isobutyl-1,3-propanediol; 2,2,4-trimethyl-1,6-hexanediol; 4,4'-sulfonyldiphenol; 4,4'-oxydiphenol; 4,4'-isopropylidenediphenol; 2,5-naphthalenediol; and other well known polyester-forming diols.

The polyester resin further comprises a diacid component. The diacid component comprises a first diacid selected from [thiobis(p-phenyleneoxy)]diacetic acid, [sulfonylbis(p-phenyleneoxy)]diacetic acid, and mixtures thereof. The diacid component further comprises terephthalic acid. Other well-known polyesterforming diacids may be employed in minor amounts. Such diacids may include, for example, isophthalic acid, 1,4-cyclohexanedicarboxylic acid, linear lower alkyl dicarboxylic acids, etc. The term "diacid", of course, is intended also to include moieties derived from acid halides and esters, particularly the lower alkyl (C.sub.1 -C.sub.4) esters of the acids mentioned.

The first diacid, which is selected from [thiobis(p-phenyleneoxy)]diacetic acid (Formula I), [sulfonylbis(p-phenyleneoxy)]diacetic acid (Formula II), and mixtures thereof, is present in the polyester in a concentration of about 5 to 60 mole percent, based upon the total amount of diacid present in the polyester. Preferably, the first diacid is present in a concentration of about 5 to 50 mole percent, and , more preferably, in a concentration of about 5 to 20 mole percent. ##STR2##

It has been observed that the gas barrier properties of the polyester of the present invention improve as the concentration of the thio- and/or sulfonylcontaining component of the polyester is increased. However, at higher concentrations, the rate of improvement in properties diminishes; that is, at high concentrations of the first diacid in the polyester of the present invention (i.e., above about 60 mole percent), no significant improvement in properties is obtained over those exhibited by polyesters containing moderate amounts (for example, about 50 to 60 mole percent ) of the first diacid.

The diacid component further comprises 40 to about 95 mole percent of terephthalic acid. More preferably, the diacid component comprises about 50 to 95 (e.g., about 80 to 95) mole percent of terephthalic acid.

When a molded container, such as a bottle, is to be formed from the polyester of the present invention, the first diacid is preferably present in an amount of about 5 to 20 mole percent and, more preferably, is present in an amount of about 10 to 20 mole percent. At higher concentrations of the first diacid, losses in crystallinity may be observed.

The polyester which is employed in the present invention preferably exhibits an I.V. of about 0.5 to 1.5. The I.V. is measured at 25.degree. C. in a 60/40 by weight mixture of phenol/tetrachloroethane at a concentration of 0.5 g/100 ml. Polymers having an I.V. within the range specified above are of sufficiently high molecular weight to be used in the formation of the containers of the present invention.

The polyesters of the present invention are synthesized by methods generally known in the art for producing polyesters. The reactants may all be reacted in a single operation or, alternatively, sequential operation may be employed. Temperatures which are suitable for forming the polyesters generally range between about 180.degree. C. and about 295.degree. C., with the preferred range being about 200.degree. to 285.degree. C. The reaction may be conducted under an inert atmosphere, such as nitrogen. Preferably, the latter stage of the reaction is conducted under a vacuum.

Conventional catalysts are employed in the preparation of the polyester. For example, polymerization can be effected in the presence of catalytic amounts of transition metal compounds, such as antimony acetate, antimony trioxide, titanium alkoxides, and organo tin compounds (for example, stannous alkoxides). Preferred catalysts are titanium alkoxides, such as titanium tetraisopropoxide, titanium tetrabutoxides, etc. Usually, the catalysts will be present in an amount of about 10.sup.-5 to 10.sup.-3 moles of catalyst per mole of total acid employed.

Of course, suitable additives, such as dyes, pigments, plasticizers, fillers, antioxidants, stabilizers, etc., may be employed in conventional amounts. Such additives may be added directly to the reaction or may be added to the final polymer.

The polyesters described above may be formed into the containers of the present invention by conventional plastic processing techniques. For example, sheets, films, and other like structures can be formed by well-known extrusion techniques.

Film or sheet material made from the compositions of the present invention is strong, flexible, and clear. It may be formed into articles such as wrappers, bags, and the like.

The polyesters may also be used to form a laminating layer between two or more permeable layers of film. The polyesters may also be used for dip coating containers from a polymer solution in order to improve the barrier properties of a package. In each of these embodiments, the present invention offers the additional advantage of not requiring the use of a tie layer.

Molded containers can be made from the above-described polyester by compression molding, blow molding, and other such molding techniques, all of which are well known in the art. Preferred molded containers are biaxially oriented blow-molded containers.

The above-described containers of the present invention are ideally suited for protecting comestibles, such as foodstuffs (especially carbonated soft drinks), medicines, and like substances. The advantages of these containers are due to low gas permeability relative to polyethylene terephthalate. Because of the decreased gas transmission rates of these polyesters, they perform well in packaging applications where improved gas barrier properties are required. Typically, the containers of the present invention exhibit an oxygen permeability of less than about 7.5 (preferably, less than about 6), measured as cubic centimeters permeating a 1 mil thick sample, 100 inches square, for a 24 hour period under a partial pressure difference of 1 atmosphere at 30.degree. C.

This invention will be further illustrated by the following Examples, although it will be understood that these Examples are included merely for purposes of illustration and are not intended to limit the scope of the invention.

This Example illustrates the improved gas barrier properties of containers formed of polyethylene terephthalate which has been modified with 10 mole percent of [thiobis(p-phenyleneoxy)]diacetic acid.

A reaction vessel was charged with 20.91 g of dimethyl terephthalate, 22.26 g of ethylene glycol, 4.0 g (10 mole percent) of [thiobis(p-phenyleneoxy)]-diacetic acid, and 100 ppm of titanium from titanium tetraisopropoxide. The reaction mixture was heated and stirred under nitrogen at 200.degree. C. for 60 minutes. The temperature was then increased to 210.degree. C. for 120 minutes until all of the methanol had distilled out of the reaction mixture. The temperature was then raised to 275.degree. C., the nitrogen was evacuated from the reaction system, and a vacuum was applied. the melt condensation was continued at 275.degree. C. for 60 minutes under 0.5 mm Hg pressure. The heating was discontinued, ;the reaction mixture was brought to atmospheric pressure with nitrogen, and the polymer was collected. The polymer exhibited an I.V. of 0.66.

The oxygen permeability of the polyester was determined in cubic centimeters permeating a 1 mil thick sample, 10 inches square, for a 24-hour period under an oxygen partial pressure difference of one atmosphere at 30.degree. C. using a MOCON Oxtran 100 instrument. The polyester was extruded into amorphous (i.e., unoriented) film using a Brabender extruder at 240.degree.-260.degree. C. The film actually used to measure permeability was 3-8 mils in thickness, but the permeability was converted to a one mil basis using conventional calculations. The glass transition temperature (Tg) of the polymer was also determined.

The results are given in Table I.

Example 1 was repeated except that [thiobis(p-phenyleneoxy)]diacetic acid was replaced with [sulfonylbis(p-phenyleneoxy)]diacetic acid. The polymer exhibited an I.V. of 0.82. The oxygen permeability and glass transition temperature of the polyester were determined as in Example 1, and the results are given in Table I.

Example 1 was repeated except that [thiobis(p-phenyleneoxy)]diacetic acid was replaced with 10 mole percent of bis(4-beta-hydroxyethoxyphenyl)sulfone. The oxygen permeability and glass transition temperature of the polyester were determined as in Example 1, and the results are given in Table I.

Example 1 was repeated except that [thiobis(p-phenyleneoxy)]diacetic acid was replaced with 4,4'-sulfonylbisbenzoic acid. The oxygen permeability and glass transition temperature of the polyester were determined as in Example 1, and the results are given in Table I.

Example 1 was repeated except that a polyethylene terephthalate control was employed. The oxygen permeability and glass transition temperature were determined as in Example 1, and the results are given in Table I.

An analysis of the data of Table I indicates the superior properties exhibited by the polyesters of the present invention which make them extremely advantageous for use in the formation of containers. It will be noted that the compositions described in Examples 1 and 2 exhibit oxygen permeabilities below those exhibited by polyethylene terephthalate as exemplified by Comparative Example 3.

The uniqueness of the polyesters of the present invention and of the properties exhibited by containers formed therefrom is indicated by a comparison of the Examples with Comparative Examples 1 and 2, which employ diacids suggested by the prior art which are structurally similar to the diacids employed as the first diacid in the composition of the present invention. The polyesters of Comparative Examples 1 and 2 exhibit poor resistance to gas permeability, whereas the compositions of the Examples of the present invention demonstrate a much improved resistance to gas permeability, as compared to polyethylene terephthalate.

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