WO2003061956A2

WO2003061956A2 - Method for improving caustic stress-crack resistance of polyester containers

Info

Publication number: WO2003061956A2
Application number: PCT/US2003/002086
Authority: WO
Inventors: Edwin Sisson; Freddie Massey; Robert Howell
Original assignee: Mossi & Ghilsophi Polimeri Italia S.P.A.
Priority date: 2002-01-24
Filing date: 2003-01-24
Publication date: 2003-07-31

Abstract

A method of improving the caustic stress-crack resistance of polyester containers for caustic applications comprises mixing between about 0.1 weight percent and about 99.9 weight percent of a nylon, with about 0.1 weight percent and about 99.9 weight percent of a polyester, wherein said percentages are based upon the combined weight of the polyester and nylon, to form a blend, and forming a container from the blend. Polyester made according to this invention has improved resistance to alkaline processing fluids, such as line lubricants and/or hydraulic fluids. A method of storing an alkaline product comprises placing an alkaline composition in a container, wherein the container comprises a blend of between about 0.1 weight percent and about 99.9 weight percent of a nylon and between about 0.1 weight percent and about 99.9 weight percent of a polyester, wherein said percentages are based upon the combined weight of the polyester and nylon.

Description

METHOD FOR IMPROVING CAUSTIC STRESS-CRACK RESISTANCE

OF POLYESTER CONTAINERS

BACKGROUND OF THE INVENTION

This invention relates to a method of improving resistance of polyester containers to caustic and other high stress materials. More particularly, this invention relates to improving environmental stress-crack resistance of polyester containers. Specifically, this invention relates to addition of nylon to polyester resins to improve caustic stress-crack (CSC) resistance.

Stress cracking may occur when a plastic is placed under tensile stress, such as that caused by carbonation of a beverage in a bottle. Polyesters are typically used in such bottles and are generally strongest in those areas where the polyester is in a highly ordered state. Conversely, they are weakest in an amorphous state, such as in areas of the base.

Small, microscopic or even visible cracks are not unusual in polyester bottles. However, if large enough, these cracks may cause the failure of the container. Stress crack resistance can be harmed by a number of factors, including exposure to heat, sunlight, or alkaline or other incompatible solutions. Caustic stress-crack resistance can be an important measure of overall strength and strength against environmental stresses, even for containers not used for alkaline products, but where, for example, the outside of a container may be accidentally or intentionally contacted by an alkaline solution such as a cleaning solution. Polyester bottles or bottle preforms may also come in contact with caustic substances during manufacture such as line lubricants. CSC resistance is an important product parameter for soft drink manufacturers, even though most soft drinks are actually acidic. A minimum requirement for caustic stress-crack mean failure time for a supplier of carbonated soft drink (CSD) containers is 2.3 hours. Because better stress-crack performance is associated with longer mean failure time, the higher the mean failure time and the fewer bottles failing during testing, the more acceptable the resin is to a typical consumer.

No polyester or nylon-modified polyester bottles are currently used in packaging containers that require use of materials with high caustic stress-crack resistance . Until the present invention, the use of polyester containers for storage of caustic materials (liquids) has been limited by the fact that the resistance of polyester containers to stress-cracking has not been acceptable. Examples of typical caustic materials are cleaning solutions, detergents, industrial additives, hydraulic fluids, pharmaceuticals and the like. It is known to add a small amount of nylon to polyester resins to form carbonated beverage containers in order to contain carbonation. However, it has not been recognized in the art that the addition of small amounts of nylon to polyester resins will significantly increase CSC resistance.

BRIEF SUMMARY OF THE INVENTION

Addition of nylon to polyester according to the present invention, has unexpectedly resulted in significant improvement in CSC average failure time and a dramatic reduction in number of bottles failing CSC testing. This improves the potential for use of polyesters in container applications requiring better stress- crack resistance than that offered by standard polyester containers.

It is, therefore an aspect of the present invention to provide a method for improving stress-crack resistance in polyester containers, especially those used in high caustic applications. It is another aspect of the present invention to improve stress-crack resistance in polyester containers by blending small amounts of nylon with polyester resin.

At least one or more of the foregoing aspects, together with the advantages thereof over the known art relating to polyester containers, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.

In general, the present invention provides a method of improving the caustic stress-crack resistance of polyester containers, the method comprising: mixing between about 0.1 weight percent and about 99.9 weight percent of a nylon, with about 0.1 weight percent and about 99.9 weight percent of a polyester, wherein said percentages are based upon the combined weight of the polyester and nylon, to form a blend; and forming a container from the blend.

It also provides a method of storing a caustic composition comprising placing an alkaline composition in a container, wherein the container comprises a blend of between about 0.1 weight percent and about 99.9 weight percent of a nylon and between about 0.1 weight percent and about 99.9 weight percent of a polyester, wherein said percentages are based upon the combined weight of the polyester and nylon.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, CSC resistance of a polyester is improved by adding a nylon to the polyester. The polyester may be any polymer containing repeating ester units.

The polyester polymers and copolymers may be prepared by melt phase polymerization involving the reaction of a diol with a dicarboxylic acid, or its corresponding diester. Various copolymers resulting from use of multiple diols and diacids may also be used. Polymers containing repeating units of only one chemical composition are homopolymers. Polymers with two or more chemically different repeat units in the same macromolecule are termed copolymers. The diversity of the repeat units depends on the number of different types of monomers present in the initial polymerization reaction. In the case of polyesters, copolymers include reacting one or more diols with a diacid or multiple diacids, and are sometimes referred to as terpolymers. Suitable dicarboxylic acids include those comprising from about 6 to about 40 carbon atoms. Specific dicarboxylic acids include, but are not limited to, terephthalic acid, isophthalic acid, naphthalene 2,6-dicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, diphenyl-4,4'-dicarboxylic acid, 1,3-phenylenedioxydiacetic acid, 1,2-phenylenedioxydiacetic acid, 1,4- phenylenedioxydiacetic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, and the like. Specific esters include, but are not limited to, phthalic esters and naphthalic diesters.

These acids or esters may be reacted with an aliphatic diol having from about 2 to about 10 carbon atoms, a cyclo aliphatic diol having from about 7 to about 14 carbon atoms, an aromatic diol having from about 6 to about 15 carbon atoms, or a glycol ether having from 4 to 10 carbon atoms. Suitable diols include, but are not limited to, 1,4-butenediol, trimethylene glycol, 1,6- hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, resorcinol, and hydroquinone.

Polyfunctional comonomers can also be used, typically in amounts of from about 0.1 to about 3 mole percent. Suitable comonomers include, but are not limited to, trimellitic anhydride, trimethylopropane, pyromellitic dianhydride (PMDA), and pentaerythritol. Polyester-forming polyacids or polyols can also be used.

One preferred polyester is polyethylene terephthalate (PET) formed from the approximate 1 : 1 stoichiometric reaction of terephthalic acid, or its ester, with ethylene glycol. Another preferred polyester is polyethylene naphthalate (PEN) formed from the approximate 1:1 to 1:1.6 stoichiometric reaction of naphthalene dicarboxylic acid, dr its ester, with ethylene glycol. Yet another preferred polyester is polybutylene terephthalate (PBT). Copolymers of PET, copolymers of PEN, and copolymers of PBT are also preferred. Specific co- and terpolymers of interest are PET with combinations of isophthalic acid or its diester, 2,6 naphthalic acid or its diester, and/or cyclohexane dimethanol. The esterification or polycondensation reaction of the carboxylic acid or ester with glycol typically takes place in the presence of a catalyst. Suitable catalysts include, but are not limited to, antimony oxide, antimony triacetate, antimony ethylene glycolate, organomagnesium, tin oxide, titanium alkoxides, dibutyl tin dilaurate, and germanium oxide. These catalysts may be used in combination with zinc, manganese, or magnesium acetates or benzoates. Catalysts comprising antimony are preferred.

Another preferred polyester is polytrimethylene terephthalate (PTT) . It can be prepared by, for example, reacting 1,3-propanediol with at least one aromatic diacid or alkyl ester thereof. Preferred diacids and alkyl esters include terephthalic acid (TPA) or dimethyl terephthalate (DMT). Accordingly, the PTT preferably comprises at least about 80 mole percent of either TPA or DMT. Other diols which may be copolymerized in such a polyester include, for example, ethylene glycol, diethylene glycol, 1,4-cyclohexane dimethanol, and 1,4- butanediol. Aromatic and aliphatic acids which may be used simultaneously to make a copolymer include, for example, isophthalic acid and sebacic acid.

Preferred catalysts for preparing PTT include titanium and zirconium compounds. Suitable catalytic titanium compounds include, but are not limited to, titanium alkylates and their derivatives, titanium complex salts, titanium complexes with hydroxycarboxylic acids, titanium dioxide-silicon dioxide-co-precipitates, and hydrated alkaline-containing titanium dioxide. Specific examples include tetra-(2-ethylhexyl) -titanate, tetrastearyl titanate, diisopropoxy-bis (acetyl-acetonato) -titanium, di-n-butoxy-bis (triethanolaminato) - titanium, tributylmonoacetyltitanate, triisopropyl monoacetyltitanate, tetrabenzoic acid titanate, alkali titanium oxalates and malonates, potassium hexafluorotitanate, and titanium complexes with tartaric acid, citric acid or lactic acid. Preferred catalytic titanium compounds are titanium tetrabutylate and titanium tetraisopropylate. The corresponding zirconium compounds may also be used. The melt phase polymerization described above may be followed by a crystallization step, then a solid phase polymerization (SSP) step to achieve the intrinsic viscosity necessary for bottle manufacture. The crystallization and polymerization can be performed in a tumbler dryer reaction in a batch-type system. Alternatively, the crystallization and polymerization can be accomplished in a continuous solid state process whereby the polymer flows from one vessel to another after its predetermined treatment in each vessel.

The crystallization conditions preferably include a temperature of from about 100 °C to about 150 °C. The solid phase polymerization conditions preferably include a temperature of from about 200 ° C to about 232 ° C, and more preferably from about 215 °C to about 232 °C. The solid phase polymerization may be carried out for a time sufficient to raise the intrinsic viscosity to the desired level, which will depend upon the application. For a typical bottle application, the preferred intrinsic viscosity is from about 0.65 to about 1.0 deciliter/gram, as determined by ASTM D-4603-86 at 30° C in a 60/40 by weight mixture of phenol and tetrachloroethane. The time required to reach this viscosity may range from about 8 to about 21 hours.

Other acceptable polyesters include those represented by formula

I below,

n where,

Z = a bond, -CH2-, -[(O-CH2)b-CH₂]_c-0-, -CO-, -COO-, -OCOO-, -CH=CH-, -

CHR'-CHR"-, -0(-CH2)b-Cycloalkyl-[(-CH2)b-O]c-, -(CH2)b-Cycloalkyl-(CH₂)b-,

-S-, -SO2-, or -NHCO-;

R= -(CH₂)_m-CH₂-, -[(CH₂)_m-CH2-O]p-CH₂-CH2- , -CH₂-CR'R"-CH₂-,

-Rl-Ar-R2-, or -Rl-Cycloalkyl-R2-; Ri and R₂ are independently -(CH₂-)_m, -(CH₂-O-)_m, or -[(CH₂)_m-CH₂-O]p;

R and R" are independentiy -H, -CH3, -(CK^m-Cr^-R*, -[(CH2)_m-CH2-0]_p-

CH2-CH2-R*, -CH2-CR3R4-CH2-R*, -Rι-Ar-R2-R* or other C1-C20 alkyl, arylalkyl, O- or S-substituted arylalkyl groups;

R* is -H or -OH; R3 and R4 are independently -H, -CH3, -(CH2)_m-CH₂-R*, or -[(CH2)_m-CH2-O]_p-

CH2-CH2-R*; b and c are independently between 0 and about 20; n is between about 100 and about 205; and m and p are independently between about 1 and about 20.

The polyester may be synthesized from polyester precursors according to methods known in the art. Acceptable precursors include precursors of polyesters and polycarbonates, and precursors of co-polymers containing other aliphatic and/or aromatic acids and esters thereof, and aliphatic or aromatic polyhydric alcohols. Specific examples of acceptable precursors include those of homopolymers and co-polymers of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyethylene isophthalate (PEI). Other acceptable co-monomer examples include those represented by formula I as described above with the exception of n being between about 40 and about 120. In order to improve the CSC resistance of polyester containers, nylon resin is added to the polyester to form a blend, which is then extruded into a suitable form, such as a pre-f orm for the manufacture of melt-blown containers . Suitable nylons include aliphatic nylons. These include A-B type nylons, such as nylon 6, nylon 11 and nylon 12, and A-A/B-B type nylons such as nylon 2,10, nylon 4,6, nylon 4,10, nylon 5,11, nylon 6,2, nylon 6,3, nylon 6,5, nylon 6,6, nylon 6,7, nylon 6,9, nylon 6,10, nylon 6,12, nylon 7,4, nylon 7,6, nylon 7,8, nylon 8,7, nylon 8,12, nylon 9,9, nylon 10,10, and nylon 10,20. Nylon 6,6, nylon 4,6, nylon 6, and poly (metα-xylylene diamine adipate) (MXD6) are particularly suitable. Mitusbishi Gas Chemical is a provider of MXD6 nylon, which has been used to exemplify the present invention. Amounts of nylon that can be added range from about 0.1 to about 99.9 percent by weight, based upon the combined weight of polyester and nylon resins. In some applications, the amount of nylon may be minimized in order to minimize cost while maintaining improved CSC resistance. In those cases, the amount of nylon may be less than 50, 40, 30, 20, or even 10 percent. In one example, the resin blend contains from about 0.1 to about 10 percent by weight of nylon. A blend containing about 1 to about 2 percent by weight of nylon' may be especially preferred.

The nylon to be used may be chosen according to the melting point of the polyester to be used. It is generally desirable for the nylon to have a melting point within about 30°C below and about 20°C above the melting point of the polyester. The nylon may be added to the polyester most readily by blending the corresponding weights of both polymers in solid (pellet) form. The mixture is fed to an extruder for blending and melt extrusion into the desired article. For containers, such as bottles, preforms are typically made, by extruding the blend into suitable molds. The preforms are thereafter placed in a container mold and heated and blown into the container shape. The manufacture and use of preforms is well known and does not form a limitation on the practice of the present invention. Similarly, the invention is not to be limited to the formation and use of bottles.

Containers formed from the polyester/nylon blends of the present invention are suitable for handling a variety of materials including caustic materials. By caustic, is meant a material having a pH of greater than about 7, up to and including 14.

To demonstrate the effectiveness of the present invention, several polyester copolymers were provided as follows: Polyester A = 98.2 percent terephthalic acid and 1.8 percent isophthalic acid reacted with ethylene glycol, with 2ppm Na; Polyester B =98.2 percent terephthalic acid and 1.8 percent isophthalic acid reacted with ethylene glycol, with 4ppm Na; Polyester C = 97 percent terephthalic acid and 3 percent isophthalic acid reacted with ethylene glycol, with 2ppm Na. The polyesters were synthesized from their precursors in the presence of a conventional catalyst, antimony glycolate, at a level of 260 ppm antimony. The polymerization was continued until the polyesters reached an intrinsic viscosity (IV) of 0.5 - 0.6 dL/g, at which point the polyesters were melt extruded and pelletized. The polyesters were then solid state polymerized until reaching an IN of 0.72-0.85 dL/g. The polyesters were dried at 150-175°C and the nylon was separately dried in a vacuum oven at 120°C before use. The nylon and the PET resins were then blended in an extruder, injection molded into industry standard preforms of 27 to 27.5 grams, and then blow-molded into 500mL industry standard contour bottles. Sample bottles were formed containing 1 and 2 percent MXD6 from Mitsubishi Gas Chemical (Grade 6007) . Control samples containing no nylon were also made. Compositions for samples 1-8 are provided in Table I. TABLE I PREFORM COMPOSITIONS

SAMPLE NO. POLYESTER MXD6

1 (Control) Polyester A

2 (Control) Polyester A

3 Polyester A 1%

4 Polyester A 2%

5 Polyester B 1%

6 Polyester B 2%

7 Polyester B 1%

8 Polyester B 2%

25 bottles made from each of the eight compositions were tested for caustic stress crack (CSC) resistance using a 3 hour test. Bottles were equilibrated at about 22°C (72 °F) for 72 hours under pressure (about 65 psi, about 4-4.5 volumes CO2 . After equilibration, the bottles were immersed to the base in 0.1 weight percent NaOH solution in water at about 22 ± 2 °C (72 ± 3 °F) and the time to failure was recorded over a 3 hour time period. The CSC testing results for samples 1-8 are presented in Table II.

TABLE II

PRESSURE TESTING

AVERAGE TIME FAILURES PER 25

SAMPLE NO. TO FAIL (HRS) BOTTLES

1 (Control) 1.59 25

2 (Control) 1.65 25

3 3.00 0

4 3.00 0

5 3.00 0

6 3.00 0

7 3.00 0

8 3.00 0

As can readily be determined from Table II, the control bottles, samples 1 and 2, containing PET resin only, failed in testing. By contrast, bottles comprising blends of PET with nylon, according to the present invention, lasted for the requisite period of 3 hours with no failures out of 25 bottles tested.

Another set of preforms and bottles were made and tested as described above. Table III provides the compositions and the CSC test results for samples 9-15.

TABLE III COMPOSITION AND PRESSURE TESTING

SAMPLE POLYESTER MXD6 AVERAGE FAILURES

TIME TO PER 25

FAIL (HRS) BOTTLES

9 (Control) Polyester B 2.67 17

10 Polyester B 1% 2.99 1

11 Polyester B 2% 2.99 1

12 Polyester B 1% 3.00 0

13 Polyester B 2% 3.00 0

14 Polyester C 1% 3.00 0

15 Polyester C 2% 3.00 0

As one can readily determine from Table III, 17 of 25 bottles made from the control sample, containing only PET resin, failed in testing. By contrast, bottles comprising blends of PET with nylon, according to the present invention, lasted for the requisite period of 3 hours with only 1 failure out of 25 bottles for samples 10 and 11 and no failures out of 25 bottles tested for samples 12-15.

Thus, it should be evident that methods of the present invention are highly effective in improving the CSC resistance of polyester containers. The method of the present invention provides a method for improving resistance of polyesters to environmental stresses that has not been previously known. Caustic materials that can be provided in containers made according to this method include cleaning solutions, detergents, industrial additives, hydraulic fluids, pharmaceuticals and the like, but are not necessarily limited thereto.

It is, therefore, to be understood that any variations evident fall within the scope of the claimed invention and thus, the selection of specific nylons and amounts blended with the polyester can be determined without departing from the spirit of the invention herein disclosed and described. In addition, polyesters according to the present invention are not necessarily limited to PET, PEN, PEI and blends and co-polymers thereof and nylons are not necessarily limited to MXD6. Thus, the scope of the invention shall include all modifications and variations that may fall within the scope of the attached claims.

Claims

CLAIMSWe claim:

1. A method of improving the caustic stress-crack resistance of polyester containers, the method comprising: mixing between about 0.1 weight percent and about 99.9 weight percent of a nylon, with about 0.1 weight percent and about 99.9 weight percent of a polyester, wherein said percentages are based upon the combined weight of the polyester and nylon, to form a blend; and forming a container from said blend.

2. The method of claim 1, wherein said polyester is the reaction product of at least one diacid containing from about 6 to about 40 carbon atoms or an ester thereof, and at least one diol selected from the group consisting of aliphatic diols having from about 2 to about 10 carbon atoms, cycloaliphatic diols having from about 7 to about 14 carbon atoms, aromatic diols having from about 6 to about 15 carbon atoms, and glycol ethers having from 4 to 10 carbon atoms.

3. The method of claim 2, wherein said polyester is the reaction product of at least one diacid selected from the group consisting of terephthalic acid, isophthalic acid, naphthalene 2, 6-dicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, diphenyl-4,4'- dicarboxylic acid, 1,3-phenylenedioxydiacetic acid, 1,2- phenylenedioxydiacetic acid, 1,4-phenylenedioxydiacetic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, and sebacic acid, and at least one diol selected from the group consisting 1,4-butenediol, trimethylene glycol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, resorcinol, and hydroquinone.

4. The method of claim 1, wherein said nylon is selected from the group consisting of aliphatic nylons.

5. The method of claim 1, wherein said nylon is selected from the group consisting of nylon 6,6, nylon 4,6, nylon 6, and poly {meta -xylylene diamine adipate).

6. The method of claim 1, wherein said nylon is present in an amount of between 0.1 and about 10 percent by weight, based on the total weight of the composition.

7. The method of claim 1, wherein the container made from the blend has improved caustic stress-crack resistance compared to a container containing the identical polyester as that present in the blend, as determined by a three hour caustic stress-crack resistance test.

8. The method of claim 1, wherein the container made from the blend has improved caustic stress-crack resistance to alkaline processing aids, line lubricants and hydraulic fluids compared to a container containing the identical polyester as that present in the blend, as determined by a three hour caustic stress-crack resistance test.

9. A method of storing an alkaline composition, the method comprising: placing an alkaline composition in a container, wherein the container comprises a blend of between about 0.1 weight percent and about 99.9 weight percent of a nylon and between about 0.1 weight percent and about 99.9 weight percent of a polyester, wherein said percentages are based upon the combined weight of the polyester and nylon.

10. The method of storing an alkaline composition according to claim 9, wherein said polyester is the reaction product of at least one diacid containing from about 6 to about 40 carbon atoms or an ester thereof, and at least one diol selected from the group consisting of aliphatic diols having from about 2 to about 10 carbon atoms, cycloaliphatic diols having from about 7 to about 14 carbon atoms, aromatic diols having from about 6 to about 15 carbon atoms, and glycol ethers having from 4 to 10 carbon atoms.

11. The method of storing an alkaline composition according to claim 9, wherein said polyester is the reaction product of at least one diacid selected from the group consisting of terephthalic acid, isophthalic acid, naphthalene 2,6-dicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, diphenyl-4,4'-dicarboxylic acid, 1,3- phenylenedioxydiacetic acid, 1,2-phenylenedioxydiacetic acid, 1,4- phenylenedioxydiacetic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, and sebacic acid, and at least one diol selected from the group consisting 1,4-butenediol, trimethylene glycol, 1,6-hexanediol, 1,4- cyclohexanedimethanol, diethylene glycol, resorcinol, and hydroquinone.

12. The method of storing an alkaline composition according to claim 9, wherein said nylon is selected from the group consisting of nylon 6,6, nylon 4,6, nylon 6, and poly (metα-xylylene diamine adipate).

13. The method of storing an alkaline composition according to • claim 9, wherein said nylon is present in an amount of between 0.1 and about 10 percent by weight, based on the total weight of the composition.

14. The method of storing an alkaline composition according to claim 9, wherein said alkaline composition is selected from the group consisting of cleaning solutions, detergents, industrial additives, hydraulic fluids, and pharmaceuticals.