EP1032491A1

EP1032491A1 - Method for production of polyester packages with improved properties

Info

Publication number: EP1032491A1
Application number: EP98961743A
Authority: EP
Inventors: Mark Edward Stewart; David Dunlap Cornell
Original assignee: Eastman Chemical Co
Current assignee: Eastman Chemical Co
Priority date: 1997-11-21
Filing date: 1998-11-20
Publication date: 2000-09-06
Also published as: WO1999026771A1

Abstract

A method for increasing the crystallinity of a thermoplastic container, such as a polyester, and more particularly a PET, PEN, or blend thereof bottle. The method includes the steps of hydrolyzing at least a portion of the inside surface of the bottle to reduce the molecular weight, Mn, of the polyester relative to the remainder of the bottle and crystallizing at least a portion of the inside surface. It is desirable for the inside surface to have a crystallinity greater than about 30 %. The invention further provides a thermoplastic container having an inside surface with a crystallinity greater than the remainder of the container wherein the inside crystallinity is increased utilizing the method of the invention.

Description

METHOD FOR PRODUCTION OF POLYESTER PACKAGES

WITH IMPROVED PROPERTIES

CROSS-REFERENCE TO RELATED APPLICATIONS

Benefit is claimed under 35 U.S.C. § 119 to the earlier filed U.S. provisional application having U.S. Serial No. 60/066,330 filed November 21, 1997, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention concerns a method for forming a container with improved properties, particularly barrier to gases. More particularly, the invention relates to blow molding a polyester container which incorporates hydrolyzing and crystallizing at least a portion of the container's interior surface.

BACKGROUND OF THE INVENTION

Rigid containers made from polyesters (such as poly(ethylene terephthalate) and its copolymers (PET) are widely used to package food, beverage and other products. However, their application is limited by the fact that gases are absorbed into and permeate through the polymer. In the case of food packages for beverages, low permeability is necessary to prevent packaged components, such as carbon dioxide in a carbonated drink, from escaping through the PET container wall. This loss of carbon dioxide from carbonated beverages results in the drink being "flat". This loss is more pronounced for small packages due to their higher surface to volume ratio. Thus, PET is not commonly used for soft drink packages below about 16 fluid ounces in size. Oxygen is also transported through PET. This limits the use of PET packages for oxygen sensitive products such as beer, fruit juices and tomato-based products.

Another example where polyester transport properties limit their use is in packaging of water. Acetaldehyde, a by-product of melt processing of PET caused by the thermal degradation of the PET, diffuses from PET into the water over time. Acetaldehyde, in very low concentrations will affect the taste of the water.

Technologies exist for improving the barrier of polyester containers. For example, another compound, such as an ethylene vinyl alcohol or nylon can be added as a barrier layer. Others have used PVDC coatings, epoxy coatings, PEN copolymers or layers , or comonomers derived from resorcinol, silicate coatings, or inorganic fillers to decrease the permeability of the PET.

Methods for increasing the barrier properties of the container by increasing the crystallinity of PET have also been developed. For example, U.S. patent no. 5,342,558 discloses a "double blow" processes where an amorphous PET preform having a body and a neck sections have dimensions greater than the dimensions of the final PET container to be formed. The body of the intermediate container are heated under specified conditions to rapidly shrink the body, to form a contracted body. The neck section is then heated to effect crystallization. The neck section is then slowly cooled while heating the contracted intermediate body. The heated contracted body of the intermediate container is then blow molded in a second mold to its final shape and dimensions.

Another method for increasing the barrier properties of a PET container is described in U.S. patent no. 5,730,914 where a PET bottle preform is preheated to soften the PET. A stretch rod is inserted through the top of the preform to stretches the preform axially until the preform is the length of the final product. The preform is expanded using compressed ambient air, and is molded against the walls of the mold. The air is vented, and dry nitrogen at a temperature below -50° C is injected into the molded preform to purge and cool the molded preform while maintaining pressure of at least 520 kPag. The molded preform is held against the mold for a predetermined time, during which the outer and inner surfaces of the molded preform are annealed, and the molded preform transforms into the final product. These methods suffer from a variety of deficiencies including added costs, increased color, brittleness, complicated processing and incompatibility with polyester recycling practices.

Accordingly, there is a need for a method for improving the barrier properties of a polyester container which is effective and compatible with polyester recycling practices and economically feasible.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a method for increasing the crystallinity of at least a portion of a thermoplastic container. The method includes the steps of hydrolyzing at least a portion of an inside surface of the container so that a portion of the inside surface has a lower molecular weight, M_n relative to the remainder of the container and crystallizing at least a portion of the inside surface. Desirably, the crystallinity of the inside surface of the container is greater than about 30 percent. Another aspect of the invention is for a thermoplastic container having a crystallinity of the inside surface greater than about 30 percent. The inside surface of the container is at least partially hydrolyzed and then crystallized to produce a container having an inside surface with a greater crystallinity relative to the remainder of the container.

It is an object of the present invention to provide a method for increasing the crystallinity of the inside surface of a thermoplastic and more particularly to increase the crystallinity of the inside surface of a thermoplastic container such as a bottle or beverage container. Another object of the present invention to provide a method for increasing the interior surface crystallinity of a container having PET and its copolyesters, polyamides PEN and its copolyesters, and blends thereof.

It is another object of the invention to provide a polyester container having a crystallinity greater than about 30 percent. These and other objects .and advantages of the invention will become readily apparent to those skilled in the art with reference to the following specification. It is to be understood that the embodiment described herein is for illustrative purposes only and the inventive concept is not to be considered limited thereto.

DETAIL DESCRIPTION OF THE INVENTION

For the purposes of illustrating the present invention in greater detail, the increase in crystallinity of a polyester and preferably a PET beverage bottle will be described. However, one skilled in the art will understand that any conventional polyester or polyamide, such as nylon MXD-6 may be used in accordance with the method described herein. Desirably, the polyester component of the present invention is a polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) resin. Copolyesters and blends of PET, PEN and polyamides may also be used. Generally the polyester resin of the present invention contains repeat units from at least 80 mole percent of a first dicarboxylic acid selected from terephthalic acid, 2,6-naphthalene- dicarboxlic acid and mixtures thereof and at least 80% ethylene glycol, based on 100 mole percent dicarboxylic acid and 100 mole percent diol. The dicarboxylic acid component of the polyester may optionally be modified with up to about 20 mole percent of one or more different dicarboxylic acids other than terephthalic acid or suitable synthetic equivalents such as dimethyl terephthalate. Such additional dicarboxylic acids include aromatic dicarboxylic acids preferably having 8 to 14 carbon atoms, aliphatic and hetero dicarboxylic acids preferably having 4 to 12 carbon atoms, or cycloaliphatic dicarboxylic acids preferably having 8 to 12 carbon atoms. Examples of dicarboxylic acids to be included with terephthalic acid are: phthalic acid, isophthalic acid, isomers of naphthalenedicarboxylic acid other than the 2,6-isomer, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, diphenyl-4,4'-dicarboxylic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, diglycotic acid, 1-3-phenylene dioxydiacetic acid and the like. Polyesters may be prepared from two or more of the above dicarboxylic acids.

It should be understood that use of the corresponding acid anhydrides, esters, and acid chlorides of these acids is included in the term "dicarboxylic acid". In addition, the polyester component may optionally be modified with up to about 20 mole percent, of one or more different diols other than ethylene glycol. Such additional diols include cycloaliphatic diols preferably having 6 to 20 carbon atoms or aliphatic diols preferably having 3 to 20 carbon atoms. The diols may further contain hetero carbon atoms. Examples of such diols to be included with ethylene glycol are: diethylene glycol, triethylene glycol, 1 ,4-cyclohexanedimethanol, propane- 1,3-diol, butane- 1,2-diol, pentane-l,5-diol, hexane-l,6-diol, 3-methylpentanediol-(2,4), 2-methylpentanediol-(l,4), 2,2,4-trimethylpentane-diol-(l,3), 2-ethylhexanediol- (1,3), 2,2-diethylpropane-diol-(l,3), hexanediol-(l,3), l,4-di-(hydroxyethoxy)-benzene, 2,2-bis-(4-hydroxy-cyclohexyl)-propane, 2,4-dihydroxy- 1,1,3, 3-tetramethyl- cyclobutane, 2,2-bis-(3-hydroxyethoxyphenyl)-propane, hydroxy ethyl resorcinol and 2,2-bis-(4-hydroxypropoxyphenyl)-propane. Polyesters may be prepared from two or more of the above diols.

The polyethylene terephthalate resin may also contain small amounts of trifunctional or tetrafunctional comonomers such as trimellitic anhydride, trimethylolpropane, pyromellitic dianhydride, pentaerythritol, and other polyester forming polyacids or polyols generally known in the art.

Preferably said PET polyesters comprise at least about 85 mole % and more preferably about 90 mole% of said first dicarboxylic acid and about 85 mole % and more preferably about 90 mole% ethylene glycol residues.

Polyethylene terephthalate based polyesters of the present invention can be prepared by conventional polycondensation procedures well-known in the art. Such processes include direct condensation of the dicarboxylic acid(s) with the diol(s) or by ester interchange using a dialkyl dicarboxylate. For example, a dialkyl terephthalate such as dimethyl terephthalate is ester interchanged with the diol(s) at elevated temperatures in the presence of a catalyst. The polyesters may also be subjected to solid state polymerization methods. PEN and PEN-T copolymers and blends may also be prepared by well known polycondensation procedures.

Many other ingredients can be added to the compositions of the present invention to enhance the performance properties of the polyesters. For example, crystallization aids, impact modifiers, surface lubricants, denesting agents, stabilizers, antioxidants, ultraviolet light absorbing agents, metal deactivators, colorants, nucleating agents, fillers, reheat aids, acetaldehyde reducing agents and the like can be included. All of these additives and many others and their use are well known in the art and do not require extensive discussion. Therefore, only a limited number will be referred to, it being understood that any of these compounds can be used so long as they do not hinder the present invention from accomplishing its objects.

In accordance with the method of the invention, it has been unexpectedly discovered that a polyester bottle having improved barrier properties can be produced by increasing the crystallinity of the inside surface of the container. To achieve this improved barrier property, it is desirable to increase the crystallinity of the inside surface of the bottle to greater than about 30 percent, preferably, greater than about 40 percent, more preferably, greater than about 50 percent and most preferably greater than about 60 percent. The crystallinity of the inside surface may be determined by using methods known to those skilled in the art. Such methods include, but are not limited to, x-ray defractions, differential scanning calorimetry and refractive index.

As is conventional in the art in producing a blown bottle, polyesters are formed into preforms using any method known to one skilled in the art. Equipment for making preforms by injection molding is readily available from Husky, Nissei, Aoki and others. In forming a blown bottle from the preform, the preforms are heated to an appropriate blowing temperature, based on its composition, predetermined size, shape and the like. Generally, the blow molding temperature is greater than about glass transition temperature, but below about the onset of crystallization of the polyester being used. For PET this temperature is generally in the range of about 90°C and about 130°C. Once the preforms are at desired blow molding temperature they are inflated with a pressurized fluid. Suitable pressurized fluids include, but are not limited to air, nitrogen, steam, any inert gas, CO2 and the like. Air is most commonly used. Generally inflation takes place in a mold of the desired size and shape.

In accordance with the present invention, at least a portion of the inside surface area is hydrolyzed so as to lower the molecular weight, M_n, of at least a portion of the inside surface area of the preform. Hydrolysis of the inside surface area of the polyester can be effected at any time, i.e., before expansion, during expansion, or subsequent to expansion. The polyester can be hydrolyzed using methods known to those skilled in the art and include, contacting the inside surface with one or more, either alone or in combination, hydrolyzing agents, such as, steam, a lower alkyl alcohol, a diol, an acid and/or a base, with steam being preferred and super heated steam being more preferred. Suitable lower alkyl alcohols include, Cι-Cι₂ alcohols with Cι-C₈ alcohols being preferred and Cι-C₄ being more preferred. Non- limiting examples of preferred lower alkyl alcohols include methanol, ethanol, n-propanol and isoropanol. Suitable diols include ethane diol; propane diol; ethylene glycol; diethylene glycol; propylene glycol; 2-ethylhexane-l,3-diol; 3-methylpentane-2,4-diol; 2- methylpentane-2,4-diol, 2,2,4-trimethylpentane-l,3-diol; 2,2-diethylprop.ane-l,3-diol; butane- 1,4-diol; hexane-2,5-diol; propane- 1,3-diol and butane-l,3-diol. Suitable acids and bases include, but are not limited to, acetic acid, hydrochloric acid, and sodium hydroxide. Desirably, the hydrolyzing agent is a generally recognized as safe food additive as defined in 21 CFR, the disclosure of which is incorporated herein by reference, or would meet the definition as a generally recognized as safe food additive. The inside surface of the polyester container desirably has from about 10% to 100%) of the surface contacted with one or more of the above hydrolyzing agents. Preferably, from about 25%> to 100% of the inside surface is contacted with the hydrolyzing agent and more preferably greater than about 50% of the inside surface is contacted with the hydrolyzing agent.

Relative to the remainder of the container, the molecular weight of at least a portion of the hydrolyzed polyester is lowered by at least about 20 %, preferably, by at least about 30%, and more preferably by at least about 50% so that the hydrolyzed polyester has a molecular weight of from about 1000 to about 15,000 and preferably from about 1000 to about 10,000.

A preform contacted (either before expansion, during expansion or after the final dimensions have been achieved) with a hydrolysis agent produces a thin film of polyester having a lower molecular weight than the untreated polyester. Surprisingly, subsequent crystalization of the lower molecular weight polyester results in a container having improved barrier properties. While the preform is in the mold, the interior of the container is treated with the hydrolysis agent. Although not wishing to be bound by any theory, it is believed that the containers should be in contact with the mold during this step to produce optimum properties. The hydrolysis agent should be at a pressure which is at least about the pressure of the fluids which are used in stage blow molding where the hydrolysis agent is introduced. For example, where steam is the hydrolysis agent, typical steam pressures include those between about 100 to about 600 lbs. The hydrolysis agent may be introduced concurrently with the pressurized fluid (which is at about 600 lbs) or sequentially, i.e., after stretching, which could be as a pressure, below about 600 lbs, to as low as about 100 lbs. Conventional processes sometimes use pulsed pressurized fluids and the hydrolysis agent of this invention may be introduced in a continuous, staged or pulsed phase, either at or slightly above the pressures which are conventionally used at that stage.

The total contact time (either continuous or the aggregation of pulses or stages) may be up to about 5 seconds, and preferably from about 0J second to about 5 seconds. It should be appreciated that higher pressure or temperature, and more acidic or basic hydrolysis agents may require shorter contact times.

The method of the invention further includes crystallizing at least a portion of the inside surface area of the container. Method for crystallizing a polyester are well known in the art and include introducing or subjecting the container to sufficient energy to overcome the energy of crystallization. The interior of the containers may also be irradiated to effect crystallization. Suitable radiation sources include any source which is capable of starting crystallization of the container interior surface. Suitable radiation sources include electromagnetic radiation having a wavelength of from about 300 nanometers (π ) to about 1 cm and preferably 700 nm - 2000 nm. Examples of suitable sources of irradiation include electron beam, electron plasma, high intensity visible light, lasers, infrared light, microwave and the like. Desirably, the irradiation time is of a limited duration sufficiently so that only a thin layer of crystallized polyester is generated. Exposure time to the radiation source will vary depending on the type and intensity of the radiation source selected. Generally exposure times of less than about 20 seconds, and preferably from about 0.2 seconds to about 10 seconds are contemplated. The process of the present invention produces a polyester container having a thin interior layer having high crystallinity. The crystalline layer provides containers of the present invention with significantly improved barrier and resistance to sorption over conventional non-treated polyester containers. Desirably, the method of the invention produces a polyester container having an inside surface with a crystallinity greater than about 30 %>, preferably, greater than about 40%, more preferably greater than about 50% and most preferably greater than about 60%.

Another aspect of the present invention is for a thermoplastic container and preferably, a polyester container having an inside surface with a crystallinity greater than the remainder of the container. The crystallinity of the inside surface is desirably greater than about 30 %, preferably, greater than about 40%, more preferably greater than about 50% and most preferably greater than about 60%. The increased crystallinity of the inside of the container is achieved using the method of the invention as described above. Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting to the invention described herein. No doubt that after reading the disclosure, various alterations and modifications will become apparent to those skilled in the art to which the invention pertains. It is intended that the appended claims be interpreted as covering all such alterations and modifications as fall within the spirit and scope of the invention.

Claims

CLAIMS We claim:

1. A method for increasing the crystallinity of at least a portion of a thermoplastic container comprising the steps of hydrolyzing at least a portion of an inside surface of said container wherein a portion of said hydrolyzed surface has a lower molecular weight, M_n relative to the remainder of the container and crystallizing at least a portion of said inside surface.

2. The method of claim 1 wherein said hydrolyzing step includes contacting said inside surface with a hydrolyzing agent selected from the group consisting of steam, a lower alkyl alcohol, a diol, an acid, a base, and mixtures thereof.

3. The method of claim 2 wherein said lower alkyl alcohol is selected from the group consisting of methanol, ethanol, n-propanal and isopropanol, said diol is selected from the group consisting of ethane diol and propane diol, said acid is selected from the group consisting of acetic acid and hydrochloric acid, and said base is sodium hydroxide.

4. The method of claim 1 wherein the molecular weight of said inside hydrolyzed surface is lowered by at least 20%, relative to the remainder of said container.

5. The method of claim 1 wherein the molecular weight of said inside hydrolyzed surface is lowered by at least 30%, relative to the remainder of said container.

6. The method of claim 1 wherein the molecular weight of said inside hydrolyzed surface is lowered by at least 50%, relative to the remainder of said container.

7. The method of claim 1 wherein the molecular weight, M_n, of said inside hydrolyzed surface is from 1000 to 15,000.

8. The method of claim 1 wherein the molecular weight, M_n, of said inside hydrolyzed surface is from 1000 to 10,000.

9. The method of claim 1 wherein said crystallizing step includes irradiating said container with electromagnetic radiation having a wavelength of from about 300 nm to about 1 centimeter.

10. The method of claim 9 wherein said electromagnetic radiation has a wavelength of from about 700 nm to about 2000 nm.

11. The method of claim 1 wherein said crystallinity of said container is greater than about 30 %.

12. The method of claim 1 wherein said crystallinity of said container is greater than about 40 %.

13. The method of claim 1 wherein said crystallinity of said container is greater than about 50 %.

14 The method of claim 1 wherein said crystallinity of said container is greater than about 60 %.

15. A method for increasing the crystallinity of at least a portion of an inside surface of a thermoplastic container comprising the steps of contacting said inside surface with a hydrolyzing agent selected from the group consisting of steam, a lower alkyl alcohol, a diol, an acid, a base, and mixtures thereof to hydro lyze at least a portion of said inside surface and wherein said hydrolyzed surface has a molecular weight, M_n, at least 20%) lower relative to the remainder of the container and crystallizing at least a portion of said inside surface.

16. The method of claim 15 wherein said thermoplastic is selected from the group consisting of polyamides and polyesters selected from the group consisting of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and blends thereof.

17. The method of claim 16 wherein said thermoplastic is selected from the group consisting of PET, PEN, and blends thereof.

18. The method of claim 15 wherein said the molecular weight, M_n, of said inside hydrolyzed surface is from 1000 to 15,000.

19. The method of claim 15 wherein said crystallinity of said container is greater than about 40 %.

20. The method of claim 15 wherein said crystallinity of said container is greater than about 50 %.

21. The method of claim 15 wherein said crystallinity of said container is greater than about 60 %.

22. A thermoplastic container comprising an inside surface with a crystallinity greater than the remainder of said container wherein said greater crystallinity is made by the method comprising the steps of hydrolyzing at least a portion of the inside surface wherein a portion of said hydrolyzed surface has a lower molecular weight, M_n relative to the remainder of the container and crystallizing at least a portion of said inside surface.

23. The container of claim 22 wherein said crystallinity is greater than about 30%.

24. The container of claim 22 wherein said crystallinity is greater than about 40.

25. The container of claim 22 wherein said crystallinity is greater than about 50.

26. The container of claim 22 wherein said crystallinity is greater than about 60.

27. The container of claim 22 wherein said hydrolyzing step includes contacting said inside surface with a hydrolyzing agent selected from the group consisting of steam, a lower alkyl alcohol, a diol, an acid, a base, and mixtures thereof.