WO1994026497A1

WO1994026497A1 - Method of making a container with crystallized inner surface

Info

Publication number: WO1994026497A1
Application number: PCT/US1994/005143
Authority: WO
Inventors: Bradley C. Dimmel; George Plester
Original assignee: The Coca-Cola Company
Priority date: 1993-05-13
Filing date: 1994-05-13
Publication date: 1994-11-24
Also published as: ECSP941083A; ZA943303B; PE9895A1

Abstract

A refillable plastic bottle (10) with low permeability and absorption to certain polar and non-polar substances is produced by a number of methods. The low permeability and absorption are achieved by providing an inner surface (14) of the bottle (10) with a crystallinity exceeding 30 %, in combination with treatment to achieve low residual stress on the inner surface (14). The crystallinity is achieved by either: applying heated fluid to the inner surface of the bottle or the preform used to form the bottle; coating the inner surface of either the bottle or preform with highly crystalline material; coating a core rod with highly crystalline material; or coating the inner surface of the preform with a temperature increasing substance. The refillable plastic bottles (10) with crystallized inner surfaces (14) can be manufactured on any orientable resin or resin blend.

Description

"METHOD OF MAKING A CONTAINER WITH CRYSTALLIZED INNER SURFACE"

BACKGROUND OF THE INVENTION The present invention relates to a method for making plastic container for beverages having an inner surface wit a relatively low permeability/absorption to certain polar an non-polar substances. More specifically, the presen 5 invention relates to a method for making a plastic containe having an inner surface with a relatively lo permeability/absorption to fluids as compared to th remainder of the container structure by providing an inne surface with special crystallinity and residual stres 0 values.

In many industries, including the beverage industries, products are packaged in containers which are returned afte use, washed and refilled. Typically refillable containers, such as beverage bottles, are made of glass which can b 5 easily cleaned. Plastic and metal containers have bee replacing glass in many applications where easy handling, lo weight and non-breakability are needed. Where metal is used, the internal metal surface of the container must often b coated with a polymer to avoid contact of the package conten 0 with the metal. Therefore, both in the case of plasti packages, and also in the case of many metal containers, th contact or inner surface of the bottle which contacts th packaged content is a polymer. Polymers are generally mor permeable to the package content than is glass. In the case of food packages such as beverages, surface inertness (low permeability) is necessary to prevent desorption of packaging material components into the food or beverage, to prevent flavor absorption, to avoid loss of food constituents through the package walls and to avoid ingress of air or other substances from outside the package.

Refillable plastic packages for beverages or the like add a further dimension to the inertness or low permeability requirements because these packages must withstand washing and refilling.

Furthermore, the permeability of plastic materials is very important for refillable containers such as beverage bottles because the higher the permeability of the inner surface of the beverage bottle the more it will absorb a variety of compounds which may later be desorbed into the product thereby potentially adversely affecting the quality of the product packaged into the container. These compounds may include certain polar and non-polar substances which may potentially contaminate the containers. These substances may be present in the containers from materials stored therein after consumption of the beverage and prior to refilling of the container with a fresh supply of beverage.

Many satisfactory systems and methods have been developed for inspecting plastic containers for contaminants before refilling the same but it would be highly desirable to minimize the penetration of any contaminants into the walls of the containers to avoid container waste and potential contamination which could affect beverage quality. Accordingly, a need in the art exists for methods for making plastic containers wherein the inner surfaces of the container have a low permeability to various fluids. Current plastic bottles for carbonated beverages generally are fabricated from polyethylene terephthalate (PET) o polycarbonate resins. PET offers the best balance o properties, cost and performance ratios. It is wel documented that the permeability or absorption of package made from PET resin is affected by crystallinity orientation, and pressurization stress. This phenomena i described in a publication entitled "Container Plastics" Eastman Kodak Company's Publication TRC-93A dated Decembe 1992. This Eastman Kodak article teaches that PET container with higher crystallinity have lower permeability to variou fluids.

However, as taught in U.S. Patent 4,725,464 to Collett issued February 16, 1988, excessive sidewall crystallizatio of a PET container for use as a refillable beverage bottl beyond 28 to 30% raises the modulas of the biaxially oriente PET matrix to the point where repeated expansion/contractio cycling causes a semi-rigid morphology to fail. In othe words if the crystallinity of the PET container is too high although permeability to various fluids is decreased, othe characteristics of the container are unsuitable for use as refillable beverage bottle. Accordingly, a need in the ar exists for a method which will permit the use of plasti beverage bottles with low permeability/absorption to variou fluids while maintaining other important characteristics an qualities needed for a suitable refillable container.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the presen invention to provide a method for decreasing at least th inner surface permeability/absorption properties of a plasti beverage bottle to certain substances such as polar and non polar fluids. It is another object of the present invention to provide a method for decreasing the inner surface permeability/absorption properties of a plastic beverage bottle while maintaining the requisite elasticity for the bottle.

It is still another object of the present invention to provide a method for decreasing at least the inner surface permeability/absorption of a plastic beverage bottle while maintaining transparency of the bottle in order to facilitate viewing of the product and/or any contaminants therein.

It is a further object of the present invention to provide a method for decreasing at least the permeability of at least the inner surface of a plastic container while maintaining the durability of the container to washing and sanitizing.

These and other objects of the present invention are fulfilled by providing a method of making a plastic container having an inner surface with a relatively low permeability/absorption to fluids as compared to the remainder of the container structure, comprising the steps of: treating said inner surface to raise the crystallinity thereof to a relatively high value as compared to said remaining container structure; and stress relaxing the inner surface of the container. It is a discovery of the present invention that if the crystallinity of the inner surface of the plastic beverage bottle is greater than 30% by volume, and if the residual container stress is reduced, that lower permeability/absorption of the inner surface of the container will result and the container will still possess all the requisite characteristics of a refillable beverage bottle.

Residual container stress can be measured by its chemical resistance to stress cracking through various types of durability test procedures associated with the toleranc of the beverage bottles to the typical washing and sanitizin process of the bottle.

In accordance with a discovery of the present invention bottles with similar crystallinity and orientation hav different permeability and absorption characteristics o certain polar and non-polar substances if they hav significantly different residual stress levels. The bottl with the lowest residual stress (similar crystallinity an orientation) has the lowest possible permeability an absorption to certain polar and non-polar substances.

The present invention includes a method fo manufacturing refillable plastic bottles with improve permeability and absorption characteristics to certain pola and non-polar substances by obtaining inside surfac crystallinity exceeding 30%, in combination with a lo residual stress. With the method of the present invention, optimized refillable plastic bottles can be manufactured fro any number of processes on any orientatable resin (or resi blend) once the above relationship is applied.

The present invention provides a method for makin refillable plastic beverage bottles with inner surface exceeding 30% crystallinity which substantially maintain lo permeability or absorption properties after sanitizin (typically 1.0% to 3.0% caustic etc. for 20 minutes to minutes, respectively at 58°C) . The achievement of highe crystallinity in combination with reduction of stress on th inside surface may be accomplished by known proces techniques to strain or thermally induce crystallizatio (increased stress ratio, increased preform heating, prope resin selection, etc.) and localized stress relaxation of th inside surface. For many of the known processes for formin refillable plastic beverage bottles, localized stres relaxation requires secondary annealing of the inside surface through high temperature air, steam, heated fluid or some other mechanism while cooling the outside of the bottle to maintain bottle shape. In accordance with the present invention higher crystallinity of the inner surface of the refillable plastic bottle may be achieved by many suitable ways.

For example, the core rod of an injection molded preform to be subsequently blown into the shape of the refillable container may be dipped, sprayed, coated or the like with a high crystalline material. Then the resin for the preform injected to form a preform with a laminated high crystalline material on the inner surface.

In another embodiment the preform may be coated on its inner surface prior to blow molding with a material to locally increase heat by conduction to the inside surface of the preform as it is being heated in the blow mold just prior to being blown into a bottle.

In other embodiments, a fully formed bottle may be provided with a coating on the inner surface thereof having a higher crystallinity than the remaining bottle structure.

In accordance with the present invention any selective method for increasing the inside surface crystallinity of the bottle above 30% and then annealing, or the like, in order to relieve or relax excess stress will achieve the objects of the present invention: namely; to obtain a bottle with low permeability/absorption to certain substances, e.g. polar and non-polar liquids, while maintaining durability of the bottle. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way o illustration only, since various changes and modification within the spirit and scope of the invention will becom apparent to those skilled in the art from this detaile description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understoo from the detailed description given hereinbelow and th accompanying drawings which are given by way of illustratio only, and thus, are not limitative of the present inventio and wherein:

Fig. 1 is an elevational view in section of a typica refillable plastic beverage bottle which may be formed by th method of the present invention; and Fig. 2 is a graph comparing the permeability/absorptio vs. percent-crystallinity of a typical refillable PE beverage bottle; a PET bottle in a non-annealed condition and a PET bottle with crystallinity above 30%, and residua stress reduction.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates in cross-section a typical refillabl beverage bottle 10 having an inner surface 14 and an oute surface 12. Typically these beverage bottles are fabricate from polyethylene terephthalate (PET) . This is the preferre material in the industry because it offers the best balanc of properties, cost and performance characteristics. I accordance with the present invention the inner surface 14 o bottle 10 is fabricated to have a percent crystallinity b volume (above about 30%) and this inner surface is als stress relaxed by appropriate treatment in order to lower th permeability/absorption properties of this inner surface. The higher crystallinity of the inner surface 14 may be achieved by various techniques. For example, if the bottle 10 is formed by blow-molding, the container may be formed as a laminate by creating a laminated preform by injection molding. This could be accomplished for example by dipping, spraying, or coating of the core rod of the injection mold with high crystalline material. The resin for forming the preform would then be injected to form a laminated preform with a high crystalline material on the inner surface. The container would then be blown in a blow-mold to achieve the shape illustrated in Fig. 1, and the inner surface 14 and outer surface 12 would consist of different layers of material corresponding to the laminated preform.

In another embodiment an existing preform could be coated on its inner surface prior to blow molding with a material to locally increase heat by conduction to the inside surface of the preform as it is being heated in the blow mold just prior to being blown into a bottle. This heat treatment would also increase the crystallinity of the inner surface 14 of the resulting bottle.

In still another embodiment a fully formed bottle such as 10 of Fig. 1 may be provided with a coating on the inner surface thereof having a higher crystallinity than the remaining of the bottle structure including outer surface 12. The coating could be applied in a variety of suitable ways.

The object of all the aforementioned techniques is to increase the crystallinity of the inner surface 14 of the bottle above 30% which decreases the permeability/absorption properties of the inner surface 14. However, it is a discovery of the present invention that if in addition the residual stress on inner surface 14 is relaxed or reduced for crystallinities from about 30% and above, that a bottle with an unexpectedly low permeability/absorption characteristics for the inner surface will result, while maintaini durability and other important characteristics of the bott 10.

The effectiveness of the method of the present inventi is illustrated in Fig. 2 wherein percent-crystallinity of t inner surface of PET bottles subjectable to vario treatments are plotted against the rate of increase permeability/absorption of the inner surface of the bottle Curve A illustrates the permeability/absorption properties the inner surface of a typical PET bottle in a non-anneal condition. That is, curve A represents the permeabili absorption properties vs. percent crystallinity of a P bottle formable which by blow-molding with no subsequent he treatment thereto. Curve B relates to a typical refillable PET bottle whi might be subjected to a secondary heat treatment f thermosetting purposes after blow-molding. It can be se that the result would be a slight decrease permeability/absorption with increased percent crystallini as compared to the non-annealed bottle of curve A.

Curve C illustrates the advantageously improv permeability/absorption characteristics of the inner surfa of a refillable PET beverage bottle in accordance with t improved method of the present invention. As illustrated 30% crystallinity there is an abrupt decrease permeability/absorption rate and with the residual stre reduction applied by the annealing process of the prese invention, there is even a further decrease in t permeability/absorption rate of the inner surface of t container. This is clearly illustrated by the increas negative slope of curve C.

Accordingly, the graphs of Fig. 2 illustrate t improved permeability/absorption rate achievable for t inner surface of a refillable PET beverage bottle utilizing the method of the present invention.

In a preferred embodiment of the present invention a refillable PET beverage bottle such as 10 of Fig. 1 having an inner surface 14 with the characteristics illustrated by curve C in Fig. 2 may be achieved by the following exemplary method.

EXAMPLE A preform to be blow-molded may be formed with a total stretch ratio for the inside surface of between 10 to 1 and

16 to 1 (preferably 13 to 1) . This may be accomplished by injecting a PET resin into an injection mold for fabricating the preform with an intrinsic viscosity (IV) between 0.70 to

1.10 (preferably 0.80 IV). The body of the preform may then be heated to achieve a temperature of between 200°F to 250°F immediately prior to blow-molding (preferably the inside surface will be nearly 250°F) .

The preform may then be blow-molded utilizing air pressures and times which are typical in the blow-molding art to form the bottle 10. Typical times and pressures for forming refillable PET beverage containers are well known in the art.

Once the container is formed in the blow-mold, further in-mold annealing and/or thermal crystallinity is performed to achieve the characteristics illustrated in curve C for the inner surface 14 through steam injection into the blow-mold at about 200°F to 400°F (preferably nearly 400°F) for about

1 to 5 seconds (preferably about 1 second) while the outside of the bottle is in contact with a cooling mold at a temperature of about 150° to 250°F (preferably nearly 200°F) for a period of about 2 to 20 seconds (preferably about 2 to

3 seconds) . The resulting container will have the optima permeability/absorption properties for the inner surface 1 as illustrated in curve C of Fig. 2 while maintainin durability, capacity stability, economics of manufacture, an recyclability. That is, the above described process wil achieve the combination of orientation crystallinity of abou 30% and above and suitable stress relaxation in order t achieve low permeability/absorption characteristics lik those illustrated in curve C of Fig. 2. It should be understood however that the above describe method for achieving crystallinity of the inner surface 14 o a beverage bottle 10 of about 30% and above in combinatio with stress relaxation of the inner surface to further lowe permeability/absorption characteristics is only an exemplar preferred embodiment, and other methods may be utilized t achieve the same results without departing from the spiri and scope of the present invention.

Claims

What is claimed is:

1. A method of making a plastic container having an inner surface with a relatively low permeability/absorption to fluids as compared to the remainder of the container structure, comprising the steps of: treating said inner surface to raise the crystallinity thereof to a relatively high value as compared to said remaining container structure; and stress relaxing the inner surface of the container.

2. The method of claim 1 wherein the step of treating raises the crystallinity of the inner surface to greater than 30% by volume.

3. The method of claim 2 wherein the step of stress relaxing is performed by annealing the inner surface.

4. The method of claim 1 wherein the step of stress relaxing is performed by annealing the inner surface.

5. The method of claim 3 wherein said step of annealing includes heating the inner surface to a temperature in a range from about 200°F to about 400°F for about 1 to about 5 seconds.

6. The method of claim 5 wherein the heating is performed by selectively applying heated fluid to the inner surface while cooling the outer surface.

7. The method of claims 1, 2, 3, 4, 5 or 6 wherein the st of treating to raise crystallinity is performed by coati the inner surface of the container with a material having crystallinity which is higher than the crystallinity of t materials forming the remainder of the container.

8. The method of claims 1, 2, 3, 4, 5 or 6 wherein t container is formed by blow molding from a preform, the st of treating to raise crystallinity being performed on t inner surface of the preform.

9. The method of claims 1, 2, 3, 4, 5 or 6 wherein t container is formed by blow molding, and the step of treati to raise crystallinity is performed by selectively coati the inner surface of the preform in the blow mold with material which locally increases the temperature of the inn surface of the preform.

10. The method of claims 1, 2 or 3 wherein the container formed by blow molding and the step of treating to raise t crystallinity is performed by applying a high crystalli coating to the exterior surface of a core rod of an injecti mold for making a preform, and forming the container prefo thereon.

11. The method of claims 1, 2, 3, 4, 5 or 6 wherein t plastic from which the container is formed is polyethyle terephthalate (PET) .

12. The method of claim 7 wherein the plastic from which t container is formed is polyethylene terephthalate (PET) a the coating applied to the inner surface is a high crystalline form of PET.

13. The method of claim 8 wherein the plastic from which th container is formed is polyethylene terephthalate (PET) an the coating applied to the inner surface is a highe crystalline form of PET.

14. The method of claim 9 wherein the plastic from which th container is formed is polyethylene terephthalate (PET) an the coating applied to the inner surface is a highe crystalline form of PET.

15. The method of claim 10 wherein the plastic from whic the container is formed is polyethylene terephthalate (PET) and the coating applied to the inner surface is a highe crystalline form of PET.

16. A product formed by the process of claim 1.

17. A product formed by the process of claim 2.

18. A product formed by the process of claim 3.

19. A product formed by the process of claim 4.

20. A product formed by the process of claim 5.