EP1132308A2

EP1132308A2 - Plastic container having improved haze resistance and a method of reducing haze in plastic containers

Info

Publication number: EP1132308A2
Application number: EP01850038A
Authority: EP
Inventors: Nils Ljungqvist
Original assignee: Rexam AB; Rexam Beverage Packaging AB
Current assignee: Rexam AB
Priority date: 2000-02-24
Filing date: 2001-02-23
Publication date: 2001-09-12
Also published as: SE0000598D0; SE0000598L; NO20010903L; NO20010903D0; EP1132308A3

Abstract

A plastic container for beverages is described having improved resistance to haziness, the plastic comprising at least one plastic component. The plastic component is PEN, and the outer surface of the container is provided with a coating imparting improved resistance to haziness to the container.

A method of reducing haze on plastic containers for beverages is also described the plastic comprising at least one plastic component comprising PEN. The container is refillable and of the type that is subjected to at least one trip during its shelf life, the trip including a washing step and a filling step. A coating is applied on the outer surface of the container between said steps or after the filling step to impart to the container an improved resistance to haziness.

Description

Technical Field

The invention is directed to plastic containers, and in particular to plastic bottles for beverages, such as carbonated soft drinks, beer, and the like. Further, the invention is related to a method for applying coatings on the outside surface of beverage bottles to make them more resistant to attacks caused by the environment and the processing thereof.

Background Art

In recent years, the market of plastic bottles for beverages has increased dramatically. In many countries, in particular the European countries, the industry of refillable plastic bottles has been very successful.
Refillable plastic bottles are disclosed in EP-A-0 247 666. In general, a refillable plastic bottle must remain aesthetically and functionally viable over a minimum of five and preferably over ten trips or loops to be considered feasible. Such a trip comprises, by way of example, (1) an empty caustic wash followed by (2) contaminant inspection and product filling, (3) warehouse storage, (4) distribution to retail locations and (5) purchase, use and empty storage by the consumer followed by eventual return to the bottler. To withstand this trip, the bottle must have extremely good chemical and physical properties. Manufacturers of packaging materials have realized this and have developed materials that approach the demands. However, there is still room for improvements.
There are today several polymers, which provide for example the clarity and physical properties as well as the barrier properties. Non-limiting examples include PET (polyethylene terephthalate), acrylonitrile, polyamides, polycarbonates, PEN (polyethylene naphthalate), etc. PEN is a member of a class of materials called NDC-based polyesters (NDC - dimethyl-2,6-naphthalenedicarboxylate). Other examples of materials within this class include, besides PEN, terephthalate/-naphthalate(T/N)copolyesters and blends of these materials (including PEN) with PET. All these materials meet the above demands to a more and less satisfactory extent. PET is rather cheap and has acceptable gas barrier properties. On the other hand, PEN has much better gas barrier properties than PET and also excellent mechanical properties.
Due to new demands from the bottlers, in particular the breweries, the leading suppliers of plastic bottles have developed thin-walled PEN bottles which compete well with conventional PET bottles. In further comparing PET with PEN, it has been found that the barrier against sunlight is better in case of PEN than in case of PET. Thus, the contents of a PEN bottle is better protected from sunlight than that of a PET bottle.
The increased light barrier depends on the fact that PEN absorbs a greater amount of light at certain wavelengths. An increased absorbance of UV light as a part of the sunlight brings about the effect that PEN is more easily degraded, which in the polymer art is called photo-oxidative degradation of polymers. UV stabilisers must be added to several of the most usual polymers, such as polypropylene (PP), if said polymers are to be used in an outdoor environment subjecting them to sunlight.
Tests have been made wherein UV-stabilisers (photostabilisers) have been admixed with compositions based on PEN but with bad results. The problem was that very high concentrations of such additive were required in order to obtain an effect at all. Such high concentrations bring about high costs and further increase the risk of migration into the beverage and thus an off-taste. Examples of known photostabilisers are hindered amines, such as TINUVIN 234 and TINUVIN P (Ciba).
In case of PEN bottles it is noticed that the degradation due to exposure to sunlight is really a problem. The sunlight degrades the outer surface layer of the plastic bottle creating an uneven surface which, when viewed under a microscope, has very small crater-like voids, which are invisible to the naked eye. Due to a diffuse reflection, this uneven surface appears as a haziness to a viewer. Many times a consumer refuses from buying a bottle having such haziness. Indeed, hazing of such bottles is disadvantageous.
Prior to the present invention, the hazing problem has been solved by using bottles having improved resistance to haziness.This in turn has been achieved by manufacturing bottles starting from polymers having a certain resistance to haziness or polymers to which substances have been added which impart resistance to haziness to the polymers.

Summary of the Invention

The main object of the invention is to provide a plastic container having improved resistance to haziness, the plastic comprising at least one plastic component, which is PEN.
Another object of the invention is to provide a method of reducing haze on plastic containers, the plastic comprising at least one plastic component, which is PEN.
Still another object is to provide a solution of the above haze problem by the use of a coating on a refillable container for beverages.
These objects are attained by the subject matter of claims 1, 6 and 10, respectively.
Preferred embodiments of the invention appear from the dependant claims.
As to the above haze problem, this is particularly accentuated in case of refillable containers or bottles wherein the bottles are passed through a certain number of trips and are exposed to diluted lye solutions when being cleaned. These trip passages bring about a more severe degradation of the outer surface layer of the bottles. This severe problem is solved by the invention.
In accordance with the invention the haze problem is solved by providing the container with a protective coating, by which the microscopic voids are "filled", thereby reinstating the even surface and reducing the haziness.

Preferred Embodiments

According to a preferred embodiment, the container is provided with a protective film-forming coating. In case of refillable containers which during their shelf lives are subjected to several washing-filling cycles, the containers are provided with such coatings at appropriate times, preferably either between washing and filling or after filling.
According to still another preferred embodiment the container comprises a plurality of layers forming a structure which is often referred to as a multi-layer structure. Preferably, the outer layer is formed of PEN, although the invention by no means is restricted thereto.
It should be noted that the expression "PEN bottle" denotes a bottle solely made of PEN or comprising at least one layer of PEN.
According to a preferred embodiment, which relates to a refillable PEN container in the shape of a bottle, as defined above, the coating is soluble in hot water and in a diluted aqueous lye solution (alkaline) and the coating is applied after the bottle is refilled. Alternatively, the coating is applied before the bottle has been refilled. By this coating, any hazed PEN bottle can return to its original appearance and can be reused, thus resulting in a less number of discarded bottles.
The coating provided on the bottles do not stop the UV initiated degradation per se but fills the voids and thereby reduces the haziness to a very high extent. Thanks to the coating, the bottles have a very nice appearance resulting in a better acceptance by the consumers.
The coating materials are not critical to the invention. Any material can be used as long as the desired reduction of the haze is achieved by filling the voids in the outer surface. Those skilled in the art can select the most feasible material for each application. As non-limiting examples may be mentioned waxes, such as vegetable waxes, animal waxes, synthetic waxes, and mixtures thereof. Vegetable waxes are preferred. Waxes have in general a melting point in the range of 50-90°C. Thus, they can be removed from a surface by hot water. Emulsions of waxes, such as Carnauba wax, Japan wax provide very good results.
According to a preferred embodiment, the coating emulsion contains conventional additives which facilitate the coating process. Such additives comprise for example surfactants, especially biodegradable surfactants. The expression "wax based composition" as used herein does not exclude such additives. As a further example of an additive in the wax composition, UV stabilisers may be mentioned. They are generally incorporated in low concentrations. As alternative coating materials various lacquer resins can be mentioned, preferably of latex type. Emulsions of natural or synthetic resins, e g epoxy resins, phenolic resins, acrylic resins, etc, with ability to form film coatings are suitable. As mentioned above, the choice of coating material is not critical and is substantially determined by other factors, such as supply, costs, coating technique, etc.
The coating is applied on the surface of the bottle using conventional processes, such as roll coating, spray coating, immersing or brushing, dried and optionally heat-treated to form a film having a thickness of up to about 0,1 mm.

Claims

A plastic container for beverages, such as carbonated soft drinks, beer and the like, having improved resistance to haziness, the plastic comprising at least one plastic component, characterised in that said at least one plastic component is PEN, and that the outer surface of the container is provided with a coating imparting improved resistance to haziness to the container.
A container as claimed in claim 1, wherein the container comprises a multi-layer structure, in which at least one layer comprises PEN.
A container as claimed in claim 1 or 2, wherein the container is refillable and provided with the coating in connection with the refilling.
A container as claimed in claim 1 or 2, wherein said coating comprises a composition based on waxes or a mixture thereof.
A container as claimed in any of claims 1-3, wherein said coating comprises a composition based on lacquer resin.
A method of reducing haze on plastic containers for beverages, the plastic comprising at least one plastic component, the container being refillable and of the type that is subjected to at least one trip during its shelf life, said trip including a washing step and a filling step, characterised in that said at least one plastic component comprises PEN and that a coating is applied on the outer surface of the container between said steps or after the filling step to impart to the container an improved resistance to haziness.
A method as claimed in claim 6, wherein the washing solution used in the washing step is alkaline.
A method as claimed in claim 6 or 7, wherein said coating comprises a composition based on waxes or a mixture thereof.
A method as claimed in claim 6 or 7, wherein said container comprises at least one layer comprising PEN.
Use of a wax or laquer based composition as an outer coating on a refillable container for beverages, said container comprising at least partly PEN said coating imparting to the container improved resistance to haziness.