Oxygen barrier properties of pet containers

A container wall of stretched plastic material has high oxygen barrier properties by incorporating an activating metal into the plastic material. The plastic material is PET in admixture with a polyamide and the metal is either added to the mixture or contained in one or both of the polymers. The material is stretched and aged to produce the container wall with the high oxygen barrier properties. The metal is preferably a transition metal and can be derived from a salt, such as a halide or acetate.

TECHNICAL FIELD 
The present invention in general relates to the provision of improved 
barrier properties in packaging containers of plastic material in which 
the plastic material comprises a mixture of polyethylene terephthalate 
(PET) and polyamide, and in particular to a method of producing a 
container having high oxygen barrier properties and to a container wall 
forming a part of such a container. 
BACKGROUND ART 
Within the packaging industry, there is a progressive change towards the 
use of containers of plastic material. This relates to both containers for 
beverages, including carbonated beverages, and containers for foods. As 
far as foods are concerned, there is an express desire in the art also to 
be able to employ containers of plastic material for the storage of 
preserved foods. In all of these fields of application, the insufficient 
barrier properties of the plastic material--and in particular its 
insufficient capacity to prevent the passage of gases, for example oxygen, 
vaporized liquids such as water vapor etc. entail that the shelf-life and 
durability of the products stored in the containers will be far too short. 
A number of proposals have been put forward in the art to solve the above 
problem, but, hitherto, the proposed technique has failed to meet 
established demands of cost in combination with barrier properties in 
order that containers of plastic material may successfully be employed 
within the above-outlined sectors. Examples of solutions proposed in the 
art are laminates in which two or more layers of plastic material are 
combined with one another and in which the material in each layer 
possesses properties which entail that, for instance, gas penetration, 
light penetration or moisture penetration are reduced. Solutions in which, 
for example, a metal such as aluminum is encapsulated between the plastic 
materials or, for instance, forms the inner surface of the container have 
also been suggested in the art. Such a solution is expensive and makes it 
difficult, if not impossible, to apply molding techniques conventionally 
employed in the plastic industry. Solutions in which barrier material 
other than metal is applied interiorly or in layers between the plastic 
material have further been proposed. Such solutions suffer from the 
drawback that they are expensive and, in addition, reduce the 
possibilities of recycling and reuse of the material, unless special 
measures are adopted in conjunction with the recovery process to remove 
the barrier material before the plastic material is reused. 
Solutions are also known in the art in which plastic materials of different 
types are mixed and thereafter molded to form containers by substantially 
conventional methods. Thus, for example, it is previously known to produce 
containers of plastic material in which the plastic material consists of a 
mixture of PET and polyamide. By way of example the polyamide is included 
in a proportion of between 4 and 10% by weight, preferably at a maximum of 
7% by weight. In the production of such containers the two materials are 
thoroughly intermixed, the thus mixed material is fed to an injection 
molding machine where the mixture is melted, and the molten mixture is 
injected to form a preform which is rapidly cooled for the formation of 
amorphous material, whereupon the preform, after heating, is expanded to 
form a container. 
In the technique described in the preceding paragraph, a certain reduction 
of the so-called permeability coefficient for oxygen will be achieved. The 
permeability coefficient is employed as a measure of the permeability of 
the material in respect of gases. For example, for containers of pure PET 
of a storage volume of 33 cl, a permeability coefficient for oxygen has 
been registered of the order of magnitude of between 3 and 4 when the 
containers are manufactured employing generally applied technology. In the 
application of the abovedescribed technology employing a mixture of PET 
and polyamide in the range of proportions stated above, a slightly lower 
permeability coefficient is obtained which, nevertheless, is relatively 
high and is of the order of magnitude of between 1 and 3, depending upon 
the amount of admixed polyamide. In real terms, this implies a 
prolongation of the shelf-life of, for example, beer from approximately 8 
weeks to approximately 16 weeks. Even though a prolongation of the 
shelf-life to 16 weeks may be of considerable importance, it is, 
nevertheless, of a marginal nature in many fields of application, in 
particular in applications within the food industry. The above-described 
technique of molding containers of PET with an admixture of a minor amount 
of polyamide has been tested repeatedly. By way of example, it might be 
mentioned that in five mutually independent trial series, the following 
results were obtained. 
______________________________________ 
Trial No. 
Weight percent polyamide 
Permeability Coefficient 
______________________________________ 
1 0 3.0 
2 2 2.4 
3 4 1.8 
4 6 1.3 
5 7 1.0 
______________________________________ 
It will be apparent from these results that, for pure PET, the permeability 
coefficient was measured at 3.0, while, with an admixture of polyamide, 
the permeability coefficient lay in the range of between 2.4 and 1.0. 
These disclosed values constitute mean values for 5 different containers 
or cans for each admixture percentage disclosed in the Table (admixture 
percentage 0 included, i.e. PET with no admixture of polyamide). For pure 
PET, the single highest value for the permeability coefficient was 3.4. At 
an admixture of 2% by weight the change in the permeability coefficient in 
relation to pure PET is essentially negligible. 
The technique for the manufacture of containers of PET and polyamide is 
conventional and corresponds to the recommendation issued by manufactures 
of raw material and adapted to suit the properties which these two 
material types possess. 
SUMMARY OF THE INVENTION 
Among the several objects of this invention may be noted the provision of a 
method of producing a container with a wall having high oxygen barrier 
properties, comprising stretching an orientable material to form a wall of 
the container, said orientable material comprising a mixture of PET and a 
polyamide in which mixture an activating metal is present which is capable 
of conferring high oxygen barrier properties to the material and aging the 
material at a determined temperature, humidity and time period to confer 
said high oxygen barrier properties to the wall. 
Another object of this invention is a container wall comprising stretched 
and aged material of a mixture of PET and polyamide containing an 
activating metal capable of conferring high oxygen barrier properties to 
the material, the components of the mixture being present in respective 
amount so that the wall has said high oxygen barrier properties. 
Other objects and features will be in part apparent and in part pointed out 
hereinafter. 
In accordance with the present invention it has, quite surprisingly, been 
found that the oxygen barrier properties in terms of the permeability 
coefficient can be highly improved (with a factor of approximately 100 or 
more) e.g. for a stretched and oriented material comprising a mixture of 
PET and polyamide in which the activating metal is present in the mixture 
and aging the material under certain conditions including temperature, 
humidity and time to confer said properties to the wall. 
The presence of the activating metal in the mixture of PET and polyamide is 
very critical in accordance with the invention and is a prerequisite for 
obtaining the highly improved oxygen barrier properties. The role of the 
metal will be elucidated in detail below. 
The presence of the metal is achieved by either adding a metal compound or 
a mixture of metal compounds to the mixture of PET and polyamide or to at 
least one of said polymers or relying on metals present in the polymer 
mixture as a result of the technique employed in manufacturing 
(polymerizing) each polymer or both. The presence of the metal as a result 
of addition is, at present, the preferred embodiment. There is a broad 
range of metal compounds that are effective in improving the oxygen 
barrier properties but quite a lot of such compounds can be excluded 
simply because they are too expensive. Another reason for excluding some 
compounds is based on lack of compatibility with the polymer or polymers. 
According to a preferred embodiment the metal of the metal compound is a 
transition metal selected from the first, the second and the third 
transition series of the periodic Table, i.e. iron, cobalt, nickel; 
ruthenium, rodium, palladium, and osmium, iridium, platinum. 
According to another preferred embodiment the metal of the metal compound 
comprises copper, manganese and zinc. 
Both aromatic and aliphatic polyamides can be used according to the 
invention. A preferred aromatic polyamide is a polymer formed by 
polymerizing meta-xylylenediamine H.sub.2 NCH.sub.2 --m--C.sub.6 H.sub.4 
--CH.sub.2 NH.sub.2 with adipic acid HO.sub.2 C(CH.sub.2).sub.4 CO.sub.2 
H, for example a product manufactured and sold by Mitsubishi Gas 
Chemicals, Japan, under the designation MXD6. A preferred polyamide of 
non-aromatic nature is nylon 6,6. According to another preferred 
embodiment copolymers of polyamides and other polymers are used. 
The invention is based on the finding that metal complexes, in particular 
of transition metals, have the capacity to bond oxygen and contribute 
thereto by reforming molecular oxygen, and on the utilization thereof in 
connection with polymers. 
The effect, which results in highly improved barrier properties, is called 
the oxygen scavenger effect or merely the scavenger effect. A prerequiste 
for this effect to occur is, in accordance with what is at present 
understood, the formation of an active metal complex, which is only 
possible if the polymer contains groups and/or atoms which have the 
capacity to coordinate to the metal ion and that the polymer chain(s) has 
the ability to occupy a conformation wherein the groups and/or the atoms 
are present in the correct positions in relation to the metal ion. Another 
prerequisite is of course that a metal ion, which has the capacity to form 
an active metal complex, is present at a location in the molecular 
structure where a forming of the complex is possible. Expressed in another 
way the ion during the formation of a metal complex "catches" or "takes 
care of" the oxygen thus forming a barrier against passage of oxygen. 
Thus, it is theorized that the key feature of the invention is the 
formation of a metal complex having the capacity to bond with oxygen and 
to coordinate to the groups and/or atoms of the polymer. 
As to the amount of metal present in the mixture of PET and polyamide this 
amount is not critical as long as the desired effect is obtained. One 
skilled in the art can without difficulty determine which concentration is 
appropriate in each case, but in general it can be said that a range of 
50-10,000 ppm (by weight), preferably 50-1000 ppm is proper. The upper 
limit is dictated by such factors as economy and toxicity. 
As metal compounds halides, in particular chlorides, of the above 
transition metals are preferred. 
As to the weight proportions between PET and polyamide in the mixture it 
may be said that an admixture of up to 10 percent by weight of polyamide 
renders the material brittle, which gives rise to problems in reshaping 
the preform into the container and insufficient mechanical strength of the 
final container. This insufficient strength gives rise primarily to 
problems in areas where the material is exposed to extreme stresses, for 
example in the discharge or mouth region when the container is sealed by 
the closing application of a metal cap. Further, the material in the 
container will become discolored or wholly or partly opaque or "hazed". In 
larger proportions of polyamide in the mixture, the material properties 
will deteriorate to such an extent that the containers can no longer be 
molded or will be become unusable for their contemplated purpose. On the 
other hand, the lowest concentration limit of polyamide amounts to 
approximately 0.5 percent by weight. 
Within said broad interval the proportion of polyamide in relation to PET 
can be varied mainly in view of the contemplated purpose of the container 
in question. At present, the preferred range is 1-7 percent by weight 
polyamide and the most preferred range is 2-4 percent by weight. 
The invention will be further described below in detail with reference to 
working examples and examples of preferred embodiments, especially 
comprising a preferred method of producing the container and the aging 
conditions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
500 g nylon 6,6 ("Ultramid" BASF) in the form of granules were refluxed for 
about 24 h with 500 ml of an ethanolic (96%) solution of cobalt chloride 
(CoCl.sub.2 .times.6H.sub.2 O) at a concentration of 0.24 g/ml. After 
refluxing during said time period the granules were dried and the cobalt 
content was determined and amounted to 7000 ppm. 
The experiment was repeated but this time poly-meta-xylylene adipamide was 
used instead of nylon 6,6. The cobalt content of the dried granules was 
4500 ppm. 
A mixture was prepared consisting of 98 percent by weight of PET and 2 
percent by weight of the above nylon 6,6 having a cobalt content of 7000 
ppm. A similar mixture was prepared consisting of 96 percent by weight PET 
and 4 percent by weight of the polyamide treated as described above and 
having a cobalt content of 4500 ppm. Prior to being mixed together the 
polyamide in question and PET were dried separately, the drying conditions 
being those recommended by the suppliers. By way of example the granules 
of PET and polyamide, respectively were held at a temperature in excess of 
approximately 90.degree. C., viz. within the temperature range of between 
100.degree.-140.degree. C. for a lengthy period of time, i.e. for at least 
8 h, and in this instance for at least 16 h. The materials were then fed, 
without being exposed to ambient atmosphere, into an injection molding 
machine where, in accordance with conventional techniques, they were 
melted and a preform was injection molded from the molten material. The 
material was held in the compression section of the injection molding 
machine at a temperature within the range of between 255.degree. and 
280.degree. C., preferably within the range of between 260.degree. and 
275.degree. C., and also in the injection nozzle generally within the same 
temperature range. The material in the preform was rapidly cooled so as to 
make the material amorphous. 
The amorphous preform was subsequently re-shaped into a container. In 
certain physical applications, this was effected in that the preform of 
amorphous material was expanded in the axial direction and/or in its 
circumferential direction into an intermediate preform which, hence, 
consisted of thinner material than the preform and preferably of at least 
monoaxially oriented material. The intermediate preform was subsequently 
subjected to further expansion so as to be finally shaped into the 
container. In other physical applications, the preform was converted into 
the container in a single forming stage. 
In one preferred embodiment, the intermediate preform was formed according 
to the technique described in U.S. Pat. No. 4,405,546 and GB 2 168 315. 
The technology described in these two patent specifications entails that 
the material in the walls of the preform passes, under temperature 
control, through a gap by means of which the material thickness is reduced 
at the same time as the material is stretched in the axial direction of 
the preform. There will hereby be obtained a monoaxial orientation of the 
material in the axial direction of the preform. As a rule, the gap width 
is selected to be sufficiently small to realize material flow in the 
transition zone between amorphous material and material of reduced wall 
thickness, i.e. oriented material. A mandrel is inserted in the thus 
formed intermediate preform, the circumference of the mandrel in its 
cross-section being greater than that of the intermediate preform, whereby 
the intermediate preform, on abutment against the mandrel, is expanded in 
its circumferential direction. By this expansion, there will be obtained 
favorably close contact between the material wall in the intermediate 
preform and the outer defining surface of the mandrel. In experiments, the 
mandrel had a surface temperature in excess of 90.degree. C., preferably 
exceeding 150.degree. C., which entailed that the oriented material 
underwent shrinkage in the axial direction of the preform. In the 
experiments, it surprisingly proved possible to carry out material 
shrinkage within a very wide temperature range, namely between 90.degree. 
and 245.degree. C. As a result of the heat treatment, the material also 
obtained a thermal crystallization in addition to the crystallization 
which occurred through the orientation of the material. Appropriately, the 
expanded and axially shrunk intermediate preform was thereafter trimmed so 
as to form a uniform discharge opening edge, in addition to which the 
discharge or mouth was, when necessary, given dimensions (by reshaping) 
which were adapted to suit a closure or seal. 
It has been surprisingly found that the low permeability coefficients are 
achieved if the material in the preform, in the intermediate preform 
and/or in the expanded intermediate preform (alternatively the container) 
is allowed to undergo an aging process. The reduction of the permeability 
coefficients will also be obtained in those cases when the aging of the 
material is accelerated by heat treatment. For reasons of production 
economy, a combination of temperature and humidity is selected which gives 
rapid aging of the material. In experiments, the material was kept at a 
temperature in the range of between 20.degree. and 100.degree. C. for 
periods of time which varied between 3 days and 10 months. The extremely 
low permeability coefficients were obtained at such a low admixture of 
polyamide as 2 percent by weight, for example on storage in an air 
atmosphere at approximately 50% relative humidity (RH) and at a 
temperature of 55.degree. C. for 3 weeks or during storage indoors with no 
special control of the air humidity, at a temperature of 22.degree. C. for 
3 months. The combination of approximately 100.degree. C. and 3 days gave 
a permeability coefficient of below 1. On both occasions, the air humidity 
was 50%. In fact, measurements made with containers formed of the mixture 
of PET and polyamide (2%) according to the invention and aged as just 
stated had permeability coefficients in respect of oxygen which have 
fallen below the lower limit of the registration capability of the 
measurement equipment which corresponded to a level of 0.05, and in 
subsequent experiments a level of 0.01. In general, it could be 
ascertained that, on storage at high temperature and during a certain 
period of time, lower permeability coefficients were obtained than if the 
material had been held at a lower temperature for an equally long period 
of time. Similarly, on longer storage at a certain temperature, a lower 
permeability coefficient was obtained than in shorter storage time at the 
same temperature. It has surprisingly proved that the contemplated effect, 
i.e. the reduction of the permeability coefficient to a certain level, is 
achieved for a shorter storage time in a heated state in applications in 
which the intermediate preform is formed and the intermediate preform is 
allowed to shrink in its axial direction at elevated temperature, for 
example by the employment of the technique described above. 
In the experiments conducted, primary use was made of granulate of 
polyamide marketed by Mitsubishi Gas Chemicals, Japan, under the 
designation MXD6, and granulate of PET marketed by Eastman Kodak, U.S.A., 
under the designation 7352. The amount of admixed polyamide was 2%, but 
experiments have shown that higher porportions of polyamide give a more 
rapid aging, but also a deterioration in mechanical properties of the 
material. At a level of 10 percent by weight, these properties become so 
poor that the container formed according to the specific process outlined 
in connection with U.S. Pat. No. 4,405,546 and GB 2 168 315 is no longer 
suitable for use in storing, after sealing, the products disclosed in the 
introduction to this specification. 
It is apparent from the foregoing description that a key feature of the 
present invention is the presence of an activating metal in the mixture of 
PET and polyamide and that said presence is responsible for the attainment 
of the high oxygen barrier properties in a container produced from said 
mixture. It should be emphasized that this improvement of the oxygen 
barrier properties is independent of whether said metal has been 
introduced by way of a positive step or the presence of the metal in the 
polymers is due to the metal catalyst added in the production of the 
polymers.