Method for producing containers

A method for providing a container with one or more layers of barrier material during the production of the container from a thermoplastic. A preform (10) is stretched in its axial direction, generally an amount corresponding to the total axial stretching required for conversion of the preform to a container, after which the stretched preform (11, 13) is provided with one or more barrier layers (12, 14), and the preform is then heated and converted to a container. In one embodiment, the axially stretched preform (15) is produced by assembling axially stretched part-preforms (13a,b) covered with one or more barrier layers. The combined preform is then converted to a container.

FIELD OF THE INVENTION 
The present invention relates to a method for producing a container of a 
thermoplastic material, provided with one or more barrier layers, in the 
production of which a preform is converted into the container, ad the 
preform is provided with a barrier layer only after the material in the 
preform has been stretched in the axial direction of the preform. 
PRIOR ART 
It has already been disclosed to produce containers, made of thermoplastic 
material, which are capable of resisting the inner pressure which exists, 
for example, when storing carbonated drinks, especially such drinks as 
fruit drinks, cola or beer. By using, for example, polyethylene 
terephthalate, hereafter referred to as PET, it is possible to produce 
containers with relatively thin walls. 
However, such PET containers with thin walls are permeable to gases such as 
carbon dioxide. Oxygen from the exterior, as well as light, can also pass 
through the walls. This behavior has an adverse effect on the contents of 
the container. 
For many applications it is therefore desirable that the container be 
provided with a layer of barrier material which only allows the passage of 
gases, or light, to a slight degree. It is known to provide the container 
with such layers by a number of different techniques. Examples of these 
are to coat the protective layer onto the finished container, to 
co-extrude a blank of which one layer consists of protective material, or 
to coat an inner or outer protective layer onto the preform. 
Especially in the case of the technique where the preform is provided with 
one or more protective layers, problems arise because each protective 
layer is greatly reduced in thickness during the conversion of the preform 
to a container. The use of protective layers is, however, necessary in the 
case of many applications, and therefore a number of solutions to the 
problem have been developed, in which protective layers of the necessary 
thickness are applied to the preform prior to its conversion to a 
container. In this alternative method, the protective layer is applied in 
a number of steps, for example through repeated dipping, where appropriate 
with intermediate drying. In another alternative, the liquid in which the 
preform is dipped is relatively viscous, whilst in a third alternative a 
coagulating material and an aqueous dispersion produce a suitable 
film-forming polymer after drying. 
A frequently used protective layer is applied as a dispersion of 
polyvinylidene chloride in water. After application, the mixture is dried 
at a relatively low temperature to avoid undesirable skinning on the 
surface of the mixture, which in turn impedes drying, ie. impedes the 
removal of water from the layer under the surface of the mixture. If a 
thick protective layer is required, the protective layer is applied in 
several steps, each new layer being dried as described above. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method for producing a 
barrier layer on a container, where the thickness of the barrier layer 
when applied to the container material is reduced. 
According to the invention, the preform is subjected to an axial 
stretching, generally corresponding to the total axial stretching required 
for the conversion of the preform to the container, before the barrier 
layer is applied. In such case, during the conversion to the container, 
the barrier layer which has been applied is reduced in thickness primarily 
only in proportion to the stretching of the material transversely to the 
axial direction. During the conversion of a PET preform to a container, 
the endeavour is to stretch the material, at least in the actual body of 
the container, ie. in the cylindrical part in the case of a 
bottle--biaxially and about 3-fold in each axial direction. As a result of 
the invention, the thinning-down of the barrier layer applied to the 
preform is thus reduced from 9-fold to about 3-fold, which greatly 
simplifies the application of the barrier layer to the preform. 
Prior to the conversion of the preform to the finished container, the 
material of the preform is heated to an appropriate moulding temperature. 
According to the invention, this heating is, in certain embodiments, 
allowed to complete the hardening of the applied barrier layer and to 
complete the bonding, associated therewith, between the material of the 
stretched preform and the material which forms the barrier layer. The 
reduction of thickness of the barrier layer on the preform, which the 
invention achieves, simplifies this procedure. 
According to one embodiment of the invention, the preform is produced by 
placing an inner part-preform into an outer part-preform, the outer 
diameter of the inner part-preform being only insignificantly less than 
the inner diameter of the outer part-preform. The barrier layer is applied 
to the outer surface of the inner part-preform and/or to the inner surface 
of the outer part-preform, before the two part-preforms are brought 
together to give the combined preform. 
The use of two part-preforms, which form a combined preform with a barrier 
layer located between the two part-preforms, has the advantages, inter 
alia, that in the filled container the contents do not come into direct 
with the barrier layers of the container, and that the barrier material, 
through being included in the container wall, is not able to detach from 
any of the surfaces of the container. Particularly in the case of a 
barrier applied as a coating to the outer surface of the container, it has 
been found that on storage of carbonated drinks, carbon dioxide passes 
through the container wall and as a result the barrier layer is prone to 
detach from the outer surface of the container wall. The inclusion inside 
the container wall, achieved by the application of the invention, makes 
this impossible, 
According to an alternative embodiment of the invention, the two 
part-preforms are each provided with a barrier layer, one layer preventing 
the passage of gases and the other the passage of light, or one preventing 
the passage of oxygen and the other the passage of carbon dioxide. 
Preferably, the oxygen barrier is applied to the outer part-preform. 
In a further alternative embodiment of the invention, one of the 
part-preforms, preferably the inner part-preform, is provided with two or 
more barrier layers on top of one another. Since, as a result of the 
heating of the combined preform, and the subsequent conversion to a 
container, the barrier layers are in good contact with, and are safely 
enclosed between, the layers of the material, the function of the barrier 
layers is safeguarded. 
In yet another embodiment, the composite axially stretched preform consists 
of more than two axially stretched part-preforms which are arranged one 
within the other. Barrier layers as above are in this case applied to one 
or more of the part-preforms which together constitute the axially 
stretched composite preform. 
In yet another embodiment, the axially stretched preform, coated with a 
barrier layer, is used as a part-preform which, together with one or more 
part-preforms which have not undergone axial stretching, constitutes the 
composite preform. The abovementioned combination of part-preforms is used 
particularly when some of the part-preforms are made of a material which 
cannot be stretched at a relatively low temperature, ie. at a temperature 
in the region of, or below, the glass transition temperature (TG). It is 
occasionally necessary to use such material in order to achieve material 
layers which, in the desired container, provide the required properties in 
respect to gas penetration, light penetration, reaction with the 
surrounding atmosphere or with the goods stored in the container, and the 
like, these properties not being achievable solely by applying barrier 
layers to the surfaces of the part-preforms or preforms. 
It is even possible to apply a print and/or decoration to the outer 
surfaces of one of the inner part-preforms. The design of the print and/or 
decoration is in that case adjusted to the stretching of the material 
which takes place during the subsequent conversion of the preform to a 
container. 
In a preferred embodiment of the invention, the material in the preform or 
part-preforms is stretched in an axial direction at a temperature close to 
or in the region of the glass transition temperature (TG). Preferably, the 
material is stretched by a mechanical procedure, wherein the preforms pass 
through one or more draw rings which reduce the wall thickness of the 
preforms and at the same time equalize the outer diameters of the 
preforms. Such a stretching procedure is described in Swedish Patent 
Application No. 8004003-3. As a result of the mechanical orientation it is 
thus easy to achieve an outer diameter for the inner part-preform which 
exactly matches the inner diameter of the outer part-preform and the 
changes in the dimensions which the application of barrier layers on the 
part-preforms entail.

DETAILED DESCRIPTION 
FIGS. 1 and 2 show a preform 10 which, after axial stretching, has resulted 
in an axially stretched preform 11a or an axially stretched preform 11b 
with a bottom portion of unstretched material. The material of preform 10 
is amorphous. For converting the preform to the axially stretched preform 
11, the cylindrical part of the preform is lengthened by a stretching 
procedure, with simultaneous reduction of the wall thickness. Preferably, 
this is carried out by passing the preform 10 through one or more draw 
rings having an internal diameter less than the diameter of the preform 
prior to such passage. In order to produce the axially stretched preform 
according to FIG. 2a, the stretching procedure just mentioned is 
supplemented by stretching the material in the bottom part of the preform. 
In FIG. 2b, an inner barrier layer 12b has been applied to the stretched 
preform, whilst in FIG. 2a an outer barrier layer 12a has been applied to 
the stretched preform. 
FIGS. 3a-3f show an embodiment of the invention where two preforms, 10a and 
10b respectively, are matched to one another so that after completed axial 
stretching they yield stretched preforms 13a,b of such dimensions that 
they fit into one another. When the smaller preform 13a is placed in the 
larger preform 13b, the two preforms yield a composite preform 15. In the 
figures, the smaller preform 13a is provided with an outer barrier layer 
14a and the larger preform 13b with an inner barrier layer 14b. This 
creates an area in the composite preform, in the transition zone between 
the two part-preforms, which consists of the two barrier layers 14 a and 
b. In FIG. 3f, the barrier layers are shown with an exaggerated thickness. 
The material in the preforms 11a and 11b or in the composite preform 15 is 
heated to a shaping temperature and thereafter the preform in question is 
converted to the container. It is of course possible, before converting 
the particular preform to a container, to impart at the mouth of the 
particular preform suitable closure means, such as screw threads and the 
like, for the finished container. It is also possible, according to the 
invention, to start with preforms which, before axial stretching, are 
already provided with such closure means. 
In the above description, the axial stretching of the preforms takes place 
by passing the preforms through draw rings, often of successively 
diminishing diameter. The invention is obviously applicable also to those 
production processes where the axial stretching of the preforms takes 
place by other means. For the embodiment of the invention where axially 
stretched part-preforms constitute a composite axially stretched preform, 
it is obviously necessary that appropriate methods of stretching should 
result in part-preforms with tolerances which allow the part-preforms to 
fit into one another. 
By applying the barrier layers to the axially stretched preforms, the 
layers are applied to a surface which, for a finished container of a given 
size, is considerably greater than the area of application in the 
techniques used hitherto, where unstretched preforms are covered with a 
barrier layer. When using, for instance, PET, it is for many applications 
desired to have biaxial orientation of the material, which is achieved by 
stretching the material at least about 3-fold in one of the axial 
directions. As a result the material acquires, inter alia, the mechanical 
properties required for the particular applications. 3-fold stretching of 
the material in each axial direction means that the ratio of the surface 
of the preform to the surface of the container is 1:9, which in turn means 
that the thickness of the barrier layer applied to the preform is reduced 
in the same ratio. In certain applications, the required properties are 
achieved by about 3-fold monoaxial stretching, followed by lesser 
transverse stretching. In applications where the stretching ratios just 
mentioned are employed, the invention has the effect that the barrier 
layer thickness is reduced at most 3-fold, which in turn has the advantage 
that the barrier layer of the preform does not have to be as thick as when 
using the technique previously employed. As the application of thick 
barrier layers is a difficult part of the production of containers, the 
invention provides a considerable simplification of the production 
process. 
Especially when using a composite preform, it is, in certain applications, 
advantageous to let the final hardening as well as the bonding of the 
barrier layer to the material of the part-preform take place after the two 
part-preforms are placed inside one another. If the barrier layer is 
hardened when the part-preforms have been placed inside one another, and 
preferably simultaneously with heating the preform to the moulding 
temperature of the material, good contact between the barrier layers is 
achieved. Even if only one barrier layer is used, good contact with the 
material of the part-preform which does not carry the barrier layer is 
achieved by the procedures as mentioned. 
According to the invention, one or several barrier layers are applied to 
the respective preforms by dipping in a solution containing a barrier 
material. In an alternative embodiment, the barrier layer is sprayed onto 
the preform. Other coating processes can of course also be employed within 
the scope of the invention. 
The production of an axially stretched preform 11, 13,15 of polyethylene 
terephthalate starts with a preform 10 a,b. The material in this preform 
will have a crystallinity less than 10% and preferably less than 5%. In 
one single operation or a number of consecutive operations the thickness 
of the material in the preform is reduced to approximately 1/3 of its 
original thickness. This reduction in thickness takes place either along 
the entire length of the preform or in one or more sections of the 
preform. Use is made of a draw ring in which the relationship between the 
internal circumference of the draw ring and the external circumference of 
the perform is such that the thickness of the material will be reduced as 
the draw ring is moved axially along the preform. The temperature of the 
material immediately before the reduction in thickness takes place must be 
in the range of or lower than the glass transition temperature (TG) of the 
material, which temperature will be abbreviated to TG in the following, 
and should deviate from TG preferably by no more than 15.degree. C. 
Although the technical effect of the present invention may be achieved at 
a much lower temperature, it is advantageous to use an initial temperature 
close to TG, for instance a temperature which is between 1.degree. to 
3.degree. C. below TG, since material at this initial temperature will 
permit the draw ring to be moved at high speed. In certain typical 
applications, the draw ring operates in conjunction with an internal 
shaping device located inside the preform, whereby the external dimension 
of the shaping device fits the internal surface of the preform. In other 
applications only the internal shaping device is used. The thickness of 
the material in the preform is reduced by contact with the draw ring 
and/or the internal shaping device as said draw ring and/or shaping device 
is moved axially along the preform. During the re-shaping operation a 
transitional zone is formed between material of the original thickness and 
material with reduced thickness, said transitional zone gradually moving 
axially along the preform. The material in the transitional zone is kept 
at a temperature close to TG during the re-shaping operation by the 
transfer of heat to the draw ring and/or the shaping device located inside 
the tubular preform. In certain applications the material in the 
transitional zone is, however, allowed to assume a temperature which 
exceeds TG by no more than 30.degree. C., and preferably by no more than 
15.degree. C. 
In certain applications the material in the area adjacent to the 
transitional zone is cooled to a temperature below TG immediately after it 
has been reduced in thickness. 
In accordance with the present invention the possibility is offered of 
producing an axially stretched preform with mainly a monoaxial orientation 
and in which the material has been reduced in thickness and whose external 
circumference has been reduced and/or the internal circumference has been 
increased compared to the circumference of the corresponding sections of 
material in the preform. 
The invention also contains the possibility of further increasing the 
crystallinity by heating the material, over and above the crystallinity 
which is produced in the material in conjunction with the monoaxial 
orientation. This crystallinity should not be allowed to continue to such 
a point that the ability of the material to undergo further re-shaping is 
impaired. The crystallinity of PET is normally allowed to reach a maximum 
level of approximately 30% when the preform is to undergo further 
re-shaping. Crystallinity is preferably allowed to lie between 10-25%, 
whereas the crystallinity produced by monoaxial orientation will achieve a 
maximum value of approximately 17%. 
The crystallinity values stated in the present application relate to the 
theories disclosed in the publication "Die Makromolekulare Chemie" 176, 
2459-2465 (1975). 
It has been assumed in the above description that the reduction in the 
thickness of the material until it reaches its final value takes place in 
a single reduction stage. The invention also contemplates the possibility 
of reducing the thickness of the material in a number of consecutive 
reduction stages, before finally reducing the thickness of the material, 
in the case of PET, to approximately 1/3 of its original thickness in a 
final stage. In this case the draw ring or draw rings will consist of a 
number of ring sections for the consecutive gradual reduction of the 
thickness of the material. The embodiment described in this paragraph is 
used mainly when the material in the preform has a large wall thickness 
and/or when the draw rings are moved at high speed. 
Tubular preforms of circular cross-section have been shown in the above 
description. The invention may, of course, also be applied to tubular 
preforms of other cross-sections. 
A large number of materials of the polyester or polyamide type are known to 
exist, however, and to have similar characteristics. The invention as such 
is also applicable either in whole or in part to these materials, provided 
that the reductions in thickness and the temperatures are adjusted to suit 
the specific requirements of the respective material. The following are 
typical materials for which the present invention is suitable, after the 
indicated adjustments have been made: polyethylene terephthalate, 
polyhexamethylene adipamide, polycaprolactam, polyhexamethylene 
sebacamide, polyethylene-2,6- and 1,5-naphthalate, 
polytetramethylene-1,2-dioxybenzoate, and copolymers of ethylene 
terephthalate, ethylene isophthalate, and other similar plastics polymers.