Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The object of the present invention is a polymeric multi-
layered structure for the packaging of products sensitive to oxygen
such as foodstuffs and pharmaceutical products.
The invention applies in particwlar to structures comprising
5 a polymeric barrier layer to oxygen intended to protect the product
sensitive to oxygen throughout the entire life-time of the package.
In view of the fact that the polymers commonly used to maintain this
barrier to oxygen, in particular the polyvinyl alcohols (PVOH) and
the copolymers of vinyl alcohol and ethylene (EVOH), are sensitive
to humidity, they are protected by barrier layers to humidity, in
particular polyolefins or polyv~nyl chloride, usually through the
intermediary of adhesive layers (GB-9-l 384 791). However, such
protective layers do not completely prevent the passage of humidity
in cases of intermediate storage in humid air and, above all, during
sterilization, a circumstance which entails the progressive loss of
the properties of the barrier layer to oxygen such as EVOH as the
relative humidity increases.
In order to remedy this disadvantage according to EP-A-O
059 274 it has been suggested that a desiccating agent be introduced
into the multi-layered structure, in particular into the adhesive
layers surrounding the oxygen barrier. However, during sterilization
(usually at about 121 C in the case of foodstuffs) the passage of
oxygen through the multi-layered structure is quite considerable
since, on the one hand, the permeability of the EVOH is not only a
function of the humidity but also of the temperature and, on the other,
the passage of water vapour may lead to the saturation of the
desiccating agent which thus becomes ineffective. In addition, the
more impermeable the external wall of the structure to water, the
slower is the removal of the water retained by the EVOH layer after
sterilization.
The present invention enables the effect of this loss of
impermeability to oxygen during the life-time of the packaging to be
reduced by placing the humidity-sensitive layer in a manner such that
it is at the lowest possible level of relative humidity and which is
the fastest possible to evaporate once the package has been formed.
The solution according to the invention consists of placing the oxygen
barrier layer on the outside of the multi-layered structure, in
particular on the external wall of the packaging in the case where
the packaged product contains water.
Thus, the object of the invention is a multi-layered
structure for the packa~ing of products sensitive to oxygen, in
particular foodstuffs, comprising a polymeric barrier layer to
humidity and a polymeric barrier layer to oxygen whose permeabiiity
to oxygen is a function of the humidity, sald layers being optionally
bound together by an adhesive layer, wherein the polymeric barrier
layer bo oxygen is located at the outside of the structure.
The invention is part;cularly useful in the case in which
the "oxygen barrier layer" com~rises EVOH or consists of EVOH.
The "humidity barrier layer" may in particular be a poly-
olefin or any other polymer usually used for packaging foodstuffs or
pharmaceutical or medical products, and be in particular a coextrudable
polymer.
In accordance with an alternative, the multi-layered structure
according to the invention may comprise two or more than two successive
barrier layers to oxygen, in particular EVOH, optionally bound by
adhesive layers.
Thus, whereas the structures according to the state of the
art are of the type :
A/adhesive/EVOH/adhesive/B
in which A, B and the adhesives are barrier polymers to
humidity for the protection of the EVOH from humidity, the invention
consists of having the structure :
A/adhesive/EVOH
A being the polymeric barrier to humidity, in particular a polyolefin,
for example polypropylene, which is in contact with the packaged product
when the latter contains water, which is in particular the case for
most foodstuffs (their relative humidity varying from 60 to 100 %).
In this case the EVOH is in contact with the ambient air which is at a
lower level of humidity.
The invention may be explained as follows.
In the case of a package sealed according to the~state of the
art the situation is the following :
Foodstuff/A/adhesive/EVOH/adhesive/B/outside air
In such a structure at equilibrium the humidity of the EVOH -
hence its efficiency as oxygen barrier - is such that it lies between
~r~
the humidity of the foodstuff and the humidity of the ambient air,
but is always higher than the humidity of the air.
By placing the EVOH layer on the outside according to the
invention , its humidity is equal to the humidity of the air, hence
the lowest possible humidity in the place where the package is stored
and, consequently, the EVOH exhibits maximal capacity with respect to
its efficiency as oxygen barrier.
The multi-layered s-tructure according to the invention may be
constructed in a known manner, in particular by coextrusion of the
polymers forming each of the layers (e.g. by flat sheet die), by
extrusion - blow moulding, by injection - blow moulding, etc.
The following example illustrates the invention.
EXAMPLE
A multi-layered structure PP/adhesive/EVOH is produced by
extrusion in the form of a rigid sheet 1.2 mm thick and having the
following composition :
thickness
of layer
PP (Transparent homopolymer from
the Amoco company) ..................... 4 % 48 lum
PP (idem., with +5 % of white
TiO2 pigment) .......................... 90.5 % 1086 lum
Grafted PP homopolymer adhesive
(Bynel from the Dupont de Nemours
company)................................ 2.5 % 30 lum
EVOH (Selar from the DuPont de Nemours
company) ............................... 3 % 36 ~m
Trays (138 mm x 138 mm x 33 mm, about 20 g weight) are made
from this sheet by thermoforming by means of a RDM37.10 machine of
the ILLIG company.
The trays are filled with cold water and then fitted with
non-strippable covers (made of aluminium/lacquer) and sealed at 220 C
for 2s at 6 bars (0.6 MPa). The trays are placed in an autoclave and
maintained there at 121 C for 30 mn with a counter pressure of 2 bars
(0.2 MPa),then they are cooled by injection of cold water into the
autoclave. S~bsequently they are stored at 23 C and 50 % relative
humidity (RH).
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The trays are opened after a lapse of time of from 1 to
30 days after the sterilization step and the permeability is measured
each time a tray is opened.
The permeability to oxygen is measured by means of an apparatus
of the Oxtran type according to the ASTM D 3985 standard at 30 C and
65-70 % RH. As an example, a permeability of 0.022 cm 02/24 h was
measured after 1 day, 0.023 cm /24 h after 7 days and 0.018 cm 02/24 h
after 28 days.
For comparison, the same tests were performed on the multi-
layered structures just mentioned but turned inside out (EVOH on the
inside of the tray in contact with the water) and on a commercially
available multi-layered structure : PP/adhesive/EVOH/adhesive/PP having
the same total thickness and a EVOH iayer of the same thickness as the
structures just mentioned. The results of the measurements are reported
on the appended graph. ( Permeability with the EVOH on the inside in
contact with-the water was measured to be 0.038 cm 02/24 h after 7 days;
0.19 cm 02/24 h after 14 days; 0.19 cm 02/24 h after 28 days).
It is observed that :
- for the structure according to the invention (curve 1),
although the EVOH layer was loaded with water at the start of the
sterilization, it loses it as soon as the air becomes dry and this
effect is magnified after removal from the autoclave;
- for the known structure (curve 2), although the EVOH layer
is separated from the water by a barrier layer to humidity (PP), it is
loaded with water which remains, as it were, trapped in the structure
for several days after sterilization, a circumstance which leads to the
loss of its barrier capacity to oxygen; and
- in the case of the inside out structure (curve 3), the EVOH
layer in contact with the water absorbs water during the sterilization
(without being saturated) and afterwards it continues to take up water,
thus losing its barrier effect to oxygen.
The invention applies to multi-layered structures comprising
at least one EVOH layer or any other similar product acting as a barrier
to oxygen and sensitive to humidity, and at least one layer of another
extrudable polymer such as polyethylene, polypropylene, polystyrene,
ethylene polyterephthalate (EPT), polyvinyl chloride (PVC), polycarbonate ~PC),
etc
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The barrier effectiveness to oxygen of the EVOH placed
as the outside layer may be reinforced or guaranteed by one or more
additiOnal layers of EVOH, for example by a structure of the type :
PP/adhesive/EVOH/adhesive/EVOH.
It should be noted that the EVOH in the outside position
offers the following additional advantages :
- an antistatic effect
- a glossy appearance
- an easier check of its presence
(its absence as a possible resul-t of scratching may be revealed
by staining the adhesive).
The multi-layer structure according to the invention, in a
rigid or semi-rigid form, may be advantageously used for the manufacture
of various recipients such as trays, cartons, tubes, and in particular
sealable containers of this type.
