Barrier film containing an ethylene vinyl alcohol copolymer, its preparation and its use for packaging.

The present invention relates to the field of films and more particularly that of so-called "barrier" films resistant to hot water and superheated water vapor intended for packaging food products.

In order to ensure the preservation of food products, it is necessary to ensure not only protection against dust but also against humidity, against certain gases such as oxygen and carbon dioxide and against odors.

To this end, polymeric resins called barrier resins are used which have properties of permeability to gases and to water vapor, that is to say which are more or less permeable to gases and to water vapor.

In general, the films are composed of one to several layers constituted by polymers impermeable to gases and polymers impermeable to water vapor.

Thus, when the oxygen permeability must be low, polymers are used whose barrier properties to this gas are very effective, such as, for example, ethylene-vinyl alcohol copolymers (EVOH) which are recognized as excellent materials for this regard.

If one wishes to obtain sufficient protection against water vapor, a barrier layer consisting of polyolefins such as polypropylene is used.

The requirements for impermeability to gases and water vapor varying from one form of packaging to another, from one food product to another, films which may be made up of one to several layers of polymer having different gas and water vapor permeability properties.

In order to improve the quality of "fresh" food products and to increase their shelf life it is necessary to condition food products aseptically.

To this end, many methods exist. Examples include pasteurization and sterilization.

In these processes the packaging - or the packaged food product - is subjected to the action of hot water (85 ° C - 100 ° C) or to the action of superheated water vapor for times ranging from a few seconds to several hours. In the case of sterilization, for example, the temperature reaches 121 ° C or even 135 ° C - 140 ° C. It is then necessary for the packaging consisting of a film having one or more "barrier" layers to mechanically resist the combined action of water and temperature as well as to pressures which can reach 3 to 4 bars.

It is noted that certain films based in particular on EVOH undergo at the time of sterilization or under the action of hot water degradations which are manifested by crumpling of the affected layer or even total delamination of the multilayer film.

We have now found a film having improved resistance to hot water and to superheated steam, characterized in that it consists entirely or partially of an alloy comprising at least one ethylene-vinyl alcohol copolymer ( EVOH), at least one polypropylene and at least one compatibilizing agent.

The film according to the invention is characterized in that the alloy comprises at least 30% by weight of EVOH. Among these films, preference will be given to those from alloys comprising:<ul id="ul0001" list-style="dash"><li>42.5% to 99.5% by weight of EVOH</li><li>42.5% to 0.4% by weight of polypropylene</li><li>0.1% to 15% by weight of compatibilizing agent.</li></ul>

The EVOH composition of the alloy is characterized in that the matrix or dispersing phase is EVOH. Polypropylene is dispersed in the form of more or less spherical nodules or cylindrical rods.

The adhesion between the phases is preferably ensured by a graft copolymer based on α-monoolefin called compatibilizing agent which is described in the European patent application published on 15.09.1989 under N ° 0 342 066 and the content of which is incorporated by reference herein.

Among the compatibilizing agents cited in the European patent application cited above with reference, preference will be given to compatibilizing agents of formula AMXP:<ul id="ul0002" list-style="dash"><li>AM represents a backbone copolymer obtained by copolymerization of propylene and α-olefins,</li><li>X is chosen from the group consisting of citraconic anhydride, fumaric acid, mesaconic acid, 3-allylsuccinic acid anhydride and maleic anhydride,</li><li>P represents a polyamide oligomer derived from caprolactam, 11-amino-undecanoic acid or dodecalactam.</li></ul>

EVOH is a copolymer consisting essentially of ethylene and vinyl alcohol units and may contain small amounts of other monomer units, in particular vinyl ester units. These copolymers can be obtained by saponification or partial or total alcoholysis of ethylene-vinyl ester copolymer. Among vinyl esters, vinyl acetate is the preferred monomer. The degree of saponification or of alcoholysis is at least equal in moles to 90% and preferably is between 94 and 99.5%. The proportion in moles of ethylene in the EVOH ranges from 3 to 75% and preferably from 10 to 50%.

Polypropylene is a polymer essentially made up of propylene units. It may contain amounts (at most 40 mol%) of another monomer unit, in particular ethylene and / or one or more α-olefins containing 4 to 6 carbon atoms. In particular, a homopolypropylene or a block copolymer of propylene with 0.5 to 30 mol% of ethylene or a random copolymer of propylene and ethylene containing 0.1 to 30 mol% of ethylene is used.

To the three main constituents which are EVOH, polypropylene and the compatibilizing agent, small amounts of other compounds of the stabilizing, antistatic, lubricating and other polymeric type can optionally be added.

The constituents being in the form of powder or granules, the preparation of the alloy can be carried out in solution or in the molten state by means of a transformation machine of the single-screw or twin-screw or counter or co-rotating extruder type or a mixer to cylinder.

The monolayer films according to the invention are made entirely of alloy.

The multilayer films consist of at least one alloy layer, the other layer or layers consist of polymers chosen from the group consisting in particular of polyethylene (PE), polypropylene (PP), polyamide (PA), polyethylene terephthalate (PET), polycarbonate (PC).

For certain polymers, it is necessary to use a coextrusion binder which ensures better adhesion between the layers.

In the multilayer films according to the invention, the alloy layer can be external or internal.

As examples of multilayer films according to the invention, mention may be made of films:<ul id="ul0003" list-style="dash"><li>PP / alloy / PP</li><li>PP / coextrusion binder / alloy</li><li>PP / coextrusion binder / alloy / coextrusion binder / PP</li><li>PA / alloy.</li></ul>

The films according to the invention can be obtained by flat extrusion or by blow molding at temperatures between 200 ° C and 260 ° C.

In addition to the coextrusion techniques in flat die or in annular die, the films according to the invention can be implemented according to techniques such as extrusion-coating and laminating-lamination, the latter technique allowing for example to associate a film and a sheet of metal such as aluminum.

The films according to the invention produced from the alloy have improved resistance to hot water and to superheated steam.

Thus, monolayer or multilayer films subjected to superheated water vapor at temperatures between 100 ° C and 140 ° C, under pressures ranging from 1 to 4 bars keep their transparency and have a practically intact surface appearance, so that the same films made entirely or partially of EVOH become opaque and are striated, deformed or delaminated.

The films according to the invention made entirely or partially of the alloy can enter into the development of various packages and all particularly they can be used to make sachets, flexible envelopes, trays, pots, ... for the packaging of food products.

The films according to the invention can also be used for making membranes used in the medical field, in techniques such as hemodialysis or plasmaphoresis.

The following examples illustrate the invention.

- <b><u style="single">Tests 1 to 5</u></b>

a / Five compositions are produced using a corotative twin screw extruder with the proportions of the constituents:<ul id="ul0004" list-style="dash"><li>EVOH: 99.3% hydrolyzed ethylene-vinyl acetate copolymer containing 32 mol% of ethylene with a melt index (Melt Index: MI) 3.2 g / 10 minutes measured at 210 ° C under 2.16 kg ,</li><li>PP: ethylene-propylene random copolymer at 3% by weight of ethylene of MI 2 g / 10 minutes measured at 230 ° C. under 2.16 kg,</li><li>compatibilizing agent: graft copolymer as indicated in Table I, prepared according to Example 1 of European patent application No. 0 342 066.</li></ul>

<u style="double">In this table</u>: <ul id="ul0005" list-style="none"><li>A is a motif derived from propylene</li><li>M is a unit derived from ethylene</li><li>X is a motif derived from maleic anhydride</li><li>P is an oligomer of caprolactam with a molecular mass of 2700 determined by potentiometric assay of the terminal amine functions.</li></ul>

b / These compositions are then extruded in the form of a film 20 μm thick using a single screw extruder at a temperature ranging from 210 ° C to 230 ° C.

These films are observed by scanning electron microscopy (SEM) after cryogenic fracture. (See Table I)

Due to the orientation due to the extrusion of the film, the dispersed phase is in the form of rods oriented in the direction of extrusion: the size of the rods is measured on the fractures after dissolving the dispersed phase in xylene .

The evaluation of the adhesion between phases is made on the fractures in a qualitative way.<tables id="tabl0001" num="0001"><img file="EP0441666A2_D0001.tif" /></tables>

vs/ <u style="double">Hot water resistance of monolayer films</u>

1/ <u style="single">Composition of films</u><ul id="ul0006" list-style="dash"><li><b><u style="single">Example 1 (comparative)</u></b> Composition of the monolayer film: 100% ethylene-vinyl alcohol copolymer with 32% ethylene (Melt Index: 3.2 g / 10 minutes measured at 210 ° C under 2.16 kg)</li><li><b><u style="single">Example 2</u></b> Composition of the monolayer film according to the invention: 100% of the alloy having the following composition by weight:<ul id="ul0007" list-style="none"><li>. EVOH of example 1:<b>80,75</b><b>%</b></li><li>. PP: ethylene-propylene random copolymer with 3% ethylene (MI: 10 - 230 ° C / 2.16 kg):<b>14,25</b><b>%</b></li><li>. compatibilizing agent A₆₆₃MX_2,91P_0,87: <b>5</b><b>%</b></li></ul></li><li><b><u style="single">Example 3 (Comparative)</u></b> Composition of the monolayer film: 100% of ethylene-vinyl alcohol copolymer at 44% by mole of ethylene (MI: 3.5 - 210 / 2.16 kg).</li><li><b><u style="single">Example 4</u></b> Composition of the monolayer film according to the invention: 100% of the alloy having the following composition by weight:<img file="EP0441666A2_D0002.tif" /></li><li><b><u style="single">Example 5</u></b> Film having the composition of the film of Example 2, but obtained by extrusion blow molding.</li></ul>

2/ <u style="single">Conditions for hot water resistance tests</u>

The films are stretched in a slide frame and are then immersed for 30 minutes in a distilled water bath, thermostatically controlled and slightly stirred by a magnetic bar.

- <u style="single">Assessment of results</u>

At the end of the test, the appearance and transparency of the films are evaluated according to the following arbitrary classification:<maths id="math0001" num=""><img file="EP0441666A2_D0003.tif" /></maths>

Thus a film of unchanged and opaque appearance will be classified A4.

The results are reported in Table II<tables id="tabl0002" num="0002"><img file="EP0441666A2_D0004.tif" /></tables>

The monolayer films according to the invention - examples 2, 4 and 5 exhibit better resistance to hot water than the monolayer films based on EVOH - examples 1 and 3. The best results are obtained with the alloy based on d EVOH containing 32 mol% of ethylene (Example 5).

d / <u style="double">Resistance to superheated steam (sterilization)</u>

1/ <u style="single">Composition of films</u><ul id="ul0008" list-style="dash"><li><b><u style="single">Example 6 (comparative)</u></b> Composition of the monolayer film: same as example 1.</li><li><b><u style="single">Example 7</u></b> Composition of the monolayer film: same as example 2.</li></ul>

2/ - <u style="single">Conditions for tests of resistance to superheated steam</u>

The tests carried out at a temperature equal to or greater than 100 ° C. were carried out in an autoclave.

The films, stretched in a stainless steel frame (5 × 5 cm) are placed in the autoclave and subjected for 30 minutes to the conditions of the test which can be carried out either in saturated vapor or in superheated water.

In the case where the test is carried out with superheated water, the water is preheated in a tank annexed to the autoclave and then rapidly introduced into the autoclave by means of a device provided for this purpose.

The pressures in the case of tests carried out with superheated steam range from 1 to 3 bars.<ul id="ul0009" list-style="dash"><li><u style="single">Assessment of results</u> The arbitrary classification mentioned above is used for the tests of resistance to hot water.<u style="double">Results</u></li><li><u style="single">Superheated steam temperature</u>: 120 ° C<dl id="dl0001"><dt>- <b><u style="single">Example 6 (comparative)</u></b></dt><dd>D4</dd><dt>- <b><u style="single">Example 7</u></b></dt><dd>B1</dd></dl></li></ul>

e / <u style="double">Resistance to superheated water vapor, three-layer films</u>

1/ <u style="single">Composition of films</u>

Three-layer films, the composition and thicknesses of which are given in Table III, are obtained by blowing coextrusion.<tables id="tabl0003" num="0003"><img file="EP0441666A2_D0005.tif" /></tables>

In this Table, Copolymer X denotes a coextrusion binder (propylene-ethylene copolymer grafted with maleic anhydride).

These tests were carried out on a three-layer blowing coextrusion equipment.

2/ <u style="single">Coextrusion conditions</u><ul id="ul0010" list-style="dash"><li><u style="single">extruder 1</u> (copolymer X)    temperature: 210 ° C - 230 ° C    screw speed: 132 rpm</li><li><u style="single">extruder 2</u> (copolymer X)    temperature: 210 ° C - 230 ° C    screw speed: 122 rpm</li><li><u style="single">extruder 3</u> (alloy or EVOH)    temperature: 210 ° C - 230 ° C</li><li><u style="single">Faculty</u>: <ul id="ul0011" list-style="none"><li>. diameter: 150 mm</li><li>. inflation rate: 8.12</li><li>. temperature: 220 ° C</li></ul></li><li><u style="single">drawing speed</u>: 8.81 m / mm</li></ul>

The results are collated in Table IV.<tables id="tabl0004" num="0004"><img file="EP0441666A2_D0006.tif" /></tables>

f / <u style="double">Resistance to superheated water vapor of five-layer films</u>

1/ <u style="single">Composition of films</u>

Five-layer films are obtained by flat coextrusion, the composition and thicknesses of which are given in Table V.<tables id="tabl0005" num="0005"><img file="EP0441666A2_D0007.tif" /></tables>

2/ - <u style="single">Coextrusion conditions</u>

These tests were carried out on five-layer coextrusion equipment.<ul id="ul0012" list-style="dash"><li><u style="single">extruder</u> (PP)    temperature: 200 ° C - 245 ° C</li><li><u style="single">extruder</u> (coextrusion binder)    temperature: 220 ° C - 245 ° C</li><li><u style="single">extruder</u> (EVOH or alloy)    temperature: 215 ° C - 245 ° C</li><li><u style="single">Faculty</u><ul id="ul0013" list-style="none"><li>. air gap: 0.7 mm. width 2 m</li><li>. temperature: 250 ° C</li></ul></li></ul>

The results are collated in Table VI.<tables id="tabl0006" num="0006"><img file="EP0441666A2_D0008.tif" /></tables>