FR2482900A1 - MULTILAYER, THERMOSCELLABLE, THERMORETRACTABLE, POLYPROPYLENE-BASED FILM AND USE IN PACKAGING FOODSTUFFS - Google Patents

Abstract

THE INVENTION RELATES TO A MULTI-LAYER FILM CLOUDABLE TO HEAT. ACCORDING TO THE INVENTION, IT INCLUDES A BASE LAYER CONTAINING A HOMOPOLYMER OF PROPYLENE, A SKIN FORMING LAYER ATTACHED TO A SURFACE OF THE BASE LAYER, AND CONTAINING A MIXTURE OF 50 TO 100 BY WEIGHT OF A PROPYLENE-ETHYLENE COPOLYMER WITH 50 TO 0 BY WEIGHT OF A HOMOPOLYMER OF PROPYLENE. THE INVENTION APPLIES IN PARTICULAR TO THE PACKAGING OF FOOD PRODUCTS AND OTHER ARTICLES.

Description

The present invention relates to thermoplastic films and

shrinkable to heat for

  packaging, and in particular to films of poly-

  propylene having better heat sealing characteristics. These films can be used for packaging a wide variety of foods

and non-food items.

  Oriented polypropylene films are useful and widely accepted as packaging films because of their good moisture barrier, stiffness, high strength and optical properties. However, polypropylene films generally do not exhibit good heat-sealing properties, which is an important consideration in packaging applications. This is due to the fact that polypropylene films have shutter temperature ranges

which are very narrow.

  The range of shut-in temperatures for a thermoplastic film may be considered as starting at point o, when applying heat and pressure, the surfaces of the film will close or seal or bond to each other. other. As the temperature continues to increase, a point is reached where the thermoplastic product melts and flows relatively easily, thus making it difficult to obtain a seal in a specific area, thus marking the upper end of the range. shutter. Similarly, at the upper end of the shutter range, the film may tend to burn and darken, leaving weak and unattractive joints. For a propylene homopolymer film, as the temperature increases, the film does not become clogged but instead flows abruptly as desired while approaching its melting point, which makes it difficult for the film to form. obtaining a smooth, continuous and satisfactory seal. This type of sealing performance or

  shutter can not be tolerated in packaging equipment.

fast lage.

  In order to obtain a good heat-sealing ability, it is common practice to apply various types of film-coating. The coatings are traditionally applied by separate operations,

  for example from emulsions an extrusion coating

  and so on, but these operations are expensive and require additional manipulation of the film. Coextrusion has been successfully used to place a heat-sealable coating on polypropylene to produce a film having good sealing resistance, but up to the present invention rolled or laminated products with coextruded polypropylene film. have a very narrow range

  heat sealing, and are expensive to produce.

  Accordingly, the present invention has as its main object a polypropylene film having a broad

shutter range.

  Another object of the present invention is a polypropylene film having acceptable seal strength for shrink wrapping applications.

or narrowing.

  Typical patents disclosing coatings for a polypropylene film include US Pat. No. 3,285,766 of November 15, 1966 in the name of

  Edward Barkis et al., Where an extrusion process is disclosed

  a coating containing at least 65% ethylene on a polypropylene sheet; U.S. Patent No. 3,671,383 of June 20, 1972 to Rikita Sakata et al. discloses a biaxially oriented polypropylene film laminated on a film of a uniaxially oriented ethylene-pzupylene copolymer with at least by weight of polypropylene; U.S. Patent No. 4,132,050, January 2, 1979 to Roger N. Young, which discloses a film having a substrate formed from a mixture of 87.5 to 60 parts of polypropylene and 12.5 to 40 parts of a block copolymer of ethylene-propylene with a heat-sealable layer; and US Patent No. 4,148,972 issued April 10, 1979 to Toshi Hiro

  Yamano et al., Where is revealed a film of poly-

  propylene obtained by laminating or laminating a polypropylene layer containing 1 to 8% of an ionomer,

with a layer of an isomer.

  It has been surprisingly found that the main objects of the present invention, that is, to increase the range of shutter temperatures of a multilayer film of polypropylene and to obtain an acceptable seal strength for such a film, could be obtained by a multilayer film comprising a base layer of a propylene homopolymer and a skin layer attached to a surface of the base layer, the skin layer comprising a mixture of 100% by weight of a propylene copolymer ethylene with 50 to 0% by weight of the propylene homopolymer. The mixture * ipo.se.ompase. 60 to 80% of the copolymer with 20% of the homopolymer. In a particularly desirable embodiment, the skin layer contains 70% by weight of the propylene-ethylene copolymer. mixed with 30% by weight of the propylene homopolymer and the copolymer contains 2 to 6% by weight of ethylene or even preferably 3 to 4% by weight of ethylene. In addition, the ratio of the thickness of the skin layer to the thickness of the base layer is preferably

4 to 1 or more.

  In another aspect, the present invention relates to a method for increasing the range of shutter temperatures of a polyethylene homopolymer film which comprises the steps of forming a base layer of a propylene homopolymer; and laminating or laminating a skin layer on at least one side of the propylene homopolymer film, the skin layer being formed from a mixture of 60 to 80% by weight of a propylene-ethylene copolymer containing 2 at 6% by weight of ethylene; and biaxially stretching the rolled film to a final total thickness of the film of 0.006-0.0355 mm. Such a film will have a shutter range of at least 20 C and will provide a seal or seal strength greater than 1.157 N / cm. A preferred method of producing the laminated or laminated film is to coextrude the diaper

forming skin and the base layer.

  Another aspect of the invention relates to a method for producing a neatly closed and shrunk package of a polypropylene film which comprises the steps of forming a base layer of a film of a propylene homopolymer; laminating a skin layer on at least one side of the film of the propylene homopolymer, the skin layer comprising a mixture of 60 to 80% by weight of a propylene-ethylene copolymer containing 2 to 6% by weight ethylene; biaxially stretching the laminated film to a final total thickness of the film of 0.006 to 0.0635 mm to thereby produce a heat-shrinkable, heat-shrinkable film; to provide a product to be wrapped; wrapping the product in the film and bringing the surfaces of the skin layer into contact with each other at points to be heat sealed; applying heat and pressure to the shutter points to thereby seal the film at these points, the range of applied heat sealing temperatures being between 135 and 1600C, the joints of the resulting package or package having a resistance minimum of 1,157 N / cm; and apply heat between 110 and 135 C to this package for a time sufficient to shrink

the film around the product.

  In all aspects of the invention, the film

can be perforated.

  The preferred embodiment of the present

  The invention relates to a two-layer film formed

  by mixing 70% by weight of a propylene copolymer

  ethylene containing 3.5 to 4.2% by weight of ethylene, with 30%

2'482900

  by weight of a propylene homopolymer for the skin layer or outer layer. This mixture is mixed in a Banbury mixer and then fed to the hopper of an extruder which feeds an annular die to extrude a tube having two layers. The inner or base layer comprises propylene homopolymer which is fed into the hopper of a second extruder which also feeds

  the annular die to form the inner layer.

  In the annular die, the two tubular extrudates are joined to form a laminated multilayer tubular product or laminate. This tubular product is rapidly cooled and then collapsed. Then, the tube is inflated and heated to the polypropylene orientation temperature range of 135-150 ° C. The heating takes place in an oven, and as the flattened film emerges from the oven, it becomes through pinch cylinders and is opened in a bubble by the well known technique of trapped lime where it is stretched about 5 times, both vertically and horizontally, which reduces the film to a thickness of between 0.0152 and 0, 02 mm. Before stretching the typical tube may have a thickness of the order of 0.457 mm with a thickness ratio of the propylene layer

  to the mixed layer of 5: 1. We can find a description

  of such a bubble technique for biaxially orienting polypropylene, in US Patent No. 3,260,776

  July 17, 1966 in the name of C.A. Lindstrom and others.

  When the film has been expanded to form the bubble and the bubble has been cooled down, it collapses, is

split and is rolled up.

  In some cases, it is desirable to perforate the film to form an arrangement of perforations or small holes therein. Depending on the desired application, the number of holes may be between 1.2 and 1955 / cm 2 of film up to several tens / o 2, and the holes may have a diameter between 0051 and 0.51, mG A device The process of film piercing employs a cooled gate on which the film passes while being exposed to a hot gas jet melting the film in uncooled areas. Such a device and such a method are described in US Patent No. 3,038,19B of June 12, 1952 to C.H. Schaar. In order to determine the sealing range and the strength of the preferred seal or film seal, two films of the film were superimposed with the surfaces of the skin layers in contact. A 5 mm wide electric resistance heated flat shut-off bar was used to press the sheets together against a support surface at a pressure of 1 bar for 0.5 seconds and the temperature of the bar was recorded . After completing the blanking, a 2.54 cm strip was cut perpendicularly and through the bar seal, and the ends of the respective strips were placed in the jaws of an Instron test machine, with the seal or closure placed approximately at the midpoint between the clamping jaws. The force was applied by separating the jaws until the seal peeled or broke. Seals having a minimum strength of the order of 1,157 N / cm, the minimum considered sufficient for packaging applications, were obtained by the preferred embodiment at a bar temperature of only 1350C, and satisfactory joints were continued with a temperature increasing to about 1600 ° C. for a shutter range of about 25 ° C. (hereinafter

  field tests indicate that the range is 300C).

  It was possible to obtain a maximum resistance of the order of 5,407 N / cm. As previously indicated, a one-ply polypropylene film will have a shutter range of substantially zero width. Since the sealing bars are cooled by contact with the film, in packaging machines it is necessary, with polypropylene, to establish the temperature of the bars slightly above the melting point so that a closed package can be obtained, but joints made in this way have a bad appearance and burned areas. With a film such as that obtained according to the invention the establishment of sealing bars and temperatures near the top of the shutter range does not present the risk of melting the film or burn and speeds fast film tends to cool the sealing bars, they will always keep enough heat to close the film

  without being cooled below the shutter range.

  A number of products can be packaged in the film according to the invention, and the various machines used are different techniques. In some cases, the product is placed on one sheet of the film and then a second sheet is placed above the product and a complete perimeter closure is then performed; in some cases, the sealing operation also makes it possible to break the excess film in order to separate the packaging. After sealing, the product passes through a heat tunnel which may be at a temperature of only 1100 ° C, but which is usually established in the range

  o The film has been stretched. In the tunnel, heat

  forces stretching stresses to relax and forces the film to shrink around the product. In other packaging methods, the product can be placed between the folds of a folded film at its center, forming

  a shutter on the remaining three unsealed sides.

  Many well known methods for packaging a product into a shrinking film will be included with those

  who are competent in this area.

  Products such as bread, soft baked goods, such as rolls, pizzas and the like, can be packaged with the film of the invention. Food products that also need to breathe through the film such as fruits and vegetables can be packaged using the perforated film.

Examples.

  In order to solve the problem presented by the virtually non-existent polypropylene heat sealing range, a number of mixed compositions for a sealing layer have been attempted. Table 1 below gives the two films having a base layer or substrate of a polypropylene homopolymer, which has been coextruded with the layer composition.

  sealing or sealing particularly designed.

  Only compositions that can be successfully treated by the technique of the bubble trapped in the film

  are shown in Table 1.

  The notes in the table explain the abbreviations for the polymers, and the range of shutter and the resistance of the joint have been determined as explained above. The mist, of course, is the percentage of transmitted light which, when it is passing through the sample, deviates from the incident beam by more than 2.5 on average, and this is determined by the method of US ASTM D1003. The measurement of the indicated thickness is that

  laminated or laminated film.

TABLE I.

  Example Composition of the sealing layer Thickness (min) Range

shutter

  (C) Shutter tempo min to have 1,157 N / cm Maximum resistance of shutter N / CM

1 1 100% PP

2 90/10 PP / PB

3 80/20 PP / PB

4 75/25 PP / EVA

50/50 PP / EVA

6 90/10 PP / SURLYN

7 80/20 PP / SURLYN

8 50/40/10 PP / P-E /

Surlyn

9 50/50 PP / P-E

40/60 PP / P-E

he 30/70 PP / P-E

12 100% P-E

0.0178

0.0180

0.0175

0.0201

0.0195

0.0202

0.0183

0.01905

0.0185

0.0188

0.0185

0.0178

ZERO

(15-20)

  at 25 to 30 PP = film-grade resin homopolymer, predominantly isotactic polymer PB = polybutyl PE = ethylene copolymer = ethylene (about 4% ethylene) SURL = Du Pont EVA type 1601 = vinyl ethyleneacetate copolymer having about 10% of Vinyl acetate Haze% 1,928, 278 1,928 1,157

1, 157

1.35 1.543 4.628, 9778, 399

4, 628

  1.0 1.1 1.3 2.8 2.9 1.5 1.9 1.1 0.31 0.70 2.0 2.4 O0re 4-s Co "o O \ O Examples 3 , 10 and 11 are expressed as ranges because these samples were used on actual equipment on site and the sealing schedule was also measured in the laboratory and on the spot, an increase of about 50 ° C in the range of temperatures was observed. shutter, thus increasing the temperature

  shutter at around 165 C. These three ichanti-

  lons were treated as perforated film for wrapping rolls and crisp bread. Each of these films had good machine-readability, i.e. they were stiff enough for proper use in equipment

standard packing.

  The film according to Example 11 has also worked well in the field of croissants packaging and

  pizzas with a non-perforated film.

  It is evident from Table I that examples 11 and 12 have very wide sealing ranges and that each of these films has an aptitude for machine use, and that the mist is not embarrassing. However, above about 80% of the propylene-ethylene copolymer in the mixture, processing problems in the trapped bubble process become pronounced while the sealant layer tends to flow and deform at high temperature necessary

  to stretch and orient the base layer of polypropylene.

  In other words, the propylene-ethylene copolymer begins to melt at the orientation temperature of the propylene homopolymer, and the sealing layer being outside the tube tends to stick to the pinch cylinder after having been heated but before being blown into a bubble. Thus, the preferred range for the copolymer

  of propylene-ethylene in the mixture of the

  The composition of Example 11 exhibits the best combination of temperature range of sealing, seal strength, capacitance of 60 to 80% by weight and this gives a range of sealing temperatures ranging from 20 to 300Co. processing in the present state of film making equipment and machinability. On a commercial packaging machine, 50 to 80 packs per minute can be formed and all indications are that these paces can be successfully exceeded. By coextruding the base layer and the skin in the form of a flat sheet from a slot die, by rapidly cooling the sheet by using an expanding frame to stretch the flat sheet when heated.

  at the orientation temperature, it is possible to obtain

  continuous filming where the skin layer mixture is greater than 80%, but the

  cutting and other factors make this process less attractive.

  In any case, it must be recognized that the widening of the heat sealing range starts remarkably with a skin layer mixture of at least 1% of the copolymer and continues up to 100% of the copolymer. The propylene homopolymer used in the present invention is predominantly a film-grade isotactic polypropylene available from a large number of known resin suppliers. Similarly, the propylene-ethylene copolymer is a film-grade random copolymer. Also available from well-known resin producers, and the weight percent of ethylene is from 2% to 6%, which gives satisfactory results. With a higher percentage of ethylene in the copolymer, it can be expected that the percentage

  copolymer in the mixture can be increased.

  Thickness ratios of layers of film

  multilayer according to the invention are those which give the combination

  optimum machine fitness for use,

  sealing ability and processability.

  If the thickness of the sealing layer is increased well beyond the ratio of 4: 1, this causes processing problems as explained above at the orientation temperature of the polypropylene. Thus, the ratio of the base layer to the sealing layer must be higher than that of the base layer to the sealing layer. To-sel [eo Ts suosTeuTqmoo sna'T enb Tsu: e sTiaOp sue.Lom sap sanbTu [qoe. sl.UTe.TTnbT sep. * qu.T. suoo suea.om sal snol. puewmoo it '.ZaTLTnoTz'cd a oTdcmecxap aq.T. Tuuop q1.a mu Tnb: .Txoap uoTq-es Taz ap apom ni eaa9. TmTt: lUa'mltinxu f.caeu UOTUa.UAuT'T 'npuel.ua ueT seaJqTlTnbe uou. uamalleanq.onaq.c seqonooL'nm saTnLoTtteed sep sue ealtnpo nd oo thu.uenod sUOT.ltnpuo sap jzet [oodmeap: nq anod e -To.o.iejqTTTnb & T- mocI e.ueTao eam-iTodomoTlT O1 ap eqonoo ml ep sa.go xnep sep, uos UOT..nmlo.qop seqonoo set ee-aTçZo eualdoad ep eaomlTodomoqtT ap eqonoo l q.se aoilueo atqonoo ml nto saeqonoo sTo.4 B selnoTllTed

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Claims (13)

  Heat-sealable multilayer film for packaging, characterized in that it comprises: a) a base layer comprising a propylene homopolymer; b) a skin layer attached to a surface of said base layer, said skin layer comprising a mixture of 50 to 100% by weight of a propylene-ethylene copolymer with 50 to 0% by weight of a homopolymer of propylene.   Film according to claim 1, characterized in that it is heat-shrinkable, in that the ratio of the thickness of the base layer to the skin layer is at least 4: 1 and in that that the mixture   60 to 80% by weight of the propylene copolymer   ethylene with 40 to 20% by weight of the propylene homopolymer, said copolymer containing 2 to 6% by weight of ethylene. 3. Film according to claim 1, characterized   in that it has a shutter range of at least 200C.   4. The film of claim 1, characterized in oe that it is perforated.   5. Film according to claim 1, characterized in that it has a skin layer which is flxée two cbtés of the base layer to balance structurally.   6. Film according to claim 1, characterized in that it can have a sealing strength of at least 1.157 N / cm at a temperature of only C. A method for increasing the range of shutter temperature of a propylene homopolymer film, characterized in that it comprises the steps of: a) forming a base layer comprising a polypropylene hoopolymer; b) laminating a skin layer to at least one side of said propylene homopolymer film, said skin layer containing a mixture of 50 to 100% by weight of a propylene-ethylene copolymer containing 2 to 6% by weight ethylene with 5 to 0% by weight of the propylene homopolymer; and c) biaxially stretching said laminated film to a final total thickness of 0.006 to 0.0635 mm, said film having a shutter range of at least 20 C to thereby produce a shrinkable propylene homopolymer film in the heat and closable to heat.   8. A method according to claim 7, characterized in that skin layers of equal thickness are laminated on both sides of the base layer.   mentioned above to balance the final film.   The process according to claim 7, characterized in that the ratio of the thickness of said base layer to the thickness of said skin layer is at least 4: 1 and that said skin layer comprises a mixture of 60 to 80% by weight of the propylene-ethylene copolymer with 40 to 20% by weight of homopolymer of propylene.   10. Process according to claim 7, characterized in that the above-mentioned leafing step is obtained by   coextruding the skin layer and the basecoat.   11. The method of claim 7, characterized in that it comprises the step of perforating the film biaxially stretched above.   A process for producing a shrunk and heat sealed package by means of a polypropylene film, characterized in that it comprises the steps of: a) forming a base layer of a film of a homopolymer of propylene; b) laminating a skin layer on at least one side of said propylene homopolymer film, said skin layer comprising a mixture of 50 to 100% by weight of 2482900-   weight of a propylene-ethylene copolymer containing 2   to 6% by weight of ethylene with 50 to 0% by weight of a homopolymer   propylene lymere; c) biaxially stretching said laminated film to a final total thickness of the film of 0.006 to 0.0635 mm to thereby produce a heat-shrinkable and heat-shrinkable film) d) providing a product to be wrapped e) enveloping said product with the film and bringing the surfaces of the skin layer into contact with each other at points to seal the heat; f) applying heat and pressure to the obturated spots to thereby seal said film at said points, the temperature range; said applied heat being between 135 and 165 C, the seals of the resulting pack having a minimum strength of 1.157 N / cm; and g) applying heat between 110 and 195 ° C to said film for a time sufficient to shrink it around said product.   13. The method of claim 12, characterized in that it comprises the step of perforating the film   after having biaxially stretched it.