DK164093B - HEAT SEALABLE FILM CONTAINING A PROPYLEN HOMOPOLYMER BASIC LAYER, PROCEDURE FOR INCREASING THE SEAL TEMPERATURE AREA OF SUCH A FILM, AND PROCEDURE FOR MAKING A MULTIPLE CIRCUIT - Google Patents

Description

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The present invention relates to a heat-sealable film comprising a propylene homopolymer base layer which has improved heat-sealing properties. Such a film can be used for packaging a large number of foods 5 and other materials. The invention also relates to a method for increasing the sealing temperature range of propylene homopolymer films as well as to a method of producing a heat-sealed and crimped packaging of polypropylene film.

Oriented polypropylene films are useful and commonly used packaging films because of their good moisture barriers, stiffness, high strength and optical properties. However, polypropylene films generally do not have good heat sealing properties, which is an important factor in packaging applications. This is because polypropylene film 15 has very narrow sealing temperature ranges.

The sealing temperature range of a thermoplastic polypropylene film can be assumed to begin at the point where, upon application of heat and pressure, the surfaces of the film will be sealed or bonded together. As the temperature 20 increases further, a point is reached where the thermoplastic material melts and flows fairly easily, making it difficult to obtain a seal in a specific region, thereby marking the upper end of the seal region. Also, at the upper end of the sealing region, the film may be prone to stinging or charring, leaving weak and unsightly seals. Propylene homopolymer films do not become sealable as the temperature increases, but the film flows rather abruptly as the melting point approaches, making it fairly difficult to obtain soft, continuous, satisfactory seals. This type of sailing relationship cannot be tolerated in connection with high speed packaging equipment.

In order to provide good heat sealability, it has been the practice to apply different kinds of coatings 35 to the films. The coatings are conventionally done in separate coating operations, e.g. from emulsions or as ex-

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However, such operations are costly and require increased handling of the film. Coextrusion has been used successfully to apply a heat sealable coating to polypropylene and to form a good film of good sealing strength, however, coextruded polypropylene film laminates have a very narrow heat sealing region. It is therefore the object of the present invention to provide a polypropylene film having a wide sealing region and having an acceptable sealing strength for shrink packaging applications.

U.S. Patent No. 3,285,766 discloses a method of extruding a coating containing at least 65% ethylene onto a polypropylene layer, and U.S. Patent No. 3,671,383 discloses a biaxially oriented polypropylene film laminated to a uniaxially oriented ethylene-propylene copolymer film with at least 75% by weight polypropylene.

X US Patent No. 4,132,050 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 ethylene-propylene block copolymer with a heat sealable layer, and U.S. Patent No. 4,148,972 discloses a polypropylene film laminate made by laminating a polypropylene layer with 1 to 8% by weight of an ionomer with an ionomer layer. A number of heat-sealable coatings on polypropylene films are also known from DK Patent Specification No. 156,549 and from GB Patent Nos. 1,168,541 25 and No. 1,145,199.

It has now surprisingly been found that the object of the present invention, viz. in particular, increasing the sealing temperature range of a multilayer polypropylene film 30 and providing acceptable sealing strength for such a film can be accomplished with a film of the kind already disclosed, characterized by having one or optionally both surfaces of the base layer attached a cover layer consisting of a mixture of 50-100% by weight of propylene-ethylene copolymer with 50-0% by weight of propylene homopolymer. The preferred mixture consists of 60 to 80 3

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wt% copolymer and 40 to 20 wt% homopolymer. In a particularly preferred embodiment, the cover layer contains 70% by weight of propylene-ethylene copolymer mixed with 30% by weight of propylene homopolymer, and the copolymer contains 2 to 6% by weight of ethylene, especially 3 to 4% by weight of ethylene. The ratio of the thickness of the cover layer to the thickness of the base layer is preferably 4: 1 or greater.

As can be seen from the experimental results set forth in the following Table I, films according to the invention have superior properties in terms of sealing area, sealing strength and optical conditions in relation to known film compositions.

The invention also relates to a method of increasing the sealing temperature range of propylene homopolymer film by providing a base layer of propylene homopolymer which is characterized by laminating a cover layer on one and optionally both sides of the propylene homopolymer. the film, the coating being a mixture of 50 to 100% by weight of propylene-ethylene copolymer, containing 2 to 6% by weight of ethylene, with 50 to 0% by weight of propylene homopolymer, and biaxially extending the laminated film to a final total film thickness in the range of 6.4 x 10 "3 to 6.4 x 10" 2 mm. Such a film will have a sealing range of at least 20 ° C and will produce a sealing 25 strength greater than 120 g / cm. A preferred method of preparing the laminated film comprises coextrusion of the cover layer and the base layer.

The invention also relates to a method of producing a heat-sealed and crimped package of 30 polypropylene films by providing a base layer of propylene homopolymer film, which is characterized by laminating a cover layer on one or optionally both sides of the propylene homopolymer. the film, the coating being a mixture of 50 to 100 wt% propylene-ethylene-35 copolymer with 2 to 6 wt% ethylene, biaxially extending the laminated film to a final total film thickness in the range of 6.4 x 10 3 to 6.4 x 10 ”2 mm to form 4

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a heat-sealable, heat-shrinkable film, provides a product to be packaged, packaging the product in the film, and brings together the surface of the cover layer at the points to be heat-sealed, applying heat and pressure at the points to be sealed, thereby sealing the film at these points, the sealing temperature range of the applied heat being in the range of 135 to 160 ° C and applying heat in the range of 110 to 135 ° C to the packaging for a time sufficient for the film to shrink around the product.

The film may be perforated for purposes of the invention; moisture impermeability is an advantage in many cases, but perforated films can e.g. advantageously used for certain purposes as set forth below.

The preferred embodiment of the invention comprises a two-layer film prepared by blending 70 wt% propylene-ethylene copolymer containing 3.5 to 4.2 wt% ethylene with 30 wt% propylene homopolymer to the cover layer or outer layer. This mixture is prepared in a Banbury mixer and then passed into the food container 20 in an extruder leading into an annular die for extruding a two-layer tube. The inner layer or base layer comprises propylene homopolymer which is fed into the food container of another extruder which also leads to the annular die to provide the inner layer.

In the annular die, the two tubular extrudates are joined to a multilayer tubular laminate. This tubular laminate is quickly cooled and then folded. Then the tube is filled with air and heated to the orientation temperature range of polypropylene, which is in the range of 135 to 150 ° C. The heating takes place in an oven, and when the flattened film emerges from the oven, it passes between pressing rollers and opens to a bubble by the well-known "trapped bubble" technique, where it is stretched approximately. 5 x 3 times in both vertical and horizontal directions, reducing the thickness of the film to the range of 0.015 to 0.020 mm. Before the stretch, the typical pipe has a thickness of approx.

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0.46 mm, the ratio of thickness of the polypropylene layer to the thickness of the mixed layer is 5: 1. A description of such a "bubble" technique for biaxially oriented polypropylene is given in US Patent No. 3,260,776.

5 After the film has been expanded in the bubble and the bubble has cooled, it is collapsed, cut into strips and wound up.

In some cases, as mentioned, it is desirable to perforate the film so as to produce a series of perforations or small holes in the film. Depending on the desired application, the number of holes may be from one to several hundred per cm2 film, and the holes may be from 0.05 to 0.5 mm in diameter. In a typical apparatus for perforating the film, a cooled grid is used over which the film passes while applying a jet of hot gas which melts the film in the uncooled regions. Such an apparatus and method is disclosed in U.S. Patent No. 3,038,198.

To determine the sealing region and sealing strength of the preferred film, two pieces 20 of the film are superimposed with the cover layer surfaces in contact. An electrically heated flat sealing piece 5 mm wide is used to compress the pieces against a background surface at a pressure of 1 kg / cm for 0.5 seconds and the temperature of the sealing piece is noted. After the seal is completed, a 2.5 cm * s strip is cut perpendicularly and transversely to the seal made of the piece and the ends of the respective strips are placed in the jaws of an Instron test machine with the seal located approximately at the midpoint between the gripping jaws. Force is applied, forcing the jaws apart until the seal is either peeled or broken. Seals with a minimum strength of approx. 120 g / cm, the minimum strength considered to be sufficient for packaging applications, is obtained by the preferred design at a temperature in the sealing piece down to 135 ° C and satisfactory formation is continued.

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sails when the temperature rises to approx. 160 ° C, corresponding to a sealing range of approx. 25 ° C (in the following, field tests indicate that the workable area is 30 ° C). A maximum seal strength of approx.

5 550 g / cm.

As previously mentioned, a monolayer propylene film has a sealing region substantially "zero" wide.

Since sealing pieces are cooled upon contact with film, it is necessary in packaging machines to practice the polypropylene temperature to keep the temperature of the seals slightly above the melting point so that sealed packaging can be obtained, but seals made in this way have a poor appearance and have burnt zones.

With a film such as that according to the invention, where the sealing pieces and temperatures are kept near the upper limit of the sealing region, without the risk of the film melting or burning, and at high speeds, where the film tends to cool the sealing pieces, For example, the sealing pieces will still retain sufficient heat to seal the film and not be cooled below the sealing region.

A large number of products can be packaged in the film of the invention and the various machines employ different techniques. In some cases, the product is placed on one piece of film, then another piece is placed over the product and a complete circuit seal is produced, and in some cases excess film is separated by the seal, thereby directing the packaging. After sealing, the product passes through a heat tunnel which may have a temperature of down to 110 ° C, but which is usually kept in the region in which the film was stretched.

In the tunnel, the heat causes the stretched tension to be relaxed, thereby shrinking the film around the product. In other packaging processes, the product can be placed between the 35 folds of a center-folded film and a seal is produced on the remaining three unsealed sides. Many 7

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0 well known methods for wrapping a product in shrink film are known to those skilled in the art.

Products such as bread, soft bakery products, including rolls, pizzas, and the like, can be packaged with the film of the invention. Also, foods that should be able to breathe through the film, e.g. fruits and vegetables can be packaged using perforated films.

Examples 10 To solve the problem caused by the non-existent heat sealing region of polypropylene, a number of mixed sealing layer compositions are tested. Table X lists the two-layer films having a polypropylene homopolymer base or substrate layer which is co-extruded with the specifically specified sealing layer composition. Table I shows only those compositions that can be successfully incorporated into the film by the trapped bubble technique.

The footnotes immediately below the table explain the abbreviations for the polymers, and the sealing area and sealing strength are determined as previously indicated. Blur indicates the percentage of transmitted light that, as it passes through the sample, deflects from the incident beam by more than 2.5 ° on average, as determined by 25 ASTM method D1003. The indicated thickness is for the total film laminate.

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For Examples 3, 10 and 11, areas are indicated because these tests are performed on available equipment in the field, as are the sealing areas measured in the laboratory and in the field an approx. 5 ° C increase in the sealing temperature range, increasing the upper sealing temperature to approx. 165 ° C. These three tests are done with perforated films that pack rolls and bread with crust. Each of these films is handled well mechanically, ie. they are sufficiently rigid to behave well on standard packaging equipment.

The film of Example 11 also behaves well in the field by packaging horns and pizzas with non-perforated films.

From Table I, it can be seen that the films of Examples 10, 11 and 15 12 have desirable wide sealing areas and these films are handled well mechanically and their obscurity is not unfortunate. However, there are more than approx. 80% propylene-ethylene copolymer in the blend, the processing problems of the blend in the trapped bubble process are pronounced as the sealing layer tends to flow and distort at the high temperature needed to stretch and orient the polypropylene -base layer. In other words, the propylene-ethylene copolymer begins to melt at the orientation temperature of the propylene homopolymer, and the sealing layer, which sits on the outside of the tube, tends to hang in the press rolls after being heated but before being blown to a bubble. . The preferred range of the propylene-ethylene copolymer in the sealing layer mixture is from 60 to 80% by weight, giving a 30 sealing temperature range ranging from 20 to 30 ° C. The composition of the film of Example 11 provides the very best combination of sealing temperature range, sealing strength, processability at the present state of the film processing equipment technique and machinability. Commercial packaging machines can produce 50 to 80 packages per per minute, and there are indications that these speeds can be successfully exceeded.

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By coextrusion of the base layer and the cover layer as a flat sheet from a slit die, rapid cooling of the sheet, and using a tension frame to stretch the planar sheet when heated to orientation temperature, a continuous processing of a film in which the coating of the cover layer is achieved can be achieved. account for more than 80%, but adjusting wastes and other factors make the tension frame method less attractive. In any case, it is to be noted that the expansion of the heat seal region begins noticeably 10 with a coating of at least 50% copolymer and proceeds up to 100% copolymer.

The propylene homopolymer used according to the invention is predominantly isotactic polypropylene of a film quality obtainable from a number of well-known resin suppliers. The propylene-ethylene copolymer is also a random copolymer of a film grade also obtained from well-known resin manufacturers, and the ethylene constitutes from 2 to 6% by weight, resulting in a satisfactory behavior. With a higher ethylene percentage in the copolymer, it is expected that the percentage of copolymer in the mixture can be increased.

The layer thickness ratios of the multilayer film of the invention are such that they provide the optimal combination of machine handling, sealability and processability. If the thickness of the sealing layer is increased so much that the ratio becomes much greater than 4: 1, processing problems arise as explained earlier at the orientation temperature of polypropylene. Thus, the ratio of the base layer to the seal layer must be greater than 4: 1, and as previously stated, the 5: 1 ratio gives a rather satisfactory behavior. In addition, a total film thickness in the range of -3 -2 6.4 x 10 to 6.4 x 10 mm provides the most satisfactory combination of film properties.

The invention also relates to three-layer films in which the central layer is the propylene homopolymer layer which is oriented and the sealing layer lies on both sides of the oriented homopolymer layer which provides balance. This prevents curling that can occur in structurally unbalanced multilayer films.

Claims (9)

A heat-sealable film comprising a propylene homopolymer base layer, characterized in that one or optionally both surfaces of the base layer has a cover layer consisting of a mixture of 50-100% by weight propylene-ethylene copolymer with 50 -0% by weight of propylene homopolymer. Heat sealable film according to claim 1, characterized in that the film is heat shrinkable, that the ratio 10 between the thickness of the base layer and the thickness of the cover layer is at least 4: 1 and that the mixture consists of 60 to 80% by weight of propylene-ethylene copolymer with 40 to 20% by weight of propylene homopolymer, the copolymer containing 2 to 6% by weight of ethylene. A heat-sealable film according to claim 1, characterized in that the film is perforated. A method of increasing the sealing temperature range of propylene homopolymer film by providing a base layer containing propylene homopolymer characterized by a) laminating a cover layer to one and optionally both sides of the propylene homopolymer film the coating being comprised of a mixture of 50 to 100% by weight of propylene: ethylene copolymer containing 2 to 6% by weight of ethylene with 50 to 0% by weight of propylene homopolymer, and b) biaxially extending it laminated films to a final total film thickness ranging from 6.4 x 10 ”3 to 6.4 x ΙΟ” 2 mm. Process according to claim 4, characterized in that the ratio of the thickness of the base layer to the thickness of the cover layer is at least 4: 1 and that the cover layer consists of a mixture of 60 to 80% by weight propylene-ethylene copolymer with 40 to 20 wt.% propylene homopolymer. Process according to claim 4, characterized in that the lamination is carried out by co-extrusion of the cover layer (s) and the base layer. DK 164093 B A method according to claim 4, characterized in that a step for perforating the biaxially stretched film is included. A method of making a heat-sealed 5 and crimped packaging of polypropylene film by providing a base layer of a propylene homopolymer film, characterized in that: a) laminating a cover layer to one or optionally both sides of the propylene homopolymer film; the topsheet consisting of a mixture of 50 to 100% by weight of propylene-ethylene copolymer containing 2 to 6% by weight of ethylene with 50 to 0% by weight of propylene homopolymer, b) biaxially extending the laminated film to a final total film thickness in the range of 6.4 x 10 ~ 3 to 15 6.4 x 10 "2 mm, (c) providing a product to be packaged, (d) packaging the product in the film and bringing the cover layer surfaces into points (e) apply heat and pressure at the points to be sealed, thereby sealing the film at these points, the temperature range of the heat supplied being in the range of 135 to 165 ° C; and (f) applying heat at range from 110 to 195 C to the packaging for a time sufficient for the film to shrink around the product 25. Method according to claim 8, characterized in that a step is taken for perforating the film after the biaxial stretch. 30 35