IE51291B1 - Heat-sealable multiply polypropylene film - Google Patents

Abstract

A heat-sealable and shrinkable, coextruded, multi-layer packaging film having a propylene homopolymer base layer and a skin layer of either a propylene-ethylene copolymer or of a blend of such a copolymer with up to 50% by weight of a propylene homopolymer, a preferred blend comprising 60% to 80% of the copolymer.

Description

This invention relates to thermoplastic, heat-shrinkable packaging films, particularly polypropylene films, having improved heat sealing characteristics. These films can be used to package a wide variety of food and non-food items .

Oriented polypropylene films are useful and widely accepted packaging films because of their good moisture barrier, stiffness, high strength, and optical properties. However, films of polypropylene do not, in general, exhibit good heat-sealing properties which is an important consideration in packaging applications . This is because polypropylene films have very narrow sealing temperature ranges.

The sealing temperature range for a thermo15 plastic film can be considered as beginning at that point where upon application of heat and pressure the surfaces of the film will seal or bond together. As temperature is increased further, a point is reached where the thermoplastic melts and flows so readily that it is difficult to achieve a seal in a specific area. This marks the upper end of the sealing range. Also at the upper end of the sealing range the film may tend to burn and char leaving weak and unsightly seals. For propylene homopolymer film, as temperature is increased, the film does not become sealable but rather suddenly flows undesirably as its melting point is approached making it quite difficult to achieve smooth, continuous, satisfactory seals. This type of sealing performance cannot be tolerated on high speed packaging equipment. »1291 In order to get good heat sealability, it has been the practice to apply various types of coatings to the films. Coatings have conventionally been applied in separate coating operations, such as from emulsions, extrusion coating, and so forth, but such operations are costly and require additional handling of the film. Coextrusion has been used successfully to put a heat sealable coating on polypropylene and produce a film of good seal strength, but until the present invention coextruded polypropylene film laminates had a very narrow heat seal range, and were expensive to produce. Accordingly, the present invention provides a polypropylene film having a wide sealing range. The invention also provides a polypropylene film having acceptable seal strength for shrink packaging applications .

Typical patents which disclose coatings for polypropylene film are U.S. Patent No. 3,285,756 (which issued on November 15, 1966 to Edward Barkis et al) which discloses a method of extruding a coating containing at least 65% ethylene onto a sheet of polypropylene; U.S. Patent No. 3,671,383 (which issued on June 20, 1972 to Rikita Sakata et al) which discloses a biaxially oriented polypropylene film laminated to a uniaxially oriented ethylene-propylene copolymer film with at least 75% by weight polypropylene; U.S. Patent No. 4,132,050 (which issued on January 2, 1979 to Roger N. Young) and which discloses a film having a substrate formed of a blend 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 (which issued on April 10, 1979 to Toshi Hiro Yamane et al) which discloses a polypropylene film laminate made by laminating a polypropylene layer having 1 to 8 weight percent of an ionomer with an ionomer layer.

It has been surprisingly discovered that the seal temperature range of a multi-layer polypropylene film can be increased and that acceptable seal strength for such a film can be provided by a multi-layer film which comprises a base layer of propylene homopolymer and a skin layer affixed to one surface of the base layer wherein the skin layer comprises a blend of 60% to 80% by weight of propylene-ethylene copolymer with 40% to 20% by weight of propylene homopolymer. In a particularly desirable embodiment, the skin layer has 70% by weight of propylene-ethylene copolymer blended with 30% by weight of propylene homopolymer and the copolymer contains 2 to 6% by weight of ethylene, or even more preferably, 3% to 4% ethylene by weight.

In addition, the ratio of the thickness of the skin layer to the thickness of the base layer is preferably 4:1 or greater.

The invention also provides a method of producing a heat-sealable heat-shrinkable multi-layer film as aforesaid which comprises (a) providing a base layer of propylene homopolymer; (b) laminating a skin layer to at least one side of the propylene homopolymer film, said skin layer comprising a blend of 60 to 80% by weight of the propylene-ethylene copolymer with 40 to 20% by weight of propylene homopolymer; and biaxially stretching the laminated film. The final total film thickness is preferably in the range of 0.25 to 2.5 mils (6.35-63.5 microns). Such a film will usually have a sealing range of at least 20°C and should produce a seal strength of greater than 300 gms/inch (118 g./cm). A preferred method of producing the laminated film is to coextrude the skin layer and the base layer.

The invention also provides a method of producing a heat-sealed and preferably shrunken package of polypropylene film which comprises overwrapping a product to be wrapped with a heat-sealable, and normally heat-shrinkable film as aforesaid, bringing skin layer surfaces into contact with each other at points to be heat-sealed; and applying heat and pressure at the points to be sealed thereby sealing said film at said points, the sealing temperature range of said applied heat being in the range of 135°C to 160°C; and then preferably applying heat in the range of 110°C to 135 °C to said package for time sufficient to shrink the said film about the said product. The film may be perforated.

The preferred embodiment of the present invention is a two-layer film made by blending 70% by weight of propylene-ethylene copolymer having 3.5 to 4.2% by weight of ethylene with 30% by weight of propylene homopolymer for the skin or outside layer. This mixture is blended in a Banbury blender and then fed into the hopper of an extruder which feeds into an annular die for extruding a tube having two layers. The inside or base layer comprises propylene homopolymer which is fed into the hopper of a second extruder which also feeds the annular die to provide the inside layer.

Xn the annular die the two tubular extrudates are joined to become a multi-layer tubular laminate.

This tubular laminate is rapidly cooled and then collapsed. Afterwards, the tubing is inflated and heated to the orientation temperature range of polypropylene which is in the range of 135°C to 150°C. The heating takes place in an oven, and as the flattened film emerges from the oven, it passes through pinch rolls and is opened to a bubble by the well known trapped bubble technique where it is stretched about five times in both vertical and horizontal directions which reduces the film to a thickness in the range of 0.60 to 0.80 mils (15.24 to S1291 .3 microns). Prior to stretching the typical tubing would have a thickness of about 18 mils (A57 microns) with the thickness ratio of the polypropylene layer to the blended layer being 5:1. A description of one such bubble technique for biaxially orienting polypropylene is given in U.S. Patent No. 3,260,776 which issued on July 17, 1966 to C.A. Lindstrom et al.

After the film has been expanded into the bubble and the bubble cooled, it is collapsed, slit, and wound up.

In some instances it is desirable to perforate the film so that an array of perforations or small holes are made in the film. Depending on the desired application, the number of holes may be from eight to ten per square inch of film up to several hundred holes per square inch and the holes may run anywhere from 2 to 20 mils (50 to 500 microns) in diameter. A typical apparatus for perforating the film employs a cooled grid over which the film is passed while it is exposed to a jet of hot gas which will melt the film in the uncooled areas. Such an apparatus and method are described in U.S. Patent No. 3,038,198 which issued on June 12, 1962 to C.H.Schaar.

To determine the sealing range and seal strength of the preferred film two sheets of the film were superposed with the skin layer surfaces in contact.

An electrical resistance heated flat sealing bar 5 mm. wide was used to press the sheets together against a backing surface with a pressure of 1 kg./sq.cm, for 0.5 second and the temperature of the sealing bar was recorded. After the sealing had been completed a one inch (25 mm.) strip was cut perpendicular to and across the seal made by the bar, and the ends of the respective strips were placed in the jaws of an Instron testing machine with the seal located at approximately the mid point between the gripping jaws. Force was applied by driving the jaws apart until the seal either peeled or broke. Seals having a minimum strength of approximately 300 gms/inch (118 g./cm.), the minimum considered sufficient for packaging applications, were achieved by the preferred embodiment at a seal bar temperature as low as 135 °C and satisfactory seals continued to be made as temperature increased up to about 160°C for a sealing range of approximately 25°C. (Hereinbelow, field tests indicate that the operable range is 30°C.).

A maximum seal strength of about 1,400 gms/inch (551 g./cm.) was achieved.

As stated previously, a monolayer polypropylene film has an essentially zero width sealing range.

Because seal bars are cooled by contact with film, in packaging machinery it is the necessary practice with polypropylene to set the temperature of the seal bars somewhat above the melting point so that a sealed package can be obtained but seals made in this way have a poor appearance and burned zones. With a film such as that of the present invention, setting the seal bars and temperatures near the top of the sealing range does not run the risk of melting the film or burning it plus at high speeds where the film tends to cool the seal bars, the seal bars will still maintain sufficient heat to seal the film and not be cooled below the sealing range.

A number of products can be packaged in the film of the present invention and the various machines use different techniques. In some instances, the product is placed on one sheet of film and then a second sheet is laid over the product and a complete perimeter seal is then made: and, in some instances the sealing operation also severs the excess film to trim the package. After sealing, the product passes through a heat tunnel which may have a temperature as low as 110°C but is usually set in the range in which the film was stretched In the tunnel the heat causes the stretch tensions to release and the film to shrink around the product. In other packaging processes the product may be placed between the folds of a center folded film and a seal on the remaining three unsealed sides is made. Many methods of over-wrapping a product in shrink film are understood by those skilled in the art.

Products such as bread, soft bakery products 10 including rolls, pizzas, and the like may be packaged with the film of the present invention. Also food products requiring breathing through the film such as fruits and vegetables may be packaged using the perforated film.

EXAMPLES In order to solve the problem presented by the virtually non-existent heat sealing range of polypropylene, a number of blended sealing layer compositions were tried. Listed below in Table 1 are the two-layer films having a polypropylene homopolymer base or substrate layer which has been coextruded with the specifically designated sealing layer composition. Only those compositions which could be successfully processed by the trapped bubble technique into film are shown in Table 1. The footnotes immediately belpw the table explain the abbreviations for the polymers, and sealing range and seal strength were determined as explained above. Haze is the percentage of transmitted light which in passing through the specimen deviates from the incident beam more than 2.5° on the average and which is determined by ASTM Method D1003. The measurement of thickness reported is of the total film laminate. in co in cn H O rH o • 0 • » • • «Η rH o o CN CN r* CN rH o rH CN in Γ* m rH in > rH in 10 in Ml Ml Ml Ml Ml Ml ol ol ol ol ol ol O o o o O o O o o in O o CN in CN rH rH rH rH rH o •H A Ό (ti +> in o m in in in O o o in o m in m m in in in in cn cn H rH rH rH rH rH rH rH rH rH H £ •Η Φ rH O'-** «j c σ φ «JO W ft*-> o CN in o cn UH M ·Η·Η O CO o tn H 10 tn cn rH 10 co 10 co • « • • • a a • • • • r- co O Oi o ω 01 CO co CO rH rH rH CN H CN H rH H H rH H \ X X X X X X X X X X X O rH Ch Ch (*· Ο CN in cn cn O Γ- Γ- 10 r- Γ- oo r- r* r- r- r- Γ- • • a a • a • • • • ο Ο O o o o o o o o o O £ g S S ra J ««·>;>§ B ra§ *? ftftHWWWQiWO) X X X X X X ft Oq fl( ft W O Oq ft ft Pi ft Λ H^ ο o m ο ο ο o rH Ol cn m h cm -O’ X X X X X X \ ο o m ο ο ο ο oi co r* m σι co in U M I I ft CU X X X ftftft ο ο o in io r* ο ο ιη <τ Φ •P iti β •H H a Φ β φ rH >< Μ c •H CO Φ M Φ Ό (ti M σ» Μ Μ a a << •H h rH ί ,β φ β ·*> Φ Μ Η φ Ο Η φ £ rH ο Οι ϋ φ β φ Η β Φ Φ I rH Φ rH >1 β Ο 4J φ 10 β Η rH Λ >< >1 ft Φ Η 0 ft 0 Pi Μ ft £ II II II II w I 1 ft § iJ O W ft o ft ra I 5 rH ft ra ft co II II copolymer having approx. 10% vinyl acetate. Φ +> (ti +) Φ ϋ (ti Η & Ή > I Φ β Φ Η ξ Μ S1391 Examples 3, 10, and 11 are expressed as ranges because these samples were run on actual equipment in the field as well as having their sealing range measured in the laboratory and, in the field, about a 5°C increase in sealing temperature range was observed thus increasing the upper seal temperature to about 165°C. These three samples were run as perforated films packaging rolls and crusty bread. Each of these films had good machinability, i.e. they were stiff enough to perform well in standard packaging equipment.

The film according to Example 11 also performed well in the field for packaging crescent rolls and pizzas with non-perforated film.

Looking at Table 1 it is evident that Examples 15 10, 11, and 12 have desirably wide sealing ranges and each of these films has good machinability and the haze is not objectionable. However, above approximately 80% propylene-ethylene copolymer in the blend processability problems in the trapped bubble process become 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 propyleneethylene copolymer is beginning to melt at the orientation temperature of the propylene homopolymer and the sealihg layer, being on the outside of the tubing, tends to stick to pinch rolls after being heated but prior to being blown into a bubble. Thus, the range for the propylene-ethylene copolymer in the sealing layer blend is from 60 to 80% by weight and this gives a sealing temperature range of from 20°C to 30°C. The composition of Example 11 is the best all around combination of seal temperature range, seal strength, processability on present state of the art film manufacturing equipment, and machinability. On commercial packaging machinery 50 to 80 packages per minute can be made and the indications are that these rates can be successfully exceeded.

By coextruding the base layer and skin as a flat sheet from a slot die, rapidly cooling the sheet, and using a tenterframe to stretch the flat sheet when heated to orientation temperature continuous manufacture of a film in which the skin layer blend is greater than 80% could be achieved but trim wastage and other factors make the tenterframe method less attractive. In any event, it should be recognized that the widening of the heat seal range begins noticeably with a skin layer blend of at least 50% copolymer and continues up to 100% copolymer.

The propylene homopolymer used in the present invention is predominantly isotactic polypropylene of film grade available from a number of well known resin suppliers. Likewise, the propylene-ethylene copolymer is a random copolymer of film grade also available from well known resin producers and the ethylene weight percentage is generally in the range of 2% to 6% which will perform satisfactorily. As the percentage of ethylene in the copolymer increases, the optimum percentage of the copolymer in the blend increases also.

The layer thickness ratios of the multi-layer film of this invention are those which give the optimum combination of machinability, sealability and processability. If the sealing layer thickness is increased much beyond the 4:1 ratio processing problems are incurred at the orientation temperature of 3D polypropylene as explained above. Thus, the base layer to sealing layer ratio should be greater than 4:1 and as noted above, a 5:1 ratio gives quite satisfactory performance. In addition, a total film thickness in the range of 0.25 to 2.5 mil (6.3 to 63 microns) produces the most satisfactory combination of film characteristics.

Also within the scope of this invention are three layer films where the central layer is the propylene homopolymer layer which is oriented and the sealing layers are on both sides of the oriented homopolymer layer to give it balance. This is to prevent curl which can occur in structurally unbalanced, multi-layer films .

Claims (5)

CLAIMS:1. A heat-sealable, heat-shrinkable multilayer film for packaging comprising: a) a base layer comprising propylene homopolymer; b) a skin layer affixed to one surface of said base layer, said skin layer comprising a blend of 60% to 80% by weight of propylene-ethylene copolymer with 40% to 20% by weight of propylene homopolymer.

1. To 7 which comprises: a) providing a base layer comprising propylene homopolymer; b) laminating a skin layer to at least one side of the said propylene homopolymer film, said skin layer conprising a blend of 60% to 80% by weight of propylene-ethylene copolymer with 40% to 20% by weight of propylene homopolymer; and c) biaxially stretching said laminated film thereby producing a heat-sealable, heat shrinkable propylene homopolymer film. 9. A method according to claim 8 wherein equal thickness skin layers are laminated to both sides of the said base layer to balance the final film. 10. A method according to claim 8 or 9 wherein the ratio of the thickness of the base layer to the thickness of the skin layer is at least 4:1. 11. A method according to any of claims 8 to 10 wherein the said laminating step is achieved by coextruding the skin layer or layers and the base layer. 12. A method according to any of claims 8 to 11 wherein the biaxially stretched film is perforated. 13. A method according to claim 8 substantially as described in Example 10 or 11. 14. A heat-sealable multi-layer film when produced by the process of any of clains 8 to 13. 15. A method of producing a heat-sealed package of polypropylene film which comprises over-wrapping a product to be wrapped with a heat-sealable film as claimed in any of claims 1 to 7 or 14, bringing skin layer surfaces into contact with each other at points to be heat sealed, and applying heat and pressure at said points, the temperature range of said applied heat being 135°C to 165 e C. 16. A method according to claim 15 in which the film is also heat-shrinkable and heat in the range of 110°C to 135°C is applied to the said package for a tine sufficient to shrink the said film about 1 or 2 wherein the ratio of the thickness of the base layer to the thickness of the skin layer is at least 4:1. 1 wherein the said copolymer comprises 2% to 6% by weight of ethylene.

2. A heat-sealable film according to claim 3. A method of producing a heat-sealable, heatshrinkable multi-layer film as claimed in any of claims

3. A heat-sealable film according to claim

4. A heat-sealable film according to any of claims 1 to 3 wherein the said film has a sealing range of at least 20°C. 5. A heat-sealable film according to any of claims 1 to 4 having a skin layer affixed to both sides of the base layer to structurally balance said film. 6. A heat-sealable film according to any of claims 1 to 5 wherein said film is perforated. 7. A heat-sealable film according to any of claims 1 to 6 having a total thickness in the range 0.25 mil to 2.5 mil (6.3 to 63 microns).

5. The said product.