GB2137931A - Laminated Packaging Film Heat-Shrinkable After Forming, and Method for the Preparation Thereof - Google Patents

Abstract

The invention relates to a laminated film comprising at least one layer of an ethylene polymer or a polymer subjected to irradiation with ionizing particles, the irradiated ethylene polymer or a polymer imparting heat-shrinkability after thermoforming, to the films. Said films are useful for packaging applications, in particular for food product packaging.

Description

SPECIFICATION Laminated Packaging Film Heat-Shrinkable After Forming, and Method for the Preparation Thereof This invention relates to the field of films mainly useful for packaging applications, in particular, food product packaging.

More specifically, the invention concerns packaging films which have improved properties as regards their heat shrinking characteristics following thermoforming.

Known are existing packaging materials which, being in the form of films, exhibit, to a greater or lesser extent, the peculiar feature of being shrinkable during or through thermal treatment.

Such materials generally comprise thermoplastic polymers and/or copolymers which, when laminated and joined together, are then stretched to impart to them those heat shrinking features which make them specially suitable for use in food product packaging.

Actually, during the stretching operations, the structures and geometries of the macromolecules making up the film components used nowadays, are so to speak "frozen" into a form such that, upon application of the necessary energy input, e.g. heat application, the macromolecules "rearrange" themselves at least in part to the original forms, thereby the previously stretched film will shrink.

The stretching operation is also carried out in standard thermoforming processes, but the materials used heretofore for manufacturing films which are heat shrinkable after forming are, in general, fairly expensive and the operation during which the heat shrinking feature of the films themselves is put to use is not always effective and does not always guarantee an effective optimum result.

It is an object of this invention, in obviating the foregoing shortcomings, to provide a packaging film, which additionally to high heat shrinking properties after forming, has good mechanical features, such as resistance to wear, abrasion, and puncturing.

A further object of this invention is to provide a packaging film which can be prepared from low cost readily avilable materials.

It is another object of the invention to provide a heat shrinkable film after forming prepared with simplified procedures.

According to one aspect of this invention, there is provided a multilayered laminated film, comprising at least one layer of an ethylene polymer irradiated with ionizing particles, at a radiation intensity in the 6 to 25 megarad range, said irradiated layer being effective to impart to said laminated film heat shrinking properties after thermoforming.

According to a further aspect of the invention, there is provided a method of manufacturing a laminated film which is heat shrinkable afte forming, comprising the steps of forming a laminated film having at least one outermost layer of a low density and/or medium density and/or high density polymer of ethylene, and of subjecting said laminate to an irradiation treatment with ionizing particles, preferably with fast electrons, at a radiation intensity in the 6 to 25 megarad range.

A third aspect of the invention provides a method of manufacturing a laminated film which is heat shrinkable after forming, comprising the step of adding, to said laminated film, at least one outer layer including low density and/or medium density and/or high density polyethylene pre-treated by irradiating it with ionizing particles at a radiation intensity in the 6 to 25 megarad range.

A rather surprising finding has been the acquisition of heat shrinking properties after forming by films wherein at least one irradiated polyethylene layer was present. Such films afford considerable cost advantage and improved characteristics over whatcould be obtained heretofore with conventional films.

Experimental trials carried out on various types of films prepared in accordance with this invention, by progressively altering the composition, number, arrangement and thickness of the component layers, have shown that the presence of at least one irradiated polyethylene layer enables packaging films to be obtained which, additionally to such satisfactory properties as resistance to wear, abrasion, puncturing, and imperviousness to gas and vapors, also have excellent characteristics of heat shrinkage after forming, without requiring any preliminary stretching operation.

The polyethylenes which are preferred for use with this invention are typically low and/or medium and/or high density polyethylenes obtained with conventional techniques and including so-called linear polyethylenes. Ethylene copolymers wherein the major constituent is ethylene are within the scope of the term "polyethylene" as used herein.

This invention already finds an embodiment thereof in subjecting to irradiation'with ionizing particles a polyethylene film obtained, as an example, by trapped bubble extrusion. According to this extrusion technique, the film is extruded in tubular form and at the exit from the extrusion head is made to travel along the vertical direction for a certain distance, with the front end closed and under slight pressure of blown in gas (trapped bubble), the pressure being just sufficient to prevent the collapsing of the tubular film, but not such as to induce any notable stretching of the film. In fact, the diameter of the tubular film along said distance is quite equal or just slightly greater than that of the final cooled film.

The irradiation iapreferably carried out with fast electrons, at a radiation intensity in the 6 to 25 megarad range. The thickness of the polyethylene film may range from 20 to 300 microns.

It has been found that the same polyethylene film, when not subjected to irradiation, is virtually non-heat shrinkable after forming.

Expediently, the low and/or medium and/or high density polyethylene acting as a layer which, upon irradiation, imparts heat shrinking properties after forming, is employed, in the laminated films according to this invention, in association with additional layers of various species and in varying numbers, in relation to the requirements of the use to which the resulting film is put.

More particularly, where the films are to be used for packaging food products, the layer(s) of polyethylene may be associated with layers capable of imparting to the final film improved gas barrier capabilities. A gas barrier layer, suitable for use with the invention, may be obtained from a polymer conventionally utilized for that purpose, such as PVOH (polyvinyl alcohol) or EVOH (ethylene/vinyl alcohol copolymer) or PDVC (polyvinylidene/chloride), etc. As a support material, polyethylene already behaves satisfactorily on its own. For this purpose, the overall thickness of the polyethylene layer(s) will be at least 20 microns.

One or more outer layers of irradiated polyethylene have shown to be useful in the implementation of this invention as regards mechanical properties of resistance to wear, abrasion, and puncturing.

The problem of weld sealing a part of the film to provide tight sealed packages may be solved with the application of inner layers of a heat sealing material. The heat sealing material may comprise possibly the same polyethylene. Other heat sealing materials are conventionally available in the form of different polymeric materials, among which the ionomers sold under the brand name, SURLYN by DuPont may be mentioned, or ethylene copolymers, or even polypropylene and copolymers thereof, etc.

One example of a simple structure for a laminated film prepared in accordance with this invention is the following: OUTSIDE INSIDE Polyethylene/EVOH/SURLYN where, EVOH is an ethylene vinyl alcohol copolymer acting as a gas barrier layer; and SURLYN is a ionomer material from DuPont acting as a heat sealing layer.

A film prepared as described in the foregoing, once it has been subjected to irradiation with ionogenic particles, and preferably with fast electrons, at an intensity in the 2 to 25 megarad range, is advantageously heat shrinkable following forming. Said film may be prepared by co-extrusion and then irradiated; Alternatively, the polyethylene layer may be first irradiated and then joined to the other layers.

Since the forming and subsequent heat shrinking operations are sometimes carried out at fairly high temperatures, in excess of 100-1 500 C, the use of adhesives may become necessary which are interposed between the layers of the laminated film; this in order to prevent possible delamination and/or wrinkling of the film. To this aim, conventional adhesives may be used, one example of a film so fabricated being as follows: OUTSIDE INSIDE Polyethylene/adhesive/EVOH/adhesive/polyethylene/SURLYN Drawing a distinction between low and medium density polyethylene, another exemplary film according to this invention may be: Medium density polyethylene/low density polyethylene/adhesive/EVOH/adhesive/low density polyethylene/SURLYN.

The adhesive layers may comprise variously modified ethylene vinyl/acetate copolymers.

Where the laminated film produced in accordance with the invention provides for the association of additional layers, such as gas barrier or heat sealing layers, with the polyethylene layer(s), the film may be irradiated after the same has been laminated in such a way as to have the outermost polyethylene layer first penetrated by the beam of ionizing particles. The depth of radiation penetration will depend on the radiation intensity and types of ionizing particles. As mentioned hereinabove, such particles would preferably comprise fast electrons, the radiation intensity ranging from 6 to 25 megarads, preferably 12 to 25 megarads.

Alternatively, irradiation may be applied to the polyethylene layer(s) prior to laminating it to the other layers.

Packaging trials using films prepared in accordance with the invention have been conducted on various conventional machines, in particular, machines have been used where the film portion intended to enclose a product to be packaged was first heated, either on a plate heater or infrared one, and then vacuum formed and/or formed under air pressure, to be then brought to suitable temperature levels to produce heat shrinking.

The heat shrinking characteristics exhibited by the inventive films are undoubtedly'such as to make said films competitive, especially in view of the important properties associated with such characteristics, which as mentioned include satisfactory resistance to wear, abrasion, puncturing, and imperviousness to gas. All of this is combined with economical convenience of the films, which are prepared from low cost materials, in particular polyethylene.

The following examples illustrate the behavior, with special regard for the heat shrinking characteristics after forming, of some laminated films prepared in accordance with this invention, on three different types of thermoforming machines: the tests illustrating the examples have been carried out operating with standard SOx 140 mm and varying depth shapes.

EXAMPLE 1 A heat shrinkage test has been carried out on a VARIFORM 970 machine (manufacturer GRACE ITALIANA S.P.A.) employing a plate heater. Samples of films produced in accordance with the provisions of this invention have been subjected to forming (to be accomplished on this machine by air pressure) at different forming depths. The films utilized were of a VSP 0194 material (as defined herebelow) having an overall thickness of 100 microns and irradiated respectively at 1 2 and 25 megarads, and of VSP 0874 (as defined herebelow) having an overall thickness of 200 microns and being irradiated at 12 megarads. VSP 0194 and VSP 0874 are products of W. R. Grace 8 Co. or Grace Italiana S.P.A. which are available in the laminated film form.

Shrinkage Percent at 980C Materials L T L T L T L T L T L T VSP 019412 MR 5 22 8 19 26 40 31 40 VSP 019425 MR 7 18 9 19 22 34 32 43 - - - VSP087412MR - - - - - - 24 28 33 40 32 42 20 mm 40 mm 50 mm 60 mm 70 mm 80 mm Forming Depth Where: VSP 0194 12 MR is a GRACE product material, VSP 0194, radiated at 12 megarads and having an overall thickness of 100 y.

VSP 0194 25 MR is a GRACE product material, VSP 0194, radiated at 25 megarads and having an overall thickness of 100 u.

VSP 0874 12 MR is a GRACE product material, VSP 0874, radiated at 12 megarads and having an overall thickness of 2008.

L and Tare the percent shrinkage in the longitudinal and transverse directions, respectively, as referred to the material coil.

In particular, with reference to the arrangement and thickness of -layers in such laminated films, the materials being investigated had the following characteristics: VSP 0194 (Overall Thickness 100 Microns): INSIDE: m.d. PE (17 17,u)/Adhesive (10 ju)/EVOH (8 y/Adhesive (10 ju/l.d. PE (40,u/SURLYN (1 5 OUTSIDE VSP 0874 (Overall Thickness 200 Microns): INSIDE: m.d. PE (38 4)/Adhesive (15 ju)/EVOH (12 (12,u)/Adhesive (15/I.d. PE ( 100 y)/EVA (20 y): OUTSIDE where: m.d. PE is medium density polyethylene, I.d. PE is low density polyethylene, EVOH is ethylene/vinyl alcohol copolymer, SURLYN is a DuPont product material, and EVA is ethylene/vinylacetate copolymer.

As may be gathered from Table I, the heat shrinking properties shown by the tested films as a function of the forming depth are proportionally increased and of considerable effectiveness.

EXAMPLE 2 A film according to the invention has been tested which was prepared from a product material (VSP-0194) by Grace, as described hereinabove, which was irradiated with fast electrons at 12 megarads. The film had an overall thickness of 100 microns.

The thermoforming machine was an ILLIG SB-53, (manufacturer Adolph ILLIG Maschinenbau, Heilbron, West Germany), where heating is accomplished by means of infrared light and forming is performed under vacuum. That machine is characterized by fairly long preheating time requirements.

The forming temperature was 800 C, for an infrared exposure time of 3-5 seconds.

The stretch ratio and shrink tension have been evaluated for different forming depths, in the 0 to 100 mm range, while ascertaining the free percent shrinkage at shrinkage temperatures of 1 O00C, 1200C, and 1 600C, respectively.

The following Table II shows in detail the results of the test.

Free Percent Shrinkage Forming Depth Stretch Shrink Tension (mm) Ratio (kg/cm2 (PSI)) 1000C 1200C 1600C L O O (O) 0 3 8 0 T O O (O) O O -2 L 1.60 3.5 (50) 3 24 38 80 T 1.62 2.1 (30) 4 16 25 L 2.26 18.3 (260) 10 41 54 100 T 3.24 13(185) 11 62 65 where L and T are, respectively, the shrink values in the longitudinal and transverse directions, as referred to the material coil.

It may be seen that the results reported in the Table indicate that already at 1000C shrink features become noticeable. At 1200 C, shrinkage is satisfactory for most applications, and at a higher temperature, in this specific instance 1600 C, such shrinkage values are achieved as to meet any practical packaging requirements.

EXAMPLE 3 A film according to the invention has been tested which was prepared from a previously described'material, VSP 0874,-a Grace product, which had been irradiated with fast electrons at 12 megarads. The film had an overall thickness dimension of 200 microns.

The thermoforming machine was a MULTIVAC M 850, (manufacturer Multivac Sepp Haggenmuller KG, Wolferstchwenden, West Germany), where preheating is accomplished by contacting the material with a hot plate for short time periods, and forming is accomplished under vacuum conditions. That machine is one of the most popular thermoforming machines currently available in Europe.

Forming Depth Stretch Shrink Tension Free Percent (mm) Ratio (kg/cm2 (PSI)) Shrinkage at 1000C L 1.42 5.1(72) 10 T 1.28 2.4 (34) 10 45 L 1.42 5.1 (72) 10 T 2.60 19.1(271) 29 where L and T are, respectively, the shrink values in the longitudinal and transverse directions, as referred to the material coil.

Claims (14)

1. A multilayered laminated film, comprising at least one layer of an ethylene polymer irradiated with ionizing particles, at a radiation intensity in the 6 to 25 megarad range, said irradiated layer being effective to impart to said laminated film heat shrinking properties after thermoforming.

2. A laminated film according to claim 1, wherein said polyethylene layer is prepared from conventional and linear types of low density and/or medium density and/or high density polyethylene.

3. A laminated film according to either claim 1 or claim 2, comprising, in addition to the one or more irradiated polyethylene layers, at least one heat sealing layer.

4. A laminated film according to any of claims 1 to 3, comprising at least one gas barrier layer.

5. A laminated film according to any of claims 1 to 4 wherein the or each said irradiated polyethylene layer also functions as steam barrier layer and wear-resisting layer.

6. A laminated film according to any of claims 1 to 5, wherein the or each said polyethylene layer has an overall thickness dimension in the 20 to 300 micron range.

7. A laminated film according to any of claims 1 to 6, wherein the or each said polyethylene layer is made heat shrinkable after forming at a temperature in the 80 to 1 600C range.

8. A laminated film according to any of the preceding claims, wherein at least some of the layers thereof are joined together by co-extrusion.

9. A laminated film according to any of the preceding claims, wherein at least some of the layers thereof are interconnected through intervening adhesive layers.

10. A laminated film according to any of the preceding claims, wherein said at least one polyethylene layer is irradiated with fast electrons at a radiation intensity in the range from 6 to 25 megarads.

11. A laminated film according to claim 10, wherein said radiation intensity is in the range from

12 to 25 megarads.

1 2. A method of manufacturing a laminated film which is heat shrinkable after forming.

comprising the steps of forming a laminated film having at least one outermost layer of a low density and/or medium density and/or high density polymer of ethylene, and of subjecting said laminate to an irradiation treatment with ionizing particles, preferably with fast electrons, at a radiation intensity in the 6 to 25 megarad range.

1 3. A method of manufacturing a laminated film which is heat shrinkable after forming, comprising the step of adding, to said laminated film, at least one outer layer including low density and/or medium density and/or high density polyethylene pre-treated by irradiating it with ionizing particles at a radiation intensity in the 6 to 25 megarad range.

14. A method according to claim 1 2 or 13, wherein said ionizing particles are fast electrons.

1 5. A method according to claim 12, 1 3 or 14, wherein said radiation intensity is in the 12 to 25 megarad range.

1 6. A laminated film substantially as hereinbefore described with reference to Examples I to Ill.

1 7. A method of manufacturing a laminated film substantially as hereinbefore described with reference to Examples I to III.