Coextruded multi-layer films for vacuum skin packaging generally are those in which upper and lower webs of the film are placed around a food product to be packaged. The space between the upper and lower webs is evacuated and the flexible webs conform to the shape of the exterior of the food product. The top web and the bottom web are then sealed together, typically by application of heat to the facing portions of the upper and lower webs about the periphery of the food product.

VSP films are designed to be thermoformable and to remain dimensionally stable at the temperatures that are applied by the vacuum skin packaging apparatus. VSP temperatures normally vary from a low of about 85 degrees Centigrade to a high of about 250 degrees, although films can be designed for use outside these ranges.

VSP films are to be distinguished from heat shrinkable films, although the components of the films can be similar. Heat shrinkable films usually are made by the "double bubble", process as described in Oberle et al.

U. S. Patent No. 4,469,742. These films are usually irradiated and are heated and stretched to become more highly orientated than VSP films. Heat shrinkable films typically are immersed in a hot liquid or otherwise heated after evacuation to shrink the film into confirmation with the exterior shape of the product, whereas VSP films are thermoformable to conform to the shape of the product on application of vacuum. VSP films may exhibit some shrinkage, but typically are not heated to a degree that causes significant shrinkage for the particular VSP film.

Typical coextruded VSP films include a sealing or bonding layer, which is the layer that is placed adjacent the food product and is used to form the seal between the upper and lower webs. One or more bulking layers are then added to the film that may have various mechanical and optical properties. Bulking layers normally are responsible for the thermoformable properties of the film.

An abuse resistant layer typically is used as the outermost external layer of the film opposite the food product. The film can be irradiated for high temperature applications.

High barrier packaging applications are those in which it is desirable for the film to have a relatively low oxygen permeability. Oxygen barrier layers, typically comprising ethylene vinyl alcohol copolymer ("EVOH") are normally included within the bulking layers of the coextruded multi-layer film structure if the film is to be used for high barrier packaging applications. It usually is necessary to include adhesive layers on each side of the EVOH barrier layer for nondelaminatable bonding of the EVOH within the interior of the VSP film structure.

For example, Botto et al. U. S. Patent No. 4,963,427 discloses a multi-layer coextruded VSP film having a bonding layer of low density polyethylene, high density polyethylene, ionomer, and mixtures thereof. Internal layers include layers of ethylene vinyl acetate (EVA). An outermost layer is coextruded with the bonding layer and the internal EVA layer. The outer layer is selected from low density polyethylene, high density polyethylene, EVA, ionomer, and mixtures thereof. In another embodiment, an oxygen barrier layer, which can be EVOH, is included as an internal laver having adhesive layers on each side thereof.

A delaminatable VSP film is disclosed in Davis et al.

U. S. Patent No. 4,886,690 in which two internal layers comprise EVOH. One of the EVOH layers is coextruded with and bonded to very low density polyethylene or ultra low density polyethylene on one side so that the EVOH barrier layers may be readily peeled from the oxygen permeable layers, whereby a red meat product can be allowed to bloom upon exposure to oxygen. The other EVOH layer is contained within the barrier portion of the film between two adhesive layers. An abuse layer of nylon, high density polyethylene, EVA, propylene, or the like is the outermost layer of the coextruded multi-layer film.

EVOH is said in U. S. Patent No. 4,886,690 to be a good gas barrier but quite moisture sensitive. It is recognized that the layer of EVOH that is bonded to very- low density or ultra low density polyethylene can lose its oxygen barrier properties. The layer of EVOH that is bonded by adhesive on each side is said to be protected from moisture attack and to retain its low oxygen permeability.

A variety of VSP films have been developed too numerous to mention. The high barrier films that include multiple tie layers sometimes suffer the disadvantage of reduced clarity and the adhesive resins typically are more expensive than the resins used for the other layers of the film. It is desirable to develop additional alternative VSP films for food packaging having improved mechanical, optical, barrier, economic and other properties.

The invention comprises high barrier, nondelaminatable VSP coextruded multi-layer film structures in which one or more barrier layers are external to the bulking layers so that the bulking layers are adjacent one another and are not interrupted by barrier layers or adhesive layers.

"Nondelaminatable"means that the coextruded layers of the VSP film are not readily separated in contrast to the delaminatable barrier and permeable layers of the film described in Davis et al. U. S. Patent No. 4,886,690 and elsewhere.

The abuse and barrier layers can be combined. For example, EVOH can be used as the outermost external layer having combined abuse and barrier functions. All but one adhesive layer can be eliminated, if desired. Additional bulking layers can be included in the film structure for improving the overall properties of the film. Various properties of otherwise similar VSP film structures can show significant improvement, particularly in thermoformability and in optical properties, where multiple bulking layers are placed in adjoining relationship rather than being separated by tie layers and intermediate barrier layers within the bulk of the film.

In a specific embodiment, the invention is a coextruded multi-layer nondelaminatable film for vacuum skin packaging applications having an inner sealing layer, one or more barrier layers, and one or more bulking layers intermediate said sealing and barrier layers. Typically, the barrier layers comprise ethylene vinyl alcohol copolymer having from about 28 to 44 percent vinyl alcohol monomer in the copolymer and being at least about 3 um thick. The EVOH layers typically are separated from the inner surface of the film by at least 60 to 95 um or more of film thickness, which reduces any impact of the high humidity package interior on the barrier properties of the film.

A film thickness separating the inner surface and the barrier of from 75 to 95 um is somewhat more typical.

However, the films can have a thickness between the inner surface and the barrier as much as 200 um while still achieving the benefits of the invention, although these thicker films tend to be too expensive and an effort is made to reduce the thickness of the films. The benefit of the invention of having the barrier layers external to the bulking layers should be achievable within the limits of thin film preparation. In particular, it is contemplated that the thickness of the film between the inner surface of the sealing layer and the barrier layers could be as low as 30 um.

The bulking layers do not have barrier layers between them. The bulking layers can be selected from the group consisting of ethylene vinyl acetate copolymer, ionomer, low density polyethylene, linear low density polyethylene, metallocene polyethylene, and blends of metallocene polyethylene and low density polyethylene. When EVA is used in the bulking layers, the copolymer typically has from about 12 to 28 percent vinyl acetate monomer and the individual layers are at least about 10 to 30 um thick.

EVA bulking layers can be irradiated to improve high temperature formability. A blend of amorphous nylon and nylon 6 can be used in a layer intermediate the bulking layers and the barrier layer to increase overall temperature resistance and to increase the oxygen barrier properties.

Adhesive tie layers typically are used intermediate multiple EVOH barrier layers and to join the barrier to the bulking layers. These tie layers are typically low density polyethylene or linear low density polyethylene that have been modified for use as adhesives in VSP films.

By practice of the invention, all but one tie layer can be eliminated to improve bulking and optical properties of the film.

If increased abuse resistance is desired, then an abuse layer can be applied to the exterior of the film while still not disrupting the integrity of the bulking layers. The abuse layer is normally ionomer, high density polyethylene, or nylon.

Thus, the invention provides an improved VSP film for high barrier applications in which the barrier layer is external to the bulking layers. The number of tie layers can be reduced and more bulking resin can be used in the film, which increases clarity, can reduce the cost of producing the film, and improve strength and thermoformability.

High barrier films of the invention normally have low oxygen permeabilities of from about 2 to 3 to about 10 to 12 cubic centimetres of oxygen per square meter per day at 75 percent relative humidity and 23°C ("cc/m2/day @ 75% rel hum. and 23°C"). Higher oxygen permeabilities may be acceptable for some packaging applications, up to about 15 or 20 cc/m2/day @ 75% rel hum. and 23°C, and yet still be low enough for the film to be considered a high barrier film. Films for which oxygen permeability is less of a concern typically have oxygen permeabilities of about 25 CC/m2/day @ 75% rel hum. and 23°C or more.

Specific film structures are shown below in Tables 1 through 5.

TABLE 1 Layer A B C D E F G I Resin EVA EVA EVA Tie EVOH Tie EVOH (23% VA) (18% VA) (18% VA) Thick- 15 20 30 5 10 5 5 ness µm Range µm 10-25 10-30 10-30 2-8 5-15 2-8 2-10 TABLE 2 Layer A B C D E F G T Resin EVA Ionomer Ionomer Tie EVOH Tie EVOH Thick- 15 20 40 5 10 5 5 ness µm Range µm 10-25 10-30 20-60 2-8 5-15 2-8 2-10 TABLE 3 Layer A B C D E F G T Resin Ionomer Ionomer Ionomer Ionomer Ionomer Tie EVOH Thick- 15 10 20 10 15 5 15 ness µm Range µm 10-20 5-15 10-30 5-15 10-20 2-10 5-25 TABLE 4 Layer A B C D E F G Resin EVA mPE/LDPE mPE/LDPE mPE/LDPE Tie 30% Amorphous PA EVOH 70% PA6 TABLE 5 Layer A B C D E F g Resin EVA EVA EVA EVA Tie 30% Amorphous PA EVOH 70% PA6 TABLE 6 Layer A B C D E F G Resin EVA Ionomer Ionomer Ionomer Ionomer Tie EVOH In the above tables, the thicknesses and ranges of thicknesses shown for the individual layers are to be regarded as suggestions. It should be recognized that thicknesses of the layers can be varied while still achieving the benefits of the invention.

Layer A is the"bonding"or"sealing"layer that is the innermost layer and is placed in contact with the product about which the film is vacuum skin packed. In the examples where layer A is ethylene vinyl acetate copolymer ("EVA"), the copolymer typically has from about 18 to 28 percent vinyl acetate monomer in the copolymer. The EVA layer normally is from about 10 to 25 um thick, but may be thinner or thicker as desired. When layer A comprises ionomer, then the ionomer layer is normally from about 10 to 20 um or more thick.

Layer G is the outermost layer, which functions as a combined abuse and oxygen barrier layer in the examples of Tables 1 through 6. In all of the above VSP films, the exterior layer is ethylene vinyl alcohol copolymer ("EVOH").

The EVOH copolymer typically has from about 28 to 44 percent alcohol monomer units in the copolymer. Layer G typically is from 3 to 25 um thick.

The examples of Tables 1 and 2 include additional EVOH barrier layers E, that are joined by tie layers, to the outermost exterior layer G of EVOH. In the example of Tables 4 and 5, the amorphous nylon/nylon 6 blend (polyamide) (layer F) beneath the EVOH outermost exterior layer G increases overall temperature resistance and increases the oxygen barrier that is already provided by the EVOH layer. The blend of 30 percent amorphous nylon and 70 percent nylon 6 is optimized for thermoformability. The blend should also be useful with from 20 to 40 percent amorphous nylon by weight, although not necessarily with equivalent results.

The bulking layers are of variable thickness. The bulking layers will normally be from 5 to 60 pm thick. The examples of Tables 1 and 5 include EVA bulking layers in addition to the EVA sealing layer. The EVA bulking layers normally have a range of vinyl acetate monomer in the copolymer of from 12 to 28 percent. The films of Tables I and 5 are irradiated for high temperature formability since they contain EVA bulking layers.

The ionomer layers, including the ionomer sealing layer A, typically are Dupont Surlyn 1601 or Exxon 3110. The film of Table 4 includes a blend of metallocene polyethylene ("mPE") and a low density polyethylene (LDPE) in the bulking layers. The metallocene polyethylene should be present in an amount from 75 percent by weight to 100 percent by weight. It has been determined that LDPE present in an amount from 10 to 15 percent by weight is useful in the blend, although the layer can be all mPE or can contain up to 25 percent LDPE by weight. Useful mPE can be obtained from Exxon in Belgium and is sold under the trade name Exceed.

The tie layers serve to adhere one layer to another and are selected to provide a strong bond between the bulking layers and the innermost exterior film layers, which are EVOH layers in Tables 1 through 3 and 6 and a nylon blend in Tables 4 and 5. Several adhesive resins are suitable for use as tie layers in the practice of the invention. For example, modified low density polyethylene (LDPE) and modified linear low density polyethylene (LLDPE) are suitable for use as tie layers and are readily available and knowr. for use as tie layers in VSP films. The LDPE and LLDPE typically are modified with maleic anhydride groups to increase bonding characteristics. The tie layers are typically from 2 to 10 um thick.

The films can be irradiated, if desired. Irradiation normally improves high temperature formability. Irradiation will normally be accomplished in the range of from about 6 to 15 Mega Rads ("MRads"). The film shown in Tables 1 and 5 have been irradiated at 12 MRads. The films of Tables 2 through 4 and 6 were not irradiated.

It has been determined that there are significant differences in some cases between the properties of a film in which the bulking layers are grouped together compared to films in which the bulking layers are separated by an internal barrier layer. For example, the film of Table 7 below, is somewhat similar in structure to the example of the invention of Table 6, except for the location of the EVOH barrier layer. Both films contain EVA sealing layers, EVOH barrier layer, and ionomer bulking layers. However, the film of Table 6 has the EVOH barrier layer as the outermost external layer of the film and in Table 7, the EVOH barrier layer is an internal layer that has adhesive tie layers on each side thereof.

TABLE 7 EXAMPLE 1 Layer A B C D E F G Resin EVA Ionomer Tie EVOH Tie Ionomer Ionomer The resin quantities for the examples of Tables 6 and 7 are similar and one would normally expect similar properties from these films. However, measurements indicate that the example of the invention of Table 6 has significantly improved tensile strength and puncture resistance when compared to the film of Table 7. The results are tabulated below in Table 8 as follows: TABLE 8 Table 6 Table 7 Tensile strenath-Machine Direction (N) 75.4 61.8 Tensile strenglh-TransVerse direction (N) 68.5 61.5 Puncture resistance (load kgf) 2.39 2.01 Puncture resistance (Diso mm) 12.7 9.6 The significant improvements in tensile strength and puncture resistance indicate that there is an advantage in grouping the bulking layers together so that none of the bulking layers are separated by tie layers or barrier layers.

Additional advantages of using a barrier layer, such as EVOH resin, as the outermost external layer of the film, are illustrated below by comparing the films of Tables 9 and 10. The films of Tables 9 and 10 are nominally identical in structure. Each film contains an EVA sealing layer, an EVOH barrier layer, and ionomer bulking layers. The film of Table 9 has an external abuse layer of high density polyethylene (HDPE). However, the film of Table 10 has an external layer that combines abuse and barrier functions and is formed from EVOH.

TABLE 9 EXAMPLE 3 Laver A B C D E F G Resin EVA Ionomer Tie EVOH Tie Ionomer HDPE Thick-15 25 5 10 5 20 20 ness um TABLE 10 EXAMPLE 4 BCLayerA D E F G IonomerIonomerIonomerIonomerResinEVA Tie EVOH Thick-15 20 20 20 10 5 10 nets hum The film of Table 9 has two tie layers. The film of Table 10 has only one tie layer. Tie layers primarily act as adhesives and do not significantly contribute to the mechanical and optical properties of the film. If a film coextruding apparatus is capable of coextruding only a fixed number of layers, then the number of tie layers directly reduces the number of layers that may be used for bulking layers. If two of the layers are tie layers, as shown in the seven-layer film of Table 9, then there are only two remaining layers for the bulking resins, B and F. Layer G is limited to the sole function of an abuse layer and layer A is the sealing layer. Therefore, the quantity of bulking resin that can be incorporated into the structure of the example of Table 9 is limited. The bulking layers contribute to the strength and thermoformability of a VSP film structure. The properties that can be obtained in a VSP film are somewhat limited in the structure of the example of Table 9.

However, the VSP structure of Table 10 has an EVOH surface layer that serves both barrier and abuse functions.

Four layers remain that can be used for the bulking resins, which are layers B, C, D, and E. The quantity of bulking resins is significantly increased and the film properties are improved. It is believed that up to four different bulking layers could be used in a VSP structure made in accordance with the example of Table 10. For example, it should be possible to use LLDPE for tear strength in one layer, mPE for tensile strength in another, ionomer for clarity in another, and LDPE for puncture resistance in another. Also, since tie resins typically are more expensive than bulking resins, then a film with only one tie layer typically is less expensive than a film that contains two tie layers.

Another benefit of the films of the invention is in film clarity. Film clarity depends partially on the loss of light transmission at interfaces between resins with different optical properties. Typically, the more interfaces in a film, then the less clarity a film will have. For example, in the film of Table 9, every interface involves two dissimilar resins and there are six such interfaces in the structure, which are represented by the lines between columns A and B, B and C, D and E, E and F, and F and G. In contrast, the example of Table 10 has only three interfaces between dissimilar resins, which are represented by the lines between columns A and B, E and F, and F and G. Accordingly, the film of Table 10 can be expected to have higher clarity.

The EVOH oxygen barrier is impaired in highly humid environments. Accordingly, it is important to ensure that the EVOH combined barrier and abuse layer is not exposed to moisture. In many VSP food packages, the most humid environment is that of the food itself. It has been determined to be advantageous to ensure that the EVOH layer is separated from the food by as many layers and as much resin as possible. There are approximately 45 microns of polymer between the inner surface of the film and the EVOH layer in Table 9, and 90 microns in Table 10. The combined barrier and abuse layer of Table 10 may therefore be somewhat better protected against the high humidity environment inside the package than the EVOH layer of Table 9.

It should be recognized that resins other than EVOH can be used as combined abuse and barrier layers in high barrier, nondelaminatable VSP film structures, although not necessarily with equivalent results. Additional resins are currently being developed in the art that have high barrier properties and may also be suitable as abuse layers and would be expected to be useful in connection with the practice of the invention.

It should be recognized that EVOH or other barrier resins can be used outside the bulking layers without being the outermost external layer of the film, although not necessarily with equivalent results. For example, a nylon, high density polyethylene, ionomer, or other abuse layer could be used over an EVOH layer or other suitable barrier.

This construction would somewhat reduce the number of bulking layers and bulking resins that could be used within the film structure. However, suitable films for nondelaminatable and high barrier VSP applications could be prepared with good results, particularly where the coextruding apparatus provided a sufficient number of layers, for example, 7 or more.