CA2155160C - Cross-linked film - Google Patents

Abstract

A cross-linkable, preferably stretch oriented film comprising a blend of a polyolefin and a diene polymer and a transition metal catalyst, wherein the blend cross-links in the presence of oxygen without irradiation to provide a film with high heat resistance.

Description

BACKGROUND OF THE II~IQ'E~NTION
The present invention pertains to compositions for making packaging film, and more particularly to a thin polymeric film suitable for replacing polyvinyl chloride film especially in packaging applications.
Polyvinyl chloride (PVC) has lone been used in many applications in the packaging art. One particularly wide-spread application for PVC is the use of such material as an overtrrap material far trayed retail cuts of meat and other l0 food products in a retail environment such as a supermarket.
. PVC has several desirable properties for this use. Far example, it has excellent burn-through resistance, optics and good elasticity and stretch properties at use temperatures.
Unfortunately, PVC also has several disadvantages, including the production of hydrogen chloride gas during heat sealing and the generally corrosive effects of such gases in the packaging room. Extractables from the PVC into the packaged food product have also become of concern.
It would be of great benefit to the packaging industry, and particular to applications requiring an instore film for overwrapping trayed food products, to provide a film with many of the advantages of PVC but without the disadvantages described above.

~- 2~~~1~a In-addition to the foregoing, it is desirable to provide a film as aforesaid without the disadvantages of PVC, and to provide such a film which is cross-linked without irradiation. In the applications described above the film material is wrapped around the product and sealed as on a hot plate. Without cross-linking the film might burn through on the hot plate and therefore cross-linking is desirable to provide high heat resistance. Irradiation is customarily used to provide cross-linking, but this procedure is inconvenient, expensive and inefficient and also difficult to perform on very thin films.
It is also desirable to provide such a film which is stretch oriented to provide improved properties. Processes for producing oriented films and oriented films themselves are known in the art.
U.S. Patent No. 3,456,044 (Pahlke) mentions thin films of thicknesses less than i mil such as 0.5 mils, and _ discloses a double bubble method for biaxially orienting thermoplastic films, including the steps of producing a primary tubing which is inflated by introducing air into the interior thereof, and a cooling ring 22, as well as squeeze rolls 34 and 28, with rolls 34 having a greater speed than rolls 28. Between the two pairs of squeeze rolls is a reinflated secondary bubble. If annealing is desired, the tubing can be reinflated to form a bubble 70.

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U.S. Patent No. 3,555,604 (Pahlke) is a patent based on a divisional application which was derived from the same priority application as the '044 patent described above, and discloses the same information described above for the '044 patent.
U.S. Patent No, 4,258,166 (Canterino et al.) discloses a uniaxially oriented plastic film material with improved strength and clarity in the direction of orientation preferably comprising homopolymers and copolymers of ethylene.
U.S. Patent No. 4,355,076 (Gash) discloses monoaxially oriented polypropylene film laminated to a monoaxially oriented high density polyethylene film, the films produced by for example tubular blowing.
U.S. Patent No. 4,440,824 (Bonis) discloses a thermoformable coextruded multilayer structure useful for thermoforming into containers, the structure having polyolefin coextruded with a high impact polystyrene layer.
A five layer structure is shown.
U.S. Patent No. 4,464,439 (Castelein) discloses a coextruded laminate having a sheet of polypropylene and a sheet of a mixture of high impact polystyrene, crystalline polypropylene, and styrene/dienic monomer block copolymer.
U.S. Patent No. 4,879,177 (Boice) discloses a monoaxially oriented shrink film having a core layer of butadiene styrene copolymer, outer layers of ethylene propylene copolymer, and intermediate binding layers of ethylene copolymer.
U.S. Patent No. 5,21,666 (Schirmer et al.) discloses polymeric,, oriented films made by the use of a combination of a hat blown process and a blown bubble process.
SUMMARY OF THE INVENTION
The present invention provides a cross-linked, stretch oriented film which can be included i.n a thermoplastic film useful as an overwrap material for trayed retail cuts of meat and other food products in a supermarket or other retail environments. Further, the present invention provides a film as aforesaid that can be cross-linked without irradiation. Ss.-.ill further, the present invention provides a film as aforesaid which has good burn-through resistance. Additiona_Lly, the present invention provides such a film with other desirable characteristics, such as good optical properties.
The inventor has discovered that a blend of a polyolefin, a dime polymer and a transition metal catalyst cross-links in the presence of oxygen. This provides a film with heat resistance, especially burn-through resistance.
The film of the present invention is a cross-linked, stretch-oriented film with heat, resistance and good optical pr~opert ies .
The film can desirably be provided as a laminate with an outer sealable layer and preferably with an intermediate adhesive layer.

The final laminate may have a thickness of from 0.5 - 5 mils, and desirably may be used a: a thin laminate in thicknesses from 0.5 - 2 mils.
In one aspect, the invention provides a cross-linkable film, comprising a) two outer layers comprising a polyolefir~ or a styrE:ne polymer or copolymer; and b) a layer intermediate the outer layers comprising a. blend of a polyolefin, a dime polymer and a transition metal catalyst, wherein the film cross-7.inks in the presence of oxygen.
In a further aspect, the invention provides a cross-linkable, oriented film, comprising: a) a core layer comprising a blend of a polyolefin, a diene polymer, and a transition metal catalyst; b) two outer layers comprising a styrene polymer or copolymer; and c) t:wo intermediate layers, bonding the core layer to respective outer layers, comprising a polymeric adhesive.
BRIEF DESCRIPTION OF THE DRAWING
The present invention may be further' understood by reference to the accompanying drawing:
which is a schematic cross-section of a preferred embodiment. of a multi-layered film in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The cross-linked, stretch-oriented film of the present invention offers considerable advantages. It is cross-linked without irradiation and obtains high heat resistance.
Further, it is stretch-oriented to achieve desirable properties and may readily be formed into an advantageous laminate.
The polyolefin is preferably VLDPE (very low density polyethylene) or LLDPE (linear, low density polyethylene), although others can readily be used, as polypropylene and copolymers of ethylene and propylene. Homogeneous ethylene alpha olef in copolymers can also be used in connection with this invention.
The diene polymer blended with the polyolefin provides unsaturated groups. Typical diene polymers include octadienes, hexadienes, 1,4-polybutadiene, I,2-polybutadiene, non-conjugated dienes and heptadienes.
Preferably, the transition metal catalyst is in the form of a salt, with the metal thereof selected from the first, second or third transition series of the Periodic _ Table. Suitable metals include, but are not limited to, manganese II or III, iron II or III, cobalt II or III, nickel II or III, copper I or II, rhodium II, III or IV, and ruthenium. The oxidation state of the metal when introduced is not necessarily that of the active form. The metal is preferably iron, nickel or copper, more preferably manganese and most preferably cobalt. Suitable counterions for the metal include, but are not limited to, chloride, acetate, stearate, palrnitate, 2-ethylhexanoate, neodecanoate or naphthenate. Particularly preferable salts include cobalt (II) 2-ethylhexanoate and cobalt (II) neodecanoate. The metal salt may also be an ionomer, in which case a polymeric counterion is employed. Such ionomers are well known in the art.
In accordance with the present invention, it has been found that a blend of a polyolefin and a di me polymer cross-links in the presence of oxygen.
The blend is preferably stretch oriented before cross-linking. For example, the blend is coextruded through a die in the conventional manner and the extruded film hot blown to form a blown bubble. In accordance with standard processing, an air cooling ring may be positioned circumferentially around the blown bubble to cool same as it exits the die. The blown bubble is melt oriented in both the machine and transverse directions using various blow up _ ratios, but preferably the bubble is hot blown to a blow-up ratio of between 1.5 and 8. If desired, one may immediately reinflate the bubble after cooling into a secondary bubble and then expand same to impart orientation of the material in primarily the transverse direction, primarily the longitudinal direction, or in both the transverse and longitudinal directions. The bubble is collapsed in a set of pinch rolls and transferred to a take-up roll. This procedure is shown, for example, in I.T.S. patent No.
5,219,66F.
Naturally, alternate methods may be utilized for providing the stretch oriented film.
The cross-linkable, stretch oriented film of the present invention may be desirably farmed into a multi-layer laminate. In a preferred constructian, outer layers 12 as shown in Figure 1 may comprise a polyalefin or desirably an ethylene polymer or copolymer such as ethylene vinyl acetate copolymer (EVA). One may also desirably use styrenic polymers and copolymers, for example, styrene butadiene copolymer, such as that commercially available from Phillips under the trade-mark KR-10 having a butadienE=_ content of TM
25o by weight of the copolymer, or KK-36 (for fatty food contact). Desirably, styrene butadiene copolymers (SBC) are used for the outer layers, especially block copolymers containing a major portion (greater than 500) of styrene and a minor portion (less than 50%) of butadiene comonomer.
These materials provide an optimum balance of stiffness and flexibility to the film.
Core layer 10 preferably comprises the bT.end of the present invention, i.e. a blend of a polyolefin and a diene polymer, and a transition metal catalyst. Mor a than one polyolefin can be used in the blend, as well as more than one diene polymer. The term "polymer" includes herein not c;4536-878 only homopolymers, but copolymers and tet-polymers as well provided they blend with the other campc~nent~s to provide a cross-linkable blend.
T_n the embodiment Shawn in the drawing, the outer layers 12 are banded to the core layer 10 by means of intermediate layers 14 comprising, for example, a polymeric adhesive and preferably a copolymer of ethylene, and more preferably an ethylene vinyl acetate copalyme:r (EVA). Other polymeric materials, including chemically modified adhesives, can be used for layers 14 provided!, that they process adequately :in processes such as those: discussed herein. Blends of polymeric materials and polymeric adhesives may also be used for intermediate layers.
For layers of the present inventian which contain SBC, TM
anti-fog/plasticizing agents, such as Atmer 645 and/or Atmer 1010 are preferably included, desirably i.n amounts between about 0.5 and 10% by weight of the layer or layers. The intermediate layers 14 of the present invention also desirably may contain such agents in the aforesaid amounts.
The multi-layer film of the present invention is preferably prepared by coextrusion techniques as discussed hereinabove.
The present invention will be more readily understood from a consideration of the following illustrative examples.
Three sample rolls of film were produced, by the process described in U.S. Patent No. 5,219,666.

The construction of each ov these three films was as follows:
Example 1 SBC/EVA/BLENLi 1/EVA/SBC
Example 2. SBC/EVA/BLE2JD 2/EVA/SBC
Example 3 (Comparatives SBC/EVA/VLDPE/EVA/SBC
In the examples, "SBC" is a styrene butadiene copolymer available from Phillips under the trade-mark KK36-2. The SBC materials of the skin layers included about 2% each, by weight of the layer, of ATMER 1010 and ATMER 645; both Atmer materials are used as antifog agents, and are available from ICI.
"EVA" is an ethylene vinyl acetate copolymer, and in the examples produced in accordance with the invention, the EVA actually comprised a blend of 50% (by weight) of an EVA
resin, and 50% (by weight) of an EVA master batch. The EVA
TM
resin was Elvax 3165 available from DuPont. This resin has a vinyl acetate content of about 18% by weight. The EVA
master batch comprised 92%, by weight of the master batch, of Elvax 3165, and 4% each, by weight of the master batch, of ATMER 1010 and ATMER 645.
The BLEND 1 of Example 1 was made up of:
TM
- 72% by weight of a VLDPE (DEFD 1015-8 available from Union Carbide) which has a density of about .900 and a melt index of about .1;

'rM
- 20°s by weight of the blend layer of RH830, a 1,2-polybutadiene available from ,7SR; anc~
- 8s by weight of an LLDPE roaster batch, i.e. a blend of 40°s by weight of a linear low densir_y polyethylene, and 30°s each, by weight, of ATI~IER 1010 and ATbtER 64540, the LLDPE master batch provided cornrnercially under the trade-mark Santec 23-222.
The BLEND 2 of Example 2 was like BLEND 1, but also included 5v, by weight of the blend, of cobalt decanoate master batch, wherein a cobalt decanoate material was compounded with EVA resin. In the cobalt decanoate master batch, EVA (9~ VA) comprises 97.7°s of the master batch, and the cobalt decanoate cornprised 2.3~ of the roaster batch.
The core layer 10 of Example 3 contained very low density polyethylene, the DEFD 1015 rnaterial available from Union Carbide.
Samples of a stretch olefin film made in accordance with the method disclosed in U.S. Patent 5,219,666, where produced to deterrnine if cross-linking would occur by an oxidation reaction. These samples were analyzed for percent gel in general accord with standard ASTM procedure for determining gel content and inferent sally t;he degree of cross-linking of the film. All samples were extracted in boiling toluene, vacuum dried and re-weighed. The samples were extracted a second 21 hours to assure complete solubility of all soluble portions. The results were as follows:
Example 1 Example 2 Exam~ale 3 Gel ~ 0 14.7 0 (ASTM D-2765-84) The cross-linkable material of the invention, as illustrated in Example 2, is very useful in producing a heat resistant material useful in over~arap applications such as those described above.
This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention .
being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

Claims (10)

1. A cross-linkable film, comprising a) two outer layers comprising a polyolefin or a styrene polymer or copolymer; and b) a layer intermediate the outer layers comprising a blend of a polyolefin, a diene polymer and a transition metal catalyst, wherein the film cross-links in the presence of oxygen.

2. A film according to claim 1, including an outer heat sealable layer.

3. A film according to claim 2, wherein the outer sealable layer is a styrene butadiene copolymer.

4. A film according to claim 2 or 3, including an intermediate polymeric adhesive.

5. A film according to any one of claims 1 to 4, wherein the blend comprises polyethylene, polybutadiene and cobalt decanoate.

6. A film according to any one of claims 1 to 5, having a thickness of 0.5-2 mils.

7. A cross-linkable, oriented film, comprising:
a) a core layer comprising a blend of a polyolefin, a diene polymer, and a transition metal catalyst;
b) two outer layers comprising a styrene polymer or copolymer; and c) two intermediate layers, bonding the core layer to respective outer layers, comprising a polymeric adhesive.

8. A film of claim 7, wherein the core layer comprises a blend of an ethylene alpha olefin copolymer, a diene polymer and a transition metal catalyst.

9. A film of claim 7 or 8, wherein the outer layers each comprise a styrene butadiene copolymer.

10. The film of claim 7, 8 or 9, wherein the intermediate layers comprise an anhydride grafted polymer.