Printable composite stretch wrap film

Printable high strength, thin stretch wrap composite films exhibiting excellent self-adherence without heat sealing are comprised of at least two layers, wherein one outside layer is comprised of a tackified ethylene-vinyl acetate copolymer and the other outside layer of a very low density polyethylene copolymer having a density below 0.915. Optionally, a third inside layer of liner low density polyethylene is present in the composite film.

BACKGROUND OF THE INVENTION 
The use of compositions based on conventional ethylene homopolymer or 
copolymers in the manufacture of stretch wrap films having good cling and 
optical properties is known, e.g., as shown in U.S. Pat. No. 4,073,782. 
This patent discloses stretch wrap films made from a ternary composition 
wherein the resin component is a polyethylene, a copolymer of ethylene and 
vinyl acetate, or a mixture of the above-mentioned polyethylene and 
copolymer. The remaining essential components of the ternary composition 
are sorbitan monooleate, which is mainly added as an antihazing agent, and 
a liquid paraffin, as an adhesiveness modifier. Although the films 
produced in accordance with the teachings of the aforementioned patent 
perform very well in some applications, their strength and toughness 
properties are generally not as good as would be desired. 
The recently developed linear low density polyethylenes have the desired 
strength and toughness properties required from stretch wrap applications, 
but do not possess the required cling, which in the case of conventional 
polyethylene, can be partially obtained by incorporation of vinyl acetate 
as a comonomer. 
U.S. Pat. No. 4,436,788 discloses a composite stretch wrap film constituted 
of a layer of ethylene-vinyl acetate copolymer containing a tackifier and 
a layer of linear low density polyethylene. Such a film exhibits a certain 
amount of adherence to itself, however, in certain applications requiring 
very thin films, acceptable adherence can be achieved only after applying 
heat to the overlapping wraps of the film. 
The patent also discloses a three-layer stretch wrap film of improved cling 
properties constituted of an inner layer of linear low density 
polyethylene and the two outer layers of ethylene-vinyl acetate copolymer 
preferably containing a tackifier. It has been found that the presence of 
tackifier is necessary in both of the outer layers in order to provide 
sufficient cling. However, because of the presence of such a tackifier in 
the outer layers, it has not been possible to use this film in the 
manufacture of printed films of acceptable print quality. The tackifiers 
work by migrating to the surface of the film. Due to the presence of these 
tackifiers on the surface, it is impossible to hold a corona treatment 
level on the film which is necessary to wet out printing inks. Also, 
because the inner linear low density polyethylene layer by necessity must 
be very thin when the composite film is required to be of thin gauge, the 
strength of the stretch wrap is less than desired. 
It is therefore an object of the present invention to provide a novel 
composite stretch wrap film which exhibits sufficient cling and strength 
without the necessity of heat treatment. 
It is another object of the invention to provide a printable composite 
stretch wrap film exhibiting excellent cling properties even at thin film 
gauges. 
Further objects of the invention will become apparent from a reading of the 
specification and appended claims. 
THE INVENTION 
In accordance with one embodiment of the present invention there is 
provided a composite film comprising 
(A) a first layer of a blend of 
(1) an ethylene-vinyl acetate copolymer (EVA) containing from about 2 to 
about 25 wt % of polymerized vinyl acetate groups and 
(2) a tackifier and 
(B) a second layer comprised of at least 50 wt % of a very low density 
polyethylene (VLDPE) having a specific gravity of below 0.915 gms/cc. 
In another embodiment of the present invention, there is provided a 
three-layer composite film further comprising an intermediate layer of 
linear low density polyethylene to provide a structure of tackified 
EVA/LLDPE/VLDPE. 
The polymerized vinyl acetate content of the EVA copolymer is preferably 
maintained between about 4 and about 16%. The melt index of the EVA should 
be in the range of from about 0.1 to about 10 g/10 min and preferably 
between about 0.25 and about 4. 
The tackifier to be used in the EVA resin can be any suitable tackifier 
known in the art. For instance, sorbitan monooleate, glycerol monooleate, 
low molecular weight polyisobutylene, amorphous polypropylene, 
polyterpenes, microcrystalline wax are among tackifiers useful in this 
invention. Typically, they are present in the EVA resin in concentrations 
between about 0.5 and about 6 wt %. 
The VLDPE is a linear nonpolar polymer of narrow molecular weight 
distribution. It is a copolymer of ethylene and at least one comonomer 
selected from C.sub.4 to C.sub.10 alpha-olefins, the copolymer having a 
density of below 0.915, such as between 0.890 and below 0.915. The melt 
index of the VLDPE copolymer is in the range of from 0.1 to about 10g/10 
min. The copolymer resins are commercially available and consequently, for 
the purpose of this invention, the process for their production need not 
be discussed. The VLDPE can be blended with from 0 to about 50 wt % of 
another ethylene polymer such as polyethylene homopolymer, ethylene-vinyl 
acetate copolymer, linear low density polyethylene, and mixtures of these 
compounds. 
The linear low density polyethylene (LLDPE) used as an optional 
intermediate layer is a copolymer of ethylene with one or more comonomers 
selected from C.sub.4 to C.sub.10 alpha-olefins such as butene-1, 
hexene-1, etc. Preferably, the density should be maintained between 0.916 
to 0.928 g/cc for film making purposes. The melt index should range 
between about 0.1 and 10 g/10 min and preferably between 0.5 and about 3.0 
g/10 min. Such polymer resins are commercially available and are 
manufactured in low pressure vapor phase and liquid phase processes using 
transition metal catalysts. 
Although the VLDPE resins used in one of the layers of the film is quite 
similar to a LLDPE resin in many respects, e.g., in linear structure, lack 
of long chain branching, etc., the difference in the catalysts and in 
process technologies used in the manufacture of the respective resins 
gives rise to an unexpected difference in cling characteristics. 
Specifically, while films produced from LLDPE resins do not posses any 
significant cling characteristics, those produced from VLDPE resins are 
sufficiently tacky that no extraneous tackifiers need to be added to 
provide the desired cling even at thin film gauges. 
Various methods may be used for producing the composite films, e.g., by 
coextrusion of tackified EVA, LLDPE (optional) and VLDPE in known fashion. 
The total thickness of the film can be between about 0.2 and about 2 mil 
and should preferably be in the range of from about 0.3 to about 1.5 mil. 
The thickness of each layer should be at least 0.1 mil. Since the films of 
these inventions have greater tear and puncture resistance, tensile 
strength and elongation compared to films prepared from conventional low 
density polyethylene or ethylene-vinyl acetate copolymers, the film 
thickness can be considerably reduced at equivalent stretch wrap 
performance. In addition, the films are printable and have generally good 
optival qualities such as low haze levels and high gloss, and are 
resistant to fogging and can, therefore, also be used with advantage for 
wrapping of food to be displayed or stored in refrigerated cases. They are 
particularly suitable as self-adherent, thin, printed wraps for vegetables 
such as heads of lettuce, cauliflower, etc. The vegetables are 
advantageously wrapped with the film at the harvesting site.

The following example further illustrates the advantages of the invention. 
EXAMPLE 
A three layer composite film of 0.5 mil thickness was produced by 
coextrusion of layers of VLDPE, LLDPE (inner) and tackified EVA. The 
relative thicknesses of the layers were respectively 20%, 40% and 40%. The 
VLDPE resin was a commercially available product UCAR.RTM. FLX Resin 
DFDA-1137 Natural 7 obtained from Union Carbide Corporation. The resin had 
a density of 0.906 gm/cc and a melt index of 0.8 gms/10 min. The LLDPE 
resin was obtained from Dow Chemical Company under the trade name 
Dowlex.RTM. 2045. It had a density of 0.920 gms/cc and a melt index of 1.0 
gms/10 min. The EVA resin was obtained from Norchem under the trade name 
3043H. The resin had a density of 0.928 gms/cc, a melt index of 2.0 and 
contained about 4.5 wt % polymerized vinyl acetate. Polyisobutene was 
incorporated at about a 4 wt % level as a tackifier. Table I below lists 
the physical properties of the coextruded film. 
TABLE I 
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Optics 
Haze % 2.3 
Gloss 68.7 
Tensiles 
Elong 
MD % 605 
TD % 820 
Pull 
MD psi 5685 
TD psi 4150 
1% Secant Modulus 
MD psi 14535 
TD psi 15885 
Coeff. of Friction 0.63 
To Teflon 
Tear 
MD grams/mil 249 
TD grams/mil 628 
Spencer Impact grams/mil 
+1935 
Melt Index grams/10 min 
1.5 
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Heads of lettuce were wrapped at ambient temperatures in 17".times.17" 
squares of the film keeping the VLDPE layer on the outside. The film of 
all test samples adhered satisfactorily around the lettuce heads and none 
of the samples became unwrapped upon refrigerated storage and repeated 
handling. The same excellent results were obtained using corona 
discharge-treated printed film samples. 
Various modificiations and alterations can be made to the films without 
departing from the scope of this invention, which is defined by the 
specification and appended claims.