This present invention relates to novel coextruded thermoplastic film and the employment of such multi-layer film as stretch-wrap material for packaging of goods, including relatively large palletized loads of material. More specifically, such coextruded stretch-wrap films comprise three-layer laminations having a relatively thin skin layer and a relatively thicker core layer. Suitable skin layers include highly-branched low-density polyethylene, and suitable core layers include linear low-density polyethylene co-polymers, such as ethylene co-polymerized with a minor amount of at least one C.sub.4 to C.sub.10 alpha-olefin, such as octene-1 and 4-methyl-pentene-1, and butene-1.

The present invention relates to thermoplastic film structures, in 
particular plastic film structures which have been formed utilizing 
coextrusion techniques. The laminate comprises a core of a linear 
low-density polyethylene having exterior skin layers of low-density 
polyethylene, i.e., conventional polyethylene prepared utilizing the prior 
art free-radical high pressure polymerization process. 
The use of thermoplastic stretch-wrap for the overwrap packaging of goods, 
in particular the unitizing of pallet loads, is a currently commercially 
developing end use application for thermoplastic films, including 
polyethylene. There are a variety of overwrapping techniques which are 
employed utilizing such stretch-wrap films, including locating the pallet 
load to be wrapped atop a rotating platform. As polyethylene film is laid 
on about the girth of the pallet load, the pallet load is rotated on its 
platform. The polyethylene stretch-wrap is applied from a continuous roll 
thereof. Braking tension is applied to the continuous roll of film so that 
the film is being continuously stretched by the rotating pallet load. 
Usually the stretch-wrap film located adjacent to the rotating pallet load 
is vertically positioned and the rotating platform or turntable may be 
operated at speeds ranging from about 5 up to about 50 revolutions per 
minute. At the completion of the overwrap operation the turntable is 
stopped completely while the film is cut and attached to the previous 
layer of film employing tack sealing, tape, spray adhesives or a 
cling-modified film whereby overlapping layers of the stretch-wrap have a 
pronounced tendency to cling together at their interface. Depending upon 
the width of the stretch film roll, the load being overwrapped may be 
shrouded in the film while the vertically positioned film roll remains 
fixed in a vertical position, or the vertically positioned film roll 
(e.g., in the case of relatively narrow film widths and relatively wider 
pallet loads) may be arranged to move in a vertical direction as the load 
is being overwrapped whereby a spiral wrapping effect is achieved on the 
packaged goods. 
Stretch films employed in the prior art have included film materials such 
as polyethylene, polyvinyl chloride and ethylene vinyl acetate. 
With respect of the ethylene vinyl acetate type of stretch film products, 
the prior art has employed a percentage by weight of vinyl acetate in the 
co-polymers of about 2% up to about 15% and preferably from about 4% up to 
about 12% by weight for stretch film applications. 
Physical properties which are particularly significant for the successful 
use of thermoplastic films in stretch-wrap applications include their 
puncture resistance, their elongation characteristics, their toughness, 
and their resistance to tearing while under tension. In particular, the 
latter physical characteristics of such film, i.e., their resistance to 
tearing and their resistance to puncture, are particularly significant. In 
general tensile toughness is measured as an area under a stress-strain 
curve for a thermoplastic film, or it may be considered as the tensile 
energy absorbed, expressed in units of ft.-lbs./in.cu. to elongate a film 
to break under tensile load. In turn, this toughness characteristic is a 
function of the capacity of such films to elongate. The process of 
stretching the film decreases that capacity. Accordingly, the stretch-wrap 
process will decrease the toughness of the film while it is in its 
stretched condition as an overwrap as compared to unstretched 
counterparts, including such materials as shrink-wrap. Generally this loss 
of toughness is proportional to the amount of stretch imparted to the film 
as it is overwrapping a load of goods. 
As hereinabove indicated, the resistance to tear characteristic of such 
films will be obviously an important physical characteristic for 
stretch-wrap applications since if the edge of the stretch film roll is 
nicked, abraded or in any way weakened before stretching or during the 
stretching operation, premature tearing of the film will usually occur 
during wrapping or subsequent handling of the load of goods. 
In practice, one commonly accepted technique for properly tensioning a film 
around a load such as a pelletized load is to adjust the braking force on 
the roll unit a significant amount of neck-in (i.e., film width reduction) 
occurs. Alternatively film may be tensioned until an initiated tear 
results in unrestricted propagation of the tear across the film width. 
It is an object of the present invention to provide a stretch film material 
which, unlike currently commercially available stretch films, is a laminar 
structure comprising at least two and preferably three film layers. The 
prior art stretch film materials hereinabove referred to, such as 
polyvinyl chloride, ethylene vinyl acetate co-polymer and low-density 
polyethylene, have been found to offer reduced resistance to tear in both 
the film's machine direction and transverse direction as well as reduced 
toughness and elongation characteristics in contrast to the laminar film 
compositions of the present invention. 
SUMMARY OF THE PRESENT INVENTION 
In accordance with the present invention, a stretch-wrap material is 
provided which comprises a primary layer of a linear low-density 
polyethylene film, which primary layer has a coextruded layer on at least 
one side thereof comprising a highly branched low-density polyethylene 
fabricated utilizing a high pressure free-radical polymerization process. 
The preferred linear low-density polyethylenes consist essentially of 
ethylene co-polymerized with minor amounts of another olefinic hydrocarbon 
having four to ten carbon atoms, including such materials as octene-1, 
4-methyl-pentene-1, hexene-1, butene-1 and decene-1. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As hereinabove discussed, the present invention comprises the formation of 
a laminar stretch-wrap thermoplastic film by initially preparing the 
coextruded stretch-wrap product utilizing conventional coextrusion 
techniques. The material construction of the laminate prepared in 
accordance with the following example comprises a core layer of linear 
low-density polyethylene, the linear low-density material comprising 
ethylene which has been copolymerized with a minor amount of octene-1. 
Linear low-density ethylene co-polymers are commercially available 
materials and are manufactured by low pressure processes employing 
stereospecific catalysts. These materials usually contain 1 to 10 wt.% of 
C.sub.4 to C.sub.8 .alpha.-olefin hydrocarbon copolymerized with ethylene, 
in sufficient amount to give 5 to 15 branches per thousand carbon atoms in 
the linear polymer. Manufacturing processes for linear low density 
polyethylenes are disclosed in U.S. Pat. Nos. 4,076,698, and 4,205,021. 
The exterior skin layers are fabricated from highly-branched low-density 
polyethylene resin produced by the high pressure process. The high 
pressure low-density polyethylene skin layer provides the requisite cling 
and gloss properties necessary for stretch film applications. The linear 
low-density polyethylene which contains the core layer imparts the desired 
tear and puncture resistance as well as the toughness which is required of 
a film in such a new use application. 
In the following Table A the physical properties of the low-density 
polyethylene and the linear low-density polyethylene resins which were 
employed to fabricate the films identified as X-1, X-2 and X-3 reported in 
Table 2 are set forth below: 
TABLE A 
______________________________________ 
LLDPE LLDPE LLDPE 
Core Core Core 
LDPE-Skins (homopolymers) 
X-1 X-2 X-3 
______________________________________ 
Density (g/cc) 0.9202 0.9228 0.9186 
Melt Index 2.3 2.1 2.4 
Molecular Weight 
Wgt. Avg. 99,100 96,300 -- 
No. Avg. 13,800 20,200 -- 
______________________________________ 
Also reported in Table 2 are the physical properties of a currently 
available LDPE laminar stretch film comprising two layers of high pressure 
(low-density) polyethylene. One layer had a density of 0.925 and a melt 
index of 1.4. The second layer had a density of 0.918 and a melt index of 
7.0.

EXAMPLE 1 
Linear low-density polyethylene as hereinabove defined was fed into the 
feed hopper of a conventional rotating screw extruder. The extruder screw 
employed has a 6" diameter and a length to diameter ratio of about 24:1. 
The satellite extruder which was employed for the extrusion of the 
hereinabove low-density polyethylene material comprised a conventional 
extruder having an extruder screw with a 3.5" diameter and a length to 
diameter ratio of about 24:1. Molten resin from the satellite extruder was 
fed into the cast film die affixed to the end of the core extruder, 
through an adapter specifically designed to join the polymer stream from 
the satellite extruder to the molten polymer core stream so that it covers 
and encompasses the molten surfaces of the core layer. A more complete 
description of this prior art process may be found in U.S. Pat. No. 
3,748,962, the disclosure of which is incorporated herein by reference. 
The specific line conditions employed in the present example are set forth 
in the following table: 
TABLE 1 
______________________________________ 
SKIN RESIN 
LDPE LDPE 
CORE RESIN 
LDPE Ethylene-octene-1 
______________________________________ 
Melt Temperature 
Skin (.degree.F.) 520 520 
Core (.degree.F.) 565 575 
Line Speed (FPM) 715 635 
Chill Roll Temperature (.degree.F.) 
75 75 
Extruder Screw Speed (RPM) 
Satellite 65 65 
Main 110 85 
Skin Percentage % by wgt. 
15 15 
Gauge of Laminate (mils) 
1.0 1.0 
% Octene-1 by Wgt. 12% 
______________________________________ 
Although the present example describes a cast film process for the 
manufacture of the present stretch film products, it will be understood 
that other conventional thermoplastic film forming techniques may be 
employed, such as the commonly employed tubular extrusion process 
utilizing an entrapped air bubble to expand the extruded film tube. The 
film produced in accordance with the present example comprises a linear 
low-density polyethylene core consisting of about 85% by weight of the 
over-all laminar product, while the exterior high pressure low-density 
polyethylene skins contributed about 71/2% by weight per side. The gauge 
of the composite laminar structure ranged from about 0.8 up to about 1.0 
mil. 
The physical properties of film produced in accordance with Example 1 and 
identified in the following Table 2 as X-1, X-2, and X-3 are set forth 
below. Additionally, in Table 2, for comparative purposes, the physical 
properties of currently commercially available stretch-wrap materials, 
including polyvinyl chloride, ethylene vinyl acetate, and a two layer 
low-density polyethylene are set forth. 
TABLE 2 
__________________________________________________________________________ 
Ethylene-.alpha.-olefin Coextrusion 
X-1 X-2 X-3 
PVC 
EVA LDPE 
__________________________________________________________________________ 
Caliper (mils) 1.0 1.1 0.93 
0.8 
1.0 1.0 
ASTM D-882 
Ultimate Tensile PSI 
MD 4200 
5400 
6542 
4900 
5400 
3600 
TD 3300 
3700 
4459 
4000 
4500 
2300 
Yield (PSI) MD 1900 
1300 
958 
1600 
900 1300 
TD 1100 
1300 
963 
1000 
800 1300 
Elongation (%) 
MD 500 650 597 
300 
450 500 
TD 900 900 907 
300 
600 700 
ASTM D-1922 
Elmendorf Tear - g/mil 
MD 150 90 130 
80 35 150 
TD 700 960 798 
120 
75 350 
ASTM D-882 
Toughness (Ft. lbs/in.sup.3) 
MD 1100 
1500 
1670 
800 
1300 
1050 
Puncture 
Instron Penetration 
Lbs. 
10 11 9.5 
12 15 8 
Energy 
Rupture In.-Lbs 
36 37 39.9 
19 44 12 
Penetration 
Instron Probe 
In. 5 5 6.2 
3 5 3 
Cling Index -- 1.0 2.4 
2.3 
3.5 2.2 
ASTM D-2457 
(Gloss (% at 45.degree.) 
87 85 89.9 
87 74 89 
ASTM D-1003 
Haze (%) 1.5 2.2 0.8 
1 2 1 
Density (g/cc) 0.9151 
0.9174 
-- 1.23 
0.9313 
0.9185 
__________________________________________________________________________ 
It has been found that the types of high pressure, low-density skin resins 
employed in the present invention may vary in physical characteristics. 
Preferred skin resins however include those with densities of from about 
0.917 up to about 0.922 and melt indices of from about 4 up to about 8. 
The preferred linear low density polyethylene co-polymer core resins 
include those with densities of from about 0.916 up to about 0.925 with 
melt indices of from about 1.0 up to about 6.0. The thicknesses of the 
skin layers may vary widely, however preferred thicknesses include those 
from about 5% up to about 40% based upon the overall thickness of the 
laminate. 
It is to be understood that the foregoing description is merely 
illustrative of preferred embodiments of the invention, of which many 
variations may be made by those skilled in the art within the scope of the 
following claims without departing from the spirit thereof.