Note: Descriptions are shown in the official language in which they were submitted.
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~6567
POI,YMER FILM HA~ING LINEAR TEAR PROPERTI~S
Background of the Invention
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~ arious polymer films are used to make bags,
pouches, packages and the like. A common practice
is to join opposed polymer sheets together along a
common line by heat sealing, whereln the sheets are
pressed together between heated bars for a sufficient
length of time and with sufficient pressure to cause
fusion therebetween.
If a bag or package composed of a polymer film
is heat sealed together on all sides, difficulties
are frequently encountered in tearing the bag open.
Many polymer films tend to stretch before tearing.
Moreover, most films will not tear along a straight
line, which presents a problem ln opening of a bag or
package.
A conventional method of forming polymer films
is by blow extrusion, wherein the polymer is melted
and f orced
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through a circular die to produc~ a thin tube, which is inflated
wi-th a gas in the vicinity of the die and drawn away in a con-
tinuous process. Widely used polymers include low density
polyethylene, usually modified with vinyl acetate, as well as
ionomers, both of which provide a film having good heat seaLing
properties and excellen-t strength. Films composed entirely of
either an ionomer or low density polyethylene, however, made
by normal blow or other extrusion methods, cannot be torn along
a straight line in the direction of extrusion. The film can be
torn in such direction only with great difficulty and not in a
straight line.
Summary_oE the Invention
The present invention is based on the discovery that a
film made from certain blends of polyethylene and an ionomer,
containing a particular minimum amount of either, unexpectedly
exhibits excellent linear tear properties in the direction of
extrusion. The polyethylene-ionomer ~ilm contains from about
12 to about 95% ionomer and from about 5 to about 8B~ poly~
ethylene, the blends in such ranges providing films having
excellent properties as well as excellent linear tear qualities
in the direction of extrusion.
By broadly speaking, therefore, the present invention
provides an extruded polymer film consisting essentially of
from about 12% to about 95~ o~ ionomer and the remainder as
polyethylene having a density of from about 0.910 to about 0.960,
the film being capable of being torn in substantially a straight
line in the direction of extrusion at a deviation of less than
2G from said straight line.
The above product may be produced by a method of imparting
linear tsar properties in the direction of extrusion to a
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polymer film comprising the steps of preparing a hlend of -~
resins containing from about 12~ to about 95~ ionomer resin and :~
from about 5% to about 88~ low density polyethylene resin, and
extruding the blend of resins into a film, whereby said film
exhibits linear tear properties in a direction parallel to the
direction o~ extrusion.
Descri tion of the Preferred Embodiment
In accordance with the preferred embodiment, ionomer
resin and polyethylene resin are mixed together in dry:orm in
the desired proportions, meltedl and then extruded through die in
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the form of a tubing or shcer, which in the case of a tubing, is
Inflated wkh a gas in the. area of extrusion.
lhe term"ionomer" as used herein is characterEzed
as a metal-containing ionic copol~mer obtained by the reaction
between ethylene or an alpha-olefin with an ethylenically un-
saturated monocarboxylic aeid such as acrylic or methacrylic
actd wherein at least 10~7o of said caxboxylic acid groups are
neutralized by an alkali metal ion, and ha-ring a melting point
range of from about 370 to 4S0F. A suitable ionomer for
making iilms is "SURLY~ 1601" sold by E. I. DuPont De Nemours
~ Company.
lhe term "polyethylene resin" iii~iludes polyethylene
resins having a density in the range of from a~out 0.91(1 to
about 0.960. ~ The polyethylene may be modifiecl to Include
from ~ero up to about 7.5% vinyl acetate as a copolymer, J ~;
without adverse effect on the tear qualities of the resulting
sheet. A suitable material is "Chemplex 304I" ma~de by the -
Chernplex Corporation.
It has been found that either the ionomer or polyethylene
may be used as a base, and beyond a particular minimum amount
of the other polymer must be added to impart the destred tear
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qualicles to the resulting Eilm. If polyethylene is the base, a
minimum of at least about 12% by weight of ionomer resin may
be blended therewith to impart linear tear qualities. If
lonomer resin i`s used as the base, a minimum o~ at least
abot!t 5% by weight polyechylene must be added. Slnce all ; ~`
Intermediate levels result in the desired characterlstic, the ~ ~
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polymer blend may con~ain from about ~2% to about 95% ionomer
and rom about 557O to abou~: 88% polyethylene, a1though up to
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about 7.5% of the polyethylene con~ribution may comprise vinyl
acetate to improve puncttlre resistance.
'rhe term "linear tear property" as used herein means
the Inherent property or ability of the filrn to tear by hand
along substantially a straight line, with less than about a 20,
and preferably less than 5 to 10 angle of deviation from said
line after Initiation of the tear. The tear i5 made by a normal
shearing or parting motion without the necessi~ to guide or
manipulate the direction of che ~ear along the line and wi~hout
using guides or other external aids. The films of the present ;
invention have the inherent guality of easily tearing in or
paxallel to the machine direction of tXe-film in substanciQlly a
stralght line. Once the tear has been initiated, preferably by
~litting or nicking the film at one edge~ continued tearLng will
result in a straight llne tear, without stretching or distortion
of the film, as if the material had been perforated or weakened
along the line. This result is totally unexpected, since neither
the pure polyethylene or ionomer films alone exhihit this
property.
The degree of force required to make the tear is also
a contributing factor of the present invention. If the tear
strength, measured by the Elmendorf procedure, is substan-
tially greater than 125 g, the film will offer excessive
resistance to tearing, whereas too low a tear scrength will
result in a film that could too easily be torn accidentally
during normal handling or use. The film of the present inven-
tion will exhibit a tear strength or a resistance to propagntion
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of tear in the order of from about 30 g to about lZS g and ;~
preferably in ~he ordcr of about 40 to about 100 grams, wh~ch
is an ideal tear strength without encountering undue weakness
or resistance to propagation of tear in the filM.
After the polymers have been blen~ed in the desired
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proportions, the mixture is preferably melted and blow extluded
by conventional methods and at temperatures in the order of
about 300 to about 42~~. ~e procedure results in the con~
tinuous production of tubular film, which may thereafter be slit
longitudinally, cut into transverse sections, and heat sealed
adjacent the transverse cuts to form an ~pen end bag or pouch.
~hereafter, any desired contents ma~ be filled into the bag
through the open end, and such end is then heat sealed.
In the alternative, the film of the present In~ention may~
be cast in the form of a sheet by convention methods, and the
linear tear proparty ~,vill still be exhibited in the machine
direction.
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In the conventional fabrication of ba~ or pouches, the -
side seams run perpendicular to the direction of extrusion, ~ -~
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and the remaining open end of the pouch is considered as ~ ~
Ihe top, with the printing, labels, etc., ~eing applied in the ~ -
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~ame orientation, i.e., along lines parallel with the direction
of extruslon. lllus, the product of the present in~ention lends
itself to the production of a pouch havin~ a linear cear property
across the top or bottom of the pouch. Ihe finished pouch
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- rnay also be provided with a notch at either side to facilitate
Inltiation of the ~ear, corresponding to the direction of extrl;sion.
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F~rom the foregoing, the importance of providing a linear
tear property in the machine direction can be reaclily appreciated
since this is ~he direction from side to side of a conventionally
fabricated pouch. Thus, the pouch rnay be grasped at a top
corner ancl easily torn open. A notch is typically provided or
one or both sides of the pouch whereby thc tear may be con-
veniently initiated from either siide.
It has also been found that polymers or blends that do
not possess the linear tear property can be coextruded with the
blends described herein, and the resulting multi~ply film will - ~-
exhibit the desired linear tear property in the m~chine
direction. ~or example, if one ply of the coextrusion comprises
a linear ~ear blend, the second ply may b-e composed OI low
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density polyethylene, polyethylene modified with vinyl acetate
or polyisobutylene, or ionomer, or a blend of ionomer Ind
polyethylene falling outside of the linear tear range described
hexein. As an example of the latter, the second ply may
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comprise a blend containing polyethylene and less than 12%
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lonomer.
A further benefit of the polymer composllion of the
present invention is that while the linear tear feature is exhibited,
there is no detraction from the other properties that would ;
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normally be expected from the individual components. Thus,
there is no loss of such properties such as stlffness modulus,
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impact strength or elongation, but rather an average of the
properties of the individual components is reali~ed. `;
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i67
In further illustration of the present in~ention, the
following examples are glven
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EXAMPLE I
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Four batches ~f identical low density polyetllylene
rontaining 07 2%, 4.S% and 7.5% vinyl acetate were dry
blended wi~h varying amounts of Dupont film grade Surlyn.
lihe blends were each e~truded through a 4 inch die at a l.S
to 1 blow-up ratio under the following extrusion conditions: -
extruder L/D, 24 to 1; gauge, 3 rnil.~ temperature at feed
section of extruder~ abou~ 300F; temperature in extruder,
from 320 to 360; average temperature at front and die of
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extruder, about 400F. The tube for each batch was flattened,
- and notched specimens tested for Elmendorf tear strength in
the machine direction (AS'~M D 1922-67, propag~tion tear
resistance). The tear strength for each sample wa~ measured
~s follows:
TABLE I
Elmendorf Tear (gm.)
,.
%Surlyn 0 10 25 50 75 100
(Remainder Polye~hylene)
~7Q Vinyl Acetate _ _ _
Homopolymer ~-~(P)120(G) 6~(E) 68(E) 82(E) 189(P)
2% Vinyl Acetate 630(P)130(G) S9~E) 71(~) lOO(E) _ _(p)
4.S% Vinyl Acetate 300(P) 255(P) 76(G) 79(E) 88(E) -~-(P3
7-5~7O Vinyl Acetate -~~(P) 194(P) 67.6(P) 62(P) 84(G) ---(p3
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EXAMPLE 11
Each of the sarn~les of Example I was also testecl for
linear tear properties in machine direction baseci on the following
cri~eria: less than 5 angular deviation from a straight line
from side to side of the fil~n, excellent (E~; less than 20 devia-
tion, good (G); and more than 30, poor (P). 'rhe ratings for
each sample are indicated adjacent the ElmendorF values in -
Table I.
EXAMPLE III
Several of the blends exhibiting excellent linearity were . ~:
~ coextruded with a blend containing 5% ionomer and 95% low
density polyethylene. 'rhe resultan~ multi-ply film exllibited
excellent linear tear properties in accordance with the criteria
o~ Example II.
EXAMP[,E IV
Samples containing various mixtures of Chemplex L.D.
3041 polyethylene containing 3-l/257o vinyl acetate and DuPont
"surlyn" 1601 were blow extruded to determine the minimum
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amount of either resin required to achieve linear ~ear properties
tn accordance with the criteria of Example II. 'Ille tests included
xuns of 5%, 7-1/2%, 10%, 12-l/2~7o and 1S% ionomer wlth the
remainder as Chemplex,~and 5%, 7-I/2%, 10%, 12-1/2% and lS~70
~hemplex~with the remainder as ionomex. It was found that the
linear tear property was attained at a minimum of 12-1/Z% ionomer
level with the remainder as polyethylene and at a minimum of 5~70
polyethylene with the remainder as ionomer.
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