CA1106567A - Polymer film having linear tear properties - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The specification describes a method of imparting linear tear properties in said direction of extrusion to a polymer film comprising said steps of preparing a blend of resins containing from about 12% to about 95% ionomer resin and from about 5% to about 88% low density polyethylene resin, and extruding said blend of resins into a film, whereby said film exhibits linear tear properties in a direction parallel to the direction of extrusion.

Description

~ `, ~6567 POI,YMER FILM HA~ING LINEAR TEAR PROPERTI~S

Background of the Invention '.
~ 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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~ 6,5~7 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 ws/~

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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 ~2a-ws/~
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L~6~ i7 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 . .
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 - 3 ~

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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 ., , . . . ~, 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 ~ -.
~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 `! .
- 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 .
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 - . .:.
comprise a blend containing polyethylene and less than 12%
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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, , . ~ . . .
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 , ................................ .
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 ~ .
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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~6S~7 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 .
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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Claims (15)

What Is Claimed Is:

1. A method of imparting linear tear properties in the direction of extrusion to a polymer film comprising the steps of preparing a blend of resins containing from about 12% to about 95% ionomer resin and from about 5% to about 88% low density polyethylene resin, and extruding said blend of resins into a film, whereby said film exhibits linear tear properties in a direction parallel to the direction of extrusion.

2. The method of claim 1 wherein said polyethylene resin contains up to about 7.5% of ethylene-vinyl acetate copolymer.

3. The method of claim 1 wherein said blend of resins is blow extruded in the form of a tube.

4. The method of claim 1 wherein the density of said polyethylene resin is in the range of from about 0.910 to about 0.960.

5. The method of claim 1 wherein a polymer lacking linear properties is coextruded with said blend of resins to provide a coextruded film having linear tear properties in the direction of extrusion.

6. An extruded polymer film consisting essentially of from about 12% to about 95% of ionomer and the remainder as polyethylene having a density of from about 0.910 to about 0.960, said film being capable of being torn in substantially a straight line in the direction of extrusion at a deviation of less than 20°
from said straight line.

7. The film of claim 6 wherein the Elmendorf tear strength of said film is less than 125 g.

8. The film of claim 7 wherein the Elmendorf tear strength of said film is from about 30 to about 125 g.

9. The film of claim 6 wherein said polyethylene contains up to about 7.5% vinyl acetate.

10. The film of claim 6 wherein said film is blow extruded.

11. The film of claim 6 wherein said film is coextruded with a second polymer film selected from the group consisting of ionomers, polyethylene and mixtures thereof.

12. A pouch composed of an extruded polymer film, said pouch having a top edge, bottom edge and side edges, said top and bottom edges being substantially parallel with the direction of extrusion of said film, the film of said pouch comprising from about 12% to about 95% ionomer with the remainder as polyethylene having a density of 0.910 to about 0.960 and con-taining from about zero to 7.5% vinyl acetate, said pouch being capable of being torn from side to side in substantially a straight line in the direction of extrusion at a deviation of less than 20° from said straight line.

13. The pouch of claim 12 wherein said film is coextruded with a second polymer film.

140 A pouch comprising a blow extruded polymer tube section having open ends, said tube being heat sealed adjacent each open end to form an enclosure, said tube being composed of from about 12% to about 95% ionomer with the remainder as polyethylene and zero to about 7 1/2% vinyl acetate, the heat sealed edges of said tube being perpendicular to the direction of extrusion, said pouch being capable of being torn in substan-tially a straight line between the sealed edges thereof in said direction of extrusion at a deviation of less than 20° from said straight line.

15. The pouch of claim 14 comprising a notch in one of the sealed edges thereof;