CA1157616A

CA1157616A - Coextruded heat sealable laminar thermoplastic films

Info

Publication number: CA1157616A
Application number: CA000313936A
Authority: CA
Inventors: Milton L. Weiner; Thomas W. Higgins; Alan M. Nahmias; Peter Breidt, Jr.
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1977-11-04
Filing date: 1978-10-23
Publication date: 1983-11-29
Also published as: EP0001898B1; JPS6411465B2; IT7829431A0; JPS5499182A; EP0001898A1; US4147827A; ES474776A1; IT1100911B; DE2861138D1; EP0001898B2

Abstract

COEXTRUDED HEAT SEALABLE LAMINAR THERMOPLASTIC FILMS

ABSTRACT OF THE DISCLOSURE

Laminar thermoplastic films are provided which exhibit improved heat seal strengths as well as heat seal temperature ranges. The laminates comprise a predominantly propylene-based polymer substrate or core, the propylene core layer being coated with continuous skins, the skins being fabricated from ethylene based resin with a density of less than 0.939.

Description

~576l`6 BACXGROUND OF THE INVENTION
Field of the Invention The present lnventlon relates to laminar thermo-plastic fllm structures which are ~ormed by coextruslon of :i the molten lamlnar layers through a single dle orlflce and partlcularly such lamlnar structures that are solldlfled a~ter ; extruslon and passed onto an orlentation or stretchlng opera-tlon to produce a biaxlally orlented lamlnate fllm. The.~llm ls characterlzed by havlng good heat seal strengths, broad heat .o temperature ranges and excellent impact reslstance~

; DescrlPtion of the Prlor Art . ' ' .
Orlented polypropylene films have become useful and wldely employed packaging fllms prlmarily because of their good molsture barrler propertles, stl~fness, lmproved machine-abillty, hlgh strength characterlstlcs and excellent optlcal propertles such as high gloss and minlmal haze. Such blaxlally orlented polypropylene ~llms are quite difficult to seal because -o~ the tendency of such films to deorlent, pucker or tear at the 0 requlslte seallng temperatures. Consequently, ln order tp achleve satisfactory heat sealablllty, coatlngs of varlous types . have been applied to the polypropylene ~llm surface to lower the . requislte heat seal temperatures~ Generally, such coatlngs are applled ln a separate operatlon a~ter the film has been formed and oriented~ Many times the coatings must be applled ~rom . . solution ln an organlc solvent medium~ This separate operation for appllcatlon o~ relatively expenslve coatlngs whlch dlctate .. .

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., 57G~6 employment of solvent recoVery systems is qulte c~stly. Examples of such coatings which have been employed ln the past to impart heat sealability and other desirable characteristics to the polypropylene film include saran and acrylic multipolymer coatings.
An alternate method for imparting ~mproved heat sealability to oriented polypropylene film, and one that is less costly than post orientation coatin~ comprises the co-~xtrusion of surface layers of a lower meltlng resin onto the surface of the hlgher melting polypropylene core prior to orlentatlon. Following orlentatlon a product ls obtained which has a relatively thick polypropylene core sandwlched between two relatively thin layers or skins of the low melting resin. However, as practiced in the prior art, this procedure has assoclated wlth it certain deflclencies, For example, ln an instance where lt ls deslrable to form a laminar structure comprising a thermoplastic core sandwiched between layers of a lower melting point material ~ch as an ethylene propylene co-polymer to provide heat sealability, the resulting laminar structure exhlbits *sirable high heat seal strengths but because of the relatively high melting point of such a copolymer layer, the heat sealing range,defined by the temperature at which usable seal strengthc are formed and that temperature where undesirable film shrinkage occurs, ~s narrow~
In other instances ln the prlor art when a coextruded fllm comprising a low melting skin materlal ls employed as the coatlng or skln layer, e.g., a medium density polyethylene resin produced by the hlgh pressure, free ~adical catalyst process, a product is produced whlch exhlbits a much wider æaling range ~576i6 , than that of the hereinabove described coextruded film, however, its seal strengths are undesirably low. Additionally, since very low machine directlon stretch ratlos must be employed, such films are oriented only to a sllght degree in the machine direction and as a result the fllm's toughness or impact strength is not as hlgh as would be deslred. Improvement of the toughness characterlstlcs of the fllm, by lmpartlng a greater machine dl-rectlon stretch to the film while malntalning the heat seal advantages of the polyethylene skln could not be achleved because the higher temperatures which would be required to permit greater machlne dlrection stretching would also cause undeslrable stick- -ing of the polyethylene skln to the stretching rolls.

SUMMARY OF THE INVENTION

The present inventlon is dlrected to the manufacture of a blaxlally orlented film that is heat sealable over a wide range of temperatures and which has hlgh heat seal strength.
Such films are obtained by the coextrusion of a surface layer of a polyethylene resin or predomlnantly ethylene based copolymer surroundlng a core of a propylene-based polymer, the core compositlon belng selected 80 that the temperature re-quired for machlne stretchlng, i.e. orientation in the machine direction, will not be so hlgh as to cause sticking of the lamlnar film skln surface to the surface of the orientation or stretching rollers. Additlonally, the laminar film structures show ex-ceptionally high heat seal strength when contrasted to prior art fllm lamlnations, superlor optical properties includlng haze and gloss, and higher strength and toughness characterlstics. Sult-able core compositlons lnclude either ~a~ a random ethylene/propylene ~:3 S7~i~

copo~r, tb) a block ethylene/propylene copolymer, (c) a mlx-ture of polypropylene with a glassy, compatible resln o~ low softenlng temperature, or td) a mlxture of a random or a block ethylene/propylene copolymer wlth a glassy, compatible resin of low softening temperature. The glassy compatible reslns of low sortening temperature may be those resins which are descrlbed ln U.S. Patent No. 3,865,903, dated Feb. 11, 1975, and U.s. Patent No. 3,937,762, dated February 10, 1976, both patents being of A. M.Na~oias and E. M. Bullard, and assigned to Mbbil Oil ~orporation. It will be noted that such glassy resin oxpositions as disclosed in those patents comprise essentially an aliphatic diene material such as pentadiene WhiCh has been capolymerized with at least one other olefinically unsaturated comonomer.
These glas~y resins serve essentially as high temperature plasticizers.
Slnce they are hlghly compat1ble wlth the core resln they do not impair the optlcal propertles of the laminar product and ln most instances they have been found to lmprove the optics of the flnal laminar product.
The present in~ention includes a method for the pro-duction of the hereinabove described heat sealable coextruded fllms, which comprises the coextrusion of a core layer which is coated on at least one or both surfaces with a lower melting, relatively thin skin layer based on polyethylene.
The core layer has a melting point which is higher than that of the surface or skin layer.
The laminate structure is subsequently oriented in both the machine and transverse direction and the resultant laminar film may be heat sealed at temperatures below which disorientation of the core material occurs.
This invention provides, then, a method for producing a heat-sealable, biaxially oriented, thermoplastic film laminate comprising a core ~,~!

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layer and at least one surface layer, said surface layer being thinner than the core layer, the core layer having a higher melting point than the ~urface layer and being orientable at a temperature at which the surface layer does not melt, which method comprises co-extruding a core layer comprising (l) an ethylene/propylene copolymer; (2) a blend of an ethylene/propylene copolymer with an aliphatic diene copolymer; or (3) 8 blend of polypropylene ho~opolymer wi~h an aliphatic diene copolymer; with a surface layer comprising an ethylene-based polymer having an ethylene content of at least aox by weight, a melt index from 0.3 to 20.0 and a specific density from 0.910 to 0.939; and biaxially orienting the resultant leminar film.

- 5a -S7~16 The preferred stretch ratios employed ln the present inventlon to obtaln satlsfactory machlne dlrectlon orientation levels are from about 3.0:1 up to about 10.0:1 and preferably from about 4.0:1 up to about 7.0:1.
Uncoated, homopolymer, polypropylene resins which are machine dlrectlon orlented to the deslred levels hereinabove de-flned must be heated to temperatures on the order of from about 285F. up to about 305F. Hence, lf an unmodified polypropylene core were coated with a low melting point resin, e.g. low density polyethylene and stretched at ratios high enough to achieve satls-factory levels of machlne dlrection orlentation (necessitatlng temperatures on the order of 285 to 305F.) the lower melting-polyethylene castlng melts and stlcks to the stretching rollers maklng hlgh level machlne directlon stretch ratlos with such a lamlnar comblnation imposslble to achleve.
The method of the present lnvention includes the em-ployment of a core or central resin layer which has a compositlon such that machine direction orlentation temperatures can be em-ployed whlch, whlIe permltting a desirable degree of machlne dl-rectlon orientatlon to be carried out, will not be so hlgh as to cause stlcking of the lower meltlng fllm surface layers to the machlne directlon stretchlng rollers. Although such machine dlrection stretching temperature may vary dependent upon the exact compositions of the lamlnar core and surface or skin layers, temperatures on the order of from about 200F. up to about 240F. have been found to be generally sultable, the pre-ferred machine directlon stretching temperature range being on ^~
the order of from about 215F, up to about 235F.

~76~6 Accordingly the thermoplastlc film laminates of the present lnvention comprise a single core layer, the core~layer being coated with relatively thin skin layers on one or both surfaces thereof, the core layer comprising a member selected from the group consisting of (1) ethylene-propylene copolymers,

(2) blends of ethylene-propylene copolymers with an aliphatic diene copolymer; and (3) blends of polypropylene homopolymer with an aliphatic dlene copolymer, said skin or surface layers comprlsing polyethylene and copolymers of ethylene with olefinlcally unsaturated comonomers.
In the case of the lamlnar film constructions of the present invention the lower melting point skin or surface layers are considerably thinner than the central or core layer, each of the skln layers constituting from about 1% up to about 10% of the overall thlckness of the lamlnate. The polyethylene homo-polymer orcopolymer which comprlses the skin layer should have a denslty of from about 0.910 up to about 0.939 and a melt index range of from about 0.3 up to about 20.0 and preferably from about 3.0 to about 6Ø In the case of the employment of ethy- -.
lene copolymers as a surface or skin layer the ethylene content of the copolymer should be at least abov~ 80% and preferably 90% or more. Typical examples of such copolymers include ethylene copolymerlzed wlth lower alkyl acrylates, bute~e, pentene, hexene, octene, ~ monoethylenically unsaturated carboxylic acids in-cluding acrylic and methacrylic acids, methyl pentene, vinyl acetate and the like.

~L~57~:16 DESCRIPTION OF SPECIFIC EMBODI~ENTS

In accordance wlth the present inventlon, the laminar fllm structures thereof a~e made by the coextrusion of.the skin resin and the core composition simultaneously, utilizing any of the prior art methods of coextrusion. The extrudate is solidified by cooling it in a water bath or on a casting roll. The solid base sheet is reheated and stretched in the machlne direction utillzing a series of rotating draw rollers, ln accordance with ; the prior ar~ and flnally the machine dlrection oriented film ls-10 ~ stretched in the transverse direction by employing weli known transverse d~rection stretching apparatus for rilm orientation such as a tenter. Specific examples of the present invention are described hereinbelow and are presented for illustrative purposes only and, accordingly, should not be construed in a limitative sense w1th respeot to the scope of the present inven-tlon.
Example 1 was produced A three layer coextrudate/comprlsing a core of a random ~^
ethylene~propylene copolymer having a melt flow rate of 5.4; a density of 0.90 and an ethylene content of 3.0% by welght, and surface layers of a copolymer of ethylene and 4-methylpentene-1, ldentlfied by the manufacturer as a linear low density resln.
The copolymer was produced by the utilization of a low pressure polymerization process and is characterlzed by having a 4-methyl-pentene-l content of 1.07 mole percent, a melt index o~ 3.0 and a density o~ 0.935. Each surface layer constituted approximately 6~ of the overall thickness. The base film was quenched on a _8--~57616 casting roll having a surface temperature of 110F. and was subsequently reheated to 218~. The preheated base sheet was drawn 5 times in the machine direction between heated rollers which were driven at an appropriate speed differential and sub-sequently tentered or stretched in the transverse direction 7.5 times at a temperature of about 285F.
The machine direction orientation assembly comprised a series of 4 sequentially positioned preheatin~ rollers which preheated the film to about 218-220F., followed by a set of two stretching rollers, the second of which is driven at a speed higher than the flrst or slow roller. Film stretching or orientatlon in the machlne dlrectlon occurs between the closely positioned stretchlng rollers. In the present example in order to achieve a stretch ratio of 5:1 the surface speed of the flrst stretchlng roll was 10 ft./min. while the second or fast roller had a surface speed of 50 ft./min.

Example 2 The procedure of Example 1 was followed except ~ that the surface layer or skin material comprised an ethylene/4-methylpentene-l copolymer skin resin having a 4 methylpentene content of 2.0 mole percent, a density of 0.925 and a melt ln-dex of 3,0, Example 3 The procedure of Example 1 was followed except that the surface layers comprised a polyethylene homopolymer resin produced by the high pressure~ free radical catalyzed process, and having a denslty of 0.935 and a Melt Index of 3Ø

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Example 4 The procedure of Example 3 was followed except that the core material comprised a mixture of 84% polypropy-lene homopolymer and 16% of a glassy, compatible, random inter-polymer resin. The interpolymer resin was prepared by anhydrous aluminum chloride catalyzed interpolymerization in toluene of a mixture comprising by weight 55.3% of a piperylene concentrate;
9.7% of a mixture comprising dipentene and ~-phellandrene present in a weight ratio of about 2:1, respectively, and 35.0% of ~-methylstyrene.

Melting Point, C. 79 - 80 (Ball and Ring) Molecular Weight 1442 (Weight Average) 1034 (Number Average) ~romine No. 6 - 10 Iodine No. 75 - 80 Acid Value ~1 Specific Gravity 0.978 - 0.980 20 . Percent Crystallinity 0 -Tg (Glass Transition Temperature) 32C.
Saponiflcation No. < 1 Vlscoaity f to g ~ln toluene - 70%) Color Gardner 5 - 7 (50% toluol.solution) Decompositlon Temperature 205C.
(in nitrogen) - The resinous interpolymer~ when heated under nitrogen at a rate of 10C. per minute, had an initial decompositlon -10- .

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temperature o~ 205C., a 0.0% weight loss at 200C. a 12.8%
weigh$ loss of 300C,; and a 90.0% weight loss at 400C.
~ The polypropylene homopolymer resin employed was identified by the manu~acturer aS~ te-6l2"* having a melt flow of 4.0 to 5.0 and a molecular weight Or 340,000 to i 380,000 tweight average) and 34,000 to 39,000 (number average).
The polypropylene resln was also characterized by the followi~g physical properties: . .
.; . . -: , : Crystalline Meltlng Point (F) . ~30-340 . Inherent Viscosity . }.4-1 6 . Density . . 0.910-0.890 .
.
Example 4-A

The procedure Or Example ~ was ~ollowed except that the ~ur~ace layers comprised an ethylene-vinyl acetate.
copolymer having a ~inyl acetate content Or ~.5 mole percent;
a melt index Or about 1.0; and an apparent.density o~ 0.927.
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ExamPle 5 For comparlson purposes a multilayer laminate . .was prepared utilizlng the process as shown in E~ample 1, ex-cept that the ~kin comprised an ethylene/propylene random co- .
polymer> havlng an ethylene content o~ approximately 3% by . ~ . . . . . .
weight, and the core comprised a polypropylene homopolymer. The ethylene-propylene copolymer and the po~propylene homopolymer resins * Trademark .' ' ' ' ~

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employed were the same as those identified in Examples 1 and 4 respectively. The Example 1 process was modified to the extent that the preheat temperature of the laminar base film prior to machine direction orientation was increased to 275F.

Example 6 For comparison purposes another coextruded film was prepared in a similar fashion to that of Example 1 except that it was only drawn 1.4 times in the machine direction prior to stretching it 7.5 times transversely. The film comprised a polypropylene homopolymer core resin tas described in Example 4) havlng surface skins consisting of a medium density poly-ethylene resin that was produced by the high pressure free radlcal catalyzed process and that was additlonally characterized by having a density of 0.935 and a melt index of 3.5. It is noted that attempts to achieve a higher machine directlon draw with this laminar combination, i.e. by increasing the machine dlrection preheat temperature up to about 275F. resulted in the polyethylene skin layers stlcking to the machine direction orlentlng rollers whereby production of satisfactory film was not possible.
The heat seal properties of the laminates which were prepared in accordance with Examples 1 through 6 are set forth in the following Table I. It can be seen that the heat seal characterlstics of the laminar films of the prior art, i.e.
films made in accordance with Examples 5 and 6, are inferlor to the laminates of the present invention as described in Examples 1, 2, 3, 4 and 4-A. While the prior art film of Example 5 provides .

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a high heat seal strength, it only does so at a high sealing temperature and therefore the temperature range over which high seal strengths can be attained with this lamination is narrow. As can be seen from the data set forth in the follow-ing Tables, the laminar film structures of the present inven-tion consistently exhibit wide heat seal ranges and toughness which are superior to the prior art film laminates. Such wide sealing ranges and toughness characteristics are essential in the packaging end use applications for the laminar film con-structions of the present invention. Additionally in certain packaging applications, high heat seal strength characteristics are particularly desirable. Utilizing the film laminates of this lnvention, in addition to the hereinbefore described tough-ness properties and wide sealing ranges, in many instances high heat seal characteristics are also obtainable. Additionally, as illustrated in Table 2, the laminated fiims of the present invention, in contrast to those of the prior art, i.e. Examples 5 and 6, provide improved optical properties, including reduced haze and higher gloss.
The beneficial effect on film tou~hnèss, e.g., impact strength which is achieved with the hlgher machine direction orientation which is made possible by the provisions o~ the present invention is shown by the data as set forth in Table 3.
It will be seen that the prior art film of Example 6 is sub-stantially less tough than the films of Examples 1 through 4-A.

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~l57G1-6 Laminates Produced In * **
Accordance ~ith: Haze Gloss Example 1 2.4 79.8 - Example 2 1.3 83 Example 3 1.2 79.5 Example 4 2.0 85 Example 4-A 2.0 80 Example 5 4,4 73.6 Example 6 . 6.5 60 ASTM - Dl003-61 . **ASTM - D2457-70 ' ~ .

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Laminates Produced In Ball Burst Accordance With: cm-kg./mil Example l 27.0 Example 2 28.1 Example 3 20.9 Example 4 20.0 Example 4-A 20.0 Example 5 22.0 Example 6 3.0 .

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. Although the present invention has been described with preferred embodiments, it is to be understood that modi-fications and variations may be resorted to, without departing from the spirlt and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing a heat-sealable, biaxially oriented, thermoplastic film laminate comprising a core layer and at least one surface layer, said surface layer being thinner than the core layer, the core layer having a higher melting point than the surface layer and being orientable at a temperature at which the surface layer does not melt, which method comprises co-extruding a core layer comprising (1) an ethylene/propylene copolymer; (2) a blend of an ethylene/propylene copolymer with an aliphatic diene copolymer; or (3) a blend of polypropylene homopolymer with an aliphatic diene copolymer; with a surface layer comprising an ethylene-based polymer having an ethylene content of at least 80% by weight, a melt index from 0.3 to 20.0 and a specific density from 0.910 to 0.939; and biaxially orienting the resultant laminar film.

2. The method according to Claim 1 wherein the melt index of the ethylene-based polymer of the surface layer is from 3.0 to 6Ø

3. The method according to Claim 1 wherein the ethylene content of the ethylene-based polymer of the surface layer is at least 90% by weight.

4. The method according to any of Claims l, Z or 3 wherein said laminar film is biaxially oriented at a temperature from about 93°C (200°F) to 116°C (240°F).

5. The method according to claim l wherein the surface layer constitutes from about 1% to about 10% of the overall thickness of the laminate.

6. The method of claim l wherein the core layer is sandwiched between two surface layers.

7. The method of claim l wherein the film laminate is biaxially oriented employing a stretch ratio of from about 3.0:1 to about 10.0:1.

8. The method of claim 7 wherein the stretch ratio is from about 4.0:1 to about 7.0:1.