Heat sealable films

Details Heat sealable films Heat sealable films

1999-12-06

2003-01-07

Tarazano, D. Lawrence (Department: 1773)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

C525S240000, C053S482000, C428S461000, C428S476100, C428S483000, C428S523000

Reexamination Certificate

active

06503637

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to films comprising metallocene polyethylene (mPE), polypropylene and free radically high pressure produced polymers.
BACKGROUND OF THE INVENTION
Polyethylene has traditionally been used as a layer in polyolefin films and packaging because of its desirable properties such as moisture impermeability good sealing behavior, good optical properties and good organoleptics. Typically polyethylene has been coextruded, laminated or otherwise bonded to other polyolefins which have better strength than polyethylene, yet do not seal as well as polyethylene. For example, in a typically multilayer film, a polypropylene layer, especially a mono or biaxially oriented polypropylene (OPP) layer, provides a high clarity, high melting, high barrier properties, combined with high stiffness while a polyethylene layer will provide extra body to the film and will allow a low sealing temperature, meaning higher packaging speeds. However, polypropylene (PP) and polyethylene (PE) have very limited compatibility and direct sealing of polyethylene onto polypropylene film is not commonly done. When a layer of PE is combined with a layer of PP, extra primer may be needed. For example, extra primer is used when polyethylene, such as low density polyethylene, is coated onto polypropylene films. In addition tie layers may also be necessary. Coextrudable tie layers such as ethylene vinyl acetate copolymers, typically having more that 8 weight % vinyl acetate, have been extruded between PP and PE to enhance adhesion between the PE and the PP. Another solution to the compatibility problem has been to blend polypropylene into the polythylene. This however has the disadvantage or creating layers that have greater haze and are thus undesirable in the industry.
Therefore there is a need in the art to provide a means to provide a polyethylene polypropylene blend for film layers that does not have haze, yet retains good mechanical properties such impact strength.
U.S. Pat. No. 5,424,362 suggests paintable polyolefin compositions comprising i) 30 to about 70 wt % of at least one polypropylene or graft modified polypropylene; ii) 0 to about 40 wt % of at least one non grafted or graft-modified substantially linear ethylene polymer, and iii) 0 to about 50 wt % of an interpolymer of ethylene and an &agr;, &bgr; unstaurated carbonyl copolymer. Molding techniques are suggested to fabricate the compositions.
WO 92/14784 suggests heat sealable compositions for film and film structures comprising (a) from 30 to 70 weight percent of a low melting polymer comprising and ethylene based sopolymer having a density from 0.88 g/cm
3
to 0.915 g/cm
3
,a molucular weight distribution no greater than 3.5, and a composition distribution breadth index greater than 70 percent; and (b) 70 to 30 weight percent of a propylene based polymer having from 88 mole percent to 100 mole percent propylene and from 12 mole percent to 0 mole percent of an alpha-olefin other than propylene.
WO 94/26816 discloses blend of metallocene polyethylene and high molecular weight high density polyethylene for use in films.
SUMMARY OF THE INVENTION This invention relates to a film comprising a blend of:
i) a homopolymer of ethylene having an Mw/Mn of 3 or less or a copolymer of ethylene and up to 50 weight % of a C
3
to C
20
olefin, wherein the copolymer has a CDBI of 50% or more, preferably 60% or more;
(ii) a homopolymer of propylene or a copolymer of propylene and up to 50 weight % of a comonomer, preferably copolymerized with ethylene and/or a C
4
to C
20
olefin; and
(iii) a polymer produced in a high pressure process using a free radical initiator (High Pressure Polymer).
This invention also relates to films as described above where one or more layers are oriented in one or more directions to the same or different extents.
DETAILED DESCRIPTION OF THE INVENTION
In a preferred embodiment, this invention relates to a film comprising a blend of:
(i) a homopolymer of ethylene having an Mw/Mn of 3 or less, preferably between 1 and 2.5 or a copolymer of ethylene and up to 50 weight %, preferably 1 to 35 weight %, preferably 1-20 weight % of one or more C
3
to C
20
olefins, (based upon the weight of the copolymer) having an Mw/Mn of 6 or less, preferably 3 or less, even more preferably between 1 and 2.5, wherein the polymer or copolymer preferably has:
a) a density of 0.86 g/cm
3
to 0.96 g/cm
3
, preferably 0.88 to 0.94 g/cm
3
, more preferably between 0.88 g/cm
3
and 0.935 g/cm
3
, more preferably between 0.88 g/cm
3
and 0.95 g/cm
3
, more preferably between 0.915 g/cm
3
and 0.935 g/cm
3
; and
b) a CDBI of 50% or more, preferably above 60%;
(ii) a homopolymer of propylene or a copolymer of propylene and up to 50 weight %, preferably 1 to 35 weight %, even more preferably 1 to 6 weight % of ethylene and/or a C
4
to C
20
olefin; and
(iii) polymer produced in a high pressure process using a free radical initiator (High Pressure Polymer).
Composition Distribution Breadth Index (CDBI) is a measure of the composition distribution of monomer within the polymer chains and is measured by the procedure described in PCT publication WO 93/03093, published Feb. 18, 1993 including that fractions having a weight average molecular weight (Mw) below 15,000 are ignored when determining CDBI. For purposes of this invention a homopolymer is defined to have a CDBI of 100%.
The C
3
to C
20
and C
4
to C
20
olefin comonomers for the polyethylene or polypropylene copolymers described above may be any polymerizable olefin monomer and are preferably a linear, branched or cyclic olefin, even more preferably an &agr;-olefin. Examples of suitable olefms include propylene, butene, isobutylene, pentene, isopentene, cyclopentene, hexene, isohexene, cyclohexene, heptene, isoheptene, cycloheptene, octene, isooctene, cyclooctene, nonene, cyclononene, decene, isodecene, dodecene, isodecene, 4-methyl-pentene-1, 3-methyl-pentene-1, 3,5,5-trimethyl hexene-1. Suitable comonomers also include dienes, trienes, and styrenic monomers. Preferred examples include styrene, a-methyl styrene, para-alkyl styrene (such as para-methyl styrene), hexadiene, norbomene, vinyl norbornene, ethylidene norbornene, butadiene, isoprene, heptadiene, octadiene, and cyclopentadiene.
Preferred comonomers for the copolymer of ethylene are propylene, butene, hexene and/or octene.
The polyethylene or polypropylene copolymers described above may also contain termonomers and tetramonomers which may be one or more of the C
3
to C
20
olefins described above, any C
4
to C
30
linear, cyclic or branched dienes or trienes and any styrenic monomers such as styrene, &agr;-methyl styrene, or para-methyl styrene. Preferred examples include butadiene, pentadiene, cyclopentadiene, hexadiene, cyclohexadiene, heptadiene, octadiene, nonadiene, norbornene, vinyl norbornene, ethylidene norbornene, isoprene and heptadiene.
The polyethylene copolymers described above preferably have a composition distribution breadth index (CDBI) of 50% or more, preferably above 60%, even more preferably above 70%. In one embodiment the CDBI is above 80%, even more preferably above 90%, even more preferably above 95%. In another particularly preferred embodiment, the polyethylene copolymer has a CDBI between 60 and 85%, even more preferably between 65 and 85%.
In a particularly preferred embodiment the ethylene homopolymer or copolymer has a CDBI of 65 to 85%, a density of 0.915 to 0.96 g/cm
3
and a Mw/Mn between 1 and 2.5.
In another preferred embodiment the ethylene homopolymer or copolymer has a density of 0.86 to 0.925 g/cm
3
and a CDBI of over 80%, preferably between 80 and 99%.
In another preferred embodiment the blend comprises a homopolymer of ethylene having an Mw/Mn of 3 or less, preferably between 2.5 and 1.
In general, the polyethylene homopolymers and copolymers described above are metallocene polyethylenes (mPE's). The mPE homopolymers or copolymers are typically produced using mono- or bis-cyclopentadienyl transition metal catalysts in combination with an activator such as alumoxane a

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