U.S. patent application number 13/263117 was filed with the patent office on 2012-02-09 for high performance sealable coextruded biaxially oriented polypropylene film.
This patent application is currently assigned to Dow Globel Technologies LLC. Invention is credited to Hongyu Chen, Yong Chen, Morgan M. Hughes, Jing Li, Rajen M. Patel, Xiao Bing Yun.
Application Number | 20120034444 13/263117 |
Document ID | / |
Family ID | 42934629 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120034444 |
Kind Code |
A1 |
Chen; Yong ; et al. |
February 9, 2012 |
HIGH PERFORMANCE SEALABLE COEXTRUDED BIAXIALLY ORIENTED
POLYPROPYLENE FILM
Abstract
A multilayer biaxially oriented polypropylene film is disclosed
which comprises at least three layers. The first layer is an outer
layer comprising a first polymer which is a homopolymer
polypropylene resin having a melting point of at least 155.degree.
C. The second layer is a core layer, having a melting point greater
than 1500C. The core layer includes a blend comprising a second
polymer which is a polypropylene homopolymer having a melting point
of at least 155.degree. C. and a third polymer having a melting
point no greater than 145.degree. C. The second polymer comprises
from 20% to 80% by weight of the core layer while the third polymer
comprises from 80% to 20% by weight of the core layer. The last
required layer in the film is a sealant layer. The sealant layer
comprises a fourth polymer having a melting point no greater than
145.degree. C.
Inventors: |
Chen; Yong; (Changchun,
CN) ; Chen; Hongyu; (Shanghai, CN) ; Yun; Xiao
Bing; (Beijing, CN) ; Li; Jing; (Shanghai,
CN) ; Hughes; Morgan M.; (Angleton, TX) ;
Patel; Rajen M.; (Lake Jackson, TX) |
Assignee: |
Dow Globel Technologies LLC
Midland
MI
|
Family ID: |
42934629 |
Appl. No.: |
13/263117 |
Filed: |
April 10, 2009 |
PCT Filed: |
April 10, 2009 |
PCT NO: |
PCT/CN2009/071236 |
371 Date: |
October 6, 2011 |
Current U.S.
Class: |
428/220 ;
428/336; 428/347; 428/517; 428/519 |
Current CPC
Class: |
B32B 2553/00 20130101;
B32B 2307/31 20130101; B32B 27/327 20130101; Y10T 428/265 20150115;
B32B 2250/04 20130101; Y10T 428/31917 20150401; Y10T 428/31924
20150401; B32B 2250/05 20130101; B65D 65/40 20130101; Y10T
428/31938 20150401; B32B 2250/242 20130101; B32B 2439/70 20130101;
B32B 7/02 20130101; Y10T 428/266 20150115; B32B 2250/03 20130101;
Y10T 428/24942 20150115; B32B 2307/518 20130101; Y10T 428/2817
20150115; B32B 27/32 20130101 |
Class at
Publication: |
428/220 ;
428/519; 428/517; 428/336; 428/347 |
International
Class: |
B32B 27/32 20060101
B32B027/32; B65D 65/40 20060101 B65D065/40; B32B 7/02 20060101
B32B007/02 |
Claims
1. A multilayer biaxially oriented polypropylene film comprising:
a. an outer layer comprising a first polymer which is a homopolymer
polypropylene resin having a melting point of at least 155.degree.
C.; b. a core layer, with a melting point greater than 150.degree.
C., comprising a blend comprising a second polymer which is a
polypropylene homopolymer having a melting point of at least
155.degree. C. and a third polymer having a melting point no
greater than 145.degree. C., wherein the second polymer comprises
from 20% to 80% by weight of the core layer while the third polymer
comprises from 80% to 20% by weight of the core layer; and c. a
sealant layer comprising a fourth polymer having a melting point no
greater than 145.degree. C.
2. The film of claim 1 wherein the film has been coextruded.
3. The film of claim 1 wherein the third polymer is a propylene
based plastomer or elastomer.
4. The film of claim 1 wherein the third polymer is an ethylene
copolymer having a density less than 0.916 g/cc.
5. The film of claim 1 wherein the third polymer is an olefin block
copolymer.
6. The film of claim 3 wherein the propylene based elastomer or
plastomer comprises units derived from propylene and units derived
from ethylene.
7. The film of claim 6 wherein the units derived from ethylene
comprise from 0.04 to 15% by weight of the propylene based
plastomer or elastomer.
8. The film of claim 1 wherein the film has a thickness of from 15
.mu.m to 80 .mu.m.
9. The film of claim 1 wherein the core layer has a thickness of
from 2 .mu.m to 20 .mu.m.
10. The film of claim 1 wherein the first polymer has a melting
point greater than 160.degree. C.
11. The film of claim 1 wherein the fourth polymer has a melting
point of at least 90.degree. C.
12. The film of claim 1 wherein the film has a heat seal strength
of at least 10 N/25 mm.
13. The film of claim 1 wherein the film has a heat seal initiation
temperature no greater than 100.degree. C.
14. The film of claim 1 wherein the first polymer and the second
polymer are the same material.
15. The film of claim 1 wherein the third polymer and the fourth
polymer are the same material.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
[0001] The present invention relates to a biaxially oriented
polypropylene film capable of being formed by coextrusion, and
characterized by having a relatively high seal strength and a
relatively low heat seal initiation temperature. The film of the
present invention is well suited for dry food packaging
applications.
[0002] The world's demand for biaxially oriented polypropylene
(BOPP) film has grown by an average of 8.7% per year since 2000.
BOPP films comprise at least one propylene-containing layer.
Flexible packaging for dry food is one of the major applications of
BOPP. At present, dry food packaging is dominated by the multi-ply
dry laminated BOPP films. These films are heat sealable and are
usually produced by joining a BOPP film with a separate sealant
film such as a polyethylene or cast polypropylene (CPP) film
through adhesive lamination techniques. Generally, these BOPP
laminated structures have heat seal strength of approximately 15-40
N/25 mm and heat seal initiation temperatures (HSIT) around
105.degree. C. (when polyethylene is used as the sealant film).
[0003] Coextruded (or "co-ex") BOPP film is another kind of heat
sealable BOPP based film which can be used for packaging. These
films are produced by coextrusion of polypropylene and sealant
resin to form a sheet and then biaxially stretching the sheet to
form the biaxially oriented film. The production of co-ex BOPP is a
more cost effective process vs. the adhesive lamination process, as
the film is made in a single step and no adhesive is required.
However, the heat seal strength of currently available co-ex BOPP
films drops to around 5-8 N/25 mm and the HSIT increases by about
10.degree. C. after biaxial stretching. Due to the reduced seal
strength, co-ex BOPP films are currently only used for packaging
small articles or articles not requiring high seal strengths, such
as cigarette packaging, or light-weight food like bread.
[0004] It would be desirable to develop co-ex BOPP films to replace
the laminated BOPP films in order to cut cost and eliminate the
adhesive involved in the lamination process and yet have comparable
heat seal strength and HSIT.
[0005] Therefore in one aspect of the present invention a
multilayer biaxially oriented polypropylene film is provided. The
film comprises at least three layers. One layer is an outer layer
comprising a first polymer which is a homopolymer polypropylene
resin having a melting point of at least 155.degree. C. A second
layer is a core layer comprising a blend comprising a second
polymer which is a polypropylene homopolymer having a melting point
of at least 155.degree. C. and a third polymer having a melting
point no greater than 145.degree. C. and a Vicat softening point of
at least 60.degree. C., wherein the third polymer comprises no more
than 80% by weight of the core layer. A third layer is a sealant
layer comprising a fourth polymer having a melting point no greater
than 145.degree. C.
[0006] Another aspect of the invention is the use of the films of
the present invention for packaging dry foods. In particular, the
films of the present invention which have a preferred heat seal
strength of at least 10 N/25 mm (even when the core layer is less
than 10 .mu.m thick), and a preferred heat seal initiation
temperature no greater than 130.degree. C., are well suited for
this application.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a graph of the heat seal strength vs. seal
temperature curves for inventive example 1 and comparative example
1 and 2.
[0008] FIG. 2 is a graph of the heat seal strength vs. seal
temperature curves for inventive examples 2, 3 and 4.
DETAILED DESCRIPTION OF THE INVENTION
[0009] Test Methods
[0010] Unless otherwise indicated, the following properties are
determined by the indicated test method throughout this
specification.
[0011] Melting point is determined using a differential scanning
calorimeter (DSC). The temperature at the maximum heat flow rate
with respect to a linear baseline was used as the melting point as
measured by DSC. The linear baseline was constructed from the
beginning of the melting (above the glass transition temperature)
and to the end of the melting. The temperature was raised from room
temperature to 200.degree. C. at 10.degree. C./min, maintained at
200.degree. C. for 5 min, decreased to 0.degree. C. at 10.degree.
C./min, maintained at 0.degree. C. for 5 min and then the
temperature was raised from 0.degree. C. to 200.degree. C. at
10.degree. C./min, and the results are taken from this second
heating cycle. The melting and cooling curves were recorded. When
plural melting peaks were present, the melting peak at the maximum
temperature was used as the melting point.
[0012] Vicat softening point is determined according to ASTM
D-1525.
[0013] Density is determined according to ASTM D-792.
[0014] Melt flow rate for propylene polymers (that is, those
polymers comprising greater than 50% by weight of units derived
from propylene monomer) is determined according to ASTM D1238,
230.degree. C., 2.16 kg).
[0015] Melt index, or I.sub.2, for ethylene polymers (that is,
those polymers comprising at least 50% by weight of units derived
from ethylene monomer) is determined according to ASTM D1238,
190.degree. C., 2.16 kg.
[0016] Heat seal strength is determined in accordance with the
following method as used on a J&B hot-tack tester model 4000
and ZWICK Z010 Universal Tensile Tester with the following
settings:
[0017] Sample width: 25 mm
[0018] Seal pressure: 0.275 N/mm.sup.2
[0019] Seal time: 0.5 s
[0020] Condition time (after heat seal): 24 hours
[0021] Peel speed: 500 mm/min
[0022] Heat seal initiation temperature is determined as the
minimum temperature at which a seal of 4.4 N/25 mm or 1 lb/inch was
obtained.
[0023] In one aspect, the present invention comprises a multilayer
biaxially oriented polypropylene film comprising: [0024] a. an
outer layer comprising a first polymer which is a homopolymer
polypropylene resin having a melting point of at least 155.degree.
C.; [0025] b. a core layer, with a melting point greater than
150.degree. C., comprising a blend comprising a second polymer
which is a polypropylene homopolymer having a melting point of at
least 155.degree. C. and a third polymer having a melting point no
greater than 145.degree. C., wherein the second polymer comprises
from 20% to 80% by weight of the core layer while the third polymer
comprises from 80% to 20% by weight of the core layer; and [0026]
c. a sealant layer comprising a fourth polymer having a melting
point no greater than 145.degree. C.
[0027] The outer layer of the film of the present invention
comprises a first polymer. The first polymer comprises a
homopolymer polypropylene resin having a melting point greater than
or equal to 155.degree. C., more preferably greater than or equal
to 160.degree. C. A particularly desirable polyolefin that may be
used as polymer in the outer layer is an isotactic propylene
homopolymer having (i) an isotacticity of from about 89 to 99% (as
measured by 13C NMR spectroscopy using meso pentads), (ii) a
melting point of from about 155.degree. C. to about 165.degree. C.,
and (iii) a melt flow rate of from about 0.5 to about 15 g/10
minutes. The homopolymer polypropylene may be produced by using
Ziegler-Natta or metallocene catalysts. Metallocene-catalyzed
isotactic polypropylenes made developmentally or commercially are
EOD 96-21 and EOD 97-09, from Fina Oil and Chemical Co., EXPP-129,
from ExxonMobil Chemical Co., and Novalen M, from BASF GmbH., and
H314-02Z from the Dow Chemical Company among others. Preferably the
resin used in the outer layer of the film comprises only
homopolymer polypropylene.
[0028] The core layer of the film comprises a blend of at least two
polymers, designated herein as the second polymer and the third
polymer. The second polymer comprises a homopolymer polypropylene
resin having a melting point greater than or equal to 155.degree.
C., more preferably greater than or equal to 160.degree. C. A
particularly desirable polyolefin that may be used as the second
polymer in the core layer is an isotactic propylene homopolymer
having (i) an isotacticity of from about 89 to 99% (as measured by
13C NMR spectroscopy using meso pentads), (ii) a melting point of
from about 155.degree. C. to about 165.degree. C., and (iii) a melt
flow rate of from about 0.5 to about 15 g/10 minutes. The
homopolymer polypropylene may be produced by using Ziegler-Natta or
metallocene catalysts. This polymer can advantageously (but not
necessarily) be the same material as the first polymer.
[0029] The third polymer (that is, the second mandatory component
of the blend of which the core layer of the film is comprised)
comprises a polymer having a melting point less than or equal to
145.degree. C., more preferably less than or equal to 130.degree.
C., or even less than or equal to 110.degree. C., more preferably
in the range of 100.degree. C. to 120.degree. C. Preferably, the
third polymer will have a Vicat softening temperature greater than
or equal to 60.degree. C. The third polymer can advantageously be
an ethylene copolymer having a density less than about 0.916 g/cc,
more preferably less than about 0.910 g/cc, a propylene based
plastomer or elastomer (or "PBPE"), or an olefin block copolymer
(or "OBC") or a blend comprising one or more of the above
materials.
[0030] The ethylene copolymers suitable for use as the third
polymer in the films of the present invention can be an
interpolymer of ethylene with at least one C.sub.3-C.sub.20
alpha-olefin, as stated in U.S. 2003/0032731. Preferably the
ethylene copolymer is a copolymer of ethylene with 1-butene,
1-hexene, or 1-octene, with 1-octene being the most preferred. The
ethylene copolymers suitable for use as the third polymer may be
linear (that is, with no long chain branching) or substantially
linear. The ethylene copolymer may advantageously be made using a
gas phase process or a solution process as is known in the art,
although solution is generally preferred in order to produce
polymer with lower densities. Similarly, the catalyst used to make
the LLDPE is not limited and includes Ziegler Natta type catalysts
as well as metallocenes. Exemplary ethylene copolymers for use in
the present inventing include EXACT.TM. polymers from Exxon-Mobil
Chemical Company, AFFINITY.TM. polymers and ENGAGE.TM. polymers
from The Dow Chemical Company, and TAFMER.TM. polymers from Mitsui
Chemicals.
[0031] The PBPE materials suitable for use as the third polymer in
the films of the present invention comprise at least one copolymer
with at least about 50 weight percent of units derived from
propylene and at least about 1 weight percent of units derived from
a comonomer other than propylene. Suitable propylene based
elastomers and/or plastomers are taught in WO03/040442, and
WO2007/024447 , each of which is hereby incorporated by reference
in its entirety.
[0032] Of particular interest for use in the present invention are
reactor grade PBPEs having MWD less than 3.5. It is intended that
the term "reactor grade" is as defined in U.S. Pat. No. 6,010,588
and in general refers to a polyolefin resin whose molecular weight
distribution (MWD) or polydispersity has not been substantially
altered after polymerization. The preferred PBPE will have a heat
of fusion (as determined using the DSC method described in
WO2007/024447) less than about 90 Joules/gm, preferably less than
about 70 Joules/gm, more preferably less than about 50 Joules/gm.
When ethylene is used as a comonomer, the PBPE has from about 0.04
to about 15 percent of ethylene, or from about 5 to about 14
percent of ethylene, or about 5 to 12 percent ethylene, by weight
of the propylene based elastomer or plastomer.
[0033] Although the remaining units of the propylene copolymer are
derived from at least one comonomer such as ethylene, a C.sub.4-20
.alpha.-olefin, a C.sub.4-20 diene, a styrenic compound and the
like, preferably the comonomer is at least one of ethylene and a
C.sub.4-12 .alpha.-olefin such as 1-hexene or 1-octene. Preferably,
the remaining units of the copolymer are derived only from
ethylene.
[0034] The amount of comonomer other than ethylene in the propylene
based elastomer or plastomer is a function of, at least in part,
the comonomer and the desired heat of fusion of the copolymer. If
the comonomer is ethylene, then typically the comonomer-derived
units comprise not in excess of about 15 wt % of the copolymer. The
minimum amount of ethylene-derived units is typically at least
about 1, preferably at least about 3, more preferably at leas about
5 and still more preferably at least about 9, wt % based upon the
weight of the copolymer. If the polymer comprises at least one
other comonomer other than ethylene, then the preferred composition
would have a heat of fusion approximately in the range of a
propylene-ethylene copolymer with about 3 to 20 wt.% ethylene. The
propylene based plastomer or elastomer of this invention can be
made by any process, and includes copolymers made by Ziegler-Natta,
CGC (Constrained Geometry Catalyst), metallocene, and
nonmetallocene, metal-centered, heteroaryl ligand catalysis. These
copolymers include random, block and graft copolymers although
preferably the copolymers are of a random configuration. Exemplary
propylene copolymers include Exxon-Mobil VISTAMAXX.TM. polymer, and
VERSIFY.TM. propylene/ethylene elastomers and plastomers by The Dow
Chemical Company.
[0035] The density of the propylene based elastomers or plastomers
of this invention is typically at least about 0.850, can be at
least about 0.860 and can also be at least about 0.865 grams per
cubic centimeter (g/cm.sup.3) as measured by ASTM D-792. Preferably
the density is less than about 0.893 g/cc.
[0036] The weight average molecular weight (Mw) of the propylene
based elastomers or plastomers of this invention can vary widely,
but typically it is between about 10,000 and 1,000,000,
alternatively between about 50,000 and 500,000 or between 100,000
and 250,000 (with the understanding that the only limit on the
minimum or the maximum M.sub.w is that set by practical
considerations).
[0037] The polydispersity of the propylene based elastomers or
plastomers of this invention is typically between about 2 and about
5. "Narrow polydispersity", "narrow molecular weight distribution",
"narrow MWD" and similar terms mean a ratio (M.sub.w/M.sub.n) of
weight average molecular weight (M.sub.w) to number average
molecular weight (M.sub.n) of less than about 3.5, can be less than
about 3.0, can also be less than about 2.8, can also be less than
about 2.5.
[0038] The PBPEs for use in the present invention ideally have an
MFR of from 0.2 to 2000 g/10min, preferably from about 0.5 to 1000,
more preferably from about 2 to 500, still more preferably from
about 2 to 40. The particular MFR selected will depend in part on
the intended fabrication methods such as blown film, extrusion
coating, sheet extrusion, injection molding or cast film processes.
MFR for copolymers of propylene and ethylene and/or one or more
C.sub.4-C.sub.20 .alpha.-olefins is measured according to ASTM
D-1238, condition L (2.16 kg, 230.degree. C.).
MFRS greater than about 250 were estimated according to the
following correlation:
MFR=9.times.10.sup.18 Mw.sup.-3.3584
[0039] Mw (grams per mole) was measured using gel permeation
chromatography.
[0040] The blend of which the core layer is comprised preferably
comprises from 20% to 80% (by weight of the blend) of the second
polymer, more preferably from 30% to 70% , and from 20% to 80% of
the third polymer. More preferably the third polymer comprises less
than or equal to 70%, or even less than or equal to about 60% by
weight of the blend. The blend may also include other polymers,
although it is preferred that the blend contain no other resins
than the ethylene based polymers and/or propylene based
polymers.
[0041] The olefin block copolymers suitable for use as the third
polymer in the films of the present invention are a relatively new
class of material which are more fully described in WO 2005/090427,
US2006/0199931, US2006/0199930, US2006/0199914, US2006/0199912,
US2006/0199911, US2006/0199910, US2006/0199908, US2006/0199907,
US2006/0199906, US2006/0199905, US2006/0199897, US2006/0199896,
US2006/0199887, US2006/0199884, US2006/0199872, US2006/0199744,
US2006/0199030, US2006/0199006 and US2006/0199983; each publication
being fully incorporated herein by reference. Olefin block
copolymers are also known as "OBCs" or olefin multi-block
interpolymers,
[0042] The OBCs may be made with two catalysts incorporating
differing quantities of comonomer and a chain shuttling agent.
Preferred olefin multi-block interpolymers are
ethylene/.alpha.-olefin multi-block interpolymers. The
ethylene/.alpha.-olefin multi-block interpolymers typically
comprise ethylene and one or more copolymerizable .alpha.-olefin
comonomers in polymerized form, characterized by multiple blocks or
segments of two or more polymerized monomer units differing in
chemical or physical properties. That is, the
ethylene/.alpha.-olefin interpolymers are block interpolymers,
preferably multi-block interpolymers or copolymers.
[0043] The ethylene multi-block polymers typically comprise various
amounts of "hard" and "soft" segments. "Hard" segments refer to
blocks of polymerized units in which ethylene is present in an
amount greater than about 95 weight percent, and preferably greater
than about 98 weight percent based on the weight of the polymer. In
other words, the comonomer content (content of monomers other than
ethylene) in the hard segments is less than about 5 weight percent,
and preferably less than about 2 weight percent based on the weight
of the polymer. In some embodiments, the hard segments comprise all
or substantially all ethylene. "Soft" segments, on the other hand,
refer to blocks of polymerized units in which the comonomer content
(content of monomers other than ethylene) is greater than about 5
weight percent, preferably greater than about 8 weight percent,
greater than about 10 weight percent, or greater than about 15
weight percent based on the weight of the polymer. In some
embodiments, the comonomer content in the soft segments can be
greater than about 20 weight percent, greater than about 25 weight
percent, greater than about 30 weight percent, greater than about
35 weight percent, greater than about 40 weight percent, greater
than about 45 weight percent, greater than about 50 weight percent,
or greater than about 60 weight percent.
[0044] The soft segments can often be present in a block
interpolymer from about 1 weight percent to about 99 weight percent
of the total weight of the block interpolymer, preferably from
about 5 weight percent to about 95 weight percent, from about 10
weight percent to about 90 weight percent, from about 15 weight
percent to about 85 weight percent, from about 20 weight percent to
about 80 weight percent, from about 25 weight percent to about 75
weight percent, from about 30 weight percent to about 70 weight
percent, from about 35 weight percent to about 65 weight percent,
from about 40 weight percent to about 60 weight percent, or from
about 45 weight percent to about 55 weight percent of the total
weight of the block interpolymer. Conversely, the hard segments can
be present in similar ranges. The soft segment weight percentage
and the hard segment weight percentage can be calculated based on
data obtained from DSC or NMR. Such methods and calculations are
disclosed in WO/2008/080111, entitled "Ethylene/.alpha.-Olefin
Block Interpolymers", with a priority date of Mar. 15, 2006, in the
name of Colin L. P. Shan, Lonnie Hazlitt, et. al. and assigned to
Dow Global Technologies Inc., the disclosure of which is
incorporated by reference herein in its entirety.
[0045] The term "multi-block copolymer" or "segmented copolymer"
refers to a polymer comprising two or more chemically distinct
regions or segments (referred to as "blocks") preferably joined in
a linear manner, that is, a polymer comprising chemically
differentiated units which are joined end-to-end with respect to
polymerized ethylenic functionality, rather than in pendent or
grafted fashion. In a preferred embodiment, the blocks differ in
the amount or type of comonomer incorporated therein, the density,
the amount of crystallinity, the crystallite size attributable to a
polymer of such composition, the type or degree of tacticity
(isotactic or syndiotactic), regio-regularity or
regio-irregularity, the amount of branching, including long chain
branching or hyper-branching, the homogeneity, or any other
chemical or physical property. The multi-block copolymers are
characterized by unique distributions of polydispersity index (PDI
or Mw/Mn), block length distribution, and/or block number
distribution due to the unique process making of the copolymers.
More specifically, when produced in a continuous process, the
polymers desirably possess PDI from 1.7 to 2.9, preferably from 1.8
to 2.5, more preferably from 1.8 to 2.2, and most preferably from
1.8 to 2.1. When produced in a batch or semi-batch process, the
polymers possess PDI from 1.0 to 2.9, preferably from 1.3 to 2.5,
more preferably from 1.4 to 2.0, and most preferably from 1.4 to
1.8.
[0046] In one embodiment, an ethylene/.alpha.-olefin multi-block
interpolymer has an ethylene content of from 60 to 90 percent, a
diene content of from 0 to 10 percent, and an .alpha.-olefin
content of from 10 to 40 percent, based on the total weight of the
polymer. In one embodiment, such polymers are high molecular weight
polymers, having a weight average molecular weight (Mw) from 10,000
to about 2,500,000, preferably from 20,000 to 500,000, more
preferably from 20,000 to 350,000; a polydispersity less than 3.5,
more preferably less than 3 and as low as about 2; and a Mooney
viscosity (ML (1+4) at 125.degree. C.) from 1 to 250.
[0047] In one embodiment, the ethylene multi-block interpolymers
have a density of less than about 0.90, preferably less than about
0.89, more preferably less than about 0.885, even more preferably
less than about 0.88 and even more preferably less than about
0.875, g/cc. In one embodiment, the ethylene multi-block
interpolymers have a density greater than about 0.85, and more
preferably greater than about 0.86, g/cc. Density is measured by
the procedure of ASTM D-792. Low density ethylene multi-block
copolymers are generally characterized as amorphous, flexible, and
have good optical properties, for example, high transmission of
visible and UV-light and low haze.
[0048] In one embodiment, the ethylene multi-block interpolymers
have a melting point of less than about 125.degree. C. The melting
point is measured by the differential scanning calorimetry (DSC)
method described in U.S. Publication 2006/0199930 (WO 2005/090427),
incorporated herein by reference.
[0049] OBCs are identified by The Dow Chemical Company by the use
of the INFUSE.TM. trademark, and also include D9100, D9500
developmental resins.
[0050] The film of the present invention also comprises a sealant
layer, the third mandatory layer in the film. The sealant layer
comprises a fourth polymer having a melting point less than or
equal to 140.degree. C., preferably less than or equal
to130.degree. C., 120.degree. C. or even 110.degree. C. The fourth
polymer preferably has a melting point greater than 75.degree. C.
The fourth polymer can advantageously be an ethylene copolymer
having a density less than about 0.925 g/cc, more preferably less
than about 0.910 g/cc , or a PBPE, or an OBC, or blends comprising
one or more of these materials. The ethylene copolymers, OBCs or
PBPEs suitable for use as the fourth polymer are the same type as
described above for the third polymer, and in fact in some
embodiments it may be preferred that the third polymer and the
fourth polymer be the same material. Even if not the exact same
material it may be preferable that the third and fourth polymer be
the same class of material (for example both be ethylene
copolymers, or both be PBPEs or both be OBCs).
[0051] As will be readily understood by practitioners in the art,
it is also contemplated that any of the resins used in the present
invention (in either the outer, core, sealant or additional layers)
may include minor amounts of additives such as antioxidants (e.g.,
hindered phenols e.g., Irganox.RTM. 1010 made by Ciba-Geigy Corp.),
phosphites (e.g., Irgafos.RTM. 168 made by Ciba-Geigy Corp.), cling
additives (e.g., polyisobutylene (PIB)), polymeric processing aids
(such as Dynamar.TM. 5911 from Dyneon Corporation, and Silquest.TM.
PA-1 from General Electric), antiblock additives, slip additives
such as Erucamide, pigments, etc. In some applications additives
may advantageously be used in skin layer to reduce stickiness in
machine direction orientation-rollers (MDO) during process and
modify the coefficient of friction (COF) to desired levels for ease
of handling.
[0052] The film of the present invention is prepared by melt
coextrusion of all the layers using the resins as mentioned above
and then by biaxial stretching as is generally known in the art,
such as via the double bubble or tenterframe biaxial orientation
processes.
[0053] Preferably the film of the present invention will have a
total thickness of from 15 to 80 .mu.m, more preferably from 20 to
50 .mu.m.
[0054] Preferably the outer layer in the films of the present
invention will have a thickness of from 10 to 70 .mu.m, more
preferably 10 to 40 .mu.m.
[0055] Preferably the core layer in the films of the present
invention will have a thickness of from 2 to 40 .mu.m, or 3 to 30
.mu.m, more preferably 4 to 10 .mu.m.
[0056] Preferably the sealant layer in the films of the present
invention will have a thickness of from 0.4 to 20 .mu.m, or 0.5 to
10 .mu.m, more preferably 1 to 5 .mu.m.
[0057] It is also contemplated that the films of the present
invention may contain additional layers, although it is generally
preferred that the films consist of only three layers.
[0058] The preferred films of the present invention will exhibit a
high seal strength, preferably greater than or equal to 8 N/25 mm,
more preferably greater than or equal to10 N/25 mm. The preferred
films of the present inventing will also exhibit low heat seal
initiation temperatures, such as less than or equal to 130, 115,
105 or even 100.degree. C. Accordingly, the preferred films of the
present invention will meet the heat seal requirements for the food
packaging market or any other application requiring these
properties.
EXAMPLES
[0059] The following materials are used in the Examples described
below.
[0060] Resin A is polypropylene homopolymer (hPP) having a density
of 0.900 g/cm.sup.3; an MFR of 2 g/10 min (230.degree. C., 2.16
Kg), and a melting point of 162.degree. C.
[0061] Resin B is propylene-ethylene based elastomer or plastomer,
with an ethylene content 5 wt %; a density of 0.888 g/cm.sup.3; an
MFR of 8 g/10 min (230.degree. C., 2.16 Kg); a melting point of
105.degree. C.; and a Vicat softening point of 90.degree. C.
[0062] Resin C is a propylene-ethylene-butene random terpolymer,
with ethylene content of 6.2 wt % and a butene content of 15.1 wt
%; a density of 0.902 g/cm.sup.3; an MFR of 6 g/10 min (230.degree.
C., 2.16 Kg); and a melting point of 133.degree. C.
[0063] Resin D is a 50/50 blend of Resin A and Resin B. The melting
point of Resin D is 162.degree. C.
[0064] Resin E is a 20/80 blend of Resin A and Resin B. Melting
point of Resin E is 162.degree. C.
[0065] Resin F is a 80/20 blend of Resin A and Resin B. Melting
point of Resin F is 162.degree. C.
[0066] The melt compounding of Resins D, E, and F is carried out in
a 30 mm twin screw extruder. The components are dry blended and
then fed into the extruder through the main feed throat. The
temperatures are set at 150-200-200-210-210-220.degree. C. (from
hopper to die). An open vent port without vacuum is used. The
polymer is extruded through a 3.2 mm 2-hole strand die with output
15 kg/hr and screw speed 300 rpm. The strands are quenched by an
ambient temperature water bath and then are strand cut by a ConAir
pelletizer.
[0067] To make the film, The pellets are converted on a coextrusion
sheet extrusion line fitted with four separate extruders (Me-100,
Me-200 Me-300 Me-400), a mini coextrusion modular quick change
feedblock assembly. The sheets are made at a gauge of 0.5 to 1.1
mm, and the die gap is set approximately to the desired gauge of
the sheet structure, and the screw speed is set at from 40 to 120
rpm. Two extruders are used to produce the two layer sheets. The
temperature control of the extruders for the three and two layer
co-ex sheets are listed in Table 1 and Table 2, respectively.
TABLE-US-00001 TABLE 1 Temperature control of the extruders for the
three layer co-ex sheets Extruder Me-100 Me-200 Me-300 Me-400 Zone
1 Temp (.degree. C.) 155 150 110 120 Zone 2 Temp (.degree. C.) 165
160 120 125 Zone 3 Temp (.degree. C.) 175 160 120 135 Zone 4 Temp
(.degree. C.) 190 160 135 Zone 5 Temp (.degree. C.) 190 160 Gate
Temp (.degree. C.) 190 160 120 135
TABLE-US-00002 TABLE 2 Temperature control of the extruders for the
two layer co-ex sheets Extruder Me-100 Me-200 Me-300 Me-400 Zone 1
Temp (.degree. C.) 155 160 Zone 2 Temp (.degree. C.) 165 170 Zone 3
Temp (.degree. C.) 175 180 Zone 4 Temp (.degree. C.) 190 Zone 5
Temp (.degree. C.) 190 Gate Temp (.degree. C.) 190 180
[0068] The co-ex sheets are then biaxially oriented in a
simultaneous manner on Bruckner Laboratory Film Stretcher Type KARO
IV. The sheet is first cut into an 85 mm.times.85 mm plaque, and
then the plaque is clamped with five clips on each edge. The sample
loading unit is moved into the oven with the plaque, and is
equilibrated for 60 seconds prior to stretching. The temperature on
the sheet is monitored and it was found that 60 seconds was long
enough to equilibrate the plaque at the target orientation
temperature (155.degree. C.). The sheet is then simultaneously
stretched at the speed of 400% s.sup.-1 to a prescribed stretching
ratio (5.times.5). The oriented film is removed from the sample
loading unit and conditioned for >7 days prior to testing.
[0069] The co-ex BOPP films were prepared according to the
following formulations:
TABLE-US-00003 Outer layer Core layer Sealant layer Seal strength
Heat seal Sample Outer layer thickness Core thickness Sealant
thickness at 110.degree. C. initiation identification Structure
(base layer) (micron) layer (micron) layer (micron) (N/25 mm) temp.
(.degree. C.) Comparative Two layer Resin A 25 / / Resin B 5 6 110
example 1 film Comparative Two layer Resin A 25 / / Resin C 5 5 110
example 2 film Inventive Three Resin A 20 Resin D 20 Resin B 5 33
93 example 1 layer film Inventive Three Resin A 20 Resin D 6 Resin
B 5 28 92 example 2 layer film Inventive Three Resin A 20 Resin E 6
Resin B 5 10 95 example 3 layer film Inventive Three Resin A 20
Resin F 6 Resin B 5 10 94 example 4 layer film
[0070] The standard heat seal and hot tack tests are conducted in
on J&B hot-tack tester model 4000 and ZWICK Z010 Universal
Tensile Tester with parameters listed below: [0071] Sample width:
25 mm [0072] Seal pressure: 0.275 N/mm.sup.2 [0073] Seal time: 0.5
s [0074] Delay time (hot tack): 0.1 s [0075] Condition time (after
heat seal): 24 hours [0076] Peel speed (heat seal): 500 mm/min
[0077] Peel speed (hot tack): 200 mm/s [0078] Five data points are
collected and averaged.
[0079] The determination of the melting point (T.sub.m) is
performed using a differential scanning calorimeter (DSC). The
temperature at the maximum heat flow rate with respect to a linear
baseline was used as the melting point as measured by DSC. The
temperature is raised from room temperature to 200.degree. C. at
10.degree. C./min, maintained at 200.degree. C. for 5 min,
decreased to 0.degree. C. at 10.degree. C./min, maintained at
0.degree. C. for 5 min and then the temperature is raised from
0.degree. C. to 200.degree. C. at 10.degree. C./min. The melting
and cooling curves are recorded. When plural melting peaks are
present, the melting peak at the maximum temperature is used as the
melting point.
[0080] The heat seal strength vs. seal temperature curves of
inventive example 1 and comparative example 1 and 2 are shown in
FIG. 1. As seen in that Figure, the heat seal strength of the two
comparative examples (two layer co-ex BOPP film produced
conventionally) is very low, averaging about 5 to 7 N/25 mm, even
at seal temperatures above 110.degree. C., and even though
different sealants are used. The heat seal initiation temperature
("HSIT") (the minimum temperature at which a seal strength of 4.4
N/25 mm was obtained) for the two comparative examples is around
110.degree. C. In contrast, the heat seal strength of the three
layer co-ex BOPP film prepared by using inventive technology
reaches a high level of ca. 30 to 40 N/25 mm at seal temperatures
above 110.degree. C., and the HSIT is around 95.degree. C.
[0081] The heat seal strength vs. seal temperature curves of the
inventive example 2, 3, and 4 are shown in FIG. 2. This Figure
demonstrates that despite downgauging the core layer to 6 .mu.m, it
is still possible to achieve seal strength values higher than 10
N/25 mm. The three films in FIG. 2 have the same film structure
except for differing ratios of resins A and B in the core layer.
FIG. 2 therefore also demonstrates the higher seal strength (around
30 N/25 mm) obtainable when using core layers containing blends
made from the more preferred ranges.
[0082] The following recitations indicate additonal aspects of the
invention. Although such recitations may not all be currently
claimed, the applicants reserve the right to add claims
corresponding to these recitations at a later date. [0083] 1. A
multilayer biaxially oriented polypropylene film comprising: [0084]
a. an outer layer comprising a first polymer which is a homopolymer
polypropylene resin having a melting point of at least 155.degree.
C.; [0085] b. a core layer, with a melting point greater than
150.degree. C., comprising a blend comprising a second polymer
which is a polypropylene homopolymer having a melting point of at
least 155.degree. C. and a third polymer having a melting point no
greater than 145.degree. C., wherein the second polymer comprises
from 20% to 80% by weight of the core layer while the third polymer
comprises from 80% to 20% by weight of the core layer; and [0086]
c. a sealant layer comprising a fourth polymer having a melting
point no greater than 145.degree. C. [0087] 2. The film of
recitation 1 wherein the third polymer is a propylene copolymer or
an ethylene copolymer. [0088] 3. The film of recitation 1 wherein
the core layer thickness is no more than 75% of the film thickness.
[0089] 4. The film of recitation 1 wherein the outer layer has a
thickness of from 10 .mu.m to 40 .mu.m. [0090] 5. The film of
recitation 1 wherein the sealant layer has a thickness of from 0.4
.mu.m to 20 .mu.m. [0091] 6. The film of recitation 1 wherein the
fourth polymer is a propylene copolymer or an ethylene copolymer.
[0092] 7. The film of recitation 1 wherein the fourth polymer
comprises a polypropylene based plastomer or elastomer. [0093] 8.
The film of recitation 1 wherein the fourth polymer comprises an
ethylene copolymer having a density less than 0.925 g/cc. [0094] 9.
The film of recitation 1 wherein the fourth polymer comprises an
olefin block copolymer. [0095] 10. The film of recitation 1 wherein
the fourth polymer has a melting point less than 130.degree. C.
[0096] 11. The film of recitation 1 wherein the fourth polymer has
a melting point less than 110.degree. C. [0097] 12. The film of
recitation 1 wherein the blend in the core layer comprises no more
than 80% of the third polymer by weight of the core layer. [0098]
13. The film of recitation 1 wherein the blend in the core layer
comprises at least 20% of the second polymer by weight of the core
layer. [0099] 14. The film of recitation 1 wherein the second
polymer is homopolymer polypropylene having a melting temperature
greater than 160.degree. C. [0100] 15. The film of recitation 1
wherein the third polymer has a melting point no greater than
145.degree. C. [0101] 16. The film of recitation 1 wherein the
third polymer has a melting point larger than 80.degree. C. [0102]
17. The film of recitation 1 wherein the film has a heat seal
initiation temperature no greater than 130.degree. C. [0103] 18.
The film of recitation 1 wherein the third polymer has a Vicat
softening point of at least 60.degree. C. [0104] 19. The film of
recitation 1 wherein the blend which comprises the core layer has
been prepared by melt compounding or simple dry
blending/tumbling.
* * * * *