Co-extruded, multilayer packaging film made from propylene...

Details Co-extruded, multilayer packaging film made from propylene... Co-extruded, multilayer packaging film made from propylene...

1999-12-21

2001-12-25

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S215000, C428S216000, C428S516000, C525S240000

Reexamination Certificate

active

06333096

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a multilayer film made from propylene polymer materials.
BACKGROUND OF THE INVENTION
Freshly cut fruits and vegetables continue to respire after being picked, washed and cut, and need to be packaged in such a way that the oxygen and carbon dioxide transmission rates of the packaging matches the respiration rate of the produce. Each fruit or vegetable has its own respiration rate, thereby requiring a unique package.
Various combinations of polyolefin films have been used for making food packaging materials. For example, U.S. Pat. No. 5,500,284 discloses composite films especially suited for the production of bags to be used for solid and/or liquid products, and/or to preserve products that must be kept at low temperatures. The film consists of (A) a single- or multilayer bioriented film, where the single layer or at least one layer of the bioriented film is a crystalline propylene homopolymer, and (B) a single- or multilayer cast film wherein the single layer or at least one layer of the film is a layer of a composition including a propylene polymer such as a copolymer of ethylene with propylene and/or a C
4
-C
8
alpha-olefin.
U.S. Pat. No. 5,318,842 discloses a biaxially oriented film or sheet article comprising (A) a base or inner layer of a film of a C
3
-C
10
alpha-olefin homopolymer or a copolymer such as a copolymer of propylene and two different olefins selected from ethylene and C
4
-C
10
alpha-olefins, and (B) at least one layer of a broad molecular weight distribution propylene polymer, optionally containing another propylene polymer such as a a copolymer of propylene and at least two different olefins selected from ethylene and C
4
-C
8
alpha-olefins. The films are described as having improved stiffness and moisture vapor transmission properties compared to conventional film grade polyolefin resins.
U.S. Pat. No. 5,741,565 discloses a multilayer cast film comprising (A) at least one layer of a propylene homopolymer, copolymer, or terpolymer with ethylene and/or a C
4
-C
8
alpha-olefin, and (B) at least one layer of a propylene homopolymer, copolymer, or terpolymer with ethylene and/or a C
4
-C
8
alpha-olefin or a composition of a random copolymer of propylene with ethylene and/or a C
4
-C
10
alpha-olefin. The preferred film is made up of three layers in which (B) is placed between two outer layers of (A), or (A) is placed between two outer layers of (B). The films are especially useful for confectionery packaging.
There is still a need for a packaging material for fresh produce that can be tailored to match the gas transmission rate of a particular fruit or vegetable being packaged and thus meet the needs of individual producers.
SUMMARY OF THE INVENTION
The co-extruded, multilayer film of this invention comprises the following layers:
(1) an outer layer of a terpolymer comprising (a) about 85% to about 96% propylene, (b) about 0.5% to about 5.0% ethylene, and (c) about 2.5% to about 10% of a C
4
-C
8
alpha-olefin, wherein the total of (b) and (c) is about 4.0% to about 15.0%;
(2) a middle layer comprising at least one of (a) 0-100% of the terpolymer described in (1), and (b) 0-100% of an olefin polymer blend comprising:
(i) about 99.5% to about 95% of an olefin polymer composition produced by sequential polymerization in at least two stages, comprising:
(A) about 5% to about 25% by weight of a copolymer of propylene with ethylene and a C
4
-C
8
alpha-olefin containing about 80% to about 96% propylene, about 2% to about 10% ethylene, and about 2% to about 10% of a C
4
-C
8
alpha-olefin, and
(B) about 95% to about 75% by weight of a linear low density polyethylene containing up to 20% by weight of a C
4
-C
8
alpha-olefin, and
(ii) about 0.5% to about 5% by weight of a low density polyethylene having a density of 0.916 to 0.922 g/cm
3
and a melt index of 0.2 to 20 g/10 min,
wherein (a)+(b)=100%, and
(3) an inner layer comprising at least one of (a) 0-100% of a copolymer of ethylene and a C
4
-C
8
alpha-olefin made with a metallocene catalyst and having a density of 0.89-0.91 g/cm
3
, and (b) 0-100% of the olefin polymer blend (b) described in layer (2), wherein (a)+(b)=100%.
The film can be used for packaging fresh produce. By varying the thickness ratios of the layers or, when both components (a) and (b) are present, the ratio of the components of the blends in layers (2) and (3), the film can be tailored to match the respiration rate of the particular type of produce being packaged while at the same time providing improved stiffness and optical clarity.
DETAILED DESCRIPTION OF THE INVENTION
Outer layer (1) of the film of this invention comprises an ethylene/propylene/C
4
-C
8
alpha-olefin terpolymer comprising about 0.5% to about 5% ethylene, preferably about 1% to about 3%; about 2.5% to about 10%, preferably about 3% to about 6%, of a C
4
-C
8
-alpha-olefin, and about 85% to about 96% propylene. This layer comprises about 15% to about 35% of the total thickness of the film.
The 4-8 C alpha-olefin is preferably selected from butene-1, hexene-1,octene-1, and 4-methyl-1-pentene. Butene-1 is preferred.
The terpolymer in layer (1) can be prepared by multi-stage sequential polymerization of monomers in the presence of stereospecific Ziegler-Natta catalysts supported on magnesium dihalides in active form. Preparation of this terpolymer is described in more detail in U.S. Pat. No. 5,486,419, which is incorporated herein by reference.
Middle layer (2) of the co-extruded film comprises at least one of (a) 0-100% of the terpolymer described in layer (1) and (b) 0-100% of a blend of (i) about 99.5% to about 95%, preferably about 98% to about 95%, of an olefin polymer composition produced by sequential polymerization in at least two stages and (ii) about 0.5% to about 5%, preferably about 1% to about 5%, most preferably about 2% to about 5%, of a low density polyethylene, wherein (a)+(b)=100%.
The olefin polymer composition produced by sequential polymerization in at least two stages in blend (b) of layer (2) comprises:
(A) about 5% to about 25% by weight, preferably about 5% about 15%, of a copolymer of propylene with ethylene and a C
4
-C
8
alpha-olefin containing about 80% to about 96% propylene, about 2% to about 10%, preferably about 2% to about 7%, ethylene, and about 2% to about 10%, preferably about 2% to about 8%, of a C
4
-C
8
alpha-olefin, and
(B) about 95% to about 75% by weight, preferably about 85% to about 95%, of a linear low density polyethylene containing up to 20% by weight of a C
4
-C
8
alpha-olefin, preferably about 5% to about 14%.
The olefin polymer composition preferably has an overall melt index (MIE) of 0.4 to 1.10, preferably 0.8 to 1.10 g/10 min, and a density of 0.905 to 0.9135 g/cm
3
.
When both (a) and (b) are present in layer (2), the blend (b) is used in an amount of about 30% to about 70% by weight.
The C
4
-C
8
alpha-olefins used for making the olefin polymer composition include, for example, butene-1, hexene-1, octene-1, and 4-methyl-1-pentene. Preferably, the alpha-olefin is butene-1.
The olefin polymer composition can be prepared using a polymerization process comprising two stages with one or more reactors in each stage, where in one stage component (A) is polymerized in one or more reactors and in another stage component (B) is prepared in one or more reactors. The order in which the components are made is not critical. However, it is preferred that component (A) is prepared in the first stage.
The polymerization is conducted in gas phase using separate fluid bed reactors in each stage and the same catalyst is used in all stages.
Hydrogen can be added as needed as a chain transfer agent for control of the molecular weight.
The reaction temperature for the polymerization of components (A) and (B) can be the same or different, and is generally from 40° to 120° C., preferably from 60° C. to 100° C.
The catalyst used in the polymerization comprises the reaction of (i) a solid catalyst component comprising a titanium compound containing at l

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