Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
2000-11-28
2003-02-04
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C525S240000, C428S220000
Reexamination Certificate
active
06514625
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to polyolefin films having certain unique properties. Specifically, it relates to clear polyolefin films that exhibit, inter alia, an increased oxygen transmission property as compared to conventional polypropylene films known to the art.
BACKGROUND INFORMATION
It is known that, for many applications, particularly packaging applications, polypropylene is less than totally satisfactory due to a relatively high permeability to oxygen. A great amount of effort has been expended in recent years to decrease the oxygen transmission characteristics of polypropylene. The best results to date have been obtained by metallization of the films.
There are, however, some applications in which a greater and controllable oxygen transmission rate is desired. This is particularly the case when fresh fruits, vegetables and flowers are to be packaged as these products continue to respire after they are packaged. Absence or insufficient levels of oxygen, which occurs as the oxygen originally present in the package is consumed, leads to premature senescence and spoilage of the products. At the same time, respiration leads to a build-up of moisture and carbon dioxide in the package, which can also lead to spoilage of the product if these materials cannot escape from the package.
For many fresh vegetables, fruits and flowers, conventional polypropylene films, whether monolayer or multilayer, of a thickness sufficient to be self-supporting, have oxygen and moisture vapor permeability values that are not sufficient to allow the optimum oxygen and moisture levels to be maintained in a sealed package. It is thus desirable to provide packaging films wherein the oxygen content can be replenished as necessary and from which moisture and carbon dioxide can escape.
It is known to prepare microporous films based on an opaque polymer mixture comprised of about 45% to 55% homopolypropylene and 55% to 45% of a copolymer of propylene and ethylene containing about 2% to 5% ethylene by weight, which films have greater oxygen permeability than films made of plain polypropylene. This polymer mixture is blended into a mixture comprised of about 40 to 60% of the polymer mixture and 60 to 40% of certain inorganic filler materials and is subjected to drawing to effect orientation. The filler material selected is one that causes voiding of the polymer matrix during the drawing operation. Exemplary of such voiding pigments are barium sulfate, calcium carbonate, silica, diatomaceous earth and titania. Calcium carbonate is said to be the preferred filler material. Anderson, U.S. Pat. No. 4,842,875, teaches the use of such films in the preparation of controlled atmosphere containers for use with fresh vegetables, fruits and flowers.
Microporous films known to the art as described above function reasonably well for preparing films having increased and, to a degree, controllable oxygen and moisture vapor permeability. However, the voiding pigments employed in the prior art are of a particle size large enough and are employed in concentrations great enough to result in formation of voids of such a size that the resultant films are almost totally opaque.
It is also known to prepare high OTR polyolefin films for some applications by laminating a polyethylene film of about one mil thickness to a polypropylene film of about 0.4 to 0.5 mil thickness. The polypropylene film acts as a stiffener to give the polyethylene sufficient stiffness that it can be used as a produce wrapper. The polyethylene, which has an inherently high or coextruding OTR, then acts as a high OTR sealant layer.
It is an object of this invention to provide transparent, high modulus polyolefin films based on polypropylene having, among other useful properties, increased oxygen and moisture vapor properties.
It is a further object of the invention to provide polyolefin films based on propylene that are useful in modified atmosphere packaging applications for packaging fresh vegetables, fruits and flowers.
It is yet another object of this invention to provide novel polyolefin films for use for packaging wherein a high modulus film is desired.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with this invention, there are provided transparent, biaxially oriented polyolefin films based on blends of olefin heteropolymers and high modulus polypropylene, which films exhibit a stiffness (i.e tensile modulus) sufficient to have self-supporting, stand-alone characteristics such that they can be employed as packaging films. Specifically, the invention is a transparent, biaxially oriented film comprised of a polyolefin blend consisting essentially of about 25 to 90% by weight of a heteropolymer of at least two a-monoolefins and 75 to 10% by weight of a high modulus species of polypropylene. Most preferably the heteropolymer is an ethylene/propylene copolymer having less than 10% ethylene, by weight, and most preferably having about 4.5-6% ethylene, by weight. Films according to the invention can be prepared in a wide range of thicknesses from about 0.4 to about 1.5 mils.
In one preferred embodiment of the invention, the films have increased oxygen and moisture vapor permeability as compared with conventional, unmodified polypropylene films of similar thickness and are substantially clearer than are polypropylene films heretofore known to the art and possessing similar oxygen and moisture vapor transmission characteristics. In accordance with this embodiment of the invention, there are provided transparent oriented polyolefin films comprised of a polyolefin blend consisting essentially of about 25 to 90% by weight of a heteropolymer of at least two &agr;-monoolefins; preferably an ethylene/propylene copolymer having less than 10% ethylene, by weight, and most preferably having about 4.5-6% ethylene, by weight, and 75 to 10% by weight of a high modulus species of polypropylene, said film being characterized by a thickness of about 40 to 80 gauge, preferably about 50 to 70 gauge, a tensile modulus of at least about 180,000 psi in both the machine direction and cross-machine direction, an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. In/day/atmosphere and a moisture vapor transmission rate (MVTR) of at least about 0.5 gram/sq. In./day. Preferred films according to this embodiment of the invention are prepared with blends comprised of 40 to 90% heteropolymer and 60 to 10% high modulus polypropylene.
In other embodiments, the films have excellent properties for use in other packaging applications where a film of high modulus is desired.
DETAILED DESCRIPTION OF THE INVENTION
In the following discussion, gas permeability properties of the films of the invention will be discussed in terms of their OTR and MVTR. Carbon dioxide transmission rate is also an important parameter of these films affecting their utility as packaging materials for fresh produce items. Carbon dioxide transmission rates can be estimated using the generally accepted factor of a 4 to 1 weight ratio of carbon dioxide to oxygen transmitted per unit of time. When used in this disclosure, the term “&agr;-monoolefin” (sometimes alpha-monoolefin) refers to a linear unsaturated hydrocarbon monomer having one carbon-carbon double bond, which double bond is located at the end of the linear chain. The term is intended to include any such monomer having 6 carbon atoms or less, including ethylene and propylene.
The term “heteropolymer” is intended to mean an olefin polymer containing at least two &agr;-monoolefins. Typical of such heteropolymers are ethylene--propylene copolymers having about 4.5 to 6% by weight of ethylene, copolymers of ethylene containing about 5 to 25% by weight of other &agr;-monoolefins of 4 to about 8 carbon atoms, butene-1-propylene copolymers containing about 5 to 34% by weight of butene-1 and ethylene-propylene-butene-1 terpolymers. Such heteropolymers can be converted to films and can be oriented within a temperature range comparable to the temperature range within which polypropylene is satisfactorily drawn to effect orientation.
The
Applied Extrusion Technologies, Inc.
Caesar Rivise Bernstein Cohen & Pokotilow Ltd.
Tarazano D. Lawrence
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