Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
2000-11-28
2002-11-26
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S515000, C428S516000, C428S910000
Reexamination Certificate
active
06485817
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to polyolefin films having certain unique properties. Specifically, it relates to clear polyolefin films that exhibit 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 in the package, which can also lead to spoilage of the product if the moisture cannot escape from the package. It is thus desirable to provide polypropylene packaging wherein the oxygen content can be replenished as necessary and from which the moisture can escape.
Conventional polypropylene films of a thickness required for most applications, whether monolayer or composite, 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 containing products of the type mentioned.
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 biaxial 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.
Polyethylene is also known to have sufficiently great oxygen transmission qualities to permit produce items to continue respiring after being packaged, but polyethylene films do not have sufficient tensile modulus to permit their use in most packaging applications.
U.S. Pat. No. 4,410,582, issued to Tsunashima et al., is directed principally to finger-tearable multilayer structures effectively usable as an adhesive tape substrate and in other applications. Although this patent discloses the use of a nucleated polypropylene in the skin, the disclosed purpose of adding the nucleating agent is to improve the transparency of the film. There is no disclosure of providing a nucleated polypropylene having any required properties for improving the OTR of the film, let alone providing the high modulus properties required in the instant invention. In fact, there is no disclosure that controlling the modulus properties of the polypropylene in the skin has any effect on OTR. It is stated in column 8, lines 15-20 that the substrate may be usable as a packaging film if a paper or metal foil is bonded to one or both surfaces. However, employing a metallized layer tends to reduce the OTR; making the films undesirable for packaging applications in which a high OTR is required.
European publication No. 0 243 965, describing an invention by Tsutomu, et al., discloses a multilayer packaging film for packaging fresh vegetables and fruits and including an antifogging additive in at least one surface layer that also has heat sealing properties. Although the surface layer is described generally as being composed of a homopolymer or copolymer of alpha-olefins, there is no suggestion that the alpha olefin should, or could by a high modulus polypropylene required for use in the instant invention. Moreover, the film disclosed in the '965 publication teaches away from forming the core of the multilayer film from a polyolefin copolymer, by itself (see page 17).
BRIEF DESCRIPTION OF THE INVENTION
In accordance with this invention, there are provided transparent biaxially oriented polyolefin films based on high OTR olefin heteropolymers, which films have a stiffness (i.e. tensile modulus) sufficient to have self-supporting, stand-alone characteristics such that they can be employed as packaging films. These films have increased oxygen and moisture vapor permeability, as compared with conventional, unmodified polypropylene film, and are substantially clearer than are films possessing these permeability characteristics that have heretofore been known to the art.
In accordance with this invention, there are provided transparent composite, oriented polyolefin films comprised of a core layer and at least one skin layer, said core layer being a heteropolymer of at least two &agr;-monoolefins, and having a thickness sufficient to give the composite structure stand-alone characteristics; said at least one skin layer being a high modulus polypropylene of a thickness such that its oxygen transmission rate (OTR) is higher than that of the core layer and the total composite structure having an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. in/day/atmosphere, a moisture vapor transmission rate (MVTR) of at least about 0.5 gram/sq. in./day, and a tensile modulus of at least 185,000 psi in both the machine direction and cross-machine direction of the film. Most preferably the heteropolymer employed in this invention is an ethylene/propylene copolymer having less than 10% ethylene, by weight, and most preferably having about 4.5-6% ethylene, by weight.
The core layer in the composite films of this invention has a sufficient thickness to be “self-supporting” (hereinafter defined), independent of the surface layers.
The composite films of the invention are characterized by having oxygen and moisture vapor permeability substantially greater than those of conventional films based on polypropylene wherein the thickest layer of the film, i.e., the core layer, is polypropylene. The films of the invention are also clear and transparent so that a product packaged therein can be visually inspected for its quality without opening the package. At the same time, they have tensile properties that make them suitable for use as packaging films.
DETAILED DESCRIPTION OF THE INVENTION
When used in this disclosure, the term “&agr;-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 less than 10% by weight ethylene and more preferably about 4.5 to 6% by weight of ethylene. Although other heteropolymers, i.e., butene--propylene copolymers containi
Applied Extrusion Technologies, Inc.
Caesar Rivise Bernstein Cohen & Pokotilow Ltd.
Nakarani D. S.
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