Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1996-10-07
2001-11-13
Reddick, Judy M. (Department: 1713)
Stock material or miscellaneous articles
Composite
Of metal
C428S411100, C428S447000, C428S451000, C428S457000, C428S463000, C428S500000, C428S515000, C428S516000, C428S518000, C428S523000, C525S056000, C525S057000
Reexamination Certificate
active
06316114
ABSTRACT:
This invention relates to biaxially oriented films comprised of polypropylene, which films exhibit improved oxygen transmission properties as compared to films of polypropylene presently known to the art. More specifically, it relates to such films that are comprised of a specific, substantially homogeneous blend of polypropylene and a copolymer of ethylene and vinyl alcohol.
BACKGROUND INFORMATION
Polyolefin films, and, in particular, polypropylene films, have become a dominant factor in the packaging film industry in recent years, thanks to a spectrum of favorable properties. However, the realization of the full potential of these films as packaging materials has been hampered by their lack of good resistance to the transmission of oxygen.
A very desirable approach to improving the oxygen and also the moisture vapor transmission rate of polypropylene films is the application of a metal, usually aluminum, coating to one or both surfaces of the film. Many journal articles and patents have been published relating to the application of metal barrier coatings and metal coating is widely practiced in the film industry at the present time. Metal coating is, however, a relatively costly technique as it involves extra surface preparation steps and costly application apparatus and processes.
It is also known to prepare films and containers of polypropylene using composite structures comprised of polypropylene layered with another material having better oxygen transmission properties, e.g. polyvinyl or polyvinylidene chloride, vinyl acrylate/acrylonitrile copolymer or a vinyl alcohol polymer or copolymer. Further, it is known to prepare films having good oxygen transmission properties in which small, discrete particles of polypropylene are dispersed throughout a vinyl alcohol polymer matrix.
BRIEF STATEMENT OF THE INVENTION
In accordance with this invention it has been found that oriented films comprised of a blend of polypropylene and a copolymer of ethylene and vinyl alcohol exhibit excellent resistance to the transmission of oxygen without the application of a metal coating. Specifically stated, the invention is an oriented film comprised of a substantially homogeneous blend of polypropylene and up to about 40% by weight, based on the total weight of the blend, of a copolymer of ethylene and vinyl alcohol. In a preferred embodiment of the invention, the ethylene-vinyl alcohol copolymer comprises about 3 to 30% by weight of the substantially homogeneous blend and in a most preferred embodiment, about 10 to 25%.
Several embodiments of the invention are contemplated. In a first embodiment, the films of the invention can exist as stand-alone films of the blend of polypropylene and ethylene-vinyl alcohol copolymer. These can be mono-layer structures or they can be composite (multilayer) structures wherein the inventive film serves as a core having one or more functional layers on at least one of its surfaces.
The invention also contemplates an oriented composite film comprised of a polypropylene core layer having, on one or both of its surfaces, as a barrier layer, a film comprised of the substantially homogeneous blend of polypropylene and an ethylene-vinyl alcohol copolymer as described hereinabove.
In a preferred embodiment of the invention, the film is an oriented composite film comprised of a polypropylene core having on one of its surfaces a substantially homogeneous blend of polypropylene and an ethylene-vinyl alcohol copolymer and, on the other surface, a sealable coating.
DETAILED DESCRIPTION OF THE INVENTION
When reference is made herein to polypropylene, it is intended to indicate a highly crystalline (isotactic) propylene homopolymer or a copolymer of propylene with another olefin or &agr;-olefin having about 2 to 5 carbons in an amount insufficient to have a significant effect on the crystallinity of the polypropylene. Typically, this is ethylene in an amount less than about 2%.
References to a “substantially homogeneous” blend of polypropylene and ethylene-vinyl alcohol copolymer are intended to refer to a mixture of the two components that has been melt blended so that each component exists as a continuous phase with the other as opposed to one of the components existing in a dispersed phase.
The present invention is based on the finding that certain ethylene-vinyl alcohol (EVOH) copolymers, within specified concentration limits, can form substantially homogeneous blends with polypropylene and such blends can be employed to form biaxially oriented films having good oxygen barrier properties. Typically, these films exhibit oxygen transmission rates, measured according to ASTM D3985-81, less than about 30 cm
3
/100 in
2
/atm/day and frequently less than about 15 cm
3
/100 in
2
/atm/day. This compares to an oxygen transmission rate on the order of about 150 cm
3
/100 in
2
/atm/day for polypropylene that has not been treated or modified to improve its oxygen barrier properties.
Films comprised of substantially homogeneous blends of an ethylene-vinyl alcohol copolymer and polypropylene according to this invention possess a significant advantage over packaging films previously known to the art in that they have high resistance to oxygen transmission without the application of metal or other barrier coating materials. Thus, costly metal application steps can be eliminated along with the costly surface preparation steps required to make the film surface receptive to the coating metal.
The films prepared from blends of polypropylene and an ethylene-vinyl alcohol copolymer according to this invention can be stand-alone, self-supporting films or they can be thin films applied as barrier layers on a polypropylene core. When the films are employed in the stand-alone embodiment, the additional expense of preparing a composite film is, of course, avoided as well as the expense involved in the application of a metal or other barrier material. Alternatively, a stand-alone film according to the invention can serve as the core layer of a composite structure having a functional layer applied to either or both of its surfaces. Suitable functional layers that can be applied include, by way of example, a heat or cold seal layer, a slip layer, a printable layer or a layer that combines two or more of such functions. Further, the second layer can be a layer suitable for lamination with yet another layer or material.
On the other hand, when the film is employed as a barrier layer on a polypropylene core, excellent barrier properties are realized with a smaller amount of the more expensive copolymer component. When employed as barrier coatings on a polypropylene core, these films exhibit an additional advantage over many other known barrier coating materials in that, due to the polypropylene content of the blends, they adhere directly to the polypropylene core without the need for an intermediate tie layer.
The ethylene-vinyl alcohol copolymer employed in this invention can contain about 40 to 50 mole % of the ethylene component. Preferably, it contains about 44 to 48 mole % ethylene. Preferred copolymers are commercially available under the Trade name “EVAL” from The Eval Company of America, Lisle, Ill. (Evalca).
The blends of ethylene-vinyl alcohol copolymer and polypropylene employed in the films of this invention can contain up to about 40% by weight of the copolymer, based on the total weight of the blend. Preferably, the copolymer is present in an amount of about 3 to 30%, and most preferably about 10 to 25%. When the blend is intended for use as a stand-alone film rather than as a layer applied onto a composite film, the amount of the copolymer in the blend will be on the lower end of the specified range, i.e, about 3 to 20%.
When the film of a blend of polypropylene and ethylene-vinyl alcohol copolymer according to the invention is applied as a barrier layer on a polypropylene core, the film can double as a functional layer for other purposes. For example, this layer can also serve as a receiving layer for decorative coatings, printed matter, or as a layer for receiving an adhesive coating.
Comer Annette Marie
DeMeuse Mark Thomas
Applied Extrusion Technologies, Inc.
Caesar Rivise Bernstein Cohen & Pokotilow Ltd.
Reddick Judy M.
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