Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1998-08-11
2001-05-08
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S518000, C428S520000, C264S177190, C264S173110, C264S171100, C264S212000, C264S288800, C264S290200
Reexamination Certificate
active
06228505
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to biaxially oriented polymeric films. Specifically, the present invention is directed to biaxially oriented polyethylene films having unidirectional tear characteristics in machine direction and methods thereof.
Polymeric films are widely used in many industrial applications. One particularly important application is the food packaging industry. Films employed in the food packaging industry are chosen to provide characteristics necessary for proper food containment. Polymeric films used for food packaging, however, usually do not have tear propagation in the film plane in one well-defined direction. For example, non-oriented polymeric films such as polypropylene (PP) or polyethylene (PE) films are difficult to tear in any direction. If a tear is made on a package of non-oriented polymeric film, the tear propagates in an uncontrolled way across the package, and destroys the entire package.
Bioriented high density polyethylene (HDPE) or polypropylene films have easy tear propagation in both machine and transverse directions. Packages produced with these bioriented polyolefins are easy to open if a tear is initiated by a notch in the pack (typically in the seal area). However, these films are not without disadvantages. Tear propagation of these films is uncontrolled. That is, if a tear is made on a package of the polymeric films, the tear propagates in an uncontrolled way across the package and the entire package is destroyed.
MD oriented polymeric films have also been developed to give controlled tearing properties in one direction. For example, Monax films manufactured by Tredegar provide controlled tear in machine direction. However, the production of MD oriented film is limited in efficiency, as only relative narrow web width can be produced and thickness uniformity is generally poor.
In addition, oriented films that give controlled tear in transverse direction are disclosed in U.S. Pat. No. 4,870,122, issued Sep. 26, 1989 to Lu; U.S. Pat. No. 4,916,025, issued Apr. 10, 1990 to Lu; and U.S. Pat. No. 5,223,346, issued Jun. 29, 1993 to Lu. These films have controlled opening in transverse direction, and thus have, unfortunately, inherent sensitivities in web resistance against tearing when the film is produced, converted, or used in packaging applications.
As a result, there exists a continuing need for bioriented films having controlled tear properties, while maintaining uniform optical properties, good mechanical properties in both machine and transverse directions, high production output with a large web to assure good material efficiencies, and uniform thickness.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits excellent unidirectional tear properties in the machine direction.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits uniform optical properties.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits good mechanical properties in both machine and transverse directions and uniform film thickness.
It is therefore an object of the present invention to provide a biaxially oriented film which exhibits high production output with a large web to assure good material efficiencies.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description and its scope will be pointed out in the appended claims.
SUMMARY OF THE INVENTION
The present invention is a biaxially oriented film having unidirectional tear characteristics in machine direction and a method for making the film. The film consists of (1) a core layer of medium density polyethylene (MDPE) and optionally a cavitating agent; and (2) at least one skin layer co-extensively adhered to at least one surface of the core layer. The core and skin layers are biaxially oriented in both machine and traverse directions.
Examples of the skin layer can be, but are not limited to, ethylene-propylene (EP) copolymer, ethylene-propylene butene (EPB) terpolymer, MDPE having a density of from about 0.90 to about 0.97, preferably MDPE having a density of from about 0.92 to about 0.95, and most preferably MDPE having a density of from about 0.935 to about 0.945.
Examples of the cavitating agent can be, but are not limited to, polystyrene, polyacrylate, polyester, polyamide, cross-linked polymeric particulates, and calcium carbonate (CaCO
3
), preferably, polystyrene, cross-linked polyacrylate powder, and CaCO
3
, and most preferably CaCO
3
.
In another preferred embodiment of the present invention, the laminated film further includes at least one tie layer co-extensively adhered to at least one surface of the skin layer.
Examples of the tie layer can be, but are not limited to, high density polyethylene (HDPE), MDPE, low density polyethylene (LDPE), preferably HDPE and MDPE, and most preferably MDPE.
The present invention is also a method for providing films with unidirectional tear properties which consists of forming a coextruded laminated film having (1) a core layer of medium density polyethylene (MDPE) and optionally a cavitating agent; and (2) at least one skin layer. The core and skin layers co-extensively adhere to each other and are biaxially oriented in both machine and traverse directions. The laminated film can further include at least one tie layer coextensively adhered to at least one surface of the skin and/or core layers.
As a result, the present invention advantageously provides laminated films having tear propagation exclusively in machine direction, and uniform opacity when cavitated. Additionally, the laminated films of the present invention have tear resistance in transverse direction, and thus making the web resistant against tearing in all production and converting operations. The resultant films also exhibit high degree of machinability, processability, and thickness uniformity resulting in better quality film and reduced manufacturing costs.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to biaxially oriented polymeric films. Specifically, the present invention is directed to biaxially oriented polyethylene films having unidirectional tear characteristics in machine direction and method thereof.
The film of the present invention consists of (1) a core layer of medium density polyethylene (MDPE) and optionally a cavitating agent; and (2) at least one skin layer coextensively adhered thereto. The core and skin layers are biaxially oriented in both machine and transverse directions.
The amount of MDPE in the base layer prior to casting the polyethylene sheet should be an amount sufficient to yield a base layer in the film having a thickness from about 0.4 mil to about 3.0 mil, preferably from about 0.6 mil to about 2.0 mil, and most preferably from about 0.75 mil to about 1.5 mil (1 mil=0.001 inch=100 gauge). The thickness value provided herein does not account for additional thickness resulting from cavitation.
The term “medium density polyethylene” (MDPE) as used herein is defined to mean an ethylene-containing polymer having a density of from about 0.910 to about 0.950, preferably from about 0.925 to about 0.945, and most preferably from about 0.930 to about 0.945. Some commercially available MDPE include Dowlex™ 2038 or Dowlex™ 2027A from The Dow Chemical Company.
The present invention also includes at least one skin layer on at least one surface of the core layer. The skin layer can be applied to the core layer by any methods known in the art.
The skin layer has a thickness from about 0 mil to about 0.4 mil, preferably from about 0.02 mil to about 0.3 mil, and most preferably from about 0.03 mil to about 0.2 mil (1 mil=0.001 inch=100 gauge).
Examples of the skin layer can be, but are not limited to, EP copolymer, EPB terpolymer, MDPE having a density of from about 0.90 to about 0.97, preferably MDPE having a density of from about 0.92 to about 0.95, and most preferably MDP
Agent Rhonda Rogers
Larter John A.
Pip Hans Joachim
Kiliman Leszek
Mobil Oil Corporation
Santini Dennis P.
Simmons T. Dean
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