Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1997-12-24
2002-05-14
Copenheaver, Blaine (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S910000
Reexamination Certificate
active
06387529
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a biaxially oriented multilayer film, and particularly to a multilayer film having a core substrate of high density polyethylene.
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 barrier characteristics necessary for proper food containment. Such barrier characteristics include water vapor barrier, oxygen and gas barrier, as well as flavor and aroma barrier properties.
High density polyethylene (HDPE) is a polymer commonly employed in the manufacture of films used in the food packaging industry. In the case of multilayer films, high density polyethylene is commonly used in the base or core layer. Often, barrier coatings, heat sealable layers, and/or layers with additional functionalities are applied on the surfaces of the HDPE layer to provide improved barrier sealant and/or other properties. For example, U.S. Pat. No. 5,500,283 to Kirk et al. discloses a biaxially oriented film made with a core layer of HDPE coated with a conventional coating such as polyvinylidene chloride polymer, acrylic acid polymer or polyvinyl alcohol polymer.
U.S. Pat. No. 5,346,763 to Balloni et al. is directed to a multilayer film structure composed of a core layer of HDPE having a skin layer of maleic anhydride modified polyethylene on one side and a heat sealable or printable skin layer on the other side.
U.S. Pat. No. 5,302,442 to O'Brien relates to a film structure made with a thermoplastic film having a heat sealable layer on at least one side thereof The thermoplastic film can be a blend of HDPE and the heat sealable layer is a blend of terpolymer of ethylene, propylene and butene.
U.S. Pat. No. 5,558,930 to DiPoto discloses a multilayer film composed of at least a barrier film having thereon at least a sealant layer. The barrier film can be an HDPE polymer and the sealant layer is a conventional heat sealable thermoplastic material, e.g., ethylene vinyl acetate copolymer, ethylene methyl acrylate polymer, copolymers of ethylene, etc.
HDPE films used in packaging are very thin, and have relatively low tensile strength. Some films would be described as somewhat brittle or “splitty”. These tend to break in manufacture and in use on packaging machines and equipment. The addition of the coating or heat sealable layers to the HDPE layer in the above patents does not produce biaxially oriented films having improved tensile properties that do not split during orientation or subsequent usage. This tendency to break hinders the machinability and processability of the films, resulting in increased manufacturing costs and/or films of inferior quality.
The present invention overcomes shortcomings of the prior art and improves the tensile properties of HDPE films as well as manufacturing capability for making biaxially oriented HDPE films.
SUMMARY OF THE INVENTION
The present invention is directed to a biaxially oriented multilayer film structure having (I) a core substrate of high density polyethylene (HDPE); and (ii) an additional layer of syndiotactic polypropylene on at least one side of the core substrate.
In one preferred embodiment, the film of the present invention includes at least one heat sealable layer on an outer surface of one or both of the additional syndiotactic polypropylene layers. The heat sealable layer can be composed of any conventional polymeric sealant material known in the art such as ethylene vinyl acetate copolymers, ethylene methyl acrylate copolymers, butene copolymers with ethylene, hexene copolymers with ethylene, octene copolymers with ethylene, ethylene acrylic acid copolymers, ethylene methacrylic acid copolymers, hexene-butene copolymers, ionomers, acid modified ethylene vinyl acetate copolymers, anhydride modified ethylene vinyl acetate copolymers, medium density polyethylenes, low density polyethylenes and mixtures thereof.
The core, additional syndiotactic polypropylene layer(s), and heat sealable layers can optionally include conventional additives. Examples of the additives include, but are not limited to, antiblocking agents, antistatic agents, anti-fogging agents and slip agents.
As a result of this invention there is advantageously provided a multilayer film exhibiting improved tensile properties.
The film of the present invention exhibits reduced water vapor transmission (WVTR) as well as other improved barrier properties and dead-fold.
The film of the present invention is heat stable and can advantageously be processed at high temperatures without undergoing degradation.
The film of the present invention also exhibits a high degree of processability, resulting in better quality film and reduced manufacturing costs, and good machinability.
For 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.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a biaxially oriented multilayer film structure having (I) a core substrate of high density polyethylene (HDPE); and (ii) at least one skin layer of syndiotactic polypropylene on at least one side of the core substrate.
The core substrate of the present invention is a relatively thin layer of HDPE. The thickness of the HDPE core substrate is from about 0.2 mils to about 10 mils, preferably from about 0.5 mils to about 3 mils, and more preferably from about 0.7 mils to about 2.5 mils.
The HDPE core substrate exhibits reduced water vapor transmission (WVTR), improved dead-fold, and other properties even when the total film thickness is reduced to less than about 1 mil (0.001 inch). The high density of the HDPE polymer provides improved stiffness in films. Films with a HDPE polymer core substrate may be used in a wide variety of packaging equipment including vertical form, fill and seal (VFFS), horizontal form, fill and seal (HFFS), and high speed horizontal slug wrapping equipment, among other types of equipment.
These packaging machines typically contain mechanical cycles in which the film goes through periods of rapid acceleration and deceleration as the film, progressing through the machine, is started and stopped. The increased MD elongation value of this film allows this film to resist snaps and breaks better than previous films.
The HDPE of the present invention includes polymers made with Ziegler-Natta or Phillips type catalysts, as well as metallocene catalysts. The HDPE of the present invention is a semicrystalline polymer available in a wide range of molecular weight as indicated by either MI or HLMI (melt index or high-load melt index).
The HDPE core substrate of the present invention can be composed exclusively of a single HDPE resin or a mixture of HDPE resins as disclosed in U.S. Pat. No. 4,870,122 issued to Lu, the disclosure of which is incorporated herein in its entirety. Films made with a blend of HDPE resins have shown some benefit in reducing the splittiness of the film, which manifests itself as a tendency of the film to break across the transverse direction (TD) during packaging on vertical, form, fill and seal (VFFS) machines. The blends of the HDPE polymers can include two or more polymers all of which preferably have densities of 0.95 g/cm
3
or greater.
The density of the HDPE polymer is in the range from about 0.94 to about 0.97 g/cm
3
, preferably from about 0.95 to about 0.965 g/cm
3
as defined by ASTM.
The melting point of HDPE polymer of the present invention, measured by a differential scanning calorimeter (DSC), is in a range from about 120 to about 150° C., preferably from about 125 to about 135° C.
The HDPE polymer of the present invention has a melt index of greater than 0.1 to about 10.0, preferably about 0.2 to about 5.0, and most preferably about 0.6 to about 2.0.
The HDPE polymers of the present invention are also commercially available as Marlex TR-130 from Phillips Chemical Company, M6211 from Lyondell Petrochem
Copenheaver Blaine
Exxon Mobil Oil Corporation
James Rick F.
Kruer Kevin R.
Santini Dennis P.
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