Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
1999-08-26
2004-08-10
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S515000, C428S517000, C428S518000, C428S519000, C428S520000, C428S521000, C428S522000, C428S523000, C428S910000, C525S240000
Reexamination Certificate
active
06773820
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to thermoplastic C
2
-&agr;-olefin copolymer resin blends and flexible films thereof having heat sealing and puncture resistance properties. Such blends are useful for making films, particularly heat shrinkable, oriented films for packaging food and non-food articles, especially fresh or frozen foods such as meat, poultry or cheese.
Manufacturers and wholesalers utilize flexible thermoplastic packaging films to provide economical, sanitary containers which help protect and/or preserve the freshness and wholesomeness of their products. These films are often sold in bag form. For example, a single or multilayer thermoplastic film may be made into bags by a packaging manufacturer using film stock comprising a tubular film or one or more flat sheets or webs of film by well known processes involving e.g. cutting, folding and/or sealing the film to form bags which may then be shipped to processors for use in packaging operations. These films and bags may be printed with customer logos, product data or other information and may also be uniaxially or biaxially oriented, heat shrinkable, irradiated, or may contain film layers which are abuse resistant or puncture resistant or which are crosslinked or which enhance or retard or prevent transmission of light, gases, or liquids therethrough. Frequently, multilayer films having one or more barrier layers to oxygen and/or moisture such as: saran (a polyvinylidene chloride copolymer); a modified saran e.g. containing methyl acrylate polymer units; ethylene vinyl alcohol copolymer; nylon; or acrylonitrile may be used with a heat sealing layer such as a copolymer of ethylene and vinyl acetate (EVA) to produce bags for packaging oxygen and/or moisture sensitive foods e.g. fresh red meat. Such bags help preserve meat in its original condition by preventing or reducing moisture loss and chemical changes in the meat structure due to oxidation reactions. A typical packaging bag produced from a tubular film stock will have one or two sides which have been heat sealed by the bag manufacturer in the bag forming process. For food packaging, bags often will have one open side to allow a food processor to insert ham, turkey, chicken, cheese, primal or subprimal meat cuts, ground beef, fruits, vegetables, bread or other food products into the bag. The food processor then makes a final seal thereby enclosing the bag. This final seal may follow gas evacuation of the bag by vacuum means or replacement of the gaseous environment within the bag by a particular gas or mixture of gases which may be inert or reactive with the enclosed product to provide some advantage such as to assist product preservation. This final seal is frequently a heat seal similar to the initial seals produced by the bag manufacturer, although the actual heat sealing equipment may vary.
Thus, commercially available bags are made by transversely sealing a tubular stock of either monolayer or multilayer film and cutting off the tube portion containing the sealed end, or by making two spaced apart transverse seals on a tubular stock and cutting open the side of the tube, or by superimposing flat sheets of film and sealing on three sides, or by end folding flat sheets and sealing two sides.
Generally, heat sealing of thermoplastic film is accomplished by applying sufficient heat and pressure to adjacent film layer surfaces for a sufficient time to cause a fusion bond between the layers.
A common type of seal used in manufacturing bags is known to those skilled in the art as a hot bar seal. In making a hot bar seal, adjacent thermoplastic layers are held together by opposing bars, of which at least one is heated to cause the adjacent thermoplastic layers to fusion bond by application of heat and pressure across the area to be sealed. For example, bags may be manufactured from a tube stock by making one hot bar seal transverse to the tube. This seal may also be referred to as a bottom seal. Once the bottom seal is applied, the tube stock may be transversely cut to form the mouth of the bag.
The strength of seals of heat shrinkable bags may be measured by determining the time it takes for a seal to fail under certain conditions. For example, the seal may be immersed in hot water at an elevated temperature such as 95° C., then the hot water seal strength (“HWSS”) may be measured by a test such as that described as the “restrained shrinkage-seal strength test” in Funderburk et al U.S. Pat. No. 3,900,635, issued Aug. 19, 1975, which patent is hereby incorporated by reference.
Once a product such as meat or poultry is inserted into the bag, the package is typically evacuated and the bag mouth sealed. At one time, the standard method for sealing a bag was to fasten a clip around the mouth of the bag. However, heat sealing techniques are now also commonly employed to produce the final closure of the bag. For example, a bag mouth may be hot bar sealed, or it may be sealed by another common type of heat seal known as an impulse seal. An impulse seal is made by application of heat and pressure using opposing bars similar to the hot bar seal, except that at least one of these bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name “impulse”), to cause the adjacent film layers to fusion bond. Following the impulse of heat, the bars are cooled (e.g. by circulating coolant) while continuing to hold the bag inner surfaces together to achieve adequate sealing strength.
Generally, impulse seals may be made faster than hot bar seals, because of the quick cool down of the impulse ribbon following the heat impulse. Impulse seals are also generally narrower than hot bar seals, which leads to an improved package appearance, but narrower seals also leave less margin for error in the production of continuous sealed edges. Since typically less area is bonded in an impulse seal relative to a hot bar seal, the performance of the sealing layer of the thermoplastic film is more critical.
One problem which occurs during impulse heat sealing of known films is that the film in the seal area often becomes extruded during sealing. This results in thinning of the film in the seal area, and therefore reduces the strength of the film at the seal; or, in extreme situations, allows the thinned film to be too easily severed or pulled apart. Those skilled in the art refer to severely extruded seals as “burn through” seals. Thus, a “burn through” seal does not have adequate strength or integrity to seal in or protect the packaged product. One attempt to solve this “burn through” problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a multilayer film made from cross-linkable polymer resins causes the various irradiated resin layers in the film to crosslink. Under controlled conditions, crosslinking by irradiation raises, and may also broaden, the temperature range for heat sealing; and, depending upon the film composition, may also enhance puncture resistance of the film. If the heat sealing layer of the thermoplastic film is crosslinked too heavily, the highly crosslinked layer is more difficult to melt or fusion bond which makes it difficult to achieve strong seals, particularly by impulse sealing the bag mouths after filling with meat or poultry. All of the bag seals (including both those made by the bag manufacturers and those made by the food processor; and those made by whatever means, including either hot bar seals, impulse seals, or both) must maintain their integrity to preserve and protect the enclosed food product.
There must be a strong continuous seal to prevent unwanted egress and ingress of gaseous, liquid or solid materials between the bag exterior and interior. This is particularly necessary when the package is made of heat shrinkable film and is to be immersed in hot water to shrink the film against the packaged article, since such shrinkage increases the stress on these seals. Thus, there is a continuing need for monolayer and multilayer films which can be made into bags having
Georgelos Paul Nick
Wilhoit Darrel Loel
Curwood Inc.
Nakarani D. S.
Richeson Cedric M.
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