Stock material or miscellaneous articles – Hollow or container type article – Paper containing
Reexamination Certificate
1998-02-12
2002-03-19
Dye, Rena L. (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Paper containing
C428S034300, C428S035900, C428S036600, C428S036700, C428S511000, C428S513000, C428S537500, C428S702000, C229S005810, C229S005840, C229S406000
Reexamination Certificate
active
06358576
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improved barrier coatings, to methods for coating substrates used in manufacturing food containers and to food containers having coatings which limit certain adverse affects of oxygen and/or moisture on the contents.
BACKGROUND
Various coatings have been applied to paperboard substrates to provide composite materials that may be used for various purposes. In particular, barrier coatings have been applied to paperboard substrates used to make food containers in order to reduce the transport of moisture and oxygen into the food that may spoil or contaminate the food or otherwise disaffect its flavor or shelf life.
Accordingly, for food container applications, paper and paperboard substrates are conventionally coated with barrier coatings selected from various polymers such as polyethylene which provides a moisture barrier and ethylene-vinyl alcohol copolymer (EVOH) to address flavor or property-affecting atmospheric changes in the containers. Such coatings can reduce transmission of moisture and oxygen through the packaging material and provide for longer shelf life for the food packaged in the container. However, some oxygen sensitive foods, such as orange juice, tomato products and fresh meat still have only a relatively short life of a few weeks despite the barrier coatings. The short shelf-life is due, at least in part, to oxidation caused by the oxygen left in the headspace during the package filling step and/or from leakage through the container or molecular diffusion through the barrier coatings on the container.
Methods have been devised for reducing the oxygen transported through food containers in an effort to increase the shelf life of the contents. One such method is the use of clay which has been recognized as an effective material to reduce oxygen and/or moisture transport through a packaging container. Conventionally, the clay in an aqueous solution or aqueous suspension is applied to the container using a water soluble hydrophilic binder or an aqueous latex binder. After applying the clay coating, an aqueous adhesive coating may be applied to the clay-coating and a barrier layer may be applied to the adhesive coating layer. The resulting clay coating is comprised of essentially two discontinuous phases, one phase containing clay and one phase containing the polymeric binder. The other polymeric layers applied over the clay coating are likewise discrete phases of material.
A description of a conventional clay coating process is found in U.S. Pat. No. 5,153,061 to Cavagna et al. which describes a barrier coating that may be applied to the inner or outer surface of a paperboard substrate in order to absorb a contaminate emitted by the substrate or provide a tortuous path so that the migration of the contaminate through the barrier layer is reduced. The material used for absorbing contaminants is activated carbon and the ingredient for providing a tortuous path is a delaminated clay pigment. In order to provide a tortuous path layer, the pigment and activated carbon are dispersed in a water soluble binder such as polyvinyl alcohol and applied to the substrate, and a polyethylene terephthalate layer is then coated over the tortuous path layer. While the coatings of Cavagna et al. are said to be effective in reducing the transport of undesirable substances into the food, the process for applying multiple coatings raises the expense of production and complicates the manufacturing process.
Another disadvantage of multiple coating steps is that the clay-coated substrate must be heated to remove water therefrom prior to applying additional layers to the clay coating. Such heating is expensive and reduces the product throughput rate.
Accordingly, it is an object of the invention to provide a relatively low cost composition for producing food containers.
Another object of the invention is to provide a polymer composition for coating paper and paperboard substrates which exhibits improved oxygen and/or water vapor barrier properties.
Another object of the invention is to reduce the manufacturing costs of food container products having improved oxygen and water vapor barriers properties.
Still another object of the invention is to provide an improved barrier layer for a paperboard food container which can be applied to a paperboard product by mono or co-extrusion coating or lamination.
Another object of the invention is to provide an improved polymeric material for use as a food container which can be blown, cast or extruded into a container shape.
Yet another object of the invention is to provide a method for applying an oxygen and water vapor barrier to a paperboard product for use as a food container.
Another object of the invention is to provide a method for applying a clay/polymer composition to a paperboard product for use in making food containers.
SUMMARY OF THE INVENTION
With regard to the above and other objects, the present invention provides a layered composite comprising a paper or paperboard substrate having opposed surfaces and a barrier layer applied adjacent at least one of the surfaces of the substrate, the barrier layer comprising a composition containing from about 1 to about 80% by weight clay homogeneously dispersed in from about 20 to about 99% by weight of a continuous phase comprising a thermoplastic material selected from the group consisting of polyamides, thermoplastic polyesters, polyolefins, nitrile polymers, thermoplastic polyurethanes and copolymers of ethylene and vinyl alcohol and, optionally, a coupling agent, compatibilizer or dispersion aid. In order to provide a continuous polymeric phase containing clay, it is preferred that the polymeric material be substantially water insoluble. By “substantially water insoluble”means a solubility of the polymer of less than 1 milligram per liter of distilled water having a pH of 6.5 to 7.5 at a temperature of 25° C. according to ASTM test method E 1148.
In another aspect, the invention provides a method for making a multi-layer composite containing at least one paperboard layer having opposed first and second surfaces which comprises dispersing from about 1 to about 80% by weight, preferably from about 10 to about 60% by weight and most preferably from about 30 to about 50% by weight particulate clay in from about 20 to about 99% by weight thermoplastic polymer selected from the group consisting of polyamides, thermoplastic polyesters, polyolefins, nitrile polymers, polyurethanes and copolymers of ethylene and vinyl alcohol and, optionally, a coupling agent, dispersion aid and/or compatibilizer under conditions sufficient to provide an essentially dry, continuous polymer phase with clay homogeneously dispersed therein without significantly degrading the polymer. The polymer phase containing clay is then extruded as a layer adjacent the first surface of the paperboard layer at rate of from about 500 to about 2000 linear feet per minute to obtain a coating weight of from about 2 to about60 pounds per 3000 square feet of substrate and thereby provide a clay/polymer barrier layer having a thickness ranging from about 1 to about 100 microns, preferably from about 2 to about 50 microns, and most preferably from about 3 to about 8 microns adjacent said first surface of said paperboard layer.
An advantage of the invention is that barrier layers composed of the clay/polymer composition exhibit significantly lower water vapor transport and oxygen transmission properties than the same polymeric material without the clay. Furthermore, the layer may be applied to the paperboard substrate in a single processing step rather than in multiple coating steps and without the need to dry the clay/polymer layer applied to the substrate.
The clay/polymer composition has the added advantage of providing relatively greater stiffness to a polymeric structure made from the composition or to a paperboard container coated with the composition as compared to the same structure or container without the clay filled polymeric material. When applied as a barrier layer coating to
Adur Ashok M.
Shih Keith S.
Volpe Raymond A.
Dye Rena L.
International Paper Company
Luedeka Neely & Graham P.C.
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