Abstract:
The present invention relates to improved metallized laminated structures for making rigid food containers and more particularly to metallized laminated structures for making rigid food containers having a lustrous metallic appearance formed from a laminate comprising a first substrate of thermoformable thermoplastic material; a first metallized layer of vapor deposited tin bonded to the substrate of transparent thermoplastic material; a second metallized layer of vapor deposited aluminum bonded to the first metallized layer of tin; a second substrate of thermoformable thermoplastic material; and a recessed cavity having a food contact surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention is concerned with metallized laminated structures for making rigid food containers and more particularly to metallized laminated structures for making rigid food containers having a lustrous metallic appearance. 
         [0002]    It is often desirable to provide a metallic reflective appearance to plastic articles, such as, rigid plastic food containers for decorative purposes. One approach to achieving this objective is to metallize a sheet or film of thermoplastic material followed by thermoforming the metallized sheet into a desired shape. Metals which impart particular brilliant reflectivity include silver, gold, copper and aluminum. However, silver, gold and copper are very expensive for large-scale manufacturing use and aluminum is relatively brittle making it difficult to thermoform. Unfortunately, the degree to which such aluminum metallized polymeric substrates can be shaped or thermoformed without the metal rupturing and/or separating from the polymer surface is generally limited to forming involving relatively small dimensional changes. The visual effect of rupturing and/or separation of the metal from the polymeric substrate is a noticeable loss of specular reflectance. 
         [0003]    Therefore, there is a need in the art for improved metallized laminate structures for fabricating rigid food containers that exhibit excellent specular reflectance after undergoing substantial dimensional changes. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is concerned with thermoformed metallized food containers formed from a laminate comprising a first substrate of thermoformable thermoplastic material; a first metallized layer of vapor deposited tin bonded to the substrate of transparent thermoplastic material; a second metallized layer of vapor deposited aluminum bonded to the first metallized layer of tin; a second substrate of thermoformable thermoplastic material; and a recessed cavity having a food contact surface. The first substrate of thermoplastic material may be a monolayer sheet or multilayer film. Preferably, the first substrate of thermoformable thermoplastic material is transparent or translucent. The second substrate of thermoformable thermoplastic material may be a monolayer sheet or multilayer film. Preferably, the second substrate of thermoformable thermoplastic is black and/or opaque. The present invention may further comprise a layer of adhesive being in direct contact with the second metallized layer of aluminum and the second substrate of thermoformable thermoplastic material. 
         [0005]    The present invention also provides an end-product of a metallized rigid food container having a food product sealed within a metallized laminate which has been formed into a recessed cavity and covered by a lidding film. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  depicts one embodiment of a thermoformed container of the present invention. 
           [0007]      FIG. 2  illustrates an enlarged cross-sectional view of one embodiment of a metallized laminate of the present invention having a monolayer first substrate and a monolayer second substrate. 
           [0008]      FIG. 3  illustrates an enlarged cross-sectional view of another embodiment of a metallized laminate of the present invention having a monolayer first substrate and a monolayer second substrate. 
           [0009]      FIG. 4  illustrates an enlarged cross-sectional view of another embodiment of a metallized laminate of the present invention having a monolayer first substrate and a monolayer second substrate where both the first and the second substrates are metallized. 
           [0010]      FIG. 5  illustrates an enlarged cross-sectional view of an embodiment of a metallized laminate of the present invention having a multilayer first substrate and a monolayer second substrate. 
           [0011]      FIG. 6  illustrates an enlarged cross-sectional view of an embodiment of a metallized laminate of the present invention having a monolayer first substrate and a multilayer second substrate. 
           [0012]      FIG. 7  illustrates an enlarged cross-sectional view of the metallized laminate of  FIG. 3  that has been thermoformed into a three-dimensional shape. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring now more particularly to  FIGS. 1-2  of the drawings, a preferred embodiment of container  10  embodying a laminate  100  of the present invention is shown. The container  10  is shaped or otherwise molded to form a recessed cavity  30  having at least one sidewall  31  and a bottom portion  32 . Container  10  may be formed from a laminate  100  which comprises a first thermoformable thermoplastic substrate  11  and a second thermoformable thermoplastic substrate  12 . For example, the first and second thermoplastic substrates may each include a material which is normally solid at room temperature and can be readily shaped or formed into a desired contour such as a bowl, cup and the like. Such thermoformable thermoplastic materials may include monolayer sheets and multilayer films formed from homopolymers or copolymers of polyethylene terephthalate, polyolefins including, but not limited to, polyethylenes and polypropylenes, polystyrenes, polyvinyl chlorides, cyclic olefins and blends thereof. In one preferred embodiment, first thermoplastic material  11  is a transparent monolayer sheet or transparent multilayer film of thermoformable polyethylene terephthalate, more preferably, a transparent monolayer sheet or multilayer film of oriented thermoformable polyethylene terephthalate, and most preferably, a transparent monolayer sheet or transparent multilayer film of biaxially oriented thermoformable polyethylene terephthalate. The thickness of the first thermoplastic material  11  may vary depending upon the desired end-use. Typically, the thickness of a monolayer of biaxially oriented polyethylene terephthalate film  11  is between about 48 gauge and about 150 gauge (about 12.2 micron and about 38.1 micron) and preferably, between about 50 gauge and 100 gauge (about 12.7 micron and 25.4 micron) and more preferably, about 75 gauge (about 19 micron). An example of a suitable commercially available thermoformable oriented polyethylene terephthalate film is 75 gauge Skyrol® SKC SL80C supplied by SKC, Inc. (Covington, Ga.). First thermoplastic material  11  forms a first outer surface of the container  10 . 
         [0014]    Laminate  100  of container  10  further comprises a first metallized layer of tin  21  which may be prepared by applying tin to the first layer of thermoplastic material  11  by any conventional metallization technique known to those skilled in the art. An especially preferred metallization technique in the practice of this invention is vacuum deposition wherein the metal is vacuum evaporated and then deposited onto the polymer layer as described by William Goldie in  Metallic Coating of Plastics , Vol. 1, Electrochemical Publications Limited, Chap. 12 (1968), the disclosure of which is incorporated herein by reference in its entirety. The first metallized layer of tin  21  may be preferably deposited by vapor deposition techniques, typically by applying the molten metal under vacuum by such techniques as electron beam evaporation, sputtering, induction heating, or thermal evaporation. A particularly preferred technique for metallization of tin is by electron beam vacuum evaporation deposition methods. 
         [0015]    The tin is deposited at a layer thickness that forms an initial continuous metal film. Later, after laminate  100  is formed into its final shape, first metallized layer of tin  21  may be distributed more evenly over first substrate of thermoplastic material  11 . The thickness of the tin layer should be in a range such that it provides good adhesion to the substrate and has good mechanical strength. Thickness of a metal layer is most easily specified by reference to the optical density measured immediately after coating of the metal onto the substrate. The amount of tin applied to the first substrate of thermoplastic material  11  should adjusted to achieve an optical density of between about 1.0 and about 4.0 higher than the optical density of the first substrate of thermoplastic material  11 . First metallized layer of tin  21  may be formed as an uninterrupted continuous layer of metal or a non-continuous layer metallic islands extending over first substrate of transparent thermoplastic material  11 . In order to promote better adhesion between first substrate of thermoplastic material  11  and first metallized layer of tin  21 , it is also contemplated that the surface of first substrate of thermoplastic material  11  may be corona treated before the metallization process occurs. In a preferred embodiment of the present invention, first metallized layer of tin  21  is formed by electron beam vacuum deposition of tin onto a corona-treated surface of a 75 gauge (about 19 micron) thermoformable oriented polyethylene terephthalate film. 
         [0016]    Without being bound to a particular theory, it is believed that the use of tin as the first metallized layer  21  in combination with conventional thermoforming processes operating at a temperature range of between 343° C. and 371° C. (650° F. and 700° F.) provides a synergistic result. Because tin has a melting point of about 232° C. (about 449° F.), it is molten during the thermoforming process and becomes more evenly distributed over the surface of first thermoplastic substrate  11 . The improved distribution of the tin provides a more lustrous metallic appearance to the formed container  10 . 
         [0017]    Following the formation of the tin layer, a second metallized layer of aluminum  22  is applied over the first metallized layer of tin  21  by the same or different metallization technique used to create the tin layer. In a preferred embodiment, the second metallized layer of aluminum  22  is provided by thermal vacuum deposition of aluminum onto a first metallized layer of tin  21 . Second metallized layer of aluminum  22  may be applied as a continuous metal film over the tin, but after the forming process, the aluminum becomes discontinuous islands of metal covering the tin. The total amount of aluminum applied to the tin is adjusted to achieve a cumulative optical density of between 2.0 and about 4.0 higher than the optical density of the first layer of thermoplastic material  11 . In one embodiment, the optical density of the first metallized layer of tin  21  is 1.0 higher than the optical density of the first substrate of thermoplastic material  11  and the optical density of the second metallized layer of aluminum  22  is also 1.0 to achieve a cumulative optical density of 2.0 higher than the optical density of the first substrate of thermoplastic material  11 . In another embodiment, the optical density of the first metallized layer of tin  21  is 2.0 higher than the optical density of the first substrate of thermoplastic material  11  and the optical density of the second metallized layer of aluminum  22  is 1.0 to achieve a cumulative optical density of 3.0 higher than the optical density of the first substrate of thermoplastic material  11 . In a preferred embodiment, the optical density of the first metallized layer of tin  21  is 3.0 higher than the optical density of the first substrate of thermoplastic material  11  and the optical density of the second metallized layer of aluminum  22  is 1.0 to achieve a cumulative optical density of 4.0 higher than the optical density of the first substrate of thermoplastic material  11 . 
         [0018]    Those skilled in the art will recognize that a typical vacuum deposited layer of aluminum having an optical density of 1.0 will be approximately equivalent to a thickness of about 364 Angstrom. In a preferred embodiment, the second metallized layer of aluminum  22  is provided by thermal vacuum deposition of aluminum onto a first metallized layer of tin  21  which was formed by electron beam vacuum deposition of tin onto a corona-treated surface of a 75 gauge (about 19 micron) thermoformable oriented polyethylene terephthalate film. Second metallized layer of aluminum  22  may be formed as a continuous layer or a non-continuous layer extending over first metallized layer of tin  21 . 
         [0019]    As depicted in  FIG. 2 , laminate  100  of container  10  still further comprises a second substrate of thermoformable thermoplastic substrate  12  which is applied over the second metallized layer of aluminum  22 . As previously mentioned, second substrate  12  may be formed from any thermoformable thermoplastic material which may include a monolayer sheet or multilayer film. In one embodiment, substrate  12  is a sheet comprising homopolymers or copolymers of polyethylene terephthalate, polyolefins including, but not limited to, polyethylenes and polypropylenes, polystyrenes, polyvinyl chlorides, cyclic olefins and blends thereof. In one preferred embodiment, second thermoplastic substrate  12  comprises a monolayer sheet or multilayer film of thermoformable polyethylene terephthalate In another preferred embodiment, second thermoplastic substrate  12  comprises a monolayer sheet or multilayer film of thermoformable polystyrene. In still another preferred embodiment, second thermoplastic substrate  12  is a black or opaque monolayer sheet or multilayer film of thermoformable polyethylene terephthalate, and more preferably, a black or opaque monolayer sheet or multilayer film of amorphous thermoformable polyethylene terephthalate. In an alternative preferred embodiment, second thermoplastic substrate  12  is a black or opaque monolayer sheet or multilayer film of thermoformable high-impact polystyrene. The thickness of the second thermoplastic substrate  12  may also vary depending upon the desired end-use. Typically, the thickness of second thermoplastic substrate  12  is between 5 and 25 mil (127 and 635 micron) and preferably, between 10 and 20 mil (254 and 508 micron), more preferably. between 12 and 15 mil (304.8 and 381 micron), and most preferably about 13.25 mil (336.6 micron). It is understood that in order to provide a black color and/or opaque appearance, the polymer or polymer blends used to form second thermoplastic substrate  12  may include dyes, fillers, pigments, carbon black and the like. An example of a suitable commercially available amorphous thermoformable polyethylene terephthalate is Performance™ PET 1708HS supplied by StarPet, Inc. (Asheboro, N.C.). A commercially available example of high-impact polystyrene is Polystyrene 825E supplied by Total Petrochemicals USA, Inc. (La Porte, Tex.). 
         [0020]    Referring now to  FIG. 3 , there is illustrated an alternative embodiment of laminate  200  which may be used to form container  10 . Laminate  200  comprises the same or similar monolayer films described above for first thermoplastic substrate  11 , first metallized layer of tin  21 , second metallized layer of aluminum  22 , and second thermoplastic substrate  12  for laminate  100 , but further includes an adhesive layer  13  which is in direct contact with the second metallized layer of aluminum  22  and the second layer of thermoplastic substrate  12 . An example of a suitable commercially available adhesive for use as adhesive layer  13  is SERFENE™ 2010 which is a polyvinylidene chloride (PVDC) emulsion adhesive system supplied by Dow Chemical Company, Inc. (Midland, Mich.). 
         [0021]      FIG. 5  illustrates another preferred embodiment of a metallized laminate  300  of the present invention having a multilayer first thermoformable thermoplastic substrate  11  and a monolayer second thermoformable thermoplastic substrate  12 . Multilayer thermoformable thermoplastic substrate  11  may be formed as a coextruded blown film comprising two or more film layers. In this particular embodiment, substrate  11  is an eleven-layer film structure having a first film layer  11   a,  a second film layer  11   b,  a third film layer  11   c,  a fourth film layer  11   d,  a fifth film layer  11   e,  a sixth film layer  11   f,  and a seventh film layer  11   g.  First film layer  11   a  may comprise polyethylene, preferably a blend of ultra-low density polyethylene and linear low density polyethylene and more preferably, a blend of about 87.7% by weight ultra-low density polyethylene. about 10% by weight linear low density polyethylene and about 2.3% by weight processing and antiblock additives. An example of a suitable commercially available ultra-low density polyethylene is ATTANE™ 4701G supplied by Dow Chemical Company (Midland, Mich.). A commercially available example of linear low density polyethylene is ExxonMobil™ 1001.32 supplied by ExxonMobil Chemical Company (Houston, Tex.). Film layers  11   b  and  11   f  serve as a tie layer bonding layers ha to  11   c,  and  11   e  to  11   g,  respectively. These layers may each comprise anhydride-modified polyethylene. Preferably, layers  11   b  and  11   f  each comprise a blend of anhydride-modified polyethylene and polyethylene, more preferably, a blend of anhydride-modified polyethylene and ultra-low density polyethylene, and most preferably, a blend of between 10% and 70% by weight anhydride-modified polyethylene and between 30% and 90% by weight ultra-low density polyethylene. Examples of suitable commercially available anhydride-modified polyethylene include Plexar® PX 3308 supplied by Equistar Chemicals, LP (Houston, Tex.) and Bynel® 41E710 supplied by E. I. du Pont de Nemours and Company (Wilmington, Del.). Film layers  11   c  and  11   e  serve as oxygen barrier layers and may each comprise polyamide, preferably, a blend of polyamides, more preferably, a blend of amorphous polyamide and crystalline polyamide, and most preferably, a blend of about 15% by weight amorphous polyamide and about 85% by weight of crystalline polyamide. A suitable commercially available example of amorphous polyamide is Grivory G21 supplied by EMS-Chemie (North America) Inc. (Sumter, S.C.). An example of a suitable commercially available crystalline polyamide is Ultramid® B36 01 supplied by BASF Polyamides (Freeport, Tex.). 
         [0022]    Substrate  11  of laminate  300  may further include a third oxygen barrier layer  11   d  positioned between layers  11   c  and  11   e.  Layer  11   d  may comprise an ethylene vinyl alcohol copolymer. The final film layer of substrate  11  is layer  11   g  which serves as an outer abuse layer and may comprise polyethylene terephthalate. An example of a suitable commercially available polyethylene terephthalate is Performance™ PET 1708 supplied by StarPet Inc. (Asheboro, N.C.). In this embodiment, metallized laminate  300  further includes a first metallized layer of tin  21  deposited on a corona-treated surface of film layer  11   a,  a second metallized layer of aluminum  22  applied to tin layer  21 , an adhesive layer  13  which is in direct contact with the second metallized layer of aluminum  22  and the second layer of thermoplastic substrate  12 . A suitable adhesive material for use as adhesive layer  13  is a polyvinylidene chloride (PVDC) emulsion adhesive system such as SERFENE™ 2010 supplied by Dow Chemical Company, Inc. (Midland, Mich.). Second substrate  12  comprises a sheet of high-impact polystyrene having a thickness approximately 20 mil (508 micron). 
         [0023]    Turning now to  FIG. 6 , there is illustrated still another preferred embodiment of a metallized laminate  400  of the present invention having a monolayer first thermoformable thermoplastic substrate  11  and a multilayer second thermoformable thermoplastic substrate  12 . Laminate  400  further includes a first metallized layer of tin  21  deposited on a corona-treated surface of substrate  11 , a second metallized layer of aluminum  22  applied to tin layer  21 , an adhesive layer  13  which is in direct contact with the second metallized layer of aluminum  22  and substrate  12 . Substrate  11  may comprise a transparent thermoformable film of biaxially oriented polyethylene terephthalate. An example of a suitable commercially available thermoformable oriented polyethylene terephthalate film is 75 gauge Skyrol® SKC SL80C supplied by SKC, Inc. (Covington, Ga.). Adhesive layer  13  may be any suitable material which bonds comprise a polyvinylidene chloride (PVDC) emulsion adhesive system such as SERFENE™ 2010 supplied by Dow Chemical Company, Inc. (Midland, Mich.). As depicted, substrate  12  includes a first film layer  12   a,  a second film layer  12   b,  a third film layer  12   c,  a fourth film layer  12   d,  a fifth film layer  12   e,  a sixth film layer  12   f,  and a seventh film layer  12   g.  First film layer  12   a  comprises a 279.4 micron (11 mil) thick high-impact polystyrene such as Polystyrene 825E supplied by Total Petrochemicals USA, Inc. (La Porte, Tex.). Film layer  12   b  may comprise any adhesive which adheres polystyrenes to polyolefins such as polyethylene. One such adhesive is Avadyne® AV5210/CA500-83 supplied by Henkel Corporation (Gulch Mills, Pa.). Film layer  12   c  includes polyethylene, preferably, a blend of ultra-low density polyethylene and linear low density polyethylene. In one preferred embodiment, layer  12   c  includes a blend of about 64% by weight ultra-low density polyethylene and about 35% by weight linear low density polyethylene. Film layers  12   d  and  12   f  serve as tie layers bonding layers  12   c  to  12   e,  and  12   e  to  12   g,  respectively, and may each comprise anhydride-modified polyethylene. A suitable material for use in layers  12   d  and  12   f  includes, but is not limited to, LyondellBasell Plexar® PX3747 supplied by Lyondell Chemical Company (Houston, Tex.). Layer  12   e  functions as an oxygen barrier and preferably includes ethylene vinyl alcohol copolymer, The seventh film layer  12   g  is a sealant layer capable of heat sealing to another material having a similar chemical composition such as, for example, a lidding film placed over recessed cavity  30 . Film layer  12   g  may comprise a polyolefin, including ethylene homopolymers and copolymers such as polyethylene copolymer and ethylene vinyl acetate copolymer. In a preferred embodiment, layer  12   g  comprises low density ethylene vinyl acetate copolymer having 5% vinyl acetate content. One such commercially available example is LyondellBasell Petrothene® NA442051 supplied by Lyondell Chemical Company (Houston, Tex.). 
         [0024]    In  FIG. 7 , there is illustrated thermoformed laminate  200  having a cup or bowl shaped recessed cavity  30 . As depicted, the first substrate  11  serves as a first outer surface  33  of recessed cavity  30 , while second substrate  12  serves as a second outer surface  34 . In the forming operation, the laminate is shaped in a manner such that at least a portion of the laminate undergoes an extension or stretching in at least one dimension in order to form a recessed cavity  30 . Any conventional forming process may be used for shaping the laminates of the present invention into its final form. Exemplary forming processes include differential air pressure thermoforming, vacuum forming, plug assist-vacuum forming, cold rolling, draw forming, impact forming, hydroforming and the like. An especially preferred conventional thermoforming process is one where the plastic laminate is formed at a temperature of between about 343° C. and about 371° C. (about 650° F. and about 700° F.), In one embodiment, the laminates of the present invention are thermoformed to provide a recessed cavity having a draw depth of at least 0.635 centimeter (0.25 inch). In another embodiment, the laminates of the present invention are thermoformed to provide a recessed cavity having a draw depth of at least 1.91 centimeter (0.75 inch). 
         [0025]    Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments disclosed to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.