Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-07-21
2002-12-31
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of metal
C156S244110, C428S520000, C428S522000, C525S221000
Reexamination Certificate
active
06500556
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an adhesive, particularly an extrudable adhesive, for bonding a polar substrate particularly a metal foil such as aluminum foil to a non-polar substrate particularly a polymer film such as polyethylene film.
2. Description of Related Art
Plastic films, particularly polyethylene films, laminated to, coated on, or co-extruded with metal foil, particularly aluminum foil, have found utility in packaging and other applications such as cable shielding. The plastic film and the metal foil may be combined with other materials to form a structure having a plurality of layers, each layer having a specific purpose. Packaging laminates, for example, often comprise a plurality of layers. Such a packaging laminate might have a configurationally rigid core layer of paper or paperboard, an outer liquid-tight layer of polyethylene and an oxygen gas barrier mid-layer of aluminum foil and, optionally, other layers depending on application needs.
To form an effective laminate, it is important that good bonding strength or intimate integrity between the layers be achieved for most applications. For materials that do not normally adhere well to each other, an adhesive binder or tie layer is used to bond adjacent layers together.
Low density polyethylene (LDPE) has become the mainstay of the foil extrusion coating industry. It is low in cost, processes well, and has adequate normal adhesion to metal foils for many applications. For demanding applications, where durability of bond strength in the presence of aggressive products, good heat seal characteristics, and excellent food adhesion are desired, LDPE alone has limitations.
Ethylene &agr;,&bgr; ethylenically-unsaturated carboxylic acid copolymers, particularly copolymers of ethylene and acrylic or methacrylic acid (E/AA and E/MAA), optionally partially neutralized, (available from the DuPont Company under the tradenames, “Nucrel”, “Bynel”, and “Surlyn”) bond particularly well to metal foils. The acid groups on such an ethylene acid copolymer resin (ACR) provide bonding sites to the basic oxides on the foil surface. By increasing the acid level, adhesion to the foil is increased. The ethylene acid copolymers, however, are more costly than the LDPE and don't bond well with LDPE particularly with increasing acid levels.
There thus remains a need for an adhesive tie layer that will intimately bond to polyethylene, particularly LDPE, and to metal foil. There is a need to form a laminate that is less costly than a foil/ethylene acid copolymer laminate and that exhibits a superior durability of bond strength in the presence of aggressive products.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to an adhesive, particularly an extrudable adhesive that has improved adhesion to both polar substrates such as metal foil and to non-polar substrates such as polyethylene. It particularly relates to an adhesive that produces a bond between metal foil and polyethylene that is durable in the presence of aggressive products such as ketchup, flavoring oils, juices and the like.
The adhesive of the present invention is an ethylene acid copolymer (ACR) blend consisting essentially of a high-acid, high-melt index (MI) acid copolymer blended with an acid copolymer that has both a lower acid level and a lower MI than the high-acid copolymer
The present invention also relates to a laminate comprising a polar substrate such as a metal foil and a non-polar substrate such as polyethylene film bonded to each other with a tie layer of the adhesive of the present invention, and to the use of such a laminate in packaging and cable shielding applications.
DETAILED DESCRIPTION
As summarized above, the present invention relates to an adhesive, particularly an extrudable adhesive, to a laminate comprising a polar substrate, particularly a metal foil such as aluminum foil, and non-polar substrate, particularly a polyethylene film bonded by a tie-layer of the adhesive, and to uses of the laminate.
The laminates of the present invention have a plurality of layers, at least one of which being a non-polar substrate being bonded to at least one layer that is a polar substrate by a tie layer of adhesive film.
The thickness of the laminates of the present invention depends on the application and the substrates used. Typical packaging laminates preferably have a thickness of about 0.1 mil (2.5 micrometers) to about 10 mil (250 micrometers), more preferably of about 0.5 mil (12.5 micrometers) to about 2 mil (50 micrometers). Cable shield laminates would be substantially thicker. A paper substrate typically will be relatively thick (about 1 to about 10 mil (25 to 250 micrometers), films will be relatively thin (about 0.2 to about 2 mil (5 to 50 micrometers), and foils will typically be thinner still (about 0.2 to 2 mil (5 to 50 micrometers).
The non-polar substrate preferably is a polyethylene film. The polyethylene film can be selected from low density polyethylene (LDPE), linear low density polyethylene (LLDPE), or high density polyethylene (HDPE). It may be made by any available process known for making such polyethylene including high pressure gas, low pressure gas, solution and slurry processes employing conventional Ziegler-Natta, metallocene, and late transition metal complex catalyst systems.
The preferred polar substrate is a metal foil, more preferably aluminum foil. Other substrates include metalized substrates such as metalized polypropylene, paper, polyester, nylon, and ethylene vinyl alcohol. The polar substrate may be surface treated by means known in the art such as corona treatment to improve the adhesion, but such treatment is not required in the present invention and preferably is not used in the case of foils and metalized substrates.
Film substrates may be primed with various primers known in the art, particularly PEI and polyurethane primers, before applying the adhesive tie layer of the present invention.
The adhesive employed in the tie layer is a blend of ethylene-&agr;,&bgr; ethylenically-unsaturated C
3
-C
8
carboxylic acid copolymers (ACR's). It particularly is a blend comprising about 5 to about 95, preferably about 15 to about 85, and more preferably about 15 to about 40, parts by weight High-Acid, High-MI ACR and about 5 to about 95, preferably about 15 to about 85, and more preferably about 60 to about 85, parts by weight Low-Acid, Low-MI ACR. Preferably, the Low-Acid, Low-MI ACR is the major component and the High-Acid, High-MI ACR is the minor component. More than two ACR's may be included so long as at least one is a High-Acid, High-MI ACR and one is a Low-Acid, Low-MI ACR and the other copolymers do not compatibilize the High-Acid, High-MI and Low-Acid, Low-MI ACR's in a way that prevents the High-Acid, High-MI ACR from moving to the surface away from a non-polar substrate when the adhesive is co-extruded with the non-polar substrate.
Both the High-Acid, High-MI ACR and the Low-Acid, Low-MI ACR (as well as other ACR's that may be in the blend) are ethylene-&agr;,&bgr; ethylenically-unsaturated C
3
-C
8
carboxylic acid copolymers, optionally having at least one softening co-monomer that is co-polymerizable with ethylene. Acrylic and methacrylic acids are preferred acid co-monomers. The softening co-monomer can be an alkyl acrylate selected from the group consisting of n-propyl-, n-butyl-,i-butyl-, n-octyl-, 2-ethylhexyl-. and 2-methoxyethyl-acrylates. The preferred alkyl acrylates are n-butyl-, i-butyl-, 2-ethylhexyl-, and 2-methoxyethyl-acrylates. The softening co-monomer can also be an alkyl vinyl ether selected from the group consisting of n-butyl, n-hexyl, 2-ethylhexyl-, and 2-methoxyethyl-vinyl ether. The preferred alkyl vinyl ethers are n-butyl vinyl ether and n-hexyl vinyl ether.
Preferably the High-Acid, High-MI ACR and the Low-Acid, Low-MI ACR are not neutralized. Either or both may be partially neutralized, however, so long as the resulting un-neutralized acid levels and the melt indices of the resulting partially neutraliz
Morris Barry Alan
Trouilhet Yves
E. I. du Pont de Nemours and Company
Evans Craig H.
Nakarani D. S.
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