Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Weight per unit area specified
Reexamination Certificate
1999-09-08
2001-05-01
Weisberger, Richard (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Weight per unit area specified
C428S341000, C428S344000, C428S354000, C428S322700, C428S339000
Reexamination Certificate
active
06224973
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a multi-layer sheet suitable as sealable sheet comprising a tie-layer between a substrate layer and a seal layer, particularly a peel-seal layer intended to be heat-sealed at the edge of a container. Such sealable sheets are specially used as lids for closing foodstuff containers or blisters.
DESCRIPTION OF RELATED ART
Sealable lids as well as peelable lids comprising a substrate layer, one side of which is coated with a seal or a peel-seal layer, are known in the art of packaging, especially for hermetically closing drug packages such as PP blisters or foodstuff containers as disclosed for example in GB 1,536,428. The seal or peel-seal layer of these peelable lids is sealed at the edge of the container either by flat sealing or, particularly for the peel-seal layer, by groove sealing. The substrate layer is either Al, oPET, or glue impregnated paper or board.
Two different kind of peelable layers exist, cohesive peel-seal layers and adhesive peel-seal layers.
The seal and adhesive peel-seal layers may comprise substantially similar polymers. However in the case of a cohesive peel-seal layer, some compounds or fillers are added to the polymer to decrease its breaking strength.
At the present time, most seal or adhesive peel-seal layers are formed by very thin lacquer layers having a thickness of only few &mgr;m, typically below 20 &mgr;m, preferably below 10 &mgr;m. These thin seal or peel-seal layers are very suitable in the field of drugs and foodstuffs packaging. However the drawback of lacquer layers results from the organic solvent which has to be eliminated in-line after applying lacquer onto a substrate layer. The elimination of organic solvents in a dryer is time consuming, so that with high production speeds, such an in-line coating process requires very large plants. Higher production speeds is a general trend observed in the packaging industry, so that the elimination of solvent involves longer dryers in order to limit the amount of residual solvent in the seal or peel-seal layer below legal requirements. Moreover, the recovery of organic solvent is also expensive.
It has already been proposed by U.S. Pat. No. 5,145,737 to obtain a lid having an Al substrate layer, a bonding layer and a sealing layer, by co-extruding the bonding and sealing layer on one side of the Al substrate layer. This technique makes it possible to assemble all three layers in one operation, and eliminates the need for using during manufacture, adhesives and/or lacquers and, hence organic solvents. The bonding layer is a layer of polypropylene, of 2 to about 20 &mgr;m thickness, on which maleic acid has been grafted and the sealing layer is a thick layer of polypropylene of 20 to about 100 &mgr;m thick, containing between 15 to 50% by weight of inert filler. The cover and container are joined by deep sealing so that the action of pulling the cover off the container causes internal breaking of the sealing layer containing the inert filler.
Usually the drawability of the bonding maleic acid grafted polypropylene layer as such would not be able to form a layer as thin as 2 to 20 &mgr;m by extrusion. It is only owing to the coextrusion with a substantially thicker sealing layer that it has been possible to form such a thin bonding layer. Such a peelable layer is not at all similar to that obtained by the wet lacquer coating process.
Another drawback is encountered with some extruded adhesive peel-seal layers of the prior art. When peeling the peel-seal lid off the container, filaments may be formed from the polymer of the peel-seal layer. In the case of a seal sheet for closing e.g. drug packages, the opening is obtained by breaking the sheet which has a breaking strength lower than that of the seal. It is also required in this case that no filaments be formed when breaking such a closure sheet.
Some compounds of polymeric materials of the prior art with thick coating, suitable to form an adhesive peel-seal layer by extrusion may be soluble in foodstuffs such as oil, so that an amount of these compounds, higher than the legal requirements, migrates into the packaged foodstuffs.
SUMMARY OF THE INVENTION
The present invention overcomes at least partially the above-mentioned drawbacks. Specifically a multi-layer sheet suitable as sealable or peelable sheet according to the present invention is defined as comprising a tie-layer between a substrate layer and a seal or peel-seal layer wherein the tie-layer comprises a polyolefin with acid or anhydride, grafted or copolymerised, said tie-layer has a thickness equivalent to 1-7 g/m
2
, the seal or peel-seal layer comprises a polyolefin, and wherein the total thickness of the combination of tie-layer and the seal or peel-seal layer is equivalent to 3-20 g/m
2
, preferably to 6-14 g/m
2
and said combination has a drawability limit of 12 g/m
2
preferably 8 g/m
2
or less at a line speed of 300 m/min.
Contrary to the above-mentioned sealing cover according to U.S. Pat. No. 5,145,737, in which a thick sealing layer allows the drawing of a substantially thinner bonding layer, according to one preferred embodiment of the present invention, the tie-layer has a thickness equivalent to 1-7 g/m
2
and surprisingly defines the drawability limit of both thin layers having a total thickness equivalent to 3-20 g/m
2
, preferably to 6-14 g/m
2
. Moreover the drawability limit of this tie-layer is 8 preferably 6 g/m
2
or less at a line speed of 300 m/min and typically allows line speeds as high as 200 to 600 m/min.
According to the invention, the substrate layer comprises at least one of aluminum foil, biaxially oriented polypropylene, polyamide or polyester film.
Preferably, the ethylene copolymer in the tie-layer is selected from ethylene acrylic acid or methacrylic acid copolymer or an ionomer thereof.
According to a more specific embodiment, the tie-layer is neutralized by zinc ionomer.
The invention also covers a tie-layer comprising polyolefin with maleic acid or maleic anhydride grafted or copolymerized.
The polyolefin forming the seal layer advantageously comprises an ethylene copolymer of at least one of vinylacetate, methylacrylate, ethylacrylate and butylacrylate.
According to another aspect of the invention, the peel-seal layer comprises a polypropylene or polyethylene or ethylene copolymer of at least one of vinylacetate, methylacrylate, ethylacrylate, butyl-acrylate, acrylic acid, methacrylic acid or ionomer thereof, and polybutylene.
Alternatively, the peel-seal layer comprises a polypropylene, a polyethylene or ethylene copolymer of at least one of vinylacetate, methylacrylate, ethylacrylate and butylacrylate, and between 5 and 30 weight % of filler.
Most preferably, according to another aspect of the invention the multilayer-sheet is prepared by extrusion or coextrusion coating.
REFERENCES:
patent: 4656068 (1987-04-01), Raines
patent: 5145737 (1992-09-01), Boiron et al.
patent: 2628071 (1989-09-01), None
patent: 1 536 428 (1975-03-01), None
E. I. Du Pont de Nemours and Company
Weisberger Richard
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