Stock material or miscellaneous articles – Hollow or container type article – Nonself-supporting tubular film or bag
Reexamination Certificate
1997-08-11
2001-03-20
Robinson, Ellis (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Nonself-supporting tubular film or bag
C383S116000, C383S207000, C426S122000, C426S127000, C426S415000, C428S035300, C428S035400, C428S043000, C428S192000, C428S195100, C428S516000
Reexamination Certificate
active
06203867
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to product packaging and in particular to a plastic material for forming a good product package having a predictable line of failure when the package is opened to prevent tearing of the bag down the body of the bag. This predictable failure path is provided through a coextruded lamination manufacturing process involving specific resins or blends or resins coextruded in three or more layers that do not inhibit processing speed, efficiency, and economy of materials used to provide this reliable openability.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Laminated films produced for making food packages are based on multiple layers of plastic film each with a specific purpose. An outer web is designed to move across a packaging machine, another web provides heat stability to prevent distortion when the package is sealed, still another provides the ability to obtain a uniform print surface, and yet another provides the ability for the laminated film to be adhered to an adhesive used to bond the laminated film to an adjacent film. When more than two film webs are used to form a laminated film, these core film webs provide additional specific properties such as moisture vapor transfer rate, gas barrier, and appearance. This part of the lamination has nothing to do with the openability of the lamination in package form. The inner web of a lamination can contribute barrier qualities, optics, and sealability, and yet provide openability that varies from “impossible” to “easy open”.
Thus, a prior art bag so constructed is illustrated in
FIG. 1
wherein the bag
10
is shown filled with product and sealed. It has a longitudinal seal
12
and end seals
14
and
16
. The bag is generally made as indicated in
FIG. 2
by forming essentially a cylindrical tube
18
with a longitudinal seal
12
and with a horizontal seal at the lower end thereof along seal
16
as shown in FIG.
3
. The package can then be filled with product when constructed as shown in FIG.
3
and sealed as illustrated in
FIG. 4
to form a bag filled with product having upper and lower sealed ends
14
and
16
, respectively, and a longitudinal side seal
12
.
The problem with these bags is illustrated in FIG.
5
. When the bag is to be opened and forces are applied in the direction of arrows
24
and
26
to the upper seal
14
, and bag may start to open at
26
and then tear downwardly into the side of the bag as illustrated at
28
. This is not a serious problem for small bags of food products because the contents of the small bag can be eaten entirely. However, with large bags containing large amounts of food products, the tear
28
prevents the bag from being reclosed to protect the contents therein; thus the contents must be transferred to another container that can be sealed to protect the freshness of the product. The reason that such bags tear as shown in
FIG. 5
is because of their construction and the interlaminar strength at each interface of the lamination. A history and description of such bag construction is helpful.
COATED FILMS
Prior to extrusion lamination with a thermoplastic adhesive, adhesives were, and still are, based on a single or two-part adhesive which “cures” to a hard bond that is very difficult to separate. With this type of adhesive, openability of a package had to be built into the inner film. The inner film's primary purpose is sealability in order to provide integrity to the package. Openability was based on either a coating on the inner film that acted as the sealant or a coextrusion film or a thin sealant ply that separated from the inner film when the package was opened. While these methods did provide “openability”, it was inconsistent and varied with the temperature of the sealing jaws, sealant ply thickness when using a coextrusion film, and with the adhesion of the coating to the inner film due to primer or treatment variations. In addition, there was a narrow hot tack (ability to keep end seals together at the hot conditions necessary to obtain a seal) and seal range with coextruded inner films. The same problems occurred with coated film if the film treatment and/or primer weight/drying was too low.
OPP FILMS
Because of cost competition, the use of coextruded oriented polypropylene films (OPP) began to replace coated films. These new films, which were and are typically two and three-layer coextrusions of homopolymer polypropylene as a core and copolymer/terpolymer skins, did not, and do not, provide consistent openability of packages without tearing of the package when it is opened. As the need for larger packages with multiple usage has expanded, the failure of packages based on laminations with oriented polypropylene coextrusions films as the inner web became unacceptable. Therefore, a solution was needed to provide reliable openability of these larger packages along a predetermined path without tearing of the package down the side when it is opened.
With the change from coated sealant films or a dissimilar thermoplastic skin such as Surlyn® coextruded with polypropylene that has some degree of openability due to built-in fracture or delamination lines to a coextruded polypropylene film that has poor openability, tearing the package is a serious problem. The increase in large packages with multiple use needs has made tearing of the package when opening unacceptable.
PROBLEMS TO OVERCOME
A process to produce a usable (opening without tearing) extrusion based lamination between an inner sealant film and an adjacent outer film in two-ply laminations, or the core film in a three-ply lamination must provide negligible loss in processing efficiency and yield loss and also enable the use of cost effective resin components to be commercially feasible. For example, if an extrusion process typically runs at 1200 ft/min, it is impractical to use a similar extrusion process that, while making a usable product, runs at 300 ft/min. In addition, the ability to produce product that has a flat profile across the web cannot be compromised by a product that meets end use requirements but has poor profile and results in poor machine performance at the end user. Also, extrusion laminating of polypropylene films (typically one web is printed) requires polypropylene films with surface treatment and/or modification with other resins to provide a surface to which polyethylene can obtain adhesion. Polyethylene, when extruded, has a non-polar surface unless it is extruded at a sufficiently high temperature and is exposed sufficiently to air through what is called an air gap (gap between the extrusion die lips and the extrusion nip) in order to provide sites which can be bonded to similar sites on polypropylene film surfaces. In addition, primers are typically used to provide adhesive to the inks on the inner surface of one of the films to be laminated. If any of the polyethylene extrusion parameters are not at a specific level, poor adhesion of the lamination will result. This manifests itself as a delamination at one or both of the film interfaces. Because of the critical nature of the polyethylene extrusion process and the speed at which the lamination process occurs (typically 1000-1500 ft/min) the bond strength at the inner surfaces of the polypropylene films must be strong and flexible to obtain a level of adhesion that provides usable handling during distribution of filled packages.
Because of the need for reliable bond strength between the polypropylene films in a polyethylene lamination and the seal strengths of coextruded polypropylene films, it is difficult for a lamination using a polypropylene sealant film to provide consistent, reliable openability within the film layers or interfaces of the lamination.
Thus, consider the lamination of a prior art film as shown in FIG.
6
. The lamination consists of outer polypropylene film layer
30
with treated surface
31
, ink layer
32
, primer layer
34
, if needed, polyethylene extrudable adhesive layer
Derkach William J.
Hawkins Robert F.
Kinsman Bruce
Jones, Day, Beavis & Pogue
Nolan Sandra M.
Recot Inc.
Robinson Ellis
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