The present invention relates to multilayered laminate films. More specifically, the present invention relates to a multilayered laminate with controlled tearability useful for forming packages or pouches.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages including melt film formation, quenching and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pennsylvania (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength.
Biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions. Biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces and tearing, leading to their greater utility in most packaging applications.
Flexible laminate packs are, however, being increasingly used for applications where a pack needs to be opened and reclosed. A current problem is that when tearing the packaging material the tearing is difficult and will propagate tear randomly in an uncontrolled way. This is particularly important when reclosure devices such as zippers, low tack adhesives and press devices, are employed. Frequently scissors and other cutting devices need to be used. Various methods have been proposed to resolve this problem including perforation, scoring and modifications to the closure device to direct the tear.
Films or laminates which can tear in a controlled direction are highly desirable for so called easy-open pouches or stand-up-pouches when a reclose device is included in the pouch. Polyethylene films or laminates are used extensively for such end uses, primarily for cost reasons. However, it appears that most proposed polyethylene films, laminates or coextrusions have, heretofore, been developed for impact and tear resistance rather than for controlled directional tearing.
Examples of such proposed tear resistant films or laminates include the lamination of two monoaxially oriented thermoplastic films, with the directions of orientation essentially at right angles to each other; a thermoplastic laminate for use as a stretch-wrap film consisting of at least two layers, one of which is a low density polyethylene and the other is a linear low density polyethylene; a laminate suitable for making heavy duty bags comprising low density polyethylene films blocked together at 80xc2x0-100xc2x0 C. and below the fusion temperature of the polyethylene, which is indicated as exhibiting greater impact and tear resistance than laminates of similar film formed by a fusion lamination process; a tear resistant laminate film comprising a) a crosslinked core layer consisting of a linear low density polyethylene and b) two crosslinked surface layers, each consisting of a blend of linear low density polyethylene, linear medium density polyethylene and ethylene/vinyl acetate copolymer, where the film has a high degree of orientation in the longitudinal and transverse directions; and a rupture resistant bag capable of being boiled which is made from an oriented multiple layer film consisting of a first layer of nylon 6, a second layer containing a blend of 50-100% linear low density polyethylene and 50-0% low density polyethylene and a third adhesive polymer layer disposed between the first and second layers which contains a substantial fraction of linear low density polyethylene.
Polymeric films or laminates, and packages made therefrom, which tear easily have also been proposed. Most of the proposed so called easy-tear films or laminates, however, generally disclose a film or laminate which can be torn easily in a particular direction.
Reissued Patent Re. 30,726 to Otten et al. discloses a film containing a blend or mixture of polyethylene and an ionomer resin which is blow extruded and stretched in the direction of extrusion. The film is indicated as having excellent linear tear properties in the direction of extrusion.
U.S. Pat. No. 5,091,241 to Lang et al. discloses a multilayer laminate consisting of a machine direction oriented linear low density polyethylene film and a sealant film made from a polyethylene resin, adhesively laminated to at least one side of the oriented film. The film, or pouches made therefrom, is indicated as being transversely tearable.
U.S. Pat. No. 5,308,668 to Tsuji discloses a multilayer film in which at least one layer contains a mixture of two or more isomeric resins which have poor compatibility with respect to each other and a large difference in melting point. The film is indicated as having an easy-to-tear characteristic in a certain direction, where the isomeric resins form numberless phases in the direction of tear.
U.S. Pat. No. 5,786,050 to Otsuka et al. discloses a multilayer laminated film pouch, wherein the film is composed of an innermost film containing linear low density polyethylene, an intermediate film made of an oriented polyolefin and an outermost film made of biaxially oriented nylon and/or PET. The pouch is indicated as having an easy opening characteristic.
It would be highly desirable to have a film with controlled tearing in the machine direction (MD) and/or the transverse direction (TD) that will allow easy and controlled pack opening. The use of such a film is particularly advantageous for stand-up-pouches (SUP) when a reclose devise is used.
According to the present invention, a laminate having controlled directional tear characteristics suitable for use in packages or pouches is provided.
More specifically, the present invention relates to providing a controlled directional tear laminate which includes:
(a) a biaxially oriented inner layer containing LLDPE and having a tensile elongation to break of less than 200% in the MD and less than 150% in the TD;
(b) an outer layer having a heat distortion temperature of at least 10xc2x0 C. higher than the inner layer and a tensile elongation to break of less than 200% in the MD and less than 150% in the TD; and
(c) an adhesive layer containing an adhesive polymer, the adhesive layer being between the inner and outer layers and adhering the inner and outer layers to each other.
In one embodiment the biaxially oriented inner layer can be a multilayer film itself having a metallizable skin layer, a sealable skin layer and a core layer, the core layer being between the metallizable skin layer and the sealable skin layer. In such an embodiment, the metallizable skin layer can be selected from an ethylene-propylene copolymer, an ethylene-propylene-butene terpolymer, a butylene-propylene copolymer, an ethylene xcex1-olefin copolymer or mixtures thereof. The sealable skin layer can be selected from a metallocene polyethylene plastomer, a metallocene LLDPE, a Ziegler Natta catalyzed plastomer, VLDPE, ULDPE and mixtures thereof. The core layer will contain LLDPE and can also contain at least one ethylene xcex1-olefin copolymer having a density from 0.91 to 0.94 g/cm3, a C3 to C12 xcex1-olefin comonomer and up to about 20 wt % of an additive selected from a propylene-ethylene copolymer, a propylene-butylene copolymer, an ethylene-propylene-butene terpolymer, polypropylene and mixtures thereof.
In another embodiment, the inner layer multilayer film can also contain a first intermediate (or tie) layer between the metallizable skin layer and the core layer and a second intermediate (or tie) layer between the sealable skin layer and the core layer. In such an embodiment, the intermediate (or tie) layers can contain a blend of 50 to 100 weight % of a material or materials useful for the core layer and up to 50 weight % of a material or materials useful for the metallizable skin layer.
In yet another embodiment, the present invention includes an inner layer which is oriented from about 1.2 to 6 times in the MD and from about 6 to 10 times in the TD.
In another aspect, the outer layer is selected from paper, foil, polyester, polyamide, polypropylene, HDPE or a lamination of these materials and the adhesive layer is selected from polyurethane, polyethylene or an ethylene copolymer.
The present invention provides laminates having the advantage of controlled tearing in the machine direction (MD) and/or the transverse direction (TD) that will allow easy and controlled pack opening. Such laminates are advantageously used in connection with pouches or other packages having a reclose device.
Additional objects, advantages and novel features of the invention will be set forth in part in the description and examples which follow, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particulary pointed out in the appended claims.
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