Coating processes – With stretching or tensioning – Running lengths
Utility Patent
1998-12-23
2001-01-02
Ball, Michael W. (Department: 1733)
Coating processes
With stretching or tensioning
Running lengths
C427S358000, C427S412300, C264S173140, C264S173150, C264S173190, C264S290200, C156S244230, C156S244240, C428S515000, C428S516000, C428S910000
Utility Patent
active
06168826
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods of preparing polymer films. Specifically, the present invention relates to methods of preparing biaxially oriented polyethylene films with improved optics and sealability properties.
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, Pa. (1992).
An optional part of the film-making process is a procedure known as “orientation.” The “orientation” of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of “orientation” 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 and toughness. 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 (T
g
) but below its crystalline melting point (T
m
), 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. Further, as a general rule, higher orientation correlates with films having improved gloss and haze characteristics in the absence of cavitation.
When a film has been stretched in a single direction (monoaxial orientation), the resulting film exhibits great strength and stiffness along the direction of stretch, but it is weak in the other direction, i.e., across the stretch, often splitting or tearing when flexed or pulled. To overcome this limitation, two-way or biaxial orientation is employed to more evenly distribute the strength qualities of the film in two directions. These biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces, leading to their greater utility in packaging applications.
Most biaxial orientation processes use apparatus that stretches the film sequentially, first in one direction and then in the other. Tenter frame orienting apparatus stretches the film first in the direction of the film travel, i.e., in the longitudinal or “machine direction” (MD), and then in the direction perpendicular to the machine direction, i.e., the lateral or “transverse direction” (TD).
The degree to which a film can be oriented is dependent upon the polymer from which it is made. Polypropylene, polyethylene terephthalate (PET), and nylon are highly crystalline polymers that are readily heat stabilized to form dimensionally stable films. These films are well known to be capable of being biaxially stretched to many times the dimensions in which they are originally cast (e.g., 5× by 8× or more for polypropylene).
High density polyethylene (HDPE), however, exhibits even higher crystallinity (e.g., about 80-95%) relative to polypropylene (e.g., about 70%). As a result, HDPE films are generally more difficult to biaxially orient than polypropylene films. U.S. Pat. Nos. 4,870,122 and 4,916,025 and U.S. application Ser. Nos. 08/715,546 and 08/940,261 describe imbalanced biaxially oriented HDPE-containing films that are MD oriented up to about two times, and TD oriented at least six times. This method produces a film that tears relatively easily in the transverse direction. Multi-layer films prepared according to this method are also disclosed in U.S. Pat. Nos. 5,302,442, 5,500,283 and 5,527,608, which are incorporated herein by reference.
The film-making process can also include extrusion coating a film to impart superior characteristics to the film and methods of extrusion coating are well known in the art. Most known methods provide for extrusion coating a film after it has been biaxially oriented. However, the gloss and haze characteristics as well as the sealability properties of the films prepared according to these known methods can be improved.
Accordingly, it is one of the purposes of this invention, among others, to provide biaxially oriented polyethylene films with improved optics and sealability properties, by providing economical and relatively uncomplicated methods of making polyethylene films that impart superior characteristics to the films, without requirement for chemical additives such as cross-linking agents, and without requirement for supplemental processing steps such as irradiation of the film.
SUMMARY OF THE INVENTION
It has now been discovered that these and other purposes can be achieved by the present invention, which provides for methods of preparing biaxially oriented polyethylene films with improved optics and sealability properties.
The methods of the present invention provide for stretching in the machine direction a multi-layer base sheet having a core layer with a first and a second side, at least one skin layer and outer surfaces; then extrusion coating at least one of the outer surfaces of the base sheet with a resin selected from low density polyethylene (LDPE), very low density polyethylene (VLDPE), linear low density polyethylene (LLDPE) and blends thereof; and then stretching the base sheet in a direction transverse to the machine direction whereby a biaxially oriented polyethylene film having improved optics and sealability properties is prepared.
The core layer of the base sheet includes a polyethylene and preferably, the polyethylene is a high density polyethylene (HDPE). The thickness of the core layer prior to film preparation is preferably from about 0.25 mil (25 gauge) to about 3.0 mil (300 gauge) (1 mil=0.001 inch=100 gauge).
The skin layer of the base sheet is coextensive with the core layer and the skin layer includes HDPE, LDPE or a copolymer of polypropylene and ethylene. The thickness of the skin layer prior to film preparation is preferably from about 0.01 mil to about 0.15 mil.
As stated above, the methods of the present invention provide for first stretching a multi-layer base sheet in the machine direction. Preferably, the base sheet is stretched in the machine direction to a degree of from 5:1 to about 8:1. Also as stated above, at least one of the outer surfaces of the base sheet is coated with a resin after orienting the base sheet in the machine direction. Preferably, the amount of resin provided should be an amount sufficient to yield a film with 3 to 10 wt % of the film in
Poirer Robert V.
Su Tien-Kuei
Ball Michael W.
Mobil Oil Corporation
Santini Dennis P.
Simmons T. Dean
Tolin Michael A
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