High density polyethylene film with high biaxial orientation

Details High density polyethylene film with high biaxial orientation High density polyethylene film with high biaxial orientation

1996-10-03

2004-02-10

Rickman, Holly (Department: 1773)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C428S500000, C428S515000, C428S516000, C428S304400, C428S307300, C428S308400, C428S910000, C264S901000, C264S902000, C264S903000, C264S299000

Reexamination Certificate

active

06689857

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to methods for preparing polymer films. Specifically, the invention relates to methods of biaxially orienting high density polyethylene films and the films prepared according to such methods.
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, 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 (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.
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 into fibers (fibrillating) when flexed or pulled. To overcome this limitation, two-way or biaxial orientation is employed to more evenly distribute the strengthalities of the film in two directions, in which the crystallites are sheetlike rather than fibrillar. 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.
From a practical perspective, it is possible, but technically and mechanically quite difficult, to biaxially orient films by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes use apparatus which stretches the film sequentially, first in one direction and then in the other. Again for practical reasons, typical 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 also dependent upon the polymer from which it is made. Polypropylene, as well as polyethylene terephthalate (PET), and nylon, are polymers which are highly crystalline and are readily heat stabilized to form dimensionally stable films. These films are well known to be capable of being 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) exhibits even higher crystallinity (e.g., about 80-95%) relative to polypropylene (e.g., about 70%), and HDPE-containing films are generally more difficult to orient biaxially than polypropylene films. U.S. Pat. Nos. 4,870,122 and 4,916,025 describe imbalanced biaxially oriented HDPE-containing films which are oriented up to about two times in the machine direction, and six times or more in the transverse direction. 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, the disclosures of which are incorporated herein by reference in their entireties.
British Patent No. 1,287,527 describes high density polyethylene films which are biaxially oriented in a balanced fashion to a degree of greater than 6.5 times in both the longitudinal dimension (i.e., MD) and the lateral dimension (i.e., TD). This method requires a specific range of orientation temperatures.
U.S. Pat. Nos. 4,891,173 and 5,006,378 each disclose methods for preparing HDPE films which requires cross-linking the film, with optional biaxial orientation of the cross-linked film. It is reported that the cross-linking process, which requires irradiation of the film, improves the film's physical properties. Other cross-linking processes, such as chemically-induced cross-linking, can have similar effects.
U.S. Pat. No. 4,680,207 relates to imbalanced biaxially oriented films of linear low density polyethylene (LLDPE) oriented by being stretched up to 6-fold in the machine direction, and up to 3-fold in the transverse direction but less than in the machine direction.
U.S. Pat. No, 5,241,030 describes biaxially oriented films of a blend of at least 75% of a linear ethylene/alpha-olefin copolymer, but no more than 25% HDPE. The film can be mono- or multi-layered, and can be biaxially oriented, i.e., stretched up to 8:1 in the machine direction, and up to 9:1 in the transverse direction.
U.S. Pat. No. 5,302,327 describes an anti-fogging, heat-sealable polypropylene film. The film includes a polypropylene core and a heat sealable layer of HDPE or ethylene copolymer. These bilayer films can be machine stretched up to 7×MD, coated or corona-treated to improve wettability, and then stretched up to 10×TD.
Blown films of HDPE having a ethylene-vinyl acetate heat seal coating used for food packaging but such films must have a thickness of about two mils to meet the water vapor transmission rate (WVTR) requirements for packaging suitable for dry foods such as cereals. Moreover, blown HDPE films do not exhibit the dead-fold properties desirable in food packages, particularly of the bag-in-box type.
In view of the above considerations, it is clear that existing methods for producing biaxially oriented HDPE films yield products which are deficient in desirable physical characteristics. Existing HDPE film-making methods generally require additional chemical components in the HDPE resin (e.g., cross-linking agents) and/or additional process steps (e.g., irradiation). Such limitations not only complicate production, but generally result in increased costs. Moreover, cross-linking tends to lower polymer crystallinity, resulting in higher W

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