CROSS-REFERENCE TO RELATED APPLICATIONS
This invention is a continuation-in-part of Ser. No. 10/016,544 which is related to U.S. patent application entitled “Tack-on Pressure Films for Temporary Surface Protection and Surface Modification” (Ser. No. 10/016,541; 3M Attorney Docket No. 56946US002), co-assigned to 3M Innovative Properties Company.
- BACKGROUND OF THE INVENTION
This invention relates to single or multilayer oriented film structures and processes for forming these structures.
Oriented film structures, particularly biaxially oriented film structures, are widely used for various purposes. They are thin and strong and generally used widely as tape backings for packaging applications.
U.S. Pat. No. 6,063,482 discloses that a problem with oriented films is that they have low tear resistance. Biaxially oriented film, if nicked, can easily tear with little force making the film difficult to use and handle for many applications. This patent addresses the problems of low tear resistance by using a very specific polymer, namely a specific isotactic polypropylene. However, a more general approach is desirable which is applicable to all resins, which resins may provide other advantages, such as barrier properties, low cost, gloss or any other advantages inherent in the oriented polymer or blend.
U.S. Pat. Nos. 6,514,597 and 6,022,612 describes a further issue with oriented films in that they have high gloss. Matte finish oriented films are desirable for aesthetics or the like. Matte finishes have been obtained by using coextruded multilayer films and/or specific blend or surface treatments such as flame treatments or embossing. Alternative, more versatile methods of obtaining matte type surfaces on oriented films are continuously being sought out in the art.
- SUMMARY OF THE INVENTION
It is desirable to develop more versatile methods of making tear resistant oriented films and/or matte finish film structures, and to provide oriented film structures suitable for a variety of applications.
One aspect of the present invention provides a method of forming an oriented film structure having controllable surface contact properties. This method comprises providing a film structure which has first and second major surfaces, partial scoring or cutting of at least one surface such that the top portion of the surface defines a plurality of separable surface elements and separating the separable surface elements across the surface of the film structure. This method forms surface elements across this first major surface of the film structure thereby creating spacings between adjacent separated surface elements, of an oriented film forming a base layer. In a preferred embodiment, both major surfaces of the film structure are partially scored or cut, preferably at angles to each other to create surface elements on both faces of the film which surface elements preferably overlap. Preferably, the surface elements are substantially continuous and overlap at an angle of from 10 to 170 degrees, preferably 45 to 135 degrees and are generally characterized by a height above the base layer of from 5 to 50 micrometers and have a width of from 100 to 1000 micrometers, preferably 100 to 500 micrometers.
If the cutting step includes cutting in more than one direction, this cutting results in discrete surface elements if the cuts on one surface are continuous and intersecting. However, if done on both faces of the film structure, overlapping surface elements preferably are formed. The surface elements prior to stretching have a width of 1000 microns or less, preferably 500 microns or less.
BRIEF DESCRIPTION OF THE DRAWINGS
In another embodiment of the above method, the stretching step includes biaxially stretching the film structure. In another embodiment of the above method, the stretching step includes simultaneously biaxially stretching the film structure. Another embodiment of the present invention provides a film structure formed by the above method.
The present invention will be further explained with reference to the appended Figures, wherein like structure is referred to by like numerals throughout the several views, and wherein:
FIG. 1 is a perspective top view of a first embodiment film structure of the invention having separable surface elements.
FIG. 2 is a bottom view of a first embodiment film structure of the invention having separable surface elements.
FIG. 3 is a side view of a first embodiment film structure after it has been stretched to form surface elements.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 4 is a side perspective view of a first embodiment film structure after it has been stretched to form surface elements.
The present invention concerns methods for making oriented film structures having an oriented base film layer and a plurality of surface structures on at least one major surface of the film structure creating controllable surface contact properties, topographies, surface finishes, ornamental appearances, strength and tear properties. The film structure has on at least one major surface predetermined surface elements which are formed by scoring or cutting the continuous film creating separable surface elements. The scored or cut film with predetermined separable surface elements is then stretched to separate the plurality of predetermined separable surface elements creating spacing, recesses or lands between the separated surface elements of an oriented film such that the resultant film assembly has different surface contact properties and film properties. The separable surface elements have different orientation properties than the spacings between them after stretching.
The stretching process generates surface features that have a controllable surface contact property. The plurality of separable surface elements are separated by stretching the film structure to achieve a desired topographical pattern. This technique makes the surface feature of the final film product predictable and easy to control. The film structure may be stretched equally along two mutually perpendicular directions (i.e., biaxial stretching) to separate the surface elements in the plane of the film. The film assembly may be stretched along two, or more than two directions, and to unequal extents in either direction, depending on the specific performance desired in the final film structure. When stretched in more than one direction, stretching in different directions may be carried out either simultaneously or sequentially. Furthermore, the film assembly may be stretched with interspersed operations. For example, the film may be stretched in one or more directions, then treated with a desirable treatment (such as heating, annealing or simply waiting), and then stretched again either in the same direction or in a different direction. In essence, any manner of stretching may be used as long as it helps to create a desirable separation of the separable surface elements as described herein. Generally, a stretch ratio of at least 1:1.05 is desired. In this disclosure, a stretch ratio of 1:X represents an amount of stretching in a certain direction where the final film length in that direction is “X” times its original length in the same direction.
The inventive method of stretching a film assembly having separable surface elements has several advantages. The stretching process according to the inventive method can be implemented in-line with conventional film-making equipment, and can therefore be accomplished in an integrated process and offers thin film capability. Since thin webs or films are usually difficult to manufacture (such as by casting, for example), it is more efficient to first form a thick web, then stretch the thick web and attenuate it to a desired final film thickness. Using the technique in accordance with the present invention, films that are less than 2 mil (0.0508 mm) thick, can be made. It is further possible to make films that are less than 0.5 mil (0.0127 mm) thick.
Another additional advantage of incorporating the invention process in-line with film lines is lower cost of production. The film manufacturing lines as used according to the present disclosure can be substantially faster than typical web or film casting and forming operations. Furthermore, film manufacturing lines in this disclosure can produce wider output rolls than most cast film processes.
In addition, biaxial or like film stretching may be carried out using standard film production equipment such as a conventional cast-tentered process. Cast-tentered films may be made sequentially (i.e., stretching in the machine direction followed by transverse stretching in a tenter), or simultaneously (i.e., using a simultaneous tenter). Either mechanical or electromechanical tenters may be employed towards this end.
Various techniques known in the art, such as solvent casting, lamination, or coextrusion, can be used to form multilayer constructions.
A textured film of the invention having separated surface elements and oriented film recesses can be used, for example, as a wrap that allows for good air bleed through the recesses. The film, either a single layer or multiple layer precursor web, can be scored or cut to an appropriate depth for desired textures. With either a single or multilayer film, a top portion containing separable surface elements is formed by scoring or cutting a top layer of the film. By scored, it is meant any line of weakness or separation.
FIGS. 1 and 2 show a perspective view of an embodiment of film structures 1 prior to stretching. The film structure 1 has a first dimension (width ‘W), a second dimension (length—as illustrated by “L” in FIG. 1) and a third dimension (thickness—as illustrated by “T” in FIG. 1) wherein the first and the second dimensions are preferably much greater than the third dimension. Either the first or second dimension could be an indefinite continuous extension. The film structure 1 has a stretchable base layer 6.
As shown in FIGS. 1 and 2, a layer of a film structure 5 and 15 is scored or cut through from the top and bottom to form scores or cuts 2 and 12, preferably in a series of parallel lines. The scores or cuts can be along first and second dimensions on only one surface, so that a layer on one surface is scored or cut into a grid of four-sided segments such as squares, diamonds, rectangles, parallelograms or rhombuses, each segment being mechanically isolated from its neighbors. This process creates a film having a matte appearance. Each segment therefore constitutes a separable surface element. There is no requirement for any particular manner or shape of scoring or cutting as long as the cutting generates desired separable surface elements 4 and 14, although different cutting mechanisms may have different efficiency or productivity. A blade cutter was used in the examples described herein, but any conventional method such as laser ablation or embossing may be used to sever the film layer into separable surface elements. Furthermore, there is no requirement for any particular shape or relative size of the separable surface elements 4 and 14 as long as the final film structure (stretched film) has the desired surface contact properties or other desired properties.
In a preferred embodiment as shown in FIGS. 1 and 2, the film structure 1 is scored or cut in a series of parallel lines 2 in one dimension or direction on a first surface of the film structure and a second series of parallel lines 12 in a second direction on a second surface of the film structure. Parallel lines can be linear or nonlinear. The directions are preferably at angles to each other so that they intersect or overlap. This intersection or overlap can be at an angle of 10 to 170 degrees; however, is preferably 45 to 135 degrees. The separable elements 4 and 14 when separated form surface elements 24 and 34 arranged in lines that reduce the surface contact of the film and increase the bulk tear resistance of the film and if an opposite surfaces of the film preferably overlap. on each surface of the film that preferably overlap. The size of the surface elements 24 and 34 formed depends on the spacing of the score lines and the degree and direction of orientation or tentering. Generally, the separated surface elements in this embodiment are substantially continuous in a predetermined direction or dimension and have a width of from 100 to 1000 micrometers, preferably from 100 to 500 micrometers where the separated surface elements comprise from about 10 to 90 percentage of the surface area of the stretched film structure, preferably 25 to 50 percent. The height of the surface elements h depends on the depth of the scoring or cutting as well as the degree of tentering or orientation. Preferably, the surface elements are 5 to 25 micrometers high with the oriented film base layer 26 thickness “T” between the separable elements being 10 to 50 micrometers thick. The film is generally stretched at an angle to the first and/or second direction of the score lines, of from about 10 to 80 degrees.
Variations of the scoring or cutting of the film layer 1 may be used by one skilled in the art. For example, cutting may be performed using a variety of schemes. Instead of using a cutter as described above, alternate cutting or surface weakening schemes such as a water-jet, laser-beam, rotary-die, or an embossing roll may be used. In general, water-jets and laser-beams may result in a wider cut swath than a cutter. Further, water-jets and laser-beams are best suited when the cutting direction is along the machine direction. One advantage with a laser beam is that intricate patterns such as waves, squiggles, predefined contours, etc. can be accomplished by programming the path into the laser scanning device. Alternatively, if overlapping lines of weakness are desired one of these lines of weakness could be created by alternative methods such as profile extruding grooves, embossing, etching or the like.
The film structure 20, when stretched constitutes an oriented thermoplastic polymer film and is prepared by methods known in the art, such as heating the polymer to a temperature near or above the softening transition temperature, followed by stretching in one or more directions. Typically, an oriented polymer film structure having separable surface elements is oriented by rapid stretching at a desired temperature to form an oriented film, followed by rapid quenching. Quenching ensures that the orientation is not lost by molecular relaxation. Orientation can occur in the direction of film motion, referred to in the art as the machine direction or the longitudinal direction. Films may be oriented in one direction only; and are referred to as uniaxially oriented films. They may also be oriented in two directions, typically orthogonal to each other, and are referred to as biaxially oriented films. The direction orthogonal to the longitudinal direction is referred to as the transverse or cross direction. Mechanical properties of oriented films vary depending upon the direction and degree of orientation. Orientation typically produces films with increased modulus, decreased elongation-at-break, increased tensile strength-at-break, and decreased tear strength.
Suitable orientable amorphous glassy thermoplastic polymers include acetates such as cellulose acetate, cellulose triacetate and cellulose acetate butyrate, acrylics such as poly(methyl methacrylate) and poly(ethyl methacrylate), polystyrenes such as poly(p-styrene) and syndiotactic-polystyrene, and styrene-based copolymers, vinylics such as poly(vinyl chloride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinylidine dichloride) and mixtures thereof. Preferred amorphous glassy thermoplastic polymers include cellulose acetate, poly(p-styrene), syndiotactic polystyrene, poly(vinyl chloride), poly(vinylidene chloride), poly(vinylidene fluoride) and poly(vinylidine dichloride).
Suitable orientable semi-crystalline thermoplastic polymers include polyolefin homopolymers such as polyethylene and polypropylene, copolymers of ethylene, propylene and/or 1-butylene; copolymers containing ethylene such as ethylene vinyl acetate and ethylene acrylic acid; polyesters such as poly(ethylene terephthalate), polyethylene butyrate and polyethylene napthalate; polyamides such as poly(hexamethylene adipamide); polyurethanes; polycarbonates; poly(vinyl alcohol); ketones such as polyetheretherketone; polyphenylene sulfide; and mixtures thereof. Preferred orientable semi-crystalline polymers include polyethylene, polypropylene, poly(ethylene/propylene), poly(ethylene/1-butylene), poly(propylene/1-butylene), poly(ethylene/propylene/1-butylene), poly(ethylene terephthalate), poly(ethylene butyrate), poly(ethylene napthalate), and mixtures thereof. Particularly preferred are linear low density polyethylene, high density polyethylene, ultra high molecular weight polyethylene, isotactic polypropylene, blends of isotactic polypropylene and substantially syndiotactic polypropylene and blends of isotactic polypropylene and polyethylene.
The oriented thermoplastic polymer film structures of the invention range in thickness from about 2 to about 250 micrometers in the base film area. Preferably, they range in thickness from about 5 to about 150 micrometers, and more preferably, from about 10 to about 75 micrometers.
The polymers forming the invention film structure may also contain fillers, plasticizers, colorants, lubricants, processing aids, nucleating agents, antiblocking agents, ultraviolet-light stabilizing agents, and other property modifiers. Typically such materials are added to a polymer before it is made into an oriented film (e.g., in the polymer melt before extrusion into a film). Organic fillers may include organic dyes, and resins, as well as organic fibers such as nylon and polyimide fibers. Inorganic fillers may include pigments, fumed silica, calcium carbonate, talc, diatomaceous earth, titanium dioxide, carbon fibers, carbon black, glass beads, glass bubbles, mineral fibers, clay particles, metal particles and the like. Filler may be added in amounts up to about 100 parts per 100 parts of the polymer forming the oriented film. Other additives such as flame retardants, stabilizers, antioxidants, compatibilizers, antimicrobial agents (e.g., zinc oxide), electrical conductors, and thermal conductors (e.g., aluminum oxide, boron nitride, aluminum nitride, and nickel particles) can be blended into the polymer used to form the film in amounts of from about 1 to about 50 volume percent.
In the invention, a layered construction, also known as a multilayered film, may be used as the film structure. Such multilayered films include, for example, layers of films that are formed by co-extrusion with one or more other polymers, films coated with another layer, or films laminated or adhered together.
If the cuts are only in one direction on a surface of the film structure, a ribbed pattern is formed in the final oriented film structure as shown in FIGS. 3 and 4. Tandem cutting is possible where multiple cuts are made along parallel directions using multiple cutting stations in order to obtain smaller cut spacing than would be possible with just a single cut in that direction. Multiple cuttings at multiple angles on one or both surfaces of the film structure would result in other shapes such as triangles and other polygons. It is, therefore, possible to achieve a wide variety of controllable shapes and sizes of the topographical features. Intermittent cutting is also possible in one or more direction resulting in discrete zones capable of elongation surrounded by separable elements. Cutting to different depths with different cuts is also possible.
- Test Methods
The present invention has now been described with reference to several embodiments thereof. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. All patents and patent applications cited herein are hereby incorporated by reference. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the exact details and structures described herein, but rather by the structures described by the language of the claims, and the equivalents of those structures.
- EXAMPLE 1
The tear strength of the films of the invention were measured using an Elmendorf Tear test per ASTM D 1922. One ply or layer of film was used and 5 replicates were tested and averaged.
- EXAMPLE 2
A structured oriented web was made using the following procedure. A relatively thick cast sheet was made using conventional profile extrusion apparatus. A polypropylene/polyethylene impact copolymer (C104, 1.3 MFI, Dow Chemical Corp., Midland, Mich.) pigmented with 1% of a TiO2/polypropylene concentrate (15100P, Clariant Corp., Minneapolis, Minn.), was extruded with a 6.35 cm single screw extruder (24:1 L/D) using a barrel temperature profile of 177° C.-232° C.-246° C. and a die temperature of approximately 235° C. The extrudate was extruded vertically downward through a die equipped with a die lip having a rectangular opening. After being shaped by the die lip, the extrudate was quenched in a water tank at a speed of 6.1 meter/min with the water being maintained at approximately 10° C. The resulting sheet was approximately 635 micrometers thick. The sheet was then advanced through a cutting station where the top (first) surface was score cut to a depth of 125 micrometers. A series of parallel score cuts were made at an angle of 23 degrees measured from the transverse direction of the sheet. The spacing of the cuts was 203 micrometers. The sheet was then turned over and advanced through a cutting station where the bottom (second) surface was score cut to a depth of 125 micrometers. A series of parallel score cuts were made at an angle of 23 degrees measured from the transverse direction of the sheet. The spacing of the cuts was 203 micrometers. The scored sheet was then biaxially stretched at a stretch ratio of approximately 1:4 by 1:4 using a KARO IV pantograph stretcher (Bruckner Gmbh, Siegfred, Germany). A 115 mm by 115 mm sample of the scored sheet was mounted into the stretcher (with an available stretch area of 100 mm by 100 mm), heated for 60 seconds at 150° C., and then stretched at a rate of 100%/second to a final dimension of 400 mm by 400 mm between grips. The resulting material, as depicted in FIG. 3, had an ornamental appearance resulting from the criss-cross diamond-like pattern imparted into the film by the score cuts. The film had a matte finish and varying levels of opacity and haze due to the varying levels of thickness in the film.
- EXAMPLE 3
A biaxially oriented film was prepared as in Example 1 except the sheet was score cut on only one side at a spacing of 305 micrometers.
- EXAMPLE 4
A biaxially oriented film was prepared as in Example 1 except the spacing of the score cuts was 305 micrometers.
- EXAMPLE 5
A biaxially oriented film was prepared as in Example 1 except the spacing of the score cuts was 610 micrometers.
A biaxially oriented film was prepared as in Example 1 except the sheet was stretched to a 5 by 5 degree of orientation and the spacing of the score cuts was 610 micrometers.
The films were tested for tear strength using an Elmendorf Tear tester. The areas of the films having increased thickness and lesser orientation resulted in significantly higher tear strength as compared to an unscored film. When the film was scored only on one side, a small improvement in tear strength resulted in the machine direction. When scored on only one side the tear tends to propagate along the score line as this is the region of lowest thickness. When scored on both sides the tear also propagates along a score line, but the tear front encounters local regions of higher thickness and lesser orientation resulting in higher tear strength. As the spacing of the score cuts decreases, the tear strength correspondingly increases due to a higher frequency of thick regions.
|TABLE 1 |
| ||Score cut spacing ||CD Tear strength ||MD Tear strength |
|Sample ||(micrometers) ||(grams/ply) ||(grams/ply) |
|Non-scored 4 ||— ||16 ||20 |
|by 4 biax film |
|Example 1 ||203 ||67 ||71 |
|Example 2 ||305 ||12 ||24 |
|Example 3 ||305 ||50 ||57 |
|Example 4 ||610 ||28 ||30 |
|Non-scored 5 ||— ||12 ||10 |
|by 5 biax film |
|Example 5 ||610 ||29 ||18 |