CA1199767A - Cross-reinforced film and method for producing a cross-reinforced film - Google Patents

Abstract

CROSS-REINFORCED FILM AND METHOD FOR PRODUCING A CROSS-REINFORCED FILM The present invention is a novel method for producing a novel cross-reinforced film which comprises trapping threads, fibers, monofilaments or the like within an extruded, self-weldable, rotating thermoplastic tubing so as to form a non-woven cross-reinforcing fiber net like structure which is located within the multi-layer thermoplastic film produced upon collapsing and internal self-welding of the tubing into a lay-flat film configuration.

Description

76'7 FIELD OF THE INVENTION
The present invention relates to film forn~ation and, more particularly, to a fiber, cross-reiniorced, strengthened film and an improved method for its production.

BACKG~OUND OF THE INVENTION
The quest for an improved method for producing a film material having improved physical characteristics such as overall strength and tear resistance has been ongoing for quite some time. ~or many years it has been known that the stretching of an orientable thermoplastic material under certain conditions, now well known in the art, would result in the orientation, i.e. alignment, of the molecules of the polymeric material in the direction of stretching. One method whereby this stretching may be accomplished is known as the "bubble" process.
The bubble p~ocess is a well known process for forming an oriented tbermo-plastic film wherein an extruded tube of thermoplastic material which is heated to its orientation temperat~re range is sequentially inflated by interDal pressure, cooled and collapsed into lay-flat configuration. The collapsed tube may subsequently be wound up in roll fashion for storage.
The tube is usually extruded vertically. After extrusion a volume of air is trapped within the tube. The internally trapped air causes the extruded tubing to ass~ne a bubble or balloon-like configuration so as to enlarge, stretch and orient the tube in both the transverse and longitu-dinal directions. The bubble may be formed through utilization of two sets of pinch rolls which may also serve to collapse the tube and form a lay-flat film. The thickness of the film may, to some degree, be con-trolled by varying the volume of the internally trapped air and hence the degree of transverse enlarging and stretching, by varying the rate of extrusion and/or by varying the speed of revolution of the pinch rolls which collapse the tube into a lay-flat configuration.

Yl]LK2/sb 7~7 The terms "oriented" and~or "orientation" are used herein to describe the process and resultant product characteristics obtained by stretching a resinous orientable polymeric thermoplastic material which is heated to its orientation temperature range and then cooled in order to lock-in or freeze the molecular alignment of the material in the direction of stretching. This action improves the mechanical properties of the film, such as, for exarnple, shrink tension and orientation release stress. Both ~f these properties may be measured in accordance with ASTI~
D 2~38 69 (reapproved 1975). The orientation temperature range for a given film will vary with the different resinous thermoplastic materials or blends thereof whic~ comprise the film. However, the orientation temperature range may generally be stated to be above room temperature and below the melting point of the thermoplastic material or blend of materials. Orie~tation temperature ranges for the materials encompassed by the present application are well known to those skilled in the art.
When the stretching force is applied in one direction uniaxial orienta-tion results. When the stretching force is applied in two directions biaxial orientation results.
The terms "polymer" and "polymeric" are used herein to include polymers, ionomers, copolymersj interpolymers, homopolymers, block or graph polymers and blends thereof.
The term "cross-oriented" is used herein to describe a multi-layer film comprising two or more layers in which at least two of the layers are orien~ed at an angle with respect to each other.
The term "cross-reinforced" is used herein to desfribe a multi-layer film having internally trapped criss-crossed fibers 9 filaments or the like which improve the overall strength and tear resistance of the film in both the transverse and longitudinal diretions.

Yll~3~sb 7~
Other methods of stretching are known to those in the art. ~or example, it has been recognized in the art that the extruded tubing may be longitudinally stretched by revolving the pinch rolls which initially collapse the tubing after extrusion at a rate in excess oE the linear velocity with which the tubing emerges Erom the extrusion die. If the temperature of the extruded tubing is maintained within its orientation temperature range during the stretching the Molecules of the tubing will be oriented in the direction of stretching. ~ilms manufactured by this method are generally referred to as hot stretched films. I$ has also be recognized in the art that the extruded tubing may be longitudinally stretched and oriented by revolving one of the pairs of pinch rolls which transport a tubular extrudate which has been cooled and reheated to its orientation temperature range at 3 rate in excess of the rate of revolu-tion of a preceding pair of pinch rolls. ~ilms manufactured by this method are generally referred to as cold stretched films. Either of these actions accomplishes some degree of orientation of the stretched tubular extruded film in the longitudianl or tubular direction. However, if a high degree of orientation is desired the later procedure should be followed since it results in a greater degree of orientation. Further-more, it is also ~ell known that the transverse stretching and expansion of an extruded tubular film which is heated to a temperature ~ithin its orientation temperature range results in the stretching and consequent orientation of the tubular extruded film in the transverse or lateral direction. ~ greater degree of transverse orientation occurs if the extruded material is first cooled and then reheated to its orientation temperature range prior to being subjected to transverse stretching and expansion. ~f the transverse stretching is coupled with longitudinal stretching, as is the case in the bubble process, a biaxial orientation is imparted to the resultant extrud~d film.

Y~lLK4/sb Stretching to orien-t a therrnoplastic material is widely utilized within -the art since i-t is well known that an oriented material exhibits increased tear resistance in the direction trans-verse to the direction of stretching and orientation. Further dis-cussion of film orientation may be found at Volume I, Chapter 10 of the Science and Technology of Films, copyrighted in 1968 by John Wiley and Sons. This book was edited by Orville Sweeting.
Unfortunately, it is also well known that an oriented material exhibits little or no increase in tear resistance in the direction of stretching and orientation. In the past attempts to overcome this problem have led to the utilization of cross-oriented films. However, the methods for processing a cross-oriented film have been somewhat complex. For example, the work of Reifenhauser et al. has resulted in United States Patent Nos. 3,726,743 and 3,926,706. Additionally, the work of Kubat et al. resulted in United States Patent No. 4,076,568. While these patents do dis-close methods for producing cross-oriented films those skilled in the art, upon reviewing these patents, will recognize tha-t these methods are quite unwieldy and cumbersome. An additional drawback inherent in any method to improve the strength and tear resistance of a film by orientation is that such a method is limited to mater-ials which are, in fact, orientable.
Yet another method by which a -tubular extrudate has been reinforced is the internal applica-tion of threads, fibersl mono-filaments or thelike to the interior surface of the -tubular extru-date. The resultant material, upon collapse and internal self-welding of the tubular extrudate exhibits, in a fashion quite analo-gous to oriented materials, increased tear resistance in the direc-;

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tion transverse to that of the threads, fibers, monofilaments orother reinforcing materials. ~nfortunately, in a manner also quite analogous to that of oriented materials, - 4a -7~7 flber reinforced tl~b~lar extrDdates do not exhibit improved tear resistance in the direction parallel to that of the fibers. In other words, the film may be torn "between" the fibers. This problem in the past has been attacked by contacting the interior surface of the tubular extrudate with a preforrned woven, cross-reinforcing net-like structure and this action has improved the overall strength and tear resistance of the resultant product. However, as those skilled in the art would acknow-ledge, the internal application of such a preformed woven, net-like structure is rather dificult and cumbersome.

_JECTS OF THE PRESENT INVE~TION
It is a principal object of the present invention to provide a method for forming a film from a tubular extrudate whereby the afore-mentioned disadvantages are obviated.
It is another object of the present invention to provide a method for forming cross-reinforced fil~s having improved strength and tear resistance from materials which need not be oriented.
It is yet another object of the present invention to provide a method for fonming a cross-reinforced film having improved strength and tear resistance iu both the transverse and longitudinal film directions.
A urther object of the present invention is to provide a cross-reinforced and strengtheDed film.
Still further objects and the broad scope of applicability of the present invention will become apparent to those of ordinary skill in the art from the details disclosed hereinafter. However, it should be understood that the Iollowing detailed description which indicates a presently preferred embodiment of the present invention is only given for purposes of illustration since various changes and modifications well Y]lLK6/sb ~ry~

within the spirit and scope of the present invention will become apparent to those of ordinary skill in the art upon review of this detailed description.
SUMMARY OF THE INVENTION
According to -the present invention there ls provided a method for forming an internally cross-reinEorced film comprising the steps of: extruding a polymeric tubular extrudate having a tacky self-weldable inner surface; applying a plurality of spaced apart reinforcing filamen-ts to said tacky inner surface while simultaneously axially ro-tating said tubular extruda-te so that said filaments are adhered to said tacky inner surface and spirally carried by said axially rotating tubular extrudate; and collapsing said tubular extrudate into a lay-flat configuration to self-weld said tacky inner surface of said tubular extrudate and foam an internally cross-reinforced film.
According to another aspect of the invention there is provided a cross-reinforced lay-flat film comprising: first and second polymeric layers self-welded together at an inner surface;
and a plurality of filaments extending entirely across said inner surface; and whereby said plurality of filaments forms a non-woven net-like cross-reinforced material trapped between said layers.
One embodiment of the present invention envisions a tubular extrudate which has been continuously cross-reinforced by threads, fibers, monofilaments, stainless steel fibers or the like to provide a film having improved strength and tear res:istance in both the transverse and longitudinal film directions. The film material involved need not necessarily be an orientable material since the cross--reinforcing occurs as a result of the internal . .

application of continuous Eilaments or the like such as fiberglass or stainless steel fibers to the tacky interior surface of an in-ternally self-weldable tubular extrudate. Simul-taneously with -the internal application of the fialmen-ts the entire extruded -tube is rotated with respect to the extrusion die orifice. Since the fila-ments are applied to the tacky internal surEace of this tubular extrudate the filaments adhere to and are thereafter carried with the rotating tube. Upon the collapse and internal self-welding of the tubular extrudate to form a lay-flat film, the internally trapped filaments form a non-woven criss-cross reinforcing pattern within the film. The final film product is provided with both longitudinal and transverse reinforcement as a result of the presence of the internally positioned criss-cross reinforcing material.

- 6a --,, 7~7 BR_F.F _ESCRIPTION OF THE DRAW_NGS
FIG. I is a schematic cross-sectional view of the apparatus utiliæed in the present invention wherein monofilaments or other thread-like reinforcing materials are trapped within the axially rotating tubular extrudate in a criss-crossing, cross-reinforcing manner.
FIG. II is a top plan view of the material formed by the process of FIG~ I.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMEN~
Turning now to the figuresl in which like reference numerals represent like materials or apparatus and, in particular, to FIG. I which represents a cross-sectional schematic view of a preferred èmbodiment of the inventive method of the present invention, it can be seen that poly-meric tubular extrudate 3 is extruded from the circular orifice of die 1 as indicated by arrow 4. The extrudate is provided to die 1 by conven-tional extruders well known to those in the art, not shown. The tubular extnldate may be a monolayer in which case the tubular extrudate 3 need only be provided from a single source as by die extrusion channel 2 or the tubular extrudate may be a coextruded multi-ply structure in which the second ply may be pro~ided by a second die extrusion channel 2a.
Other plies may be provided through utilization of additional die extru-sion channels and extruders as is well known in the art. Shortly after its extrusion and prior to its collapse-as at 5 by deflate pinch rolls 6 the hot tubular extrlldate is cooled by conventional quenching means, not shown. Quenching means are well known in the art. An example o~ such means is the cascading water means.
An important feature of the present invention is that the monolayer or, in the case of a coextruded multi-ply structure the inner-most ply, comprises a self-weldable adhesive resinous polymeric thermo-plastic material having a low melting point, preferably between 160~F and YllIK8/sb q

2~F. A preferred self-weldable material is a copolymer of ethylene and vinyl acetate which has from 10 to 40%, by weight, of vinyl acetate derived units. Oth~ particularly effective adhesive self-weldable resinous polymeric tbermoplastic materials broadly include unsaturated ester polymers such as ethylene/unsaturated ester copolymers; e.g., ethylene/ vinyl acetate copolymers, ethylene/vinyl propionate copolymers, ethylene/ methyl methacrylate copolymers, ethylene/ethyl methacrylate copolymers, ethylene/ethyl acrylate copolymers, ethylene/ isobutyl acrylate copolymers, and the like; unsaturated carboxylic acid polymers, e.g., ethylene/unsaturated carboxylic acid copolymers, ethylene/acrylic acid copolymers, ethylene/methacrylic acid copo~y~ers, ethylene/maleic acid copolymers, ethylene/fumaric acid copolymers, ethylene/itaconic acid copolymers and the like; low molecular weight polyethylene, low mo]ecular weight polypropylene and other low molecular weight olefin polymers.
Other useful adhesive self-weldable materials include ionomer resins, and vinylidene chloride polymers e.g. vinylidene chloride/vinyl chloride copolymers. Non-crystalline materials such as un wlcanized rubber may also be utilized.
Returning to ~IG. I it can be seen that the tubular polymeric extrudate 3 having an interior surface comprising a self-weldable adhesive material as described above is, simultaneously with the extrusion of the tubular extrudate 3 from die 1, interiorly contacted by filaments 11.
Filaments 11 are coDtinuously provided from source 10 and are substan-tially equidistantly spaced about the circumference of the inner surface of tubular extrudate 3. The greater the degree of cross-reinforcing desired the greater the number of filaments which should be applied. The filaments 11 are provided in such a manner that the filaments 11 contact and adhere to the inner surface of the tubular extrudate 3 shortly after its formation by die 1. The filaments 11 readily adhere to the iDterior surface of tubular extrudate 3 since the self-weld;ng materials described YllLK~/sb ~

above are also qllite t.acky immediately a~ter their formation by die 1.

Examples of filament ma~terials which may be utilized by the present 62 ` 5 /C~ SS
invention are f~beF~ rayon, cotton and stainless steel. Polyester and nylon may also be utilized if the temperature of extrusion is maintained below the degradation temperature of these latter materials.
Other suitable materials are well known to those skilled in the art.
FIG. I also illustrates that tubular extrudate 3 is col].apsed as indicated at 5 by deflate rolls 6 which revolve as indicated by arrows 6a. Deflate rolls 6 rev~lve at a speed substantially equal to the rate of extrusion of tubular extrudate 3 from die 1. The co].lapsing of-tubu-lar extrudate 3 by deflate rolls 6 brings the tacl~ self-weldable inner surface of tubular extrudate 3 into intimate pressuri.zed contact with itself. The collapsing and pressure applying action of deflate rolls 6, when utilized with tubular extrudates such as those disclosed above having an inner surface which is tacky, adhesive and self-weldable, results in the selfwelding of the interal-surface-of the ~ubular extru-date 3 to itself so as to produce a unitary lay-flat multi-layer film as indicated at ~. The lay-flat ~ulti-layer film 8 comprises two outer layers with the rei~forcing fibers or filaments trapped therebetween in a non-woven fashion. If necessary, deflate rolls 6 ~ay be heated to further assist in the self-welding of the internal surface of tubular extrudate 3 to itself. After collapse and self-welding of the tubular extrudate 3 into the unitary lay-flat film as at 8, the lay-flat film may be transported to and stored on roll 9 which revolves as indicated by 9a.
Of principal importanee to the present invention is that the deflate rolls 6 are constructed so as to continuously axially rotate as indicated by arrow 7 with respect ~o die 1. Additionally, it is preferred that any accompanying transporting and storing apparatus~ for example 9, is also constructed to continuously axially rotate in unison ~ith deflate rolls 6 as indicated by arrow 7. The continuous circular rotati.on of the YllLK10/sb c~ G1 7~7 dellate rolls 6 imparts a spiral character to the downwaxd path of fila-ments 11 after the~ have contacted and adhered to the interior surface of tubular extrudate 3 as at, for example, point 12- The spiral path of filaments 11 results from the fact that, upon contacting the tacky inner surface of tubular extrudate 3, the filaments are carried spirally down-ward by the tubular extrudate 3 which~ as a resl~lt of the axial rotation of deflate rolls 6, is, likewise, rotating. Since the reinforcing fila-ments are carried spirally downward by the rotating polymeric tubular extrudate, the reinforcing material, upon collapse and self-welding of the tubular extrudate by deflate rolls 6, will assume a non-woven criss-crossing cross-reinforcing pattern. Accordin~ly, the final unitary lay-flat film 8 will comprise first and second polymeric layers with the criss-crossed reinforcing material trapped between the two layers.
ThDse skilled in the art will readily recognize that the rate of circular rotation of deflate rolls 6 and storage roll 9 can be varied to some degree depending on the thermoplastic materials utilized. In any event, the rate of rotation ~ust be sufficient to effect a criss-cross reinforcing pattern while not be so great as to cause an actual physical t~isting of the tubular extrudate 3.
~ I~. II is a top plan view of the final lay-flat film 8 produc~d by the preferred process of FIG. I. This view clearly illustrates the criss-crossing pattern 13 which the filaments assume upon collapse and self-welding of the tubular extrudate 3. This criss-crossing pattern 13 provides both longitudinal and transverse reinforcement to the final film product.
~ rom the above it should be recognized that an orientable material need not be utilized since the tubular extrudate 3 does not have to be strengthened by stretching to orient. However, the imIer surface of the tubular extrudate must be capable of self-welding upon being ': /
YllLK11/sb /0 7~

brought into intimate pressurized contact with itself by deflate rolls 6.
As stated above deflate rolls 6 may be heated to assist in the internal self-welding.
While it is not necessary for the thermoplastic resinous materials of the present invention to comprise an orientable material utilization of this type of material may be preferred if an even stronger and/or heat shrinkable final product is desirable. In this instance the tubular extrudate would initially be longitudinally andtor transversely stretched to orient as is well known in the art and as was discussed above. Thereafter, the oriented tubing would be transported in tubular form so that the tube passed over and enclosed filament source 10. At that point filaments 11 would contact the tacky inner surface of the oriented tubing in the manner discussed above. Depending on the thermo-plastic material utilized, some slight reheating may be necessary to render the inner surface sufficiently tacky. The remainder of the process would, likewise~ coincide with the steps discussed above with regard to the unoriented fiber cross-reinforced tubular extrudate.
Orientable thermoplastic materials which may be utilized with this embodiment of the present invention are well known in the art.
Examples of such orientable thermoplastic materials are polymers and polymeric blends of the following monomers: the mono-olefins and conju-gated di-olefins, e.g., ethylene, propylene, butene-l, isobutene, 1,3 butadiene, isoprene and other ~liphatic mono and di-olefins; $he halogen substituted olefins, e.g. vinyl chloride, vinylidene chloride; the mono/
vinylidene aromatic compounds, e.g., styrene, alpha methylstyrene, chlorostyrene9 other aromatic olefins; and other unsaturated monomers such as acryonitrile, acrylamide and the like. Ionomer resins and nylon may also be utilized. A preferred orientable thermoplastic material lS a polymer of vinylidene chloride and vinyl chloride comprising at least YllLK12/sb ~l 5~,~, by weight, of vinylidene chloride derived units. This material is well kDown in the art for its oxygen impermeability characteristics.
If utilization of a particular orientable material which is not self-weldable is desired, this material may be coextruded as an outer ply to an inner ply of a self-weldable material. In this instance the final film product could, depending upon whether the inner ply of self-weldable material was, itself, orientable, result in a structure where the inner self-welded ply is not oriented and the outer ply is oriented or both plies could be oriented.
~ lile the above-discussed preferred embodiment of the present invention envisions the continuous application of the reinforcing material to the inner surface of the tubular extrudate, such application should not be considered as limiting the scope of the present invention since a degree of cross-reinforcing is obtained by applying discrete, non-continuous, segments of reinforcing material to the interior surface of the tubular extrudate. In this embodiment continuous filament source 10 would be replaced by a mechanism, not shown, whereby the segments of discontinuous reinforcing material, which may have been chopped up, are sprinkled, dropped or lightly blown onto the tacky inner surface of the tubular extrudate in a somewhat random manner. Those skilled in the art will reco~nize that, upon collapse and self-welding of the tubular extrudate 3 to itself, some of the randomly applied discontinuous fila-ments will be criss-crossed and a degree of cross-reinforcement will be obtained. In spite of the random application, the total amount o~
randomly applied reinforcing material is still controlable on a by weight basis and thus, tbe degree of cross-reinforcement may still be somewhat varied.
Wbile this disclosure has general1y discussed the resultant product as a reinforced material, other uses for the material are also envisiolled. For example, if stainless steel or other electrically con-Y1lLK]3/j / ~

ducting reinforcing materials are utilized -the produc-t may be used as an antenna or as a means for attenuating radio frequency (R.F.) signals. Of course, numerous other modifications of the present inventive concept will be readily discernable to those of skill in the art in view of the present disclosure. For example, the film materials may be irradiated, as is well known in the art, if a cross-linked final structure is desired. Modifications of this sort axe intended to be within the scope and spirit of the present invention.

Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for forming an internally cross-reinforced film comprising the steps of: extruding a polymeric tubular extrudate having a tacky self-weldable inner surface; applying a plurality of spaced apart reinforcing filaments to said tacky inner surface while simultaneously axially rotating said tubular extru-date so that said filaments are adhered to said tacky inner sur-face and spirally carried by said axially rotating tubular extru-date; and collapsing said tubular extrudate into a lay-flat con-figuration to self-weld said tacky inner surface of said tubular extrudate and foam an internally cross-reinforced film.

2. The method of claim 1, wherein said reinforcing material is continuously applied.

3. The method of claim 1, wherein said reinforcing material is discontinuously and randomly applied.

4. The method of claim 1, wherein a multi-ply tubular extrudate is coextruded.

5. The method of claim 1, comprising the further step of orienting said tubular extrudate prior to contacting said tacky inner surface with said reinforcing material.

6. The method of claim 1, wherein said cross-reinforcing material is selected from the groups of: fiberglass or rayon or cotton or polyesters or nylon or stainless steel.

7. The method of claim 1, wherein said cross-reinforcing material is an electrically conducting material.

8. A cross-reinforced lay-flat film comprising: first and second polymeric layers self-welded together at an inner surface;
and a plurality of filaments extending entirely across said inner surface; and whereby said plurality of filaments forms a non-woven net-like cross-reinforced material trapped between said layers.

9. The cross-reinforced film of claim 8, wherein said cross-reinforcing material is selected from the group of: fiber-glass or rayon or cotton or polyesters or nylon or stainless steel.

10. The cross-reinforced film of claim 8, wherein said cross-reinforcing material is an electrically conducting material.

11. The cross-reinforced film of claim 8, wherein said layers comprise a copolymer of ethylene and vinyl acetate having from 10 to 40%, by weight, of vinyl acetate derived units.

12. The cross-reinforced film of claim 8, wherein said layers are multi-ply layers.

13. The cross-reinforced film of claim 8, wherein said layers are oriented.

14. The cross-reinforced film of claim 8, comprising a continuous, non-woven, net-like cross-reinforcing material.

15. The cross-reinforced film of claim 12, further com-prising inner self-welded plies of a copolymer of ethylene and vinyl acetate having from 10 to 40%, by weight, of vinyl acetate derived units.

16. The cross-reinforced film of claim 12, wherein at least one of said plies is oriented.

17. The cross-reinforced film of claim 15, further com-prising outer oriented plies of a copolymer of vinylidene chloride and vinyl chloride having at least 50%, by weight, of vinylidene chloride derived units.