US3730821A - Interconnected network structure - Google Patents

Abstract

A LONGITUDINALLY STRIATED FILM IS CONTACTED WITH A ROTATABLE PRECISION FIBRILLATION APPARATUS CONTROLLED TO CONTACT THE FILM BETWEEN ABOUT 0.5 TO 2 IMPACTS PER LINEAR INCH BETWEEN EACH PAIR OF ADJACENT STRIATIONS. THE RESULT IS A PLURALITY OF CONTINUOUS BACKBONE FFILAMENTS INTERCONNECTED BY SUBSTANTIAL LENGTHS OF UNSPLIT WEB SECTIONS. THIS INTERCONNECTED WEB CAN BE OPENED TO FORM A UNIQUE NETWORK STRUCTURE USEFUL IN A VARIETY OF LAMINAR, NON-WOVEN FABRIC APPLICATIONS AND AS A REINFORCING WEB IN PAPER OR PLASTIC FILMS.

Description

May 1, 1973 D. B. JACKSON 3,730,821

INTERCONNECTED NETWORK STRUCTURE Filed Nov. 23, 1970 2 Sheets-Sheet 1 DAVID B. JACKSON INVENTOR BY [J T (TQM ATTORNEY y 1, 1973 D. B. JACKSON 3,730,821

INTERCONNECTED NETWORK STRUCTURE Filed Nov. 23, 1970 2 Sheets-$heet 2 DAVIID B. JACKSON INVENTOR BY MQQM'M 52"" (LQL LMQQLN ATTORNEY United States Patent 3,730,821 INTERCONNECTED NETWORK STRUCTURE David B. Jackson, Loveland, Ohio, assignor to Hercules Incorporated, Wilmington, Del. Filed Nov. 23, 1970, Ser. No. 92,099

Int. Cl. B32b /12 U.S. Cl. 161-57 3 Claims ABSTRACT OF THE DISCLOSURE A longitudinally striated film is contacted with a rotatable precision fibrillation apparatus controlled to contact the film between about 0.5 to 2 impacts per linear inch between each pair of adjacent striations. The result is a plurality of continuous backbone filaments interconnected by substantial lengths of unsplit web sections. This interconnected web can be opened to form a unique network structure useful in a variety of laminar, non-woven fabric applications and as a reinforcing web in paper or plastic films.

This invention relates to the preparation of a novel continuous interconnected monofilament structure and to the use-of such a product in the fabrication of non-woven fabrics.

In recent work of Kim and his coworkers, described in a series of United States patents issued over the past several years, there have been described a number of techniques for preparing filaments and yarns by the fibrillation of striated films and a number of products resulting from such fibrillation. By a striated film is meant a film product having an irregular surface characterized by a plurality of relatively thick filament-like rib sections interconnected by alternating relatively thin web sections the ribs being in the machine direction. When such a film is longitudinally drawn by about 100 to 1000% of its original length and subjected to a fibrillating stress, the thin web sections constitute its weakest portion and substantially the entire fibrillating stress operates therein, leaving the relatively thick ribs intact in a form resembling continuous, low denier monofilaments.

In US. Patents 3,495,752 of Kim and Samluk, there is described a fibrillation method by which a number of ditferent products can be formed. The method, briefly described, comprises contacting a longitudinally striated film with a rotatable beater bar having serrated edges adapted to contact the striated film in the thin web sections between the ribs and cause rupturing of those web sections. By varying the linear rate of advancement of the film with respect to the rotating speed of the beater bar, i.e., by varying the number of contacts of the beater bar against the film, per linear inch, it was found possible to prepare completely fibrillated products comprising single filaments, completely fibrillated products comprising two filaments joined by a web, incompletely fibrillated products comprising single filaments interconnected via hair-like fibrils or incompletely fibrillated products comprising web-joined single filaments interconnected via hair-like fibrils.

In accordance with the present invention, it has been determined that, by carrying out a fibrillation or splitting operation to a very limited degree, it is possible to prepare an entirely different type of product. The method according to the invention includes advancing a longitudinally oriented striated film having a plurality of relatively thick filament-like rib sections interconnected by alternating relatively thin web sections. The web sections are split about 0.5 and 2.0 times per linear inch in each of said intervening web sections, with the splits on adjacent web sections being staggered or linearly displaced from one another. The film is transversely expanded to 3,730,821 Patented May 1, 1973 form the split web sections into transverse rows of openings with each opening in a row being separated from the adjacent opening by at least two or more rib sections which are interconnected by a web section or sections. The product thus formed has a plurality of continuous backbone filaments interconnected by substantial lengths of the unsplit web sections rather than by fine denier hairlike fibrils, and can be used alone as a network structure or used in the formation other unique network structures, inter alia.

The invention is illustrated in the attached drawing in which:

FIG. 1 depicts a fibrillation process and apparatus in operation:

FIG. 2 depicts a fibrillated film according to the invention;

FIG. 3 depicts a network structure derived from the film of FIG. 2;

FIG. 4 depicts another fibrillated film according to the invention which has been fibrillated to a lower level than that of FIG. 2;

FIG. 5 depicts a network structure derived from the film of FIG. 4; and

FIG. 6 depicts a process and apparatus for spreading the fibrillated films to form a net-like structure.

With reference to the drawings, there is illustrated schematically in FIG. 1 apparatus for practicing the method of this invention comprising a feeding mechanism including a pair of feed rolls 1 and a pair of draw rolls 2 for feeding aribbon 3 of striated thermoplastic film. The space between the feed rolls 1 and draw rolls 2 defines the fibrillation zone and the rolls 1 and 2 are driven at speeds such as to maintain the section ofribbon 3 in this zone under tension. The tensioned length of theribbon 3 in the fibrillation zone is engaged by a fibrillating means which comprises a beater 4 having a cross section defining an equilateral triangle in the illustrated embodiment, though this is not a limitation. The edges 5 of the beater are serrated. to define teeth 6 and valleys 7. The beater ft is journaled for rotation about its axis by means of ashaft 8 mounted at itsopposite ends insupports 9. R0- tation in the direction of the arrow A is imparted to the heater 4, for example, by a belt 10' entrained about a pulley 11 on one end of theshaft 8. The beater 4 is mounted with its axis substantially parallel to the pinch lines defined by the feed rolls 1 and draw rolls 2 and is oifset from a straight line between the pinch lines so that theribbon 3 is deflected over the edges 5 of the heater, 4.

The ribbon 3 (FIGS. 2 and 4) comprises a thin, striated strip of thermoplastic material such as polypropylene, provided with a series of substantially uniformly spaced parallel ribs orstriations 12 running longitudinally thereof and interconnected bywebs 13 of reduced thickness. Theribbon 3 is oriented uniaxially in the direction parallel to thestriations 12. With uniaxial orientation, the tensile strength of the ribbon in the direction of the axis of orientation is greatly increased but the strength transversely is reduced so that the ribbon can be readily split lengthwise.

In comparison with thewebs 13, thestriations 12 have a relatively high resistance to splitting, so lengthwise splitting of the ribbon is generally confined to thewebs 13 and the resulting filaments correspond generally to thestriations 12.

In the operation of the apparatus of FIG. 1, theribbon 3 is advanced by the draw rolls 2 from the feed rolls 1, with the section of the ribbon between them, that is, the section in the fibrillation zone, being under tension and angled over the beater 4. As the beater 4 is rotated in the direction of the arrow A, the edges 5 are successively brought into engagement with the ribbon along lines transversely thereof with each successive line of engagement spaced upstream of the ribbon from the preceding line of engagement. After engagement, the edge is advanced along the ribbon and then carried out of engagement with it. With theribbon 3 under tension, the teeth 6 of the edges 5 penetrate the ribbon.

In accordance with this invention, the process is carried out at a rate such that the penetrating means of the fibrillation apparatus contacts the striated film at a rate of about 0.5 to 2.0 impacts (with resultant penetrations) per linear inch of each intervening web section. With specific reference to the apparatus shown in FIG. 1, the number of penetrations per linear inch is determined by the rotational speed of the beater bar, the number of serrated edges thereon, and the number of teeth per inch of width of each edge of the bar relative to the linear rate of advancment of the film and the spacing of the striations on the film. Thus,

where N is the number of penetrations per linear inch of film, R is the number of serrated edges on the beater bar, W is the rotational speed (revolutions/minute) of the beater bar, T is the number of teeth per inch on each edge of the beater bar, V is the velocity of the film in inches per minute and S is the number of striations per inch of film width.

In addition to limiting the number of fibrillating impacts to the level recited, it is also required, for reasons which will become more apparent hereinbelow, that the points of impact in adjacent webs be staggered. With specific reference again to the fibrillating apparatus shown in FIG. 1, this staggering of impact points is accomplished by a helical positioning of the teeth on succeeding serrated edges of the beater bar. By this is meant that the teeth on successive edges are laterally displaced from those on the preceding edge by a fraction of the distance between the teeth on the preceding edge. Further, the teeth on each serrated edge are spaced at a greater distance than are the backbone filaments of the striated film, so that each serrated edge contacts only a specific fraction of the webs across the film. Thus, each succeeding serrated edge will contact a different group of webs from that contacted by its predecessor. Since the film is being continuously advanced the points of contact are linearly displaced in successively contacted webs.

The striated films are depicted in FIGS. 2 and 4 after they have been subjected to the action of the fibrillation apparatus and, as stated, are still comprised of a plurality of relatively thick ribs orstriations 12 separated by interveningoriented web sections 13. As a result of the fibrillating impacts, however, the webs contain a plurality ofslits 14. These slits are uniformly spaced within each web, i.e., they repeat at a constant spacing so that the number of penetrations per linear inch is constant, and from one web to the next their locations are uniformly displaced along the length of the film.

In comparing FIG. 2 and FIG. 4, it will be observed that, in a given line across the width of the film in FIG. 2, the fibrillating penetrations are separated by two interconnected striations whereas in FIG. 4 the penetrations are separated by three interconnected striations. There are two principal distinctions between these products. The first is that the penetration frequency of the FIG. 4 product is less than that of FIG. 2, i.e., the number of penetrations per linear inch of intervening web is less. Equally important, however, the penetrations are staggered across the width of the film so that their pattern repeats at three-web intervals rather than on every other web. This staggering is brought about by employing a fibrillating apparatus having a greater number of serrated edges and, therefore, a different helical displacement of the teeth on succeeding serrated edges. By further manipulation of this parameter of the fibrillation process, products having even greater numbers of interconnected striations can be prepared.

As a rule of thumb, the number of interconnected striations between perforations across the web will be a function of the number of striations per inch of film width and the effective number of teeth per inch on the beater. (The product of the number of teeth per inch on one edge times the number of edges.) In fact, when the number of striations per inch of film width is equal to the effective number of teeth per inch on the beater, the number of interconnected striations between slits will, in an idealized situation, be the same as the number of serrated edges on the beater. This rule does not consider, however, the possibilities of punctures being propagated Wl'thlIl a web. When propagation occurs, the splitting and perforations assume a more random, less predictable pattern. Such propagation can occur during fibrillation or when the partially fibrillated film is expanded for use as specified hereinafter.

When the partially fibrillated films of FIGS. and 4 are subjected to a lateral stress, the slits in the intervening webs are caused to open, forming network structures as are depicted in FIGS. 3 and 5. The openings in the network structures correspond to the slits. The network structure illustrated in FIG. 3, joined predominantly by two interconnected striations represents approximately the upper limit of penetration frequency along any one web which is employed according to this invention and the lower limit of cross-web displacement of penetrations, i.e., the lower limit of the number of interconnected backbones between perforations along a transverse axis of the film. The FIG. 5 product, joined predominantly by three interconnected backbones has a lower linear frequency of penetrations and a higher cross-web displacement.

The preceding discussion is based on the assumption of an ideal fibrillation procedure wherein one perforation of the Web results from each impact of the fibrillation device on the film and such perforations are of uniform size and location. Thus, in FIGS. 3 and 5, the network structures are shown with the slits opened to a symmetrical, hexagonal shape. With more or less lateral stress, they can assume a substantially square shape or a diamond sha e.

'l he symmetry of the networks as shown in the drawing, however, is exaggerated for the sake of the examples they represent. Such symmetry results from an ideal fibrillation procedure as mentioned above. In more normal practice, the fibrillation will not be so precise, even though the number of fibrillating impacts may be as calculated from the equation set forth herein'above, because the equation does not consider the possibility of the fibrillating perforations being propagated within a web during fibrillation. The degree of such propagation can vary considerably depending upon at least two additional factors. These factors are the tension on the film as it passes over the fibrillation device and the amount of orientation which has been imparted to the film prior to fibrillation. As either or both of these parameters increases the probability of the perforations being propagated so that a randomly split web will result also increases. However, the unique structural characteristic remains, i.e., a network structure whose backbone element is the striations on the original film interconnected by unperforated sections of that film between two or more consecutive striations.

Propagation of the fibrillating perforations can also take place during lateral spreading of the web to form the opened network structure. The amount of such propagation which will take place, which is the ultimate control on the extent to which the web can be opened, is affected by the cross-web displacement of the slits relative to the linear penetration frequency and the degree of orientation to which the film has been subjected.

As the degree of film orientation is increased, the oriented webs become more splittable and accordingly tear more easily under the influence of the stress to which the film is subjected in spreading the same to an opened network. Ata specificfilrnorientation level, it will be apparent that, as the magnitude of the cross-web displacement of the fibrillating penetrations is increased, the linear penetration" frequency must be decreased in order to allow spreading of the film to open the network without an undue amount of propagation of the perforations by tearing. H l

The lateral'stress required to open orspread the network structure can be applied in any convenient way such as by means of a tenter as illustratedin FIG. 6. Partially fibrillatedfilm 17 from the fibrillation device is fed into the tenter indicated generally at 18 and gripped byendlesschains 19 which diverge in, the area designated S at a rate calculatedto effect the desired degree of opening of the web to form anetwork structure 20. As a general rule, the amount of lateral spreading is about 2 to times and preferably about 4 to 7 times.

Other techniques for accomplishing the opening of the fibrillated film will readily occur to those skilled in this art. For example, the film can be drawn over a crown idler roll under tension.

It will be apparent that the'method of the invention is applicable to anythermoplastic film and fiber-forming material. Exemplary but not all inclusive example of such materials are polypropylene, polyethylene, acrylic polymers and copolyiners, polyesters, polyvinyl chloride and nylon.

The network structures prepared by spreading and opening the films are characterized, by high machine direction tensile strength and high tear strength in the crossdirection. They are usefulas reinforcing materials in many applications where high linear strength is desirable, such as, e.g., in paper or plastic tape and strapping materials and as reinforcing materials in nonwoven fabricsor scrims. Laminates of several of the open network structures can be used as webbing and strapping in furniture upholstery.

In applications such as the aforementioned tape and strapping material reinforcement, the network structures can be employed per se using known techniques for incorporating strand-like reinforcement into paper or plastic sheets. Thus, e.g., in making a paper tape, the fibrillated film can be stressed to open up the network structure and while it is held open or fixed in the open position, the paper is formed around it and calendered thereon. In the case of plastic tape, the plastic material can be extruded onto the opened network structure in such a way that the network is embedded in the plastic without losing its useful structural characteristics.

In preparing non-woven fabrics based on the opened network structures, several approaches are possible. In one such approach, two or more of the opened networks can be laminated to one another 'with their longitudinal axes parallel to form a scrim. The films are preferably stressed to different levels of expansion, so that striations or backbone filaments on adjacent layers do not coincide. In this way, one film tends to hold the other in the opened or spread configuration and the films reinforce each other to improve both the longitudinal and transverse strength of the resultant fabric or scrim.

In another method of preparing a non-woven fabric or scrim from these films, the opened film can be overlaid with a beam of parallel filaments aligned predominantly in the machine direction and bonded to the opened net. The parallel filament beam can be multifilament yarns or a monofilament yarn beam such as is prepared by complete or substantially complete fibrillation of a striated film as is taught in the above mentioned Kim et al. US. Patent 3,495,752.

The laminated films can be bonded by adhesive means, by means of heat or ultrasonically. A very effective means of accomplishing bonding is to employ conjugate striated films, wherein the film is prepared in layers, one of which is lower melting than the other and, upon application of heat, melt preferentially to form a fusion bond. In the same way, one component of the laminate can be a lower melting polymer than the other. Thermoplastic adhesive coatings, applied to either component from an emulsion, from solution, or by hot-melt can also be used for heatbonding.

Scrims prepared by the parallel lamination of films as described above have their tensile strength predominantly in the longitudinal direction since that is the direction of the continuous filaments. There is a small amount of tensile strength in the transverse direction due to the reinforcing effect of the bonded filaments upon one another. However, the'tear strength of the scrims in the transverse direction is quite high.

As stated above, scrims of this type are useful in ap plications .where only linear strength is required. A good example of such a utility is the plastic tape employed as webbing in lawn chairs and other casual furniture.

Fabrics or scrims of the type discussed can be of a wide range of fabric weights, e.g., from about 0.1 to 20 ounces/ sq. yd. Fabric weights vary according to the amount of spreading of the network structure, the d.p.f. of the striations and the number of layers which are laminated. The fabric weight selected will depend upon the utility for which the scrim is intended.

In another method of preparing non-woven scrims, two or more of the opened networks can be cross-overlaid at an angle of about 45 to about A structure of this type exhibits significant cross direction tensile strength as well as longitudinal strength since there are continuous filaments aligned in each direction. A method of accomplishing this cross-overlaying is taught in copending US. application Ser. No. 885,595, of Kim, filed Dec. 16, 1969.

Any of the scrims prepared from opened network structures according to the invention can also be used as reinforcing materials for other types of non-Wovens. For example, a randomly oriented batt of staple fibers can be laid on either side of an opened network and bonded by known methods to form a non-woven. The fabric will have substantially the same bulk and aesthetics as it normally would have, but will be stronger in the longitudinal direction and have greater transverse tear strength due to the presence of the reinforcing network.

EXAMPLE 1 Polypropylene was extruded through a fiat die onto an embossing roll with grooves S-mils wide and 4.5-mils deep, machined at right angles to the roll axis on l0-mil centers. The cast embossed film had a profile corresponding to the embossing roll with about 300 striations per inch of width and webs of 1.5-mils thickness. This film was then linearly oriented by drawing about 6 to 1, yielding a film with ribs about 2-mils thick by 1.7-mils wide, and separated by web 0.7-mil thick and 1.6-rnils wide.

Fibrillation of the oriented film. was effected by a 12- edged bar with 40 teeth/inch on each edge rotating at 800 r.p.m. with the film traveling at 160 ft./minute. The penetration frequency was about 0.66/inch. The resulting net structure was joined together by substantial lengths of unsplit webs between a plurality (two, three, or more) of ribs extending in the machine direction.

EXAMPLE 2 A bicomponent striated film containing 80% of a propylene homopolymer and 20% of an ethylene-propylene copolymer, with a melting point 5-7 C. lower than the homopolymer, was formed by extrusion and casting onto an embossing roll. The film was linearly drawn 6 times in a heated oven to a thickness of 0.003 inch with 200 striations per inch of width. After cooling, this film was fibrillated at ft./minute by a l2-edged beater bar, 3 inches in diameter, with 40 teeth per inch on each edge, rotating at 400 r.p.m. The fibrillation frequency was about 0.57/inch. The fibrillated film was then laterally spread to 7 times its width to form a net interconnected by unsplit Webs between two or three 18 d.p.f. filaments. This net was bonded under 75 p.s.i. pressure at 340 F. to another net formed by spreading a similar fibrillated film to twice its width. The laminated network structure, with a weight of 0.75 oz./ sq. yd. had a strength in the machine direction of 10.2'lbs./inch of width. While the transverse strength was only 0.03 lb./ inch of width, it was dimensionally stable and could be readily handled without deformation.

EXAMPLE 3 The fibrillated film of Example 2 was spread to 7 times its Width and laminated under the same conditions to a similar fibrillated film opened to 4 times its original width. The resulting laminate had a Weight of 0.4 oz./ sq. yd. with a machine direction strength of 5.0 lbs/inch of width and 0.025 lb./inch in the transverse direction.

EXAMPLE 4 Example 2 was repeated with a striated film formed by extrusion through a profile die. The laminated net of 22 d.p.f. filaments had a weight of 0.9 oz./sq. yd., a machine direction strength of 12 lbs./inch, and a transverse strength of 0.035 lb./inch.

In addition to their use in preparing the opened network structure shown in FIGS. 3 and 5, the lightly fibrillated films described herein can be employed to prepare yarn. This is done by twisting narrow ribbons of the films, without their being opened to form a network tially fibrillated striated film of a thermoplastic polymer having alternating relatively thick longitudinal rib sec-. tions and relatively thin longitudinal web sections, saidweb sections being split between each pair of rib sections about 0.5 to 2.0 times per linear inch with the splits in adjacent web sections being linearly displaced from one another, said film being transversely expanded to form the split web sections into transverse rows of openings with each opening in a row being separated from the adjacent opening by. at least two or more rib sections which are interconnected by web sections. 1

2. A laminated network structure according to claim 1 wherein at least one of said layers is cross-laid at an angle of about to with respect to the longitudinal axis of at least one other of said layers.

3. A laminated network structure according to claim 1 wherein said layers are superimposed longitudinally and are expanded to different levels of expansion whereby corresponding rib sections on adjacent superimposed layers do not coincide.

References CitedUNITED STATES PATENTS 8/1965 Scragg 57157 (WILLIAM A. POWELL, Primary Examiner J. J. BELL, Assistant Examiner U.S. C1. X.R.

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