US3489643A - Sheet material of improved tear strength including long undrawn polyamide fibers - Google Patents

Description

Jan. 13, 1-970 H. A. HOFFMAN 35:89.643

SHEET MATERIAL OF IMPROVED TEAR STRENGTH INCLUDI LONG UN-DRAWN POLYAMIDE FIBERS Filed Feb. 17, 1967 $2400- [I 2000- 35 I600 2% E m I200- D g 800 O P 400- NYLON 3200- g 2800- g 2400- UNDRAWN [i :FmZ g 2000 I600- J DRAWN 8 I200- Z 9 800 O I I I I l r IO 6O %NYLON 3200- U) 2 2800 UNDRAWN 52400- 52000- i I600- DRAWN 5 I200- s00- INVENTOR. I T HERBERT A. HOFFMAN 0 IO 20 30 4O 5O NYLON ATTOR NEYS United States Patent 3,489,643 SHEET MATERIAL 0F IMPROVED TEAR STRENGTH INCLUDING LONG UNDRAWN POLYAMIDE FIBERS Herbert A. Hotfman, Hartford, Conn., assignor to The Dexter Corporation, Windsor Locks, Conn., a corporation of Connecticut Continuation-impart of application Ser. No. 543,108, Apr. 18, 1966. This application Feb. 17, 1967, Ser. No. 626,652

Int. Cl. D21f 11/00; D21h /12 US. Cl. l62ll46 14 Claims ABSTRACT OF THE DISCLOSURE When cold drawable undrawn fibers are incorporated in nonwoven fibrous structures improved properties such as added resistance to tear propagation are imparted to the structures.

This application is a continuation-in-part of the earlier filed copending application Ser. No. 543,108 filed Apr. 18, 1966, now abandoned.

The present invention relates generally to the art of nonwoven fabrics and is more specifically directed to a new and improved nonwoven fibrous structure having outstanding tear strength characteristics.

For some time attempts have been made to improve the tear resistant properties of nonwoven fibrous materials, such as cellulosic paper produtcs and the like, by incorporating into the papers various types and amounts of synthetic fibers. Recently man-made synthetic fibers such as nylon have been successfully added to paper-making furnishes and used for producing tear resistant papers which take advantage of the high strength, toughness, tenacity and durability of these fibers. The nylon fibers employed are generally supplied as conventional textile tows or bundles of filaments to which have been applied special finishes for permitting the adaptation of the fibers to the wet or dry process being used. These filaments or bundles of filaments are generally formed by extrusion through spinnerettes in accordance with various known techniques and then cold drawn or stretched to orient or align the molecules along the filament axis and produce the strength and durability characteristics normally associated with textile filaments. The drawing or elongation to which the filaments are subjected tends to more closely pack the molecules and increase the intermolecular forces of attraction therebetween, thereby producing greater tensile strength and increased tenacity and modulus within the drawn filaments. Understandably, papers incorporating fibers cut from these drawn filaments exhibit proportionally greater strength, toughness, and fold endurance than is normally possessed by papers made solely of cellulosic fibers. However, papers utilizing these manmade fibers have not experienced Widespread acceptance, primarily because of the increased cost factor associated with the incorporation of drawn man-made synthetic fibers.

Accordingly, it is an object of the present invention to provide a new and improved nonwoven fibrous structure which not only retains the advantages of the drawn synthetic fibers while obviating the disadvantages thereof but also exhibits a substantial and unexpected improvement in the tear strength of nonwoven materials at the same and lower synthetic fiber concentrations. Included in this object is the provision for the incorporation into nonwoven fibrous materials of a lower percentage of fib'ers to produce a structure having greater resistance to tear propagation including greater elongation to break and improved stitchability, softness and hand.

Another object of the present invention is to provide a new and improved specialty paper product exhibiting improved tear strength and prolonged structural integrity while being characterized by exhibiting along torn edges thereof fibers of appreciably greater length than those in the body of the product, said paper incorporating a specific type of fiber whereby the paper may be used in fabrication processes subject to high speed operations and some of the resultant materials may even be laundered and/0r dry cleaned.

A further object of the present invention is to provide in a facile, efiicient and economical manner a new and improved nonwoven Web material exhibiting the characteristics of high tear resistance, low tear propagation, high tear energy absorption, and good stitchability whereby should the material sutfer a slight rip or hole it will have a substantial number of fibers traversing the rip or hole so as not to render the product unusuable.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly comprises the several features, properties, characteristics, uses and relation of elements which are exemplified in the following detailed disclosure, the scope of the application of which will be indicated in the appended claims.

In the drawing FIGS. 1-3 are graphs illustrating typical improved tear strength characteristics obtained in sheet materials made according to the present invention.

In accordance with the present invention, it has been unexpectedly found that nonwoven fibrous structures exhibit substantially greater tear strengths when cold drawable, undrawn fibers are employed in place of the more conventional drawn, albeit stronger, fibers of the same material. Inherent in undrawn, cold drawable material is the predominantly nonelastic or nonrecoverable deformation which occurs during elongation coupled with the substantial extensibility or elongation exhibited by the materials prior to failure, the latter quality being measured as the ultimate elongation to break. Drawn materials also exhibit a measurable ultimate elongation to break. However, it should be understood that the elongation of drawn materials is relatively small and is predominantly the result of recoverable or elastic stretch by the fibers. As a point of reference for comparative purposes, cold drawable materials generally exhibit an ultimate elongation to break of about 200 percent or more, that is, they are drawable to greater than about three times their original length before breaking. On the other hand, drawn fibers employed heretofore generally exhibit an ultimate elongation to breaking of about 20 percent or less, with some drawn nylon fibers reaching as high as 40 -45 percent ultimate elongation. Accordingly, drawn fibers may also be referred to as those which have an ultimate elongation to break of less than about 200 percent.

The improvement in tear strength etfectuated by the invention, which improvement may be as great as threefold or more, is graphically illustrated in the drawing wherein the tongue tear strength is plotted against the amount of drawn and undrawn nylon fibers within otherwise identical sheet materials. Although in practice cold drawability is almost always accompanied by lower tenecity, it is recognized that a high tenacity not only can be permitted but will be quite beneficial if it coexists with cold drawability in the same material. Accordingly, the present invention employs fibers which, despite their low tenacity, product fibrous structures of improved tear characteristics.

These nonwoven fibrous materials are of particular importance with respect to products having uses wherein high resistance to tear propagation and/or the other improved characteristics of the present invention are especially desirable. For example, these materials may be used as base sheets for stencils such as those used with mimeograph or addressing machines, as a base for industrial and domestic tapes; as substitutes for body imparting buckram interliners, or as disposable items such as towels, wash cloths, wipe or dust cloths, headrest covers, table cloths, aprons, diapers, or disposable sheets, pillow cases and dressing gowns for hospital use, especially those employed in operating rooms and the like such as surgical packs and drapes. The improved fibrous structures may also be advantageously employed in more permanent items as carpet backings, books and bookbindings, electrical insulation, laminated structures including reinforcing layers for plastic film and low strength leather, industrial and domestic clothing such as costumes and novelty clothing including interlinings for clothing, aisle runners, tarpaulins and tents, curtains and draperies, upholstery for home furnishings and automobiles, sporting equipment such as hunting or bullet-proof vests and athletic pads and linings, protective wrappings such as food wrappings including meat casings and the like, industrial wrappings and packaging such as ditch liners or underground pipe wrappings and fabric-like structures for filtration or infusion including vacuum bags, tea or coffee bags, dust collectors and supports therefor. Many other uses including currency, cover stock for sanitary and personal products, cordage, photographic paper, interliners,

inflatable structures, shrubbery protectors and pool covers will also be apparent as ways in which these materials may be advantageously utilized.

The nonwoven fibrous materials of the present invention which exhibit the improved properties, characteristics and uses set forth herein are generally made by forming a fluid dispersion of fibers including the highly extensible, cold drawable, undrawn fibers of the present invention, and then depositing the dispersed fibers in the form of a fibrous sheet-like structure. The fiber dispersion may be formed in a conventional manner using water as the dispersant or by employing other suitable dispersing media such as air. Preferably, aqueous dispersants are employed in accordance with known papermaking techniques and accordingly, for clarity and ease of understanding, the following description will deal primarily with water-laid webs. Accordingly, a fiber dispersion is formed as a dilute aqueous suspension of papermalcing fibers, i.e., fibers having a length conventionally well below one inch, which is then conveyed to the webforming screen or wire such as a Fourdrinier wire of a paper-making machine and deposited on the wire to form a fibrous web or sheet which is subsequently dried in a conventional manner. The sheet or web thus formed may be treated either before, during or after the drying operation to properly bond the fibers, particularly the cold drawable, undrawn fibers, within the sheet and thereby take full advantage of their unique properties.

As will be appreciated, the undrawn but cold drawable fibers preferably used in accordance with the present invention need only constitute a minor portion of the total fiber mixture from which the sheet is formed in order to produce the beneficial effects of the present invention. In fact, as illustrated in FIGS. 1 and 3 of the drawing, the beneficial properties are evidenced with even small amounts such as only a few percent and seem to level off at about 70 percent so that the use of more than 70 percent undrawn fibers within these sheets gives no corresponding improvement in properties. It will, of course, be appreciated that the improved properties are substantially evident up to and including an undrawn fiber content of 100 percent even though the improvement is not as dramatically illustrated above a 70 percent undrawn fiber content when used with bonding systems similar to those employed with the materials of FIGS. 1 and 3. However, as evidenced by FIG. 2, when better bonding systems are employed the improvement of the undrawn, cold drawable fibers shows a sharper rise with no evidence of ap- 4 preciable leveling off as the 70 percent undrawn fiber content is approached. The remainder of the fiber pulp within these mixtures may be conventional cellulosic pulp prepared by conventional pulp producing methods including mechanical, semi-mechanical or chemical pulp preparation or may be mixtures of cellulosic fibers and drawn, man-made synthetic fibers of the same or diffierent chemical compositions as the undrawn but drawable fibers. The fiber stock may be unrefined or refined in a beater or other device and may include bleached or unbleached stock. The undrawn extensible fibers, on the other hand, generally do not need beating or other preparatory operations but are merely cut to the desired length and dispersed in the furnish after which the fiber dispersion is diluted to the proper consistency prior to web formation. Conventional paper-making additives such as fillers, wet strength resins and the like may also be utilized, if desired.

The fiber length of the materials employed will depend on the characteristics of the materials as well as on the method of fiber dispersion and deposition. It is believed that most fibers of a length suitable for use in the particular process employed will be elfective. However, it is generally preferred that longer cold drawable fibers be employed where practical. Thus, as illustrated hereinafter in the specific examples, the fiber length of the undrawn fibers is greater than the length of conventional papermaking fibers, i.e., about A inch in length or longer.

The fibers suitable for use in accordance with the present invention generally are synthetic man-made fibers formed from organic thermoplastic materials capable of producing fibers in an undrawn yet drawable condition. That is, the fibers exhibit a low molecular lateral order or orientation, relatively low tenacity or modulus and high elongation. Typical of such materials are the melt-spinnable thermoplastics generally produced in the form of continuous filamentary tows for the textile industry. These materials are utilized essentially as spun, that is, they are not subjected to deliberate drawing operations. However, even if the filaments are drawn somewhat they may still be employed in the present invention so long as they continue to exhibit the requisite cold drawability. These materials include but are not limited to synthetic organic polymers and copolymers of polyamides, acrylics, polyesters such as polyethylene terephthalate, vinyls such as polyvinylidene chloride, and polyolefins including polyethylene and polypropylene. Among these materials polyamides such as nylon 66 [poly (hexamethylene adipamide)] are preferred due to their combined beneficial strength characteristics and relative ease of bonding as compared with the other materials mentioned.

As indicated hereinbefore, the undrawn filamentary material and therefore the fibers cut therefrom should primarily be capable of high, nonrecoverable elongation and should possess a substantially constant order of molecular orientation along their entire length. It will, of course, be appreciated that although the degree of molecular orientation is less than that exhibited by the fully drawn material and substantially constant along the individual fibers, their external configuration need not be uniform. Rather the fibers may exhibit fluctuations, such as varying denier and/or varying cross-sectional dimensions whereby the fibers are more readily retained within the structures, e.g., the fibers may possess a knobby or a dumbbell configuration. Additionally, rough or irregular surface characteristics which will enhance the mechanical bonding of the drawable material within the fibrous structure are also contemplated by the present invention. As mentioned, the fibers also should be capable of being drawn to at least three times their original length without breaking. Typically, the percent elongation of the undrawn material is more than five times that of the corresponding drawn material. For example, in the case of polyethylene terephthalate the ultimate elongation to breaking is about 450 percent for the undrawn fibers as compared with only 45 percent for the drawn fiber-a ten-fold difference, while the corresponding elongations for nylon 66 are about 900 percent and 35 percenta twenty-five-fold difference.

The tenacity of presently available undrawn but cold drawable materials is quite low as compared with that of the same materials when drawn. It has been found that the improved results of the present invention are achieved when the undrawn tenacity is less than 75 percent of that exhibited by the same materials when drawn. In fact, satisfactory results are obtained at undrawn tenacities of only about percent of that possessed by their drawn counterparts with the preferred materials now commercially available having tenacities ranging from to percent of the tenacity of the drawn materials. Exemplary of the preferred range are the undrawn tenacity percentages for nylon 66 and polyethylene terephthalate fibers which are about 22 percent and 29 percent, respectively, the tenacities of the undrawn fibers being 1.0 and 1.4 grams per denier, respectively. However, as mentioned hereinbefore, it is recognized that undrawn fibers which possess the requisite drawability and have appreciably higher tenacities will be quite advantageous when used in accordance with the present invention. Unfortunately, however, such materials are not presently available on a commercial basis.

Due to the low tenacities possessed by the undrawn but drawable, man-made, synthetic fibers, they have heretofore been considered inferior to drawn fibers made from the same synthetic materials, especially when it was de sired to improve the strength of the sheet materials produced therefrom. However, it has been unexpectedly found in accordance with the present invention not only that these lower tenacity fibers produce sheet structures of tear resistance equal to that possessed by sheets containing the same percentage of drawn fibers but that the tear resistance is appreciably enhanced when low tenacity, cold drawable fibers are employed. This result is strikingly demonstrated in the graphs of the drawing where, in each figure, it can be easily seen that the undrawn fibers produce a remarkable improvement in tear strength over the drawn fibers at the same fiber concentration.

The undrawn man-made synthetic fibers of the present invention are used as formed with the exception that water-dispersing and anti-static additives may be applied thereto in order to provide greater adaptability of the fibers to the wet or dry-laid process employed. It will be appreciated that these materials may frequently be crimped or twisted after they come from a spinnerette in order to hold all the filaments together as a tow and accordingly, will not necessarily result in uniformly straight fibers when cut to length for use in accordance with this invention.

The improved result obtained in accordance with the present invention will vary depending on the binder system employed. Suitable bonding may be effectuated by adhering the extensible fibers to the other components of the sheet and/or to themselves through a conventional paper bond with cellulosic fibers or by employing resins, solvents or heating techniques. Resin binders used should be compatible with the synthetic fibers employed and may themselves contribute somewhat to the tear strength characteristics of the resultant sheet structure as evidenced by the information set forth in the drawing, Such binders may be applied by conventional binder application techniques including uniform applications such as dip coating and continuous or discontinuous applications such as patterned or spot bonding. Soft or weak binders are generally employed to produce softer, more flexible structures with the particular binder used being dictated by both the end result desired and the fibers being bonded. However, it will be appreciate that the present invention does not depend on the use of any particular binder or binder system and most of the conventional binders normall used in nonwoven fibrous structures may be advantageously employed.

It has long been recognized that the force required to initiate a tear is substantially greater than that necessary to continue or propagate the tear. Accordingly, the resistance to tear propagation is used herein to illustrate the beneficial results of the present invention. The tearing strengths of the sheet materials are measured according to a tongue or single rip method similar to the ASTM method designated D2262-64T. In the method used a constant-rate-of-traverse tensile testing machine, such as the Scott Tensile Tester is employed. The tearing strength is measured by holding the long sides of a rectangular 2 x 3 inch specimen, cut in the shorter edge to form two tongues. The tongues are held by a pair of clamps and the specimen is pulled to simulate a rip. Thus, the tearing strength measured in this method is the maximum force required to continue or propagate a previously started tear in the test specimen. The force registered in the test is the highest peak load recorded during travel of the rip a measured distance, usually about 1% inches.

Observation of the sheet material during the testing procedure will provide a far more appreciative understanding of the invention since the unusual form and appearance of the rip or tear thus produced is a predominnant characteristic of the nonwoven fibrous materials of the present invention. Consequently, in addition to the measurable resistance to tear propagation evidenced by these materials there are visual improvements in tear resisfance. These include substantial increases in the length of the zone of tear propagation, i.e., the zone where fibers continue to traverse the tear and are held by both tongues of the sheet material, and substantial increases in the length of the fibers forming that zone. In conventional materials this zone is so small that it is frequently unobservable upon unaided visual examination. However, in accordance with the present invention, the zone of tear propagation has been extended to such an appreciable degree that zones of one inch length and more are common.

From the foregoing it can be appreciated that as the nonwoven material rips or tears, the drawable fibers therein will elongate or extend until they either pull away from their points of contact within the body of the fibrous structure or reach their ultimate elongation and break. Accordingly, in any given nonwoven fibrous structure, substitution of cold drawable fibers in place of drawn fibers of comparable size will generally result in longer exposed fibers along the edge of rupture. Although the theory of the present invention is not fully understood, it is believed that the large number of extensible fibers traversing the line of tear together with the progressive increase in the tensile strength of the individual fibers as they are subjected to elongation provide a combined and possibly synergistic resistance to the tearing force. A far greater number of fibers resist the tearing force or load applied to the sheet along a substantially lengthened zone of tear propagation. At the same time the elongation of the highly extensible fibers tends to orient and align the fiber molecules along the fiber axis to thereby increase the resistance to elongation of the individual fibers and further resist the tearing force. The elongation of the fibers along their length will, of course, help to distribute the force throughout the fibers. However, regardless of the theory or possible explanation it is abundantly clear that substantially improved results are obtained in accordance with the present invention.

Of additional significance is the appearance of the nonwoven material as well as the impression imparted by the material as it is ruptured. The elongated fibers at the zone of rupture leave the impression that the fibers originally possessed a length at least equal to that of the fibers bridging that zone. It will, of course, be appreciated that this impression is incorrect since the bridging fibers are of substantially greater length than the fibers actually employed to make the sheet material. This impression is in itself beneficial as it imparts to even the knowledgeable observer the concept of improved strength long associated with elongated filament-like fibers, especially since the incorporation of such fibers in wet-laid webs has long evaded the art.

In order that the present invention may be readily understood the following examples are given by way of illustration but are not intended to be in any way a limit on the practice of the invention.

EXAMPLE I This example illustrates the eifect which small amounts of highly extensible undrawn fibers have on the tear strength of sheets containing them.

A fiber dispersion was prepared by beating bleached kraft wood pulp for about 13 minutes at a consistency of 1.5 per cent in a l' /2# Valley laboratory beater with a 5 pound weight and counterweight on the bedplate lever arm. Handsheets were made from a portion of this dispersion which contained a slight amount of wet strength resin and dried both in air at room temperature and on a drum dryer at 220 F. No post formation treatment was used. The tongue tear of these sheets as measured according to ASTM D2262-64T is set forth in Column A of Table I.

The above dispersion was then divided into two portions. To each of these portions was added percent by weight inch, denier nylon fibers. The nylon added to one portion was of the drawn variety exhibiting a tenacity of 4.5 grams per denier and an ultimate elongation of about 35 percent while the nylon fibers added to the other portion were undrawn, had a tenacity of 1.0 grams per denier and an ultimate elongation of 900 percent. The tongue tear of sheets made from these portions is given in Table I. The sheets were not treated with a binder and all sheets had a basis weight of about 47 pounds. As can be seen, the tear strengths of the sheets made with undrawn fibers showed an improvement of about 100 percent over those made with the stronger fibers.

About 300 grams of bleached kraft wood pulp was beaten at a consistency of 1.5 percent for about 13 minutes in a 1' /2# Valley laboratory beater with a 5 pound weight and counterweight on the bedplate lever arm. To this pulp was added inch length, 15 denier nylon fibers. A number of handsheets were formed with nylon to wood fiber ratios varying from 0 to 70 percent nylon, namely, at 0, 5, 10, 20, 30, 40, 50, 60, and 70 percent nylon. In half of the sheets undrawn nylon was employed having a tenacity of 1.0 grams per denier and an ultimate elongation of 900 percent. The other half of the sheets were formed from pulps containing drawn nylon 66 fibers having a reported tenacity of 4.5 grams per denier and an ultimate elongation of 35 percent.

The sheets were couched between blotters and dried on a drum dryer at 220 F. All of the dry sheets were then treated with 6 percent by weight of cross-linked polyvinyl alcohol (Elvanol 72-60E. I. du Pont de Nemours & Co.) and dried. The tear strengths of the resultant sheets were then tested in accordance with ASTM D2262-64T and the results plotted on the graph of FIG. 1. As illustrated in FIG. 1, the sheets made from undrawn nylon fibers exhibited substantially greater tear strengths at all levels of nylon concentration despite their lower fiber strength. In fact, it can be readily observed that only 5 percent of the extensible undrawn nylon gave results comparable to ten times that amount of drawn nylon.

EXAMPLE III The procedure of Example II was repeated except that the polyvinyl alcohol binder was replaced by 25 percent 8 by weight butadiene-acrylonitrile latex (Hycar 1561- B. F. Goodrich Chemical Co.). The tearing strengths when plotted against the percent nylon content resulted in the graph labeled FIG. 2, again depicting the beneficial results obtained from the lower tenacity, highly drawable, undrawn fibers.

EXAMPLE IV Example 111 was repeated using an acrylic latex (Resyn X-Link 2873National Starch and Chemical Co.) in place of the butadiene-acrylonitrile. The results are charted in FIG. 3.

EXAMPLE V Sheet materials similar to those of Example IV were made keeping the nylon content at 30 percent while varying the ratio of drawn to undrawn fibers. All the nylon fibers were inch in length, the drawn fibers being 15 denier while the cold drawable fibers were reported as being 19 denier. The tongue tear measured for each mixed fiber sheet is set forth in Table II.

TABLE II Percent of drawn vs. undrawn Tongue tear, g. :0 1270 75:25 1320 50:50 1390 25:75 1706 0:100 1988 EXAMPLE VI Nonwoven webs made from 100 percent synthetic fibers were prepared. One web was made entirely from nonextensible drawn nylon 66 of inch length, 15 denier fibers while another web was made from cold drawable undrawn nylon 66 of 4 inch length, 19 denier fibers. The webs were produced by slowly feeding dry nylon fibers into a Buchner funnel under vacuum. The unbonded webs each had a basis weight of about 50 pounds per 2880 square feet.

The webs were saturated with the self-cross-linking acrylic latex of Example IV (Resyn X-Link 2873) and dried at 270 F. for 5 minutes. Each of the bonded airlaid webs had a basis weight of about 65 pounds. Upon testing for tongue tear in accordance with ASTM D2262- 64T the web made from extensible undrawn nylon exhibited a 30 percent improvement over the web made from drawn fibers.

EXAMPLE VII Fiber furnishes of 70 percent wood and 30 percent nylon were prepared in accordance with the procedure of Example I, the furnishes containing 0.2 percent based on the weight of the fiber of an epichlorohydrin-polyamide wet strength resin (Kymene 557Hercules Powder Co.). The nylon fibers in one furnish were cut from drawn filaments while the nylon fibers in the other furnish were undrawn. Sheets having a basis weight of 47 Pounds/ 2880 square feet were produced from each furnish and dried at 250 F. They were then heat bonded by subjecting the sheets to 400 F. for 15 seconds at 300 psi. The tongue tears of the undrawn fibrous sheets were appreciably higher than those of the corresponding drawn fiber sheets both before and after bonding.

EXAMPLE VIII Sheets having a basis weight of 20 pounds/ 2880 square feet were made in accordance with the procedure of Example VII and after drying were saturated with the acrylic latex resin of Example IV (Resyn X-Link 2873). The sheet made with drawn nylon fibers exhibited a tongue tear of 635 grams while the sheet made using extensible undrawn nylon fibers gave a tongue tear of 975 grams,

9 EXAMPLE IX A fiber dispersion was prepared by defibering bleached kraft wood pulp in a laboratory beater. About 30 percent by weight of 4 inch polypropylene fibers were added to two aliquots of the pulp dispersion which contained no wet strength resin. The first aliquot received drawn 3 denier fibers having an elongation of about 32 percent and a tenacity of 6 grams per denier while undrawn 6 denier polypropylene fibers of about 500 percent ultimate elongation and about 1.4 grams per denier tenacity were added to the other aliquot. Handsheets having a basis weight of 47 pounds/2880 square feet were prepared from each aliquot and treated with toluene solutions of an acrylic ester resin (Acryloid K-7004Rohm & Haas) to provide differing levels of resin retention. The tongue tear of the resultant sheets is set forth in Table III.

TABLE III Tongue Tear, g. Resin retention, Drawn Undrawn percent fiber fiber EXAMPLE X The procedure of Example IX was repeated using polyester fibers in place of the polypropylene. The drawn polyester used was inch, 2.5 denier polyethylene terephthalate having a tenacity of about 5.5 grams per denier and an elongation of 30 percent while the cold drawable fibers were 4 inch, 3 denier polyethylene terephthalate having a tenacity of 1.5 grams per denier and an elongation of 450 percent. The handsheets prepared were treated with resin to a retention of 24 percent. The sheet made from undrawn polyester gave a tongue tear of 494 grams while the drawn fiber gave a tongue tear of 320 grams.

I claim:

1. A nonwoven sheet material comprising paper-making fibers and up to about 70% by weight cold drawable, undrawn polyarnide fibers of at least about %1 length, said undrawn fibers having an ultimate elongation of more than 200% and possessing a substantially constant order of molecular orientation along individual fiber lengths, said undrawn fibers being unfused and being present in an amount sutficient to impart to the sheet material an ability to provide an extended zone of tear propagation with an appreciable number of said undrawn fibers elongatably bridging said zone to resist tear propagation therealong.

2. The sheet material of claim 1 wherein the tenacity of the undrawn fibers is about 2 grams per denier or less.

3. The sheet structure of claim 1 wherein the undrawn fibers have an ultimate elongation of at least 500%.

4. The sheet structure of claim 3 wherein the polyamide fibers are nylon having an undrawn to drawn tenacity ratio of less than 0.3 to 1.

5. The sheet structure of claim 1 wherein the undrawn fibers are poly (hexamethylene adipamide) fibers having a tenacity of about 1.0 gram per denier and the sheet includes about 45% by weight of a resin binder.

6. The sheet structure of claim 1 including about 45% by weight or less of a resin binder.

7. A nonwoven fibrous sheet structure comprised of organic, synthetic, and thermoplastic cold drawable un drawn fibers and a binder for the undrawn fibers, said binder being selected from the group consisting of resins and conventional paper-making fibers, said undrawn fibers having a length of about A" and longer, an ultimate elongation of more than 200% and a substantially con stant order of molecular orientation along individual fiber lengths, the undrawn fibers being unfused and being present in an amount sufiicient to impart to the sheet structure an ability to provide an extended zone of tear propagation with an appreciable number of cold drawable fibers elongatably bridging said zone to resist tear propagation therealong.

8. The material of claim 7 wherein the ultimate elongation of the drawable fibers is at least five times the ultimate elongation of the same fibers when fully drawn.

9. The sheet structure of claim 7 wherein the undrawn fibers are selected from the group consisting of polyarnide and polylefin fibers.

10. The sheet structure of claim 7 wherein the undrawn fibers are polyarnide fibers.

11. The sheet structure of claim 10 wherein the sheet includes about 45% by weight and less of a resin binder.

12. The sheet structure of claim 7 wherein the cold drawable undrawn fibers comprise essentially and less of the fiber content of the sheet.

13. The sheet structure of claim 7 wherein the tenacity of the cold drawable fibers is less than the tenacity of the same fibers when fully drawn and the ultimate elongation is greater than three times the ultimate elongation of the same fibers when fully drawn.

14. The sheet structure of claim 7 wherein the fiber length of the undrawn fibers is about and longer.

References Cited UNITED STATES PATENTS 3,053,609 9/1962 Miller 162157 X 3,081,519 3/1963 Blades et a1. 162157 X 3,186,897 6/1965 Hochberg 162-157 X HOWARD R. CAINE, Primary Examiner US. Cl. X.R. 162-157, 169,

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