US3912567A - Stabilized nonwoven web and method of preparation - Google Patents

Abstract

A process is described for the intermittent autogenous bonding of a continuous filament web wherein the filaments have a low degree of crystallinity. In one embodiment, the process involves passing the web directly through a nip formed by a smooth hard surfaced roll and a roll containing raised points on its surface with both rolls being maintained at a temperature near the softening point of the filaments. The speed of the web through the nip, the roll size and the configuration of the raised points are coordinated such that the surface temperature of the web is not substantially increased before maximum pressure has been developed in the nip, but then is rapidly raised to a point where surface fusion is effected before a significant increase in filament crystallinity occurs. Webs prepared as illustrated possess a desirable combination of surface abrasion resistance and strength.

Description

United States Patent [191 Schwartz Oct. 14, 1975 4] STABILIZED NONWOVEN WEB AND 1,245,088 9/1971 United Kingdom METHOD OF PREPARATION 799,933 11/1968 Canada [75] Inventor: J. Schwartz, Wmneconne, i y Examiner seorge F. Lesmes Assistant Examiner-Ellis P. Robinson [73] Assignee: Kimberly-Clark Corporation, Attorney, Agent, or FirmWolfe, Hubbard, Leydig,

Neenah, Wis. -Voit & Osann, Ltd.

[22] Filed: May 14, 1973 21 Appl. No.2 359,673 [57] ABSTRACT A process is described for the intermittent autogenous bonding of a continuous filament web wherein the fila- [52] 0 ments have a low degree of crystallinity. In one em- [sn Int C12 6 3/16 bodiment, the process involves passing the web di- 58] d 306 324 rectly through a nip formed by a smooth hard surfaced o c 81 18;), 1 6 roll and a roll containing raised points on its surface with both rolls being maintained at a temperature near the softening point of the filaments. The speed of the [56] References Cited web through the nip, the roll size and the configura- UNITED STATES PATENTS tion of the raised points are coordinated such that the 3,542,634 I l 1970 Such 161/148 surface temperature of the web is not substantially in- 3692-618 10/1969 Dorschnel' l61/150 creased before maximum pressure has been developed 31695985 10/1972 Brock 156/29 in the nip, but then is rapidly raised to a point where 32 2 3 g surface fusion is effected before a significant increase 32 232 2x974 156/291 in filament crystallinity occurs. Webs prepared as il- 3:855:046 12/1974 Hansenm: .2: 156/306 i a desirable cmbination Surface abrasion resistance and strength. FOREIGN PATENTS OR APPLICATIONS 2,081,057 2 1971 France 10 Clam, Drawmgs STABILIZED NGNWOVEN WEB AND METHOD OF PREPARATION The present invention relates to stabilizing nonwoven webs into coherent structures. More particularly, it concerns a method by which webs of molecularly oriented, thermoplastic fibers can be autogenously bonded to provide fabric-like materials possessing a desirable combination of physical and aesthetic characteristics. The invention is especially concerned with the stabilization of nonwoven fiber webs of substantially continuous thermoplastic filaments.

Nonwoven webs comprising a plurality of substantially continuous and randomly deposited, molecularly oriented filaments of a thermoplastic polymer are now widely known. The following patents illustrate a variety of methods for preparing such webs and for bonding them into coherent structures: Kinney (U.S. Pat. Nos. 3,338,992 and 3,341,394), Levy (U.S. Pat. No. 3,276,944), Petersen (U.S. Pat. No. 3,502,538), Hartmann (U.S. Pat. Nos. 3,502,763 and 3,509,009), Dobo et al. (U.S. Pat. No. 3,542,615), Dorschner et al. (U.S.

Pat. No. 3,692,618), Vosburgh (U.S. Pat. Nos. 3,459,627 and 3,368,934), Harmon (Canadian Pat. No. 803,714, and Cumbers (British Pat. No. 1,245,088).

What has heretofore been considered to be a particularly desirable method of obtaining bonded nonwoven continuous filament webs, particularly at low basis weights, is described in Belgian Pat. No. 763,570, issued on Apr. 15, 1971. As illustrated in this patent, the nonwoven web is initially prepared by the method described in the above-identified U.S. Pat. No. 3,692,618 and thereafter autogenously bonded in an intermittent fashion.

The method of initial web formation involves extruding a thermoplastic polymer through a multiple number of downwardly directed spinning nozzles, preferably extending in a row or a multiple number of rows. The filaments, as they are spun, are then gathered into a straight row of side-by-side, evenly spaced apart, untwisted bundles, each containing at least about and preferably from 50 up to 1000 filaments. These filament bundles are simultaneously drawn downwardly at a velocity of at least three thousand meters per minute, and preferably from 3,500 to 8,000 meters per minute in individually surrounding gas columns flowing at supersonic velocity and directed to impinge on a substantially horizontal carrier. The gathering of the filaments into the untwisted bundles and their drawing and directing to impinge on the carrier is preferably effected by passing the bundles to air guns which surround the filaments with a column or jet of air which is directed downwardly at supersonic velocity. The air guns are arranged so as to extend in one or more rows extending across the carrier at right angles to its direction of movement, so that the bundles confined in the gas columns as the same strike the moving carrier extend in a line or row at right angles across the carrier.

In accordance with the aforementioned Belgian Pat. No. 763,570, after formation of the nonwoven web on the carrier, web bonding is then effected by passing the web from the carrier into contact with a smooth surfaced heated roll and, after a given degree of prewrap and accompanying preheating on the roll, passing the web between a high pressure nip formed by the smooth surface roll and a second heated roll containing raised points on its surface. The web so prepared contains intermittent compressed regions of autogenous filament bonds, is soft and drapable, possesses a desirable tensile strength and capacity to absorb energy under strain and, with respect to the webs illustrated therein, desirable surface abrasion resistance.

While the manner of web bonding illustrated in the Belgian patent is quite suitable for preparing materials having basis weights up to about 1 oz./yd.'-' or so, as interest developed in higher basis weight materials, it was noted that it became more difficult to achieve optimum surface abrasion resistance on both sides of the web while maintaining the other desirable characteristics. In particular, the problem was in obtaining abrasion resistance on the side of the web which was prewrapped around the smooth surfaced roll prior to passage through the nip. And, while abrasion resistance of this surface could be increased through the use of more intense bonding conditions (e,g. by increasing the nip pressure and temperature of the heated rolls), this was accompanied at the sacrifice of optimum tensile strength, energy absorption and drape.

The present invention is particularly addressed to an improvement in the bonding process disclosed in the aforementioned Belgian Pat. No. 763,570 whereby, with respect to higher basis weight webs, the combination of desirable two-sided surface abrasion resistance and physical strength properties can be achieved. However, as will be apparent, the technique described herein is broadly applicable to the preparation of nonwoven webs of molecularly oriented thermoplastic filaments which have a particular combination of crystallization dependent bonding characteristics For the purposes of the present invention, the following definitions are applicable: Continuous filament web is a nonwoven web of substantially continuous and randomly arranged, molecularly oriented filaments of a crystallizable thermoplastic polymer wherein substantially all of the filaments have about the same softening point. lntermittent autogenous bonding? is a process wherein bonding is accomplished simply by applying heat to a substantially unbonded web at intermittent areas which define the upper and lower surfaces of intermittent regions of the web which are compressed under a pressure of at least about 2000 psi. Stabilized web is a continuous filament web bonded by intermittent autogenous bonding which is characterized by having a surface abrasion resistance (on both sides) of at least about 20 (determined as hereinafter described) and a basis weight normalized grab tensile strength (average of MD and CD) of at least about 10 lbs/(oz.- /yd. (also determined as hereinafter described).

Briefly stated, the present invention is based on the discovery that, with respect to high basis weight webs prepared in a manner such as illustrated in U.S. Pat. No. 3,692,618, eliminating thermal pretreatment prior to bonding is necessary in order to fashion a stabilized web. In its broadest aspect, the present invention is believed to reside in the discovery that the preparation of stabilized webs by intermittent autogenous bonding is dependent to a significant extent on not permitting the crystallinity of the web filaments to exceed a certain low level prior to the bonding process.

As will hereinafter become apparent, the present invention is especially applicable to the preparation of stabilized continuous filament webs having a basis weight of at least about 1 oz./yd. of polypropylene filaments having a crystallinity of less than about 45 percent which exhibit a very rapid rate of crystallization over an intermediate temperature range which is above ambient temperature and substantially below the filament softening temperature. With respect to such webs and in accordance with the present invention, stabilization is accomplished by intermittently autogenously bonding the web such that the areas thereof defining the surfaces of the compressed regions are substantially instantaneously raised to a temperature near the filament softening point before filament crystallinity is increased to a significant degree.

In more detail and in accordance with one embodiment, the process described herein involves intermittent autogenous bonding of a continuous filament web wherein the filaments have a low degree of crystallinity by passing the web at about ambient temperature directly through a nip formed by a smooth surfaced roll and a roll containing raised points on its surface with both rolls maintained at a temperature near the softening point of the filaments. The speed of the web through the nip, the roll size and the configuration of the raised points are coordinated such that the surface temperature of the web is not substantially increased before maximum pressure has been developed in the nip, but then is rapidly raised to a point where surface fusion is effected before a significant increase in filament crystallinity occurs. Due to the thermal gradient across the web, there is no substantial filament fusion within the interior thereof.

In fully appreciating the present invention, it is believed necessary to first understand the nature of fila ment bonding present in a stabilized web prepared by intermittent autogenous bonding. The filaments on the web surfaces are fused together over the intermittent bond areas with the areas having a film-like appearance. Thus, under the action of an abrading force the filaments are unable to pull free in a continuous manner though localized filament breakage, such as at where a filament enters a bond area, may occur. The principal'aspect, however, is that abrasion does not result in the creation of continuous filament spans on the web surfaces which would present an undesirable, fuzzy, pile-like appearance.

While filament fushion is desirable on the web surfaces for abrasion resistance, directly the opposite is so within the interior of the web insofar as obtaining desirable strength and energy absorption. As explained in Belgian Pat. No. 763,570, these latter characteristics are present when bonds between filaments have a strength such that, as strain is progressively applied to the web, filaments release from each other when the strain approaches the filament breaking strength. Such bonds have been termed release bonds and are characterized by an absence of substantial fusion between filaments the nature of the bond being more of mechanical or cohesive attachment due to filament deformation and the increase in effective contact area between filaments which accompanies compression of the web in the bonding nip.

Therefore, it is believed that a stabilized continuous filament web is characterized as having fused autogenously bonded filaments within intermittent areas on the web surfaces with releasably autogenously bonded filaments within the interior of the web adjacent the fused areas. And, with respect to high basis weight continuous filament webs prepared such as described in US. Pat. No. 3,692,618, obtaining this combination of filament bonds by the procedure described in Belgian Pat. No. 763,570 was quite difficult. The principal problem, as now understood, apparently residing in the fact that preheating of the web raised filament crystallinity and, in turn, softening temperature .thus necessitating very intense bonding conditions to achieve surface filament fusion for two-sided abrasion resistance. As a result, over-bonding and accompanying excessive fusion was created within the interior of the web which adversely affected the web's strength characteristics.

In contrast, by eliminating preheating in accordance with the present invention, low crystallinity can be maintained and surface filament fusion effected under thermal conditions which do not lead to detrimental fusion within the web interior. Two reasons, and probably a combination of both, are believed to be responsible for the absence of such interior fusion. The first reason is simply that less intense bonding conditions, both with respect to pressure and temperature, are needed to achieve necessary surface fusion. Therefore, due to the thermal gradient from the web surfaces to the center, there is less likelihood of fusion in the web interior. Secondly, due to the thermal gradient, crystallinity of the interior filaments and particularly those close to the web surfaces increases before an appropriate softening temperature is reached, thus diminishing the possibility of fusion.

Why preheating as illustrated in the Belgian patent adversely affects web stabilization is not known for sure. However, as has been indicated, it is believed to be related to an increase in filament crystallinity. And, in this respect, it should be noted that the filaments contained in webs prepared such as illustrated in the Belgian patent are believed to have several distinctive crystallinity characteristics. These characteristics are a low degree of crystallinity as prepared, a very rapid rate of crystallization on exposure to a moderate elevated temperature, and a stiffness at a temperature near the softening point which increases as the degree of crystallinity increases. The advantages in obtaining a stabilized web by means of the present process are believed to be especially applicable with respect to webs having these crystallinity characteristics.

Turning to the aspect of using webs containing filaments with a low degree of crystallinity, this, as has been mentioned, is believed to be important since, so long as such low crystallinity can be maintained during bonding, the possibility of effecting filament fusion at a lower temperature exists (see e.g. the aboveidentified Levy patent). The degree of crystallinity can be determined by well-known X-ray diffraction techniques such as described by Weidinger and Hermans, Makromol Chem. 50 98 (1961). For the purpose of the present invention, the term low crystallinity refers to a level of crystallinity below the equilibrium value and generally at least about 5% below the equilibrium value. The equilibrium value is the level of crystallinity present after annealing for an extended periof of time at a temperature near but below the melting point. Particularly useful polypropylene filaments are those wherein crystallinity is about 45 percent or less (55 percent or higher being the equilibrium value.) As is well known in the art, the crystallinity present in a filament depends to a significant extent on the thermal history which the filament experiences after spinning and drawing. Filaments with low crystallinity can be prepared by rapid quenching to room temperature or TABLE 1 Continued below after pneumatic, melt drawing or by unheated mechanical drawing of quenched solidified filaments. Mall lndeX of pfllymer Greater than 13 As indicated, a second characteristic of filaments z ggg 'igg f ggi contained in webs which can be advantageously stabi- 5 2160 grms.) lized by the present process in a rapid rate of crystalli- 3 :3 at Spmnem m/mm' zation. The fact that the rate of crystallization of ther- Filament Rate at Air Gun moplastic filaments is temperature dependent being g fz 4000 m/mlnslow at both ambient temperatures and at temperatures QuenCHAir .857 RH approaching the melting point and reaching a maxi- 10 a raw o n (f om Spmneret to Final Filament) mum at an intermediate temperature between these In cross secnonalarea From 2500,01

two is recognized. However, in contrast to many 1n diameter From 8.50 micron to 17 micron crystallizable filaments, the process of the present invention is especially applicable to webs fashioned with In order to illustrate the present invention, a web 1.5

filaments having an extraordinarily rapid rate of cryso2./yd. filaments having a denier of about 2.0 tenactallization, such that, even with a short pre-heat, the deity of about 2.8 g.p.d., elongation of about 150 percent,

gree of crystallinity is significantly increased. and crystalline melting point of about 165C.) prepared Melt drawn filaments as illustrated in U.S. Pat. No. under the above conditions was stabilized by passing it 3,692,618 wherein high shear is present during drawing at a speed of 32 ft./min. directly through a nip formed followed by a rapid quench to room temperature, are by two heated steel rolls under the following condibelieved to typify filaments having an especially high tions:

Pressured on each point diamond shaped with each side 0.0285 inch 0.04 in. high, 200 points/inf. arranged in a diamond pattern with diagonal of pattern and points in machine direction through nip. effective nip area determined by direct measurement from imprint on impression paper obtained while rolls are loaded under operating pressure but not rotating.

rate of crystallization. It has been noted that the differ- Thereafter, the abrasion resistance and the grab tenential thermal analysis (DTA) curve of a filament samsile strength of the stabilized web were measured as folple prepared in such a manner exhibits a significant lows:

thermal response at atemperature below that attributa- Abrasion measurements were made using the Stan-- ble to crystallite melting. It is believed that filaments dard Taber abrasion method. The results are obtained having such DTA curves will possess an extraordinarily in abrasion cycles to failure. For purposes of the presrapid rate of crystallization, ent invention, failure is deemed to occur at that point The last characteristic of filaments contained in webs Where a noticeable Portion of the Web Surface especially suitable for stabilization by the present projected to abrasion in U16 Hist eXhibilS a f zzy, P k

cess is that their stiffness increases with filament crysappearance p m y due to Web fi ts be g P led tallinity. As has been indicat d, b strength i b out of compacted areas although some filament breaklieved to depend on releasable filament bonds which g y also AS so determined, the failure Point are fashioned in part by attachment due to filament de- Occurs Prlor to filament Piling the Web Surfaceformation. In turn, under a given pressure release 19 in copendlng Hansen et application, Sen bonding should be enhanced with filaments having a 177,077 now U.S. Pat. No. 3,855,046, illustrates the lower stiffness. Therefore, if deformation is effected Surface appearance of a yp Web at failurebefore an increase in crystallinity occurs, enhanced Suremems are made using a Taber Standard Abrader strength should be obtainable. With respect to a poly- (Model 140 P with rubber callbrase propylene, at least a doubling in stiffness at a temperawheel on the fight abradmg head and a 125 gram coun' ture of about 140 C. accompanying a crystallinity terwelght (Fetal load of 125 g change of about 45 to 60 percent is believed to be Grab tensile strength (lbs/in.) was determined using dicativg of this type of characteristic a conventional lnstron tensile testing machine in accor- Table 1 describes useful parameters for preparing dance with ASTM D4117 part part continuous filament webs which are especially applicagg sgg z s ss 3 :22: :2 :5 Z 2; ble for stabilization in accordance with the present inwe prepare 6 c e p vention. The web forming apparatus and procedure ilgivfin m Table iustrated in U.S. Pat. No. 3,692,618 wherein spun fi1a TABLE 2 ments are melt drawn using supersonic air guns are applicable. Abrasion Resistance Tensile Side in Con- Side in Con- Strength TABLE 1 tact with tact with Roll 1 Roll 2 MD CD In General Sample Web A 80+ 80+ 34.8 30.1

Polypropylene Polymer 1. t t ci :23; MW LESS lha 5 In order to illustrate the effect ofa heat pretreatment No. Ave. M.W. on the above characteristics other webs were passed through the above-identified nip after having experienced various degrees of preheating. Preheating was accomplished by contacting a surface of the web with a heated smooth surfaced roll at about l45-l47 C.

desirable textile-like qualities with respect to drape and hand. On visual examination, the webs are seen to have intermittent compressed regions extending through their thickness corresponding to the raised points on weight normalliz ed tensile strength of at least about 20. Also, while not illustrated in the foregoing tables. the webs prepared in the manner described above possess over the following time intervals: the roll 2. Webs A-D do not have a glazed surface and,

0.09 sec.; 0.1 1 sec.; 0.15 sec.; and 0.4 sec. Abrasion particularly with respect to web A, on being held up to resistance and grab tensile measurements are given in a light sourc th r is a m k d contrast b tw th Table 3 for webs wherein the preheated Side Of the Web bond areas and the regions disposed therebetween with was in COhtaCt with the F011 with raised Points (Roll bond areas being noticeably more shiny and film-like. in the bonding P- In addition to the above-discussed attributes. webs TABLE 3 ABRASION RESISTANCE TENSILE STRENGTH Side in Contact Side in Contact Web Preheat (Sec.) with Roll l with Roll 2 MD CD Table 3 illustrates that both abrasion resistance and prepared in accordance with the illustrated technique strength diminish with increasing preheating. And, for and having a basis weight of about 1 oz./yd. 3 02.- the purpose of correlating this behavior with filament /yd. and, particularly 1.25 oz./yd. 2.5 oz./yd. have crystallinity, the percent crystallinity of filaments in good delamination resistance and tear strength. While Webs A, B and E was determined by X-ray analysis it is believed that the latter of these properties is after the indicated preheating for Webs B and E and achieved through the same mechanism which contribafter bonding for Web A. As prepared, the filaments in utes to the desirable tensile strength characteristics, the all Of these Webs had a crystallinity 0f about 40 Percent level of delamination resistance achieved is considered The values of crystallinity so obtained are given in to b an ected ben fit Tab Since in the absence of preheating it would be ex- TABLE 4 pected that the centervportion of the web would experience only a very slight increase in temperature on pas- CRYSTALUNITY sage through the bonding nip, very little physical at- WEB AFTER BONDlNG AFTER tachment between filaments in this region would be an- PREHEAT ticipated and, in turn, it would be expected that the A About 55% web could be peeled apart. However, in fact, such can B bo 5 not be easily accomplished thus permitting the elimina- E About 65% tion of preheating with the accompanying benefits dis- 40 cussed above without a substantial loss in delamination Similarly for the purpose of illustration, the effect of resistaflcewhile as higher basis w h F than percent crystallinity on stiffness was determined for those luustrateq f F P l l reslstance polypropylene using compression molded samples wl ll pr obably diminish, it IS believed that such can be Specimens were die cut (ASTM 33 type v dumb minimized by instantaneously raising the temperature bell shape) and the stiffness moduli (lOOx force reofthe web while itisin the bonding p- One Suggested quired f 1 percent extension) was d i d using manner of doing this is in the direct introduction of a an lnstron at a strain rate of 0.2 in./min. At about 140 thin j of Steam into the web coincident with t5 C., samples with about 45 percent and 60 percent crysdUCtiOh into the ptallinity exhibited a moduli of about 100 lbs/in and AS mentioned above, webs prepared in accordance about 250 lbs./in respectively. with the present invention possess a desirable textile- Referring to Tables 2 and 3, it will be seen that webs like drape and hand. These attributes are believed to be A-D, wherein preheating was for less than about 0.15 principally due to the fact that intermittent bonding is seconds, exhibit a desirable combination of abrasion employed to effect web stabilization. And, in this re- 'resistance and tensile strength. In particular, the abraspect, it is believed that webs having discrete bond sion resistance on both sides of the webs is at least areas occupying about 550 percent of the web surface about 20 and the basis weight normallized tensile area and disposed in a density of about 50 3200 areas strength (the average of the MD and CD values divided /in. are useful. As should be appreciated, in achieving by the web basis W ight f L5 0z./yd. is at lfiaSt about textile-like qualities the use of higher bond densities 20 lbs./(0z./yd- Ful'thfifmol'e, as can be and total bond area is associated with the use of lower Table 2, the elimination of any preheating results in a basis weight webs with finer filament deniers. And, as dramatic increase in abrasion resistance and, accordbasis weight and filament denier increase, the density ingly, webs can be prepared in accordan with th of the bond areas should be correspondingly reduced. preferred aspects of the present invention having an With respect to webs having a basis weight of about abrasion resistance of at least about 50 and a basis -2.5 oz d. containing filaments havingadenier of about 0.5 l0, and particularly l-5, a total bonded area of about 10-25 percent and a bond density of about l0O5O0/in. are preferred.

As a further point, it will be recalled that the desirable strength characteristics achieved by the presently illustrated process are believed to reside in achieving filament deformation in the high pressure nip which contributes to the discussed "release bonding. It is believed that the extent of deformation and in turn the degree of release bonding is dependent on the shear which the web experiences as it passes through the bonding nip.

With respect to the illustrated process, a high degree of shear is believed to be present. Both of the rolls em-' ployed for bonding are hard surfaced, thus insuring filament deformation in the nip rather than mere conformity with the roll pattern. Also, the fact that the rolls have a small radius of curvature is believed to increase shear accompanying passage through the nip. And in this respect it is believed that as roll diameter increases, correspondingly higher nip pressures should be used. Thus, while as is illustrated in Table l with about 7 inch diameter rolls a nip pressure on raised points of about 3500 psi. is adequate, a higher nip pressure, for example, about 5000 psi., would be more apprpriate for larger rolls such as those having a 16 inch diameter.

Similarly, with higher basis weight webs greater nip pressures are necessary to achieve adequate bonding.

I In general, nip pressures in excess of 50,000 psi. should be avoided so as not to overly deform the filaments to a point where they are materially weakened. However, as a practical matter, bonding in accordance with the present invention will generally require the use of pressures in excess of about 1500 psi. With the above in mind, it should also be apparent that other means for increasing shear can also be used such as employing rolls with matching or slightly offset raised points or by using variably driven rolls.

While the invention has been described in connection with certain preferred embodiments, it is to be understood that the invention is not to be limited to those embodiments. On the contrary, all alternatives, modifications, and equivalents as can be included within the scope and spirit of the invention defined in the appended claims are intended to be covered.

1 claim as my invention:

l. A process for stabilizing a nonwoven web of molecularly oriented crystallizable thermoplastic fibers, said web having a basis weight of at least about 1 oz.- /yd. and containing fibers having a crystallinity below the equilibrium value, said process consisting essentially of compressing in the absence of any preheating step said web in intermittent regions under a pressure of at least about 2000 psi. to effect fiber deformation within said regions and substantially instantaneously raising the temperature of the areas of the web defining the surfaces of the compressed regions to effect fiber fusion of the surface fibers before crystallinity is substantially increased, to combined effects of fiber deformation within the compressed regions of the web and fiber fusion on the surfaces thereof providing desirable two-sided abrasion resistance and strength characteristics.

2. The process of claim 1 wherein the fibers are polypropylene.

3. The process of claim 2 wherein the nonwoven web is a continuous filament web having a basis weight of about l.25-2.5 oz./yd*.

4. The process of claim 3 wherein the filaments in the continuous filament web have a crystallinity of less than about 45 percent.

5. In a process for preparing a continuous filament having a crystallinity below about 45 percent web of polypropylene filaments comprising extruding polypropylene through a multiple number of downwardly directed spinning nozzles in the form of filaments, pneumatically melt drawing the filaments at a velocity of at least 3000 meters/min. followed by rapid quenching and collection as a continuous filament web having a basis weight of at least about 1 oz./yd. the improvement wherein the continuous filament web is thereafter stabilized by compressing in the'absence of any preheating step said web in intermittent regions under a pressure of at least about 2000 psi. to effect filament deformation within said regions and substantially instantaneously raising the temperature of the areas of the web defining the surfaces of the compressed regions to effect filament fusion of the surface filaments before crystallinity is substantially increased, the combined effects of filament deformation within the compressed regions of the web and filament fusion on the surfaces thereof providing desirable two-sided abrasion resistance and strength characteristics.

6. The process of claim 5 wherein the continuous filament web is stabilized by being passed directly through a nip formed by hard surfaced rolls maintained at a temperature near the softening point of the filaments with at least one of the rolls having raised intermittent regions on the surface thereof and with the nip pressure on the raised regions being at least 2000 psi.

7. The process of claim 6 wherein the continuous filament web has a basis weight of about l.25-2.5 oz./yd.".

8. The process of claim 7 wherein at least one of the hard surfaced rolls is so configured such that the stabilized web contains compressed regions in a density of about 50-3200/in. which occupy about 550% of the web surface area.

9. The process of claim 7 wherein at least one of the hard surfaced rolls is so configured such that the stabilized web contains compressed regions in a density of l00-500/in. which occupy about 10-25 percent of the web surface area. I

10. The process of claim 9 wherein the temperature of the hard surfaced rolls is about C. and the nip pressure is about 3500 psc.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3 12 5 7 DATED October 14, 1975 INVENTORG) 2 Robert J. Schwartz It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 10, line 13, delete "having a crystallinity below about 45 percent";

line 14, after "filaments" insert .having a cry stallinity below about 45 percent.

Signed and Scaled this tenfh Day Of February 1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN 8 /7 (mnmr'ssionvr of Patents and Trademarks

Claims (10)

1. A PROCESS FOR STABILIZING A NONWOVEN WEB OF MOLECULARLY ORIENTED CRYSTALLIZABLE THERMOPLASTIC FIBERS, SAID WEB HAVING A BASIS WEIGHT OF AT LEAST ABOUT 1 OZ./YD.2 AND CONTAINING FIBERS HAVING A CRYSTALLINITY BELOW THE EQUILIBRIUM VALUE, SAID PROCESS CONSISTING ESSENTIALLY OF COMPRESSING IN THE ABSENCE OF ANY PREHEATIG STEP SAID WEB IN INTERMITTEN REGIONS UNDER A PRESSURE OF AT LEAST ABOUT 200 PSI. TO EFFECT FIBER DEFORMATION WITHIN SAID REGIONS AND SUBSTANTIALLY INSTANTANEOUSLY RAISING THE TEMPERATURE OF THE AREAS OF THE WEB DEFINING THE SURFACES OF THE COMPRESSED REGIONS TO EFFECT FIBER FUSION OF THE SURFACE FIBERS BEFORE CRYSTALLINITY IS SUBSTANTIALLY INCREASED, TO COMBINED EFFECTS OF FIBER DEFORMATION WITHIN THE COMPRESSED REGIONS OF THE WEB AND FIBER FUSION ON THE SURFACES THEREOF PROVIDING DESIRABLE TWO-SIDED ABRASION RESISTANCE AND STRENGTH CHARACTERISTICS.

2. The process of claim 1 wherein the fibers are polypropylene.

3. The process of claim 2 wherein the nonwoven web is a continuous filament web having a basis weight of about 1.25-2.5 oz./yd2.

4. The process of claim 3 wherein the filaments in the continuous filament web have a crystallinity of less than about 45 percent.

5. IN A PROCESS FOR PREPARING A CONTINUOUS FILAMENT HAVING A CRYSTALLINITY BELOW ABOUT 45 PERCENT WEB OF POLYPROPLENE FILAMEENTS COMPRISING EXTURDING POLYPROPLENE THROUGH A MULTIPLE NUMBER OF DOWNWARDLY DIRECTED SPINNING NOZZLES IN THE FORM FILAMENTS PNEUMATICALLY MELT DRAWING THE FILAMENTS AT A VELOCITY OF AT LEAST 300 METERS/MIN. FOLLOWED BY RAPID QUENCHING AND COLLECTION AS A CONTINUOUS FILAMENT WEB HAVING A BASIC WEIGHT OF AT LEAST ABOUT 1 OZ./YD.2, TTHE IMPROVEMENT WHEREIN THE CONTINUOUS FILAMENT WEB IS THEREAFTER STABILIZED BY COMPRESSING IN THE ABSENCE OF ANY PREHEATING STEP SAID WEB IN INTERMITTEN REGIONS UNDER A PRESSURE OF AT LEAST ABOUT 200 PSI. TO EFFECT FILAMENT DEFORMATION WITHIN SAID REGIONS AND SUBSTANTIALLY INSTANTANEOUSLY RAISING THE TEMPERATURE F THE AREAS OF THE WEB DEFINING THE SURFACE OF THE COMPRESSED REGIONS TO EFFECT FILAMENT FUSION OF THE SURFACE FILAMENT BEFORE CRYSTALLINITY IS SUBSTANTIALLY INCREASED THE COMBINED EFFECTS OF FILAMENT DEFORMATION WITHIN THE COMPRESSED REGIONS OF THE WEB AND FILAMENT FUSION ON THE SURFACE THEREOF PROVIDING DESIRABLE TWO-SIDED ABRASION RESISTANCE AND STRENGTH CHARACTERISTICS.

6. The process of claim 5 wherein the continuous filament web is stabilized by being passed directly through a nip formed by hard surfaced rolls maintained at a temperature near the softening point of the filaments with at least one of the rolls having raised intermittent regions on the surface thereof and with the nip pressure on the raised regions being at least 2000 psi.

7. The process of claim 6 wherein the continuous filament web has a basis weight of about 1.25-2.5 oz./yd.2.

8. The process of claim 7 wherein at least one of the hard surfaced rolls is so configured such that the stabilized web contains compressed regions in a density of about 50-3200/in.2 which occupy about 5-50% of the web surface area.

9. The process of claim 7 wherein at least one of the hard surfaced rolls is so configured such that the stabilized web contains compressed regions in a density of 100-500/in.2 which occupy about 10-25 percent of the web surface area.

10. The process of claim 9 wherein the temperature of the hard surfaced rolls is about 145* C. and the nip pressure is about 3500 psc.