US3546062A - Unbonded nonwoven web of polypropylene fibers - Google Patents

Description

United States Patent 3,546,062 UNBONDED NONWOVEN WEB 0F POLYPROPYLENE FIBERS Arthur John Herrmau, Nashville, Tenn., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Continuation-impart of application Ser. No. 390,134, Aug. 17, 1964. This application Dec. 9, 1969, Ser. No. 883,652

Int. Cl. 133% /02; D04h 1/04 U.S. Cl. 161-169 4 Claims ABSTRACT OF THE DISCLOSURE Nonwoven webs of polypropylene fibers having low sensitivity to fluctuations in heat-bonding conditions are particularly useful as precursors for carpet backings. To possess these qualities, the fibers of the web should have a crystallinity index of between about and 70, a co efiicient of variation in fiber-birefringence of at least 5% and greater than 75% by weight of the fibers should have a birefringence of at least 0.02.

DETAILED DESCRIPTION OF THE INVENTION This application is a continuation-in-part of my application Ser. No. 390,134, filed Aug. 17, 1964 now abandoned.

Bonded nonwoven sheets of like oriented polypropylene fibers have been developed with a combination of properties which has made them of value in such applications as primary and secondary backings for carpets, furniture foundation sheets, shoe fabrics, coated and laminated structures, filters, molded sheets and electrical and sound insulation. In particular, the combination of high tear strength and high tensile strength, which is important in these applications, has been obtained by self-bonding the polypropylene fibers under carefully con trolled conditions of temperature, time of exposure to bonding conditions, and maintenance of the Web under restraint while under bonding conditions. In the production of such sheets, careful temperature control of the bonding operation is required to avoid underbonding, which leads to low tensile strength and overbonding which leads to low tear strength. In addition, the self-bonding operation must of necessity be carried out near the softening point of the polypropylene fibers. Since heating the fibers at this temperature causes deorientation and an accompanying decrease in fiber tenacity, the bonding temperature must be precisely controlled to avoid an excessive loss in orientation. Loss in orientation is minimized by maintaining the web under restraint during the entire time it is at a temperature at which shrinkage of relaxed fibers will occur. By so minimizing loss in orientation and by careful temperature control, it is possible to self-bond polypropylene fibers while confining the drop in fiber birefringence to less than and preventing a drop in birefringence to a level below about 45% of the maximum birefringence (0.04 for polypropylene). In this way adequate fiber properties are retained in the bonded nonwoven sheet.

Even more precise temperature control is required in the self-bonding of nonwoven webs of continuous polypropylene filaments which are to be used as primary carpet backings; for not only must the bonded sheet have high tensile and tear strength, it is also necessary that the strength of the self-bonds be such that, during the tufting operation, the bonds break rather than the filaments. It has been found that if the bonds break during tufting, the filaments have sufficient mobility to move under the stress of the tufting needles without excessive filament breakage. Excessive filament occurs with overbonded sheets and leads to tufted carpets which are deficient in grab-tensile strength.

If high grab-tensile strength were the only requirement of the tufted carpet, a lightly bonded sheet which could be easily prepared would be satisfactory. However, not only must the fibers be so bonded that they are free to move with a minimum of fiber breakage during tufting, but a sufficient number of bonds must also be retained after tufting to give the sheet dimensional stability during subsequent processing. For example, with an inadequate number of self-bonds, the tufted nonwoven sheet undergoes excessive shrinkage in the cross-machine direction during passage through the dye beck. Thus the self-bonding must be carefully controlled to avoid both overbonding, which leads to excessive fiber-breakage and low tufted-grab-tensile strength, and underbonding, which gives high dye-beck-width losses. Temperature control within the range of 05 C. and preferably operation within a range of only 0.1 C. in all dimensions of the nonwoven web has been required for such self-bonded sheets.

This critical temperature control and the maintenance of the required restraint can be obtained by exposure of the web, while held between a porous belt and a solid belt, to a nonsolvating saturated atmosphere, for example steam, under pressure. By maintenance of a differential steam pressure on the two sides of the solid belt, an adequate restraining force can be applied to the web to hold filament shrinkage within about 20%, and preferably within 5%. As mentioned above, the preciseness of temperature control required is difficult to obtain, and is a disadvantage of these self-bonded nonwoven sheets. The problem is particularly severe with wide sheets, for example those 15 ft. (4.6 m.) or more wide, which are used as primary carpet backings.

The purpose of this invention is to provide a nonwoven web of polypropylene fibers which has low sensitivity to bonding conditions.

Another purpose is to provide a nonwoven web of polypropylene fibers which can be bonded to a strong and tear-resistant sheet.

Still another purpose is to provide a nonwoven web which can be readily bonded in wide widths to form a nonwoven sheet having a good combination of properties for use as a primary carpet backing.

These and other purposes of this invention are attained by providing a nonwoven web comprising polypropylene fibers having a crystallinity index of 30 to and having a distribution of orientation levels such that the coefiicient of variation of the fiber-birefringences is at least 5% with the proviso that less than 25% by weight of the fibers have a birefringence of under 0.02.

Coefficient of variation (CV) is used herein in accord with its customary meaning in statistical analysis and is defined as follows:

0v: it? 1 where AN is the arithmetic mean of the birefringence values,

An is the measured value of the birefringence of individual fibers, and

m is the number of measurements.

In order to obtain the required coefficient of variation, the nonwoven web comprises either a combination of low-oriented fibers and high-oriented fibers derived from the same polymer, or fibers having alternating highand low-oriented segments. In the case where a combination of low-oriented and high-oriented fibers are employed, the former will be called binder fibers and the latter, matrix fibers. When such a sheet is properly bonded, it will differ from the bonded sheets of like polypropylene filaments, which contain only self-bonds between matrix fibers, in having self-bonds between the binder fibers, self-bonds between the matrix fibers and interbonds between the matrix and binder fibers. This ability to form a variety of bonds is an important characteristic of the nonwoven webs of this invention. Since the bonds can and do have different bond strengths, the bonded nonwoven sheet has a wide distribution of bond strengths. It has been found that a wide distribution of bond strengths permits wide limits on the average bond strength and bond concentration to give good products, thus less critical control of bonding conditions is required. The nonwoven webs of this invention may comprise staple fibers, continuous filaments or combinations of the two. The nonwoven webs can be formed from polypropylene fibers having the required coefficient of variation in birefringence by the standard techniques known in the art for manufacturing nonwoven fabrics. As mentioned previously, the binder fibers can be incorporated into the web as separate low-oriented fibers or as the low-oriented segments of fibers having both highand low-oriented segments along their length.

Preferred nonwoven webs are composed of continuous polypropylene filaments which have a random distribution throughout the web and are separate and independent of each other, except at filament crossover points. They exhibit a low level of parallelism between filaments, thus indicating a relative freedom from aggregation or ropiness. Such nonwoven structures have especially high tear and tensile strength because of the continuous filaments and exhibit isotropic properties because of the random arrangement of the filaments. Structures of this type can be made by the general procedure of British Pat. 932,482. The process described in this patent involves an integrated spinning, orientation, and laydown of the filaments to give a random nonwoven web which is essentially free from filament aggregates. In this process, the freshly spun filaments are electrostatically charged, forwarded toward a web-laydown zone and then permitted to separate due to the applied electrostatic charge before web-laydown.

This process can be readily used to prepare nonwoven webs in which the binder filaments are either separate low-oriented filaments or are low-oriented segments of filaments having highand low-oriented segments along their length. Separate binder and matrix filaments can be produced by using two separate spinning and drawing machines and combining the filaments prior to or during web-laydown. They can also be produced by splitting the threadline from the spinneret so that a part bypasses the drawing operation. Another method involves the use of a spinneret with varying capillary geometry which produces filaments with varying responses to the drawing operation. Nonwoven webs of filaments with segments having different levels of orientation can be produced by pulsing the throughput of polymer going to the drawing operation, by pulsing the draw ratio in the drawing operation, or by variation of the drawing temperature. This latter method can be carried out by passing the filaments over a heated fluted feed roll in the drawing step. It has been found that these various techniques can be used to produce nonwoven webs of polypropylene filaments in which the distribution of orientation (as measured by filament-birefringence) among or along the filaments is characterized by a coefficient of variation of at least The oriented polypropylene matrix fibers in the unbonded nonwoven web will normally have a birefringence in the range of 0.020 to 0.040, while that of the binder fibers will normally be in the range of from less than 0.01 to about 0.030 and will always be less than that of the matrix fibers. Less than and preferably no more than about 20% by weight of the fibers in the web have a birefringence below 0.02.

In addition to the birefringence distrbution, another important characteristic of the nonwoven webs of this invention is the crystallinity in both the binder and matrix fibers. In order to obtain the three types of bonds which are needed for good overall sheet properties and low bonding sensitivity, it is required that the web can be heated to a sufficiently high temperature to obtain some degree of bonding between the matrix fibers without destroying the structure of the binder fibers. If the binder fibers were amorphous or had a significantly different crystallinity index than the matrix fibers, their melting point would be much below that of the crystalline matrix fibers, and thus they could melt and lose their fibrous structure before the matrix fibers could self-bond.

The 5 to 10 C. differences in softening point between the matrix and binder fibers of the web which can be obtained by having differences in levels of orientation such that the coefficient of variation is at least 5%, are ideally suited to obtaining the variety of bonds and distribution of bond strengths which are needed for lowbonding sensitivity. Accordingly, it is desired that the crystallinity index of the matrix and binder fibers be at about the same level and within the range of to to minimize the effect of crystallinity on the softening point differences between the matrix and binder fibers.

The present invention makes use of the discovery of an unusual relationship to bonding sensitivity possessed by nonwoven webs of polypropylene fibers having a crystallinity index in the range of 30 to 70. The finding that the bonding sensitivity of such webs unexpectedly drops when the percent coefficient of variation in fiber-birefringence is at least 5% permits the production of acceptable product over a relatively wide range of bonding conditions.

As indicated above, the bonding of the nonwoven webs of this invention must be carried out under controlled conditions to obtain the various types of desired bonds; but compared with the production of self-bonded nonwoven sheets of like polypropylene fibers, the bonding conditions are much less critical, and therefore, it is easier to produce a uniform product of acceptable properties. The aforementioned self-bonding technique as applied to polypropylene fibers of like orientation requires a control of temperature within the range of about 0.5 C. and preferably within 0.1 C. A change in bonding temperature greater than this amount would result in product with properties so different as to be unacceptable in tensile, tear or tufted-grab-tensile or as would require different tufting or other processing conditions. However, bonding of webs having the required percent coefficient of variation in fiber-birefringence can be carried out within a range of about 3 C. and preferably within a range of 1 C. This wider latitude of operable bonding conditions is of great significance to the commercial manufacture of bonded nonwoven sheets, particularly for the Wide widths required in primary carpet backings.

Bonding of the webs of this invention (comprising fibers of varying orientation) can be carried out in a manner similar to that described previously for self-bonding webs in which the polypropylene fibers are all alike. Thus, exposure of the mixed-orientation web to a saturated vapor atmosphere, for example steam, under sufficient pressure to give the desired bonding temperature while maintaining the web under restraint to minimize fiber shrinkage is a preferred method. The actual temperature used will be lower than that required for self-bonding a polypropylene web in which the fibers have the same level of orientation as the matrix fibers of the mixed-orientation Web. This lower temperature is advantageous since the matrix fibers under these conditions show less or no tendency to undergo fiber deorientation. In many instances, the matrix fibers in mixed-orientation webs actually show an increase in birefringence (greater orientation) after bonding, in contrast to the usual loss in fiber birefringence during self-bonding of polypropylene webs. The binder fibers in mixed-orientation webs lose much of their orientation during bonding, thus the birefringence of the binder fibers in typical mixed-orientation webs will drop from about 0.02 to less than 0.001.

The temperature at which the nonwoven webs of this invention are bonded depends on the rate the fibers are heated and the fiber crystallinity. While it is not intended to be limited by any particular theory of operation, it is believed that bonding occurs when the polymer as it exists in the fibers softens. A slow heat-up rate permits the crystallinity to increase so that the softening point is raised and bonding can take place only at a higher temperature corresponding to the softening point of the higher crystalline form of the polymer. The bonding temperature will approach the crystalline melting point of the binder fibers in the mixed-orientation webs of this invention. In the case of rapid heat-up rate, the bonding temperature is reached before the polymer in the fiber has an opportunity to increase substantially in crystallinity and hence a lower temperature can achieve bonding. Starting with the same nonwoven web of isotactic polypropylene fibers, the faster the heat-up rate, the lower will be the bonding temperature necessary to achieve a desired level or bonding.

Heat-up rate is dependent on both the bonding method and the apparatus. In the operation of a typical batchbonding process in an autoclave, the pressure of the saturated steam may be increased so that the heat-up rate of the web will be of the order of C. per minute. When operating a typical continuous process, the web is passed continuously through a bonding chamber which is essentially closed to the atmosphere. The chamber is pressurized with saturated steam at a constant pressure and under these conditions the heat-up rate of the web may be of the order of 250 C. per second. Thus, the heat-up rate, when operating continuously may be as much as 500 times greater than when operating batch-wise. The bonding temperature employed in the continuing process can be as much as 10 C. below that used in batchbonding.

Typical steam pressures used in the preferred continuous bonding of the nonwoven Webs of this invention, under conditions wherein the exposure time of the web in the bonding chamber is about 6.2 seconds, are in the range of about 65 to 85 p.s.i.a. (4.6 to 6.0 kg./cm. which corresponds to a temperature of about 148 to 158 C. Using the same procedure with a nonwoven web of polypropylene fibers, all of which have approximately the same level of orientation as the matrix fibers in the mixed-orientation web, pressures of about 85 to 102 p.s.i.a. (6.0 to 7.2 kg./cm. which correspond to temperatures of about 158 to 165 C., are used. Within these pressure ranges, the grab-tensile strength of a tufted 4 oz./yd. (136 g./m. sheet varies by about 6 lb. per lb./in. change in bonding pressure (39 kg. per kg./cm. for the bonded sheet of mixed-orientation polypropylene filaments and by about lb. per lb./in. change in bonding pressure (226 kg. per kg./cm. for the self-bonded nonwoven sheet of uniformly oriented polypropylene filaments. This variation is determined by measuring the slope of the curve of tuftedgrab-tensile strength versus bonding pressure at the tuftedgrab-tensile strength of 120 lb./( kg). This indicates that the nonwoven webs of this invention are much less sensitive to bonding conditions than the previously known polypropylene nowoven webs.

The lower bonding temperature and bonding sensitivity permits the production of useful products from these webs by calender bonding. Further evidence of this lower sensitivity of the nonwoven webs of this invention to bonding conditions is illustrated in the figure, in which bonding sensitivity, expressed as the variation in strip tensile strength with bonding pressure, is plotted against per cent coefficient of variation in fiber-birefringence. Bonding sensitivity in the figure is obtained by calculating the slope of the curve of strip tensile strength versus bonding pressure within the range of 4 to 8 lbs./in.// oz./yd. (21 to 42 g./cm.//g./m. for the strip tensile strength of nonwoven sheets of continuous polypropylene filaments. This range was selected since it represents the optimum combination of tear and tensile properties. It is seen that the bonding sensitivity rises rapidly as the coefficient of variation falls below 5%. Above 5%, the bonding sensitivity decreases, that is, falls below 0.5 lb./in.// oz./yd. /p.s.i.a. (37 g./cm.//g./m /kg./cm. providing that greater than 75% by weight of the fibers in the web have a birefringence that is at least 0.02. It will be noted that where 25% or more by weight of the fibers in the web have a birefringence of under 0.02 as in Examples 13-15, the bonding sensitivity rises substantially. This is evident from the distance that the points representing Examples 13-15 are above the curve in the figure. With a typical nonwoven Web of polypropylene filaments having the same level of orientation, the bonding sensitivity is about 0.8 lb./in.//oz./yd. /p.s.i.a. (60 g./cm.//g./m. kg./cm.

The polypropylene fibers used in the nonwoven webs of this invention are of textile denier, varying from about 1 to about 15 (0.1 to 1.7 tex). Higher deniers up to about 25 (2.8 tex) may be used for special applications. These filaments may be crimped or straight and may be round in cross section or have different shapes such as trilobal, elliptical, etc. The unbonded nonwoven webs may have a unit weight from as low as about 0.5 oz./yd. (17 g./m. to 20 oz./yd. (680 g./m. or higher, with webs having a unit weight of 2 to 5 oz./yd. (68 to 170 g./m. being preferred as intermediates for preparation of primary carpet backings.

Fiber-birefringence in the nonwoven webs of this invention can be measured by standard techniques known in the art. It is necessary to determine the birefringence of an adequate number of the fibers for the measurement to be representative of the entire nonwoven web. A measure of the crystallinity index of the polypropylene fibers can be obtained by means of X-ray diffraction techniques.

The invention will be further illustrated by the following examples.

A series of nonwoven webs is prepared as described in Examples 1l6. Each of the webs is then bonded continuously by passing the web at a speed of 10 yd./min. (9 m./min.) while under restraint between one porous metal belt and one solid belt, each faced with cloth, for a distance of 37 inches (94 cm.) through a steam chamber in which saturated steam is maintained at superatmospheric pressure. A differential steam pressure of 2 p.s.i. (0.14 kg./cm. is maintained on opposite sides of the solid belt. The time of exposure of the web to saturated steam in the bonding chamber is about 6.2 seconds and the restraint on the web while in the chamber is 0.75 lb./in. //oz./yd. (1.56 g./cm. //g./m. The steam temperature is varied to give a bonding pressure profile. The strip tensile strength of the various bonded sheets is then determined according to ASTM method D1117, except that a 5-in. (12.7 cm.) jaw separation is used With a strain rate on a 0.5-in. (1.27 cm.) wide sample. The strip tensile strength is plotted against the pressure of the steam on the web side of the solid belt. The bonding sensitivity is obtained by calculating the slope of the curve of the strip tensile versus bonding within the range of 4 to 8 lb./in.//oz./yd. (21 to 42 g./in.//g./m. The data on the bonded webs are summarized in Table l. The birefringence of the fibers in the unbonded nonwoven webs was determined with a Berek compensator known in the art. The crystallinity index is the percentage ratio of the crystalline X-ray diffraction intensity to the total diffraction intensity. Absolute values will vary according to the specific details of the method and corrections used. For the purpose of this description and the following claims, values of birefringence are determined using a Berek compensator and crystallinity index determined by the method described below.

A Phillips (Norelco) diffractometer is employed for the crystallinity index determination. The radiation is CuKu from a high intensity diffraction tube operated at 40 milliamps and 40 kilovolts. The X-ray beam is defined by 4 divergence and scatter slits, a 0.006" receiving slit and a 4 take-off angle. A 0.001 Ni filter is used to eliminate CuKp radiation and the detector is a Nal(Tl) scintillation counter with pulse-height-analysis to minimize background from continuous radiation. The diffracted intensity is recorded in the usual manner with a rate motor and strip-chart recorder.

The sample consists of a inch thick layer of the fibers to be examined. They are placed in the Philips sample spinner which is mounted such that the plane of rotation of the spinner bisects the incident and detected X-ray beams. A mask having a inch diameter hole covers the sample. The X-ray beam strikes the sample, while the sample is being rotated at 77 rpm. The transmitted diffraction pattern is recorded between the angles of 4 and 34 20, where equals the Bragg angle with scale factor settings such as to make the intensity of w the 110 peak near 14 about 70 to 80% full scale. The scanning rate is 1 20 per minute with a time constant of 2 seconds. The chart speed is /2" per minute.

The diffractometer scan obtained by the above procedure is used in the following manner in order to determine a crystallinity index. A base line is drawn to inter- 8 peaks but bounded by the crystalline peaks, A That is,

A =A +A A crystallinity index, C, is defined as:

C AT

In practice, it is more convenient to measure A and A in which case the equivalent expression is:

The fibers may be removed from the web prior to bonding for the determination of birefringence and crystallinity, or these characteristics may be determined on fibers prior to the formation of the web. In either case, a representative sampling should be effected.

The relationship between bonding sensitivity and percent CV of filament-birefringence is shown in the figure. From the figure, it is seen that the use of webs having at least 5% CV in fiber-birefringence is the key to obtaining bonded sheets of reproducible properties over a relatively wide range of bonding temperatures, such sheets having the combination of high tear and tensile strength and preferably high grab-tensile strength after tufting. Points 1-4, 10, 12 and 16 representing the corresponding examples are illustrative of such webs. Points 1315 corresponding to Examples 13-15 represent webs with the required percent CV, however, such webs have poor bonding sensitivity because 25% or more by weight of the fibers have a birefringence of below 0.02. Examples 5-9 and 11 are unsuitable as evidenced by their points on the figure falling below 5% CV.

TABLE 1 Lo\voriented filaments High-oriented filaments Birefringence Crystal- Average Crystal- Average Bonding Tenacity linity birefrin- Tenacity linity birefrin- Web Percent sensig.p.d index gence g.p.d. index gence average V tivity X 4 03 0. 0288 0. 0208 17. 3 0. 40 4 30 37 0. 0203 O. 0277 14. 0 0. 37 4. 34 3.) 0. 0294 0. 0285 9. 0 0. 47 4. 10 38 0. 0291 0. 0284 7. 7 0. 4. 00 40 0. 0287 0. 0287 4. 1 0. 63 4. 07 44 0. 0280 O. 0280 4. (i 0. "I. 00 47 0. 0201 0. 0291 3. 0 0. 00 3. 96 36 0. 0279 0. 0270 3. 1 0. 82 3. 82 42 0. 0280 0. 0282 4. 0 0. 71 4. 11 42 0. 0200 0. 0277 15. 5 0. 46 3. 75 40 0. 0276 0. 0270 3. 4 0. 70 4. 03 38 0. 0303 0. 0302 5. 0 0. 44 3. 86 43 0. 0330 0. 0280 10. 3 0. 55 4. 48 42 0. 0208 0. 0264 '13. 1 0. 56 3. 05 41 0. 030 0. 0252 24. 4 0. 73 5. 83 34 0. 0330 0. 0317 20. 1 0. 25

Inlb./in.//oz./yd. /p.s.i.a. bonding pressure; to convert to g./em.//g./n1. /kg./cm. multiply by 75.

sect the scan at 7 and 32 26. This base line should appear tangent to the scan at these angles. The intensity below this base line is considered background and is ignored. The intensity above this base line is assumed to consist of coherent diffracted radiation from both amorphous and crystalline regions. The following steps are employed to resolve the crystalline peaks from the underlying amorphous peak. First, a line is drawn tangent to the scan at both sides of the peak which occurs at about 14 26. The points of tangency will be near 12 and 16 20. This line is extended beyond the point of tangency near 16 and is terminated at 20. From this point, another line is drawn tangent to the scan on the high-angle side of the 131, 041, 111 triplet peak which occurs in the region 21 to 23 20. This point of tangency will be near 24 20. Other tangents to the scan are drawn under the less intense crystalline peaks located at 25 /2 and 20 20. This completes the resolution of the pattern. The amorphous peak defined by this procedure is a roughly triangular area with its apex at 165 20. It is defined in the above manner because it is uniquely determined by the diffraction profile.

The areas determined by the above procedures are measured with a planimeter. The total area above the background line, A consists of the area within the amorphous peak, A and the area above the amorphous EXAMPLE 1 A nonwoven web of 14% low-oriented and 86% highoriented crystalline polypropylene filaments is prepared as follows: isotactic polypropylene (melt flow rate (MFR) 12, by method of ASTM D-1238 at 230 C. with a loading of 2.16 kg.) is spun through a 30-h0le spinneret at a rate of 18 g./min. total and through a 5-hole spinneret at a rate of 3 g./rnin. total. Each spinneret hole for both spinnerets is 0.015 in. (0.038 cm.) in diameter and the temperature of the 30-hole spinneret is 242 C. and the 5-hole spinneret, 220 C. The filaments from the 30-hole spinneret are led to a heated feed roll operating with a surface temperature of 118 C., and advanced by means of an idler roll canted with respect to the heated roll. A total of 5 wraps is used on the heated feed roll, which is operated with a surface speed of 243 yd./min. (222 m./min.). The filaments leaving the heated feed roll are then passed 5 Wraps around an idler roll/ draw roll system operating cold with a surface speed of 858 yd./ min. (785 m./min.). These filaments are drawn 3.5X, are 7.48 denier (0.83 tex) per filament and have a tenacity of 4.03 g.p.d. The filaments from the 5-hole spinneret are led to a heated roll operating with a surface temperature of 95 C. and a surface speed of 703 yd./min. (642 m./rnin.). The filaments are in contact with the heated roll for 180. The filaments leaving the heated roll are then passed to a draw roll operating cold with a surface speed of 852 yd./min. (779 m./min.). The filaments are in contact with the draw roll for 180. These filaments are drawn 1.21X, are 7.73 denier (0.86 tex) per filament and have a tenacity of 1.62 g.p.d. The filaments from both spinnerets meet and are guided so the low-oriented fila- 'rnen ts are dispersed uniformly throughout the highoriented filaments. The filaments are then electrostatically charged with a corona discharge device, passed into a draw jet and subsequently deposited on a moving belt to form a nonwoven web of randomly distributed continuous filaments.

When this web is bonded by the procedure described above at a saturated steam pressure of 75 p.s.i.a. (5.3 kg./cm. prepared for tufting by application of a polysiloxane lubricant; and then tufted under the following conditions:

Gauge (distane between need1es)-0.188 in. (0.48'cm.)

Speed400 tufts/min; 7 tufts/in. (2.8 tufts/cm.)

Pile yarn-3700 denier (410 tex) continuous filament nylon Type pile-Loop a tufted carpet with a grab-tensile strength of 133 lb. (60 kg.) and a dye-beck-width loss of 3.8% is obtained.

EXAMPLES 2-5 Four nonwoven webs of continuous polypropylene filaments are prepared in the same general method as described in Example 1 except that the draw ratioof the low-oriented filaments is varied. The surface speed of the draw roll used for the high-oriented filaments is maintained at 858 yd./min. (785 m./min.), and that used for the low-oriented filaments, at 852 yd./min. (779 m./

10 examples do not give the required percent CV for the nonwoven webs to qualify as products of this invention.

TABLE 3 Total throughp g./min.

27 110 z 9 0.018 in. X 0.250 in 18 inlet...

1(0.1046 cm. x 0.685 cm.)....

es. 0.0131 in. x 0.0655 in inlet-.. (0.0333 cm. x 0.166 cm.)

+ Spinneret hole dimensions as shown are diameter x length.

EXAMPLES 10-1 1 Two nonwoven webs of oriented polypropylene filaments having low-oriented segments along the filament length are prepared as follows: polypropylene filaments are spun through a 30-hole spinneret at a rate of 18 g./min. total. Each spinneret hole is 0.015 in. (0.038 cm.) in diameter and the temperature of the spinneret is 234 C. The filaments are led to a heated feed roll having a circumference of 18.75 in. (47.7 cm.) with three grooves cut out 120 apart. The grooves are 1.25 in. (3.2 cm.) wide in Example 10 and 0.25 in. (0.64 cm.)

min.). Identification of the variables is made in Table 2. as in Example 11. The filaments are in contact with the TABLE 2 High-oriented filaments Low-oriented filaments Draw Tenacity Draw Tenacity Example ratio g.p.d. Denier Textile ratio g.p.d. Denier Textile EXAMPLE 6 roll for 220 The surface temperature of the roll is A nonwoven web of 100% high-oriented polypropylene fibers is prepared as follows: polypropylene filaments are spun through a SO-hole spinneret at a rate of 18 g./min. total. Each spinneret hole is 0.015 in. (0.038 cm.) in diameter and the temperature of the spinneret is 234 C. The filaments are led to a heated feed roll operating with a surface temperature of 118 (3., and advanced by means of an idler roll canted with respect to the heated roll. A total of 5 wraps is used on the heated feed roll which is operated with a surface speed of 243 yd./min. (222 m./ min.). The filaments leaving the heated feed roll are then passed 5 wraps around an idler roll/draw roll system operating cold with a surface speed of 858 yd./min. (785 m./min.). The filaments are drawn 3.6X, are 7.2 denier (0.8 tex) per filament and have a tenacity of 4.07 g.p.d. The drawn filaments are then electrostatically charged with a corona discharge device, passed into a draw jet, and subsequently deposited on a moving belt to form a nonwoven web of randomly distributed continuous filaments.

EXAMPLES 7-9 Three nonwoven Webs of crystalline polypropylene filaments were prepared in the same general manner as that described in Example 6, except the geometry of the holes in the spinneret is varied. Identification of the variables is made in Table 3. The results in Table 1 indicate that the variations in spinneret geometry in these C. and is operating with a surface speed of 243 yd./min. (222 m./min.). The filaments leaving the heated feed roll are then passed 3 Wraps around an idler roll/draw roll system operating cold with a surface speed of 858 yd./min. (785 m./min.). As the filaments pass over the grooved feed roll, the portions of the filaments over a groove remain relatively cool and the portions of the filaments between the grooves come in contact with the roll and are heated by conduction. The hot portions orient more than the cold portions, thus the filaments consist of thick and thin segments of low and high orientation. In Example 10, the high-oriented segments are about 18 in. (46 cm.) long and have a denier of about 7 (0.8 tex), and the low-oriented segments are about 2 in. (5 cm.) long and have a denier of about 15 (1.7 tex). The low-oriented segments comprise about 20% by weight of the filaments. After drawing, the filaments are electrostatically charged with a corona discharge device, passed into a draw jet. and subsequently deposited on a moving belt to form a nonwoven web of randomly distributed continuous filaments.

EXAMPLES 12-15 Four nonwoven webs of continuous polypropylene filaments are prepared in the same general manner as described in Example 1, except that the amount of loworiented polypropylene filaments is varied by changing the number of those filaments, keeping the number of 11 high-oriented filaments constant. Identification of the variables is made in Table 4.

1 Spinneret holes are 0.015 in. (0.038 cm.) in diameter x 0.075 in. (0.100

m.) long.

1 Spinneret holes are 0.020 in. (0.051 cm.) in diameter x 0.080 1n. (0.203 cm.) long.

EXAMPLE 16 A web is prepared as in Example 1 except that the feed roll speed for the high orientation filaments (from the 30-hole spinneret) is 155 yd./min. (142 m./min.), and the draw roll speed is 839 yd./min. (767 m./min.). The low-orientation filaments (-hole spinneret) are made with a feed roll speed of 667 yd./min. (610 m./min.) and a draw roll speed of 852 yd./min. (779 -rn./min.). Properties of the fibers and the web are presented in Table 1.

EXAMPLE 17 TABLE 5 Matrix fiber Bonding pressure birefringence After Web P.s.1.a. Kg./em. As spun bonding Mixed-orientation 68 4. 8 0. 0312 0. 0335 74 5. 2 0. 0312 0. 0326 78 5. 5 0. 0312 0. 0327 100% oriented 95 6. 7 0.0317 0. 0280 97 6. 8 0. 0317 O. 0278 98 6. 9 0. 0317 0. 0255 What is claimed is: 1. An unbonded nonwoven web of polypropylene fibers, said fibers having a crystallinity index of between about 30 and and having a coefiicient of variation in fiber-birefringence among the fibers of at least 5% and greater than by weight of said fibers having a birefringence of at least 0.02.

2. The nonwoven web of claim 1 wherein all the fibers have substantially the same level of crystallinity.

3. The nonwoven web of claim 1 wherein fibers are present having segments of varying orientation along their length thus providing the required coefiicient of variation in fiber-birefringence.

' 4. An unbonded nonwoven web of polypropylene fibers, said fibers having a crystallinity index of between about 30 and 70 and having a coefficient of variation in fiber-birefringence among the fibers of at least 5% and no more than about 20% by weight of said fibers having a birefringence below 0.02.

References Cited UNITED STATES PATENTS 3,193,442 7/1965 Schulz et a1. l6ll69 3,396,071 8/1968 Couzens 161-170 US. Cl. X.R.

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