US3567569A

US3567569A - Synthetic fibers having novel shape

Info

Publication number: US3567569A
Application number: US570325A
Authority: US
Inventors: Terumichi Ono; Shigeru Mizutani; Hazime Arai
Original assignee: Toyo Rayon Co Ltd
Current assignee: Toray Industries Inc
Priority date: 1965-08-09
Filing date: 1966-08-04
Publication date: 1971-03-02
Anticipated expiration: 1988-03-02
Also published as: BE685219A; GB1124380A; SE333038B; DK119269B; DE1660646A1; ES330003A1; NL6611192A

Abstract

SYNTHETIC, MOLECULARLY ORIENTED FIBERS OF FINE PITCH AND HAVING LARGE CONCAVITIES AND CONVEXITIES COMPRISING A MELT-SPINNABLE SYNTHETIC POLYMER HAVING A SINGLE CONTINUOUS MALE SCREW SHAPE WHEREIN THE RATIO OF PITCH TO OUTER

DIAMETER OF THE SCREW THREAD IS WITHIN THE RANGE OF 0.3 TO 15, AND THE RATIO OF HEIGHT TO OUTER DIAMETER OF SAID THREAD IS IN THE RANGE OF FROM 0.03 TO 0.4.

Description

March 2 1971 TERUMICHI ONO ETAL SYNTHETIC FIBERS HAVING NOVEL SHAPE Filed Aug. 4, 1966 United States Patent US. Cl. 161-179 6 Claims ABSTRACT OF THE DISCLOSURE Synthetic, molecularly oriented fibers of fine pitch and having large concavities and convexities comprising a melt-spinnable synthetic polymer having a single continuous male screw shape wherein the ratio of pitch to outer diameter of the screw thread is within the range of 0.3 to

15, and the ratio of height to outer diameter of said thread is in the range of from 0.03 to 0.4.

This invention concerns novel synthetic fibers having a unique form, and particularly concerns synthetic fibers having a male scre-W shape structure of a specific dimension.

Synthetic fibers have excellent physical properties and durability which are not seen in natural fibers, and because of these properties, they have found wide applications in clothing and as industrial fibers. However, synthetic fibers obtained by melt-spinning method have a grave drawback in that they have a smooth surface and a strong waxy feeling.

It has been known to improve the feeling and luster of fibers by making the cross sections of fibers into those of different shapes such as fiat or triangular shape or changing the thickness of fibers along these lengths in the manufacture of synthetic fibers. However, none of the prior art methods of spinning has been able to produce fibers having male screw shape in an extruded state.

An object of this invention is to provide novel synthetic fibers which have a shape of male screw and because of this, have novel properties which cannot be attained by the conventional fibers.

A specific object of this invention is to provide synthetic fibers which have no waxy feeling and a surface having varying states of large concavities and convexities at close pitches, which have large resistance to the deformation of the structures when woven or knitted, and which exhibit a complicated appearance with brilliant portions and delustred portions intermingled with each other.

Another specific object is to provide monofilamentary synthetic fibers which are excellent in knot properties, have a large surface friction, and which are used conveniently as brushes and fishing nets.

Still another object of this invention is to provide a method of continuously producing synthetic fibers of male screw shape having the aforesaid properties without using any particular apparatus.

Other objects and advantages of this invention will become apparent from the following descriptions.

The aforesaid objects can be achieved by synthetic fibers comprising a melt-spinnable synthetic polymer which has a structure of a male screw wherein the ratio (p/d) of the pitch (p) to the outer diameter of the thread (d) being in the range of from 0.3 to 15, and the ratio (h/ d of the height of the thread (h) to the outer diameter (d) being in the range of from 0.03 to 0.4.

This invention can be widely applied to melt-spinnable synthetic polymers, but can especially preferably be applied to linear polyamides. The linear polyamide suited 3,567,569 Patented Mar. 2, 1971 for this invention includes polycaprolactam (nylon-6), polyhexamethylene adipamide (nylon-6,6), polyhexamethylene sebacamide (nylon-6,10), poly-w-aminoheptanoic acid (nylon-7), poly-w-aminononanoic acid (nylon-9), polyundecanamide (nylon-ll), polylauriclactam (nylon- 12), polyamide prepared from paraxylene diamine and dodecandionic acid, copolymerised polyamides composed of two or more of these polyamide components, modified polyamides graft or block copolymerized with other monomers within a range wherein the properties of a linear polyamide are not substantially changed, a mixture of at least two of the said polyamides, and a mixture of the said polyamide with other polyamides capable of being melted. Among these various linear polyamides, polycaprolactam and polyhexamethylene adipamide are especially suited for this invention.

It is of course possible to incorporate into such polymers an ordinarily used additive such as stabilizers for heat and light, delustrants and pigments.

In order to facilitate the understanding of this invention, reference is to be had to the accompanying drawings in which:

FIG. 1 is an enlarged side view of the synthetic filaments of this invention having a male screw structure;

FIG. 2 is illustrative of various dimensions of the synthetic filaments of this invention having a male screw structure;

FIG. 3 is a diagrammatical view showing the range of conditions for the extrusion of a nylon-6,6 melt in accordance with the method of this invention; and,

FIG. 4 is a diagrammatical view showing the range of conditions for the extrusion of a nylon-6 melt in accordance with the method of this invention.

As shown in FIG. 1, the synthetic filaments of this invention have a male screw structure having regular pitches in the direction of the filaments. In FIG. 2, (d) represents the outer diameter of a screw thread; (p), the distance between the screw threads, i.e., pitches; and (h), the height of the thread.

It is essential in this invention that the ratio of the pitch (p) to the outer diameter of the thread (d) should be in the range of from 0.3 to 15 The ratios outside the specified range lead to disadvantages. If this ratio is smaller than 0.3, it is impossible for the filaments to engage closely with each other, and desired effects of this invention cannot be expected. If, on the other hand, this ratio (p/d) is larger than 15, the engagement of fibers with each other become loosened, and the diffused reflection and feeling become deteriorated. It is preferable in this invention that the said p/ d ratio should be in the range of 0.5 to 10.

Likewise, the height of screw thread should be in an appropriate range in order to obtain serviceable fibers which have excellent luster and feeling, and are well engaged with each other. If the height (h) of the thread of the fibers is too small for the outer diameter (d) of the thread, the engagement between fibers is not sufiicient, and the fibers do not exhibit excellent luster and feeling. The lower limit of a ratio of the height of the thread to the outer diameter of the thread (h/d) is 0.03. On the other hand, if the height of the thread is large as compared with its outer diameter, the properties intended by the invention are prominently exhibited. But if it is too large, the resistance of the fibers against outer force is lowered and the fibers become unserviceable. The upper limit of a ratio of the height to outer diameter of the thread is 0.4. It is preferable for the purposes of this invention that the said h/d ratio should be in the range of 0.05 to 0.35.

Suitable synthetic fibers of this invention are drawn in the longitudinal direction of fibers and molecularly oriented. However, when the denier of the said fibers is large, for instance, when the fibers are monofilaments having more than 100 denier, the said fibers have a sufficient strength even in an undrawn state, and can be used as such.

In accordance of the method of this invention, the synthetic fibers having the said male screw shape can be provided by the steps of melting melt-spinnable synthetic polymer, extruding it under extruding conditions with a shear stress capable of producing a melt fracture, thereafter quenching the extruded filamentary melt to set the male screw form of the said melt, and if desired, drawing the extruded filaments in their longitudinal direction.

The melt fracture is known as a phenomenon in which the outer shape of an extrudate becomes irregular when a ditficultly fusible polymer or a polymer with a very high melt viscosity is extruded through a capillary. However, in the past, when a polymer having a fiber-forming ability was subjected to melt-spinning, there was always employed such a condition that shear stress to be exerted onto a melt at the time of extrusion is lowered as much as possible and the flow of a molten polymer becomes smooth.

Unexpectedly, however, we have found that when a remarkably well melt-spinnable synthetic polymer such as a linear polyamide is extruded under a specific extruding condition, especially under very high shear stress, there occurs a melt fracture and filaments having a male screw shape can be prepared stably, that by quenching the melt extruded under such a condition, the male screw shape produced by the melt fracture can be set; and that the so formed synthetic fibers having a male screw shape while in a spun state, have excellent physical properties and even when in a drawn state, their male screw structure is maintained.

Such a method of this invention is contrary to the teaching of prior melt-spinning which specifies the carrying out of spinning under a condition with a shear stress rendered as small as possible. It is a surprising discovery that under such a severe extruding condition, there can be extruded stably continuous filaments having regular male screw shape.

The shear stress (nv), as used in the specification and claims, is a value determined by the method described in E. B. Bagley, J. Appl. Phys., 28, 624 (1957), and is expressed by the following formula 'rW=P'R/Z(L+NR) (1) (where P is an extruding pressure, R is the radius of an orifice, L is the length of the orifice, and n is a tubelength compensating coefficient).

In accordance with the method of this invention, the said melt-spinnable synthetic polymer is melted, and extruded under a shear stress such that melt fracture can occur. The value of the shear stress usable varies depending upon the type and degree of polymerisation of the polymer to be used, but generally it should preferably be in the order of at least We also found that the shear stress to produce the melt fracture has an upper limit according to the type and degree of polymerisation of a polymer, and when it exceeds the limit, there is a tendency that a utilisable melt fracture does not come into existence. This upper limit can easily be determined experimentally with respect to each polymer.

When polyhexamethylene adipamide is used as the synthetic linear polyamide, it is preferable that a melt of said polyamide should be extruded under a shear stress expressed by the formula (where 'rW represents a shear stress (dyne/cm. and '11) is a relative viscosity measured in a 98% sulphuric acid at C.).

If the value of nr is less than 2.2, it is extremely difiicult to obtain fibers having a male screw structure of the previously s ecified dimension. Even if they are obt in d,

their physical properties such as tenacity are so poor that they cannot be used commercially. On the other hand, when the value of 17) exceeds 6.0, it is difiicult to carry out melt-spinning specified in this invention.

When polycaprolactam is used as the synthetic linear polyamide, it is preferable to extrude a melt of the said polyamide under the shear stress expressed by the formula (where 1w represents a shear stress (dyne/cm. and 1;) shows a relative viscosity measured in a 98% sulphuric acid at 25 C.).

For the same reasons as mentioned with respect to polyhexamethylene adipamide, the value of mof polycaprolactam should preferably be in the range of from 2.0 to 9.0.

FIGS. 3 and 4 show regions operable with respect to polyhexamethylene adipamide and polycaprolactam. In these diagrams, the axis of ordinates represents a shear stress in terms of dyne/cm. and the axis of abscissa represents a relative viscosity (7 of the synthetic linear polyamide. In these diagrams, the regions surrounded by a solid line show regions within which the preparation by melt-spinning of synthetic fibers having a male screw shape can be carried out.

In accordance with this invention, the fracture of a melt comes into existence at a frequency in the order as high as 10 to 10 per second when a melt-spinnable polymer is extruded under the said specific shear stress condition. Therefore, the under the shear stress condition capable of producing melt fracture as used in this invention can be expressed the other way round as under an extrusion condition to produce the melt fracture at a frequency in the order as high as 10 to 10 per second. This capability of producing a melt fracture at such a high order of frequency in this invention is also quite unexpected from the prior information in the field of plastics.

It has also been found that in the method of this invention, the relative viscosity of a polymer to be used affects the dimension of the male screw structure. Specifically, the higher the relative viscosity of the polymer is, the greater the height of the screw thread becomes.

In accordance with the method of this invention, the so extruded filamentary melt is quenched and the male screw shape of the said melt is set The filamentary melt having the male screw structure in that form travels to some distance from the spinning hole, and then is subjected to draft by the tension of take-up rolls whereby the filaments in a molten state are drawn. It is preferable according to the invention to quench the melt at a position under the spinneret and above the point at which the drawing of filaments in a molten state is started, and then to set the male screw structure of the melt.

The position where a melt extrudate is quenched varies depending upon the amount of a polymer melt extruded, the take-up speed, the temperature of a melt extrudate, its relative viscosity, and the denier of the intended fibers, but it is advantageous that generally it is a distance within 50 cm. from the spinneret.

The melt can be quenched by a known means used for the quenching of a molten polymer. For example, a filamentary melt having a male screw structure is contacted with a cooling medium having a large heat capacity before a substantial drafting occurs, whereby a latent heat of melting is deprived of. A suitable cooling medium is a liquid inert to synthetic filaments, such as water, an aqueous solution of methanol, an aqueous solution of ethanol and an aqueous solution of ethylene glycol.

The filaments whose male screw structure has been so set have excellent physical properties in a spun state, and especially when they are the monofilaments with more than 100 denier, they can be used without subsequent drawing.

In a preferred embodiment of this invention, the filaments whose male screw structure has been so set are subjected to a known drawing operation to make molecularly oriented filaments having especially excellent physical properties. The drawing should preferably be carried out at a temperature from room temperature to below the melting point of the polymer at a draw ratio of from 3 to 4.5 X

Fabrics woven from the so prepared synthetic fibers of this invention do not give a waxy feeling often seen in fabrics woven from conventional synthetic fibers, because of small pitches and large convexities and concavities, and show diffused reflection like that of a pear because the surface is varied in its state. The fabrics are also strong in resistance to deformation of the structure as often seen in conventional fabrics. The synthetic fibers also give knitted goods having a large resistance to run and large resistance to expansion and contraction.

The monofilaments in accordance with this invention have excellent knot properties, a large surface resistance,

a unique surface appearance, an excellent dyeability, fiexibility and physical tenacity, and because of these excellent characteristics, they are very useful as brush bristles, nets, guts for rackets, ropes, cords, wigs, etc. In addition, the products of this invention are used in the form of batt, felt, paper and other non-woven fabrics either by cutting them to certain lengths or without cutting them. The male screw structure gives a high stability of form to products.

The invention will hereafter be explained by way of examples which, it should be understood, only serve to explain the invention and in no way limit the invention.

EXAMPLE 1 Nylon-6 having a relative viscostiy in sulphuric acid of 3.4 was extruded under the following conditions. Namely, the spinning temperature is 280 C.; the diameter of the spinneret hole is 0.2 mm.; the holes are cylindrical with a length of 0.2 mm. and 0.6 mm.; and the amount of polymer extruded is 2, 4, 6, 8, 10, 12, 14, 16 and 18 g./min., respectively. The extrusion condition with various shear stress is obtained as shown in Table 1. When an extruded polymer is cooled with water at a position immediately under the spinneret, there is obtained male screw shaped extrudate as shown in FIG. 1. To determine the outer shape of the obtained filaments, the pitches, the height of the screw thread, and the outer diameter of the male screw were measured. The results are shown in Table 1. It is understandable from the table that when nylon-6 having a relative viscosity in sulphuric acid of 3.4 is used, it is necessary and sufiicient to adjust the shear stress (value measured by the Formula 1) to 5 10 to 2 10 (dyne/ cm?) to obtain male screw shaped fibers.

TABLE 1 Extrusion condition Screw shape of fibers Height of screw Outer diam- Arnount of extruded Shear stress Pitch thread eter of screw polymer (g./min.) (dyne/emfl) (mm.) (mm.) thread (mm.)

3. 4X10 O O 3. 8X10- 5. 9X10 2. 5X10 3X10 4. 5X10- 6. 9X10 2. 9X10- 6X10 5. 1X10- 7. 8 10 3. 6X10 10 1()- 5. 3X10- 9. 3X 10 3. 5X10 12x10- 5.4)(10- 1.1)(10 4. X10- 11x10- 5. 6X10- 1. 2X 10 8. 4X10- 8 (10 6. X10 1. 7X 11. 4X10 4X10 6. 8X10- 2. 2X 10 0 0 7. 0X 10- EXAMPLE 2 Nylon 6 having a relative viscosity in sulphuric acid of 2.7 was extruded under the following conditions. Namely, the spinning temperature is 255 C.; the diameter of the spinneret hole is 0.2 mm.; the spinneret hole is cylindrical with a length of 0.2 mm. and 0.6 mm.; the amount of the polymer extruded is 2, 4, 6, 8, 10, 12 and 14 g./min., respectively. The extrusion condition with various shear Stresses was obtained as shown in Table 2. The extruded polymer was cooled and solidified with water at a position immediately under the spinneret to form extruded filaments as shown in FIG. 1. The pitch, height, outer diameter of a male screw were measured to determine the outer shape of the filaments obtained. The results are shown in Table 2. It is understandable that when nylon-6 having a relative viscosity of 2.7 in a sulphuric acid is used, it is necessary and suflicient to adjust the value of shear stress (value determined by Formula 1) to 8 10 to 2 10 (dyne/cm?) to obtain male screw shaped filaments.

TABLE 2 Extrusion condition Screw shape of fibers Height of screw Outer diam- Amount of extruded Shear stress Pitch thread eter of screw polymer (g./ mm.) (dyne/cmfi) (mm.) (mm.) thread (111111.)

6. 4X10 0 2. 8X1O- 9. 3X10 2. 8X10- 2 10- 4.1)(10- 1. 2X10 2. 8 l0' 4 10- 4.4X10" 1. (5X10 3.0)(10- GXIO- 4. 8X10- 1. 9x10 4. 2X10 3X10 5. 0X10" 2.1)(10 0 0 5. 2X10- 2. 5x10 0 0 5. 3X10- EXAMPLE 3 Nylon-6,6 having a relative viscosity in sulphuric acid of 3.1 was extruded under the following conditions. Namely, the spinning temperature is 290 C.; the diameter of the spinneret hole is 0.2 mm.; the spinneret hole is cylindrical with a length of 0.4 and 0.8 mm.; and the shear stress is varied as shown in Table 1 by the change of the extrusion speed. When the extruded polymer was cooled and solidified with water at a position immediately under the spinneret, there were obtained filaments. With respect to the so obtained filaments, the pitch, height and outer diameter of the male screw were measured. The results are shown in Table 3. It is understood from the table that it is necessary and sufiicient to adjust the shear stress within the range shown in Formula 2 for the purpose of developing melt fracture advantageously and stably, and that by quenching the extruded melt at a position immediately under the spinneret, filaments having stable male screw shape are obtained.

EXAMPLE 4 Nylon-6,6 having a relative viscosity in sulphuric acid of 2.8 was extruded under the following conditions. Namely, the spinning temperature is 275 C.; the diameter of the spinneret hole is 0.2 mm.; the spinneret hole is cylindrical with a length of 0.2 and 0.4 mm.; the shear stress is varied with the change of the extrusion speed as shown in Table 4. When the extruded polymer was cooled and solidified with water at a position immediately under the spinneret, there were obtained male screw shaped filaments as shown in FIG. 1. With respect to the obtained filaments, the pitch, height, and the outer diameter of the male screw were measured. The results are shown in Table 4. It is understood from the table that it is necessary and sufficient to adjust the shear stress within the range shown in Formula 2 for the purpose of developing melt fracture advantageously and stably, and that by quenching the extruded melt at a position immediately under the spinneret, filaments having stable male screw shape are obtained.

Nylon-6,6 having a relative viscosity in sulphuric acid of 2.8 was spun under a variety of conditions as shown in Table 5. The used spinneret hole is 0.25 mm. in diameter and 0.25 mm. in length. The oiling and cooling were carried out at a position one meter below the spinneret. The so obtained synthetic fibers of this invention when woven into gauze fabrics, showed excellent properties as shown in Table 5. The product had 30 warps per centimeter and 27 weft per centimeter.

TABLE 5 Experiment 1 1 2 3 4 Temperature 0.). 300 300 290 290 Amount of extrusion (g./min.) 3 5 15 20 Take-up speed (m./min.) 206 300 900 1,050 Draw ratio 4. 4. 0 4. 0 4. 0 Denier 37 40 41 47 Outer diameter d (mm.) 0.070 0.087 0. 082 0.080 Pitch p (mm. 3.2 1. 1 0.031 0.021 Height h (mm. 0. 003 0. 010 0. 005 0. 003 p/d ratio 45. 7 14. 1 0. 38 0. 26 h/d ratio 0. 04 0. 21 0. 06 0. 04 Knot slip of woven fabrics. Much Little Little Much Luster of woven fabrics. Touch of woven fabrics 1 Control.

1 Luster like that of flat surface. 3 Luster like surface of pear.

4 Strong waxy feeling.

5 Strong linen feeling.

6 Weak waxy feeling.

7 Slightly strong waxy feeling.

Experiments Nos. 2 and 3 refer to the fibers in accordance with the invention, and it was found that they have little knot-slip of woven fabrics, have a gloss like that of the surface of a pear, have weak waxy feeling or have strong linen touch. The experiments Nos. 1 and 4 refer to the fibers which fall outside the invention, and these fibers have much knot slip in woven fabrics and strong waxy feeling. The fibers of the experiment No. 1 have a gloss like that of a flat surface.

EXAMPLE 6 Nylon-6 having a relative viscosity in sulphuric acid of 3.2 was extruded under the conditions as shown in Table 6, quenched with water at a position 30 cm. under the spinneret, and taken up. The obtained filaments were drawn and used as guts for rackets for use in badminton. It was found as shown in FIG. 6 that the monofilaments within the range of this invention are not subject to the knot-slip of the guts, and had increased friction against shuttlecock, showing excellent properties.

The used spinneret was round in Experiment A with a diameter of 1.0 mm. and a length of 1.0 mm.; round in Experiment B with a diameter of 2.0 mm. and a length of 1.0 mm.; and Y-shaped in Experiment C with a slit width of 0.4 mm., a slit length of 1.0 mm., and a length of one wing of 2.0 mm.

TABLE 6 Experiment Temperature C.) Amount of extrusion (g./min.)

Take-up speed (m./n1in.) Draw ratio Denier Outer diam Pitch p (mm.). Height h (mm.). p/d ratio h/d ratio Knot slip of guts Friction 1 A little. 2 Much. 3 High. 4 Small. 5 Slightly high.

EXAMPLE 7 Nylon-6 having a relative viscosity in sulphuric acid of 3.2 was extruded under the following conditions, quenched with water at a position 30 cm. under the spinneret, and taken up. The obtained monofilaments were drawn at a ratio of 40X, and the drawn filaments were used as fishing nets. It was found as shown in Table 7 that the nets within the range of this invention (2, 3 and 4) have only a little deviation and showed excellent properties. The used spinneret had a hole diameter of 0.5 mm., and a length of 0.5 mm., and the flow-in portion is round-bottomed With one hole.

TABLE 7 Sample number 1 2 3 4 Conditions for production:

Polymer temperature C.) 290 290 290 290 Amount of extrusion (g./mi 50 100 Take-up speed (m./min.) 50 80 100 100 Draw ratio 4.0 4.0 4.0 4.0 Filaments propert Denier ,340 2,370 2,360 2, 380

Outer diameter of screw thread (mm.) 50 0. 64 0. G5 0. 63

Pitch (mm.) 0 2. 0 2. 5 0. 2

Height (n1m.) 0 0. 20 0.15 0.03

Frequency oftip ovcr(percent) 61.0 71.0 68.0 65.2

Frequency of knot slip (percent) 30 0 0 0 Control.

Frequency of tip over is expressed by the following formula:

Frequency of tip over: (a/ b) X 100 (wherein a is a tension measured the moment a knot formed by binding filaments A and B is deformed by pulling, and b shows a tenacity at the breakage of the filaments that occurs by a continuous pulling).

By the term frequency of knot slip is meant a frequency in percentage of knot slips occurring in the samples at the time when the tenacity of the filaments is measured by pulling the meshes of the net.

EXAMPLE 8 Nylon-6,6 having a relative viscosity in sulphuric acid of 3.0 was spun under various conditions, quenched with water at a position 30 cm. below the spinneret, taken up and drawn to 3.5 times the original length. The obtained filaments were used as fishing nets. It was found as shown in Table 8 that the fishing nets within the range of this invention exhibit excellent properties. The used spinneret was 0.2 mm. in diameter and 0.2 mm. in length, and the flow-in portion is round-bottomed with 10 holes.

TABLE 8 Sample number 1 2 3 Production condition:

Polymer temperature C.) 280 280 280 Amount of extrusion (g./min.) 50 80 Take-up sped (m./min.) 67 108 Draw ratio 3. 5 3. 5 3. 5 Properties of filaments:

Denier 2,100 2,106 2,100

Outer diameter (mm.) 0.08 0. 11 0. O9

Pitch (mnr) 0. 00 0.17 0. 45

Height (mm.) 0. 005 0. 03 0.01

p/d ratio 120 1. 5 5.6

h/d ratio 0. 00 0. 25 0. 10 Net: Frequency of knot slip 5 O 0 What We claim is:

1. Synthetic fibers of fine pitch and having large concavities and convexities comprising a melt-spinnable synthetic polymer having a single continuous male screw shape wherein the ratio (p/d) of pitch (p) to outer diameter (d) of the screw thread is in the range of from 0.3 to 15, and the ratio (h/d) of height (h) to outer diameter (d) of the screw thread is in the range of from 0.03 to 0.4, said fibers being drawn and molecularly oriented along their longitudinal axis.

2. The fibers of claim 1 wherein the melt-spinnable synthetic polymer is a linear polyamide.

3. The fibers of claim 2 wherein the linear polyamide is polyhexamethylene adipamide having a relative viscosity, measured in a 98% sulphuric acid at 25 C., of of 2.2 to 6.0

4. The fibers of claim 2 wherein the linear polyamide is polycaprolactam having a relative viscosity, measured in a 98% sulphuric acid at 25 C., of 2 to 9.

5. The fibers of claim 1 wherein the said p/d value is in the range of from 0.5 to 10.

10 6. The fibers of claim 1 wherein the said h/d value is in the range of from 0.05 to 0.35.

References Cited 5 UNITED STATES PATENTS 2,190,770 2/ 1940 Carothers l6lPolyamide Digest 2,434,533 1/ 1948 Wurzberger 57140I 2,578,743 12/1951 Rosenthal 161179 10 2,743,511 5/1956 Genovese 57140I FOREIGN PATENTS 760,179 10/1956 Great Britain 161173 15 ROBERT F. BURNETT, Primary Examiner R. L. MAY, Assistant Examiner US. Cl. X.R.