US3558420A

US3558420A - Hollow filaments

Info

Publication number: US3558420A
Application number: US661348A
Authority: US
Inventors: James E Opfell
Original assignee: Allied Chemical Corp
Current assignee: Allied Corp
Priority date: 1967-08-17
Filing date: 1967-08-17
Publication date: 1971-01-26
Anticipated expiration: 1988-01-26

Abstract

A HOLLOW FILAMENT OF SYNTHETIC POLYMER MATERIAL IS DISCLOSED CONSISTING OF A SHEATH SURROUNDING A LINGITUDINALLY EXTENDED CAVITY. THE SHEATH COMPRISES A CONTINUOUS PHASE POLYAMIDE POLYMER CONTAINING DISPERSED THEREIN A POLYESTER IN THE FORM OF MICROFIBERS LYING PREDOMINANTLY IN THE DIRECTION OF THE AXIS OF THE FILAMENT.

Description

Jan. 26, 1971 J. OPFELL I HOLLOW FILAMENTS Filed Aug. 17. 1967 INVENTOR JAMES E.OPFELL Raimi 61' Huh/ W F'IG.| I.

AT TOR N EY United States Patent (1) 3,558,420 HOLLOW FILAMENTS James E. Opfell, Colonial Heights, Va., assiguor to Allied Chemical Corporation, New York, N.Y., a corporatloll of New York Filed Aug. 17, 1967, Ser. No. 661,348 Int. Cl. D01d 5/24 US. Cl. 161176 3 Claims ABSTRACT OF THE DISCLOSURE A hollow filament of synthetic polymer material is disclosed consisting of a sheath surrounding a longitudinally extended cavity. The sheath comprises a continuous phase polyamide polymer containing dispersed therein a polyester in the form of microfibers lying predominantly in the direction of the axis of the filament.

BACKGROUND OF THE INVENTION This invention relates to novel hollow synthetic filaments and to the production thereof. More particularly, it relates to the production of said filaments by extruding a molten synthetic polymer material through a spinnerette containing at least one unobstructed orifice followed by controlled cooling of the freshly spun filaments to effect solidification thereof.

It is recognized that hollow synthetic filaments have certain advantages over solid filaments having the same outside diameter. Some of the advantages which hollow filaments have compared to solid filaments include improved insulating properties, increased bouyancy, and greater covering power (e.g., in carpet yarn) per unit weight. Hollow filaments composed of polymer blends also have less tendency to fibrillate under flexing conditions than corsponding solid filaments.

However, it has proved to be extremely difficult to manufacture hollow filaments in a commercially feasible manner by melt-spinning. Considerable time, effort and money have been spent on attempts to adapt existing methods to the production of hollow filaments on a commercial scale. Processes which have been devised for this purpose have necessitated the use of special and often expensive processing conditions and equipment. Such improvements as have been made have generally been related to the spinnerette. Unfortunately, the spinnerettes that have been designed thus far are difficult to construct and are subject to frequent breakdowns which may be attributed at least in part to their complex construction.

One type of spinnerette employs orifices containing an internal obstructing member which causes the orifice to function as an annulus, said obstructing member being joined to the spinnerette body by internal support members upstream from the extrusion face of the spinnerette. This type of spinnerette and others which have been designed to prod-nee hollow filaments are not only hard to make but are extremely difiicult to maintain in a good state of repair and cleanliness.

Other spinnerettes which have been studied employ a multitude of simple, unobstructed orifices grouped in a perimeter. They are so closely spaced that, upon extrusion, molten polymer emerging from each of the orifices" coalesces with extrudates from the adjacent orifices of the group to form a continuum of the polymer substrate which, after rapid cooling, forms a hollow-shaped filament. The filament is hollow because the area encompassed by the perimeter of orifices contains no openings and thereby blocks passage of polymer. These spinnerettes require very close spacing between adjacent orifices to permit coalescence of extrudate streams with the result that the thickness of the intervening metal between ori- Ice fices is so small as to cause structural weakness and difficulties of fabrication. The weakened nature of these spinnerettes is particularly significant in the melt-spinning of synthetic fibers because the high extrusion pressures required will often cause distortion or actual rupture of spinnerettes of inadequate strength. Another disadvantage of orifices which are too closely spaced in forming a perimeter is that polymer coalescence may occur too close to the face of the spinnerette, thereby preventing the entrance of air into the hollow cavity of the filament. The resultant vacuum within. the filament causes internal coalescence of the molten polymer, thus minimizing or eliminating the central cavity.

SUMMARY -OF THE INVENTION Therefore, it is an object of the present invention to provide a filament having an internal cavity extending along its length.

Another object is to provide an improved melt-spinning process for the production of hollow synthetic filaments.

Yet another object of the present invention is to provide a melt-spinning process for the production of hollow synthetic filaments using a spinnerette having unobstructed orifices which provide at least one gap in an otherwise continuous periphery of openings encompassing a polymer occluding area.

Other objects and a fuller understanding of the present invention may be had by reference to the following description, drawings, and appended claims.

The objects of this invention are accomplished in general by:

(a) Extr-uding a molten synthetic polymer composition having a die swell factor greater than about 2.5, and preferably between about 2.5 and about 10, through a spinnerette containing at least one circumscribing orifice arrangement. Said orifice arrangement comprises at least one unobstructed orifice and defines, without completely surrounding, a polymer occluding area which is devoid of orifices. This occluding area communicates with the area outside the orifice arrangement by means of at least one passage through said arrangement. The average diameter, D of the polymer occluding area is greater than AD wherein A is the die swell factor and D is the maximum orifice width in a direction radial to the center of said occluding area; and

(b) Cooling the polymer extrudate to effect solidification thereof.

In the accompanying drawings:

FIG. 1 is a vertical sectional view of part of a spinnerette useful in the practice of the present invention;

FIGS. 2 to 8 are plan views of spinnerette orifice arrangements according to the present invention comprising more than one discrete unobstructed orifice, each of said orifice arrangements having a perimetric configuration about a polymer occluding area. The spinnerette illustrated in FIG. 8 is disclosed and claimed in copending U.S. application Ser. No. 687,170, :filed Dec. 1, 1967.

FIG. 9 is a plan view of a spinnerette orifice arrangement according to the present invention comprising a single discrete orifice conformed into an encircling configuration about a polymer occluding area.

FIG. 10 is a partial sectional view showing the coalescence of polymer extrudates downstream from the spinnerette; and

FIGS. 11 and 12 are cross-sectional views of hollow filaments produced in accordance with the present invention.

In FIG. 1, spinnerette 1 has an upstream face 3 and downstream extrusion face 5. Spinnerette 1, being typical of those employed in melt-spinning of synthetic fibers, is generally about 0.2-1.0 inch thick and may be of monolithic or laminated construction. It is generally made of steel or other high strength metal or alloy. Counterbore depression 7 may be formed in the upstream face of spinnerette 1 in order to minimize the length of capillaries 9 leading to orifices 11 which define polymer occluding area 13.

The capillaries of spinnerettes useful in the practice of the present invention are generally of constant cross section throughout their length, i.e., the capillary wall is a cylindric surface as may be formed by circular movement of a straight line parallel to a given fixed centrally located straight line. The capillary may however, be slightly chamfered at either or both ends. The length of a typical capillary is preferably between about 8 and about 70 mils (l mil=0.00l inch). If the capillary length is appreciably below 8 mils, the spinnerette plate may be undesirably weakened, while capillary lengths substantially greater than about 70 mils may cause difficulties in obtaining satisfactory hollow filaments by the process of this invention. The capillaries may be made by standard techniques which include: drilling with rotary drills; drilling with pneumatic devices or electrodes; punching techniques; and insertion of shaped wires into differently shaped capillaries (as disclosed in US Pat. 3,174,364). The capillaries are preferably perpendicular to the extrusion face but may be angled to direct extrudate streams toward one another for improved coalescence.

In FIG. 2, portion 15 of spinnerette .1 contains a circumscribing arrangement of six individual orifices 17, each having diameter, D Orifices 17 are perimetrically disposed in symmetrical arrangement about central point 19, thereby defining central polymer occluding area 21, having average diameter D taken as the diameter of the circle which tangentially contacts the orifices at points closest to central point 19. Spacings 23 provide passages through which the occluding area 2.1 communicates with the rest of the spinnerette face. As will be explained more fully hereinafter, adjacent polymer streams coalesce just below these passages. If, for a given set of extrusion conditions, the following relationship holds:

then separate polymer streams from adjacent orifices will coalesce to form the desired hollow filaments.

In preferred embodiments of the present invention, D is greater than 2AD In FIGS. 3 through 8, D is the maximum distance within an orifice in a direction radial to the center of the polymer occluding area. D may be taken as the average length of all the straight lines which can be drawn between the inside walls of the orifices through the center of the polymer occluding area.

FIG. 9 illustrates an orifice arrangement consisting of a single discrete orifice having a spiral shape. For the purpose of ascertaining the value of D since there is no center of symmetry, a center of gravity may be chosen for the configuration, and D may be taken as the average length of straight lines which can be drawn through said center of gravity between opposing orifice boundaries.

The individual orifices may be of any cross-sectional shape, e.g., circular, rectangular, crescent shape, or other curvilinear or polygonal shape. Elongated orifices are preferred, especially those having a shape factor of from 1.4 to 20. The shape factor is defined as the ratio of the longest straight line which can be drawn within a cross section of the orifice to D Accordingly, circular orifices are not preferred since the shape factor in this case is one. The cross-sectional area of each orifice is preferably between about 12x10" and about 25 10 in. In the case of circular orifices, this would correspond to a diameter of between about 4 and about 55 mils. The orifice arrangement, which may consist of one, two, three, or more discrete orifices serving to produce a single hollow filament, generally has an axis of symmetry, and preferably a point of symmetry on the face of the spinnerette.

A spinnerette may contain any feasible number of 4 such orifice arrangements. The hollow, centrally positioned cavity of the resultant filament is produced by the absence of polymer extrudate within the perimeter of the orifice arrangement.

There is at least one and preferably several passages leading from within the otherwise complete annulus formed by the circumscribing orifice arrangement. The passages between the orifices, consisting essentially of material of which the spinnerette is composed, are made small enough to permit coalescence of the individual extrudate streams, yet far enough apart to preserve the strength of the spinnerette structure and to permit the atmosphere or gas to enter between individual extrudate streams prior to their coalescence. In general, the minimum distance of separation between the orifices should be greater than 3 mils to insure spinnerette strength, and less than about 15 mils to secure satisfactory interstream coalescence. Such spinnerettes are made useful in the practice of the present invention by virtue of operating conditions which provide a polymer die swell factor greater than 2.5.

The die swell factor of filament-forming synthetic polymers is a known measureable value and is defined as the ratio of the maximum diameter of the extrudate stream to the diameter of the orifice opening when employing a circular orifice removed from the influence of coalescing streams. The extrudate stream diameter may be measured by photographic or visual methods. In the case of noncircular orifices the die swell factor is measured on a circular orifice having the same area as said noncircular orifice, all other conditions being the same.

The die swell factor in the course of melt-spinning of homogeneous thermoplastic polymers is dependent upon several factors. It can be determined by simple tests, and the spinnerette for producing the best hollow core filaments selected on the basis of the factor as determined. Alternatively, the die swell factor can be varied to accommodate a particular spinnerette in accordance with known procedures for varying said die swell factor. Thus, holding all other variables constant, the die swell factor can be increased by:

(a) Decreasing the speed of the take-up roll.

(b) Decreasing the orifice extrusion temperature.

(0) Decreasing the residence time within the capillary (i.e., increasing the throughput). In this connection it should be noted that increased throughput even when compensated by increased take-up speed to maintain constant denier of the wound filament, still increases the die swell factor.

(d) Increasing the polymer melt viscosity; and

(e) Decreasing the diameter of the orifice.

The site of maximum swelling or stream diameter is generally within the first inch downstream from the spinnerette. The actual distance, however, tends to increase with higher throughput values.

In the case of the usual production of synthetic fibers by melt-spinning from polymers such as polyamide, polyesters, and polyolefins, the die swell factor rarely exceeds 1.5. The occurrence of die swell factors above 1.5 has generally been considered undesirable since it is normally associated with the production of filaments of varying diameters along their lengths. The phenomenon of the swelling or expansion of a freshly extruded polymer stream is attributable to the elastic characteristics of the polymer and to the tendency of the aligned polymer molecules to become disoriented upon emerging from the capillary.

In accordance with the present invention, when synthetic polymers are spun under conditions such that the die swell factor exceeds 2.5, a sufficiently large bulge is formed in the still molten extrudate to permit location of the separate orifices sufiiciently far apart to permit ambient air or gas to enter the space between individual extrudate streams prior to coalescence, thereby preventing collapse of the individual streams into a solid filament. The location of the orifices at greater distances of separation has the further advantage of giving increased strength to the spinnerette. In prior art methods, deficiencies in spinnerette strength have manifested themselves in a bulging or actual rupture of the central polymer occluding area.

With polymers spun under conditions such that the die swell factor is appreciably below 2.5 or above 10, the behavior of the molten extrudate becomes such that the spinnerettes described herein could not be used for making hollow filaments. It is possible, however that the polymer could be spun by modifying the spinning conditions so as to bring the die swell factor within the desired range.

Thermoplastic polymers suitable for use in the present invention include:

(a) Polyesters such as polyethylene terephthalate and polyhexahydro p-xylylene terephthalate.

(b) Polyamides such as polyhexamethylene adipamide (nylon-66) and polycaproamide (nylon-6).

(c) Polyolefins.

(d) Polyurethanes.

(e) Polyesteramides.

(f) Polyethers; and other synthetic polymers and mixtures thereof which may be spun under conditions which ensure a die swell factor within the preferred range. When only one polymer entity is employed it must be fiber-forming. If a mixture of polymers in which one polymer is in the form a dispersed phase within a continuous phase of the other polymer, the dispersed phase is not necessarily fiber-forming.

Suitable nylon polymers having satisfactory fiber-forming properties generally have molecular weights which are preferably in the range of about 15,000 to 40,000. Such polymers will have formic acid relative viscosities of 30 to 150, and preferably between 30 and 100, as determined at a concentration of 11 grams of polymer in 100 ml. of 90% formic acid at 25 C. (ASTM D-789-62T).

Suitable polyester polymers having satisfactory fiberforming properties generally have a reduced viscosity above 0.50. The reduced viscosity of polymers useful in the compositions employed in this invention is determined by viscosity measurements carried out at 25 C. on a 0.5% by weight solution of polymer in purified orthochlorophenol containing 0.1% water. Employing a standard Cannon-Fenske 150 bore viscometer, the flow time of the polymer solution (t is measured relative to the flow time of the solvent (t and the reduced viscosity iscalculated using the following equation:

N red: (11,-1 /C where:

N red=reduced viscosity C=concentration of dissolved polymer in grams/ 100 ml. n,.= relative viscosity=tp/t We have found that a particularly preferred class of thermoplastic compositions for use in the practice of this invention comprises heterogeneous compositions consisting of a dispersion of discrete regions or particles of one polymer within a continuous phase of a second polymer. Such heterogeneous compositions afford unexpectedly higher die swell factors under conditions which are not adverse to the general requisites of good spinning practice. For example, whereas many single-component polymer systems require lower than normal extrusion temperatures to obtain an adequate die swell factor, for the purpose of this invention the aforesaid heterogeneous compositions provide suitable die swell factors at higher temperatures, a factor which also favors interstream coalescence and good fiber formation. Although it is not intended that the present invention be bound by theory, it is felt that the high values for the die swell factor obtained with such heterogeneous systems may be due to the tendency of the minute globules of dispersed phase polymer, which emerge from the orifice in an elongated molten condition, to reassume a spherical shape in order to decrease their free energy content. In tending toward the spherical form, the dispersed phase polymer globules tend to slow down the polymer stream or cause a shortening thereof, which causes a bulge immediately beneath the spinnerette. Particularly desirable heterogeneous polymer compositions comprising polyester particles dispersed in polyamides are disclosed in copending US. patent application Ser. No. 368,028, filed May 18, 1964, in the name of I. C. Twilley, now Pat. No. 3,369,057.

FIG. 10 illustrates the melt spinning process of this invention employing a heterogeneous polymer composition. As depicted therein, a molten composition comprising a continuous phase polymer 25 and dispersed discontinuous phase polymer 27 is extruded through closely

adjacent orifices

29 and 31 of spinnerette 33. Under the shear conditions within the capillary a dispersed polymer globule assumes an elongated form 35. Upon emerging from the capillaries, the elongated dispersed polymer globule assumes the more spherical form 37. Concurrently, the diameter of the extrude composition expands to a maximum value at a site 39 which leads to coalescence of the two streams of polymer emerging from

spinnerette orifices

29 and 31. Downstream from this point, the generally spherical globules again become elongated as exemplified by

globules

37a, 37b and 37c. At the same time, the diameter of the extrudate (now a hollow filament) decreases as it comes under the influence of drawing and stretching forces exerted by the take-up roll.

In the preferred heterogeneous compositions, both polymers should be capable of elongation at temperatures above their melting point and preferably also capable of elongation under cold-drawing conditions below the melting point. The dispersed polymer need not be fiberforming, i.e., capable of forming a continuous filament of useful strength by a standard melt-spinning operation. It should also be sufiiciently incompatible with the continuous phase polymer so as to remain a distinct, discontinuous phase dispersed within the continuous phase during melt-spinning. It is preferable that the dispersed phase polymer have a melting point in the range of between about 50 C. below to about C. above the melting point of the continuous phase polymer. A particularly preferred range for the polyester-polyamide blends described in the aforementioned Twilley application is for the melting point of the polyester to be between about 10 C. to about 90 C. higher than the melting point of the polyamide. For the purposes of the present invention, polymer melting points may be determined by using a hot-stage microscope. For those polymers having indistinct melting points, the melting point is taken as the temperature at which the polymer becomes fiowable at atmospheric pressure.

Hollow filaments prepared from said preferred heterogeneous polymer mixtures are found to present a visual appearance characterized as having aesthetically pleasing sparkle or glitter. The novel filaments are also found to have improved strength and rigidity in view of the reinforcing eifect of the dispersed phase polymer which, in the drawn fiber is in the form of microfiber reinforcing elements dispersed within the continuous phase polymer structure in a direction generally parallel to the axis of the filament.

Polymer die swell factors greater than 2.5 may also be secured by incorporating a gas or gas-forming material within the polymer melt prior to extrusion. Said gasforming material may be in the nature of a gaseous or volatile liquid material which, upon extrusion and consequent sudden decrease in pressure, will have a tendency to increase in gaseous volume. Such gaseous or gas-liberating substances may conveniently be employed in amounts ranging from about 0.1% to about 2% by weight based on the total weight of the polymer composition prior to extrusion. There may also be incorporated within the polymer composition finely divided and dispersed particles which serve as nucleation sites for bubbles and which therefore control the uniformity of bubble size and distribution upon extrusion. The resultant hollow filament obtained when employing polymer compositions containing gas-forming agents may or may not retain bubbles, depending upon the rate of bubble formation and quenching of the extrudate in the course of forming the solidified hollow filament. Suitable gas-forming agents include, for example, methanol, methylene chloride, hexane, benzene, fluorocarbons, water, and the like.

For extrusion, the molten polymer composition is metered to the spinnerette at pressures which vary somewhat with the composition, but generally are in the range of about 1,000 to about 10,000 p.s.i.g. The composition is preferably passed through a filtration device such as a sand pack prior to entering the capillaries. The temperature of the polymer mixture at the spinnerette is maintained generally at between about C. and about 50 C. above the melting point of the highest melting polymer component of the composition. In the case of heterogeneous polymer blends, melt-spinning is preferably conducted under conditions which are interadjusted to maintain the value of the dimensionless ratio R/Pu in the range between about 0.0001 and 0.002, where R is the rate of throughput per orifice, P is the orifice diameter or average orifice diameter in the case of noncircular openings (which may be approximated by the average diameter of a series of circles inscribed within the orifice on the plane of the spinnerette), and u is the viscosity of the molten polymer blend at the extrusion temperature all factors being expressed in self-consistant units. In this manner, uniform hollow filaments will be consistently produced. If the value of R/Pu is greater than 0.002, then discontinuities or drips may occur during spinning. If, on the other hand, the value of R/Pu is less than 0.0001, then a pulsing effect may occur, which results in erratic control of filament denier.

In accordance with the usual melt-spinning techniques, the plastic extrudates of this invention are cooled to solidification by contact with a gaseous medium of controlled thermal properties. In such processes, the spinnerette usually forms the upper end of an enclosed cylinder known as a quench stack wherein controlled cooling to solidify the extruded polymer is effected. Cooling is usually brought about by contacting the extrudate with air or other gas which is chemically inert to the polymer under controlled conditions of temperature, fiow rate, and flow pattern. The cooling gas flow pattern may be cocurrent, countercurrent, or crosscurrent to the filaments. Gas temperatures may be as high as about 50 C. or more above the temperature of the extruded polymer at the face of the spinnerette and may decrease in a predetermined manner down the stack thereby controlling the length of time or traveling distance during which the extrudate is in a flowable form. Gas flow rates in the range of about to about 1,000 ft. min. may generally be employed.

The solidified filaments are removed from the quench stack by means of a driven take-up roll which is generally located below the quench stack. By suitable adjustment of the peripheral speed of said take-up roll, controlled elongation of the molten extrudate stream prior to solidification may be effected, the extent of said elongation being known as stack draw-down." The speed of the take-up roll may also effect the die swell ratio. In order to obtain satisfactory coalescence so as to produce hollow fibers, it is preferred that the polymer remain flowable for at least 5 mm. downstream from the spinnerette orifices and that the stack drawn-down be maintained between about 10 and about 1,000.

The extrudate, while still molten may be subjected to ultrasonic energy, electrostatic radiation or other physical treatment to secure specialized effects such as periodically closing the hollow filament core to form cells, imparting a crimpability to the filaments, and other effects.

After being taken-up below the quench stack, the filaments may be subjected to a drawing operation at temperatures below their melting point to confer molecular orientation along the filament axis and increase the strength of the filaments. Dra w ratios of between about 2 and about 12 have been found to impart maximum yarn strength. The optimum draw ratio, however, will vary with the selected polymer or polymer mixture. The drawing operation is preferably carried out by standard methods which may use a draw pin to localize the region of drawing. The yarn may be drawn in either a single or successive stages, and at least one of said drawing stages may be carried out while the yarn is heated, e.g., to a temperature between about C. and about C. for nylon 6 yarns. The optimum temperature will, of course, vary with the selected polymers. The heat can be applied to the yarn by known means such as stationary or rotating contact heaters, steam chambers, heated liquid sprays or baths, infrared, radio frequency heating, and other means.

Finishing compositions comprising lubricating ingredients and/or Wax can be applied to the yarn prior to drawing to facilitate the drawing operation. Prior to packaging, or in subsequent operations, the drawn yarn can be subjected to annealing at constant length or to relaxation with controlled shrinkage in order to reduce the shrinkage and/ or ultimate elongation of the yarn. One suitable method of effecting such treatments is disclosed in Wincklhofer US.

Pat. 2,859,472, issued Nov. 11, 1958. The drawn yarn can be subjected to treatment with ionizing radiation, ultrasonics, crosslinking agents such as formaldehyde and isocyanates, and other finishing treatments to secure various desired effects, none of which, however are to be considered as limiting the scope of the invention.

The product of this invention is a synthetic filament consisting essentially of a sheath and an internal, longitudinally extended cavity. The cavity may occupy between about 10% and about 80% of the entire cross-sectional area of the filament. The sheath may be of circular or noncircular shape. The cavity may be centrally or eccentrically disposed with respect to the filament axis, and the peripheral contour of the cross section of the cavity may be of the same or different from the shape of the cross-sectional periphery of the sheath. The shapes of both the cavity and sheath will, however, be essentially constant along the length of the filament. The sheath portion, consisting of the extruded polymer composition, will contain, in the case of heterogeneous polymer blends, the dispersed polymer in the form of microfibers. In preferred drawn filaments these microfibers will have an average diameter generally not greater than about 1 micron, and an average length of at least about 5 times their diameter. The microfibers lie predominantly in the direction of the filament axis, and there may be from 500 to more than 150,000 microfibers per 1,000 square microns of transverse area of the drawn filament. The presence of microfibers in the hollow filaments of the present invention imparts improved bending modulus, or stiffness characteristics which are especially appreciated in textile apparel and in carpeting where improved resiliency is usually a desirable feature.

FIG. 11 represents a cross-sectional view of filament 41 comprising annular round sheath portion 43 of uniform thickness and circular cavity 45 which is substantially centrally disposed with respect to sheath 43.

FIG. 12. illustrates a cross-sectional view of a filament 47 of this invention containing dispersed microfibers 49 within continuous phase polymer 51 of the noncircular sheath 53. Cavity 55 also is noncircular, but it is differently shaped than the outer periphery of the sheath 53.

The hollow filaments of this invention may be made to contain various additive ingredients which impart specialized properties. For example, ingredients which may be added to the filaments either by incorporation within the polymer prior to spinning, or by aftertreatment of the yarn or fabric include: flame retarding agents such as compounds of antimony, phosphorous, and halogens; delustrants such as titanium dioxide, calcium acetate and other opaque metal compounds; antistatic agents; adhesion promoting agents such as isocyanates and epoxides, heat and light stabilizers such as inorganic reducing ions, metal ions such as manganese, copper and tin, phosphites, and organic amines such as alkylated aromatic amines and keto-aromatic amine condensates; thermally stable organic and inorganic pigments; fluorescent agents and brighteners; crosslinking agents; bacteriostats such as phenols and quaternary amines; colloidal reinforcing particles; antisoiling agents such as colloidal silica and boehmite; dyeing modifiers; lubricants such as molybdenum disulfide and silicones; lasticizers; dispersing agents to facilitate and maintain dispersion of one polymer of a heterogeneous blend within the other; and other additives and treatments. Gaseous mateirals may also be incorporated within the internal cavity of the fibers of this invention. Such additives may be incorporated by serving as the quenching media for the filaments. In this respect, fibers of this invention may serve asuseful containers for the controlled use of such gases.

Thermally stable flow controlling agents or surface active agents which decrease surface free energy may be included within the polymer prior to extrusion to increase the extent of extrudate coalescence, and thereby improve the circularity of the sheath structure where such is desired. Examples of such agents include metal salts of long chain aliphatic carboxylic acids, long chain aliphatic alcohols, long chain aliphatic amides, and other surface active agents.

The filaments of the present invention are useful in numerous textile applications in the form of monofilament and multifilament yarn and tow, cords, and staple spun yarns. The filaments may be blended with other fibrous materials, and may be employed in crimped or uncrimped conditions. Typical textile applications include apparel products such as woven suitings, shirtings, sheeting and lingerie, tricot, circular knitted fabrics, broadcloths, satins, and the like. In view of their relatively high stiffness, strength, and low weight, the blended polymer filaments of this invention are further useful in textile applications such as sewing thread, tire cord, fiber-reinforced laminates upholstery, carpeting, drapery, curtains, ducks, parachutes, reinforced belts and hoses, marine lines, ropes and netting, and other applications. The filaments may be admixed with solid core filamentary structures of various modified cross section, of the same or different denier and the same or different chemical composition to produce various special effects.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following examples, all parts and percentages are by weight unless otherwise indicated.

Example 1 A thermoplastic polymer composition comprising a uniform heterogeneous molten blend of 30% polyethylene terephthalate of 0.7 6' reduced orthochlorophenol viscosity, and 70% polycaproamide of 50 formic acid relative viscosity is prepared in the presence of trace amounts of sebacic acid which serves as a chain length terminator, whereby fewer than 40% of the terminal groups are amino groups. The blended composition is prepared by meltblending the two polymer components under conditions of high shear so that the dispersed polyester in the composition has an average particle size of 0.8 micron. The composition is then extruded at 280 C. at a rate of 4.6 pounds per hour, and a residence time within the capillary of 3.8 seconds through a spinnerette having 16 groups of rectangular orifices essentially as shown in FIG. 4 wherein each orifice is 0.004 inch wide (average diameter) and 0.032 inch long, and the capillaries are 0.020 inch deep. The three rectangular orifices, having a shape factor of 8.0, are arranged about the center of a circle having a radius of .020 inch. The effective value of D is about 0.050 inch. The closest spacing between the orifices is 0.004 inch. Under these conditions, the polymer has a die swell factor of 3.5 as measured on a circular orifice having a diameter of 0.013 (which provides the same orifice area as one of the rectangular orifices). Upstream from the surface of the spinnerette is a sandand-screen filter pack. The pressure on the polymer at the upstream face of the spinnerette is about 1900 p.s.i.g. The value of R/Pu is 0.00137. The value of AD is 0.004 3.50.014".

The filaments are extruded into an atmosphere of air at 72 C., flowing concurrently to the extrudate at a rate of 20 cubic feet per minute. The solidified yarn is wound up at a rate of 1170 feet per minute. The yarn thus obtained is cold-drawn at a draw ratio of 3.83 between feed rolls and draw rolls, employing a snubbing pin of /2 inch diameter operating at ambient temperature to localize the draw zone.

The resultant yarn has a denier of 210. The individual filaments have a cross section resembling that shown in FIG. 12 wherein the hollow core occupies about 12% of the over-all cross-sectional area of the filament. The filaments have a sparkling luster, particularly when observed after fabriaction into a carpet sample, and have good resiliency. Microscopic examination of the fibers reveals the presence of microfibers of the polyester component, said microfibers having an average diameter of 0.18 and average length of 165,u., there being about 9,500 of such microfibers in a given cross section.

For purposes of comparison, the same spinnerette is employed for the spinning of a composition consisting entirely of the polycaproamide employed above. The polymer temperature, extrusion rate, and all other process conditions are maintained the same as above. Under such conditions the die swell factoris only 1.5 and the extrudate streams do not coalesce to form hollow filaments.

By way of further comparison, the above experiment using the polymer blend composition is repeated using a spinnerette of essentially the same configuration but wherein the minimum spacing between the individual orifice is 0.002 inch. The filaments obtained have no hollow core. Furthermore, after several hours of use, the polymer occluding areas within the orifice groups of the spinnerette are observed to distort or bulge outwardly. This causes spinning discontinuities and makes it impossible to wipe the spinning face of the spinnerette, i.e., to scrape it clean for restarting satisfactory extrusion.

Example 2 A sample of linear polyethylene having a density of 0.960 and melt index of 2.0 (determined according to ASTM D1238-52T) is extruded at 190 C. through a spinnerette having twenty seven groups of six circular orifices whose centers are equidistantly spaced on a circle having a diameter of 0.072 inch generally as shown in FIG. 2. Each orifice has a diameter of 0.023 inch and a capillary length of 0.046 inch. The diameter D of the occluding area is thus 0.049 inch. The entrance angle to the capillary from the countcrbore is i.e., the countcrbore has a fiat bottom. The polymer is extruded at a rate calculated to give a capillary shear rate of 800 secsaid capillary shear rate being given by the formula where Q is the flow rate in m/sec., R is the radius of the orifice in cm. and n is a constant expressing the degree to which the flow deviates from the Newtonian condition. In this test, n is taken as equal to 1.0. The die swell factor, measured on an orifice isolated from the regular groups is 3.0. The value of AD, is 0.069 inch.

The extrudate is solidified by contact with a countercurrent flow of heated nitrogen in a confining column. The rate of quenching is such as to maintain the polymer in molten or plastic form for at least 12 mm. downstream from the spinnerette. The solidified, hollow core filaments are taken up on a winder positioned below the column.

The extrudates from the individual orifices of each group coalesced, forming an integral molten structure which upon cooling yields hollow filaments. The yarn is drawn at a draw ratio of six at ambient room tempera ture between standard feed rolls and draw rolls. The resultant drawn filaments retain their hollow configuration, the area of the hollow interior being about 40% of the complete cross-sectional area of the filaments.

The die plate, after continued use, shows no evidence of bulging or rupture of the occluding area within the groups of orifices.

Example 3 A sample of Hoechst Hostalen GF-5200 polyethylene having a melt index of 0.4 (determined as in Example 2), and a density of 0.947 is extruded at 220 C. through a spinnerette having 16 groups of three orifices as shown in FIG. where each orifice has a width of 0.004 inch, and a cross-sectional area of 0.000128 square in. The distance of separation between the orifices is 0.004 inch. The diameter, D of the polymer occluding area is 0.0304 inch, and the shape factor of the orifices is 5.6. The capillaries are perependicular to the extrusion face of the spinnerette and have a length of 0.030 inch. The polymer is extruded at a rate such that the capillary shear rate is 350 secr' the shear rate being determined from the equation of Example 2. Here R denotes the radius of a circular orifice of comparable area, and n is equal to 2.4. Under these conditions, a die swell factor of 3.3 is observed. The value of AD is 0.013 inch. This swell factor is sufl'icient to cause the polymer streams from the separate orifices to contact each other and coalesce to form an integral structure which yields a hollow fiber on cooling.

Example 4 A sample of polycaproamide having a melting point of 215 C. and relative formic acid viscosity of 50 is extruded at 250 C. through a spinnerette of the same general type employed in Example 3. The spinnerette in this case, however, has 12 groups of three orifices wherein each orifice has a width of 0.005 inch, a cross-sectional area of 0.000303 square inch and a capillary length of 0.018 inch. The distance of separation between the orifices is 0.006 inch. The diameter D of the polymer occluding area is 0.060 inch, and the shape factor of the orifices is 8.0. At an extrusion pressure of 5,000 p.s.i., a die swell factor of 3.4 is obtained. The value of AD is 0.0175 inch. Under these conditions coalescence of adjacent extrudate streams gives hollow filaments. The filaments are cooled to solidification by countercurrent contact with heated air, and are wound up at a rate of 1580 feet/min. The yarn is drawn at a draw ratio of 4 between feed and draw rolls using a draw pin to localize the draw point. The drawn yarn retained a continuous hollow cavity which occupied about 40% of the total cross-sectional area. Fabrics woven from the drawn yarn in this example are light in weight, have high covering power, and additionally possess the usual desirable characteristics of a nylon fabric.

Example 5 A sample of polyethylene terephthalate having a reduced orthochlorophenol viscosity of 0.83 is extruded at 12 270 C. through the spinnerette of Example 4. A die swell factor of 3.6 is obtained with proper adjustment of flow rate, and under these conditions satisfactory coalescence is obtained to secure hollow filaments. The value of AD is 0.018 inch.

Example 6 A sample of polycaproamide terminated with acetic acid, and having a relative formic acid viscosity of 52 with about 23 milliequivalents of amine end groups per kilogram of polymer is extruded at 260 C. at a throughput of 0.96 lbs/hr. through a spinnerette having four groups of trislots as shown in FIG. 8. Each orifice is 0.003 inch wide. The closest spacing between the orifices is 0.0035 inch. The orifices are arranged about a circle having a diameter, D of 0.026 inch. Upstream from the spinnerette is a filter pack containing sand having an average particle size of 400 U.S. mesh.

The filaments are extruded into an atmosphere of air at a take-up speed of 1600-1625 feet per minute, followed by drawing at a draw ratio of 3.4-to-1 at 1000 feet per minute.

The resultant drawn hollow filament has a denier of 41, with a tensile strength of 4.8 am./denier. The hollow portion of the filament occupies 911% of the cross-sectional area of the product filament.

The foregoing examples were presented for the purpose of illustrating the melt-spinning process and hollow synthetic filaments of the present invention. It is, of course, understood that variations in the procedures described in those examples as well as changes in the materials used therein may be made without departing from the spirit of the invention and scope of the appended claims.

I claim:

1. A hollow filament consisting essentially of a sheath containing an internal longitudinally extended cavity occupying at least 10 percent of the cross-section of said filament, said sheath comprising a continuous phase polyamide polymer containing dispersed therein a polyester in the form of microfibers lying predominantly in the dirrection of the axis of said filament.

2. The filament of claim 1 wherein said microfibers have a length-to'diameter ratio greater than about 5.

3. The filament of claim 1 wherein the dispersed phase is a polyethylene terephthalate and the continuous phase is a fiber-forming polycaproamide.

References Cited UNITED STATES PATENTS 3,160,193 12/1964 Baggett et al 152359 3,369,057 2/1968 Twilley 260857 3,382,305 5/1968 Breen 16lMicrofiber Dig.

ROBERT F. BURNETT, Primary Examiner R. O. LINKER, JR., Assistant Examiner U.S. C1. X.R.