US3720741A

US3720741A - Melt spinning process

Info

Publication number: US3720741A
Application number: US00863707A
Authority: US
Inventors: R Cunningham; W Privott; L Rakestraw
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1969-10-03
Filing date: 1969-10-03
Publication date: 1973-03-13
Anticipated expiration: 1990-03-13

Abstract

A low stress spinning process wherein disruption of low viscosity streams due to the viscous drag forces imposed upon the extruding body is minimized by maintaining the propagation velocity of drag-sustained deviations less than the stream velocity to prevent the upstream migration of such deviations into the low strength molten region. This interrelationship of propagational and spinning velocities may be accomplished by a variety of techniques for controlling the interaction of viscous drag and tensional forces upon the stream.

Description

United States Patent 1191 [111 3,720,741 Cunningham et al. 45 March 13, 1973 MELT SPINNING PROCESS 2,976,590 3/1961 Pond ..22/200.1 75 E h W) 3,214,805 11/1915 McKenica ..22/200.1 1 Z 3,216,076 11 1965 2116616: al ..1 ..22/2001 1 Lawrence Rikestraw, 3,218,681 11 1965 Ditto ..22 57.2 all of Ralelgh, 3,429,722 2 1969 Economy etal... ...106/55 7 I 3,461,943 8/1969 Schile "164/39 I 3] Asslgnee Mmsamo Company St Lows Mo 3,481,390 12 1969 Veltrietal. ..9/86 [22] Filed: Oct. 3, 1969 3,490,516 l/l970 Basche et a1. .....9/273 3,516,478 61970 D t l ..7 281 211 Appl. No.2 863,707 I a FOREIGN PATENTS OR APPLICATIONS Related U.S. Applicatmn Data I 1,069,472 5/1967 Great Britain ..264/176.2 [63] Cont1nuat1on-1n-part of Ser. No. 596,286, Nov. 22,

1966, abandoned. OTHER PUBLICATIONS 52 U.S.Cl. ..264/82 106/39 106/55 Prepamm" Thin wires by Smldificati'" 164/82 264/40 262N176 F 264/232' Liquid Metal Jets by Tammann et al. Zeitschrift fur 51 Int. (:1. ..0286 3/20, B22d 11/00 Mefallkunde, 27 (5); 1117115 (1935) Translatim 5 [58] Field ofSearch ..264/176F, 82,40; 106/55, Pages Attorney-Vance A. Sm1th, Russell E. Wemkauf, John 5 References Cited D. Upham and Neal E. Willis UNITED STATES PATENTS [57] ABSTRACT 3,543,831 12/1970 Schile ..164/82 A low stress s innin rocess wherein disruption of P g P 3,583,027 6/1971 Garrett et a1. ..164/82 low viscosity streams due to the viscous drag forces 3,593,775 7/1971 Pl'lVOtt ..164/82 imposed upon the extruding is minimized 18/1971 1 1 "164/82 maintaining the propagation velocity of drag-sustained 0/1971 Mottem et deviations less than the stream velocity to revent the 2,825,108 3/1958 Pond 22/200 1 p 2 879 566 3/1959 Pond "zz/zoo'l upstream m1grat1on of such dev1at1ons mto the low 2:900:708 8/1959 x: strength molten region. This interrelationship of 2,907,082 10 1959 Pond ....22 57.2 propagation? and Spinning velodties y be accom- 0,886 6/1960 Nachtman ..154/91 plished by a variety of techniques for cont o i the interaction of viscous and tensional forces upon the stream.

1 1 Claims, 14 Drawing Figures Vertical Downward Forge (F F dynes/crn Angle of Filament with Vertical PATENiEUMimi-ms 3,720,741

' sum 10F 5 "a FIG. 2

Spinning Velocity (V O cm/sec e degrees INVENTORS Robert E. Cunningham Wilbur J. Privoii Jr.

ATTORNEY cm/sec PAIENIEDIIAR I 3 I973 V crn/sec SHEET 2 BF E I0 dynes/cm g 3.1 x 10- 9/00 l l I 0 2O 4O 6O 80 I00 r x I0 cm Effect of SIreum Radius on Drag Propagation l I I l 0 I.O 2.0 3.0 4.0 5.0

E x I0 dynes/cm EHecI 0f SIreom Modulus on Drug F I G. 4

F I G. 5

37.5 x ro' cm 600 E l0 dynes/cm p 3.1 x lo' g/cc 400 .02 cp Effect of Filament Density 0n Drag Propagation FIG. 6

INVENTORS Rober? E. Cunningham Wilbur J. PrIvoH Jr.

ATTORNEY FATENIEUMAM 3191a SHEET 5 BF- FIG.

FIG.

I N VEN TORS Robert E. Cunningham H Jr.

ivo

ilbur J MELT SPINNING PROCESS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of the copending and commonly assigned application Ser. No. 596,286, filed on Nov. 22, 1966 and now abandoned. Related copending applications are Ser. No. 829,216, filed June 2, 1969 and Ser. No. 838,603 filed July 2, 1969 both of S. A. Dunn, L. F. Rakestraw and R. E. Cunningham and assigned to the same assignee as the present invention.

FIELD OF THE INVENTION The present invention relates generally to the form ation of shaped articles from materials of low melt viscosity and, more particularly, to a process for forming shaped articles directly from streaming low viscosity melts wherein the viscous drag forces generated by the interaction of such streams with the atmosphere through which they pass are so controlled as to avoid disruption or permanent deformation of such streams.

BACKGROUND OF THE INVENTION In undertaking to form shaped articles directly from streaming melts of low viscosity, the problem initially encountered is one of stabilizing the liquidous portion of such streams, pending their solidification, against an intrinsic tendency to undergo local fluid mass transfer which, if unchecked, normally culminate in stream disruption. The above-referenced applications Ser. No. 829,216 and Ser. No. 838,603 are addressed to the multifaceted aspects of this problem in disclosing a practical low viscosity stream stabilization technique. Quite briefly, this approach involves the concept of film-stabilized melt spinning wherein a low viscosity melt is free-streamed under carefully controlled conditions through selected atmospheres productive of a rapid film formation about the liquid region of the nascent stream to thereby stabilize same against breakup pending solidification by normal heat transfer phenomena. Though the present invention is of particular advantage in the practice of such a film-stabilized spinning technique, it will become apparent that it may also be utilized with low viscosity melt spinning techniques generally, no matter the mode of stabilization of the fragile liquid region. For example, the present invention will be found to have important application to low viscosity melt spinning techniques employing stabilization of the liquid region by the imposition of magnetic or electrostatic fields, or by the application of hydrodynamic principles to suppress disruptive mass transfer.

Aside from the manner in which the liquid region of a low viscosity -stream is initially stabilized, a further problem of stream disruption or distortion is encountered. As distinguished from the behavior of highly viscous materials, such as the glasses and high molecular weight polymers that are readily accommodated by conventional melt spinning practices, the liquidous portion of a low viscosity, high surface tension stream is most fragile, even though initially stabilized against fluid migration pending substantial solidification. As herein disclosed, it has been discovered that, though the liquid portion of the stream is successfully stabilized against local fluid migrations, the stream may yet undergo distortion or disruption by action of viscous drag forces encountered in the passage of the stream through a given atmosphere.

Contrary to conventional high viscosity melt spinning, the viscous drag factor becomes one of primary importance in the spinning of low viscosity melts due to the inability of such melts to undergo attenuation without disruption. Because of the non-extendable character of the molten region of low viscosity streams, they must be conveyed under relatively low tension. It is this necessity for low tension spinning which raises the problem of drag-induced stream distortion.

It has now been established that, if the net force imposed upon the stream through the interaction of tensional forces (normally gravity) with viscous drag forces reaches a certain magnitude, the stream is subject to a drag-sustained displacement from its normal vertical, straight line path; further, such drag-sustained deviations from the vertical, straight-line path of the stream will, if not controlled according to the teachings herein set forth, migrate upstream to a weaker region where permanent distortion, if not total disruption, will result. Having thus ascertained the mechanism of draginduced stream deviations, we have discovered that their disruptive effect can be avoided by controlling the magnitude of the drag force relative to the stream, or extrusion velocity such that the velocity of propagation of the drag-sustained stream deviations is maintained less than the stream velocity, with the result that such deviations, when formed, are caused to pass harmlessly downstream.

It is emphasized that the present invention is directed to the control of drag-sustained stream deviations initiated within a relatively solid region in a manner to prevent their upstream migration into the weaker, liquidous region. The drag-sustained straight line path deviations are distinguishable from sinuous-like deviations which may be generated within the liquid region of a stream by drag forces. The disruptive effect of both such drag-driven phenomena may be similar, but their mechanisms and means of control are not to be equated.

This concept of controlling the propagation velocity of drag-induced stream deviations relative to stream velocity by controlling viscous drag force factors becomes one of increasing criticality with decreasing stream diameter and/or density. In attempting very small diameter production (on the order of less than microns from low density melts), the tensional force due to gravity becomes so small as to offer little or not net resistance to the initiation and propagation of stream deviations. Even upon optimizing to a practical extent, those spinning atmosphere properties affecting viscous drag, one may still encounter drag-induced disruption. The present teachings not only define this problem but disclose its solution. 0n the other hand, in the case of extruding relatively large diameter streams from melts of relatively high density, the present concept affords a measure of the extent to which tensile breaking of low viscosity streams may be counteracted by viscous drag without distortion.

OBJECT OF THE PRESENT INVENTION BRIEF SUMMARY OF THE INVENTION We have found that the viscous drag between the solidified portion of an extruded stream and the spinning atmosphere can cause the stream to be deviated from the desired vertical extruding path which deviations sustained by the viscous drag may propagate upstream into the liquid stream portion thereby disrupting the stream. We have further found that the disruptive effect of viscous drag may be minimized by extruding the stream at a greater velocity than the upstream velocity of the drag induced deviations. More specifically, in one embodiment of our invention, this is accomplished by providing a film-stabilizing atmosphere which has a low gas density. In still another embodiment, a co-current gas flow is provided to reduce viscous drag.

The term film-stabilizing atmosphere is defined as an atmosphere containing one or more gaseous components at least one of which reacts or decomposes to form a film about the molten stream to stabilize the stream against surface tension disruptions pending solidification thereof by heat exchange. The reactive and/or decomposing gaseous component is called the film-forming component. The film-stabilizing atmosphere may also include an inert component which aside from reducing the film-forming reaction may be utilized as hereinafter explained in more detail to reduce the density and/or viscosity of the film-stabilizing atmosphere.

The materials employed in the instant invention to form fibers and filaments are generally those normally solid metals and inorganic non-metals having a melt viscosity below about poises. The term metals is intended to include not only essentially pure metals, but also their alloys and intermetallic compounds. The term inorganic non-metals includes ceramics and metalloids.

Among the normally solid metals (solid at 25C) which can be spun according to this invention are beryllium, cobalt, aluminum, thorium, nickel, iron, copper, gold, uranium, zinc, manganese, magnesium, tin and alloys made from such metals.

Representative of the low melt viscosity ceramics useful for fiber and filaments by the process of this invention are alumina, calcia, magnesia, zirconia, and mixtures of these and other oxides wherein such mixtures exhibit a melt viscosity of less than about 10 poises.

Metalloids, such as boron and silicon, and a variety of other inorganic solids having a melt viscosity below about 10 poises under the spinning conditions can be employed to make fibers and filaments according to the process of this invention as hereinafter described.

It should be understood that this invention is particularly concerned with the manufacture of fine diameter fibers and filaments. As a practical matter, the process herein described is applied to the manufacture of fibers and filaments having diameters of less than about mils and is particularly attractive technique for making fibers and filaments having diameters less than 35 mils from low viscosity melts.

DESCRIPTION OF THE DRAWINGS The invention may be best understood with reference to the following description taken in connection with the appended drawings in which:

FIG. 1 is a graph illustrating total force/unit length acting on the stream as a function of the angle of stream deviation.

FIG. 2 depicts the normalized propagation velocity of the deviations as a function of spinning velocity for various spin gas velocities.

FIG. 3 graphically illustrates the force/unit length acting upon the stream as a function of the angle of stream deviation for various stream velocities.

FIGS. 4, 5, 6, 7, 8 and 9 depict stream velocity as a function of stream radius, filament density, stream modulus, spin gas density, spin gas viscosity, and spin gas velocity, respectively.

FIG. 10 illustrates the normalized vertical stream force as a function of spinning gas velocity for various stream velocities.

FIG. 11 is a vertical cross-section of a typical spinning apparatus which may be utilized in the practice of this invention.

FIG. 12 is a photograph of a fiber spun where the drag-sustained deviations propagated upwards at a rate greater than the stream velocity. A

FIGS. 13 and 14 are photographs of fibers spun at successively lower stream velocities.

DETAILED DESCRIPTION To facilitate a better understanding of the present in vention and the possible variations in its practice, reference is now made to a discussion of the mechanism by which drag-induced deformation of low viscosity streams is understood to occur. For the sake of simplicity, the following discussion will be made with the assumption that the stream decends vertically through a homogeneous atmosphere maintained at a uniform velocity along the length of the stream. It is to be understood, however, that the same considerations will apply in the same manner in cases of other than vertical streaming through other than homogeneous atmosphere.

The stream will initially be observed to decelerate at a downstream portion at a rate faster than the remaining upstream portion which might result in a deviation from its original vertically straight path. As distinguished from the general configuration that is characteristic of liquid stream deviations, the present invention is concerned with drag-sustained deviations initiated in the relatively higher modulus, solid downstream region. In the case of deviations in the liquid region, the configuration has been observed to approach that of a planar sine wave if conditions are sufficiently adverse to generate such disturbances. However, as the stream increases in modulus with increasing solidification, a point is reached beyond which the increased rigidity of the stream results in the formation of a three-dimensional, substantially helical (as opposed to sinuous) configuration. Further, we have found that the level of the viscous drag beyond which drag-sustained deviations are found to occur in the solid region is less than the level of the drag force required to sustain deviations in the liquid region; which is to say that, though a given set of spinning conditions is such as to avoid drag-sustained deviations within the liquid region, the stream may yet suffer distortionthrough helical deviations initiated within the higher modulus, solid region.

Starting from literature studies of the viscous drag forces acting upon streaming bodies (see for example, Sakiadis, B. C., AIChEJ, 7, 467 (1961) and Knudsen, J. G. and D. L. Katz, Fluid Dynamics and Heat Transfer, McGraw-Hill, New York, 1958), we have derived a reasonably close approximation of the total downward force acting upon a streaming body, as given by:

F,,/unit length the force acting upon the stream/unit length, dynes/cm r= stream radius, cms.

g gravitational acceleration, 980 cm/ser p8 stream density, gm/crn )t spin gas viscosity, poise V, stream velocity, cm/sec V, velocity of the gas into which the stream is extruded, cm/sec p, spin gas density, gm/cm I) angle of stream deviation between axis of the stream and the vertical, degrees The stream and gas velocity are considered positive downward or away from the point of origin of the stream.

As may readily be appreciated, it is impossible, at

least within the realm of practicality, to constrain a freely streaming body to move in a precisely straight path; even though one takes elaborate precautions, it is virtually impossible to isolate the stream from at least initially minor disturbances engendered, for example, by uncontrolled, though minor, fluctuations in extrusion conditions, eddy currents and the like due to such as minor orifice imperfections, as well as external vibrations. As shown by Equation (1) above, the total downward force per unit length on the stream varies with the angle of deviation of the stream from the vertical, FIG. 1 depicts a curve for a solution of Equation (1) as a function of the angle of stream deviation from the vertical. As there indicated, any deviation of the stream from the vertical less than 0, leaves the entire length of the stream in a positively tensioned state, with the result that there is no tendency for the stream to undergo further deviation or bending. That is, because a net tensional force remains upon the stream at deviations of a magnitude less than the angle 0 the stream tends to return to its original vertically straight path even though initially deviated by some stream disturbing force. Once some portion of the filament, by whatever event, achieves an angle greater than 0, from the vertical, this portion will be decelerated because the interaction between gravity and the increased drag force acting upon the deviated portion results in the imposition of a net upward, or negative, force upon the stream. Further, the force of oncoming upstream portions will tend to increase the angle of this portion of the stream and the filament will be urged to move to an angle b with the vertical. As shown by the curve of FIG. 1, the only two equilibrium configurations of he stream supplied at a rate V, are a straight, vertically falling stream with a constant speed V, or a stream falling at an angle of deviation 0, with the vertical at a velocity of V, cos 0,. Any other magnitude of deviation will be found unstable, with the stream returning to either the configuration accompanying an angle of deviation of 0 or a vertically straight path. Thus, it is seen that, for a given net tensional force acting upon a stream passing in a state of equilibrium along a straight, vertically downward path, there exists an angle of deviation 0,, hereafter termed the angle of threshold deviation, below which the stream is, under the influence of gravity, returned to the equilibrium state of its original, straight-line path, and above which a net upward, or negative, force comes to act upon the deviated portion urging it to an angle of deviation 0 hereafter termed the angle of equilibrium deviation. Further, as the net tensional force acting upon a vertical portion of the stream decreases with, for example, an increase in the force of viscous drag acting thereon, the angle of threshold deviation (9 decreases; which is to say that the magnitude of stream disturbance necessary to initially deviate the stream to 0, decreases-"with decreasing stream tension.

In the case where a stream portion has come to occupy the configuration of equilibrium deviation 62, it is clear that oncoming upstream portions must be decelerated in the vertical direction from a velocity V, to a velocity V, cos 0, as successive portions undergo helical bending to the angle 0 The effect of this successive bending of the stream is the propagation of a helically configured deviation along the stream. It is with this understanding of the mechanism of dragsustained stream deviations that we have been enabled to discover that, byv manipulation of those spinning conditions afi'ecting the magnitude of the viscous drag force, one can control the formation and propagation velocity of such deviations. We have further discovered that, if permanent stream distortion is to be avoided, the velocity of propagation must be controlled so as not to exceed the stream velocity.

It has now been experimentally confirmed that a reasonable approximation of the propagation velocity, relative to the stream, of a drag-sustained stream bending or deviation is givenby the relationship: ps

wherein:

V, velocity of propagation, relative to the stream of a drag-sustained stream bending or deviation, cm/sec F, maximum upward or negative force/unit length acting upon the stream as given by equation (I), dynes/cm.

E stream modulus at the point of stream bending, dynes/cm r= stream radium, cm.

p, stream density, gm/cm.

V, velocity of the stream, cm/sec.

0, angle of equilibrium stream deviation from the vertical degrees 6 threshold angle of stream deviation above which the stream undergoes further deviation to the angle and below which such deviation is non-sustaining degrees.

The magnitude and direction of the velocity of propagation relative to the orifice, hereinafter referred to as the normalized velocity of propagation (V,,,,) is obtained from the following relationship:

V,,,=V,,+V,, 3 wherein V, the normalized velocity of the velocity of propagation relative to the orifice and being negative in value when propagating toward the orifice, cm/sec.

V, the velocity of propagation of the stream deviation relative to the stream as given by equation (2), cm/sec.

V, the velocity of the stream, cm/sec. where the velocity of propagation matches spinning velocity, the deviation propagates at a velocity relative to the orifice, or normalized velocity, of zero and the deviation would remain a fixed distance from the orifice. A plot is shown in FIG. 2 for the cases of countercurrent, stagnant and co-current spin gas flow. As indicated, for spinning velocities greater than a critical velocity V any disturbance which causes a stream deviation to an angle greater than 0 would propagate upstream to the fragile liquid region to result in either total breakup of the stream or a jointed appearance in the resulting filament. In other words, the critical velocity may be defined as that velocity of the stream which results in the propagation of the stream deviation at a velocity equal in magnitude but opposite in direction to the stream velocity. For stream velocities less than V any stream deviation would be swept harmlessly downstream.

It should here be noted that the representation in FIG. 2 is made assuming the stream is of constant modulus along the length under consideration. In reality, the effective stream modulus may be found to decrease with increasing stream temperature encountered at progressive points upstream so that, where conditions are such that the disturbance propagates at a velocity to move upstream relative to the orifice, the length over which such stream bending acts becomes less and less until a point is reached where the propagation becomes arrested by a sharp bend, resulting in either broken lengths or filaments exhibiting knee-like joints.

To gain a better understanding of the interrelationship of the vertical force system acting upon a filamentary stream and the velocity of propagation of drag-induced deformations, reference is now made to the showing of FIG. 3 where, employing Equation (1), the variation in vertical force per unit length with spin velocity is plotted. Forces acting in the downstream direction are designated as positive. The FIG. 3 showing is for the case of an 86 micron diameter 62 wt. percent lead/38 wt. percent tin stream passing through a stagnant film-forming atmosphere of 93 percent by volume helium and 7 percent by volume oxygen.

As shown in FIG. 3 for the zero spin velocity (V,) curve, the vertical force per unit length acting upon the stream equals approximately the stream weight when the stream is stationary. Up to a spinning velocity of about 625 cm/sec., the net force acting upon the fiber is always in the downward, or positive direction, regardless of the angle of the filament with the vertical, with the result that any incipient stream deviation is immediately overcome and the stream returns to its original path. Consequently no propagation of dragsustained deviations is possible. As the spinning velocity is increased, a point is reached at which, once the stream suffers a deviation to an angle greater than 0,, the resultant drag-sustained propagation velocity due to the net upward, or negative, force acting upon the stream exactly equals the spinning velocity with the result that the deformation is stationary relative to the orifice as it rides the stream in treadmill fashion. According to Equation (2), a spinning velocity of approximately 2,050 cm/sec. is required to satisfy this condition. As previously discussed, it has been discovered that, under the conditions above specified, this constitutes the maximum allowable spinning velocity or critical spinning velocity V when non-jointed, continuous lengths are desired. At higher spinning velocities, an upstream force is imposed upon the stream of such magnitude as to cause any stream deviation initially exceeding (9 to propagate at a velocity greater than the spinning velocity to move upstream relative to the orifice to a point where stream strength has decreased sufficiently to arrest such propagation by permanent distortion or breakage. At lower spinning velocities, any 0 deviation of the stream will, due to the reduced upward force, propagate at a velocity less than the spinning velocity to thereby be passed harmlessly downstream. It is to be recognized that, in general, a precise balance between propagation and spinning velocities is not likely of observation since very minor variations in spinning conditions, particularly as regards spinning velocity or gas conditions, would result in movement of the deformation either upstream to a point of ultimate disruption or harmlessly downstream.

As will be understood, as the upstream-acting drag force increases with, for example, increasing spinning velocity, the threshold or minimum initial stream deviation 0,, beyond which the drag force is sufficient to further deviate the stream to the stable 0 position, decreases. So long as a stream is conveyed with such precision as to avoid any initial deviation greater than 0 deviations and their propagation will not be sustained. We have observed, however, that, in the case of low density melts and/or small diameter streams, deviations greater than 0 are virtually impossible to avoid. Also, where it is found necessary to resort to counter-current gas flows in order to avoid stream breakage due to gravity (as may be the case with relatively large diameter streams of high density), the extent to which gravity, or stream weight, may be counteracted by a drag force is limited out of the same considerations. It will often be found desirable to impose drag forces of such magnitude as to result in a value for 0, so small that it will be found most difficult to isolate the stream from incidental 6 deviations. Again, once such a magnitude of initial deviation occurs, it is critical that the propagation of the resultant drag-sustained deviation be controlled not to exceed the spinning velocity if permanent distortions are to be avoided.

As determined by calculation from Equation(2), a still further increase in the spinning velocity above the critical spinning velocity (V,,) of 2,050 cm/sec. results in a propagation velocity of drag-sustained deviations greater than the spinning velocity, with the result that a deviation moves upstream to a point of ultimate stream disruption. For example, at a velocity of about 2,170 cm/sec., the gravitional and viscous drag forces are of equal magnitude at an angle of zero deviation and the net force of drag versus gravity acts upstream at any other angle. Under these conditions, even the most minor stream deviation is drag-sustained and propagated at a very rapid rate.

It may readily be appreciated that, for other stream and gas compositions and other spinning conditions, a force diagram such as depicted in FIG. 3 may reflect considerably different values, but will generally conform to the pattern there indicated.

To afford a more thorough understanding of the interrelationship of various spinning parameters upon the formation of viscous drag-sustained deviations and their propagation, reference is now made to Examples I through VII, taken in conjunction with the data plots shown in FIGS. 4-10.

In this series of examples, the critical spinning velocity V, (above which deviation propagation velocity V exceeds spinning velocity) is plotted against each of those spinning parameters having a major effect upon spinning results, insofar as drag-sustained propagations are concerned. For simplicity, these investigations were conducted on the assumption that each of the designated parameters was an independent variable to better isolate its effect upon drag propagation. Save for the parameter which constitutes the variable under consideration in each example, the stream properties of density (p and modulus (E) are those of stainless steel, the gas properties of density (p,) and viscosity (p.) are those of 93 percent by volume helium/7 percent by volume oxygen mixture; similarly, gas velocity (V,,) was maintained stagnant and stream radius (r) and 37.5 times 10' cm.

EXAMPLE I In FIG. 4 there is plotted the effect of varying stream radius upon the maximum or critical spinning velocity (V,) beyond which the propagation velocity (V,,) of drag-sustained stream deviations exceeds spinning velocity to therebyresult in stream distortion or disruption. At very small stream radii it is seen that the propagation of such deviations, once initiated, exceeds any reasonable spinning velocity and successful spinning is possible only by effecting a co-current flow ofa gas having an appropriate density and viscosity. At intermediate stream radii, it is seen that drag-induced disruption is within the range of realistic spinning velocities so that spinning may, if desired, be carried out in stagnant atmospheres. At larger stream radii, the critical spinning velocity increases rapidly to thereby enable one to resort to a considerable viscous drag support by utilizing counter-current gas flow. Even in this range, however, the counter-current flow must be controlled so as not to induce a drag force productive of propagation velocities greater than the chosen spinning velocity.

EXAMPLE 11 FIG. 5 depicts the effect of a variation in stream density on permissible spinning velocity (V with the given properties and conditions. Here, it is seen that low density melts, such as boron and aluminum, are disrupted at less than normally desired spinning velocities (i.e., velocities below which the stream undergoes disruptions due to localized fluid transfer within the liquid region), higher density melts, as typified by stainless steel and lead/tin, are easily accommodated. In the case of lower density melts, as typified by boron and aluminum (2.3 and 2.37 gms/cm at 20C, respectively), co-current gas flows would, under the conditions specified in FIG. 5, be required to achieve what have been normally found to be the minimum permissible spinning velocities for even medium size streams, i.e., streams of a few thousandths inch diameter.

EXAMPLE III In FIG. 6 is shown the effect of stream modulus (E) on permissible spinning velocity (V,). Contrary to what one might expect, a fairly wide variation in stream modulus is seen to have little efi'ect upon the maximum spinning volocity.

EXAMPLE IV According to the graphical data set out in FIG. 7, a variation in stabilizing atmosphere density (g) has a significant effect upon permissible spinning velocity, assuming that gas viscosity remains constant. This assumption is warranted for several of the gases, but not for all. For example, the helium inert component and oxygen film-forming component have approximately the same viscosity, but, as shown in FIG. 7, spinning into a primarily helium atmosphere can be successfully executed at much higher velocities than into an atmosphere having the density of oxygen. It is advantageous therefore to spin into film-forming atmospheres having an inert component which is less dense than the film stabilizing component. For example, the densities of helium and oxygen at standard temperature and pressure are 0.178 gm/l and 1.42 gm/l. respectively. Thus, by utilizing helium as the primary component, the deviation propagation velocity is reduced.

EXAMPLE V As seen from the data set out in FIG. 8, the effect of gas viscosity (IL) is also quite pronounced. It can be appreciated that this effect would be even greater in the case of a lower density and/or smaller diameter stream than that designated.

EXAMPLE VI The parameter which is normally most readily constream. In this case it is seen that spinning may be accomplished at any co-current gas flow, as well as at counter-current gas flows up to approximately 430 cm/sec. At the lower spinning velocities, stream support can be gained by the use of counter-current gas is carefully drilled to provide either a small diameter flows. At higher spinning velocities (above approxiorifice 16, or a carefully machined hole in which a mately 1,200 cm/sec.) it is seen that co-current gas watch-sized jewel having an orifice formed therein is flows must be used, though the flow here can be conseated. Although the crucible illustrated is provided siderably less than the spinning velocity, again with the with only a single spinning orifice, production versions result that a considerable fraction of the stream weight would of course be provided with a plurality of similar may be supported by the action of viscous drag. such orifices. The crucible rests upon a supporting and To establish optimum spinning conditions for a given insulating cylinder 18 of quartz construction, which, in melt issued at a given stream diameter, it is possible to turn, rests upon a support plate 20. The crucible, thus modify only the gas properties and flow. As borne out mounted, may be enclosed by a conventional susceptor in the foregoing examples, in many cases, significant 22 which encircles the crucible when it is desired to improvement may be achieved by varying gas composiprocess materials in non-conductive/non-coupling tion to get a less dense and/or viscous stabilizing atcrucibles, i.e., crucibles which cannot be directly mosphere. When the allowable changes in composition heated inductively. The susceptor may be held in place are found not to concord i th required spinning by a suitable refractory cording, not shown. The cruciconditions, the situation becomes one wherein high ble and susceptor are enclosed within a heavy-walled quality spinning performance can only be accomhousing 24 of Pyrex or quartz construction. plished by circulation of the stabilizing atmosphere. Mounted at the upper end of the housing 24 is a fitting 26 suitable for connection to a source of pres- EXAMPLE sure, which fitting is clamped in gas-tight relationship This example illustrates a further aspect of the interbetween pp plate 28 and pp plate 20 y means relationship between the critical spinning velocity (V of suitable tu bOltS 30 By this arrangement, an inert, and the gas (stabilizing atmosphere) velocity (V,) pressurized gas may be imposed upon the materials along the stream. As set out in FIG. 10, for each being melted within the crucible to effect its extrusion spinning velocity there exists either a maximum through the orifice 16. An induction coil 32 ofsuitable counter-current flow or a minimum co-current flow electrical characteristics encircles the crucible to whi h an b om d t d ith t n i d supply heat to the contents thereof according to the disturbances to propagate u stream into the weak, principles of magnetic induction. Suitable flexible molten region. Further, for a given spinning velocity, gaskets 38 are provided between the base plate 36, supthe limit upon gas flow also corresponds to a definite port plate 20 and spin chamber 12 to assure a gas-tight amount of viscous support of the free-falling stream. assembly. The spin chamber or chimney 12 is provided The measure of this viscous support is indicated along with two or more spaced inlet-outlet ports 40 which are the ordinate axis of FIG. 10 in terms of a normalized used to introduce and extract the desired atmosphere vertical stream force (F, (l' F,,)/F,, where F and within chamber 12, as well as to control the direction F,, are the forces of gravity and viscous drag, respecand magnitude of its circulation in accordance with the tively), taken as positive in the downward direction. teachings herein set forth. I The locus of the limit on gas velocity as it varies with It is to be understood that the above-described spinning velocity defines the curve shown in FIG. 10. 40 spinning apparatus merely represents a typical as- Stream disruption due to the propagation of dragsembly which may be employed in the practice of the sustained deviations into the molten region occurs present invention, which latter is in no way limited to below and to the left of this line and does not occur the details of the apparatus. For example, where it is above and to the right. Thus, at a spinning velocity of desired to spin low melting point materials, a reapproximately 500 cm/sec., the maximum counter-cursistance-heated spinning assembly may be employed to rent gas flow which can be tolerated without propagagood advantage.

tion or disruption is approximately 400 cm/sec.; Utilizing the apparatus shown in FIG. 11, Examples similarly, at a spinning velocity of approximately 1,700 VIII and IX serve to further demonstrate the cm/sec., the minimum co-current flow is approximately mechanism by which viscous drag-sustained stream- 400 cm/sec. Further, it is seen that, at a spinning deviations are initiated and propagated, as well as the velocity of 1,700 cm/sec. and a co-current gas flow of manner in which such propagation is controlled. By

400 cm/sec., approximately nine-tenths of the steam employing a m lt Source (Such as the Alumla-9391cia weight may be supported without propagational disrup mixture of Example VIII) which solidifies without tion. crystallization into a glassy filament, it is possible to ob- An illustrative apparatus which may be employed in tain filaments wherein stream contortions due to drag the spinning of the low viscosity melts with which the propagation become fixed or set during the process of present invention is particularly concerned is shown in solidification. This is due to the fact that, compared to FIG.- ll of the drawings wherein there is depicted a substances which undergo crystallization during simplified, partially sectionalized vertical view of an insolidification, glassy substances undergo a spaced induction-heated spinning .assembly. crease in viscosity which increases resistance to bend- As there shown, such an assembly, generally ing to give the effect that would be observed in the designated by arrowed numeral 10, is mounted upon an presence of increasing modulus. In that the propagaelongated, cylindrical spinning chamber or chimney tion of drag-sustained stream deviations decreases with 12.The spinning assembly comprisesamelt crucible 14 stream modulus and since modulus decreases at which may be formed from any suitable refractory progressive upstream point (rather than the sudden material which is found compatible with the melt it is decrease exhibited by crystallizable mediums), up-

desired to process. The bottom surface of the crucible stream propagations relative to the orifice ultimately reach a point of such reduced modulus that the stream is either severed or bent upon itself, resulting in permanently contorted filamentary lengths.

EXAMPLE VIII The apparatus of FIG. 1 1 was provided with a molybdenum crucible having a 6 mil diameter, 2 mil long cylindrical orifice formed in the bottom surface. A stagnant stabilizing-atmosphere gas mixture of 75 vol. percent helium/25 vol. percent propane was maintained in spin chamber 112 at 1 atmosphere pressure.'A charge of 1:1 molar ratio 35 weight percent calcia/65 weight percent alumina was supplied to the crucible and brought to an extrusion temperature of about 1,685C. Extrusion was effected under argon pressures of 100. 60,and 35 psig. to obtain 6 mil diameter streams.

Under an extrusion pressure of 100 psig., the stream was observed to contort into a helical-like configuration with a number of knee-like joints or kinks occurring at randomly spaced points therealong. Such a filament is shown in the photograph constituting FIG. 12, wherein several rather obvious kinks are indicated by the letter (K). The helical configuration is obvious. Such contortion resulted by virtue of the fact that, under the conditions specified, the resultant viscous drag force generated was of such magnitude as to cause drag-sustained deviations to propagate at a velocity greater than the spinning velocity obtained under the specified extrusion pressure of 100 psig. As previously indicated, one mode of mitigating this adverse relationship of propagation and stream velocities may be accomplished by reducing stream velocity to thereby effect an even greater reduction in the velocity of propagation. As indicated by the showings of FIGS. 13 and 14, further reductions in extrusion pressure to 60 to 35 psig., respectively, resulted in the stream undergoing first an intermediate level of propagational distortion and, finally, a state wherein the straight glassy ceramic fibers shown in FIG. 14 were obtained. Though, in the present example, viscous drag-sustained stream distortion was controlled by a reduction in a spinning orstream velocity (V -L), a similar result may also be achieved by manipulations of the several other parameters affecting the magnitude of the viscous drag force (see Examples I-VI).

EXAMPLE IX s The apparatus of Example VIII was modified by employing an electrical resistance heating unit and a boron nitride crucible having a watch-sized ruby (A1 jewel seated in the bottom surface. The jewel insert was drilled to provide a 100 micron diameter, 100 micron long cylindrical orifice. A stagnant stabilizing atmosphere mixture of 67 vol. percent helium/33 vol. percent oxygen was supplied at room temperature At an extrusion pressure of psig., providing a stream velocity of approximately 1,410 cm/sec., highly jointed fibers of substantially reduced length (less than 1 foot) were obtained. The calculated critical spinning velocity (as given by Equation (2)) of 1,350 cm/sec. is seen to be in good agreement with the above observations. At spinning velocities within and above this region, it is clear that long, smooth fiber lengths cannot be obtained due to the upstream migration of drag-sustained stream deviations. Again, it is to be emphasized that the critical spinning velocity above which stream distortion and breakage becomes excessive varies according to variations in the parameters determinative of the viscous drag force.

It may now be appreciated that there has been herewith disclosed a unique and most beneficial process, the practice of which enables one to obtain the production of shaped articles from low viscosity melts by a stabilized free-stream extrusion wherein distortions due to the propagation of viscous drag-sustained deviations are avoided. Further, by employing the present teachings, one is enabled to counteract any tendency toward a tensile breaking of the stream without introducing aerodynamic disruptions. Having ascertained the mechanism by which drag disruption occurs, it has now been discovered, as herein disclosed, that low viscosity streams which are conveyed at the low levels of tension necessary to preserve their continuity may be protected against drag distortion by maintaining the velocity of propagation of any drag-sustained stream deviation at a value less than the stream velocity. Thus enlightened, many obvious variations, modifications and substitutions may readily occur to those skilled in the art. It is to be understood, therefore, that the invention herein set forth, particularly as regards the many alternative process manipulations, is limited only by a proper construction of the appended claims.

We claim:

1. In a method for forming fibers and filaments from a normally solid inorganic melt having a viscosity not greater than 10 poises by extruding the melt through an orifice into a film-stabilizing atmosphere as a free stream having a velocity such that where V,, p,.D, y are the stream velocity, melt density, stream diameter, and melt surface tension respectively, and wherein deviations from the vertical in the stream path subsequent to solidification are sustained and propagated upward toward the liquid portion of the stream due to drag interaction with the stabilizing atmosphere,

the improvement which comprises extruding the melt as a molten stream such that V, Z to V,

where the stream velocity V, and the propagation velocity V of the drag-sustained deviations are measured as positive in a direction away from the orifice.

2. The method of claim 1 including providing a cocurrent column of gas along the stream.

3. The method of claim 1 including providing a filmstabilizing atmosphere having a film-forming component and an inert component, said inert component being less dense and comprising a larger volume percent of said film-stabilizing atmosphere than said filmforming component.

4. The method of claim 3 in which said inert component comprises at least 80 volume percent of said film-stabilizing atmosphere.

5. The method of claim 3 in which said inert component is helium.

6. The method of claim 3 in which said film-forming component is a non-oxidizing gas.

7. The method of claim 1 wherein said melt is a metal or alloy thereof.

8. The method of claim 1 wherein said melt is a metalloid.

9. The method of claim 1 wherein said melt is a ceramic.

10. A method of preventing the propagation of mosphere comprising extruding the stream at a velocity V, measured positively in a direction away from the point of origin of the stream such that PI I cos 1) where V the velocity of propagation relative to the stream of a drag-sustained deviation, cm/sec.

F, maximum upward force/unit length acting upon the stream, dynes/cm.

E stream modulus at the point of stream deviation,

dynes/cm.

r stream radius, cm.

p, stream density, gm/cm.

V, velocity of the stream, cm/sec.

0 angle of equilibrium stream deviation from the vertical, degrees.

0 threshold angle of stream deviation above which the stream undergoes further deviation to the angle 0, and below which the angle is non-sustaining, degrees.

11. The method of claim 7 wherein the metal is stainless steel.

Claims

1. In a method for forming fibers and filaments from a normally solid inorganic melt having a viscosity not greater than 10 poises by extruding the melt through an orifice into a film-stabilizing atmosphere as a free stream having a velocity such that where Vs, Rho , D, gamma are the stream velocity, melt density, stream diameter, and melt surface tension respectively, and wherein deviations from the vertical in the stream path subsequent to solidification are sustained and propagated upward toward the liquid portion of the stream due to drag interaction with the stabilizing atmosphere, the improvement which comprises eXtruding the melt as a molten stream such that Vs > or = -to Vp where the stream velocity Vs and the propagation velocity Vp of the drag-sustained deviations are measured as positive in a direction away from the orifice.

2. The method of claim 1 including providing a co-current column of gas along the stream.

3. The method of claim 1 including providing a film-stabilizing atmosphere having a film-forming component and an inert component, said inert component being less dense and comprising a larger volume percent of said film-stabilizing atmosphere than said film-forming component.

5. The method of claim 3 in which said inert component is helium.

7. The method of claim 1 wherein said melt is a metal or alloy thereof.

8. The method of claim 1 wherein said melt is a metalloid.

9. The method of claim 1 wherein said melt is a ceramic.

10. A method of preventing the propagation of deviations along a film-stabilized free stream which is subjected to the drag interaction between the solid portion of the stream and surrounding stabilizing atmosphere comprising extruding the stream at a velocity Vs measured positively in a direction away from the point of origin of the stream such that Vs > or = -Vp, Vp being approximately equal to the expression, where Vp the velocity of propagation relative to the stream of a drag-sustained deviation, cm/sec. Fv maximum upward force/unit length acting upon the stream, dynes/cm. E stream modulus at the point of stream deviation, dynes/cm2. r stream radius, cm. Rho s stream density, gm/cm2. Vs velocity of the stream, cm/sec. theta 2 angle of equilibrium stream deviation from the vertical, degrees. theta 1 threshold angle of stream deviation above which the stream undergoes further deviation to the angle theta 2 and below which the angle is non-sustaining, degrees.