Low viscosity melt spinning process

BACKGROUND OF THE INVENTION

Materials having melts with large viscosities in relation to surface tension are particularly suitable for spinning or extruding into fiber form as evidenced by the multitude of glass and polymer spinning techniques. In attempting to spin fibers directly from streaming melts having viscosities measured in poises less than the surface tension measured in dynes/cm., the problem initially encountered is one of stabilizing the molten streams, pending solidification. There is an intrinsic tendency for molten streams of the above viscosity-surface tension ratio to undergo local fluid mass transfers which, if unchecked normally culminates in stream disruption.

It is thought that inherent vibrations in the spinning process cause minor disturbances in the stream configuration. Because the molten stream has a low viscosity, the natural tendency for a liquid under the influence of surface tension to become spherical in shape is increased. These spherical shapes are well known in the melt spinning art and are called “shot.”

While mention in the prior art has been made of extruding fibers from melts of the above characteristics (particularly the metals) through a cooled atmosphere or impinging the molten stream against a rotating chill 5 block, the results have not been entirely satisfactory. For most free streaming inorganic molten materials, the time required to effect stream solidification through heat exchange is significantly larger than the time to breakup the stream due to surface tension.

The U.S. Pat. 3,216,076, issued to Alber et al. on Nov. 9, 1965 and the aforementioned copending application Ser. No. 829,216 are addressed to the aspects of this problem in disclosing stream stabilizing techniques which have proved to be most practical. In quite general 15 terms, these approaches involve the concept of filmstabilizing melt spinning wherein a melt having a low viscosity-surface tension ratio, i.e. equal to or less than one, is streamed into an atmosphere capable of rapidly forming a film about the stream of sufficient strength to 20 stabilize the stream against breakup pending solidification by normal heat transfer phenomena. In addition to the above, the stabilized stream portion of the stream is highly susceptible to disruption by forces engendered by the impact or deceleration incident to supporting or taking 25 up the streaming body after solidification. The forces generated during such deceleration are normally of sufficient magnitude as to propagate upstream to weaker regions where stream deformation and disruption may occur. A recognition of the nature of the deceleration 30 forces has brought forth the realization that such streams conveyed under carefully controlled conditions may result in fibers of predetermined lengths. More precisely, we have now discovered that in extruding a free falling stream from an inorganic melt having a viscosity-surface tension ratio of not greater than one, there is a point (D_o), as measured from the stream origin, above which any given deceleration of the molten stream results in the formation of a non-fibrous mass. Also, there exists a point (D_a) as measured from the stream origin, above ⁴⁰ which any given deceleration of the stabilized stream causes disruptions in the continuity of the now solidified stream or fiber.

We have further found that by manipulating the force system acting upon the stream at given points therealong, ⁴⁵ a low stress spinning operation may be established wherein the fragile, low strength upper regions of a low viscosity stream, initially stabilized against disruption due to mass transfer within the liquid region, is effectively utilized for the formation of staple fiber of preselected lengths. ⁵⁰ Further, as will be disclosed, the location of the point D_aalong the stream may readily be varied, both relatively and absolutely, by appropriate manipulation of one or more spinning conditions to thereby facilitate preferred operating conditions and results.

⁵⁵ It is therefore a primary object of the present invention to provide a method of forming staple fibers from an inorganic melt having a viscosity-surface tension ratio not greater than one.

BRIEF STATEMENT OF THE INVENTION

The foregoing and other objects are attained in the practice of the present invention, wherein the production of fibers of desired length from a low viscosity melt is accomplished by (1) creating an initially stabilized stream from the melt and (2) intercontrolling the forces acting upon the stream relative to the stream state to decelerate the stream at a selected point intermediate of a point (D_o) upstream of which attending deceleration causes ₇₀ the formation of a non-fibrous mass and a point (D_a) upstream of which attending deceleration causes the disruption in fiber continuity.

3,715,418

DESCRIPTION OF THE DRAWINGS

In undertaking a more detailed discussion of the present invention and the manner of its practice, reference shall be had to the accompanying drawing as being illustrative, but not limitative, thereof and in which:

FIG. 1 is a diagramatic graph depicting the typical interrelationship of fiber length to stream free-fall distance;

FIG. 2 is a vertical, partially sectionalized view of a simplified apparatus which may be employed in the practice of the instant process;

FIG. 3 is a schematic circuit diagram of conventional design which may be employed to electrically monitor and record stream continuity, and

FIGS. 4-7 are graphical presentations of pertinent data obtained in carrying out the reported examples which show the effect of variations in selected spinning parameters upon the interrelationship of fiber length with stream free-fall distance.

DETAILED DESCRIPTION

Generally, it has been noted that melts of materials having viscosity-surface tension ratios not greater than one have a tendency to break into shot prior to solidification when extruded as a stream. Examples of inorganic materials having the above tendency are ceramics, metals and alloys thereof, metalloids, and intermetallic compounds. Typically, a metallic melt has a viscosity-surface tension ratio on the order of 1 χ 10~⁵. Another characteristic of the above molten materials is the viscosities thereof are not greater than approximately 10 poises. Conversely, materials such as glass, polymers, and materials of large molecular size have viscosity-surface tension ratios significantly greater than one and viscosities much greater than ten poises. Thus, the disruptive effect of surface tension is prevented by the viscous inertia of the molten material.

Until recently, attempts to produce fibers from materials such as metals were typified by such techniques as drawing relative large diameter wire through successively smaller dies until the desired diameter was achieved, or encasing the low viscosity melt within a vitreous sheath to confine and support the melt during its attenuation. In general, wire drawing becomes highly expensive when wire of small diameters (10 mils or less) is desired.

As previously mentioned, the problem initially encountered in the streaming of low viscosity-surface tension ratio melts is one of stabilizing the liquid region of the stream against a surface tension-driven mass transfer mechanism. As set forth in the hereinbefore referenced Alber patent and the copending application Ser. No. 829,216, and proceeding from a proper recognition of the controlling mechanisms causing liquid jet breakup, it has recently been discovered that breakup of the jet, prior to solidification, may successfully be suppressed by the generation of a stabilizing film of minute thickness about the nascent, essentially inviscid stream prior to its disruption and pending solidification by normal heat transfer phenomena. Briefly, this has come about through an appreciation of the following considerations: if the stream velocity of the material is not sufficient, surface tension-driven amplification, and propagation of normally unavoidable though initially minor, stream disturbances prevent the formation of an efficient jet; at intermediate velocities, the jet is disrupted by varicose breakup, wherein slightly attenuated portions of a liquid cylinder tend to further attenuate to ultimate disruption under the urge of surface tension forces. With increasing velocity, sinuous breakup and undue aerodynamic deceleration (in which the molten stream becomes contorted by interaction with the atmosphere) become the limiting considerations. Such disturbances are resisted by stream inertia and viscosity, but the viscosity of the materials of interest is negligible to the point that breakup of the stream normally occurs well before it can be solidified.

According to the teachings of the above-identified applications, the liquid portion of such streams may be successfully film-stabilized by spinning into suitable atmosphere which, ether by reaction, decomposition, or deposition, result in the rapid formation of thin films about the nascent stream to thereby suppress the above referred disruptive forces pending solidification of the stream into wire-like form.

Another aspect of the film-stabilized spinning technique is in the proper utilization of the dimensionless quantity

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hereinafter called the Rayleigh parameter (wherein V, p, D, and 7 are stream velocity, density, diameter, and surface tension respectively). The Rayleigh parameter (abbreviated Ra) is the square root of the well known hydrodynamic expression known as the Weber number. As stated in the prior mentioned copending application Ser. No. 829,216, the Rayleigh parameter should lie in the range of 1 to 50, preferably 2 to 25. It has been discovered that, where the velocity of the stream for a given stream density, diameter, and surface tension does not satisfy this condition that effective film stabilization cannot be established. For example, when molten material having a density of 4 gms./cm.³ and a surface tension of 1000 dynes/cm. is extruded into a stream having a 0.03 cm. diameter, it is necessary that the stream velocity be in a range of approximately 90 to 4500 cm./sec. (which gives a Rayleigh parameter range of 1 to 50).

The optimum velocity within the Rayleigh parameter range of 1 to 50 (for a melt composition of known surface tension and density extruded as a free stream at a given diameter) may normally be determined experimentally.

In general, actual propagation of surface tension breakup determines the lower limit of the Rayleigh parameter range, while either sinuous breakup or aerodynamic deceleration determine its upper limits. The upper limit of the range is increased as the density of the melt relative to the spin atmosphere increases; i.e. the greater the density of the melt and/or the less the density of the spin atmosphere, the higher the Rayleigh parameter value (taken as a measure of extrusion velocity) at which successful spinning may be accomplished, though optimum performance may dictate a somewhat lower level.

In exemplifying the practice of the present invention, a simplified spinning assembly, as schematically depicted in FIG. 2, was employed. As there indicated, such an apparatus essentially comprises a melt crucible 10 which, in the case of the examples which follow, was fabricated from stainless steel. The crucible is provided with an upper header plate 12 and a lower orifice plate 14, both of which are maintained in sealing engagement with crucible 10 to provide a gas-tight melt chamber 26. The orifice plate 14 has seated centrally thereof a watch-sized jewel 18 formed of any suitable material chemically compatible with the melt being processed; the jewel is drilled to provide a suitable spinning orifice 20. In the examples which follow, a ruby jewel having an orifice diameter of 100 microns and a length/diameter ratio of unity was employed. Melting of the spin charge within chamber 16 was accomplished by means of electrical resistance heating elements 22 and the charge temperature was monitored by means of a thermocouple arrangement 24. Preferably, the spinning charge was melted under a vacuum prior to effecting extrusion under an inert gas pressure; this may readily be accomplished by the two-way valve and conduit arrangement indicated at 26, whereby chamber 16 may be alternately evacuated and pressurized to effect the desired extrusion rate. In order to confine the various reactant and cooling gas mixtures employed, a glass (Pyrex) spinning column 27 is arranged to receive the stream extruded through orifice 20. The spin gas mixture is supplied through conduit 28 to be gently deployed

3.715.418 throughout the spinning column by means of a gas distribution ring 30 provided with equi-spaced gas orifices 32. Prior to extrusion into the desired gas mixture, the spinning column is preferably swept out under vacuum, which may be provided by means of a valved connection 34. Of course, during evacuation, the lower end of the spinning column may be temporarily sealed by means of any suitable plate arrangement, not shown. The extruded stream was collected at selected distances down the column by means of a collection surface 36, which may take the form of a metal plate. As indicated by the twoway arrow heads, the collection surface 36 is adjustable in the vertical direction to establish the desired catch distance. Normally, the plate was maintained horizontal, but we have found that it may be inclined at widely varying angles without any substantial effect upon the fiber lengths obtained.

It is to be understood that the above-described spinning assembly merely represents a typical apparatus which may be employed in the practice of the present invention, which latter is in no way limited to the details of the apparatus. For example, where it is desired to spin high melting point materials, an induction-heated spinning assembly would be found preferable, if not essential.

In the following discussion and examples, the term freefall distance shall be taken to denote that vertical distance between the spinning orifice and the collection surface. However, it is to be emphasized that free-fall distance is not necessarily defined by the position of a solid collection surface such as depicted in FIG. 2. Such a surface is merely to be taken as symbolic of that point below the orifice at which a decelerating force is brought to bear upon the stream. Deceleration may arise by the sudden impact of the solidified stream upon a solid surface, or more gradually, by causing the solidified stream to pass through a significantly denser and/or counterflowing spin atmosphere; similarly, a more gradual stream deceleration may be achieved by the imposition of a suitable electrostatic field. It is only in service of simplicity that the following discussion is made largely in terms of effecting stream deceleration by collecting upon a solid surface. Similarly, the chosen length of fiber may be found experimentally. That is, the collection surface may be moved up and down the stream until a level is reached wherein the resulting fiber is of the desired length. Interruption of the solidified stream so as to impart a deceleration thereto at a level intermediate D_a and D„ generally results in the formation of continuous filament as described and claimed in the parent copending application Ser. No. 599,539. The point “D_b” may be defined as a point along the stream at which the stream must be decelerated to avoid repeated tensile breaking. Interruption intermediate D_a and D_o results in staple fiber the length of which we have found to be a function of the free-fall distance. This may be seen in FIG. 1.

More specifically, the process embodying the present invention is based on our discovery that, in the case of streams issuing from low viscosity-surface tension ratio melts, there is a point D_a above which such streams cannot undergo a given deceleration without disruption in the stream continuity. We have also found that spinning conditions such as, for example, spinning velocity, heat transfer, and melt temperature may be varied so as to cause the point D_a to move upward and downward as desired for a particular material. FIG. 1 illustrates diagrammatically the relationship between the free-fall distance of the stream and fiber length. As seen therein, four distinct regions appear on this curve. For free-fall distances less than the distance D_o (denoting the freeze point of the stream, as hereinbefore defined) the stream is still molten and any attempt to collect the stream at lesser distances results in but a molten, non-fibrous mass. For distances intermediate the points D_o and D_a, fiber length is seen to increase approximately exponentially with increasing freefall distance. Although the stream is at least partially solid in this region, disruption thereof may be effected by deceleration due to impingement upon the collection surface. When the stream is allowed to fall through distances equal to or greater than D_b, the tensile force due 5 to the increased stream length (and, therefore, weight) is sufficient to cause stream breaking. Thus, at collection points at or below D_b, relatively uniform fiber lengths are obtained independently of the free-fall distance.

As the examples will illustrate, the relative and absolute 10 positions of the points D_o, D_a, and D_b may be manipulated as desired by proper variations of the process variables. A circumstance may arise wherein D_b occurs upstream of D_a. This may be caused by numerous combinations of factors affecting the tensional force upon the j5 stream but is largely present when spinning high density melts, especially in the larger diameter range. In such cases, the force system acting upon the stream must be modified in light of the present teachings such as, for example, the use of counter-current gas flow so that the 20 Point D_b is caused to be shifted downstream relative to the point D_a when indefinite length production is desired.

An effective aid in determining stream continuity relative to free-fall distance may take the form of a very simple electrical continuity tester circuit, such as sche25 matically diagrammed in FIG. 3. As there indicated, such a tester serves to electrically interconnect the collection surface with the melt crucible to thereby sense electrical continuity, or lack of it, because the collection surface and the spinning head where an electrically conductive 30 melt is being processed. Similar determinations are, of course, made by directly measuring the fiber lengths obtained at varying free-fall distances, but use of the continuity tester allows one to monitor stream continuity continuously and to modify spinning conditions accord35 ingly.

As previously indicated, the fiber lengths obtained under various process conditions will vary in a characteristic manner with free-fall distance, as shown in FIG. 1. Indications of the mechanism by which stream breakup occurs 40 in the regions above point D_a are obtained by relating fiber length to free-fall distance to indicate where and when stream breakage occurred, while microscopic examination of the ends of broken lengths serve to indicate stream state at the position of break as well as the 45 rapidity of the break. In the lower portion of FIG. 1 is shown a recording of the continuity tester for the typical graph appearing there-above. In relating this recording to the free-fall distance curve, it may be observed that the freeze point D_o of the stream is the point below which 50 fibrous shaped, as opposed to a molten mass, may be collected. Thus, when the stream is caught at distances less than D_o, there is electrical continuity between the collection surface and the spinning head via the liquid stream, as indicated by the continuous positive deflection 55 on the recorder curve. For free-fall distances between D_oand D_a, the on-off shape of the continuity curve indicates that the impingement of the stream upon the collection surface initiates a disturbance sufficient to effect stream breakage at some point upstream of the collection sur60 face. An examination of the fiber ends obtained by collection within the region indicates that stream disruption occurs very rapidly, but after a solidification sufficient to retain a fibrous shape. Thus, the disturbances set up in the stream due to deceleration as, for example, impingement 35 on the collection surface at these free-fall distances are sufficient to physically disrupt the stream and short fibers are obtained. It has also been observed that the configuration of the ends of the staple vary according to the material employed, the temperature thereof, and the 70 ^raPidity of the break. For example, if deceleration occurs when a melt material such as a lead-tin is very hot but solid, the fiber ends are relatively square due to the rapidity of the break.

The exponential increase in fiber length as the point

D_a as approached is highly suggestive of a dampened

3,715,418 disturbance superimposed upon increasing stream strength as it passes downstream, which factors combine to determine the fiber lengths obtainable in the region intermediate the points D_o and D_a.

As indicated by the graph and recording of FIG. 1, continuous fibers are obtained for free-fall distances between points D_a and D_o; the continuity tester of course indicates electrical continuity within this region.

The recorded continuity curve of FIG. 1 clearly illustrates that, for distances greater than D_b, the stream undergoes breakage before impingement upon the collection surface. Because the force due to gravity has, beyond this point, become greater than the sum of stream strength and the upward drag force generated under the chosen spinning conditions, the stream is caused to break prior to contacting the collection surface; thus, the recording indicates a continuously open circuit.

For a more detailed understanding of the present invention, reference is now made to the following examples which are illustrative of the effects of the more pertinent spinning parameters upon the force system imposed upon an extruded stream at varying free-fall distances. In all of the following examples, stream deceleration was effectuated through collection of the solidified stream upon a metal plate although deceleration through other techniques such as counter-current gas flow may have been used equally as well. The spinning assembly previously described with reference to FIG. 2 was employed and the spinning charge was in the form of a mixture of 62 weight percent lead/38 weight percent tin. The stream was film-stabilized by extrusion into an oxygen-containing atmosphere to form an oxide film. Unless otherwise specified, the spinning conditions were as follows: extrusion pressure of 20 p.s.i.g. argon at an extrusion temperature of 400° C.; a reactant/cooling gas mixture of 7 vol. percent oxygen/93 vol. percent helium at room temperature and atmospheric pressure; extrusion through a 100 micron diameter orifice formed in a watchsized ruby jewel, the orifice having an L/D ratio of unity; and a Pyrex spin column of 6 in. inside diameter and 300 cm., length. The melt crucible was fabricated from stainless steel having an inside diameter of 1¾ in. and a depth of 6 in. Preferably, heating to the melt state was accomplished under a vacuum of below 100 microns of mercury. Also extrusion was commenced prior to introducing the spin gas into the spinning column.

EXAMPLE I

This example illustrates the effect of a variation in the reactant gas concentration upon the fiber length obtainable at given free-fall distances. Utilizing the apparatus of FIG. 2, a helium/oxygen spin gas mixture was supplied to the spin column through the gas distribution ring 32 positioned approximately 50 cm. below the orifice, in a manner to effect a gentle gas motion in the vicinity of the orifice. The helium flow rate, which functions as a coolant gas, was maintained constant at 2.5 c.f.m. to maintain a relatively constant heat transfer rate, the oxygen flow rate being varied to obtain the desired volume percent oxygen in the mixture. Oxygen functions as the reactant gas in forming an oxide stabilizing film about the lead/tin melt as it is given issue as a free stream through the orifice. The variation in fiber length with free-fall distance was determined at oxygen concentrations of 2, 7 and 15 volume percent, the resultant data being presented in the graph of FIG. 4. As there shown, under the conditions specified, an oxygen concentration of 2% is virtually the minimum level at which indefinite length filaments can be obtained. That is, at a free-fall distance, as measured downstream from the orifice, of approximately 95 cm., substantially continuous lengths are obtainable, whereas a small decrease in free-fall distance (i.e. shifting the collection surface upstream) results in impact breakage of the stream; alternatively, a small increase in free-fall distance is seen to result in tensile breakage. At somewhat lesser oxygen concentrations, the impact and tensile break points D_a and D_b woud come to occupy positions wherein the tensile breaking point D_b occurs upstream of the impact breaking point D_a, with the result that continuous lengths would not be obtainable at any free-fall distance As the oxygen concentration is increased to the levels of 7 % and 15%, it is seen that the free-fall range over which continuous lengths may be obtained increases; that is, the distance downstream of the tensile break point D_b from the impact break point D_a increases with increasing oxygen concentrations, the stabilizing film is of increasing thickness and consequently, strength, resulting in both an upstream shift of the impact break point D_a as well as a downstream shift of the tensile break point D_b

It is possible that, even when employing optimum orifice configurations as above discussed, too high a concentration of reactant gas may result in such an early formation of the stabilizing film that the velocity of the extruding stream has not had sufficient time to relax to a flat profile. This results in a film which is either unduly attenuated or entirely disrupted thereby preventing the formation of fibers of useful length. As shown by the data of FIG. 4, an oxygen concentration of 15% is not sufficient to precipitate stream disruption under the conditions specified. It has been observed, however, that spinning melts into high reactant gas concentrations (viz. 100% oxygen) results in stream breakage into short lengths, very likely due to premature formation of the stabilizing film.

EXAMPLE II

This example illustrates the effect of coolant gas properties, particularly as regards viscosity, density and coefficient of heat transfer, upon the variation in fiber length with free-fall distance. As graphically recorded in FIG. 5, two series of runs were conducted employing spin gas mixtures of 3.3 c.f.m. helium/0.3 c.f.m. oxygen and 1.7 c.f.m. nitrogen/0.3 c.f.m. oxygen.

The molecular weight of the coolant gas affects the stabilizing film formation through its mass transfer effects on the rate of diffusion of the reactant gas to the stream surface; the freeze point and temperature history of the stream through its heat transfer effects; finally, the force system on the stream through its momentum transfer or viscous drag effect. Nitrogen is not as an effective heat transfer agent as helium; it provides a greater viscous drag; finally, it retards the diffusion of oxygen to the stream surface necessary to form the stabilizing film. These effects combine to reflect the data set out in FIG. 5, wherein it is seen that, comparing nitrogen to helium, larger distances are required for freezing the fibers (the freeze point D_obeing approximately 20 cm. below the orifice for the helium mixture and approximately 40 cm. for the nitrogen mixture), larger distances are required before disturbances due to deceleration upon the collection surface cease to have an effect upon stream continuity (as indicated by an impact break point D_a of approximately 90 cm for the helium mixture and 120 cm. for the nitrogen mixture), and finally, even longer stream lengths are required before tensile breakage occurs due to the difference in the downward acting force of gravity in stream weight and upward acting viscous drag force stream plus film strength being substantially constant in both cases (the tensile break point value D_b being approximately 160 cm. for the helium mixture and 190 cm. for the nitrogen mixture).

It should be pointed out that, for higher melting materials, viz. 1500-1600° C., heat transfer by radiation becomes more important so that the type or coolant gas employed will have less effect on the overall heat transfer rate at such higher temperatures.

EXAMPLE III

This example illustrates the effect of spinning velocity upon the interrelationship between fiber length and freefall distance. The rate at which the melt is extruded

3,715, 9 through an orifice determines the free, unbroken jet length of the stream in the absence of a reactant gas, as well as affecting the viscous drag force and heat transfer of the stabilized stream through its influence upon the relative velocity of the stream with respect to the spinning atmos- 5 phere. These factors combine to provide the net result set out in the graph of FIG. 6 when following the procedure and conditions set out in Example I (using a reactant gas of approximately 7% oxygen/93% helium supplied at a rate at approximately 3 c.f.m., extrusion being carried out 10 at a temperature of approximately 400° C.).

It is seen that at an extrusion pressure of 12 p.s.i.g., the resulting stream velocity of 365 cm./sec. is too low to provide continuous lengths under these conditions, probably because of an insufficient free jet length. Further, it is seen 15 that, at least up to an extrusion velocity of approximately 900 cm./sec. (under a pressure of approximately 60 p.s.i.g.), a significant increase in the range of the free-fall distance over which staple, fibers and continuous lengths thereof can be collected are achieved by increasing the 20 extrusion velocity. This is accounted for by an increase in the distance from the orifice of the tensile break point D_b resulting from an increasing viscous drag force on the stream with increasing stream velocity. The freeze points D_o and impact break points D_a are also seen to shift 25 downstream with increasing spinning velocity due to the fact that mass flow rates increase faster than heat transfer rates, while the downstream migration of the tensile, break point D_b is greater than that of the impact break point D_a, resulting in the observed increased in the free- 30 fall distance range over which continuous lengths may be collected.

EXAMPLE IV

It is the purpose of this example to demonstrate the 35 effect of spinning temperature upon the variation in fiber length with free-fall distance. The temperature at which a low viscosity melt is extruded directly affects the heat transfer requirements necessary to solidify the stream and indirectly affects the rate of formation of the stabiliz- 40 ing film through the temperature-dependence of the filmforming reaction. Following the procedure of Example I, two series of runs were conducted at extrusion temperatures of 300 and 400° C. and an extrusion pressure of 42 p.s.i.g. the results obtained being graphically set forth 45 in FIG. 7. The decrease in heat transfer requirements at the lower temperatures are clearly reflected in the significantly decreased distances to the freeze point D_o and impact break point D_a due to a more rapid increase in stream strength. The change in the rate of formation in 50 the stabilizing film for these two temperatures is apparently not sufficient to cause significant changes in the tensile strength of the film as reflected by the small changes in tensile break point D_b. It is thus observed that a greater-free fall distance range for collection of con- 55 tinuous lengths obtains at the lower extrusion temperature while the reverse is true for collection of fiber above D_a.

As may now be appreciated, the position of the tensile break point D_b relative to the orifice is determined by the balance between the force of gravity, on the one 60 hand, and stream strength and viscous drag force, on the other hand. As discussed in the previously referenced copending application Ser. No. 680,898 where the force due to gravity is decreased (as by spinning lower density melts and/or smaller diameter streams) and/or the viscous 65 drag force is increased (as by an increase in spin gas density and viscosity, a lower cocurrent or higher countercurrent gas flow, or a higher spinning velocity), a point is reached where the viscous drag force overcomes gravity to the extent that, upon the initiation of normally ufl- 70 avoidable deviations of the stream from a straight-line path, the drag force is sufficient to effect a progressive bending of the stream. When the velocity of such dragsustained bending exceeds stream velocity due to high viscous drag forces, the bending migrates upstream rela- 75

418 ¹⁰ tive to the orifice into the hot weak region of the stream, causing stream disruption or total severance.

Under the above conditions, the tensile breaking point D_b is shifted a finite distance downstream. Thus when it is desired to obtain staple or indefinite length fiber, the force system imposed upon the stream must be modified by reducing the viscous drag force to a level at which the propagation velocity of drag-sustained deviations is less than the stream velocity. Under these conditions, the tensile break point D_b occupies a position of finite distance from the orifice and, when this distance is greater than the distance of the impact break point D_a, deceleration of the stream at a point below D_o but above D_b results in fiber of desired length.

It may now be appreciated that there has been herewith disclosed a novel process for spinning staple from inorganic melts having a viscosity-surface tension ratio not greater than one by intercontrolling the force system imposed upon the stream relative to that point to which a predetermined deceleration is brought to bear. 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 proper construction of the appended claims.