US3437725A - Melt spinning apparatus and method - Google Patents

Description

April 8, 1969 N. c. PIERCE MELT SPINNING APPARATUS AND METHOD Sheet Filed Aug. 29, 1967 INVENTOR NORWIN CALEY PIERCE FIG-2 ATTORNEY April 8, 1969 N. c. PIERCE MELT SPINNING APPARATUS AND METHOD Sheet Filed Aug. 29, 1967 FIG. 4a

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INVENTOR NORWIN CALEY PIERCE ATTORNEY US. Cl. 264176 2 Claims ABSTRACT OF THE DISCLOSURE Improvements in a filter pack are disclosed for melt spinning synthetic linear polymer of high relative viscosity without heating the polymer sufliciently to cause significant degradation. The polymer passes from a conventional filterholder to a spinneret assembly having a top plate attached to the filterholder and a spinneret plate mounted parallel to the top plate wtih an insulating space between the two. The polymer flows into generally tubular inserts which extend through the two plates and have spinning capillaries at the spinneret-plate end. The spinneret plate is heated to provide a steep temperature gradient in the polymer so that shear stress is reduced momentarily at the wall of the capillary without degrading the polymer. The insulating space keeps the top plate and topplate end of the insert from being heated to a temperature at which polymer degradation will occur.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to the melt-spinning of synthetic polymers and particularly to a method of spinning polymers having higher melt viscosities without polymer degradation and with reduced orientation of the polymer in the spun yarn. It is especially useful in the formation of filaments which have a final denier of less than 50 after they have been drawn.

Description of the prior art and problems In melt-spinning of yarns, synthetic linear polymer is melted, forced through the filter of a spinneret pack, then through spinnerette capillaries, and quenched to form filaments which are conveged into a bundle. The filament bundle is processed to provide the desired physical properties and finally the yarn product is packaged for shipment. Most melt-spinnable polymers are temperature sensitive and degrade raidly at elevated temperatures. The lower the spinning temperature the higher the pressure required to force the molten polymer through the spinneret. Nonuniform flow, and melt fracture, will occur if the spinning temperature is too low. Consequently, a compromise is always made between a high spinning temperature for good spinning and a low spinning temperature for a minimum of polymer degradation.

Another approach to reducing spinning pressures is to increase the size of the capillary. If the same filament size (denier) is desired, this approach requires an increase in the attenuation or stretching of the molten material leaving the spinneret and, ideally, before it solidifies. Unfortunately, this stretching can seldom be done under sufiiciently ideal conditions to avoid a spun orientation which adversely alfects the final yarn properties. Further, when a heavy filament is spun, the inner portion is not quenched properly due to the insulating properties of the outer portion. This causes nonuniformities across the fiber as well as along the fiber. Frequently, it is necessary to use hot-gas annealing directly below the spinneret to counteract these adverse effects and obtain a more uniform yarn. However, this is costly, both in initial cost and operating costs.

ted States Patent Yet another approach has been tried in which the molten polymer is kept at a relatively low temperature until it is heated as it passes through a heated spinneret which is maintained at a temperature much higher than the melt. In practice, this has not worked because polymer in contact with the top face of the spinneret stagnates, becomes too hot and degrades.

The search for better and better yarns goes on continuously. It is now known that improved physical pr0perties, e.g., higher tensile strength, can be provided by polymers with higher melt viscosities, either from high molecular weight polymers or from polymers with stiff chain structures. However, this further complicates the compromise among high spinning temperatures, high spinning pressures, and capillary size. In some instances, it has been impossible to make commercial quality yarns from high viscosity polymers. The improved process of this invention makes possible the spinning of such polymers. For other polymers, it reduces the spun orientation and enables improved properties to be produced in the drawn yarn products.

SUMMARY OF THE INVENTION This invention is an improvement over the above proccesses, and spinneret packs, of the type wherein molten synthetic linear polymer is supplied to a filterholder at sufficient pressure for the subsequent extrusion into filaments, the polymer is passed through the filter medium in the filter and is extruded through spinning capillaries in a spinnerette plate. In accordance with the present invention, the spinning pltae is separated from the filterholder by an insulating medium and the spinning capillaries are formed by hollow inserts in the spinneret plate which provide passages through the insulating medium from the filterholder. The spinning process includes (a) passing the molten polymer through the filterholder at initial temperatures within a temperature range below that at which significant polymer degradation will occur, (b) passing the polymer into a plurality of passages each of which leads to a different spinning capillary in the spinerett plate and has an entrance temperature within said initial temperature range, (c) heating the spinneret plate to increase the temperature along the passages from said temperature at the entrance to a temperature at least 60 higher at the spinning capillary, and (d) extruding the polymer from the spinning capillary after a maximum of 4 seconds of travel through the heated passage.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a sectional view of a preferred spinning pack assembly, the cross section being taken in a plane through the central axis of the assembly;

FIGURE 2 is an enlarged view of a portion of FIG- URE 1;

FIGURE 3 is a schematic illustration of temperature profiles in the polymer at various locations in the tubular passageway of a spinneret insert;

FIGURE 4a shows schematically the bulge and temperature profile for a prior art spinning; and

FIGURE 41; shows schematically the bulge and temperature profile for the improved spinning of this invention.

As shown in FIGURE 1, lid 10, filterholder 12 and spinneret assembly 14 comprise the main elements of the spinneret pack assembly. Lid 10 is attached to filterholder 12 by a number of bolts 16, while the spinneret assembly 14 is similarly attached to the filterholder by a number of bolts '18. The entire pack assembly is mounted into the spinning machine by bolts 20. Inlet port 22 connects the top face of lid 10 to a plurality of distribution channels 24 which exit on the lower face of lid 10'. Sands or screens, or a combination of the two, are placed in cavity 3 26 in filterholder 12. Distribution channels 28 connect cavity 26 with the downstream face of filterholder 12. Gasket 30, placed between filterholder 12 and spinneret assembly 14, is of sufiicient thickness to form distribution space 32.

As shown in more detail in FIGURE 2, the spinneret assembly 14 includes top plate 34, heating plate 36, and lower plate 38. An electric resistance heating element 40, spirally wound, is embedded in lower plate 38. Spacer 42 provides an air space between top plate 34 and heating plate 36, which acts as a thermal barrier between these two plates. The assembly is bolted together by bolts 44. Hollow inserts 46, one for each filament to be spun, are placed in top plate 34 and extend to the bottom face of lower plate 38. The relatively long tubular passageway 48 of the insert connects the top face of plate 34 to the final capillary 50.

In operation, molten polymer is supplied under presssure by a metering pump (not shown) to inlet port 22 (FIG. 1) and is distributed equally to the top of cavity 26 by distribution channels 24. After being filtered and sheared by passing through the sand, screens, etc., it is distributed to distribution space 32 by distribution channels 28. Temperature conditions maintained up to this point are such that the temperature of the molten polymer in the distribution space 32 is essentially the same as the temperature of the adjacent metal. The molten polymer at this temperature is then forced into the top-plate ends of the inserts for spinning through capillaries 50, where the polymer exits in filament shape. Electrical heater 40 supplies heat to maintain the lower plate 38, heating plate 36, and the lower portions of insert 46 at a temperature at least 60 C. higher than the temperature of the supplied molten polymer. Because of the insulation efiects of the air space between the plates, very little of this heat reaches top plate 34, which remains essentially at the same tem perature as the molten polymer. Due to the resulting temperature difference, the poor heat transfer characteristics of the polymer, and the extremely short time (a maximum of 4 seconds) that the polymer stays in contact with the high temperature portions of insert 46, a steep temperature gradient is given to the polymer flowing through each insert 46.

FIGURE 3 illustrates temperature profiles for polymer flowing through a portion of an insert 46. An upward direction of polymer flow is shown so that the temperature scale indicates increasing values from the bottom to the top of the figure. Initially, the temperature profile across the polymer in insert 46 is essentially constant as shown by T (corresponding to point a in FIGURE 2); while at point b, the beginning of a temperature gradient is shown by curve T Just before the polymer enters capillary 50, that is, at point 0, there is a steep temperature gradient as illustrated by curve T With a higher temperature on the walls of the entrance hole, the outer layer of polymer has a lower apparent viscosity. Consequently, the pressure required to force the polymer from the entrance hole 48 into capillary 50 and through capillary 50 is considerably reduced even though the temperature of the bulk of the polymer flowing through capillary 50 has not been increased enough to have an appreciable effect on the polymer.

FIGURE 4a shows the carrot or bulge below a spinneret capillary of sufficient size to spin a high viscosity polymer with a reasonable pressure drop. Line 60 indicates the temperature profile across the polymer just below the capillary. Notice that the temperature of the center portion is slightly higher than that of the outer portion. Line 62 indicates the temperature profile at a short distance from the capillary. Since the exterior of the polymer is being cooled by a quenching medium, the center portion now has a considerably higher temperature than the exterior. It has been found, and is well known in the art, that this temperature profile causes nonuniformities in a filament.

FIGURE 4b shows a relatively smaller capillary which can be used with the same polymer when spinning with a heated capillary spinneret according to the present invention. It illustrates the reduced carrot or bulge that results because of less shearing action and less pressure required to force the polymer through the capillary. Furthermore, in order to get the same filament size or denier, less spin stretch is required under these conditions. Line 64 indicates the temperature profile just below the capillary exit. Note that the outer portions of this filament are hotter than the inner portions. A short distance below, line 66 illustrates the profile after some cooling has taken place. Notice that, by this time, the temperature gradient across the filament is very small. This leads to improved product uniformity.

The heated capillary which generates the steep temperature gradient reduces the shear stress at the wall of the capillary without excessive thermal degradation of the polymer. Further, since smaller capillaries can be used without excessive pressure drops, lower spin-stretch ratios can be used to spin low birefringent yarns.

In alternate designs, each insert could have a plurality of long entrance holes and capillaries. This is particularly advantageous when the capillaries are arranged in a straight row. Further, it may be helpful to use heat shields or barriers in the air space between the top plate and the lower plate.

The following examples are cited to illustrate the advantages of this invention. They are not intended to limit it in any manner. The first two examples illustrate the spinning of high molecular weight polymers with the method of this invention which cannot be spun with conventional spinnerets. The second two examples illustrate the reduction in spun filament orientation, and improvement in drawn yarn properties, obtained by the method of this invention; they also demonstrate good spinning without the requirement of hot gas annealing.

EXAMPLE I Polyethylene 2,6-naphthalenedicarboxylate was prepared as disclosed in Hogsed US. Patent No. 3,123,587 dated Mar. 3, 1964. A series of small batches were polymerized to high molecular weight polymer with the object of preparing fibers with as high a molecular weight as possible (consistent with good processing performance) to achieve superior fiber tensile properties. Inherent viscosities of the various polymer batches for each melt spinning experiment are listed in Table 1.

Inherent viscosity (n is defined as:

where C is the concentration of polymer in the solvent, in g./ 100 ml. (nominally 0.32 g./1OO ml.). The solvent is trifluoroacetic acid/methylene chloride (25/ vol./vol.). 0 is the drop time of polymer solution through the capillary viscometer at 25 C. in seconds. 0 is the drop time of solvent through the capillary viscometer at 25 C. in seconds.

Polymer viscosities which could be melt processed by conventional melt spinning techniques had an upper limit of molecular weight of about stLSS. Melt fracture occurred at higher molecular weights, preventing continuous spinning. In the examples of Table 1 with conventional spinnerets, a high spinning block temperature of 320-325" C. was used, which alleviated melt fracture to some degree but was not sufiicient to eliminate the problem. Higher block temperatures degraded the polymer melt. When using the heated capillary spinneret of this invention, no melt fracture problems were encountered at a significantly higher molecular weight level. Spun yarn extensibility at high molecular weights and the resulting physical properties were good for the heated capillary spinneret. No direct comparison with conventional yarn could be made because melt fracture prevented yarn processing at these higher molecular weight levels. At the lower molecular weight levels for conventional samples shown in Table 1, extensibility was quite low and physical properties were inferior.

Flex life is defined as the number of flex cycles to failure of the median of single filament samples as tested on a Masland Flex Tester at F. and 62 /2% RH. Filament, under a load of approximately 0.33 gpd. are

TABLE 1 Block Capillary Inherent Viscosity Birefrin- Draw Tenac- Elong., Mod- Spinneret Quench Zone Annealer Temp., Temp., gence Ratio ity, percent ulus, C. C. Flake Yarn g.p.d. g.p.d. 1A Conventional. 390 C. Hot Wall 325 325 0.84 0. 0.0045 4.7 4. 9 11 1-B. do None 320 320 0. 78 0.72 0.0176 4.8 6.0 8 188 1-C- Heated cap. do 312 394 1. 26 0. 0.0135 4. 8 7. 3 12 173 1-D- do 6 (15.2 cm.) Delay Baflle... 315 392 1. 26 0. 87 0.0057 5. 7 7.0 6. 4 218 1-E. do do 310 393 0. 93 0.87 0.0008 6. 1 6. 1 9. 2 151 1 Approximate.

EXAMPLE H Poly(bicyclohexyl 4,4 dimethylene-4,4'-bibenzoate) polymer was produced as. disclosed in British Patent No. 979,401 granted Apr. 21, 1965, to E. I. du Pont de Nemours and Company. Its molecular weight was increased by solid phase polymerization. The polymer was heated and spun through a conventional spinneret, and through a heated capillary spinneret of this invention. The important parameters are detailed in Table 2 below. Drawable, smooth yarn is produced with small heated capillaries (Test 2b). With conventional capillaries, large enough to avoid melt fracture (Test 2a), the yarn was essentially undrawable and no low denier-per-filament yarn having good tensile properties could be made.

When the test with the heated capillary spinneret was repeated with the capillary temperature reduced to the range of 360-375 C., slight melt fracture was observed. 35

TABLE 2 Test 2A 2 B Spinneret Conventional Heated Capillary 4O .017 (.43 mm.) .009 (.23 111111.). .042" (1.07 mm.) .012 (.30 mm.).

Draw ratio 2 1.15. Tenacity, g.p.d

Elongation, percent 11.1- Modulus, g.p.d 41-.- Flex life 224 1 With 5% phenyl ether plasticizer. Maximum draw ratio obtainable when using 0. hot shoe and 175 C. draw rolls.

' flexed over a 0.001" (0.025 mm.) diameter tungsten wire,

oscillating through a arc at 150-154 cycles/minute.

EXAMPLE III Conventional polyethylene terephthalate polymer melt was spun through a conventional spinneret, and a heated capillary spinneret of this invention, to ultimately form a staple product. With the conventional spinneret, Test 3-A, the filaments were spun into 360 C. inert gas in an annealing zone 6 inches long (15 cm.), followed by a delay bafiie 16 inches long (41 cm.), a 6-inch 15 cm.) open space, and thence to a radial quench unit. The spun yarn was wound up at 1030 y.p.m. (940 meters per min.), at 38 pounds per hour (17 kilograms per hour) throughput. Several undrawn yarn bobbins were then combined and drawn on a staple draw machine at 4.0X draw ratio to properties shown in Table 3.

With the heated capillary spinneret, Tests 3-B and 3-C, a small amount of inert gas (1 to 2 cubic feet per minute) was used to blanket the spinneret face, but no heat was supplied to the gas stream. The yarn was spun through a delay baflle 18 inches long (46 cm. a 6-inch 15 cm.) open space, to the radial quench unit, and wound up at 1030 y.p.m. (940 meters per min), at 29 pounds per hour (13 kilograms per hour) throughput. Two tests were run, 3-B and 3-C, with different molecular weight polymers. The products were drawn the same as in Test 3-A. The properties are shown in Table 3.

Comparison among the test items shows that the yarn from the heated capillary spinneret of this invention is much more uniform in diameter and orientation, and could be drawn to a high draw ratio and to superior physical properties without evidence of nonuniformity (e.g., broken filaments). This was accomplished with the heated capillary spinneret without hot gas annealing.

TABLE 3 Test 3-A 3-B 3-C Spinneret Conventional Heated capillary Number of capillaries 250 192 192. Capillary dimensions:

Diameter 0 012" (.30 mm 0.110 (.28 mm.) 0.011 (.28 mm.).

Length 0.019 (.48 mm.) 0.019 (.48 mm); Block temperature".-. 290 C 290 C. Capillary temperature--. 420 C 290 C. Fiber relative viscosity. 8.5. Spun yarn birefringence 51.

Percent cost. of var. of biref. Percent coat. of var. of diameter: Draw ratio Tenacity, g.p.d Elongation EXAMPLE 1V degrading the polymer, wherein the improvement comprises (a) passing the molten polymer through the filter- Using the same heated capillary spinneret as used in holder at initial temperatures within a temperature range Example. 111, with the same type of annealing and quenchbelow that at which significant polymer degradation will ing system for the control, and a small amount of 11n occur, (b) passing the polymer into aplurality of passages heated inert gas when operating in accordance with this each of which leads to a different spinning capillary in the invention, polyethylene terephthalate industrial yarn was spinneret plate and has an entrance temperature within produced. said initial temperature range, (c) heating the spinneret Three pairs of tests were run, varying the polymer RV plate to increase the temperature along the passages from and the draw ratio. The spinning details and yarn propersaid temperature at the entrance to a temperature at least ties are given in Table 4. These data clearly demonstrates 60 C. higher at the spinning capillary, and (d) extruding that the heated capillary spinneret produces spun yarn the polymer from the spinning capillary after a maximum with improved properties without hot gas annealing and of 4 seconds of travel through the heated passage. that a smaller capillary can be used. 2. The improvement in a spinneret pack having a filter- TABLE 4 Test 4-A 4-H 4-0 4-D 4-E 4-1 spinneret Conventional Heated capillary Conventional Heated capillary Conventional Heated capillary No. of capillaries 192 102 192 192 192 192. Capillary dimensions:

Diameter 0.020 (0511111110-- 0.011" (0.28 mm.) 0.020" (0.51 mm.).. 0.011" (0.28 rnm.) 0.020" (0.51 mm.).- 0.011" (0.28 mm.). Length 0.030" (0.76 mm.) 1 0 48 mm.) 0.030" (0.76 rnm.) 0.019" (0.48 mm.) 0.030 (0.76 mm.) 0.019 (0.48 mm.). Block temp., 0.-...." 315 2 315 285 08 295. Capillary temp., C 430.. Approx. 310 430 Approx. 305 430. Hot gas anneal temp, 0.- 460... Not heated 458 Not heated 440 Not heated.

Relative viscosity: 1

Polymer supply 58 Yarn 51 Draw ratio 6.52.

Tenacity, g.p.d. 9.4... Elongation, percent 2 11.6 Uniformity, percent coci. 8.7

var. of diameter.

1 Approximate. 2 Measured after free relaxing yarn for 24 hours.

While the examples illustrate polyester polymers, this holder and a spinneret plate for melt spinning synthetic invention is also advantageous for other high melt viscosilinear polymer, wherein the improvement comprises a top ty polymers such as high molecular weight poly(hexaplate attached to the filterholder, a spinneret plate mounted methylene adipamide), poly(hexamethylene isophthalparallel to the top plate with an insulating space between amide), poly(Z-methyIhexamethylene terephthalamide), the two, hollow inserts extending through the top plate the polyamide from p-xylylene diamine and azelaic acid and the spinneret plate, means for conducting polymer and the polyamide from bis(4-aminocyclohexyl) methane from the filterholder into the top-plate end of each insert,

and azelaic, sebacic, dodecane dioic or hexadecane dioic a capillary for spinning the polymer into filaments at the acid, polyethylene, polypropylene, polyvinylidene chlospinneret-plate end of each insert, and means for heating tilde, and polyvinyl chloride. the spinneret plate to provide increasing temperatures Since many different embodiments of the invention may along each insert.

be made without departing from the spirit and scope thereof, it is to be understood that the invention is not References Cited limited by the specific illustrations except to the extent UNITED STATES PATENTS igiggf clams 1,984,472 12/1934 Friederich et a1.

1. In the melt-spinning process of supplying molten syn- 3360597 12/1967 Jones at thetic linear polymer to the filterholder of a spinneret pack JULIUS FROME, Primary Examiner.

under suflicient pressure for subsequent extrusion, passing the polymer through the filterholder and. filter medium Asslstam Examiner therein, and extruding the polymer through spinning capil- US. Cl. X.R.

laries in a spinneret plate to form filaments; the improve- 188 ment for spinning polymer of high melt viscosity without

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