METHOD OF EXTRUDING POLYPROPYLENE YARN

Abstract: A method of extruding multi-filament polypropylene yarn in which the polypropylene is extruded at a temperature below 218°C, preferably in the range 213°C to 177°C, particu­ larly about 205°C, into a hot zone (26) having a temperature sufficiently high to retard cooling of the extruded polyp­ ropylene yarn. The temperature of the hot zone can be within 16°C of the temperature of extrusion. The yarn is then passed through a quenching zone (28) across which air is blown to cool the yarn. The swell value of the polypropylene can be less than 3 and its melt flow may be greater than 30. The yarn is drawn down in the hot zone and the filaments may be drawn down to an undrawn denier of less than 40.

Description: METIIOD OF EXTRUDING POLYPROPYLENE YARN Polypropylene yarns, particularly continuous filament textile face yarns, are usually produced with conventional 'down-the-stack' air quench extrusion annaratus. These are housed in a building several stories high with an extruder on an upper floor, air quench cabinets on the floor below, and inter-floor tubes extending down to a lower floor where the yarn is taken up onto packages. Cooled air is blown through the quench cabinets to solidify and cool the yarn. One disadvantage tliat occurs is resonance in the ionnation of the filaments of the yarn. As the polyprony- lenc melt is extruded through a capillary in a spinnerette, it swells out on the underside of the sninnerette and then the filament is drawn-dowrn from such swelling. However this drawing-down occurs non-uniformly and to exaggerate the filament forms in the manner of a string of sausage links: this effect is termed resonance. Subsequently when the filaments are being fully drawn, this resonance tends to cause draw breaks in tlle filaments. The more pronounced the resonance, tlle greater the frequency of draw breaks. Also the point at which a filament comnletes its drawing-down in the quench cabinet to its undrawn denier, varies. This can be seen as a rain drop effect when looking into the quench cabinet. This contributes to further non-uniformity. The temperature at which the polypropylene melt is extruded is usually of the order of 260 C although lower temneratures have been tried. It is known that in general, as the temnerature is-lowered the swell on the underside of the spinnerétte gets greater with an increase in reson ancc, and even the occurrance of spin breaks at or near the sninnerette face. The problem of resonance and subsequent draw breaks gets more acute with finer denier per filament yarns, for examplw yarns having an undrawn denier per filament less than 30, say less tlian 10 denier per filament in the finally drawri yarn. Also with finer denier yarns, the problem of denier variation from filament to filament as well as along the length of the filament, becomes more noticeable. The invention is based unon the realization that if the fi laments are extruded into a relatively short hot zone, at or slightly below the temperature of extrusion, before the are contacted by the cooling air, then the extrusion temperature can be decreased without the usual increase in the volume of swell at the spinnerette face. It has bcen found that as the extrusion temperature de creases the resonance in the filaments decreases; an ontimum noint is reached around 2050 C. When the temnera ture goes much lower than this optimum point, resonance starts increasing again and then spin breaks occur. The ortcise optimum point is believed to be influenced b the swell value of the polypropylene and its. melt flow. It is considered possible that as the temperature of the melt decreases, the melt becomes more Newtonian in its behaviour. This s is believed to be further helped as the swell value of the polypropylene is decreased for examnle to below 2.5. According to one aspect of the invention there is provided a method of producing a plurality of polypropylene filaments comprising heating polypronylene having a narrow molecular weight distribution with a swell value of less than 3 to a temperature at which it is molten; extruding ie molten volypropylene at a temperature less than 21S0C into the plurality of filaments; passing the filaments through a first zone having a temperature sufficiently high to retard cooling of the filaments therein; drawing down the filaments to their undrawn denier in said first zone; massing the thus-formed filaments through a second zone; and directing cooling gas over the filaments in said second zone to cool them; the combination of the swell value of the polypropylene, the temperature of extrusion, - and the temperature of said first zone interacting to sub stantially eliminate the occurrence of resonance in the filaments as they are drawn down in said first zone. The polypropylene preferably has a swell value less than 2.5. The nelt flow of the polypropylene may be greater than 20, and is preferably greater than 30. Said first zone is preferably short relative to said second zonc, and preferably contains gas in a quiesceni state. The temperature of said first zone may be less than 21 C below the temperature of extrusion; it may be above 177 C. It may be within 16 C of the extrusion temperature. The filaments may be drawn down in said first zone to an undrawn denier per filament of less than 40, for example less than 30. In said second zone, cooling gas may be blown transversely over the filaments to cool them. The tempera ture of this cooling gas is preferably less than 32 C as it enters this quenching zone. The extrusion temperature may be less than 215 C, such as ill the range 213 0C to 177 C, or in tlic range 210 0C to 1840C Preferably it is about 2050C. At least two multifilament yarns may be simultane ously produced by extruding a matered stream of said molten polypropylene through a spznnerette having at least two grouts of orifices. A specific embodiment of tlie invention will now be described in greater detail with reference to the accompanying drawings, Figure 1 is a schematic vertical section of an apparatus for carrying out the method of the invention; Figure 2 is a diagrammatic section, on a larger scale, on the line 2 - 2 of Figure 1; Figure 3 is a diagrammatic sectional view on line 3 - 3 of Figure 1 but on the same scale as Figure 2; Figure 4 is an illustration, on an enlarged scale, of a filament being produced; and Figure 5 is an illustration, on an enlarged scale, of another filament being produced uniformly. In Figure 1 an extruder 10 has an infeed hopper 11, a screw 12, and band heaters 13a, 13b, 13c and 13d. A transfer tube 14 connects the discharge end of the extruder 10 to a metering pump 15. The transfer tube 14 and the nzetering pump 15 are surrounded by band heaters 16 and 17, respectively. The discharge side of the metering pump 15 is connected by a tube 18 to a spin pack 19 mounted in a spin bloc 20 which is surrounded by a band heater 21. The spin mack 19 has a cover plate 22, à body 23, a breaker plate 24, and a spinnerette 25. For simplicity, the usual heat insu lotion that covers the band heaters and other parts of the apparatus is not shown. A shroud 26 is attached by bolts 27 (see Figure 2) to the underside of the spin block 20. Below the shroud 26 is mounted an air quench cabinet 28 at the bottom of which are finish applying guides. 29. Just below the guides 29 is a denier control roll 30. The shroud 26 defincs a rectangle in hbrizontalsection, see Figure 3. At its upper end is a flange 31 through which the bolts 27 pass. At the lower end of tie shroud 26 is an inwardly directed collecting trough 32. The spinerette 25 has capillaries 33 arranged in three grouns 34, 35 and 36, resnectively, to produce three nulti-filament.yarns 37, 38 and 39, respectively. To produce yarns having various filament counts, different spinnerettes can be used having a different number of capillaries. The quench cabinet 28 has a top cover 40 which fits closely around the outside of the trough 32. One wall of the quench cabinet 28 is formed of wire mesh 41 supported in a frame 42. The opposite wall is formed of slotted sheet metal 43 supported in a frame 44. A cooling air plenum 45 registers with the wire mesh 41. In cross-section the quench cabinet is rectangular, similar to the shroud 26 and the face of the spinnerette 25 with the groups of capillaries 34, 35 and 36 spaced apart in a direction parallel to the longer sides of these rectangles. Tlie shroud 26 is relatively short and fits closciy around tile groups 34, 35 and 3G of capillaries but with i;cient clearance so that the yarns 37, 38 and 39, if they sway, do not come in contact with the inner edge of the trough 32. As seen in Figure 3, the longer side of this shroud 26 is 12 inches and the shorter side 7 inches; the length of the face of the spinnerette 25 is 8 inches and the width 4 inches. The height of tile shroud 26, as seen in Figure 2, is 9 inches. With the method according to the invention, pellets oi polypropylene resin and pellets of colour concentrate are fed via the hopper 11 into the extrude 10. The poly- propylene has a melt flow of 30 aiid has a narrow molecular weight distribution with a die swell or swell value below 2, in this illstance 1.9. The resin and colour are melted and heated by the extruder heaters to a temperature of 205 and mixed by the screw 12. The heaters 13a, 13b, 13c and 13d are set to control their zones at 1480C , 1770C 1910 C and 2050C , respectively. TIle downstream heaters 1G, 17, 21 are set to control their zones at 205 C . The melt is fed by the screw 12 through the transfer tube 1'4 to the metering pump 15 which delivers a metered stream of melt through the tube 18 to the spin pack 19. Inside the spin nack this metered stream is hydraulically split and extruded downwards through the capillaries 33 into the multitude of filaments forming the three spaced,apart yarns 37, 38, and 39. The number of capillaries in the spinnerette is chosen to determine the number of filaments in each yarn, in this instance 70 filaments. These yarns pass through the shroud 26, which defines a hot zone, and are then cooled as they pass through the quench cabinet 28. The cooling of the yarns is effected by blowing air transverse ly across them, the air from the plenum 45 entering the quench cabinet through the wire mesh 41 and being exhausted to Atmospllere through the slots in the sheet metal 43. The cooled yarns then pass through the guides 29 which apply spin finish to them before 1hey are brought together around 1t denier control roll 30, after which tie three yarns are sc%arated and wound onto separate packages 47, 48 and 49. The denier control roll pulls the yarns down from the capil lanes 33 at a controlled rate, in this instance 600 meters Fer minute, to determine their undrawn denier, in this instance 900 denier. The air inside the shroud 26 is trapped there and remains quiescent. This air is heated by the metal above i t, namely the face of the spinnerette 25, the lower end of the pack body 23 and part of the spin block 20, these being heated by the spin block heater 21. Tlic molten filaments leaving the capillaries 33 also heat this air. ln this way, the air inside the shroud 26 remains hot at a temperature close to or just below, the temperature of the melt being extruded and prevents substantial cooling of the filaments as they pass therethrough. The tempera ture in the lower portion of the shroud 26 may. be at ct lower temperature than in the upper portion, but is sufficiently high to retar cooling of the filaments. Figure 4 shows in. an exaggerated manner a poly propylene filament being extruded from a capillary 50 directly into an air quenching zone 51 by a conventional air quench process. The molten polypropylene swells out at 52 under the.face of the spinnerette and then forms a series of diminishing swellings 53, 54 before the drawndown to the size of the filament is completed. This series of swellings is not completely drawn out and results in the filament exhibiting resonance to some degree. Figure 5 illustrates tha way in which the swell draws down in the present inventioit. An initial swell 55 occurs under the face of the spinnerette, but ten due to the combination of the low temnerature of extrusion and the extrusion of the filament into a hot quiescent zone 56, the draw down occurs quicker over a shorter distance to a uniform filament 57. As can be seen, the total I volu; c ol' the swell 55 is less than the volume of the elongated swell 52, 53, 54 shown in Figure (1. The 900 undrawn denier 70 filament yarn produce; by tlie method of the invention, wllen subsequently drawn at a draw ratio of 3 : 1 to a continuous filament 300 denier 70 filament yarn, produces a uniform yarn with substantially no resonance symptoms and improved uniformity of denier from filament to filament. The yarn also draws with a high efficiency with substantially no draw breaks. This further makes possible multi-end drawing, for example drawing eight yarns together on the same drawframe For j he production of finer denier per filament yarns i t is preferable to use narrow molecular weight distribu- tion polypropylene with a higher melt flow, for example in the range 35 to 45, and with a lower swell value, for example in the range 1.2 to 1.7. Narrow molecular weight distribution polypropylene is usually made by thermal degradation of reactor resin, although this can be done chemically. The object is to degrade the high molecular weight material. The swell value is the ratio of the diameter of the extrudate just below the face of the spinnerette divided by the diameter of the capillary through which it is being extruded. This should be measured using a capillary with basically zero land (length to radius ratio not greater than 0.221) at a temmerature of l900C and at a shear rate of one thousandth of a second. Shear rate.equals four times the volumetric flow rate (q in cubic centimetres per second) divided by times the third power oi the capillary radius (in centimetres) i.e. Shear rate = 4g X X radius 3

Claims: CLAINS 1. A method of producing a plurality of polypropylene filaments, characterized in that it comprises heating polypropylene having a narrow molecular eight distribution with a swell value of less than 3 to a temperature at which it is molten; extruding the molten polypropylene at a temperature less than 218 C into a plurality of filaments; passing the extruded filaments into a first zone having a temperature sufficiently high to retard cooling of the filaments therein; drawing down the filaments to heir undrawn denier in the said first zone; passing true drawn fil ments through a second zone and directing cooling gas ovtr the filaments in the said second zone to cool them; tilt combination of the swell value of the polypropylene the temperature of extrusion and the temperature of said fi, t zone interacting to substantially eliminate the occurrence of resonance in the filaments as the, are draw' down ill the said first zone. 2. A method as claimed in Claim l, characterized in thjt said swell value is less than 2.5 an the polypropy- lene has a melt flow greater than 30. 3. A method as claimed in Claim 1 or Claim 2 character zed in that the said first zone is short relative to said second zone and contains gas in a quiescent state. 4. A method as claimed in any of Claims s 1 to 3, c.laracterized in that the said first zone contains gas at a temperature less than 21 C below the temperature at which the molten polypropylene is extruded. A method as claimed in Claim 4, characterized in that the temperature of the said first zone is within 16 C of tile temperature at wliicli the molten polypropylene is extruded. 6. A method as claimed in any of Claims 1 to 5, characterized in that the molt, ' polypropylene is extruded at about 205 C. 7. A method as claimed in any of Claims 1 to 6, characterized in that the-filaments are drawn down in the said first zone to a denicr per filament of less than 30. 8. A method as claimed in any of Claims 1 to 7, characterized in that a metered stream of said molten moly- propylene is extruded through a spinneratte having at least two grounds of orifices into at least two multifilament yarns. 9. Polypropylene filaments produced by the method of any of Claims 1 to 8.

Inventors: ROBERTS, JOHN S.
Applicants: KLING-TECS,INC.
Application year: 1979
Main classification: D01D5
IPC codes: D01D5, D01F6