US3706826A

US3706826A - Melt spinning process

Info

Publication number: US3706826A
Application number: US39525A
Authority: US
Inventors: James Duncan Bremner; Iain Alexander Mclellan
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1969-05-23
Filing date: 1970-05-21
Publication date: 1972-12-19
Anticipated expiration: 1989-12-19
Also published as: FR2048728A5; GB1268908A; DE2025109A1; NL7007336A; BE750515A; JPS494014B1

Abstract

A METHOD FOR DECREASING THE BIREFRINGENCE OF A MELT SPUN FILAMENT BY APPLYING A FORWARDING FORCE TO A FILAMENTARY STREAM IN A REGION WHERE THE FILAMENT IS IN AN INTERMEDIATE PLASTIC STATE, THE FORWARDING FORCE BEING SUFFICIENTLY SMALL SUCH THAT THE FILAMENT IS UNDER POSITIVE TENSION THROUGHOUT THE PROCESS SEQUENCE.

Description

Dec. 19, BREMNER ETAL MELT SPINNING PROCESS 2 Sheets-Sheet 1 Filed May 21, 1970 Dec. 19, 1972 D. BREMNER ETAL $706,826 H m MELT SPINNING PROCES S Filed May 21, 1970 2 SheetsSheet 2 32 W 1 Q 33 N P /34 INVENTORS JAMES DUNCAN REMNER IAI QJ ALEXAND MCLELLAN FW "(W7 United States Patent 3,706,826 MELT SPINNING PROCESS James Duncan Bremner and Iain Alexander McLellan, Harrogate, England, assignors to Imperial Chemical Industries Limited, London, England Filed May 21, 1970, Ser. No. 39,525 Claims priority, application Great Britain, May 23, 1969, 26,429/ 69 Int. Cl. B28b 3/20 US. Cl. 264-176 F 6 Claims ABSTRACT OF THE DISCLOSURE A method for decreasing the birefringence of a melt spun filament by applying a forwarding force to a filamentary stream in a region where the filament is in an intermediate plastic state, the forwarding force being sufficiently small such that the filament is under positive tension throughout the process sequence.

This invention relates to melt spinning processes for aromatic polyesters in which 'a fluid stream of polymer is extruded at a controlled speed from a spinnert, and cooled to form a solidified filament which is hauled oif at a higher controlled speed; such filaments being subsequently drawn.

in such spinning processes the filamentary streams have to stretch somewhere in the threadline between the initial extrusion speed and the final higher haul olf speed. Such stretching tends to orient the polymer molecules, and after solidification some degree of orientation usually remains which may be measured in the resultant filaments as an optical birefringence. This birefringence increases with increasing haul oif speed, and the ratio to which a filament may subsequently be drawn decreases with increasing birefringence, with the unfortunate consequence that if it is desired to make a drawn filament of a certain denier, then any increase in productivity deriving from a higher spinning speed is at least partially offset by the need to spin a lower denier and draw to a lower ratio. There are'also circumstances, for example in the production of high tenacity filaments, where a low birefringence in the spun filaments is beneficial quite apart from productivity considerations.

It is known that increasing extrusion temperature and decreasing cooling rate can decrease the birefringence, but these effects are not sutncient to counter the effect of haul off speed and permit the production of filaments with low birefringence at high haul olf speeds.

After extrusion, filamentary streams cool rapidly but remain molten for a substantial initial distance from the spinneret, within which region any chance contacts cause them at least temporarily to coalesce. -In this region the bulk of the stretching occurs. This molten region is followed by a short plastic region in which the viscosity rises between the previous molten region and the subsequent solidified region. In this plastic region occasional contact between filaments does not cause the problems due to coalescence which characterise the molten region but some stretching continues and the bulk of the birefringence is introduced. The plastic region is followed by a solidified region in which no significant further stretching or birefringence changes occur.

According to the present invention we provide a method of decreasing the birefringence of such melt spun and solidified aromatic polyester filaments comprising:

(1) applying a forwarding force to such filamentary streams in a region in which they are in such mtermediate plastic state Patented Dec. 19, 1972 We find that, within the intermediate plastic region, the filaments are so nearly solidified that they are not very prone to coalescence, and it is possible to apply the force according to this invention by a small arc of contact round one 101' more driven rollers preferably with roughened surfaces, or by a fluid force applicator, either gaseous or liquid even though each method tends to increase filament contacts before convergence in the solidified region. An air ejector device is particularly convenient as a force applicator, providing an increased tension upstream in the threadline and a decreased tension downstream, the ejector action being readily controllable to keep the downstream tension positive in the interests of process control and uniformity.

The air used in such an ejector device may be, but need not be heated. The rate of heat transfer between filaments and a roller is however much higher and a driven roller forwarding device should normally therefore be heated to maintain the filament temperature in the required intermediate plastic range.

'Melt spinning processes using ambient cooling, delayed cooling and accelerated cooling are all known. The present invention may be applied to each of these types of spinning process and indeed delayed and accelerated cooling may with benefit be used in sequence in the same threadline. In one form of the process of this invention, threadline cooling is delayed between the spinneret and the region in which the force is applied, and threadline cooling is accelerated thereafter. Using an air ejector as a force applicator it is particularly convenient to use the ejector also to provide accelerated cooling. A surprising aspect of this invention is that, although a more broadly applied increased cooling rate normally increases the birefringence a locally applied high rate of cooling coupled with forwarding action can reduce birefringence.

For the better understanding of this invention, some specific examples will now be described by way of illustration with reference to the accompanying drawings in which FIG. 1 is a schematic diagram of a melt spinning process according to the invention and FIGS. 2 and 3 are cross-sections of suitable forms of air ejector.

In FIG. 1, a spinning pack 11 with a spinneret 12 at its base extrudes filamentary streams 13 which at first are subject to delayed cooling in the region 14 which is partially enclosed in the thermostated box 15 used to heat the pack 11. An air ejector 16 surrounds the threadline at a point between the spinneret 12 and a spin finish applicator 17. The filaments are wound up on a bobbin 18. FIG. 2 illustrates one suitable form of air ejector. A short wide bore venturi tube 21 is held by a screw thread in a. casing member 22 with an air inlet duct 23 connected to a compressed air supply line 24 by a valve 25. Air is distributed round the venturi tube in the chamber 26 and escapes through annular slot 27.

In operation a high speed inward air flow through the annular slot 27 tends to bend downwards parallel with the wall 28 of the venturi tube 21, thus entraining with it air in the upper throat 29 of the venturi tube 21. This entrained air in turn applies a downward force to filaments passing through the venturi tube.

FIG. 3 illustrates another suitable form of air ejector. Two

members

30 and 31 are engaged in a threaded bore in a member 32, forming an annular chamber 33 connected by an annular passageway 34 to a yarn passageway 35. Compressed air is fed from means not shown through a bore 36 in member 32. In operation compressed air passes through the passageway 34 to apply a downward force on a yarn in passageway 35. The following examples illustrate but do not limit the invention.

EXAMPLE I Using the apparatus of FIGS. 1 and 2, polyethylene terephthalate with an intrinsic viscosity of 0.67 in orthochlorophenol at 25 C. was extruded at 288 C. through spinneret holes each 9 thou. diameter to form 40 filaments which were wound up at 4140 feet per minute as 8 denier filaments. With no ejector present the filament birefringence was 0.0073. When the ejector was positioned 15" from the spinneret, using air pressures of 2, 6 and p.s.i.g. the resultant filament birefringence was reduced to 0.0059, 0.0049 and 0.0032 respectively. Operat ing the ejector at 10 p.s.i.g. it was then positioned at 15, 20 and 27" below the spinneret, the resultant birefringences in each case being 0.0032, 0.0040 and 0.0067

respectively.

EXAMPLE II Polyethylene terephthalate with an intrinsic viscosity of 0.67 in orthochlorophenol at 25 C. was extruded at 290 C. through spinneret holes each 9 thou. diameter to form 20 filaments which were wound up at 4800 f.p.m. as 8 denier filaments. The apparatus of FIG. 3 was placed 19 inches below the spinneret.

It was found that the variability of filament denier depended on the yarn passagway length and diameter and best results were achieved with a jet diameter in member 30 or 0.5 inch, a passageway 35 of total length 1.25 inches and an air passageway 34 of width 0.01 inch. The air pressure in chamber 33 was adjusted to provide an air pressure reduction of 5.4 cm. water at a point inch above the entrance to passageway 35.

Under these conditions a coefiicient of denier variability of 0.8% was achieved at a birefringence of 0.0073 compared with 0.5% and 0.010 without the ejector present.

EXAMPLE III In place of an air ejector, two driven unheated capstans were positioned 17.5 inches below the spinneret. Polyethylene terephthalate with an intrinsic viscosity of 0.67 was extruded at 295 C. to form five filaments wound up at 3700 f.p.m. at a denier of 12. When the threadline passed the capstans without touching them the resultant birefringence was 0.0085. When the threadline took one complete turn round the capstans rotating at 4075 f.p.m. the birefringence fell to 0.0080. Since the capstan speed was slightly higher than the wind up speed it will be clear that a downward force was applied to the threadline but the tension between the air ejector and the wind up means was positive.

This relatively small reduction in birefringence may be increased by heating the driven capstans.

A melt spinning threadline modified according to this invention is particularly advantageous in processes involving separate spinning and drawing, such as are common in the production of staple fibre. In these processes, the productivity advantage of a high spinning speed can be greatly enhanced by the suppression of spun filament birefringence according to this invention. The

invention is however, also useful in integrated spin draw processes because it permits control of birefringence other than by first take up roll speed, so that birefringence and draw ratio may 'be adjusted at will.

What we claim is:

1. In a process for spinning an aromatic polyester comprising (a) extruding a filamentary stream at a controlled speed in the molten state,

(b) cooling such stream from its initially molten state through an intermediate plastic state to a solid state, and

(c) hauling off such solidified filament at a higher controlled speed a method of reducing the optical birefringence of such solidified filament comprising (i) applying a forwarding force to such filamentary stream in a region in which it is in such intermediate plastic state (ii) the forwarding force being sufficiently small such that the filament tension before hauling off is always positive.

2. A method according to claim 1 in which the forwarding force is applied by at least one driven capstan.

3. A method according to claim 2 in which such capstan is heated.

4. A method according to claim 1 in which the forwarding force is applied by a fluid drag.

5. A method according to claim 4 in which the for-' warding force is applied by an air ejector.

6. The method of claim 1 wherein said polyester is polyethylene terephthalate.

References Cited UNITED STATES PATENTS 2,296,202 9/ 1942 Hardy 264168 2,604,667 7/1952 Hebeler 264168 3,361,859 1/1968 Cenzanto 264210F 3,527,862 9/1970 Shimosako et al. 264290 3,553,305 1/1971 Au 264-210 F 2,880,057 3/1959 Cuculo 264210 F 2,957,747 10/1960 Bowling 264-210 F 2,987,373 6/1961 Bezemer et a1. 264178 F 3,011,215 12/1961 Alley 264176 F 3,291,880 12/1966 Pitzl 264176 F 3,346,684 10/1967 Gosden 264176 F 3,382,307 5/1968 Ciceri et a1. 264210 3,414,646 12/1968 Pitzl 264210 3,426,754 2/ 1969 Bierenbaum et a1. 128156 3,511,905 5/1970 Martin 264176 F FOREIGN PATENTS 44/ 13,772 6/1969 Japan 264210 F IAY H. WOO, Primary Examiner US. Cl. X.R. 26440; 42572.