EP0724029A1 - Yarns from melts using cold gas jets - Google Patents

Abstract

In the prodn. of filaments from molten polymers, the cold gas stream flow below the outlets for the molten filaments, at a small gap from the openings, has a speed greater than the molten filament speed. The gas flow gives a constant acceleration to the filaments so that they are drawn and cooled. Also claimed is a melt spinning assembly with an outlet slit below the centre axis of the spinarette opening rows, with a smaller cross section than the side flow cross sections. This gives a constant acceleration in the flow from the arrival of the gas at the filaments to the narrowest section of the slit. Pref. the technique is for melt spinning of molten polymers into filaments such as polyamide, polyester, polystyrene, polyurethane, polypropylene, and the like. The gas, after compression, is fed directly to the spinning station, without heat applied to it or taken from it, with pref. air for the gas flows. The gas is delivered to the filaments, at their setting point, at sonic or ultrasonic speeds. The slit has a converging shape, as a Laval jet structure. The spinarette has a lower thermal insulation, or has additional lower heating, to prevent heating of the gas streams.

Description

The invention relates to a method and an apparatus for producing threads of finite and endless length from thread-forming melts, preferably from polymers, with the aid of gas jets, primarily cold air jets.

It is known to extrude thermoplastic polymer melts from spinnerets and to extract them by means of hot gas jets, mostly air jets, these air jets flowing out of a series of melt outlet openings on both sides. This method was first known by A. Van Wente in a publication by the Naval Research Laboratory, Washington DC, USA in 'Industrial and Engineering Chemistry', 48 (1956), 1342-46. A large number of subsequent technical designs up to the recent past can essentially be traced back to the principle shown there, namely with the characteristics that it is a series of melt outlet openings and the air streams pulling out the melt must be heated to at least melt temperature and mostly above it. Since the outlet slots for the air flows are about 1 mm, often below, from the melt bores, this heating is necessary in order not to solidify the melt in the outlet openings. These blow stream or blow nozzle spinning processes (English meltblown) were created in order to produce threads that are as thin as possible, but also of finite length, ie with tears, for which purpose a higher air temperature gives the necessary heat supply for drawing to the finest possible diameter.

These processes are energy-intensive due to the necessary heating of the gas on the one hand, on the other hand the highest gas velocity is at the outlet of the air from the slits and is continuously reduced in the outlet jet by mixing with the ambient air. An increasingly lower air speed hits the melt thread and since this is accelerated by the thrust forces of the air jet acting on it, the effective difference in momentum between air and thread speed becomes smaller and smaller.

The object of the invention is to provide a method and devices suitable therefor in which these two essential disadvantages are avoided. The gas, preferably air streams are not heated, since they no longer hit the threads directly next to the melt bores, but just below the outlet openings. These cold air jets, which attack a row of threads on both sides, accelerate increasingly until the thread reaches its final diameter has reached. It has then cooled so far that it can no longer be warped.

Since the air streams only hit the threads at a certain distance below the melt outlet openings, more than just one row of melt outlet bores can be arranged, for example two or three, which increases the throughput for a given nozzle length and makes better use of the gas jet energy due to a higher material load can be. There is also no need for expensive injection molds, only normal spinnerets and parts for the gas flow that are not technically demanding.

In conventional melt spinning, the deformation of the thread from the exit from the melt bore to its solidification is caused by tensile forces which are applied aerodynamically mechanically by take-off or winding rollers or in the spunbond process. After the thread has solidified, the tensile forces act on the lower end of the spinning line, cf. DOS 37 36 418 and 43 12 419. In the upper area, the thread is cooled by specially introduced cooling air, usually regulated according to temperature, sometimes also humidity. The cooling air can be blown in transversely or with a component in the thread running direction. In blow spinning with heated air, the meltblown process, the hot air mixes with the cold ambient air after it emerges from the slots next to the melt holes, and the threads cool and solidify.

In the method according to the invention, the deformation of the threads is generated by shear forces which generate the air flowing in laterally, which is not particularly heated, on the thread jacket. The shear forces acting on the thread after its solidification also act as tensile forces on the deformation area, but mostly less than the shear forces on the not yet solidified part. Apart from a range of a few mm directly below the melt outlet, the speed of these air flows is greater than the thread speed. They accelerate steadily towards a Laval nozzle, in the narrowest cross section of which the speed of sound can be reached.

The process according to the invention thus stands between the known spunbonded nonwoven processes with a drawdown on the solidified thread and the meltblown process with hot air streams from the spinneret. Such a method has not yet become known.

Similar to the method according to the invention, a method for the production of powders from metal and ceramic melts has proven itself (DE 33 11 343), in which usually a single outflow opening of the melt stands just above a rotationally symmetrical Laval nozzle and the radially inflowing gas quantity is constantly accelerating, also up to the speed of sound and above, and thus for the extraction of the melt monofilament favorable conditions due to increasing gas flow with increasing monofilament speed. In the case of metal melts, the surface tension is usually so great that the thread bursts in the area of the lower pressure, approaching the narrowest point of the Laval nozzle, and balls result from the individual particles, which solidify into solid powder. In another embodiment, DE 35 33 964, the melt monofilament is kept warm longer by radiant heating in order to pull it out further and thus to obtain even finer powder.

The aim of the present invention is to produce threads which are as fine as possible from thread-forming, ie higher-viscosity melts getting produced. Unlike metal melts, the influence of viscosity outweighs that of surface tension. This leads to threads and not to powders. Solely the production of threads of finite or endless length is intended with the method according to the invention and its characteristic devices. However, for threads made of melt-spinnable polymers such as polyamides, polyester, polystyrene, polyurethane, polypropylene and other melt-spinnable polymers that can be converted into synthetic fibers, it is not important to have threads of a certain thickness and in special cases such as filters, fine-capillary nonwovens, textiles with a soft feel, for example to produce particularly fine threads, but in many cases they should also have the highest possible strength with low elongation and certain, usually as low as possible shrinkage, which is important for the subsequent post-treatment. For this, a molecular orientation during the transition from the melt to the solid thread is the basic requirement. In the present case, this is achieved primarily by the action of shear forces on the molten monofilament. General gravity and tensile forces on the solidified thread part help.

It has been shown that with a rapid reduction in the diameter with high shear forces and consequently high gas velocities, particularly good strength values of the threads with a high initial modulus can be generated force to elongation. In special cases, such as polyester, the shrinkage is also low in this case. The present invention fulfills the requirement for rapid stretching to the final diameter due to the basically short stretching length between the melt outlet nozzle and the Laval nozzle. Compared to the conventional spinning process for endless threads with mechanical take-off and subsequent winding or the spunbonded nonwoven process with aerodynamic take-off and subsequent filing of the threads into a nonwoven, the molecular orientation between the spinneret and the solidification point in the short warping distance and the associated high shear gradients in the thread are higher, because in addition to the tensile forces from below, strong shear stresses attack directly in the deformation area. These also attack the meltblown spinning process, but only at the very beginning, then increasingly weaker due to the unfavorable ratio of the decreasing gas speed to increasing thread speed.

The high thread qualities of polymer threads add to the energy saving advantage. Compared to other known methods for spinning threads from the melt with the aid of aerodynamic forces, the devices for implementing the method according to the invention are simpler.

The device for the present process for the production of threads and fibers with cold blowing streams consists of a longitudinal nozzle from which the melt emerges from openings in one or more rows. Below the longitudinal nozzle there is a gap of a few mm in width, the contour of which has the convergent-divergent shape of a Laval nozzle and which extends over the entire length of the row of spinning bores. With appropriate pressure ratios, the speed of sound can be generated in the narrowest part of the gap and even the supersonic speed in the expanded part. The gap is only a few mm wide in order to get by with as little air volume as possible, but is sufficiently wide to avoid the threads striking its walls. With ratios between ambient pressure p ₀ = 1 bar and pressure in the space below the spinneret with opening downwards through the longitudinal gap of p ₁ = 2 bar designed as Laval nozzle, the speed of sound already results in the narrowest part of the longitudinal gap.

The part below referred to as the blowing nozzle below the spinneret can be designed as a Laval nozzle with a convergent-divergent flow cross-section. With a corresponding pressure ratio p ₁ / p ₀ greater than about 1.9, supersonic is created behind it by jet expansion to the ambient pressure. In the case of a non-expanded blow nozzle, the so-called Borda mouth, the speed of sound at the outlet is at most.

Instead of letting hot air escape right next to the melt bores, unheated air hits the threads at a certain distance from the melt outlet openings and at ever increasing speed, pulls them out and cools them down at the same time. Everything takes place on a very short delay line. The outlet bore itself is only flowed around at a low air speed, and can also be substantially shielded from any flow in a known manner in that the outlet bores are arranged in a longitudinal groove and are thus somewhat reset. In addition, the nozzle body can be isolated next to the row of holes. These relatively simple measures make it possible to significantly reduce the energy expenditure for the air jets due to the heating which is no longer necessary and the better use of the pressure energy to accelerate the threads and thus to pull them out to a smaller diameter than the conventional meltblown processes.

Fig. 1

Querschnitt durch eine Spinnvorrichtung mit Laval-Blasspinndüse

Fig. 2

Ausführung mit Gasströmung schräg von oben

Fig. 3

Spinndüse mit zusätzlicher elektrischer Heizung in der Nähe der Schmelzeaustrittsbohrungen

The invention is illustrated by the following drawings.

Fig. 1

Cross-section through a spinning device with Laval blow spinneret

Fig. 2

Version with gas flow diagonally from above

Fig. 3

Spinneret with additional electrical heating near the melt outlet holes

1 shows a spinning device 1, as is usually used for polymer melts, above a blowing nozzle 2. The melt passes from a metering device (not shown), for example a gear pump, through an opening 3 into the spinning device and is distributed over a longitudinal channel 4 over the width of the nozzle. Known filters 5 for filtering out foreign bodies and for forming resistance for the purpose of uniform distribution of the melt over the nozzle length reach the melt through the pilot holes 6 of the nozzle to the melt openings 7 in the nozzle plate 8. This is in a nozzle screw connection 9 consisting of several parts with the aforementioned Parts summarized and placed in a heated box 10, from which it can be removed at 3 for exchange by dissolving the connection to the melt line.

The unheated air along the arrows 11 over the entire length of the melt outlet openings 7 arranged one behind the other in one or more rows via resistors 17 for equalization and expediently also for rectification, i.e. Vortex reduction, fed from the pressure chamber 15 to the Laval nozzle gap 12. This is formed from the two plates 13a and 13b, which extend over a little more than the row of nozzle bores. Due to the corresponding shaping at the ends, they form a gap 12, the Laval nozzle, which initially converges, then diverges.

The air flow is preferably laminar in order to allow quiet warping without turbulent fluctuations in the threads 25. A laminar flow allows a narrower gap at 12 and thus less use of gas (air) amount. A laminar flow in the warping area of the threads is also advantageous for reasons of energy saving.

In room 15 there is a higher pressure p ₁ than below in the room labeled 16. The space 16 can be under ambient pressure p ₀ or, if the threads are to be collected into a fleece or the threads are to be transported further, they can also have a pressure above the environment. If air as the flowing medium in room 15 is at least 1.9 times more pressurized than room 16, the narrowest point is 12 at the speed of sound.

Depending on the polymer or other melt-spinnable mass, temperatures are set slightly above the melting point. In order to obtain particularly thin threads, the temperature can be increased significantly above the melting point, which is necessary in any case with higher-viscosity polymers such as polypropylene; with polyester and polyamides, a lower overheating above the melting point is sufficient. The width of the gap 12 is chosen so narrow that just striking and thus clogging and jamming of the fibrous material in the blowing nozzle 2 is prevented. In order to be able to adjust the gap both in its distance to the outlet openings of the melt and in its width, the plates 13a and 13b, which form the lower end of the pressure chamber 15, can be moved via pivot points 18 and sliding openings 19, this type of adjustment only should be a possibility for others. The pressure chamber is closed at the end by plates 20. The sealing with the plates 13a and 13b can be carried out metallically by means of flat surfaces and pressing together; sealing elements in the plates 13a and 13b are also possible in a known manner. The tension between these and the plates of the end wall is easily released for piecing and adjusting the gap 12.

In Fig. 2 a similar device consisting of spinning device 21 and blowing nozzle 22 is shown. The air streams 23a and 23b, with the component directed downward from the start, step laterally onto the threads 25 from above the nozzle outlet bores 24. The heating by means of a heat transfer medium 27, in vapor or liquid form, located in chambers 26 is brought as close as possible to the spinneret plate 28 to reduce their cooling due to the air flow, which is still slow in this area. Unlike in FIG. 1, in this exemplary case the spinneret must be installed and removed from above.

In Fig. 3 it is indicated that an electrical, flat rod-shaped heater 29 provides heat to the spinneret and counteracts the cooling of the air streams. Insulating plates 30 can act in the same sense as shown here and in FIG. 1. With the help of these side heaters, the melt can be overheated shortly before it emerges in order to obtain finer threads. Because of the short exposure time of the higher temperature, the damage to the polymer can be kept low and a drop in the mechanical quality values of the threads remains small or cannot be determined at all.

The air necessary for spinning the melt is fed to the atmosphere or the pressure p ₂ in the space 16 of the blowpinning device at pressures of about 0.5 to about 3 bar above the pressure p ₀ . If there is a negative pressure in room 16, for example if a thread deposit is connected to a fleece, the pressure in room 15 can be lower and it can still be achieved at 12 sonic speeds if the critical pressure ratio 1.9 is reached or exceeded in air .

It is expedient to maintain the heat generated in the gas by the compression of the air, ie the supply lines to keep it short and to isolate it well in order to use this otherwise lost energy for spinning, in that the difference in speed between air and thread can be smaller if the thread can be kept at a higher temperature for longer during the warping. Keeping the thread warm also by radiant heating as in DE 35 33 964 is also possible here. Heating the entire blown air is not necessary and could only be carried out to a small extent (if, for example, cheap heat is available and particularly fine fibers are to be produced), since the threads would otherwise not be cooled down quickly enough.

In the production of threads from synthetic polymers, the process according to the invention succeeds in producing threads with the quality of staple fibers, although their essential properties of strength, elongation, shrinkage and of course also the thread diameter fluctuate more due to the not completely constant tensile effect of the air flow on the threads than in conventional thread production with mechanical withdrawal of the threads from spinnerets. Some examples should show that.

example 1

Carefully dried granules of polyester (polyethylene terephthalate) were melted in an electrically heated vessel of about 0.5 l. There was a nitrogen atmosphere above the melt level. The melt temperature was 270 ° C. By increasing the nitrogen pressure, the melt was brought to the bottom out of a bore with a diameter of 2 mm. At a distance of about 5 mm below this outlet opening, air approached the emerging thread in a radially symmetrical manner and flowed through a Laval nozzle of 3 mm diameter parallel with the threads down. The narrowest cross section of the Laval nozzle was about 8 mm from the melt outlet. The air was taken from an industrial compressed air network.

With an air pressure in the feed of 2.5 bar above atmosphere, threads with a fineness of 1.1 den, corresponding to 11 μ, and a strength of 2.7 g / den, elongation of 71%, boiling shrinkage 9% - each resulted Average values from 10 measurements. The throughput was 0.9 to 1 g / min.

Example 2

A plastic granulate mixture of polyester and polypropylene in a weight ratio of 50:50 was melted in an elongated vessel, also with electrical heating and under a nitrogen atmosphere, and brought to an outlet by means of an overpressure in nitrogen from three outlet openings with a diameter of 0.8 mm arranged in series. The temperature of the melt was 285 ° C, the pressure of the compressed air 1.8 bar above atmosphere and the throughput 0.75 g / min per hole. The Laval nozzle had a rectangular cross section with a width of 3 mm at the narrowest point. Their distance from the melt outlet openings was approximately 25 mm. The result was silky threads with an average of 0.76 den, 1.60 g / den strength, 272% elongation, 35% cooking shrinkage.

The threads were caught on a sieve belt moved underneath the device, where they were laid down to form a 5 cm wide fleece.

In the same device, various plastic mixtures and pure plastics such as polystyrene, polyamide, polyethylene were spun into fine threads and it was found that with these simple devices, surprisingly high fiber values in terms of strength, elongation and low shrinkage could be produced, which would otherwise require a higher level of equipment and especially energy.

The present invention is applicable to all substances which can be converted into threads in the molten state. In the case of polymers, the melt monofilaments should be warped quickly and under the influence of tensile and shear stresses in order to obtain the highest possible molecular orientation and thus the quality of the threads. This also applies if the threads are particularly fine and are not endless, but tear off at irregular intervals and a new piece of thread then forms. The production of polymer threads and nonwovens thereon is a preferred application of the method according to the invention, without being restricted thereto.

In the production of nonwovens, the device consisting of spinneret and blowing part extends over a certain length, which specifies the width of the nonwoven produced. The threads - endless, partially endless or predominantly finite length - are placed on a catch belt, the direction of which is the longitudinal axis of the spinneret with blowing part at a certain angle, usually at 90 °. The thread can be deposited by having an open space between the Lavalblower nozzle and the catch belt. However, laying methods known in spunbonded nonwoven technology can also be used for swiveling the thread sheet emerging from the Laval nozzle. The suction process, in which the laying part is separated from the surroundings and suctioned off under the belt, can also be used in that the space below the Laval blower nozzle is closed in a known manner up to the collecting belt and the threads laid into a fleece seal this closed laying room via sealing rollers leaves with the sieve belt. Nonwoven webs of endless threads, referred to as spunbonded webs, when produced directly from the spinneret, can be a preferred one Field of application of the present invention. Apart from special cases, the threads should be distributed as evenly as possible in all directions in order to obtain isotropic materials. In accordance with the statistical character of the turbulence that forms in the flow below the Lavalblas nozzle, the use of the method according to the invention and the associated device is particularly suitable for the production of such products.

Claims (8)

A process for producing threads from melts with the aid of a gas flow, characterized in that the gas flow just below the outlet openings for the melt reaches a speed above the speed of the melt threads and continues to accelerate, whereby the threads are warped and cooled. A method according to claim 1, characterized in that it is thread-forming polymer melts. A method according to claim 1 or 2, wherein the gas after compression to the desired pressure is used directly for spinning the melt without heat being supplied to or removed from the gas. Method according to one of the preceding claims, wherein the flowing gas is air. Method according to one of the preceding claims, wherein the gas flow in the area of solidification of the threads reaches the speed of sound or also supersonic speed. Apparatus for carrying out the method according to claim 1 or one of the following claims, wherein the melt emerges from one or more parallel rows of spinning bores of a spinneret into which gas, preferably air, enters and enters a space below the spinneret from both sides of the rows of spinning bores through a slit-like cross-section below the central axis of the row of spinnerets, characterized in that that it is much smaller than the lateral inflow cross sections in order to achieve a constantly accelerated flow from the impact of the gas on the threads to the narrowest cross section of the slot. Apparatus according to claim 6, according to which the slot in its cross section in the flow direction of the gas is formed convergent-divergent in the form of a Laval nozzle. Apparatus according to claim 6 or 7, characterized in that the spinneret is covered in a heat-insulating manner to reduce the outflow of heat to the gas flow or is additionally heated from below.

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