GB2053629A - Process and Device for the Heat Treatment of Filiform Elements - Google Patents

Abstract

Moving filaments 10 are heat treated by passing them through a UHF resonator cavity 4 in which the electric field is parallel to the moving filament 10 to be treated. The overvoltage coefficient of the cavity is damped by increasing its losses, and this lost damping energy is converted into infrared energy produced in the vicinity of the filament to be treated by a tubular element 8 mounted on a quartz support tube 7. A plurality of such resonator cavities, fed by a common microwave generator may be provided in series or parallel. <IMAGE>

Description

SPECiFICATION Process and Device for the Heat Treatment of Filiform Elements The invention relates to a process and a device for the heat treatment of moving filliform products, in particular those moving at high speed.

In the remainder of the description, the terms "filliform product", "filiform element" or filament" denote a long, thin, flexible or rigid element.

These expressions encompass textile filaments of any type (filament of chemical, mineral or organic origin) and in any form (monofilaments or multifilaments, spun filaments, assembled filaments, twisted filaments and the like), which can be used not only for textile applications (woven fabrics, knitted fabrics and the like), but also in other fields, such as, for example, for the manufacture of optical fibres.

In all the processes for the treatment of moving filaments requiring a heat treatment, the main problem which arises is the rapid transmission of heat, it being necessary for the heat to penetrate uniformly into the core and in the same manner over the whole length of the filament. In fact, as is known, the temperature of the treatment and its uniformity greatly influence the qualities of the filament.

It is well known that the heat treatment varies in accordance with the material treated, the gauge of the filament and its speed of travel.

Thus, it is easy to understand that the core of a thin filament will be reached more rapidly than that of a thicker filament. Likewise, it is known that it is not possible to treat a filament above a certain limiting temperature without degrading it.

This question of heat exchange is a very important point in the field of texturising or covering by means of coating or polymerisation, and those skilled in the art have envisaged various solutions to this problem by applying one of the three main principles of heat exchanger, namely convection, radiation or conduction, or a combination thereof.

However, because of the speeds which are currently used, it is necessary considerably to increase the length of the treatment ovens, which increases the bulk of the machines, and there is the fact that these solutions necessitate an appreciabie energy expenditure.

It has also been proposed, in the textile industry, to use the well-known principle of heating by means of high frequency or ultra-high frequency. Although the application of this principle gives good results in certain cases, for example for drying spools of filaments and drying pieces of woven fabric or flock, it must be recognised that this process has hardly been developed in the field of the treatment of filaments, although its application in texturising was envisaged more than twenty years ago (U.S.

Patent 2,823,513).

The lack of development of this technique, despite the advantages which it provides, namely the rapidity, the high energy efficiency and the absence of atmospheric pollution, is doubtless explained by the fact that it is very difficult to regulate the temperature and to obtain a precise treatment, this being an essential condition for the production of a filament of good quality.

Moreover, the use of ultra-high frequencies necessitates high overvoltage coefficients and a limitation is then imposed by the stability of the currently available generators, which lead to substantial energy losses.

As is known, the term ultra-high frequency, which is sometimes also referred to as UHF, denotes vibrations of between 900 and 30,000 Megahertz, that is to say between 0.9 and 30 GHz. In practice, a frequency of 2.45 GHz is used, this being the standard band in France, but is selfevident that other frequencies can also be used, in particular if they are better suited to the process.

It has already been proposed to heat filaments by means of electromagnetic or microwave radiation, in particular at 2.45 GHz, as described in U.S. Patent 3,557,334 of E.I. Du Pont de Nemours.

The problem which arises in using this process is the wave-filament coupling factor, which is low because the volume is small and because it is known that the dissipated power corresponds to the formula Pdis=27rfEt'VE2, in which f denotes the wave frequency, Ett denotes the dielectric losses in the material, v denotes the volume of the material and E denotes the electric field on this material.

To compensate the low value of v, attempts have been made to increase either frequency or the electric field E. The increase in the frequency is limited by the following disadvantages: the reduction in the dimensions of the cavity, which leads to a lowering of the breakdown field, the absence of generators suitable for these applications, and the high price of the equipment.

To increase the electric field E, a high-overvoltage cavity can be used. However, this raises two major difficulties. Firstly, there is the requirement of having a very frequency-stable microwave energy source, and there is the requirement of not exceeding a critical field, beyond which a plasma is excited.

The invention relates to a new process which is at one and the same time rapid, economic, precise and efficient and which does not exhibit the disadvantages of the process used hitherto.

According to one aspect of the present invention there is provided a process for the heat treatment of moving filaments which includes the steps of passing the filaments through at least one ultra-high frequency resonator cavity, and damping the over-voltage coefficient of the cavity by increasing its losses, the lost damping energy being converted to infrared energy produced in the vicinity of the filaments being treated.

With the process of the invention, the moving filaments to be heated are thus subjected simultaneously to microwave radiation and to infrared radiation.

Preferably, the electric field inside the UHF resonator cavity is parallel to the moving filaments, but it is possible to have an electric field perpendicular to the movment of the filaments.

In order to damp the overvoltage coefficient of the cavity by increasing its losses, and to convert this lost damping energy into infrared energy produced in the vicinity of the filament to be treated, there may be a body of material having ultra-high frequency dielectric losses arranged in the cavity with its longitudinal axis parallel to the longitudinal axis of the cavity, the body having at least one longitudinal channel, and the filaments passing through the channel without contacting the wall thereof.

Thus, by suitably choosing the material, part of the UHF energy dissipated can be recovered in the form of infrared radiation. Any products having large ultra-high frequency (UHF) losses can be used as the material having dielectric losses.

Examples which may be mentioned are ceramics, glass or other silica-based products. Those skilled in the art can easily choose the material in accordance with the price and the desired performance. Likewise, they can easily determine experimentally, in accordance with these data, the exact dimensions which the element having large dielectric losses, and also the resonator cavity in which it is arranged, should have. Good results are obtained by using a material based on ceramics containing 30% of silica.

The invention also provides a device for the heat treatment of moving filaments, such device including an ultra-high frequency resonator cavity, a microwave generator, a waveguide and a coupling component connected to said cavity, means to pass filaments through the cavity and, within the cavity, converter means to damp the overvoltage coefficient of the cavity by increasing its losses and converting the lost damping energy to infrared energy in the vicinity of the filaments.

Preferably, the converter means includes, inside the cavity, a body of a material having a large ultra-high frequency dielectric losses, the body having at least one channel of which the axis is parallel to the longitudinal axis of the cavity and inside which channel the moving filaments pass without contacting the walls of the channel.

In the remainder of the description, the element based on a material having large UHF dielectric losses will be designated by the expression "tubular element" by way of simplification.

Furthermore, in the remainder of the description, this element will be described for cases in which it possesses only one longitudinal channel, the said element also being arranged along the longitudinal axis of the resonator cavity, but, of course other types of tubular elements could be used, for example it is possible to use a tubular element possessing a plurality of channels over its length, which would make it possible to treat several filaments simultaneously, or to use several elements.

As stated previously, the damping tubular element is made of a material having large UHF dielectric losses. Its cross-section is arbitrary, for example cylindrical or the like.

Advantageously, this tubular element is mounted on a support tube made of a material which does not conduct heat, for example quartz, which support tube is arranged along the longitudinal axis of the cavity, it being possible for the said tubular element to slide coaxially on the said support tube.

The manner in which the invention is put into effect can be summarised in the following way.

As is known, a hollow cavity of given diameter has a given resonance frequency f, which is calculated experimentally by applying Maxwell's laws. If this frequency is excited, all the energy enters the cavity in which undamped stationary waves are then produced. If a frequency f,, which is different from f, is sent into the cavity, the energy is reflected to an extent proportional to the difference between f, and f.

If a foreign body, for example a moving filament, is introduced into the cavity, there is a tendency to lower the resonance frequency.

Likewise, when the foreign body becomes hot, the dielectric losses increase and there is hence a further lowering of the resonance frequency. In continuous operation, the frequency emitted by the generator must therefore correspond to that of the cavity in order to permit heating, and this requires continuous adjustment of the frequency of the cavity to that of the generator, and vice versa.

The invention consists in introducing, into the cavity, a tubular material having dielelctric losses, so that the energy rediated by the generator is absorbed by this tubular material. In other words, the introduction of this material modifies the shape of the curve of the reflected energy by widening the pass-band of the cavity.

Consequently, the majority of the energy is consumed by the dielectric material and is converted into infrared, and this makes it possible to heat the moving filament both by the action of the infrared radiation emitted in this way and also by the action of the microwave radiation.

The manner in which the invention can be put into effect and the advantages which result therefrom will become more clearly apparent from the following illustrative embodiments, which are given by way of indication but without implying a limitation, and which are illustrated by the attached figures in which: Figure 1 schematically shows a resonator system produced according to the invention, Figure 2 is a simplified representation of a filament-coating installation using these systems, Figure 3 illustrates a modified embodiment of a resonator system produced according to the invention and comprising two resonator cavities arranged in series, and Figures 4 to 11 illustrate different embodiments of devices permitting the heat treatment of moving filaments and produced according to the invention.

In the remainder of the description, the same elements will be denoted by similar references, if appropriate with an added index a, b and so on, by way of simplification.

Figure 1 illustrates an embodiment of a resonator system produced according to the invention, which system makes it possible to treat a moving filament.

According to this embodiment, the resonance system is composed of: a microwave generator 1 of a type which is in itself known, for example a 1 KW/2.45 GHz generator such as those marketed by the Sociétés SNEA or SAREM, an excitation waveguide 2 connected to the said generator 1 for the purpose of transmitting the energy emitted by this generator 1; in the present case, this excitation waveguide 2 has a cross-section in the shape of a rectangular parallelepiped, a coupling component 3 of a known type, which consists, for example, of a coupling diaphragm, sometimes also referred to as a "coupling window", the purpose of which is to connect the waveguide 2 to a resonator cavity 4 and to adjust the impedance of the cavity of the applicator to the impedance of the waveguide, a cylindrical applicator 4 forming a cavity, which resonates, for example, in the Two10 mode at 2.45 GHz (this is to say it does not have a maximum electric field on a transverse radius, but only a maximum at the centre, which is constant over the whole length of the applicator); this cylindrical applicator 4 possesses two openings 5, 6 which are arranged, in the present case, along the longitudinal axis passing through its centre, and the purpose of which is to allow the passage of the filament 10 to be treated, a support tube 7 made of quartz, which passes through the cavity 4 in the longitudinal direction via the openings 5 and 6, and a cylindrical tubular element 8 which is made according to the invention, of a material having dielectric losses, for example sintered alumina, and is coaxial with the support tube 7; in this embodiment, the tubular element 8 is mounted so as to slide on the tube 7, the latter possessing a bulge 9 forming a stop.

In this embodiment, the cavity 4 (or applicator), which is excited, as stated previously, in the Two10 mode at 2.45 GHz, is made of stainless steel or any other suitable material, such as brass, duralumin, copper or any other conductive material, and possesses the following characteristics: internal diameter: 80 mm length: 160 mm sintered alumina tube 8: internal diameter: 13 mm external diameter: 1 6 mm quartz support tube 7: ~~internal diameter: 10 mm external diameter: 12 mm The length of the tube 8 is adjusted to a suitable value so that the cavity can be tuned to the generator 1 by a simple translation movement of the quartz tube 7, which then carries the alumina tubular element 8 along by means of the stop 9.

It is self-evident that it is possible to use other types of cavity than the cylindrical cavity described, provided that, according to the invention, a tubular element based on a material having large UHF dielectric losses is arranged inside each cavity, the said tubular element possessing at least one longitudinal channel, the axis of which is parallel to the longitudinal axis of the cavity and advantageously coincides with the latter axis, and the filament to be treated passing inside this channel.

Thus, by way of example, so-called "Groove mode" cavities, described in French Patent Application No. 77/13903 of 6th May 1977, could be used. Likewise, other dimensions of the alumina tubular element described above, and of its quartz support tube, can be chosen in order to modify the volume of the use chamber. In practice, it suffices to determine these dimensions and those of the cavity in order to achieve tuning of the unit to the frequency of the generator, for example to 2.45 GHz.

The installation also comprises, upstream and downstream, members for feeding and taking up the filament 10 to be treated, which members are not shown in Figure 1 by way of simplification.

By way of indication, this device makes it possible to achieve temperatures of between 800 and 1,500 Centigrade on the tubular element 8, and, with this equipment, a synthetic poyester textile multifilament, having a gauge of 1 50 deniers, comprising 46 strands and travelling at à speed of 700 metres per minute, was heated at 2.45 GHz by means of a temperature gradient of 1 ,0000C per second for a UHF power of 300 to 500 W.

A UHF and infrared radiation oven was thus constructed, of which the thermal stability is high and can be controlled, and of which the electrical over-voltage under load is low. The thermal efficiency is excellent and the treatment temperature can be high. This oven would advantageously replace a conventional texturising oven of five to ten times the length.

Figure 2 shows an installation for covering filaments with polymer, which comprises: a feed member 11 for the filament 10 to be treated, idler rollers 12, 13, 14, an impregnating tank 1 5 for receiving a solution of polymer, for example, through which tank the moving filament 10 passes, two applicators 4a, 4b, arranged in series, which are both produced in the same manner as in the example described for Figure 1, the first applicator being used to evaporate the solvent from the polymer solution present in the tank 15, and the second applicator making it possible to harden the coating or to polymerise it on the filament 10, and a take-up member 23 for the polymer-covered filament obtained after this treatment.

Figure 3 illustrates a device which is produced according to the invention and which makes it possible to subject the filament to two successive heat treatments, for example a treatment such as that described above. In accordance with this embodiment, the two cavities 4a, 4b, arranged in series, are excited by means of a single UHF generator 1, for example a generator with power of 1 KW, such as the generator marketed by the Société SNEA. This generator 1 is connected, according to the invention, to a waveguide 2 which then divides into two elementary waveguides 21 a, 21 b, which possess, at their junction, a flap 22 of adjustable orientation, the purpose of which is to channel and distribute the energy, as required, either into the guide 21 a or into the guide 21 b.

Installations such as those illustrated in Figures 2 and 3 are particularly suitable for covering filaments, for example for the production of polyvinylidene-covered silica filaments which can be used as optical fibres. It has thus been possible easily to achieve production speeds of 50 to 60 metres per minute and a power distribution of 300 Watts at 21 a and 400 Watts at 21 b.

Figures 4 to 11 illustrate various modified embodiments produced according to the invention, in accordance with which embodiments several treatment cavities 4 are fed by a single microwave generator 1, these cavities 4 being coupled to a waveguide fed by the said generator.

These embodiments are of great value by virtue of the fact that, at the present time, the manufacturers of ultra-high frequency generators construct modules having a power of 1 KW, which modules are inexpensive because they are mass-produced. However, this power is four to five times greater than the energy required for a single applicator. Furthermore, for certain applications, it can be advantageous to subject a filament to a plurality of successive heat treatments or to treat a web of filaments in parallel. The embodiments illustrated in Figure 4 to 11 make it possible to achieve these objectives.

In the first type of assembly illustrated in Figures 4 to 7, the applicators 4a, 4b are coupled to the large face of the waveguide, the axis of the cavities 4a, 4b being perpendicular to the axis of the excitation guide 2. In this embodiment, the coupling is produced in a known manner by means of a diaphragm.

Figure 4 illustrates an embodiment which makes it possible to treat two filaments simulataneously, although this does not of course imply a limitation, the cavities 4a, 4b being fed by a single source 1.

Figure 5 illustrates, in partial section, an embodiment which makes it possible, for example, to carry out a treatment on a filament moving at high speed. In this embodiement, the two cavities 4a, 4b are arranged in series, on either side of the excitation waveguide 2 which is fed by a single generator t. As in the example illustrated in Figure 4, the axis of the cavities is perpendicular to the axis of the waveguide 2.Of course, according to the invention, the cavities 4a, 4b comprise, in their central part, a tubular element 8 based on a material having UHF dielectric losses, Figures 6 and 7 illustrate modified embodiments in which the waveguides 2 are not rectilinear but have a configuration which is such that they make it possible to feed a plurality of resonator cavities 4 produced according to the invention, these resonator cavities being arranged in their mutual extension.

Figures 8 to 11 illustrate a second modified embodiment of an assembly.

According to this embodiment, the cavities 4 are coupled to the large face of the waveguide 2 in such a way that the axis of the said cavities lies in a plane parallel to the plane containing the axis of propagation of the waves in the guide 2.

These various cavities 4 are coupled by means of an antenna in the form of an open loop, such as that which is illustrated schematically in Figure 8.

The orientation of the cavities 4 is such that it favours the excitation of the desired heating mode.

If appropriate, as illustrated schematically in Figure 11, the applicators 4 can be orientated by any angle a in one and the same plane, provided, however, that this complies with the orientation of the antenna. Such a configuration makes it possible, if appropriate, to treat a web of filaments in parallel, with any spacing (see Figure 7), or, if appropriate, to subject one and the same filament to a plurality of successive treatments (see Figure 10).

These embodiments produced according to the invention make it possible, as stated above, to use inexpensive ultra-high frequency generators, and, furthermore, the fact that the overvoltage factors are relatively low for each applicator viratually eliminates the problems of mutual couplings between cavities. In fact, the frequency mistuning in one cavity has virtually no effect on the others and thus makes it possible to obtain different temperatures, if desired, on each element having dielectric losses (for example ceramic).

The above examples clearly show the advantages provided by the process and the device produced according to the invention.

Amongst all these advantages, the following may be mentioned in comparison with the solutions used hitherto: the possibility of precisely determining the temperature of the tubular heating element, and hence that of the filament, by virtue of the control of the power emitted by the generator, the high temperature gradient between the inside and the outside of this tubular element, and hence the concentration of the infrared heating inside this tubular element, the possibility of treating a moving filament both by infrared and UHF or by any combination of these heating means, since the infrared radiation permits a better coupling with the residual UHF field, because of the increase in temperature of the filament and hence the increase in its dielectric losses; moreover, there is the possibility of heating in several stages by using cavities arranged in series, it being furthermore possible for these cavities either to be adjusted differently or to be designed in such a way that, for example, the first cavity is equipped, according to the invention, with an element based on a material having dielectric losses, which makes it possible to preheat the filament by means of infrared, whilst the second cavity can be arranged so as to heat the filament directly by means of ultra-high frequency in a known manner, the possibility of treating non-polarfilaments, that is to say filaments which do not have dielectric losses, the possibility of creating a controlled atmosphere inside the heating element, of modifying the pressure (overpressure or vacuum) therein, or of sweeping it with a gas, and the possibility of using the quartz tube, seving as a support in the embodiment illustrated in Figure 1 , for injecting or collecting a complementary treatment gas.

Thus, this technique can be used successfully for the heat treatment of moving filaments. By way of indication, the following applications may be mentioned: the covering or coating of filaments, for example optical fibres, the thermofixing of filaments, for example during texturising, and the manufacture of carbon fibres.

Claims (13)

Claims

1. A process for the heat treatment of moving filaments which includes the steps of passing the filaments through at least one ultra-high frequency resonator cavity and damping the overvoltage coefficient of the cavity by increasing its losses, the lost damping energy being converted to infrared energy produced in the vicinity of the filaments being treated.

2. A process according to claim 1 ,wherein the electric field inside the ultra-high frequency resonator cavity is parallel to the moving filaments.

3. A process according to claim 1 or 2, wherein a body of a material having large ultra-high frequency dielectric losses is arranged in the cavity with its longitudinal axis parallel to the longitudinal axis of the cavity, the body having at least one longitudinal internal channel and the filaments passing through the channel without contacting the wall thereof.

4. A device for the heat treatment of moving filaments, such device including an ultra-high frequency resonator cavity, a microwave generator, a waveguide and a coupling component connected to said cavity, means to pass filaments through the cavity, and, within the cavity, converter means to damp the overvoltage coefficient of the cavity by increasing its losses and converting the lost damping energy to infrared energy in the vicinity of the filaments.

5. A device according to claim 4, wherein the converter means includes, inside the cavity, a body of a material having large ultra-high frequency dielectric losses, the body having at least one channel of which the axis is parallel to the longitudinal axis of the cavity and inside which channel the moving filaments pass without contacting the walls of the channel.

6. A device according to claim 5, wherein the body is made of ceramic, glass or a silica-based material.

7. A device according to claim 5 or 6, wherein the body is axially slidable on a support tube made of a material which does not conduct heat, which support tube is arranged along the longitudinal axis of the cavity.

8. A device according to claim 7, wherein the support tube is made of quartz.

9. A device according to any one of claims 4 to 8, wherein the resonator system includes a first waveguide which divides into two elementary waveguides, which are each connected to a respective resonator cavity, the two elementary waveguides having, at their junction, a flap of adjustable orientation, the purpose of which is to distribute the energy between the elementary waveguides.

10. A device according to any one of claims 4 to 8, wherein the microwave generator feeds a single waveguide on the large face of which a plurality of cavities are coupled by means of a diaphragm, the axes of the cavities being perpendicular to the waveguide.

11. A device according to any one of claims 4 to 8, wherein the microwave generator feeds a single waveguide on the large face of which circular cavities are coupled, the axes of the cavities lying in a plane parallel to the plane containing the axis of propagation of the'waves in the guide, and each cavity being coupled by means of an antenna in the form of an open loop, the orientation of which can be adjusted.

12. Processes for the heat treatment of moving filaments substantially as hereinbefore described with reference to the accomvanying drawings.

13. Devices for the heat treatment of moving filaments substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.