US3242248A - Process for the thermal treatment of thermoplastic fibres - Google Patents

Description

March 22, 19.66 P. TERRA 3,242,248

THERM Filed June 11, 1962 United States Patent O 3,242,248 PRCESS FUR THE THERMAL TREATMENT F THERMOPLASTIC FIBRES Pierre Terra, Lyon, France, assignor to Societe Rhodiaceta, Paris, France, a corporation of France Filed .Tune l1, 1962, Ser. No. 201,552 Claims priority, application France, .lune 13, 1961,

1 claim. (ci. 264-290) This invention relates to the thermal treatment of elongated thermoplastic articles such as threads, libres, tows, cables and films, and more particularly to such thermal treatment as a step in the stretching and shrinking of artificial and synthetic libres.

It is known to carry out such thermal treatments by sliding a thread over a fixed heating member having a plane or curved surface, in such manner as to ensure continuous contact between the thread and the heating surface. Plane, polyhedral, c g., prismatic or pyramidal, or curved, e.g., cylindrical or conical, heating surfaces have been employed. These heating surfaces may be in the form of pins, plates `or solid or hollow bodies heated by any known means, such as heating fluid, electrical resistance or electrical induction.

In order to improve the output of the installations, the speeds of travel are often high, and in order that the period of contact between the thread and the heating surface may not be too short, it has been found desirable to increase the length of this contact. F-or example, in the case of a cylindrical pin and a thread, this length of contact may be increased by so Winding the thread as to form a number of noncontiguous turns round the pin. However, it is obvious that if the number of turns is increased, the friction between the thread .and the pin also increases and the tension downstream of the pin may be come so excessive as to result in breakage, while the upstream tension becomes insufficient, which results in slack on the upstream side.

If, with a thread of given titre, the thermal treatment is satisfactorily performed with a given number of turns on -a pin of given diameter and given temperature, and if it is desired to effect the same operation under the same conditions with a thread of larger titre, it is found that a limit is set by the surface occupied by the thread, which spreads lout on the pin and will thus obviously occupy more space than a finer thread; the turns will then tend to overlap, which will result in breakage of the thread.

There are -a number of ways of avoiding this effect:

(a) The angle of the thread `and of the axis of the pin may be modified so as to increase the pitch between the turns, but this has the disadvantage of also increasing that component of the tension which is parallel to the axis of the pin and of thus favouring slipping of the thread, which thus tends to slide off the end of the pin.

(b) The diameter of the pin may be increased without modifying the angle between the thread and the pin axis. The pitch between the turns increases porportionally with the diameter and the thread is thus enabled to spread out to a greater extent. This increases the length of contact between the thread and the surface. In circumstances when a thread of fine titre is Wound in a number of turns or only in one turn, it is possible with a thread of larger titre to work with a reduced number of turns or to retain the single turn for, with the larger titre thread, it is diflicult to reduce the winding .angle without reducing the contact and Without lowering the heat transmission.

Some spreading-out of the thread is essential, If the thread does not spread out a temperature gradient is set up between the libres in contact with the heating surface and the outermost fibres, which results 4in breakage of the thread. This arises because, if the temperature of the fibres in contact with the surface is correct, the outer iiibres are at a lower temperature and therefore break, and if the temperature of the fibres in contact with the surface is increases so that the outer fibres are brought to the correct temperature, the libres in contact with the surface are -then at too high a temperature and break for that reason.

It is an object of the present invention to provide a new solution to the problem of thermal treatment of threads and the like materials which makes it possible to use larger surfaces on which threads of higher count can spread out, while avoiding excess friction and excessive or insufficient heat exchange.

According to the present invention, there is provided a process f-or the thermal treatment of threads and like lamentary materials of thermoplastic nature which cornprises causing the said threads and like materials to pass in rubbing contact over a fixed heating member which is so shaped that the thread and like material rubs only on certain parts of its surface, the parts with which the thread and like material makes contact being of the same nature and at the same temperature as those with which the thread and like material does not come into contact. Such a device makes it possible for the heat supplied by the heating member to transfer readily within the thread. It may be observed that it has been found that if the parts in contact with the thread are not of the same nature as the remainder of the fixed member, the heat transfer does not take place satisfactorily, especially if such contacting part-s -are not directly heated.

In this method of the invention where the whole surface is of the same material, i.e., the heating member is of homogeneous composition, an excellent transfer of the heat from the heating member to the thread is achieved.

A heating member for use in the invention may have a plurality of plane regular surfaces or may have alternate concave and convex surface portions (for example corrugations, grooves and iiutings), or the heating surface may be interrupted in any other geometrical pattern.

Preferably the surface is made up of `a plurality of convex surfaces with sufficiently large interruptions to avoid excessively sharp edges which might cause breakage or deformation of the thread.

Specifically, the heating member may be a cylindrical pin having grooves -or ribs on its surface lying substantially in the same direction as the longitudinal axis of the pm.

In order further to explain the nature of the invention reference will now be made to the accompanying drawings which relate to the use of a cylindrical pin, this being the commonest and simplest type of heating surface for use in this operation. However, it is to be understood that the improvements achieved by the process of the present invention are obtained also with other surface forms and, more especially, using plane surfaces, flat or curved plates, cylinders, cones, prisms, pyramids, polyhedra, and regular surfaces produced in the form of plates or solid or hollow bodies. In the following description reference will be made to threads, but the method of the invention is equally applicable to the treatment of toWs, cables or the like.

In the accompanying drawings, FIGURES l, 2 and 3 diagrammatically illustrate smooth heating pins of known type, and FIGURE 4 a heating pin for use according to the invention. FIGURES 5, 6, 7 and 8 diagrammatically illustrate the mathematical developments of the surfaces of the pins of FIGURES l, 2, 3 and 4, respectively.

Referring to FIGURES 1 and 5, there is shown a cylindrical heating pin. The pin may conveniently have a heating core, not shown. In a specific case the pin is mm. in diameter and heated to a surface temperature of C.; a SOO-denier polyvinyl alcohol thread is correctly stretched in a ratio of 7:1 between a train of rolls situated upstream of the pin and another train situated downstream, whose peripheral velocity is 7 times as high as that of the first. If T is the stretching tension, in grams per denier, and if a is the angle of arrival of the thread in relation to the axis of the pin, the component R of the tension parallel to the said axis tends to cause the thread to slip on the pin. If a is in the neighbourhood of R is small and the thread does not tend to leave the pin. If the titre of the thread is increased, while the same stretching conditions and the same angle a are retained, the threads commence to overlap, as shown in FIGURES 2 and 6, and the threads break at a titre of the order of 2000 deniers.

In order to avoid this breakage, the angle a may be reduced, i.e. a position similar to that of FIGURES 3 and 7 may be produced. However, R then increases and the thread slips and leaves the end of the pin.

If a pin of larger diameter is employed, it is possible to adopt an angle a closer to 90 and a Component R which will not produce the slip.

It is necessary that the pitch between the turns taken normally to the thread should be slightly greater than the Width of the thread. It is therefore desirable to increase the diameter with the width of the spread-out thread, but this increases the friction between the thread and the heating surface and produces breakage downstream and slack upstream.

According to the form of device shown in FIGURES 4 and 8, use yis made `of a channelled surface which has the advantage of oiering to the thread a smaller friction surface. By adjusting the width of the channels, the spacing between them, and their arrangement, it is possible in addition to adjust the heat exchange between the thread and the surface. Thus, using the form of device shown in FIGURE 4, using a cylindrical pin having a diameter of mm. and formed with parallel longitudinal channels of rectangular cross-section, of a width of 1 mm. and a height of 0.5 mm. and spaced apart by 4 mm., it is possible to stretch polyvinyl alcohol threads of up to 8400 deniers and more in a ratio of 7:1 at a heating surface temperature of C. without breakage. On a simple pin of cylindrical surface it is difficult to go beyond a titre of the order of 2000 deniers under similar stretching conditions without breakage occurring.

In the apparatus shown in FIGURE 4, channels of rectangular cross-section are used. However, channels of different forms may be adopted (triangular, polygonal, semi-circular, elliptical or other cross-section). They may be obtained by any appropriate means, e.g., by machining the surface.

It is to be noted that the present inventive concept applies to instances wherein the yarn to be heat-treated is comprised of at least a plurality of separate filaments and to instances wherein said yarn is looped around a draw pin to form a loop of at least in excess of It is under these two conditions that the aforementioned overlapping of threads is liable to occur since it is only under these conditions that the upstream side (i.e., the top half in each of FIGURES 5-8) will intersect with the downstream side (i.e., the bottom half in each of FIGURES 5-8) of the filaments. This intersecting Condition exists in FIGURES 2 and 6. In FIGURES l-4, the loops are shown as constituting substant-ially 360 arcs. In such instances, as is shown in FIGURE 6, the threads being drawn away from the pin pass between those threads being fed to the pin. In FIGURE 7 this condition has been overcome by winding the filaments around the pin in helical loops having such a helix angle that the axial distance (i.e., the pitch) between the beginning and the end of each loop is greater than the total axial distance occupied by all the filaments in the yarn (i.e., the width of the yarn). This means that the leftmost tilament in FIGURE 7 can be fed to the pin (top half of FIGURE 7) and then make a 360 loop and be drawn away from the pin (bottom half of FIGURE 7) without its passing between any of the other filaments in the top half of FIGURE 7, these all being the filaments which are being parallelly fed to the pin. In FIGURE 8 this same result is achieved with smaller helix angle but by providing a larger diameter pin since, the larged the diameter and the larger is the axial pitch.

In all cases, it is desirable to ensure that the heating is very regular and that no prohibitive temperature gradients are produced. The pins are preferably made of materials having good mechanical strength and thermal conduc tivity, eg., of metals or alloys such as copper, brass, bronze and steel.

I claim:

A process for uniformly heat-drawing continuous thermoplastic synthetic yarns which are composed of a plurality of separate filaments parallelly spaced alongside each other, said process comprising the steps of:

(a) providing a stationary draw element adapted to have said laments looped around the outer periphery thereof, said element having on its periphery a series of Contact faces spaced apart from each other,

(b) looping said plurality of laments transversely around said element so as to form respective helical loops contacting said faces, said loops circumscribing an arc of greater than 180 around said element, and with said helical loops having a helix angle which provides a pitch spacing between the beginning and the ending of each loop greater than the yarn width which is defined by said plurality of filaments,

(c) uniformly heating said faces,

(d) drawing said lilaments across said faces.

References Cited by the Examiner UNITED STATES PATENTS 2,244,281 6/1941 Alibert 18-8 2,778,058 l/1957 Gabler 18k8 2,948,583 8/1960 Adams et al. 264-210 2,975,474 3/1961 Smith 18-8 ALEXANDER H. BRODMERKEL,

Primary Examiner. WILLIAM I. STEPHENSON, MORRIS LIEBMAN, Examiners.

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