Heater For Use In The Manufacture Of Plastics Filaments

Abstract

A heater for use in the manufacture of spun plastics filaments which compes two portions each having a truncated right polygonal pyramidal or truncated right conical internal surface that is open ended, the internal surface of one of the portions is heated while the internal surface of the other portion is thermally reflective and at the smaller end of the heated portion a screen is arranged to reduce the effective area of the said smaller opening. With the use of the heater spun filaments can be produced having a low degree of preorientation and a satisfactory uniformity.

Description

The present invention relates to a heater for use in the manufacture of macromolecular plastics filaments having a low degree of pre-orientation for the production of high strength threads.

High strength threads can only be produced when the material spun to make the threads is drawn to a very high degree. A high degree of drawing can only be obtained, however, with material which, when spun, has a low pre-orientation. In the manufacture of high strength threads, as used, for example, for tire cord, the spinning process has to be carried out in such a manner that the spun filaments should have as low a preorientation as possible. A further problem arises in achieving the necessary uniformity, because the filaments can only be drawn to a high degree when, in the spinning process, each of the many capillaries has been uniformly treated. Irregularities which may, for example, occur by unsuitable cooling cannot be remedied by further processing and detrimentally affect the quality of the finished thread.

It has been discovered that the pre-orientation of the filaments during spinning can be reduced when a heating zone is provided below the spinneret to retard cooling of the filaments. British Pat. No. 580,832 describes a process and a device for heating freshly spun filaments produced by the dry spinning process. The spun filament is drawn off in a direction parallel to the axis of a tube having a vertical axis and heated by means of horizontal radiation. In one embodiment the tube is ellitical in cross section and the inner wall of the tube has good reflecting properties. To achieve good focusing of the rays, the source of heat radiation is situated at one principal focus of the ellipse while the filament is at the other principal focus. It is stated that the heating of the spun filaments thus obtained proved to be advantageous also in melt spinning polyamides. In this manner the filaments are maintained in a plastic or semi-plastic state so that drawing is facilitated. The device described in the aforesaid patent is, however, very large and difficult to handle.

According to German Offenlegungsschrift No. 1,435,512 a long, heated, cylindrical or rectangular tube serves as heating zone which surrounds for a considerable distance the freshly spun filaments below the spinneret. In this specification the minimum temperature T.sub.G around the filaments is given by the equation

T.sub.G = T.sub.D - 9X + 30

where:

T.sub.D stands for the temperature of the spinneret

X = 10.sup.4 . D/F .sup.. V.sub.sp

D represents the distance in feet from the spinneret

F indicates the denier value of the filaments

V.sub.sp is the winding speed in feet per second and

T.sub.G .ltoreq. T.sub.D + 100.degree. C

In this process the degree of pre-orientation could be kept low in multifilament yarns.

French Pat. No. 1,347,986 provides a process according to which the freshly spun polyester or polyamide filaments pass through a cylindrical heated tube the gas temperature around the filaments being determined according to the following condition

0.001 .ltoreq..sub.-.sub.X.sup.Y .ltoreq.0.08

where:

Y = (T.sub.GO - T.sub.G)/T.sub.D

X = 10.sup.4 . D/V.sub.sp .sup.. F

T.sub.GO = gas temperature directly on the spinneret in .degree. C with 270.degree. C.ltoreq.T.sub.GO .ltoreq.700.degree. C

T.sub.G = gas temperature in .degree. C at distance L vertically below the spinneret

T.sub.D = temperature of spinneret in .degree. C

D = distance from the spinneret in meters

V.sub.sp = draw-off speed of the spun filaments in m/sec

After having left the cylinder the filaments are rapidly cooled by a horizontal air current. Subsequently, they are treated with a preparation, hot steam is blown on to them for warming, they are drawn and wound off.

In the known processes heating devices are used which heat the filaments over a long distance after they have left the spinneret. In this manner cooling and solidification of the filaments takes place very slowly so that their pre-orientation is reduced. It has been found, however, that this method does not give filaments of optimum quality. The regularity of the filaments obtained is not satisfactory and the blowing step carried out after the filaments have passed the radiator involves sticking together of the capillaries because with the construction of the heating radiator used the blowing air gets into the heating zone where it whirls together the still plastic filaments. A further drawback resulting from the irregularity is the reduction of the tensile strength.

It is an object of the present invention to provide a heater which permits the production of spun filaments having a low degree of pre-orientation and having satisfactory uniformity, which filaments are suitable for the manufacture of threads of high tensile strength.

The invention provides a heater for use in the manufacture of spun plastics filaments which comprises two portions each of which has a truncated right polygonal pyramidal or truncated right conical internal surface that is open ended, the perimeters of the larger ends of the internal surfaces being congruent and the two portions meeting at their larger ends with the perimeters of the larger ends of the internal surfaces in register with one another, wherein the internal surface of one of the portions is heated and the internal surface of the other portion is thermally reflective.

Advantageously, there is provided at the smaller end of the said one portion a screen arranged to reduce the effective area of the said smaller opening. The internal surface of each of the said portions is, preferably, a truncated right conical surface.

One form of heater constructed in accordance with the invention will now be described in detail by way of example with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of the heater;

FIG. 2 is an axial cross-section of the heater shown in FIG. 1; and

FIG. 3 illustrates the variation in temperature along the axis of the heater.

Referring to the accompanying drawings, a heater comprises two parts 1 and 2, each of which has the shape of a hollow truncated cone, which are attached to each other at their larger circular openings. The lower part 2 is heated while the inside wall of the upper part 1 reflects the heat emitted by the lower part. Hence, part 1 has the function of a reflector while part 2 has the function of a radiator. The lower opening 3 of the heater is protected by an annular screen 4 preventing air blown onto filaments after they have left the heater entering the space below the spinneret and disturbing the course of the capillaries while they are still plastic. The diameter d.sub.1 of the opening in the screen is larger by only 5 to 30 millimeters than the diameter d.sub.F of a bundle of capillaries, that is to say,

d.sub.F + 5.ltoreq.d.sub.1 .ltoreq.d.sub.F + 30

The diameter d.sub.4 of the upper opening of upper part 1 is larger than the diameter of the bundle of capillaries. The height L of the radiator is in the range of from 1.0 to 2.5 times the diameter d.sub.4 of the opening of the upper part. That is to say:

1.0 d.sub.4 .ltoreq. L .ltoreq. 2.5 d.sub.4

Between the bundle of capillaries and the walls of the heater air streams upward and replaces the air entrained by the bundle of capillaries. The cross sectional area of flow should be as large as possible so that the compensation of air can take place at a low speed. The largest cross sectional area of the heater should be at least twice the cross sectional area of the bundle of capillaries, that is to say,

d.sub.3.sup.2 >2 d.sub.F.sup.2

The side line H of the heated part 2 of the heater and the cone angle .phi. should be chosen in such a manner that the said perpendicular 5 drawn on the wall of part 2 points to the opposite wall of part 1; and

arcsin H/2d.sub.3 <.phi./2 < arcsin (H/2.cuberoot.(d.sub.3 + d.sub.4 /2).sup.2 + L.sub.1.sup.2) + arccot d.sub.3 + d.sub.4 /2 L.sub.1

The biconical shape of the heating radiator allows concentric thermal radiation. The reason why this shape was chosen is that only a minor part of the thermal rays hits the spinneret while the major part is reflected by the conical reflector into the space below the spinneret. In a preferred embodiment of the radiator according to the invention the reflector 1 is, therefore, provided with a highly polished surface or coated with a reflecting foil. The heating elements of radiator 2 preferably consist of ceramic plates with inserted heating spirals.

The device according to the invention can be used in melt spinning, preferably spinning of high molecular weight polyesters, more preferably polyethylene terephthalate, and copolyesters, the acid components of which preponderantly consist of terephthalic acid. With the use of the radiator of the invention spun filaments of a very low degree of pre-orientation can be obtained, which permit the production of high strength threads. It is likewise possible to increase the throughput of molten polyester since the higher pre-orientation resulting from a higher draw-off speed of the spun filaments can be compensated for in the heater. The device according to the invention is also suitable for the continuous spin drawing of filaments from highly viscous material.

With the aid of the short biconical heating radiator according to the invention a narrow temperature variation with respect to time and space as indicated in FIG. 3 can be obtained below the spinneret in the solidification zone of the filaments, whereby the solidification is very favourably influenced. The temperature is within the indicated limits

110 - 1.7 .times. 10.sup.3 .vertline.D/L -0.4 .vertline..sup.3 <T.sub.G -T.sub.D <125 - 2 .times. 10.sup.3 (D/L - 0.5).sup.4

at a distance 0 < D/L.ltoreq.1

where:

L = height of the heating radiator, measured in the same units of length as the vertical distance D from the spinneret

T.sub.G = the gas (air) temperature and

T.sub.D = the temperature of the spinneret

In the direct vicinity of the spun filaments from the spinneret in downward direction the air temperature first increases, it passes a maximum and then decreases rapidly with a growing distance from the spinneret. Owing to the fact that a screen narrows the lower opening of the heating radiator to such an extent that it has just the size necessary for an undisturbed running of the filaments, the air blown onto the filaments for cooling them does not enter the space below the spinneret, this being extremely important for obtaining an optimum filament quality. The still soft and very sensitive capillaries could otherwise be entangled by air whirls so that they would stick together and uniform cooling would be impossible.

Moreover, a heater as described above permits a higher draw-off speed of the spun filaments whereby the production rate can be considerably increased. As compared with known heating devices used in the manufacture of filaments, the short biconical heating radiator according to the invention is not only more effective but also considerably smaller and thus more handy and easier to use.

The following examples illustrate the invention.

EXAMPLE 1

Polyethylene terephthalate having an intrinsic viscosity of 1.23, measured at 25.degree. C in a mixture of phenol and tetrachloroethane in a ratio of 3 : 2, was spun at 304.degree. C at a rate of 220 g/minute through a spinneret with 200 orifices each having a diameter of 0.5 mm and the filaments were wound off with a speed of 320 m/minute. Directly below the spinneret a biconical heater was mounted having the following dimensions:

d.sub.1 = 140 mm, d.sub.2 = 170 mm, d.sub.3 = 225 mm, d.sub.4 = 135 mm, H = 70 mm, L = 150 mm, L.sub.1 = 85.6 mm, .phi. = 46.degree.

It had an installed filament power of 2,000 watts at 220 volts and was operated with 150 volts. Immediately after having left the heater, the filaments passed an air blowing zone having a length of about 2 meters with a blowing speed of the air of 0.8 m/sec.

As measurement for the molecule orientation the spun filaments produced in this manner have a double refraction DR of (1.6 . . . 2.0). 10.sup..sup.-3, measured according to the compensation method of Ehringhous with quartz or calcite compensators. The DR value is calculated from the ratio of the path difference and the capillary diameter.

The filaments had a very good uniformity over their length. For the variation of titer of the spun filament composed of 200 capillaries there is given as measurement of non-uniformity U the average linear deviation of the titer T from the means titer value T: ##SPC1##

For this purpose the titer T is measured as a function of the filament length 1. L is the total filament length measured. The mean titer value is ##SPC2##

With the use of the heating radiator according to the invention filaments were produced having a titer non-uniformity U of 0.8%. The spun filaments did not stick together.

The spun filaments obtained in this manner could be drawn in a ratio of 1 : 6.5, their strength then being 82 g/tex.

COMPARATIVE EXAMPLE

In an analogous spinning process a known cylindrical heater was used. The heater had a diameter of 205 millimeters, a length of 875 millimeters and a wall temperature of 245.degree. C. Into the upper part of the heating tube air having a temperature of 245.degree. C was concentrically blown in through an annular slit. The amount of air could only be increased to 80 litres per hour, as otherwise a large degree of sticking of the capillaries did occur.

Double refraction DR = (1.6 . . . 2.0).10.sup..sup.-3 uniformity of titer U = 1.6% possible draw ratio 1 : 6.4 strength obtained 79 g/tex

EXAMPLE 2

A heater according to the invention was used in the spinning process of high strength filaments from a material having an intrinsic viscosity of 0.73. In this case it can be used either for reducing the molecule orientation with the same draw-off speed of the spun filaments or for maintaining the degree of molecule orientation with an increased draw-off speed. The present example is intended to illustrate the former possibility. The positions of the heater and spinning chamber were the same as in Example 1.

Spinning temperature 290.degree.C Spinning rate 325 grams/minute number of orifices in spinneret 200 diameter of orifice 0.35 millimeter winding speed 500 meters/minute heating radiator (2,000 watts with 220 volts) 150 volts blowing length 2 meters speed of air current 0.8 meter/second non-uniformity of titer 0.7% double refraction 1.2-10.sup..sup.-3

COMPARATIVE EXAMPLE

The same material as used in Example 2 was spun under the conditions of Example 2, with the exception that no heating radiator according to the invention was used. The filaments obtained had the same degree of titer non-uniformity but a higher degree of pre-orientation characterized by a double refraction of 1.6 .sup.. 10.sup..sup.-3.

Claims

What is claimed is:

1. A heater for use in the manufacture of spun plastics filaments which comprises two portions each of which has a truncated right polygonal pyramidal or truncated right conical internal surface that is open ended, the perimeters of the larger ends of the internal surfaces being congruent and the two portions meeting at their larger ends with the perimeters of the larger ends of the internal surfaces in register with one another, wherein the internal surface of one of the portions is heated and the internal surface of the other portion is thermally reflective.

2. A heater as claimed in claim 1, wherein there is provided at the smaller end of the said one portion a screen arranged to reduce the effective area of the said smaller opening.

3. A heater as claimed in claim 1, wherein the internal surface of each of the said portions is a truncated right conical surface.

4. A heater as claimed in claim 1, wherein the largest cross sectional area is at least twice the cross sectional area of the bundle of filaments.

5. A heater as claimed in claim 1, wherein the diameter d.sub.4 of the smaller opening in the said other portion is in the range of from 0.4 to 1.0 times the total height L, of the heater.

6. A heater as claimed in claim 1, wherein the length H of the internal surface of the said one portion and the apex angle .phi. are such that a perpendicular drawn from the said point of the internal surface of the said one portion meets the internal surface of the said other portion, and

arcsin H/2d.sub.3 < .phi./2 <arcsin (H/2.cuberoot. (d.sub.3 + d.sub.4 /2).sup. 2 + L.sub.1.sup.2) + arccot d.sub.3 +d.sub.4 /2L.sub.1

where:

L.sub.1 is the total height of the heater,

d.sub.3 is the largest diameter of the heater taken at right angles to the axis, and

d.sub.4 is the diameter of the smaller opening in the said other portion.

7. A heater as claimed in claim 1, wherein the internal surface of the said other portion is highly polished or coated with a reflective foil.