Yarn Texturing

This invention relates to the texturing of yarns by false twist crimping. According to the present invention there is provided a texturing process in which a yarn advances through one or more heat transfer zones to a false twisting means wherein heat is transferred to and/or from the yarn by a transverse fluid flow which also supports the yarn in a curved path.

The preferred fluid is air though other gaseous mixtures or single gases may be employed including steam.

In particular the yarn may be cooled by a transverse fluid flow.

The curved path may comprise an arc so that the yarn direction after the curved path is different from the yarn direction before the curved path. Alternatively the curved path may conveniently take the form of a 360.degree. loop so that the yarn leaves the curved path travelling in the same direction as that in which it entered the curved path. Depending upon the relative positions of heating and/or false twisting means other shapes of curved path may also be used such as, for example, an arc in one direction of curvature followed by an arc in another direction of curvature, or a succession of such alternating arcs providing other 2 or 3-dimensional sinuous yarn paths.

If the logarithm of the difference between the yarn temperature and the ambient air temperature at a point in the cooling zone is plotted against the distance along the yarn path from the end of the heating zone, then a roughly straight line is usually obtained with a slope of - k/v where v is the yarn velocity and k is the cooling rate. It is found that in a process according to the invention the cooling rate is greater and more consistent between different yarns, and that the yarns are more stable and yield a more uniform textured yarn product than in processes in which no air guide is used to assist in yarn cooling before the false twisting means.

The following examples illustrate but do not limit the present invention.

A 250 decitex 45 filament polyester yarn derived from polyethylene terephthalate was passed at 440 meters per minute over a 1.8 meter flat plate with a surface temperature of 250.degree.C to a false twister, and the following tables (I and II) gives some results from 16 experiments under different cooling conditions. Experiments 1-4 and 13-16 are examples of the invention, while 5-12 are comparative examples. In each of the examples of the invention a circular air guide was used providing a yarn path loop of 360.degree. at a diameter of about 9.5 cm in a continuous slot 0.0038 cm wide. An air flow of 220 liters per minute was provided at the slot and this was sufficient the cushion the yarn in its circular path so that it did not come into contact with any solid guiding surfaces.

In example 5 an unheated aluminium cooling plte 0.97 meters long was used to stabilise and support the yarn. In examples 6, 7 and 8 no such stabilising plate was used and the yarn vibrated between the heater and the false twister. Under these vibrating conditions the cooling rate in ambient air was higher but less consistent. In examples 1-5 the cooling rate in the air guide was both very high and very consistent, and the yarn was quite stable so the cooling rate in the ambient air before and after the guide, unassisted by a cooling plate, was quite low. It is common to use a second heater to stabilise the yarn after crimping. Table II shows that the resultant yarn shrinkage is much less dependent on primary and secondary heater temperatures when there is efficient cooling by an air guide before the false twister.

Using the same false twist crimping process described in the previous examples, the circular air guide was replaced by a linear air guide comprising a continuous slot 0.018 cm wide, 3.5 om deep and 20 cm long. Ceramic guides had to be used before and after this linear device in order to keep the yarn in the slot while deflected by the air into an arcuate path. This device was broadly similar in its cooling effect to that of the circular air guide, as illustrated by Table III.

In examples 18 and 19 the air guide used was similar to that described in example 17, but in example 20 the guide was V-shaped in cross-section. This guide had a continuous slot 0.008 inches wide at its base which diverged at an angle of 10.degree. for 3.5 cm. All three examples were of draw texturing processes.

It has also been found that similar types of air guides may be very advantageously used for heating the yarn before the false twister.

It is known that as yarn speed is increased longer heating paths are required to maintain the same twist setting time. Also, it is known that for efficient twist setting it is necessary for all or virtually all of the twist to run back right through the heating zone so twist snubbing must be avoided in the heating zone. Thus it has been proposed to reverse the direction of yarn travel across a heater so that it can pass back and forth and by using cushions of air to support the yarn at the reversal points instead of solid surfaces or idler rolls, there is virtually no twist snubbing action.

However in the present invention the need for long heaters and for yarn reversals between passes over heaters is avoided.

Thus, the present invention provides a process for false twist crimping an advancing yarn by twisting, heating, cooling and detwisting in which the yarn is heated by fluid flowing transversely thereto and supporting it in a curved path. The heat transfer between fluid and yarn under these circumstances is high and twist snubbing action is negligible.

The guides of this invention may be used on their own to heat the yarn or alternatively may be used in conjunction with conventional heaters, e.g. flat plate heaters. Where appropriate, hot air issuing from the guides may be recirculated.

The experiments listed in Table V relate to draw texturing a 520 decitex 30 filament polyester yarn derived from polyethylene terephthalate of birefringence 6.2 .times. 10.sup.-.sup.3 at a take off speed of 440 meters per minute. The draw ratio in each case was 3.16 and the apparatus comprised feed roll, heater, friction bush false twister and draw roll. In the Table experiments A to F are examples of the invention while experiments G to H are control experiments using conventional flat plate heaters.

Preferably the yarn is both heated and cooled while being supported in curved paths by fluids flowing transversely to the yarn.