US2346208A - Treatment of high tenacity yarn of synthetic origin - Google Patents

Description

Patented Apr. 1 1, h h

, UNITED-STATE TREATMENT or HIGH Tammi YARN or H srN'rnE'rIcomGm Rollin F. Conaway, WilmingtomDeL, assignor to E. I.- du Pont de Nemours & Company, Wilmington, DeL, a. corporation of Delaware No Drawing. Application April'8,1941, Serial No. 387,549 r 6 Claims.

This invention relates to improvements in filaments, yarns, threads and ribbons of artificial thermoplastic materials. More particularly, it relates to a method for increasing the elongation and reducing the brittleness of filaments, yarns, threads, and ribbons of cellulose derivatives, the tenacity-of which has been increased by stretching, and to the products of the method.

It has long been known in the textile field that the tenacity or tensile strength of filaments of artificial thermoplastic substances is increased by subjecting the filaments to a stretching operation. The increase in tenacity through stretching, however, is attended by a decrease in elongation. This is especially true in the case of cellulose acetate yam, the tenacity of which has been enhanced by the usual solvent swelling methods, or by the known thermal procedures for stretching. These yarns may have an elongation as low as 2-3%. High tenacity cellulose acetate yarns possessing such low elongations have a tendency to be brittle and give difficulty in manufacturing operations throughout the textile processes, and on this account the utility of stretched yarns of high tenacity has heretofore been seriously limited.

It is an object of this invention to produce high tenacity filaments. yarns, threads, and ribbons of artificial thermoplastic substances, which filaments, etc., in addition to high tenacity, are characterized chiefly by improved elongation and softness. A further object is the provision of a method for increasing the elongation of filaments, yarns, threads, and ribbons of artificial thermoplastic materials, the tenacity of which has been increased by a stretching operation, A still further object is to substantially increase the elongation of stretched, high tenacity cellulose acetate yarn, and to provide a high tenacity cellulose acetate yarn of increased elongation. These and other objects will more clearly appear hereinafter.

These objects are accomplished by the following invention which, broadly stated, comprises bringing in a substantially relaxed condition, a

' moisture-free, high tenacity yarn of an artificial thermoplastic material, for example, cellulose acetate, to its softening temperature and cooling the yarn immediately thereafter.

By "high tenacity yarn" is meant a yarn having a tensile strength of at least 2.0 g./den. By moisture-free" is meant that the yarn may have its natural amount of moisture but that any addit onal wetting must be avoided, By substantially relaxed condition is meant that, during the heating treatment, the yarn is either under,

no tension at all, or under a slight tension sufficient to maintain it substantially taut, but not sufiicient to stretch the yarn appreciably.

As usually practiced, the process consists primarily inpassing high tenacity yarn uniformly through a suitable heating chamber under a slight but positive tension at such a speed that the yarn is heated to its softening temperature during its exposure in the heating chamber. The "softening temperature need not necessarily be exactly that temperature at which a sample of the yarn visibly softens when heated.- Precise determination of the softening point is at best'a difflcult matter with the thermoplastic materials used in this process. For this reason it is intended that the expression softening temperature" be taken to mean a temperature between the visible softening point and 15 C. below it. In the case of cellulose acetate, this would be approximately to 205 C., the upper limit (softening point) varying somewhat depending upon the amount of combined acetic acid and extent to which the yarn has been previously stretched.

The process in its preferred form is carried out in a continuous manner in which high tenacity yarn of, for example, cellulose acetate which has been stretched sufflciently to increase its tenacity to at least 2.0 g./den., and which is substantially moisture-free, is passed continuously from a positively-driven bobbin through a suitable heating chamber at such a speed that the temperature of the yarn is increased up to, or at any rate within 15 C. of, the softening temperature of the yarn and withdrawn from the chamber continuously, such as by means of a positively-driven bobbin operated at the same speed as the feed bobbin. The heating chamber may be either an air chamber maintained at a constant temperature by conduction heat, such as an oven, or it may be an air chamber containing heating elements which are maintained at such a temperature that the yarn is heated to a considerable extent by radiation. The latter type of heating chamber is to be preferred for the reason that yarns with higher elongations are obtained when the yarns are heated by radiation rather than by conduction.

' than'one gram total tension. At higher tensions such, as 4-5 grams, particularly with yarns in the order of 50-100 denier, no substantial increase in the elongation is obtained. Furthermore, the

tensionon the yarn must be controlled accurate-.

ly since the yarn is suificiently plastic that variations in tension caus diflerences inishrinking or stretching and result in the production of yarn having non-uniform denier characteristics; When the speed at which the yarn is removed from the heating chamber is not so great as the speed of the yarn entering. the chamber, the yarn shrinks during the relaxation treatment with a corresponding increase in denier. When the feed and wind-up speeds are th same, no change in denier of the yarn occurs; and when the wind-upspeed is slightly faster than the feed speed the yarn is stretched slightly by the treatment with a corresponding decrease in denier.

The temperature of the heating chamber must be sufficiently high that the yarn is heated to its softening temperature (as defined above) during the length of time it is in contact with the heating chamber. Ordinary cellulose acetate yarn containing 54.5% of combined acetic acid, for example, softens sufficiently under staticconditions at 205 C. However, when cellulose acetate yarn is passed continuously through a relatively short heating chamber, the temperature of the chamber must be maintained above 205 C. in order for the yarn to be heated to its softening temperature during its time of contact with the heated zone. In other words, in a heating unit of a given length it is possible to maintain the temperature of the unit above the softening temperature of the yarn, such as at 225 C. for cellulose acetate, and pass the yarn through the heating chamber at such a speed that the yarn is only heated to its softenin temperature. At faster speed the yarn does not reach its softening temperature, and at slower speeds the yarn melts in the stretching chamber. While it is possible that, with a suificiently long heating chamber, its temperature be maintained below the softening point of the yarn (though not more than 15 C. below it) with satisfactory results, in practice the temperature of the heating chamber should be above the softening point of the yarn.

It is essential in the process of this invention that the yarn be cooled. i. e. removed from the heating chamber, immediately, or as soon as possible, after it has reached its softening temperature. The reason for this is that, when the softening temperature is reached, the yarn is so plastic that continued standing at that temperature, even for a very short period, would cause deformation, and any increase in temperature above the softening temperature would cause the yarn to melt. It is therefore obvious that the speed of the yarn and the temperature of the chamber must be adjusted in relation to one another, if necessary by preliminary experiments, so that the required balance is attained.

The softening temperature under static conditions can be determined by suspending a piece of high tenacity cellulose acetate yarn in the heating chamber and gradually increasing the temperature of the chamber until the yarn undergoes spontaneous shrinking. The minimum temperature at which the dry high tenacity thermoplastic yarn shrinks spontaneously may be regarded as the softening temperature. Under operating conditions where the yarn is passed through the heating chamber continuously the softening temperature can be determined by decreasing the wind-up speed so that it is slightly slower than the feed speed. If the yarn is being heated to its softening temperature it will remain taut, due to shrinkage, whereas excess yarn will collect between the two rolls if the yarn is not being heated to its softening temperature.

The resulting treated yarns are washed to remove sizes that may have been added to the yarn, dried, twisted, and processed in the same manner as regular textile fibers.

The following examples further illustrate the invention.

Example I This example illustrates the relaxation of hiBh tenacity cellulose acetate yarn by heating in conduction-type heaters.

A 61 denier-100 filament cellulose acetate yarn possessing a tenacity of 2.7 g./den., an elongation of 6%, and a knot strength (which is a measure of brittleness) of 0.73 g./den., obtained by stretching 300 denier-100 filament cellulose acetate yarn approximately 500%, was passed through a heated tube 9 mm. in diameter and 18 inches in length maintained at a temperature of 202 C. at a feed speed of 15 ft./min. under a tension just sufilcient to keep the yarn taut during its passage through the heater. The resulting yarn possessed a denier of 65, a tenacity of 2.65 g./den. and an elongation of 9%. When the temperature of the heating chamber was increased to 207 C. with all other conditions remaining the same, the resulting treated yarn possessed a denier of 72, a tenacity of 2.5 g./den.. an elongation of 11%, and a knot strength of 1.3 g./den. Under the latter conditions of treatment the elongation and knot strength of the yarn were nearly doubled, whereas the tensile strength was only reduced from 2.7 g./den. to 2.5 g./den.

Example II This example illustrates the relaxation of high tenacity cellulose acetate yarns by means of radiant heat.

A 50 denier-100 filament cellulose acetate yarn possessing a tenacity of 3.0 g./den. and an elongation of 4%, obtained by stretching heat-softened 300 denier-100 filament cellulose acetate yarn 600%, was passed through a radiation heater approximately 24 inches in length and of the type described in copending application Serial No. 387,552, filed April 8, 1941, maintained at a temperature of approximately 240 C. at a yarn speed of 100 ft./min. and under sufficient tension (less than one gram) to stretch the yarn slight- 1y. The resulting 45 denier yarn possessed a tenacity of 3.2 g./den. and an elongation of 7%. In another run the yarn speed was reduced to 60 ft./min., and the tension on the yarn reduced by having the feed and windup speeds the same so that the yarn was not permitted to stretch or shrink during the treatment, although the temperature of the heating chamber was the same. The resulting 50 denier- 100 filament cellulose acetate yarn possessed a tenacity of 3.0 g./den. and an elongation of 20%.

These runs show that it is possible to increase the elongation of high tenacity cellulose acetate yarn many-fold without substantial loss in tenacity by subjecting the yarn to a thermal treatment under a tension of less than one gram.

Example I]! This example illustrates the relaxation of a piled high tenacity cellulose acetate yarn by means of a heating until that heats the yarn partly by radiation and partly by conduction.

A 100 denier-200 filament high tenacity cellulose acetate yarn, obtained by plying two ends of 50 denier-100 filament stretched cellulose acetate yarn possessing a tenacity of 2.9 g,/den, and an elongation of 6%, was passed through a combination radiation-conduction heater of the type described in application Serial No. 387,552, filed April 8, 1941, maintained at a temperature of approximately 240 C. at a speed of ft./min. and a total yarn tension of about 0.5 gram. The resulting treated denier yarn possessed a tenacity of 2.9 g./den. and an elongation of 10%.

Example IV This example illustrates the relaxation of a plied high tenacity cellulose acetate yarn coated with an anti-sticking finish to prevent sticking of the individual filaments during the thermal treatment.

A 90 denier-300 filament cellulose acetate yarn possessing a tenacity of 3.0 g./den. and an elongation of obtained by plying three ends of previously stretched 30 denier-100 filament cellulose acetate yarn was passed-through a radiation heater at a speed of 100 ft./min., temperature of approximately 240 0., and a total yarn tension of approximately one gram. The yarn had been sized with an aqueous solution, described in application Serial No. 387,551, filed April 8, 1941, containing 2 parts of saponin and 4 parts of sodium chloride, and dried before entering the heating chamber. The resulting relaxed yarn possessed a tenacity of 3.0 g./den., an elongation of 9%, and a soft, pleasant hand indicating the absence of stuck filaments. In most cases, the surfaces of the individual filaments become sufficiently soft during the thermal relaxation treatment that a slight sticking of filaments is obtained which results in a yarn with harsh properties. The application of the aqueous salt solution containing a water-soluble colloid eliminates the sticking of filaments dur-' ing the thermal treatment.

Example V This example illustrates a continuous process for stretching and relaxing cellulose acetate yarn.

A 300 denier-100 filament cellulose acetate yarn was coated with an aqueous solution containing 2 parts of saponin, 6 parts of sodium chloride and 92 parts of water, and passed continuously in a moist condition into a molten solder (50:50 leadztin) bath at atmospheric pressure maintained at a temperature of 240 C. The yarn entered the bath at a linear speed of 33 ft./min. under a total yarn tension of 8 grams, and was removed from the bath and passed around a positively-driven roll at a speed of 200 ft./min. The yarn was removed continuously from this roll and passed through a heating chamber 24 inches in length maintained at a temperature of approximately 254 C. by means of radiation and conduction heat under a tension of approximately 0.5 gram, and wound on a second positively-driven roll at a speed of 200 ft./min.

The 50 denier-100 filament yarn resulting from the initial stretching operation possessed a tenacity of 3.0 g./den., an elongation of 4%, and a knot strength of 1.4 g./den. The washed 50 denier- 100 filament yarn resulting from the secondary relaxation treatment possessed a tenacity of 3.0

g./den., an elongation of 8%, a knot strength of 2.0 g,/den., and a soft, pleasant hand indicating the absence of stuck filaments. This continuous relaxation treatment doubled the elongation of the initial stretched yarn and increased the knot strength from 1.4 g./den. to 2.0 g./den. without loss of tensile strength.

Example VI This example illustrates the relaxation of high tenacity cellulose acetate propionate yarn by means of a combination radiation-conduction heater.

A 30 denier-38 filament cellulose acetate propionate yarn, containing 2.5% of combined proplonic acid and 52% of combined acetic acid and possessing a tenacity of 2.5 g./den. and an elongation of 4%, was sized with a 2% saponin-6% sodium chloride aqueous sizing solution, dried. and passed through a heating chamber 24 inches in length maintained at a temperature of 250 0., at a speed of 200 ft./min. under a tension of approximately 0.5 gram. The yarn was heated in the heating chamber partly by radiation and partly by conduction. The resulting purified 30 denier yarn possessed a tenacity of 2.5 g./den., an elongation of 8%, and a soft, pleasant hand indicating the absence of stuck filaments.

Example VII This example illustrates the relaxation of high tenacity cellulose nitrate yarn by means of a thermal treatment in which the yarn is heated partly by radiation and partly by conduction.

A 105 denier-38 filament cellulose nitrate yarn, containing 11.06% nitrogen, which had been stretched previously 600% was coated with an aqueous solution containing 2% saponin and 4% sodium chloride, dried, and passed continuously through a heating chamber approximately 24 inches in length and maintained at a temperature of 190 C. at a linear speed of 100 ft./min. The tension on the yarn during the thermal treatment was approximately 0.5 gram. The yarn was heated in the chamber partly by radiation and partly by conduction. The original yarn possessed a tenacity of 2.6 g./den. and an elongation of 3%. whereas the relaxed yarn possessed a tenacity of 2.5 g./den. and an elongation of 8%. The relaxed yarn also possessed a soft, pleasant hand indicating the absence of stuck filaments.

It is understood that the above examples are for purposes of illustration only, and that the invention is not limited to the exact materials and conditions recited therein but is susceptible rather to wide variations. Thus, while the examples have specific reference to cellulose acetate and cellulose nitrate, the process is applicable generally to all filament-forming artificial thermoplastic substances, including esters, ethers,

mixed esters, mixed ethers, and ether-esters of cellulose, such as cellulose propionate, butyrate, acetate propionate, ethyl cellulose, benzyl cellulose, ethyl cellulose acetate, vinyon, and nylon, and linear polymeric materials such as polymerized vinyl compounds, polyesters, and polyamides.

Best results are obtained with yarns possessing tensile strengths in the order of 2.0 g./den. since these generally possess lower elongations and contain more structural strains than do the lower tenacity products which are generally less oriented as indicated by X-ray patterns.

The yarn must be relatively free of moisture in order to obtain relaxation, but the yarn prior to heating can contain a normal amount of water, such as 46%. Attempts to relax cellulose acetate yarn in the presence of steam and heated water under pressure were not successful.

The yarn can be heated either by conduction, such as with hot air or other inert gases, by radiation such as infra-red heaters, or by a combination of conduction and radiation. Due to the greater penetrating power of radiant energy, heating the yarn by radiation rather than by conduction is to be preferred. Likewise, the combination heaters in which the yarn is heated partly by conduction and partly by radiation give more satisfactory results than do the air-type conduction heaters alone. Any of the commercial methods for heating articles uniformly either by conduction. radiation, or a combination of these two methods of heating can be employed.

It is desirable to control the temperature of the heating chamber to within i2 C. of the desired temperature. The temperature at which the heating unit is maintained depends primarily on the length of the unit, the speed of operation and the nature of the original yarn. It is possible to maintain a given heating chamber at a temperature above the softening temperature of the yarn, but still not have the yarn reach its softening point, by decreasing the time of exposure of the yarn in the chamber. With cellulose acetate yarn, which softens at approximately 205 C., relaxation temperatures in the order of 200 C. to 275 C. will suffice for yarn'speeds from a few feet a minute to several hundred feet a minute. The relaxation treatment by the method of this invention is, in general, a delicate balance temperature and time of exposure and must be adjusted so that the yarn is only heated sufficiently to permit the structure to become sufilciently plastic to release the strains developed in the material by previous stretching or mechanical operations. If the yarn is heated beyond this temperature, it either melts or becomes so soft or plastic that the material loses its shape and desirable tensile strength properties. With the cellulose derivatives the temperature at which the yarn must be heated in order to obtain relaxation is slightly below and within C. of the observable softening point of the cellulose derivative. The softening temperature may be defined as the temperature at which a material melts or becomes sufficiently plastic that it can be extended almost infinitely, o" as the minimum temperature at which a highly oriented cellulose acetate yarn shrinks spontaneously in the absence of tension.

The amount of tension on the yarn during the treatment is a very important factor. In general, the tension should be low, such as in the order of a few hundredths of a gram to one gram on yarns ranging in denier from to 200. With larger yarns, such as in the order of 2000 denier, it would be possible to increase the tension to several grams total tension and still obtain satisfactory results. With cellulose acetate yarn the best results are obtained when the yarn is not permitted to change denier during the treatment, which can be realized by having the feed and wind-up speeds the same and the total tension on the yarn in the order of 0.5 to 1.0 gram. Under these conditions, the elongation and knot strength are increased appreciably without a corresponding sacrifice in tensile strength.

The treatment of this invention can be carried out as either a separate operation or in conjunction with a stretching operation or some aftertreatment such as twisting, saponification, etc. The final yarn can be washed, dried, twisted, and processed in general by any of the commercial methods employed for carrying out these operations with similar types of yarn.

Stretched yarns, in general, have two extremely desirableproperties, namely, high tensile strength and a finer denier than can be obtained in regular yarns by regular spinning procedures. These yarns also have a tendency to possess a low elongation and brittle characteristics which may make them unsuitable for many textile operations. The process herein described overcomes this dimculty by increasing the elongation and knot strengths of the yarns. In the'case of high tenacity cellulose acetate yarns, weaving and knitting tests have shown that relaxed yarns can have been stretched while in the plastic state, to

a temperature just below the softening temperature of said material while maintaining said filaments, yarns, threads, and ribbons under a tension of less than one gram, and allowing said fila ments, yarns, threads, and ribbons to cool under said tension of less than one gram.

2. The process which comprises dry-heating, predominantly by radiant energy, substantially moisture-free filaments, yarns, threads, and ribbons of thermoplastic cellulose derivatives which have been stretched while in the plastic state, to a temperature just below the softening temperature of said derivative while maintaining said filaments, yarns, threads, and ribbons under a tension of less than one gram, and allowing said filaments, yarns, threads, and ribbons to cool under said tension of less than one gram.

3. The process which comprises dry-heating, predominantly by radiant energy, substantially moisture-free filaments, yarns, threads, and ribbons of cellulose acetate which have been stretched while in the plastic state, to a temperature lust below the softening temperature of said cellulose acetate while maintaining said filaments, yarns, threads, and ribbons under a tension of less than one gram, and allowing said filaments, yarns, threads, and ribbons to cool under said tension of less than one gram,

4. The process which comprises dry-heating, predominantly by radiant energy, substantially moisture-free filaments, yarns, threads, and ribbons of cellulose acetate propionate which have been stretched while in the plastic state, to a temperature Just below the softening temperature of said cellulose acetate propionate while maintaining said filaments, yarns, threads, and ribbons under a tension of less than one gram, and allowing said filaments, yarns, threads, and ribbons to cool under said tension of less than one gram.

5. In the process for producing high tenacity filaments, yarns, threads, and ribbons of thermoplastic cellulose derivatives wherein preformed filaments of the derivatives are softened and stretched while in the softened state, the im provement which comprises dry-heating, predominantly by radiant energy, to within not less than 15 C. of the softening point, substantially moisture-free, stretched filaments of thermoplastic cellulose derivatives while maintaining the filaments under a tension of less than one gram.

6. In the process for producing high tenacity filaments, yarns, threads, and ribbons of cellulose acetate wherein preformed filaments of cellulose acetate are softened and stretched while in the softened state, the improvement which comprises dry-heating to within not less than 15 C. of the softening point and by radiant energy, substantially moisture-free, stretched filaments of cellulose acetate while maintaining the filaments under a tension of less than one gram.

ROLLIN F. CONAWAY.

CERTIFICATE or CORRECTION. Patent No. 3h 8- April 11, 19th.

ROLLIN F. CONAWAY.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, line 62, for "until' read --unit--; -page l -first column, line 19, before 'temperature insert --between--; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 6th day br June, A. 1:. 19%.

Leslie Frazer (seal) Acting Commissioner of Patents.