US2053766A - Production of filaments, yarns, fabrics, and like materials - Google Patents

Description

Patented Sept. 8, 1936 PATENT OFFICE I PRODUCTION OFFILAMENTS, YARNS, FABRICS, AND LIKE MATERIALS lienry Dreyfus, London, England No Application February 8, I933, Serial No. 655,773. In Great Britain February This invention relates to improvements in the production of filaments, yarns, fabrics and like materials, and is directed to the production of filaments, threads, yarns and the like, or fabrics or like materials containing the same, which con-.

sist of or substantially consist of regenerated cellulose, or which at least have an ailinity for the cotton colours, and which have a high tenacity.

In recent years considerable attention has been directed to the production of filaments, threads and yarns of artificial silk, and particularly of artificial silk of the viscose or regenerated cellulose type, which have high, dry and wet tenacities, and in particular dry tenacities of 3 to 5 l5 grams or more per denier. While some success acids. Not only are such acids diflicult to work with on a commercial scale, lint in addition they involve the use of expensive apparatus, and moreover in general yield products which possess the serious disadvantages of being brittle and lacking the flexibility desirable in textile products. The present invention aims to produce filaments, threads, yarns and the like and fabrics containing the same which have high tenacities as referred to above and are of the regenerated cellulose type, or consist substantially of regenerated cellulose, or at least have an aflinity for the cotton 5 colours. 4

The prior processes referred to involve the use of aqueous spinning solutions, and notably solutions of viscose as the starting material. The present invention, on the other hand, uses as 4 starting material cellulose acetate or other organic esters of cellulose.

I have found that products of the character referred to may be obtained by saponification of filaments, threads, yarns or fabrics containing the same made of or containing cellulose acetate or other organic esters of. cellulose which, at some stage during their manufacture, have been subjected to a stretching operation, and especially an operation involving the application of a relatively' high degree of stretch, for example a stretch 'of 200% and upwards of their original length. Such yarns of cellulose acetate or other organic esters of cellulose have a relatively high tenacity, for example a tenacity exceeding 1.5 grams per denier, and preferably exceeding 2 or even 2.5 grams per denier. The best results are obtained according to the present invention by saponifying yarns of cellulose acetate or other organic esters of cellulose which have been stretched so as to attain a tenacity of 3-4 or even more 6 grams per denier. The saponification of yarns and other materials containing cellulose acetate by means of caustic soda or caustic potash has, been known for many years, but in general the application of such saponification processes to the i0 ordinary cellulose acetate yarns results in a decrease, and in some cases a very serious decrease,

in the dry tenacity of the yarn under treatment.-' In thepresent invention, on the other hand, it

is found that provided the conditions of saponifil5 cation are not unduly drastic the tenacity of the stretched yarn may not only benialntained but may even increase very considerably as a result of the saponification treatment, and it is found that the higher the tenacity of the unsaponified 29 but stretched yarn the greater may be the gain intenacity upon saponification. This is a very remarkable observation, since owingto the fact that stretched yarns of cellulose acetate or other organic esters of cellulose are less amenable to' 25 saponification than ordinary unstretche'd yarns, and hence have to be subjected to conditions during saponification which are somewhat more drastic than those applicable to ordinary unstretched yarns to obtain a similar degree of 30 saponification, or have to be subjected to the ordinary conditions for longer periods, it would be expected that 'as a result the tenacity, far from being increased by the saponification treatment, would be decreased'to a still greater extent 5 than in the case .of ordinary unstretched yarn. Again while in the treatment of ordinary unstretched yarn with caustic alkalies for the purpose of saponification particular attention has been paid to restricting the concentration of al- 0 kali, for example to under 1% solution, for the purpose of maintaining as far as possible the tenacity, it is found that in the treatment 0! stretched yarns according to the present invention concentrations considerably-above 1% may 5 be applied, and still substantial increases in tenacity are obtained, Thus, concentrations of 5% or more of caustic soda and temperatures up to C. have been used with success and yield products of' very high tenacity.

The stretching of the yarns or other products for saponification according to the present invention may be carried out by any suitable process, as for example those specifically described in U. S. Patent No. 1,709,470 and'U. S. applica-- The stretching is facilitated by means of solvents,

latent solvents or swelling agents for the cellulose acetate or other cellulose esters, for example aqueous solutions of thiocyanates, such as those of the alkali metals, ammonium or alkaline earth metals, or aqueous solutions of zinc chloride; acetic acid, formic acid, lactic acid, diacetone alcohol, acetone, the ethers and esters and etheresters of olefine and polyolefine glycols, for instance glycol mono-acetate, methyl glycol monoacetate, the acetins, dioxane and its homologues and substitution products, methylene ethylene ether and its homologues and substitution products and similar cyclic ethers, dichlorethylene, methylene chloride and the like. Such organic solvents, latent solvents or swelling agents may be'applied in conjunction with any suitable diluent, for example water, benzene and its homologues, petroleum fractions or other aliphatic hydrocarbons and chlorinated hydrocarbons, for example carbon tetrachloride, which have precipitating action or at least little or no solvent action upon the cellulose acetate or other cel-.

lulose ester treated. As previously indicated, the stretching forthe purposes of the present invention should involve a very substantial extension of the filaments, yarns or other products, and in particular extensions of the order of 200, 300 or 500% or more. Such "processes of stretching may be applied either to wet or to dry spun yarns. Instead of or in addition to stretching the yarn after its formation, either continuously or discontinuously with its production, the yarn may be stretched considerably during its formation. In dry spinning operations this involves a softening treatment during the dry spinning process so as to enable substantial stretch to be imparted to the materials, while in the case of wet spinning the nature of the coagulating bath or of the spinning solution may be such that a very considerable stretch may be imparted to the materials during their actual formation. Processes of this character are described in U. S. applica tions S. Nos. 402,785 filed 26th October, 1929, 418,414 filed 3rd January, 1930, 437,423 filed 20th March, 1930, 469,622 filed 21st July. 1930 and 601,043 filed 24th March, 1932. -In general the stretching of the materials either during, continuously with or after their formation should be such as to impart thereto a relatively high tenacity, which, as previously indicated, should preferably exceed 2 or 2.5 grams per denier, and may attain values of 3-4 grams per denier or more. The advantage of stretching to a considerable extent so as to obtain such high tenacities willbe appreciated from the fact referred to above that the higher the tenacity of the stretched yarn the greater the increase in tenacity as a result of saponification treatment.

The saponification treatment may be carried out by means of any suitable basic reagent. The most important for this purpose are caustic soda and caustic potash. When using causticsoda or caustic potash in aqueous solution or in mixed aqueous-alcoholic solution it is advisable, partlcularly when the concentration of the caustic alkali is relatively high, for example over 1%, to include in the saponifying medium a. buffer salt, for example sodium chloride, sodium acetate, or sodium salts of other fatty acids, including the soaps or sodium salts of higher fatty acids, or other substance adapted to repress any destructive action which the caustic alkali may have upon the cellulose material under treatment. Thus, it is highly advantageous to have present in the saponifying medium sodium chloride in a high concentration, for example in a concentration of 20%-25% or more depending upon the solubility of the sodium chloride at the temperature of the saponification treatment. Sodium acetate may be dissolved in similar high concentrations. In fact it is of great advantage to use as a buffer salt in the saponifying medium the salt which is produced by the action of the saponifying agent, for instance caustic soda, upon the cellulose ester, for instance cellulose acetate, since such a salt is always present in the spent saponifying liquor, and its use as a butler salt considerably facilitates regeneration of the spent liquor. The concentration of the caustic alkali in the aqueous or aqueous-alcoholic medium may be adjusted in accordance with the required degree of saponification, the temperature used in the saponification and the time available for treatment. I have obtained very good results by employing as saponifying medium a 4-5 or even 5 /;-6% solution of caustic soda in a 25% aqueous solution of sodium chloride or sodium acetate at temperatures ranging from I5-90 C., the time of treatment being of the order of 1 minute, this time being very suitable for treatment of travelling filaments, threads or yarns in a bath at a speed of about 40 metres per minute. Again goods results are obtainable by employing concentrations of the order of 10-12% caustic soda or caustic potash together with about 18% of sodium chloride or as much sodium chloride as will dissolve in the caustic alkali or corresponding concentrations or even higher concentrations of sodium or potassium acetate at temperatures ranging from 75-90 C., the time of treatment being correspondingly shorter, for example of the order of 20 seconds. This treatment may be applied with filaments, threads or yarns travelling in a bath at a speed of the order of metres per minute. However, the invention is not limited to these specific concentrations, and the caustic soda may be used in lower concentrations preferably at higher temperatures or in higher concentrations preferably at lower temperatures.

The application in a bath treatment of relativelyhigh concentrations of caustic alkali, i. e. concentrations of about 2% or over, ranging upwards to 10-12 or even 15% or more, particularly when combined with the use of high concentrations of buffer substances, for instance sodium or potassium chloride or acetate or soaps, for example concentrations of buffer substances of the order of 10%, and particularly 15-25%, or to saturation, at high temperatures, and particularly temperatures above 65' C. up to the boiling point, for example the temperature range mentioned above of 75-90" C. for relatively short periods, such as periods ranging from 10 seconds to 1 minute, constitutes an entirely new saponification process, and is applicable not only to the stretched filaments, threads, yarns and other products with which the present invention is particularly concerned, but also to unstretched materials, for example ordinary dry-spun cellulose acetate yarn, with highly satisfactory results.

terial, but in "a substantial increase in the tenacity. The invention therefore includes the application of these saponiflcation conditions in a bath treatment whatever be the character of the textile material under treatment. Thus, the cellulose ester contained in the textile material may be of any viscodty, for example relatively low viscosities of 6-10 or ordinary viscosities of 10-20 or 30 or high viscosities of 40 or 50100 or 200 or more as measured by comparing the viscosity of a 6% solution of the cellulose acetate in acetone at a temperature of 25 C. with that of glycerine at thesame temperature taken as a standard of 100. .Further, as previously indicated, the yarn or"'other material may be stretched or unstretched. In this special process, instead of using the above concentrations of caustic alkali solutions, other alkaline substances, for instance sodium silicate, trisodium phosphate, may be used in concentrations giving similar degrees of alkalinity.

In addition to including saponification by caustic alkali in aqueous or aqueous-alcoholic media the invention further includes saponification in a purely alcoholic medium. As is known in such a process the caustic soda may be present in the alcoholic medium, for example in a medium of methyl or ethyl alcohol, in a proportion less than that theoretically required to produce the required degree of saponification. Thus; in order to produce substantially complete saponification the caustic soda may be present, for example, in a proportion ranging from about 23-35% or more of that theoretically necessary to eliminate all the ester groups. The concentration of the caustic alkali in the alcoholic medium lends itself more particularly to batch processes in which, for example, the yarns, threads or the like are treated in the form of hanks or cakes. Similarly instead of employing methyl alcohol or ethyl alcohol as the medium in which to dissolve the saponifying agent other alcoholic or even phenolic media maybe employed, for example mono-hydric alcohols higher in the series than ethyl alcohol, or dior polyhydric alcohols, for example the glycols, glycerine or the like. With the higher monohydric alcohols and still more with the diand polyhydric alcohols the best results are obtained at temperatures exceeding atmospheric, for example temperatures of the order of 50-80 or even 90 C. Higher temperatures do not in general lead to the best results according to the pre"ent invention. The term "alcoholic medium is to be understood to include the use as medium of certain cyclic alcohols. As already indicated, phenolic media may be-used, but it is desirable to use the phenols in very low concentration in aqueous solution and to employ relatively low temperatures.

While in the above description caustic soda and caustic potash have alone been referred to as the saponifying agents, the invention is not limited to these agents, but includes quite broadly the use of alkaline saponifying agents. Other suitable agents include sodium silicate, trisodium phosphate and ammonia. Such reagents may be applied in aqueous, alcoholic or aqueous-alcoholic media. Ammonia or volatile organic amines maybe used in the vapour state, particularly when mixed with steam or water vapour to effect the saponiflcation.

The saponifying mixture may be applied to the threads by any suitable method, for example by bath treatments, padding methods particularly followed by a batching process during which the saponifying mixture has time to effeet the requisite. degree of tie spraying methods followed by batching, gaseous or vaporous application of the saponifying agent as previously referred to, or the methods described in U. S. Patents Nos. 1,884,622 and 1,884,623 in which the saponifying mixture is applied to the materials and immediately afterwards the materials are heated, for example by passage over heated drums or like devices to effect a quick saponificatidn. In methods involving batching or subsequent heating to eifect saponification, the concentration in which the caus tic alkali or othersaponifying agent is applied is relatively immaterial provided that there is enough of the medium to enable uniform application qf the saponifying agent.

The extent of the saponification may vary considerably depending upon the object in view. An affinity for the cotton colours is acquired with a comparatively small loss in weight, such as 5-10 or 20%, though such low degrees of saponification are in general only eii'ective in conferring ailinity for the cotton colours when the saponification is mainly superficial. If, as in the case of using an alcoholic medium for the saponifying agent, the saponification is relatively uniform, a somewhat higher degree of saponification, for example'up to 50% or more of the estergroups, is generally necessary to confer affinity for the. cotton colours. The invention particularly contemplates complete or substantially complete elimination of ester groups,

since, as already indicated, the main object of the invention is to produce filaments, yarns, threads or the like which are of high tenacity and are of the regenerated cellulose type.

The saponifying treatment may be effected continuously with the stretching operation which in turn may be efle'cted continuously with the production of the yarns. Whether carried out continuously with the stretching operation or discontinuously therewith the best method of carrying the present invention into effect is to use a bath treatment and to treat a warp of yarns in one operation. The stretching of a warp of yams in a single operation is described 1932. Such a warp of yarns may be carried directly into the saponifying bath, preferably after washing ofi the softening agent. However, the invention also includes intermediate reeling or beaming of the yarns between the stretching and saponification treatments. It is found that great advantages accrue from treating the travelling threads, particularly when travelling in the form of a warp or sheet of threads. Thus, the threads from a creel of bobbins may be carried through a reed, round one or more rollers, for example nip rollers or feed rollers, into the saponifying bath, preferably through a further reed, traverse an appropriate length of bath, the length depending upon the desired degree of saponification, the speed of travel, the temperature of the bath and the concentration and character of the saponifying agent, then through a further reed, round a feed roller, out of the bathand finally to a washing device,- and preferably to a dryingand reeling or beaming device. It is desirable in order to utilize the full length of the saponifying bath to wet out the materials thoroughly either on entering the saponifying bath or before entering. For this purpose the yarns may be carried into a short wetting out bath in which they are carried round a pair of nip rollers, one at least of which dips into a wetting out liquid. The nip rollers are preferably so arranged that one is'in advance of the other so that their rotation causes a trough of liquid to collect in the nip of the rollers. By this means veryeflicient wetting out may be obtained. The rollers may be metal rollers or may be covered with fabric or with rubber or similar materials to assist wetting out. The wetting medium may be water, or may be an aqueous solution of a wetting agent, as for example the soaps, for example sodium, potassium or ammonium oleates, palmitates or stearates, sulphonated soaps, for example Turkey red oil or Monopol soap, the naphthalene sulphonic acid wetting agents, and particularly the'propyl and butyl naphthalene sulphonic acids or their salts, or the w-amino-alkylamides of oleic, stearic, palmitic or other fatty acids. A small proportion of the saponifying agent itself may be included in the wetting out bath.

Instead of applying a continuous treatment as described above batch treatments, for example with the yarn in the form of hanks or cakes, may be applied. Such batch processes are particularly useful where it-is desired to extend the saponification treatment over a considerable period of time, as for example from 10 minutes to many hours. Thus, for instance, the yarn proceeding from a stretching process, as for example the stretching process carried out with the yarns in warp formation described in U. S. application 8. No. 602,844 filed 2nd April, 1932, maybe separately collected and washed in a centrifugal spinning box. These boxes may themselves be used for the saponifying treatment in which case they should be made of or lined with a material resistant to alkali, as for example copper, or the cake may be removed from the centrifugal spinning box, and while being supported on the outside or inside or both be subjected to the saponifying treatment. Thus, for example, the cake removed from the box may be supported from the interior by means of a swift which may be made expansible for insertion in the cake. Thus, for instance, the swift may comprise a small end plate, the periphery of which is perforated with holes for the insertion I of a number of wires which after insertion in the interior of the cake may be expanded umprella-fashion by means of a conical piece sliding on a shaft fitted to the end plate. Alternatively the centrifugal spinning box before winding of the yarn therein may be provided with a removable copper gauze lining in which the cake is built up and in which it can be removed from the spinning box. The cake with its supporting gauze outside may if necessary be fitted with top and bottom plates to enable saponifying liquor to be circulated through the cake. the cake is supported from the interior, as for example by means of the expansible swift referred to. Another means of supporting the cake from the interior consists in constructing a num- Such a gauze may be used whether or not ber of small segmental gauzes to fit inside the cake, for example at opposite ends of two perpendicular diameters, the gauzes being attached to each other in pairs by a spring distance piece. Such a device is of value for supporting the interior of the cake whether or not the cake is removed from the spinning box for saponification. The saponifying liquor may be circulated through the cake, as for example by means of a pump, for the requisite time. In a similar man ner the stretched yarn may be wound upon perforated bobbins or on a perforated spool tube and the .liquor circulated through the bobbin or other package. In all such packages, whether of the bobbin, spool or cake type, it is sometimes advantageous to employ a cross wind with a quick traverse so as to obtain a relatively porous package.

The following examples illustrate the invention but are not to be considered as limiting it in any way:-

Example 1 Cellulose acetate dry-spun yarn as spun or after having been stretched to 500% of its originalidry-spun length is wound on bobbins from which a creel is built up. The yarns are led from the creel through a reed so as to constitute a warp, over the top roller, through the nip and under the bottom roller of a pair of nip rollers, the lower one of which is in advance of the upper one in the direction of travel of the yarn and which dips into water acting as a wetting out liquid. Thence the warp of yarns is carried out of the wetting out bath,.over a further roller, down into the saponifying liquor, through a reed and under a roller, thence the whole length of the saponifying bath which may be of the order of feet long, through a further reed, close to' Example 2 The saponifying treatment is carried out as in Example 1 with the exception that the saponifying liquor consists of a 5% solution of caustic soda in a 25% aqueous solution of sodium chloride and is maintained at 75 C.

Example 3 The saponifying treatment is carried out as in Example 1 with the exception that the sodium chloride is replaced by sodium acetate.

Example 4 The saponifying treatment is carried out as, in Example 1 with the exception that the saponifying bath consists of a 5.5% solution of caustic soda in a 25% aqueous solution of sodium chloride and is maintained at 75-90 C.

Example 5 A warp of dry-spun cellulose acetate yarns fed forward at 8-10 metres per minute is stretched to 400-500% by treatment in a 47.5% aqueous dioxane bath, and the warp, after emergence from the stretching bath, is carried directly into a washing bath and thence into any of the saponifying baths of Examples 1-4. may remove the whole or part of the dioxane, or, if desired, may be omitted altogether.

, Example 6 A warp of stretched yarns constituted as described in Example 1 is carried through a 2 or 3 bowl padding mangle, wherein itis impregnated with a- 10% aqueous solution of caustic soda. The nip of the mangle is adjusted so that the yarn retains about its own weight of liquor. is carried directly from the padding mangle to a series of hollow steam heated rollers which serve to eflect the saponification as described in U. 3. Patents Nos. 1,884,622 and 1,884,623.

- Example 7 Stretched yarn, for example dry-spun cellulose acetate yarn which has been stretched to 400- 500% of its original length, is put up into hank or cake form and is then treated with any of the,

following baths. under the conditions stated:-- (a) 0.25% solution of caustic alkali in methyl alcohol in a bath volume of 20:1 for 16 hours at 25 C.

(b) 0.5% solution of caustic alkali in methyl alcohol in a bath volume of 20:1 for 16 hpurs at 25 C.

(c) 0.5% solution of caustic alkali in methyl or ethyl alcohol in a bath volume of 40:1 for 4 responds closely with the amount of caustic soda with which the yarn is impregnated, and in Example 7 the degree of saponification varies with the concentration of caustic soda and the time and temperature of treatment.

In a similar manner the invention may be applied to the production of filaments, threads,

\ viscosity characteristics.

yarns and other materials from other saponifiable cellulose esters, for instance cellulose formate, cellulose propionate or cellulose butyrate.

The cellulose esters may be present in the threads, yarns or other materials eitheralone or mixed with each other or mixed with other materials not deleteriously affected by the treatment, for instance natural or artificial cellulosic materials. The cellulose esters present in the materials may be either simple cellulose esters as in the case of cellulose acetate, or mixed cellulose esters, for instance cellulose nitro-acetate or mixedether-esters of cellulose, for example oxyethyl cellulose acetate or ethyl cellulose acetate. A further advantage accrues as described in my U. S. application S. No. 655,774 filed on even date herewith by employingfor the production of the cellulose ester materials saponified in accordance with the present invention cellulose esters of high Thus, while the invention contemplates the treatment of materials containing cellulose acetate of low or ordinary viscosity characteristics, for example a viscosity of 6-10, upto 20-30, as measured by comparing the viscosity of a 6% solution of the cellulose acetate in acetonegat a temperature of 25 C.

The washing bath The warp taken as standard of 100, further advantages accrue by the use of cellulose acetates 0! higher viscosity characteristics, for example showing containing such high viscosity cellulose esters is claimed in my U. 8. application 8. No. 655,774.

When, however, such yarns have been stretched and saponified as described above the process thus carried out is included within the ambit of the present specification.

The materials in the form of filaments, threads, fabrics and the like may in addition to the above treatments be subjected to a treatment with shrinking agents to improve their extension or for the productioniof special efiects, and in this connection reference is made broadly to the processes described in U. S. applications 8. Nos. 607,- 667 filed 26th April, 1932, 609,255 filed 4th May, 1932 and 611,240 filed 13th May, 1932. Uniform shrinkage for the purpose of increasing extension is particularlyadvantageous in the case of stretched yarns which usually have quite low extensions. The shrinking treatment may be applied after saponification, but is preferably applied between the stretching and the sa'ponification. When applied between the stretching and the saponification the agents specified in the specifications referred to above may be used with advantage. When the shrinking is applied subsequent to saponification the nature of the reagent will depend upon the degree of saponification. Thus, if a relatively small degree has been applied, such as up to 10 or 20% loss in weight, swelling agents for the cellulose derivative may still be eiIective in producing the desired shrinkage. When a high degree of saponification or substantially complete saponification has been carried out swelling agents for cellulose should be used to eifect the shrinkage.

What I claim and desire to secure by Letters Patent is:-

1. Process for the production of strong filaments, threads, yarns and like textile materials having an aihnity for cotton colors, which comprises saponifying filaments, threads, yarns'and like textile materials of an acetone-soluble organic ester of cellulose and having a tenacity of at least 2.5 grams per denier, which tenacity is produced by stretching the said textile materials to at least 200% of their original length, so as to cause the initial tenacity of the materials to be increased.

2. Process for the production of strong filaments, threads, yarns and like textile materials having an ailinityior cotton colors, which comprises saponifying filaments, threads, yarns and like textile materials of an acetone-soluble cellulose acetate and having a tenacity of at least 2.5 grams per denier, which tenacity is produced by stretching the said textile materials to at least 200% of their original length, so as to cause the initial tenacity of the materialsto be increased.

3. Process for the-production of strong filaments, threads, yarns and like textile materials having an aflinity for cotton colors, which comprises saponifying. by means of a bath treatment with. an aqueousalkaline saponifying agent, filaments, threads, yarns and like textile materials of an acetone-soluble organic ester of cellulose and having a tenacity of at least 2.5 grams per denier, which tenacity is produced by stretching the said textile materials to at least 200% of their original length, so as to cause the initial tenacity of the materials to be increased.

' 4.v Process for the production of strong filaments, threads, yarns and like textile materials having an aflinity for cotton colors, which comprises saponifying, by means of a bath treatment with an'aqueous alkaline saponifying agent, filaments, threads, yarns and like textile materials of an acetone-soluble cellulose acetate and hav-- ing a tenacity of at least 2.5 grams per denier, which tenacity is produced by stretching the said textile materials to at least 200% of their original length, so as to cause the initial tenacity of the materials to be increased.

5. Process for the production of strong filaments, threads, yarns and like textile materials having an aflinity for cotton colors, which comprises saponifying, by means of a bath treatment with an alcoholic solution of a mineral base, filaments, threads, yarns and like textile materials of an acetone-soluble organic ester of cellulose and having a tenacity of at least 2.5 grams per denier, which tenacity is produced by stretching the said textile materials to at least 200% of their original length, so as to cause the initial tenacity of the materials to be increased.

6. Process for the production of strong filaments, threads, yarns and like textile materials having an affinity for cotton colors, which comprises saponifying, by means of a bath treatment with an alcoholic solution of a mineral base, filaments, threads, yarns and like textile materials of an acetone-soluble cellulose acetate and having a tenacity of at least 2.5 grams per denier, which tenacity is produced by stretching the said textile materials to at least 200% of their original length, so as to cause the initial tenacity of the materials to be increased.

HENRY DREYF'US.