US2371233A - Rayon - Google Patents

Description

Patented Mari 13, 1945 Wanda K; Farr, Yonkers, N. Y.,-, assignor toThe- Incorp orated, a corporahemica'l Foundation tion' of Delaware:

No Drawing. Applicationxlune 6,1939, Serial No. 2774625 3 Claims. (01. 260 -217"! This invention relates "to' the production of an improved rayon.

The commercialproduction of cuprammonium silk, as is known,uisbased upon the original dis-' coveries of" John- Mercer. Mercer, in common with his contemporaries; suchas Schweizer, thought that cellulose was actually' dissolved in cuprammonium solutions. For example, Mercer reportedythat cellulose fibers were soluble ina liquid: obtained by dissolving, hydrated cupric 'oxide to saturation in ammoniaof sp. gr. 0.920 and dilutingvwith three measures-of water. On neutralizing with-sulphuriaacid, the organic matter was precipitated and later freed from copper by an exceess of the acid. Schweizer similarly taught the artthat witha sufiicient excess of cuprammonium solutionatrue dissolutionof cellulose occurred.

. The commercial production of cuprammonium rayon'did not begin until some thirty years after the original observation of the behavior of cotton fibers in ouprammonium hydroxide: This protracteddelay was due to-the technical diificulties inhandlinglthe spinning mixtures. Suchmixtures were foundto it be unstable and tended to separate. lI-n theearly efforts inthis field evenunderthe mostfavorable: conditions,= the regenerated fibers did-not possess-that degree of uniformity requiredby the trade;

Full scale commercial production of" cuprammonium rayon was instituted onlyafter the observation thatfpretreatment of the fibers over-' came theearlier difiicultiesof separation. Such pretreatment consisted of a merceri'zation treatmentusually by treatment with 18 percent. sodium hydroxide-at roomtemperature. It was found that such pretreated' fibers disperseduniformly in the cuprammonium hydroxide' solution and the resulting spinning mixture,.even' in-thehig-h dilutions required forthe finer: fibers, remained stable and homogeneous throughout the entire spinning process.. Eollowing-the originalprecepts of the art as to the solubility of cellulosedn cuprammonium solution,.this enhanced stability was attributed to anincreased solubility of themercerized fibers in the solvent. This concept of increased solubilit'yhas been substantially ac-x cepted down-to the present time It'hasnow been discovered that this concept is erroneous, It has definitely been established that; contrary to the generally accepted theory, true cellulose-isnot soluble in thetypicalcuprammoniumsolutions; and-that enhanced stabilityoi cuprammonium spinning mixtures is not due-es; senti'ally to the modificationflof the cellulose. It

has been round, as aresult of intensive, prolonged experimentationthatthe stability and spinability of c'upr'ammo'nium vehicles cmi-tai-ning cellulose 1 is primarily junction of the characteristics of the non cellulosic constituents-of the fiber.- In a' of cuprammonium spinning mixtures can be technique; involving. among. others, such-factors as-criticalillumination, the' slit ultr'a greatly enhancedfnot bymodification of the-cellulose component but by positively inhibiting theswelling of the non-cellulosic component of the fibrousstarting.material. v 1

The present invention therefore is based on the discovery that inhibition or such swelling markedly modifies the stability of the described spinning mixtures and specifically relates to improved methods of attaining or positively estab* lishing suehcontro'lled inhibition, thereby mg: the-concomitant or attendantadvantage'sicommercial production' of cuprammoniurn rayon The improved procedure which forms the basis of the present invention is bas'edupondiscoveries of the nature of the cellulose orystal'lite and its correlation with; the amorphouscementitious material with which iii-is naturally associated in" the cellulose membrane. To this-extent the-pres ent application is a continuation in part of co-- pendingapplication" of Wanda'K. Farr, SerialNo. 270,157; filed April"26;.1939;

As' is fully described in the application referred to, it has been discovered that th'e cellulose com-' pohent of the cell membrane or the? cotton" fiber, which is in. the form ofind'ividua'lly' distinct, ellipsoid particles;.does notdissolve'in a stanclard solutionotcup'ramm'onium hydroxide; as has been'popularlywsupposed; vIn experiments record-' ed in the cross-referenced; application; cellulose membranes of diiferent types: were dispersedin standard cupra'mmonium solutions; By special the use of microscopic and: extremely small cross sections of specimens, it definitely was established that the action insuch' solutions involved essentially the extenuationof the cementitious non-ce1iulosicmaterial-andthe concomitantdisintegration pending application, the cementitious material may be separated and recovered from the cellulose.

It was found that the true cellulose itself had little if any effect upon the viscosity of cuprammonium solutions; that such cellulose was a precise or definite entity and that, however the cellulose was produced, it had substantially no effect upon the viscosity of cuprammonium hydroxide. It was found conversely, and most unexpectedly in view of the teachings of the art, that the determinant in establishing the increased viscosity of cuprammonium solutions was not the quantity of the cellulose in the solution but was the cementitious material, and that the viscosity of a cuprammonium system was a direct function of the quantity of this cementitious material contained in the solution. As a result of microscopic examination as well as chemical analysis, as for example by uronic anhydride determinations, it was determined that the characteristics of this cementitious material placed it in the category of what is generally known as pectic substance.

The results described in the application referred to fundamentally alter the previous conception of'the behavior of both untreated and purified cell membranes in. solutions of cuprammonium hydroxide. It is obvious from the earlier literature that this viscosity-producing characteristic of the pectous material has been entirely overlooked, as well as the fact that the cellulose, per se, exists in a cuprammonium solution, not as a dissolved substance but as a dispersed colloidal phase.

Upon these findings the present invention is based- Under the present invention the stability of cuprammonium spinning solutions may be enhanced and definitely established by suitably modifying the component which engenders instability, namely,the non-cellulosic pectous component. In brief, the present invention relates to a method of increasingthe stability of cuprammonium spinning solutions not by any attempt at modification of cellulose itself but by inhibiting the swelling action of the intercrystalline, cementing material.

In experimentations in this field, itwas observed that when very thin cross sections (2 to 3 mu) of cotton fibers were treated with cuprammonium hydroxide, some striking effects were produced. If these sections were previously untreated the plastic and viscous cementing material, existingexample. swelled very gradually and three stages of the action were readily discernible: first, an early stage in which therewas a general increase in the diameter of the cross. section of the specimen due to the imbibition or expansion of the cementing substance; a later stage in which the lamellae making up the fiber separated from each other to the extent that they were readily discernible; and a final stage in which the lamellae themselves disintegrated into cellulose particles of colloidal dimensions which are randomly dispersed in the swollen colloid matrix. These cellulose particles, which had the approximate dimensions 1.5 x 1.1 mu, exhibit the 'I'indall efiect, Browning movement, and other characteristics of colloidal dispersions; and as a result of these findings it was determined that the stability of a spinning mixture of cuprammonium hydroxide is due, not to increased solubility of the cellulose, or in fact any solubility of the cellulose whatever, but to one-of increased colloidal stability of the cementing material. It was definitely established, for example, that the cellulose particles, as such, are present as a dispersed phase in the cuprammonium solution whether they had been 7 previously mercerized or not. Under the present invention, this discovery is utilized to improve cuprammonium spinning solutions by the simple expedient of pretreating the fibers, not with a reagent which insures mercerization or necessarily under mercerizing conditions but with a reagent and under conditions which are chosen so as selectively to inhibit the swelling function of the pectic substance.

In one preferred modification it has been found that such inhibition may be securedwith cheap reagents, e. g. sodium hydroxide or potassium anhydride in relatively low concentrations and at relatively low temperatures as distinguished from the relatively elevated temperatures which had been thought necessary in the past in order to secure mercerization.

In a typical case, the desired inhibition of the rapid swelling of the cementitious material and the attainment of the concomitant desirable stability of the colloidal cementing phase, may read- 10% and lower, which solutions are preferably maintained at a low temperature, 'i. e., of the order of 4 C. It will be observed that this type of treatment is essentially different from the prior art methods in which sodium hydroxide solutions of 18% or higher were employed at elevated temperatures so as chemically to modify the cellulose. Under the conditions of'the present invention, while sodium hydroxide may be employed, it is used under conditions controlled preferentially to modify a physicochemical characteristic of a non-cellulosic component of the starting material and not as in the prior art, to chemically change the cellulose.

Since the fibrous or cellulosic starting materials which are used in the production of cuprammonium silk may Vary considerably, the conditions of the swelling inhibition treatment may be modified to secure the desireddegree of 001- loidal stability for the particular material used.

The relationship of concentration'of the reagent and the time and temperature of thetreatment may thus be definitely correlated with the particular starting material.

After the desired degree of pretreatment, the fibrous material may be freed of the alkali hydroxide, washed, and then 'dispersed ina typical cuprammonium solution. The resulting disper-' sion may then beejected into water, stretched, and then treated in the acid coagulating bath in the usual manner.

It will be understood concept of the invention, a number of specifically different materials or reagents may be employed to effect the desired modification of the external or cementitious phase. Any material which functions to operate on the non-cellulosic material to reduce its swelling tendency and to colloidally stabilize it, is suitable.

In experimentation in this field it has been found that a particularly efficacious method of producing improved cellulose dispersions, as for example a dispersion of cellulose in cuprammonium hydroxide, comprises a treatment with a relatively weak solution of nitric acid followed by treatment with a caustic soda solution. In

' I a preferred mode of operation, for the production of cuprammonium rayon, the cellulosic starting material is treated with a solution of 1% nitric acid at about room temperaturefor a period of from approximately to 20 hours. During the period the cellulosic material may be agitated continuously or intermittently. After this period of contact the nitric acid solution is drained, the cellulose Washed, with Water and then treated with a caustic soda solution of approximately that within the broad 10% concentration at a temperature from room temperature down to about 4 C. Thereafter the cellulose may be dispersed in a cuprammonium hydroxide solution in the usual manner.

The treatment with nitric acid appears to have a critical or specific beneficial effect. The outer limiting membrane of cellulose fibers, as is known, is of a different chemical constituency from the remainder. This outer membrane contains a protective substance, generally designated as cutin. The material is insoluble in the reagents generally employed in the pretreatment and dispersion of cellulose and in many instances appears in the final product, that is to say, the extruded rayon filament or sheet. Such material occurs in the filament as a dispersed impurity and causes considerable difliculty in dyeing in that it does not respond or react to the dye as does the regenerated cellulose thus giving an imperfect product. The nitric acid treatment appears to selectively extract this constituent or fraction from the cellulosic material while at the same time beneficially modifying the colloidal characteristics of the intercrystalline or pectous phase insuring greater colloidal stability.

It has been ascertained that the described pretreatment with Weak nitric acid is of wide-utility in the rayon field. As noted above the described chemical constituent which occurs in the outer limiting membrane of cellulose fibers is quite refractory. As a result of numerous tests, it has been discovered that in many circumstances this constituent appears in certain viscose rayon fibers. In other words this particular substance is resistant even to the action of the 18% caustic solution used in preparing the alkali cellulose for xanthation, Under one phase of the present invention viscose rayons may considerably be improved, especially in respect to uniform dyeing, by selectively extracting this characterizing constituent of the outer limiting membrane of the cellulose fiber prior to dispersion of the cellulose. A simple and effective method of accomplishing this is to subject the cellulosic material to the describedextraction with nitric acid. Preferably this is done prior to the formation of the alkali cellulose. The extraction with the nitric acid solution may be effected by a simple batch immersion treatment or by continuously recirculating the weak nitric acid solution through a mass of the cellulosic material.. When such a treatment is employed it is found that the undesirable constituents are removed from the cellulose. Viscose rayons produced from such pretreated cellulose are characterized by an improved uniformity with respect to dyeing.

It will be appreciated that the term cutin herein applied to the undesired constituent of the cellulose membrane is used in a generally descriptive rather than a strictly scientific sense. This material probably i not a single chemical individual but may be a mixture of compounds. Re-

searches to date indicate that it is comprised largely of suberin. With respect to the present disclosure, the term cutin is intended to comprehend those constituents such as the water resistant wax-like components which characteristically distinguish the outer limiting membrane from the remainder of the fiber.

Hence while preferred pretreating agents have been described, it is to be understood that these are chosen to illustrate an effective though not the exclusive method of procedure. Theinvention resides not so much in the use of particular reagents at definite concentrations and temperature,but primarily in the discovery of the essential role of the non-cellulosic phase and for the practical methods of modifying its colloidal characteristics so as to insure the proven new results.

I Having thus described my invention, I claim:

1. Inv the production of viscose rayons that improvement which comprises extracting cotton cellulose, prior toalkalization at room temperature, with a solution of nitric acid of approximately 1% strength for a time sufficient to selectively extract the cutin-like component of the material.

2. In the production of viscose rayons that improvement which comprises extracting cotton cellulose at substantially room temperature with a 1% solution of nitric acid for a period of the order of 15 hours to extract cutin-like substances without substantial decomposition of the other components of the starting material.

3. In the production of viscose rayons that improvement which comprises extracting cotton fibers with a 1% solution of nitric acid at room temperature for a period of from approximately 10 to 20 hours.

WANDA K. FARR.