US3397944A - Deposition of alkaline material on viscose-fluoroethylene fibers prior to sintering - Google Patents

Description

United States Patent C 3,397,944 DEPOSITION OF ALKALINE MATERIAL ON VISCOSE-FLUOROETHYLEN E FIBERS PRIOR TO SINTERING Hideji Kitagawa, Shigenobu Kinoshita, and Hiroshi Uchiyama, Ohtsu-shi, Shiga-ken, Japan, assignors to Toyo Rayon Kabushiki Kaisha, Tokyo, Japan, a corporation of Japan No Drawing. Filed Jan. 15, 1964, Ser. No. 337,730 Claims priority, application Japan, Feb. 7, 1963,

38/4,825; May 8, 1963, 38/23,523 4 Claims. (Cl. 8137.5)

ABSTRACT OF THE DISCLOSURE In a process for the production of fibers of fiuoroethylene resin wherein a yarn is spun from a mixture containing a viscose matrix and an emulsion of said fluoroethylene resin and the mixture is thoroughly purified by washing to reduce the amounts of residual acids and salts with subsequent heat treatment of the purified yarn, the improvement which comprises depositing on the yarn before heat treatment an alkaline treating agent in an amount of 0.001 to 2 weight percent based on the dry fiber.

The present invention relates to the method of manufacturing polytetrafiuoroethylene fiber with remarkably excellent fiber properties and separability and freedom from irregularities of heat treatment which, in the emulsion spinning of fluoroethylene resin in a viscose matrix bath, comprises water washing the yarn to reduce the acid and salt contents below a certain level, and immersing the 'said yarn in a dilute aqueous solution of alkali for further purification and also for deposition of certain amounts of alkali, followed by heat treatment.

Particularly important is the water washing of the yarn, for acids and salts remaining in the fiber and colloidal sulfur deposited thereon considerably affect the quality of polytetrafiuoroethylene fiber after heat treatment. The test results point out that water washing should be carried to such extent that the acid and salt contents of the yarn are reduced below 0.05 and 5.0 wt. percent respectively of the air-dried fiber, although the latter percentage vary somewhat depending on the kind of salt. Among the acids, particularly sulfuric acid, and among the salts,- particularly ammonium sulfate are considerably harmful to the yarn quality, even though they may be deposited in minor amounts, as shown in Tables 1 and 3. The present invention has discovered that uniform heat treatment free from spotty sintering patterns can be conducted by treating the water-washed yarn in a less than 1% aqueous solution of alkali such as caustic soda, caustic potash, or sodium sulfide to reduce the deposited materials below 2 to 0.001% of the air-dried fiber, instead of subjecting the said yarn to heat treatment directly after mere water washing. Of course, immersion in a caustic alkaline solution may be practised in the course of water washing the yarn, which more facilitates the removal of colloidal sulfur than by mere water washing. Thepoint is that alkaline matter is deposited in proper amounts prior to heat treatment.

It has previously been known to carry out the emulsion spinning of tetrafluoroethylene in the viscose matrix. Public information is also available on rough particulars of viscose concentration, blending ratio, preparation of 3,397,944 Patented Aug. 20, 1968 ment, where viscose is used as matrix. It is also a public knowledge to perform heat treatment after the yarn is water washed and dried. Although the knownmethod, too, commonly uses heat treatment after water washing, yet it has not always been capable of obtaining such fibers as have sufiicient strength and separability to meet industrial requirements and are also free from defects arising from irregularities of heat treatment. This has been one of the major technical problems with emulsion spinning of tetrafluoroethylene resin from the matrix. A proposal (British Patent No. 831,331) has been advanced as to additional incorporation of salts in the fiber when the films and fibers of tetrafluoroethylene resin are drawn. This proposal also suggests addition of platicizers, dispersants, pigments, and such salts as sodium sulfate and Zinc chloride or of clay and silica as components of the spinning bath, or their incorporation in the polymer later in the process. The said patent further describes that the tetrafluoroethylene fiber may even contain as much as more than 20% of a filler like titanium oxide or talc.

The inventors have studied a method of obtaining a fiber which has good properties and separability and permits uniform heat treatment in the emulsion spinning of tetrafluoroethylene resin using the viscose matrix. As a result they have discovered that even if a separating agent, for example, talc is added agglutination in heat treatment cannot essentially be avoided, and that the yarn obtained by emulsion spinning should be water washed prior to heat treatment to the extent that the residue acids and salts are reduced below a certain level. It has also been disclosed that any process that will increase the amounts of residue acids and salts obstructs the strength and separability of the fiber and the object of ensuring the uniformity of heat treatment, and so should be avoided by all means.

The inventors have made further intensive study of the effects on the yarn quality and fiber separability of the acid and salt contents. As a result they have discovered that no amount of mere water washing involved in the spinning process as described in the previous literature could provide under any conditions such type of tetrafluoroethylene fiber as had excellent separability, extensibility and heat treatment properties. Particularly where viscose in its early stage of curing (having an ammonium chloride value of over 7) was used, the yarn had large amounts of residue salts and deposited colloidal sulfur. They were extremely difficult for water washing, so that in addition to warm water washing, an aqueous solution of alkali or sodium sulfide had to be used in further purification before commercially useful tetrafluoroethylene fiber could be obtained. Practically, it is necessary to reduce by water washing the acid and salt contents of the yarn at least below 0.05 and 5.0 wt. percent respectively of the air-dried fiber. It has been disclosed that to obtain the kind of yarn which has particularly excellent fiber properties and separability and permits uniform heat treatment, it is necessary to immerse before heat treatment the water washed yarn while still wet in a dilute aqueous solution of alkali in order to deposit alkaline matter on the yarn within a range of at least 0.001 to 0.2 wt. percent of the air-dried fiber.

Therefore, the object of the present invention is to provide a heat treating process for the manufacture of fluoroethylene fiber by emulsion spinning of fluoroethylene resin from the viscose matrix which comprises heat treating the yarn of which the acid and salt contents have been sufiiciently removed and on which an alkaline treating agent has been deposited in proper amounts, thereby producing the said yarn with extremely excellent fiber properties, separability and readiness for uniform heat treatment.

The further object of the present invention is to disclose the desired limits to the aforesaid acid and salt contents and the required amount of an alkaline treating agent deposited and thus to provide an advantageous means to attain the above-mentioned object.

Many other objects and advantages of the present invention are obvious from the following description.

The process of the present invention is not only concerned with the emulsion of the polytetrafluoroethylene, but also applicable to all cases where emulsified particles are continuously formed into fiber by thermal coagulation as included among the emulsion spinning processes in which the viscose matrix can be employed. For example, the emulsion of trifiuoromonochloroethylene and that of a copolymer consisting of tetrafiuoroethylene and other monomers copolymerizable therewith are covered by'the present invention. It is therefore understood that the fluoroethylene resin or fiber as used in the present invention is defined to include all resins or fibers of such fluoroethylene 'base.

For further illustration, a detailed description follows of the conditions of attaining the object of the present invention using polytetrafluoroethylene.

(a) SPINNING MATRIX The matrix consists of viscose and emulsion (about 60% aqueous solution) of polytetrafluoroethylene. The viscose is composed of 3 to cellulose, 2 to 12% caustic soda and 27 to 32% carbon disulfide. Generally speaking, however, the viscose composition as is commonly used in the manufacture of rayon is preferable. For example, a composition containing 6 to 8% cellulose, 6 to 9% caustic soda and 23 to 30% (as against cellulose) carbon disulfide can be profitably used.

The curing degree of viscose (ammonium chloride value, hereinafter abbreviated as HZ) ranges between 1.5 to 20. An advanced level of curing of less than H2 13, for example, HZ 2 to 6 gives particularly good results.

The polytetrafiuoroethylene emulsion is commonly used in an aqueous solution. This solution contains to 75% polytetmfiuoroethylene and 3 to 10% (as against poly tetrafluoroethylene) non-ionic or anionic active agent as stabilizer.

Generally, a higher concentration of polytetrafluoroethylene gives better results. The commercially available grade usually contains 60% of the material. The promoter usually represents a non-ionic type.

Ne definite method of measurement has yet been established as to the molecular weight of polytetrafiuoroethylene, which is estimated at 2 to 6 million. In any case, however, a molecular weight of at least more than 10,000 is required. The size of particles runs 0.005 to in most of them generally being 0.3 to 0.6,u.. Where particles are coagulated together, the mass is particularly preferred to have a diameter of less than 200a.

I The composition of a mixed solution of viscose and polytetrafluoroethylene emulsion is particularly desired to contain at least 60 to 96% polytetrafiuoroethylene is the mixed high molecular substances (cellulose+polytetra fiuoroet-hylene). At less than 60% concentration of polytetrafiuoroethylene, it is difiicult to form the material into fiber even by heat treatment. If the said concentration runs higher than 96% it obstructs spinning, causing the choking up of the mouth piece and the breakage of single yarns. While the optimum concentration of polytetrafiuoroethylene is about 70 to 93%, it should be raised to as high a level as about 85 to 93% in order to obtain strong polytetrafiuoroethylene fiber. The viscosity of the mixed solution varies with the HZ of viscose and polytetrafluoroeth- 4 ylene concentration actually involved in the process. Generally speaking, however, the viscosity of about 50 to 140 pois at 20 C. is preferable.

Preparation of the mixed solution of viscose and polytetrafiuoroethylene emulsion may be made by various types of apparatus so as to effect uniform mixing. For instance, a stirrer, kneader, emulsifier and, in some cases, colloid mill may be available for use. It is advisable to conduct the mixing while cooling the solution below 20 C. in order to avoid the progress of viscose curing due to the mixing heat. Cooling below 10 C.-is particularly effective for this purpose. Although the dispersion of polytetrafiuoroethylene emulsion in the viscose is mostly stable, additional use maybe made for better mixing'of a dispersant such as a non-ionic active agent, glyce'rine, or sodium hexamethaphosphate in amounts equal to 3 to 1 0% of the polytetrafiuoroethylene content of the mixed solution. If, however, the solution is defoamed the emulsion generally tends to get coagulated into coarse particles due to the evaporation of Water. For this reason, low temperature instant defoaming, or previous supplementary addition of water to compensate for'its evaporation may be practised. For practice purposes, however, gas-tight mixing (for example, onlator of the blend type), or vacuum mixing is preferable. Those processes are capable of preparing the matrix which contains small amounts of foams.

(b) SPINNING METHOD A wet spinning method is employed in which the mixed solution is forced into a viscose-solidifying bath. An ideal solidifying bath is an aqueous solution of inorganic mineral acids and/or inorganic salts. In some cases it is advisable to incorporate further spinning in a secondary bath regenerated in inorganic acids after the primary spinning in a saturated aqueous solution of salts. The latter process is particularly effective, where viscose having a highHZ value is used. The saturated aqueous solution of salts may include ammonium sulfate [(N-H SO sodium sulfate (Na SO etc. The inorganic acid usually represents sulfuric acid (H 80 and, in some cases, nitric acid, hydrochloric acid, and phosphoric acid. A bath temperature of less than 60 C. is generally preferable in the case of inorganic mineral acids and/or inorganic salts, though it may vary with the kind of bath in actual practree.

The point of spinning is to avoid the growth of large air bubbles during the process and thoroughly remove by water washing the acid and salt contents of the fiber or deposited colloidal sulfur. A mouth piece available for common use has 50 to 300 orifices, each having a diameter of 0.08 to 0.2 mm. On the air-dried basis, the yarn spun from to 93% polytetrafiuoroethylene generally has the strength of about 0.05 to 0.25 g./d. and the extensibility of about 15 to 35%. In common practice an air-dried yarn of about 0.05 to 0.2 g./d. is easily obtained. In the case of the air-dried yarn, a lower percentage of polytetrafluoroethylene naturally produces better fiber properties.

(c) HEAT TREATMENT AND HOT DRAWING The water washed fiber consisting of a mixture of cellulose and polytetrafluoroethylene is immersed in an aqueous solution of alkali for deposition of a specified amount of an alkaline treating agent and then subjected to heat treatment at a temperature of 300 to 400 C, in that state or after drying. During this step the cellulose is burnt off, and the polytetrafluoroethylene particles in the cellulose are thermally coagulated into fiber. While the time of heat treatment varies with the concentration of polytetrafluoroethylene, the dried condition, thickness and temperature of the tow, etc., the'said time usually runs about 5 to 20 seconds at a temperature of about 330 to 400 C. For example, the time is of the order of 6 to 12 seconds at 340 to 350 C. in the case of a tow of 1300 to 1500 deniers. Heat treatment is conducted over a heating plate or a metal heating roller. If heat treatment continues too long at ,elevated temperatures fibers tend to coagulate rather tight and become lessseparable. Consequently lower temperature and longer time is more preferable than higher temperature andshorter time. Another tendency is that a lower concentration of polytetrafluoroethylene in the .fiber gives better separability after heat treatment. It is, however, not desirable in respect of the subsequent extensibility and yarn quality. Furthermore, it is important to avoid the irregularities of heat treatment between filaments. The present invention favorably prohibits the occurrence of such irregularities.

Heat treated fibers, for example, multifilaments are in the form of brown or black tape having the single fiber strength of about 0.15 to 0.28 g./d., and the extensibility of about 200 to 800%; thus the strength and extensibility of the'heat treated fiber are respectively about 2 and 15 to 40 times higher than those of the air-dried material. High extensibility indicates that the polytetrafiuoroethylene particles are continuously coagulated, and uniform heat treatment improves the extensibility. When the heat treated yarn is cold or hot drawn the fibers are separated from each other'to form individual filaments, The yarn which has been heat treated unevenly and incompletely presents difficulties in the separation of fibers. Although polytet rafiuoroethylene fibers may be obtained by cold drawing, crystallization does not reach a sufiicient level, nor is the strength satisfactory. Consequently such fibers are more subject to thermal contraction. For this reason it is preferable either to carry out hot drawing generally at a temperature of 300 to 400 C. or to supplement the cold drawing by the subsequent heat treatment at the above-mentioned temperature. Hot drawing involves a heating roller or salt bath consisting of a mixture of potassium nitrate, sodium nitrate, sodium nitrite, etc.

The present invention comprises purifying the yarn obtained by spinning as described in the preceding paragraph (b) by water washing carried to the specified extent orby alkali scrubbing, depositing a certain amount of alkali on the yarn and subjecting the said yarn to heat treatment- More particularly, the present invention involves specially purifying the fiber consisting of a mixture of polytetrafluoroethylene emulsion and cellulose to such extent that the acid and salt contents of the said fiber prior to heat treatment are reduced to a least 0.05 and 5% respectively and depositing on the said fiber an alkaline treating agent, particularly caustic soda or potash, in amounts of 0.001 to 2 Wt. percent of the dry fiber and subjecting the said fiber usually in the wet state or after drying to heat treatment at a temperature of 330 to 400 C. This purification may be made by a watersoluble interfacial active agent, or repeated washing by very warm water. However, a single cycle of water washing is not enough. It is therefore recommended to supplement ordinary water washing by a subsequent purification using a dilute aqueous solution of alkali or sodium' sulfide. The latter process is effective to remove the deposited colloidal sulfur, particularly where immature viscose isiused as matrix.

Such a thorough-going purification and pretreatment followed by heat treatment has made it possible to offer the practically useful fiber which is free from irregularities of heat treatment (or uneven coloring) and has excellent separability and drawability as has previously not been obtained. Irregularities of heat treatment are actually reflected in uneven coloring (ranging from black to brown). For instance the heat treated fiber is generally in the form of a dark brown tape. If this fiber is cold or hot drawn it will be formed into a uniformly colored filament in the case of the material which has undergone even heat treatment and into a material spotted with different shades, where irregular heat treatment has been conducted. Thus the latter grade is not only commercially valueless, but also seriously defective in that the properties of eachsingle fiber present wide variations,

The inventors have discovered that the fibers consisting of a mixture of polytetrafluoroethylene and, cellulose obtained by spinning in various kinds. of viscose. solidi: fying-baths presented wide differences in. the separability of heat treated yarns, depending on the HZ value ofv viscose used, the concentration of polytetrafluoroethylene and the kind of bath. To trace this cause, the inventors made a microscopic observation of the cross sectional and lateral structures of the yarn to check the presence of air bubbles in the fiber, abnormalities of the fiber forms and those of polytetrafiuoroethylene particles in the fiber. Furthermore, they'tested the compactness of the fibrous structure from the point of swelling, linear con traction and dyeing properties. No definite variations were observed in the interrelationship between the fiber and heat treatment. Hitherto no report has been available on this type of study, and a large number of unknown factors still remain to be solved. The inventors, however, have discovered that the residue salts in the fiber greatly alfects the properties of the heat treated yarn, and-pursued their research work from this angle. For'instancQ-the mixed matrix solution of the same composition wasspun under the same conditions in two typesof baths consisting of H 50 and Na SO at the percentage ratio of- 10 to 22 and H 50 and (NH SO at the percentage ratio of 10 to 25 respectively. The fibers thus produced were water washed and heat treated under the same conditions. Then it was found that the material spun in the bath of H SO :Na SO l0:22% presented good separability and drawability, whereas the material using the bath of H SO :(NH SO =10:25% often developed agglutination and poor separability and drawability. This reason was accounted for by the amounts or residue salts after water washing and their thermal stability. As pointed out by the results in Table 3 given later, it was confirmed that in the case of (NH SO poor thermal stability was rather responsible for agglutination than its residue amount after washing. In other words it was discovered that (NH SO decomposed while absorbing the atmospheric moisture at a heat treating temperature of 300 to 400 C. and left H thus causing the polytetrafiuoroethylene particles to be carburized. 7

Table 1 below shows the effect on the heat treated arn of the amounts of residue acids and salts in the TABLE L-EFFECT ON HEAT TREATMENT OF DEPOSITE AMOUNT OF H2504 D polytetrafiuoroethylene fiber] Amount Aqueous solution deposited 2 04, (percentage Condition of heat treatment concentration per g.) of (percent) dry fi bar 6. 6 The yarn broke at the 1st roll. 0.66 Do. 0.066 Do. 0.0066 No yarn breakage but poor strength and separability. 0. 00066 Satisfactory strength and separability 0 an?) no acid effect.

The above Table 1 represents the results of heat treatment which comprised first thorough warm water washing of tows of 2440 d./ 60 containing 90% polytetrafluoroethylene for complete removal .of residue acids (qualitative analysis by methyl orange) and residue salts (quantitative analysis by uranyl zinc acetate) in the fiber, 24 hr. immersion of the said tows at room temperature in each bath, passing them through press rolls operated at a certain pressure to reduce the water content to and then subjecting them to heat treatment by roll under the same conditions. With H 50 a deposition of even 0.007 had an adverse effect on heat treatment.

Next the relation of the HZ values-of the viscose used 7 and the residue salts is shown in Table 2 below. The figures represent-the results where the matrices having different HZ values which would produces fibers containing 90% polytetrafiuoroethylene were spun in a bath of H 50, to Na SO =10:22% all under the same conditions.

OF HZ VALUES OF THE VISCOSE USED TABLE 2.EFFECT polytetratluoroethylenc] Deposition of N 02804 per 100 g. of dried tow Sampling point of 2440 d./60 pieces HZ o1 viscose used NOTE .-The quantitative analysis of NazSOi was made by measuring Na ion by uranyl zinc acetate.

As clearly seen from Table 2 above, lower HZ produced less deposition of Na SO At HZ of 10.5, Na SO was found difiicult to remove even by water washing. This fact could be explained by assuming that high HZ viscose contains many sodium cellulose xanthate groups, which in turn retards the decomposition and regeneration of viscose in the solidification and regeneration bath, thus entraining the residues into the fiber. This accounts for one of the reasons why the use of high HZ viscose generally obstructs heat treatment. It was also observed that the higher concentration of polytetrafiuoroethylene tended to produce less residue salts. This is a natural outcome if it is considered that the cellulose which was ready to absorb salts was all the more reduced.

When the kinds and concentration of salts in the spinning bath are varied, the residue salts in the fiber logically present corresponding amounts to the original salt contents of the said bath. Table 3 given later shows the results of tests made on the effects of those salts. The table represents the test process which comprises preparing the mixed matrix using the viscose having HZ of 4.0 in such a way that the polytetrafluoroethylene concentration (in the fiber) accounted for 90%, wet spinning the same in a 8 bath of H SO ':Na SO =10:22%, repeating warm water washing carefully, preparing the wet fiber in which complete absence of Na SO was confirmed by the uranyl zinc acetate method, immersing overnight samples taken from the master batch of this fiber in solutions of various salts, pressing the said samples by certain press rolls to the extent that the contained water indicated 135% and immediatelysubjecting them to heat treatment by four-step heating rolls all under the same conditions. The total denier of the fiber involved in the test was 2440 d./ 60 pieces.

The surface temperature of each roll was as shown below and the treating time ran 23 seconds. I

Rolls: Surface temperature C.) No. 1 350 No. 2 340 No. 3 360 As to salt depositions on the fiber, there were some variations between thereoretical values arrived at by computations from the water "content as practised depending on the kind of salt and the amounts of deposition determined by the quantitative analysis. The actually measured values, however, averaged as low as 20%.

The terms separability and Irregul'arities of Heat Treatment as indicated in Table 3 are defined as follows:

Separability.-This represents the percentage number of single fibers included in the total volume of the tow as obtained at the time of cutting after the heat treated yarn has been drawn at room temperature. Therefore, the higher the separability, the more preferable it is.

Irregularities of heat treatment.This is a percentage indication of the number of fibers bearing abnormal coloring as against that of the fibers which present uniform coloring at the time of separation of fibers after they have been heat treated and subjected to room temperature drawing to the maximum extent. Consequently the higher the value, the less satisfactory it is.

TABLE 3.EFFECT OF RESIDUE SALTS ON HEAT TREATMENT Heat treated fiber Percentage Bath consalt deposition Salts in immersion bath centratlou per 100 g. of Fiber condition after heat treatment Extensibility separability lrregularitles (percent) fiber (percent) (percent) of heat treatment (NaPOQa 0. 155 0. 2 Good (brown) 470 100 15 1. 2.0 Both ergzensibility and separability de- 400 80 27 crease 15. 5 20. 0 The same tendency increased 370 40 38 N0050 ZnSO4=10 1 0. 015 0.02 490 100 20 0.076 0.10 Extensibility sli tly decreased 410 100 25 0. 76 1.03 Both ergaensibility and separability 316 0 40 crease 7.65 10. 30 The same tendency became prominent 258 0 (NHQ SO; 0.007 0.009 Fairly good (brown) 350 o 0.013 0.018 Both extensibility and separability tend- 306 0 o enced to decrease.

0.006 0. 09 The same tendency increased 216 0 0 N NO 0. 11 0.015 Mostly good (black) 410 100 20 1.11 0. 15 Both eigensibility and separability de- 350 50 26 crease 11. 1 1. 5 The same tendency increased 250 0 27 Z 50, 0. 008 0. 011 Mostly good (black) 480 80 0 0. 016 0. 022 separability was poor 408 0 0 0. 08 0. 11 Extensibility decreased. 2'21 0 0 NQ SO; 0. 035 0. 047 Mostly good (black) 506 26 0. 071 0. 005 Both ergacnsibility and separability dc- 430 80 15 crease 0. 71 0. The same tendency increased 210 50 0 N H 01 0.027 0. 0036 Good 430 15 0. 27 0. 036 Both eigzcnsibility and separability de- 360 100 0 crease 2. 7 0.36 The same tendency increased 305 60 0 MgSO 0.006 0. 008 Good .l 496 100 10 0.06 0. 08 Both exdtensibility and separability de- 440 80 0 crease 0. 6 0. 8 The same tendency increased 348 50 0 6.0 8.1 .do 209 0 0 50, 0. 0000 0. 0012 498 100 18 0. 009 0. 012 500 100 20 0.087 0. 12 I .d 450 90 16 v 0. 87 1. 17 Extensibility slightly decreased 290 70 26 8. 7 11. 7 Poor 200 0 0 9 Referring to the irregularities of heat treatment, the treatment, the conventional operating conditions are likely to result in fibers bearing spotty coloration as seen from the above Table 3. The present invention, however, is considered capable of removing deposited colloidal sulfur probably by alkali scrubbing and accomplishing uniform heat treatment due to the combined effect of the said scrubbing and the even deposition of alkaline matter on the yarn surface. The alkaline treating agent may be arranged to deposit during or after the purification step which is conducted for thorough removal of the residue acids and salts, in terms of operation, however, the latter process facilitates the control of alkali deposition.

In either case alkali deposition naturally takes effect only after the residue salt content of the fiber has been reduced at least below This is also true of the residue acids; the effect of alkali deposition becomes more prominent when the fiber contains less residue acids. The reason is that the higher acid content is more likely to produce Na SO through alkali treatment which is further entrained in the form of residue salt.

The alkaline treating agent may include hydroxides, carbonates and bicarbonates of alkali metals in the single form mixtures thereof. The commonest and most effective, however, are caustic soda and potash, followed in importance by sulfides such as sodium sulfide and potassium sulfide.

Carbonates of alkali metals such as sodium carbonate, and potassium carbonate and bicarbonates of alkali metals such as sodium bicarbonate and potassium bicarbonate are also desirable treating agents, butv less-prefer,- able than caustic alkalis. The purified yarn is immersed in aqueous solutions of the single forms or mixtures of those alkaline treating agents,-which are then deposited on the said yarn. Preferable alkali deposition ranges between 0.001 and 2 wt. percent of the air-dried fiber. The concentration of the alkaline immersion bath necessary to obtain such amount of deposition varies with the yarn denier, acid content, kind of alkaline treating agent, pressing rate and other related factors. The common preferable practice, however, is to provide a deposition of the order of 0.05 to 0.0005 mol/l. and set the pickup of the aqueous solution of an alkaline treating agent for the tow after its immersion at about 100 to 200%. In addition'to the uniform heat treatment as previously referred to, the effectofalkali treatment lies in preventing the polytetrafluoroethylene fibers coagulated by heat treat--- ment from being damaged by alkali deposition and offering excellent mutual separability among them; If, instead of the alkaline treating agent, any other materials are evenly coated on the untreated fiber or fiat solid particles are uniformly deposited thereon which are ready to get brittle through oxidation it will not be desirable, because heat treatment is usually conducted at a temperature of 300 to 400 C. In this respect the alkaline treating agent as claimed in the present invention is not only harmless but also eifective. As previously described, irregularities of heat treatment represent spotty colorings. They are divided into two types: (1) color variations in the lateral direction (expressed in the number of differently colored filaments) and (2) those in the longitudinal direction (or the axial direction of the ,fiber). The alkali treatment of the present invention is very effective in that both types of color variations are substantially eliminated.

Next several features of the present invention are further explained by examples.

Example 1 A vacuum type mixer was charged with 250 g. of viscose containing 8% cellulose, 6% caustic soda, and 29% (as against cellulose) carbon disulfide with a HZ value of 4.0 and a viscosity of 40 p./20 C.., 300 g. of polytetrafiuoroethylene emulsion and 9 g. of glycerine after they had been cooled below 15 C. The composition of the mixed solution represented the 90% concentrat-ion of polytetrafluoroethylene in a mass of high molecular substances. The charged materials were stirred in a vacuum of 10 mm. Hg for 20 min. by a stirrer rotating at 100 r.p.m. Then the mixedv solution was filtered through a filter which contained 30 mesh sand grains. After the filtrate was mixed with the matrix, the mixture was passed through a gear pump under pressure and spun through a mouth piece having 100 orifices each of 0.15 mm. ,dia. in a solidification bath at 25 C. consisting of sulfuric acid, sodium sulfate and water at the ratio of 10:22:68%. The spinning speed was at 18 m ,/m in. The solidified fiber was thoroughly purified by passing it through the cold and warm water washing baths. The H deposited on the fiber leaving th esaid baths were quantitatively analyzed by N/ NaOH, and the amount of sodium sulfate deposited was precipitated by uranyl zinc acetate and quantitatively analyzed by the chelate titration. The results are shown in Table 4 below.

TABLE 4 Next the yarns leaving the baths were pressed by roll until the water content was reduced to and then passed over and below four heating rolls which were kept at temperatures 340, 330, 360" and 350 C. respectively. The takeup speed was 3 m./min., and the treating time 23 sec. The yarn from the bath No. 1 not only stuck to the first roll when dried, but also had its single fibers considerably broken to obstruct heat treatment. While the yarns from the baths No. 3 and No. 4 practically had the same tendency, they were gradually improved in respect of heat treatment. The yarn from the bath No. 4 suffered less breakage of the single fibers, but still was unsatisfactory as to strength, separability, and uniform heat treatment. In contrast, the yarn from the bath No. 5 showed no defiective heat treatment. When hot drawn, it presented no practical difficulties in point of strength and separability. Still, however, it indicated some irregularities of heat treatment both in the longitudinal and lateral directions. Consequently the same yarn from the bath No. 5 was further immersed in a 0.05% aqueous solution of caustic soda and pressed to the 135% water content, followed by heat treatment. Then no irregularities of such treatment appeared. When this yarn was drawn 8 times the original length in a salt bath at 340 C. polytetrafiuoroethylene fiber of excellent separability and uniform quality was obtained which had a fiber rating of 3.98d, strength of 2.71 g./d., extensibility of 27.7%, and initial tensile strength of 26.9 g./ d.

The heat treated yarn from the bath No. 4, when similarly hot drawn, was diflicult of 8-fold extension at once, and could only be down 1.5 to 2 times at best. Even when drawn in two or three steps, this material only had a total drawability of less than 4 times. Beyond that, the single fibers began to be broken.

Example 2 The yarn (fiber rating 43d; strength 0.07 g./d.; extensibility 15.5%) from the bath No. 5 which was prepared in Example 1 was further washed in a warm water bath at 80 C. twice in 10 min. The washing speed was 3/ min. each time. When the quantitative analysis of Na+ content in the fiber was tried by uranyl zinc acetate it showed no analytical results, which in turn confirmed that there remained no traces of Na SO in the fiber. Next 30 m. samples were taken from this fiber while still wet, and immersed for about 1 hr. in ammonium sulfate solutions of 0.000066%; 0.00066%, 0.0066% and 0.066% respectively; Then each sample was pressed by rolls operated at a certain pressure and treated by four-step heating'rolls. The water content of the yarn after passage through the press rolls was 140%. The heating roll temperature,"takeup speed and'treating time were the same as in Example 1'. As a result it was found that the heat treated yarn fromthe bath of 0.000066% solution had no practical difiiculties in respect of hot drawability and separability. However, the yarns from the baths of more concentrated solutions than 0.00066 were observed tostick to the first roll when dried. Moreover, "they were less satisfactory in point of uniform heattreatrnent and hot drawability andeven presented agglutination, so that they were unsuitable for practical use..The aforesaid yarn from the 0.000066% bath was more improved in uniform heat treatment and drawability when further heat treated after being immersed in a 0.05% aqueous solution of NaOH.

Example 3 The yarn spun in the same way as in Example 1 from the matrix which also had the same composition as in Example 1 was passed through a cold water washing bath (2 min.) and warm water washing bath at 180 C. (10 min). The amount of residue acids accounted for and that of Na so less than 0.01%. The yarn was immersed in aqueous solutions of NaOH having different concentrations and pressed to the 160% water content. Then while still wet, it was heat treated at 340 C. for 10 sec. each time by eight-step heating rolls. The results are shown in Table below.

TABLE v(i Density of heat treated ,7 V am 25C.) Example 1- 2.135 Example 2 2.123 Example 3 2.1'02 Control Test 1 L 7.073

, Example 4 l The tow. prepared in the same way as in Example 3 was thoroughlywater washed to substantially reduce the contents of residue acids and salts, and immersed in aqueous solutions -.of sodiumcarbonate having different con: centrations. and. pressed -.to the water content of 130%. Then each sample was heat treated for 1 0 sec. each time bypassing it through the heating rolls which had a surface temperatureof 340 C. The results are shown in Table 7 below. The yarn'subjected to heat treatment with sodium carbonate deposited increased in density and permitted a uniform and quick treatment.

' Example 5 Viscose for synthetic fiber having a cured rate of 5.0 and '60 polytetrafiuoroethylene emulsion were mixed in vacuum to form a =r'nixed solution containing 85% concentration of polytetrafluo'r'oethylene in a high molecular mass. This mixture was passed through a gear pump under certain pressure and'spun in a bath consisting of H 50 Na SO and H 0 at the ratio of 10:21:69% through a mouth piece having 100 orifices each of 0.1 mm. After discharged intoa solidification bath, and then washed by cold and warm water, the yarn was purified suflicien'tly to reduce the amounts of residue acids and salts. Thenit was shaken into a box at the spinning TABLE 5 NaOH Properties of heat treated yarn NaOH bath deposition (percent) per 100 g. of Strength Extensibility separability. Spotty air dried fiber (g./d.) (percent) Color (percent) coloring (percent) (percent) Control test:

0.2 0. 32 0.21 600 .80 0 0. 02 0. 032 0. 22 000 80 0 0. 002 0. 0032 0.23 730 do 80 0 0. 0002 0. 00032 0. 21 650 Slightly colored. 80 10 TABLE 7 NaOH Properties of heat treated yarn v NaOH bath deposition (percent) per 100 g. of Strength Extensibility 1 1 separability Spotty air dried fiber (g./d.) (percent) Color (percent) coloring (percent) (percent) 0 0 0. 21 570 Brown 100 23 5. 3 6.9 0.23 r 480 Black 30 17 0. 53 0.69 0.21 s10 -.'-do 100 0- 0.053 0. 069 0.22 580 do 100 0 0. 0053 0.0 9 0.21 500 7 Slightly colored... 100 5 i As seen from the above table, the yarn which had NaOH deposition ranging from 0.32 to 0.0032 wt. percent of the air-dried fiber was preferable as to uniform heat treatment and separability. As shown in Table 6, those of the heat treated yarns in Table 5 which had previously undergone alkali treatment had a relatively high density, which in turn means that the said heat treatment had been successfully carried out.

speed of 18 ml/min. The single'fiber denier of the airdried yarn was d. The said yarns were immersed as a tow in baths containing aqueous solutions of caustic potash having'diiferent concentrations, and pressed to the 160% water content by press roll and directly 'passed through the heating rolls.'These yarns and the others which were heat treated under the same conditions were further hot drawn to the 8-fold size at 380 C. The results are shown in Table 8 below.

TABLE 8 KOH Properties of heat treated yarn Properties of hot drawn yarn K011 bath deposition Coloring (percent) per 100 g. of Strength Extensibility Strength Extensibility separability spotty air dried fiber (g./d.) (percent) (g./d.) (percent) (percent) (percent) (percent) Control test:

The yarn which had KOH deposition of 4.5% was impossible of being drawn to the eight-fold size, a maximum, drawing ratio being only 3 times. The control tests 5 and 7 presented a maximum drawing ratio of 6 times.

We claim:

1. In a process for the production of fibers of fluoroethylene resin wherein a yarn spun from a mixture containing a viscose matrix and an emulsion of fluoroethylene resin is thoroughly purified by washing to reduce the amounts of residue acids and salts below 0.05 and 5 Wt. percent respectively based on the dry fiber with subsequent heat treatment of the purified yarn, the improvement which comprises depositing on said yarn before heat treatment an alkaline treating agent in an amount of 0.001 to .2 wt. percent based on the dry fiber.

2. The process of claim 1 wherein the fluoroethylene resin is polytetrafiuoroethylene,

3. The process of claim 1 wherein said alkaline treating References Cited UNITED STATES PATENTS 1,977,533 10/1934 Thurmond 8-137.5 2,074,076 3/1937 Schrenk 8137.5 2,208,965 7/1940 Dousma 8137.5 2,488,667 11/1949 Jacokes 8-137.5 2,686,103 8/1954 Church 264195 2,772,444 12/ 1956 Burrows et al. .264-205 NORMAN G. TORCHIN, Primary Examiner.

J. CANNON, Assistant Examiner.