US 3148234 A
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United States Patent 3,148,234 METHOD OF PREPARING FILAMENTS CONTAIN- gTSNSPOLYTETRAFLUOROETHYLENE EMUL- Clarence Boyer, Swarthmore, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware N0 Drawing. Filed Dec. 24, 1956, Ser. No. 630,081 3 Claims. (Cl. 264-211) The present invention relates to the formation of films, fibers and other shaped articles from intractable polymers. Specifically, the invention is directed to the making of con tinuous structures, e.g., fibers and film from polytetrafluoroethylene.
A fiber having the desirable properties of polytetrafluoroethylene, e.g., chemical inertness, high temperature stability, non-adhesiveness, low modulus, low wettability with water and organic liquids, low coefiicient of friction and unique electrical properties, has promising utility for many industrial purposes. However, the chemical inertness of polytetrafluoroethylene (apparent from its low solubility) and its high temperature stability (apparent from its high melting point) have made the processing of this polymer difiicult. It is, therefore, obvious that polytetrafluoroethylene cannot be spun into fibers by conventional spinning methods, e.g., by wet or dry spinning from solution or by melt extrusion. A newer method involving the spinning of polytetrafluoroethylene in the form of a lubricated paste, requires enormous pressures, and yields only coarse fibers of uneven denier and ex tremely low strength even before the sintering involved in the making of the final product; the high pressure, the
unadaptability of the polymer to the spinning of multi-.
filament yarns composed of low denier fibers, and the practical necessity of using the batch process in the application of the lubricated paste method imposes serious limitations on the use of this method. Other known techniques for forming continuous structures of polytetrafluoroethylene have resulted in very weak filaments of no practical value; the filaments Were not well formed and their weakness before they were to be subjected to the sintering step prevented their handling by conventional methods. An approach to the problem of the dry spinning of polytetrafluoroethylene dispersions by transferring the aqueous polytetrafluoroethylene dispersion into an organic liquid has hitherto been unsuccessful; this attempt resulted in the formation of a coagulated non-filterable paste which could not be diluted with additional dispersant.
, It is, accordingly, an object of the present invention to provide a process for making shaped articles from polytetrafluoroethylene and similar intractable polymers. A further object is to provide new compositions of matter for use in the preparation of shaped articles, particularly fibers and films of polytetrafluoroethylene. A still further object is to provide a process for making strong textile filaments from polytetrafluoroethylene. Other objects will appear hereinafter.
The objects of this invention are accomplished, in one form, by forming a spinning composition comprising a co-dispersion of discrete particles of polytetrafluoroethylene and discrete particles of a water-insoluble, selfcoalescing matrix-forming material, shaping the resulting mixture by extruding it through an orifice into a setting medium which promotes spontaneous coalescence of the matrix material, and then coalescing or fusing the tetrafluoroethylene polymer particles in the presence of the matrix without destroying the shape of the extruded article. The shaped article will thus comprise a continuous phase of the polyetetrafluoroethylene fiber-forming polymer, together with, at the most, minor quantities "ice of the matrix-forming material. The matrix material can, in some cases, be left in the shaped structure or it can be withdrawn from it by adequate means, such as by heat destruction or extraction.
The term matrix-forming materials, as used herein, refers to those readily-coalescible polymeric materials serving primarily as temporary supporting structures.
The term intractable polymer (also called primary polymer), as used herein, refers only to those polymers, such as polytetrafluoroethylene, which (1) cannot be spun into fibers and filaments by conventional melt spinning processes, either because they rave too high a melting point or no melting point or decompose at practical melt-spinning temperatures, e.g., at 300 C. or below, or (2) cannot be spun into filaments or fibers by dry spinning from solutions in solvents of low boiling point, or (3) if wet-spun, require the use of heat and special coalescing agents in the spinning bath. These intractable polymers are initially present in the aqueous medium as particulate solids and remain, after sintering, as the major or sole polymer constituents of the solid shaped structure.
Contrary to known processes, the process of this invention starts with discrete particles of polymer dispersed in an aqueous medium and does not involve the usual steps of dissolving or melting and resolidification in order to form the desired shape. Instead, the shaped article is conveyed into a region where coagulating and simultaneous or subsequent coalescing is exerted on the polymer. The coalescing action is so regulated that it serves to fuse or coalesce the polymer particles. These features, in conjunction with the use of a matrix-forming material, define a process which is distinctly different from previous proc esses.
In the examples, which are given for illustrative purposes only, and are not limitative, the parts, proportions and percentages are to be understood as percentages by weight unless otherwise indicated and all processes were carried out at room temperature (about 25 C.) unless otherwise stated.
Example l 25 cc. of a 60% aqueous polytetrafluoroethylene dispersion, i.e., the polymer is 60% of the dipsersion, containing 6% of an alkyl aryl polyether alcohol marketed as Triton X-lOO by Rohm & Haas Co. were mixed with 25 cc. of an aqueous polyisobutylene dispersion containing 52% solids. The dispersions were hand mixed, yielding a brownish co-dispersion. No increase in viscosity was observed. 3 gm. methyl cellulose were added as a thickener and the mass extruded at C. through a 20 mil diameter spinneret hole into air heated to 130 C. Upon extrusion, the polyisobutylene coalesced to form a continuous filament which could be wound up on a bobbin. From this bobbin, the filament was rewound by leading it over a pair of hot rollers heated to 380 C. to sinter the discrete particles contained in the polyisobutylene threadline, thereby forming a continuous polytetrafluoroethylene threadline with vaporization of the decomposition products. The filament was brownish and had attractive physical properties after it had been drawn 2.5x (150% permanent stretch).
Example [I A smooth, uniform spin-dope is prepared by intimately mixing equal parts of a 60% aqueous polytetrafluoro ethylene dispersion containing 6% Triton X-" polyether alcohol, with a 10% aqueous dispersion of polystyrene plasticized with Koppers Emulsion P plasticizer sold by the Koppers Co., Inc., followed by the addition of 3% (based n the total dope Weight) of sodium alginate as a thickener for the dispersion. After deaereating this co-dispersion, a film is cast by passing it over a drum heated to 80 C. into air heated to C.
Subsequent sintering of the polystyrene film containing the discrete particles of the intractable polymer, by a salt bath heated to 375 C. yields a tough brownish transparent film. This film is almost free of matirx polymer, since polystyrene decomposes at the sintering temperature of polytetrafluoroethylene into volatile decomposition products.
Example III Another spinnable dope is prepared by mixing equal parts of a 60% aqueous polytetrafluoroethylene dispersion with a dispersion of polyethylacrylate in water. To increase the viscosity somewhat, 2% sodium pectinate is added and the co-dispersion extruded through a 3-hole vbpinneret of 20 mil hole diameter at 70 C. into a hotair chamber at 125 C. A brown filament is obtained after sintering the discrete polytetrafluoroethylene particles over 2 electrically heated rollers at 400 C. with a contacting time of 7 to 8 seconds. The brown discoloration comes from charred remains of polyethylacrylate and possibly from 'the sodium pectinate surfactant. The filament could be drawn to 500% of its original length.
Example IV A warm 60% aqueous polytetrafluoroethylene dispersion was mixed with a hot concentrated polyvinyl acetate aqueous dispersion in such a manner that the co-dispersion has the following composition:
The latter is merely added to the co-dispersion as a thickener. The smooth, creamy spin dope was extruded at 85 C. through a 10 mil spinneret hole into air of 125 C. The fiber is strong enough to support 5 feet of its length. It is then sintered to a dark brown filament, which can be drawn to 2.6x (160% draw).
From the above examples, it can be seen that novel fibers and films comprising the water-insoluble synthetic polymer and the water-insoluble self-coalescing matrix material are produced. It is surprising that these can be led through long paths in an unsupported fashion when it is realized that the synthetic polymer particles constituting the major portion of the filament solids are in an uncoalesced form. These composite filaments, without removal of the matrix polymer, may be used in the manufacture of woven or knitted textile products, particularly where resistance to heat, melting or burning is desired. It is preferred, however, that these products be sintered either in yarn form or after being formed into woven or knitted products.
While the production of the tetrafluoroethylene polymer dispersions is not a part of the present invention, they may be prepared by any suitable process described in the prior art, for example, according to the procedures of Llewellyn and Lontz US. Patent No. 2,685,- 707, issued August 10, 1954, Berry US. Patent No. 2,559,750, issued July 10, 1951, Renfrew US. Patent 2,534,058, issued December 12, 1950, or Berry US. Patent 2,478,229, issued August 2, 1949.
While the particle size of the tetrafiuoroethylene polymer in a dispersion may vary over a wide range, it is preferred that the polymer particles be of a size sufiiciently small to pass through the holes of a spinneret; normally a polymer, the particles of which are included within the range of 0.05 to 5 microns and preferable within the range of 0.1 to 2 microns, is suitable for the practice of the invention.
The primary polymers can vary widely as to molecular weight. Generally speaking the preferred molecular weight range for the tetrafluoroethylene polymer is 8000 or higher. Several processes for preparing satisfactory 4 tetrafluoroethylene polymers are described in Lontz US. Patent No. 2,685,707.
For convenience, much of the foregoing discussion has been limited to the preparation of films and filaments. It should be readily apparent, however, that this new invention applies equally well to the formation of fibers, threads, foils, tapes, ribbons, bristles, coatings (particularly on a heat resistant base when sintering is used) and like shaped articles.
The above process can also be employed to convert intractable fiber-forming polymers other than polytetrafiuoroethylene into shaped structures from a single aqueous dispersion. Cross-linked polymer particles can be used as a part or all of the fiber-forming polymer provided a dispersion of the cross-linked polymer is compatible with the dispersion of the linear polymer particles used as a matrix material; the term compatible signifies that the polymer particles in the matrix dispersion are not precipitated by the addition of the dispersion containing the cross-linked polymer, also, the degree of tightness of the cross-linking should not be so great as to prevent coalescence during sintering. Materials which can be used as the intractable fiber-forming polymer (as distinct from the matrix polymer) in the present invention are polymers which are water-dispersible, non-self-coalescible but coalescible by heat or a liquid treatment, such as polytrifluorochloroethylene, polyacrylonitrile and acrylonitrile copolymers (particularly those of very high molecular weight, e.g., 1,000,000 or more) containing a high percentage, e.g., or more, of acrylonitrile combined in the polymer molecule, polyacrylic acid and aryl or higher alkyl esters of polymethacrylic acid, poly(vinyl chloride) and copolymers of vinyl chloride with vinylidene chloride and the like, vinylidene chloride polymers, poly(methylvinyl ketone), poly(vinyl carbazole), poly (vinyl acetals), poly(methylene bis[p-cyclohexylene]- adipamide), polyureas such as poly(pentamethylene urea), polyurethanes such as those described in Wittbecker US. Patent Nos. 2,731,445 and 2,731,446, polyesters such as poly(ethylene terephthalate) and copolyesters, polysulfonamides, polysulfones, polyethers, cellulose derivatives such as cellulose acetate and many others. As illustrated above, copolymers of all types can be used, as well as the homopolymers listed above. The term copolymer is intended to include all types, such as random, altered, segmented, or block, and graft copolymers.
It can be seen from the examples that complete coalescence of the polytetrafluoroethylene particles is achieved by sintering. Development of optimum mechanical properties is dependent in part upon the sintering conditions, since incomplete sintering results in weak spots with attendant poor mechanical properties. The optimum temperature for the developing of maximum properties for polytetrafluoroethylene fibers and films appears to be approximately 350 to 400 C. At this temperature, yarns have to be sintered about 7 seconds before maximum physical properties can be developed. While higher sintering temperatures naturally require shorter sintering times, and sintering temperatures up to 430 C. have been used successfully, at the temperature below about 375 C. the contact times required to develop maximum properties becomes excessive. Many suitable heating media, such as molten salt or metal baths, heated rolls or plates, hot air, or radiant heat may be used. Other polymers can be sintered by a similar method or they can be coalesced by other means, i.e., polyacrylonitrile coalesces by a treatment with calcium thiocyanate solution.
Suitable tensile properties for commercial applications are obtained by drawing the filaments after sintering, preferably at temperatures between the crystalline melting point (327 C.) and the decomposition temperature of the polymer. Polymer temperatures of approximately 430 C. represent a practical upper limit, since polymer degradation begins to become appreciable at this tem- 5 perature. Where sintering and drawing are combined into a single operation, temperatures of approximately 400 C. represent about the best balance between sintering rate, drawability, decomposition, and the yarn properties. Where drawing is performed as a separate operation, it is preferably carried out at temperatures between 330 C. and 400 C. for polytetrafluoroethylene.
In the above examples, a thickener was used to increase viscosity of the dispersion, since even a codispersion containing 40% solids or more is not sufiiciently viscous to be extruded through an ordinary spinneret. Generally, this thickener is a water-soluble high molecular weight compound, such as sodium pectinate, sodium alginate, polyvinyl alcohol, methyl cellulose, and the like. Ordinarily, an addition of 0.5 to 5% by weight based on the total weight of the composition of such a thickener is sufficient to increase the viscosity of the spin dope enough. It has been demonstrated that this water-soluble thickener is insufficient to form a threadline by itself, and, therefore, cannot be used as a matrix material alone when dryspinning. However, it helps considerably in the formation of the threadline immediately upon extrusion through the orifice. Apart from the thickener, a solids content of 30% or more of the combined weights of the dispersed companents in the continuous water phase has been found satisfactory. The preferred solids content, however, is above 35% since dispersions of this concentration can easily be dry spun. Dispersions having a lower solids content can, without departing from the scope of the present invention, be wet spun into a setting medium for the matrix polymer, this setting medium being at the same time, a solvent for the water.
An important selection has to be made concerning the composition of solids in the codispersion. It has been established that the operable range for the ratio of polytetrafluoroethylene to matrix polymer is from 1:1 to about 10:1, the preferred range being from 1:1 to 6:1. A low polytetrafluoroethylene content yields poor fibers after sintering. At higher ratios, the threadline obtained gets weaker for obvious reasons. A high solids ratio in the co-dispersion can be attained by the addition of an inert surfactant. A great many surfactants, sold as emulsifiers, have been tried and found very satisfactory; among these are polyethylene glycol esters of fatty alcohols, diaryl sulfonates, alkyl aryl polyether alcohols, polyglycol esters, fatty alkoyl amide condensates, aliphatic ester sulfates, glycerol monoesters from fatty acids, polyoxyethylene alkyl ethers, soya bean lecithin, phosphates of fatty alcohols, tertiary amine salts, etc. In addition to 1 to 4% (by weight based on the total Weight of the composition) of such a surfactant allows a combined solid Weight in the co-dispersion up to about 60%.
The strength-providing, self-coalescible matrix materials useful in practicing the present invention should preferably meet the following requirements: (1) their queous dispersion should be compatible with the dispgfl'SlOl'l of the intractable polymer at some pH between 1 nd 14 and at some temperature between and 100 d (2) they should be dispersible at the percentage hi h! forms the self-sustaining structure. The preferred matrix materials for practicing the invention have a decomposiition temperature at least 20 C. below the melt temperature (melting point) of the main polymer and flash off at such a temperature without leaving charred remains. Examples of suitable matrix materials are, among others, plasticized polystyrene, polyisobutylene, copolymers from butadiene and styrene, polyethyl acrylate, plasticized polyvinyl acetate, a copolymer made from 5 mol. ethylene glycol, 1 mol. terephthalic acid, and 4 mols. sebacic acid and the like. The matrix materials are preferably linear polymers. Furthermore, a low concentration of this matrix material must be capable of producing a self-supporting fiber or film which can easily be handled and further processed; this characteristic is readily determined by casting a solution of the matrix polymer (in low concentration) on a plate and removing the solvent with the production of a self-sustaining sheet or alternatively a solution of the matrix polymer can be spun with removal of the solvent to form a coherent filament which can be handled readily. The preferred matrix material is the one which can be prepared by a method yielding its aqueous dispersion.
The new compositions of matter of the present invention distinguish clearly from the formerly known dispersion spinning mixtures. In the case of these new compositions, non-water-soluble organic polymers can be used as matrix materials. Many of these matrices, in contrast to water-soluble polymers, decompose into Volatile material at sintering temperature of the intractable polymer. A volatile matrix material has the valuable and desirable characteristic of flashing off during sintering without leaving any charred remains, thus yielding a white or almost white polytetrafluoroethylene structure which cannot be obtained by using polymeric water-soluble matrices. The compositions of the present invention can be kept for months and no agglomeration occurs; they are fiber-forming with about the same ease as a cellulose acetate spindope, and they are readily dry spun. Fibers, films, rods, bristles and the like can also be obtained by extruding the new composition into a liquid coagulant for the matrix material; examples of suitable liquid coagulants are acetone, alum solutions, in general, any aqueous solutions of highly polar salts, e.g., CaCl Na SO etc., will effectively coagulate these dispersions. This liquid coagulant process seems to be particulraly attractive, if the co-dispersion has a relatively low content of solids, for example, less than 30% A great advantage of the present invention is the possibility of producing shaped articles from the polytetrafluoroethylene dispersion without separating it from its preparation mixture. This invention, therefore, otters a great improvement for the process described in US. 2,413,498 to Hill, issued 1946, according to which the said polymer, in the dry state, has to be comminuted first and can only be shaped in this form with the use of a matrix material.
Any departure from the above description which conforms to the present invention is intended to be included within the scope of the claims.
1. The novel process which comprises commingling in an aqueous medium between about 30% and about 60% of solids consisting essentially of polytetrafiuoroethylene and a matrix polymer selected from the group consisting of polystyrene, polyisobutylene, butadienestyrene copolymers, polyethyl acrylate, polyvinyl acetate and the copolymer of 5 mols ethylene glycol, 1 mol terephthalic acid and 4 mols sebacic acid, said polytetrafluoroethylene and matrix polymer being present in a ratio between about 1:1 and about 10:1, respectively, the polytetrafiuoroethylene being present as particles of between about 0.05 and about 5 microns average diameter; thoroughly stirring the resulting mixture until said polymers are colloidally distributed in said medium; incorporating between about 0.5% and about 5%, based on the total weight of the said composition, of a thickener selected from the group consisting of sodium pectinate, sodium alginate, polyvinyl alcohol, and methyl cellulose; incorporating between about 1% and about 4%, based on the total weight of the said composition, of a surfactant; extruding the resulting dispersion through a spinneret and coalescing the matrix polymer to obtain a self-supporting filamentary structure.
2. The novel process of claim 1 wherein the dispersion is extruded at a temperature between about 70 and about C. into a chamber containing hot air at a temperature between about and about C., whereby the matrix polymer coalesces.
3. The process of claim 1 wherein the said filamentary structure is thereafter heated to a temperature from about 350 C. to about 430 C. for a period of at least about 7 seconds.
References Cited in the file of this patent UNITED STATES PATENTS Alfthan et al Mar. 19, 1946 Bernard Aug. 10, 1948 Bacon et a1 Sept. 21, 1948 Borcherdt et a1 May 30, 1950 Crouch Apr. 22, 1952 Buckley Febi17, 1953 Graham Apr. 28, 1953 Hochberg June 15, 1954 8 Llewellyn et a1 Aug. 10, 1954 Herzog May 31, 1955 Craemer et a1 Oct. 4, 1955 Caroselli July 10, 1956 Hendricks Sept. 10, 1957 Smith-Johannsen Sept. 17, 1957 Orth Oct. 15, 1957 Green June 24, 1958 Chambers et a1 Jan. 13, 1959 FOREIGN PATENTS Great Britain Apr. 8, 1953 Australia Aug. 2, 1954