US3813219A - Process for the thermal stabilization of polyacrylonitrile fibers and films - Google Patents

Abstract

AN IMPROVED PROCESS IS PROVIDED FOR THE PRODUCTON OF STAILIZED ACRYLIC FIBERS AND FILMS. STANNOUS CHLORIDE IS INCORPORATED IN A SOLUTION OF AN ACRYLIC POLYMER PRIOR TO FORMING FROM SAID SOLUTION A FIBROUS MATERIAL OR FILM WHEREIN THE PENDANT NITRILE GROUPS PRESENT IN THE ACRYLIC POLYMER ARE SUBSTANTIALLY UNCYCLIZED, AND THE ACRYLIC FIBROUS MATERIAL OR FIL HAVING 0.5 TO 10 PERCENT BY WEIGHT STANNOUS CHLORIDE INCORPORATED THEREIN IS HEATED IN A GASEOUS ATMOSPHERE CONTINING 30 TO 100 PERCENT BY WEIGHT MOLECULAR OXYGEN UNTIL A STABILIZED FIBROUS MATERIAL OR FLM IS FORMED. THE PRESENCE IF THE STANNOUS CHLORIDE IN COMBINATION WITH THE GASEOUS ATMOSPHERE CONTAINING MORE THAN THE USUAL CONCENTRATION OF MOLECULAR OXYGEN (E.G. 10 TO ABOUT 20 PERCENT BY WEIGHT) HAS BEEN FOUND TO RESULT IN A SUBSTANTIALLY IMPROVED PROCESS. MORE SPECIFICALLY, THE RESULTING STABILIZED ACRYLIC FIBROUS MATERIALS AND FILMS EXHIBIT ENCHANCED PHYSICAL PROPERTIES (I.E. STRENGTH AND MODULUS), AND THE STABILIZATION REACTION IS ACCELERATED IN A CONTROLLED MANNER IN THE SUBSTANTIAL ABSENCE OF UNDESIRABLE FIBER COALESCENCE. THE RESULTING STABILIZED FIBROUS MATERIAL OR FILM IS NON-BURINING, AND MAY BE UTILIZED AS A FIRE RESISTANT FIBER, FABRIC OR FILM, OR OPTIONALLY CARBONIZED AND CARBONIZED AND GRAPHITIZED TO FORM A CARBONACEOUS FIBROUS MATERIAL OR FILM.

Description

United States Patent O 3,813,219 PROCESS FOR THE THERMAL STABILIZATION OF POLYACRYLONITRILE FIBERS AND FILMS Andrew H. Di Edwardo, Parsippany, and Klaus H. Gump, Gillette, NJ., assiguors to Celanese Corporation, New York, NY. No Drawing. Filed Apr. 28, 1972, Ser. No. 248,371 Int. Cl. C01b 31/07; D06c 7/04 US. Cl. 8--115.5 19 Claims ABSTRACT OF THE DISCLOSURE An improved process is provided for the production of stabilized acrylic fibers and films. Stannous chloride is incorporated in a solution of an acrylic polymer prior to forming from said solution a fibrous material or film wherein the pendant nitrile groups present in the acrylic polymer are substantially uncyclized, and the acrylic fibrous material or film having 0.6 to 10 percent by weight stannous chloride incorporated therein is heated in a gaseous atmosphere contining 30 to 100 percent by weight molecular oxygen until a stabilized fibrous material or film is formed. The presence of the stannous chloride in combination with the gaseous atmosphere containing more than the usual concentration of molecular oxygen (e.g. 10 to about 20 percent by weight) has been found to result in a substantially improved process. More specifically, the resulting stabilized acrylic fibrous materials and films exhibit enhanced physical properties (i.e. strength and modulus), and the stabilization reaction is accelerated in a controlled manner in the substantial absence of undesirable fiber coalescence. The resulting stabilized fibrous material or film is non-burning, and may be utilized as a fire resistant fiber, fabric, or film, or optionally carbonized or carbonized and graphitized to form a carbonaceous fibrous material or film.

BACKGROUND OF THE INVENTION In the past procedures have been proposed for the conversion of fibers formed from acrylic polymers to a modified form possessing enhanced thermal stability. Such modification has generally been accomplished by heating a fibrous material in an oxygen-containing atmosphere at a moderate temperature for an extended period of time.

US. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda disclose processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) a chain scission and oxidative cross-linking reaction of adjoining molecules, (2) dehydrogenation reactions, as well as (3) a cyclization reaction of pendant nitrile groups. It is generally recognized that the rate at which the stabilization reaction takes place increases with the temperature of the oxygen-containing atmosphere. However, the stabilization reaction must by necessity be conducted at relatively low temperatures (i.e. below about 300 0.), since the cyclization reaction is exothermic in nature and must be controlled if the original fibrous configuration of the material undergoing st-abilization is to be preserved. Accordingly the stabilization reaction tends to be time consuming, and economically demanding because of low productivity necessitated by the excessive time requirements. Prior processes proposed to shorten the period required by the stabilization reaction include that disclosed in US. Pat. No. 3,416,874. See also the processes of commonly assigned U.S. Ser. Nos. 777,- 902, filed Nov. 21, 1968, of K. H. Gump and D. E. Stuetz (now US. Pat. No. 3,647,770) wherein a solution of an acrylic polymer containing a Lewis acid is heated e.g. at

3,813,219 Patented May 28, 1974 140 to 160 C.) to produce cyclization of pendant nitrile groups, and the resulting solution is formed into a cyclized acrylic fibrous material; 109,669, filed Jan. 25, 1971, of E. C. Chenevey and R. M. Kirnmel; and 200,184, filed Nov. 18, 1971, of K. H. Gump and D. E. Stuetz.

US. Pat. No. 3,242,000 to I. A. Lynch discloses an unrelated process for producing carbonized textile products from acrylic textile products wherein a refractory metal oxide barrier coating is formed upon the surface of fabric employing a heat treatment atmosphere which contains at least some oxygen (e.g. about 10 to about 20 percent oxygen).

While stabilized acrylic fibrous materials may be used directly in applications where a non-burning fiber is required, demands for the same have been increasingly presented by manufacturers of carbonized fibrous materials. Carbonized fibrous materials are commonly formed by heating a stabilized acrylic fibrous material in an inert atmosphere, such as nitrogen or argon, at a more highly elevated temperature. During the carbonization reaction elements such as nitrogen, oxygen, and hydrogen are substantially expelled. Accordingly, the term carbonized as used in the art commonly designates a material consisting of at least about percent carbon by weight, and generally at least about percent carbon by weight. Depending upon the conditions under which a carbonized fibrous material is processed, it may or may not contain graphitic carbon as determined by the characteristic X-ray diffraction pattern of graphite. See, for instance, commonly assigned U.S. Ser. No. 777,275, filed Nov. 20, 1968, of Charles M. Clarke (now abandoned) for a preferred procedure for forming carbonized and graphitized fibrous materials from a stabilized acrylic fibrous material.

It is an object of the invention to provide an improved process for forming thermally stabilized acrylic shaped articles.

It is an object of the invention to provide an improved process for forming fiame-proofed fibrous materials or films derived from acrylic polymers.

It is an object of the invention to provide an improved process for forming thermally stabilized acrylic shaped articles wherein the thermal stabilization of an acrylic fibrous material or film is accelerated in a controlled and non-deleterious manner.

It is an object of the invention to provide an improved process for forming thermally stabilized acrylic fibrous materials or films which exhibit enhanced physical properties, i.e. strength and modulus.

It is an object of the invention to provide an improved process for forming thermally stabilized acrylic fibers which employs an enriched oxygen-containing atmosphere on an expeditious basis in the absence of expected fiber coalescence.

It is another object of the invention to provide an improved process tor forming thermally stabilized acrylic fibrous materials and films wherein a superior product is produced which is suitable for carbonization, or carbonization and graphitization.

These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.

SUMMARY OF THE INVENTION It has been found that an improved process for the production of stabilized acrylic fibers and films exhibiting enhanced physical properties comprises:

(a) Providing a solution of (1) an acrylic polymer selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about 85 mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith wherein the pendant nitrile groups present in the acrylic polymer are substantially uncyclized, (2) a minor quantity of stannous chloride, and (3) a solvent for the acrylic polymer and the stannous chloride,

(b) Forming from the solution an acrylic fibrous material orfilm wherein the pendant nitrile groups present in the acrylic polymer are substantially uncyclized having incorporated therein about 0.5 to percent by weight of stannous chloride, and

(c) Heating the acrylic fibrous material or film having the stannous chloride incorporated therein in a gaseous atmosphere containing 30 to 100 percent by weight molecular oxygen provided at a temperature of about 260 to 350 C. until a stabilized fibrous material or film is formed which retains its original configuration substantially intact and which is non-burning when subjected to an ordinary match flame.

DESCRIPTION OF PREFERRED EMBODIMENTS The acrylic polymer utilized as the starting material is formed primarily of recurring acrylonitrile units. For instance, the acrylic polymer should generally contain not less than about 85 mol percent of acrylonitrile units and not more than about mol percent of units derived from a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such monomers. The pendant nitrile groups present within the acrylic precursor are substantially uncyclized.

The preferred acrylic precursor is an acrylonitrile homopolymer. Preferred acrylonitrile copolymers contain at least about 95 mol percent of acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith.

The stannous chloride which is dissolved in the solvent for the acrylic polymer may be either anhydrous [e.g. SnCl or hydrous [e.g. SnCl -2H O]. When stannous chloride is present in the hydrous form, the water of hydration is not included when calculating the concentration of stannous chloride for the purposes of the present specification and appended claims.

Suitable solvents which may be utilized in the present process are capable of dissolving both the acrylic polymer and the stannous chloride. Representative organic solvents include N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, butyrolactone, and N-methyl-2-pyrrolidinone. The preferred solvents are those which are commonly utilized during the spinning of fibers of acrylonitrile homopolymers and copolymers. The particularly preferred solvents are N,N-dimethylformamide and N,N dimethylacetamide. The solvent may be dried by running through a bed of drying agent (e.g. Linde sieves, activated alumina, etc.), and moisture excluded by blanketing with dry air or nitrogen.

The concentration of the acrylic polymer in the solvent may be varied widely, e.g. about 5 to about 30 percent by weight based upon the weight of the solvent. Preferred concentrations range from 10 to 25 percent acrylic polymer by weight based upont the weight of the solvent.

The stannous chloride is present in the solution of acrylic polymer in a minor concentration, i.e. about 0.5 to percent by weight based upon the weight of the acrylic polymer. In a preferred embodiment of the process the stabilization promoting agent is present in a concentration of about 1 to 10 percent by weight based upon the weight of the acrylic polymer.

In a preferred embodiment of the process employing an N,N-dimethylacetamide solvent the solution of acrylic polymer and stannous chloride additionally contains 0.1 to 5.0 percent by weight based upon the total weight of the solution (0.5 to 2.0 percent in a particularly preferred embodiment) of lithium chloride dissolved therein. The incorporation of lithium chloride serves the function of lowering and preserving upon standing the viscosity of the solution. The desired solution fluidity and mobility for spinning or casting are accordingly efiiciently maintained even upon the passage of time.

The solution of the acrylic polymer and stannous chloride suitable for extrusion may be formed by any convenient technique. For instance, the acrylic polymer while in particulate form together with stannous chloride may be added to the solvent with stirring while maintained at about 50 to C. It is recommended that any heating of the solution in excess of about C. be of limited duration, i.e. no more than a few minutes, so that no substantial degree of cyclization of pendant nitrile groups within the acrylic polymer occurs while dissolved in the solvent.

The solution is preferably filtered, such as by passage through a plate and frame press provided with an appropriate filtration medium, prior to forming a fibrous material or film.

The solution containing the acrylic polymer and the stannous chloride is preferably converted into a fiber or film through the substantial elimination of the solvent following extrusion through a shaped orifice employing conventional solution spinning techniques (i.e. by dry spinning or wet spinning). As is known in the art, dry spinning is commonly conducted by passing the solution through an opening of predetermined shape into an evaporative atmosphere (e.g. nitrogen) in which much of the solvent is evaporated. Wet spinning is commonly conducted by passing the solution through an opening of predetermined shape into a suitable coagulation bath. Acrylic films may also be formed by casting wherein a layer of the solution is placed upon a support and the solvent evaporated.

When wet spinning is utilized in the fiber or film forming step of the process, a coagulation bath is selected which is capable of preserving the requisite catalytic quantity of stannous chloride within the resulting fibrous material or film. More specifically, the bath preferably exhibits no propensity to leach out and dissolve the stannous chloride below the minimum level required for catalysis during the subsequent heat treatment step (described hereafter). Such coagulation bath may inherently possess no substantial tendency to dissolve the stannous chloride. Alternatively, the coagulation bath which is selected may have its inherent tendency to dissolve stannous chloride diminished by preliminarily dissolving a substantial quantity of stannous chloride, or other compound therein. The coagulation bath is preferably substantially anhydrous. A preferred wet spinning technique is disclosed in commonly assigned U.S. Ser. No. 28,545, filed Apr. 14, 1970 (now US. Pat. No. 3,657,409), which is herein incorporated by reference.

The shaped orifice or spinneret utilized during the extrusion may contain a single hole through which a single filament is extruded, and preferably contains a plurality of holes whereby a plurality of filaments may be simultaneously extruded in yarn form. The spinneret preferably contains holes having a diameter of about 50 to microns when producing relatively low denier fibers having an as-spun denier of about 8 to 24 denier per filament. Alternatively, acrylic films of relatively thin thickness, e.g. about 1 to 10 mils, may be formed, when the extrusion orifice is a rectangular slit. Generally stated, the solution may be formed into an acrylic fibrous material or film having stannous chloride incorporated therein utilizing conventional fiber or film forming techniques with a minor quantity of the stannous chloride being merely added to the polymer dope.

The resulting as-spun fibrous material or film is preferably maintained in a continuous length configuration throughout the process. At an intermediate point prior to heat treatment the fibrous material may alternatively be transformed into another fibrous assemblage, e.g. a tow, fabric, or yarn of greater total denier.

When the fibrous material is a continuous multifilament yarn, a twist may be imparted to the same to improve the handling characteristics. For instance, a twist of about 0.1 to 5 t.p.i., and preferably about 0.3 to 1.0, t.p.i. may be utilized. Also a false twist may be used instead of or in addition to a real twist. Alternatively, one may select bundles of fibrous material which possess substantially no twist.

The fibrous material may be drawn in accordance with conventional techniques in order to improve its orientation. For instance, the fibrous material may be drawn by stretching while in contact with a hot shoe at a temperature of about 140 to 160 C. Additional representative drawing techniques are disclosed in US. Pat. Nos. 2,455,- 173; 2,948,581; and 3,122,412. It is recommended that fibrous materials prior to the heat treatment (described hereafter) be drawn to a single filament tenacity of at least about 3 grams per denier. If desired, however, the fibrous material may be more highly oriented, e.g. drawn up to a single filament tenacity of about 7.5 to 8 grams per denier, or more. Additionally, the acrylic films optionally may be either uniaxially or biaxially oriented prior to the heat treatment described hereafter.

Immediately prior to the heat treatment step employing a gaseous atmosphere of enriched molecular oxygen content the acrylic fibrous material or film contains stannous chloride substantially uniformly incorporated therein in a concentration of about 0.5 to 10 percent by weight, and preferably in a concentration of 1 to 5 percent by weight.

The resulting acrylic fibrous material or film containing stannous chloride incorporated therein is heated in a gaseous atmosphere containing 30 to 100 percent by weight molecular oxygen provided at a temperature of about 260 to 350 C. until a stabilized fibrous product or film is formed which retains its original configuration substantially intact and which is non-burning when subjected to an ordinary match flame. The portion of the gaseous atmosphere other than molecular oxygen, if any, is preferably substantially unreactive with the acrylic fibrous material or film during the stabilization treatment, e.g. it may include nitrogen, hydrogen, carbon dioxide, carbon monoxide, argon, helium, etc. In a preferred embodiment of the process, the oxygen-containing atmosphere is air enriched with molecular oxygen. Molecular oxygen is preferably present in the gaseous atmosphere in a concentration of 35 to 100 percent by weight, and most preferably in a concentration of about 40 to 60 percent by weight. Preferred temperatures for the oxygen-containing atmosphere range from about 290 to 310 C. If desired, the fibrous material or film may be exposed to a temperature gradient wherein the temperature is progressively increased. The presence of an enriched oxygen atmosphere in combination with the presence of stannous chloride has been found to be of prime importance in accomplishing the improved stabilization results discussed hereafter.

For best results during the stabilization reaction uniform contact with the gaseous atmosphere throughout all portions of the stannous chloride containing acrylic material is encouraged. Such uniform reaction conditions can best be accomplished by limiting the mass of fibrous material or film at any one location so that heat dissipation from within the interior of the same is not unduly impaired, and free access to molecular oxygen is provided. For instance, the acrylic fibrous material or film may be placed in the gaseous atmosphere While wound upon a support to a limited thickness. In a preferred embodiment of the invention, the stannous chloride containing acrylic fibrous material or film is continuously passed in the direction of its length through the heated gaseous atmosphere. For instance, a continuous length of the acrylic fibrous material or film may be passed through a circulating oven or the tube of a muflle furnace. The speed of passage through the heated oxygen-containing atmosphere will be determined by the size of the heating zone and the desired residence time.

The period of time required to complete the stabilization reaction within the gaseous atmosphere is generally inversely related to the temperature of the atmosphere, and is also influenced by the denier of the acrylic fibrous material or the thickness of the film undergoing treatment, and the concentration of molecular oxygen in the atmosphere. Treatment times in the oxygen-containing atmosphere accordingly commonly range from about 6 minutes to 60 minutes.

Thhe stabilized acrylic fibrous materials or films formed in accordance with the present process are black in appearance, retain substantially the same configuration as the starting material, are non-burning when subjected to an ordinary match flame, commonly have a bound oxygen content of at least 7 (e.g. 7 to 12) percent by weight as determined by the Unterzaucher, or other suitable analysis, commonly contain from about 50 to 65 percent carbon by weight, and commonly contain about 0.4 to 8 percent tin by weight.

The theory whereby the presence of stannous chloride in combination with a greater than usual oxygen concentration in the gaseous atmosphere produces improved stabilization results is considered complex and incapable of simple explanation. The results achieved are considered to be surprising and unexpected. While it has been suggested in the past that acrylic stabilization reactions can be conducted in an atmosphere of air enriched with oxygen, the results of such stabilization conditions have tended to be less than optimum particularly it relatively high stabilization temperatures (e.g. 260 C. and above) are selected because of the increased tendency for an explosive exotherm to occur under such conditions. Such an exothermic reaction at the very least produces a weak and brittle product and may result in a complete breakage of the acrylic fiber or fragmentation of the acrylic film. It has now been found after extensive experimentation that While stannous chloride has the ability to accelerate the kinetics of the cyclization portion of the stabilization reaction in air, that the presence of this compound has the concomitant tendency to retard the dehydrogenation and oxidative cross-linking portions of the stabilization reaction. Additionally, even when the oxygen concentration of the gaseous stabilization atmosphere is increased, the mechanical properties of the resulting product are surprisingly not diminished and even more surprisingly are enhanced. For instance, stabilized products formed in the present process in the presence of stannous chloride exhibit a higher tenacity and modulus than if produced in the presence of stannous chloride in air. Not only is the stabilization reaction accelerated, but no substantial fiber coalescence occurs and less fiber weight loss results. The process of the present invention proceeds at an expeditious rate in a controlled fashion with the fiber temperature during the stabilization reaction more closely approximating that of the gaseous atmosphere while eliminating a deleterious exothermic reaction.

In our commonly assigned Ser. No. 248,372, filed concurrently herewith, is disclosed a related process wherein stannous chloride is incorporated into a previously formed acrylic fibrous material or film by contact with a solution of the same provided at a moderate temperature, and the resulting stannous chloride impregnated fiber or film stabilized in a gaseous atmosphere containing more than the usual concentration of oxygen.

The stabilized fibrous material resulting from the stabilization treatment of the present process is suitable for use in applications where a fire resistant fibrous material is required. For instance, non-burning fabrics may be formed from the same. As previously indicated, the stabilized acrylic fibrous materials are particularly suited for use as intermediates in the production of carbonized fibrous materials. Such amorphous carbon or graphitic carbon fibrous products may be incorporated in a binder or matrix and serve as a reinforcing medium. The carbon fibers may accordingly serve as a lightweight load bear- 7 ing component in high performance composite structures which find particular utility in the aerospace industry.

The stabilized film resulting from the stabilization treatment is suitable for use in applications where a fire resistant sheet material is required. Such stabilized films may also be utilized as intermediates in the production of carbonized films. Carbonized films may be utilized in the formation of lightweight high temperature resistant laminates when incorporated in a matrix material (e.g. an epoxy resin).

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

EXAMPLE I A solution of an acrylic polymer is formed while employing N,N-dimethylformamide as a solvent while maintained at 50 C. Particulate acrylonitrile homopolymer is added to the solvent with stirring in a concentration of 25 percent by weight based upon the weight of N,N-dimethylformamide. Stannous chloride is dissolved in the acrylic polymer solution with stirring in a concentration of 7.5 percent 'by weight based upon the weight of the acrylic polymer. The pendant nitrile groups of the acrylic polymer while dissolved in the N,N-dimethylformamide are substantially uncyclized.

Following filtration the solution is fed while at a temperature of 35 C. to a standard cup type spinneret having a circle of 40 holes each having a diameter of 50 microns. The solution is extruded through the spinneret into a coagulation bath of approximately 60 parts by weight of ethylene glycol and approximately 40 parts by weight of N,N-dimethylformamide provided at 25 C. to form a continuous length of acrylic yarn having stannous chloride substantially uniformly incorporated therein. The resulting yarn fibers possess a denier per filament of about 4, are subsequently washed in water to remove residual solvent, are dried, and are drawn at a draw ratio of about :1 by passage over a hot shoe at a temperature of about 140 C. to produce an acrylic fibrous material exhibiting a single filament tenacity of about 4.5 grams per denier. The pendant nitrile groups of the resulting acrylic fibrous material are substantially uncyclized.

A portion of the yarn containing 5 percent by weight stannous chloride is next stabilized on a continuous basis by heating in a circulating gaseous atmosphere of air encircled with molecular oxygen provided in a muffle furnace. The total oxygen concentration in the atmosphere is 40 percent by weight. The gaseous atmosphere is provided at a temperature of 300 C. and the residence time therein is 8 minutes. The yarn is maintained under a longitudinal tension sufiicient to maintain a substantially constant length during the stabilization reaction.

The resulting stabilized yarn is black in appearance, non-brittle, flexible, has a textile-like hand, retains its original fibrous configuration substantially intact, is nonburning when subjected to an ordinary match flame, retains strength after glowing in a match flame, and has an oxygen content in excess of 8 percent by weight as determined by the Unterzaucher analysis.

In a control run a sample of acrylonitrile homopolymer yarn is passed through the muflie furnace in an identical manner with the exception that the yarn was formed from a N,N-dimethylformamide solution which contains no stannous chloride. The resulting yarn is coalesced, extremely brittle, and non-flexible.

The resulting stabilized yarn of Example -I is carbonized and graphitized in accordance with the teachings of US. Ser. No. 777,275, filed Nov. 20, 1968, of Charles M. Clarke (now abandoned) which is herein incorporated by reference. The resulting graphite yarn exhibits satisfactory tensile properties.

8 EXAMPLE 11 Example I is repeated with the exception that portions of the stannous chloride containing yarn are stabilized for 15 minutes in air enriched with molecular oxygen atmospheres containing 40 and percent by weight molecular oxygen by weight provided at 290 C.

In each instance the resulting stabilized yarn is black in appearance, non-brittle, flexible, has a textile-like hand, retains its original fibrous configuration substantially intact, is non-burning when subjected to an ordinary match flame, retains strength after glowing in a match flame, and has an oxygen content in excess of 8.5 percent by weight as determined by the Unterzaucher analysis.

The stabilized yarn produced in the 40 percent by weight molecular oxygen gaseous atmosphere possesses a single filament tenacity of 2.5 grams per denier, and a Youngs modulus of 300 grams per denier.

The stabilized yarn produced in the 80 percent by weight molecular oxygen gaseous atmosphere possesses a single filament tenacity of 2.5 grams per denier, and a Youngs modulus of 350 grams per denier.

For comparative purposes the process of Example II is repeated with the exception that the gaseous atmosphere is air only and contains 20.9 percent by weight molecular oxygen. The resulting fibers burn when subjected to an ordinary match flame, exhibit a single filament tenacity of only 0.5 gram per denier, and a single filament Youngs modulus of only 150 grams per denier.

EXAMPLE I11 Example I is repeated with the exception that a portion of the solution of the acrylonitrile homopolymer containing stannous chloride dissolved therein was placed upon a glass support in a thickness of 10 mils and the solvent evaporated in a circulating air oven providedat 110 C. to form a flexible film. The pendant nitrile groups of the acrylic polymer of the resulting film are substantially uncyclized. Stannous chloride is substantially uniformly incorporated within the film in a concentration of approximately 7.5 percent by weight. The resulting stannous chloride containing film is next suspended for 7 minutes in a circulating air enriched with molecular oxygen atmosphere containing 40 percent by weight molecular oxygen provided in at 300 C. wherein it is converted to a stabilized form while retaining its original configuration substantially intact. The resulting stabilized film is black in appearance, non-brittle, flexible, non-burning when subjected to an ordinary match flame, and contains a bound oxygen content in excess of about 7 percent by weight as determined by the Unterzaucher analysis.

In a control run a sample of an acrylonitrile homopolymer film is processed in an identical manner with the exception that the film is formed from a N,N-dimethylformamide solution which contains no stannous chloride. The resulting film is brittle and falls apart.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in theart. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

We claim:

1. An improved process for the production of thermally stabilized acrylic fibers and films exhibiting enhanced physical properties comprising:

(a) providing a solution of (1) an acrylic polymer selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith wherein the pendant nitrile groups present in said acrylic polymer are substantially uncyclized, (2) a minor quantity of stannous chloride, and (3) a solvent for said acrylic polymer and said stannous chloride, (b) forming from said solution an acrylic fibrous material or film wherein the pendant nitrile groups present in said acrylic polymer are substantially uncyclized having incorporated therein about 0.5 to

10 percent by weight of said stannous chloride based upon the weight of said acrylic polymer, and (c) heating said acrylic fibrous material or film having said stannous chloride incorporated therein in a gaseous atmosphere containing 30 to 100 percent by weight molecular oxygen provided at a temperature of about 260 to 350 C. until a thermally stabilized fibrous material or film is formed which is black in apearance, retains its original configuration substantially intact, contains a bound oxygen content of at least 7 percent by weight, and which is non-burning when subjected to an ordinary match flame, with any portion of said gaseous atmosphere other than molecular oxygen being substantially unreactive with the material undergoing stabilization.

2. An improved process of claim 1 in which said acrylic polymer present in said solution is an acrylonitrile homopolymer.

3. An improved process of claim 1 in which said acrylic polymer present in said solution is an acrylonitrile copolymer containing at least about 95 mol percent of acrylonitrile units and up to about mol percent of one or more monovinyl units copolymerized therewith.

4. An improved process of claim 1 in which said acrylic polymer is present in said solution in a concentration of about 5 to 30 percent by weight based upon the weight of the solvent.

5. An improved process of claim 1 in which said stannous chloride is present in said solution in a concentration of about 0.5 to 20 percent by weight based upon the weight of said acrylic polymer.

6. An improved process of claim 1 in which said stannous chloride is present in said solution in a concentration of about 1 to percent by weight based upon the weight of said acrylic polymer.

7. An improved process of claim 1 in Which said solvent for said acrylic polymer and said stannous chloride is selected from the group consisting of N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, butyrolactone, and N-methyl-Z-pyrrolidinone.

8. An improved process of claim 1 in which said solution is extruded through a shaped orifice to form a fibrous material.

9. An improved process of claim 8 in which said fibrous material is drawn to a single filament tenacity of at least about 3 grams per denier prior to heating in said gaseous atmosphere containing 30 to 100 percent by weight molecular oxygen.

10. An improved process of claim 9 in which said acrylic fibrous material is a continuous multifilament yarn.

11. An improved process of claim 1 in which said solution is extruded through a shaped orifice to form a film.

12. An improved process of claim 1 in which said fibrous material or film contains said stannous chloride in a concentration of about 1 to 5 percent by wegiht based upon the weight of said acrylic polymer immediately prior to heating said gaseous atmosphere containing 30 to 100 percent molecular oxygen by weight.

13. An improved process of claim 1 in which said gaseous atmosphere contains about 35 to 100 percent molecular oxygen by weight.

14. An improved process for the production of thermally stabilized acrylic fibers exhibiting enhanced physical properties comprising:

(a) providing a solution of (1) an acrylic polymer selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers containing at least about mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith wherein the pendant nitrile groups of said acrylic polymer are substantially uncyclized, (2) a minor quantity of stannous chloride, and (3) a solvent for said acrylic polymer and said stannous chloride,

(b) extruding said solution through a shaped orifice to form an acrylic fibrous material wherein the pendant nitrile groups present in said acrylic polymer are substantially uncyclized having stannous chloride incorporated therein,

(c) drawing said acrylic fibrous material to a single filament tenacity of at least about 3 grams per denier,

(d) heating said acrylic fibrous material having incorporated therein about 1 to 5 percent by weight stannous chloride based upon the weight of said acrylic polymer in a gaseous atmosphere containing 35 to 100 percent molecular oxygen provided at a temperature of about 290 to 310 C. until a thermally stabilized fibrous material is formed which is black in appearance, retains its original configuration substantially intact, contains a bound oxygen content of at least 7 percent by weight, and which is nonburning when subjected to an ordinary match flame, with any portion of said gaseous atmosphere other than molecular oxygen being substantially unreactive with the material undergoing stabilization.

15. An improved process of claim 14 in which said acrylic polymer is an acrylonitrile homopolymer.

16. An improved process of claim 14 in which said acrylic polymer is an acrylonitrile copolymer containing at least about mol percent of acrylonitrile units and up to about 5 mol percent of one or more monovinyl units copolymerized therewith.

17. An improved process of claim 14 in which said solvent for said acrylic polymer and said stannous chloride is selected from the group consisting of N,N- dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, butyrolactone, and N-methyl-Z-pyrrolidinone.

18. An improved process of claim 14 in which said acrylic fibrous material is a continuous multifilament yarn.

19. An improved process of claim 14 in which said gaseous atmosphere contains about 40 to 60 percent molecular oxygen by weight.

References Cited UNITED STATES PATENTS 3,647,770 3/1972 Gump et al. 423-447 X 3,539,295 11/1970 Ram 8-115.5 X 3,427,120 2/1969 Shindo et al. 8115.5 3,242,000 3/ 1966 Lynch 8Acrylo 3,729,549 4/1973 Gump et al. 8115.5

LEON D. ROSDOL, Primary Examiner H. WOLMAN, Assistant Examiner US. Cl. X.R. 423-447