US3376253A - Flame retarding acrylonitrile polymer compositions - Google Patents

Description

Unite States Patent 3,376,253 FLAME RETARDING ACRYLGNITRILE POLYMER COMPOSITIONS Edward V. Buruthall and Julian I. Hirshfeld, Decatur, Ala, assignors to Monsanto (lampany, St. Louis, M0., a corporation of Delaware N0 Drawing. Filed May 5, 1966, Ser. No. 547,742 12 Claims. (Cl. 260-457) ABSTRACT OF THE DISCLOSURE The instant invention relates to a novel method of producing flame retardant synthetic compositions and to the novel compositions so produced. More particularly, the instant invention provides a novel method for producing acrylic fibers and films having a high degree of permanent flame resistance, and to the novel acrylic articles themselves which remain flame resistant over long periods of time despite use and laundering.

Flame and heat resistant fabrics in general have been commonplace for many years. One method of achieving flame resistance in fibers and films has been to introduce a chlorine or phosphorus-containing monomer in the synthetic polymer chain. Although such specialty polymers are useful in specific applications, in general such polymers do not possess the all-around qualities of dyeability, resiliency, texture and hand which is demanded of a commercial synthetic fiber to be used in consumer textile applications. Moreover, in many of these applications, although flame resistance would be desirable from safety and other standpoints, it is not mandatory and hence the disadvantages of specialty polymers have essentially precluded them from this market.

Another approach to flame resistance has been to impregnate the fiber with a flame resistant composition, as for example by immersion, spraying or the like. Such techniques suffer from two prime disadvantages. Initially, such treatment usually adversely affects the appearance and hand of the fiber and hence is not usually desirable, and secondly, the degree of flame resistance available by such techniques is generally only temporary inasmuch as, at least in the case of fibers, the flameproofing agent is usually gradually removed as a result of wear and laundering.

Fluoboric acid and the metal fluoborates have been recognized in the past as having beneficial flame retardant properties and accordingly have been applied to fabrics in the past with an eye toward flameproofing. However, fluoboric acid itself is extremely soluble in water, and likewise are many of the metal fiuoborates, such as sodium fluoborate and the like. Accordingly, despite the thoroughness with which the fiber is impregnated with these compounds, eventually laundering, particularly at high temperatures, leaches the fluoboric acid or fluoborate from the fiber, thus eliminating the flame resistant effect. Attempts to substitute insoluble or sparingly soluble fluoborates as flameproofing agents have been equally unsuccessful inasmuch as these compounds must be applied to fabrics from a dispersion in water. Hence, the application is extremely diificult and results only in a surface 3,376,253 Patented Apr. 2, 1968 coating of flameproofing agent which usually adversely alfects the texture and hand of the fabric, and is invariably at least partially removed during the use of the treated article.

The difiiculties in fabric texture which arise as the result of flameproofing treatment consequently provoke further countermeasures in an attempt to restore fabric resiliency and hand. Quite often, the flameproofing of fabrics is accompanied by the addition of plasticizers and like compounds to the fabric in order to combat the harshness usually imparted by the flameproofing treatment. Thus, despite the apparent desirability of flame retardant properties in everyday fabrics, flameproofing is not usually available in fabrics where such a requirement is not demanded because of a special end use due to the numerous measures and countermeasures which must be undertaken to finally produce a fabric of satisfactory quality.

It is accordingly an object of this invention to provide a novel process for spinning flame resistant acrylic fibers which possess a pleasing texture and hand and which at once retain their flame retardant properties after prolonged use and laundering. It is a further object of this invention to provide a process for producing flame retardant acrylic fibers which are impregnated with a metal fluoborate which is not leached from the fiber during washing, even at high temperatures. It is a further object of this invention to provide such novel acrylic fibers so impregnated. It is another object of this invention to provide a novel wet spinning technique for producing acrylic fibers which do not support a flame. Although the instant specification will be primarily directed to the production of flame resistant fibers from acrylic polymers, it is to be understood that the instant novel process is equally eflicient for producing novel acrylic films and other articles which are extruded by wet extrusion techniques, i.e., those articles produced by extruding an acrylic dope solution into a coagulant.

These and other objects are achieved by the novel process of the instant invention wherein acrylonitrile polymer is dissolved in fluoboric acid and then coagulated as a filament or a film in a coagulating bath containing metal salts, the metal portions of which form insoluble fluoborates upon reaction with fluoboric acid. Accordingly, there is formed in situ within the fiber fluoborate salts which are relatively insoluble in water and hence which are not subject to leaching during subsequent laundering of the fiber. Furthermore, inasmuch as these insoluble fluoborates are formed within the fiber at a point when the fiber is still in a gelatinous state the uniformity of dispersion of the fluoborates through the fiber is remarkably uniform. Furthermore, the in situ formation of these fluoborate salts result in the thorough impregnation of the fiber with a flameproofing agent without adversely affecting the hand or texture of the ultimate fiber.

In the practice of the instant invention the acrylonitrile polymer dope is prepared in much the same manner as is employed in ordinary Wet extrusion techniques. Initially, the polymer in a finely divided solid state is mixed with the fluoboric acid to form a dope solution. The solution may be heated for example from 25 up to C. to raid dissolution. Generally the acrylonitrile polymer will be dissolved in the fluoboric acid in such amount as to provide from about 5 to about 35 percent by weight of polymer in the solvent. Of course, optimum values of concentration will be determined by the molecular weight of the particular polymer being spun, and accordingly the ultimate viscosity of the polymer solution. Although the low concentrations of polymer in solvent can be used in extrusion operations, such concentrations are undesirable because they necessitate the removal and recovery of too much solvent from the extruded solution, thereby increasing solvent recovery cost and reducing spinning speeds by reason of the longer period required for coagulation. Particularly in the instance of the present invention, the expense of fiuoboric acid necessitates its recovery from the coagulating bath, and accordingly the concentration of polymer in the fluoboric acid solution is preferably maintained between about 10 and 25 percent.

Fluoboric acid will function adequately as a solvent for acrylonitrile polymer at aqueous concentrations ranging from 35 percent to about 60 percent by weight. Typically, for example, a 48 percent solution of fluoboric acid in water will perform satisfactorily as a solvent for the acrylic polymer, though higher and lower concentrations may be used.

Fluoboric acid need not be the sole polymer solvent used in preparing the dope solution. To minimize the expense and the need for recovery, it is preferred that the acrylic polymer solution be prepared using a cosolvent system comprising fluoboric acid and a miscible compatible polyacrylonitrile solvent such as dimethylformamide or dimethylacetamide. Generally the fiuoboric acid should be present in such amount as to provide at least about percent by weight based upon the Weight of the acrylic polymer in the dope solution. However, the amount of fluoboric acid employed in the dope system will be highly dependent upon the ultimate desired concentration of fluoborate in the coagulated fiber. Hereinafter, reference to the fiuobon'c acid solution of the acrylonitrile polymer is deemed to include solutions wherein fluoboric acid is employed as a cosolvent with a second miscible and compatible acrylonitrile polymer solvent.

The fibers are spun by extruding the fluoboric acid so lution of the acrylonitrile polymer through an orifice or a spinneret having a plurality of orifices into a liquid medium which at once results in the formation of insoluble or sparingly soluble fluoborates in situ in the fiber and also extracts from the gelatinous fiber the remainder of the polymer solvent. In accordance with this invention the coagulating bath contains dissolved amounts of metal salts, the metal portion of which will combine with the fluoboric acid to form sparingly soluble fiuoborates in situ within the fiber. The metals, potassium, rubidium, and cesium have been found to be effective for this purpose, and accordingly a salt of one or more of these metals must be present in the coagulating bath to effect the proper in situ formation of the insoluble fluoborate fiameproofing agents. A liquid medium employed in the coagu lating bath is not critical so long as it functions properly to remove the solvent from the filaments and is capable of dissolving the metal salt used in the formation of the in situ insoluble fluoborates. Typical of liquidmediums which are useful for this purpose are simple aqueous solutions of soluble potassium salts. When an organic cosolvent is used, the coagulating bath may contain up to about 65% of the organic solvent.

As stated above, the potassium, rubidium and cesium salts of the fluoborates are sufficiently water insoluble to function properly as permanent fiameproofing agents in accordance with the instant invention.

In this regard, the terms insoluble and sparingly soluble as used herein refer to fluoborate salts having a maximum solubility in water of about one-half mole per liter. In general, any of these metals may be introduced into the coagulating bath solution as any suitable soluble salt otherwise inert to the system. Thus, suitable salts of the above metals include the phosphates, sulfates, halides, acetates, tartrates, and the double salts with aluminum called alums. Exemplary of the metal salts which may therefore be employed are potassium aluminum sulfate, potassium phosphate, cesium aluminum sulfate, rubidium aluminum sulfate, rubidium tartrate, cesium chloride, potassium acetate, potassium chloride, potassium tartrate and the like. The comparative availability of potassium salts renders the salts of this metal particularly preferred.

Although any of the above metal salts may be employed in the coagulating bath solution, the use of phosphate salts in particular is preferred inasmuch as the phosphates impart an additional degree of flame resistance to the fiber. Although the flame resistance so attained is of primarily a temporary nature, the mere coagulation of a filament in the presence of phosphate necessarily implies the inclusion of some phosphate within the filament, hence increasing the initial, if not the ultimate, flame resistance of the fiber. In any case, it is preferred to employ such metal salts which have the tendency to improve fiameresistance, rather than employing salts which might tend to have the opposite effect, such as the nitrates or perchlorates.

In general, the metal salts are present in the solution in amounts ranging from about 5 percent by weight based upon the coagulating bath solution to amounts which completely saturate the bath with salt. However, the absolute amount of salt present in the coagulating bath solution is not critical. The comparative concentration of fluoboric acid in the polymer dope solution and of metal salt in the coagulating bath solution will however determine the ultimate amount of fiuoborate fiarneproofing agent which is formed in situ in the fiber. In general, it is preferred that the amount of insoluble fiuoborate salt formed in situ in the fiber range from about 4 to about 10 percent by weight based upon the ultimate weight of the dried fiber. The

concentration of fluohoric acid in the spinning dope solu tion and of metal salts in the coagulating bath can accordingly be adjusted to achieve the desired amount of fluoborate in the ultimate fiber.

Inasmuch as the metal salt is consumed from, the coagulating bath as the fiuoboric acid containing dope solution is fed into the coagulating bath, it is desirable to continually supplement the concentration of metalsalt in the coagulating bath by constant addition of metered amounts of metal salt. Such a procedure aflords the opportunity to closely control the fluoborate and to minimize the amount of insoluble fluoborate formation outside the fiber. Since fiuoboric acid recovery from the coagulating bath is desirable, avoidance of unnecessary insoluble fluoborate precipitation outside the.

fiber should be avoided if possible.

Due to the extremely corrosive nature of fiuoboric acid it is necessary in conducting the novel process of this invention to thoroughly wash the coagulated fiber to remove any free fluoboric acid remaining in or on the fiber. The sparingly soluble fiber are not corrosive but small amounts of free fiuohoric acid may remain on the fiber after passage through the coagulating bath. Accordingly a Washing procedure involving, for example, passing the coagulated fiber through a water cascade at atemperature of about to C.

is subsequently employed. Generally, such a washing, operation is performed on all acrylonitrile fibers to remove residual solvent after the coagulating bath. In conducting the instant novel process such standard washing procedures may be adequate to remove the fiuoboric acio,

particularly when the amount of fiuoboric acid in the dope solution had been low. However, lengthen the standard washing operation or provide an additional washing stepto control the fluoboric acid concentration on the fiber. Typically the coagulated fiber is subjected to washing conditions as above for a period of 12 seconds to upwards of a minutee or more.

The novel process of this invention is employed in conjunction with the spinning of acrylonitrile polymers, including homopolymers of acryonitrile, copolymers of acrylonitrile with copolymerizable monomers, as ,well. as polymer blends, said polymers containing at least about 75 percent and preferably at least 85 percent by weight polymerized acrylonitrile. Other monomeric polymerizable compounds containing ethylenic unsaturation which are copolymerizable with acrylonitrile may form part of the polymer which may be used in the practice of this invention. Examples of ethylenically unsaturated monomers copolymerizable with acrylonitrile include the vacrylates and alkyl arcrylates such as methyl acrylate, ethyl acrylate, butyl arcrylate, Z-ethyl hexyl acrylate, methyl concentration in the fiber.

fluoborate salts formed in situ in the it may be necessary to i Example 1 To a flask containing 90 grams of a 48 percent by weight solution of fluoboric acid there was added 10 grams of an acrylonitrile polymer having a composition of 94 percent polymerized acrylonitrile and 6 percent of polymerized vinyl acetate. The mixture was heated at 80 to 85 C. while stirring. A clear yellow solution resulted. The solution was coagulated as a filament in water containing 20 percent by weight potassium aluminum sulfate and percent by weight dimethyl formamide. The resulting filament was scoured by boiling in fresh water for 5 minutes. The filament obtained was dried in an oven until completely dry. The filament could not be ignited and did not support burning upon being placed in a flame.

Example 2 The procedure of Example 1 was followed save that the potassium aluminum sulfate was omitted from the coagulating bath. The filament so obtained ignited upon being placed in a flame and continued to support burning.

Example 3 The procedure of Example 1 was repeated coagulating a mass of the same acrylonitrile polymer to form a casting. The coagulating bath in this example consisted of water containing 20 percent by weight of tribasic potassium phosphate. After being washed and dried in an oven as in Example 1 the casting so obtained did not ignite upon being placed in a flame and did not support burning.

Example 4 The procedure of Example 3 was repeated save that the tribasic potassium phosphate was omitted from the coagulating bath solution. After drying the casting so obtained burned upon being placed in a flame and supported burning upon removal from the flame.

Example 5 To a flask containing 90 grams of a 48 percent by weight solution of fluoboric acid there was added grams of an acrylonitrile polymer having a composition of 94 percent by weight polymerized acrylonitrile and 6 percent by weight of polymerized vinyl acetate. The mixture was heated at 80 to 85 C. with stirring to produce a clear yellow solution. This solution was coagulated as a filament in a Water bath. The filament so obtained was divided in two parts to provide two samples. The first sample of the filament was removed from the coagulating bath and directly dried. The second sample of the filament was first rinsed well and then scoured by boiling in fresh water for five minutes after which it was rinsed and dried.

The first sample which was directly dried did not ignite and did not burn upon being placed in a flame. The second sample ignited upon being placed in a flame and supported burning.

Example 6 An acrylonitrile polymer solution prepared as in Example 5 was coagulated as a filament in 500 cubic centimeters of water containing 20 percent by weight aluminum sulphate and containing 25 grams of dimethyl formamide. The filament so obtained was divided into two portions to provide two samples which were treated separately.

The first sample of filament was directly dried while the second sample of filament was first rinsed. then scoured by being placed in boiling water for five minutes, after which it was again rinsed and then dried. Upon being placed in a flame neither the filament which was directly dried nor the filament which was subjected to a scouring procedure ignited or burned.

Example 7 The procedure of Example 6 was repeated save that the coagulating solution contained 20 percent by weight of potassium phosphite and did not contain dimethyl form-amide. A filament was obtained and divided into two samples to be treated as in Example 6.

It was found that upon being placed in a flame neither the filament which was directly dried nor the filament which was subjected to the scouring procedure ignited or burned.

What is claimed is:

1. A process for uniformly impregnating an acrylic polymer with Water-insoluble fluoborate fireproofing agent comprising the steps of:

(a) dissolving an acrylic polymer in an inert solvent comprising fluoboric acid, said polymer containing at least by weight acrylonitrile, any balance being another ethylenically unsaturated monomer which is copolymerizable with acrylonitrile;

(b) extruding said polymer into a coagulating bath to effect coagulation of said polymer, said bath containing a dissolved metal salt which reacts with the fluoboric acid of the polymer solution to form an insoluble metal fluoborate salt in situ within the polymer during coagulation thereof, said fluoborate salt being characterized in having a solubility in water of less than one-half per liter; and

(c) recovering the fluoborate-containing coagulated polymer from the coagulating bath and removing any free fluoboric acid therefrom.

2. The process of claim 1 wherein said coagulated polymer is in the form of a filament.

3. The process of claim 1 wherein said mer is in the for-m of a film.

4. A process for uniformly impregnating an acrylic polymer with water-insoluble fluoborate fireproofing agent comprising the steps of (a) dissolving an acrylic polymer in an inert solvent comprising fluoboric acid, said polymer containing at least about 75 by weight acrylonitrile, any balance being another ethylenically unsaturated monomer which is copolymerizable with acrylonitrile;

(b) extruding said polymer solution into an aqueous bath to effect coagulation of said polymer in the form of a filament, said bath containing a dissolved metal salt selected from the group consisting of potassium, cesium and rubidium salts which reacts with the fluoboric acid of the polymer solution to form the corresponding fluoborate salt in situ within the coagulated filament; and

(c) recovering the filament from the water bath and removing any free fluoboric acid therefrom.

5. The process of claim 4 wherein the fluoboric acid is present in the polymer solution in an amount corresponding to at least 5 percent by weight based upon the weight of the acrylic polymer.

6. The process of claim 4 wherein said salt is present in the bath in an amount ranging from about 5 percent to about 25 percent by weight based upon the weight of the aqueous solution.

7. The process of claim 4 wherein said metal salt is potassium phosphate.

coagulated poly- 8. The process of claim 4 wherein said metal salt is potassium aluminum sulphate.

9. The process of claim 4 wherein the acrylic polymer contains about 94% by weight acrylonitrile and the balance of the polymer is vinyl acetate.

10. A fire resistant acrylic film obtained by the process of claim 4.

11. A fire resistant :acrylic filament obtained by the process of claim 4.

12. The filament of claim 11 wherein said fluoborate 1 salt is potassium fluoborate.

8 References Cited FOREIGN PATENTS 560,993 7/1958 Canada.

DONALD E. CZAJA, Primary Examiner.

M. I. WELSH, Assistant Examiner.