390
INDUSTRIAL AND ENGINEERING CHEMISTRY
(13) Hoogsteen, H. AI., Young, A. E., and Smith, 11.K., IND. EBG. CHEM., 42,1587 (1950). (14) Kappelmeier, C. P. A., Paint, O i l , Chem. Ret., 114, S o . 3, 16 (1951). (15) McKabb, J. W., and Payne, 11. F., IND. ENG.CHEM.,44, 2394 (1952). (16) Patrick, W. H., and Trufisell, E. H., Ofic. Dig. Federation Paint & Vamish Production Clubs, KO. 309, 767 (1950). (17) Petit, J., and Fournier, P., I b i d . , No. 307, 609 (1950).
Vol. 46, No. 2
(18) Powers, P. O., IND.ENG.CHEM.,42, 2096 (1950). (19) Bchroeder, H. RI., a n d Terrill, R.L., J . Am. Oil Chemists' SOC., 26,153 (1949). (20) Tess, R. W., Jakob, R . H., and Bradley, T. F., U. 8. P a t e n t 2,596,737 (May 13, 1952).
RECEIVED for review M a y 8, 1952. ACCEPTED November 4, 1953. Presented before Division of Paint, Varnish, and Plastics Chemistry at the 121st lfeeting, . ~ W . X C A S C1i~mc.41SOCIETY, h,Iilwaukee, Wis., >larrh 1952
Spinning Solvents for Polyacrylonitrile GEORGE E. HAM The Chemstrand Corp., Decatur, A h .
P
OLYACRTLONITRII~Ewas first described as a vinyl polymer of high softening point in several patents issued to 1. G. Farbenindustrie over 20 years ago ( 2 0 ) . For many years, polyacrylonitrile defied commercial utilization because of its insolubility in common solvents and high softening point. I n 1942 Kern and Fernow (18) attempted t o relate these characteristics with polymer structure. In the period between 1920 and 1940 the use of acrylonitrile in copolymers with butadiene, vinyl chloride, and other common monomers was extensively described, but polyaciylonitrile is conspicuous by its absence from hhe literature. In 1946 E. I. du Pont de Kemours & Co., Inc., was granted broad patent coverage on solvents for polyacrylonitrile, and in 1948 announced the original development of a polyacrylonitrile fiber. This development and the research preceding it have been adequately summarized by €Iout,z ( 1 4 ) . Research on polyacrglonitrile solvents was also conducted by €1. Rein a t I. G. Farbenindustrie in the period 1930-40. One solvent for polyacrylonitrile which was reported by I. G. Farbenindustrie is sulfuric acid (21). Fibers were actuallv produced from these solutions. However, considerable hydrolysis to polyacrylamide and polyacrylic acid occurred, and fibers of little utility resulted. Kern and Fernow (18) also reported that sulfuric acid was a solvent for polyacrylonitrile. I n addition, concentrated salt, solutions (zinc chloride, sodium thiocvanate, etc.) and quat,ernary ammonium compounds were found t o be solvents by R.ein, but molecular weight degradation of the polymer and unsatisfactory phase equilibria prevented the preparation of good fibers by techniques then known. From work on polyvinyl chloride, Rein knew that tetrahydrofuran and cyclopentanone were much better solvents than t,heir aliphatic counterparts, diethyl et,herand diethyl ketone. Two compounds which had shoxn other promising solvent characteristics were r-butyrolactone and 2-pyrrolidonej which were available from acetylene and formaldehyde, via the Reppe synthesis. These compounds were found to be solvents for polyacrylonitrile (62). These compounds were not complet~ely suitable because of their instability, so a more stable analog, N-formylpiperidine, was prepared. This compound was found to be a good solvent for polyacrylonitrile. Lower open chain homologs, in particular dimethylformamide and diethylformaniide, were tried. The former compound was found to be an excellent solvent. Numerous other compounds were found by Rein to be solvents, some of which will be discussed later. In 1946 t,he -4nierican patents issued to Du Pont showed the use of a wide variety of solvents for polyacrylonitrile. Dimethylformamide was the most conspicuous of these solvents, but a wide variet,y of dimethylamides (19), dinitriles (16), dithiocyanates ( l 7 ) ,sulfones ( 1 6 ) . and sulfoxides were described. Our work on solvents for polyacrylonitrile was undert'aken in 1946. The first solvent,s for polyacrylonitrile which \yere
studied were maleic anhydride (melting point 5 7 ' l o 60" C.) and succinic anhydride (melting point 119.6" C.) (7'). Related nonsolvenw were caitraconic. anhydride, chloromaleic anhydride, and acetic anhydride. The solutions were not entirely satisfactory, however. The cyclic anhydrides crystallized from the solution on cooling. In addition, fibers could not be precipitated direct,ly into water. It was necessary to use alcohol or caustic solutions as precipitating baths. Rein had previously described lactones as solvents ($8). The use of maleic and succinic anhydrides as solvents for polyacrylonitrile is also described in Du Pont (6') and Imperial Chemicals Industries ( I S ) patents. AMIDEI'YPE SOLVENTS
Another solvent for polyacrylonitrile is cyanoacetamide ( 8 , 14). Thie is unusual, since acetamide is not a solvent for polyacrylonitrile. I n general, compounds possessing free amide groups exhibit sufficient int,ermolecular hydrogen bonding to prevent solvent action toward acrylonitrile polymers. The high melt,ing point of cyanoacetamide (118" to 119' C.) is undesirable. Tris( dimethylamido) phosphate dissolves polyacrylonitrile at 150" C. to yield solutions mhic-h are stable at room tempcrat,ures:
0 f (CH~)2;1'--P--p\' (CHI)* I
N(CHz)2 These solutions exhibit spinning characteristics which are similar to those of dimethylformamide solutions. Fibers are readily precipitated in water from the solutions. Morcover, the solutions are stable and fluid a t room temperature. A patent on this subject has been ismed to Eastman Kodak Co. (5). Tris(dimethy1amido) phosphate has also been found to I)(: the only known solvent, for pulyvinylidene chloride which yields solutions which do not revert to gels on cooling t o room temper:iture. A clear, colorless solution was obtained on heating 1 gram of polyvinylidine chloride and 7 grams of t,ris(dimethylamido) phosphate to 60" C. By contrast, dimethylformamide did not dissolve polyvinylidine chloride unt,il a temperature of 125 C. was reached. A gel was obtained on cooling the solution to 100 ' C. Presumably, dipole interaction rather than h.vdrogen bonding accounts for solvency in these cases. Some similarity between dimethylformamide and tris(dimethylamido) phosphate as solvents for polyacrylonitrilc exists. However, the phosphorus-oxygen bond in the case of the phosphate is a coordinate covalent bond, whereas the carbon-oxygen bond in dimethylformamide is a simple covalent bond. It appears that the solvent action of these compounds arise3 from enhanced electronegativity of the oxygens, which results from the electron-repelling tendency of the dimethylamino groups. The unshared electrons on the oxygen atom are, accordingly, loosely
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1954
been required.
39 1
These solutions will not precipitate in water to
SOLVENTS FOR POLYACRYLONITRILEyield satisfactory fibers. It is necessary to use alcohol or some TABLE I. AMIDE-TYPE Solventa 0
I1
Dimet hylformamide, H C S ( C H s ) z 0
/I .V,n'-Dimethylacetamide, C H a C S ( C H d !
00
It
N,S,N',N'-Tetramethyloxamido, (CHa)iN CK(CHs)z Nonsolvents 0
I1
Diethylformamide, H C S (CzHs!?
0
I/
S,,~'-Diniethyl-a,a,a-trifluoroacetamide,CIisCN(CHd2 0 11 N,N-Diethylacetamide, C H ~ C K ( C ' ~ H I ) I 0
I1
Formamide, HCPiHn
0
n
N,N-Dimethylpropionamide, CHaCHtC N( CHI)? 0
'i
Aoetamide, CI-IaCNHz
0
'I
X-llethylformamide, H C S H C H a
9
,Y-Methylacetamide, CHsCKHCHa
held and niore readily available for bonding with the hydrogens which are alpha to the nitrile groups in polyacrylonitrile. The bonds so formed appear to be stronger than the hydrogen bonds already existing in the polymer between the alpha hydrogen and the nitrile groups of neighboring polymer molecules and solution occurs. Certain techniques have been devised for producing acrylonitrile copolymers of improved uniformity (8, 9). It was found t h a t LV,.V-dimethylacetamide was a solvent for a copolymer of 97% acrylonitrile-3% vinyl acetate prepared according to these techniques. I n fact, polyacrylonitrile prepared in the presence of sulfur dioxide was soluble in dimethylacetamide. Insoluble polymers may otherwise be obtained. Industrial Rayon Corp. has also reported the use of dimethylacetamide as a solvent for acrylonitrile polymers (4). Solvents and nonsolvents for polyacrylonitrile related to dimethylamides are shown in Table I. Another phosphorus derivative, tetrakis(dimethylamid0) pyrophosphate was found to be a solvent for
0
t
(CH&NPN(CH&
a copolymer of 95% acrylonitrile and 6% vinyl acetate (11). A solution formed a t 160' C. from one part of copolymer and seven parts of solvent was stable on cooling. The solution formed fibers on spinning into water. NITRO DERIVATIVES
Since numerous compounds containing electronegative substituents were found to be solvents, nitro derivatives were studied. Surprisingly, wet nitromethane, but not dry nitromethane, is a solvent for polyacrylonitrile. When a sample of wet polymer is heated with nitromethane, a solution is obtained. The experiment could not be repeated until it was realized that water had
other organic solvent which is miscible with nitromethane, but not with the polymer. Approximately 6% water is required to render nitromethane a good solvent for polyacrylonitrile. 4 copolymer of 85% acrylonitrile-l5% vinyl acetate is soluble in dry nitromethane. As the acrylonitrile content of the copolymer increases, the water content of the mixture must be increaseJ to produce a polymer solution. Generally, these solutions must be prepared a t a temperature above 50' C. When the solutions are cooled below this temperature, a reversible gel results. A British patent has been issued on the subject ( 1 ) . The possibly explosive nature of nitromethane under certain conditions detracte from its utility as a solvent. There is considerable variation, depending on structure, in the proportion of solvent such as dimethylacetamide which can be replaced with a nonsolvent in preparing polyacrylonitrile solutions. As much as 50% -pvalerolactone, diethyl phosphite, and N,N-dimethylpropionamidecan be used in such formulations to dissolve a copolymer of 95% acrylonitrile-5% vinyl acetate, whereas less than 10% water, methanol, diethyl ether, etc., results in polymer precipitrttion. I t was also found that dimethyl phosphite, a liquid with a boiling point of 167' C , was a solvent for polyacrylonitrile One gram of polymer and 10 grams of solvent were heated to 100" C. in a test tube. A clear, viscous solution, which was stable on cooling, was obtained. I n another case, 1 gram of a copolymer of 93% acrylonitrile-7% vinyl acetate and 9 grams of dimethyl phosphite was heated to 100" C. and a clear, colorless solution was also obtained. The solution was stable on cooling. These solutions could be spun directly into water to yield fibers, but the solvent was readily hydrolyzed. Diethyl phosphite was found to be a solvent for copolymers containing up to 90% acrylonitrile (10). In a typical example, 1 gram of a copolymer of 85% acrylonitrile-15% vinyl acetate and 9 grams of diethyl phosphite were heated to 100" C. A transparent solution was obtained which yielded clear, transparent fibers on spinning into water. Apparently, clear fibers are obtained because of the relatively slow rate of diffusion of solvent into water. Since it had been reported (3) that acetonitrile was a solvent for copolymers containing up to 80% acrylonitrile studies of related mononitriles were undertaken. It appeared that a possible reason for the failure of acetonitrile to dissolve copolymers containing higher amounts of acrylonitrile was the opportunity for intermolecular hydrogen bonding in the molecule It was believed that replacement of the hydrogens with other groups might lead to improved solvents. Accordingly, chloroacetonitrile was tried as a solvent, but was found to be ineffective. Addition of sufficient water to saturate the solution a t 80 C., however, rendered chloroacetonitrile an effective solvent for a copolymer of 97% acrylonitrile and 3% vinyl acetate. I n a typical experiment, a mixture of 2 grams of a copolymer of 97% acrylonitrile and 3% vinyl acetate, 10 grams of chloroacetonitrile (boiling point 123" to 124" C.), and 2 grams of water was heated to SO" C. A homogeneous solution was obtained. Additional solvents which have been recently found are dioxanone, glycolide, and ethylene oxalate. COHESIVE ENERGY DENSITIES
A recent article by TVallter ( 2 4 ) describes in some detail partial success in the correlation of cohesive energy densities of solvents with their ability to dissolve polyacrylonitrile. He employs the following equation in determining cohesive energy densities (C.E.D.)of solvents in calories per cc.:
C.E.D. =
-. I'
Lzo - pRT 20
INDUSTRIAL AND ENGINEERING CHEMISTRY
392
0
Ethylene carbonate, CH?---0
I
'
I
C-C
h l a l i ~ i ra n h y d r i d e ,
CH-0'
cp;O CH- 0
\o
0
t
Dimethyl phorphitr, CH,OPOCR:~
Sit voiiwtliane-axirr.
H
0
I1
' h i s (dimr:thylamido) phosphate,
C J x n o a c e t a m i d e , CNCITCSH;
Vol. 46, No. 2
mate distribution between the phases. I t has been found h i polyacrylonitrile solvents which rontain nitro, cyano, Iactonc. and anhydride groups produce solution8 which cannot he spun into water to yield fibers. One exception is dioxanone, solutionr of rrhioh may he spun into \rattar.. Polyacrylonitrile solut,ions made wit,h solvents containing dirnethylcarbamyl, cyclic cwhonate, phosphite. and dimethylamido phosphate group? yield fibers readily on spinning into water. Polyacrylonitrile solutions may be classified as shown in Tahlt. 11. Rutyrolactonc and maleic anhydride are miscihle with water i ~ all i proportions and yet wili not yield polymer filament,s on spinning into water. Nitromethane, cyanoacetamide: and chlororrcetonitrile are also wppr('cialily water soluble and show the same behavior. En no instance n-ith which the author. is acquainted h:ts there* been found a maximum equilibrium soluhilitv of polyarrylonitrile or copolymers in solvents. In every instance, misrildity in all proportiong under equilibrium conditions appears t,o Of course, as the concentration of E)ii13'itcrylonitrile of fil~wforming molecular weight exceeds a.hout 25'% of the soliiiion the high viscosity of the solutions C:IU t>herate of aolution of the undissolved a8 is knoirn, all classes of high polymers exhibit, complctct niiscihility of this type with the lone exreptions of polyethylrnr* (v3";i and certain fluorinated polyet1 -1enes. It is believed t,li:ti t,he latter polymers possess limikd oluhility in solvents, sin(:(!t,liey are comparatively nonpolar in nature. I n the case of polyacrylonitrile in solution. the solvent ivould coordinate as rctadily with mgment,s of the same polymer niolecule as \yit,h tliftorcnt, ecules. Accordingly, in the presence of s Iiauciflp gments of all polymer molecules rather than Cti~tiiict, polymer molecules would tend to he coordinated with solvent. It has hceu ohserved t>hat polgwrylonitrile solvents, suoti as dimethylform:tmide, lrhich produce water-spinnable solutions, are difficult to extract, from water xvith toluene. Butyrolactone, which produces solutions which are not spinnahle into wai,er, may hc readily extracted from wrtit>erwith toluene. Di~net,hylformamide and related dimet,hylaiiiides may he extracted from :tqueous solutions .ivit,hchloroform :til ti related chlorinated hydroc~arl>ons( 1 2 ) . ~
(:liloroacetonitrilc-~~.ater. Surcinonitrilc C I I P C S
CIIrCS
where Lz0= latent heat of vaporization :it 20" C. in calories per mole, V2"= molecularvolumcat 20" C,] and fi = PV/I22'. The latent heat of vaporization of i,inir o f thc eolvents is estinyat,ed from the following equat~ion: L20
= 5.075
where
71b =
+ 2.2!87'b2 x 10-5 + 1 . 2 0 2 ~ ~x3
+ 3.3432'bx hoiling point,
O
io--.i
(2)
('.
Kalker finds that good polyacrylonit,rile solvents possess cohesive energy densities of around 220 to 230 calories per cc. (approximately the cohesive cnergy densitmyof polyacrylonitrile). Thus, ethylene carbonate has a value of 21 7 and malononitrile possesses R value of 220 calories per cc. Utilizing t,hese equations, the cohesive cnergy densities of some of the solvents have been estimat,ed. Organophosphorus deriva tivex wore found t o behave anomalously. Cohesive energy densities of dimethyl phosphit,e ( J 5 5 ) and tris(dimethy1amido) phosphate (110) were considerably lower than thc range proposed hy Kallrer. for good ~olvents. Howover, it should he rcport,ed that Walker also reports a, low valur of 1.17 for dimethylformamide, a good solvent. Walke ibes solvency in such cases to specific intera,ct,ion of solvent ant1 polymer. Sincc t,his type of solvenia\- is perhaps the most important type, the utility of \i'alker's concept, is somewhat restricted. The application of the concept of cohesive energy density to the nit,roinethane-water combination is enlightening. Nitrornetliane, with a cohesive energy density of tfjl, and wat,er, with a wlue of approximately 500, appear to Iw halanced in the solvent cwiibinaiion to a value closer t>o 22h. the preferred range for polyacrylonitrile solvent's. h similar explanation of the solvent poirer of chloroacetonitrile-water mixtures is possible. The crtimated cohesive energy density of c.hloroacetonitrile ( I 58 (1:iIorip per oc. ) is remarkably (*lose 1o that, of nitromethane ( 161 caloriw per cc.). In a eimilar \rit\-. water increases the cohesive energy density of chloroa,cctoiiitrile to that required for dipsolving :mylonitrile polymer?. Thew appears to be litt,lc si)lverit-pol?.Iiier interacttion in t>hiscase. SPINNING CIT&RACTERISTICS
In t w t spinning, the distribution of wlvcnt between fiber and water ip the importitnt featurc i n ol)taiiiing satisfactory fiber setup. The rate of diffusion of solvtint from the fiber also plays an important part. The rato 01' ( l i f h i o n is w i'uirt,ion of the ulti-
ACKVOM IAI)C7WENT
Ln the synthesis, testing. a n d sliinning evaluation of t h v w -01vents, the aid of E. L Ringwald, \T T Dye A. €3 Craig, M' ld Hammond, and B. R Itobci t a 1" qrntefnlly acknowlcdgcd
I x r m -\TUKE c r r m ;,I) Uesman, €1. G., Brit. It'ateiit (iti5,004 (1951). (2) (:haney, D. W., C. 3. I'ateril 2,406,267 (Feb. 7, 1950). (3) Ibid., 2,537,031 ( J a n . 9 , 1951). (4) D'Alelio, G. F., I.-,P. Parerit 2,531,407 (Tov. 28, 1950). (5) Dickey, J. B.. ct a!., Ihi'd., 2,487,859 (Nov. 15, 1949). (6) Flanagan, J. V., I b i d . , 2,607,751 (Aug. 19, 1952). (7) Ham, G. E., Ibid., 2,498,605 (E'eh. 21, 1950). (8) Ibid., 2,522,445 (Sept. 12,1940). (9) Ibid., 2,559,154 (July 3 , I 110) Ibid., 2,595,847 (May ti, (11) Ibid., 2,606,168 (dug. 5, 1952). (12) Heider, R. L., U. S.Patent 2.602,817 (July8, 1952). (13) Hill, R., et al., Brit. Patent 838,:331 (1950). (14) Houts, 11. C., Textile nvch J . , 20,786-801 (1950). t, 2,404,718 (,July23, 1946). (15) Houta, R . C., U. 8.I (16) I b i d . , 2,404,727 (July 23, 1948). (17) Illid., 2,404,722 (July 2 3 , 1948). (18) Kern, W., and Fer P, H . , J . [irekl. CAenz., 160, 281-95 (1942). Patetit 2,404,714 (July 23, 1946). (19) Latham, G. H., U (20) l l a r k , H., and Fikentscher, H., C+er. Patent 580,351 (1929). (21) Rein, H., Angew. Chern., 60, 159 til (1948). (22) Ibid., 61,241-5 (1949). (23) Richards, R. B., Tmnu. Fi1,aduy Soc.. 42, 10-20 (1946). , 470 (1952). (24) Walker, E. E., J . A p p l . Ciior?.( I m ~ d o n )11, RECEIVEDfor review Decenibrr 17. 195%. ACCEPTEDSeptember 2 9 , 1953. Presented at the Southeastern Kegioiid Alerting of the AMERICAU( : i t ~ . h f t CAI, SOCIFJTY, Auburn, .41a., Oct. 2:. 1952.
