High-Strength/High-Modulus Fibers from Aromatic Polymers J. Preston' Monsanto Triangle Park Development Center, Inc., Research Triangle Park, NC 27709 Almost two decades have passed since the introduction of fibers from wholly aromatic polymers. The first fiber of this type was that from poly-m-phenyleneisophthalamide (Nomex"),
ome ex@
and this fiber was probably intended for use as a nonflatspotting tirecord. T h e cost of this fiher was, however, far too expensive for use as a conventional tirecord. However, some commercial applications hased on the high heat resistance and flame resistance of Nomex were found which could hear the relatively high cost of this unusual fiber. The quest to extend the heat stability of aromatic polymers was joined by many workers worldwide, and the route taken was to include as many para-oriented rings as possible into the aromatic polyamides while still maintaining solubility in organic solvents so that fihers could be spun from solution. At the DuPont Company and a t Monsanto Company, some wholly para-oriented polymers were spun into fibers which were found to have, in addition to high beat resistance, remarkable strength, and stiffness. The fiber under study a t DuPont was called Fiber B and was based on poly-p-henzamide; the fiber under study a t Monsanto was called X-500 and was based on the polyamidehydrazide from terephthaloyl chloride and p-aminobenzhydrazide.
Fiber B
The work on Fiber B was terminated in favor of another fiher, KevlaF, based on N~,+CO-NH-W
+ CI-co *-a
X-500 poly-p-phenyleneterephthalamide,which was spun from a sulfuric acid solution.
PPD-T
~evlar@
The efforts to prepare heat resistant fihers were reviewed ( 1 , 2 )elsewhere by the author and by others in numerous reviews and hence will not be discussed extensively here. The 'Current address: Monsanto Textiles Company. Pensacola. FL 32575.
most important result of these efforts was the unexpected discovery that rod-like p o l h e r s , e.g., Kevlar", in similar fashion to Nomexa, yield fibers having ultra highstrength coupled with ultra high-modulus. In this review, some of the lessons learned from the work on fihers from the rodlike polymers of the polyamide class will be summarized and the extensions of the early work to fibers from other rod-like polymers will be discussed. Dlscusian - ...-. . .. .. As regards structurelproperty relationships for ultra high-strengthhigh-modulus (H-SIH-M) fibers, the following conclusions can be reached based on the experience of the puhlished work of the last several years: (a)
Essentially rod-like polymers are required which, in practice, means that p-phenylene rings must be used or that other rings,
used. (h) Copolymers are useful for enhancing solubility and hence processability; thus, copolymers based on PPD-Thave been prepared having 5-55 mole percent substitution of various diacids for terephthalic acid, depending on the stiffness of the diacid moieties. Similar copolymers have been made with &25 mole percent substitution of various diamines for PPD. But nonrod-like moieties (e.g., meta-oriented rings) must be limited to about 5-10 mole percent of the total polymer composition to maintain H-SIF-M fiber orooerties.
which case elongation-to-breakvalues may drop to unacceptably low levels. In addition to the above considerations, the single most important development which led to a considerable advancement in techhology was in the area of how the polymers were spun. Indeed, the spinning method has been shown to be almost as critical as polymer composition to obtaining H-SIH-M fiher properties. For example, fibers spun from about a 10-12% solution of poly-p-phenylene-terephthalamide (Kevlar") in sulfuric acid have about one-half the tensile strength of fibers spun from a >20% solution. Similarly, fibers from poly-p-benzamide spun from a 6% solids solution in an oreanic solvent have about one-half the as-snun streneth and AoYdulus (216 gpd versus 435 gpd) of fiher spun from a 12%solids solution (3).Spinning of fibei from high solids solution in sulfuric acid results in a doubling of tenacity compared to fiber spun from an organic solvent (Table 1). The reason for the relatively large differences in tensile properties can be found in the fact that the higher solids solution exhibit an anisotropic state, that is, they show liquid crystalline behavior. Thus, the rod-like polymers align in oriented packets a t the high solids level whereas a t the lower solids level the polymers are in the isotropic (i.e., unoriented) state. The preorientation of the molecules in solution means that less orientation must be developed by means such as hot drawing u.hirh tends to lead to exc&sik crysrsllinity and results in fihers that are too brittle for someappliuations, for example, tirecord. ~
Volume 58
~
Number 11 November 1981
935
Table 1.
H-SIH-M Flbers Vla Solutlon Splnnlng of Aromatlc Polymers Hotdrawn fiber horn organic solvent
T/E/Mla As-spun fibw from sulfuric acid
Hatdrawn fiber from sulfuric acid
Polyamides
+*4-
Fiber B
Av.
BN-N 4
16.013.51725 (21.813.71691)~
C-N-NH-C
T = tenacity, g.iden: E = elongation-to-break. %;MI = initial madulus, glden. ORef(4. 'Ref. (5). dRef. (6). a
Table 2.
H-SIH-M Flbers Vla Men Splnnlng of Aromatlc Polymers
Table 3.
H-SIH-M Flbers Vla Llquld Crystalline Splnnlng of Cellulose a
TIEIMma Polyesters (-A?;
--6**)' yarn:
20.014.41 365 (30.414.71 527)'
boilWff:
7.119.9/121(4.9) 7.716.51129 (4.8)
tiydmlysis40f Ceilulosehiacetate
Polyuamelhines (-A?: --CH=N-)"
10.617.31284 (3.1) Single filament:
28.013.21 916 138.014.411012~r
T = tenacity, g./dsn; E = elongation-tMreak. %:Mi = innid modulus, g.lden.
The liquid crystal state can be obtained in melts as well as in solutions. in which case the consideration of solids level need not be taken into account. Fibers (Table 2) from liquid crvstalline melts of aromatic polvmers have been reported and . . nromatic polyesters, in particulnr, hold considerable promise .for the future development uf H - S l H - M fibers. In theselections that follow, spinning from organic solvents, sulfuric acid, and polymer melts will be reviewed. Spinning from Organic Solvents
Two of the first H-SIH-M fibers to be reported were Fiber B and X-500 (Table 1). When spun from organic solvents these fibers have comparable properties. The liquid crystalline state can be obtained in an organic solvent for poly-p-benzamide (PPB) but not to a sufficiently high degree, because of poor solubility in an organic solvent system, to yield fiber having the optimum tensile properties ( 1 ) . Sulfuric acid solutions of PPB can be used to make H-S/H-M fibers (Table 936
TIEIM.idenlb Asspun Cellulosshiacetater
Journal of Chemical Education
eDaakmRef.(lfl. T = IenacQ, g.lden.: E = elongation-Wreak, %;Mi = initial modulw, g.lden: denierlfllament, 1.e.. w. in grams of 9000 meterr of f l t a -Spun hom 28% rolMD solvtion in blllwroacetio aoalmethyienechloride (95:5). Sodium m e ~ x l d e l m e ~solmion. ol
1) because liquid crystalline dopes can be formed in this solvent. The polymer is, however, apparently more readiIy degraded than poly-p-phenyleneterephthalamide (Kevlar"), which may explain why the latter fiber was developed instead of the former. The X-500 composition likewise is not sufficiently soluble in an organic solvent to obtain the liquid crystalline state to any significant degree. Hence, hot-drawing is required to develop the H-S/H-M fiber properties (Table 11, but the fiber oroduced conseauentlv suffers from a relativelv low eloneation-to-break vaiue. while liquid crystalline sol&ions can-be formed from the X-500 composition in sulfuric acid solutions. the polymer degrades and H-S/H-M fiber from such solution; have not been reported. A polyhydrazide copolymer, H - 2 0 2 , has been reported (7) whirh has a rather good balance of H - S / H - M fitwr r m ~ e r t i e s despite the fact that this fiber is produced from an isotropic solution and the fiber is hot-drawn. The polymer shows. predictably, relatively low solubility in organic dolvents, and i t is degraded by sulfuric acid.
Recently, H-S/H-M fibers (Table 3) based on cellulose spun via the liquid crystalline state have been reported (11). Although these fibers do not have as high strength or modulus as KevlaF, these properties may be adequate for many applications. Moreover, these initially reported properties may well improve as the process for these fibers is developed. Spinning from Sulfuric Acid
The polymer used to make KevlaF is essentially insoluble in organic solvents so that fiber spun from solution in the latter cannot he compared with fiber spun from sulfuric acid. Based on the foregoing discussions, the fiber obtained from sulfuric acid would in any event represent better H-SIH-M fiher properties than one spun from an organic solvent. For reasons discussed earlier, the ability to obtain very high solids levels (>20%) of this polymer in sulfuric acid solutions accounts for the excellent tensile properties which can be achieved. Another fiber, based on an oxadiazole-N-methyl hydrazide copolymer and spun from sulfuric acid, has been reported (8, 9) that bas rather good properties despite the fact that the fiher has to he hot-drawn. The oolvmer . . used for making this fiher apparently does not show liquid crystalline behavior in solution nor would it be expected to be based on the polymer structure which exists in solution. Thus, the polymer exists as a polyoxidazole in solution and is converted a t the time of spinning into a polyoxadiazole-N-methyl hydrazide.
N-N
r
case of Kevlafl where the ~ . o l.v m eisr ~. r e.n a r e din an oreanic solvent (which must he recovered), precipitated into water, isolated. dried. and redissolved in sult'uric acid fu,hirhmust be disposed offollowing spinning) prior to spinning. Some Conclusions from Recent Work
The results of some of the newer work confirm the experience of the older work regarding the use of copolymers for H-S/H-M fibers. Moreover, whereas use of copolymers of the amide class made spinning marginally easierand did not detract in properties (usually improved properties marginally), the use of copolymers have proved to be the key to the successful preparation of the newer fibers. Thus, for two new fibers (Table 1) containine hvdrazide erouns. eood tirecord areobtained evenwhen hotrdra&hgis used. This is orobablv the case because the cooolvmers crvstallize less readily than do polymers such as'pdly-p-be&amide and poly-p-phenyleneterephthalamide. For the melt spun aromatic fibers, copolymers are a must because only copolymers melt low enough to be spun (i.e., highly regular and rod-like polyesters melt with decomposition or at too high a temperature to he within the liquid crystalline state). Further, even if highly regular aromatic polymers could be spun from a melt they probably would crystallize too readily while cooling from the melt and become too brittle for use as a tirecord. Finally, the use of cellulose for the preparation of H-S/H-M fibers makes the point that almost any very stiff, rod-like polymer should he capable of making H-SIH-M fibers provided that the polymers can he processed to fibers under suitable conditions, which in the case of cellulose, requires an intermediate (the triacetate) that exists in the liquid crystalline state in solution. End Use Applications
(The 1,3,4-oxadiazolerings are oriented in such a manner that they are more nearly like meta- than para-oriented rings, hence they would not be expected to yield anisotropic solutions.) Spinning lrom Melts
The ahilitv to melt soin aromatic nolvmers of the rod-like type is whofiy dependknt on the use df copolymers. Thus, mixtures of rod-like units are emoloved . - because the derived nonregular structures have lower melting points (i.e., eutectic m.o.s.) com~aredto hiehlv reaular rod-like structures which w o h d prohihly melt with decomposition. Fibers (Table 3) have been re~ortedfrom two tvues of uolnners: uolvesters (12, 13,14) and polyazomethines~i5).itb bough the patents disclosing these fibers describe processes involving heat treatment of the fibers to achieve high molecular weight, it would appear possible to spin even high molecular weight polymers because the melt viscosity of these materials should he decreased due to their liquid crystalline state as contrasted to the very high melt viscosity which prevails for, e.g., polyesters of the nonrod-like type. The H-SIH-M orooerties of fibers snun from liauid crvstalline melts compare quite favorably with those fibers spun from liauid crvstalline solutions. The economics of . ureuarina . polymer in a melt and spinning from the melt would appear to favor this route compared to the preparation of polymer in solution followed by spinning from solution, especially in the
Some significant end use applications for K e v l f l have been develooed, and i t has been announced that oroduction capacity willbe increased to 45 Tii Ibslyear. ~ h e . e n duse applications include tirecord. convevor belts. V-belts, roues and cables, body armor, and ieinforcement in rigid cokp&es in general (skis, golf clubs, surf boards, fan blades, aircraft structures, filament-wound vessels radomes, circuit boards, etc.). Some use is made of flame resistant garments where high dimensional stability in a fire is required. Fibrils (-2 mm in length) of Kevlae pulp can he used as an asbestos replacement in gaskets and brake linings. Literature Cited (11 Plp8ton. J.,Polym. Eng. S e i , 15,199 (1975). (2) Preston, J.,Polym. EnE. Sci.. 16, (19761. (3) Preston, J.. and W. L. Hofferberl. Jr.. J. Polymer Sci: Polymer Sympasio. 65. 13
\.-,",. ,>""O
