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Chemical Modification of Carbon Fiber Surfaces with Organic Polymer Coatings Joan H. Cranmer, Glullana C. Tesoro,' and Donald R. Uhlmann Massachusetts Institute of Technology, CambrMge, Massachusetts 02 139
Modifications of the carbon fiber-matrix interface for the purpose of improving the performance of composites require consideration of the microstructure, chemistry and geometry of fiber surfaces, as well as of molecular interactions at the interphase. Several approaches to tailoring of the interphase by coating carbon fibers with organic polymers have been reported. The present paper concerns one such approach, namely, the coating of carbon fibers by interfacial polycondensation. The conditions required for the formation of uniform adherent films on the fiber surfaces have been investigated for selected substrates and monomer pairs, and the feasibility of the approach has been demonstrated. Further research is in progress to establish correlations between the chemical structure and properties of the organic polymer coatlng and its effectiveness in improving the properties of composites in which the modified fibers are used.
Introduction and Background The importance of carbon and graphite fiber composites has increased at a rapid pace since the 1960's, when the technology of manufacturing carbon fibers from organic precursors was first implemented on a commercial scale. During this period, the advantages in composite performance which could be realized through the uniquely high stiffness combined with low density of the fibers were explored in depth. The structural requirements of the aircraft and aerospace industries motivated extensive research efforts on the development of carbon/graphite fibers with enhanced properties, and on investigations of advanced composites in which the use of these fibers as reinforcing elements contributes superior performance and significant reductions in weight compared to metals. To date, high performance carbon/graphite fiber composites have been used almost exclusively for aerospace and aircraft applications. However, a great deal of work now in progress is aimed at the development of advanced composites for the replacement of steel parts in automobiles. It is evident that the demand for carbon/graphite fibers will increase dramatically when their utilization in the automobile industry is implemented. With increased use, the price of the fibers, currently ranging from $8.00 to $20.00, depending on properties, will decrease, and further efforts towards technological advances and new applications will be stimulated. Recent estimates place U.S. production of carbon/graphite fibers in 1980 at about 1OOO OOO pounds-and manufacturing capacity is expected to rise to 4 000 000 pounds by 1985. Major producers in the U.S. are Hercules, Inc., Union Carbide Corporation, and Celanese Corporation. Fibers manufactured in Japan (Toray) and in Great Britain (Courtalds) are also distributed in the U S . by Union Carbide and by Hercules, respectively. Carbon fibers are manufactured by the thermal treatment (pyrolysis) of fibrous organic precursors, specifically, rayon (regenerated cellulose), polyacrylonitrile (PAN), or from liquid crystalline (mesophase) pitch, spun into oriented fibers. Processes employing rayon and PAN have been reviewed (Goodhew et al., 1975, for example), and the conversion of mesophase pitch to carbon fibers is documented in the literature (Bright and Singer, 1979; Chwastiak et al., 1979) and in patents. The general process scheme for the production of carbon fibers from organic 0196-4321/82/1221-0185$01.25/0
Table I. Repreeentative hoperties of Carbon/Graphite Fibers tensile tensile density, strength, modulus, fiber type" glcm' hi mi low modulus (A) (rayon precursor) PAN precursor (A-S) PAN precursor
1.35
100
8
1.75
410
30
1.77
410
34.37
PAN precursor
1.91
340
50.55
(HM-S) ultra-high modulus
1.96-2.02 270->300
(HR-S)
70-100
(UHM-S) (PAN or pitch precursor)
E. glass 2.54 500 10-12 steel wire I.8 500 25-30 " Designations: A, high strain, low modulus; HT, high tensile strength; HM, high modulus; UHM, ultra-high modulus; s, surface treatment.
Table 11. Advantages of Interfacial Polycondensation (IFP) for Coating of Carbon Fibers high reaction rates at room temperature stoichiometric ratio of monomers not required control of wetting by selection of solvent system control of coating thickness and distribution broad range of molecular structures and polymer properties
covalent bonding (grafting) on oxygen bearing groups precursors consists of a thermal stabilization step (200-400 "C), carbonization (1000-2000 "C), graphitization (2000-3000 "C),and, finally, a surface treatment or finish application (Bacon, 1980). The properties of carbon/graphite fibers are determined primarily by their microstructure, which in turn depends on the organic precursor, and on the conditions of processing. The range of properties for commercially available fibers is illustrated by data from the literature, summarized in Table I. With decreasing importance of low modulus fibers, commercial fibers fall into two major classes: those of intermediate modulus and high tensile strengthgenerally made from PAN precursors and used extensively 0 1982 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 Table V. Surface Chemical Analysis (ESCA)
Table 111. Monomers
~~
H,NRNH,
+ OCNR'NCO
-+
f HNRNHCONHR'NHCO j j j pol yurea
diisocy anates O=C=N( CH,),N=C=O
HDI TDI
diamines
Table IV. Fiber Properties fiber designation
VYB
T300
T50
manufacturer
Union Union Union Carbide Carbide Carbide rayon PAN rayon precursor cross section serrated serrated round modulus ( X 10' psi) 6.3 33.5 57 filament diameter, pm 9.5 7.0 6.5 filaments per yarn 3600 1000 1400 fiber density, g/cm3 1.53 1.75 1.67
in high-performance composites-and those exhibiting high modulus and somewhat lower tensile strength-made from either PAN or pitch (Chwastik and Bacon, 1981). The relationship of fiber properties to purity and structure is of critical importance in the study of carbon/graphite fibers and in the development of improved processes, fibers, and composites. Thus, the structural characterization of a wide range of carbon/graphite fibers and the definition of relationships between microstructural features and properties has been the subject of extensive research for at least a decade and major progress has been made. Salient aspects of the structural characterization of carbon/graphite fibers have been summarized by Diefendorf and Tokarsky (1975). These authors have described the basic structural unit as resembling a continuous ribbon or sheet of graphite, microns in length, and have stressed the concept that: "A fiber must not be thought of as a homogeneous anisotropic entity, but rather a collection of anisotropic units linked such that the fiber may be viewed as a composite system in its own right. As such, the moduli of the individual fibers will depend on the details of the axial preferred orientation in each fiber structure, while the strength will be a function not only of surface flaws and related effects, but also of the axial, as well as radial, textures and gradients present". These investigators have proposed three-dimensional models of microstructures for carbon fibers differing in origin and properties, and have diecussed the consequences of differences in axial and radial structure on the properties of the fibers per se, and of the composites made from them. In the context of the problem addressed in the present paper, some of the structural differences have particular significance. For example, the centrosymmetric ("onion skin") structure in which surfaces of basal planes are exposed, observed in high modulus fibers from PAN precursors, may be associated with difficult wetting by the resin matrix in composites, and with relatively poor in-
fiber VYB (rayon precursor) as received HF scoured polyurea coated (TDI/HDA) T-50 (rayon precursor) as received HF scoured polyurea coated (TDI/HDA) polyurea (calculated) (TDI/HDA)
%N
%O
11.4
13.9 15.6 13.4
87.0 77.7 68.2
17.5
13.0 15.4 14.3
70.0
20.0
10.0
%C
86.1 70.1 69.2
terfacial strength. On the other hand, bonding to the resin may be more easily attained for fibers exhibiting radial orientation of basal planes, with edges exposed on fiber surfaces, as in the case of fibers from pitch precursors. The critical problems of bonding and interfacial strength in carbon fiber reinforced composites have been studied by many investigators. It has been generally recognized that the major factors affecting the mechanical properties of carbon fiber composites include the fiber structure, the nature of the fiber-resin bond, and the mode of stress transfer at the interface. The underlying mechanisms and principles have been discussed in recent publications (e.g., Ehrburger and Donnet, 1980; Drzal et al., 1980), which reflect the tentative character of current hypotheses and generalizations. It is certain, however, that molecular interactions between the resin matrix and the surface of the reinforcing carbon fibers play a major role. For a given matrix, reinforced by carbon fibers of specific structure and mechanical properties, adhesion of the resin to the fiber surface is a dominant fador in the interlaminar shear strength of the composite. Improvements in adhesion of matrix resins to fibers and concomitant improvementa in the interlaminar shear strength of the composite have been sought through modifications of the fiber surfaces and/or of the interphase region. Conceptual approaches have included coupling agents, methods for increasing surface roughness or introducing asperities on the fiber to enhance mechanical interlocking, deformable interface polymer layers designed to adhere to matrix and fiber, and approaches to covalent bonding through functional groups on the fiber surface (Ehrburger and Donnet, 1980). Oxidative treatments of carbon fibers to introduce oxygen-bearing groups have been investigated in depth (Ehrburger et al., 1978, Hammer and Drzal, 1980) and are used commercially. The particular functional groups formed on fiber surfaces (hydroxyl, carboxyl, carbonyl) and the extent of modification depend on the fiber and on the specific oxidative treatment employed. Significant increases in interlaminar shear strength of composites have been obtained by oxidative treatment, but no simple relationship has been established between the concentration of oxygen-bearing groups, or the surface area, or other surface properties of the oxidized fibers, and the increases in shear strength attained in the composite (Donnet and Ehrburger, 1977). Other consequences of oxidative modification on fiber properties-including weight loss, reduction in fiber strength, and possibly the formation of voids or flaws near fiber surfaces (Drzal et al., 1979)depend on fiber structure and properties as well as on the reaction conditions. I t may be said that oxidation of carbon fiber surfaces provides a viable approach to improved interfacial bonding in composites, but the mechanism of effectiveness and the
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I A.
i
+-L
t?
-
A
B.
Figure I. HF scoured fiber surfaces: (A), V Y B (B), T 300: (C), T 50; (D), GY 70.
optimum systems and conditions have not been defined. Other methods for the chemical modification of surfaces in carbon fibers have been explored, including treatmenu with reducing agents, with sulfuric acid. and with other chemical solutions. The development of crystalline structures (e.g., deposition of silicon carbide fihers or 'whiskers") on the surfaces of carbon fibers has also heen proposed as an effective approach to improve mechanical bonding of resin to fiber in composites. In many instances, the objecrive of the studies cited has been tu gain new insight into mechanisms of bonding and failure modes at the carbon fiber-resin interface in composites. On the other hand, more pragmatic approaches to the modification of fiber surfaces-and of the fihe-resin interface-have been focused on methods for coating the fihers with organic polymers, designed to provide a -tie coat", or flexible interlayer. or uptimal interphase hetween fiber and matrix, in order IQ improve composite properties. Many approaches in this CIHSS have been documented with varying degrees of depth and sophistication, and promising reiults ha\,e been reported in some instances. For example, Riess et al. (1974) have proposed a prncess for grafting a copolymer benring flexible segments (polyisoprene), and segments compatible w i t h the expoxy matrix (styrenemaleic anhydride), in which bonding between the carbon fiher surface and the elastomer segment is attained by ionic (dipole-dipole! interactions. Improvements in interlaminar shear strength were reported for epoxy composite samples prepared from carbon fibers which had heen oxidized in air to introduce pnlar groups (primarily carboxyl) and then treated with solutions of the block copolymer. Extensive studies of electrodic processes for the modification of carbon fiher surfaces with organic polymers have been reported by Suhramanian and co-workers, and recently reviewed (Subramanian and Jakuhowski, 1980). These have included polymerization of monomers on commercial graphite fiber electrndes in an electrolytic cell (electropolymerization)and also migration of preformed polymers carrying ionized groups to the oppositely charged
Figure 2. Polyurea coated VYB fibers: (A), TDIJEDA; (B), TDI/HDA.
fiber electrode under an applied voltage (electrodeposition). The major advantage claimed for these techniques for coating carbon fibers is the possibility of obtaining a uniform layer of controlled thickness, and variable polymer strcture and properties, providing a tailor-made interphase in the composite. The use of carbon fibers as electrodes in the system entails practical limitations which are evident. However, depending on the monomer or preformed polymer used, interphase modification through electropolymerization, or electrodeposition (e.g., electropolymerization of N-(2-hydroxyethyl)ethylene imine) has been shown to improve shear strength and toughness of carbon fiber reinforced epoxy resins. There has been no dearth of thoughtful and creative effort in pursuing modifications of carbon fiber surface and approaches to obtain optimum bonding between fiber and matrix and improved properties in carbon fiber reinforced epoxy resins. However, effective, practical solutions to the problems of the interphase have not been identified, and the search for new approaches is continuing. The present paper reports the results of a feasibility study of a novel and potentially practical technique for modifying the surfaces of carbon fibers and for tailoring the interphase in carbon reinforced resins. The approach entails the coating of carbon fibers with or without prior oxidative treatment with polymers formed by interfacial polycondensation (IFP) techniques. I t is postulated that the chemical structure and properties of the interphase may be varied over a broad range by appropriate selection of monomers and conditions of treatment. Furthermore, by employing fihers in which oxygen bearing groups have
Ind. Eng. Chem. Prod. Res. Dev.. VoI. 21. No. 2. 1982
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..,,
,
7 i 4
!
A.
B.
Figure 3. Polyurea coated VYB fibers: (A), TDI/PDA; TDI/MDA.
(B),
been introduced by prior oxidation treatment, covalent honding or grafting of the organic polymer onto fiber surfaces may be obtained, and the conditions for optimum bonding of fiber to matrix may be met. Experimental Results and Discussion Interfacial polycondensation came into prominence with work carried out in the du Pont Laboratories in the 1950's (Morgan and Kwolek, 1959; Morgan, 1965). It is an effective method for the preparation of many polymers on a small scale, and it is of particular significance for the synthesis of polymers for which solution processes are not successful or convenient. In unstirred liquid-liquid systems, a thin film of polymer is formed rapidly a t the interface of two immiscible monomer solutions. If the film is removed from the area of the interface, more polymer forms a t once. Many features of IFP reactions are significant for the treatment of carbon fibers with organic polymers. For a hroad range of reactive monomer systems, reaction rate is extremely rapid a t room temperature. For example, in the reaction of a diamine in water with a diacid chloride in an immiscihle organic solvent, high molecular weight polymer is formed near the interface in a fraction of a second. Thus, coating of a moving substrate is feasible and has, in fact, been practiced commercially for the dimensional stabilization of wool (Fong and Pardo, 1971). The high reaction rate in the IFP formation of polyamides is further illustrated by a patented process in which protective coatings have been successfully applied to glass fibers moving at speeds of 5000 to 10000 feet per minute during the forming process (Wong, 1964). In IFP reactions,
Figure 4. Polyurea coated T 50 Bhers TDIIHDA
(A) TDI/EI)A,
(B),
it is not necessary to employ precise stoichiometric ratios of monomers-and purity of the reacting species is not critical. Control of molecular weight and of film thickness can be attained through variations in solvent, solution concentration, and other processing parameters. Many classes of polymers can he synthesized by the IFP technique, and, within each class, chemical structure may be systematically varied with ease by selecting monomers in which specific molecular features are designed. Thus, the relationship between the molecular structure of the polymer and its effectiveness in improving the adhesion of IFP-treated carbon fibers to a given matrix as well as the shear strength of composites made from treated fibers can, in principle, be investigated experimentally. I t is postulated that under optimum conditions, including monomer system, solvents, additives, and processing, IFP reactions afford the possibility of modifying the surfaces of carbon fibers with continuous and uniform films of predetermined thickness, composition, and properties without requiring specialized equipment or technology. An additional advantage of the IFP approach in coating carbon fibers with organic polymers resides in the possibility of controlling wetting of the carbon surfaces by the reacting monomer species by judicious selection of the solvent system used, thus minimizing the problems of wetting by solvent and by polymer, which are inherent in the deposition of preformed polymer solutions on low energy surfaces. A summary of the advantages of the IFP technique for coating carbon fibers with organic polymers is shown in Table 11. The selection of a monomer system for the feasibility study reported in this paper bas been guided by several
Ind. Eng. Chem. Prod. Res. Dev.. Vol. 21. No. 2. 1982
b
Y
I
189
A.
I
B.
4 Figure 5. Polyurea coated TDIJMDA.
'I
50 fihers:
( A ) , TDIJPDA;
(B),
considerations, including availability, solubility characteristics, and reactivity. Diamines and diisocyanates were chosen as illustrative in preference to other reactants (e.g., acid chlorides) because they yield polyureas without formation of byproducts, and the presence of acid acceptor is not required. The prototype monomers investigated are shown in Table 111. These were commercial compounds of 98% (or better) purity, used without purification or characterization. The source and properties of the fibers used in the experimental work are shown in Table IV. These include commercial fibers from rayon and from PAN precursors, encompassing a range of moduli and structures. The method of treatment and sequential wetting of the carbon fibers by immiscible monomer solutions presented some problems. Several techniques were examined, using diamine in water and diisocyanate in toluene, varying the sequence of application with and without intermediate partial drying. Results obtained by impregnation and by sequential dipping in monomer solutions of varying concentration were reproducible, showing good consistency of weight gains, elemental analyses, and visual observations. It was difficult, however, to obtain uniform distribution of polymer deposits and to avoid flakiness of the polymer formed on fiber surfaces. The appearance of the coatings was significantly improved by changing the solvent system to carbon tetrachloride (for the diisocyanate) and methanol (for the diamine) and by spraying the monomer solutions evenly on fiber samples until predetermined amounts of solution were applied. While carbon tetrachloride and methanol are miscible, they form layers when not stirred because of a large difference in specific
i
Figure 6. Polyurea coated T 50 fihers: (A), HDIJEDA: HDl/HDA.
(8)
gravity. Increased miscibility of the phases as compared to the preferred aqueous system may impair film continuity (Morgan, 1965, p 37). and a systematic study of nonaqueous solvent pairs will be required to establish an optimum balance between wetting of the fiber by monomer solutions and film integrity. To date, the following experimental procedure has given the best results among those investigated. For scouring the carbon fibers with hydrofluoric acid (HF) to remove surface coatings added a t the time or their manufacture, coils of fibers were placed in a polyethylene beaker, covered with a 10% H F solution at room temperature for 20 min, rinsed by placing in distilled water for 20 min, and then under running water, blotted with paper towels and air dried. For coating, samples ranging in weight from 10 mg to 5 g were sprayed with an aerosol unit designed for use in chromatography (Gelman Chromist, Fisher Scientific). A 1% solution of diisocyanate in carbon tetrachloride was sprayed first, followed by a 1%solution of diamine in methanol without intermediate drying. The amounts of solution applied and of polymer deposited were monitored during the experiment by weighing on an analytical balance. Samples of hydrogen fluoride (HF)-scoured VYB and T-50 fiber, spray-coated with several monomer pairs by this technique were examined in the scanning electron microscope (SEM). The surface appearance of the HFscoured fibers is shown in Figure 1. Representative polyurea coated fibers are shown in Figures 2 and 3 (VYB fibers), and Figures 4 to 7 (T-50 fibers). An examination of the micrographs of coated fibers
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_-
---
4,
7
+.-,
!
B,
gases. The chemical composition of the coating would he more accurately revealed hy milling into the surface. Initial probing by this technique showed no change in nitrogen content and a decrease in oxygen content with distance into the coating. It is evident from these preliminary results that the IFP technique can he employed for coating carbon fibers with organic polymers of varying chemical structure and physical properties. The preparation of sufficient quantities of fibers treated with representative polyureas for analytical characterization of the coating, preparation of composite specimens, and laboratory testing of interlaminar shear strength is currently in progress. The present report demonstrates the feasibility of the approach, with initial results suggesting the roles of substrate microstructure, conditions of treatment, and molecular structure of polymers formed in the distribution and smoothness of the coatings obtained. Much additional work will he required to establish systematic correlations of fiber structure, polymer composition, and conditions of treatment with the properties of the coating and, ultimately, with its effect on interlaminar strength of composites. However, the modification of carbon fibers by the interfacial polycondensation technique offers exceptional flexibility and breadth in the range of polymers and in their properties, as well as an approach which may be used under practical processing conditions.
Acknowledgment
Figure
7.
l'olyurea coated T 50 fibers: (A), HDI/PDA;
(B),
HDI/MDA.
Financial support for the present work was provided by the National Science Foundation through the Center for Materials Science and Engineering a t Massachusetts Institute of Technology. This support is gratefully acknowledged.
Literature Cited obtained in these experiments showed that coatings formed from aliphatic diamines (EDA, HDA) were in all instances more continuous and uniform than those made with aromatic diamines (PDA, MDA). With the same diamine, TDI yielded a. smoother coating than the aliphatic diisocyanate (HDI). For both fibers (VYB and T-50), the appearance of the coatings in the micrographs suggested that the polymer formed from HDA and TDI was more evenly distributed than others. In the case of the T-50 fiber, the TDI/HDA coating yielded a smooth and uniform surface, seemingly free of polymer clumps or flakes. On the other hand, the wholly aromatic TDI/PDA polyurea coating resulted in a rough surface-which might he desirable in that it provides high surface area and an enhanced opportunity for mechanical interlocking of fiber to matrix. On HF-scoured T-300 fibers, the polyurea coatings deposited by the technique described above gave a ilaky and discontinuous appearance in comparable micrographs. Samples of VYB and T-50 fibers, as received from the manufacturer, after HF scouring, and after spray coating with the polyurea from HDA and TDI, were examined by ESCA (Electron Spectroscopy for Chemical Analysis). In the present case, the technique was used to determine the chemical composition of the first -30 8, below the surface. Results are shown in Table V. The expected elemental composition was confirmed for the polyurea-coated samples. The higher-than-expected oxygen content reflects the presence of adsorbed impurities such as atmospheric
Bacon. R. Proceedings 01 lhe Nstbnal Symposium on Pokmers in the Sewb e 01 Man. June 1980, Washington. DC. American Chemlcal Saciaty. DIvisbn 01 Industrial and Englneerlng Chemistry. W t . A. A,: Slnger. L. S. Carbon 1979. 17. 59. Chwastlak, S.: Barr. J. 8.: Didchenko. R. C a h m 1979. 17. 49. Chwastlak, S.: Bacon. R. Pot,". Prep,.. A m . Chem. Soc., Dh.. Pa&". Chem. 1981. 222. Dielendwt. R. J.: Tokarsky. E. P w m . Eng. Sci. 1975. 15. 150. Chmnet. J. E.; Ehrburpr. P. Carbon 1977, 15. 143. Drral. L. T.: Mescher. J. A,: Hall. D. L. Carbon 1979, 17. 375. DrraI. L. T.: RICh. M. J.: Camping. J. D.; Pa*. W. J. Proceedings 01 lhe 35th Annual Technical Conterence-Rehlaced PlastkslcOmpaiies Insthie. society 01 me plastic Industw. NBW orieans. LA. 1 s section ~ ~ 206. Ehrburgef. P.; H e r q ~ J. ~ .J.: Omnet. J. E. Proceedings 01 the Filth London lnternatbnal Carbon and eaphne Conference. London. 1978. p 398. Ehrburger. P.: Donnet. J. E. P h l . T r m s . R . Soc. London 1980. A294. 495. Fong. W.: Pardo, C. E. Appl. Polym. Symp. 1971. I S . 839. Goodhew. P. J.: CieMe. A. J.: Bailey. J. E. Mater. S d . Eng. 1975. 17. 3. Hammer, 0. E.: DrraI. L. T. Appl. Surl. Scl. (980. 4 , 340. k m n . P. W. '"CondensationPotvmers bv Interfacial and Sciuibn Memods": herscienm: New YO&. 1965: pp 19-83. MMgan P.W.: Kwolek. S. L. J . Polym. Sei. 1959. 40. 299-327. Reiss. 0.; Bourdeaux; Brie. M.; Jouquet. G. Proceedings 01 the International conierence on carbon FI~.WS, n e plastics Inmute ondo don): London. 1974. Paper NO. 8. Subramanian. R. V.: Jakubowokl. J. J. ACS Symp. Ser. 1080. No. 132, 257-274. Wong, R. (to Owens-Caning Fikgias Corp.) US. Patent 3 143405. 1964.
Receiued for reuiew November 25, 1981 Accepted February 8, 1982
Presented at the Fourth Intemational Conference on Surface and Colloid Science, Jerusalem, Israel, July 5-11, 1981.
