Interphase Resin Modification in Graphite ... - ACS Publications

20 Interphase Resin Modification i n Graphite Composites R.

V.

SUBRAMANIAN

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J A M E S J.

JAKUBOWSKI

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Downloaded by MONASH UNIV on August 26, 2015 | http://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch020

D e p a r t m e n t of M a t e r i a l s Science a n d E n g i n e e r i n g , W a s h i n g t o n State U n i v e r s i t y , P u l l m a n , WA 99164

A comprehensive program of research i s being conducted in our l a b o r a t o r i e s t o i n v e s t i g a t e the applicability of electrochemcial processes f o r interphase resin m o d i f i c a t i o n in graphite composites (1-6). Research on asbestos and glass fiber composites has shown t h a t the i n t r o d u c t i o n of a polymer i n t e r l a y e r between the fiber and the polymer matrix leads t o significant alterations i n the mechanical p r o p e r t i e s of composites (7-11). Since carbon fiber is an electrically conducting m a t e r i a l , it was coated w i t h polymer by e l e c t r o p o l y m e r i z a t i o n or by e l e c t r o d e p o s i t i o n and the e l e c t r o c o a t e d fiber was used as reinforcement i n an epoxy matrix t o evaluate r e s u l t i n g improvements in composite shear and impact s t r e n g t h s . The results of this i n v e s t i g a t i o n of particular i n t e r e s t to aerospace a p p l i c a t i o n s a r e discussed here. Furthermore, t h e extension of this research t o minimize t h e hazards of release of e l e c trically conductive f i b e r fragments from composites is a l s o included in this d i s c u s s i o n . The concept of interphase modification has been expanded thus t o deal with not only mechanical, but a l s o other p r o p e r t i e s of composites. E l e c t r o p o l y m e r i z a t i o n i n v o l v e s t h e polymerization o f monomers i n an e l e c t r o l y t i c c e l l . E l e c t r o d e p o s i t i o n u t i l i z e s t h e migration of preformed polymers c a r r y i n g i o n i z e d groups t o t h e o p p o s i t e l y charged e l e c t r o d e under an a p p l i e d voltage. The major advantage of u t i l i z i n g these e l e c t r o d i c processes f o r c a r bon f i b e r coating i s that a uniform l a y e r of c o n t r o l led t h i c k ness and v a r i a b l e polymer s t r u c t u r e and p r o p e r t i e s can be expected t o be formed. I t i s u s e f u l , i n t h i s context, t o recognize t h a t the f i b e r and matrix are not bridged i n the composite by a well defined i n t e r f a c e but by an interphase polymer. The p r o p e r t i e s of the interphase polymer can be s i g n i f i c a n t l y d i f f e r e n t from the bulk matrix polymer p r o p e r t i e s . When a polymer c o a t i ng i s formed on the f i b e r by e l e c t r o d e p o s i t i o n o r e l e c t r o p o l y m e r i z a t i o n ,

1

Current address:

Dow Chemical Co., Midland, MI 48640

0-8412-0567-l/80/47-132-257$05.00/0 © 1980 American Chemical Society

In Resins for Aerospace; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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the p r o p e r t i e s of the interphase are f u r t h e r modified by the polymer c o a t i n g . Such m o d i f i c a t i o n i s manifested i n c o r r e sponding changes i n composite p r o p e r t i e s , mainly shear and toughness. Prompted by a n t i c i p a t e d A i r Force requirements of a thermosetting polymer which could be f a b r i c a t e d i n t o high s t r e n g t h f i b e r r e i n f o r c e d s t r u c t u r a l composites, acetylene terminated polyimides have been developed r e c e n t l y (12) as shown i n the f o l l o w i n g example.

H R 6 0 0

The major problem addressed i n t h i s program was the thermal cure, through an a d d i t i o n mechanism, without e v o l u t i o n of v o l a t i l e s , of the polyimide intermediate. The e l e c t r o p o l y m e r i z a t i o n of acetylene terminated polyimides and of a model compound, v i z . , phenyl acetylene was t h e r e f o r e undertaken. I t was necessary t o e s t a b l i s h t h a t the p o l y m e r i z a t i o n of the p o l y i mide intermediate on g r a p h i t e f i b e r s d i d i n f a c t proceed through the terminal a c e t y l e n i c groups. The r e s u l t s would have considerable s i g n i f i c a n c e i n view of the development of a new f a m i l y of polyphenylquinoxalines with terminal acetylene groups which o f f e r great p o t e n t i a l f o r use i n high temperature composite and adhes i v e a p p l i c a t i o n s (13,14). A p a r a l l e l development i n high temperature r e s i s t a n t matrix r e s i n s f o r composites i s the s y n t h e s i s of n i t r i l e c r o s s l i n k e d polyphenylquinoxaline (15). The p a r t i c i p a t i o n of n i t r i l e groups i n e l e c t r o i n i t i a t e d a d d i t i o n p o l y m e r i z a t i o n was t h e r e f o r e studied through the use of b e n z o n i t r i l e as monomer. Experimental Monomers of 99 percent or b e t t e r p u r i t y were used as rec e i v e d from the s u p p l i e r s . Obtained from Cardova Chemical N-(2hydroxyethy1)ethyleneimine (HEEI) was used as received. D i methyl formamide s o l v e n t , M a l l i n c k r o d t chemical, a n a l y t i c a l grade was r e f l u x e d over calcium hydride f o r 24 hours before d i s t i l l -


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a t i o n at reduced pressure. Epoxy matrix r e s i n , Epon 8 2 8 was obtained from S h e l l Chemical Co. HR6U0 was obtained from G u l f ChemicalCo. Graphite f i b e r s i n t h e form of continuous tow were s p e c i a l l y obtained f r e e of commercial treatments f o r use i n e l e c t r o p o l y m e r i z a t i o n . The usual commercial f i b e r s were used as c o n t r o l f o r comparison with the experimental treatment. The f i b e r s were al 1 PAN based f i b e r s , Hercules AS and AU from Hercules Inc., F o r t a f i l 3T, 4T, 5T, CG3 and CG5 from Great Lakes Chemical Co., and Thornel 300 from Union Carbide. Electropolymerization.

G e n e r a l l y , e l e c t r o p o l y m e r i z a t i o n s were conducted i n a three compartment eel 1 i n which the two end compartments were sep arated from t h e middle one by two f r i t t e d glass di ses. Graphite f i b e r e l e c t r o d e s were placed i n the c e n t r a l com partment containing monomer and two platinum counter e l e c t r o d e s were placed i n each of t h e two end compartments which con t a i n e d only s o l v e n t and e l e c t r o l y t e . The use of f r i t t e d di ses as a b a r r i e r minimized monomer and polymer migration i n t o the end compartments. E l e c t r o p o l y m e r i z a t i o n s were conducted a t constant DC voltage provided by a Hewlett Packard Model 6 4 3 8 B power supply. C e l l voltage and eel 1 c u r r e n t , as monitored by measuring t h e voltage across a f i x e d 10 ohm, 100 watt, w i r e wound r e g i s t e r placed i n s e r i e s with t h e eel 1, were recorded using a HP 7128A, two channel, s t r i p chart recorder. Composite F a b r i c a t i o n .

A f t e r p o l y m e r i z a t i o n , t h e g r a p h i t e f i b e r s were r i n s e d w i t h water o r acetone and then d r i e d under vacuum t o determine weight increase due t o polymer coating and f o r f u r t h e r t e s t i n g . When composite specimens were d e s i r e d , Hercules AU f i b e r tow was wound around Η-type frames and immersed i n the e l e c t o l y t i c c e l l . Prepregs were prepared by brushing an epoxy r e s i n c a t a l y z e d by m-pheny 1enedi ami ne and heating a t 80°C f o r an hour. S t r i p s were cut from the prepregs p a r a l l e l t o f i b e r * a l i g n m e n t and stacked i n s t e e l molds t o o b t a i n composite bars by compression molding a t 150°C/30 min, 200 p s i . Test specimens f o r impact strength and shear t e s t s were cut from these pieces. ASTM standard methods were adopted t o measure d e n s i t y , void content ( l e s s than 0.2%), and f i b e r content. Short beam shear t e s t s were conducted i n an Instron model TTCML t e s t i n g machine at a crosshead speed of 1.0 mm/min, and span t o t h i c k n e s s r a t i o of 4:1 (ASTM D2344-76). At l e a s t f i v e specimens were t e s t e d f o r each composite; and



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only the values obtained f o r shear f a i l u r e were used. A T i n i u s Olsen p l a s t i c impact t e s t e r was used t o measure impact strengths by Method A, Izod of ASTM D256-73. A r i t h m e t i c averages of two or t h r e e specimens f o r each volume f r a c t i o n were obtained. Measurements at nine d i f f e r e n t f i b e r volumes were conducted f o r each e l e c t r o c o a t i n g treatment. A simple device was set up f o r continuous e l e c t r o d e p o s i t i o n of polymer on graphite f i b e r tow from a spool drawn s u c c e s s i v e l y through an e l e c t r o l y t i c eel 1 and r i n s e bath before being rewound on a drum (2J. E l e c t r o d e p o s i t i o n s were c a r r i e d out from a 2.5 percent s o l u t i o n of the s e l e c t e d polymer at 10 v o l t s f o r a period of 1 minute. Epoxy prepregs and t e s t specimens were prepared from the coated tow f o l l o w i n g procedures described above. A v a r i e t y of maleic anhydride copolymers p a r t i a l l y hydrolyzed were used f o r c o a t i ng the f i b e r s by electrodeposition. E l e c t r o d e p o s i t i o n s on a smal1er seale were c a r r i e d out as f o l lows. Weighed lengths of carbon f i b e r tow, i n the form of bundles 12.0± 0.5 cm long, t i e d at both ends, were placed i n the center of a s i n g l e compartment eel 1 contai ni ng the e l e c t r o d e p o s i t i o n s o l u t i o n . C e l l dimensions were 8 χ 7 χ 12 cm. The carbon f i b e r bundle was immersed t o a depth of 10.0± 0.2 cm. Platinum e l e c t r o d e s were placed on both sides of the bundle at a d i s t a n c e of 3.0 cm. Constant DC voltage was a p p l i e d t o the eel 1 f o r a s e l e c t e d p e r i o d of time, a f t e r which the f i b e r s were removed, r i n s e d , and d r i e d at 50°C f o r 18 hours i n a va euurn oven. The increase i n weight of the f i b e r bundle was then measured and the average weight i n c r e a s e of at l e a s t two specimens was recorded. E l e c t r o d e p o s i t i o n of polyamic a c i d s was c a r r i e d out by t h i s procedure. Pyre-ML (Du Pont) i s a s o l u t i o n of polyamic acids formed by the r e a c t i o n of aromatic diamines with aromatic dianhy d r i d e s . When Pyre-ML i s baked, i t i s converted to an i n e r t polyimide. Received as a 16.5 percent polymer s o l i d s s o l u t i o n i n N-Methy1-2-pyrrolidone and aromatic hydrocarbons, a c o l l o i d a l d i s p e r s i o n of Pyre-ML (#RC-5057) i n acetone was prepared as fο 11ows (22). Twentyfive m i l l i l i t e r s of Pyre-ML was mixed i n 100 ml of dimethyl s u l f o x i d e . Five m i l l i l i t e r s of t r i ethyl ami ne was added and the s o l u t i o n was heated t o 40°C, with s t i r r i n g , f o r 15 minutes. The s o l u t i o n was then slowly added t o 500 ml of acetone i n a Waring blender. Pyre-ML was e l e c t r o d e p o s i t e d from t h i s s o l u t i o n on carbon f i b e r anodes. Upon completion of the d e p o s i t i o n , the coated f i b e r s were d r i e d at 15U°C f o r one hour and were then placed i n a vacuum oven f o r 18 hours at 5U°C. No r i n s i n g procedure was employed s i n c e i t was found e x p e r i m e n t a l l y t h a t r i n s i n g i n water washed away almost al 1 of the e l e c t r o d e p o s i t e d Pyre-ML on the f i b e r surface.


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Thermal A n a l y s i s .

Al1 thermogravimetric analyses (TGA) were performed w i t h a Perkiη Elmer TGS-1 Thermobalance. An atmosphere of flow ing dry a i r was d e l i v e r e d from a c y l i n d e r of dry a i r at a r a t e of 25 ml per minute. Dynamic thermal analyses were conducted at a heating r a t e of 10°C per mi nute. The v a r i a t i o n of weight with i n c r e a s i n g temperature was recorded on a s t r i p chart recorder. Data obtained as a c t u a l weights as a f u n c t i o n of temperature were converted t o show the percentage of r e s i d u a l weight and p l o t t e d as a f u n c t i o n of temperature. Isothermal a n a l y s i s was per formed by r a i s i n g the temperature of the sample to 500°C at a heating r a t e of 80°C per minute. The sample was a11 owed to decompose at constant temperature and the v a r i a t i o n of weight was recorded as a f u n c t i o n of time. Cured r e s i n s and mixtures of epoxy (EPON 828-mPDA) r e s i n and polyimide intermediate were ground i n t o a f i n e powder i n a mortar and p e s t l e and screened through a #40 (0.417mm mesh) T y l e r Standard screen and s u b j e c t e d t o thermal a n a l y s i s as described. P r e p a r a t i o n of the cured r e s i n s p r i o r to thermal a n a l y s i s was as f o l lows: S t o i c h i o m e t r i c amounts of EPON 828 and meta-pheny1enedi ami ne (mPDA) were heated t o 80°C, thoroughly mixed, and precured at 80°C f o r one hour. F i n a l curing was done at 15U°C f o r one hour. Themrid 600 (HR 600) or Pyre-ML as r e c e i v e d , was placed i n an aluminum cup and cured at 315°C f o r t h r e e hours, o r two hours respec t i v e l y . F o r t a f i l 5U carbon f i b e r s t r e a t e d by e l e c t r o p o l y m e r i z a t i o n and e l e c t r o d e p o s i t i o n were subjected to thermal a n a l y s i s as short l e n g t h s , cut from the t r e a t e d f i b e r bundle. Untreated F o r t a f i l 5U f i b e r s were run as received i n the same manner. Al 1 t r e a t e d f i b e r s were subjected t o a 60 s, 24 VDC treatment p r i o r to a n a l y s i s . Composites prepared from t r e a t e d and untreated F o r t a f i 1 5U carbon f i b e r s i n an EPON 828-mPDA matrix were subjected t o a n a l y s i s i n the form of a s i ngle sol i d chunk cut from the composite specimen. As b e f o r e , al 1 t r e a t e d f i b e r s were exposed t o a 60 sec, 24 VDC treatment p r i o r t o i n c o r p o r a t i o n i n t o a composite. Composite specimens were prepared i n the usual manner by compression molding as described e a r l i e r . Results and D i s c u s s i o n D e t a i l e d r e s u l t s of the screening of a va r i ety of monomersol v e n t - e l e c t r o l y t e systems i n e l e c t r o p o l y m e r i z a t i o n , and i n v e s t i g a t i o n s of polymer g r a f t i n g and s t e r e o r e g u l a r i t y , have been described elsewhere ( J J . Not only v i n y l monomers but others containing a v a r i e t y of c y c l i c f u n c t i o n groups such as epoxy or a z i r i d i n y l groups were found t o e l e c t r o -



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polymerize on g r a p h i t e f i b e r s which thus proved t o be a good s u b s t r a t e f o r polymer c o a t i n g by e l e c t r o i n i t i a t i o n . The p r i n c i p a l e f f e c t of e l e c t r o p o l y m e r i z a t i o n of monomers on graphite f i b e r s was expected t o be i n a l t e r i n g t h e i n t e r f a c i a l bond s t r e n g t h of t h e coated f i b e r s when incorporated i n a composite. The short beam shear t e s t i s a popular compromise between p u r i t y of s t r e s s and ease of procedure and was employed here t o measure i n t e r l a m i n a r shear strength ( I L S ) . The corresponding values f o r impact strength were a l s o measured t o f o l l o w t h e t r e n d i n toughness of t h e composite specimens. These e x t e n s i v e r e s u l t s {I) can be summarized as f o l l o w s : The ILS of composites prepared from Hercules type AU carbon f i b e r s coated by e l e c t r o p o l y m e r i z a t i o n o f a s e r i e s of d i f f erent monomer systems were measured. Systems were s e l e c t e d t o i n c l u d e r e p r e s e n t a t i v e s of various types of monomers i n both aqueous and nonaqueous sol v e n t - e l e c t r o l y t e systems encountered during t h e screening process. Composites prepared from Hercules type AU untreated, and type AS, commercially t r e a t e d graphite f i b e r s without f u r t h e r treatment were t e s t e d f o r comparison. I t was seen that i n c o r p o r a t i o n of a polymer i n t e r l a y e r on g r a p h i t e f i b e r s p r i o r t o embedding them i n an epoxy matrix caused t h e strength of the composite, t o vary over t h e range 45 t o 82 MPa a t Vf = 50%, which i s s i g n i f i c a n t i n r e l a t i o n t o t h e standard d e v i a t i o n s ( l e s s than ±5 MPa) i n v o l v e d i n each s e t o f measurements. Although more d e t a i l e d s t u d i e s a r e needed t o standardize t h e e l e c t r o p o l y m e r i z a t i o n technique i n order t o o b t a i n optimum r e s u l t s , composite mechanical p r o p e r t i e s were found t o be dependent upon monomer, s o l v e n t , e l e c t r o p o l y m e r i z a t i o n time, and post treatment of t h e coated f i b e r s . Since s i g n i f i c a n t l y d i f f e r e n t shear strengths a r e obtained using d i f f e r e n t types o f monomers i n e l e c t r o p o l y m e r i z a t i o n , i t would appear t h a t t h e shear strength i s q u i t e s e n s i t i v e t o t h e chemical and s t r u c t u r a l p r o p e r t i e s of t h e polymer interphase. The types of polymers formed v a r i e d from very f l e x i b l e v i n y l terminated b u t a d i e n e - n i t r i l e copolymer t o r i g i d ( a c r y l o n i t r i l e ) polymers, and from those which had f u n c t i o n a l groups capable of r e a c t i n g with t h e epoxy matrix t o those which had none such. However, from the 1imited amount of data a v a i l a b l e , no c o r r e l a t i o n could be made of t h e trend i n shear s t r e n g t h with the chemical o r mechanical p r o p e r t i e s o f the polymer c o a t i n g . The p r o p e r t i e s of t h e polymer f i l m formed on the graphite f i b e r need t o be c h a r a c t e r i z e d before any such c o r r e l a t i o n can be made. The r e s u l t s of impact t e s t s on notched specimens provided f u r t h e r evidence t h a t the carbon fiber-polymer matrix i n t e r face could be modified by e l e c t r o p o l y m e r i z a t i on. As was observed i n shear t e s t s , s i g n i f i c a n t d i f f e r e n c e s occur i n the impact strength as well as i n strength v a r i a t i o n with volume f r a c t i o n of f i b e r s t h a t had undergone d i f f e r e n t treatments.



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When the trends of impact strength are compared t o trends of shear s t r e n g t h , i t was seen t h a t u s u a l l y an increased impact strength r e s u l t e d whenever a decreasing shear strength was observed. This i s i n agreement with the general obser v a t i o n i n composites t h a t excessive i n t e r f a c i a l bondi ng causes b r i t t l e f a i 1 u r e and lowered impact s t r e n g t h . However, i n exceptional instances, an increase i n both shear and impact strength was observed. I t i s useful t o point out here t h a t the composite specimens were prepared from f i b e r s which were coated under one set of e l e c t r o p o l y m e r i z a t i o n c o n d i t i o n s , and t h a t these c o n d i t i o n s were not optimized with respect t o composite p r o p e r t i e s . The r e s u l t s thus i n d i c a t e the p o t e n t i a l of e l e c t r o p o l y m e r i z a t i o n f o r interphase m o d i f i c a t i o n i n graphite f i b e r composites and the need t o standardize e l e c t r o p o l y m e r i z a t i o n c o n d i t i o n s and monomer systems t o c o n t r o l polymer f i l m p r o p e r t i e s and, through them, composite p r o p e r t i e s . The e l e c t r o d e p o s i t i o n technique a l s o has proved e f f e c t i v e f o r interphase t a i l o r i n g i n carbon f i b e r composites (2,5)· The copolymers employed f o r anodic e l e c t r o d e p o s i t i o n on carbon f i b e r s were a s e r i e s of polymers with car boxy 1 groups, v i z . , copolymers o f maleic anhydride with s t y r e n e , α-olefins and methyl vi nyl ether. The i n t r o d u c t i o n of the polymer interphase r e s u l t s i n s i g n i f i c a n t improvements i n composite p r o p e r t i e s , and the extent of improvement i s c o n t r o l led by the nature of the polymer interphase. The molecular weight, chemical composition, and c r o s s l i nki ng of the interphase polymer are some of the molecular parameters modifying the e f f e c t s observed. The most s t r i k i ng f e a t u r e of the r e s u l t s was t h a t , i n most cases, i n c r e a s e i n shear strengths was not accompanied by a corresponding l o s s i n impact strength as i s g e n e r a l l y observed i n various methods of surface treatment. N- i 2 - h y d r o x y e t h y l j e t h y l e n e i m i n e .

The r e s u l t s obtained with HEEI are presented i n Figure 1. The e l e c t r o p o l y m e r i z a t i o n of HEEI was conducted a t 12 VDC, from a s o l u t i o n of the monomer i n DMF c o n t a i n i n g NaN0 as supporting e l e c t r o l y t e (O.lg HEEI per ml o f 0.2N NaN0 /DMF). The polymer coated f i b e r s were washed with acetone, and vacuum d r i e d a t 6U°C f o r 18 hours before preparation of composite specimens. A number of i n t e r e s t i n g general f e a t u r e s described above are seen t o be i l l u s t r a t e d i n Figure 1. Polymer c o a t i n g of Hercules All f i b e r s by e l e c t r o i n i t i a t i o n of HEEI, a t the anode, c l e a r l y r a i s e s the ILS of the epoxy composite t o values comparable t o those of specimens prepared from commercial l y a vai1 able surface t r e a t e d Hercules AS f i b e r s . 3

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40 I

ι 50

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Figure 1. The interhminar shear strengths (ILS) of composites prepared from Hercules AS and AU graphite fibers, and AU fibers coated by electropolymeriza tion of N-(2-hydroxyethyl) ethyleneimine (HEEI) under conditions indicated, at various fiber volume fractions (V ); (O) HEEI, 3 sec, anode; (A) HEEI, 10 sec, anode; (Q) HEEI, 3 sec, cathode; ( Q ) HEEI, 10-sec dip, no current passed f


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The d u r a t i o n of p o l y m e r i z a t i o n i s seen as an important v a r i a n t , changing from a 3 s t o 10 s p o l y m e r i z a t i o n time has produced n o t i c e a b l e changes i n ILS values. Dipping the f i b e r s i n HEEI monomer f o r 10 s without e l e c t r o i n i t i a t i o n does not r a i s e the ILS of composites above t h a t of untreated Hercules AU f i b e r s . This observation i s i n t e r e s t i n g s i n c e i t emphasizes the importance of the interphase polymer, i n t h i s case poly-HEEI, i n improving the ILS of the composite. Monomer HEEI c a r r i e s the azi r i d i ne f u n c t i o n a l group which can r e a d i l y react with carboxyl groups present on the f i b e r s u r f a c e as f o l lows:

/ψ

1

HUCH CH —Ν 2

+ -CUUH

Z

> -COO

- CH

2

- CH

2

- NH - CH CH 0H 2

2

The r e s u l t i n g f u n c t i o n a l groups, amino and hydroxy1, are both capable of i n t e r a c t i n g with the epoxy m a t r i x , thus formiηg a molecular bridge from t h e f i b e r surface to the matrix r e s i n . Apparently, such a s u r f a c e m o d i f i c a t i o n t o produce r e a c t i v e f u n c t i o n a l groups, without the formation of a polymer i n t e r phase, i s not the cause of the observed improvement i n ILS. The absence of any s i g n i f i c a n t change i n ILS when HEEI monomer i s present at the f i b e r cathode ( F i g . 1) a l s o supports the r o l e of the polymer interphase i n the observed property improvement. I t has been observed i n other experiments not reported here (6,16), t h a t HEEI i s polymerized only at the anode, and not at the cathode. The anodic p o l y m e r i z a t i o n proceeds by a c a t i o n i c mechanism, l e a d i n g t o the polymer of the s t r u c t u r e : 4 Ν - CH

2

- CH f 2

n

CH CH 0H 2

2

The observed e f f e c t i v e n e s s of the anodic treatment and the lack of i t i n cathodic treatment ( F i g . 1 ) , must be a t t r i buted t o the formation of the polymer of HEEI i n the former case, and the absence of i t i n the l a t t e r . The s t r u c t u r e of the polymer formed c a r r y i n g t e r t i a r y amino, and hydroxy1 f u n c t i o n a l groups, i s f a v o r a b l e f o r subsequent p a r t i c i p a t i o n i n the epoxy c u r i n g r e a c t i o n , and a l s o f o r adhesion to the epoxy matrix formed. Polyimi des.

In view of the above r e s u l t s , and c o n s i d e r i n g the importance


266

RESINS

FOR

AEROSPACE

of high temperature r e s i s t a n t g r a p h i t e composites, the e l e c t r o p o l y m e r i z a t i o n of HR-600, the o l i g o m e r i c polyimide i n t e r mediate was conducted on graphite f i b e r s . A polymer was r e a d i l y formed by cathodic p o l y m e r i z a t i o n , as i n d i c a t e d by the weight increase of f i b e r s i 1 1 u s t r a t e d i n F i g u r e 2. Since a v a r i e t y of f u n c t i o n a l groups were found to be poly merized by e l e c t r o i n i t i a t i o n (1_), i t was sought to e s t a b l i s h the p a r t i c i p a t i o n of the a c e t y l e n i c terminal groups i n the p o l y m e r i z a t i o n r e a c t i o n . Using phenylacetylene as model monomer, i t was found t h a t i t r e a d i l y polymerized i n NaN0 / DMF i n the cathode compartment t o y i e l d a polymer of average molecular weight 3000 (17). Carbon-hydrogen analyses agreed well with c a l c u l a t e d values f o r polypheny1 acetylene ( c a l c u l a ted f o r CoH : C,94.08; H,5.92. Found: C,92.14-93.05; H, 5.75-5.84;. The i r and nmr s p e c t r a l data and other evidence confirmed the polymer t o be a l i n e a r polymer with a polyene s t r u c t u r e formed by a n i o n i c a d d i t i o n of C^C bonds. I t i s t h e r e f o r e t o be expected t h a t the polyimide c o a t i n g formed on carbon f i b e r during e l e c t r o p o l y m e r i z a t i o n of HR-600 i s the r e s u l t of a d d i t i o n of a c e t y l e n i c terminal groups. The compression moldi ng of e l e c t r o c o a t e d f i b e r s , with or without a d d i t i o n a l impregnation with polyimide oligomer, was adopted to produce composite specimens. The preparation of specimens has proved more d i f f i c u l t than with an epoxy matrix probably because of the flow p r o p e r t i e s of polyimide. Therefore, composite specimens were prepared from the p o l y i mide coated Hercules-AU g r a p h i t e f i b e r s using an epoxy matrix. The r e s u l t s shown i n Figure 3, confirm the m o d i f i c a t i o n of shear strength by the e l e c t r o p o l y m e r i z e d coating (3 s p o l y m e r i z a t i o n of HR-600 i n 0.2N NaN0 /DMF at 24 VDC). I t i s s u r p r i s i n g t o observe the large change i n shear s t r e n g t h w i t h f i b e r volume f r a c t i o n . Perhaps i t can be a t t r i b u t e d to the f a c t t h a t with i n c r e a s i n g "volume f r a c t i o n " of the poly imide coated f i b e r s the matrix changes from a predominantly epoxy matrix t o one c o n t a i n i n g i n c r e a s i n g amounts of polyimide. As indi cated a l r e a d y , the p o l y m e r i z a t i o n of n i t r i l e groups i s a l s o r e l e v a n t t o composite research i n view of the n i t r i l e c r o s s l i nked p o l y q u i n o x a l i nes being developed (14). Poly m e r i z a t i o n of b e n z o n i t r i l e has shown t h a t a l i n e a r polymer i s formed w i t h the repeating u n i t being (CgHgC = N). The conjugated s t r u c t u r e formed by a d d i t i o n of C Ν groups was supported by i n f r a r e d and mass s p e c t r a l data which showed the absence of the c y c l i c t r i m e r 2 , 4 , 6 - t r i p h e n y l - s - t r i a z i n e (17,18). From these r e s u l t s i t can be expected t h a t the p o l y m e r i z a t i o n of n i t r i l e terminated polyimide and p o l y q u i n o x a l i n e intermed i a t e s can a l s o be e l e c t r o i n i t i a t e d on graphite f i b e r s . E l e c t r o p o l y m e r i z a t i o n s through C^C and C=H bonds have great s i g n i f i c a n c e t h e r e f o r e to high temperature r e s i s t a n t polyimide and p o l y q u i n o x a l i n e composites and improvement of t h e i r shear


3

6

3



20.


8.0

Resin

267

Modification

r

L 0

ι

ι

ι

ι

30

60

90

120

TIME,S

Figure 2. Electropolymerization of acetylene terminated polyimide (HR-600) on Fortafil carbon fibers: HR 600/DMF-NaNO (24 VDC); (·) Fortafil 3U; (•) Fortafil 5U 2

120 -

100 ο Ο

ΙΕ 80 -

If) _j

—

60 -

401 40

ι

ι 50

ι

ι 60

ι

» 70

ι

« 80

Figure 3. The interlaminar shear strengths (ILS) of composites prepared from Hercules AS and AU graphite fibers, and AU fibers coated by electro polymerization of HR-600, (O) at vari ous fiber volume fractions (V ). f


RESINS FOR

268

AEROSPACE

s t r e n g t h and toughness.


E l e c t r i c a l Hazards of Conductive F i b e r Fragment Release.

The e l e c t r o c h e m i c a l techniques of c o a t i n g graphite f i b e r s described above are a p p l i c a b l e to addressing another import a n t p o t e n t i a l problem attendant upon the use of composites. The problems t h a t may a r i s e when carbon f i b e r s are accident a l l y released i n t o the environment are well p u b l i c i z e d by now. The Department of Commerce issued a press r e l e a s e i n January 1978, concerning p o t e n t i a l e l e c t r i c a l problems assoc i a t e d w i t h carbon f i b e r s c u r r e n t l y being used i n composites (19). NASA simultaneously r e l e a s e d a t e c h n i c a l memorandum concerning the observed e f f e c t s on e l e c t r i c a l systems of a i r borne carbon f i b e r s ( 2 0 ) . In these r e p o r t s , the high e l e c t r i c a l c o n d u c t i v i t y of the carbon f i b e r s was i d e n t i f i e d as the prime f a c t o r i n t h e i r e f f e c t s on e l e c t r i c a l equipment, w i t h other p r o p e r t i e s such as smal1 f i b e r diameter, general l y short l e n g t h , and low d e n s i t y being important c o n t r i b u t o r y f a c t o r s . Because of t h e i r l i g h t weight, carbon f i b e r s can f l o a t i n the a i r l i k e dust p a r t i c l e s and, i f they come t o r e s t on e l e c t r i c a l c i r c u i t s , can cause power f a i l u r e s , blackouts, s h o r t s , or a r c i ng t h a t can damage equi pment. A s e r i o u s c o n s i d e r a t i o n of these apsects of g r a p h i t e comp o s i t e s emphasizes the need t o evaluate new techniques of d e v i s i n g organic coatings f o r carbon f i b e r s which can* provide, i n case of f i r e , a r e l a t i v e l y nonconducting l a y e r of char or other material that might r e s u l t i n f i b e r "clumping" preventing f i b e r r e l e a s e o r , i n the event of f i b e r r e l e a s e , prevent e l e c t r i c a l contact with e l e c t r i c a l components. In t h i s c o n t e x t , research was conducted t o apply the techniques of e l e c t r o p o l y m e r i z a t i o n and e l e c t r o d e p o s i t i o n developed f o r i n t e r phase m o d i f i c a t i o n of carbon f i b e r composites toward a s o l u t i o n to the problems of a i r b o r n e carbon f i b e r fragments ( 6 ) . The behavior of e l e c t r o l y t i c a l l y formed organophosphorous and polyimide coatings at high temperature was t h e r e f o r e invest i gated ( 6 ) . The f i r s t r e s u l t s obtained with polyimide coatings are presented here. The use of g r a p h i t e f i b e r s , e l e c t r o l y t i c a l l y coated with polyimide r e s i n s , as reinforcement f o r epoxy matrix r e s i n i s seen to o f f e r p o t e n t i a l advantage. On combustion, such a composite would be expected t o r e s u l t i n increased f i b e r clumping because of the high temperature r e s i s t a n t polymer c o a t i ng and thus show reduced tendency t o r e l e a s e f i b e r fragments. The r e s i d u a l charred coating on the f i b e r s i s a l s o 1 i k e l y t o be l e s s conductive than graphite f i b e r fragments. In order to evaluate these p o s s i b i l i t i e s , polyimide coatings on graphite f i b e r s were formed not only from ace-



20.


Resin

Modification

269

t y l e n e terminated polyimides as d e s c r i b e d e a r l i e r , but a l s o from polyamic a c i d s by e l e c t r o d e p o s i t i o n . The thermal o x i d a t i v e behavior of the f i b e r s , matrix r e s i n s and com p o s i t e s was i n v e s t i g a t e d by thermogravimetric a n a l y s i s based on a study by Wentworth and coworkers (21) on the TGA o f graphite f i b e r composites. The a b i l i t y o f d i f f e r e n t types of precursor coatings t o reduce the p o t e n t i a l f o r a c c i d e n t a l r e l e a s e of carobn f i b e r s was sought t o be compared. The weight increase of carbon f i b e r s as a f u n c t i o n of time at constant a p p l i e d voltage during e l e c t r o d e p o s i t i o n s and e l e c t r o p o l y m e r i z a t i o n s are shown i n F i g s . 2 and 4. I t can be seen from these r e s u l t s t h a t the amount of polyimide incorporated i n t o a coating i s a f u n c t i o n , o f the a p p l i e d voltage and the exposure time of the treatment. Little d i f f e r e n c e i s observed between f i b e r s having d i f f e r e n t e l a s t i c moduli ( F o r t a f i l 3U and 5U, Ε=210 and 330 GPa, r e s p e c t i v e l y ) . Dynamic TGA of Resins and M i x t u r e s .

The thermal o x i d a t i v e behavior of the cured polyimide r e s i n and polyimide-epoxy r e s i n mixtures i s shown i n F i g . 5. As seen F i g . 5, the neat epoxy has t h r e e major breaks i n the TGA curve. One s t a r t i n g a t 275°C and another at 350°C, corresponding t o r e s i n decomposition t o char, and a t h i r d s t a r t i n g about 450°C f o r the o x i d a t i o n of the char residue. As expected, the p o l y i m i d e s , are c l e a r l y shown t o be more thermally s t a b l e than the epoxy r e s i n . In f a c t , the poly imides d i d not show any major decomposition below 500°C. At t h i s temperature, the char from the epoxy r e s i n had already begun t o decompose. In the context of the purpose of t h i s study, t h i s would imply t h a t a polyimide coating on the carbon f i b e r s c o u l d s u r v i v e t o a higher temperature. That i s , i n the composite, the epoxy matrix r e s i n and the r e s u l t i n g char could be completely consumed before the polyimide c o a t i ng would begi η t o decompose. Thi s would not only r e s u l t i n h o l d i n g the f i b e r s t o g e t h e r , but would a l s o provide an i n s u l a t i n g l a y e r on any r e l e a s e d f i b e r s , thereby preventing e l e c t r i c a l contact. The r e s u l t s of the thermal decomposition of the poly imide coated f i b e r s lead t o an i n t e r e s t i n g o b s e r v a t i o n . F i b e r s coated w i t h polyimides appear to decompose more r a p i d l y than the untreated f i b e r . The r e s u l t s are shown i n F i g . 6. HR 6U0 coated f i b e r s showed a decomposition f o r the p o l y i m i d e , beginning a t about 500°C, f o l lowed by a very r a p i d f i b e r decomposition a t about 760°C. Since HR 600 was e l e c t r o p o l y m e r i z e d i n a NaNO^-DMF s o l u t i o n , i t i s poss i b l e t h a t a s a l t , codeposited with the HR 600, c a t a l y z e d the the o x i d a t i o n of the f i b e r s . Pyre-ML, F i g . 6, 1ikewi se appears t o i n f l u e n c e the f i b e r decomposition. The decomposition of



270

RESINS F O R A E R O S P A C E

60 TIME, S

Figure 4.

Electrodeposition of polyamic acid Pyre-ML on Fortafil carbon fibers: PYRE-ML (24 VDC); (M) Fortifil 517, (Φ) Fortafil 3U

0

200

400

600

800

1000

TEMP°C

Figure 5.

Dynamic TGA of epoxy and polyimide resins



20.


Resin

Modification

271

the Pyre-ML, beginning about 550°C does not show any c l e a r break before f i b e r o x i d a t i o n begins. I n s t e a d , a smooth l i n e i s observed. I t appears the polyimide has some c a t a l y t i c e f f e c t on f i b e r o x i d a t i o n , decomposing the f i b e r at a lower temperature than i s observed f o r the untreated f i b e r . F i n a l l y , the thermal behavior of composites prepared from polyimide coated f i b e r s i s shown i n F i g . 7 . Several observations are worth n o t i n g . The f i b e r decomposition i n both polyimide-coated f i b e r composites occur a t temperatures somewhat lower than i s observed i n the untreated f i b e r composite. This again suggests t h a t the decomposition of the polyimides i n some manner c a t a l y z e s the o x i d a t i o n of the f i b e r , as was observed w i t h the coated f i b e r s themselves. On the other hand, HR 600 and Pyre-ML r e s i n s correspond, almost e x a c t l y , t o the observed r e s i n decompositions i n the composites. This o b s e r v a t i o n adds support t o the occurrence of an i n t e r a c t i o n between the polyimide r e s i n and carbon f i b e r decomposition behavior. In summary, t h e thermal o x i d a t i v e behavior of the neat r e s i n s , coated f i b e r s , and composites have shown t h a t e l e c trochemical treatments r e s u l t i n g i n the decomposition of polyimides have a s i g n i f i c a n t e f f e c t on the behavior of carbon fiber-epoxy matrix composites. Polyimides are not only more t h e r m a l l y s t a b l e than the epoxy r e s i n s , but a l s o , appear to reduce the thermal s t a b i l i t y o f the carbon f i b e r ' s u b s t r a t e . Other s i m i l a r and s i g n i f i c a n t e f f e c t s of the coatt i n g s on the decomposition of the carbon f i b e r s were observed with organophosphorus coatings a l s o ( 6 J . Al 1 of these e f f e c t s could be seen as e i t h e r preventing t h e r e l e a s e of carbon f i b e r s i n t o the environment o r , perhaps, as r e s u l t i n g i n a f i b e r having a reduced c o n d u c t i v i t y , thereby preventing e l e c t r i c a l contact once r e l e a s e d . It i s obvi ou s t h a t a great deal of f u r t h e r study wi11 be necessary t o answer the questions r a i s e d by t h i s work. Future study should be di rected toward determi ning t h e e f f e c t these coatings have on the e l e c t r i c a l c o n d u c t i v i t y of t h e f i b e r , both before and a f t e r thermal decomposition of the matrix r e s i n . A l s o , s i n c e the coated f i b e r s are t o be used i n composites, a study of the composite mechanical p r o p e r t i e s would be e s s e n t i a l . Further study of t h e mechanical and thermal o x i d a t i v e p r o p e r t i e s of carbon f i b e r s t r e a t e d by electrochemi c a l means would be very useful i n gai ni ng an understandi ng of the behavior of the f i b e r i n the composite. Summary.

Interphase m o d i f i c a t i o n through e l e c t r o p o l y m e r i z a t i o n and e l e c t r o d e p o s i t i o n , such as t h a t o f N-(2-hydroxyethyl)



272

RESINS F O R A E R O S P A C E

Figure

6.

Dynamic

TGA of untreated, Pyre-ML Fortafil 5U carbon fibers

coated, and

HR-600

coated

TEMP°C Figure 7. Dynamic TGA of Fortafil 5U fiber and epoxy composites prepared from untreated, HR-600 coated, and Pyre-ML coated Fortafil 5U fibers



20. SUBRAMANIAN AND JAKUBOWSKI Resin Modification

273

ethyleneimine, has been shown t o be e f f e c t i v e i n improving composite shear s t r e n g t h and toughness. S t u d i e s w i t h model compounds such as phenyl a c e t y l e n e and b e n z o n i t r i l e confirm the occurrence of e l e c t r o i n i t i a t e d p o l y m e r i z a t i o n on g r a p h i t e f i b e r s through the C E C or C E N bonds i n n i t r i l e or a c e t y l e n e terminated polyimide i n t e r m e d i a t e s . The concept of i n t e r phase m o d i f i c a t i o n has been expanded by the a p p l i c a t i o n of e l e c t r o c h e m i c a l c o a t i ng of carbon f i b e r t o reduce the p o t e n t i a l f o r r e l e a s e of conductive f i b e r fragments from g r a p h i t e compos i t e s . Polyimide precursors were formed e l e c t r o c h e m i c a l l y on carbon f i b e r s . Thermogravimetric a n a l y s i s was used to measure the s i g n i f i c a n t e f f e c t s of the c o a t i n g s on the thermal o x i d a t i v e behavior of the system components. E l e c t r o c h e m i c a l p o l y m e r i z a t i o n thus o f f e r s a new route f o r c o a t i ng carbon f i b e r s p r i o r to embeddi ng them i n a polymer m a t r i x . The p o t e n t i a l value of these techniques t o composite property m o d i f i c a t i o n was amply demonstrated. Acknowledgement This research was supported by g r a n t s from the O f f i c e of Naval Research and from the Washington S t a t e U n i v e r s i t y Research Foundation. Literature Cited.

1.

R. V. Subramanian, James J. Jakubowski, Polym. Eng. Sci., 18. 590 (1978).

2.

R. V. Subramanian, V. Sundaram, A. K. P a t e l , 33rd Ann. Tech. Conf. SPI R e i n f . Plastics/Comp. I n s t . , 20F (1978).

3.

R. V. Subramanian, James J. Jakubowski, F. D. J . Adhesion, 9, 185 (1978).

4.

James J . Jakubowski, " I n t e r f a c e M o d i f i c a t i o n of Carbon F i b e r Composites by E l e c t r o p o l y m e r i z a t i o n " M.S. T h e s i s , M a t e r i a l s Science and E n g i n e e r i n g , Washington S t a t e U n i v e r s i t y , Pullman, WA (1976).

5.

V. Sundaram, " E l e c t r o d e p o s i t i o n of Polymers on Carbon F i b e r s : E f f e c t s on Composite P r o p e r i t i e s " T h e s i s , M a t e r i a l s Science and E n g i n e e r i n g , Washington S t a t e U n i v e r s i t y (1977).

6.

James J. Jakubowski, " E l e c t r o c h e m i c a l P o l y m e r i z a t i o n and D e p o s i t i o n on Carbon F i b e r s " , Ph.D. T h e s i s , M a t e r i a l s Science and E n g i n e e r i n g , Washington S t a t e U n i v e r s i t y , Pullman, WA (1979).

Wi1liams,



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RESINS FOR AEROSPACE

7.

A. S. Kenyon, H. J. Duffey, Polym. Eng. S c i . , 7, 189 (1967).

8.

A. S. Kenyon, J. Colloid I n t e r f a c e Sci., 27, 761 (1968).

9.

G. J. Fallick, H. J. Bixler, R. A. M a r s e l l a , F. R. Garner, Ε. M. F e t t e s , Mod. P l a s t . 45, 143 (1969).

10.

J . L. Kardos, F. S. Cheng, T. L. T o l b e r , Polym. Eng. Sci., 13, 455 (1973).

11.

M. Xanthos, R. T. Woodham, J. Appl. Polym. Sci., 16, 381 (1972).

12.

N. B i l o w , A. L. L a n d i s , Proc. 8th National SAMPΕ Conf. S e a t t l e , WA (1976), p. 94.

13.

R. F. Kovar, G. F. L. E h l e r s , F. E. A r n o l d , AFMLTR-76-71 (June 1976); ibid, p. 106.

14.

A. Wereta, AFML-TR-75-214 (1976).

15.

W. B. A l s t o n , Proc. 8th National SAMPE Conf. S e a t t l e , WA (1976), p. 114.

16.

J . Jakubowski

17.

R. V. Subramanian, J. J. Jakubowski, Β. K. Garg, U. S. NTIS AD-Rep, AD-A047492 (1977), Chem. A b s t r . 88, 170537q, (1978).

18.

R. V. Subramanian and Β. K. Garg, Polymer (1979).

19.

U. S. Department of Commerce News, ITA 78-13, J a n . 20, (1978).

20.

NASA TM 78652, January

21.

S. E. Wentworth, R. J. Shuford, and A. O. K i n g , Eighth North East Regional Meeting, ACS, Boston, June (1978).

22.

D. C.

Phillips,

and R. V. Subramanian, Polymer,

J.

Bull.

(communicated).

1, 421

(1978).

Electrochem. Soc., 119, 1645 (1972).

RECEIVED January 8, 1980.


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