Interaction of carbon fibres with epoxide resins

2448

I. ~T. YEBMOLENKO et al. REFERENCES

1. W. V. SMITH a n d R. H. EWART, J. Chem. Phys. 16: 592, 1948 2. S. S. MEDVEDEV, K i n e t i k a i mekhanizm obrazovaniya makromolekul (Kinetics a n d Mechanism of F o r m a t i o n of Maeromolecules). p. 5, Izd. " N a u k a " , 1968 3. L. G. MELKONYAN, Ueh. zap. EGU, No. 1, 111, 1970 4. A. A. SHAGINYAN, Yu. E. NALBANDYAN, O. M. AIVAZYAN, L. G. MELKONYAN a n d Sh. A. MARKARYAN, Arm. khim. zh. 9: 743, 1976 5. A. A. SHAGINYAN, O. M. AIVAZYAN, Yu. E. NALBANDYAN, L. G. MELKONYAN a n d Sh. A. MARKARYAN, Kolloid. zh. 3: 605, 1977 6. A. A. SHAGINYAN, O. M. AIVAZYAN, Yu. E. NALBANDYAN and A. Zh. K H A N DANYAN, KoUoid. zh. 39: 610, 1977 7. N. M. BEILERYAN and D. D. GRIGORYAN, Vysokomol. so'yed. B16: 540, 1974 (Not t r a n s l a t e d in P o l y m e r Sci. U.S.S.R.) 8. N. WHITBY (Ed.), Sinteticheskii kauchuk (Synthetic Rubber). p. 242, Goskhimizdat, 1957 (Russian translation) ~). S. S. I V A N C H E V , N. I. SOLOMKO, V. V. K O N O V A L E N K O a n d V. A. Y U R Z H E N K O , Dokl. Akad. N a u k SSSR 191: 593, 1970

Polymer Science U.S.S.R. Vol. 20, pp. 2448-2457. ~(~) Pergamon Presa Ltd. 1979. Printed in Poland

0~32-3950178/1001-24~8507.50/0

INTERACTION OF CARBON FIBRES WITH EPOXIDE RESINS* I . N . YERMOLENKO, V. I . DUBKOVA a n d I . P . LYUBLINER I n s t i t u t e of General a n d Inorganic Chemistry, B.S.S.R. A c a d e m y of Sciences (Received 29 November 1977)

A comparative investigation of the mechanism of interaction of carbon fibres containing phosphate and carboxyl groups, with epoxide resins, has been m a d e b y potentlometric titration, I R spectroscopy and electron microscopy. I t is shown t h a t when carbon fibres containing phosphate groups in their structure, in contrast to carbon fibres containing carboxyl groups, or graphite fibres, as mixed w i t h the resin t h e l a t t e r becomes completely and irreversibly cured, without addition of o r d i n a r y hardeners. The use in composites of carbon fibre containing phosphate groups, as a simultaneous filler and h a r d e n e r , is proposed.

ONE of the more important problems in production of carbon reinforced plastics is attainment of the necessary bonding between the carbon fibres (CF) and the binder [1, 2]. The information in the literature on this problem is concerned mainly with production of composites from high-modulus CF, obtained from polyacrylonitrile fibre [3]. * Vysokomol. soyed. A20: No. 10, 2180-2187, 1978.

Interaction of carbon fibres with epoxide resins

2449

One of the major hindrances to extensive use of high modulus CF is its high cost. Meanwhile cheaper, low modulus CF, obtained from cellulose hydrate, is available. Because of their low cost these fibres are widely used ir~ various branches of technology [4, 5]. The use of low modulus CF in production of compositea 100

80

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"

•

r

I

10

~G

I

20

i

I

30

T[me , rain

:Fro. 1. Dependence on the duration of heat treatment, of the epoxide group content of composites comprising 40 p.b. w of CF and 100 p.b.w of UP-632, heat treated at 160° (1-3), 40 p.b.w of CF and 100 p.b.w, of ED-20, heat treatod at 180° (4-6), with the following concentrations of phosphorus in the fibre: 4-25% (1, 4), 6"25% (2, 5) and 8.20% (3, 6). would be advantageous economically if the necessary combination of propertiea of the composites could be attained, and this could be achieved b y introduction into the CF of functional groups capable of reacting with the binder. The introduction of reactive groups into CFs was studied in [6]. I t must be mentioned t h a t the problem of reaction between modified, low modulus CFs and polymeric binders has been discussed extremely inadequately in the literature. Meanwhile it m a y be assumed t h a t CFs containing reactive groups will fulfil the function of hardener as well as t h a t of filler. The aim of the work reported in this paper was study of the processes occurring in interaction of low modulus, modified CFs, with epoxide resins. The material studied was CF containing phosphate groups. I n the form of fabric and skeins, obtained by carbonization of cellulose phosphate by the routine described in reference [7]. The final temperature of carbonization was 500 ° . The phosphorus content of the CFs was 1-9%, determined by the photocolorimetric method, and their exchange capacity with respect to sodium was 0.704.25 mg-equiv/g. Carbon fibre containing carboxyl groups was prepared by t r e a t m e n t with concentrated nitric acid at 70°C for 5 hr, of CF obtained b y carbonization of cellulose at a final temperature of 500 °. The concentration of carboxyl groups, determined by the calcium acetate method [8], was 19-0%.

2450

I.N. YERMOLENKOet al.

For comparison, high temperature graphite fibre, without deliberately introduced functional groups, was also used. The polymeric binders used were the diphenylolpropane epoxide resin ED-20, containing 20.2 % of epoxide groups, and the eycloaliphatie epoxide resin UP-632, containing 27.0% of epoxide groups. To find the effect of the type and degree of modification of the CF on the curing of an epoxide resin, two component, resin fibre composites were moulded, containing 50-300 parts by weight of resin to 100 parts by weight of fibre, and these were heat treated in air, using various temperature routines. The kinetics of polymer formation were followed by means of the degree of conversion of the epoxide groups of the resin. This was determined by potentiometrie titration [9]. The degree of conversion was calculated from the equation Ce.g.= (%--C,)/% X 100%

where co and c, are the concentrations of epoxide groups initially and at time v. The change in the number of epoxide groups during heat treatment of the two component systems, was also determined from IR spectra, obtained by the method of reference [10]. The I R spectra of the cured composites were recorded in a UR-20 spectrometer, using test specimens in the form of pellets with KBr [11]. The degree of curing of the epoxide resin was determined as the quantity of gel fraction, not extractable from the surface of the CF by boiling solvent. The extraction was carried out with acetone in a Soxhlet apparatus for 24 hr. The distribution of the cured resin on the CF surface was studied by electron microscopy, using an MSM-2 scanning electron microscope. When CF containing phosphate groups is incorporated in an epoxide resin matrix and the system is then heated, the concentration of free epoxide groups falls. Figure 1 shows the variation in epoxide group content of resin-CF composites, the fibres containing different concentrations of phosphate groups. I~ is seen from the graphs that already in the first few minutes of heat treatment a marked reduction in the quantity of epoxide groups occurs in both the ED-20 and UP-632 resins. The rate of decrease in epoxide group concentration on the surface of phosphated CF is higher for ED-20 than for UP-632. It is also seen from Fig. 1 that the degree of modification of the fibre has a substantial effect on the rate of polymerization of the resins. Increase in the phosphorus content of the fibre brings about more rapid combination of the epoxide groups in both resins. For example, at phosphorus contents o f 4.25% and 8-20% the conversion of the epoxide groups of UP-632 resin after heat treatment of the composites for half an hour was 37% and 75% respectively (Fig. l, curves 1 and 3). The decrease in the number of epoxide groups in the resin by reaction with CF containing phosphate groups, is confirmed by the I R spectra (Fig. 2), which show a reduction in the intensity of the 920 cm -1 absorption band, assigned to vibration of the epoxide ring. The sharp fall in epoxide group content gives grounds for assuming that

Interaction of carbon fibres with cpoxide resins

2451

CF containing phosphate groups, reacts chemically with the resin through its epoxide groups. The hydroxyl value of the system increases uniformly from its initial value, as the epoxide equivalent falls. I t m a y therefore be supposed t h a t hardening of the epoxide resin in contact with CF containing phosphate groups, involves opening of epoxide rings and formation of new hydroxyl groups. 5

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FIG. 2. IR spectra of CF containing phosphate groups (1), resins ED-20 (5) and UP-632 (3), and of cured composites comprising 100 p.b.w, of phosphated CF and 300 p.b.w, of ED-20, cured at 160° for 10 hr (4); 30 p.b.w, of CF and 100 p.b.w. of UP-632 cured at 160° for 40 min (2). On the other hand it is known [13] t h a t in carbonization of phosphorylated cellulose fibre in the given range of temperatures, in addition to formation of the structures characteristic of ordinary CF, condensation reactions also occur, OH / with formation of structure of the type C F - - P , which contains labile hydII\ O OH rogen capable of taking part in an exchange reaction. Figure 3 shows the variation with resin content, of the concentration o f ion exchanging groups on fibre t h a t had previously had ED-20 resin cured

~2452

I. 1~. YERMOLENKO 8t a~.

.on its surface. I t is seen t h a t with increase in the q u a n t i t y of hardened fraction o f the resin, not extractable from the surface of the phosphated CF, the con~ e n t r a t i o n of ion exchanging grQups on the fibre decreases. This indicates t h a t l~ '200

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170

,

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FIG. 4

FxG. 3. Dependence of the quantity of insoluble resin fraction on phosphated CF (1) and '4on exchanging groups in the CF (2),on the quantity of ED-20 per 100 p.b.w, of CF in the ~original composite. Concentration of phosphorus in the CF 3.6%. Curing temperature 180°, time 10 hr. FIG. 4. Dependence of the quantity of UP-632 (1) and ED-20 (2) not extractable from the surface of phosphated CF, on the temperature of treatment of composites comprising 100 p.b.w, of resin and 100 p.b.w, of fibre. Duration of heat treatment 1 hr, phosphorus content of CF 3.6~/o. t h e CF reacts with the epoxide resin through its phosphate groups. I n order to examine this assumption some of the labile hydrogen atoms in phosphate groups of the CF were replaced by nickel, cobalt and chromium ions, by ion exchange from 0.1 N solutions of salts. As a result the degree of curing of an e p o x i d e resin on the metal phosphated CF was much reduced. Whereas the insoluble fraction of ED-20 on CF containing phosphate groups was 96~/o, on m e t a l phosphated CF in the form of nickel, cobalt and chromium salts, the quantities of resin not extractable from the surface of the fibre were 15.7~o , 18-4 and 14-6~o respectively. F r o m this investigation of the k~ne~ics of change in the epoxide group cont e n t of phosphated CF-resin composites, and of the reaction between epoxido resins and OF, it m a y be concluded t h a t reactive phosphate groups are of definite significance in polymerization of epoxide resins on the surface of the CF. For •example, when composites are made from 100 parts by weight of CFs containing

Interaction of carbon fibres with opoxido rosins

2453

1% and 9% by weight of phosphorus, with 300 parts by weight of epoxide resin ED-20 and then heated at 190° for 10 hr, the quantity of gel fraction of the resin, not extractable from th6 surface of the fibre is 17-5% and 26-3% respectively. These results enable the following most probable scheme to be put forward for reaction between the surface of CF containing phosphate groups and an epoxide resin. First an epoxide resin molecule adds to a phosphate group of the CF F l 0-C--C--R

OH CF--P=0+--C~C--R I \ /

OH

-, CF--P=0 I

0

0it

OH

(R is the rest of the epoxide resin molecule. Then formation of a crosslinked product occurs, resulting in production of an insoluble residue of resin on the fibre

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- - c\-/- c - ~

0H

o

CF,--

=0

I

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)

CF--

=0

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1 1 OH Study of the degree of conversion of epoxide groups on CF with the same phosphorus content, showed that this differs in the two resins chosen (Fig. 1, curves 3 and 6). The resin UP-632 reacts at a high rate with the phosphate groups of the fibre, even at a lower temperature. When CF containing phosphate groups is mixed with UP-632 the concentration of epoxide groups falls even before heat treatment, which cannot be said of the CF-ED-20 system. Increase in the proportion of phosphated CF in the CF-UP-632 system causes increase in the rate of conversion of epoxide groups. When 20, 40, 60, 80 and 100 parts by weight of CF (8.20% of phosphorus) are mixed with 100 parts of UP-632, the degrees of conversion of epoxide groups at room temperature were 18.8, 27.2, 36.5, 48.4 and 61.3% respectively. This again shows that the phosphate groups are involved in hardening of the epoxide group on the surface of the fibre, though under these conditions formation of insoluble polymer does nob occur, or it occurs only to a very small extent. The dependence on temperature of the degree of curing of resin UP-632 and ED-20 on CF containing 3.6% of phosphorus, in two component, resin-CF systems, is shown in Fig. 4. It is seen from the graphs that an insoluble gel-fraction is formed from both resins at temperatures above 100°. The increase in the weight of insoluble fraction on the CF as the temperature is raised, is grea~or in the case of resin UP-632 and it reaches a maximum (85%) at 180°. In UP-632

2454

I . N . YERMOLENKO et aZ.

gel formation begins at lower temperatures. This investigation shows that a l o n g e r period at 170-190 ° is required for complete hardening of ED-20 on the

carbon fibre. Experiments on interaction of graphite fibre with epoxide resins showed that when this fibre is mixed with an epoxide resin and the two component system is then heated under the conditions described above, cured resin does not become attached to the fibre and the change in epoxide group concentration is commensurate with the change that occurs when the epoxide resin itself is heated. When CF containing carboXyl groups is mixed with epoxide resins, binding of epoxide groups of the resin occurs during heat treatment. When a composition containing 100 parts by weight of CF and 100 parts of ED-20 is heated, the degree of conversion of epoxide groups is 11.5%, and for a composition comprising 100 parts by weight of CF and 100 parts of UP-632 the conversion is 16°/o. Carboxyl groups are not found on the fibre after the composite has been heated. The quantity of insoluble resin on the fibre is small (in the region of 17%), however, even after prolonged heat treatment. From the change in the carboxyl group content of the fibre and of the epoxide-group content of the resin, it may be assumed that the following addition reaction occurs between them 0 II J CF--C--OH ~ - - - C \ / 0

0 I N I I C--R -~ C F - - C - - 0 - - C - - C - - R I I OH

Thus the presence of functional groups containing oxygen on the surface of CF, brings about chemical reaction with the epoxide groups of the resin under t h e conditions of heat treatment. Moreover, in the case of CF containing phosphate groups, complete, irreversible curing of the resin occurs on the fibre, without t h e use of traditional hardeners. In such two ~component system the CF filler is also the resin hardener [14]. The possibility was examined of curing a multicomponent system, using CF containing phosphate groups as the sole hardener of the epoxide resin. For this purpose the following model system was moulded: 100 parts by weight of ED-20 with 50 parts of graphite fibre and various amounts of the CF, from 50 parts to 300 parts, at intervals of 50 parts by weight. The graphite and carbon fibres were chopped to lengths of 1 mm or less and added to the epoxide resin, t h e mixture then being thoroughly compounded and finally heated at 180 °. T e s t samples were removed from the reaction mixture at hourly intervals and analysed for the degree of curing. It was found that the system became irreversibly cured to the extent of 96-99°//o in 1-9 hr, depending on the quantity of CF used, being converted to a solid, infusible proauct. Cured composites consisting of CF containing phosphate groups and an epoxide resin, have densities of 1.15-1.25 g/cm a and compressive strengths up to 1600 kg/cm ~, and they are free resistant.

I~lteraction of carbon fibres with epoxide resins

2455

FIG. 5. Electron photomicrographs of the original, phosphated CF (a), of CF with resins cured on its surface, with a degree of curing of UP-632 85% (b), ED-20 11.2°/o (c) and E D - 2 0 20.4% (d), and of transverse sections of composites comprising chopped CF and resirm ED-20 (e, g) and UP-632 (f, h). Magnification 100 (a-d), 2000 (e,f), 3000 (g, h).

2456

I. ~NT.YERI~OLENKO et al.

Figure 5 shows electron photomicrographs of CF containing phosphate groups (Fig. 5a), and of fibres containing epoxide resin hardened on their surface. It is seen that the cured resin is distributed uniformly over the entire exterior surface of the monofilaments (Fig. 5b and c). At the same time, the original, star-shaped cross section of the fibre is preserved (Fig. 5e,f and h). When the quantity of resin fraction that is not extractable from the CF surface, is small, the hardened polymer covers the fibre in the form of a thin, isolating sheath, with free space remaining between the fibres. In some places, where the monofilaments are close together, merging of the insoluble layers of polymer around separate fibres occurs, and aggregates are formed (Fig. 5b and c). As the quantity of unextractable resin increases, the number of such aggregates increases, so t h a t the space between the fibres becomes filled with polymer and the permeable, three dimensional network structure is converted to a solid, monolithic structure (Fig. 5d). Note that the distribution of the two resins studied here, on the surface of phosphated CF, is different. Resin UP-632 coats the carbon fibre with a uniform layer of film (Fig. 5b and lt), whereas the surface of CF covered with hardened ED-20 resin is roughened by numerous associations (Fig. 5c and g). A possible reason for this is difference in the wettability of CF by the resins. Study of the wettability of phosphated CF by these resins showed that resin UP-632 flows out completely over the surface of the fibre, whereas ED-20 forms an angle of contact with the fibre of about 44 °. Resin UP-632, which thoroughly wets the exterior surface of CF, can also penetrate to a considerable depth into the fibres, in defects, pores and fissures, giving high strength to the composite. Figure 5e-h shows transverse sections of composites, but the boundary between the phases is diffuse and estimation of the depth of penetration of the cured resin into the fibre is difficult. Thus the evidence presented above shows that in highly filled composites in which the filler is low modulus carbon fibre, the major part of the resin is present as a surface layer. Capillaries, pores and fissures in the fibres promote penetration of the resin into the latter. In these circumstances, where curing of the composite involves phosphate groups that were already introduced into the composition of the fibre in the stage of chemical modification of the cellulose, and after carbonization are present on the external and internal surface of the fibre, a crosslinked structure can be formed through the entire volume of the system. The reaction, after beginning on active centres, extends also into the free resin phase, converting the composite to a monolithic structure, as has been shown. It is probable that hardening of the resin in the depth of the composite proceeds with the aid of intermediate products formed in the primary reaction, but this requires further investigation. The authors thank P. A. Vityaz and V. Sazanovets for producing the electron photomicrographs. Translated by E. O. PHILLIPS

I n t e r a c t i o n of carbon fibres with epoxide resins

2457

REFERENCES

1. A. A. KONKIN, Uglerodnye i drugie zharostoikie voloknistye materialy (Carbon a n d Other H e a t - R e s i s t a n t Fibrous Materials). p. 286, Izd. " K h i m i y a " , 1974 2. A. T. TUMANOV (Ed.), Monokristal'nye volokna i armirovannye imi materialy (SingleCrystal Fibres and Materials Reinforced W i t h Them). Izd. "Mir", 1973 3. V. Ya. VARSHA¥SKII, Khim. tekhnol, vysokomol, soyed. 8: 92, 1976 4. S. OTANI, Gidzyupu sire Mitsubisi sekiyu kabusiki kaisa, No. 51, 43, 1973 5. I. N. YERMOLENK0, A. A. MOROZOVA and I. P. LYUBLINER, Sorptsionno-aktivno voloknistye ugol'nye m a t e r i a l y i perspectivy ikh ispol'zovaniya v narodnom khozyaistve (Fibrous Carbon Materials with Activity as Sorbents and the Prospects for Their Use in the National Economy). p. 10, B e l N I I N T I , 1976 6. R. N. SVIRIDOVA, M. Z. GAVRILOV and I. N. YERMOLENKO, Kolloid. zh. 35: 305, I973 7. I . N . YERMOLENKO, I. I. VYGOVSKII and I. P. LYUBLINER, Izv. Akad. N a u k BSSR, ser. khim. nauk, No. 4, 78, 1974 8. A. B. PAKSHVER (Ed.), Spravochnik po analitieheskomu kontrolyu v proizvodstve iskusstvennykh i sinteticheskikh volokon (Handbook of Analytical Control in P r o d u c tion of Artificial and Synthetic Fibres). p. 47, Gizlegprom, 1957 9. A. G. KULICHEV, M. S. TRIZNO and A. F. NIKOLAYEV, Plant. massy, No. 10, 58, 1969 10. A. M. NOSKOV a n d V. N. GOGOLEV, Zh. prikl, spektroskopii 20: 88, 1974 l l. H. KLEIN, Analiticheskaya k h i m i y a polimerov (Analytical Chemistry of Polymers). p. 143, Foreign Literature Publishing House, 1963 (Russian translation) 12. Kh. LI and K. NEVILL, Spravochnoye rukovodstvo po epoksidnym smolam (Guide to Epoxide Resins). p. 20, Izd. " E n e r g i y a " , 1973 13. I. S. SKORININA, S. S. GUSEV, N. K. VOROB'EVA and I. N. YERMOLENKO, Izv. Akad. N a u k BSSR, ser. khim., No. 3, 29, 1970 14. I. N. YERMOLENKO, V. I. DUBKOVA and I. P. LYUBLINER, Rus. Pat. (Author'sCertificate). 537953; Byul]. izobret., No. 45, 84, 1976