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The effect of coupling agent on electrical and mechanical properties of


JPOL 4236

Polymer 41 (2000) 3243–3252

The effect of coupling agent on electrical and mechanical properties of carbon ber/phenolic resin composites
M.H. Choi, B.H. Jeon, I.J. Chung*
Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong, Yusong, Taejon 305-701, South Korea Received 8 March 1999; received in revised form 22 April 1999; accepted 29 July 1999

Abstract Carbon ber/phenolic resin composites were prepared by changing the content (5–10 wt%) of short carbon bers. To investigate the effect of carbon ber treatment on the electrical and mechanical properties of the composites, three specimens were prepared: the short carbon ber treated to remove size (called USCF); the carbon ber oxidized with nitric acid (called NAOCF); and the ber oxidized with nitric acid and treated with coupling agent glutaric dialdehyde (called GTDACF). The GTDACF composite had higher electrical conductivity and better mechanical property than the other composites with the same content of carbon bers. The surface treatment methods affected the dielectric behaviors of the composites with short carbon bers while they did not affect those of the composites with fabric type carbon bers. From these observations, the coupling agent improved adhesion between the carbon ber and the phenolic resin by forming a chemical bond between ber and resin. The coupling agent also affected the ow and dispersion of the short carbon ber in the phenolic resin during compression molding, resulting in the higher electrical conductivity and better mechanical property of GTDACF composite. 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Phenolic resin; Carbon ber; Coupling agent

1. Introduction Over the last four decades, carbon bers have emerged as the main reinforcement ller for high performance composite materials. They have various useful properties: high strength and high modulus, fatigue resistance and vibration damping, corrosion resistance, good friction and wear qualities, low thermal expansion, thermal and electrical conductivity. So they are an attractive substitute for various metals, alloys and other materials [1]. They offer versatile applications in aerospace, robots, sporting leisure goods, solar cell, resistor, semiconductor elements, antistatics and electromagnetic shielding materials etc. [2,3]. The properties of a composite, such as strength and modulus, etc. are important factors to get the high quality of the composite. The properties of ber and matrix make a critical contribution to the quality of a ber-reinforced composite. In addition, the physical–chemical interaction at the ber–matrix interface plays an important role in improving the mechanical properties of a ber-reinforced composite. Nowadays many researchers have tried to improve the
* Corresponding author. Tel.: 82-42-869-3916; fax: 82-42-869-3910. E-mail address: chung@cais.kaist.ac.kr (I.J. Chung). 0032-3861/00/$ - see front matter PII: S0032-386 1(99)00532-7

adhesion between carbon ber and matrix resin by chemical reaction using a coupling agent. But there is almost no functional group on the carbon ber surface that is able to react with matrix or coupling agent. Some functional groups can be generated by treating the carbon ber surface. In general there are three ber surface modication methods: chemical method, electrochemical method and plasma treatment method. The chemical treatments have been preferred since they afford improvement without degradation of the ber properties. Electro-oxidative techniques have been developed and are conventionally used in the treatment of carbon ber surfaces. These treatments remove weak boundary layers from the surface and produce the increased surface activity with formation of both acidic and basic moieties [4,5]. Low energy plasma processes have demonstrated the potential to control the acidity of the surface for the optimization of adhesion between the carbon ber and the resin [6,7]. The plasma technique functionalizes the basal plane sites on the ber surface and increases the level of surface activity. X-ray photoelectron spectroscopy (XPS) has been widely used to study carbon ber surfaces after oxidative treatments. Ishitani has pointed out that the usage of XPS in the surface study of carbon ber is advantageous, since carbon bers are not disturbed by the strong absorption of

2000 Elsevier Science Ltd. All rights reserved.

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M.H. Choi et al. / Polymer 41 (2000) 3243–3252

between hydroxyl group on the carbon ber surface and the coupling agent. As shown in Fig. 1, the formation of acetal and hemiacetal linkages between phenolic resin and carbon ber by the glutaric dialdehyde can improve the adhesion between phenolic resin and carbon ber. In this study, carbon bers were oxidized chemically in nitric acid. The oxidized carbon ber was also treated with a coupling agent of glutaric dialdehyde. Carbon ber/phenolic resin composites were prepared with these surface treated carbon bers. The electrical and mechanical properties of carbon ber/phenolic resin composites were investigated.

2. Theoretical background Dielectric storage 1 H is a measure of the degree of charge polarization, or the energy involved for the separation between charges. On the other hand, dielectric loss 1 HH is a direct measure of the energy dissipated irreversibly in the relaxation of polarization. The dielectric loss 1 HH is divided into two terms.

1 HH 1 HH AC

1 HH DC

1

Fig. 1. Interfacial reaction involving a hemiacetal linkage between the carbon ber treated in nitric acid solution using chemical oxidation method and the phenolic resin in the presence of glutaric dialdehyde (GTDA).

where 1 HH represents the contribution from DC conduction, DC i.e. the long-range electronic transport, and is well tted by the following empirical relationship reported by Ngai et al. [21]

the electromagnetic wave but is selective to the surface structure [4]. It is extremely difcult to analyze the carbon ber surface by FT-IR spectroscopy because of the absorption and the severe scattering of the infrared radiation. However, Sellitti et al. succeeded in obtaining the IR spectra of oxidized carbon ber surfaces using the internal reection technique [8]. XPS studies have conrmed that oxidative treatment of carbon ber increases the surface functionality by creating additional carbonyl groups, carboxyl groups, carboxyl/ester groups and/or hydroxyl/ ether groups at the surface [9–14]. Different functional groups are obtained from different oxidation methods. Various surface treatments of carbon ber have been reported to improve the ber–matrix adhesion, interfacial shear strength, and toughness [15–18]. However they are mostly restricted to epoxy-based composites. Sherwood et al. reported XPS studies of carbon ber surfaces including interfacial interactions between phenolic resin and carbon ber oxidized electrochemically in nitric acid, phosphoric acid, and ammonium carbonate solutions, accompanying ab initio calculations to interpret spectral features in the core and valance band spectra [19,20]. They showed the generation of various functional groups such as –CyO, –C–OH, – COOH, –NH2, depending on various kinds of solution, but the generation of hydroxyl groups for most solutions. Using XPS they studied the chemical reactions between phenolic resin and glutaric dialdehyde (i.e. coupling agent) as well as

1 HH Av DC

n

0

n

1

2

where v is the frequency of the applied AC electric eld, A is a frequency-independent constant and n is a coarse morphological parameter relating to the transport pathway of the electronic charge. Ngai et al. reported that Eq. (2) is a universal response and can be applied to ionic interaction, dipolar interaction and electronic hopping system etc. The morphological parameter n varies from 0 to 1 depending on various transport situations. n 1 corresponds to the ideal case that charge-conducting paths span the sample dimension in a direct fashion, and charge drift is dominant. n 1=2 indicates a totally tortuous and random pathway as in Brownian motion. Of course, a hybrid situations between drift and random diffusion-dominant cases can be envisioned and may yield the effective values of n neither 1 nor 1/2.

3. Experimental 3.1. Matrix resin Resol type of phenolic resin used in this study was obtained from Kolon Chemical Co., Ltd. and its commercial name is KRD-HM2, which is a mixture of 60 wt% phenolic resin and 40 wt% methanol. Its density was 1.0630 g/cm 3 and gel time was 52 s at 150 C.

M.H. Choi et al. / Polymer 41 (2000) 3243–3252 Table 1 Description of samples and sample codes Surface treatment method Carbon ber content (wt%) Temperature at which electrical property is measured ( C) Sample code

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Untreated (USCF) 5 8 10 Nitric acid treated (NAOCF) 5 8 10 GTDA (coupling agent) treated (GTDACF) 25 70 25 70 25 70 25 70 25 70 25 70 N5-25 N5-70 N8-25 N8-70 N10-25 N10-70 G5-25 G5-70 G8-25 G8-70 G10-25 G10-70 25 70 25 70 25 70 U5-25 U5-70 U8-25 U8-70 U10-25 U10-70

5 8 10

3.2. Carbon ber surface treatments The carbon ber used was TZ-307 epoxy sized highstrength PAN-based carbon ber (pre-heat temperature, 350 C; carbonization temperature, 1500 C) produced from Taekwang Ind. Co., Ltd. It was a short ber with the following characteristics: diameter, 6.8 mm; length, 2 mm; and electrical conductivity, 6:67 × 102 S=cm at 25 C. To investigate the effect of carbon ber surface treatments on the properties of carbon ber composites, epoxy sizing was removed from the carbon ber with acetone in sonicator. The unsized carbon ber was named as USCF. The USCF was oxidized for 1 h in 60% nitric acid (Junsei Chemical co., Ltd) at 25 C, washed with distilled water and dried in a vacuum oven. This oxidized carbon ber was named as NAOCF. A coupling agent, glutaric dialdehyde (GTDA) purchased from Aldrich, was used in an attempt to produce interfacial chemical bonds. The NAOCF was immersed in an aqueous coupling agent solution (25 wt% of coupling agent) for 1 h at 25 C. Just before the NAOCF was immersed into the coupling agent solution, this solution was acidied by adding a drop of concentrated sulfuric acid purchased from Aldrich. The NAOCF treated with coupling agent was washed with distilled water and dried in a vacuum oven. This carbon ber was named as GTDACF. 3.3. Infrared spectroscopy (FT-IR) The chemical bonding between the NAOCF and the coupling agent was veried in the report of Sherwood et

al. [20] in which the carbon ber was oxidized electrochemically. In this study chemical rather than electrochemical oxidization of the carbon ber surface was performed. The surface functionality resulting form the oxidation and the coupling agent treatment process was determined by FT-IR spectroscopy. Infrared spectra on KBr pellets were averaged over 20 scans taken at 4 cm 1 resolution using a FT-IR spectrometer (Bomem 102 model). 3.4. Preparation of carbon ber/phenolic resin composites Surface treated carbon bers (5–10 wt%) and phenolic resin were mixed and stirred in a beaker at 30 C. Then the composite plate was prepared with 2 mm thickness. It was dried in a convection oven at 40 C to remove methanol. The carbon ber/phenolic resin composite was prepared by stacking 4 plies of plate and pressing with a hot press isothermally at 150 C for 30 min. The composite had dimensions of 12 mm × 12 mm with 1 mm thickness. In order to measure its electrical and mechanical properties, it was cut in dimensions of 5 mm × 2:5 mm: 3.5. Measurement of electrical properties Electrical conductivity, impedance and dielectric constant of the composite were measured over a frequency range from 100 Hz to 10 MHz at 25 and 70 C using a Solatron SI1255 frequency response analyzer (FRA). The power of FRA was 1000 V/m and the surface area of electrode was 1.13 cm 2. Table 1 lists a description of the samples and the sample codes.

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indicating a little –OH functional groups in the USCF. In the NAOCF case, the –OH peak increases. It is also well known that when the USCF is treated chemically in the nitric acid solution, only –OH functional groups are formed on the surface of the carbon ber. In Fig. 2(c), the –OH peak decreases a little but the –CO peak increases clearly around 1200 cm 1. This reveals that the coupling agent reacts with the –OH groups on the surface of the NAOCF to form hemiacetal linkages. Thus, the FT-IR spectra indicate that the coupling agent reaction in this study is similar to Sherwood’s results when the case is conned to the –OH functional groups formed on the surface of the carbon ber. 4.2. Electrical conductivity of the carbon ber/phenolic resin composite Fig. 3 shows the Cole–Cole plots of various composites as a function of surface treatment, carbon ber mass fraction and measurement temperature. ZR and ZI are the real and imaginary parts of the complex impedance Z , respectively. The radius of semicircle ZR indicates the electrical resistance of the composite. The Cole–Cole plot shows a semicircle for each case, which indicates a single relaxation mode and a resistive and capacitive network structure of the composite. The electrical conduction in the carbon ber/phenolic resin composite would occur mainly through the carbon ber because the carbon ber is a conductor and the phenolic resin is an insulator. Thus, ionic or dipolar interaction in the composite, if ever, is not signicant, resulting in the single relaxation mode. The radius of the semicircle decreases as the amount of carbon ber increases and temperature decreases. Also it is interesting to note that the radius of the semicircle decreases in the sequence of NAOCF, USCF and GTDACF composites with the same carbon ber mass fraction and at the same temperature. The electrical conductivity of a composite can be calculated from the radius of the semicircle using the following equation.

Fig. 2. FT-IR spectra of various surface treated carbon bers: (a) USCF; (b) NAOCF; and (c) GTDACF.

3.6. Measurement of mechanical properties and investigation of morphology A three-point exural test was performed to measure the mechanical properties of the composites using an Instron test machine (model 4201). The test was carried out according to the procedure described by the ASTM specication (ASTM D 790M). At least, ve specimens were tested for each set of samples and the mean values were reported. The fracture surfaces of the samples were examined by means of scanning electron microscope (SEM). Secondary electron images of the fracture surface (sputter-coated with gold) were analyzed.

4. Results and discussion 4.1. FT-IR spectroscopy It is well known that the interface between carbon ber and matrix resin of a composite plays an important role in affecting the mechanical property of the composite. The surface treatment of carbon ber with a nitric acid solution called chemical oxidation is one method to improve the interfacial strength. This method is similar to the electrochemical oxidation reported by Sherwood et al. Fig. 1 shows the chemical reaction proposed by Sherwood et al. They also reported that a few functional groups were formed including –OH group on the surface of the carbon ber. In this study, FT-IR is used to detect the functional group. The FT-IR spectra of USCF, NAOCF and GTDACF are illustrated in Fig. 2. As shown in Fig. 2(a), there is no peculiar peak except around 3400 cm 1 associated with –OH peak on the surface of the USCF. It reveals that the epoxy sizing is almost removed. The peak is not so intense

s S=cm

sample thickness cm electrode area cm2 × resistance V

3

where s is the electrical conductivity. The resistance in V is obtained from the radius of the semicircle. Fig. 4 shows the electrical conductivity as a function of carbon ber content and temperature for various composites. The electrical conductivity increases with the carbon ber content, but decreases with temperature for each composite. The electrical conductivity is about 10 3 –10 1 S/cm, which is a fairly high value. Thus, it is well understood that the randomly oriented short carbon bers in the all composites are well connected themselves. The decrease of electrical conductivity with the temperature is considered to be caused by the thermal expansion which reduces the contacts between the bers and the rotation of graphite crystallite axes [22–24]. In Fig. 4, the GTDACF composite has higher electrical conductivity than the other composites at the same

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Fig. 4. Electrical conductivity of the carbon ber/phenolic resin composites as a function of carbon ber content and temperature. N stands for NAOCF; U for USCF; and G for GTDACF composites.

carbon ber content. This is an unexpected result. It can be thought that the dispersion of the short carbon ber or the various surface treatments may cause this anomalous behavior. This behavior is explained later in the analysis of the dielectric constant data. 4.3. Phase angle change versus frequency Fig. 5(a) shows the dependence of phase angle upon frequency at room temperature for the NAOCF composite as a function of carbon ber content. The phase angle is dened as arctangent ZI =ZR : The phase angle of N5-25 changes from 0 to 90 as the frequency increases. Nevertheless, N8-25 and N10-25 have the maximum phase angles of 80 and 70 , respectively. The phase angles of the perfect resistor and capacitor are 0 at a low frequency and 90 at a high frequency, respectively. Thus, only N5-25 can function as a perfect resistor and a capacitor. The others can only function as perfect resistors. In Fig. 4, it can be seen that the electrical conductivity of the composite increases with the carbon ber content. Consequently, the higher the electrical conductivity, the lower the absolute value of the maximum phase angle the composite shows. USCF and GTDACF composites show a similar behavior to the NAOCF composite even though the data are not given here. In Fig. 5(b), the maximum phase angles are 70, 60 and 45 for NAOCF, USCF and GTDACF composites, respectively. Thus, the GTDACF composite shows more resistive behavior than the other composites, indicating
Fig. 3. Complex impedance diagrams of the carbon ber/phenolic resin composites as a function of carbon ber content and temperature: (a) USCF composites; (b) NAOCF composites; and (c) GTDACF composites.

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Fig. 5. Phase angle versus frequency plots for various surface treated carbon ber/phenolic resin composites: (a) as a function of carbon ber content at the room temperature for the NAOCF composite and (b) as a function of surface treatment and temperature for NAOCF, USCF and GTDACF composites containing 10 wt% carbon ber.

Fig. 6. Dielectric constant versus frequency plots at room temperature for the NAOCF composites as a function of carbon ber content: (a) dielectric storage and (b) dielectric loss.

equation

higher electrical conductivity (see Fig. 4). Fig. 5(b) shows that the phase angle does not signicantly change with increasing temperature. From Figs. 4 and 5(b), we conclude that the electrical conductivity is more sensitive to temperature than phase angle in the carbon ber/phenolic resin composite. 4.4. Dielectric constant of the carbon ber/phenolic resin composite The complex impedance Z can be converted to the complex dielectric constant 1 using the following

1

l 1H iv10 AC Z

1 HH

4

where l is the thickness of the specimen, 1 0 is the dielectric constant of free space and AC is the surface area of the electrode. Fig. 6(a) and (b) show the variation of dielectric constant versus frequency at the room temperature for the NAOCF composites. The dielectric storage spectra show the monotonic decrease with increasing frequency in Fig. 6(a). This is due to the fact that the mobile electronic transport such as an electronic hopping becomes less able to follow the applied electric eld reversal, which is the typical behavior of

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Fig. 7. The variation of dielectric constant versus frequency as a function of surface treatment and temperature for various surface treated carbon ber/ phenolic resin composites containing 10 wt% carbon ber: (a) dielectric storage and (b) dielectric loss.

all dielectrics regardless of the relaxation mechanism. Also the linear decrease in 1 HH up to about 10 6 Hz is shown in Fig. 6(b). This linear behavior contains the 1 HH part, which is related to the long-range electronic DC transport and is dominant in the low frequency range. In Fig. 6(b), the 1 HH starts to deviate from the linearity at 2 × 106 Hz: 1 H spectra are almost constant in the range of 10 3 –10 4 Hz but show a little decrease above 10 4 Hz in Fig. 6(a). This small change of 1 H with frequency may be due to electronic interfacial polarization (the Maxwell–Wagner process) of the carbon bers [24]. With the results of 1 H and 1 HH , some mixed effects of long- and short-range relaxation are considered to

appear above 10 4 Hz. Eq. (2) with n 1 ts the data in the range 10 3 –10 4 Hz indicating the electronic transport in direct manner. In this region the change of 1 H is small and 1 HH shows the linearity. It is not yet clear why the change in 1 H is smaller than that of 1 HH . The behavior is also shown in the ionic system [25]. The phenolic resin is an insulator, but the carbon ber has the high electrical conductivity. The electronic transport in direct manner means that the short carbon bers are well connected themselves in the composite. USCF and GTDACF composites show a similar dielectric behavior to NAOCF composite. The dielectric constants show similar shapes of curves and the decrease with temperature for the different composites with 10 wt% of carbon ber. It is noted that as the temperature increases, both the electrical conductivity and the dielectric constant of the composite decrease. Because the coarse morphological parameter has the value n 1 for different temperatures, the temperature change does not affect the electronic transport pathway but the rate of the electronic transport. This behavior is partly due to good thermal stability of phenolic resin and partly due to the fact that the overall electronic transport is limited to only the carbon ber with high electrical conductivity. Mauritz et al. have recently introduced an ionic cluster to ionic systems and showed various dielectric behaviors [26,27]. They also reported that the DC conduction part of 1 HH spectrum at a lower frequency involved the long-range intercluster ionic hopping, while the intracluster-conned ions contributed to the interfacial polarization at the interface between hydrophilic and hydrophobic parts at a higher frequency [26]. From these mechanisms, the relaxation times derived from the relaxation peak positions at a high frequency in 1 HH spectrum are thought to represent the natural time scales during which intracluster electronic charge relaxations occur. In this study, the electronic charge hops through the carbon bers. Hence, we can assume an electronic charge relaxation within some boundary and some clusters of carbon bers (i.e. intercluster and intracluster). But it is very difcult to dene the boundary because an electronic charge has a much shorter relaxation time scale than an ionic charge. In Fig. 7(b), a small relaxation peak is shown above 2 × 106 Hz for every specimen. The relaxation peak shifts to a lower frequency in the order of NAOCF, USCF and GTDACF composites. We rationalize that the GTDACF composite has the lowest electronic mobility and the longest relaxation time. Recall again the anomalous behavior that the GTDACF composite has higher electrical conductivity than the other composites with the same carbon ber content. Generally the electrical conductance is represented as the product of the charge carrier density and the charge mobility. The GTDACF composite has the smallest charge mobility because of the longest relaxation time, but the highest

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Fig. 8. Dielectric loss spectra versus frequency plots at room temperature as a function of diamine content in the fabric type of carbon ber composite.

Fig. 9. The dielectric loss spectra change of the composite with various surface treated fabric type carbon bers.

electrical conductivity. Thus, we can conclude that the GTDACF composite has higher charge carrier density than other composites. This different behavior may be explained by a difference of electronic transport affected by carbon ber dispersion and coupling agent. There is a possibility that the unreacted or physically adsorbed coupling agent on the carbon ber in the GTDACF composite as an impurity can increase the charge carrier density. However, such charge carriers have a difculty in the movement in the composite because the carbon ber/phenolic resin composite has a very rigid morphology. Thus we can exclude the impurity effect. To identify the difference in the charge carrier density among NAOCF, USCF and GTDACF composites, the fabric type carbon ber/phenolic resin composites were prepared following the same surface treatments used in this study. And to conrm again no effect of impurity on the charge carrier density, diamine was added in the fabric type carbon ber composite. Fig. 8 shows the dielectric behavior of the composite treated with the coupling agent of GTDA as a function of diamine content. It is recognized that diamine content does not affect the relaxation time and the charge carrier density of the carbon ber composite. Fig. 9 shows the dependence of the dielectric loss on the surface treatment. The spectra almost coincide and the relaxation time does not change no matter how the fabric type carbon bers are treated. The surface treatment of fabric type carbon ber does not affect the charge carrier density as well as the relaxation time. Thus, it is concluded that the different dielectric behaviors between short carbon ber composites are caused by the dispersion of the carbon ber rather than the surface treatment methods. The GTDACF composite has the highest electrical conductivity but the lowest mobility as shown in the

dielectric behavior. Thus, the GTDACF composite must have the highest charge carrier density if the GTDACF composite have the highest electrical conductivity because the electrical conductivity is the product of charge carrier density and charge mobility. This anomalous behavior cannot be accurately explained here, but the clue may be suggested. The GTDACF treated with the coupling agent has a strong adhesion between carbon ber and phenolic resin. So the GTDACF ows with the phenolic resin and is dispersed very well during the hot pressing. 4.5. Mechanical property of the carbon ber/phenolic resin composite The exural strength of the composite regardless of the method of surface treatment increases with carbon ber content in Fig. 10. At a xed carbon ber content, the GTDACF composite has higher exural strength than the other composites, indicating that the coupling reaction improves the interfacial strength between the carbon ber and the phenolic resin. The GTDACF composite is found to have 2–2.5 times the exural strength of NAOCF composite. The USCF composite has higher strength than the NAOCF composite. Manocha reported the similar phenomenon that carbon ber composite treated with nitric acid solution had a lower strength than the untreated carbon ber composite [28]. The exural strength of the composite follows the same order as GTDACF USCF NAOCF composites in electrical conductivity. 4.6. Surface morphology of the carbon ber/phenolic resin composite Fig. 11 shows the SEM images of the fractured surface of the composites. The carbon bers are randomly dispersed in

M.H. Choi et al. / Polymer 41 (2000) 3243–3252

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Fig. 10. The mechanical properties of various surface treated carbon ber/ phenolic resin composites as a function of carbon ber content.

the matrix resin. There are some holes in the composite formed by pulling carbon bers out of the matrix resin during the mechanical test as shown in Fig. 11(a). It means that the adhesion between the carbon ber and the phenolic resin is very poor. But it is easy not to nd a hole in the GTDACF/phenolic resin composite but to nd some attached matrix resins on the surfaces of the carbon bers in Fig. 11(b). Thus, it reveals that the interfacial adhesion between carbon ber and phenolic resin is improved by treating carbon bers with the coupling agent.

Fig. 11. The scanning electron micrographs of the fractured surfaces of the carbon ber/phenolic resin composites containing 8 wt% carbon ber: (a) NAOCF composite and (b) GTDACF composite.

5. Conclusions The effect of coupling agent on electrical and mechanical properties of carbon ber/phenolic resin composites was studied. The electrical conductivity of the carbon ber/ phenolic resin composite was about 10 3 –10 1 S/cm. The composite had the morphological parameter n of unity, which means that the carbon bers connected themselves well in the matrix resin. The relaxation peak maxima at a high frequency in the 1 HH spectra shifted to a lower frequency in the order of NAOCF, USCF and GTDACF composites with the same carbon ber content. Therefore, it was recognized that the GTDACF composite had the longest relaxation time of electronic charge. From the test of the fabric type carbon ber composite, it was found that the electrical conductivity did not depend on the surface treatment methods. But the composite with short carbon bers showed increasing electrical conductivity in the order NAOCF USCF GTDACF composites. It could be concluded from these observations that the

electrical conductivity of the composite prepared from short carbon bers depended upon the dispersion of the carbon bers in the matrix. It also showed that the exural strength increased in the order NAOCF USCF GTDACF composites. It was conrmed that the interfacial strength of the composite was improved by using the coupling agent. The use of a coupling agent could improve the electrical and mechanical properties of the composite by the ow and dispersion of carbon bers in the phenolic resin and chemical bond between carbon ber and phenolic resin during compression molding.

Acknowledgements We would like to thank Kolon Chemical Co. Ltd for the phenolic resin support as well as KOSEF (Korea Science and Engineering Foundation) and CAFPoly (Center for Advanced Functional Polymers) for nancial support.

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M.H. Choi et al. / Polymer 41 (2000) 3243–3252 [14] Kozlowski C, Sherwood PMA. Carbon 1984;24:357. [15] Subramanian RV, Crasto AS. Polym Compos 1986;7:201. [16] Dujardin S, Lazzaroni R, Rigo L, Riga J, Verbist JJ. J Mater Sci 1986;21:4342. [17] Dagli G, Sung NH. Polym Compos 1989;10:109. [18] Wimolkiatisak AS, Bell JP. Polym Compos 1989;10:162. [19] Wang T, Sherwood PMA. Chem Mater 1994;6:788. [20] Wang T, Sherwood PMA. Chem Mater 1995;7:1020. [21] Ngai KL, Jonscher RK, White CT. Nature 1979;277:185. [22] Ahmad MS, Zihilif AM. Polym Compos 1992;13:53. [23] Ramadin Y, Jawad SA, Musameh SM, Ahmad M, Zihilif AM. Polym Int 1994;34:145. [24] Jawad SA, Ahmad M, Ramadin Y, Zihlif A. Polym Int 1993;32:23. [25] Kim HT, Park JK, Lee KH. J Membr Sci 1996;115:207. [26] Mauritz KA. Macromolecules 1989;22:4483. [27] Deng ZD, Mauritz KA. Macromolecules 1992;25:2369. [28] Manocha LM. J Mater Sci 1982;17:3039.

References
[1] Mark HF, Gaylord NG. Encyclopedia of polymer science and technology, 2. New York: Wiley, 1969. [2] Bigg DM. Polym Engng Sci 1979;19:1188. [3] Jana PB, Mallick AK, De SK. Polym Compos 1991;22:1. [4] Ishitani A. Carbon 1981;19(4):269. [5] Fitzer E, Weiss R. Carbon 1987;25(4):455. [6] Wesson SP, Allred RE. J Adhes Sci Technol 1980;4(4):277. [7] Donnet JB, Brendle M, Dhami TL, Bahl OP. Carbon 1986;24(6):757. [8] Sellitti C, Koenig JL, Ishida H. Carbon 1990;28:221. [9] Harvey J, Kozlowski C, Sherwood PMA. J Mater Sci 1987;22:1585. [10] Nakayama Y, Soeda F, Ishitani A. Carbon 1990;28:42. [11] Takahagi T, Ishitani A. Carbon 1984;22:43. [12] Kozlowski C, Sherwood PMA. J Chem Soc Faraday Trans 1984;80:2099. [13] Kozlowski C, Sherwood PMA. J Chem Soc Faraday Trans 1984;81:2745.


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