Grafting Maleic Anhydride onto Carbon Fiber via a Solid-phase Grafting Method①

LIN Guo-Liang ZHENG Yu-Ying

(College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350116, China)

1 INTRODUCTION

Carbon fiber-reinforced polymer (CFRP) is a kind of advanced composite materials, which combines the outstanding properties of carbon fiber (CF) with the matrix. The strong, light and chemically stable CFRPs have applications in diverse fields, including aviation, spaceflight,automobile, medical treatment and chemical industry[1]. When the CFRPs are used in certain devices, the performance of the device is not only dependent on the properties of the reinforced fiber and the matrix, but also on the strength of the interface bonding. In a CFRP system, the nature of the interface bonding plays an important role in improving properties of the resulting materials.Therefore, chemical modification of CF has attracted extensive studies to improve the proper-ties of CFRPs[2-4]. Many approaches have been employed to achieve appropriate chemical modifications of CFs, including acid treatment[5,6], air oxidation[7,8], rare earth treatment[9,10], plasma surface processing[11,12]and graft modification[13-14].Among these methods, graft modification using polymer and small molecules is a widely used strategy. After grafting modifications, the CFs usually have an excellent compatibility, which can improve the interfacial adhesion between the CFs and the matrix.

In this work, we developed a robust solid-phase functionalization technique to link chemical functional groups onto the surfaces of CFs using the graft modification concept. Briefly, pre-oxidation treatment of CF was first carried out to destroy the weak interface layer on the surface of the fiber using mixed acids. Then, the maleic anhydride(MAH) was harnessed as a grafting monomer to modify the treated CF by a solid-phase grafting method to enhance the bonding force between the matrix and the CF surface, allowing for linkage of a high density of chemical functional groups onto the CF surfaces. Compared with the solution and melting methods, the solid phase method can be carried out at much lower reaction temperature,with simpler equipments and a more convenient post-treatment[15]. Moreover, the structure and surface feature of the resultant maleic anhydride grafted carbon fibers (CF-g-MAH) were investigated by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) thermogravimetric (TG) and contact angle analyzer. The effect of the solid-phase grafting reaction conditions on the structure of CFs was investigated systematically.

2 EXPERIMENTAL

2. 1 Materials

Carbon fibers (CF) were supplied by Nanjing Weida Composite Material Co., Ltd., China.Maleic anhydride (MAH) was obtained from Xilong Chemical Co., Ltd., China. Benzoyl peroxide (BPO) was purchased from Sinopharm Medicine Holding Co., Ltd, and acetone from the Changlian Chemical Reagent Industries, Ltd.,Chengdu, China.

2. 2 Pre-oxidation treatment of carbon fibers using mixed acids

The CFs were immersed in acetone for an appropriate time, followed by repeated washing with distilled water, and dried in an oven. The treated CFs were pre-oxidized with a mixed solution of nitric acid and sulfuric acid at suitable temperature. Then the pre-oxidized CFs were washed with distilled water and dried in an oven at 66 ℃ for 2 h prior to the grafting reactions.

2. 3 Preparation of CF-g-MAH

The pre-oxidized CFs were added in an acetone,MAH and initiator solution. The mixture solution was agitated by ultrasonic waves to distribute the MAH and initiator evenly. The solid-phase grafting reaction was carried out in a water bath for certain time. Pumping filtration was used to purify the reaction mixture using acetone to remove the non-reacted reactants and the by-products such as poly (maleic acid ester).

2. 4 Determination of the grafting efficiency

16 mL of a standard NaOH solution (about 0.01 mol/L) was added into a conical flask containing a suitable amount of the synthesized CF-g-MAH.The suspension was stirred for 24 h, followed by filtration using a microporous membrane. The solution was titrated using a standard HCl solution(about 0.001 mol/L), and a phenolphthalein solution was used as an indicator. The grafting degree of MAH was calculated by the following equation:

Where m and m′ are the weights (g) of CF-g-MAH and CF, respectively; C1and C2are the molar concentration (mol/L) of the standard NaOH and HCl solutions, respectively; V1and V1′ are the consumed volumes (L) of NaOH solution of CF-g-MAH and CF samples, respectively; V2and V2′ are the consumed volumes (L) of HCl solution of CF-g-MAH and CF samples, respectively.

2. 5 Characterization

The infrared spectra (IR) of CFs before and after grafting of MAH were obtained on a Perkin-Elmer PE-983G Fourier transform infrared spectrometer(FTIR). The surface morphology of the samples was characterized using a JEOL JSM-7500F scanning electron microscope (SEM). X-ray photoelectron spectroscopy (XPS) analyses were carried out on a Thermo Scientific ESCALAB 250 spectrometer to determine the surface composition and the chemical states of CF in different con-ditions.Thermogravimetric (TG) analyses were performed on a NETZSCH STA449C Instruments (Germany)in the range of 10～750 ℃ under a heating rate of 20 ℃/min. The contact angle of CF was measured using a SL200A/B/D type Contact Angle Analyzer(Shanghai SOLON Information Technology Co.,Ltd, China).

3 RESULTS AND DISCUSSION

3. 1 FTIR

As shown in Fig. 1, the peaks located at 3660 and 1406 cm-1originating from the stretching vibration of -OH as well as 2984 and 2901 cm-1arising from the asymmetric and symmetric stretching vibrations of C–H bond of the -CH2groups appeared in both pre-oxidized CFs and CF-g-MAH.Three peaks situated at 1852, 1775, and 1704 cm-1appearing in Fig. 1b can be attributed to the characteristic peaks of anhydride. These results demonstrate that the carboxyl groups have been successfully bound onto the CF surfaces. The presence of carboxyl groups probably is caused by the hydrolysis of the grafting MAH.

Fig. 1. FTIR spectra of (a) the pre-oxidized CFs, and (b) CF-g-MAH

3. 2 SEM

The SEM images of CFs, the pre-oxidized CFs and the CF-g-MAH are shown in Fig. 2. Both the CFs and pre-oxidized CFs have a smooth surface with some shallow grooves. However, some small cuticle-like substances are found on the surface of the pre-oxidized CFs, suggesting its surface structure is destroyed by the mixed acids for connecting carboxyl and hydroxyl groups. The cuticle-like substances become larger in CF-g-MAH due to the linkage of MAH onto the CF surfaces.

Fig. 2. SEM images of (a) CFs,(b) pre-oxidized CFs and (c) CF-g-MAH

3. 3 XPS

Fig. 3. XPS spectra of C1s for (a) CFs,(b) pre-oxidized CFs, and (c) CF-g-MAH

The XPS spectra of C1s in CFs, the pre-oxidized CFs and the CF-g-MAH are shown in Fig. 3. For the CFs without any treatment, the C1s spectrum displays a peak ranged from 284.31 to 284.59 eV,indicating the primary composition of the graphite carbon. However, for the pre-oxidized CFs, the C1s spectrum turns to be a little cross in the main cap, which was attributed to the increasing amount of the oxygenic groups in the pre-oxidized product of CFs. The binding energy of C–C bond in the graphite carbon falls in the range of 284.31～284.59 eV. The binding energy of carbon present as phenolic hydroxyl and/or ether groups (C–OH)is located between 285.48 and 285.76 eV. The binding energy of carbon in carbonyl groups (C=O)ranges from 286.27 to 286.99 eV, and that in carboxyl and/or ester functions (-COOH or -COOR) is from 287.96 to 288.57 eV. As shown in Fig. 3, the peak of the C1s spectrum of the oxidized CFs intended to move to the high binding energy direction. According to the main chemical composition of the sample, the peak shift was attributed to the increased amount of C–O groups.

In order to study the percentage of each group in the samples, the C1s spectrum was deconvoluted into four peaks as demonstrated in Table 1.The portion of C–O groups increased after being treated with mixed acids, and then increased prominently after grafting MAH. All of these results prove that the MAH has grafted onto the CF surfaces by the solid-phase grafting.

Table 1. Percentage of Oxygen Functional Groups on the Surface of CFs (C1s)

Table 2. Contact Angles of CFs

3. 4 TGA

Fig. 4 shows the TGA curves for the CFs, preoxidized CFs and CF-g-MAH, and their weight loss is 0.32%, 1.23% and 9.48%, respectively after thermal treatment at 550 ℃. The weight loss is suggested to be caused by the removal of some functional groups or organic compounds on the CF surfaces. For CFs without any treatment, there are few organic compounds and functional groups on the surfaces, resulting in less weight loss. However,for the pre-oxidized CFs, the treatment by mixed acids increases the amount of carboxyl and hydroxyl groups on the CFs surface, leading to an increment in the weight loss. For CF-g-MAH, the poly (maleic anhydride) grafted on the CF surfaces decomposes at 330～550 ℃, leading to the highest weight loss after thermal treatment.

Fig. 4. TGA caves of (a) CFs,(b) Pre-oxidized CFs, and (c) CF-g-MAH

3. 5 Wettability of carbon fiber

Table 2 shows the contact angles for CFs, preoxidized CFs and CF-g-MAH. The contact angle of CFs to water decreases remarkably after the mixed acid treatment and the MAH grafting due to the increment of oxygen functional groups.Improved wetting behavior of CF-g-MAH is caused by its high surface energy, thus improving the interfacial adhesion between CF-g-MAH and the matrix.

Fig. 5. Effect of the reaction time on the grafting efficiency of CF-g-MAH

Fig. 6. Effect of the reaction temperature on the grafting efficiency of CF-g-MAH

3. 6 Effect of preparation conditions on the grafting efficiency of CF-g-MAH

3. 6. 1 Reaction time

The CF-g-MAHs were obtained at a CF/MAH/initiator weight ratio of 1/3/0.048 at 87 ℃for different reaction time. The effect of reaction time on the grafting efficiency is shown in Fig. 5.The grafting efficiency of CF-g-MAH is increased from 7.7% to 10.4% after increasing the reaction time from 3 to 4 h due to the presence of a higher monomer concentration and a larger number of free radicals in the initial reaction process. The grafting yield is decreased from 10.4% to 9.5%,8.5% and 8.0% after further increasing the reaction time to 5, 6 and 7 h, respectively. The reduced grafting efficiency indicates that the grafting reaction stops when the initiator is exhausted. In this case, the by-product of poly (maleic anhydride)forms. Therefore, the grafting reaction time is set to be 4 h in the following experiments.

3. 6. 2 Reaction temperature

The effect of the reaction temperature on the grafting efficiency of CF-g-MAH is summarized in Fig. 6. The grafting efficiency of CF-g-MAH increases when the reaction temperature increases from 80 to 90 ℃, and then decreases after further increasing the reaction temperature. At lower temperature, the activity of the BPO initiator and the concentration of free radicals increase with the temperature, benefiting the grafting. However, at higher temperature, the movement of the polymer chain segment becomes violent and the opportunity of the collision chance between the molecules increases. The steric hindrance increases and the rate of self-polymerization becomes faster than the rate of grafting. As a result, the by-product of poly (maleic anhydride) forms. Thus, the optimal reaction temperature is about 90 ℃.

3. 6. 3 Dosage of initiator

Fig. 7 shows the effect of the initiator BPO dosage on the grafting efficiency of CF-g-MAH. In Fig. 7, the grafting efficiency increases when increasing the BPO dosage to 2 wt%, and then it decreases if further increasing the BPO amount.There is a sufficient amount of initiator for the grafting sites on the surface of the pre-oxidized CFs to react with MAH at the initial reaction stage.However, when the dosage of the initiator reaches a constant value, the number of the initiated free radicals increases dramatically, leading to the free radical coupling and inactivating. Moreover, the homopolymerization rate of MAH increases, with the grafting efficiency of CF-g-MAH decreases.Hence, the optimal BPO dosage for CF-g-MAH is about 2 wt%.

Fig. 7. Effect of the initiator dosage on the grafting efficiency of CF-g-MAH

Fig. 8. Effect of the MAH amount on the grafting efficiency of CF-g-MAH

3. 6. 4 Dosage of the monomer Fig. 8 shows the effect of MAH dosage amount on the grafting efficiency of CF-g-MAH. It is found that the grafting efficiency increases to the maximum level and then decreases with raising the dosage amount of MAH. There are numerous active points on the CF surfaces to react with MAH. When the MAH/CF molar ratio reaches 3/1,the grafting efficiency gets to the maximum value.However, the MAH tends to homopolymerize when further increases the dosage of MAH in the reaction system. The formation of MAH homopoymer prohibits MAH from approaching the active points on the surfaces of CFs. Therefore, the optimal molar ratio of MAH to CF is 3/1.

4 CONCLUSION

The solid phase grafting method has been developed to graft maleic anhydride onto the surfaces of CFs, which can significantly modify the CF’s surface to enhance the adhesion force between CFs and the polymer matrix. After the surface modification of CFs, the composite materials composed of CFs and the polymers show extraordinary mechanical properties. Several methods have been used to characterize the structure and surface chemistry of the functionalized CFs, and all results indicate that the MAH has been grafted onto CFs by the solid-phase grafting method. The FT-IR results demonstrate that carboxyl groups have been successfully bound onto CF, which is also evidenced by the increased proportion of C–O groups by XPS. Compared with CFs and pre-oxidized CFs,the CF-g-MAH decomposes at 330～550 ℃,which has the highest weight loss in the TGA curve. Cuticle-like substances become larger in CF-g-MAH, suggesting that the surface structure of CFs is destroyed. The contact angle test reveals improved wetting of CF-g-MAH induced by the high surface energy, which significantly improves the interfacial adhesions between CFs and the polymer matrix. The optimal reaction conditions have been determined as the MAH/CF molar ratio being 3/1, the dosage of initiator 2 wt%, the reaction temperature 90 ℃, and the reaction time 4 h. This investigation develops an effective way to improve the interfacial strength between CFs and the polymer matrix by the solid phase grafting method, which can supply super-strong composite materials to satisfy the special industrial applications.

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