Surface Treatment of Carbon Fibers

Surface treatments of carbon fibers can in general be classified into oxidative and non-oxidative treatments. Oxidative treatments are further divided into dry oxidation in the presence of gases, plasma etching and wet oxidation the last of which is carried out chemically or electrolytically. Deposition of more active forms of carbon, such as the highly effective whiskerization, plasma polymerization and grafting of polymers are among the non-oxidative treatments of carbon fiber surfaces. 

Wet oxidation Several types of liquid-phase oxidizing agents, such as nitric acid, acidic potassium permanganate, acidic potassium dichromate, dichromate permanganate, hydrogen peroxide, ammonium bicarbonate and potassium persulfate, have 

Electrolytic or anodic oxidation is fast, uniform and best suited to mass production. This process is most widely used for treatment of commercial carbon fibers. The oxidation mechanism of most carbon fibers is characterized by simultaneous formation of CO2 and degradation products that are dissolved in the electrolyte of alkaline solution or adhere onto the carbon fiber surface in nitric acid. Only minor changes in the surface topography and the surface area of the fiber are obtained with a small weight loss, say, normally less than 2%. 

In the light of the foregoing experimental evidence, the following can be summarized regarding the effects of carbon fiber surface treatments, depending on the methods and media employed  

More recently Drzal and coworkers (Madhukar and Drzal, 1991a, b, 1992a, b Drzal and Madhukar, 1993) have spent significant research efforts to establish the 

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Effect of size and surface treatment of carbon fibers on mechanical properties of magnesium matrix composites ... 

Ehrburger, P., Donnet, J.B. (1985). Surface treatment of carbon fiber for resin matrices. In Strong Fibers, Handbook of Composites, Vol. 1 (W. Watt, and B.V. Perov, eds.). Elsevier Sci., Amsterdam, pp. 577-603. 

Norita, T., Malsui, J., Matsuda, H.S. (1986). Effects of surface treatment of carbon fiber on mechanical properties of CFRP. In Proc. ICCI-I, Compo.site Interfaces (H. Ishida and J.L. Koenig, eds.), Elsevier Sci. Pub. New York, pp. 123-132. 

Surface treatments of carbon fibers include oxidative treatments. This results in cleaning of the surface impurities and debris, formation of chemical surface groups (largely acidic) at the carbon fiber surface and a rougher surface morphology, all of which result in enhanced mechanical interlocking with the polymeric matrix. 

The comparison of (a) and (b) shows the effect of adding untreated milled carbon fibers, which is no improvement. The comparison of (a) and (c) shows the effect of the surface treatment of X-ray-opaque powders on fatigue cycles, which is a significant (nearly threefold) increase. The comparison of (c) and (d) shows the effect of adding untreated carbon fibers on bone cement with treated Zr02, which is an appreciable negative effect. The comparison of (d) and (e) shows the effect of the surface treatment of carbon fibers, which is a roughly twofold increase. 

Polymer composites. The composite research at the Institute is led by Prof. Wu Renjie, Deputy director of the Institute. Chen, et al. (14), studied the effect of oxidation of carbon fiber on the wettability by the binder resin. With the aid of ESCA, they showed that the Q/C ratio on the fiber surface increased with the oxidation time. The interlaminar shear strength also increased correspondingly. Cai Weizhen and her colleagues showed me their exceptional setup for a carbon-fiber composite study. They built their own torsion pendulum for the study of dynamic mechanical properties of the composite and a contact angle goniometer for the study of the composite interface. It was apparent that surface treatment of carbon fiber was their major concern. 

Wan Y.Z., Wang Y.L., Li Q.Y., Dong X.H., Influence of surface treatment of carbon fibers on interfacial adhesion strength and mechanical properties of PLA-based composites, J. Appl. Polym. Sci., 80, 2001, 367-376. 

Donnet JB, Dhami TL, Dong S, Brendle M, Journal of Physics D Applied Physics, 20, 269, 1987. Edie ED, Cano RJ, Ross RA, 20th Biennial Conference on Carbon, Santa Barbara, Jun 1991. Peng JCM, Rebouillat S, Surface Treatment of Carbon Fibers, Donnet JB, Wang TK, Peng JC, Rebouillat S eds.. Carbon Fibers, Marcel Dekker, New York, 187, 1998. 

Figure 10.24 A typical unit cell for electrolytic surface treatment of carbon fiber.

Hall IW, Manrique F, Surface-treatment of carbon-fibers for aluminum-alloy matrix composites, Scripta Metallurgica et Materialia, 33(12), 2037-2043, 1995. 

Lukjanova and Lovzova [1] have described how carbon fibers occur in nature. Certainly, more research on surface treatment should help to apportion a specific treatment for a given matrix. The introduction of better sizes would be expected, probably applied by electrodeposition, or electropolymerization, to provide better control during apphcation. The surface treatment of carbon fiber may be tailored for a given resin matrix and similarly, the size can be chosen to perform with a given resin matrix or production process. Hydrosize Technologies Inc. has developed a special size for vinyl esters [2]. 

Adhesion between fiber and matrix can be altered by surface treating the fiber [16,17], Surface treatments of carbon fibers have been developed for epoxy systems, but the optimum surface treatment for epoxy systems may be inadequate for newer resin systems. It may also be possible that by tailoring the interface between fiber and matrix, the mechanical properties of the composite can be controlled. To tailor the interface, it is necessary to understand the nature of carbon fiber surfaces and their reactions when surface treated. 

SURFACE TREATMENT OF CARBON FIBERS TO OPTIMIZE FIBRE-MATRIX ADHESION... 

Fu Yuan-Fei, Xu Kan, Sun Zhao-Yao, Zhang Fu-Qiang, Chen De-Min, and Li Jing. The effect of silane surface treatment of carbon fiber on the tribological properties of bis-maleimide (BMI) composite. Polym. Plast. Technol. Eng. 51 no. 10 (2012) 1068-1072. 

The surface properties of carbon fibers are intimately related to the internal structure of the fiber itself, which needs to be understood if the surface properties are to be modified for specific end applications. Carbon fibers have been made from a number of different precursors, including polyacrylonitrile (PAN), rayon (cellulose) and mesophase pitch. The majority of commercial carbon fibers currently produced are based on PAN, while those based on rayon and pitch are produced in very limited quantities for special applications. Therefore, the discussion of fiber surface treatments in this section is mostly related to PAN-based carbon fibers, unless otherwise specified. 

Delmonte, J. (1981). Surface treatments of carbon/graphite fibers and their effect on composites. In Technology of Carbon and Graphite Fiber Composites. Van Nostrand Reinhold, New York, pp. 171-197. 

Figure 5.12. Specific surface area of carbon fibers vs. treatment time in oxygen plasma. [Adapted, by permission, from Byung Suk Jin, Kwang Hee Lee, Chul Rim Choe, Polym. Int., 34, No.2, 1994,181-5.]...

The specific surface area depends on filler treatment. The treatment of carbon based materials is one of such examples (Figure 5.12). Surface oxidation increases the specific surface area of carbon fibers. 

These differences in the chemical structure of the surface depend not only on the process of manufacture but also on additional treatments or processing conditions. In oxidized carbon fibers, the concentration of carbonyl and, more particularly carboxyl groups, is substantially increased at the expense of hydroxyl groups. In the treatment of carbon fibers, several methods of oxidation are used. Liquid phase oxidation is carried out by the electrochemical and chemical methods whereas gaseous oxidation is carried out in air, oxygen or in the presence of catalysts. Plasma treatment is also used for the surface oxidation of formed fibers. Different methods of oxidation produce different surface characteristics. For example, interlaminar strength is improved by a factor of 10 by electrolytic oxidation over crude oxidation in air. 

Fibers of high moduli of elasticity are used to make thermoplasts and elastomers more rigid. Since the moduli of elasticity of typical textile fibers are only about 4-40 GPa, glass, steel, and carbon fibers, whose values can reach 350 GPa (Table 38-4), have been used for this purpose in the past. The poor adhesion of these fibers to thermoplasts and elastomers can be improved to various extents by surface treatment of the fibers with binding aids. 

Lin SS, Yip PW, Surface adhesion of carbon fibers after chemical treatments, 79 Biennial Conference on Carbon, Penn State University, Session 2A, 244-245, Jun 25-30 1989. 

Figure 9.1 ILSS of Type I carbon fiber composite as a function of fiber weight loss. Source Reprinted with permission from Clark D, Wadsworth NJ, Watt W, The surface treatment of carbon fibres for increasing the interlaminar shear strength of cfrp, Carbon Fibers—Their Place in Modem Technology, The Plastics Institute, London, 44-51, 1974. Copyright 1974, Maney Publishing (who administers the copyright on behalf of lOM Communications Ltd, a wholly owned subsidiary of the Institute of Materials, Minerals Mining).

No consensus has been reached on the roles of physical absorption and chemical bonding when investigating the surface chemistry of carbon fibers and made more difficult by the buried interface. Jones [47] claims that the electrolytic surface treatment process produces a surface on which the known concentration of chemical functionalities cannot be accommodated on the surface of a smooth cylinder. Absorption studies [48] support the fact that erosion could occur and active species can be deposited in the vicinity of intercrystallite voids. Types A and HT fibers have more basal planes that emerge directly to the surface than is the case with HM fiber and hence are more readily surface treated. Hence, it was suggested [49,50] that HM fiber would require an active epoxy group of smaller dimensions that could be accommodated within the micropore. 

Clark D, Wadsworth NJ, Watt W, The surface treatment of carbon fibres for increasing the interlaminar shear strength of cfrp. Carbon Fibers—Their Place in Modern Technology, The Plastics Institute, London, 44—51, 1974. 

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