Surface Modification of Carbon Fibers for Advanced Composite

Chapter 27

Surface Modification of Carbon Fibers for Advanced Composite Materials

Hybrid Organic-Inorganic Composites Downloaded from pubs.acs.org by UNIV OF LEEDS on 11/13/18. For personal use only.

Yuechuan Wang1 and Roderic P. Quirk2 Maurice Morton Institute of Polymer Science, University of Akron, Akron, O H 44325

Carbon fibers were treated with a strong oxidizing agent, 2 % K C l O 3 in concentrated H 2 S O 4 , to introduce polar phenol and/or hydroxyl groups on the fiber surfaces. A convenient benzoate labeling and analysis method for surface hydroxyl and phenol groups was developed to evaluate this treatment. Both E S C A and the benzoate labeling analyses indicated that oxidation for only 3-10 minutes at room temperature increased the oxygen content to a maximum of 2-3 times the original amount. The concentration of phenol and/or hydroxyl groups on the surfaces of treated fibers corresponded to approximately 23 phenol and/or hydroxyl groups/ nm2. The contact angle of water (pH=7) on highly ordered pyrolytic graphite after oxidation for 30 sec. was 5 3 - 5 8 ° , compared with 8 7 ° for the original surface. There was no obvious damage to the surface morphology of the treated fibers as indicated by S E M . The tensile breaking strength of single filaments of AS-4 carbon fibers was not affected by oxidation treatments for up to 1.25 hours; a decrease of only 3 % was observed after oxidation for 10 hours. Treatment of oxidized carbon fibers with trimellitic anhydride chloride formed anhydride-labeled surfaces at a concentration of approximately 1-2 anhydride groups /nm2.

1Visiting scientist from Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, China 2Corresponding author

0097-6156/95/0585-0348$12.00/0 © 1995 American Chemical Society

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Carbon fibers, with their low density, high strength and stiffness, are useful materials in combination with matrix resins for the fabrication of high performance composite materials (1-3). It has been generally recognized that the mechanical properties of these composites are strongly dependent upon the interfacial bonding between reinforcing fibers and the resin matrix (4,5). Effective bonding promotes the efficient transfer of the applied forces among the fibers and the resin matrix (5). In the case of the composites of carbon fibers with polyimides, effective bonding also increases the thermal stabilities of the composites; strong interfacial bonding prevents oxygen from penetrating into the interface, where carbon fibers are more easily oxidized than the polyimide matrix (6). The interaction between the fibers and matrices depends on both physical and chemical interactions. The chemical interaction component relies on the chemical bonding interactions of functional groups in the matrix resin with complementary functional groups on the fiber surface. Physical interactions depend on the mechanical interlocking and wetting of the matrix on fiber surfaces. Processes which increase surface functional groups and surface roughness or eliminate surface defects on carbon fibers can improve interfacial bonding (4,5). Native carbon fibers have relatively smooth surfaces, and only limited amounts of surface functional groups, which are mainly located on the edges of the exposed graphitized layers, and at the imperfection in the graphite structure, like vacancies, dislocations, and steps in the outer planes. Such surfaces do not promote the wetting of resin matrices on the fibers. Surface treatment of carbon fibers is often necessary in order to achieve the desired properties for advanced composites. A variety of methods have been investigated for the surface treatment of carbon fibers for composite materials (2,3,7). Treatment methods include using plasmas of various gases, high temperature treatment with gases, chemical treatment with oxidizing agents or anodic treatment in basic or acid media. Anodic oxidation is generally recognized as the best method and has been used commercially (8,9). However, there are controversial reports on whether fiber surfaces are damaged by anodic treatment using an alkaline electrolyte, for example with N a O H (8,10-12). Furthermore the effects of anodic treatment are sensitive to the p H value of the medium, and sodium ions are difficult to wash out from the fiber surface with distilled water (77). Recent work using scanning tunneling microscopy found that almost any of these treatments of carbon fibers caused physical damage such as pitting and increased surface roughness (13); however, most of these treatments improve the mechanical properties of the corresponding carbon fiberreinforced composites (7,8).

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Herein we report a rapid and convenient solution oxidization method to introduce phenol and/or hydroxyl functional groups on the surfaces of carbon fibers to improve the interfacial bonding to matrix resins for use in advanced composite materials.

Experimental Oxidation of fibers. About 0.4g of carbon fibers (AS-4, Hercules 3K unsized, or T650, Amoco 12K, unsized) cut into 1 inch lengths was added to a yellow - red, 2 % solution of 1 g of KC103 (Fisher) in 50 m L of concentrated sulfuric acid at room temperature. The contents were stirred intermittently. The reaction was stopped after various times by removing the fibers and washing them with 40 m L portions of distilled water until no absorption at 358 nm was detected by UV-visible spectroscopic analysis of the wash water. The fibers were dried at 120 150 ° C under high vacuum (less than 10-4 mm Hg) for 12 hours. Oxidation of highly ordered pyrolytic graphite (HOPG). A thin sheet (10 mm χ 7 mm) of highly ordered pyrolytic graphite (HOPG, Union Carbide) was peeled from the surface of a plate with adhesive tape, which was then attached to a glass slide. The graphite sheet was oxidized by adding one drop of the 2 % solution of KCIO3/H2SO4 at room temperature. After 30-60 seconds, the plate was rinsed with distilled water for 1 min. and dried under vacuum at room temperature.

Benzoyl labeling of the oxidized fibers. The oxidized and vacuum dried fibers (0.4g) were introduced into a 100 m L dry flask containing 50 m L of T H F (distilled after drying over CaH2) under dry nitrogen pressure, followed by addition of 1 m L of triethylamine (Aldrich, distilled after drying over K O H ) and 1 m L of benzoyl chloride (Aldrich, used as received) under nitrogen. The benzoylation of the fibers was effected for 6-8 hours with occasional mild stirring. The decanted fibers were then washed twice with 40 m L of acetone and eight times with 40 m L of water until no residual absorption (A < 0.002) at 230 nm was detected by UV-visible spectroscopic analysis. The fibers were used directly for the hydrolysis reaction or dried under vacuum.

Hydrolysis of benzoyl-labeled fibers. The benzoyl-labeled fibers were mixed with 100 - 150 m L of water and 3-5 m L of concentrated HC1 in a 200 m L beaker. The hydrolysis of the fibers was effected at 65 ° C for approximately 8 hr as determined by a constant UV-visible absorption reading for the solution at 230 nm.

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Anhydride labeling and analysis of carbon fibers. Oxidized and dried fibers (about 0.4 g) were reacted with 0.1 g of trimellitic anhydride chloride (Aldrich) and 0.1 m L triethylamine in 50 m L of T H F at room temperature for about 6 hr. with intermittent shaking. The decanted fibers were then washed with acetone six times (6x 40 mL), and then washed three times with distilled water until no absorption at 240 nm was detected by UV-visible spectroscopic analysis. These anhydride-labeled fibers were hydrolyzed with a mixture of 100 mL of water and 3 m L of concentrated HC1 at 60 ° C overnight.

Determination of the absorption coefficient of 1,2,4benzenetricarboxylic acid. The triacid was synthesized in-situ by the reaction of 19.0 mg of trimellitic anhydride acid chloride with distilled water in a 100 m L volumetric flask at room temperature for 2.5 days. The solution was diluted by removing 1 m L of solution with a 1/100 ml pipette and diluting to 10 mL, followed by removal of 0.5 m L of this solution and diluting to 10 ml using 10 m L volumetric flasks. The absorption coefficient at 288 nm was determined with the first solution. The stronger absorption bands near 240 nm and 210 nm were determined with the second solution.

Measurements. UV-visible absorption spectra were recorded on an HP 8452 Diode Array Spectrophotometer with a 10 cm quartz cell for quantitative analyses. Contact angles for water at various p H on graphite were determined with a Rame-Hart 100-07-00 contact angle goniometer equipped with an environmental chamber, which was saturated by filling the wells in the sample chamber with distilled water. Scanning Electron Micrographs (SEM) were obtained on a J E O L 3 U microscope at 25 k V . Electron spectroscopy for chemical analysis (ESCA) was performed on a Perkin-Elmer E S C A 5400 electron spectrometer with a magnesium Κ α X-ray source operated at 400 W . For E S C A many fibers were coaxially mounted and analyzed simultaneously. Quantification was based on computer-calculated peak areas. The breaking strengths of individual carbon fibers were measured on a laboratory built microinstron with 0.2g force sensitivity. The fiber was attached to a strip of adhesive tape cut as shown below. The narrow adhesive strip was cut

fiber

2 cm

352


after mounting the sample on the instron. Ten samples were measured to calculate the average breaking strength of a given sample of treated fibers.

Results and Discussion. Oxidation of carbon fibers. The solution oxidation of carbon fibers by strong oxidative agents has been used for a long time (2,3,14). The goal of this research was to develop a mild, efficient and rapid oxidation method to produce polar, reactive functional groups on the surface of carbon fibers for use in promoting interfacial bonding in composites. McCarthy and coworkers (75) have reported that graphite fibrils could be oxidized using a dilute solution of potassium chlorate in concentrated sulfuric acid. They reported that hydroxyl and phenol groups accounted for most of the increased oxygen content of the oxidized fibrils, based on results of analysis using a variety of labeling methods which were specific for each different type of functional group; this conclusion is also in accord with other data in the literature regarding the nature of the functional groups on oxidized carbon fiber surfaces (8,16). Therefore, it was of interest to determine the usefulness of this oxidation method to generate phenol and hydroxyl groups on the surface of carbon fibers for use in composites. The oxidation of carbon fibers was effected at room temperature with a 2% solution of potassium chlorate for various periods of time as illustrated in Scheme 1 where only the formation of phenol-type oxygen functionality is indicated for simplicity.

Benzoate labeling and analysis.

In order to quantitatively determine the extent of oxidation of the fibers to form phenol and/or hydroxyl groups on the surface, it was necessary to develop an indirect method based on (a) labeling the functional groups by converting them to a suitable derivative, (b) removing the label by hydrolysis and (c)

Scheme 1

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353

analyzing the amount of label in the hydrolysis solution using a sensitive UV-visible spectroscopic method. For this purpose, the preparation of benzoate derivatives was utilized since this is a recommended procedure for analysis of both alcohols and phenols (77). Samples of fibers were reacted with benzoyl chloride in the presence of triethylamine to convert the alcohol and phenol groups on the surface to the corresponding benzoate derivatives as shown in equation 1. After 8 hours, the fibers were washed repeatedly with water to remove unreacted benzoyl

chloride. This washing step to remove unreacted benzoyl chloride is a critically important step in this procedure because benzoyl chloride could be physically adsorbed on the surface. Therefore, the water washing process was monitored by UV-visible spectroscopy until the absorbance at 230nm was less than 0.002. Then the benzoate-labeled fibers were treated with dilute, aqueous hydrochloric acid to hydrolyze the benzoate esters and form benzoic acid. The resulting aqueous solution was analyzed for the presence of benzoic acid by UV-visible spectroscopy (Xmax = 230 nm, ε = 1.16 χ 10 L/mol-cm) (75). From the reproducibility of these analyses, the error limits on the analytical results are estimated to be approximately ± 20%. The results of analysis of surface hydroxyl and phenol groups using this benzoate-labeling method are shown in Figure 1. These results indicate that this chlorate oxidation procedure is both rapid and mild. Within 3 minutes at room temperature, 1.5 χ 10~6 moles of phenol and/or hydroxyl groups per gram of fiber are formed, and the number of these phenol and/or hydroxyl groups does not increase significantly with increasing oxidation time up to a period of two hours. From control experiments, it was established that the untreated fibers contained approximately 0.5 χ 10"6 mol/g of hydroxyl/phenol functional groups. Thus, using this mild oxidation procedure it was possible to increase the concentration of surface hydroxyl/phenol groups by a factor of approximately 2-3 within a few minutes of contact with the chlorate oxidizing solution. Preliminary experiments indicate that this procedure utilizing a bath of this oxidant is adaptable to a fiber-spinning process. The surface concentration of phenol and hydroxyl groups was estimated from the surface area of the fibers. The diameter and density of AS-4 fibers are 7 mm and 1.8 g/cm3, respectively; thus, the specific surface area was calculated to be 0.317 m2/g; the actual measured 4

354


2.5

Oxidation Time (min.) Figure 1. Plot of number of phenol/hydroxyl groups versus oxidation time determined using the benzoate labeling method. specific surface areas of carbon fibers are reported to be in the range of 0.33-0.56 m^/g depending on surface treatment (13). Therefore, the concentration of 1.5 χ 10~6 mol of phenol and hydroxyl groups per gram of fiber corresponds to approximately 2.8 hydroxyl/phenol groups per nm2 of surface area. For structurally perfect graphitic carbon, it can be calculated that an area of 1 nm^ contains approximately 38 carbon atoms (79); therefore, the chlorate oxidation procedure results in the formation of approximately 1 hydroxyl/phenol group for every 13 carbon atoms.

Trimellitic anyhydride labeling and analysis. In order to promote chemical reactions between functional groups on the carbon fibers and a variety of matrix resins, it was of interest to attach other reactive functional groups to the surface of the oxidized carbon fibers. For this purpose, trimellitic anhydride chloride was chosen to react with the oxidized fibers and to introduce the phthalic anhydride-type of functional group on the surface. The reaction is shown in equation 2; it was envisioned that the acid chloride would react with surface hydroxyl and phenol groups to generate the corresponding anhydride-

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functionalized surface. The procedure for analysis of the efficiency of anhydride labeling was via an indirect labeling and hydrolysis method analogous to the benzoate labeling procedure. Thus, samples of oxidized fibers were treated with trimellitic anhydride chloride in the presence of triethylamine to convert phenol and hydroxyl groups on the surface to the corresponding trimellitic anhydride ester groups. After six hours the fibers were washed with acetone and water to remove unreacted acid chloride and its hydrolysis products. To analyze the extent of anhydride labeling of the oxidized fibers, the anhydride-labeled fibers were treated with dilute, aqueous hydrochloric acid to remove the anhydride groups. The resulting acidic aqueous solution was analyzed for the presence of 1,2,4-benzenetricarboxylic acid by UV-visible spectroscopy at 249 nm (in water, X a x = 210 nm, 249 nm and 290 nm,e = 26,900, 10,400 and 1,900 L/mol-cm, respectively; in dilute aq. HC1 (pH < 4), the absorption at 210 nm is shifted to 202 nm, ε = 35,900 L/mol-cm, and the £ 2 4 9 increased to 11,200 L/mol-cm). For a sample of carbon fibers which had been oxidized for 30 minutes, the results of anhydride labeling and analysis indicated that the concentration of reactive hydroxyl and phenol groups was approximately 1 χ 10"6 mol/g compared with a value of 1.5 χ 10~6 mol/g based on benzoyl labeling. These results are in reasonable agreement and confirm the expected reactivity of the surface hydroxyl and phenol groups with acid chlorides and the ability to produce reactive anhydride groups on the surface for subsequent reactions with a variety of other functional groups in matrix resins. m

ESC A analysis of oxidized carbon fibers. The effect of chlorate oxidation on carbon fibers was also investigated by E S C A and the results are shown in Table 1. After oxidation with the chlorate solution for only 10 minutes, the oxygen content increased by a factor of approximately two compared to the untreated fibers. Furthermore, in accord with the titration data (see Figure 1), the oxygen content does not further increase significantly upon prolonged oxidative treatment for up to 2 hours. The atomic compositions which correspond to the E S C A data are Cl00O8.5 for the untreated fibers and C l 0 0 O l 8 for the fibers

356


Table 1. ESCA Analysis of Oxidized Fibers Ox.time

(min.) 0

C

0

atom % Na

Ν

86.46

9.75

1.39

2.4

10

77.4

19

0.48

2.23

120

76.6

18.45

0.21

2.34

which were chlorate oxidized for 10 minutes, i.e. the increase in surface oxygen content corresponds to a factor of two. These results can be compared to the hydroxyl and phenol content as determined by the chemical labeling methods. Thus, it was determined that the amount of surface hydroxyl and phenol groups increases by a factor of approximately 2-3 as determined by benzoate labeling and by a factor of 1-2 as determined by trimellitic anhydride labeling. Considering the difficulty in measuring the concentrations of surface functional groups, all of these results are in reasonably good agreement.

SEM analysis of oxidized carbon fibers.

In addition to the

requirement of rapid, efficient introduction of surface functional groups on carbon fibers for composites, a useful chemical oxidation procedure must not seriously damage the fibers. In order to evaluate the effect of chlorate oxidation on the physical nature of the carbon fibers, the surface morphologies of treated fibers were compared with untreated fibers using scanning electron microscopy (SEM). The results are shown in Figure 2, which shows micrographs (x 10,000) for T-650 carbon fibers before (A) and after (B) chlorate oxidation (30 minutes). No evidence for surface damage was observed. However, the oxidized fiber surfaces appeared to be smoother than the untreated fibers; the striations along the length of the untreated fibers are much less prominent in the micrograph of the treated fibers. The micrographs for untreated AS-4 fibers exhibited smooth, non-structured surfaces and they remained smooth after 10 hours of oxidation at a magnification of x20,000.

Single filament breaking strength of oxidized carbon fibers. The most important criterion to evaluate the effectiveness of an oxidation procedure is that the process does not significantly reduce the tensile strength of the fibers. Single filament breaking strengths of oxidized fibers were measured and compared with untreated fibers and

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Figure 2. Scanning electron micrograph (x 10,000) for T 650 carbon fibers before (A) and after (B) oxidation with 2% KC103 in H2SO4 for 30 min.

357

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HYBRID ORGANIC-INORGANIC

COMPOSITES

Table 2. Breaking Strength of Oxidized Fibers Breaking Strength (GPa)

Ox. Time 0

3.8

1.25 h

3.8

10h

3.7

the average results for at least ten samples are shown in Table 2. After a prolonged oxidation for 1.25 hours, there was no observable change in the single filament breaking strengths. Even after an oxidation time of 10 hours, only a decrease of 3% in tensile breaking strength was observable. These tensile breaking strengths are in good agreement with the value of 3.6 GPa for AS-4 single filaments which was reported by Drzal (5).

Oxidation of highly ordered pyrolytic graphite (20). The oxidation of the surface of highly ordered pyrolytic graphite (HOPG) proceeded very rapidly; when a few drops of the KCIO3/H2SO4 solution were dropped onto the surface of H O P G , the red color discharged immediately. If another drop of oxidant solution was added after the first one was washed away, the interface at the liquid and graphite became colorful, rainbow reflective; the original mirror-flat graphite surface with a metallic luster changed into a metal-gray color. The contact angle of water (pH 7) on the original H O P G surface was 8 7 ± 1 ° ; the contact angle of water on the surface of the oxidized H O P G sheet was 5 3 - 5 7 ° . S E M analysis of the untreated and oxidized H O P G surfaces are shown in Figure 3. In contrast to the smooth original surfaces, after oxidation the surfaces exhibited domain-like structures surrounded by oxidized edges.

Chemical Safety of KCIO3/H2SO4 The reaction of KCIO3 and H2SO4 forms perchloric acid and the active oxidizing agent, chlorine dioxide, as shown in equation 3(27).

3 KCIO3 + 3 H S 0 2

4

•

HC10 + 3 KHS0 + H 0 + 2 C10 4

4

2

2

(3)

The chemical reactivity of perchloric acid depends upon the temperature and concentration. At room temperature and at low concentrations, its oxidation ability is very limited (22). On the other

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Figure 3. Scanning electron micrograph of highly ordered pyrolytic graphite before (x 100) (A) and after (x 160) (B) oxidation with 2% KCIO3 in H2SO4.

359

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hand, chlorine dioxide is much more reactive. In fact, when its concentration is over 10%, or when chlorine dioxide comes in contact with easy oxidized organic chemicals and reducing agents, it is explosive (27,22). The chosen concentration was about 2 g/L in this work, which is well below the amount specified by the safety requirement. We have processed over 80 meters of T650 carbon fibers safely using this method. However, it is necessary to only use fibers which are free of organic chemicals, and they should be unsized. Chlorine dioxide is not stable; it can be decomposed by heat and sunlight. The KCIO3/H2SO4 solution has an orange-red color, and this color could be used as a buildin indicator: when the solution stands, especially under sunlight, for several hours, or when it reacts with carbon fibers, the color will fade. The patented method using the same chemicals was carried at high temperature for a long time, possibly because the strong oxidizing agent, C 1 0 2 , was destroyed at the high temperature used (14). Under the conditions described in this patent, therefore, it is probable that only the HCIO4 was available to oxidize the fibers.

Conclusions The oxidation of carbon fibers using a 2% solution of potassium chlorate in concentrated sulfuric acid reacts rapidly to double the oxygen content on the surface. Analysis using labeling with both benzoyl chloride and trimellitic anhydride chloride is consistent with the formation of primarily hydroxyl and phenol groups on the surface. Both S E M analysis and single fiber tensile breaking strength measurements show that this oxidation procedure does not damage the fibers.

Acknowledgment The financial support from the N S F Center for Molecular and Microstructure of Composites ( C M M C ) at Case Western Reserve University and the University of Akron is greatly appreciated. The authors are grateful to Professor Galiatsatos and his research group for the help in measuring the single filament breaking strengths of fibers. The authors are also grateful to Professor Gary M . Michal in the Department of Metallurgy and Materials Science at Case Western Reserve University for his help in obtaining the E S C A data.

Literature Cited 1. "Carbon Fibers. Properties and Application", J. Physics, D, Applied Physics, Carbon fiber conference issue, 1987, 20(3), pp 245-322.

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2. Riggs, J. P. in Encyclopedia of Polymer Science & Engineering; Kroschwitz, J. , Ed.; John Wiley & Sons: New York, 1985; V o l . 2; pp 640. 3. Ehrburger, P. ; Donnet, J.B. in Handbook of Composites,V1, Strong Fibers; Watt , W.; Perov, Β . V . , Ed.; Elsevier Science Publishers B . V : Netherlands, 1985; pp 577. 4. Drzal, L . T. in Controlled Interphases in Composite Materials; Ishida, H . , Ed.; Elsevier: New York, 1990; pp 309-320. 5. Drzal, L . T. Adv. Polym. Sci., 1986, 75, pp. 1. 6. Scola, D. Α . ; Vontell, J. H . Polym. Eng. Sci., 1991, 31, pp 6. 7. Jang, Β . Z . Composites Sci. Technol., 1992, 44, pp 333-349. 8. Fitzer, E . ; Rensch, H.-P. in Controlled Interphases in Composite Materials; Ishda, H . , Ed.; Elsevier, New York, 1990; pp 241. 9. Harvery, J.; Kozlowski, C ; Sherwood, P. M . A . J. Mater. Sci., 1987, 22, pp. 1. 10. Nakahara, M.; Shimizu, K . J. Mater. Sci., 1992, 27, pp 1207. 11. Kozlowski, C . and Sherwood, P. M . A . J. Chem. Soc., Faraday Trans. 1, 1985, 81, pp 2745. 12. Chun, Byong-Wa; Davis, C . R.; Quan He; Gustafson, R R. Carbon, 1992, 30, pp 177. 13. Hoffman, W. P.; Hurley, W. C.; Owens, T.W.; Phan, H.T. J. Mat. Sci., 1991, 26, pp 4545. 14. Goan, J. C.; Joo, L . A . U. S. Pat. 3,746,560 (July 17,1973). 15. Bening, R. C . ; Ivatury, S. R.; McCarthy, T. J. Polymer Prepr.

(Am.Chem.Soc., Div.Polym.Chem.) 1990, 31(1), pp 420. 16. Ehrburger, P.; Herqte, J. J.; Donnet, J. B. in Petroleum Derived Carbons; Deviney, M . L . ; O'Grady, T., Eds.; ACS

-Symposium Series, 1976, 21, pp. 324. 17. Knapp, D. R. Handbook of Analytical Derivatization Reactions; Wiley: New York, 1979, pp 55. 18. Jaffe, H . H . ; Orchin, M . Theory and Applications of Ultraviolet Spectroscopy; Wiley: New York, 1962. 19. Hugh O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerenes, Noyes Publications, Park Ridge, 1993, Chapt. 3, p 43. 20. Chang, H . P.;Bard, A . J. J.Am.Chem.Soc, 1991, 113, 5588. 21. Durrant, P-J. and Durrant, B.; Introduction to Advanced Inorganic Chemistry; 2nd ed., John Wiley & Sons: New York, 1970; pp 937. 22. Downs, A . J.; Adams, C . J. in Comprehensive Inorganic Chemistry; Pergamon Press, Great Brittain; 1973; Vol. 2, pp 1366, 1442. R E C E I V E D October 26, 1994

Surface Modification of Carbon Fibers for Advanced Composite

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