Full Length ArticleAn investigation into the surface heterogeneity of nitric acid oxidized carbon fiber
Graphical abstract
Introduction
The carbon fiber (CF) surface has a significant impact on the final properties of CF reinforced composites. The interactions between the matrix polymer and the surface of the CF have been found to play a role in delamination, fracture propagation and poor interfacial shear strength [1], [2], all of which are major challenges that need to be overcome in the development of superior CF composites. A thorough understanding of the heterogeneity of the CF surface is crucial for continued improvement in the properties of CF composite materials.
Surface oxidation enables bonding between the fiber and sizing and or matrix in composites by introducing hydroxyl (-OH), carbonyl (CO), and carboxyl (COOH) functional groups [3], [4], [5], as well as by etching and modifying the surface topography of the CF [6], [7], [8]. While various surface treatments have been used in industry that all lead to changes in the functionality and topography of the fiber surface, very little is known about the true heterogeneity of the surface after these treatments. Whilst oxidation by nitric acid is not commonly used in industry, it has been found that wet chemical oxidation using strong solutions of mineral acids such as nitric acid is a very effective method for the uniform surface modification of CF [9]. Also, as it is a method that can be easily applied and controlled within a laboratory environment on a range of sample sizes, it has been studied previously by other researchers [3], [7], [10], [11], [12], [13], [14], [15]. Techniques, including X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Raman spectroscopy have all been used to gain an understanding of the CF structure and surface after nitric acid oxidation [3], [7], [10], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Whilst all of these techniques, as utilized, provide valuable information, none offer the combination of chemical information and the high spatial resolution required to investigate surface heterogeneity.
In one of the first studies, Brewis et al. used XPS to analyze the effect that refluxing CF in nitric acid (60% w/w) for 3 h had on the surface [3]. The increase in surface oxygen atoms observed was attributed to the formation of alcohols, ethers, esters and carboxyl groups on the fiber surface. In another study, SEM has shown that nitric acid (68%) oxidation of Intermediate Modulus (IM) CFs at room temperature altered the surface topography of the fiber by etching the surface [7]. It was also noted that the number of surface particulates was reduced after a 90 min treatment and treatment times in excess of 90 min were found to cause significant etching of the CF surface. Nohara et al. have found that treatment of IM CF in hot (103 °C) nitric acid (97% (w/w)) for ten minutes produced similar changes [21], whereas Xu et al. concluded that one hour in hot (110 °C) nitric acid (68%) did not significantly alter the appearance of the IM CF surface [23]. Clearly significant variations have been observed for similar CFs that have undergone very similar treatments.
Vautard et al. used SEM, AFM and XPS to compare the effect of treatment time in boiling concentrated nitric acid on the CF surface [15]. For treatment times over 45 min, SEM imaging revealed the disappearance of the striations inherent from the manufacturing process and nanoscale analysis by AFM revealed the creation of a grainy surface. XPS analysis detected the presence of hydroxyl, carboxyl and lactone groups not present in the untreated fibers.
Raman spectroscopy has been used to assess changes to the carbon − carbon bonding of CFs [10], [21]. A typical analysis has involved the collection of data from one or more micron scale spots from the fiber surface. This analysis approach, which does not result in uniformly spatially resolved information, will be referred to as spot probe Raman spectroscopy in this article. Such analysis has suggested that a more ordered or graphitic structure is produced after 12 h of oxidation in hot (90 °C) nitric acid (65%) [10]. An increase in the disordered carbon (D) content relative to graphitic carbon (G) content, as evident by changes in the D/G Raman band ratio, while a reduction in the amorphous carbon was also observed [10]. Others have concluded that similar treatments have little or no effect on the CF structure and the D/G ratio [21].
One of the most extensive studies of nitric acid treated CF utilizes SEM, AFM, XPS and spot probe Raman spectroscopy to characterize the fiber surface [21]. The treatment, which resulted in a 1.4 fold increase in the number of functional groups detected, was found to increase etching and roughness on the fiber surface without any significant effects on the graphitic structure.
Based on the literature reported above, there is a poor understanding of the effect of nitric acid oxidation and the evenness of treatments across the CF surface. Some of these issues could be due to the variation in starting materials used, as some work was carried out on sized CFs, which could also have been oxidized prior to the application of the size.
Confocal Raman mapping has recently been used to compare the heterogeneity of polyacrylonitrile (PAN) based CF to that of pitch based CF [24]. This technique enabled the collection of spatially related spectra at sub-micron resolution across a defined area. The information is typically obtained from the top 50 nm of the CF surface [25] and thus gives valuable insight into the structural variation of the CF surface. A detailed investigation of the heterogeneity as a function of surface treatment on a given CF type has not however been reported in the literature.
In this work we look to assess the potential of confocal Raman spectral mapping to provide a detailed assessment of the heterogeneity of the treated CF surfaces. A series of nitric acid oxidized CFs were prepared and characterized by SEM, AFM and XPS to confirm the addition of functional groups and evaluate any changes in surface topography and roughness as a function of treatment level. These results are compared to those in the literature. Raman spectral maps were then collected and various approaches to map creation were investigated. The most robust method was used to assess the chemical variation between treatment levels as well as between areas on the same fiber and between fibers of the same treatment level.
Section snippets
Materials and treatments
The IM fibers used in this study were PAN based 50 K automotive grade, Panex 35 carbon fibers, supplied by Zoltek Hungary. The fibers had not been subjected to surface oxidative treatment and were size free.
Prior to acid treatment the fibers were ultrasonicated in AR grade acetone (Ajax Finechem) for 10 min, filtered through a sintered glass filter and after a final rinse in fresh acetone, dried at 80 °C for 2 h. These fibers will be referred to as “untreated” fibers.
Acid treatments were carried
Results
Prior to undertaking extensive Raman mapping the CFs used in this study were assessed by a range of surface analysis techniques and the results were compared to those presented in the literature. As an initial assessment of the morphological effects of the acid treatments, the CFs were examined using the SEM. Typical images from an untreated and nitric acid treated fibers are shown as Fig. 1. There is some material, possibly residual debris from the manufacturing process, on the surface and in
Discussion
The general trends observed for the effect of nitric acid treatment level for the various bands are presented in the last column of Table 2. In terms of Raman spectra, the cleaning/etching effect could induce changes in the spectrum if the etching removes surface amorphous carbon and/or weakly bound graphene surface sheets, thus exposing a slightly different structure. Melantis et al. found differences in crystalline structure when comparing the surface and core of high modulus (HM) CF produced
Conclusion
A physically and chemically consistent set of nitric acid treated CFs were prepared. The results of various techniques were consistent with the removal of amorphous material from the fiber surface, along with the introduction of moieties that include CO and NOx functionalities. The largest effect was observed for the highest level of treatment.
The use of Raman mapping to explore the chemical effect of these treatments was developed. This enabled the collection of spectra from an area 47 × 3 μm on
Acknowledgements
We would like to acknowledge Chris Easton for his expertise in collecting the XPS data. ALW acknowledges the assistance of Colin Veitch with SEM training, Debbie Hamilton for AFM training and Mickey Huson for discussion on roughness measurement analysis.
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