Influence of fibre surface oxidation–reduction followed by silsesquioxane coating treatment on interfacial mechanical properties of carbon fibre/polyarylacetylene composites
Introduction
Polyarylacetylene (PAA) is going through increasing development in the field of advanced heat resistant composites owing to its outstanding heat resistance and excellent ablative properties [1], [2], [3], [4], [5], [6], [7]. The advantages of PAA resin over the state-of-the-art heat resistant resin, for example phenolic resin, have been reviewed by Huang [8]. The main potential applications of PAA resin are used in conventional resin matrix composites with ultralow moisture outgassing characteristics and improved dimensional stability suitable for spacecraft structures, as an ablative insulator for solid rocket motors, and as a precursor for carbon–carbon composites. Carbon fibre reinforced PAA composites (CF/PAA) undoubtedly play a very important role in all these fields [5], [7], [8]. Unfortunately, the mechanical properties of the CF/PAA material are not yet sufficiently satisfactory to replace the heat resistant composites used widely currently such as carbon or graphite reinforced phenolic resin. The mechanical properties of carbon fibre reinforced resin matrix composites depend on the properties of CF and matrix, especially on the effectiveness of the interfacial adhesion between CF and matrix [9], [10], [11], [12], [13]. PAA has high content of benzene ring and hence a highly cross-linked network structure, which render the material brittle [3], [8]. Moreover, the chemical inert characteristics of the CF surface lead to weak interfacial adhesion between fibres and non-polar PAA resin [8], [14].
To ensure that the material could be used safely in complicated environmental conditions and to exploit the excellent heat resistant and ablative properties more effectively, it is necessary to improve the mechanical properties of the CF/PAA composites. To achieve this purpose, two kinds of methods can be used. One kind of method is to improve the properties of PAA resin by structural modification [15] or by intermixing other resins, such as phenolic resin [14], with PAA. The other is treatment of CF surface. The treatment of carbon fibre surface has been studied for a long time and several methods such as heat treatment [16], wet chemical or electrochemical oxidation [9], [17], [18], plasma treatment [10], [12], [19], gas-phase oxidation [20], and high-energy radiation technique [13] have been demonstrated to be effective in the modification of the mechanical interfacial properties of composites based on polar resins such as epoxy. Huang has studied the influence of fibre surface ozone oxidation treatment on the interfacial mechanical properties of the CF/PAA composites [8].
As an important hybrid material, silsesquioxane (SSO) has been studied for some years. The name of SSO derives from the non-integer (one and one-half or sesqui) ratio between oxygen and silicon atoms [21]. It relates to a class of compound that contains a silicon–oxygen nanostructural skeleton with intermittent siloxane chains (general formula RSiO1.5), R can be H, alkyl, alkylene, aryl, aromatic alkylene or their derivative groups [22], which are reactive or inert functional groups. SSO can exist in several structural types such as random, ladder, cage, and semi-cage structures. It can be prepared by the hydrolytic condensation of an organic siloxane (RSi(OR′)3), where R and R′ are different organic groups [23]. SSO with a cage structure is also called polyhedral oligomeric silsesquioxane (POSS, or (RSiO1.5)n, where n = 6, 8, 10, …). POSS has been used as a new chemical feedstock technology for the preparation of organic–inorganic hybrid materials [24]. Literature concerning incorporating dispersed SSO into traditional organic polymer systems has been published and the amount of them increases year by year. New hybrid inorganic–organic polymer systems have been produced with remarkable enhancements in mechanical and physical properties, including dramatic increases in both glass transition and decomposition temperatures [25], [26], reduced flammability [27], increased moduli [28], [29], and oxidation resistance [30], [31].
In the present work, carbon fibres were oxidized with oxygen plasma and then were deoxidized with LiAlH4 followed SSO coating treatments. The effects of the treatment on the fibre surface and the mechanical interfacial properties of the CF/PAA composites were studied. The fibre surface composition and functional group were examined by using X-ray photoelectron spectroscopy (XPS). The surface morphology of the fibre was observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The interfacial mechanical properties of the CF/PAA composites were characterized by short-beam bending test method.
Section snippets
Materials
PAA resin, a brown liquid with the density of ∼1.05 g cm−3 and the viscosity of 200–3000 mPa s (corresponding to a temperature changing from 90 to 30 °C), was supplied by the Aerospace Research Institute of Material and Processing Technology (Beijing, China). PAA resin is the mixture of m-diethynylbenzene, p-diethynylbenzene, and phenylacetylene. Poly(acrylonitrile) (PAN) carbon fibres (3 × 103 single filaments per tow, tensile strength was 3.38 GPa, average diameter was 7 μm, density was 1.76 g cm−3,
ILSS of CF/PAA composites
The CF treatment process could be divided into three phases, i.e., oxygen plasma oxidation, LiAlH4 reduction, and VMS–SSO coating treatment. The time of every treatment phase could affect the CF surface treating effect which resulted in the difference of the adhesion between CF and PAA resin. The LiAlH4 reduction time was decided according to the experimental parameter of Lin [32]. The VMS–SSO coating concentrations and treatment time were decided according to Huang [33] who had optimized
Conclusions
Carbon fibres were treated with oxidation–reduction followed by VMS–SSO coating method to improve the interfacial mechanical properties of the CF/PAA composites. Polar functional groups, including carboxyl and hydroxyl, on carbon fibre surface were imported after the oxygen plasma oxidation treatment. The quantity of carboxyl on carbon fibre surface was decreased and that of hydroxyl on carbon fibre surface was increased after the LiAlH4 reduction treatment. The VMS–SSO coating was grafted onto
Acknowledgements
The authors would like to thank the National Natural Science Foundation of China (No. 50333030) and the Natural Science Foundation of Heilongjiang for Distinguished Young Scholars (No. JC04-12) for financial supports.
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