X-ray photoelectron spectroscopic studies of carbon fibre surfaces
The type and extent of surface oxidation of carbon fibres has been determined after electrochemically treating fibres in a variety of electrolyte solutions. The chemical and physical characteristics of these fibres have been evaluated using XPS, SEM, FTIR and UV spectroscopy. The fibres were anodically treated, both in a laboratory and in a commercial type cell. Fibres that have undergone commercial treatment were then incorporated into epoxy composites, the ILSSs of which were then measured. The extent of oxidation and type of surface functionality produced as a result of electrochemical treatment is shown to depend upon several factors, ie the nature of the electrolyte, the anodic potential, reaction time, the structure of the fibre surface, the pH of the electrolyte solution, and the electrolyte concentration. Surface nitrogen functionality is not produced as a result of polarising the fibres in nitric acid. It is produced, however, with treatments in solutions containing ammonium ions. The amount of surface nitrogen depends upon the concentration of these ammonium ions in the solution. In most cases, polarisations in salt solutions produce similar changes in the fibre surfaces as treatment in the acid alone. The presence of bicarbonate ions tend to inhibit fibre surface oxidation. In acidic solutions the fibres are shown to be extensively oxidised. Although the functionality of the oxide layer produced is very similar Ge. consisting of keto- and carboxyl/ester groups) after all the acidic treatments studied, the surface topography of the oxide layer produced is very different. In all cases this oxide layer is loosely bound to the bulk fibre. In general, as reaction time increases, oxidation of the fibre surfaces also increases. Surface oxidation also increases with potential. However at high potentials (-3V) and long reaction times (>15mins) the detected functionality of type 11 fibres decreases. This is thought to be due to the formation of gaseous products such as carbon dioxide. The reactivity of type I and type II fibre is shown to be different. The amount of carboxyl/ester functionality produced is far greater for type 11 fibres. It is concluded that carboxyl functionality is produced at the edge sites and keto-type functionality on the basal planes. The amount of oxidation decreases as the pH increases. In alkaline solutions carboxyl and alcohol groups are produced, (the former being in greater quantities). The physical mechanism of oxidation is also different. Instead of an overall oxide layer being produced (in acidic solutions), holes are produced in the fibre surfaces. These holes are thought to be areas of localised attack. It is also shown, using a small pilot plant, that both galvanostatic and. potentiostatic control of electrolysis are satisfactory in producing treated fibres, which when incorporated into resins form composites with a high ILSS. The ILSS of the composites produced are dependent neither upon the amount of surface oxygen present nor upon the number of carboxyl groups present.