(133e) Energetic Heterogeneity of Carbon Fibers with Different Surface Treatments and the Effects On Wettability

Carbon fiber composites are showing increased uses in marine, aerospace, automobile, and sports industries.  The quality and performance of carbon fiber composites depends strongly on the interaction of the components at their interface. If interfacial adhesion is sufficient, then stress can be effectively transferred from the matrix to the fibers.  To enhance the adhesion properties at the interface, fibers are often exposed to surface treatments such as oxidation and sizing. However, sizing agent and matrix have to be compatible to achieve an improved interaction and composite performance. In this study, surface energies of different carbon fibers with and without sizing agent have been determined by Inverse Gas Chromatography (IGC). 

EXPERIMENTAL

IGC is a well-known tool for the characterization of particulates, fibers and films. IGC inverts the traditional roles of traditional GC experiments and involves the sorption of a known adsorbate (probe molecule) onto an unknown adsorbent stationary phase (solid sample).  IGC experiments were carried out using iGC Surface Energy Analyzer (iGC-SEA; Surface Measurement Systems, Alperton, U.K.) with a flame ionization detector. All data were analyzed using both standard and advanced iGC-SEA Analysis Software. Each sample column was conditioned in-situ for 2 hours at a constant temperature and at 0% RH with 10 sccm of helium carrier gas flow rate, prior to any surface energy measurements.

For the study of heat treatment, sized and unsized AS4 carbon fibers sample columns were pre-conditioned in-situ at 30 °C and 120 °C. As for the study of sizing effect, four sized carbon fibers were conditioned in-situ at 70 °C.

RESULTS

Dispersive surface energy profiles for the unsized and sized AS4 carbon fibers show that both unsized samples are energetically more active and heterogeneous in surface sites, than the sized samples. Also, heat treatment has more significant influence on the unsized carbon fibers. The elevated temperature of 120 ºC activates the higher energy sites of the oxidized surface.  Sized carbon fibers on the other hand exhibited a much lower and fairly homogenous dispersive surface energy profile. Even though the heat-treated, sized sample has marginally higher values, the 120 °C pretreatment condition was found to have little effect on the sized carbon fibers, which may be due to surface passivation by the formation of an epoxy layer. 

It has been reported that high concentrations of sizing agent will enhance the wetting performance of carbon fibers which can be attributed to the presence of hydrophilic groups in its molecules. This is demonstrated with a considerably large acid-base surface energy contribution, implying that there was a higher concentration of polar surface groups or different surface groups with higher specific surface energy on these surfaces. However, too high concentration of a sizing agent can also provide challenges in the processing of carbon fibers.  IGC can estimate wettability by dividing the acid-base contribution by the total surface energy.  This was done for a series of silanized carbon fibers.  Performance data shows that wettability in fact correlated to tensile strength and inter-laminar shear strength (ILSS).

CONCLUSIONS

Sized and unsized AS4 carbon fibers and four different sized carbon fibers with varying concentrations of sizing agent were investigated in this work, using the IGC. It is evident that surface energy distributions can provide valuable information on the surface properties of these carbon fibers, thus relating to its wetting and processing performances. It has been shown that heat treatment has significant influence on unsized carbon fibers. Surface energy measurements on different sized carbon fibers show a relationship between the inter-laminar shear strength and wettability of the surfaces.