Page 218 - Engineered Interfaces in Fiber Reinforced Composites

200 Engineered interfaces in fiber reinforced composites
interface with epoxy resins (Allred et al., 1985). The high nitrogen content in the
form of an amine group is usually accompanied by a decrease in oxygen content
with increase in treatment time. The treatment also increases the ILSS of epoxy
matrix composites by 30% to 50% with associated dominant failure mechanisms of
fiber splitting and matrix cracking, in contrast to the apparent interface debonding
in untreated counterparts (Brown et al., 1991). Fig. 5.21 shows the increase in the
amine group and the corresponding ILSS with treatment time.
In addition, the increase in flexural strength of treated fiber composites can be
attributed to a lower extent of fiber buckling in the compressive face as a result of
improved interfacial bonding (Brown et al., 1992a). A similar improvement in
mechanical properties is also reported for vinylester matrix composites (Brown et al.,
1992b), which is associated with enhanced fiber wettability rather than chemical
functionality. Improvements in retainability of composite flexural strength and ILSS
have also been shown under water aging condition for plasma treated Twaron
aramid fibers (Verpoest and Springer, 1988; Janssens et al., 1989).
Coupling agents: The application of coupling agents to aramid fibers has not been
particularly successful. Vaughan (1 978) applied onto Kevlar fibers several silane
coupling agents that were developed originally for glass fibers with limited
improvement in the composite strength under both dry and water aging conditions.
Application of organotitanate and organozirconate coupling agents (Sugerman
et al., 1989) also shows only a slight improvement in flexural strength of epoxy
matrix composites. The addition of coupling agents after oxygen plasma treatment
appears to be a good combination to maximize the benefits of improvement in
interface bonding, as evidenced by the 250% increase in the fiber pull-out force for
Kevlar 49 fiber reinforced silicon rubber matrix composites, Table 5.11 (Inagaki

Treatment time in min

Fig, 5.21. Surface amine concentration (0) of aramid fiber and ILSS (0) of epoxy matrix composites as a
function of ammonia plasma treatment time. After Brown et al. (1991).