Page 310 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 7. Improvement of transverse fracture toughness with interface control 29 1

residual thermal stresses caused by differential shrinkage between the fiber and
matrix upon cooling from the processing temperature (Arridge, 1975; Marom and
Arridge, 1976); and as a crack inhibitor or arrester, allowing large debonding and
fiber pull-out to take place, thus making substantial contributions to the total
toughness of the composites.
Apart from the discrete layers that form at the fiber-matrix interface, reactive
functionality of the coating material has been studied for CFRP systems (Rhee and
Bell, 199 1). Two different coating materials were used, namely acrylonitrile/methyl
acrylate (AN/MA) and glycidyl acrylate/methyl acrylate (GA/MA) copolymers
which represent, respectively, non-reactive and reactive systems. These coatings
were applied to fiber bundles by electrochemical copolymerization which allows
accurate control of the coating thickness. The reactive coating system showed 10-
30% simultaneous improvement in impact fracture toughness and ILSS when
appropriate combinations were used, as illustrated in Fig. 7.8. In contrast, the non-
reactive coating system improved the impact toughness with a concomitant loss in
ILSS, due to the weak interface between the coating and the matrix material.
In view of the foregoing discussion, the effectiveness of coating materials can be
summarized and some general conclusions can be drawn. The principal aim of the
fiber coating is to optimize the interfacial characteristics, which, in turn, allows
desired failure mechanisms to take place more extensively during the fracture
process. Depending on the specific combination of fiber and matrix materials, the
thermo-mechanical properties and the thickness of the coating material are the
predominant parameters that limit the performance of the coating. Polyurethane
coatings are found to be effective for improving the fracture toughness of BFRPs
and KFRPs. Silicone rubbers on CFRPs and GFRPs, PVAL coatings on CFRPs
and KFRPs, and liquid rubber coatings on CFRPs have also shown to be quite
promising. However, the selection of an appropriate coating material for a given
composite has relied entirely on the trial and error method, there are apparently no
established principles to determine which coating materials are most suited for a
specific combination of fiber and matrix materials. Even so, some points of
generalization may still be made with respect to the criteria required for a potential
coating material to improve the fracture toughness of brittle polymer matrix
composites. According to Kim and Mai (1991a) these are:
(1) If the coating remains fluidic or becomes rubbery at the fiber-matrix interface
after cure, such as SVF and Estapol, a coating having a high viscosity is
preferred because the frictional shear work during the fiber pull-out is
proportional to the coating viscosity (Sung et al., 1977).
(2) Tf the coating forms a discrete, rigid interlayer after cure, it should be more
ductile and compliant than the matrix material, such as some thermoplastic
coatings for thermoset-based matrices. At the same time, it should also provide a
weak bonding at the interface while retaining sufficiently high frictional bonding.
(3) Coating thickness should be chosen to optimize the benefit in toughness and
minimize the loss in strength and some other properties. As a rule of thumb, the
thickness of the coating should be kept minimum compared to the fiber diameter
in order to eliminate any reductions of composite stiffness and strength in both