Page 306 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 7. Improvement of transverse fracture toughness with interface control 287

nylon matrix system (Jao and McGarry, 1992a, b); ethylene-propylene elastomers
for glass fiber-epoxy matrix composite (Mascia et al., 1993).
Many researchers have shown promising results with a range of different polymer
coatings for many different types of composites: polysulfone, polybutadiene and
silicone rubber on CFRP (Hancox and Wells, 1977; Williams and Kousiounelos,
1978); latex coatings, e.g. polybutyl acrylate, polyethyl acrylate, etc. on GFRPs
(Peiffer, 1979; Peiffer and Nielson, 1979); polyvinyl alcohol (PVAL) on KFRPs and
CFRPs (Kim and Mai, 1991b; Kim et al., 1993a); anhydride copolymers, e.g.
polybutadiene-co-maleic anhydride and polymethylvinylether-co-maleic anhydride
(Crasto et al., 1988) and acrylonitrile copolymers, e.g. acrylonitrile/ methylacrylate
and acrylonitrile/glycidylacrylate (Bell et al., 1987) on CFRPs; polyamide coating on
CFRPs and carbon-Kevlar hybrid composites (Skourlis et al., 1993; Duvis et al.,
1993). Particularly, Peiffer and Nielsen (1979) achieved a significant 600% increase
in impact toughness of GFRPs with a negligible strength reduction using colloidal
latex particles that were attracted to glass fibers by electrostatic forces to form a
rubbery acrylic polymer layer of uniform thickness. The impact toughness was
shown to be a function of both thickness and glass transition temperature, T', of the
coating: the toughness was maximum when the coating had a low Tg and a thickness
of about 0.2 pm.
Kim and Mai (1991b) have made an extensive study on CFRPs and KFRP with
PVAL coated fibers. The coating increased the composite impact toughness by more
than loo%, particularly at sub-zero temperatures, without causing any significant
loss of flexural strength and interlaminar fracture toughness. These promising
results are highlighted in Figs. 7.5 and 7.6, and Table 7.2. The thermoplastic coating
reduced the bond strength at the fiber-matrix interface significantly as indicated by
the average interlaminar shear strengths (ILSSs) obtained in short beam shear tests.
High resolution scanning electron microscopy (SEM) of the fracture surface further
supports the weak interfacial bonding due to the PVAL coating. For KFRP, the
uncoated fibers most often split into small fibrils longitudinally due to the weak
bond between the fibrils and the skin-core heterogeneity of the fiber (see Fig. 5.20).
In contrast, the PVAL coated Kevlar fibers debonded clearly from the matrix with
little fibrillation. Clear distinction was also evident between the interlaminar
fracture surfaces of CFRPs, as shown in Fig. 7.7. The composite without coating
consisted of substantial deformation of the matrix material which covered the
majority of the surface and tiny matrix particles adhering to the debonded fiber
surfaces. However, the coated fiber composite displayed a relatively clean fiber
surface, with partial removal of the rugosity generated by the surface oxidative
treatment, which effectively deteriorates the mechanical anchoring of the resin to the
fiber. The above findings support the appreciable difference in surface chemical
composition and functional groups of CFRPs that have been revealed by X-ray
photoelectron spectroscopy (XPS) (Kim et al., 1992). The uncoated fiber composite
showed a significant amount, say about 6 at. wt%, of silicon associated with the
epoxy matrix, whereas the coated fiber composite had little trace of silicon with a
larger amount of C-0 group, which is a reflection of the PVAL coating. All these
observations strongly suggest that the coating acts as a physical barrier to the