Page 198 - Engineered Interfaces in Fiber Reinforced Composites

180 Engineered interJaces in .fiber reinforced composites

1.5 I

_ _
ITS SBS @'Flexure 900Flexure
Fig. 5.5. Normalized interfacial shear strength of unsized (bare) and sized E-glass fiber-epoxy matrix
composites measured from the interfacial testing system (ITS, equivalent to fiber push-out test), short
beam shear (SBS) test, Oo flexural test and 90° flexural test. After Drown et al. (1991).

mechanical performance and structural integrity of the composite as a whole. Since
the details of the interface reaction is specific to each combination of fiber and
matrix materials with totally different chemical and atomic compositions and
morphological nature, no general conclusions can be drawn regarding the ductility
and fracture toughness of the interphase relative to the surrounding matrix.

5.2.2.3. Effects of water
Apart from the chemical reaction and the IPN discussed in the foregoing, another
important characteristic of silane treatment is its ability to provide the glass fibers
with a water resistant bond. The effect of water degradation on untreated glass
fiber-resin matrix interface is found to be much pronounced. Small molccules of
water penetrate into the interface of untreated fibers by diffusion and filtering
through voids and cracks of the resin or by capillary migration along the fibers, that
are eventually absorbed by the glass fiber. The randomly distributed groups of
oxides on the surface of glass, such as SiOz, Fe203 and Alz03, absorb water as a
hydroxyl group. The water then forms a weak hydrogen bond with these oxides.
Other oxides also absorb water and become hydrated. Water hydrolyzes the existing
physical bonds at the interface and destroys the adhesion, which ultimately results in
mechanical failure of the composite system (Ishida and Koenig, 1978, 1980).
Immersion of untreated fiber composites in hot water for a long period causes the
polymer resin to swell, followed by shrinkage due to leaching out of low molecular
weight materials from the resin, in addition to the above water absorption processes.
When glass fibers are treated with hydrolyzed silane solution, multi-layers of the
silane coupling agent are deposited on the fiber surface. The thickness and
orientation of the layers are determined by a number of factors, such as conditions
of deposition, topology of the glass surface, concentration of the solution and the
length of the treatment time (Ishida and Koenig, 1979, 1980). Schrader (1970) has
proposed that there are three different structural regions in the deposited layer: (i)