Page 197 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treatments of fibers and efects on composite properties 179

501 I I 1 i 1 I 1 I
0 5 10 15 20 25
Amount of size (wt%)

Fig. 5.4. Glass transition temperature, Tg, measured by dynamic mechanical analysis as a function of
wt% epoxy-compatible PPG size. After Drown et a]. (1991).

epoxy-compatible size added to the stoichiometric mixture. It is clearly seen that the
polymer created by the addition of the commercial size exhibits a monotonic
decrease in Tg, suggesting that the silanes and other ingredients present in the size
act to reduce the crosslink density of mixtures. When the fiber was treated with
epoxy-compatible sizing containing silane agents, thc composite shows a higher
interlaminar shear strength (ILSS), and flexural strengths in both the longitudinal
and transverse directions than the composite without silane sizing, as shown in
Fig. 5.5 (Drown et al., 1991). This finding is attributed to the improved interface
bond quality due to the silane size. However, the brittle interphase material
promoted matrix cracks near the broken fiber ends as observed in fiber fragmen-
tation tests.
On the contrary, a completely opposite result was reported by Chua and Piggott
(1992). The presence of large amounts of siloxane y-MPS in a polyester resin was
found to reduce the modulus and compressive strength, while increasing the fracture
toughness of the interphase material. This anomaly appears to be associated with
plasticization of the inherently brittle resin by the silane size, making the interphase
material softer and more ductile than the bulk matrix. As a result, the debonding
and the fiber pull-out forces were reduced substantially, suggesting that the
chemisorbed layer on the fiber surface constituted the debonding and sliding surface
(Chua et al., 1992a). Whether the interphase material created by interdiffusion of
silane sizing is more ductile or brittle than the bulk matrix material is an issue of
great importance because the interphase properties often dictate the gross