Page 200 - Engineered Interfaces in Fiber Reinforced Composites

I82 Engineered interfuces in fiber reinforced composites

30

25

20
6
11 15
+
t
m
C
W
3 10
v)
5
55

0 2000 4000 6000 8000
Immersion time (hrsl

Fig. 5.7. Effect of immersion in hot water on interfacial bond strength of silane treated glass fiber-poxy
matrix composite. After Koenig and Emadipour (1985).

under dry condition is found to be highest when the multilayer silanes on the glass
fibers are washed with boiling water (Emadipour et al., 1982). It appears that the
amount of silanes needed for protection against hygrothermal condition is different
from that for dry condition (Liao, 1989). It is repeatedly confirmed that a thicker
silane layer does not necessarily result in improvement in hydrothermal stability of
the interface bond; but on the contrary, it may have an adverse effect on the bond
strength of the interface. An excessive amount of coupling agent is not effective,
rather impairing the properties of the interphase (Chua et al., 1992b). Koenig and
Emadipour (1985) also suggested that there is an optimum concentration of silane
which would produce the most favorable result on interfacial shear strength, for
example approximately 0.5% concentration of N-2-aminoethylene-3-aminopropyl
trimethoxysilane (AAPS) for glass fiber-epoxy matrix composites. For this purpose,
partial removal of the thick silane layer is suggested prior to fabrication of the
composite to enhance the mechanical performance, as demonstrated in Fig. 5.8. The
interlaminar fracture toughness of glass fiber-polyester matrix composites is also
influenced by the type of silane and solution concentration used (Suzuki et al., 1993).
There is an optimum amount of silane required to achieve the maximum fracture
toughness as measured in double-cantilever-beam tests. An excessive amount of
silane decreases the fracture toughness with unstable crack propagation, as
evidenced in force-displacement curves as shown in Fig. 5.9.