Page 196 - Engineered Interfaces in Fiber Reinforced Composites

178 Engineered interfaces in fiber reinforced composites

indication of interpenetration was also observed at the y-aminopropyl-triethoxysi-
lane (APS)/polyethylene interface (Sung et al., 198 1). The coupling agent-resin
matrix interface is a diffusion boundary where intermixing takes place, due to
penetration of the resin into the chemisorbed silane layers and the migration of the
physisorbed silane molecules into the matrix phase (Schrader, 1970).
The synergism of these two major bonding mechanisms with a silane coupling
agent, Le., the chemical reaction and the IPN theories, is of particular importance in
composites containing thermoset matrices. It is yet to be shown, however, to what
extent chemical bonding contributes to the total interface bond strength in
thermoplastic matrices, although there are appreciable improvements in flexural
strength of composites containing silane treated fibers, particularly those fabricated
by compression molding, see Table 5.5. The compatibility between the silane and
the matrix resin appears to be more important than chemical bonding in
thermoplastic matrix composites, although chemical reaction can add additional
strength. The reactivity may be improved by tailoring the unreactive molecules in
the thermoplastic so that it consists of special functional groups capable of bonding
with the coupling agent. Another approach is to include chemicals in the size that
may cause local chain scission of the molecules near the fiber, allowing chemical
reaction to take place so that coupling occurs directly with the molecules.
The mechanical properties of the blend of silane/size and bulk epoxy matrix (at
concentrations representing likely compositions found at the fiber-matrix interface
region) also suggest that the interaction of size with epoxy produces an interphase
which is completely different to the bulk matrix material (Al-Moussawi et al., 1993).
The interphase material tends to have a lower glass transition temperature, Tg,
higher modulus and tensile strength and lower fracture toughness than the bulk
matrix. Fig. 5.4 (Drown et al., 1991) presents a plot of Tg versus the amount of

Table 5.5
Improvement in flexural strength due to silane treatments in glass fiber thermoplastic matrix compositesa
~~ ~~
Polymer-silane system Percentage strength improvement
Compression molded Injection molded

Dry Wet Dry Wet

Nylon-aminosilane F 55 115 40 36
Nylonxationic silane H 85 133 40 45
PBT-aminosilane F 21 - 23 24
PBT-cationic silane H 60 47 28 11
Polypropylene-silane F 8 18 7 10
Polypropylene-silane H 86 89 16 16
aAfter Plueddemann (1988).
Wet, after 2 h in boiling water; PBT, polybutylene terephthalate.