Page 193 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treatments of Jbers and effects on composite properties I75

their effects on composite mechanical properties (Plueddemann 1981, 1982, 1988;
Ishida, 1984) are useful references on this subject.
Several theories have been proposed to explain the interfacial bonding mecha-
nisms of silane coupling agents which are responsible for the improvement of
mechanical performance and hygrothermal stability of composites. Among these,
the most widely accepted is chemical bonding (Schrader et al., 1967; Schrader and
Block, 1971; Koenig and Shih, 1971; Ishida and Koenig, 1980). Other theories
include those associated with preferential absorption (Erickson, 1970), restrained
layer (Hooper, 1956), coefficient of friction (Outwater, 1956), and wettability and
surface energy effect (McGarry, 1958; Bascom, 1965). Although all of these theories
have some merits, the chemical bonding theory has been well established and
confirmed many times. Therefore, development of silane coupling agents have been
based on the concept of chemical reactivity between the inorganic substrate and the
organic resin. A large variety of silanes containing different organofunctional groups
have been developed for different resin chemistry (e.g. epoxy, vinyl and amino).
Representative commercial coupling agents are listed in Table 5.4, according to
Plueddemann (1982). Among the various silane agents with vinyl, hydroxy, thio,
carboxy, amine, alkyl and ester substitutions, y-methacryloxypropyl trimethoxysi-
lane (y-MPS) in particular has established wide commercial applications for
polyester resin composites today.
In the chemical bonding theory, the bifunctional silane molecules act as a link
between the resin and the glass by forming a chemical bond with the surface of the
glass through a siloxane bridge, while its organofunctional group bonds to the
polymer resin. This co-reactivity with both the glass and the polymer via covalent
primary bonds gives molecular continuity across the interface region of the
composite (Koenig and Emadipour, 1985). A simple model for the function of silane
coupling agents is schematically illustrated in Fig. 5.3, according to Hull (1981). The
general chemical formula is shown as X3Si-R, multi-functional molecules that react
at one end with the glass fiber surface and the other end with the polymer phase. R is
a group which can react with the resin, and X is a group which can hydrolyze to
form a silanol group in aqueous solution (Fig. 5.3(a)) and thus react with a hydroxyl
group of the glass surface. The R-group may be vinyl, y-aminopropyl, y-
methacryloxypropyl, etc.; the X-group may be chloro, methoxy, ethoxy, etc. The
trihydroxy silanols, Si(OH)3, are able to compete with water at the glass surface by
hydrogen bonding with the hydroxyl groups at the surface (Fig. 5.3(b)), where M
stands for Si, Fe, and/or A1 (see Table 5.1). The type of organofunctional group and
the pH of the solution dictates the composition of silane in the dilute aqueous
solution. When the treated fibers are dried, a reversible condensation takes place
between the silanol and M-OH groups on the glass fiber surface, forming a
polysiloxane layer which is bonded to the glass surface (Plueddemann, 1974)
(Fig. 5.3(c)).
Therefore, once the silane coated glass fibers are in contact with uncured resins,
the R-groups on the fiber surface react with the functional groups present in the
polymer resin, such as methacrylate, amine, epoxy and styrene groups, forming a
stable covalent bond with the polymer (Fig. 5.3(d)). It is essential that the R-group