Page 199 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treatments offibers and effects on composite properties 181

physisorbed region; (ii) chemisorbed region; and (iii) chemically reacted region. The
physisorbed region is the outermost layer, and consists mainly of the bulk of the
deposited silane. The layer of weak oligomeric siloxanols hydrolyzes easily and is
extracted with water even at room temperature. The chemisorbed region is the next
layer which can only be extracted by boiling water after prolonged immersion. It
consists mainly of higher oligomeric siloxanols that possess better resistance to
hydrolysis than the lower siloxanols.
The innermost region next to the glass surface is stable and resistant to extraction
by hot water and may be regarded as the chemically reacted region. The
interconnecting cross-linking exists in this region in the form of a three-dimensional
network of siloxane. The extent of cross-linking is found to increase from the outer
layers to the glass surface with corresponding increase in the mechanical and
hydrothermal stability (Ishida and Koenig, 1980). Fig. 5.6 shows the schematic
structure of the silane remaining on the glass surface after extractive hydrolysis with
hot water, according to Cheng et al., (1993). The individual characteristics of each of
these silane regions play a major role in controlling the interface stability and the
mechanical properties of the composites under both dry and hot/wet conditions. The
chemically reacted region is most likely responsible for the high resistance of the
interfacial bond of silane treated composites to hygrothermal attack. Fig. 5.7
exemplifies the shear strength measured as a function of immersion time in water. It
is also suggested that the silane agent, when present as a chemisorbed layer, not only
provides protection against attack by water, but also restores, to some extent, the
damage produced along the fiber-matrix interface once dried at a high temperature
(Chua et al., 1992b).
Schrader (1974) reported that the interface shear strength in a hygrolherrnal
environment is at its maximum when the multi-layer silanes on the glass fibers
remain after being washed in boiling water. On the other hand, the pull-out strength

HO - -si -O-A~-O~-H

Fig. 5.6. Schematic structure of the silane remnant remaining on the glass fiber surface after extractive
hydrolysis with hot water. After Cheng et al. (1993).