Page 201 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treatments 0fJ;ber.F and effects on composite properties 183

Q
P
I
.-
C
n
t -0
c1

0 2 4 6 8 10 12
Silane concentration in 'YO

Fig. 5.8. Effect of diamino-silane solution concentration and modification on interfacial bond strength ~h
of GFRP: (0) silane treated; (0) silane treated and partially removed. After Koenig and Emadipour
(1 985).

5.3. Carbon fibers
5.3.1. Structure and properties of carbon fibers

The surface properties of carbon fibers are intimately related to the internal
structure of the fiber itself, which needs to be understood if the surface properties are
to be modified for specific end applications. Carbon fibers have been made from a
number of different precursors, including polyacrylonitrile (PAN),, rayon (cellulose)
and mesophase pitch. The majority of commercial carbon fibers currently produced
are based on PAN, while those based on rayon and pitch are produced in very
limited quantities for special applications. Therefore, the discussion of fiber surface
treatments in this section is mostly related to PAN-based carbon fibers, unless
otherwise specified.
The properties of a carbon fiber are a direct reflection of the structure of graphite
which is highly anisotropic on a nanoscopic scale. The basic structure of the carbon
fibers is the graphite crystallites which, in turn, are composed of turbostatically
layered basal planes, as schematically shown in Fig. 5.10. The high bond strength
between the carbon atoms in the basal plane gives an extremely high modulus along
the fiber axis, while the weak van der Waals type of bonding between the
neighboring layers produces a low modulus along the edge plane. The edges and
corners of these crystallites intersect the fiber surface. A schematic three-dimensional
representation of the structure of a PAN-based carbon fiber is shown in Fig. 5.11,
where irregular space filling and the distortion of the graphite basal planes are seen.