Page 207 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surjace treatments offibers and effects on composite properties I89

imperfections. The plasma polymerization process employs polymerizable organic
vapors, such as polyamide (Goan and Prosen, 1969), polyimide (Marks et al., 1975),
organosilanes (Goan and Prosen, 1969), other alternating and block polymers like
styrene and maleic anhydride (Riess et al., 1974), propylene (Jang et al., 1988), and
acrylonitrile and styrene monomers (Sung et al., 1982; Dagli and Sung, 1989).
Plasma polymerization is shown to increase the polar component of surface free
energy of the carbon fiber (Dagli and Sung, 1989; Donnet and Guilpain, 1991). The
polymers used for plasma treatment should possess not only the capability of being
fixed on the fiber surface by covalent or ionic bonds, but also on the compatibility
with the resin matrix. Further details of fiber coating techniques with appropriate
polymeric materials, including plasma polymerization to improve the transverse
composite fracture toughness, are presented in Section 7.2.

5.3.2.2. Eflects of surface treatment on fiber properties
Improvements in ILSS, flexural, tensile, compressive and off-axis strengths of the
composite which arise from various surface treatments are attributed to the changes
in surface area, rugosity and surface functional groups and the removal of weak
outer layers. Nevertheless, there is no simple, direct relationship of the interfacial
bond strength with these factors. The increase in fiber surface area by the formation
of pits or enhancement of longitudinal striations certainly improves mechanical
anchoring of the matrix and interpenetration between fiber and matrix. However,
because the rugosity develops mainly in the fiber axial direction, the improvement of
interfacial bonding is realized only in the same direction. This is evidenced by the
preferential increase in the adhesion when a force is applied parallel to the fiber axis
(Donnet et al., 1974). Moreover, the rugosity becomes an ameliorating factor only
when the fiber is perfectly wetted by the liquid resin. This means that unless the resin
penetrates into all the asperities present on the fiber surface, cure or polymerization
of the resin always results in the formation of interfacial cavities, causing premature
failure of the interfacial bond. Another important characteristic of chemically
treated carbon fibers is that an outer weak layer containing various types of defects
is removed by oxidation, which, in turn, results in a surface capable of supporting a
high shear load (Drzal et al., 1983a). An incidental improvement of fiber strength is
also reported with light anodic treatment (Bader et al., 1991). However, plasma
etching in nitrogen or oxygen causes excessive removal of the fiber surface layer,
reducing the fiber diameter by up to 22% (Jang et al., 1988).
Significant attention has been devoted to characterize the nature of the chemical
groups produced by surface treatments as discussed above. However, there is still
much controversy as to whether chemical bonding actually takes place at the
interface region, and if so, to what extent it contributes to the fiber-matrix interface
bonding of carbon fiber composites. A schematic model for chemical reaction
between oxidized fibers and an epoxy resin is presented in Fig. 5.13, according to
Horie et al. (1977). The strong covalent bonds could be either ether (-COH) or ester
(-COOH) bonds which are produced by the reactions between the hydroxyl and
carboxyl groups present at the fiber surface and the epoxy group of the resin.
Oxidative treatments increase the oxygen (often more than double) and nitrogen