Page 217 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Sugace treatments ofjibers and effects on composite properties 199

Fig. 5.20. Scanning electron microphotograph of a fibrillated Kevlar 49 fiber. After Kim and Mai (1991b).

diepoxide molecule may be attached. A long exposure time causes direct mechanical
damage to the fiber which may outweigh any advantages gained by improved
interface bonding. In a similar study, Andreopoulos (1989) has also treated Kevlar
49 fibers with reactive chemicals, such as acetic acid anhydride, methacryloyl
chloride, sulfuric acid and acrylamide, to ensure grafting in the aramid chain while
avoiding excessive loss in fiber weight. Tensile strengths of both fiber and polyester
matrix composite decrease as a result of fiber etching by most of the above chemical
agents used. Only methacryloyl chloride is effective in producing an intermediate
surface roughness and chemical grafting that are favorable for a strong interface
bond.
Chemical surface treatment influences the properties of semicrystalline thermo-
plastic matrix composites in a different manner. Yu et al. (199121, b) reported that a
treatment with suberoyl chloride promotes the growth of a trans-crystalline zone
around the Kevlar fibers which is considered favorable for good fiber-matrix
interface bonding. A wide-angle X-ray diffraction study has indicated that chemical
treatment modifies the nucleating ability of the fiber. Takayanagi et al. (1982) also
modified Kevlar 29 by polymer reaction via the metalation reaction in dimethyl
sulfoxide to provide the fiber surface with several functional groups such as n-
octadecyl, carboxymethyl and acrylonitrile. They have shown that the treatment
increases the roughness of the fiber surface, hence improving the mechanical
properties of the composites made with ionomer matrices. Bromine water attack on
the fiber surfaces has produced similar roughening effects on the composite ILSS,
but with a slight loss in fiber tensile strength due to splitting of the fiber skin layer
from the core (Lee-Sullivan et al., 1994).
Plasmu treutment: More promising results have been obtained by modifying the
aramid fiber surface with cold plasma in the presence of vacuum, ammonia or
argon. The improvement in bond strength varies between 50% to a remarkable
400Y0, depending on the exposure time and atmosphere (Wertheimer and Schreiber,
1981). Plasma treatment in ammonia increases the amine concentration on fiber
surfaces which is thought to be responsible for strong covalent bonding at the