Page 312 - Engineered Interfaces in Fiber Reinforced Composites

Chapter I. Improvement of transverse fracture toughness with interface control 293

chemical reaction between the coating and matrix could enhance the frictional
shear stress (Mai and Castino, 1984; Rhee and Bell, 1991). Partial or complete
mixing of the coating material during the curing process with the matrix, for
example, CTBN rubber in an epoxy (Gerard, 1988; Kim and Mai, 1991b),
produces composites with hardly modified interfaces that may not be desirable
as it only changes the matrix properties.

7.2.3. Fiber coating techniques

Several processing methods have been developed to apply organic polymer
coatings to both continuous and short fibers for applications in PMCs. They can be
classified into three broad categories: solution dip coating and roll coating;
electrodeposition techniques, including electrochemical deposition, electropolymer-
ization and electrostatic deposition; and polymerization techniques. A summary of
the reviews (Hughes, 1984; Wicks et al., 1992; Labronici and Ishida, 1994) on the
application techniques of organic coatings is presented below.

7.2.3.1. Solution dip coating and roll coating
The solution dip coating technique has been most widely used for fiber coatings
because of the ease of application and the simplicity of principle (Sung et al., 1977;
Dauksys, 1973; Hancox and Wells, 1977; Mascia et al., 1993; Tomlinson and Barnes,
1992; Kim and Mai, 1991a, b; de Kok, 1995; Jao and McGarry, 1992a, b). Almost
every type of polymer, ranging from thermoplastics, thermosets to elastomers, has
been successfully applied with the aid of appropriate solvents. The continuous
immersion coating process involves drawing of a fiber tow or yarn through the
coating solution bath and complete evaporation of the solvent, before being
embedded into a matrix material. The thickness of the coating layer may be
controlled by varying the solution concentration and the drawing speed. Maintain-
ing a uniform thickness in a batch of fiber is a critical aspect of this process. When
bundle fibers or tows are immersed in a polymer solution, the individual filaments in
a bundle tend to stick together, making it difficult to wet or coat them thoroughly.
Good impregnation of the individual filaments can be achieved by using a low
viscosity solution; and ultrasonic stirring of the solution bath was helpful in
dispersing the filaments from the bundle (Gerard, 1988). It may also be necessary to
separate the fiber bundles by using techniques such as gas jets, ultrasonic horns and
mechanical combs (Sung et al., 1977), during the drying process after immersion. In
this respect, care must be exercised in selecting volatile solvents for dip coating
because of the changes in viscosity of the solution, resulting from evaporation of the
solvent, in addition to flammability hazards. Viscosity can increase not only by loss
of solvent, but also by chemical reactions of the coating components.
Roll coating is widely used for uniform, whether flat or cylindrical, surfaces
including fiber bundles. In a roll coating process, fibers are coated between two
rollers, an applicator roller and a backup roller: coating is fed continuously to the
applicator roller by a feed roller which runs partially immersed in a coating bath;
and the backup roller pulls the fibers by rotating in opposite directions. Slow