Page 313 - Engineered Interfaces in Fiber Reinforced Composites

294 Engineered interfaces in fiber reinforced composites

evaporating solvents must be used to avoid viscosity buildup on the rollers. The
coating thickness on the fiber is controlled mainly by the clearance between the feed
roll and applicator roll and by the viscosity of the coating solution. The roll coating
process has a major advantage over other coating techniques in that the coating
solution is uniformly applied to the individual filaments as they are forced to
disperse between the two rollers when being pulled. This technique has been
successfully used (Atkins, 1975; Mai and Castino, 1984, 1985) to apply polyurethane
and silicon rubber coatings onto carbon and Kevlar fiber tow surfaces, with
resulting intermittently coated and uncoated regions along the fiber.

7.2.3.2. Electrochemical processes
Most suitable for electrically conducting materials such as carbon fibers, the
electrochemical processes involve deposition of polymer coatings on the fiber
surface through electrodeposition or electropolymerization techniques. The major
advantage of these processes is that a uniform layer of controlled thickness and
variable polymer structure and properties can be obtained by controlling the current
and the solution concentration.
The electrodeposition process utilizes the migration of polymer carrying ionized
groups to the oppositely charged electrode under an applied voltage. In anionic
systems, negatively charged particles of coating in an aqueous dispersion are
electrochemically attracted to a substrate which is the anode of an electrochemical
cell. In cationic systems, the substrate is made the cathode, and positively charged
particles of coating are attracted to the cathode and precipitated on its surface by
the hydroxide ions generated there. The system must be designed so that it allows all
coating components to be attracted to the electrode at the same rate; otherwise the
composition will change with time. In the process employed by Subramanian and
Crasto (1986) and Crasto et al. (1988), carbon fibers acted as the anode of an
electrolytic cell containing solutions of ionic polymers, such as butadiene-maleic
anhydride and ethylene-acrylic acid copolymers. As the polymer is formed, the
increased electrical resistance of the coating directs film formation to uncoated
regions which are more conducting. This enables a film of uniform thickness to be
deposited. Even so, the deposit growth process is not completely uniform, and it
rather becomes faceted, resulting in surface discontinuities, because the process
involves the condensation of polymer atoms at rough sites on the substrate surface.
Organic additives are used to modify the nucleation process and thus to eliminate
undesirable deposition modes. Another critical requirement for the electrodeposi-
tion process is that the coating solution be closely monitored to maintain a constant
particle concentration. The dispersion must also have a high level of stability against
coalescence by continuous stirring and recirculation.
The electrochemical polymerization process is achieved by polymerization of
monomers in an electrolytic cell (Subramanian and Jakubowski, 1978). The
electrode is the source of active species that initiates the polymerization. It is
necessary to select a solvent electrolyte system which is capable of forming a
solution with the monomer and having sufficient current-conducting properties. In
the process employed by Bell and coworkers (Bell et al., 1987; Wimolkiatisak and