Page 314 - Engineered Interfaces in Fiber Reinforced Composites

Chapter I. Improvement of transverse fracture toughness with interface control 29 5

Bell, 1989; Rhee and Bell, 1991), random copolymers of methyl acrylate and
acrylonitrile were directly polymerized onto the carbon fiber surface. Dimethyl
formamide, dimethyl sulfoxide and distilled water proved to be useful as solvents for
this process. Polymerization can take place on the carbon fiber electrode, with initial
wetting of the fiber surface leading to better adhesion of the polymer formed. The
structure and properties of the polymer can be varied by employing different vinyl
and cyclic monomers in homopolymerization. Chemical bond can also be formed,
such as polymer grafting to the carbon fiber surface.

7.2.3.3. Electrostatic deposition
Glass fibers are coated with a uniform layer of acrylic latex polymer by using
electrostatic forces (Peiffer, 1979; Peiffer and Nielsen, 1979). This method is based
on the earlier work of Iler (1966) where cathodically charged particles, such as ion,
polar molecules, lattices, are attracted to the anionic surface of glass. Because
further deposition is inhibited by electrostatic repulsion after a monolayer of
charged particles are formed, the formation of multi-layers requires layers of
oppositely charged particles between each layer of like charges. As such, alternate
layers of negatively and positively charged colloidal particles can be deposited from
dilute sol to form coating layers. Since the acrylic polymer particles are normally
negatively charged, the neutral coupling agent must be removed before the
deposition process to expose the glass surface, so that the particles can be attracted.
In this process, pH control of the coating solution is of prime importance as it
determines the ability of the particle attraction of the glass surface.
7.2.3.4. Plasma polymerization and condensation polymerization
The plasma polymerization technique (Benatar and Gutowski, 1986) utilizes
polymerizable organic vapors, producing a highly cross-linked thin film on the fiber
surface with good adhesion. This technique is very flexible for treating carbon fibers,
but is limited to the use of monomers having a low surface energy to ensure
thorough wetting of fiber surface. Many different polymer coatings have been
successfully applied to carbon fibers using this technique (see Section 5.3).
The condensation polymerization process, employed recently by Skourlis et al.
(1993) and Duvis et al. (1993), involves immersion of carbon fibers in a solution
containing hexamethylenediamine and sodium carbonate. Dried carbon fibers are
then immersed in a dipolychloride solution in carbon tetrachloride where the
interfacial polycondensation reaction takes place. The result is that a thin layer of
polyamide (nylon 6,6) coating is deposited on the continuous carbon fiber, whose
thickness is controlled though by varying the diamine concentration.

7.3. Theoretical studies of interphase and three engineered interphase concepts

The term ‘interphase’ has been used to refer to the region which is formed as a
result of the bonding and reaction between the fiber and matrix. The morphological
or chemical composition and thermo-mechanical properties of the interphase are