Page 25 - Engineered Interfaces in Fiber Reinforced Composites

8 Engineered interfaces in jber reinforced composites

The surface energy of a solid (i.e. reinforcement in composites), ysv, must be greater
than that of a liquid (Le. matrix resin), yLv, for proper wetting to take place.
Table 2.1 gives values of surface energies for some fibers and polymer matrix
materials. Thus, glass and carbon fibers can be readily wetted by thermoset resins
like epoxy and polyester resins at room temperature unless the viscosity of the resin
is too high (Hull, 1981), and by some thermoplastic resins (e.g. Nylon 6.6, PET,
PMMA and PS). In contrast, it is difficult to wet polyethylene fibers (of surface
energy approximately 31 mJ/m2) with any of these resins unless the fibers are surface
treated. For the same reason, carbon fibers are often coated with Ti-B (Amateau,
1976) using a chemical vapor deposition process to allow wetting by an aluminum
matrix.
Combining Eqs. (2.1) and (2.2) yields the familiar Young-Dupre equation

The values of WA reflect directly the significance of energetics between the liquid and
solid phases, i.e. the higher the work of adhesion the stronger the interactions. WA
can be determined in experiments by measuring the surface energy of the liquid, yLv,
and the contact angle, 8. Details of the measurement techniques of the contact angle
are discussed in Section 2.3.11.
It should be noted that, in the above equations, the effects of adsorption of vapor
or gas on the solid surfaces are completely neglected. The amount of adsorption can
be quite large, and may approach or exceed the point of monolayer formation at
saturation. The spreading pressure, ns, which is the amount of the reduction in
surface energy on the solid surface due to the adsorption of vapor in equilibrium, is
given by (Adamson, 1982)

ns = Ys - Ysv . (2.5)

The subscript s indicates the hypothetical case of a solid in contact with a vacuum.
The importance of impure surfaces is well recognized in areas like brazing where the
difficulty of brazing aluminum is associated with the presence of an oxide film on the
surface. Therefore, Eq. (2.5) can be substituted in Eqs. (2.1) and (2.2) by introducing
the spreading pressure. The Young-Dupre equation is then modified to

Although the discussion of wettability presented above has focused on the
thermodynamics between the fiber surface and the liquid resin, real composite
systems consist of an extremely large number of small diameter fibers embedded in a
matrix. Adding to the issue of proper wetting of fiber surfaces by the resin, a key to
creating good adhesion at the fiber-matrix interface is infiltration of the resin into
the fiber tow during the fabrication process. The minute gaps present between the
fibers can create very large capillary forces, which are often characterized by a
pressure drop due to the surface energy acting in the small capillaries. If the liquid