Composite Materials Research Progress

134
Yuanxin Zhou, Hassan Mahfuz, Vijaya Rangari et al.
Acoustic cavitation accelerates heat and mass transfer processes such as diffusion, wetting,
dissolution, dispersion and emulsification [15-16].
Optimization of the solution prepregging process begins with the appropriate choice of
solvent. A high degree of wetting can only be expected from solvents that possess favorable
thermodynamics regarding wetting of the particular solid material (carbon filaments, in this
case) [17-18]. The process of wetting entails the contact and spreading of the solvent over the
surface of the solid, i.e., liquids that possess a low contact angle for a particular solid show
considerable wetting behavior (as opposed to liquids that display high contact angles). This
solvent should be chosen from a list of candidate solvents capable of dissolving the matrix
polymer. The differences in wetting action, coupled with other relevant parameters such as
boiling point and general practicality of the particular solvent choice usage, will lead to an
appropriate choice of solvent. In particular, solvent characteristics should include a much
lower boiling point than melt flow point of the resin and a lower density then that of the resin
for ease of residual solvent removal [19]. An example of the preceding contact angle analysis
can be found in a study by Patel and Lee [17]. In their study, fiberglass tows were subjected
to contact angle analysis using the Wilhelmy plate method. A series of liquids was used (not
polymer solutions), each having differing values of viscosity and surface tension. The
equilibrium contact angles for all of these liquids were not observed to be a function of
solvent viscosity (viscosity range = 0.33 mPa – 1499.0). Furthermore, the liquid surface
tension was found to be positively correlated with the contact angle, i.e., increases in surface
tension generally yielded larger contact angle measurements. It should be stressed that these
results only indicate trends in contact angles; they may not imply favorable conditions for
capillary flow (in addition to wetting), which is another important consideration in the
prepreg process [15]. Once the appropriate solvent is identified for solution prepregging,
prepregged tapes can be manufactured. The objective in solution prepregging is to prepare a
uniform tape in which every fiber surface is uniformly wetted with the polymeric matrix
material. Another objective in solution prepregging is maximizing the amount of matrix
material pick-up. This is easily quantifiable as the amount of matrix material adhering to the
fiber surface after a single immersion into the resin bath. The nature of the relationship
between fiber dispersion and matrix pick up is expected to be competitive. This can be
inferred from the extremes of the process. In a polymer solution with a concentration
approaching zero, every filament can be expected to be wetted (resulting in a good fiber
dispersion), assuming that the thermodynamics are favorable. But the matrix pick up in this
case is nearly zero since there is no polymer in solution. At the other extreme, the polymer
weight fraction in solution approaches one. In this case, the fiber wetting upon dipping will be
very poor given the extremely high viscosity of the resin (kinetic limitation). But upon
wetting, a large amount of polymer will remain on the fiber surface (high matrix pick up).
Therefore, intuition states that there will exist an intermediate polymer solution concentration
in which a balance is obtained between the fiber dispersion and matrix pick up. The concepts
in the preceding paragraph can be more easily visualized by using a model that approximates
the wetting process of a fiber tow by a polymer solution. By combining the Kelvin equation,
which describes wetting of a solution in micro-capillaries and Darcy’s Law, which describes
flow in porous media, the following equation is obtained:
f
void
{ 2S
( 2 / R)
γ θ}
2
t =
l μV /
cos
b
sizing