Composite Materials Research Progress

An Experimental and Analytical Study of Unidirectional Carbon Fiber… 135
where: tf = tow wetting time
l = tow thickness
µ�= solution viscosity
Vvoid = tow void volume
Sb = tow permeability (perpendicular to fiber direction)
R = fiber-fiber separation
γsizing= solution surface tension
θ= contact angle
A quick survey of the equation reveals the following three trends:
• As the solution viscosity increases, the time of tow wetting increases.
• As the surface tension of the solution increases, the time of tow wetting decreases.
• As the contact angle increases from 0 0 �(complete wetting) to 90 0 �(mostly nonwetting),
the cosine term decreases and thus increases the time of tow wetting.
Prepreg residence time is also known to influence both the fiber dispersion and
efficiency. In a study by Lacroix et al. [20], ultra-high modulus polyethylene fiber bundles
were prepregged with a xylene/ low-density polyethylene solution. For a prepregging time
range of 8 min. – 19.5 hours, it was noted that increasing prepreg time increased the layer
thickness of deposited polymer around the fiber surfaces. Similar results were obtained in a
study by Moon et al. [19] in which solvent prepregged fiber bundles were prepared from glass
fibers and a high-density polyethylene/ toluene solution.
After the fiber tapes are prepregged with the nano-phased resin, the solvent has to be
driven off. In this case, since the tapes are not to be wound around a storage spool following
prepregging, solvent elimination should be complete. This represents a crucial step in the
overall composite manufacturing process, as residual solvent can result in voids during the
melt consolidation process. How the solvent interaction with the fiber/matrix/nanoparticle
interface is an important consideration, given the influence of the quality of the interface in
determining the final mechanical properties of the composite. The presence of solvent is
generally known to reduce the quality of the matrix/fiber interface. The reasons for this
phenomenon are unclear, but can be explained by the following hypothesis [21]:
• Solvent extraction can cause separation of the fiber/matrix interface
• Solvent concentration at the interface will interfere with fiber/matrix contact; and
• Phase separation of low molecular weight species at the interface may form a weak
interface between the fiber and matrix.
Solvent removal, in part, is regarded to proceed by solvent concentration at the interface,
followed by solvent traversing the fiber surface and escaping from the ends of the composite.
Obviously this will result in poor interfacial quality if this is to occur during melt
consolidation or autoclave processing, as the case maybe. A study conducted by Wu et al.
[22] illustrates how residual solvent negatively affects composite mechanical property
quality. Solution prepregged carbon fiber reinforced polyethersulphone composites were
prepared and compared with strictly hotmelt processed composites of the same nominal fiber