Composite Materials Research Progress

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W.H. Zhong, R.G. Maguire, S.S. Sangari et al.
For aramid and UHMWPE fibers, silane coupling treatments (effective for glass fibers)
and oxidation treatments (effective for carbon fibers) are not effective in improving the
interfacial strength [1-6]. For UHMWPE fibers, many treatment methods including nitrogen
ion implantation, nitrogen plasma, fast atom beams, laser ablation, chain disentanglement,
high power ion beam treatments, and cold plasma [7-13] have been used. Such approaches
show some improvement in interfacial properties, but also can degrade the mechanical
properties of the fibers by damaging the chain structure of the UHMWPE fiber and result in
formation of amorphous hydrogenated carbon. Recently, the effectiveness of an atmospheric
plasma was demonstrated for dramatic improvement in the adhesion of polyetheretherketone
(PEEK) composites to epoxy [14]. This atmospheric plasma was shown to be an important
potential strategy for improving the interfacial adhesion between organic fibers and polymer
resins.
A nanotechnology approach was recently developed by Dr. Zhong’s group in which
conventional epoxy resins are converted into reactive nano-epoxy resins. Unlike the
conventional epoxy resins that require carbon fibers to be surface oxidized/treated before
being impregnated, the nano-epoxy resins contain reactive graphite nanofibers which can
improve the wettability and adhesion properties between UHMWPE fibers and the resin
matrix [15-19].
Next Generation Carbon Fibers – Continuous Nanoscale Carbon Fibers Traditional Carbon
fibers have high strength, high modulus and attractively low density. The high strength-toweight
ratio combined with superior stiffness has made carbon fibers the material of choice
for high performance composite structures in the aerospace, defense and other industries.
Polymer fibers, which leave a carbon residue and do not melt upon pyrolysis in an inert
atmosphere, are generally considered candidates for carbon fiber production. It is known that
the structural perfection of precursor fibers is the most crucial factors on the strength of
carbon fibers. Imperfections (such as surface defects, bulk defects and others) in the precursor
fibers are likely to be translated to the resulting carbon fibers, and the amounts and sizes of
structural imperfections directly determine the final fiber strength. The fundamental
approach/solution for improving the strength of carbon fibers is to reduce the amounts and
sizes of numerous types of defects in the precursor and there has been a clear and continuing
trend among commercial carbon fiber suppliers in achieving higher strengths through this
approach.
There is also a growing interest in having thinner plies of composite material without loss
of strength or stiffness and thinner plies means thinner fibers. Historically the means of
producing very small diameter (down to submicron range) has been electrospinning. Since the
1930s electrospinning has been used on nylon and other polymers to achieve small fibers to
provide filtering media and other applications. In the process a strong electric field acts on a
polymer solution resulting in a polymer stream which solidifies through the evaporation of
the solvent.
This can also be applied to a polyacrylonitrile (PAN) copolymer (precursor) to produce
nanofibers with diameters in the nanoscale range with the potential of ultimately producing
continuous nano-scaled carbon fibers with strengths and stiffness much higher than
conventional micro scale carbon fibers. Additionally, since diameters of the electrospun PAN
nanofibers can be further reduced by stretching and carbonization processes, the resulting
nano-scaled carbon fibers can have diameters of less than 100 nm. When incorporated into a