Page 219 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surface treatments ofjibers and effects on composite properties 20 1

Table 5.11
Effect of surface modification on pull-out force of PPTA silicone rubber composite"

Surface modification Pull-out force (g/cm)
Plasma treatment' Coupling agent treatment'

No No 458 & 5.8
No Yes 1240 =k 63
Yes No 1250 f 50
Yes Yes 3120 f 34

"After Inagaki et al. (1992).
bPlasma treatment: exposed to oxygen plasma at 150 mA, 26.6 Pa for 2 min.
'Coupling agent treatment: coupled with silicone adhesive TSE322 at 150°C for 1 h

et al., 1992). It is suggested that oxygen plasma treatment removes some carbon-rich
surface layers of the Kevlar fiber, hence exposing the nitrogen-rich layer. The high
carbon content on the fiber surface is considered to be detrimental to chemical
bonding with the liquid resin.
In summary, novel techniques have yet to be evolved to further improve the
interfacial bonding with aramid fibers. Modification of aramid fibers to produce
chemical functionality or improved wettability based on chemical agents or plasma
treatment warrants much further research before applications to commercial
production can be realized. In particular, the importance of wettability is evidenced
by the fact that higher interface bond strengths are obtained with epoxy resins in the
order of uncoated (Le. chemically reactive and wettable), gold-coated (chemically
inert but thermodynamically wettable) and silicone coated (inert and non-wettable)
Kevlar fibers (Kalanta and Drzal, 1990b). An optimum surface condition should
always be chosen after a compromise with the mechanical properties of fiber, as the
fibers are sensitive to severe damage by the attack received during the surface
modification process. This is particularly true with aramid fibers which are
characterized by the skin-to-core inhomogeneity coupled with extremely low
transverse and compressive strengths. This has a practical implication in that a
strong interface bond does not always guarantee the best mechanical performance,
unless the inherent microstructure is properly modified to produce a more
homogcneous and isotropic material (Kalanta and Drzal, 1990a).

5.4.2. Ultrahigh modulus polyethylene ,fibers

Several high modulus, high strength polyethylene fibers are commercially
available, including Spectra (Allied Signals), Dyneema (DSM/Toyobo), Tekmilon
(Mitsui Petrochemical), Snia (Snia Fibers) and Celanese (Celanese Research). They
are produced via a gel-spinning process where the low concentration solution of
ultrahigh molecular weight polyethylene (UHMWPE) (M > 2 x IO6) is extended to
form gel precursor fibers. The precursor fibers are then hot drawn to produce very
highly oriented fibers with an extended chain fibrillar microstructure, as schemat-