Page 54 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2. Characterization of interfaces 37

K = dl - (!y (2.21)

= sin-ld- n2 - 1 .
(2.22)
n2 - a2

F(4, K) and E($, K) are elliptical integrals of the first and second kind, respectively.
n can be plotted versus L for a range of small values of contact angle, 8. By
measuring the relative dimensions of the droplet, XI and x2, as illustrated in
Fig. 2.21, 8 can then be evaluated (Carroll, 1976).
Nardin and Ward (1987) successfully used this method to evaluate surface
treatment for polyethylene fibers. A linear correlation was observed between the
fiber ys measured by this method using a glycerol contact angle and the interfacial
shear strength measured from fiber pull-out tests for chemically treated fibers.
Wagner and coworkers (Wagner, 1990; Wagner et al., 1990) extended the droplet
aspect-ratio method by introducing a computer program based on an initial estimate
of 8, which allows the above equations to be solved iteratively, giving a more
accurate value of 8.
Experimental evidence on composite materials has shown that the methods of
contact angle measurement are useful in detecting changes in carbon fiber surface
energy due to oxidative treatments, which can enhance the composite interlaminar
shear strength (ILSS). The surface energy values increase with increasing surface
concentrations of oxygen and nitrogen-containing groups, as determined from AES
and corresponding ILSS of the composite laminates (Gilbert et al., 1990), as shown
in Table 2.7. In a similar study, differences in wetting behavior of sized/unsized and
surface treated/untreated carbon fiber immersed in a number of different thermo-
plastics (Weinberg, 1987) as well as in commercial silicon oil and epoxy resin (Lee
et al., 1988) were observed by a wettability study. There is excellent correlation
between the contact angle and composite transverse flexural strength for carbon
fiber-PEEK matrix composites (Bucher and Hinkley, 1992).
In contrast to carbon fibers, no simple correlation has been reported between the
work of adhesion to various polymer resins determined from the contact angle

-

Fiber' 'I

Fig. 2.21. A liquid droplet attachcd to a monofilament. Gilbert et al. (1990).