Anisotropy of Composites: Mechanical and Solvent-Swelling Characteristics

INTRODUCTfONHE SWELLING BEHAVIOR of unidirection short fiber/elastomer composites in T some aromatic solvents has been reported by Coran et al. [ I ] using rectangular shaped specimens.The nature of swelling was interpreted in terms of t h e mechanical properties (e.g., Young's Modulus) of composites as a restrictive force against the swell induced by the solvent/composite interaction.Halpin and Pagano [2] have shown that it is possible to predict the dimensional strain of a nylon-reinforced elastomer swollen in benzene within the experimental limits by employing Schapery's [3] formulae.In this paper, the swelling behavior of well-bonded and oriented composites containing various levels of treated cellulose fibers imbedded in a vulcanized EPDM matrix will be studied in an aromatic solvent.Circular shaped specimens were used in order to observe the characteristic anisotropy.Then the resulting oblong shape of the swollen specimen is indicative of the directional reinforcement, and may be expected to correlate with the mechanical behavior of the unswollen materials in a quantitative manner.The swelling ratio, measured by t h e eccentricity of the elliptical shape, can be empirically correlated with the mechanical anisotropy.A mathematical derivation based on t h e elasticity equations applied to composite materials will also be compared with the experimental results.The mechanical properties will be further correlated to fiber concentrations using the well-known Halpin-Tsai equation 141 as well as an empirical form based o n Takayanagi's equation 151.Since the configuration of such a swollen specimen will be approximately elliptical, a mathematical equation describing an ellipse may be employed to estimate the angle dependence of the mechanical properties.The calculation can be made once the major/minor axes of the swollen sample, as well as the corresponding mechanical properties before swelling, are known.A more appropriate method using the transformation of swelling expansion as a second order tensor will also be described.