Effects of material properties and particle morphology on toughening of plastics.

Abstract

The effects of material properties and particle morphology were studied both phenomenologically and analytically, using finite element method (FEM). The objective was to establish quantitative guidelines/model for prediction of toughening performance. Polycarbonate (PC) and DGEBA type of epoxy were used as matrices. Paraloid$\sp\circler$ EXL-2691 and EXL-3300 core-shell rubbers, as well as microgel particles made from mixtures of butylacrylate, methyl methacrylate, and ethyl methacrylate, were used as tougheners. Additionally, polyethylene was also used for its poor miscibility with PC to represent an extreme case. Fracture tests were performed to evaluate the fracture toughness. Toughening mechanisms were determined by optical and electron microscopy techniques. Then, based on the phenomenological results, FEM models were constructed for subsequent studies. A unit-cell model was used for studying the effects of material properties on toughening mechanism. Also, a modified unit-cell, which encompasses two particles in the unit cell, was developed to probe the effects of particle morphology on local stress state and energy absorption. A first neighbor interaction concept and a strain energy model were proposed for the correlation of morphology with toughening performance. Along the way, two spatial parameters were defined for particle dispersion characterization. Our phenomenological results confirmed that (1) the intrinsic matrix properties dictate the extent of toughening and the choice of mechanism, and (2) the final toughness enhancement is determined by the matrix and the toughener-related parameters. The analytical results showed that, during voiding, both the hydrostatic and deviatoric stresses increase. However, the deviatoric stress component builds up much more quickly than the hydrostatic counterpart. But voiding alone only changes the local stress state, and does not guarantee shear yielding and/or ductile failure behavior. The strain energy model was demonstrated to be able to correlate morphology with toughening performance. Consequently, the model can be used for toughness prediction and optimization. The two spatial parameters were proven capable of characterizing the particle dispersion pattern. Moreover, the relative angle index is suitable for probing directional anisotropy.