Multiscale modeling of thermoplastic PC/ABS blends

Abstract

Rubber-toughened thermoplastic polymers such as PC/ABS blends exhibit a pronounced plastic dilatancy in their overall deformation behavior which results primarily from damage mechanisms in the ABS phase while PC behaves plastically incompressible, e.g. [1]. In this contribution, two commercial PC/ABS blends of different composition and morphology are approximated through appropriate unit cell models featuring the two constituents PC and ABS as distinct phases. Different visco-plasticity models are utilized to capture the specific deformation behavior of each phase. Numerical studies of the unit cell models looking at the effect of the blend composition on the overall response are compared with experimental data. 1 PC/ABS blends, blend morphologies and unit cell models PC/ABS blends are composed of the thermoplastic polymers polycarbonate (PC) and acrylonitrile butadiene styrene (ABS). These blends combine the strength of PC with the fracture toughness of the rubber-toughened ABS [1]. ABS is overall plastically dilatant due to the underlying micromechanisms rubber-particle cavitation, void growth and crazing. PC, in contrast, behaves plastically incompressible.Two commercial PC/ABS blends, Bayblend T45 and Bayblend T85, of covestro AG are considered in this work. Bayblend T45 features a PC/ABS ratio of 45/55 wt% while T85 has a 70/30 wt% PC/ABS ratio. The different compositions are reflected in the uniaxial stress-strain response as well as the dilation behavior. Bayblend T85 exhibits a higher overall stress response which can be attributed to its higher PC content (Fig. 1). A more pronounced dilation is found for Bayblend T45 caused by its higher ABS content (Fig. 2).Owing to differences in composition PC/ABS blends display different morphologies. In PC-rich blends (e.g. Bayblend T85) typically the ABS is dispersed as particles in the PC matrix whereas a co-continuous (interpenetrating) phase morphology is observed in blends with approximately equal PC and ABS content such as Bayblend T45 [2]. Assuming idealized periodic microstructures in the present work, the different morphologies are approximated by the unit cell models shown in Fig. 3. For the matrix-particle structure of the PC/ABS = 70/30 blend a body-centered cubic (BCC) arrangement was chosen (Fig. 3 a). The co-continuous morphology of the PC/ABS = 45/55 is approximated with an interpenetrating phase (IPP) structure (Fig. 3 b). 2 Constitutive modeling of the individual phases and parameter calibration For modeling of the individual PC and ABS phases two qualitatively different visco-plasticity models are utilized. The meanwhile standard model for neat glassy polymers by Boyce and coworkers [4] is chosen for the PC phase. Due to the purely deviatoric plastic part of the rate of deformation tensor this material model behaves plastically incompressible. The model's parameters were calibrated to experimental data fom [5] and the characteristic softening upon yield as well as the progressive hardening for large strains are well reproduced (Fig. 4).The behav...