| Polycarbonate (PC) is a high-performance amorphous engineering thermoplastic with exceptionally high impact strength, clarity, heat resistance, and dimensional stability. Makrolon 3208 PC, which is produced by Bayer-Miles, is known to change its mechanical and physical properties with thermal history, particularly when it has been heat-aged at 120°C. The purpose of this work is to study the stress relaxation behavior of Makrolon PC heat-aged at 120°C. Stress relaxation tests were conducted for unaged and heat-aged Makrolon PC using a Dynamic Mechanical Analyzer (DMA) to determine the change in relaxation modulus and characteristic relaxation time. Thermo-mechanical dynamic tests were also performed using the DMA to determine the change in the primary (alpha) and secondary (beta,gamma) transitions. The stress relaxation results were modeled using the Kohlrausch-Williams-Watts (KWW) relation and superimposed into TTS (Time-Temperature Superposition) and ATS (Aging time-Time Superposition) master curves using the principle of time-temperature and time-aging time equivalence. The application of TTS and ATS methods has shown that curve fitting using the three-parameter KWW expression is more accurate than the two-parameter Maxwell expression. These master curves can predict long term relaxation behavior of the Makrolon PC at any elevated temperature. The results show that after heat-aging at 120°C, the stiffness of Makrolon PC increases and the stress relaxation rates decrease. The secondary relaxation (beta) intensity decreased significantly without affecting the alpha, beta and gamma transition temperatures. |
