Effect of crystalline structure of polypropylene random copolymers on mechanical properties and thermal degradation kinetics

Polypropylene-random copolymers are a relatively new type of polypropylene (PP) modified plastics, which were introduced over the last years on the market. The comonomer which is most commonly used is ethylene at random concentrations for the improvement of mechanical and thermal properties of the final material. For the present work, we characterized and compared the mechanical properties and the thermal degradation kinetics results of polypropylene-random copolymer (PP-R) with 7% ethylene content and polypropylene-random copolymer/enhanced crystalline structure/improved temperature resistance (PP-RCT), which is formed by using a special beta-nucleation process, which leads to a homogenous dispersion of equal sized beta-crystals. The tensile strength properties of PP-RCT are improved due to the presence of both alpha- and beta-crystals, making the final product more ductile than neat PP or its random copolymer (PP-R), which both consist exclusively of alpha-crystals (monoclinic system). Characterization with XRD and DSC confirmed the two crystalline phases of PP-RCT. Thermogravimetric (TG) studies of PP-RCT showed that the new material exhibits better thermal stability than neat PP-R, since 1% mass loss of PP-RCT appeared at temperatures 10 degrees C higher than neat PP-R. The activation energy of degradation of PP-RCT and PP-R was calculated using the Kissinger, Akahira and Sunose (KAS) method. Degradation took place into two stages; the first stage which corresponded to an initial small mass loss was simulated with an n-th order model (F-n), while the second stage was attributed to the main degradation mechanism of the material and was simulated with an n-th order model with autocatalysis (C-n). When the calculated activation energies with isoconversional and model-fitting methods are compared, PP-RCT has much higher activation energy than PP-R, a fact which indicates the higher thermal stability of PP-RCT. (C) 2012 Elsevier B.V. All rights reserved.