Industrial applications of fast scanning dsc: new opportunities for studying polyolefin crystallization

Abstract

This overview attempts to motivate and address the background of industrial interest in fast scanning thermal analysis methods with some of the most frequently applied methods and protocols being summarized. Although the utilized approaches and protocols are generic with respect to industrial type polymer resins, particular emphasis will be on polypropylene systems. The latter are characterized by relatively low crystallization rates and nucleation density, and hence, experimental fast scanning calorimetric techniques which are available on a (more) routine basis can provide significant added value. Of particular interest is in this case: To reach the isothermal crystallization temperature without any ‘disturbance’ of seed structures which can be present in efficiently nucleated systems during the early stages of crystallization before reaching the isothermal crystallization temperature. To obtain a disordered PP glass by applying cooling rates far above the crystallization rate, thus enabling studies on the crystallization mechanism under controlled conditions even during cold crystallization from low temperature. The insights provided by these type of approaches have been illustrated, based on examples of nucleated and non-nucleated polypropylenes, as well as a polypropylene/polyethylene blend and polypropylene impact copolymers. Fast scanning calorimetry enables to fully identify crystallization kinetics of the different formulations via the double clock-curve, in which the low-temperature curve is specifically attributed to homogeneous nucleation and mesophase formation whereas the high-temperature side is attributed to heterogeneous crystallization, as illustrated by the dominant effect of nucleating agents. The described results suggest in fact that the influence of heterogeneous nucleation in polypropylene is dominant, in comparison to influences arising from variations in molecular structures. In non-nucleated systems, on the other hand, the influence from molar mass and molar mass distribution on the crystallization process is moderate, compared to the influence of microstructure (tacticity). Since sample preparation provides some limitations with respect to applicability of the method, more recent developments involve the use of ultramicrotomed samples, permitting studies on samples displaying different thermal histories or in confined geometries, thus opening up new applications which are of high relevance for industrial practice. While polyethylenes fundamentally are more difficult to study by the currently available techniques owing to their exceptionally high crystallization rates, we address a relevant development in this area which in principle enables to study the microstructure of polyethylene copolymers. This is illustrated via a series of resins with identical macroscopic composition, but employing different catalyst systems.