Peroxide-controlled degradation of polypropylene using a tetra-functional initiator

Time/Molecular Weight Superposition to Describe the Behavior of Controlled-Rheology Polypropylenes

Polypropylene resins issuing from polymerization reactors are degraded by peroxides in subsequent reactive extrusion processes to improve their processability. This operation reduces their molecular weight and, thus, their viscosity and elasticity. In a previous study, a series of homo- and copolymer polypropylenes of different molecular weight distributions were prepared by twin-screw extrusion and characterized by oscillatory rheometry. It was shown that their behavior could be described by Carreau–Yasuda equations, possibly with a yield stress, in which all parameters depended on the weight average molecular weight. By using these experimental data, it is show in the present study that a time/molecular weight superposition allowed for a drastic reduction in the number of parameters to be considered in order to precisely describe the viscous behavior of these materials. This concept was then validated by applying it to various experimental data from the bibliography.

Rheological Properties and Melt Spinning Application of Controlled-Rheology Polypropylenes via Pilot-Scale Reactive Extrusion

Based on pilot-scale twin-screw reactive extrusion, the structural and rheological properties of controlled-rheology polypropylenes (CR-PPs) are investigated, where the effects of peroxide content and extrusion conditions such as screw configuration, extrusion temperature, and screw speed are prioritized. The active chain cleavage reaction by a small peroxide content of less than 600 ppm inside the extruder gradually increases the melt index and narrows the molecular weight distribution of CR-PPs, thereby affording favorable properties that are applicable to the fiber spinning process. The mechanical properties of CR-PPs are slightly degraded owing to the generation of unsaturated chain ends during the reactive extrusion, which suppresses crystal growth. Under all extrusion conditions, the chain scission and thermal degradation of polypropylene samples occur actively in the harsh twin-screw extruder compared with those in the mild twin-screw extruder. Finally, it is confirmed that CR-PPs can be suitably applied to the melt-spinning process for staple fiber production, thereby guaranteeing a more stable spinning process window against draw resonance instability.

Impact of rapid ozone degradation on the structure and properties of polypropylene using a reactive extrusion process

Schematics of rapid ozone degradation of polypropylene using a reactive extrusion process.

Rheological and Molecular Investigations of Polyethylene Degradation in a Batch Mixer

In this study, polyethylene degradation is related to its thermo-mechanical history from processing in a batch mixer. A focus is made on the rheological and molecular properties of the polyethylene subjected to different processing conditions by changing the temperature, the rotor speed and the time of process. The results indicate that while shear rheological properties changed slightly, the elongational properties were substantially modified, especially the maximum stress and the elongation at break. Due to the formation of macroradicals while processing, macromolecular changes were characterized in terms of molecular weight distribution and the results represent a balance between rates of chain scission and chain branching. It was found that the main parameter to characterize the degradation behaviour was the input power.