International audienceIn the present work, experimental studies of the free-radical-initiated molecular weight degradation of polypropylene in a modular self-wiping corotating twin-screw extruder are investigated. The control of the molecular weight distribution of polypropylene resins by peroxide degradation is widely used in the polymer industry. It allows one to adjust the viscosity of these resins to the level required for processing applications. The purpose of this work was to characterize the influence of peroxide degradation on the rheological behavior of a polypropylene homopolymer and a block polypropylene/polyethylene copolymer, which includes an addition of a low percentage of polyethylene (around 7%). The homopolymer exhibits a classical behavior: When the peroxide amount is increased, we observe a decrease in the viscosity corresponding to a decreasing molecular weight and a pronounced shift toward more Newtonian behavior. The rheological behavior of the copolymer is influenced by the presence of the polyethylene phase which greatly modifies the viscoelastic properties and increases the viscosity when the polypropylene matrix is highly degraded

This work describes a systematic study of the compounding conditions by twin-screw extrusion on the defibrization process that modulates the fiber aspect ratio and in turn the mechanical properties. Composites made of polycaprolactone reinforced by 20% hemp fibers were prepared by melt blending. The influence of the extrusion parameters (screw speed, feed rate, barrel temperature, and screw profile) and the initial fiber moisture content on both the fiber dimensions and the mechanical performance of the composite was investigated. The fiber aspect ratio increased when the fibers were water plasticized, principally at a higher feed rate and under a moderated extrusion temperature. The screw speed slightly influenced the fiber dimensions. Flow modeling was used to estimate the specific mechanical energy provided to the fibers, which ranged from 300 to 1700 kWh/t. Independent of the screw profile, a decrease in fiber length with an increase in energy was observed. The evolution of the fiber length and aspect ratios with respect to energy can be accurately described by an exponential function.The defibrization process concomitantly results in decohesion and fragmentation phenomena of the fiber elements. When fracture occurs in the interfiber cement (i.e., by degumming of the middle lamella connecting fibers), the process is called decohesion. Fragmentation occurs when the intrafiber structure integrity, i.e., the cell wall, is damaged. Defibrization is therefore related to fracture, and initiation sites and propagation pathways can be influenced by intrinsic parameters related to the quality of the fibers as well as by external factors (e.g., machining, 9 stress mode, 10,11 etc.)At the molecular level, some factors that influence the viscoelastic properties, such as the water (moisture) content and

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In this work, experimental and theoretical studies of the free‐radical initiated molecular weight degradation of polypropylene in a modular self‐wiping corotating twin screw extruder have been investigated. Our objective was to build a model that would be able to predict the evolution of the average molecular weight along the screws, in relation to the processing conditions and the geometry of the twin screw extruder. Modeling the process involves resolving interactions occurring between the various flow conditions encountered in the extruder, the kinetics of the reaction and the changes in viscosity with changes in molecular weight. We have studied the influence of operating parameters such as the initial peroxide concentration, the feed rate and the screw speed on the degradation reaction. Good agreement was found between theoretical results and experimental values obtained by size exclusion chromatography measurements.

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