Abstract
In this work, virgin as well as thermally degraded branched polypropylenes were investigated by using rotational and Sentmanat extensional rheometers, gel permeation chromatography and different constitutive equations. Based on the obtained experimental data and theoretical analysis, it has been found that even if both chain scission and branching takes place during thermal degradation of the tested polypropylene, the melt strength (quantified via the level of extensional strain hardening) can increase at short degradation times. It was found that constitutive equations such as Generalized Newtonian law, modified White-Metzner model, Yao and Extended Yao models have the capability to describe and interpret the measured steady-state rheological data of the virgin as well as thermally degraded branched polypropylenes. Specific attention has been paid to understanding molecular changes during thermal degradation of branched polypropylene by using physical parameters of utilized constitutive equations.
Highlights
Polyolefins are one of the most globally produced and widely used polymeric materials in the marketplace
Newtonian model; local increase in shear viscosity, elasticity, relaxation time and molecular weight of local increase in shear viscosity, elasticity, relaxation time and molecular weight of the the coil at 5 h of thermal degradation could be explained by the presence of a lower number of coil at 5 h of thermal degradation could be explained by the presence of a lower number of branches branches where some of them have higher molecular weight in comparison with the 3 h degraded where some of them have higher molecular weight in comparison with the 3 h degraded sample
3 h, increase to the local maximum decrease the degradation time withintime first 3within h, thenfirst increase up to the localupmaximum at five hours at five hours and again indicatingoccurrence simultaneous occurrence of theand chain scission and and decrease againdecrease indicating simultaneous of the chain scission recombination recombination reactions
Summary
Polyolefins are one of the most globally produced and widely used polymeric materials in the marketplace. The most widespread representative of polyolefins is polypropylene (PP) for its higher melting point, lower density, chemical resistance, rigidity, impact strength and low cost For these properties, the PP can be used in many manufacturing processes such as compression, foaming, thermoforming, blow moulding, injection moulding, rotational moulding and extrusion coating. Model parameters’ identification process is rather complicated due to high mathematical complexity and huge number of adjustable parameters, which usually require their manual adjustment Both models are based on the assumption that branched macromolecules take the “H” shape, which can result in poor model predictions, especially for PPs having different, e.g., star-like, structures [60]. Specific attention was paid to quantification and interpretation of the measured data by using four different constitutive equations allowing manual-adjustment-free determination of their parameters as well as quantification/characterization of molecular changes during thermal degradation of branched PP
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