Extensional rheology, cellular structure, mechanical behavior relationships in HMS PP/montmorillonite foams with similar densities

Abstract

The main goal of this work is to analyze the relationships between the extensional rheological behavior of solid nanocomposites based on high melt strength polypropylene (HMS PP) and montmorillonites (MMT) and the cellular structure and mechanical properties of foams produced from these materials. For this purpose two systems have been analyzed. The first one incorporates organomodified MMT and a compatibilizer and the second system contains natural clays and is produced without the compatibilizer. Results indicate that the extensional rheological behavior of both materials is completely different. The strain hardening of the polymer containing organomodified clays decreases as the clay content increases. As a consequence, the open cell content of this material increases with the clay content and hence, the mechanical properties get worse. However, in the materials produced with natural clays this relationship is not so clear. While no changes are detected in the extensional rheological behavior by adding these particles, the nano-filled materials show an open cell structure, opposite to the closed cell structure of the pure polymer, which is caused by the fact of having particle agglomerates with a size larger than the thickness of the cell walls and a poor compatibility between the clays and the polymer.

Effect on the Impact Properties of Adding Long‐Chain Branched Polypropylene in Polypropylene‐Polyolefin Elastomer Cellular Polymers Produced by Core‐Back Injection Molding

“…For example, Zhao observed strain hardening for a binary blend of LCB‐PP and POE, but they used a much higher amount of LCB‐PP; the minimum content was 50 wt%. [ 59 ] Our results prove that in a ternary blend, with only 20 wt% LCB‐PP content, the addition of the same amount of POE only reduces slightly the strain‐hardening coefficient. As a result, blends with a significant strain hardening and a high extensional viscosity were obtained when LCB‐PP and POE were blended with L‐PP.…”

“… where is the extensional viscosity as a function of time and Hencky strain rate, and is the extensional viscosity in the linear viscoelastic regime, which is obtained by extrapolation of the superimposed portion of the curves for different elongation rates. [ 59,60 ] Figure shows the strain‐hardening coefficient for the two materials including LCB‐PP in its composition.…”

Linear viscoelasticity, extensional viscosity, and oxygen permeability of nanocomposites based on propylene copolymer and organoclay

“…However, when the network is deformed by the flow, at ɛ > 1.2–1.5, the degree of interactions between particles may be reduced by deformation and orientation of the relatively flexible tactoids. Moreover, the displacement and deformation of the tactoids by the flow may impose some shear flow in the polymer surrounding the filler particles, reducing the extensional deformation of the macromolecules and contributing to the observed decrease in viscosity at large Hencky strains …”

“…Furthermore, it has been generally observed that nanoparticles also increase the melt strength and induce strain hardening of molten PPs . On the other hand, a few authors have reported a decrease of the melt strength of PP and/or strain softening at large strains …”

Sorption behavior and microcellular foaming of low-density polyethylene/clay nanocomposites by gas dissolution under sub-critical conditions

“…35,36 However, an opposite behavior, where a reduction of the strain hardening is observed, has been also found in literature, mainly when working with branched polymers. 26,37 The cellular structure of the foamed materials is highly affected by the extensional rheological properties of the polymer matrix. By increasing the polymer strain hardening, it is possible to increase the expansion ratio, to reduce the average cell size, and to decrease the open cell content.…”