Rheology in the Presence of Carbon Dioxide (CO2) to Study the Melt Behavior of Chemically Modified Polylactide (PLA)

Dominik Dörr; Tobias Standau; Svenja Murillo Castellón; Christian Bonten; Volker Altstädt

doi:10.3390/polym12051108

Rheology in the Presence of Carbon Dioxide (CO₂) to Study the Melt Behavior of Chemically Modified Polylactide (PLA)

Polymers (Basel). 2020 May 13;12(5):1108. doi: 10.3390/polym12051108.

Authors

Dominik Dörr¹, Tobias Standau¹, Svenja Murillo Castellón², Christian Bonten², Volker Altstädt^{1

3}

Affiliations

¹ Department of Polymer Engineering, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.
² Institut für Kunststofftechnik, University of Stuttgart, Pfaffenwaldring 32, 70569 Stuttgart, Germany.
³ Bavarian Polymer Institute, Universitätsstraße 30, 95447 Bayreuth, Germany.

Abstract

For the preparation of polylactide (PLA)-based foams, it is commonly necessary to increase the melt strength of the polymer. Additives such as chain extenders (CE) or peroxides are often used to build up the molecular weight by branching or even crosslinking during reactive extrusion. Furthermore, a blowing agent with a low molecular weight, such as carbon dioxide (CO₂), is introduced in the foaming process, which might affect the reactivity during extrusion. Offline rheological tests can help to measure and better understand the kinetics of the reaction, especially the reaction between the polymer and the chemical modifier. However, rheological measurements are mostly done in an inert nitrogen atmosphere without an equivalent gas loading of the polymer melt, like during the corresponding reactive extrusion process. Therefore, the influence of the blowing agent itself is not considered within these standard rheological measurements. Thus, in this study, a rheometer equipped with a pressure cell is used to conduct rheological measurements of neat and chemical-modified polymers in the presence of CO₂ at pressures up to 40 bar. The specific effects of CO₂ at elevated pressure on the reactivity between the polymer and the chemical modifiers (an organic peroxide and as second choice, an epoxy-based CE) were investigated and compared. It could be shown in the rheological experiments that the reactivity of the chain extender is reduced in the presence of CO₂, while the peroxide is less affected. Finally, it was possible to detect the recrystallization temperature T_rc of the unmodified and unbranched sample by the torque maximum in the rheometer, representing the tear off of the stamp from the sample. T_rc was about 13 K lower in the CO₂-loaded sample. Furthermore, it was possible to detect the influences of branching and gas loading simultaneously. Here the influence of the branching on T_rc was much higher in comparison to a gas loading.

Keywords: carbon dioxide; chain extender; chemical modification; peroxides; polylactide; polylactide acid; pressure cell; rheology.

Abstract

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