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 Trc of the unmodified and unbranched sample by the torque maximum in the rheometer, representing the tear off of the stamp from the sample. Trc 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 Trc was much higher in comparison to a gas loading.
Highlights
Polylactide (PLA) is often referred as one of the most promising biopolymers as it possesses similar properties to polystyrene, but has a lower carbon footprint [1,2]
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
The specific effects of CO2 at elevated pressure on the reactivity between the polymer and the chemical modifiers 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
Summary
Polylactide (PLA) is often referred as one of the most promising biopolymers as it possesses similar properties to polystyrene, but has a lower carbon footprint [1,2]. By the use of these modifiers, chain extension, branching and/or expected, depending on functionality and concentration [4,5,6]. The recycling industry uses chemical modification during reactive extrusion to counteract chain degradation at high temperatures and the resulting change of properties [7]. A change of the polymer topology, caused by extension, branching or crosslinking, influences the rheological behavior of the polymereither melt. Unlike the other parameters the solved gas affects the vertical shift factor b, due to change of the concentration [17] This vertical shift factor is defined as: with the concentration c, fractional free volume f and the contribution of the dissolved gas to the increase of free volume θ. PLA is modified with two different commercially available modifiers and analyzed via rotational rheology with and without the presence of high-pressure CO2
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