Resumen de Preparation and characterization of reactive extrusion modified pla/abs blends and its foams
Mohammad Reza Kamrani Moghadam
The current thesis takes place within the context of the projects MAT2016-80045-R "Aplicaciones industriales de compuestos y mezclas basados en REX-PLA" and the project "PLAIABS Blends: Recydability and weight reduction" financially supported by the Ministerio de Economía y Competitividad of the Spanis h Government and SEAT res pectively.
The main objective was to improve the properties of poly lactic acid (PLA) in arder to make it suitable for long-lasting applications where emphasis is de\Qted to the reduction of carbon dioxide emissions and the renewable raw materiaIs content. Among the different routes to enhance PLA properties, blending was used, as it is easily scalable bythe industry. Dueto its good balance of properties, ABS was considered a suitable polymer to be used together with PLA However, dueto their immiscibili ty, a coupling agent (ABS-g -MAH) was introduced.
Apart of brittl ness, another PLA problem is its relatively easy degradation during process ing. Previous studies of the Group indicated that PLAdegradation could be limited using a multi epoxide reactive agent that a110ids the loss in molecular weight and increases the PLA melt visc osity, which, on the other hand, facilitates processing . Moreover, it was believed that the unreacted chain extender epoxide groups could also form covalent bonding with the coupling agent improving ABS and PLA compatibility.
Using foams both the total weight and carbon dioxide generation of components can be diminished. For these reasons, different techniques for physicallyfoaming the blends either during injection (MuCell) or through a batch process were object of research. The effect of the above mentioned chain extender and the effect of a tale filler on the batch-foaming behavior of PLAwas studied.
The first step of the research consisted in obtained the base materiaIs by reactive extrusion (REX) at a pilot plant seale (50 kg.). A conventional injection molding process was then employed to obtain standard samples that were used for the thermo mechanical characterization ofthe blends.
The thermal stabilitywas studied bythe Kissinger 's method. The results showed that it was improved when the amount of ABS-g-MAH was increased. From the DSC analysis, it was found that in comparison to more perfect REX-PLA structures, crystals with a smaller lamellar thickness were formed in the blends. The differences in crystallinitywere small and do not influence the mechanical properties. The presence of ABS in the compositions led to a decrease ofthe blend 's storage modulus. Nonetheless, the ABS phase was found to playthe role of scaffold for the PLA phase that can undergo re crystallization, being thus able to support a certain amount of loading during heating.
The physical foaming ofthe different injected blends was difficult to be achieved. The optimization ofthe foaming conditions required several triaIs and a large quantityof material. As the amount of each blend was limited, only a partial number of the proposed experiments were possible. The obtained blends contained a few large cells as well as a certain population of small sized cells. Although a complete micro-cellular structure was not achieved, the obtained one was similar to that of previous works found in literature. lt was, however, demonstrated that this technique can be used byindustryto generate lighter components.
Batch foaming using CO2 in supercritical conditions resulted in successful foams with cell sizes below 100 microns and very high cell densi ties. In comparison to PLA, dueto its higher viscosity and melt resistance, using REX-PLA resulted in improved level offoaming which prevents from cell coalescence and collapse. The presence of ABS in the blends allowed broadening the range of pressure and temperatures in which foams can be produced. Tale was found to actas a nucleating agent leading to smaller cell sizes and higher cell densities in the foamed blends.
