Development of Polylactide (PLA) and PLA Nanocomposite Foams in Injection Molding for Automotive Applications

RESUME Les materiaux plastiques sont largement utilises dans l'automobile, car ils permettent la fabrication de vehicules qui consomment moins d’energie. Actuellement, l'industrie automobile cherche des solutions de rechange a base de matieres bio-ressourcees et renouvelables comme alternative aux plastiques issus du petrole et ce, afin de reduire la dependance aux combustibles fossiles. Parmi les polymeres issus de sources renouvelables, le polylactide (PLA) a suscite un vif interet en tant que polymere vert tres prometteur. Aussi, l'utilisation de ce polymere dans les industries durables estelle en plein essor. Par ailleurs, les automobiles economiques en carburant sont aujourd'hui tres demandees en raison de l'inquietude croissante sur les questions environnementales. La consommation de carburant peut etre amelioree en reduisant le poids de l'automobile et en utilisant un materiau leger. Une methode potentielle pour atteindre cet objectif est l'utilisation de la technologie des mousses. La mousse est un materiau ou une phase gazeuse est encapsulee par une phase solide. Le processus de moussage consiste en l’incorporation de gaz, la formation d'une solution uniforme polymere-gaz, la nucleation cellulaire, la croissance cellulaire et enfin la stabilisation des cellules. La technologie de moussage permet de fabriquer des pieces allegees avec des proprietes superieures en comparaison avec leurs homologues solides. Cependant, le moussage du PLA s’est revele etre une tâche difficile principalement en raison des faibles proprietes rheologiques, d’une fenetre de mise en forme restreinte et d’une cinetique de cristallisation lente. Le but ultime de ce projet est de reduire de 30 % le poids des nanocomposites de polylactide (PLA)-argile par la fabrication de pieces en mousse moulees par injection. Afin d’utiliser un equipement de mise en forme standard, un agent de soufflage chimique (ASC) a ete employe dans le cadre de cette etude. La technique de moulage par injection a ete employee, car c’est le procede de fabrication le plus largement utilise dans l'industrie et qui peut produire des pieces de formes complexes. Cependant, ce procede est plus difficile que d'autres procedes de moussage car il possede de nombreux parametres supplementaires a controler. Dans la premiere partie de ce projet, nous avons illustre la facon dont la ramification a longues chaines (RLC) et la structure moleculaire influencent la rheologie a l’etat fondu, la cristallisation et le comportement moussant en discontinu du PLA. ----------ABSTRACT Plastic materials are extensively used in automotive structures since they make cars more energy efficient. Recently, the automotive industry is searching for bio-based and renewable alternatives to petroleum-based plastics to reduce the dependence on fossil fuels. Among polymers originating from renewable sources, polylactide (PLA) has attracted significant interest. The use of this polymer in durable industries is promising. Fuel-efficient automobiles are nowadays demanded due to the increasing concerns about environmental and fuel issues. The automobile fuel efficiency can be improved by using a lightweight material and, thereby, reducing the automobile weight. A potential method to achieve this objective is the use of the foaming technology. Foam is a material where a gas phase is encapsulated by a solid phase. Foaming technology helps to manufacture lightweight parts with superior properties in comparison with their solid counterparts. The basic mechanisms of foaming process normally consists of gas implementation, formation of uniform polymer-gas solution, cell nucleation, cell growth and, finally, cell stabilization. PLA foaming has, however, proved to be difficult mainly due to poor rheological properties, small processing window, and slow crystallization kinetics. The ultimate purpose of this work is to reduce by 30 % the weight of polylactide (PLA)-clay based nanocomposites by manufacturing injectionmolded foamed parts. To use standard processing equipment, a chemical blowing agent (CBA) was employed. The injection molding technique was utilized in this project because it is the most widely used fabrication process in industry that can produce complex shaped articles. This process, however, is more challenging than other foaming processes since it deals with many additional controlling parameters. In the first part of this project, we illustrated how long chain branching (LCB) and molecular structure impact the melt rheology, crystallization and batch foaming behavior of PLA. To this end, LCB-PLAs were prepared in the presence of a multifunctional chain extender (CE) using two different processing strategies. In the first strategy, the dried PLA was directly mixed in the molten state with various quantities of CE (the formation of LCB structure). To further examine the impact of CE and molecular topology, a LCB-PLA was also prepared using a second approach, strategy S2. In this approach, a highly branched PLA was first prepared and then mixed with the neat PLA at a weight ratio of 50:50 (the introduction of LCB structure).