Authors: F. Pla, R. Rached, S. Hoppe, C. Fonteix
Affilation: Nancy University, France
Pages: 885 - 888
Keywords: PA12/PDMS nanosturured blends, reactive extrusion, process design, multiobjective optimization
The important demand of innovative materials implies the development of new processes and concepts for their design and optimization. This work deals with the manufacture, by reactive extrusion, of Polyamide 12 (PA12)/ Polydimethylsiloxane (PDMS) nanostructured blends, with the purpose of improving the mechanical properties of PA12. In this process, the continuous phase (PA12) is synthesized in the extruder in the presence of (i) the dispersed phase (PDMS), (ii)a new and particularly efficient catalytic system composed of aluminium hydride as catalyst and MDI caprolactam as activator and (iii) a macro-activator (functionalized PDMS: dicarbamoyloxy caprolactam-PDMS) well suited to develop in situ a tri-block (PA12-PDMS-PA12) copolymer acting as compatibilizer. The originality of the process is based on the simultaneous synthesis of both PA12 and the compatibilizer by anionic polymerization of lauryl lactam (LL). This gives rise to blends in which the elastomer particles (PDMS) are dispersed and stabilized at a nano-scale. The mechanism of formation of the resulting dispersion differs completely from the classical ones in which polymers are directly mixed in the presence of a compatibilizer. Figure1 clearly shows the morphological difference between two PA12/PDMS blends of the same composition obtained by these two different processes. Before implementation of the extrusion procedure, the study comprised (i) the synthesis and the characterization of the macroactivator, (ii) the kinetics analysis of the two anionic polymerizations which, owing to the catalytic system used, allowed obtaining a complete conversion of the monomer in a reaction time similar to the residence time in the extruder. Preliminary reactive extrusion experiments were then carried out to study the effect of the operating conditions on the final properties of the blends. The process was finally studied owing to a D-optimal experimental strategy, completed by using a rotatability criterion. The experimental results allowed then elaborating empirical models able to predict the effect of the main operating variables [Screw rotation speed (N), PDMS feed rate (Q), concentration (F) and number average molar mass (Mn)], on the corresponding blends properties data [LL conversion (X), average diameter of the dispersed phase (Dn), Young modulus (E), melting temperature of the matrix (Tf), Charpy impact energy (Eimp). These models were then used in a multiobjective optimization procedure which allowed, (figure 3), determining the best operating conditions for the production of blends for which all specific properties were maximized except the diameter of the dispersed phase which was minimized. This optimization used genetic algorithms and developed: (i) Pareto’s concept, which allowed determining a large number of non-dominated solutions, (ii) ranking of these solutions according to the preferences expressed by decision-makers. This work led finally to a significant improvement of the mechanical properties of the resulting blends , particularly their impact energy at low temperature.