STEFANI Pablo Marcelo
capítulos de libros
P.M. STEFANI; S.M. MOSCHIAR; M.I. ARANGUREN
Homolytic and heterolytic reactions. Problems and solutions
Nova Science Publishers
Lugar: NY; Año: 2004; p. 233 - 259
The reaction kinetics and the curing process of an epoxy-urethane copolymer in a heated mold were modeled. The phase separation process was studied and used to analyze the final morphologies. The final thermal, mechanical and fracture properties were correlated to the corresponding morphologies. The system selected for the study was an epoxy resin (DGEBA, Araldyt GY9527, Ciba Geigy), co-reacted with an urethane prepolymer (PU, Desmocap 12, Bayer) through an amine that acted as crosslinking agent (mixture of cycloaliphatic amines, Distraltec). The kinetic study focused on the effect of the curing conditions and PU concentration on the reaction rate. The global kinetic parameters included the initial chemical control and further diffusional step as the sample gets close to the glass transition temperature. The copolymerization with PU reduced the activation energies of the chemical and diffusional controlled parts of the reaction and increased the final conversion of the epoxy amine system. The kinetics model was applied to the analysis of the curing of a plaque, time-temperature profiles measured in the mold and the consequent final morphologies obtained. Phase separation is a function of the conversion and temperature reached in the curing part and was analyzed using experimental data and a mathematical model that predicts temperature and conversion throughout the thickness of the mold. The size of the dispersed phase increased with the initial PU concentration while there were practically no differences as a function of position in the mold or oven. The superposition of the phase diagrams with the conversion-temperature trajectories during cure allowed to explain the morphologies generated. Moreover, the addition of PU proved to lead to the usual trend of improved toughening and reduced modulus and compression yield stress. This result is correlated to a morphology consisting in a fine rubber particle dispersion, but also to the ductilization effect of the flexible elastomer copolymerized to the epoxy amine system. Key Words: urethaneepoxy copolymers, curing kinetics, diffusion control, phase separation, mechanical properties.