Resumen

ALBANO, C; PAPA, J; BARE, W  y  GONZALEZX, J. Avrami equation inadequacy for modeling the hdpe isothermal crystallization ratesFallas de la ecuación de avrami en el modelado de velocidades de cristalización isotérmica de pead. Rev. Fac. Ing. UCV [online]. 2007, vol.22, n.1, pp.71-78. ISSN 0798-4065.

The temperature dependence of the crystallization rate of polymers has been related to the classical nucleation and growth process. At temperatures slightly below the melting point, the controlling step is the nucleation process while, with largeundercoolings, the rate control shifts toward transport restrictions. With increasing degrees of crystallinity, a shift of the controlling step from nucleation towards transport resistances was also observed, which is frequently associated with a secondary crystallization process. This behavior has also been explained in terms of a separate geometric spreading of the semi-crystalline superstructure followed by an increasing local crystallinity. The Avrami equation is known to be a good model for the first process but rapidly fails with the appearance of a secondary crystallization process and the development of transport resistances. In this work, results of the crystallization rates of HDPE from 118 to 115°C are presented and analyzed of the light of the Avrami equation. It was observed that this model begins to fail with increasing degrees of undercooling and crystallinity. The degree of fitting of the Avrami model, was analyzed processing different ranges of relative crystallinity at four different temperatures, and results are interpreted in terms of the possible rate controlling process. With the Avrami equation a good data fitting at 118ºC was obtained, but it was observed that for increasing degrees of undercooling and increasing degrees of crystallinity the degree of fitting deteriorates. It was found that restricting the use of the Avrami equation within the region of experimental data where it is more likely that the nucleation and the growing process of the semicrystalline superstructure are the controlling steps, very consistent values for Avrami parameters were obtained. Simulation results suggest the presence of a secondary crystallization process for which the Velisaris-Seferis parallel model was found satisfactory.

Palabras clave : HDPE; crystallization; Avrami model; data fitting; Velisaris-Seferis model.