A kinetic study on crystallization of an amorphous lubricant

Abstract

Purpose: The purpose of the work was to study the crystallization kinetics of amorphous sterotex K at different temperatures and with different methods, and to compare the obtained kinetic and thermodynamic results.

Methods: Isothermal microcalorimetry was the principal technique to determine the crystallization behavior of sterotex K. X-ray powder diffraction and differential scanning calorimetry were employed to interpret the microcalorimetric heat flow curves and compare the kinetics and the calculated value for the transition enthalpy.

Results: The best way to analyze the x-ray diffractograms was the statistic Wakelin's correlation method. The method gave consistent quantitative results with microcalorimetry and both methods showed crystallization to consist of two partially parallel part-processes, which attached probably to the different growing rates of the forms consisting of different lattice planes having the same spacing. The quantitative analysis of differential scanning calorimetry scans was troublesome to perform but the obtained average value of -51 J/g for the enthalpy of the crystallization was in good agreement with the values obtained with the other methods, the average being -52 J/g. Calculating heat flow versus released energy plots from the microcalorimetric data the crystallization mechanism was observed to change as a function of temperature.

Conclusions: A sensitive isothermal microcalorimeter can give precise and fast knowledge about possible solid state transition mechanism, but the source of the heat flow signal must be verified with other analytical methods to avoid incorrect conclusions. Due to the sensitivity and the real-time data the microcalorimetry can show up the mechanism of the transition for varying as a function of temperature and reveal the uselessness of the conventional Arrhenius relationship for extrapolations.