Kinetics of perlite glasses degassing: TG and DSC analysis

Abstract

Kinetics of dehydration is one of the important characteristics of natural water-bearing glasses. This study deals with the simultaneous TGA/DSC of perlites. The dehydration profiling of natural perlite samples from Eastern Rhodopes (Bulgaria) and a pitchstone from the Eastern Transbaikal region (Russia) has been obtained by the use of a thermal balance. Initial water content in samples varied between 4.0 to 6.5 wt% H₂O. In pitchstone TG and DTG signals indicate two degassing maxima at temperatures of 220 and 290 °C. The peaks in the DTG signal mimic those in DSC signal. Perlitic water is driven out smoothly under heating over a wide ränge of temperatures from 140 to 700°C up to the glass transition temperature (Tg). The glass transition temperature has been determined independently from viscosity measurement using a micropenetration method. In slowly cooled perlites DTG and DSC Signals have one distinct peak which extends from 140 up to 800 to 850 °C. In fast cooled perlites DTG and DSC signals often exhibit two distinct peaks at around 200 to 450 and 600 to 700 °C. The temperature ränge of DTG and DSC peaks is wider than in slowly cooled perlites. The enthalpy effect of perlite glass dehydration at 200 to 300 °C has the enthalpy value of 55 to 75 kJ/mol, which is much larger than the enthalpy value (30 to 25 kJ/mol) of water evaporation at these temperatures. The additional thermal effect of periite dehydration may be associated with the water desorption reaction and water vapour expansion in microcracks.