New aspects on the mechanism of the solid state polyamidation of PA 6,6 salt

Hexamethylenediammonium adipate (PA 6,6 salt) was solid state polymerized in the temperature range of 158-190 °C, in a thermogravimetric analysis (TGA) chamber, which simulates a polyamidation reactor. A mechanism based on the role of the volatile component of the salt (i.e. hexamethylenediamine) is found to predominate: the diamine escapes along with polycondensation water, meanwhile this volatilization occurs earlier than the water formation, apparently resulting in an increase of the vacancy defects and of the nucleation sites in the salt crystals. In addition, critical reaction parameters, such as reaction time, temperature, surrounding gas and presence of catalyst in the starting material were investigated, so as to discern the rate-controlling mechanism of the process. Finally, proper SSP kinetics were studied taking into consideration the diamine loss occurred and SSP rate constants were calculated through a suitable rate expression.