Initial Thermal Decomposition Mechanism of Un-degummed Silk

Abstract

A mechanism affecting the initial stage of thermal decomposition of Bombyx mori silkworm silk in an un-degummed state was elucidated using thermogravimetry (TG) and temperature programmed desorption – mass spectrometry (TPD-MS) for silk samples of three kinds, in which the amount of sericin relative to fibroin increased in the order of raw silk, silk floss, and sericin cocoon. Thermal decomposition occurred along with weight loss consistently at 170 °C in all three samples. Considering the TPD-MS results indicating simultaneously observed onset of H₂O evolution, the initial thermal decomposition was attributed to dehydration of serine (Ser) and asparatic acid (Asp) side-chains, which are predominant in sericin. The evolved H₂O can induce amide-bond hydrolysis, triggering main-chain decomposition. The progress of main-chain decomposition was monitored via TPD-MS by measuring the evolution of CO₂ and NH₃. Both the extent of the previously described H₂O evolution and the extent of amide-bond hydrolysis by the evolved H₂O affected the thermal decomposition rate during the initial stage. Such a decomposition rate was evaluated from the TG weight loss rate. The Ser and Asp residue contents increase along with the sericin ratio. This increase leads to accelerated H₂O evolution and amide-bond hydrolysis, resulting in a steeper decline gradient in the TG curve.