99℃. Propylene glycol is physical and chemical, with boiling point 188.2℃, melting point 59℃, closed cup flash point 99℃ and open cup flash point 107℃. Closed cup flash point refers to the closed cup flash point when the sample begins to generate steam after being heated in a closed cup. Under certain conditions, ignite the fire source on the cover, and record the temperature when the combustion stops.