Phase behavior and reaction of polyethylene in supercritical water at pressures up to 2.6 GPa and temperatures up to 670°C

Phase behavior and reaction of polyethylene (PE) in supercritical water were studied with a diamond anvil cell (DAC) technique with visual and Raman spectroscopy. When PE+water (12–30% PE ) mixtures were rapidly heated at initial pressures ranging from 110 to 690 MPa, PE first melted and formed a liquid spherule PE phase. The spherule began to expand at above 450°C and underwent a color change to red at about 570°C. At higher temperatures, the red color disappeared and the PE molten phase turned transparent. Upon further heating, the red color returned and other material underwent homogenous reaction as evidenced by a dark color which appeared throughout the cell. Volatile liquids were formed on the surface of the liquid PE phase spherule. For reactions run at higher temperature (645–671°C) at pressures ranging from 1.9 to 2.6 GPa, thin films formed on the anvils after quenching which had CC, OH, and CC Raman bands, which indicated that hydrolysis products formed even though the reaction times were relatively short (290–475 s). Reactions performed at a constant temperature of 423°C and at an initial pressure of 850 MPa showed only a slight decrease (0.03 MPa/s) in pressure with time. The results of this study show, conclusively, that PE and water remain as a heterogeneous system over the polymer (12–30% PE) compositions studied during heating and reaction in supercritical water. Only after PE decomposes to lower molecular weight hydrocarbons, above about 565°C, can homogeneous reaction conditions result.