Phase behavior and reaction of nylon 6/6 in water at high temperatures andpressures

The phase behavior and reaction of nylon 6/6 in water were studied with a d iamond anvil cell (DAC) technique and visual microscopy. Nylon 6/6 concentr ations in water and cell temperatures were varied from 11 to 46% and from 2 64 to 425 degrees C, respectively. The pressures studied ranged from 30 to 900 MPa. When an aqueous solution of 27% nylon 6/6 was rapidly heated (2.6 degrees C/s) to 372 degrees C at 30 MPa, the solution became homogeneous at 331 degrees C. Upon cooling, the final pressure was 30 MPa and both partic les and gas were observed, Analysis of the particles by Raman indicated dec omposed nylon 6/6 solid. When an aqueous solution of 31% nylon 6/6 was rapi dly heated (2.9 degrees C/s) to 425 degrees C at 58 MPa, the solution becam e homogeneous at 323 degrees C. Upon cooling, the final pressure was 143 MP a, and, remarkably, only a second Liquid precipitated and no gas or solids were observed. From the experiments, we concluded that the reaction pathway s are completely different between the subcritical and supercritical water conditions. For the case of subcritical conditions, the final products were solid particles having a nylon character along with a considerable amount of gas. At supercritical water conditions, the final products were liquids having little nylon character and no gas. Experiments were performed at, a constant temperature of 272 degrees C at initial pressures ranging from 87 to 400 MPa. As the reaction proceeded, the pressure was measured at 30-s in tervals. At average pressures less than 300 MPa, the nylon 6/6 samples melt ed and appeared to become homogeneous, At average pressures higher than 520 MPa, the nylon 6/6 samples remained heterogeneous, From these results, the rate of hydrolysis was concluded to increase with pressure. The reaction v olume was found to be -21.1 cm(3)/mol, which can be explained by the overal l formation of water-soluble products. (C) 2000 John Wiley & Sons, Inc.