Kinetic Modeling of Nylon 6 Depolymerization Under Hydrothermal Conditions

Abstract Nylon is the earliest developed material in the plastics industry and is currently the most widely produced engineering plastic. However, nylon is chemically stable and does not readily degrade in the natural environment. This study aims to study the depolymerization of nylon 6 under hydrothermal conditions using an autoclave reactor with variation of the reaction temperature and reaction time in the range of 260–340 °C and 0–60 min, respectively. The monomer products (caprolactam and aminocaproic acid) were obtained at 8:1, showing the equilibrium between these two molecules. The monomer yield increased with the reaction temperature and time, with negligible decomposition of the product. Because polymer has a repeating structure of the same unit, it is natural to consider the probability of decomposing one peptide bond to be the same as another one. This same probability results in the bond-breaking frequency in proportion to the number of remaining bonds. Then the rate of the bond breaking should be of first order. The first-order reaction model for the breakage of the peptide bonds adequately explained the depolymerization behavior. The reaction rate equation was expressed as an explicit function of time. The reaction rate was not affected by the degree of polymerization. The pre-exponential factor and activation energy for depolymerization of the bond were determined to be 0.002096 /s and 4.002 kJ/mol, respectively.