Unraveling the multi-scale pyrolysis mechanism in the melt-pyrolysis coupling process of polyamide 4: A high-melting bio-based plastic with green chemical recycling potential, Chemical Engineering Journal, 2026,546,179908.

Kai Si, Mingda Wang, Guanfeng Lyu, Jiayu Mou, Tao Chen, Yongjun Qiu, Liming Zhao*

Polyamide 4 (PA4) is entirely bio-based and fully biodegradable, offering significant environmental advantages. However, its exceptionally narrow melt-pyrolysis processing window poses a critical challenge that limits thermal processing and hinders large-scale applications. To achieve this goal, the overlapping process of physical phase transformation and chemical depolymerization was decoupled. This clarification of the microscopic pyrolysis path and reaction mechanism establishes a theoretical foundation for advancing thermal processing. Gaussian deconvolution of the DTG curves resolved the main mass-loss process into three overlapping apparent kinetic contributions. The findings reveal a decreasing trend in the average activation energies of these pseudo-components, measuring 102.41, 94.56, and 74.80 kJ/mol, respectively. Combined with in situ variable-temperature XRD and kinetic analysis, these contributions were associated with progressive structural relaxation before the main mass loss, rapid degradation following the substantial loss of long-range crystalline order, and the subsequent degradation of residual species, respectively. The pyrolysis reaction pathway of PA4 was analyzed using density functional theory (DFT) for the first time to investigate its microscopic chemical essence of multi-stage degradation. The analysis revealed that the energy barrier of the intramolecular interlocking ring path (Path 2) is notably lower than that of the hydrolysis-induced link breaking path (Path 1). Thus, it was determined that the intramolecular interlocking ring pathway serves as the primary route for the decomposition of PA4. This research delved into the thermal decomposition mechanism within the coupling process of phase transformation and depolymerization across various scales, offering a theoretical foundation for realizing large-scale thermal processing.

https://doi.org/10.1016/j.cej.2026.179908.