Effects of Nickel on The Acceleration of Radiation-Induced Decomposition of Polytetrafluoroethylene (PTFE)

<b>Introduction</b><br/>High-performance plastics, such as PTFE (Polytetrafluoroethylene) and PEEK (Polyether ether ketone), have extensive applications in various industries. However, recycling and reusing these plastics pose challenges due to their exceptional heat resistance. This study presents our findings on a new method to reduce the decomposition temperature of PTFE. The method involves utilizing the catalytic effects of Nickel (Ni) for the radiation-induced decomposition of PTFE.<br/><br/><b>Materials and Methods</b><br/>PTFE (Mw=5,000~20,000) and Ni/NiO/Al₂O₃/SiO₂ powders were purchased and used. The PTFE or the mixture of PTFE with Ni was irradiated with an electron beam under air or argon gas. The maximum dose of the electron beam was 15 MGy, and the dose rate was measured using a cellulose triacetate (CTA) film dosimeter. Mixtures containing different mixing ratios of PTFE and Ni catalyst were prepared to investigate the impact of proportion of catalyst. The mixing time was varied before and after irradiation to observe the sequential effects of irradiation and catalyst.<br/>To perform quantitative analysis, the weight change of samples resulting from radiation was measured, and the solid residue remaining after irradiation was collected. The thermal decomposition of the samples with and without irradiation was measured using thermogravimetric differential thermal analysis (TG-DTA) from room temperature to 1000°C under argon gas. The gaseous products via thermal treatment were collected and introduced into a gas chromatograph-mass spectrometer (GC-MS) for their identification. Furthermore, the chemical groups were measured using Fourier transform infrared spectroscopy (FT-IR), the crystal structure changes were measured using X-ray Powder Diffraction (XRD), and the binding energy changes were measured using X-ray Photoelectron Spectroscopy (XPS), as a function of irradiation and temperature.<br/><br/><b>Results and Discussions</b><br/>Both the use of Ni catalyst and irradiation accelerated the reduction of the decomposition temperature of PTFE. Increasing the proportion of Ni catalyst and irradiation dose had a positive effect on decomposition. In addition, the synergistic effect of Ni catalyst and irradiation had the greatest impact on the decomposition. In the presence of oxygen, when mixing was followed by irradiation, the 5% weight loss temperature of the solid residue was 103°C, which is 419°C lower than the pyrolysis temperature of pure PTFE (522°C). Under 15 MGy irradiation, approximately 40% of the PTFE equivalent experienced weight loss below 200°C.<br/>During the irradiation, several changes were observed in the PTFE. In PTFE alone, the presence of carbonyl groups (-CO-) was confirmed, and GC-MS analysis indicated the occurrence of oxidation (C_nF_2n to C_xF_yO_z). These oxidative substances are responsible for the decomposition of irradiated PTFE at lower temperatures. There were no changes in crystal structure or binding energy (C 1s, F 1s). In the presence of Ni, the presence of -CO- was also confirmed, but the gaseous products detected in GC-MS decomposed into smaller fragments (C_xF_yO_z to CO₂/COF₃). Additionally, new crystalline structures were confirmed in the irradiation process involving oxygen. The binding energy of C 1s, Ni 2p3/2 remained unchanged, but a new peak of F 1s (F^-, 685 eV) was confirmed, indicating the production of F^- from the original PTFE (p(CF₂-CF₂), 689 eV) during irradiation. In summary, the presence of oxygen played a crucial role in the degradation of PTFE, and we propose that Ni acts as a catalyst to accelerate the decomposition of PTFE.<br/><br/>Acknowledgment: This work was supported by JST, CREST Grant Number JPMJCR21L1, Japan. The authors would also like to thank the operators of the Takasaki electron beam irradiation system for their cooperation.