Characteristic of low-temperature pyrolysis of printed circuit boards subjected to various atmosphere

Low-temperature pyrolysis (LtP) has always accompanied to the recycling and treatment of printed circuit board (PCB), which would generate pollutants. In order to better understand this process and develop methods of mitigating its deleterious environmental consequences, a thermo-analytical and kinetics study on PCBs was conducted by thermo gravimetry (TG) analysis under various atmospheres and focused on low-temperature scope. Meanwhile, this paper compares literature sources about how the characteristics are exhibited of LtP on PCBs scrap using TG or TG-IR (infrared spectroscopy) experiment. The LtP process consisted mainly of three distinct phases: (a) minor mass loss but without chemical reaction, (b) slight mass loss with adhesives decomposition, and (c) rapid degradation and char decomposition from resin. The initial temperature of third phase for PCBs pyrolysis occurred in the range of 290–300 °C using a heating rate of 10 °C min−1 under N2 or air atmosphere. While the activation energy resulting in mass loss (second phase) are found to be 127.87 and 115.36 kJ mol−1 under N2 and air atmosphere, the activation energy is 53.59 kJ mol−1 under vacuum by kinetics analysis, of which the initial temperature leads to rapid degradation is decreased into 270 °C. Vacuum condition significantly improves the outcome of the pyrolysis process. With a comparison on literature sources subjected to N2 or air atmosphere, while the products released during the first phase were mainly H2O and CO2, the flame retardants and other plastic additives were decomposed or volatilized, releasing small-molecule products within the second phase, including HBr, H2O, CO2 and molecular hydrocarbons (methane, ethane, and butane). Gases and liquid substances were released from resin decomposition during the third phase; it is indicated that complete pyrolysis and carbonization occurred. The gases products are also very rich in hydroxyl-benzene and other brominated aromatics. Liquid by-products consist mainly of phenol, alkyl phenol, bisphenol A, water, and various brominated phenols.