Raman study of the microstructure changes of phenolic resin during pyrolysis

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Abstract
After curing, phenol‐formaldehyde resins were post‐cured at 160°C, and then carbonized and graphitized from 300°C to 2400°C. The structure of the resulting carbonized and graphitized resins were studied using X‐ray diffraction and Raman spectroscopy. Thermal fragmentation and condensation of the polymer structure occurred above 300°C. The crystal size of the cured phenolic resins increased with an increase in temperature. The crystal size increased from 0.997 nm to 1.085 nm when the heat‐treatment temperature rose from 160°C to 500°C. Above 600°C, the original resin structures disappeared completely. Below 1000°C, the stack size (Lc) increased very slowly. The values increased from 0.992 to 1.192 nm when the heattreatment temperature rose from 600°C to 1000°C. Above 1000°C, the stack size showed an increase with the increase in heat‐treatment temperature. The values increased from 1.192 to 2.366 nm when the temperature rose from 1000°C to 2400°C. The carbonized and graphitized resins were characterized using Raman spectroscopy. The Raman spectrra were recorded between 700 and 2000 cm−1. Below 400°C, there were no carbon structures in the Raman spectra analysis. Above 500°C, G and D bands appeared. Raman spectra confirmed progressive structure ordering as heat‐treatment temperature increased. The frequency of the G band of all carbonized and graphitized samples shifted to 1600 cm−1 from the 1582 cm−1 of graphite. At the same temperature, the D band shifted to 1330 cm−1 from the 1357 cm−1 of the imperfect carbon. In the curve fitting analysis of the Raman spectra, a Gaussian shaped band centered at 1165 cm−1 was included. This band has not been described before in the literature and is attributed to disordered structures, which are formed from the original polymeric structures. These polymeric structures formed unknown disordered structures and remained in the carbonized phenolic resins. Above 1800°C, this band disappeared completely. But, a weak peak is present near 1620 cm−1. This indicated that those disoriented molecules and some disordered carbons were removed as volatiles or repacked into the glassy carbon structures during graphitization. The carbonized and graphitized phenolic resins were found to correspond to low order sp2 bonded carbon, but cannot be considered as truly glassy or amorphous carbon materials since they have some degree of order in the basal plane.