Raman study of the microstructure changes of phenolic resin during pyrolysis

After curing, phenol-formaldehyde resins were post-cured at 160 degreesC, a nd then carbonized and graphitized from 300 degreesC to 2400 degreesC. The structure of the resulting carbonized and graphitized resins were studied u sing X-ray diffraction and Raman spectroscopy. Thermal fragmentation and co ndensation of the polymer structure occurred above 300 degreesC. The crysta l size of the cured phenolic resins increased with an increase in temperatu re. The crystal size increased from 0.997 nm to 1.085 nm when the heat-trea tment temperature rose from 160 degreesC to 500 degreesC. Above 600 degrees C, the original resin structures disappeared completely. Below 1000 degrees C, the stack size (L-c) increased very slowly. The values increased from 0. 992 to 1.192 nm when the heat treatment temperature rose from 600 degreesC to 1000 degreesC. Above 1000 degreesC, the stack size showed an increase wi th the increase in heat-treatment temperature. The values increased from 1. 192 to 2.366 nm when the temperature rose from 1000 degreesC to 2400 degree sC. The carbonized and graphitized resins were characterized using Raman sp ectrocopy. The Raman spectra were recorded between 700 and 2000 cm(-1). Bel ow 400 degreesC, there were no carbon structures in the Raman spectra analy sis. Above 500 degreesC, G and D bands appeared. Raman spectra confirmed pr ogressive structure ordering as heat-treatment temperature increased. The f requency 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 ban d centered at 1165 cm(-1) was included. This band has not been described be fore in the literature and is attributed to disordered structures, which ar e formed from the original polymeric structures. These polymeric structures formed unknown disordered structures and remained in the carbonized phenol ic resins. Above 1800 degreesC, this band disappeared completely. But, a we ak 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 an d graphitized phenolic resins were found to correspond to low order sp(2) b onded carbon, but cannot be considered as truly glassy or amorphous carbon materials since they have some degree of order in the basal plane.