In-line monitoring of resorcinol–formaldehyde gelation using fiber optic UV-vis spectroscopy in real time: part I

The sol-gel transition of resorcinol-formaldehyde (RF) solutions in the presence of sodium carbonate as a catalyst was monitored using in-line UV-vis measurements in real time by observing the absorbance changes at various gelation temperatures using a fiber-optic probe. The two absorbance maxima at around 340-400 and 500 nm were caused by different resorcinol derivatives and <i>O</i>-quinone methide intermediates, respectively, which were formed during the gelation. The absorbance was measured until visible turbidity appeared. The turbidity time (<i>t</i> _turb) was determined, and it was found to decrease with increasing gelation temperatures. The general absorbance behaviour and the trend in absorbance value variations in correlation with time and at three different wavelengths (460, 485 and 510 nm) were studied for different gelation temperatures, and the observed correlation was presented and discussed for each temperature. The research method developed in this investigation helps in understanding and optimizing the synthesis process of RF aerogels on a laboratory scale. A simple transfer to process monitoring in industrial production seems possible.