engleski

Radiation crosslinking of unsaturated polyester resins

Radiation and thermally induced crosslinking of unsaturated polyester resin with styrene were followed by measuring DC electrical conductivity. The viscosity increased during the course of the reaction due to the formation of a three-dimensional network. The accompanying significant decrease of electrical conductivity was high enough to enable good sensitivity. Simplicity and good reproducibility of measurements are advantages of this method. Contrary to most conventional kinetic methods, physical changes of the system, (i.e., vitrification) did not limit the range of the measurements. The determination of gel content and free styrene content by extraction analysis, as a non-electrical method, showed that the electrical conductivity change is directly related to the extent of the crosslinking reaction. The obtained results proved that the electrical conductivity is a very promising method for following the course of radiation-induced polymerization and/or crosslinking reaction, especially because it is suitable for the direct in-source monitoring. The electrical field enhanced the crosslinking of unsaturated polyester resin, probably due to the favorable orientation of polar resin chains. That influence was confirmed by extraction analysis. It has to be pointed out that upper liquid-liquid transition Tlr, that is caused by breaking of intramolecular interactions in the polymer coil, was detected in the same temperature range where radiation-induced reactions were conducted. Liquid-liquid transitions in unsaturated polyester resins were investigated by electrical conductivity method. The influence of electrical field strength, solvents and crosslinking on liquid-liquid transitions made it possible to distinguish between lower, Tll, and upper, Tlr, liquid-liquid transitions and gave evidence on their inter- and intramolecular nature. Upper liquid-liquid transition was confirmed by differential scanning calorimetry (DSC). Near-infrared spectrophotometry indicate changes in hydrogen bonding of polymer chains in the temperature region of the transition where the collapse of short range intramolecular ordering based on hydrogen bonding took place. The results of DC-electrical conductivity monitoring of radiation- and thermally-initiated crosslinking of unsaturated polyester resins are interpreted using conductivity data themselves instead of commonly used logarithmic data form. Main setbacks of the logarithmic form of conductivity data were the shift towards longer reaction time as compared to non-logarithmic conductivity data and extraction analysis results. and pronounced scattering at the end of the reaction, so that it was difficult to detect vitrification point. By a revision of the approach to the analysis of results, full sensitivity of electrical conductivity method to structural changes in a reacting system was shown. The apparent rate constants calculated from conductivity itself showed the influence of upper liquid-liquid transition on the rate of radiation-induced reaction that could not be seen if the logarithm of conductivity were used. The influences of dose rate and electrical field on reaction rate were detected too and confirmed by DSC measurements.

unsaturated polyesters; viscosity; electrical conductivity; crosslinking; radiation initiation; electrical field effect; data processing upper liquid-liquid transition; local order; DSC; near-infrared spectroscopy; hydrogen bonding; extraction analysis

nije evidentirano