Thermally stimulated current in glass-laminated unsaturated polyester resins

Experimental results are reported on the electrical properties of glass-laminated unsaturated polyester (UP) resins. Measurements of the poling current as a function of temperature reveal two different activated processes. Above 50°C the activation energy is 160 kJ/mol and has been associated with ionic condition, possibly by protons, while at lower temperatures (below −70°C) the activation energy is very low (less than 40 kJ/mol) and is associated with dipole relaxation. It is concluded that the laminated UP resin undergoes a phase transformation between low and high temperatures, which gives rise to a completely different electrical response. This phase transformation can be associated with the glass transition of UP resin phase. In view of the low activation energy, giving rise to a polarization peak, it is concluded that the low temperature range (glassy state) favors strictly localized motions of charges—most likely electrons. At higher temperatures, the behavior corresponds to d.c. conductivity and is associated with extended motions of charge carriers with limitations at contacts and at internal barriers. According to the air gap measurements, the pronounced dip found in the temperature dependence of the poling current (in the range from 20 to −30°C) may be due to separation of conduction and dipole relaxation current. Thermally stimulated depolarization (TSD) current measurements reveal the presence of high internal fields due to space charges accumulated at the phase boundaries of glass fibers.