Y. Li et J. Unsworth, EFFECT OF PHYSICAL AGING ON DIELECTRIC, THERMAL AND MECHANICAL-PROPERTIES OF CAST-EPOXY INSULATORS, IEEE transactions on dielectrics and electrical insulation, 1(1), 1994, pp. 9-17
Samples of cast bisphenol-A and cycloaliphatic epoxy insulation were q
uenched from a temperature above T(g) (170-degrees-C) to room temperat
ure. Variations in dielectric, thermal and mechanical properties were
examined during subsequent sub-T(g) annealing (physical aging). It was
found that the impact strength of the epoxy insulation decreased cons
iderably with physical aging. Moisture intake and subsequent dielectri
c loss at room temperature were also found to decrease with physical a
ging. Glass transition temperature and excess enthalpy measured by DSC
demonstrated significant increase with increased sub-T(g) annealing.
On the other hand, dielectric properties showed different changing pat
terns during sub-T(g) annealing in different epoxy systems. It is conc
luded that epoxy insulating materials should be properly annealed befo
re assessing their dielectric and other physical properties because of
the possibility of unknown rapid cooling histories.