Thermal analysis of zinc oxide-eugenol cements during setting.

It has been accepted traditionally that the setting of zinc oxide-eugenol (ZOE) cements is the result of the formation of zinc eugenolate crystals.1 A recent study pointed out that at least seven hours elapsed before zinc eugenolate crystals precipitated from a clear solution of zinc acetate in eugenol.2 Accelerated ZOE, however, set in as short a time as three minutes.3 Electron microscopy, electron diffraction,2 and X-ray diffraction4 studies indicated that a set ZOE mix may contain less than 2% zinc eugenolate. Even that low concentration is doubtful because the studies did not identify clearly whether the diffraction patterns were due to a crystalline or amorphous phase. Differential thermal analysis of set commercial ZOE did not show the characteristic transition of zinc eugenolate on melting.5 These studies suggest that the setting of ZOE cements may not be because of the formation of a zinc eugenolate phase. Various studies on the same system indicated that the addition of o-ethoxybenzoic acid,6 large amounts of zinc acetate dihydrate (accelerator),3 or zinc eugenolate powder to standard ZOE mixes improved the mechanical properties of the hardened mass. The purpose of this investigation was to establish the characteristic thermal transitions of ZOE materials, to compare them to those of zinc eugenolate, and to study the effect of adding o-ethoxybenzoic acid, large amounts of accelerator, and rosin on the formation of zinc eugenolate.