Physical Properties of the Zinc Oxide-Eugenol Impression Pastes

¹ University of Michigan, School of Dentistry, Ann Arbor, Michigan

Several physical and clinical properties of the commercially available zinc oxide impression pastes were studied. An attempt was made to correlate the values obtained from bench tests for initial set and consistency with the clinical behavior of the pastes in regard to these properties. Correlation of the values obtained in the final set test using the Krebs penetrometer and the Gillmore needle was examined. The qualities of the set impression were investigated in relation to hardness, flow properties, separation from the cast, and detail reproduction. Initial set times of the fifteen products tested varied from 1.5 to 4.0 minutes. Nine of the fifteen products had an initial set value of less than 3 minutes. The final set times of the fifteen products determined with the Gillmore needle ranged from 3.0 to 15.0 minutes. Comparison of the values obtained in the rod test for initial set with the clinical behavior of these pastes showed a close degree of correlation. The initial set times as determined represented the time available in which the paste has to be mixed, loaded into a tray, and the impression registered. A comparison of the final set times determined with the Krebs penetrometer and the Gillmore needle shows a correlation sufficiently close for the purpose to which the values are applied. The values obtained in the consistency test varied from 27 to 50 mm. When these values were compared with the clinical behavior of the pastes, it was found that pastes with a disk size in the consistency test greater than 37 mm. flowed freely from the spatula and that those with a disk size less than 37 mm. did not flow from the spatula. Initial set time may influence the disk size obtained in the consistency test. A short initial set time may give a smaller disk size than the consistency merits, as compared with a product of similar consistency but having a longer setting time. For this reason, disk sizes in the consistency test cannot be used as a basis of comparing the relative consistency of two pastes unless they have similar initial set times. From the clinical point of view, the zinc oxide impression pastes can be divided into three groups according to the nature of the flow from the spatula: (a) flows freely, (b) flows slowly, and (c) does not flow. As measured with the Krebs penetrometer, the hardness of the set impression paste in seven products varied from 0.5 to 3.2 mm. Those products described as hard set had values of 1.0 mm. or less, whereas those known as soft-set products had a value of 1.5 mm. or more. The percentage flow of the set impression paste measured 5 minutes after final set for eight products ranged from 3.8 to 47.3 per cent. The hard-set products had values of 7 per cent or less, and the soft-set products had values of 8 per cent or greater. At the end of 1 hour the percentage flow values were considerably reduced, and after 24 hours a further hardening was noted. All fourteen products tested separated satisfactorily from the stone cast. All fourteen products reproduced the finest line on the test impression block satisfactorily, and when the impressions were poured in stone, the lines were all reproduced on the cast.