Evaporation in addition impression silicones

Abstract

Summary. --The effect of weight loss on the dimensional stability of some addition-polysiloxanes used as impression materials in dentistry is studied. We show that polymerization is not the only factor affecting the shrinkage of these materials. An explanation for the time-dependent weight functions is given.PACS 81.70 -Materials testing. PACS 87.90 -Other topics in biophysics, medical physics, and biomedical engineering. Polysiloxane-based impression materials, which cure by addition reaction, are considered materials with superior properties in dentistry [1]. Several works report that they show little dimensional change with time [2][3][4], when compared with the polysulphides and condensation silicones. This is because the addition polymerization does not produce volatile by-products [5,6]. Recently, it has been suggested [7] that the shrinkage is a complicated function of time: the early shrinkage, that proceeds exponentially for very few hours, is affected by the polymerization reaction. However, depending on the sample thickness, the long-term shrinkage, which is not exponential, finishes after one or more days. The knowledge of all the factors affecting the dimensional variation is closely related to the accuracy of the impressions and is a condition required to improve the material quality.In this work the possible evaporation of volatile components was tested by monitoring the weight of addition impression silicones. Their weight loss with time can be ascribed to evaporation. To confirm this phenomenon, we tested the weight variation of samples having similar shapes but different thicknesses. In contrast with the polymerization, which does not depend on thickness, the ratio W/W1 (where W = = W1 -W2, W1 is the sample weight measured at zero-time, W2 is the weight after t time) is larger for thin samples, compared with the thick ones, in the early stages of the evaporation.The influence of evaporation on the dimensional stability was assessed by examining the linear shrinkage as a function of time, for materials showing different

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