Wiktor L. Gambin
Polymerization rate, thermal analogy, dental layers, C-factor
Test results show that shrinkage stresses in cured dental layers depend on C-factor (bounded/unbounded surface ratio). To check it, a polymerization of set of thin resin-composite layers earlier tested by Alster et al. was modelled in the present work. To find shrinkage stress fields, the Maxwell model with time dependent parameters was used . Values of linear shrinkage, Young's modulus, and viscosity, were taken as the input data from the work by Dauvillier et al. The calculated shrinkage stresses were compared with the test results. To find a reason of the discrepancies between the calculated and tested stress values, the assumptions of the model were reconsidered. Based on the thermal analogy, it was taken that a degree of polymerization of the layers depends on their C-factor. The new assumption alludes to case of non-uniform temperature drop due to a faster heat transfer at the free surface of cooled element. As a consequence, the input data given by functions a(t) take the form a(t)ยทf(C), where f(C) is an unknown parameter. The stresses calculated on the basis of the modified input data compared with the test results enable to get parameter f(C). In this way, the measured and calculated stresses became the same. The method was used for the analysis of shrinkage stresses in dental layers filling the Class I cavities. It was found that for the restorations of small diameters the shrinkage stresses can be relatively high.
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