06-09-2012, 05:28 PM
BASIC SCIENCES AND APPLIED PHYSICS
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INTRODUCTION
The goal of dentistry is to maintain or improve the quality of life of dental patients and this requires the replacement or alteration of tooth structure and selection of biocompatible, long lasting , prosthetic material that can withstand most of the conditions in oral cavity.
Amorphous / non crystalline phase:
No regular crystalline arrangement.
Higher viscosity and plastic deformation.
on heating softens to form liquids.
Eg: dental impression compound,fused porcelain ,glass,waxes.
do not have a definite melting temperature ,but rather they gradually soften as the temperature is raised.
Glass transition temperature:
The temperature at which there is abrupt increase in the expansion coefficient, indicating increased molecular mobility .
characterized by the particular glassy structure. Below this, the glassy structure loses its fluid characteristics and has significant resistance to shear deformation.
Viscoelasticity
Materials that have mechanical properties dependent on loading rate .
these materials have characteristics of an elastic solid or a viscous fluid.
agar, alginate, elastomeric, impression materials and waxes
increasing the loading (strain) rate produces a different stress-strain curve with higher rates giving higher values for the elastic modulus, proportional limit and ultimate strength.