Polymeric materials light sources

With the light curing mechanism, there is a limitation to the penetration of the light. The dentist may have to place a restoration that is 6 + mm thick, whereas the light may penetrate only 2 mm [182]. Factors that affect this penetration are the translucence of the material, the color or shade used to match the tooth, the ability to place the light source close to the material being polymerized, and the intensity of the source. Under relatively ideal conditions, the mean depth of cure is approximately 4-5 mm. Thus, the dental application requires that the material be placed in layers. Due to the oxygen inhibition of the outside surface of the resin layers, additional layers can be laminated and cured with the appearance of uniformity of the final restoration. 

The object is the second part of our triad. Here we will discuss how materials interact with the energy from the light source. In this book we consider objects made of polymeric materials. Generally, polymers are colorless or at best weakly colored, and the aim always seems to be to cover up the undesirable color of the polymer in favor of the more desirable color selected by the designers. This requires the addition of colorants (pigments and dyes) to the polymer. The subject of colorants will be covered in depth in another chapter. However, because colorants become part of the object, we will first discuss some aspects of how they interact with light. 

UV radiation originating from sunlight or artificial light sources results in substantial degradation of many commercial polymeric substrates. Degradation manifested as discoloration, embrittlement, cracking, and loss of other physical properties seriously reduces the useful lifetime and potential applications of polymeric materials. 

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