Light-cured materials, dental resins

For light-cured materials, the initiator system can be based on camphorquinone, so that cements can be cured with a conventional dental cure lamp emitting at a maximum wavelength around 470nm. Unlike formulations of composite resin, these materials cannot deploy amines as activators, because they would react with the carboxylic acid groups on the polymer, forming salts. Instead, a substance such as sodium p-toluene sulfinide is used as the activator. In addition, a photo-accelerator such as ethyl 4-NJ -dimethylamino benzoate is included [10]. 

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 resin component is ethylene urethane dimethacrylate, UDMA [40], a substance of the type used in conunercial composite resins. It contains two carbon-carbon double bonds and these are capable of undergoing addition polymerization. This particular reaction is triggered by the photo-initiators present when the material is exposed to light from a dental cure lamp. The calcium hydroxide component effectively serves as filler and is completely enclosed in the polymeric UDMA matrix... 

IKE 08] Ikemura K., Ichizawa K., Yoshida M. et al, UV-VIS spectra and photoinitiation behaviors of acylphosphine oxide and bisacylphosphine oxide derivatives in unfilled, light-cured dental resins , Dental Materials Journal, vol. 27, no. 6, pp. 765-774, 2008. 

IKE 10] iKEMURA K., Endo T., A review of the development of radical photopolymerization initiators used for designing light-curing dental adhesives and resin composites , Dental Materials Journal, vol. 29, no. 5, pp. 481-501, 2010. 

RUE 97] Rueggeberg F.A., Ergle J.W., Lockwood P.E., Effect of photoinitiator level on properties of a light-cured and post-cure heated model resin system . Dental Materials, vol. 13, pp. 360-364, 1997. 

The resin matrix of dental materials has important influence on the chemical and physical properties of light cure resins. The organic formulations also include photoiniti-ating systems that absorb light. From there free radicals start the conversion of the oligomer blend to a polymeric cross-linked network. Camphorquinone (CQ) is widely used in dental resin mixed with an amine. However, CQ is a solid, yellow compound with an unbleachable chromophore. 

The majority of the polymerization of a dental composite resin occurs very quickly, typically during the 20-40 s or so of light irradiation from the dental cure lamp. However, free radicals within the material do not terminate immediately the lamp switches off. Hence they are able to continue their propagation steps for some time after this initial cure, as growing polymer molecules containing free radical centres continue to incorporate extra monomer molecules [24]. Shrinkage, which is associated with polymerization, has been shown to continue for up to 24h after initial setting [25] in a process known as post-polymerization [26]. 

Acrylic resins, because of their desirable esthetics, ease of processing, optical clarity that can duplicate in appearance the oral tissues it replaces, satisfactory mechanical properties and excellent biocompatibility, are the materials of choice wherever plastics have found applications in dental practice. The ready acceptability of these materials is the result of the ease with which they can be converted into their final state even under clinical conditions. In practically all dental applications a liquid monomer-solid mixture is cured by a free radical initiated polymerization that is generated by heat, light, an initiator, or a redox initiator-accelerator system adapted to the constraints imposed by the oral environment. 

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