Polymerisation kinetics in a fibre reinforced resin-based composite

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Bluephase 20i ( G2 ) High power mode

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Figure 3 . a ) Spectral distribution ( exposure distances 0 , 2 , 4 , 6 , 8 and 10 mm ) and b ) Variation in irradiance delivered at exposure distances up to 10 mm ( Bluephase 20i ( G2 ), High power mode , n = 3 ).

in contrast , strongly dependent on specimen thickness , being about 4 times lower in 6-mm thick increments in comparison to the thin , 100- µ m increments ( Fig . 2 and Table 1 ). As for the mechanical properties , the flexural strength ( mean ± standard deviation ) amounted ( 128.30 ± 8.38 ) MPa , while the flexural modulus ( 8.38 ± 0.87 ) GPa . The properties measured at microscopic scale amounted ( 92.00 ± 15.86 ) N / mm ² for HV , ( 17.82 ± 1.82 ) GPa for Y HU and

( 3.35 ± 0.84 ) % for Cr . 3.2 . Curing unit characteristics The spectral distribution identified the used light curing unit as a violet-blue LED LCU , with two distinct peaks at 407 nm ( violet ) and 454 nm ( blue ). The spectral distribution measured at exposure distances of 0 , 2 , 4 , 6 , 8 and 10 mm is indicated in Fig . 3a . The incident irradiance ( Fig . 3b ) was identified as ( 1415.3 ± 22.2 ) mW / cm ². The irradiance increases significantly up to an exposure distance of 2 mm , when it reaches a value of ( 1496.3 ± 16.3 ) mW / cm ². Subsequently , the irradiance decreases exponentially with the exposure distance , amounting 78 % of the maximal irradiance at an exposure distance of 5 mm , while only 32 % at an exposure distance of 10 mm .

4 . Discussion This study analyzed the curing performance of a FRC for direct restorations , indicated to be used particularly in large posterior cavities , including cavities with 3 surfaces or more as well as deep cavities or such with missing cusps . Although the

FRC it is not specifically advertised as a bulk-fill resin composite , the manufacturer specifies that the material may be cured in larger increments . This statement can only partially be confirmed by the present study . While the DC measured at the end of the observation period ( 5 minutes ) revealed statistical similar values at the bottom of 2 mm and 4 mm thick increments , the situation changes when including in this comparison the top of a restoration . In this latter case , which was simulated in the present study by evaluating a 100- µ m thin increment , a decrease in DC was identified in 4-mm depths , while values measured in 2-mm depths were similar to the top . The attenuation in polymerization degree with depth is even clearly evidenced when involving the calculated polymerization kinetic parameters and the maximal rate of polymerization , as it will be discussed below . The analyzed material is a methacrylate-based dental restorative , containing monomers with two carbon-carbon ( C-C ) double bonds , namely the Bowen monomer bisphenol-Aglycidyldimethacrylate ( Bis-GMA ) and triethylene glycol dimethacrylate ( TEGDMA ), both able to build a three-dimensional polymer network . As generally known , the light-induced polymerization process is a radical one that initiates when photons emitted by the LCU activates the photo-initiator system . Applied exposure time and radiant exposure are both in line with manufacturer recommendations . The curing conditions involved in the present study an exposure time of 20s by using a modern LED LCU with an irradiance of ( 1415.3 ± 22.2 ) mW / cm ², which was placed directly and perpendicularly to the specimen ’ s surface . The radiant exposure (= incident irradiance x exposure time ) reaching the specimen ’ s surface was accordingly high , and amounted 28.3 J / cm ². These curing conditions are comparable or even superior to radiant exposure values identified for adequate polymerisation in conventional ( 21-24 J / cm 2 ) 11 , 12 or bulk-fill resin-based composites ( 20 J /

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cm ²). The second stage of the polymerization process , the propagation , is directed by the radical attack on Bis-GMA and TEGDMA monomers , leading to larger molecules ( chain growth ) by preserving the free radical . 15 This stage of polymerization was intended to be monitored in the present study by following the development of the DC over time . It must however be noted that the presented DC measurements evaluated the functional group conversion ( C-C double bond ) and not the monomer conversion . This stage of the polymerization process is described as a sequence of three distinguished phases : the quasistatic process , the gel phase , and the glass phase . The quasistatic process is described by a normal chain growth , while the number of high molecular weight chains increases and , consequently , also the viscosity of the composite . Next , the reaction rate increases during the gel phase while at a

Stoma Edu J . 2017 ; 4 ( 3 ): 164-170 http :// www . stomaeduj . com