Polymerisation kinetics in a fibre reinforced resin-based composite

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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