STOMATOLOGY EDU JOURNAL 2017, Volume 4, Issue 3 SEJ_3-2017_Online - Page 14

DENTAL MATERIALS Polymerisation kinetics in a fibre reinforced resin-based composite Nicoleta Ilie 1a* Department of Operative Dentistry and Periodontology, University Hospital, Ludwig-Maximilians-Universität München, DE-80336 Munich, Germany 1 Dipl. Eng, PhD, Professor a Received: July 14, 2017 Revised: July 28, 2017 Accepted: August 24, 2017 Published: August 25, 2017 Academic Editor: Jean-François Roulet, DDS, PhD, Dr hc, Prof hc, Professor, University of Florida, Gainesville, FL, USA Cite this article: Ilie N. Polymerisation kinetics in a fibre reinforced resin-based composite. Stoma Edu J. 2017;4(3):164-170. Abstract DOI: 10.25241/stomaeduj.2017.4(3).art.1 Objective: The study aimed to evaluate the potential of a commercial fiber reinforced resin composite (FRC) to be cured adequately also in increments larger than 2-mm. Material and methods: One FRC (EverX Posterior, GC) was investigated by assessing in real-time the degree of conversion (DC) and polymerisation kinetic at increasing depths (100-µm, 2-mm 4-mm and 6-mm). In addition, a battery of mechanical properties - flexural strength, flexural modulus, Vickers hardness, indentation modulus, creep – and the characteristics of the used curing light were determined. Results: One-way ANOVA revealed no significant difference in DC measured 300 s post-irradiation in a depth of 100-µm and 2-mm (p = 0.281). Similarly, no significant difference was identified between DC measured at 2-mm and 4-mm (p = 0.724), while the DC measured at 6-mm depth was significantly lowest (p<0.001). The polymerisation kinetic was well described (R²>0.95) by a double exponential sum function, distinguishing between the gel and the glass phase of the polymerisation process. It allowed identifying a slower start of polymerization in depth, associated with a lower maximal rate of C-C double bond conversion. The mechanical properties amounted (128.30±8.38) MPa (flexural strength), (8.38±0.87) GPa (flexural modulus), (92.00±15.86) N/mm² (Vickers hardness), (17.82±1.82) GPa (indentation modulus) and (3.35±0.84) % for creep. Conclusions: While DC recorded in 2-mm and 4-mm depths were statistically similar, there is evidence that the quality of curing in a depth of 4-mm is lower compared to the top of the specimen. The mechanical properties were within the range of high viscosity bulk-fill resin-composites. Keywords: fiber reinforced resin-based composite, degree of cure, polymerisation kinetics, bulk-fill, mechanical properties. 1. Introduction Fiber-reinforced resin-based dental composites (FRC) are advanced restoratives, particularly designed to be placed in load bearing areas. The denomination FRC implies that the material is composed of dissimilar constituents, involving a homogeneous polymer matrix that is reinforced by a stronger and stiffer fibrous constituent. The shape, dimension, orientation and volume amount of fibers are used to modulate the mechanical properties of the FRC. The shape of fibers is usually described by its aspect ratio R, and is defined as the proportional relationship between their length and width. To achieve a high material strength, a high aspect ratio is sought. In dental materials, an optimum was estimated at ca. 5.2, 1 thus a fiber must be 5.2 time longer as it is wide. In addition to the aspect ratio, the strength of a FRC is directly related to the length of the reinforcing fibers. The critical fiber length in dental FRC was shown to lie between 0.5 mm and 1.6 mm, 2 while fibers below these values are inducing a lower reinforcement effect and are considered to act similarly to fillers present in particulate micro hybrid resin-composites. 3 A third aspect influencing the strength of a FRC is the fiber orientation that can be unidirectional or randomly distributed. In continuous, unidirectional fiber composites, high strength and stiffness is found in the fiber longitudinal axis with very minor changes from the matrix properties in the transverse direction. In contrast, the material behaves macroscopically homogenous and isotropic in randomly orientated, discontinuous FRC. 4 It must however be considered that, as demonstrated by Karbhari and Strassler, a dental FRC having the highest strength does not necessarily have the highest flexural stiffness or the greatest capacity for energy absorption, 5 thus the mechanical behavior of this particular material category requires a battery of different tests to be understood. Beside improvements in mechanical properties *Corresponding author: Prof. Dr. Dipl. Eng. Nicoleta Ilie, Department of Operative Dentistry and Periodontology, University Hospital, Ludwig-Maximilians-Universität München, Goethestr. 70, DE-80336 Munich, Germany Phone: +49-89-44005-9412, Fax: