Minimization of polymerization shrinkage effects on composite resins by the control of irradiance during the photoactivation process

Abstract

High levels of shrinkage stress caused by volumetric variations during the activation process are one of the main problems in the practical application of composite resins. Objective The aim of this study is to reduce the shrinkage stress and minimize the effects caused by composite resin volumetric variation due to the photopolymerization. In this way, this work proposes a systematic study to determine the optimal dimming function to be applied to light curing processes.Material and Methods The study was performed by applying mathematical techniques to the optimization of nonlinear objective functions. The effectiveness of the dimming function was evaluated by monitoring the polymerization shrinkage stress during the curing process of five brands/models of composites. This monitoring was performed on a universal testing machine using two steel bases coupled in the arms of the machine where the resin was inserted and polymerized. The quality of the composites cured by the proposed method was analyzed and compared with the conventional photoactivation method by experiments to determine their degree of conversion (DC). Absorbance measurements were performed using Fourier-transform infrared spectroscopy (FT-IR). A T-test was performed on DC results to compare the photoactivation techniques. We also used scanning electron microscopy (SEM) to analyze in-vitro the adhesion interface of the resin in human teeth.Results Our results showed that the use of the optimal dimming function, named as exponential, resulted in the significant reduction of the shrinkage stress (~36.88% ±6.56 when compared with the conventional method) without affecting the DC (t=0.86, p-value=0.44). The SEM analyses show that the proposed process can minimize or even eliminate adhesion failures between the tooth and the resin in dental restorations.Conclusion The results from this study can promote the improvement of the composite resin light curing process by the minimization of polymerization shrinkage effects, given an operational standardization of the photoactivation process.

Effect of radiant exposure and UV accelerated aging on physico-chemical and mechanical properties of composite resins

“… 6 Moreover, the use of high radiant exposure has been associated with the development of higher polymerization shrinkage stress levels. 7 …”

In Vitro Evaluation of Bond Strength to Primary Molar Dentine and Microleakage Properties of Low Polymerization Shrinkage Silorane Based Composite Resin

“…The most important parameter in the formation of microleakage is polymerization shrinkage. Therefore the bonding strength of adhesives to dentine must be able to withstand the tension of polymerization shrinkage [28][29][30][31] . The elasticity module, shrinkage and thermal expansion coefficients of composite resins are the determining factors for preventing microleakage [32][33][34] .…”

Shrinkage Stress and Temperature Variation in Resin Composites Cured via Different Photoactivation Methods: Insights for Standardisation of the Photopolymerisation

“…For the conventional, soft-start and pulse-delay protocols, the light-curing device emits constant irradiance as a function of time (or time intervals) and total photoactivation energy given by the product of irradiance and exposure time. The parameters for the exponential photoactivation method, however, were determined using the model of the exponential function proposed by Guimarães et al [ 10 ], which allows determining the instantaneous irradiance during the photoactivation ( i ) as a function of the time ( t ): where a = 0.58309, b = 4.928 and i 0 = 300. The last term in Equation (1) ( i 0 ) corresponds to the irradiance at the start point of the photoactivation process (300 mW/cm 2 ).…”

“…Recent studies have demonstrated that control of irradiance during the polymerisation process can contribute to an improvement in the mechanical properties of the cured biomaterials, and one of the main goals in this research area is to identify an optimal protocol for photoactivation of resins composites [ 17 ]. Guimarães, et al [ 10 ] developed an alternative photoactivation protocol named the exponential protocol, in which the LED irradiance along the photoactivation was determined and modelled by a mathematical function (this function being an exponential growth of irradiance in time domain). Their results showed that the proposed photoactivation method minimised the polymerisation shrinkage stress and its effects, without affecting the effective conversion of monomers into polymers.…”

“…Nowadays, there are devices able to emit light with irradiance up to 1800 mW/cm 2 [8]. However, high irradiance does not necessarily lead to better curing results obtained by the polymerisation processes [9,10].…”