Delayed post-curing stage and oxygen inhibition of free-radical polymerization of dimethacrylate resin

: Mikke K. Aromaa, Pekka K. Vallittu

Publisher: ELSEVIER SCI LTD

: 2018

: Dental Materials

: DENTAL MATERIALS

: DENT MATER

: 34

: 9

: 1247

: 1252

: 6

: 0109-5641

: 1879-0097

DOI: https://doi.org/10.1016/j.dental.2018.06.019

Objective. It is known that after light-initiated free radical polymerization of a dimethacrylate monomer system, the curing continues for some period of time after the curing light emission has stopped (so-called delayed post-curing stage, DPCS). It is also known that during free radical polymerization, the presence of oxygen effectively inhibits polymerization of monomers. However, less is known of the influence of oxygen inhibition of light initiated polymerization during the DPCS. The aim of this study was to determine some polymerization related properties of a resin system during the DPCS.

Methods. Monomer systems of BisGMA-TEGDMA (60/40%) with light sensitive initiator activator (camphorquinone-amine) system were polymerized by light-initiation (wavelength average 430-480 nm) with a radiation intensity of 1200 mW/mm(2) for 20 s on the ATR sensor of the fourier transform infrared (FTIR) spectrometer. After light curing, the samples were divided into two groups: the DPCS stage was allowed to continue in air (O-2-exposed group) or the samples were protected from the direct effect of air (O-2-protected group). The degree of monomer conversion (DC%) was monitored from the sample surface up to 360 min from both groups of samples (n = 6). Sample surfaces were additionally analyzed for surface microhardness (VHN) at four time-points corresponding to time-points of the DC% measurement (n = 6).

Results. After ending the light-curing of 20 s, i.e. during the DPCS, the DC% still increased from 50% to 65% in the O-2-protected group, whereas no increase was seen in the O-2-exposed group. Surface microhardness increased from 2.99 to 9.10 VHN of the O-2-protected samples and to 4.80 of the O-2-exposed samples during a 6-h period. Surface microhardness differed significantly between the groups (p < 0.005). There was significant correlation between the microhardness and DPCS (O-2-protected r = 0.950; O-2-exposed r = 0.940, p < 0.001). A correlation was also found between degree of conversion values and DPCS time (O-2-protected r = 0.941; for O-2-exposed r = 0.780, p < 0.001).

Significance. The results of this study suggested that O-2-inhibition of free radical polymerization of dimethacrylate resin occurred after ending the curing light emission. This correlated with a lower surface microhardness of the polymer when the DPCS continued under air exposure. (C) 2018 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.