Effect of light intensity and irradiation time on the polymerization process of a dental composite resin

Abstract

Polymerization shrinkage is a critical factor affecting the longevity and acceptability of dental composite resins. The aim of this work was to evaluate the effect of light intensity and irradiation time on the polymerization process of a photo cured dental composite resin by measuring the Vickers hardness number (VHN) and the volumetric polymerization shrinkage. Samples were prepared using a dental manual light-curing unit. The samples were submitted to irradiation times of 5, 10, 20 and 40 s, using 200 and 400 mW.cm -2 light intensities. Vickers hardness number was obtained at four different moments after photoactivation (immediate, 1 h, 24 h and 168 h). After this, volumetric polymerization shrinkage values were obtained through a specific density method. The values were analyzed by ANOVA and Duncan's (p = 0.05). Results showed increase in hardness values from the immediate reading to 1 h and 24 h readings. After 24 h no changes were observed regardless the light intensities or activation times. The hardness values were always smaller for the 200 mW.cm -2 light intensity, except for the 40 s irradiation time. No significant differences were detected in volumetric polymerization shrinkage considering the light intensity (p = 0.539) and the activation time (p = 0.637) factors. In conclusion the polymerization of the material does not terminate immediately after photoactivation and the increase of irradiation time can compensate a lower light intensity. Different combinations between light intensity and irradiation time, i.e., different amounts of energy given to the system, have not affected the polymerization shrinkage.

Exposure Reciprocity Law in Photopolymerization of Multi‐Functional Acrylates and Methacrylates

“…The applicability of ERL has been tested in the field of dental materials with regard to a number of material properties, such as degree of double bond conversion (DC), [9][10][11][12][13] flexural strength and modulus, [8,[14][15][16] hardness, [13,17,18] and depth of cure. [1,11,19,20] In many cases, the ERL holds reasonably well.…”

“…[1,11,19,20] In many cases, the ERL holds reasonably well. [9][10][11]14,[16][17][18] It is probable that because of this finding, the relationship has been proposed as a guideline for selecting the proper t irr based on the available I inc in the photo-cure mode of a dental resin composite. [1][2][3][4][5] There are, however, issues remaining to be resolved before the ERL can be applied to most (meth)-acrylate-based dental materials (adhesives, composites, sealants, etc.)…”

“…The previously-cited studies only examined a limited number of commercial products. [1,[8][9][10][11][12][13][14][15][16][17][18][19][20] Without knowing the composition, one cannot assume that the conclusions drawn from one product can automatically apply to another. In addition, most of the experiments only varied the I inc several folds.…”

“…[10,11,[14][15][16][17][18] The conclusions may not be extrapolated to some extreme I inc , such as a high I inc from a plasma-arc curing unit or a low I inc in a curing box. Finally, a number of research projects used a radiant exposure that was higher than the minimum in all combinations of I inc and t irr , [2,14,17,18] where the DC had reached the plateau value due to the reactiondiffusion-controlled process under continuous irradiation. [21][22][23] Those radiant exposure numbers would have eventually produced a similar conversion and other properties, making the ERL irrelevant.…”

Effect of different light-curing techniques on hardness of a microhybrid dental composite resin

“…T he generally preferred mode of polymerization in dental composite resins is photo-activation method [1]. Effectiveness of the polymerization is one important meaning to obtain adequate physical properties.…”