Indian Journal of Dental Research

: 2013  |  Volume : 24  |  Issue : 2  |  Page : 225--228

The effect of photoactivation time and light tip distance on the degree of conversion of light and dual-cured dentin adhesives

Fatemeh Maleknejad1, Hamideh Ameri1, Safa Manafi2, Joseph Chasteen3, Marjaneh Ghavamnasiri1,  
1 Department of Operative Dentistry, Mashhad Dental School and Dental Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
2 Department of Operative Dentistry, Kerman Dental School, Kerman University of Medical Science, Iran
3 Department of Oral Medicine of the University of Washington in Seattle, Washington, USA

Correspondence Address:
Hamideh Ameri
Department of Operative Dentistry, Mashhad Dental School and Dental Research Center, Mashhad University of Medical Sciences, Mashhad


Background: The degree of conversion of dental adhesive is an important parameter since poor mechanical properties are related to low percentage of monomer-to-polymer conversion within resin-based materials. Objectives: To evaluate the influence of polymerization time and light guide distance on the degree of conversion (DC) of three contemporary dental adhesives. Materials and Methods: The spectral data of ExciTE DSC, Single Bond ® , and Adper ® Prompt L-Pop were analyzed using FTIR spectroscopy after 20 s, 40 s, and 60 s of photoactivation times. Light tip distances were kept at 1, 3, and 6 mm during the exposures. Statistics: Data were analyzed using ANOVA and Tukey«SQ»s test (α = 0.05). Results: Within groups, greater DC values were found using a tip distance of 1 mm or a 60-s curing time for Single Bond ® (59%) and Adper ® Prompt L-Pop (65%). No statistically significant difference (P > 0.05) was found using either 1 mm or 3 mm tip distances after 20 s, 40 s, and 60 s of light curing time for Single Bond ® . ExciTE ® DSC showed the greatest DC values with light tip distances of 1 mm (90%) and 3 mm (89%), using 60 s of light curing. Conclusion : The self-etch adhesive Adper ® Prompt L-Pop could be applied in shallow cavity preparations and must be light cured for at least 40 s. The light-cured total-etch adhesive ExciTE ® DSC could be applied in every restorative scenario if the curing time is extended up to 60 s or if the tip distance is extended up to 3 mm.

How to cite this article:
Maleknejad F, Ameri H, Manafi S, Chasteen J, Ghavamnasiri M. The effect of photoactivation time and light tip distance on the degree of conversion of light and dual-cured dentin adhesives.Indian J Dent Res 2013;24:225-228

How to cite this URL:
Maleknejad F, Ameri H, Manafi S, Chasteen J, Ghavamnasiri M. The effect of photoactivation time and light tip distance on the degree of conversion of light and dual-cured dentin adhesives. Indian J Dent Res [serial online] 2013 [cited 2021 Apr 11 ];24:225-228
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Full Text

The degree of conversion (DC) of dental adhesives refers to the ratio of the double bond content of monomer to that of polymer. [1] Low conversion may result in premature failure of the resin-dentin bond. [2] Different studies have confirmed an increase in the level of polymerization with all adhesive systems when a prolonged photoactivation time is used. [3],[4],[5] The polymerization of simplified adhesives has been shown to be severely compromised when the bonding agent is combined with either the primer or the etching/primer solution. [3],[6] The objective of this study was to determine the effect of different light tip distances and photoactivation times on the DC of different adhesive resins.

 Materials and Methods

The three commercially available adhesive systems used in this study and their compositions are shown in [Table 1].{Table 1}

A single drop of each adhesive was spread between two polyethylene sheets to study the vinyl bond conversion under anaerobic conditions. The spectra of the uncured and cured adhesives were obtained using a Bomem MB154 (ABB Bomem Inc., Quebec, Canada) Fourier transform infrared spectrometer (FTIR) equipped with an attenuated total reflectance in the 5000-400 cm−1 region. The FTIR spectrum collected from each specimen was the result of 40 scans at a resolution of 2 cm−1 .

The spectra of each unpolymerized resin material were first collected. The adhesives were then light cured with the commercially available Starlight (Mectron, SPA, Loreto, Italy) visible light-curing LED unit. The unit operated in the wavelength range of 440-480 nm (peak of 460 nm) at an intensity of a 1000 mW/cm 2 , using a LED light guide diameter of 8 mm. The intensity (mW/cm 2 ) of the unit was measured using a Cure Rite digital radiometer (Efos Inc., Ontario, Canada).

Spectra data were collected from the adhesive materials after 20, 40, and 60 s of light curing, using three different light guide distances (1, 3, and 6 mm) from the surfaces of the adhesives. These distances were standardized using opaque ring spacers placed between the light guide and the polyethylene sheets that covered the materials. This method has been previously described by Salgado et al. [7] [Table 2].{Table 2}

The collection of the infrared spectra was repeated three times for each bonding system. Additionally, FTIR spectra were obtained from photoactivated samples stored in distilled water for 1 week. [8]

Statistical analysis

Data were subjected to analysis using a three-way ANOVA relating to the three variables: type of dentin adhesive, light tip distance, and photoactivation time. Further analysis of the photoactivation time and curing distance was performed for each material using a two-way ANOVA in order to better assess some of the interactions. The pair-wise comparisons were carried out using Tukey's test (α = 0.05).


The influence of the light tip distance on the DC (%) of each adhesive system following 20, 40, and 60 s exposures within and between groups are displayed in [Table 3].{Table 3}

The three-way ANOVA showed a statistically significant (P < 0.05) interaction among the three variables (photoactivation time, tip distance, and type of adhesive). A two-way ANOVA was used to evaluate the statistical interaction between two variables (tip distance and photoactivation time) only for the dual-cured ExciTE ® DSC (P < 0.05).

Statistical analysis within groups demonstrated greater DC values using a tip distance of 1 mm or a 60-s curing time for Single Bond ® (59%) and Adper ® Prompt L-Pop (65%) (P < 0.05). No statistically significant difference was found using either 1 mm (51%, 54%, and 59%) or 3 mm (50%, 51%, and 56%) tip distances, respectively, after 20, 40, and 60 s light curing times for Single Bond ® (P > 0.05).

ExciTE ® DSC showed greater DC values with tip distances of 1 mm (90%) and 3 mm (89%) in combination with 60 s of light curing in comparison with 20 s and 40 s (60% and 75%) (P < 0.05). However, when a 6-mm tip distance was used there was no statistically significant difference (P < 0.05) between photoactivation times of 40 s (60%) and 60 s (64%) in terms of the DC.

Among three materials, the greatest DC was found in ExciTE ® DSC and the least amount in Adper ® Prompt L-Pop (P < 0.05) when using the three different tip distances and curing times [Table 3].


The most commonly used distance between the tip of the curing light and the resin to be cured is approximately 1 mm. [9] The tip distance for deep proximal boxes in class II cavity preparations could be extended gingivally up to 6 mm. Therefore, in the present study, we examined the effect of different light tip distances (1, 3, and 6 mm) and different curing times (20 s, 40 s, and 60 s) on the DC of a dual-cured adhesive system (ExciTE ® DSC), a self-etch adhesive (Adper ® Prompt L-Pop), and a light-cured total-etch adhesive system (Single Bond ® ).

The first null hypothesis of this study, which was that 'in each group of dentin adhesive, different light-curing times and different distances of light tip have a statistically similar effect on DC' was not accepted, because photoactivation of Single Bond ® at 60 s resulted in a DC of about 55% on average, while 20 s and 40 s of photoactivation resulted in DC of 48% and 51%, respectively. Ye et al. [10] reported the highest DC values for Single Bond ® following 20 s and 40 s of photoactivation to be 72% and 74.8%, respectively. In another study, Single Bond ® showed 13.5% DC values after 10 s exposure using a LED light-curing unit. [8]

Adper ® Prompt L-Pop showed the least amount of DC. The greatest amount of DC achieved with Adper ® Prompt L-Pop was 51% using a 1-mm tip distance, and 50.6% when a 60-s light-curing time was used. This probably means that the self-etch adhesive Adper ® Prompt L-Pop can be used only in shallow cavity preparations, under veneers, or in other restorative situations that allow only a small light tip distance. According to the findings of the present study, the Adper ® Prompt L-Pop self-etch adhesive requires at least 60 s of irradiation to achieve an adequate polymerization at a 1 mm tip distance. Further studies need to be done to evaluate the DC of some other self-etch adhesives using different tip distances.

Ye et al. reported DC values of 42% and 82.9% for Adper ® Prompt L-Pop when 20 s and 40 s photoactivation times, respectively, were utilized. [10] These values are higher than those obtained in the present study, but according to Craig [9] they could be desirable for adhesive materials. This difference between the studies is likely due to the use of a LED light-curing unit at a higher intensity (>1200 mW/cm 2 ) in the study by Ye et al., compared to an intensity of 1000 mW/cm 2 used in the present study. Furthermore, different application methods of the adhesives were used in the two studies. In the study by Ye et al. the adhesives were applied and cured directly on the horizontal face of the internal reflectance crystal of the spectrophotometer, while in the present study transparent polyethylene sheets were used for light curing the samples. [10] Franz et al. [11] found the DC of Adper ® Prompt L-Pop after 20 s of photoactivation to be about 63.4% using a Quartz Tungsten Halogen light source at 500 mW/cm 2 , and without an oxygen inhibited layer. This is higher than the 22% found in the present study despite the fact that the application method of the adhesive was identical in both studies. The findings of a couple of previous studies about the DC value of Adper ® Prompt L-Pop were in agreement with the result of present study. [12],[13]

With ExciTE ® DSC we found that the greater the light tip distance the lower the DC value, even when using 40 s and 60 s curing times. This result confirms the necessity of light exposure, regardless of the chemical curing process, for obtaining higher DC values while curing dual-cured adhesives. The greatest DC value for all dual-cured materials has been reported to be about 80%. [1] In the present study, DC values obtained in ExciTE ® DSC were 90%, 89%, and 64%, respectively, at 1, 3, and 6 mm tip distances. There are no earlier studies that have examined the DC of dual-cured dentin adhesives with which we can compare the results of the present study.

The three adhesive materials in the present study showed the greatest DC with a curing time of 60 s at a 1 mm tip distance. However, the DC of ExciTE ® DSC (90%) was significantly greater than that of Single Bond ® (55%) and Adper ® Prompt L-Pop (50%). Therefore, the second hypothesis of the present study which was that within each photoactivation time and/or distance there is no difference among light-cured, dual-cured and self-etch adhesives in terms of the DC. was not accepted, namely that within each curing time and/or distance there is no difference between light-cured, dual-cured etch-and-rinse adhesive resins and self-etch adhesives in terms of the DC.

ExciTE ® DSC and Single Bond ® contain 24.5% and 35% ethanol, respectively. According to Cadenaro et al., the greater ethanol content (>30%) in Single Bond ® could be responsible for its lower DC. [3] The presence of ethanol in Single Bond ® decreases the reaction rate initially, but enhances the DC at a later stage. The effect of solvent on the polymerization behavior is different between Single Bond ® and Adper ® Prompt L-Pop. [14] This behavior is easily explained by the mobility of the system. Ethanol dilutes viscous monomers so that the reaction occurs in a less restricted environment. The decreased viscosity of the system allows propagation to continue longer, without constraining diffusion. By comparison, the most extensively used solvent in Adper ® Prompt L-Pop is water, which plays a different role. Wang et al. [15] claimed that the presence of water within the resin and a continuous supply of water within dentin/tubules might cause incomplete polymerization of Adper ® Prompt L-Pop.

Cadenaro et al. claimed that in contemporary dental adhesives the permeability of simplified adhesives is correlated with incomplete polymerization of resin monomers and that these adhesives may be rendered less permeable by using longer curing times than that recommended by the respective manufacturers. [3] High concentrations of relatively hydrophilic methacrylate monomers (i.e., HEMA, BPDM, PENTA) are generally blended with relatively hydrophobic adhesive monomers (i.e., bis-GMA, UDMA) to enhance bonding to wet dentin.

To facilitate the mixing of hydrophilic with hydrophobic monomers and to avoid phase separation between these components, manufactures have also added volatile solvents such as ethanol and acetone when formulating dental adhesives. However, the presence of residual solvent/water before the photoactivation of adhesives and formation of hybrid layers has been thought to be responsible for producing localized areas of incomplete monomer polymerization. [16]

Clinical significance

Regarding the degree of adhesive monomer conversion into polymer, depth of cavity preparation could be an important factor for selection of adhesive system.


This study was supported by a grant from the Research Council of Mashhad University of Medical Sciences, Mashhad, Iran.


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