UNIVERSIDADE ESTADUAL PAULISTA
“JÚLIO DE MESQUITA FILHO”
Instituto de Ciência e Tecnologia
Campus de São José dos Campos
ORIGINAL ARTICLE DOI: https://doi.org/10.4322/bds.2023.e3704
1
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Impact of photoinitiator quality on chemical-mechanical properties
of dental adhesives under different light intensities
Impacto da qualidade de fotoiniciadores nas propriedades químico mecânicas de adesivos dentais sob diferentes
intensidades de luz
Tânia Mara da SILVA
1
, Nícolas de Faria PETRUCELLI
1
, Rafael Pinto de MENDONÇA
1
,
Jefferson Pires da SILVA JÚNIOR
1
, Tiago Moreira Bastos CAMPOS
2
, Sérgio Eduardo de Paiva GONÇALVES
1
1 - Universidade Estadual Paulista Júlio de Mesquita Filho - UNESP, Instituto de Ciência e Tecnologia de São José dos Campos,
Departamento de Odontologia Restauradora, São José dos Campos, SP, Brazil.
2 - Universidade de São Paulo - USP, Faculdade de Odontologia de Bauru, Departamento de Prótese e Periodontia, Bauru, SP, Brazil.
How to cite: Silva TM, Petrucelli NF, Mendonça RP, Silva JP Jr, Campos TMB, Gonçalves SEP. Impact of photoinitiator quality on
chemical-mechanical properties of dental adhesives under different light intensities. Braz. Dent. Sci. 2023;26(1):e3704. https://doi.
org/10.4322/bds.2023.e3704
ABSTRACT
Objective: Evaluate the mechanical properties of experimental adhesive models with different photoinitiators
(PI) polymerized by LED units of different power densities. Material and Methods: Three groups of adhesive
models based on HEMA/BisGMA (45/55) were prepared in association with different PI combinations: G2
(control) – 2 PI: 0.5% CQ, 0.5% EDMAB; G3 - 3 PI: 0.5% CQ; 0.5% DMAEMA, 0.5% DPIHP; G4 - 4 PI: 0.5%
CQ; 0.5% EDMAB; 0.5% DMAEMA; 0.5% DPIHP. The three formulations were polymerized at two different
LED power densities: 550 mW/cm
2
and 1200 mW/cm
2
. The degree of conversion (DC) of adhesive monomers
was monitored
in situ
through the FTIR for 600 s. Specimens were prepared for each formulation for analysis of
exural strength (FS), modulus of elasticity (ME), sorption (SOR) and solubility (SOL). Data were submitted to
two-way ANOVA and Tukey tests (5%). Results: DC: there is a signicant difference among adhesive systems
(G2<G3<G4). FS and ME: signicant differences were found between densities, with the lowest average for
550 mW/cm
2
. SOR and SOL: adhesives polymerized at 1200 mW/cm
2
presented higher sorption and solubility.
Conclusion: The mechanical properties of the adhesive models are directly related to the types of photoinitiatiors
and the LED power densities.
KEYWORDS
Dental adhesive; Irradiance; Light-curing; Photoinitiators; Physicochemical phenomena.
RESUMO
Objetivo: Avaliar as propriedades mecânicas de modelos adesivos experimentais com diferentes fotoiniciadores
(PI) polimerizados por unidades de LED de diferentes densidades de energia. Material e Métodos: Três grupos
de modelos adesivos baseados em HEMA/BisGMA (45/55) foram preparados em associação com diferentes
combinações de PI: G2 (controle) – 2 PI: 0,5% CQ, 0,5% EDMAB; G3 - 3PI: 0,5% CQ; 0,5% DMAEMA, 0,5% DPIHP;
G4 - 4 PI: 0,5% CQ; 0,5% EDMAB; 0,5% DMAEMA; 0,5% DPIHP. As três formulações foram polimerizadas em
duas densidades de potência de LED: 550 mW/cm2 e 1200 mW/cm
2
. O grau de conversão (DC) dos monômeros
adesivos foi monitorado
in situ
através do FTIR durante 600 s. Amostras foram preparadas para cada formulação
para análise de resistência à exão (FS), módulo de elasticidade (ME), sorção (SOR) e solubilidade (SOL). Os
dados foram submetidos aos testes ANOVA 2-fatores e Tukey (5%). Resultados: DC: houve diferença signicativa
entre os sistemas adesivos (G2<G3<G4). FS e ME: foram encontradas diferenças signicativas entre as densidades,
com as menores médias para 550 mW/cm
2
. SOR e SOL: adesivos polimerizados a 1200 mW/cm
2
apresentaram
2
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Silva TM et al.
Can the quality of photoinitiators compensate for the lack of light intensity on the mechanical properties of adhesive models?
Silva TM et al.
Can the quality of photoinitiators compensate for the lack
of light intensity on the mechanical properties of adhesive
models?
INTRODUCTION
Adhesion represents the main mechanism
by which resin materials bond to a dental
substrate, and it remains the most fragile link in
long-term clinical performance. The durability
of the adhesive system at the interface may
be affected by incomplete polymerization,
prevention of inltration of demineralized dentin
by the presence of dentin uids, phase separation
and hydrolytic-enzymatic degradation of the
adhesive [1-4]. Knowledge of the behavior of
light-polymerized adhesives and the generation of
more effective adhesive models that enhance the
longevity of restorations in a wet environment are
necessary to improve current dental practice [5].
Adhesive systems comprise a resinous
phase, an aqueous phase, photoinitiators, and
co-initiators. The resinous phase is rich in Bis-
GMA, a highly hydrophobic monomer and
polymerizable because it has branched chains
that provide good mechanical properties for the
adhesive system but inadequate penetration into
the demineralized and moist dentin. The aqueous
phase is rich in HEMA, an aggregate monomer
for increasing water compatibility and adhesive
inltration on the demineralized and wet dentin
substrate. However, HEMA has a linear chain
that does not have the same polymerization
potential as Bis-GMA, which provides a poorly
polymerized adhesive interface favoring
longitudinal degradation [5-9].
Photoinitiators, of which camphorquinone
is the most commonly used in current adhesive
systems, are added to the adhesive formulations
for polymerization reaction. With hydrophobic
characteristics, camphorquinone remains
associated with Bis-GMA in the resinous phase,
ensuring excellent polymerization [2,7].
However, in view of the moist substrate and
adhesive phase separation, the polymerization
rate and degree of conversion are affected,
inducing a nonhomogeneous polymerization
structure of the adhesive. This may lead to
a mechanism for degradation. Therefore,
in addition to camphorquinone, other
photoinitiatiors with hydrophobic characteristics,
such as ethyl-4-(dimethylamino) benzoate
(EDMAB) and hydrophilic characteristics,
such as 2-(dimethylamino) ethyl methacrylate
(DMAEMA), are used in adhesive systems to
improve polymerization in the presence of water.
In addition, a new co-initiator, DHIHP (salt)
has been introduced into adhesive systems to
replace inactive radicals with active radicals of
both camphorquinone and phenyl, optimizing
polymerization [7,10].
Analysis of mechanical properties of the
new adhesive systems
in vitro
offers a signicant
reference for their behavior under clinical
conditions [5]. These mechanical properties are
also directly related to the effectiveness of the
photopolymerizing units and the intensity and
spectrum of light emission [11,12]. Currently,
LED devices are the most commonly used for
photopolymerization [13]. Recent developments
have increased the output of LED devices from
a power density between 300 mW/cm
2
and
650 mW/cm
2
to more than 1200 mW/cm
2
.
The increase in light intensity may result in a
higher degree of conversion and, consequently,
better mechanical properties [14,15]. However,
it may generate higher tensile stresses from
polymerization contraction.
The present study optimized the
incorporation of a combination of hydrophilic
and hydrophobic photoinitiators to improve the
degree of conversion of the adhesive systems,
mainly in the hydrophilic phase. The evaluation
of different photopolymerization power densities
should provide better adhesiveness and strength,
both of which are important to the maintenance
of restorations in the oral environment. Thus, the
aim of this study was to evaluate the mechanical
properties of experimental models of adhesive
systems polymerized by LED units of different
maior sorção e solubilidade. Conclusão: As propriedades mecânicas dos modelos adesivos estão diretamente
relacionadas com os tipos de fotoiniciadores e as densidades de potência LED.
PALAVRAS-CHAVE
Fotoiniciadores; Fotopolimerização; Irradiação; Propriedades físico-químicas; Sistemas adesivos.
3
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Silva TM et al.
Can the quality of photoinitiators compensate for the lack of light intensity on the mechanical properties of adhesive models?
Silva TM et al.
Can the quality of photoinitiators compensate for the lack
of light intensity on the mechanical properties of adhesive
models?
power densities. The null hypotheses tested
were that no signicant differences would be
found among the photoinitiators regarding the
mechanical properties of the adhesive systems
tested and that no signicant differences would
be found among the LED intensities used.
MATERIAL AND METHODS
Model adhesive compositions
Experimental models of adhesive systems
similar to commercially available adhesive
systems were fabricated. The model adhesive
consisted of hydroxyethyl methacrylate (HEMA,
Sigma-Aldrich, St. Louis, MO, USA) and 2,2-bis[4-
(2-hydroxy-3-methacryloxypropoxy) phenyl]-
propane (BisGMA, Sigma-Aldrich, St. Louis,
MO, USA) with a mass ratio of 45/55 (HEMA/
BisGMA) as monomers widely used in dentin
adhesives [7]. In association, 2, 3, and 4 different
photoinitiators were added:
•
G2
- 2 photoinitiators: 0.5% camphorquinone
(CQ, Sigma-Aldrich, St. Louis, MO, USA);
0.5% ethyl-4-(dimethylamino) benzoate
(EDMAB, Sigma-Aldrich, St. Louis, MO, USA).
•
G3
- 3 photoinitiators: 0.5% CQ; 0.5%
2-(dimethylamino) ethyl methacrylate
(DMAEMA, Sigma-Aldrich, St. Louis,
MO, USA); 0.5% diphenyliodonium
hexauorophosphate (DPIHP, Sigma-Aldrich,
St. Louis, MO, USA).
•
G4
- 4 photoinitiators: 0.5% CQ; 0.5%
EDMAB; 0.5% DMAEMA; 0.5% DPIHP.
The experimental adhesive systems were
prepared in a brown glass vial and shaken (Orbit
300, LabNET International Inc., Woodbridge, NJ,
USA) for 48 h to yield well-mixed adhesive resin
solutions.
Degree of conversion
The photopolymerization of the experimental
adhesive models was monitored
in situ
with an
infrared spectrometer (FT-IR, PerkinElmer,
Waltham, MA, EUA) with a resolution of 4cm
-1
in
the attenuated total reection (ATR) mode and a
transmission range of 650 cm
-1
to 4000 cm
-1
[7].
The technique consisted of collecting the reected
radiation from the interface between the adhesive
and the crystal (ATR), providing evidence for the
transformation of carbon double bonds (C=C)
in the intensity range of 1638 cm
-1
and of single
bonds (C-C) in the 1608 cm
-1
range [4,16].
A volume of 10 µL of experimental adhesive
system was placed on the ATR crystal, and
a transparent coverslip was attached with a
piece of tape placed on the specimen to prevent
the evaporation of components [15]. Three
formulations were evaluated according to the
power density used for photopolymerization [13],
as measured with a radiometer (Curing Radiometer
Model 100, Demetron Research Corporation,
Danbury, CT, USA):
• Group 550 - LED unit (Emitter A, Schuster,
Santa Maria, RS, Brazil), with light intensity
of 550 mW/cm
2
.
• Group 1200 – LED unit (Demi Light Curing
System, Kerr Corporation, São Paulo, SP,
Brazil), with light intensity of 1200mW/cm
2
.
The LED units were positioned at a distance
of 2 mm perpendicular to the horizontal platform
where the ATR was located. A time-resolved
spectrum collector (Spectrum TimeBase, Perkin-
Elmer, MA, USA) was used for the continuous
and automatic collection of spectra during
polymerization.
The decrease in band ratio prole for intensity
at 1638 cm
-1
to that at 1608 cm
-1
was monitored
continuously during polymerization [16-18].
The degree of conversion (DC) was determined
using the following formula, which was based on
the intensity band ratios before and after light-
polymerization [10,16]:
( )
1638 1
1608 1
% 1 1 0 0
1638 1
1608 1
cm
cured
cm
DC
cm
uncured
cm
−
−
=−×
−
−
(1)
All experiments were carried out in triplicate,
and the results were averaged.
Adhesive exural strength and modulus of
elasticity
Ten specimens were prepared for
each adhesive system formulation (n=60).
The specimens were made in rectangular silicone
molds (12 mm length × 2 mm width × 2 mm
height) [19]. Unpolymerized adhesive was
dropped onto the molds, covered with a mylar strip,
and light polymerized for 20 s according to the
different LED power densities (550 mW/cm
2
and
4
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Silva TM et al.
Can the quality of photoinitiators compensate for the lack of light intensity on the mechanical properties of adhesive models?
Silva TM et al.
Can the quality of photoinitiators compensate for the lack
of light intensity on the mechanical properties of adhesive
models?
1200 mW/cm
2
). The specimens were stored in
brown glass until testing [18].
The exural properties were evaluated using
a three-point exural strength test performed with
a universal testing machine (EMIC DL-2000MF,
São José dos Pinhais, PR, Brazil) at a crosshead
speed of 0.5 mm/min using a 10 kgf load cell.
Flexural strength was obtained by measuring the
load at the fracture point, and the modulus of
elasticity was calculated based on the recorded
load deection curves [20].
Adhesive sorption and solubility
For each experimental adhesive system
formulation, ten disc-shaped specimens (n=60)
were fabricated using a silicone mold (6 mm
diameter × 2 mm height). Unpolymerized
adhesive models were placed in the silicone
mold, a mylar strip and a glass slide were placed
onto the silicone mold, and the adhesive was
light-polymerized for 20 s [20], according to
the LED power densities (550 mW/cm
2
and
1200 mW/cm
2
).
The specimens were stored in a desiccator
containing freshly dried silica gel. After 24 h,
the specimens were weighed using a 0.0001 mg
precision scale (Mettler, Toledo, OH, USA). This
cycle was repeated until a constant mass (M1) was
obtained. The specimens were immersed in 1 ml
of distilled water at 37°C for 28 days [21]. Every
24 h, the specimens were removed, blotted dry,
reweighed (M2 - sorption), and returned to the
water. After 28 days, the specimens were again
dried in the desiccator and weighed daily until
a constant mass was achieved (M3 - solubility).
Water sorption was calculated by the ratio
between the difference M1 and M2 by the
specimen volume [22], according to the following
equation:
( )
% 100 2 1 / .
SOR M M V= −
(2)
Water solubility was calculated by the
ratio between the difference M1 and M3 by the
specimen volume of the according to the formula:
%SOL = 100 (M1 - M3 / V).
Statistical analysis
Data collected on degree of conversion
(%DC), exural strength (in MPa), modulus of
elasticity (in MPa), water sorption (%SOR) and
solubility (%SOL) were statistically analyzed
using two-way ANOVA (adhesive models; LED
power densities) and the Tukey test (5%).
RESULTS
Table I shows the mean values and standard
deviation of exural strength (FS), modulus of
elasticity (ME), degree of conversion (%DC),
sorption (%SOR), and solubility (%SOL) obtained
in the groups. The highest means of FS, ME, and
DC were observed in the G4 group, independently
of the LED power density. For sorption and
solubility, the greatest mean values were obtained
in the G4 group, which was photopolymerized
with 1200 mW/cm
2
.
According to two-way ANOVA, the LED
power densities showed a statistically signicant
effect (
p
= 0.0095; F = 7.934) for the modulus
of elasticity. Flexural strength presented a
statistically significant difference according
to LED intensity (
p
= 0.0125; F = 7.293).
Water solubility showed statistically signicant
differences according to adhesive model
(
p
= 0.0053; F = 6.58) and LED power density
(
p
= 0.0001; F = 23.27). For water sorption,
the adhesive model (
p
= 0.0001; F = 19.0) and
LED intensity (
p
= 0.0005; F = 16.43) were
statistically signicant.
For the degree of conversion, the adhesive
models had statistically signicant differences
(
p
= 0.0001; F=201.15). The representative results
from kinetic study for the adhesive formulations,
according to the degree of conversion, are
presented (Figure 1). The G2 adhesive groups
exhibited a lower degree of conversion means
than the formulation of G3 and G4 groups.
The measurements of the conversion and
the polymerization rate as a function of time
are shown in Figure 1. Figure 1A shows the
conversion of the monomers of the G2 adhesive
subjected to 550 and 1200 W, where it is
possible to notice that all the curves present
the same behavior, with an abrupt increase
in conversion, in 25 seconds, followed by a
saturation in 45 seconds. The saturation values
were different for the two conditions, the highest
with 550 W with ≈ 57% and the lowest with
1200 W with ≈ 55%. The samples from groups
G3 and G4 showed a curve like G2 with a reaction
start at 20 and 25 seconds respectively, followed
5
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Silva TM et al.
Can the quality of photoinitiators compensate for the lack of light intensity on the mechanical properties of adhesive models?
Silva TM et al.
Can the quality of photoinitiators compensate for the lack
of light intensity on the mechanical properties of adhesive
models?
by a saturation at 30 and 35 seconds, respectively,
these reactions being faster than that observed in
the G2 sample. Another relevant point observed is
that the change in light intensity did not result in
a signicant difference in monomer conversion.
When analyzing the monomer conversion
rate by time, shown in Figure 2, it is possible to
conclude that the G2 sample for both LED powers
densities presented the same curve prole, and
500 W presented a higher polymerization rate
when compared to 1200 W. On the other hand,
G3 showed similar polymerization rates, but
with the highest potency, it generated a shift in
the curve for shorter times. As for G4, it is not
possible to observe a signicant visual difference
for the two potencies used.
Table II presents the kinetic constants
referring to the auto-catalytic model adjusted to
the experimental results. This model lists four
constants:
k
(speed constant),
m
(exponent of the
autocatalytic term),
n
(exponent of the reaction
order term), and
a
(reaction yield). G2 did not
show a signicant increase in its rate constant
as a function of the LED power densities used
to initiate the reaction. However, the constant
m that correlates with the propagation of chains
reduced and the constant
n
that correlates
with the termination of chains increased. This
suggests that at 1200 W it favors the chain
termination mechanisms and at 500 W it favors
the propagation mechanism and therefore there
was an increase in the conversion of monomers.
The G3 did not show significant changes in
the values of
m
and
n
as a function of the light
power used, however, there was an increase in
the values of
k
with 1200 W, indicating that this
Table I - Mean (SD) and Tukey test: Flexural strength (FS), Modulus of elasticity (ME), Degree of conversion (DC), Water sorption (%SOR), Water
solubility (%SOL) of the adhesive systems evaluated
FS (MPa) ME (GPa) DC (%) % SOR % SOL
550 W 1200 W 550 W 1200 W 550 W 1200 W 550 W 1200 W 550 W 1200 W
G2
86.3
(10.72) Ab
116.7
(5.73) Aa
0.74
(0.25) Aa
1.48
(0.40) Ab
55.09
(0.94) Aa
51.58
(2.70) Ab
8.53
(0.26) Aa
9.03
(0.51) Aa
-1.41
(0.63) Aa
-1.84
(0.38) ABa
G3
96.1 (23.99)
Aab
125.0
(19.58) Aa
1.22
(0.55) ABa
1.66
(0.26) Aa
68.21
(1.66) Ba
69.22
(0.87) Ba
9.12
(0.34) ABa
9.91
(0.49) Bb
-1.21
(0.41) Aa
-2.35
(0.31) Bb
G4
114.6
(28.62) Aa
113.8
(20.58) Aa
1.43
(0.32) Ba
1.46
(0.46) Aa
72.91
(1.27) Ca
72.26
(1.78) Ba
9.63
(0.49) Ba
10.16
(0.24) Bb
-1.97
(0.20) ABa
-2.53
(0.33) Bb
Different letters show statistically significant differences (
p
<0.05); capital letters refer to columns; lowercase letters refer to lines.
Figure 1. Polymerization kinetics of adhesives with different formulations, varying the light power and relating the conversion of monomers
over time and the polymerization rate by time.
6
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Silva TM et al.
Can the quality of photoinitiators compensate for the lack of light intensity on the mechanical properties of adhesive models?
Silva TM et al.
Can the quality of photoinitiators compensate for the lack
of light intensity on the mechanical properties of adhesive
models?
condition favored the polymerization process in
relation to 500 W. The G4 presented the higher
values of
k
having a small percentage reduction
with increasing power and a small reduction in
the constant
m
, however due to the high values
of
k
it was not possible to notice a signicant
change in the conversion.
DISCUSSION
In vitro
studies represent an important
tool for understanding adhesive models and
for predicting and validating the behavior of
materials before clinical use. The hydrolytic
degradation caused by phase separation of the
adhesive under clinical conditions of the presence
of intrapulpal uid requires adhesive systems
with improved performance in hydrophilic
environments. Therefore, the evaluation of the
behavior of new combinations of hydrophilic
and hydrophobic photoinitiators is relevant
to optimize resistance to degradation with no
prejudice to the mechanical properties [10].
Commercially available adhesive systems
typically have a composition based on cross-
linking polymerizations between the hydrophobic
monomers (Bis-GMA) and hydrophilic (HEMA)
monomers. The Bis-GMA molecule has two
binding sites, whereas HEMA has only one,
which helps raise the rate of polymerization of
the adhesive systems. After polymerization, Bis-
GMA shows two outstanding hydroxyl radicals,
which allows water sorption and increases the
risk of degradation [6,7]. Due to the moisture
from the demineralized dentin, adhesive phase
separation occurs, with Bis-GMA tending to
migrate to the resin phase (hydrophobic) and
HEMA to the aqueous phase (hydrophilic).
The polymerization of the resin phase is more
efcient than the aqueous phase, weakening the
adhesive bond [3,7,23].
Table II - Kinetic constants referring to the autocatalytic model adjusted to the experimental results presented in Figure2
Groups k* m n a
G2
550 W 14.15 ± 1.8 0.46 ± 0.03 0.43 ± 0.05 0.46 ± 0.003
1200 W 13.40 ± 1.2 0.40 ± 0.01 0.53 ± 0.04 0.42 ± 0.002
G3
550 W 20.92 ± 0.6 0.14 ± 0.01 0.15 ± 0.01 0.51 ± 0.005
1200 W 38.85 ± 2.42 0.15 ± 0.02 0.17 ± 0.02 0.48 ± 0.005
G4
550 W 99.80 ± 7.6 0.53 ± 0.03 0.47 ± 0.05 0.55 ± 0.002
1200 W 79.84 ± 10.4 0.38 ± 0.03 0.47 ± 0.05 0.55 ± 0.002
*k (speed constant), m (exponent of the autocatalytic term), n (exponent of the reaction order term), and a (reaction yield).
Figure 2. Polymerization kinetics of adhesive models with different formulations.
7
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Silva TM et al.
Can the quality of photoinitiators compensate for the lack of light intensity on the mechanical properties of adhesive models?
Silva TM et al.
Can the quality of photoinitiators compensate for the lack
of light intensity on the mechanical properties of adhesive
models?
In case of phase separation, an improved
photoinitiator system should be able to generate
sufcient hydrophilic and hydrophobic radicals
to improve the integrity of the adhesive interface.
Under
in vivo
conditions, photopolymerization is
usually carried out in the presence of endogenous
water and other factors that limit the durability
of the adhesive layer in dentin. Problems such
as incomplete polymerization, partial inltration
in the demineralized dentin matrix, phase
separation and enzymatic degradation of adhesive
system and demineralized collagen have been
reported [1,7].
Most current photoinitiator systems use
a combination of camphorquinone with an
aromatic amine co-initiator (EDMAB) and exhibit
a highly hydrophobic prole that compromises
their performance in humid environments.
New photoinitiators, such as DMAEMA and
DPIHP (hydrophilic prole), have been associated
with formulations that could improve the degree
of conversion in the presence of moisture [1,10].
Therefore, this study compared three
photoinitiator systems, G2 formulation with
hydrophobic characteristics, G3 with hydrophilic
photoinitiators, and G4 with a combination of
hydrophilic and hydrophobic components to
verify the physical-mechanical prole of each
adhesive model. The results of the present study
support the hypothesis that the combination of
hydrophilic and hydrophobic photoinitiators
would affect the polymerization of adhesive
systems based on the BisGMA/HEMA model,
independently of the energy intensity applied
during photopolymerization, similar to the
previous study [24]. For the degree of conversion,
the G4 formulation produced statistically better
results compared with G3 (hydrophilic) and
G2 (hydrophobic), as shown in Figure 1.
The influence of DPIHP on adhesive
polymerization provided a higher degree of
conversion to G3 and G4 in relation to G2 (which
was not present). This showed that a combination
of hydrophobic and hydrophilic photoinitiators
can provide effective polymerization in an
adhesive system. Thus, the rst null hypothesis
was rejected.
The benecial effects of DPIHP were observed
in another study with respect to the mechanical
properties [7]. The authors reported that
formulations with hydrophobic photoinitiators
showed poor mechanical durability but that a
combination of hydrophilic and hydrophobic
co-initiators significantly improved these
properties. Considering the model of flexural
strength and modulus of elasticity according to
ISO 4049, the results of this study did not show
statistically signicant differences in the exural
strength and modulus of elasticity in relation
to type of photoinitiator, despite the greater
mean values presented by G4 and G3 regarding
G2 (Table I). The LED power density used for
photopolymerization showed a signicant effect
on adhesive models. As a result, the second null
hypothesis was rejected.
Different intensities of LED power density
(550 mW/cm
2
and 1200 mW/cm
2
) were
evaluated to determine the influence of the
light source on the presence of photoinitiators
in the physical-mechanical properties of the
adhesive models [11,12]. Statistically signicant
differences were observed for G2, which had a
higher exural strength, modulus of elasticity,
and degree of conversion when photopolymerized
with an LED intensity of 1200 mW/cm
2
. A higher
light intensity is required to obtain adequate
mechanical properties because of the hydrophobic
characteristics of adhesive systems. In the
G3 and G4 groups, the presence of different
photoinitiators reduced the inuence of light,
which may have practical value as clinicians may
use defective or aging LED units.
Also, statistically significant differences
were observed for the LED intensity factor for
water sorption and solubility (
p
< 0.05). It was
found that 1200 mW/cm
2
promoted higher
sorption and greater solubility, which present
highly hydrophilic characteristics. Probably, the
higher intensity promoted faster polymerization
of the resin monomers, reducing the gel phase
of the material and favoring the degradation
of the polymer matrix and the stability of
the material. The results showed that LED
intensity had a direct inuence on the physical-
mechanical properties of the adhesive models
tested; however, the interaction may depend on
the adhesive composition.
The results obtained for water sorption
showed the best behavior for G2, with lower
mean values when compared with those of
G3 and G4 (Table I). Studies have suggested
that the water sorption of adhesive systems is
inuenced by the composition and hydrophilicity
of the material [9,25-27]. This fact was observed
8
Braz Dent Sci 2023 Jan/Mar;26 (1): e3704
Silva TM et al.
Can the quality of photoinitiators compensate for the lack of light intensity on the mechanical properties of adhesive models?
Silva TM et al.
Can the quality of photoinitiators compensate for the lack
of light intensity on the mechanical properties of adhesive
models?
in this study, since the hydrophobic composition
of G2 favored lower water sorption in relation to
the adhesives with hydrophilic composition such
as G3 and G4. Consequently, higher sorption
promoted the greater solubility of the unreacted
hydrophilic monomers [18,28] and monomers for
G3 and G4 groups, relating the composition of
the adhesive models to the effects of the presence
of water in the oral environment. Although
the combination of different photoinitiators
improved the degree of conversion of the
experimental adhesive models, the water sorption
characteristics deteriorated.
The addition of other initiators to the
adhesives impacted the propagation and
termination steps of the reaction as seen by the
changes in the values of
m
and
n
. However, its
main effect is to increase the speed constant,
which is one of the factors that most inuence
the degree of conversion.
The evaluation of the physical properties of
sorption and solubility is relevant to the prole
of adhesive systems [9], as both can lead to
extensive chemical and physical processes with
deleterious effects on mechanical properties
such as exural strength, modulus of elasticity,
and degree of conversion. These can affect the
deformation mechanisms [29] and the structure
and function of the polymers [27,28].
The results of this study indicate that the
selection and combination of photoinitiator
components should be based on the polymerization
behavior of the resin monomers under conditions
of higher or lower moisture, as in the previous
studies [5,7,11,12]. Further research should
be conducted to nd the optimal percentage of
photoinitiator components that can provide a
homogeneous and stable blend and maintain
mechanical properties in the long term.
CONCLUSION
Within the limitations of this study, it can
be concluded that the mechanical properties of
the experimental adhesive systems were directly
related to the type of photoinitiator and the LED
density.
Acknowledgements
The authors would like to thank Prof. Dr.
Ivan Balducci for his assistance in statistical
analysis.
Author’s Contributions
TMS, SEPG: Conceptualization. TMS, NFP,
SEPG: Methodology. RPM: Software. TMS,
SEPG: Validation. TMS, TMBC: Investigation.
TMS, SEPG: Resources. TMS, NFP, TMBC: Data
Curation. TMS, SEPG: Writing – Original Draft
Preparation. RPM, JPSJ, TMBC, SEPG: Writing
– Review & Editing. NFP, RPM, JPSJ, TMBC:
Visualization. SEPG: Supervision. TMS, SEPG:
Project Administration.
Conict of Interest
No conicts of interest declared concerning
the publication of this article.
Funding
This work was partially nanced by FAPESP
grant 20/12874-9.
Regulatory Statement
This study was conducted in accordance with
all the provisions of the local research subjects
oversight committee guidelines and policies: no
human or animals was used in this research.
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Date submitted: 2022 Nov 20
Accept submission: 2022 Nov 29
Sérgio Eduardo de Paiva Gonçalves
(Corresponding address)
Universidade Estadual Paulista Júlio de Mesquita Filho-UNESP, Instituto de Ciência
e Tecnologia de São José dos Campos, Departamento de Odontologia Restauradora,
São José dos Campos, SP, Brazil.
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