further progress of the polymerization process, the
organic matrix becomes a gelatinous state, and
the reaction rate decreases (glass effect). 15
The variation of DC during irradiation of the
FRC was well described by the superposition of
two exponential functions. The first exponential
function, characterizing the gel phase, is described
by the parameters “a” and “b”, while the second
exponential function is characterizing the glass
phase and is described by the parameters “c” and
“d”. The experimental data reveal that parameter
“a” decreases with increased specimen thickness,
but only in increments thicker than 2 mm, while
parameter “b” decreases with increased specimen
thickness already starting from thin layers. Both
effects demonstrate that the decrease of the C-C
double bonds in the gel phase is slower in thicker
layers compared to the top, due to the exponential
decrease (Lambert’s Law) of light transmission with
the composite’s thickness. 16 It has been previously
shown that merely 24% ÷ 44% of the incident blue
light and 9 ÷ 14% of the incident violet light of a
LCU is transmitted through 2 mm commercially
available bulk-fill resin composite increments.
These values are yet consistently reduced in 4 mm
increments (9 ÷ 24% and 3 ÷ 9%, respectively) 7
and may further change to the detriment of curing
quality when LCUs with low irradiances are used
or, as usual in clinical practice, when the LCU
cannot be applied perpendicularly and directly on
the restoration surface.
While the effect of light attenuation was not directly
reflected in DC, as evidenced by the statistically
similar DC values measured in 2 and 4 mm
increments, the polymerization kinetic describes a
slower polymerization (lower parameter a and b)
as well as a lower maximal rate of polymerization
(Rate max = 20.0÷22.1 and 13.5÷15.7, respectively).
The lower amount of photons reaching 4-mm
layers compared to 2-mm will activate less
efficiently the photo-initiator, inducing less nuclei
of polymerization and therefore longer polymer
chains and a lower cross-linking. This can have as
a result a lower modulus of elasticity, although the
amount of C-C double bond conversion (DC) is
similar. This effect might be in accordance with data
presented by Omran et al. by evaluating the bond
strength of various resin composites to dentin,
revealing that EverX Posterior can be safely applied
in bulks of 4-mm increments, similarly to other
analysed bulk-fill composites, but its performance
was better in 2-mm thick increments. 17
In addition to that, parameters “c” and “d”
describing the glass phase reveal that “c” is not
altered up to 4-mm thick increments, while “d” has
a very low value and is of less relevance. The low
variation of parameters “c” and “d” demonstrates
that the reaction kinetic is less thickness-dependent
in the glass phase compared to the gel phase.
Since the analyzed material is indicated to be
used in large posterior cavities in optional larger
incremental thickness compared to conventional
resin-composites layered in 2 mm increments, a
direct comparison with the material category of
high-viscosity bulk-fill resin-composites seems
pertinent. This comparison is possible owing
to identical specimen geometry, test type and
test parameters as well as specimen storage
conditions. Accordingly, EverX Posterior may
be ranged for the flexural strength (128.30 ±
8.38) MPa in the middle of the above specified
material category, characterised by values varying
among 99.9±10.7 MPa (Admira Fusion x-tra) and
142.8±12.9 MPa (SonicFill). Similar considerations
apply to the Vickers hardness (92.00 ± 15.86 N/
mm²) when compared to the high-viscosity bulk-
fill resin-composite category, delimited by the
values 77.1±5.6 N/mm² (Admira Fusion x-tra)
and 133.5±32.0 N/mm² (X-tra Fil). The modulus
of elasticity measured in either macro and micro
scale was rather interrelated to the upper values of
the material category (8.38± 0.87) GPa vs 4.5±0.8
(Tetric EvoCeram Bulk Fill) to 9.5±0.6 (X-tra Fil) and
for Y HU (17.82±1.82) GPa vs 13.4±0.8 GPa (Tetric
EvoCeram Bulk Fill) to 22.2±1.7 GPa (X-tra Fil). 9
Polymerisation kinetics in a fibre reinforced resin-based composite
5. Conclusions
In the present study and considering the
tested FRC, the polymerization kinetic allowed
identifying a slower start of the polymerization
process depending on material’s depth. This was
associated with a lower maximal rate of C-C double
bond conversion, although DC values recorded in
2 and 4 mm depth were statistically similar. There
is, however, evidence that the quality of curing at
4-mm depth is lower compared to the top of the
material. The null hypothesis must therefore be
rejected.
The analyzed FRC revealed mechanical properties
situated within the range of the high viscosity bulk-
fill resin-composites category, with a high modulus
of elasticity (upper range of the mentioned
material categories.
Acknowledgement
The author kindly acknowledges the company GC
for supporting ca. 20% of the costs involved in the
presented study.
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