by integrating fibers as a reinforcing phase in
resin-composites, their use as direct restorative
materials underlie modern requests for fast and
save application. This implies that a cavity may be
restored in larger increments (> 2 mm), helping
thus to reduce both the chair time and the risk to
insert defects or contaminants between layers, in
comparison to an incremental placement technique
with conventional resin composites. 6
Currently, a FRC that is indicated to restore
large posterior cavities, owing to its enhanced
mechanical properties, is also conferred by the
manufacturer right to use in larger increments (up
to 4 or 5 mm) similarly to a bulk-fill resin composite.
Whether a resin-composite is adequately cured
in its deeper layers is essentially influenced by
the amount of photons reaching these areas
during polymerization, and consequently by the
translucency of the material. It has been shown
that bulk-fill resin composites may become either
progressively opaquer or transparent during
polymerisation, allowing hence for less or more
light to reach deeper areas in relation to the light
transmitted at the beginning of irradiation. 7 The
change in light transmission vs. time is determined
by the increasing or decreasing mismatch between
the refractive indexes of monomer and filler, as
the resin polymerizes. 8 It has been evidenced in
commercially available bulk-fill resin composites,
however, that the mode of light transmission does
not alter the degree of conversion or polymerisation
kinetics in depths up to 4 mm, 7 providing that the
radiant exposure specified by the manufacturer was
applied properly.
The aim of the present study was therefore to
evaluate the potential of a commercial FRC to be
adequately cured in increments larger than 2 mm,
and to describe the kinetic of polymerization as
a function of incremental thickness. In addition,
several mechanical properties, measured at
micro and macro scale, were evaluated, to allow
a comparison of the FRC with several particulate
high viscosity bulk-fill resin composites that were
measured under identical conditions and published
previously. 9
The null hypotheses tested were that: a) the depth
(100-µm to 6-mm) has no effect on polymerisation
kinetic parameters and degree of conversion (DC)
measured 300s post-irradiation
2. Materials and Methods
One FRC (EverX Posterior, GC, Lot 1310242)
was investigated by assessing the degree of
conversion (DC) and the polymerisation kinetic at
various depths (100-µm, 2-mm 4-mm and 6 mm),
as well as the macro (flexural strength and flexural
modulus) and micro-mechanical properties (Vickers
hardness, HV, indentation modulus, Y HU and creep,
Cr). In addition, the curing characteristics of the
light curing unit (LCU) which was used in all tests
for polymerisation (LED-LCU Bluephase 20i, Ivoclar
Vivadent, Schaan, Liechtenstein, High power mode)
were considered.
As indicated by the manufacturer, the organic
Stomatology Edu Journal
matrix of the analysed FRC is methacrylate-based
(Bis-GMA, TEGDMA, PMMA), while the inorganic
fillers (77 % by weight) consist of E-glass fibers (5-
15 wt%) with an average length between 1-2 mm,
barium borosilicate glass filler (60-70 wt%) with an
average size between 0.1-2.2 µm and silicon dioxide
(1-5 wt%) (http://www.dibateb.com/wp-content/
uploads/2016/01/FAQ-everx-posterior.pdf).
2.1. Degree of conversion (DC) and polymerisation
kinetic.
DC was measured in a real-time profile (5 minutes,
with 2 spectra/s) with an FTIR-Spectrometer with an
attenuated total reflectance (ATR) accessory (Nexus,
Thermo Nicolet, Madison, USA). Four different
specimen geometries were considered. Therefore
100 µm and 2-mm, 4-mm and 6-mm high molds (4
mm diameter) were filled in bulk. Specimens were
cured by applying the curing unit for 20 s directly
on the top (0 mm exposure distance) of the mold,
covered by a transparent polyacetate sheet. For
each thickness, six specimens were measured (n
= 6). The non-polymerized composite paste was
applied directly on the diamond ATR crystal in the
mold as described above. DC was measured on
the bottom of the specimens and was calculated
by assessing the variation in peak height ratio of
the absorbance intensities of methacrylate carbon
carbon (C-C) double bond (peak at 1634 cm −1 ) and
that of an internal standard (aromatic C-C double
bond, peak at 1608 cm −1 ) during polymerization, in
relation to the uncured material.
Polymerisation kinetics in a fibre reinforced resin-based composite
(1634cm -1 /1608cm -1 ) Peak height after curing
DC Peak% = [1− ______________________________
] x100
(1634cm -1 /1608cm -1 ) Peak height before curing
The DC at the end of the observation time (300
seconds) is indicated, while the increase in DC
(= decrease of the C-C double bonds) until 300
seconds post-irradiation was described by the
superposition of two exponential functions as:
y=y 0 +a*(1−e −bx )+c*(1−e −dx )
The term y 0 presents the y-intercept, depending
e.g. on the thickness of the specimen and the
composition of the material. The parameters: a,
b, c, d are modulation factors of the exponential
function to optimize the exponential sum function
on the measured curve.
In addition, the change in carbon-carbon double
bond conversion with respect to time was
calculated and plotted against DC. The maximal
rate of C-C double bond conversion (Rate max ) and
the corresponding D C are indicated.
2.2. Macro mechanical properties - Flexural
strength (FS) and flexural modulus (FM)
The flexural strength (FS) and flexural modulus
(FM) were determined in a three-point-bending
test (n = 20) in analogy to ISO/DIN 4049:1998. 10
The specimens were made by compressing the
composite material between two glass plates with
intermediate polyacetate sheets, separated by a
steel mould having an internal dimension of 2 x
165