Performance Assessment of Three Similar Dental Restorative Composite Materials via Raman Spectroscopy Supported by Complementary Methods Such as Hardness and Density Measurements

Abstract

(1) Background: A widespread problem in oral health is cavities produced by cariogenic bacteria that consume fermentable carbohydrates and lower pH to 5.5–6.5, thus extracting Ca2+ and phosphate ions (PO43−) from teeth. Dental restorative materials based on polymers are used to fill the gaps in damaged teeth, but their properties are different from those of dental enamel. Therefore, a question is raised about the similarity between dental composites and natural teeth in terms of density and hardness. (2) Methods: We have used Raman spectroscopy and density and microhardness measurements to compare physical characteristics of several restorative dental composites at different polymerization intervals. (3) Results: XRVHerculite®, Optishade®, and VertiseFlow® showed the very different characteristics of the physical properties following four polymerization intervals. Of the three composites, OptiShade showed the highest polymerization rate. (4) Conclusions: Only fully polymerized composites can be used in teeth restoring, because incomplete polymerization would result in cracks, pitting, and lead finally to failure.

Full text

Citation: Iordache, S.-M.; Iordache,
A.-M.; Gatin, D.I.; Grigorescu, C.E.A.;
Ilici, R.R.; Luculescu, C.-R.; Gatin, E.
Performance Assessment of Three
Similar Dental Restorative Composite
Materials via Raman Spectroscopy
Supported by Complementary
Methods Such as Hardness and
Density Measurements. Polymers 2024,
16, 466. https://doi.org/10.3390/
polym16040466
Academic Editor: Sufyan Garoushi
Received: 20 December 2023
Revised: 1 February 2024
Accepted: 2 February 2024
Published: 7 February 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
polymers
Article
Performance Assessment of Three Similar Dental Restorative
Composite Materials via Raman Spectroscopy Supported
by Complementary Methods Such as Hardness and
Density Measurements
Stefan-Marian Iordache 1, Ana-Maria Iordache 1, Dina Ilinca Gatin 2, Cristiana Eugenia Ana Grigorescu 1,
Roxana Romanita Ilici 2, Catalin-Romeo Luculescu 3, * and Eduard Gatin 4, 5, *
1Optospintronics Department, National Institute for Research and Development in
Optoelectronics—INOE 2000, 077125 Magurele, Romania; stefan.iordache@inoe.ro (S.-M.I.);
ana.iordache@inoe.ro (A.-M.I.); krisis812@yahoo.co.uk (C.E.A.G.)
2Faculty of Dentistry, University of Medicine and Pharmacy “Carol Davila”, 020021 Bucharest, Romania;
dinailinca@yahoo.com (D.I.G.); roxana.ilici@umfcd.ro (R.R.I.)
3National Institute for Laser, Plasma and Radiation Physics, CETAL, 077125 Magurele, Romania
4Faculty of Medicine, University of Medicine and Pharmacy “Carol Davila”, 020021 Bucharest, Romania
5Faculty of Physics, University of Bucharest, 077125 Magurele, Romania
*Correspondence: catalin.luculescu@inflpr.ro (C.-R.L.); masterdent2009@yahoo.com (E.G.)
Abstract: (1) Background: A widespread problem in oral health is cavities produced by cariogenic
bacteria that consume fermentable carbohydrates and lower pH to 5.5–6.5, thus extracting Ca
2+
and
phosphate ions (PO
43−
) from teeth. Dental restorative materials based on polymers are used to fill
the gaps in damaged teeth, but their properties are different from those of dental enamel. Therefore, a
question is raised about the similarity between dental composites and natural teeth in terms of density
and hardness. (2) Methods: We have used Raman spectroscopy and density and microhardness
measurements to compare physical characteristics of several restorative dental composites at different
polymerization intervals. (3) Results: XRVHerculite
®
, Optishade
®
, and VertiseFlow
®
showed the
very different characteristics of the physical properties following four polymerization intervals. Of
the three composites, OptiShade showed the highest polymerization rate. (4) Conclusions: Only fully
polymerized composites can be used in teeth restoring, because incomplete polymerization would
result in cracks, pitting, and lead finally to failure.
Keywords: dental materials; microhardness; density of dental composites; Raman spectroscopy;
cavity treatment
1. Introduction
Cavities are an omnipresent problem in oral health, developed by children (60–90%)
and adults alike (90–100%) [
1
]. Tooth decay is produced by cariogenic bacteria that consume
fermentable carbohydrates and lower the pH to 5.5–6.5, thus extracting Ca
2+
and phosphate
(PO
43−
) ions from teeth [
1
,
2
]. The reversible process, deposition of Ca
2+
and PO
43−
ions
from saliva tries to compensate the loss and reconstitute hydroxyapatite (Ca
10
(PO
4
)
6
(OH)
2
)
in enamel. If the process is maintained in equilibrium, the cavities do not form; however, an
imbalance is present most of the time, and small caries initiate advanced demineralization
and lead to tooth decay.
To arrest the demineralization process, clinical restorative polymer materials (e.g., resin
composites, adhesives, and dental primers) are used to replace missing enamel/dentin tis-
sue. However, all replacement polymers contain either bisphenol A glycidyl dimetacrylate
(bisGMA) or triethylene glucol dimethacrylate (TEGMA), which promote mineral imbal-
ance and sustain biofilm development on the restored tooth [
1
]. Moreover, the recurrence
Polymers 2024,16, 466. https://doi.org/10.3390/polym16040466 https://www.mdpi.com/journal/polymers

Polymers 2024,16, 466 2 of 12
rate for this type of cavities is around 60% [
1
]. In a systematic review analyzing the materi-
als used for the treatment of carious tissue, the American Dental Association recognized
the superiority of amalgam in terms of durability, longevity, and affordability [
3
], but the
use of mercury has raised concerns about health security and environmental harms. Thus,
using dental composites for the treatment of damaged teeth was a better alternative than
amalgam. To achieve the best results in terms of durability, the physical properties of the
dental composites should be closely matched to those of the enamel.
There are a series of tests that can be conducted on the dental restorative materials:
stress–strain ratio, elastic modulus, Poisson ratio, flexural strength, resistance to fatigue,
and hardness [
4
] to name a few. In terms of mechanical properties, Chun and Lee [
5
] tested
six dental restorative materials (amalgam, dental ceramic, gold alloy, dental resin, zirconia,
and titanium alloy). Apart from dental ceramic, all the other materials exhibited maximum
stress and strain values higher than natural enamel and dentin. Zirconia was particularly
interesting because its hardness value was 4.5 over that of enamel, which implies poor
biocompatibility. Since dental restorative materials have to sustain prolonged biting wear
(vertical and side compression forces) they have to be compatible with the surrounding
natural teeth components (from the Vickers’ hardness values, dental resin and titanium
alloy were best matched with dentin and enamel, respectively). Chung et al. [
6
] has
proposed the measurement of Poisson’s ratio for dental composite materials to determine
their mechanical characteristics including biaxial flexural strength and indentation modulus.
A strong mismatch between the elastic values of the restorative materials and natural tooth
will cause micro-fractures which could lead to composite failure.
This is seen in the case of resin composites that more commonly undergo surface
degradation and margin/bulk fractures [
7
]. The authors conducted a study on four resin
composites assessing the fracture toughness, Vickers hardness, and color change, among
others, in dry and wet environments. Their conclusion was that moisture had a direct
influence on the hardness and fracture propagation of the resin materials due to the
polymerization shrinkage stress (materials that contain a polymerization modulator are
less capable of leaching their un-polymerized monomers).
Yadav et al. used statistical analysis and different decision-making processes to rank
dental restorative composite materials [
8
,
9
] or tribological behavior [
10
] to assess the
compatibility between hybrid dental restorative composite materials and natural teeth.
Tribological characterization has also been employed by Amanda Carvalho et al. [
11
] to
evaluate the shear stress in restorations and to compare the wear behavior of these materials
with patients’ teeth. Other authors [
12
–
14
] evaluated the wear behavior by using a dual-axis
chewing simulator and/or an ACTA wear machine in order to establish a relationship
between the physical parameters and wear of dental composites [15].
Besides the mechanical and esthetical properties, dental materials should also exhibit
antimicrobial activity. An excellent review [
16
] showed that the preferred materials are the
positively charged nanoparticles which can interact with the negatively charged membrane
of the bacteria, penetrating the cellular wall and destroying the cell. The nanoparticles
do not interfere with all the other properties (flexural strength, modulus of elasticity, and
hardness, self-curing, radiopaque, etc.), but complete this list with their sustainable bacterial
resistance. A good example is the use of strontium phosphate glass microfiller, which can
inhibit the microbial growth of S. mutans while maintaining the mechanical properties [
17
].
A major drawback is, however, the release of Ca, P, and Sr in the oral environment (its
release increases with the increase in the strontium phosphate concentration in the filler).
The physical characteristics of the restorative materials change over time and af-
ter harmful repetitive actions such as aesthetic bleaching and intense ultrasound clean-
ing
[18,19]
. These actions could result in damages to both natural teeth and restorative
materials. Thus, assessing the compatibility between filler and natural teeth is important
during yearly checkups. The main drawback of these methods is the requirement of spe-
cialized instruments. Moreover, they are sensitive to the variation in porosity and density
of the dental filling materials.

Polymers 2024,16, 466 3 of 12
Our group has been investigating the compatibility between several commercially
available dental restorative materials and natural enamel [
20
–
25
]. In this paper, we aim to
evaluate the physicochemical properties of three commercially available and widely used
dental materials and rank them according to those properties. In our investigations, we used
simple and affordable instruments and correlated the chemico-structural characteristics
with the behavior of the composite. We selected three dental restoration materials produced
by the same company (Kerr Corp., Orange, CA, USA) to maintain a similar composition
among the samples. The manufacturer aimed to improve (or at least to maintain) the
mechanical properties of the old dental restoration materials (e.g., Herculite), but in the
meantime was interested in adding esthetic features and easy handling for dental cavity
application (filling)—according to the release date of the products in Table 1. We focused
on highlighting the Raman spectroscopy results because it corroborated the results from
the microhardness and density measurements and provided further insights into the
photopolymerization process, allowing us to calculate the degree of conversion of the
monomer and to rank the materials in terms of stability.
Table 1. Composition and release date of the dental composite materials used in this study.
Dental Materials Composition Filler Loading
(wt%)
Release Date
(On the Market) Manufacturer
XRV Herculite®
Bis-GMA (bisphenol glycidil dimethacrylate)
TEGDMA (triethylene glycol dimethacrylate)
Prepolymerized filler (silica nanofiller-20–50 nm
nanoparticles, barium submicron fillers-0.6 µm
average size)
Titanium Dioxide (TiO2)
Organic pigments for shading
59% October 2017
Kerr Corp.,
Orange, CA, USA
OptiShade®
BisGMA (bisphenol glycidil dimethacrylate)
BisDMA (bisphenol A dimethacrylate)
TEGDMA (triethylene glycol dimethacrylate)
Filler (spherical silica and zirconia particles with
effective particle size is 5–400 nm,
400 nm barium glass particles)
80% June 2021
VertiseFlow®
Bis-GMA (bisphenol glycidil dimethacrylate)
GPDMA (glycerolphosphoric acid dimethacrylate)
HEMA (hydroxyethyl methacrylate)
Pre-polymerized filler (silanated barium glass,
nano-sized colloidal SiO2, YF3)
70%
November 2021
2. Materials and Methods
2.1. Materials
We selected three of the most widely used dental restorative materials in Romania and
ranked them in terms of microhardness, density, and degree of polymerization (curing).
Thus, XRV Herculite
®
, VertiseFlow
®
, and Optishade
®
composite fillers were purchased
from Kerr (Kerr Corp., Orange, CA, USA) [
26
] and each sample was photopolymerized for
four different time intervals (5 s, 10 s, 15 s, and 20 s) before testing.
According to the datasheet, XRV Herculite
®
contains 41% methacrylate ester monomers
and 59% inert mineral fillers (average particle size of 0.6 microns). VertiseFlow
®
is com-
posed of about 30% methacrylate monomers and less than 10% ytterbium trifluoride.
OptiShade
®
contains 80% fillers and 20% monomers, activators, and stabilizers. The com-
plete composition, based on the datasheet and references [27–29], is presented in Table 1.
Curing was performed with a Denjoy DY400-4 LED lamp (
λ
= 420–480 nm, from Kerr
Corp., Orange, CA, USA), which delivers a power of 720 mW and has a light intensity at
the irradiation plane in the 1500–2000 mW/cm2range.

Polymers 2024,16, 466 4 of 12
2.2. Methods
Density measurements were performed at 18.6
◦
C by the pycnometer method (py-
cnometer from Paul Marienfeld Gmbh, Lauda-Königshofen, Germany) using a six-digit
microbalance. For this measurement, a plastic mold of 5 mm diameter and 2 mm thickness
was used for the preparation of the disc samples of each restorative material; the disc
samples were cured with the LED lamp at four curing durations (5 s, 10 s, 15 s, and 20 s).
Our reason behind producing samples of this geometry is that clinical gross dental cavities
are about 5 mm (width) and 2 mm (depth), hence the geometrical dimensions (5
×
2 mm)
for our mold. For larger cavities, the restoration material must be added in layers to overlap
with the correspondent irradiation time exposure on photo polymerization lamp in order
to have a final good quality dental restoration work.
Microhardness was tested by means of the Vickers method and was measured on the
back side of the samples as the face surface was deformed due to shrinkage (the surface at the
sample/air interface released the volatile compounds faster than the bottom surface and the
samples became concave). The tester was equipped with a square based diamond pyramid
as indenter; we set the value of the load to 25 g and the time duration of 10 s. The sample
was held firmly in position and the indentation performed using the set parameters. The
equipment used in our investigation was a microhardness tester Model FM-700 and Serial
number XM0190 (Future-Tech Corp, Kawasaki, Japan). The obtained results are given by unit
of hardness that are known as the Vickers pyramid number (HV). The Vickers test is often
easier to use than other hardness tests since the required calculations are independent of the
size of the indenter, and the indenter can be used for all materials irrespective of hardness.
The Raman spectra were obtained with a research-grade dispersive micro-Raman
spectrometer (NRS-7200, from JASCO Corp, Tokyo, Japan) using a 531.94 nm laser beam as
a probing source. The laser light was focused on the sample surface by means of a long
working distance 10
×
magnification objective lens with a numerical aperture (NA) = 0.25
(Olympus, Tokyo, Japan) to a spot of about 20 microns. The nominal power of the laser was
set to 5.6–5.7 mW and spectra were collected in a backscattering geometry. The spectra were
nonlinearly calibrated with PP bands. In some cases, a fluorescence correction was applied
using JASCO Spectra Analysis 2.10 software using a circle type with 3–4 intervals and
radii of 100–800 points. All spectra were analyzed in the OriginLab Origin Pro 9 software
(version 9.6, OriginLab Corp, Northampton, MA, USA).
3. Results and Discussion
3.1. Density Measurements
Density measurements took into consideration the mass of the sample and the curing
time and the results were adjusted for water density correction. Table 2shows the results
of the measurements. XRV Herculite had the highest density with a 20 s curing time.
This value is similar to that of human enamel (2.84–3.00 g/cm
3
, according to [
30
]). The
composition of the material—Herculite was composed of up to 59% mineral fillers—is
responsible for this value. OptiShade, which had 80% nanofillers dispersed inside the
monomeric matrix, has the lowest values for density. Even at 20 s curing time, the density
barely approaches that of liquid water.
Table 2. Summary of the density measurements (n = 3) for the three dental materials at four curing intervals.
Time (s) P VertiseFlow (g/cm3)
±SD
P Optishade (g/cm
3
)
±
SD
P Herculite (g/cm3)
±SD
5 1.0146 ±0.05 0.4184 ±0.02 0.5497 ±0.02
10 0.8641 ±0.04 0.733 ±0.03 1.7716 ±0.08
15 1.8606 ±0.09 0.4527 ±0.02 1.8408 ±0.09
20 0.6367 ±0.03 0.7442 ±0.03 2.7015 ±0.1

Polymers 2024,16, 466 5 of 12
For the third composite, VertiseFlow, its behavior was curious: it reached the maximum
density at 15 s of curing, while at 20 s the density was lowest. Ytterbium triflouride (YbF
3
)
could be responsible for this anomaly, but the exact mechanism is not clear. Initially, YbF
3
has been used as radiopacifying agent in composite resin restorative materials, but the
additional benefits in terms of the lower setting time and fast hardening process have led
to its widespread use in root-sealing cements. An excellent work on the intricate properties
of this additive in dental restorative materials is given by John W. Nicholson [
31
]. Here, the
author noted that the hardness of the surface of the restorative materials containing YbF
3
and BaSO4decreases at higher percentage.
This could explain the density measurements in VertiseFlow. YbF
3
is a Lewis acid that
releases fluoride when incorporated in polymer-based materials and leads to the formation
of fluorinated compounds. The reaction is triggered by laser irradiation which causes
local heating and accelerates the release of fluoride ions. In turn, fluorinated compounds
typically have a low density [
32
]. For VertiseFlow, it is recommended that the curing time
is limited to 15 s.
An important observation is that only for Herculite the density of this materials
increases with the increase in the curing interval. For the OptiShade and VertiseFlow, the
density does not vary in a homogeneous manner. This is due to the composition of the
two composites: VertiseFlow has YbF
3
as radiopacifier material and OptiShade has 80%
nano-sized fillers finely dispersed in a monomer matrix, which becomes lightweight upon
photopolymerization.
3.2. Microhardness Test
The Vickers hardness test produced hardness numbers (Vickers pyramid
number—HV
)
that indicate the load over the indentation surface area. An important relation established by
Equation (1) occurred between HV and the other materials’ constants, such as the E and YS:
HV =2
3YS 2+lnE
3YStanα (1)
where,
- HV is hardness;
- E is the elastic modulus;
- YS is the yield strength;
- A is the semi-angle of the conical indenter.
According to the experiments and finite element analysis (FEA), Equation (1) is sat-
isfied for E/Y tan
α
< ~30; in case of E/Y tan
α
> ~30, the relation is simple as H > 3YS
known as Tabor ’s relation) or H~3YS (in this case, because our materials are polymers). HV
behavior can be easily interpreted by the YS in the case of polymers [33].
Microhardness was measured on the back side of the samples as the face surface was
deformed due to shrinkage. The results are presented in Figure 1. The highest HVs were
obtained for OptiShade (the highest number was 75.7 HV for 15 s curing time). Compared
with the HV for natural dental enamel, which is 273–374, OptiShade has a HV that is quite
low. Herculite showed a decrease in hardness with the increase in curing time. Thus,
its lowest HV was obtained after a 20 s curing time (48.48 HV) and its highest was for
10 s curing time (71.125 HV). This is unusual since Herculite comes the closest to natural
enamel density. The lower hardness could be due to a lower concentration of fillers (only
59% compared to OptiShade, which has 80%) or to the size of the fillers (Herculite has
micro-fillers, while OptiShade contains nano-sized fillers). Another relation was found
between the monomer concentration and the HV, and is further sustained by the results
for VertiseFlow. For the VertiseFlow composite, the HVs were the lowest of the set. This
corresponds to a composition of 30% methacrylate. Moreover, these results come to support
the density measurements where VertiseFlow showed an abnormal behavior for the 20 s
curing time. We hypothesize that the YbF
3
released during curing reacts with the cured

Polymers 2024,16, 466 6 of 12
methacrylate polymer chains, breaking their bonds, and weakening the structure (this
justifies the low YS and low HV according to Equation (1) and the anomaly in density).
Polymers 2024, 16, x FOR PEER REVIEW 6 of 12
curing time. We hypothesize that the YbF3 released during curing reacts with the cured
methacrylate polymer chains, breaking their bonds, and weakening the structure (this jus-
tifies the low YS and low HV according to Equation (1) and the anomaly in density).
Figure 1. Vickers hardness results of the three dental materials after four polymerization intervals.
Herculite and OptiShade reach the maximum hardness after 10 s and 15 s of polymerization, re-
spectively, while VertiseFlow has an unusual behavior: the hardness value drops significantly at
15 s of curing compared to the neighboring values. This abnormal behavior could be due to the
presence of YbF3, which reacts with the cured methacrylate polymer chains, breaking their bonds
and weakening the structure.
Corroborating these results with the density measurements we can conclude that
OptiShade composite is lightweight and resistant, which makes it a very good material
for dental enamel restorations. The results of the two measurements imply that the con-
centration of methacrylate polymers and the size of the fillers induce the level of hardness
(the higher the concentration of fillers, the higher the HV) and the density (cross-polymer-
ization produces low density composites). Unreacted monomer acts as plasticizer and
weakens the matrix.
3.3. Raman Spectroscopy
The Raman spectra were recorded after each polymerization interval on the back of
the sample (Figure 2) because it provided a smooth surface. The spectra of the VertiseFlow
and Herculite were similar due to their close methacrylate concentration: VertiseFlow has
a 30% methacrylate loading while Herculite has a 41% monomer loading. Optishade, how-
ever, showed different behavior: because of the 20% methacrylate loading of the compo-
site, the spectrum displayed intense vibrational activity in the 600–3200 cm−1 range. A pos-
sible explanation was given by Willis et al. in 1969 [34]: the polymerized methacrylate
Figure 1. Vickers hardness results of the three dental materials after four polymerization intervals.
Herculite and OptiShade reach the maximum hardness after 10 s and 15 s of polymerization, respec-
tively, while VertiseFlow has an unusual behavior: the hardness value drops significantly at 15 s of
curing compared to the neighboring values. This abnormal behavior could be due to the presence of
YbF
3
, which reacts with the cured methacrylate polymer chains, breaking their bonds and weakening
the structure.
Corroborating these results with the density measurements we can conclude that
OptiShade composite is lightweight and resistant, which makes it a very good material for
dental enamel restorations. The results of the two measurements imply that the concentra-
tion of methacrylate polymers and the size of the fillers induce the level of hardness (the
higher the concentration of fillers, the higher the HV) and the density (cross-polymerization
produces low density composites). Unreacted monomer acts as plasticizer and weakens
the matrix.
3.3. Raman Spectroscopy
The Raman spectra were recorded after each polymerization interval on the back of the
sample (Figure 2) because it provided a smooth surface. The spectra of the VertiseFlow and
Herculite were similar due to their close methacrylate concentration: VertiseFlow has a 30%
methacrylate loading while Herculite has a 41% monomer loading. Optishade, however,
showed different behavior: because of the 20% methacrylate loading of the composite, the
spectrum displayed intense vibrational activity in the 600–3200 cm
−1
range. A possible
explanation was given by Willis et al. in 1969 [
34
]: the polymerized methacrylate molecule
is both asymmetrical and amorphous, which lead to several conformational states.

Polymers 2024,16, 466 7 of 12
Polymers 2024, 16, x FOR PEER REVIEW 7 of 12
molecule is both asymmetrical and amorphous, which lead to several conformational
states.
(a) (b)
(c)
Figure 2. (a) Raman spectra at 532 nm for non-cured composites after luminescence correction; (b)
Raman spectra for XRV Herculite samples during curing; (c) details of Raman spectra for XRV Her-
culite samples during curing showing 1608 and 1638 cm
−1
bands. The inset shows the calculated
degree of conversion.
To avoid any misleading data and bias from our part, we collected the Raman spectra
of unpolymerized samples to identify the primary compositions of the three materials in-
vestigated here (Figure 2a). It can be clearly seen that the spectra have a similar shape for
all the composites, showing the typical 638 cm
−1
medium-intensity peak aributed to the
C-COO in plane symmetrical deformation vibration, a 1113 cm
−1
weak-intensity peak cor-
responding to the skeletal breathing mode (C-C-C), a 602 cm
−1
peak aributed to the C-C-
O symmetric vibration, a 810 cm
−1
strongly polarized C-O-C bond, a 1456 cm
−1
medium-
400 600 800 1000 1200 1400 1600 2800 3000 3200
0
200
400
600
800
Intensity (arb. units)
Raman Shift (cm
-1
)
Optishade
Vertise
XRVHerculite
(602.6)
(638.6)
(810.8)
(1638.3)
(1718.1)
(1607.7)
(2929)
(2954.6)
(3068)
(3103)
(1456.7)
(1403.6)
(1188)
(1113.5)
(1225)
(2990.4)
(1295)
400 600 800 1000 1200 1400 1600
0
50
100
Intensity (arb. units)
Raman Shift (cm
-1
)
0s
5s
10s
15s
20s
Figure 2. (a) Raman spectra at 532 nm for non-cured composites after luminescence correction;
(b) Raman spectra for XRV Herculite samples during curing; (c) details of Raman spectra for XRV
Herculite samples during curing showing 1608 and 1638 cm
−1
bands. The inset shows the calculated
degree of conversion.
To avoid any misleading data and bias from our part, we collected the Raman spectra
of unpolymerized samples to identify the primary compositions of the three materials
investigated here (Figure 2a). It can be clearly seen that the spectra have a similar shape
for all the composites, showing the typical 638 cm
−1
medium-intensity peak attributed
to the C-COO in plane symmetrical deformation vibration, a 1113 cm
−1
weak-intensity
peak corresponding to the skeletal breathing mode (C-C-C), a 602 cm
−1
peak attributed to
the C-C-O symmetric vibration, a 810 cm
−1
strongly polarized C-O-C bond, a 1456 cm
−1
medium-intensity peak corresponding to the C-H vibrational mode in
α
-CH
3
, a 1403 cm
−1
peak attributed to the CH
2
bond, and a 1718 cm
−1
peak attributed to the polarized C=O
bond [34]. The assignment of the Raman bands is presented in Table 3.

Polymers 2024,16, 466 8 of 12
Table 3. Observed Raman bands and their assignments.
Raman Band Assignment Reference
602.6 νs(C–C–O) [34]
638.6 ν(C–COO) [35]
810.8 νs(C–O–C) [34]
1113.5 νa(C–O–C) and C–C skeleton backbone [34,35]
1188 νa(C–O–C) and C–C skeleton backbone [34,35]
1225 ν(C–O) [34,36]
1295 ν(C–COO) [34,36]
1403.6 CH2twist or wag [34]
1456.7 δa(C–H) of α–CH3δa(C–H) of O–CH3 [34,35]
1607.7 Aliphatic C-C [37]
1638.3 Aromatic C=C [37]
1718.1 ν(C=O) [34]
2929 Combination band [34]
2954.6 νsC–H stretching [35]
2990.4 νaC–H stretching [35]
To calculate the polymerization rate for the three composites, we needed to select two
representative Raman bands: (1) one that corresponds to the polymerized composite (a
peak that is attributed to a
σ
-bond characterized by the stretching or deformation mode
of the polymer backbone
→
saturated chain) and a second band that corresponds to the
un-polymerized composite (a peak attributed to a
π
-bond
→
un-saturated bond). Based
on Figure 2c, we selected the 1608 cm
−1
peak corresponding to the saturated C-C bond as
“marker” of the cured composite [
34
] and the 1638 cm
−1
band attributed to the un-saturated
C=C bond [
38
]. By translating these peaks to the cured composites (e.g., Figure 2b and
particularly 2c-Herculite), we observed that their ratio shifted during curing compared to
the un-polymerized composite. This shift is given by the variables in the composition of
the dental materials and the time of curing.
An important observation is that the uncured dental materials have a certain amount
of polymerized material that act as polymerization sites and are responsible for the initial-
ization of the curing (to kick-start the radical polymerization process) [39].
The polymerization rate was calculated based on Equation (2) [38], as follows:
DC(d egree o f curin g %) = (1−Rcured
Runcured
)·100 (2)
where the ratio of the cured/uncured composite is given as follows:
Rcured =I1638 cm−1
I1608 cm−1
(3)
The results of the calculations are provided in Table 4, for each composite and for each
curing time. Moreover, Table 4also contains the methacrylate loading of each dental material.
The highest DC value was calculated for OptiShade, which also indicated a stable
polymerization process in time. At 20 s, the curing was slightly decreased because the
reaction rate slowed due to shrinkage and breaking of the surface lattice (Figure 3) [
40
].
This high stability as a function of curing time is given by its composition of 80% nano-
sized filler and 20% methacrylate monomer. Increasing the concentration of monomer
to 41%, as it is the case of Herculite induces a curious behavior: the DC decreases from
74.8% (at 5 s curing time) to 72% (at 15 s) and increases sharply to 80.4% (at 20 s curing

Polymers 2024,16, 466 9 of 12
time). These results could be explained by the high content of inert mineral fillers, which
interfere with the linear polymerization process of the resin [
40
,
41
]. By decreasing the
amount of methacrylate in the dental material to 30% as it is the case for VertiseFlow, the
DC value showed that the polymerization process was constant for the first 10 s, after
which it increased slowly to 85%.
Table 4. Calculation of the degree of curing for each dental composite at various curing times (5 s,
10 s, 15 s, and 20 s).
Composite Name Monomer
Composition
DC (% ±SD)
(5 s)
DC (% ±SD)
(10 s)
DC (% ±SD)
(15 s)
DC (% ±SD)
(20 s)
Vertise Flow 30% methacrylate 80.89 ±4.04 80.87 ±4.04 82 ±4.1 85 ±4.25
Herculite 41% methacrylate 74.8 ±3.7 74.3 ±3.7 72.08 ±3.6 80.4 ±4.0
Optishade 80% methacrylate 89.4 ±4.47 91.9 ±4.5 90.8 ±4.54 79.2 ±3.96
Polymers 2024, 16, x FOR PEER REVIEW 9 of 12
filler and 20% methacrylate monomer. Increasing the concentration of monomer to 41%,
as it is the case of Herculite induces a curious behavior: the DC decreases from 74.8% (at
5 s curing time) to 72% (at 15 s) and increases sharply to 80.4% (at 20 s curing time). These
results could be explained by the high content of inert mineral fillers, which interfere with
the linear polymerization process of the resin [40,41]. By decreasing the amount of meth-
acrylate in the dental material to 30% as it is the case for VertiseFlow, the DC value showed
that the polymerization process was constant for the first 10 s, after which it increased slowly
to 85%.
Table 4. Calculation of the degree of curing for each dental composite at various curing times (5 s,
10 s, 15 s, and 20 s).
Composite Name Monomer Composition DC (% ± SD)
(5 s)
DC (% ± SD)
(10 s)
DC (% ± SD)
(15 s)
DC (% ± SD)
(20 s)
Vertise Flow 30%methacrylate 80.89 ± 4.04 80.87 ± 4.04 82 ± 4.1 85 ± 4.25
Herculite 41% methacrylate 74.8 ± 3.7 74.3 ± 3.7 72.08 ± 3.6 80.4 ± 4.0
Optishade 80% methacrylate 89.4 ± 4.47 91.9 ± 4.5 90.8 ± 4.54 79.2 ± 3.96
Figure 3. Variation of the degree of curing as a function of time for the three dental composites.
The amount of filler as well as the type of filler added to each product has a direct
impact on the polymerization process. OptiShade has the highest amount of filler and ex-
hibits the highest degree of curing, while VertiseFlow and Herculite show a lower
polymerization grade. The difference comes from the size of the fillers: Herculite has a
micro-range for its filler while OptiShade is composed of nano-sized fillers. Another dif-
ference that can explain the results obtained from the microhardness and density is the
make-up of the filler: Herculite contains TiO2 and OptiShade contains only glass ceramics,
while VertiseFlow contains small amounts of YbF3. Although fillers promote mechanical
reinforcement and most importantly low polymerization shrinkage [42] for the dental
Figure 3. Variation of the degree of curing as a function of time for the three dental composites.
The amount of filler as well as the type of filler added to each product has a direct
impact on the polymerization process. OptiShade has the highest amount of filler and
exhibits the highest degree of curing, while VertiseFlow and Herculite show a lower
polymerization grade. The difference comes from the size of the fillers: Herculite has
a micro-range for its filler while OptiShade is composed of nano-sized fillers. Another
difference that can explain the results obtained from the microhardness and density is the
make-up of the filler: Herculite contains TiO
2
and OptiShade contains only glass ceramics,
while VertiseFlow contains small amounts of YbF
3
. Although fillers promote mechanical
reinforcement and most importantly low polymerization shrinkage [
42
] for the dental
composite, their performance is dependent on filler–polymer contact; if there are any kind
of interfacial flaws, the stress point results in a decrease in flexural strength and fracture of

Polymers 2024,16, 466 10 of 12
the composite. In turn, as it ages and the fracture propagate in the bulk of the material, it
produces breakdown of the restoration.
Comparing our data with the literature, we found that Taher reported similar DC
values, in the range of 77–83% for Herculite, measured by Raman spectroscopy [
43
]. Un-
fortunately, the Optishade and VertiseFlow composites are relatively new and we could
not find data for their DC values from Raman measurements. However, we found some
reports for similar composites measured by FTIR with DC values in the 90% range [
44
]. We
agree that are a lot of differences in DC data reported in the literature. The main sources of
errors are related to differences in investigation method, sample preparation, Raman laser
power or wavelength on sample, or even the post-curing polymerization effects [45].
Based on the Raman spectra, the optimal curing time for the composites are as follows:
20 s for Herculite and VertiseFlow, and 10 s for Optishade. By comparing these results with
the ones from the density and microhardness measurements, one can observe that they
have similar behaviour. Herculite has the highest density for all the materials investigated,
coming close to that of human enamel after 20 s of curing, while VertiseFlow has the highest
density after 15 s of curing time. OptiShade is the lightest material irrespective of the curing
time (its density is lower than that of water). The microhardness measurements indicated
that OptiShade has the best Vickers number after 15 s of curing, the same as VertiseFlow,
while Herculite has the best microhardness number after just 10 s.
4. Conclusions
Three dental materials have been analyzed in terms of density, Vickers’ microhardness,
and degree of curing by spectral monitoring via Raman spectroscopy at four different curing
intervals. We attempted to rank them in order to establish their capacity to treat cavities
in damaged teeth. Following the density measurements, we rated from best to worst, as
follows: Herculite (20 s curing time) > VertiseFlow (15 s curing time) > Optishade. After the
Vickers’ microhardness measurements, the rank became Optishade (15 s)
≥
VertiseFlow
(15 s) > Herculite (10 s). The degree of curing indicated that Herculite (20 s)
≥
VertiseFlow
(20 s) > Optishade (10 s). In conclusion, 10–15 s of curing time is optimal for Optishade to
reach its best characteristics, while a curing time of 20 s is advised when using Herculite and
VertiseFlow. Future studies will focus on the stability of the resin composites in different
oral conditions (immersion of the polymerized resins in acidic/alcoholic synthetic plasma,
or fracture induced by thermal shock).
Author Contributions: Conceptualization, E.G.; methodology, C.-R.L. and
S.-M.I.
; software, E.G. and
S.-M.I.
; validation, R.R.I.; formal analysis, E.G. and D.I.G.; investigation, S.-M.I., D.I.G. and C.-R.L.; re-
sources, R.R.I.; data curation, C.E.A.G. and A.-M.I.; writing—original draft preparation, A.-M.I.;
writing—review
and editing, e.g., C.E.A.G. and C.-R.L.; visualization, S.-M.I.; supervision, C.E.A.G.;
funding acquisition, R.R.I. All authors have read and agreed to the published version of the manuscript.
Funding: Publication of this paper was supported by the University of Medicine and Pharmacy Carol
Davila, through the institutional program Publish not Perish.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are contained within the article.
Acknowledgments: C.R. Luculescu was supported by the Core Program LAPLAS VII—contract
no. 30N/2023 funded by the Romanian Ministry of Research, Innovation and Digitization. A.M.
Iordache, S.M. Iordache and C.E.A. Grigorescu were supported by the CORE Program with the
National Research Development and Innovation Plan 2022–2027, carried out with the support of
MCID, project no. PN 23 05 and by the Ministry of Research and Innovation through Program
I—Development of the National R&D System, Subprogram 1.2—Institutional Performance—Projects
for Excellence Financing in RDI, contract no. 18PFE/30.12.2021.
Conflicts of Interest: The authors declare no conflicts of interest.

Polymers 2024,16, 466 11 of 12
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