D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
871–878
SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE
USED FOR RESIN TEETH
SINTEZA PMMA/ZnO NANODELCEV KOMPOZITOV ZA
IZDELAVO ZOB IZ UMETNIH SMOL
Danica Popovi}1, Rajko Bobovnik2, Silvester Bolka2, Miroslav Vukadinovi}1,
Vojkan Lazi}1, Rebeka Rudolf3,4
1University of Belgrade, School of Dental Medicine, Dr. Suboti}a 8, 11000 Belgrade, Serbia
2Faculty of Polymer Technology, Ozare 19, 2380 Slovenj Gradec, Slovenia
3Zlatarna Celje d.o.o., Kersnikova ulica 19, 3000 Celje, Slovenia
4University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia
d.popovic984@gmail.com
Prejem rokopisa – received: 2017-02-24; sprejem za objavo – accepted for publication: 2017-03-31
doi:10.17222/mit.2017.025
Wear resistance is one of the most important physical properties of the artificial teeth used in acrylic dentures. The goal of this
research was to synthesize a new composite material made of matrix Poly-(methyl methacrylate)-PMMA with different
percentages (2 % and 3 % of volume fractions) of zinc-oxide nanoparticles (ZnO NPs) as reinforcing elements, to improve its
mechanical properties. The dynamic mechanical behaviour of this composite was studied through the DMA method in
comparison to the pure PMMA supported by the characterization of their microstructures. Then the wear resistance was
analysed on the samples, which were prepared in the form of teeth. In this context their vertical height loss was measured after
100,000 chewing cycles on a chewing simulator, before and after the artificial thermal ageing. Investigations showed that the
PMMA/ZnO NP composites dampened the vibrations better than the pure PMMA, which could be assigned to the homogenous
distribution of ZnO NPs in the PMMA matrix. It was found that the mean vertical height loss for the pure PMMA teeth was
significantly higher (more than 4 times) compared to composite teeth made with ZnO NPs. Introducing the thermal artificial
ageing led to the finding that there was no effect on the height loss by the composite material with 3 % of volume fractions of
ZnO NPs. Based on this it was concluded that PMMA/ZnO NPs composites showed improved in-vitro wear resistance
compared to acrylic-resin denture teeth, so this new composite material should be preferred when occlusal stability is
considered to be of high priority.
Keywords: poly-(methyl methacrylate)–PMMA, zinc-oxide nanoparticles, composite, resin teeth
Odpornost proti obrabi je ena izmed najbolj pomembnih fizikalnih lastnosti umetnih zob pri akrilnih protezah. Cilj te raziskave
je bil sintetiziranje novega kompozitnega materiala, izdelanega iz matrice poli-(metil meta akrilata) – PMMA z razli~nimi
volumskimi odstotki (2 % in 3 %) nanodelcev cinkovega oksida (ZnO NPs), uporabljenimi kot oja~itveni element za izbolj{anje
mehanskih lastnosti materiala. Dinami~no mehansko obna{anje teh kompozitov smo raziskali z metodo DMA v primerjavi s
~istim PMMA, ter s karakterizacijo njihovih mikrostruktur. Nato je bila analizirana odpornost proti obrabi na vzorcih, ki so bili
pripravljeni v obliki zob. Izmerjena je bila njihova navpi~na izguba vi{ine po 100.000 `ve~ilnih ciklih na `ve~ilnem simulatorju,
pred in po umetnem toplotnem staranju. Preiskave so pokazale, da so PMMA/ZnO NPs kompoziti bolje bla`ili vibracije kot ~isti
PMMA, kar lahko pripi{emo homogeni porazdelitvi ZnO NPs v PMMA matrici. Ugotovili smo, da je bila povpre~na navpi~na
izguba vi{ina za ~isti PMMA zob znatno vi{ja (ve~ kot 4×) v primerjavi s kompozitnimi zobmi z ZnO NPs. Umetno toplotno
staranje ni imelo u~inka na izgubo navpi~ne vi{ine zoba iz kompozitnega materiala s 3 volumskimi % ZnO NPs. Na podlagi tega
smo sklenili, da imajo kompoziti PMMA/ZnO NPs izbolj{ano in vitro odpornost proti obrabi, v primerjavi z zobno protezo iz
akrilne smole, zato naj bi imel ta novi kompozitni material prednost pri uporabi, kadar je potrebna okluzalna stabilnost.
Klju~ne besede: poli-(metil meta akrilat)-PMMA, nanodelci cinkovega oksida, kompoziti, zobje iz umetnih smol
1 INTRODUCTION
Teeth loss in the human population is associated with
numerous problems such as chewing difficulties,
alteration of speech and facial expressions, which all
leads to de-socialization. According to the Oral
Health-Healthy People 2010: Objectives for Improving
Health 26 % of the US population and 33 % of the
Central Europe population aged between 65 years and 74
years are toothless; but some dramatic data and a wide
variation in edentulism prevalence among adults aged 50
and above was found in the following countries: 48.3 %
in Ireland, Malaysia 56.6 % and Netherlands 65.4 %,
which indicates an international problem.1–3
The problems of toothless people in the region of the
Balkan peninsula and South-Eastern Europe are usually
solved with a complete or partial acrylic denture, which
is the easiest and the cheapest solution, because more
than 90 % of these patients are in a bad financial
situation and incapable of buying dental implants and
hybrid bridges above them. With an acrylic denture the
wear resistance represents one of the most important
physical properties because, in the case that the material
has excessive wear, that might cause loss of the vertical
dimension of occlusion (VDO) which is associated with
decreased occlusal forces, loss of masticatory efficiency,
improper tooth relationship, and fatigue of masticatory
muscles.4 Recently, a study examined how VDO can
Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878 871
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.3:616.314-77:678.6:67.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)871(2017)
affect brain function in complete denture wearers, and
also measured occlusal forces when there is a change in
VDO in which electroencephalograms were used.5 Many
authors have pointed out the necessity of creating
complete new dentures, or relining of the denture base,
which is not a simple procedure and presents financial
pressure for the patients.6 So, finding the new combi-
nation of materials with improved mechanical properties,
which include better wear resistance, would be helpful
for many reasons for those patients.
Many basic researches based on polymer materials
represent the challenge. One of these are made of
supramolecular polymers as a new high-tech material.7,8
Over the past two decades a great effort has been made
by the community of researchers for the synthesis and
utilization of nanomaterials in the field of medical
materials.9 Using fine nanoparticles as reinforcing
elements could improve the functional properties of
conventional materials such as PMMA, which has been
used for dentures since 1937. The ZnO NPs are small
objects that behave as an entire unit regarding their
transport and properties. The introduction of ZnO NPs
represents a new approach in dentistry by the production
of a denture, because they could decrease the level of
residual monomer and, besides this, they also have anti-
microbial activity. This was shown in recent studies.10,11
Namely, ZnO NPs are environmentally friendly materials
in the nanometre size range, and could be synthesized in
a wide range of particle shapes and structures.12–14 ZnO
in bulk form is a largely inert, white powder compound
that has a very broad application, from the chemical
industry through cosmetic and medical products. In
dentistry they have an important place as a medicament
for healing injures after periodontal surgical treatment, in
the cementation of temporary, as well as definitive,
crowns etc. ZnO is also a biocompatible material that
exhibits antimicrobial properties against gram-negative
as well as gram-positive bacteria.15 A recent study also
investigated, proved and determined the minimal
inhibitory concentration of ZnO NPs against Candida
Albicans.16
Nanostructured materials with unique and fascinating
properties motivate scientists tremendously to explore
and understand their formation and growth processes and
following the critical volume in several studies we tried
to find the right percentage of ZnO NPs in a PMMA
matrix which could advance the properties of this new
composite, because increasing the percentage of added
NPs could downgrade some of them.17,18
According to the presented state-of-the-art study, the
aim of this research was:
(i) To synthesize a composite PMMA/ZnO NPs material,
to test its mechanical properties, and evaluate the
developed microstructures;
(ii) To investigate the wear resistance of resin teeth in a
chewing simulator with two combinations, before and
after thermal artificial ageing. This investigation
included not only the classical testing samples, but
also the real model of the shapes of first upper molars
(testing resin teeth) and showed possible practical
applications in dentistry.
2 EXPERIMENTAL PART
2.1 Materials for composite PMMA/ZnO NPs
Commercially available ZnO nanopowder (purity
99.99 %, density 5.606 g/cm3, insoluble in water, for-
mula weight 81.37, melting point 1975 °C) was pur-
chased from Interdent (Belgrade, Serbia). The average
size of the ZnO NPs was 30 nm and they were in the
form of a sphere. The typical shape and morphology of
characteristic ZnO NPs are shown in Figure 1, which
was made on a transmission electron microscope (TEM).
PMMA matrix (polymethylmethacrylate powder with
benzoyl-peroxide, pigments and for a liquid-methyl-
methacrylate HQ 60, ethylene glycoldimethacrylate,
methylmethacrylate composite) was distributed by
Galenika (Belgrade, Serbia). This material was used for
Biogal® acrylic teeth (Galenika, Belgrade, Serbia) and
has already passed through the applied ISO Standards.
2.2 Measurement of ZnO nanoparticles’ size distri-
bution and zeta potential in water and MMA
Because of the frequent problem with agglomeration
among nanoparticles, especially ZnO NPs in water, we
used the DLS technique to confirm or deny this fact
indirectly and compare them with ZnO NPs in suspen-
sion with MMA. Initially, ZnO NPs powder (0.05 g) with
water and MMA monomer (15 mL by volume in both
cases) was mixed through magnetic stirring for the time
duration of 45 min. The size distributions and surface
charge of the nanoparticles were determined with a
Malvern Zetasizer Nano ZS (Malvern Instruments Ltd.,
U.K.) by the DLS technique. The suitable parameters for
ZnO NPs Absorption: 0.1 and Refractive index: 2.0.
were chosen according to 19.
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
872 Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Micrograph of shape and morphology of characteristic ZnO
NPs (TEM, 10 000×)
2.3 Preparation of samples
Samples were made in two shapes: Lamina (dimen-
sion: 45 mm × 3 mm × 7 mm) for characterization of the
new composite and then in the form of the first upper
molar for direct potential use in dentistry.
The ZnO NPs in the PMMA matrix were mixed with
PMMA powder and then free radical chain poly-
merization was performed of MMA monomer in bulk.
The new composite was synthesized at the selection of
the appropriate technological regime, which started in
the moment of adding monomer liquid PMMA/ZnO NPs
powder for 1 h and 45 min at 100 °C. The procedure for
both samples followed by shaping the samples firstly in
wax, then duplicated and changed in to the pure PMMA,
PMMA filled with 2 % of volume fractions and 3 % of
volume fractions of ZnO NPs. The volume % were
chosen according to our previous experiences and based
on 7,16,18. All samples in the form of the first upper molar
were identical in form, plan parallel and highly polished.
They were prepared for a wear test in a chewing
simulator. Schematic presentations of the composite
PMMA/ZnO NPs synthesis and their corresponding
testing are shown in Figure 2. A list of samples’ names,
their mixing ratio PMMA/ZnO NPs and technical
descriptions are given in Table 1. PMMA1 was chosen
for a comparison with the new composites.
2.4 Characterization
Thermo-mechanical properties were examined using
a Perkin Elmer DMA 8000 dynamic mechanical anal-
yser. The TT_DMA software, Version 14310, was used
for the evaluation of the results. The viscoelastic proper-
ties of the samples were analysed by recording the
storage modulus (E’), loss modulus (E’’) and loss factor
(tan ) as a function of temperature. The height of the
peak of the loss factor determined the damping beha-
viour – with decreasing peak height the damping beha-
viour was increasing. For the analyses of test samples,
the DMA instrument was operated in the dual cantilever
mode. The viscoelastic analyses were carried out on
samples with dimensions of approximately 42 mm ×
5 mm × 2 mm. The samples were heated at 2 °C/min
from room temperature to 180 °C under an air atmo-
sphere. A frequency of 10 Hz and amplitude of 20 μm
were used.
A flash differential scanning calorimeter (Flash DSC)
was used for the identification of the glass transition and
reorganisation of the polymers for all the samples. Flash
DSC works by ultra heating and cooling rates that induce
physical transitions and chemical processes. This charac-
terization was very important for dentistry practice
because it allowed us to follow the level of polymeri-
zation in the residual monomer, which could produce an
allergic reaction by the use of the new composite for the
resin teeth in removable dentures in the mouths of
patients.
2.5 Determination of wear resistance
The wear tests of the samples in the shape of upper
first molar were performed in a chewing simulator
CS-4.2 economy line (SD Mechatronics, Germany). The
simulator has two identical sample chambers and two
stepper motors which allow computer-controlled vertical
and horizontal movements. The masticatory cycle in this
research consisted of three phases: contact with a vertical
load of 5 kg, horizontal sliding of 0.4 mm, vertical
sliding of 0.2 mm and separating the teeth and machine
antagonist. In our investigation, two types of tests were
made: before and after thermal artificial ageing including
all samples from Table 1. The machine was set at
100.000 cycles, which simulated chewing over a period
of one year. The loss of substance was measured with an
electronic digital calliper – Globathronics GmbH (Ger-
many). Accelerated thermal ageing was carried out by
immersing the samples in a water bath at a temperature
of 70 °C.20,21
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878 873
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 1: Composition, mixing ratio and forms of samples for different testing
Samples/
abbreviation
Composition Mixing ratio Form forcharacterization Form for wear test
PMMA1 Pure PMMA 15 mL MMA + 23.4 gPMMA Lamina Tooth model
PMMA2 PMMA/2vol%ZnO NPs 15 mL + 22.93 g PMMA+ 0,47 g ZnO NPs Lamina Tooth model
PMMA3 PMMA/3vol%ZnO NPs 15 mL + 22.7 g PMMA+ 0,7 g ZnO NPs Lamina Tooth model
Figure 2: Schematic presentation of synthesis composite PMMA/ZnO
NPs and samples for testing
affect brain function in complete denture wearers, and
also measured occlusal forces when there is a change in
VDO in which electroencephalograms were used.5 Many
authors have pointed out the necessity of creating
complete new dentures, or relining of the denture base,
which is not a simple procedure and presents financial
pressure for the patients.6 So, finding the new combi-
nation of materials with improved mechanical properties,
which include better wear resistance, would be helpful
for many reasons for those patients.
Many basic researches based on polymer materials
represent the challenge. One of these are made of
supramolecular polymers as a new high-tech material.7,8
Over the past two decades a great effort has been made
by the community of researchers for the synthesis and
utilization of nanomaterials in the field of medical
materials.9 Using fine nanoparticles as reinforcing
elements could improve the functional properties of
conventional materials such as PMMA, which has been
used for dentures since 1937. The ZnO NPs are small
objects that behave as an entire unit regarding their
transport and properties. The introduction of ZnO NPs
represents a new approach in dentistry by the production
of a denture, because they could decrease the level of
residual monomer and, besides this, they also have anti-
microbial activity. This was shown in recent studies.10,11
Namely, ZnO NPs are environmentally friendly materials
in the nanometre size range, and could be synthesized in
a wide range of particle shapes and structures.12–14 ZnO
in bulk form is a largely inert, white powder compound
that has a very broad application, from the chemical
industry through cosmetic and medical products. In
dentistry they have an important place as a medicament
for healing injures after periodontal surgical treatment, in
the cementation of temporary, as well as definitive,
crowns etc. ZnO is also a biocompatible material that
exhibits antimicrobial properties against gram-negative
as well as gram-positive bacteria.15 A recent study also
investigated, proved and determined the minimal
inhibitory concentration of ZnO NPs against Candida
Albicans.16
Nanostructured materials with unique and fascinating
properties motivate scientists tremendously to explore
and understand their formation and growth processes and
following the critical volume in several studies we tried
to find the right percentage of ZnO NPs in a PMMA
matrix which could advance the properties of this new
composite, because increasing the percentage of added
NPs could downgrade some of them.17,18
According to the presented state-of-the-art study, the
aim of this research was:
(i) To synthesize a composite PMMA/ZnO NPs material,
to test its mechanical properties, and evaluate the
developed microstructures;
(ii) To investigate the wear resistance of resin teeth in a
chewing simulator with two combinations, before and
after thermal artificial ageing. This investigation
included not only the classical testing samples, but
also the real model of the shapes of first upper molars
(testing resin teeth) and showed possible practical
applications in dentistry.
2 EXPERIMENTAL PART
2.1 Materials for composite PMMA/ZnO NPs
Commercially available ZnO nanopowder (purity
99.99 %, density 5.606 g/cm3, insoluble in water, for-
mula weight 81.37, melting point 1975 °C) was pur-
chased from Interdent (Belgrade, Serbia). The average
size of the ZnO NPs was 30 nm and they were in the
form of a sphere. The typical shape and morphology of
characteristic ZnO NPs are shown in Figure 1, which
was made on a transmission electron microscope (TEM).
PMMA matrix (polymethylmethacrylate powder with
benzoyl-peroxide, pigments and for a liquid-methyl-
methacrylate HQ 60, ethylene glycoldimethacrylate,
methylmethacrylate composite) was distributed by
Galenika (Belgrade, Serbia). This material was used for
Biogal® acrylic teeth (Galenika, Belgrade, Serbia) and
has already passed through the applied ISO Standards.
2.2 Measurement of ZnO nanoparticles’ size distri-
bution and zeta potential in water and MMA
Because of the frequent problem with agglomeration
among nanoparticles, especially ZnO NPs in water, we
used the DLS technique to confirm or deny this fact
indirectly and compare them with ZnO NPs in suspen-
sion with MMA. Initially, ZnO NPs powder (0.05 g) with
water and MMA monomer (15 mL by volume in both
cases) was mixed through magnetic stirring for the time
duration of 45 min. The size distributions and surface
charge of the nanoparticles were determined with a
Malvern Zetasizer Nano ZS (Malvern Instruments Ltd.,
U.K.) by the DLS technique. The suitable parameters for
ZnO NPs Absorption: 0.1 and Refractive index: 2.0.
were chosen according to 19.
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
872 Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Micrograph of shape and morphology of characteristic ZnO
NPs (TEM, 10 000×)
2.3 Preparation of samples
Samples were made in two shapes: Lamina (dimen-
sion: 45 mm × 3 mm × 7 mm) for characterization of the
new composite and then in the form of the first upper
molar for direct potential use in dentistry.
The ZnO NPs in the PMMA matrix were mixed with
PMMA powder and then free radical chain poly-
merization was performed of MMA monomer in bulk.
The new composite was synthesized at the selection of
the appropriate technological regime, which started in
the moment of adding monomer liquid PMMA/ZnO NPs
powder for 1 h and 45 min at 100 °C. The procedure for
both samples followed by shaping the samples firstly in
wax, then duplicated and changed in to the pure PMMA,
PMMA filled with 2 % of volume fractions and 3 % of
volume fractions of ZnO NPs. The volume % were
chosen according to our previous experiences and based
on 7,16,18. All samples in the form of the first upper molar
were identical in form, plan parallel and highly polished.
They were prepared for a wear test in a chewing
simulator. Schematic presentations of the composite
PMMA/ZnO NPs synthesis and their corresponding
testing are shown in Figure 2. A list of samples’ names,
their mixing ratio PMMA/ZnO NPs and technical
descriptions are given in Table 1. PMMA1 was chosen
for a comparison with the new composites.
2.4 Characterization
Thermo-mechanical properties were examined using
a Perkin Elmer DMA 8000 dynamic mechanical anal-
yser. The TT_DMA software, Version 14310, was used
for the evaluation of the results. The viscoelastic proper-
ties of the samples were analysed by recording the
storage modulus (E’), loss modulus (E’’) and loss factor
(tan ) as a function of temperature. The height of the
peak of the loss factor determined the damping beha-
viour – with decreasing peak height the damping beha-
viour was increasing. For the analyses of test samples,
the DMA instrument was operated in the dual cantilever
mode. The viscoelastic analyses were carried out on
samples with dimensions of approximately 42 mm ×
5 mm × 2 mm. The samples were heated at 2 °C/min
from room temperature to 180 °C under an air atmo-
sphere. A frequency of 10 Hz and amplitude of 20 μm
were used.
A flash differential scanning calorimeter (Flash DSC)
was used for the identification of the glass transition and
reorganisation of the polymers for all the samples. Flash
DSC works by ultra heating and cooling rates that induce
physical transitions and chemical processes. This charac-
terization was very important for dentistry practice
because it allowed us to follow the level of polymeri-
zation in the residual monomer, which could produce an
allergic reaction by the use of the new composite for the
resin teeth in removable dentures in the mouths of
patients.
2.5 Determination of wear resistance
The wear tests of the samples in the shape of upper
first molar were performed in a chewing simulator
CS-4.2 economy line (SD Mechatronics, Germany). The
simulator has two identical sample chambers and two
stepper motors which allow computer-controlled vertical
and horizontal movements. The masticatory cycle in this
research consisted of three phases: contact with a vertical
load of 5 kg, horizontal sliding of 0.4 mm, vertical
sliding of 0.2 mm and separating the teeth and machine
antagonist. In our investigation, two types of tests were
made: before and after thermal artificial ageing including
all samples from Table 1. The machine was set at
100.000 cycles, which simulated chewing over a period
of one year. The loss of substance was measured with an
electronic digital calliper – Globathronics GmbH (Ger-
many). Accelerated thermal ageing was carried out by
immersing the samples in a water bath at a temperature
of 70 °C.20,21
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878 873
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 1: Composition, mixing ratio and forms of samples for different testing
Samples/
abbreviation
Composition Mixing ratio Form forcharacterization Form for wear test
PMMA1 Pure PMMA 15 mL MMA + 23.4 gPMMA Lamina Tooth model
PMMA2 PMMA/2vol%ZnO NPs 15 mL + 22.93 g PMMA+ 0,47 g ZnO NPs Lamina Tooth model
PMMA3 PMMA/3vol%ZnO NPs 15 mL + 22.7 g PMMA+ 0,7 g ZnO NPs Lamina Tooth model
Figure 2: Schematic presentation of synthesis composite PMMA/ZnO
NPs and samples for testing
2.6 Microstructure analysis
Microstructural characterizations of all the samples
were carried out with scanning electron microscopy
(SEM-Sirion 400 NC and Quanta 200 3D). The
specimens, without previous preparation, were broken in
an atmosphere of N2 to minimise the influence of
metallographic preparation regarding changes in the
structure of the composite. After breaking, the samples
were sprayed with Au (Jeol JSM 8310 appliance), which
enables the observation of the non-conductive surfaces of
the samples with an electron beam. The samples were
positioned into the chamber of the microscope and
observations were performed with an accelerating
voltage of 15 kV.
2.7 Statistical analysis
The statistical analysis included all the parameters
involved in the laboratory tests and it was performed in
the statistical package SPSS® 17.0. Multivariate linear
regression analysis was used to determine the predictors
of differences between the analysed groups of
samples. The threshold value for accepting the working
hypothesis was set at p < 0.05.
3 RESULTS AND DISCUSSION
3.1 Macroscopic view
A macroscopic view of both samples’ shapes
confirmed the homogenous structure of PMMA and
PMMA/ZnO NPs. There was only an acceptable diffe-
rence in colour. Adding ZnO NPs to the mixture with
PMMA gave a brighter colour as the percentage of
nanoparticles grew. This could be solved in the future by
the addition of proper pigment, which results in a similar
colour as in the teeth or oral mucosa, which are
replicated in resin teeth and denture bases.
3.2 Size distribution and zeta-potential by DLS
The statement of the homogeneous distribution of
ZnO NPs in samples was also confirmed by DLS
measurement of the results, which are shown in Table 2.
In ZnO NPs with MMA suspension, the average particle
size was 2.21±0.10 μm, while 42.1 % of the particles
were in the size range of 21.04 nm, whereas in ZnO NPs
with water suspension, the average particle size was
4.01±0.4 μm, while 38.1 % of the particles were in the
size range of 68.06 nm. Therefore, the average size of
the particles was greater in ZnO NPs with water
suspension with more presence of larger-sized clusters of
particles. Distribution of ZnO NPs in MMA suspension
was homogenous, which showed additionally the
insolubility of ZnO NPs in water. ZnO NPs with MMA
had a sharp and narrow intensity vs. size curve, as com-
pared to ZnO NPs with water; thereby, it can be stated
that there is a higher degree of agglomeration in the ZnO
NPs with water suspension (Figure 3). The present study
showed how it is possible to predict, simulate and
characterize the composite material, which is aimed for
use in dentistry. The DLS technique could confirm
indirectly that ZnO NPs in an MMA suspension give a
smaller agglomeration, which was also proven through
the calculated large hydrodynamic diameters. ZnO NPs
which have a zeta-potential between minus 30 mV and
plus 30 mV show a tendency for coagulation. Therefore,
ZnO NPs suspensions with water were unstable solutions
and showed the tendency of agglomerate, while adding
MMA in the ZnO NPs did not show much difference in
the stability of the suspension as compared with the
water. The samples in our work were made by mixing
first ZnO NPs nanopowder and PMMA powder matrix in
vacuum, at room temperature for 2 h then adding MMA,
which reduced the aggregation among ZnO NPs. This
may allow us to prepare homogenous nanocomposites
with organic matrices without additional surface
modification. The chosen average ZnO NPs size showed
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
874 Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Number vs. size distribution curve of ZnO NPs with MMA
and water
Table 2: Composition and the type of mixing used in the preparation of the solvent with the DLS measurements
Exp.
No. Composition Methodology
DLS Measurement
Run of
measurement Average size range (ìm) Zeta potential (mV)
1 0.05 g ZnONPs+ 15 mL H20
Magnetic stirring
(45 min)
I 3.50
4.01 ± 0.40
–3.45
–3.38 ± 0.81II 4.08 –2.35
III 4.48 –4.35
2 0.05 g ZnONPs+ 15 mL MMA
Magnetic stirring
(45 min)
I 2.28
2.21 ± 0.10
–3.40
–3.48 ± 0.07II 2.28 –3.57
III 2.06 –3.49
that is of vital importance for the final properties of the
new developed composite PMMA/ZnO NPs since it
affected the UV-vis absorption and thermal stability.
During the synthesis it was found that smaller ZnO NPs
showed a higher degree of agglomeration, which is in
accordance with the literature.22–24
3.3 Determination of properties
The results obtained by the DMA (Figure 4) showed
the following results. The difference in the elastic
modulus at low temperatures which may be due to a
slight decrease in the cross linking of PMMA by adding
ZnO NPs. At elevated temperatures, the incidence of
chain transfer increased (at termination). With chain
transfer, the propagating polymer radical reacts with
another molecule by proton abstraction and this leads to
branching and cross-linking.
Partly, the E modulus increases due to the addition of
ZnO NPs, but this increase is minimal. Because of the
absence of good interactions between the ZnO NPs and
the PMMA matrix, the loss factor increases with the
amount of addition of ZnO NPs in the PMMA matrix.
ZnO NPs added to the PMMA increased the E modulus
and reduced the level of cross-linking. At the same time,
the poor interaction between the filler and the matrix
results in a less elastic response, which means that the
composite of PMMA/ZnO NPs dampen the vibrations
better than pure PMMA. With the increasing of the loss
factor (tan delta) the damping of the material also
increases. In our case, the height of the loss factor peak
of the sample PMMA3 was the highest; therefore, the
damping of the vibrations of the sample PMMA3 is the
highest (Figure 5).
Characterization using the DMA method showed that
changes in the vibration-damping behaviour of the new
composite could mean that at the gingival a more gentle
feeling appears, which leads to this material being more
friendly to oral mucosa due to vibration damping. This
should also be proven through practice. Composite
PMMA/ZnO NPs would require longer curing times in
comparison with the pure PMMA. Based on this finding
it is expected to obtain a better material with the surface
modification of ZnO NPs where good interactions
between filler and matrix can be formed.
From the Flash DSC measurements (Figure 6), the
lowest glass-transition temperature was measured in the
PMMA3 sample (129.6 °C), followed by PMMA2
(131.0 °C) and the highest glass-transition temperature
was in the pure PMMA1 (131.1 °C). This fact is also
confirmed by the position of the peaks of the loss factor
(tan ), where the lowest glass-transition temperature
was measured in the PMMA3 (146.1 °C), followed by
PMMA2 (147.2°C) and the highest glass-transition
temperature in the pure PMMA1 (147.8 °C). In this
context it was found that the degree of cross-linking was
reduced by the addition of ZnO NPs. With increasing
glass-transition temperature the loss factor decreased,
which indicates that the material PMMA3 (tan  =
1.2003) has the best vibration-damping behaviour,
followed by PMMA2 (tan  = 1.1856). The pure
PMMA1 (tan  = 1.1839) showed the most elastic
response. From the height of tan  it can be concluded
that the 3 % addition of ZnO NPs in PMMA is needed to
obtain a significant change in the vibration-damping
behaviour of the composite. Despite the small differen-
ces measured, as well on DMA (E modulus, tan ) as on
Flash DSC (glass-transition temperatures), all the results
had the same tendency.
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878 875
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Results of flash DSC analysis for all samples, heating
1.000 °C/s (diagram heat flow (mW) – T (°C))
Figure 4: DMA results- diagram E – modulus (N/mm2) – T (°C) for
all samples
Figure 5: DMA results-loss factor vs. temperature for all samples
2.6 Microstructure analysis
Microstructural characterizations of all the samples
were carried out with scanning electron microscopy
(SEM-Sirion 400 NC and Quanta 200 3D). The
specimens, without previous preparation, were broken in
an atmosphere of N2 to minimise the influence of
metallographic preparation regarding changes in the
structure of the composite. After breaking, the samples
were sprayed with Au (Jeol JSM 8310 appliance), which
enables the observation of the non-conductive surfaces of
the samples with an electron beam. The samples were
positioned into the chamber of the microscope and
observations were performed with an accelerating
voltage of 15 kV.
2.7 Statistical analysis
The statistical analysis included all the parameters
involved in the laboratory tests and it was performed in
the statistical package SPSS® 17.0. Multivariate linear
regression analysis was used to determine the predictors
of differences between the analysed groups of
samples. The threshold value for accepting the working
hypothesis was set at p < 0.05.
3 RESULTS AND DISCUSSION
3.1 Macroscopic view
A macroscopic view of both samples’ shapes
confirmed the homogenous structure of PMMA and
PMMA/ZnO NPs. There was only an acceptable diffe-
rence in colour. Adding ZnO NPs to the mixture with
PMMA gave a brighter colour as the percentage of
nanoparticles grew. This could be solved in the future by
the addition of proper pigment, which results in a similar
colour as in the teeth or oral mucosa, which are
replicated in resin teeth and denture bases.
3.2 Size distribution and zeta-potential by DLS
The statement of the homogeneous distribution of
ZnO NPs in samples was also confirmed by DLS
measurement of the results, which are shown in Table 2.
In ZnO NPs with MMA suspension, the average particle
size was 2.21±0.10 μm, while 42.1 % of the particles
were in the size range of 21.04 nm, whereas in ZnO NPs
with water suspension, the average particle size was
4.01±0.4 μm, while 38.1 % of the particles were in the
size range of 68.06 nm. Therefore, the average size of
the particles was greater in ZnO NPs with water
suspension with more presence of larger-sized clusters of
particles. Distribution of ZnO NPs in MMA suspension
was homogenous, which showed additionally the
insolubility of ZnO NPs in water. ZnO NPs with MMA
had a sharp and narrow intensity vs. size curve, as com-
pared to ZnO NPs with water; thereby, it can be stated
that there is a higher degree of agglomeration in the ZnO
NPs with water suspension (Figure 3). The present study
showed how it is possible to predict, simulate and
characterize the composite material, which is aimed for
use in dentistry. The DLS technique could confirm
indirectly that ZnO NPs in an MMA suspension give a
smaller agglomeration, which was also proven through
the calculated large hydrodynamic diameters. ZnO NPs
which have a zeta-potential between minus 30 mV and
plus 30 mV show a tendency for coagulation. Therefore,
ZnO NPs suspensions with water were unstable solutions
and showed the tendency of agglomerate, while adding
MMA in the ZnO NPs did not show much difference in
the stability of the suspension as compared with the
water. The samples in our work were made by mixing
first ZnO NPs nanopowder and PMMA powder matrix in
vacuum, at room temperature for 2 h then adding MMA,
which reduced the aggregation among ZnO NPs. This
may allow us to prepare homogenous nanocomposites
with organic matrices without additional surface
modification. The chosen average ZnO NPs size showed
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
874 Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Number vs. size distribution curve of ZnO NPs with MMA
and water
Table 2: Composition and the type of mixing used in the preparation of the solvent with the DLS measurements
Exp.
No. Composition Methodology
DLS Measurement
Run of
measurement Average size range (ìm) Zeta potential (mV)
1 0.05 g ZnONPs+ 15 mL H20
Magnetic stirring
(45 min)
I 3.50
4.01 ± 0.40
–3.45
–3.38 ± 0.81II 4.08 –2.35
III 4.48 –4.35
2 0.05 g ZnONPs+ 15 mL MMA
Magnetic stirring
(45 min)
I 2.28
2.21 ± 0.10
–3.40
–3.48 ± 0.07II 2.28 –3.57
III 2.06 –3.49
that is of vital importance for the final properties of the
new developed composite PMMA/ZnO NPs since it
affected the UV-vis absorption and thermal stability.
During the synthesis it was found that smaller ZnO NPs
showed a higher degree of agglomeration, which is in
accordance with the literature.22–24
3.3 Determination of properties
The results obtained by the DMA (Figure 4) showed
the following results. The difference in the elastic
modulus at low temperatures which may be due to a
slight decrease in the cross linking of PMMA by adding
ZnO NPs. At elevated temperatures, the incidence of
chain transfer increased (at termination). With chain
transfer, the propagating polymer radical reacts with
another molecule by proton abstraction and this leads to
branching and cross-linking.
Partly, the E modulus increases due to the addition of
ZnO NPs, but this increase is minimal. Because of the
absence of good interactions between the ZnO NPs and
the PMMA matrix, the loss factor increases with the
amount of addition of ZnO NPs in the PMMA matrix.
ZnO NPs added to the PMMA increased the E modulus
and reduced the level of cross-linking. At the same time,
the poor interaction between the filler and the matrix
results in a less elastic response, which means that the
composite of PMMA/ZnO NPs dampen the vibrations
better than pure PMMA. With the increasing of the loss
factor (tan delta) the damping of the material also
increases. In our case, the height of the loss factor peak
of the sample PMMA3 was the highest; therefore, the
damping of the vibrations of the sample PMMA3 is the
highest (Figure 5).
Characterization using the DMA method showed that
changes in the vibration-damping behaviour of the new
composite could mean that at the gingival a more gentle
feeling appears, which leads to this material being more
friendly to oral mucosa due to vibration damping. This
should also be proven through practice. Composite
PMMA/ZnO NPs would require longer curing times in
comparison with the pure PMMA. Based on this finding
it is expected to obtain a better material with the surface
modification of ZnO NPs where good interactions
between filler and matrix can be formed.
From the Flash DSC measurements (Figure 6), the
lowest glass-transition temperature was measured in the
PMMA3 sample (129.6 °C), followed by PMMA2
(131.0 °C) and the highest glass-transition temperature
was in the pure PMMA1 (131.1 °C). This fact is also
confirmed by the position of the peaks of the loss factor
(tan ), where the lowest glass-transition temperature
was measured in the PMMA3 (146.1 °C), followed by
PMMA2 (147.2°C) and the highest glass-transition
temperature in the pure PMMA1 (147.8 °C). In this
context it was found that the degree of cross-linking was
reduced by the addition of ZnO NPs. With increasing
glass-transition temperature the loss factor decreased,
which indicates that the material PMMA3 (tan  =
1.2003) has the best vibration-damping behaviour,
followed by PMMA2 (tan  = 1.1856). The pure
PMMA1 (tan  = 1.1839) showed the most elastic
response. From the height of tan  it can be concluded
that the 3 % addition of ZnO NPs in PMMA is needed to
obtain a significant change in the vibration-damping
behaviour of the composite. Despite the small differen-
ces measured, as well on DMA (E modulus, tan ) as on
Flash DSC (glass-transition temperatures), all the results
had the same tendency.
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878 875
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Results of flash DSC analysis for all samples, heating
1.000 °C/s (diagram heat flow (mW) – T (°C))
Figure 4: DMA results- diagram E – modulus (N/mm2) – T (°C) for
all samples
Figure 5: DMA results-loss factor vs. temperature for all samples
The difference in the E modulus at low temperatures
may have been due to the slight decrease in the cross-
linking of the PMMA by adding ZnO NPs. The degree of
cross linking by adding ZnO NPs decreased, as shown
by the measurements on the Flash DSC and the level of
the peak of tan . This results, at temperatures up to
about 45 °C, in a minimum E modulus in the PMMA3
and a maximum E modulus in the PMMA2 (the differen-
ce between the glassy transition of PMMA2 and pure
PMMA1 was minimal). Partly, the E modulus increased
due to the addition of ZnO NPs, but this increase was
minimal. Because of the absence of good interactions
between the ZnO NPs and the PMMA matrix, the loss
factor increased with the amount of addition of ZnO NPs
in the PMMA matrix.
3.4 Wear resistance
The mean vertical dimension of each tooth sample
was measured (from the top of the field where the wear
test was performed to the bottom) before the artificial
chewing action and after. All the samples were subjected
to artificial thermal ageing (the samples were in warm
water at 70 °C for one month) and, after that, to mecha-
nical ageing on the chewing simulator. The results of
mean height loss of the control (PMMA1) and tested
group (PMMA2, PMMA3) are shown in Figure 7.
Recently, a study showed that the median vertical wear
of polymer denture teeth, made by different materials,
has been reported to be above 0.2 mm after 2 years of
observation and, in over 50 % of these, variability of
wear. This could be attributed to specific patient factors
such as biting force, nutrition habits and other unknown
factors. Gender differences were also found in the spatial
and temporal parameters of masticatory movement path
and rhythm.25–27 On the other hand, we used the same
material and improved these mechanical properties by
adding a different percentage of ZnO NPs; we also
checked the height loss of the material only after one
year of mechanical ageing on a chewing simulator.
The tested PMMA2/3 samples showed a higher wear
resistance than the tooth sample PMMA1 by 4 times.
Artificial thermal ageing had an effect on the pure
PMMA1 and PMMA2, but there was no effect in the
group with samples made of PMMA3. There was no
statistically significant difference in the loss of height
between samples made of PMMA2 with PMMA3.
Occlusal wear values of the samples made of pure
PMMA1 and PMMA2 after thermal artificial ageing
were 2-times as big compared to the samples made from
PMMA3. Wear resistance of restorative materials under
clinical conditions is a rather complicated phenomenon
compared to other mechanical and physical properties of
materials.25 Furthermore, the cause of thermal stability
and its effect on the wear resistance of both PMMA2 and
PMMA3 samples could be linked significantly. Long-
term polymerization at 70 °C, after polymerization at the
usual 100 °C for 1 h and 45 min can be recommended,
because of its positive effect on the wear resistance of
PMMA1 and PMMA2 after artificial ageing (70 °C, one
month). PMMA3 had lower height loss before, as well as
after, thermal artificial ageing compared to the other
samples.
3.5 Microstructure
The microstructure of the fracture surface of the
newly developed PMMA2 is shown in micrographs
(Figure 8), where the visible fracture is brittle. Detailed
observation by higher magnification revealed that ZnO
NPs (coloured in white) are distributed homogeneously
through the PMMA matrix with some small evidence of
ZnO NPs’ agglomeration (about 1 μm) and signs of
de-polymerized dark fields around them. This led to the
deteriorated final properties of the composite. In the
future, it is necessary to avoid the de-polymerization pro-
cess by the adequate preparation of ZnO NPs’ surface
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
876 Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 8: Micrographs of characteristic surface fracture in PMMA2
Figure 7: Loss of height of resin teeth after wear test on chewing
simulator before and after thermal artificial aging
modification.23 On the other hand, from the literature it is
known that homogeneously dispersed ZnO NPs in the
PMMA matrix can be explained by the theory of the
different kinds of integration by grafting copolymer
chains.28,29
4 CONCLUSIONS
Based on the methodology applied in this study, and
by considering the obtained results, the following con-
clusions can be drawn:
• The newly developed composite of PMMA/ZnO NPs
has a better vibrations damping effect than pure
PMMA1. This could lead to the gingival’s more
gentle feeling of appearance.
• Composite resin denture PMMA teeth reinforced by
the ZnO NPs showed better wear resistance by about
4 times compared to the pure PMMA1.
• The microstructure of PMMA/ZnO NPs consists of a
PMMA matrix and homogenously distributed ZnO
NPs.
• Combinations of different characterization techniques
in the designing of polymer composites reinforced by
nanoparticles with in vitro chewing simulation enable
the determination of functional composite’s behavior
in dentistry.
Conflicts of interests
There are no conflicts of interests to declare.
Acknowledgment
This study was supported by the research project
"Development of PMMA composite enriched/enhanced
with nanoparticles (Ag, ZnO) and biocompatibility
testing", number of project 451-03-2802-IP Type 1/144
and by the Infrastructure Programmes I0-0046 and
I0-0029 financed by the Slovenian Agency ARRS.
Thanks to the Ministry of Education, Science and Sport,
Republic of Slovenia (Programme MARTINA, OP20.
00369), which enabled the research with co-financing.
Special acknowledgements go to Mohammed Shariq
for work on DLS.
Note:
The responsible translator for the English language is
mag. Shelagh Hedges, Faculty of Mechanical Engineer-
ing, University of Maribor, Slovenia.
Abbreviations:
PMMA – Poly-(Methyl-Methacrylate)(powder)
MMA – Methyl-Methacrylate(liquid)
ZnO NPs – Zinc-Oxide Nanoparticles
DMA – Dynamic mechanical analysis
PMMA2 – PMMA + 2 vol. %ZnO NPs
PMMA3 – PMMA + 3 vol. %ZnO NPs
Flash DSC – Flash differential scanning calorimeter
DLS – Dynamic light scattering
TEM – Transmission Electron Microscope
5 REFERENCES
1 Healthy people, 2010, vol. 2, section 21, Oral Health, www.healthy-
people.gov/Publications, 21–18 to 21–19, 25.11.2008
2 F. Müller, M. Naharro, G. E. Carlsson, What are prevalence and
incidence of tooth loss in the adult and elderly population in
Europe?, Clin. Oral Implants Res., 18 (2007) 3, 2–14, doi:10.1111/
j.1600-0501.2007.01459.x
3 D. A. Felton, Edentulism and comorbid factors, J. Prosthodont., 18
(2009) 2, 86–88, doi:10.1111/j.1532-849X.2009.00437.x
4 J. Zeng, Y. Sato, C. Ohkubo, T. Hosoi, In vitro wear resistance of
three types of composite resin denture teeth, J. Prosthet. Dent., 94
(2005) 5, 453–457, doi:10.1016/j.prosdent.2005.08.010
5 M. Risa, Y. Yoshikazu, M. Masakazu , O. Chikahiro, The influence
of vertical dimension of occlusion changes on the electroencephalo-
grams of complete denture wearers, J. Prosthodont. Res., 58 (2014)
2, 121–126, doi:10.1016/j.jpor.2014.01.003
6 A. L. Machado, E.T. Giampaolo, C. E. Vergani, A. C. Pavarina, D.
Salles, J. H. Jorge, Weight loss and changes in surface roughness of
denture base and reline materials after simulated tooth brushing in
vitro, Gerodontology, 29 (2012) 2, 121–127, doi:10.1111/j.1741-
2358.2010.00422.x
7 M. @igon, G. Ambro`i~, Supramolecular polymers, Mater. Tehnol.,
37 (2003) 5, 231–236
8 B. M. Geilich, T. J. Webster, Reduced adhesion of Staphylococcus
aureus to ZnO/PVC nanocomposites, Int. J. Nanomed., 8 (2013),
1177–1184, doi:10.2147/IJN.S42010
9 I. Bilecka, M. Niederberger, New developments in the nonaqueous
and/or non-hydrolytic sol–gel synthesis of inorganic nanoparticles,
Electrochim. Acta, 55 (2010) 26, 7717–7725, doi:10.1016/j.electacta.
2009.12.066
10 K. Memarzadeh, M. Vargas, J. Huang, J. Fan, R. P. Allaker, Nano
metallic-oxides as antimicrobials for implant coatings, Key Eng.
Mater., 493–494 (2012) 489–494, doi:10.4028/www.scientific.net/
KEM.493–494.489
11 M. Bitenc, Z. Crnjak Orel, Synthesis and characterization of
crystalline hexagonal bipods of zinc oxide, Mater. Res. Bull., 44
(2009) 2, 381–387, doi:10.1016/j.materresbull.2008.05.005
12 Y. Li, J. Z. Zhang, Hydrogen generation from photoelectron chemical
water splitting based on nanomaterials, Laser Photonics Rev., 4
(2010) 4, 517–528, doi:10.1002/lpor.200910025
13 C. Buzea, I. Pacheco, K. Robbie , Nanomaterials and nanoparticles:
Sources and toxicity, Biointerphases, 2 (2007) 4, 17–71, doi:10.1116/
1.2815690
14 G. R. Patzke, Y. Zhou, R. Kontic, F. Conrad, Oxide nanomaterials:
synthetic developments, mechanistic studies, and technological
innovations, Angew. Chem. Int. Edit., 50 (2011) 4, 826–859,
doi:10.1002/anie.201000235
15 J. Sawai, Quantitative evaluation of antibacterial activities of metallic
oxide powders (ZnO, MgO and CaO) by conductimetric assay, J.
Microbiol. Meth., 54 (2003) 2, 177–182
16 M. Cierech, J. Wojnarowicz, D. Szmigiel, B. B¹czkowski, A. M.
Grudniak, K. I. Wolska, Preparation and characterization of
ZnO-PMMA resin nanocomposites for denture bases, Acta Bioeng.
Biomech., 18 (2016) 2, 31–41
17 P. Patil, G. Gaikwad, D. R. Patil, J. Naik, Synthesis of 1-D ZnOnano-
rods and polypyrrole/1-D ZnO nanocomposites for photocatalysis
and gas sensor applications, Bull. Mater. Sci., 39 (2015) 3, 655–665,
doi:10.1007/s12034-016-1208-9
18 M. Demir, M. Memesa, P. Castignolles, G. Wegner, PMMA/Zinc
Oxide Nanocomposites Prepared by In-Situ Bulk Polymerization,
Macromol. Rapid Comm., 27 (2006) 10, 763–770, doi:10.1002/marc.
200500870
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878 877
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
The difference in the E modulus at low temperatures
may have been due to the slight decrease in the cross-
linking of the PMMA by adding ZnO NPs. The degree of
cross linking by adding ZnO NPs decreased, as shown
by the measurements on the Flash DSC and the level of
the peak of tan . This results, at temperatures up to
about 45 °C, in a minimum E modulus in the PMMA3
and a maximum E modulus in the PMMA2 (the differen-
ce between the glassy transition of PMMA2 and pure
PMMA1 was minimal). Partly, the E modulus increased
due to the addition of ZnO NPs, but this increase was
minimal. Because of the absence of good interactions
between the ZnO NPs and the PMMA matrix, the loss
factor increased with the amount of addition of ZnO NPs
in the PMMA matrix.
3.4 Wear resistance
The mean vertical dimension of each tooth sample
was measured (from the top of the field where the wear
test was performed to the bottom) before the artificial
chewing action and after. All the samples were subjected
to artificial thermal ageing (the samples were in warm
water at 70 °C for one month) and, after that, to mecha-
nical ageing on the chewing simulator. The results of
mean height loss of the control (PMMA1) and tested
group (PMMA2, PMMA3) are shown in Figure 7.
Recently, a study showed that the median vertical wear
of polymer denture teeth, made by different materials,
has been reported to be above 0.2 mm after 2 years of
observation and, in over 50 % of these, variability of
wear. This could be attributed to specific patient factors
such as biting force, nutrition habits and other unknown
factors. Gender differences were also found in the spatial
and temporal parameters of masticatory movement path
and rhythm.25–27 On the other hand, we used the same
material and improved these mechanical properties by
adding a different percentage of ZnO NPs; we also
checked the height loss of the material only after one
year of mechanical ageing on a chewing simulator.
The tested PMMA2/3 samples showed a higher wear
resistance than the tooth sample PMMA1 by 4 times.
Artificial thermal ageing had an effect on the pure
PMMA1 and PMMA2, but there was no effect in the
group with samples made of PMMA3. There was no
statistically significant difference in the loss of height
between samples made of PMMA2 with PMMA3.
Occlusal wear values of the samples made of pure
PMMA1 and PMMA2 after thermal artificial ageing
were 2-times as big compared to the samples made from
PMMA3. Wear resistance of restorative materials under
clinical conditions is a rather complicated phenomenon
compared to other mechanical and physical properties of
materials.25 Furthermore, the cause of thermal stability
and its effect on the wear resistance of both PMMA2 and
PMMA3 samples could be linked significantly. Long-
term polymerization at 70 °C, after polymerization at the
usual 100 °C for 1 h and 45 min can be recommended,
because of its positive effect on the wear resistance of
PMMA1 and PMMA2 after artificial ageing (70 °C, one
month). PMMA3 had lower height loss before, as well as
after, thermal artificial ageing compared to the other
samples.
3.5 Microstructure
The microstructure of the fracture surface of the
newly developed PMMA2 is shown in micrographs
(Figure 8), where the visible fracture is brittle. Detailed
observation by higher magnification revealed that ZnO
NPs (coloured in white) are distributed homogeneously
through the PMMA matrix with some small evidence of
ZnO NPs’ agglomeration (about 1 μm) and signs of
de-polymerized dark fields around them. This led to the
deteriorated final properties of the composite. In the
future, it is necessary to avoid the de-polymerization pro-
cess by the adequate preparation of ZnO NPs’ surface
D. POPOVI] et al.: SYNTHESIS OF PMMA/ZnO NANOPARTICLES COMPOSITE USED FOR RESIN TEETH
876 Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 8: Micrographs of characteristic surface fracture in PMMA2
Figure 7: Loss of height of resin teeth after wear test on chewing
simulator before and after thermal artificial aging
modification.23 On the other hand, from the literature it is
known that homogeneously dispersed ZnO NPs in the
PMMA matrix can be explained by the theory of the
different kinds of integration by grafting copolymer
chains.28,29
4 CONCLUSIONS
Based on the methodology applied in this study, and
by considering the obtained results, the following con-
clusions can be drawn:
• The newly developed composite of PMMA/ZnO NPs
has a better vibrations damping effect than pure
PMMA1. This could lead to the gingival’s more
gentle feeling of appearance.
• Composite resin denture PMMA teeth reinforced by
the ZnO NPs showed better wear resistance by about
4 times compared to the pure PMMA1.
• The microstructure of PMMA/ZnO NPs consists of a
PMMA matrix and homogenously distributed ZnO
NPs.
• Combinations of different characterization techniques
in the designing of polymer composites reinforced by
nanoparticles with in vitro chewing simulation enable
the determination of functional composite’s behavior
in dentistry.
Conflicts of interests
There are no conflicts of interests to declare.
Acknowledgment
This study was supported by the research project
"Development of PMMA composite enriched/enhanced
with nanoparticles (Ag, ZnO) and biocompatibility
testing", number of project 451-03-2802-IP Type 1/144
and by the Infrastructure Programmes I0-0046 and
I0-0029 financed by the Slovenian Agency ARRS.
Thanks to the Ministry of Education, Science and Sport,
Republic of Slovenia (Programme MARTINA, OP20.
00369), which enabled the research with co-financing.
Special acknowledgements go to Mohammed Shariq
for work on DLS.
Note:
The responsible translator for the English language is
mag. Shelagh Hedges, Faculty of Mechanical Engineer-
ing, University of Maribor, Slovenia.
Abbreviations:
PMMA – Poly-(Methyl-Methacrylate)(powder)
MMA – Methyl-Methacrylate(liquid)
ZnO NPs – Zinc-Oxide Nanoparticles
DMA – Dynamic mechanical analysis
PMMA2 – PMMA + 2 vol. %ZnO NPs
PMMA3 – PMMA + 3 vol. %ZnO NPs
Flash DSC – Flash differential scanning calorimeter
DLS – Dynamic light scattering
TEM – Transmission Electron Microscope
5 REFERENCES
1 Healthy people, 2010, vol. 2, section 21, Oral Health, www.healthy-
people.gov/Publications, 21–18 to 21–19, 25.11.2008
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878 Materiali in tehnologije / Materials and technology 51 (2017) 5, 871–878
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
ERRATUM
In Materiali in tehnologije/Materials and Technology 50 (2016) 5, 797-804, doi: 10.17222/mit.2015.307
in the article entitled:
Measurement of bio impedance on an isolated rat sciatic nerve elicited with specific current
stimulating pulses – Meritev bioimpedance na izoliranem `ivcu ischiadicusu podgane vzbujene s
posebnimi tokovnimi stimulacijskimi impulzi
written by Janez Rozman1,3, Monika C. @u`ek2, Robert Frange`2, Samo Ribari~3
1Center for Implantable Technology and Sensors, ITIS d. o. o., Lepi pot 11, 1000 Ljubljana, Slovenia
2Institute of Physiology, Pharmacology and Toxicology, Veterinary Faculty, University of Ljubljana, Gerbi~eva 60,
1000 Ljubljana, Slovenia
3 Institute of Pathophysiology, Zalo{ka 4, Medical Faculty, University of Ljubljana, Republic of Slovenia
two additional authors are missing. The correct authorship of this article is:
Robert Brajkovi~1, Dejan Kri`aj1, Janez Rozman2,3, Monika C. @u`ek4, Robert Frange`4, Samo Ribari~3
1Faculty of Electrical Engineering, University of Ljubljana, Tr`a{ka 25, 1000 Ljubljana,
2Center for Implantable Technology and Sensors, ITIS d. o. o. Ljubljana, Lepi pot 11, 1000 Ljubljana,
3Institute of Pathophysiology, Medical Faculty, University of Ljubljana, Vrazov trg 2, 1000 Ljubljana,
4Institute of Physiology, Pharmacology and Toxicology, Veterinary Faculty, University of Ljubljana, Gerbi~eva 60,
1000 Ljubljana, Republic of Slovenia
MIT Editorial
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS