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A Comparative Study on Mechanical Properties of Some Thermoplastic and
Thermo Set Resins Used for Orthodontic Appliances
Article    in  MATERIALE PL ASTICE · Sept ember 2015
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MATERIALE PLASTICE ♦ 52 ♦ No. 3 ♦ 2015 http://www.revmaterialeplastice.ro 364A Comparative Study on Mechanical Properties
of Some Thermoplastic and Thermo Set Resins
Used for Orthodontic Appliances
LIGIA VAIDA1, LIVIU MOLDOVAN2*, IOANA ELENA LILE3*, BIANCA IOANA TODOR1*, ANCA PORUMB1, IOAN TIG1,
DANA CRISTINA BRATU4
1 University of Oradea, F aculty of Medicine and Pharmacy, Department of Dentistry, 1 Universitãii Str., 410087, Oradea, România
2 University of Oradea, Faculty of Electrical Engineering and Information Technology, Department of Electronics and
Telecommunications, 1 Universitãtii Str., 410087, Oradea, România
3 V asile Goldis” Western University of Arad, F aculty of Medicine, Pharmacy and Dental Medicine, Department of Dentistry, 94-96
Revolutiei Blv., 310025, Arad, România
4 Victor Babes” University of Medicine and Pharmacy, Dental Medicine, 2 E. Murgu Sq., 300041, Timisoara, România
The orthodontic appliances can be built in a great diversity of resin that could be classified by one of the
criteria, in thermoplastic and thermo set. This paper has as purpose to compare three resins used for orthodontic
appliances, one thermoplastic and two thermo set –Biocryl C (Scheu Biostar), Orthocryl (Dentaurum),Orthoplast (Vertex Dental) – from the mechanical properties respect such as surface hardness and indentation
elastic modulus. We tested six specimens; two from each type of these resins. Each specimen was fixed on
a support and
they were tested with Nano Indenter G200 from Keysight (Agilent). We obtained as a result
that the thermoplastic material has a lower elastic modulus than the other thermo set.
Keywords: thermoplastic resin, thermo set resin, elastic modulus, hardness, nanoindentation
* email: [anonimizat]; [anonimizat]; [anonimizat] orthodontic appliances have a great diversity of
resins they can be made of and therefore, for practitioneris important to wisely choose the advantages and
disadvantages of each type. There are many ways of
classifying these resins, but starting from their thermalbehavior they can be thermoplastic and thermo set.
The thermoplastic resins, as the name explains, are
shaped at heat and pressure without chemical changes.They get hard after cooling and can be softened again at
heat and are soluble in solvents. The thermo set resins are
obtained by the chemical reaction of two substances (forexample a powder and a liquid), cannot be resoftened by
exposure to heat and are not soluble in solvents [1, 2].
Examples of thermoplastic resins are: polymethyl
methacrylate, polyvinyl acrylics and polystyrene, and
examples of thermo set resins are: cross-linked polymethyl
methacrylate, silicones.
Pros for thermo-cured orthodontic appliances are: lower
cost, the ability to introduce multiple active elements in
the acrylic base, large offer of models.
Cons are represented by the irritating and allergenic
potential due to the presence of unpolimerized monomer,
which give also their porosity, increased working time,water uptake, more liable to break, uneven thickness.
The vacuum-thermoformed orthodontic appliances
have no residual monomer, even thickness and betterfinishing but cost more, reduced color offer and fewer active
elements possible to be introduced in their structure.
This paper has as purpose to compare three resins used
for orthodontic appliances, one thermoplastic and two
thermo set: Biocryl C (Scheu Biostar), Orthocryl
(Dentaurum), Orthoplast (Vertex Dental).
Biocryl C – Scheu consists in break-resistant acrylic
blanks, made of pure thermoplastic PMMA material,without monomer, which bonds to acrylic resins and which
can be used for a large variety of removable appliances
designed for retention of minor tooth movement,
orthodontic plates and retainers [3].
Orthoplast (Vertex Dental) is a self curing acrylic resin
available as powder and liquid (curing time 20 min at 55degrees and 2.5 bar, dough time 9 min, working time 6
min) [4].
Orthocryl (Dentaurum) is another self curing acrylic resin
available as powder and liquid (curing time 20 min, 40-46
degrees at 2.2 bars) [5].
The instrumented indentation technique is a method
used to measure mechanical properties of a material like
surface hardness and indentation elastic modulus (e-
modulus). The method can be used on resins and polymersat micro or nano scale, materials with qualities that can
vary from point to point [6].
In a traditional indentation test (macro or micro
indentation), a hard tip whose mechanical properties are
known (frequently made of a very hard material like
diamond) is pressed into a sample whose properties areunknown. The load placed on the indenter tip is increased
as the tip penetrates further into the specimen and soon
reaches a user-defined value. At this point, the load maybe held constant for a period or removed. The area of the
residual indentation in the sample is measured and the
hardness is defined as the maximum load, divided by the
residual indentation area.
In nanoindentation small loads and tip sizes are used,
so the indentation area may only be a few squaremicrometers or even nanometers. This presents problems
in determining the hardness, as the contact area is not
easily found. Atomic force microscopy or scanning electronmicroscopy techniques may be utilized to image the

MATERIALE PLASTICE ♦ 52♦ No. 3 ♦ 2015 http://www.revmaterialeplastice.ro 365
Fig. 1. The support for the
specimen exposed to
heat, while the adhesive
resin is meltingFig. 2. The specimen
glued on the support with
the melted glue resinFig. 3. Three specimens
and reference specimen
of the Nano IndenterFig.4. The Nano Indenter
G 200 used during testingFig. 5. Those 25
indentations on each
specimen
Fig. 6. Results of the testindentation, but can be quite cumbersome. Instead, an
indenter with a geometry known to high precision (usually
a Berkovich tip, which has three-sided pyramid geometry)is employed. During the course of the instrumented
indentation process, a record of the depth of penetration is
made, and then the area of the indent is determined usingthe known geometry of the indentation tip. While indenting,
various parameters such as load and depth of penetration
can be measured. A record of these values can be plottedon a graph to create a load-displacement curve. These
curves can be used to extract mechanical properties of
the material. The indentation curves have often at leastthousands of data points. The hardness and elastic modulus
can be quickly calculated by using a programming
language or a spreadsheet.
Experimental part
Materials and method
Three resins were put in test. From each of them were
made two squared specimens, one transparent and one
coloured: Biocryl transparent, Biocryl blue, Orthoplasttransparent, Orthoplast pink, Orthocryl transparent,
Orthocryl yellow. The resin specimens were prepared
according to the manufacturer recommended instructions.All of those six specimens were designed at the same
dimensions size 20x20x4 mm, using a sandblaster. Each
specimen was fixed on a support and
they were tested
with Nano Indenter G200 from Keysight (Agilent) (fig. 1-
4). For each specimen it was analyzed the mean modulus
at maximum load and the mean hardness at maximumload.
The method uses a single load/unload cycle to a
specified depth. Hardness and modulus are determinedusing the stiffness as calculated from the slope of the load
displacement curve during the unload cycle. The indenter
tip begins approaching the surface from a distance above
the surface of approximately Surface Approach Distance.
The approach velocity is determined by Surface ApproachVelocity. When the indenter determines that it has
contacted the test surface, according to the criteria Surface
Approach Sensitivity, the indenter penetrates the surfaceat a rate determined by Strain Rate Target. When the
surface penetration reaches the Depth Limit, the load on
the indenter is held constant for Peak Hold Time. The loadon the indenter is then reduced by an amount defined by
Percent to Unload at a rate equal to the maximum loading
rate. Then, if Perform Drift Test Segment is set to “1”, theindenter is held in contact with the sample under constant
force for 75 seconds (otherwise, this test segment is
skipped). Each specimen had received 25 indentations (fig.5), and for each indentation were around 800
measurements of load on sample (mN) versus
displacement on surface (nm). The mean e-modulus andhardness for the specimens were obtained with the
software of the Nanoindenter according to these data pairs
(fig. 6-8) [7, 8].
Statistical analysis
Data for each mechanical property were analyzed to
compare statistical difference using Student t-test,
summary statistics and Student Newman Keuls test with
Med Calc software (table 1).
Results and discussions
Biocryl got the closest values of mean e-modulus to
those presented in technical sheet by the producer for both
samples, the transparent and blue one (3.3 GPa given by
producer, 3.202 GPa for Biocryl transparent, 3.329 GPa forBiocryl blue, p>0.05). Orthoplast had received bigger
values in test comparing with those given by the producer
(3.628 GPa for Orthoplast transparent and 3.719 GPa forOrthoplast pink comparing with 1.985 in technical sheet,
p<0.001). Orthocryl had intermediate values (3.543 GPa
for Orthocryl transparent and 3.608 GPa for Orthocrylyellow). The producer has not given values in this case.
The statistical difference for Orthoplast can be explained
by the different testing of e-modulus. The producer has not
given details about the method used for testing.
The mean e-modulus is bigger in both thermo set resins
than in thermoplastic one. For all resins, the coloured
specimen had similar values with the transparent one
(p>0.05).

MATERIALE PLASTICE ♦ 52 ♦ No. 3 ♦ 2015 http://www.revmaterialeplastice.ro 366Table 1
MEAN RESULTS FOR E-MODULUS AND
SURFACE HARDNESS
Fig. 8. E-modulus for tested resins Fig. 9. Hardness for tested resins
The literature written about mechanical properties of
materials used for orthodontic appliances determined withnanoindentension or other methods is not very extensive.
The results are obtained with different methods, so they
can not be in all cases compared.
One study has measured flexural modulus for 6 acrylic
PMMA resins cold and heat cured and found values
between 1.7-2.5 GPa [9]. The method used was three pointbending test in water bath at 37 degrees under the
conditions specified by ISO 1567:1999.
Another study observed three resins obtained with
thermal polymerization and two with microwave
polymerization. The e-modulus for thermo polymerized
resins was 3724, 2925, 2613 MPa and 2945 and 3737 MPafor microwave group microwave. The method used was
nanoindentation [10]. Our data for e-modulus and working
method are close to these results.
Elastic modulus was significantly higher for heat-
polymerized acrylic resins than for auto polymerized acrylic
resins in a study conducted by American researchers [11].
According to the brand used, flexural (e) modulus in
another research ranged from 2550 MPa for heat-curing
denture base polymer to 2418 MPa for auto polymerizingdenture base polymer, so lower results than in our study
[12].
Lower results had also Ali et al. using 3-point bending
test. The flexural modules (MPa) were 1969 +/- 55, and
1832 +/- 89 for Meliodent (heat-cured PMMA), and Probase
Cold (auto-cured PMMA) materials, respectively [13].On the other part, Danesh et al. had a larger range of
results for auto and light polymerized PMMA then our study,
using a different method. Flexural modules lay between
1.3 and 5.3 GPa [14].
Hardness is also a mechanical property tested by several
studies. It has many facets, regarding the measuring
characteristics. Hardness Vickers, Rockwell, Barcol,nanoindentation hardness are just a few of the parameters
found in research.
In our study the mean surface micro hardness was the
biggest in Orthoplast (169 MPa for Orthoplast transparent
and 159 MPa for Orthoplast pink), followed by Biocryl (134
MPa for Biocryl transparent and 130 MPa for Biocryl blue)and with very closed values by Orthocryl (130 MPa for
Orthocryl transparent and 129 MPa for Orthocryl yellow).
Only Scheu Gmbh has provided values for surface hardnesswithout explaining the type of test used 195 MPa. The values
obtained by our test for both specimens of Biocryl are
smaller.
Biocryl was also tested by some Serbian authors [15]
for parameters like tensile strength, 3 point bending
strength, fracture toughness and hardness Vickers. Thehardness Vickers was 22.43 HV0.3 and 21.96 HV0.3 but
the data are not comparable because of different work
method and type of formula used comparing to our study.
Other authors have studied the hardness of light cured
resins and auto polymerized resins (including Orthocryl)
and the auto polymerized resins had lower hardness thanthe light polymerized resins [14].*

MATERIALE PLASTICE ♦ 52♦ No. 3 ♦ 2015 http://www.revmaterialeplastice.ro 367Another study that tested five resins with nanoindentation
found 236, 156 and 145 MPa micro-surface hardness for
thermo polymerized ones and 158 and 226 MPa formicrowave polymerized resins. The results are for three of
them close to our study, but still two of them were higher
[10]. Other materials tested were Meliodent (heat-curedPMMA), and Probase Cold (auto-cured PMMA) with
hardness Vickers (VHN) values 17.0 +/- 0.4, and 16.0 +/-
0.4 [13].
Zafar et al. made another evaluation of micro-hardness
with a nanoindenter. The absolute hardness was recorded
297.72±19.04 MPa and 229.93±18.53 MPa for heat curedPMMA and cold cured PMMA, so values higher than in our
study [16].
A group of six heat and auto cured PMMA resins got
hardness Vickers between 11.88 HV0.3 and 17.24 HV0.3,
12.16-16.48 HV0.03 and 23.6-39.1 hardness Barcol [9].
Conclusions
The thermoplastic material examined had a lower e-
modulus than the other thermo set PMMA. The micro-surface hardness values are different in the case of all
tested PMMAs and they are not influenced by the type of
resin-thermoplastic or thermo set.
Acknowledgment: Our gratitude to “Hungarian-Romanian Research
Platform for Smart-Materials Research Project Support”, cod HURO/tto1/1g1/2.2.1, Hungarian-Romanian Cross-border Cooperation
Programme 2007- 2013, for the help in acquiring the device used.References
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