ORIGINAL RESEARCH

Evaluation of Impact Strength of Dental Acrylic Resins by

Incorporation of TiO2 Nanoparticles Using Two Different
Processing Techniques
Chippalapally Arun Kumar1​, C Ravi Kumar2​, Kanchiradasu Vamshikiran3​, Gaddam Deepthi4​, G Naveen Kumar5​, M Akhilesh6​

A b s t r ac t
Aim: The present study aims to evaluate the impact strength of PMMA incorporated with TiO2​nanoparticles by two different processing
techniques i.e. water bath and microwave processing techniques.
Materials and methods: A total of 80 samples made of PMMA were divided into four groups. Each group includes 20 samples with group I and
II comprising of samples made of normal acrylic resin and acrylic resin reinforced with 1 wt% TiO2​nanoparticles processed with conventional
water bath technique respectively, groups III and IV include normal acrylic resin and acrylic resin reinforced with 1 wt% TiO2​processed using
microwave technique respectively. The specimens were tested for impact strength using IZOD pendulum impact tester. The impact energy
values obtained were analyzed statistically.
Results: The results through one-way ANOVA showed a high mean impact strength with group IV samples (23.13) and lowest with respect
to group I (19.42) with highest statistical significance (p​ < 0.001). A post hoc​Tukey test intergroup analysis showed a statistically significant
difference between group I and other groups, whereas there was no statistical significance associated with other intergroup comparisons.
Conclusion: PMMA with its current drawbacks in the physical and mechanical properties requires modifications to make it an ideal denture
base material. The current investigation evaluates that a high mean impact strength with samples made of PMMA incorporated with TiO2​
nanoparticles processed by microwave technique was obtained when compared with normal acrylic resins processed by water bath technique.
Clinical significance: The methodology of current study can be used while processing of denture bases for patients to evaluate the effect of
oral environment on inclusion of TiO2​nanoparticles with microwave processing in a clinical setup. This could help in reducing the amount of
fractures associated with heavy load masticatory stresses and improving the mechanical properties in denture bases.
Keywords: Acrylic denture base, Impact strength, Microwave, TiO2​ nanofillers.
The Journal of Contemporary Dental Practice (2019): 10.5005/jp-journals-10024-2655

Introduction 1,3,5
​Department of Prosthodontics and Crown and Bridge, Government
A breakthrough in the field of prosthetics was the invention Dental College and Hospitals, Hyderabad, Telangana, India
of poly(methyl methacrylate) (PMMA) and is considered as 2
Department of Prosthodontics and Crown and Bridge, Mamatha
​
an ideal denture base material compared to many. PMMA is a Dental College, Khammam, Telangana, India
routinely used material in the fabrication of removable partial 4
​Department of Oral and Maxillofacial Pathology, Government Dental
and complete dentures as it possesses a wide range of beneficial College and Hospitals, Hyderabad, Telangana, India
properties like ease of handling, inexpensive fabrication, and 6
Department of Orthodontics, Government Dental College and
​
stability in oral environment, lack of toxicity, color matching Hospitals, Hyderabad, Telangana, India
ability, biocompatibility and accurate fit. Despite the favorable Corresponding Author: Gaddam Deepthi, Department of Oral and
properties, masticatory forces have a deforming effect on PMMA Maxillofacial Pathology, Government Dental College and Hospitals,
during function and it lacks particularly in terms of transverse Hyderabad, Telangana, India, Phone: +91 7799357588, e-mail:
and impact strength. So, any factor that increases the distortion drdeepthireddy999@gmail.com
of the denture base and alters the stress distribution may lead to How to cite this article: Kumar CA, Kumar CR, Vamshikiran K, et al.​
denture fracture.1,​2​ ​Even though PMMA is an excellent denture Evaluation of Impact Strength of Dental Acrylic Resins by Incorporation
base material, it has few disadvantages like low in strength of TiO2 Nanoparticles Using Two Different Processing Techniques.
resulting in denture fractures. Fracture may occur because of J Contemp Dent Pract 2019;20(10):1184–1189.
unsatisfactory impact strength, transverse strength, or fatigue Source of support:​ Nil
resistance. 3​ Conflict of interest:​ None
However, due to its poorer physical and mechanical properties,
PMMA structure is further modified by rubber copolymerization or
by reinforcement with different types of fillers like metallic wires, dioxide (TiO2​) are used along with PMMA, among which TiO2​ is
fibers (glass, aramid, nylon, polyurethane and carbon)4,​5​ ​and use routinely used as they could improve the flexural strength, fracture
of metallic oxides to reduce the drawbacks and to improve the toughness, hardness, thermal conductivity, antimicrobial property,
fracture resistance of PMMA denture based resins.6,​7​ ​Metal oxides impact strength and a substantial decrease in water sorption and
such as aluminum oxide, zinc oxide, zirconium oxide and titanium solubility.8​

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.
org/licenses/by-nc/4.0/), which permits unrestricted use, distribution, and non-commercial reproduction in any medium, provided you give appropriate credit to
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 Effect of TiO2​Nanoparticles on PMMA Using Microwave Processing

Nanotechnology is a growing field which determines the Table 1: Study groups

future of science and technology. It is based on the idea of creating Group Method Sample size
functional structures by controlling molecules on a structural basis Group I Normal acrylic resin processed with 20
and manipulating the matter at billionth of meters or nanometers.9​ conventional water bath technique
According to the definition of the National Nanotechnology
Group II Acrylic resin reinforced with 1 wt% 20
Initiative, nanotechnology is the direct manipulation of materials TiO2​nanoparticles processed with
at the nanoscale.10​This term defines a technology that enables conventional water bath technique
almost complete control of the structure of matter at nanoscale
Group III Normal acrylic resin processed using 20
dimensions. It will give us the ability to arrange atoms as we desire microwave technique
and subsequently to achieve effective, complete control of the
Group IV Acrylic resin reinforced with 1 wt% 20
structure of matter. These advances in nanotechnology provided
TiO2​processed using microwave
suitable changes in dentistry from diagnosing the disease to varying technique
treatment options.11​
The approaches in which nanotechnology can be used in
dentistry can be on therapeutic basis, regeneration of dental tissues M at e r ia l s and Methods
or by incorporating them in dental materials.12​ As mentioned In the present study, the concentrations of TiO2​ incorporated
earlier, the usage of these nanoparticles in PMMA denture base into PMMA was restricted to 1 wt%. A total of 80 samples divided
is by alteration and addition of TiO2​ filler size to improve the into four groups with 20 samples each with dimensions of
performance of the material. The nanomaterials present with variety 60 mm × 12 mm × 3 mm were prepared as per ASTM (American
of morphologies such as particles, flowers, cubes, rods, tubes etc.8​ Standards for Testing and Material). The sample size and
A number of processing techniques like compression molding, methodology in each group are summarized in Table 1.
injection molding or fluid resin techniques are proposed in A total of 40 metal sample analogs with the above-mentioned
the fabrication of denture bases using acrylic resins. Variable dimensions were invested into flasks through two pour technique
dimensional changes tend to occur in PMMA, which is attributed to using dental plaster as the investing medium in conventional
the type of processing technique.13​Recently, various studies were water bath technique (groups I and II). After the investing medium
conducted using microwave processing, the principle of which has reached its final set, the two portions of the flask were gently
depends on the effect of microwave energy on the monomer separated. The metal sample analogs were also lifted from investing
components promoting an uniform and immediate heating of medium along the beveled edges, thereby creating mold space
the polymer mass, that activates the decomposition of benzoyl into which acrylic resin was packed (Fig. 1A). For microwave
peroxide, and quickly yields free radicals for the polymerization oven processing (groups III and IV), separate microwavable flasks
process, which decreases in the same proportion as polymerization were used. The above mentioned procedure was repeated with
increases.14​ microwavable flasks (Fig. 1B).
Studies conducted on TiO2​nanoparticles incorporation into To pack acrylic resin into the mold space, a selected acrylic
denture base resins showed improved physical and mechanical material was mixed according to manufacturer’s recommended
properties. The other studies conducted on comparison of polymer: monomer ratio and packing was done. Then curing of
various processing techniques have shown improved mechanical samples in water bath technique was done in an acrylizer at 74°C for
properties of denture bases with microwave curing technique by 8 hours with no terminal boiling. Following which, the flasks were
decreasing the residual monomer content.15​Hence, considering allowed to cool to room temperature and removed from the water
the above said facts, the present study was conducted to evaluate bath and bench, cooled for 30 minutes. Subsequently the flask was
the effect of TiO2​nanoparticles on impact strength of dental acrylic immersed under cold tap water for 15 minutes. The same procedure
resins using microwave processing and conventional water bath was repeated for microwavable flasks and cured at 540 watts for
methods. 10 minutes in microwave oven (Fig. 2).

Figs 1A and B: (A) Metal analogs invested in brass varsity flask; (B) Nonmetallic (microwavable) flasks

The Journal of Contemporary Dental Practice, Volume 20 Issue 10 (October 2019) 1185
 Effect of TiO2​Nanoparticles on PMMA Using Microwave Processing

Fig. 2: Microwave oven Fig. 3: Amalgamator

Fig. 4: Test samples from various groups

TiO2​nanoparticles were incorporated into the polymer with a clamped upright in the anvil with a v​-notch at the level of clamp.
concentration of 1 wt% i.e., 1 g of nanoparticles to 99 g of polymer The dimensions of the sample were entered in the display box.
and mixed using an amalgamator (Fig. 3) to achieve a homogeneous The pendulum was released after entering the data. The test piece
mixture. Polymer and monomer were mixed according to was hit by pendulum which was aerodynamically controlled. It
manufacturer’s recommendations (3:1 by volume). was allowed to fall freely from fixed height, to give a blow of 120
After completing the process, the cured samples were retrieved ft lb. energy. After fracturing the test piece, the height to which
from the flask by deflasking. The samples were trimmed of the the pendulum rises was recorded by a friction pointer from which
excess resin flash using tungsten carbide burs and fine abrasive of absorbed energy was read. The impact energy of the test specimen
grit size 120/240 microns without altering the specimen surfaces and was displayed in J/m. The results obtained for each test specimen
dimensions. Samples from all the groups are depicted in Figure 4. were analyzed statistically using SPSS version 22. Descriptive
Specimens from all the four groups were tested for impact analysis for scale data, one-way ANOVA and independent t test
strength using a pendulum impact tester—IZOD (Tinius Olsen for intergroup comparison are the statistical analysis done with
IZOD/Charpy Impact Tester Model IT504, USA) (Fig. 5). It conforms a p​value of <0.05 was considered significant and <0.01 as highly
to ASTM standards D256 and D6110; ISO standards 179 and 180. It significant.
has a pendulum, a digital display and an anvil for supporting the
specimen Model IT 504.
The fabricated samples were notched in the middle, with R e s u lts
a notch cutter (Fig. 6) as in ISO 179:1993 to a depth of 0.2 mm The present study was conducted to evaluate the impact strength
leaving a residual depth of 2.8 mm beneath the notch. In this test, of PMMA incorporated with TiO2​nanoparticles by two different
the sample piece was used as a cantilever i.e., the specimen was processing techniques. The test samples of different groups

1186 The Journal of Contemporary Dental Practice, Volume 20 Issue 10 (October 2019)
 Effect of TiO2​Nanoparticles on PMMA Using Microwave Processing

Fig. 6: Notch cutter

Fig. 5: Izod impact tester

Table 2: Mean and SD of impact strength in various groups

Group N​ Minimum Maximum Mean Std. deviation
Group I 20 17.000 27.398 19.42 2.47
Group II 20 19.200 27.398 22.53 2.06
Group III 20 19.300 27.000 21.73 1.92
Group IV 20 19.333 32.400 23.13 2.96

one-way ANOVA (Table 3), which showed a highly significant

difference between the groups with a “p​” value of 0.001.
The comparisons between the individual groups was
statistically using post hoc​Tukey test (Table 4). Post hoc​ Tukey
analysis showed statistically significant differences in the mean
impact strength values between group I with all other groups
individually (p​ < 0.05). The analysis of mean impact strength
between group II with group III (p​ = 0.716), group II with group IV
(p​ = 0.856), group III and group IV (p​ = 0.256) showed no statistical
significance. A negative value (−) in the mean difference indicates
that the second variable has higher mean value than first variable
in that comparison.

Discussion
PMMA is widely used to fabricate denture bases due to various
desirable properties. Despite having these, it lacks physical
and mechanical strength that could support the masticatory
Fig. 7: Mean impact strengths between the four groups
stresses.16​A study conducted by Johnston et al. on flexure fatigue
of 10 commonly used denture base resins concluded that 68%
mentioned in Table 1 were subjected to pendulum impact tester. of dentures break within their first year of clinical service.17​ Few
The mean values and standard deviation obtained in various groups other studies also showed varied reports on fractures of denture,
were summarized in Table 2 and Figure 7. The mean impact strength with fatigue and impact resulting in maxillary dentures to break
was highest in group IV with mean value of 23.13 and lowest in in the midline and with impact as a cause in 80% of mandibular
group I with 19. 42. The results were analyzed statistically through denture fractures.18​Due to the above said facts, there are various

Table 3: Overall comparison of mean impact strength in various groups by one way ANOVA test
Comparison Sum of squares df Mean square F​ Sig.
Between groups 158.423 3 52.808 9.276 <0.001*
Within groups 432.658 76 5.693
Total 591.081 79
*Highly significant

The Journal of Contemporary Dental Practice, Volume 20 Issue 10 (October 2019) 1187
 Effect of TiO2​Nanoparticles on PMMA Using Microwave Processing

Table 4: Post hoc​Tukey test for individual intergroup comparisons themselves. Nishii et al. in 1968 first proposed the use of microwave
Comparison Mean difference p​ value energy as an alternative PMMA processing method.25​Kimura et al. in
Group I–Group II −3.106650* 0.001**
their studies concluded that a similar behavior is exhibited by acrylic
resins processed through both conventional water bath method
Group I–Group III −2.307950* 0.016*
and using microwave technique , but with early processing time
Group I–Group IV −3.707800* <0.001**
using microwave processing.15​Jadhav et al. investigated the impact
Group II–Group III 0.798700 0.716 NS
strength of denture base resins polymerized by various processing
Group II–Group IV −0.601150 0.856 NS techniques. Their results showed denture bases polymerized by
Group III–Group IV −1.399850 0.256 NS microwave processing technique had highest impact strength
*Significant; **Highly significant compared to water bath technique.24​In microwave processing, the
NS, nonsignificant polymer and monomer mixer is heated directly before the flask is
new interventions put forth into PMMA by adding nanofillers and heated, so the heat generated gets dissipated to the investment and
also introduced newer processing techniques alternative to the flask. In water bath method, water gets heated first followed by the
conventional methods. TiO2​nanofillers compared to others are heating of the flask, investment polymer and monomer mass. This
proved to have good mechanical and antimicrobial properties.19​ delay in the heating of polymer and monomer mass results in longer
In the present study, the evaluation of impact strength of PMMA processing times.25​The limitations of the present study includes:
material incorporated with TiO2​nanofillers was done and also even though the dimensions of the specimens were standardized,
comparison between acrylic resins with/without TiO2​ nanofillers the strength of the reinforced PMMA orally in the presence of the
using two different processing techniques (microwave processing saliva cannot be anticipated and the simulation with the clinical
and conventional water bath methods) was done. The test samples conditions is not done. Another limitation was that surface of acrylic
were divided into four groups (Table 1) and all the specimens were samples were flat when compared to any removable prostheses
tested for impact strength. The obtained results showed that impact which has curved surfaces.
strength was increased by adding TiO2​ nanoparticles. Highest
impact strength was observed for group IV (23.13) followed by C o n c lu s i o n
group II (22.53), group III (21.73), group I (19.42). i.e. PMMA reinforced Specimens of heat cured acrylic resin materials fabricated with
with 1 wt% TiO2​using microwave technique showed higher impact incorporation of TiO2​nanoparticles showed high impact strength
strength compared to others because in this technique microwaves compared to the normal acrylic resins. Among the groups tested,
are used to generate heat inside the resin. the acrylic resins processed with microwave technique with
The concentration of TiO2​in the present study was restricted incorporation of TiO2​particles showed highest impact strength.
to 1 wt%, because in a study conducted by Shirkavand et al. Further tests can be done to evaluate the effect of oral environment
on the effect of various concentrations of (0.5, 1 and 2 wt%) on inclusion of TiO2​nanoparticles with microwave processing of
TiO2​ nanoparticles on tensile strength of dental acrylic resins, denture bases in a clinical setup.
incorporation of 1 wt% of TiO2​ to PMMA showed improved
mechanical properties. 20​Increased concentrations of TiO2​ (5%)
will lead to worsening in impact strength of the resin material. On References
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