|Year : 2014 | Volume
| Issue : 4 | Page : 459-463
|Effect of Titanium dioxide nanoparticles on the flexural strength of polymethylmethacrylate: An in vitro study
P Harini1, Kasim Mohamed2, TV Padmanabhan2
1 Final Year Undergraduate, Faculty of Dental Science, Sri Ramachandra University, Porur, Chennai, Tamil Nadu, India
2 Department of Prosthodontics, Faculty of Dental Science, Sri Ramachandra University, Porur, Chennai, Tamil Nadu, India
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|Date of Submission||15-Oct-2013|
|Date of Decision||03-Nov-2013|
|Date of Acceptance||25-Feb-2014|
|Date of Web Publication||10-Oct-2014|
| Abstract|| |
Context: To improve the flexural strength of polymethylmethacrylate (PMMA).
Aim: To evaluate whether the incorporation of titanium dioxide nanoparticles in polymethylmethacrylate (PMMA) increases the flexural strength and to compare the different concentrations of titanium dioxide nanoparticles and its relation to flexural strength.
Settings and Design: Study was conducted in Sri Ramachandra University utilizing 40 specimens manufactured from clear heat polymerizing acrylic resin.
Materials and Methods: Forty specimens of clear heat polymerizing acrylic resin of dimensions 65 Χ 10 Χ 3 mm as per ISO 1,567 standardization were fabricated and were grouped into A (CONTROL) with no titanium dioxide (TiO2) nanoparticles, B with 0.5 gms of TiO 2 nanoparticles, C with 1 gm of TiO 2 nanoparticles and D with 2.5 gms of TiO 2 nanoparticles added.The concentrations of titanium dioxide in each group were 1 wt%, 2 wt% and 5 wt%. Universal testing machine INSTRON was used to load at the center of the specimen with a cross head speed of 1.50 mm/min and a span length of 40.00 mm.
Statistical Analysis Used: ANOVA and multiple comparisons are carried out using the independent t-test.
Results: The ANOVA result shows that there is a significant difference between the groups with respect to the mean flexural strength. Highest mean flexural strength is observed in Group D, while the lowest is seen in Group A. Independent t-test revealed that there was a statistical significance between Group A and Group D (0.041) and between Group B and Group D (0.028).
Conclusions: The results concluded that polymethylmethacrylate reinforced with different concentrations of titanium dioxide nanoparticles showed superior flexural strength than those of normal PMMA.
Keywords: Flexural strength, heat polymerizing acrylic resin, polymethylmethacrylate, titanium dioxide nanoparticles
|How to cite this article:|
Harini P, Mohamed K, Padmanabhan T V. Effect of Titanium dioxide nanoparticles on the flexural strength of polymethylmethacrylate: An in vitro study. Indian J Dent Res 2014;25:459-63
Polymethylmethacrylate (PMMA) is one of the most commonly used materials in dentistry. It has many desirable properties such as stability in the oral environment, ease of manipulation, polishability, and can be used with inexpensive equipment.  The PMMAs are extensively used for fabrication of prosthesis, and orthodontic appliances. Since its introduction there has been a continuous attempt to improve the mechanical properties of acrylic resins specially the flexural strength.  The poor flexural strength is was attributed to the brittle nature at glass transition temperature of approximately 110°C  and its susceptibility to cyclic loading.  Fracture of denture material is the primary mode of clinical failure. 
|How to cite this URL:|
Harini P, Mohamed K, Padmanabhan T V. Effect of Titanium dioxide nanoparticles on the flexural strength of polymethylmethacrylate: An in vitro study. Indian J Dent Res [serial online] 2014 [cited 2018 Dec 9];25:459-63. Available from: http://www.ijdr.in/text.asp?2014/25/4/459/142531
Flexural strength, also known as modulus of rupture, bend strength, or fracture strength, a mechanical parameter for brittle material, is defined as a material's ability to resist deformation under load. The transverse bending test is most frequently employed, in which a rod specimen having either a circular or rectangular cross-section is bent until the specimen fractured using a three point flexural test technique. The flexural strength represents the highest stress experienced within the material at its moment of rupture. It is measured in terms of stress (σ).
Several experiments have been conducted in order to try and improve the flexural strength and basic mechanical properties of PMMA. PMMA reinforced with glass fibers, ,,,, sapphire whiskers, , aramid fibers, , carbon fibers, ,, metal wires, ,, nylon,  polyurethane fibers , and zirconia,  showed a improved fracture resistance. These experiments have led to a significant improvement in the flexural and tensile strength of PMMA but there was always need for improvement.
Nanotechnology is a growing interest that will determine the future of science, medicine and technology. 'Nano' is the Greek word that means 'dwarf'.  Nanotechnology is the science of manipulating matter measures in the billionth of meters or nanometers roughly the size of two or three atoms.  Nanoparticles have been increasingly used in material science for its wear and tear resistance and anti-corrosion abilities. The principle behind the usage of nanoparticles is that alteration of filler size is considered responsible for the performance of the material (PMMA) in aspects of both polishability and fracture resistance.  Of the few experiments which have been conducted, it has been found that the addition of metal nanoparticles to materials is known to increase the surface hydrophobicity and reduce the adherence to biomolecules.  Many metals such as aluminum dioxide,  cobalt-chromium,  silver,  zinc oxide,  zirconia  and most commonly titanium dioxide  have been used in experiments to improve the mechanical properties of PMMA. Titanium dioxide is most commonly used since it acts as a coloring agent and can improve the antimicrobial properties of PMMA resins.  Experiments using the different shapes of nanoparticles such as nanotubes  and fibers  have also been conducted.
The few studies conducted on the effect of nanoparticles on the flexural strength have been more or less inconclusive and unclear. These experiments showed a marginal improvement of flexural strength but had several shortcomings and limitations that restricted immediate clinical application such as evaluation of fatigue life, fatigue crack, propagation resistance and long term wear.  This developing novel science of nanotechnology requires more experimentation to be employed in clinical scenarios and thus this study was undertaken.
· To evaluate whether the incorporation of titanium dioxide nanoparticles in PMMA increases the flexural strength
· To compare the different concentrations of titanium dioxide nanoparticles and its relation to flexural strength.
| Materials and methods|| |
The test specimens utilized in the current study were manufactured from clear heat polymerizing acrylic resin. Forty specimens of dimensions 65 Χ 10 Χ 3 mm as per ISO 1567 standardization  are prepared. Forty wax patterns of the mentioned dimensions were prepared and invested in flasks through capping technique (2 pour technique) with dental plaster. After the investing materials had set, the flasks were placed for dewaxing in conventional water bath. Following the dewaxing they were opened and cleaned to remove any traces of wax to facilitate the application of separating medium (cold mold seal) in order to protect the gypsum surface. A mixture of 54 gms 50 gms and 20 ml of polymer and monomer were mixed. The nanoparticles (procured from Reinste Nano Ventures Pvt. Ltd.) were incorporated into the monomer by ultrasonic dispersion.  Ten specimens of each group containing different concentrations of titanium dioxide (TiO 2 ) were fabricated similar to the procedure mentioned. The concentrations of titanium dioxide in each group were incorporated as 1 wt%, 2 wt% and 5 wt% as follows,
· Group A (CONTROL): No TiO 2 nanoparticles are added
· Group B: 0.5 gms of TiO 2 nanoparticles are added
· Group C: 1 gm of TiO 2 nanoparticles are added
· Group D: 2.5 gms of TiO 2 nanoparticles are added.
Once in dough stage the flasks were packed with PMMA and a polythene sheet was placed over the mold space resin and trial closures in the hydraulic press were repeated until no flash was observed to ensure even flow of the resin throughout the mold space. After processing the flasks were cooled to room temperature. Rapid cooling for 30 minutes subsequently placed in cool tap water for 15 minutes is ideal. The strips thus obtained were ground and polished to the required size. The 40 specimens were stored in water at a temperature of 37 °C for 50 hours prior to flexural testing. 
Flexural strength calculation
The specimens were mounted on the designed part of the Universal Testing Machine INSTRON (three point loading and testing equipment). The load was applied at the center of the specimen with a cross head speed of 1.50 mm/min and a span length of 40.00 mm. The maximum load before fracture was measured. The results were recorded through the three point bending test. Flexural strength was calculated as follows 
S - Stress
F - Load/Break at heat (N)
L - Span of the specimen (65 mm)
b - Width (10 mm)
d - Thickness (3.3 mm).
The statistical analysis was carried out with the SPSS.16.0 version. To describe about the data mean and SD was used. The multi variate analysis with analysis of variance (ANOVA) shows the significant increase in the flexural strength with the increase in titanium dioxide with the P < 0.05. To verify the better rate of change in flexural strength within the four groups the independent t-test was implemented.
| Results|| |
In this study the comparison of flexural strength was done between the control group and the specimens containing different concentrations of titanium dioxide nanoparticles. [Table 1] shows the materials and methods used in the group. [Table 2] shows the flexural strength and flexural load of different groups. [Table 3] shows the mean and standard deviation values of flexural strength for each of the experimented groups.
Highest mean flexural strength is observed in Group D, while the lowest is seen in Group A [Graph 1]. The ANOVA result [Table 4] shows that there is a significant difference between the groups with respect to the mean flexural strength. The ANOVA showed the significant increase in the flexural strength of 0.038 (P < 0.05) [Table 4].
In order to find out among which pair of the groups there exist a significant difference, multiple comparisons were carried out using the independent t-test. There was a statistical significance between Group A and Group D of 0.041 (P < 0.05) and between Group B and Group D of 0.028 (P < 0.05) [Table 5].
| Discussion|| |
Polymethylmethacrylate (PMMA) is the most commonly used material in the fields of prosthodontics. But is shows weak physical and mechanical properties. Many experiments have been undertaken to improve the flexural strength of PMMA in order to the prevent the fracture and clinical failure. Since these tests showed inconclusive results which were clinically inapplicable, there was a scope for improvement and experimentation. This study was undertaken to investigate whether the flexural strength of PMMA changed with the addition of nanoparticles.
Nanoparticles are used based on the principle that reduction of filler size is known to increase the mechanical properties of resins. In this research the spherical particles of titanium dioxide are used to improve the flexural strength as spherical particles increase the polishability and mechanical properties and polishability. , Other structures such as nanotubes and fibers having been recently discovered, show much better properties. They show good dispersion and good adhesion to polymers, indicating chemical compatibility and creation of chemical bonds between the surface and polymer.
Titanium is used since it increases the surface hydrophobicity, reduces the adherence of biomolecules, aids in coloring, has antimicrobial properties and improves mechanical properties  of PMMA resins. Tests that have been conducted previously with titanium dioxide incorporated in PMMA showed increased mechanical properties with extended strong interfacial adhesion. 
The amount of titanium dioxide concentration used in the present study was restricted to 0.5 gm (1 wt%), 1 gm (2 wt%), 2.5 gm (5 wt%). Previous study done using ZrO2 have shown, the addition of modified nano-ZrO 2 nanoparticles powder increased the value of the impact strength and transverse strength compared to the control group, 5 wt% group has the highest impact strength and transverse strength, but increasing the percentage of modified nano-ZrO 2 to 7 wt% lowered the impact strength and transverse strength due to agglomeration nano-ZrO 2 .  Hence nanoparticles were limited to 5 wt %.
The resins were processed by the conventional water bath technique in this research. But recently researches have proven the microwave polymerization to be as effective as water bath technique and have advantages such as reduced porosity and time consumption over the water bath technique.
Principal factor that governs the rate of polymerization is the rate at which the free radicals of the benzoyl peroxide are released and this is largely controlled by the temperature. Once the resin had reached the dough stage the powder and the liquid should be thoroughly mixed to ensure that they have proper polymer monomer balance, the failure of which will result in low strength, porosity and poor color of the processed acrylic resin. As polymerization is exothermic if the temperature exceeds the boiling point of the unreacted monomer the components tend to boil resulting in porosity. Thus an ideal time saving technique of polymerization which involves processing the resin at 74°C for two hours and increasing the temperature of the water bath to 99°C and processing for one hour is ideal. 
Incorporation of titanium dioxide is done using the ultrasonic dispersion of the spherical nanoparticles in the monomer since other methods apart from ball milling shows reduced homogenous dispersion. The nanoparticles fail to dissolve completely in the monomer. 
A total of four groups were made [Table 1]. The flexural strength between the groups was measured and compared. The result showed that flexural strength had increased with the addition of titanium dioxide nanoparticles. Group A had a mean of 176.06 ± 47.06, Group B had a mean of 182.51 ± 22.29, Group C had a mean of 204.75 ± 29.42, and Group D had a mean of 223.43 ± 49.27. There was a statistical difference when the samples of normal acrylic resin were compared with modified acrylic resin (P < 0.05).
This increase in the flexural strength is attributed to the reduction in filler size, which is responsible for the improved fracture resistance of PMMA. The functionalized nanoparticles thus bind to the polymer matrix leading to increased adhesion thereby increasing the mechanical properties of acrylic resin. But since nanotechnology is a growing interest it will determine the future of dentistry. In the near future nanotechnology could create a revolution in fields such as oral anesthesia, tooth repair, renaturalization, hypersensitivity, orthodontics, dentrifrobots and dental durability. 
| Limitations of study|| |
Despite the increase in the flexural strength of PMMA, there are a few limitations in the research conducted.
- Incorporation of titanium dioxide by ultrasonic dispersion showed restriction of homogenous dispersion of the nanoparticles. Other methods of incorporation are mortar and pestle, high energy ball milling and silination of which ball milling seems to be the most effective.  Future experimentation will address the above limitations and will investigate an easier method of incorporation
- The lack of homogenous dispersion is also attributed to the morphology of the nanoparticles. Non spherical nanoparticles like nanotubes and nanorods show more surface area and hence might result in better bonding to the surface of the PMMA 
- The time required for water bath technique can be prevented by using microwave technique instead, which will yield more or less the same results as water bath technique.
| Conclusion|| |
From the present study it could be concluded that the specimens of heat cured polymethylmethacrylate reinforced with different concentrations of titanium dioxide nanoparticles showed superior flexural strength than those of normal PMMA. But studies with greater concentrations must be done.
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Final Year Undergraduate, Faculty of Dental Science, Sri Ramachandra University, Porur, Chennai, Tamil Nadu
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
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