The Open Dentistry Journal




ISSN: 1874-2106 ― Volume 13, 2019
RESEARCH ARTICLE

TiO2-Nanofillers Effects on Some Properties of Highly- Impact Resin Using Different Processing Techniques



Hawraa Khalid Aziz*
Department of Dental Technology, Prosthetic Dental Technology, College of Health and Medical Technology, Middle Technical University (MTU), Baghdad, Iraq

Abstract

Background:

The criteria of conventional curing of polymethyl methacrylate do not match the standard properties of the denture base materials.

Objectives:

This research was conducted to investigate the addition of TiO2 nano practical on impact strength, thermal conductivity and color stability of acrylic resin cured by microwave in comparison to the conventional cured of heat-polymerized acrylic resin.

Materials and Methods:

120 specimens made of high impact acrylic resin were divided into two main groups according to the type of curing (water bath, microwave), then each group was subdivided into two groups according to the addition of 3% TiO2 nano-fillers and control group (without the addition of TiO2 0%). Each group was subdivided according to the type of test into 3 groups with 10 specimens for each group. Data were statistically analyzed using Student t-test to detect the significant differences between tested and control groups at significance level (P<0.05).

Results:

According to curing type methods, the results showed that there was a significant decrease in impact strength of microwaved cured resin, but there was no significant difference in the thermal conductivity and color stability of resin. In addition, by using nanofiller, there was a significant increase in the impact strength and color stability with the addition of 3% TiO2 nanofillers, but no significant difference was found in the thermal conductivity of the acrylic resin.

Conclusion:

The microwave curing of acrylic resin had no change in the color stability and thermal conductivity in comparison to the water bath, but the impact strength was decreased. The addition of 3% TiO2 improved the impact and the color stability, but the thermal conductivity did not change.

Keywords: Acrylic denture base, TiO2 nanofillers, Microwave, Impact strength, Thermal conductivity, Color stability.


Article Information


Identifiers and Pagination:

Year: 2018
Volume: 12
First Page: 202
Last Page: 212
Publisher Id: TODENTJ-12-202
DOI: 10.2174/1874210601812010202

Article History:

Received Date: 31/12/2017
Revision Received Date: 05/02/2018
Acceptance Date: 16/02/2018
Electronic publication date: 26/03/2018
Collection year: 2018

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© 2018 Hawraa Khalid Aziz.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: (https://creativecommons.org/licenses/by/4.0/legalcode). This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


* Address correspondence to this author at the Department of Dental Technology, Middle Technical University (MTU),B.D.S., M.Sc. Prosthetic Dental Technology, Baghdad, Iraq; Tel: +9647703294204; E-mail: hawraakhalidazizaziz@yahoo.com , lawa2246@gmail.com




1. INTRODUCTION

The substance which is employed broadly for dentures manufacturing is the PMMA resin, because of its characteristics such as simple laboratory using, low weight, cheap material, good stability in oral condition, low toxicity and appropriate aesthetic and color matching ability [1Sakaguchi RL, Powers JM. Craig’s restorative dental materials 2012; 163-176, 192-94.]. In order to get better physical properties and to simplify the technique, new resins and processing methods have been introduced as an alternative to conventional water-bath [2de Souza Júnior JA, Garcia RC, Moura JS, Del Bel Cury AA. Influence of a cobalt-chromium metal framework on surface roughness and Knoop hardness of visible light-polymerized acrylic resins. J Appl Oral Sci 2006; 14(3): 208-12.[http://dx.doi.org/10.1590/S1678-77572006000300012] [PMID: 19089075] ]. Several types of modified poly-methyl methacrylate have been introduced for denture base construction. These include self-cured resins, high impact strength resins, pour type resins, microwave-cured resins and light-cured resins [3Von Fraunhofer JA. Dental materials at a glance 2013; 143-8.].

Acrylic resin is traditionally cured with a water bath method. Microwave energy processing was first reported as an alternative technique [4Rasmy AH. Effect of microwave cured acrylic resin on candidal growth in complete denture. Doctoral dissertation, ain shams university. 2009.]. Processing with dry heat, steam and infra-red induction or dielectric heating has also been used. Microwaves are important additions to this list. The American dental association specification indicated that microwave energy used for curing the acrylic resin further withstands to mechanical foundering than conventionally polymerized acrylic materials [5Rizzatti-Barbosa CM, Ribeiro-Dasilva MC. Influence of double flask investing and microwave heating on the superficial porosity, surface roughness, and knoop hardness of acrylic resin. J Prosthodont 2009; 18(6): 503-6.[http://dx.doi.org/10.1111/j.1532-849X.2009.00469.x] [PMID: 19432756] ]. In comparison, the polymerization of heat-cured PMMA is usually carried out in a temperature-controlled water bath for at least 9 hours, while the processing by microwave reduced the period to just three minutes, resulting in the resin with the same finesse as those cured by water bath [6Shibayama R, Gennari Filho H, Mazaro JV, Vedovatto E, Assunção WG. Effect of flasking and polymerization techniques on tooth movement in complete denture processing. J Prosthodont 2009; 18(3): 259-64.[http://dx.doi.org/10.1111/j.1532-849X.2008.00421.x] [PMID: 19210609] ]. The method is fast and clean, and the final resulting product has better accuracy of fit, with a better adaptation of denture base [7Vuorinen AM, Dyer SR, Lassila LV, Vallittu PK. Effect of rigid rod polymer filler on mechanical properties of poly-methyl methacrylate denture base material. Dent Mater 2008; 24(5): 708-13.[http://dx.doi.org/10.1016/j.dental.2007.07.003] [PMID: 17888507] ].

On the other hand, there is a drawback of the use of PMMA as a denture base material with a low impact strength that leads to the fracture of the denture commonly. So the increase of the impact strength is important to get rid of the denture fracture as a result of its fall accidentally [8Yilmaz C, Korkmaz T. The reinforcement effect of nano and microfillers on fracture toughness of two provisional resin materials. Mater Des 2007; 28(7): 2063-70.[http://dx.doi.org/10.1016/j.matdes.2006.05.029] , 9Yadav P, Mittal R, Sood VK, Garg R. Effect of incorporation of silane-treated silver and aluminum microparticles on strength and thermal conductivity of PMMA. J Prosthodont 2012; 21(7): 546-51.[http://dx.doi.org/10.1111/j.1532-849X.2012.00873.x] [PMID: 22672686] ]. In addition, declined thermal conductivity of the polymer will have an effect on the wearers’ palatability and taste rating [10Aly AA, Zeidan ES, Alshennawy AA, El-Masry AA, Wasel WA. Friction and wear of polymer composites filled by nano-particles: A review. World J Nano Sci Engin 2012; 2(01): 32.[http://dx.doi.org/10.4236/wjnse.2012.21006] ].

On the other hand, the huge advancement and development of nanotechnology caused the nano-fillers to reinforce the denture materials and improve mechanical and physical properties of the polymer [11Mowade TK, Dange SP, Thakre MB, Kamble VD. Effect of fiber reinforcement on impact strength of heat polymerized polymethyl methacrylate denture base resin: in vitro study and SEM analysis. J Adv Prosthodont 2012; 4(1): 30-6.[http://dx.doi.org/10.4047/jap.2012.4.1.30] [PMID: 22439098] , 12Al-Hartomy OA, Al-Ghamdi A, Dishovsky N, et al. Dielectric and microwave properties of natural rubber based nanocomposites containing graphene. Mater Sci Appl 2012; 3(7): 453-9.[http://dx.doi.org/10.4236/msa.2012.37064] ]. Polymer nanocomposites are commonly defined as the combination of a polymer matrix and some particulate additives that have at least one dimension in the nanometer scale [13Adhikari R, Michler GH. Polymer nanocomposites characterization by microscopy. J Macromolecular Sci®, Part C. Polym Rev 2009; 49(3): 141-80.[http://dx.doi.org/10.1080/15583720903048094] ].Therefore, several attempts were made to change and improve the strength, thermal properties, and hardness of PMMA. These attempts included the addition of filler particles such as zirconia, glass fiber, alumina, tin and copper to resin [14Chaijareenont P, Takahashi H, Nishiyama N, Arksornnukit M. Effect of different amounts of 3-methacryloxypropyltrimethoxysilane on the flexural properties and wear resistance of alumina reinforced PMMA. Dent Mater J 2012; 31(4): 623-8.[http://dx.doi.org/10.4012/dmj.2012-056] [PMID: 22864216] ], which depend on the incorporated nano-particles type, size, and shape [15Jordan J, Jacob KI, Tannenbaum R, Sharaf MA, Jasiuk I. Experimental trends in polymer nanocomposites: A review. Mater Sci Eng A 2005; 393(1): 1-11.[http://dx.doi.org/10.1016/j.msea.2004.09.044] ]. There is high interest in using TiO2 because of its so many applications, for example as chemical sensor and as a biomedical material. Nano-sized structured TiO2 has proved to have antimicrobial properties [16Liu XL, Han Y, Gao G, Li ZY, Liu FQ. Effect of silane coupling agent on the mechanical, thermal properties and morphology of tremolite/PA1010 composites. Chin J Polym Sci 2008; 26(03): 255-62.[http://dx.doi.org/10.1142/S025676790800290X] ]. Moreover, it is a biocompatible material, cheap with chemical stability and free toxicity, and the two most useful advantages of TiO2 are resistance to corrosion and highest strength [17Emsley J. Nature’s building blocks: An A–Z guide to the elements 2011; 452.]. It acts as a photo-catalyst that has been used extensively for killing different groups of microorganisms including bacteria, fungi, and viruses [18Natarajan S, Kumari J, Lakshmi DS, et al. Differences in antibacterial activity of PMMA/TiO2/Ag nanocomposite on individual dominant bacterial isolates from packaged drinking water, and their consortium under UVC and dark conditions. Appl Surf Sci 2016; 362: 93-101.[http://dx.doi.org/10.1016/j.apsusc.2015.11.223] ]. Many studies conduct the use of TiO2 in different percentages such as 1% of TiO2 nanoparticles which may be incorporated into color-modified acrylic resin powder to enhance its tensile and impact strength, given that they have no adverse effect on other properties [19Ghahremani L, Shirkavand S, Akbari F, Sabzikari N. Tensile strength and impact strength of color modified acrylic resin reinforced with titanium dioxide nanoparticles. J Clin Exp Dent 2017; 9(5): e661-5.[PMID: 28512543] ].

Further researches revealed that the addition of 3% TiO2 nanoparticles to heat cured resin improved the impact strength, transverse strength and surface hardness of acrylic resin and decreased water sorption and solubility [20Alwan SA, Alameer SS. The effect of the addition of silanized Nano titania fillers on some physical and mechanical properties of heat cured acrylic denture base materials. J Baghdad Coll Dentistry 2015; 27(1): 86-91.[http://dx.doi.org/10.12816/0015269] ].

Chen et al., 2017 revealed that the use of nano-sized 3% TiO2 had antibacterial activities compared to the control and blank groups with no significant differences in the cytotoxicity and found no significant differences in the mechanical properties compared to the control group within the range of an appropriate ratio [21Chen R, Han Z, Huang Z, et al. Antibacterial activity, cytotoxicity and mechanical behavior of nano-enhanced denture base resin with different kinds of inorganic antibacterial agents. Dent Mater J 2017; 36(6): 693-9.[http://dx.doi.org/10.4012/dmj.2016-301] [PMID: 28701638] ]. The different percentages of superinducing TiO2 nanofiller 1% and 5% were added to the acrylic which showed that 5% of TiO2 nanoparticles can adversely affect its flexural strength [22Ahmed MA, El-Shennawy M, Althomali YM, Omar AA. Effect of titanium dioxide nano particles incorporation on mechanical and physical properties on two different types of acrylic resin denture base. World J Nano Sci Engin 2016; 6(03): 111-9.[http://dx.doi.org/10.4236/wjnse.2016.63011] ].

Therefore, this study is oriented to evaluate the effect type of curing methods water bath and microwaved cured on the impact strength, thermal conductivity, and color stability of high impact acrylic resin and evaluate the addition of 3% TiO2 on the same properties.

2. MATERIALS AND METHODS

A total of 120 specimens were prepared from heat-cured denture base resin. High impact acrylic (Vertex-Dental, Netherlands) was divided into two groups according to the processing technique: Group I: water bath, and Group II: microwave curing. Each group was divided into 2 subgroups: the control group with non-additive filler and the experimental group treated with 3% TiO2 Nano-fillers (Nanoshel, Stock no. NS6130-03-352, USA), TiO2 nanopowder of 30 nm particle size according to the recommendation of manufacturer instruction. The samples of each group were subdivided depending on the tests (Impact test, thermal conductivity, and color stability test) (N=10).

2.1. Sample Preparation

The wax patterns (Polywax, Bilkim chemical company, Turkey) were constructed into the desired shape and dimensions for each test (Figs. 1 and 2) as follows:

Fig. (1)
The design and the dimensions for each test,(A) impact strength;(B) thermal conductivity; (C) color stability test.


Fig. (2)
The wax pattern for each test, (A) impact strength; (B) thermal conductivity; (C) color stability test.


  • 1- Impact strength test: a bar-shaped specimen with the dimensions of 80mm x 10mm x 4mm length, width, and thickness respectively [23ISO E. 179-1. Plastics: Determination of Charpy impact properties: Part 1: Non-instrumented impact test European Committee for Standardization CEN, Bruxelles, Belgium 2000.] according to ISO.179,2000.
  • 2-Color stability test: a rectangular shaped specimen with dimensions of 35mm x 15mm x 0.5mm length, width and thickness respectively aand according to ADA,1999 [24American dental association specification No. 12 for denture base polymer guide to dental materials and devices.7th ed. Chicago Illinois 1999.].
  • 3-Thermal conductivity test: disc with dimensions of 40mm in diameter and 2.5mm in thickness according to instrument specification (Thermal constant apparatus).

The mould preparation was utilized by covering the wax pattern with separating medium and left to dry. The mould was made by bagging the lower portion of the flask with dental stone type IV(Elite stone, Zhermack, Germany), that was mixed according to the manufacturer's instructions. The wax patterns were placed into 1/2 of their thickness. After the stone was set, another layer of separating medium was coated and then allowed to dry. After that, the opening of the flask was carefully done and the patterns were removed from the mould.

2.2. Addition of TiO2 Nano-Fillers

The addition of 3% TiO2 nanofillers to the monomer was done through the extremely sonication of the fillers which had been well dispersed in the liquid by a probe of sonication equipment (Soniprep150, England) at 120W and 60 KHz to split them into nano-crystals for 3 minutes individually (Fig. 3) [25Atai M, Solhi L, Nodehi A, et al. PMMA-grafted nanoclay as novel filler for dental adhesives. Dent Mater 2009; 25(3): 339-47.[http://dx.doi.org/10.1016/j.dental.2008.08.005] [PMID: 18829096] ]. The monomer with nanoparticles was mixed with acrylic powder immediately to reduce the possibility of particle aggregation and separation. The proportion of acrylic mixing was 2.5:1 (P/L) according to the manufacturer's instruction and left to stand until a dough stage was reached. An electronic balance with 0.0001g accuracy (Staroius BP 30155, Germany) was used to measure the weight of materials that were utilized in this study.

Fig. (3)
TiO2 Nanofillers were well dispersed in the monomer by a probe sonication apparatus.


2.3. Packing and Curing

The dough stage of the mixture was packed in the mould after lining with a separating medium and become ready for curing. According to the type of curing, the conventional brass metal flask (Broden, Sweden) was used for water bath curing, while for microwave cured the special type of flask fiber reinforced plastic (FRP Flask, G C America) was used.

There are two methods for curing: the water bath curing which was carried out by placing the clamped flask in the water bath (Memmert, Germany) and processed by heating at 74°C for an hour and a half. The temperature was increased to the boiling point for 30 minutes according to ADA Specification No.12, 1999 [24American dental association specification No. 12 for denture base polymer guide to dental materials and devices.7th ed. Chicago Illinois 1999.].The metal flask was allowed to cool at room temperature for 30minutes after completing the curing, followed by complete cooling of the metal flask for 15 minutes before deflasking. Then the specimens were removed from the stone mould. While the microwave curing was carried out by placing the flask in the microwave (Samsung TDS, Korea) and processed by heating at(500W)for 3minutes (Fig. 4) [26Singh S, Palaskar JN, Mittal S. Comparative evaluation of surface porosities in conventional heat polymerized acrylic resin cured by water bath and microwave energy with microwavable acrylic resin cured by microwave energy. Contemp Clin Dent 2013; 4(2): 147-51.[http://dx.doi.org/10.4103/0976-237X.114844] [PMID: 24015000] ]. After overnight bench cooling, the specimens of microwave-cured acrylic were deflasked [27Barbosa DB, Monteiro DR, Barão VA, Pero AC, Compagnoni MA. Effect of monomer treatment and polymerisation methods on the bond strength of resin teeth to denture base material. Gerodontology 2009; 26(3): 225-31.[http://dx.doi.org/10.1111/j.1741-2358.2008.00262.x] [PMID: 19702820] ]. Eventually, finishing and polishing for all samples had been completed.


Fig. (4)
The flask inside the microwave.


2.4. Testing Procedures

2.4.1. Impact Strength Test

Acrylic specimens for impact strength test (Fig. 5) were conditioned by storing in distilled water at 37°C for 48hours before testing [24American dental association specification No. 12 for denture base polymer guide to dental materials and devices.7th ed. Chicago Illinois 1999.]. Impact testing device N.43-1, (INC, USA) (Fig. 5) was used to test impact strength by placing the specimen horizontally where a free-swinging pendulum (2 joules capacity) in the middle wa in a stable height. The reading of the impact energy absorbed to specimen fracture from the scalar was obtained in joules. The “Charpy impact strength” was recorded in KJ/m2:

Fig. (5)
(A) Impact test specimens; (B) Impact strength device.


Impact strength= E/ b.d×103 [28Anusavice KJ, Shen C, Rawls HR. Phillips’ science of dental materials 2013; 143-166, 721-56.]

Where E: is the impact absorbed energy in Joules.

b: is the width in mm of specimens.

d: is the thickness in mm of specimens.

2.4.2. Thermal Conductivity Test

The disc was made following the specification of the instrument (Thermal constant apparatus) (Fig. 6).To measure properties of thermal transport, the analyzer hot disc thermal constant was used with thermal conductivities extending from 0.005w/m°c (Evacuated powders) to 500 w/m°c (graphite) [29Muklif OR, Ismail IJ. Studying the effect of addition a composite of silanized Nano-Al2O3 and plasma treated polypropylene fibers on some physical and mechanical properties of heat cured PMMA denture base material. J Baghdad Coll Dentistry 2015; 27(3): 22-7.[http://dx.doi.org/10.12816/0015029] ]. On both sides of the conducting pattern, there was a thin electrically insulating material for supporting. The equipment was connected to computers that were programmed for the test. The experiment was called TPS (transient plane source).

Fig. (6)
(A)Thermal conductivity specimens;(B) Thermal conductivity test machine.


2.4.3. Color Stability Test

The spectrophotometer device (Macbeth, USA) was used for measuring the light absorption of each specimen at 500 nm wavelength in order to detect the light surface color absorption, the differences in the light absorption of samples scoring as well as the degree of changes in the color of the specimens between the control and experimental groups [30Ihab NS, Hassanen KA, Ali NA. Assessment of zirconium oxide nano-fillers incorporation and silanation on impact, tensile strength and color alteration of heat polymerized acrylic resin. J Baghdad Coll Dentistry 2012; 24: 36-42.] (Fig. 7).

Fig. (7)
(A) Color stability specimens; (B) Spectrophotometer device.


Data were analyzed to conclude the descriptive results as means and standard deviation tables and figures. Student t-test was used to detect the significant differences between tested and control groups at a level of significance (P<0.05).

3. RESULT

3.1. Impact Strength Test

Descriptive statistics of impact strength values showed that the heat-cured had higher mean values than the microwaved-cured samples. The impact strength for those without the addition of 3%TiO2 showed a lower mean value than those with an addition of 3%TiO2 for both curing methods as shown in Table 1 and Fig. (8).

Table 1
Mean, minimum values, maximum values, and Std. Deviation of Impact strength test, thermal conductivity, and color stability for all groups.


Fig. (8)
Bar chart showed the Impact strength.


On comparing the effect of the addition of 3%TiO2 nano-filler on the impact strength of the heat cured acrylic, the student t-test showed that there was a significant difference between the tested groups (Table 2).

Table 2
T-Test was used for impact strength, thermal conductivity, and color stability between groups according to the addition of 3% of TiO2.


In comparison, the effect of the curing methods on the impact strength of the heat cured acrylic, the student t-test showed that there was a significant difference in the impact strength between water bath and microwave cured of acrylic resin in the tested groups (Table 3).

Table 3
T-Test of impact strength, thermal conductivity, and color stability between groups according to curing methods.


3.2. Thermal Conductivity Test

Descriptive statistics of the results of the thermal conductivity values showed that heat-cured specimens without addition of 3%TiO2 had lower mean values than the specimens with addition of 3% TiO2, and that microwaved-cured specimens with addition of 3% TiO2 had higher mean values than those without addition of 3% TiO2 asshown in Table 1 and Fig. (9).

Fig.(9)
Bar chart showed the Thermal conductivity test.


For comparison of the effect of 3% TiO2 addition on the thermal conductivity of the heat-cured acrylic, the student t-test showed that there were non-significant differences between the test groups (Table 2).

On comparing mean values of the thermal conductivity of the acrylic according to the curing methods, student t-test showed that there were non-significant differences in thermal conductivity between water bath and microwave cured specimens of acrylic resin in the test groups (Table 3).

3.3. Color Stability Test

Descriptive statistics of the color stability value results showed that the heat-cured specimens without addition of 3% TiO2 had lower mean values than the specimens with addition of 3% TiO2, and that microwaved-cured specimens with addition of 3%TiO2 had higher mean values than those without addition of 3% TiO2 as shown in Table 1 and Fig. (10).

Fig. (10)
Bar chart showed the color stability test.


On comparing the effect of the addition of 3% TiO2 on the color stability of the acrylic, the student t-test showed that there were significant differences between the tested groups (Table 2).

Furthermore, depending on the curing methods, the mean values of the color stability of the acrylic resin were compared using student t-test which showed that there were non-significant differences in color stability between water-bath and microwave-cured acrylic resin in the tested groups (Table 3).

4. DISCUSSION

The decision was made to choose the percentage of TiO2 nanoparticles about 3% depending on the previous study that was conducted different concentration of TiO2 nanoparticles was added to acrylic resin, that was obtained the greatest values of impact strength, flexural strength and surface hardness of heat cured acrylic resin at 3% of nanofiller and these properties decreased with increase of this percentage [20Alwan SA, Alameer SS. The effect of the addition of silanized Nano titania fillers on some physical and mechanical properties of heat cured acrylic denture base materials. J Baghdad Coll Dentistry 2015; 27(1): 86-91.[http://dx.doi.org/10.12816/0015269] ], as well as when the percentage increase above 5% that lead to massive changes occurred in the color of acrylic [31Shi JM, Bao YZ, Huang ZM, Weng ZX. Preparation of poly (methyl methacrylate)/nanometer calcium carbonate composite by in-situ emulsion polymerization. J Zhejiang Univ Sci 2004; 5(6): 709-13.[http://dx.doi.org/10.1631/jzus.2004.0709] [PMID: 15101107] ]. On the other hand, higher concentrations (5% TiO2) will lead to impact strength deterioration of the resin material, thus causing a decrease in the strength of reinforced specimens that was reported by different authors [31Shi JM, Bao YZ, Huang ZM, Weng ZX. Preparation of poly (methyl methacrylate)/nanometer calcium carbonate composite by in-situ emulsion polymerization. J Zhejiang Univ Sci 2004; 5(6): 709-13.[http://dx.doi.org/10.1631/jzus.2004.0709] [PMID: 15101107] , 32Asopa V, Suresh S, Khandelwal M, Sharma V, Asopa SS, Kaira LS. A comparative evaluation of properties of zirconia reinforced high impact acrylic resin with that of high impact acrylic resin. Saudi J Dental Res 2015; 6(2): 146-51.[http://dx.doi.org/10.1016/j.sjdr.2015.02.003] ].

Another study reported that the use of 3% of TiO2 resulted in antibacterial activity without comprising the mechanical properties and cytotoxicity of PMMA [21Chen R, Han Z, Huang Z, et al. Antibacterial activity, cytotoxicity and mechanical behavior of nano-enhanced denture base resin with different kinds of inorganic antibacterial agents. Dent Mater J 2017; 36(6): 693-9.[http://dx.doi.org/10.4012/dmj.2016-301] [PMID: 28701638] ]. Another research on the addition of TiO2 into PMMA showed that the incorporation of nano-sized TiO2 adversely affected the mechanical properties and the flexural strength values with increase in TiO2 concentration [33Sodagar A, Bahador A, Khalil S, Shahroudi AS, Kassaee MZ. The effect of TiO2 and SiO2 nanoparticles on flexural strength of poly (methyl methacrylate) acrylic resins. J Prosthodont Res 2013; 57(1): 15-9.[http://dx.doi.org/10.1016/j.jpor.2012.05.001] [PMID: 23200530] ].

The result of the present study revealed that the use of nano-TiO2 resulted in higher impact strength, which is attributed to the good bonding between nano-filler and matrix as result of increased interfacial shear strength with decreased crack propagation [34Sun L, Gibson RF, Gordaninejad F, Suhr J. Energy absorption capability of nanocomposites: A review. Compos Sci Technol 2009; 69(14): 2392-409.[http://dx.doi.org/10.1016/j.compscitech.2009.06.020] ]. The other cause was the very small size of TiO2nano-fillers used in this study (about 30 nm) that led to increase in surface area, which was useful for energy dissipation [35Chen JK, Huang ZP, Zhu J. Size effect of particles on the damage dissipation in nanocomposites. Compos Sci Technol 2007; 67(14): 2990-6.[http://dx.doi.org/10.1016/j.compscitech.2007.05.020] ]. The use of TiO2 also may form a good mesh between the resin and TiO2 resulting in slow segmental motion [36Hu Y, Zhou S, Wu L. Surface mechanical properties of transparent poly (methyl methacrylate)/zirconia nanocomposites prepared by in situ bulk polymerization. Polymer 2009; 50(15): 3609-16.[http://dx.doi.org/10.1016/j.polymer.2009.03.028] ]. These results coincided with the research which found that the addition of silanized nano-TiO2 increased the value of impact strength [34Sun L, Gibson RF, Gordaninejad F, Suhr J. Energy absorption capability of nanocomposites: A review. Compos Sci Technol 2009; 69(14): 2392-409.[http://dx.doi.org/10.1016/j.compscitech.2009.06.020] ] and also agreed with the results of the study that showed that the use of 2% nano ZrO2: TiO2 caused higher impact strength [37Salman TA, Khalaf HA. The influence of adding of modified ZrO2-TiO2 nanoparticles on certain physical and mechanical properties of heat polymerized acrylic resin. J Baghdad Coll Dentistry 2015; 27(3): 33-9.[http://dx.doi.org/10.12816/0015032] ].

This study revealed no change in the thermal conductivity by using the TiO2, a finding that agreement with the study which concluded that the incorporation of 2% by weight of ZrO2: TiO2 showed no difference in this property [37Salman TA, Khalaf HA. The influence of adding of modified ZrO2-TiO2 nanoparticles on certain physical and mechanical properties of heat polymerized acrylic resin. J Baghdad Coll Dentistry 2015; 27(3): 33-9.[http://dx.doi.org/10.12816/0015032] ], while it disagreed with the study which concluded that the thermal conductivity increased with the use of alumina [29Muklif OR, Ismail IJ. Studying the effect of addition a composite of silanized Nano-Al2O3 and plasma treated polypropylene fibers on some physical and mechanical properties of heat cured PMMA denture base material. J Baghdad Coll Dentistry 2015; 27(3): 22-7.[http://dx.doi.org/10.12816/0015029] ].

For the color changes, the studies have been carried out to confirm and to investigate the color alteration by using apparatus because the human eyes are not sensitive to specific devices [38Ayad NM, Badawi MF, Fatah AA. Effect of reinforcement of high-impact acrylic resin with zirconia on some physical and mechanical properties. Rev Clin Pesq Odontol 2008; 4(3): 145-51.]. Therefore, color alterations objectively measured the amount of light absorption using a spectrophotometer [39Kiat-amnuay S, Beerbower M, Powers JM, Paravina RD. Influence of pigments and opacifiers on color stability of silicone maxillofacial elastomer. J Dent 2009; 37(Suppl. 1): e45-50.[http://dx.doi.org/10.1016/j.jdent.2009.05.004] [PMID: 19505752] ]. The tested acrylic specimens showed a noticeable color change with the addition of TiO2. This result in agreed with the study which showed that the increase in light absorption is statistically significant as a result of light absorption with the increasing of modified nano-ZrO2 concentration [30Ihab NS, Hassanen KA, Ali NA. Assessment of zirconium oxide nano-fillers incorporation and silanation on impact, tensile strength and color alteration of heat polymerized acrylic resin. J Baghdad Coll Dentistry 2012; 24: 36-42.]. This is obviously because of the presence of nano-TiO2 powder in the matrix which is opaque and absorbs more light energy than polymer matrix and appears more opaque. These findings are attributed to the high atomic number of titanium compared to the low atomic number of the chemical constituent of acrylic which is dependent on the cube of its atomic number [30Ihab NS, Hassanen KA, Ali NA. Assessment of zirconium oxide nano-fillers incorporation and silanation on impact, tensile strength and color alteration of heat polymerized acrylic resin. J Baghdad Coll Dentistry 2012; 24: 36-42.].

According to the curing methods, the result viewed that there was a significant decrease in the impact of acrylic resin that was cured with microwave, and this may be attributed to the residual monomer that affects the strength of specimens because the low levels of residual monomers increased the strength of resin [40Matos AO, Costa JO, Beline T, et al. Effect of disinfection on the bond strength between denture teeth and microwave-Cured acrylic resin denture base. J Prosthodont 2016; 00: 1-8.[PMID: 26999386] ]. Another reason may be that, conventional acrylic resin polymerized in water bath exhibited fewer porosities, unlike the acrylic that was cured by microwave which exhibited a higher change in porosity [26Singh S, Palaskar JN, Mittal S. Comparative evaluation of surface porosities in conventional heat polymerized acrylic resin cured by water bath and microwave energy with microwavable acrylic resin cured by microwave energy. Contemp Clin Dent 2013; 4(2): 147-51.[http://dx.doi.org/10.4103/0976-237X.114844] [PMID: 24015000] ]. The result of this study showed that there was no significant change in thermal conductivity, and this may be due to the microwave heating which is independent of the thermal conductivity. The benefits of curing resin denture base by microwave energy include less cumbersome equipment, greatly reduced curing time and a cleaner method of processing [41Hasan RH. Comparison of some physical properties of acrylic denture base material cured by water bath and microwave techniques. Al-Rafidain Dent J 2003; 3: 144-7.]. On the other hand, the result of this study showed that there was no change in the color of the acrylic specimens by microwave curing, which may be due to a shortened dough forming time and more homogeneous resin dough that causes minimal changes in the color of resin base [42Rizzatti-Barbosa CM, Cury AA, Garcia RC. The use of microwave energy in dental prosthesis. In: Grundas S, Ed. Advances in Induction and Microwave Heating of Mineral and Organic Materials. In Tech 2011; p. 766.[http://dx.doi.org/10.5772/562] ].This result is in agreement with Assunçao et al., 2009 who found that the color of denture teeth was not affected by the polymerization methods, as there was no significant difference between conventional polymerization and microwave curing [43Assunção WG, Barão VA, Pita MS, Goiato MC. Effect of polymerization methods and thermal cycling on color stability of acrylic resin denture teeth. J Prosthet Dent 2009; 102(6): 385-92.[http://dx.doi.org/10.1016/S0022-3913(09)60200-6] [PMID: 19961997] ].

CONCLUSION

It seems that the curing of high-impact acrylic by microwave had not changed the color stability and thermal conductivity in comparison to the water bath, but it decreased the impact strength. The incorporation of 3% of TiO2 improved the impact and the color stability, however, the thermal conductivity was not changed.

ETHICS APPROVAL AND CONSENT TO PARTICIPATE

Declared none.

HUMAN AND ANIMAL RIGHTS

No Animals/Humans were used for studies that are base of this research.

CONSENT FOR PUBLICATION

Not applicable.

CONFLICT OF INTEREST

The authors declare no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

Declared none.

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