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Research Article

The Assessment of Some Physical and Mechanical Properties of PMMA Added Different Forms of Nano-Zro2

Zeynep SAHIN and Gulfem ERGUN

Correspondence Address :

Zeynep SAHIN
DClinDent, Private Practice
Ankara
Turkey
Tel: +90 312 316 3269
Email: dtsahinzeynep81@gmail.com

Received on: February 06, 2017, Accepted on: February 13, 2017, Published on: February 20, 2017

Citation: Zeynep SAHIN and Gulfem ERGUN (2017). The Assessment of Some Physical and Mechanical Properties of PMMA Added Different Forms of Nano-Zro2

Copyright: 2017 Zeynep SAHIN, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract
Background: The need to repair the acrylic prosthesis with chemically polymerized PMMA, which is less durable than the original prosthetic base materials, poses a clinically serious problem and causes re-fractures in a short time.
Objective: The aim of this study was to evaluate the effect of resin structures, formed by adding different forms of (tetragonal/cubic) zirconium oxide nanoparticles (nano-ZrO2) to two chemically polymerized PMMAs on transverse strength, modulus of elasticity, surface roughness, color stability, water sorption and solubility. 
Method: After two different forms of nano-ZrO2 were modified with silane coupling agent (APTES), it was added to PMMA powder by 1 % in weight. The chemical bonding of nano-ZrO2 with silane coupling agent was evaluated by Fourier Transform Infrared Spectroscopy (FT-IR) analysis. All tests were performed and then, two test specimens from each subgroup were selected. The microstructure of the specimens was evaluated by Scanning Electron Microscope (SEM) analysis. 
Results: Statistical analysis showed that the transverse strengths of two PMMAs were decreased by the addition of the tetragonal and cubic nano-ZrO2 (p<0.001). The surface roughness values of the tetragonal nano-ZrO2-added group of the Paladent (p=0.008) and the cubic nano-ZrO2 added groups of Meliodent (p=0.024) showed the lowest values. However, color change, water absorption and solubility values of two chemically polymerized PMMA test materials with tetragonal and cubic nano-ZrO2 added showed no statistically significance (p<0.005).
Conclusion: The different forms of nano-ZrO2 that were added to both chemically polymerized acrylic resin of the test specimen reduced transverse strength. 
Keywords: Nano-ZrO2, Polymethyl methacrylate (PMMA), Chemically polymerized, Transverse strength, Surface roughness, Color stability, Water sorption and solubility 
This study was presented as an oral presentation at the 2016 FDI Annual Dental Congress (Abstract ID: 1093)

Fulltext
Introduction 
Today, the most frequently preferred denture base materials are the polymethyl methacrylate (PMMA) based acrylic resins [1,2]. 
Fractures in the prostheses generally stem from the fatigue due to bending and impact forces [3,4]. In the denture bases, it is necessary to increase the transverse strength to be able to resist the high flexural stresses occurring during the function [5]. 
The choice of repair material in the acrylic resin fractures is an important issue for the clinicians [6]. The time necessary for the process, transverse strength of the repair and the dimensional accuracy of the repair material have a significant role in the choice of repair material [7]. Acrylic resins polymerized with light, heat, chemicals and microwave are used in the repair of the prostheses [8]. 
Chemically polymerized acrylic resin is one of the materials in dentistry most frequently used in the repair of the prosthesis fractures [6]. The greatest problem faced when the acrylic prosthesis is repaired with the chemically polymerized acryl is that the repair is mechanically less endurable than the original prosthesis base material, and re-fracture incidents may happen after a short time [9]. Acrylic resin should have sufficient repair strength when there is a repair with the chemically polymerized acryl [6]. The transverse strength of the repairs made with this acrylic resin varied between 40-90% [7].
There are options such as the chemical modification of PMMA, addition of filling materials and development of the materials possible to be an alternative to PMMA for the development of the mechanical properties of polymethyl methacrylate [10,11].
The studies on the nanomaterials conducted for increasing the mechanical properties of PMMA have recently increased. There are some studies conducted on the development of the mechanical properties of the acrylic resins by the addition of zirconium oxide (ZrO2) as filler [12]. In these studies, it was aimed to improve the mechanical properties of ZrO2 by its addition to the structure of the acryl due to its various advantages such as chemical inertness, thermal stability, high fracture strength [13,14], high hardness and mechanical resistance, abrasion resistance, resistance to physical corrosion and biocompatibility [5,15,16].
There are not sufficient studies in which the impact of the addition of ZrO2 nanoparticle (nano-ZrO2) on the mechanical and physical properties of the acrylic resins is seen in the literature.
In this study, it was planned to assess the impacts of the addition of nano-ZrO2 powder in different forms (tetragonal/cubic) on the ultimate flexural strength, surface roughness, color stability, water sorption and solubility of two chemically polymerized PMMA.
Materials and Methods
Two chemically polymerized acrylic resins, reinforcement materials and chemical agents used in the study and their properties are given in Table 1, 2 and 3.
'Cubic and tetragonal nano-ZrO2 was added to the powder of the chemically polymerised test materials after some chemical processes were applied'. For this purpose, 0,5 ml silane coupling agent (APTES) was dissolved with 49,5 ml washing agent (toluene) and a solution with the volume of 1% was prepared. Nano-ZrO2 powder was added in this prepared solution and they were put in the magnetic mixer. The solution we prepared after keeping in the magnetic mixer for 24 hours was sieved with funnel. With the washing agent (toluene), washing process was completed. It was kept in the washing agent for 5 minutes and in the ultrasonic mixer for 10 minutes in total. Whether there was any bonding with ZrO2 powder silane coupling agent was assessed with Fourier Transform Infrared Spectroscopy (FT-IR) analysis after keeping in room temperature for 24 hours. Spectrum was taken from the modified cubic and tetragonal nano-ZrO2 in FT-IR spectrometer (Nicolet İS 5, Madison, WI, USA). 400-4000 cm-1 wave number range was used.
Powder / liquid ratio of the acrylic resin was determined according to the suggestions of the manufacturer company. It was mixed by adding 1% tetragonal/cubic form nano-ZrO2 in weight to the PMMA powder by using precision scale depending on the determined powder weight.
Ultimate Flexural Strength and the Modulus of Elasticity
The International Standard no. ISO 20795-1 [17] was taken as the basis during the application of the tests for ultimate flexural strength and modulus of elasticity. Test specimens were prepared in the dimensions of 64x10x3,3 mm and with the use of stainless stain moulds. Before testing, the prepared specimens were kept in the distilled water at 37◦C for (50 +/- 2) hours. The properties of ultimate flexural strength and modulus of elasticity were assessed with three point bending test. The tests were conducted with universal testing machine (Lloyd Instruments, Fareham, Hampshire, England). The values of transverse strength and modulus of elasticity were calculated according to the formula below:
S=3FI/2bh2 and E= FI3/4bh3d
In these formulas, S= Transverse strength (MPa), E=Modulus of Elasticity (MPa), F= maximum load at the moment of fracture (N), l=distance between the supports, b=specimen width and h= specimen thickness (mm).
Surface Roughness
To conduct the surface roughness test, profilometer device (Perthometer M2, Mahr Federal, Germany) was used to measure the surface roughness of the specimens prepared as discs (with the diameter of 10 mm and thickness of 1,5 mm). Roughness measurements were conducted by the same researcher from three separate zones of the surfaces. After keeping the specimens in a fixed position, the averages of these values were calculated and the average surface roughness values of each specimen (Ra) were recorded.
Water Sorption and Solubility
A total of 72 samples (n=12 for each sub-group) were prepared with the diameter of (50 +/- 1) mm and thickness of (0.5 +/- 0.1) mm for the test method of water sorption and solubility. The upper and lower surfaces of these specimens were sanded with 400, 600 and 800 grit silicon carbide water sandpaper. The prepared five group test specimens were placed in the desiccator (ILDAM Ltd., Ankara, Turkey) that includes newly dried silica gel (Sigma, Aldrich). The desiccator was kept in the drying oven with the temperature of 37 +/- 1 Degree Centigrade for 24 hours. This drying cycle was repeated until the specimens reached a constant mass. The specimens were kept in distilled water for 7 days at 37 +/-1 Degree Centigrade after they reached a constant mass (M1). Each specimen was taken out of the water at the end of this period and they were dried with a clean and dry paper towel until the visible humidity was gone. They were shaken in air for 15 seconds and each specimen was weighed 1 minute after they were taken out of the water. This weighed mass was recorded as M2. After this weighing, the test specimens were subjected to process again until they reached a constant mass in the desiccator as expressed before. This renewed mass, namely the reconditioned mass of the specimens were recorded as M3. Water sorption and solubility were calculated with Wsp (g/mm3) = M2 (g) - M3 (g) / V (mm3) and Wsl (g/mm3) = M1 (g) - M3 (g) / V (mm3) formula.
Color Stability Test
Test specimens were prepared as 72 discs in total with the diameter of 10 mm and thickness of 2mm from each sub-group [the one in which tetragonal and cubic nano- ZrO2 was not added (control), the one in which cubic nano-ZrO2 was added, the one in which tetragonal nano-ZrO2 was added] for the measurements of the color stability. Spectrophotometer device (Minolta Chromascop CR-321, Osaka, Japan) was used for the assessment of the color stability. The color difference between the control group and the specimens to which nanoparticle was added was detected with the use of the related formula (ΔΕ* = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2) after the attainment of CIE Lab coordinate values.
Color change (ΔΕ) values were also converted to National Bureau of Standards (NBS) units and clinical assessment of the in vitro assessment results was also ensured. NBS ratios are the color changes that could be assessed with human eye [18]. NBS values were attained by multiplying the ΔΕ values with 0.92 (NBS Unit = ΔΕx0, 92). 
Two specimens were selected from each sub-group after the transverse strength test was applied. The selected specimens were covered with aurum-palladium film layer by using 10-1mbar/Pa combustion room pressure values and a current of 10 mA for 180 seconds in the aurum-palladium coating unit (Sputter Coater SC7620, Polaron, VG Microtech, England). In SEM analysis, SEM (JEOL, JSM-6060LV, Scaning Electron Microscope, Tokyo, Japan) images were attained with x50, x100, x500, x1000, x 1 500 zooms.
The analysis of the data was done via IBM SPSS statistics version 17.0 software program (IBM Corporation, Armonk, NY, USA). The average differences between groups were compared with Student's test. In addition, Mann Whitney U test was applied in the comparison of the data not having any normal distribution. The moment p value was found as important in one-way ANOVA or Kruskal Wallis test statistics results, post hoc Tukey HSD or Conover's multiple comparison test were used to show which group was different from others. Unless a contrary situation was detected, the results were accepted as statistically important if they show the value: p<0.05. However, Bonferroni Correction was made in this study for all multiple comparisons in order to be able to control Type I error.
Results
Statistically no significant difference was observed between the tetragonal/cubic forms within the Meliodent group in terms of average transverse strength (p=0.257) (Table 4). There was a statistically significant difference between the tetragonal and cubic forms of the Paladent group in terms of average transverse strength (p<0.001). Their transverse strength results were found to be less in statistically significant level when compared to the control group (p<0.001) (Table 4).
The average and standard deviation values related to elasticity modulus values of the test specimens were given in Table 5. No statistically significant difference was seen between the tetragonal/cubic forms within the Paladent group in terms of the average modulus of elasticity (p=0.074). Statistically significant difference was shown between the tetragonal/cubic forms within the Meliodent group in terms of the average modulus of elasticity (p=0.005). Moreover, the elasticity modulus of the cubic form added test specimens significantly gave a higher value when compared to the control group (p=0.004) (Table 5).
As a result of the profilometric measurements, Ra values of all of the tested materials and control groups were obtained, and then the statistical assessment results related to the surface roughness are given in Table 6.
No statistically significant difference was found in terms of Ra between the Paladent control group and meliodent control group (p=0.242 and p=0.551). Cubic form was reinforced with ZrO2 and no statistically significant difference was observed in terms of Ra between the paladent group and meliodent group (p=0.198 and p=0.160). Ra level of the paladent test group reinforced with tetragonal form ZrO2 was found to be less in statistically significant level when compared to the meliodent test group to which tetragonal form ZrO2 was added (p=0.004) (Table 6).
Statistically no significant difference was detected between two acrylic resin (Paladent/Meliodent) control groups, the one to which 1% cubic form nano-ZrO2 were added (p=0.032) and the one to which tetragonal form nano-ZrO2 were added (p=0.071) according to the average water sorption values of Bonferroni Regulation (Table 7).
Moreover, statistically no significant difference was detected between two acrylic resin (Paladent/Meliodent) control groups, the one reinforced with 1% cubic form nano-ZrO2 and the one reinforced with 1% tetragonal form nano-ZrO2 according to average water solubility values of Bonferroni (Paladent p=0.179/ Meliodent p=0.336) (Table 8).
No statistically significant difference was found in terms of the average ΔE values between the Paladent control group and meliodent control group (p=0.551) Also, the color stability assessments of the test materials conducted after the addition of ZrO2 in different forms did not display a statisitically significant difference between the paladent group reinforced with cubic form and the meliodent group (p=0.160). Similar results were revealed between the paladent group and meliodent group reinforced with tetragonal form (p=0.713) (Table 9).
NBS values of the test specimens were attained by using the formula NBS= ΔEx0.92 (Table 10). NBS values of the chemically polymerized control group specimens (Paladent/meliodent) and the test group specimens to which nano-ZrO2 were added in tetragonal/cubic form were found to be within the clinically acceptable limits.
FT-IR analysis was conducted for the purpose of assessing the chemical bond formation between the ZrO2 particles in different forms and the bonding type with silane coupling agent. Three strong absorption bands were seen in 3396, 1635 and 588 cm-1 in tetragonal and cubic nano-ZrO2 (Figure 1). Two bands in 3396 and 1635 cm-1 were stemmed from the tension vibration of the hydroxyl groups on ZrO2 surface. The absorption band in 588 cm-1 occurred from the vibration of Zr-O bond. Three new strong absorption peaks were monitored at 2926, 1558 and 1024 cm-1 in FT-IR spectrum modified with silane coupling agent (Figure 1). The formation of these peaks respectively showed the stress vibration of C-H bond, symmetrical bending of -NH3+ and stress vibration of Si-O-Zr bond. Two new absorption bands being the stress vibration of C-H bond in the range 2800-3000 cm-1 and the stress vibration of Si-O-Zr bond seen in the range 800-1200 cm-1 showed that silane coupling agent was successfully bonded to nano-ZrO2 (Figure 1). The asymmetrical and symmetrical stress vibration of N-H bond in the wave number in the range 3366 and 3296 cm-1 in the spectrum of silane coupling agent was not observed in the modified ZrO2, because it coincides with the stress vibration of the hydroxyl groups.
Silane coupling molecules were chemically bonded on the surface of ZrO2 with Zr-O-Si bond according to the FT-IR results. Zr-O-S bond occurred with the condensation reaction between the hydroxyl groups of nano-ZrO2 and the cylanol groups of Silane agent.
In our study, the structural images of the specimens were done with the use of surface scanning electron microscope. A more decent and compact structure was observed in meliodent control group test specimens when compared to the test specimens to which nano-ZrO2 were added (Figures 2a and 2b). It was observed in the meliodent cubic form nano-ZrO2 added test specimens, the layer providing adhesion to the polymer matrix did not occur (Figures 3a and 3b). It was observed in the tetragonal form nano-ZrO2 added meliodent test specimens that the nano-ZrO2 show accumulation and clustering in PMMA matrix (Figures 4a and 4b).
A visible line was followed between the PMMA powders and the surrounding matrix in the Paladent control group test specimens. This situation showed that adhesion to the polymer network occurred. It showed a decent and compact structure similarly with the meliodent control group test specimens (Figure 5a and 5b). It was monitored that nano-ZrO2 were not distributed homogenously in the matrix in the cubic form nano-ZrO2 added Paladent test specimens (Figure 6a and 6b). It was observed in the Paladent test specimens to which tetragonal form nano-ZrO2 were added that the penetration of nano-ZrO2 was not provided in the resin matrix, the surfaces of nano-ZrO2 remained smooth and the adhesion to resin matrix was weak (Figures 7a and 7b).
Discussion
A transverse and impact resistance value of PMMA used as prosthesis base material is low. This negative property causes the frequent observance of prosthesis fractures inside or outside the mouth [5].
Because the preparation of a new prosthesis is expensive and time-consuming, permanent or temporary repair works are conducted to solve the problems of prosthesis fractures [19]. The repair of the fractured prostheses could be made with the light, heat, microwave and chemically polymerized acrylic resins [8]. Chemically polymerized acrylic resin is the most preferred (by 86%) for the repair of the prosthesis [20]. The mechanical properties of the resins polymerized with heat are higher when compared to the chemically polymerized acrylic resins. In addition, taking to lab muffle and boiling processes are time-consuming and the prostheses have distortion risk with heat [7]. The popularity of the chemically polymerized acrylic resins still continues due to easy usage, short period of waiting at the chair for the patient, no requirement for lab processes and short period of a prosthesis which is of no use, by the patient during the repair process [21]. For this reason, chemically polymerized acrylic resin material was used in this study.
Many methods have been tried until today for the purpose of developing the mechanical properties of the chemically polymerized acrylic resins that could both be used as base material and repair acrylic [19,21]. With the addition of ZrO2 as filling material recently, some studies were conducted regarding the development of the mechanical properties of the acrylic resins [5,12,15]. Zirconium is biocompatible and it has positive impacts on the development of mechanical properties. As zirconium powder is white, it also meets the aesthetic expectations [5]. Nanoparticles tend to cluster and cause the decrease in the interaction of particle-polymer due to the specific surface area, high surface energy and active chemical properties. This situation decreases the reinforcement activities [12,13]. The adhesion in the polymer and filler interface as well as the filler type, magnitude, distribution and bonding affects the mechanical properties of the acrylic resin [5]. Silane coupling agents decreasing the surface stresses of the fillers are used for the purpose of providing the chemical bonding between ZrO2 and acrylic resin and preventing the clusters of the nanoparticles [5,12]. These silane coupling agents are the ligands widely used in the functioning (operationalization) of the oxide nanoparticles [22].
Silane coupling agent has the highest bonding ratio via condensation among the hydroxyl groups on the surface of nano-ZrO2 and naturally the triethoxy group. In addition, rapid formation of the hydrogen bond between the amino main group and hydroxyl group may contribute to the rapid transformation of nano-ZrO2 into transparent ZrO2 distribution [22]. For this reason, 3-Aminoprophiltriethoxysilane (APTES) was used as the silane coupling agent in our study.
In the studies, the dispersibility of ZrO2 nanocrystals was examined by using the ligands such as 3-glycidoxypropyltrimethoxysilan (GPTMS), APTES and 3-isocyanatopropyltriethoxysilan (IPTES) among the organic dissolvers such as toluene, pyridine, ethane nitryl, acetone and isopropyle [13,22]. It was observed in the studies that transparent ZrO2 distribution was prepared in the organic dissolvers such as pyridine and toluene with the use of APTES agent and it was also expressed that it was not prepared in other dissolvers [22]. For this reason, toluene was used as the organic dissolver in our study in the functioning of nano-ZrO2 via APTES agent.
FT-IR analysis was preferred in our study in the assessment of the chemical bonding of nano-ZrO2 and APTES agent. The reason why FT-IR analysis was preferred is that it is one of the cheapest and fastest spectroscopic techniques. It is easy to prepare specimen. Any IR spectrums of composites are same to each other except for optic isomers [23]. 400-4000 cm-1 wave number was used in our study for FT-IR analysis.
Transverse strength test is widely used because it is the one that resembles the forces applied to the prosthesis inside mouth a lot [10]. Transverse strength of the material is the composition of compression, stress and shearing strength [24]. Another property that is important in addition to the transverse strength in the assessment of the mechanical properties of the acrylic resins is the value of the modulus of elasticity. This value is the indicator for the resistance of the material against deformation [25]. For this reason, three point bending test was preferred in our study for the purpose of attaining the elasticity modulus value and transverse strength values. 
There are many studies examining the transverse strength of the acrylic base resins [5,15,26,27]. Ayad, et al. [15] reported that 5% and 15% zirconia addition significantly increases the transverse strength in the high-resistant acrylic resins. Asopa, et al. [5] revealed that 10% and 20% zirconia addition increases the transverse strength, but it decreased the surface hardness. In our study, the transverse strength values of the test specimens to which cubic and tetragonal form nano-ZrO2 being 1 % in weight were added were observed as lower when compared to the control group. Transverse strength of the control group test specimens were found as 99.49 MPa in paladent branded acrylic resin and as 85.94 MPa in meliodent branded acrylic resin. The transverse strengths of the paladent branded test specimens to which cubic and tetragonal nano-ZrO2 was added were found respectively as 79.17 MPa and 76.99 MPa. Furthermore, the same strengths were found as 82.75 MPa and 76.89 MPa in the meliodent branded test specimens to which cubic and tetragonal nano-ZrO2 was added. It is considered that these differences may stem from the difference in the percentage of the nano-ZrO2 used and difference in the used acryl type or non-homogenous distribution of the nano-ZrO2 within PMMA matrix. At the same time, our findings may evoke the probability that the transformation of ZrO2 nanoparticles into monoclinic phase from the tetragonal phase in the structure of the ZrO2 in the event that it is treated with APTES coupling agent may cause to the weakening of mechanical properties. Furthermore, micro fractures may occur on the surface due to the increase in volume. This condition also evokes the probability that the process of keeping the specimens in distilled water at 37 Degree Centigrade for 50 +/- 2 hours before the application of transverse strength test may decrease the mechanical properties of the material due to the absorbance of the water molecules from these micro fractures.
While ZrO2 is expected to show good mechanical properties and contribute to the improvement of the mechanical properties in the nanoparticle added acrylic resins, addition to autopolymerizane PMMA decreased the transverse strength values. This condition could be explained as the insufficiency of the layer which provides adhesion between the polymer matrix and ZrO2 filler. Our findings show similarities with the study conducted by Vuorinen, et al. [28].
The elasticity of the materials is known as the modulus of elasticity or Young modulus. This property is in an indirect relation with the mechanical properties [29]. The modulus of elasticity shows the hardness of the materials within the elastic limits [25]. Values of the modulus of elasticity (Young modulus) as well as the transverse strength are also a significant criterion in the assessment of the mechanical properties of the acrylic resins.
In the study conducted by Gul, et al. [4] in which the mechanical properties of PMMA prosthesis base were assessed after coating with different ceromers (3-lycidoxy propyl tri metoxy silane (GLYMO)- tetraehtoxylane (TEOS),GLYMO-TEOS-TiO2, GLYMO-TEOS-ZrO2 and 3-metacryloxypropyltrimethoxysilane (A174)-TEOS), they concluded that coating with GLYMO-TEOS-TiO2 and A174-TEOS increased all the mechanical properties of PMMA, but coating with GLYMO-TEOS-ZrO2 decreases the flexural and impact strengths while it increases the modulus of elasticity. In our study, decrease in transverse strength after reinforcement with cubic and tetragonal form ZrO2 and increase in the modulus of elasticity after reinforcement with cubic form ZrO2 in meliodent branded acrylic resin showed similarities with the above mentioned study.
It was observed that in our study in the SEM images that the impregnation of the cubic and tetragonal nano-ZrO2 inside the resin matrix could not be ensured and adhesion to the resin matrix is weak. At the same time, our control group transverse strength values revealed higher values. In the study conducted by Zhang, et al. [11], PMMA flexural strength value to which 2% silane was applied and aluminum borate whisker (ABWs) was added was notified as 76.85 MPa. In our study, the flexural strength values in the groups to which tetragonal nano-ZrO2 were added (76.89 MPa) are similar to the values of Zhang, et al.
The surface roughness of the prosthesis materials is important because the tissues having direct contact with the prosthesis affect the oral health. Bridges, implant abutments and the rough surfaces in the prosthesis bases cause more dental plaque accumulation and adhesion as compared to smooth surfaces. Ideally, the surface with possibly the lowest roughness which decreases the microorganism adhesion and prevents the early prosthesis distortion is suggested [30].
The threshold value that is clinically acceptable for the surface roughness was expressed as Ra 0.2 m in a conducted study [31]. It was specified that 0.2 m threshold value decreases the plaque accumulation in PMMA base materials [30]. If surface roughness value is more than 0.2 m, bacterial colonization significantly increases [32]. The characterization of the smooth surface of the acrylic resin may show differences between the values 0.03 m and 0.75 m depending on the techniques used for finishing and polishing [33]. In our study, surface roughness values were found respectively as 0.50 +/- 0.10 / 0.44 +/- 0.97 / 0.32 +/- 0.09 in the test specimens to which control, cubic and tetragonal form nano-ZrO2 of the paladent branded acrylic resin was added. Surface roughness values were found respectively as 0.56 +/- 0.28 / 0.36 +/- 0.14 / 0,47 +/- 0,20 in the test specimens to which control, cubic and tetragonal form nano-ZrO2 of the meliodent branded acrylic resin was added. The surface standardization was provided by applying 400, 600 and 800 grit water sandpapering process in our study. Ra values attained when only applied sandpapering process is above 0.2 μm threshold value without the application of any other process such as traditional lab polishing process or polishing kits on the acrylic resin surface. Ra results of our study were found higher than clinically acceptable Ra threshold value. This condition makes us think that only the sandpapering process is not sufficient for the clinical usage of the prosthesis and it also reveals that polishing process is necessary especially after the repair process.
One of the problems commonly seen regarding the acrylic resins is the color instability. This condition affects the acceptation of the prosthesis by the patient, their satisfaction and aesthetics. Acrylic resins that change their colours may cause aesthetic problems. The oxidation of tertiary amine accelerator may cause change in the color of the prosthesis base polymer in the chemically polymerized acrylic resins [34].
The color change determined by colorimeter and spectrophotometer could be turned into National Bureau of Standards (NBS) units (NBS unit=0.92x ΔE) to relate the amount and quality to the clinical environment [18,35]. NBS parameters are important for the color comparisons and quality control functions [36]. ΔE values were turned into NBS values and their clinical validities were assessed in our study. NBS values were respectively observed as 0.0368 and 0.0276 in the control group of Paladent and Meliodent branded chemically polymerized acrylic resins. NBS values were respectively found as 0.0368 and 0.0368 in the tetragonal form nano-ZrO2 added paladent and meliodent branded test specimens. NBS values were respectively found as 0.046 and 0.0368 in the cubic form nano-ZrO2 added paladent and meliodent branded test specimens. These values are "very little" in NBS assessments and they are within the clinically acceptable limits.
In our study, the addition of nano-ZrO2 changed the ΔE color change values between 0.04-0.13 values. In the study conducted by Tuncdemir, et al. [35] in which they reinforced the composite resins with glass fiber and polyethylene fiber, they expressed that the addition of composite resin fiber changed the ΔE color change between 0.32-1.03 values. The reason why we attained different results from this study stems from the difference in the type, color and reinforcement of the used material. Another reason is that there is a difference of the color measurement device.
Most of the studies related to the color stability in the acrylic resins assessed the color change after a certain period of dipping in any disinfectant solution [37,38], cleaning agent [39,40] or drinks [41,42]. It cannot be possible to compare the results of our study with these data. There are not sufficient studies in the literature regarding the impact of the addition of nano-ZrO2 reinforcement agent on the color stability.
Water sorption is determined depending on the mass increase in the unit volume and water solubility is determined depending on the mass loss in the unit volume [17,43]. Since it is known that the shapes and thickness of the test specimens are very efficient in terms of the provision of standardization, water sorption and solubility specimens were prepared in accordance with the standards no. ISO 20795-1 in our study. PMMA absorbs water slowly in long term [44]. For this reason, period of keeping in water is important. The specimens were kept in distilled water for seven days in our study and the aim was to equalize the differences considered to exist among all the specimens. According to ISO 20795-1 standards, water sorption should not exceed the value of 32 g/mm3 and water solubility should not exceed the value of 8.0 g/mm3 in chemically polymerized acrylic resins [17]. The water sorption and solubility values we found in our study took place within the clinically acceptable limits foreseen by ISO standards (Table 7 and Table 8).
Negative water solubility values were also found in the paladent branded test specimens to which tetragonal nano-ZrO2 was added in our study. This condition makes us think that these materials may absorb the water molecules and they could not be oscillated back. The values of negative water solubility illustrates the result that these materials or their contents have chemically bonded with the water molecules. This result shows similarities with the results of the study conducted by Tuna, et al. [43]. 
The effect of nano-ZrO2 additions were adverse to the transverse strength of the acrylic denture base resin. However, if the non-homogenous distribution nano-ZrO2 within PMMA matrix and the poor adhesion between nano-ZrO2 and acrylic resin problems can be solved, nano-ZrO2 addition PMMA acrylic resin may become an alternative method to reduce the fracture of dentures. To increase the transverse strength, it could be evenly coated on to the surface of the ZrO2 by using electrolytes or the silanization method. Furthermore, since ZrO2 is biocompatible and white, it does not adversely affect the esthetic appearance of denture base.
Our study is limited with only the use of tetragonal or cubic nano-ZrO2. Nano-composites of boron, titanium, aluminum with ZrO2 can also be used for PMMA reinforcement. Further studies are recommended to evaluate the effect of reinforcement with ZrO2 on surface hardness such as mechanical properties, thermal property and biocompatibility. Additionally these studies should focus on simulation of clinical conditions when specimens are stored in water for a long time and artificial aging. 
Conclusion
Consequently, although a successful bonding has been observed between the nano-ZrO2 and silane coupling agent in FT-IR analyses, non-homogenous distribution of nano-ZrO2 in PMMA matrix in SEM analysis has negatively affected the transverse strength results. These results revealed the insight for us that the nanoparticle ratios added to the test materials should be increased. More comprehensive studies should be conducted and it is necessary to conduct new applications that shall provide a homogenous distribution and bonding of nano-ZrO2 within PMMA matrix.
Conflict of Interest
The authors declare no conflict of interest.
Ethical Statement
There is no ethical issue regarding this study.

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Tables & Figures
Table 1: Acrylic denture base materials used in the study


Table 2: Reinforcement materials used in the study


Table 3: Chemical agents used in the study


*The data have been shown in +/- standard deviation, the results were accepted as statistically significant for p<0.017 according to the comparisons made between paladent and meliodent groups within forms, Student's t test and Bonferroni Correction, the results were accepted as statistically significant for p<0.025 according to the comparisons made between the groups among Paladent and Meliodent groups, One-Way ANOVA and Bonferroni Correction, a: The difference between Control and Cubic was statistically significant (p=0.003), b: The difference between Control and Tetragonal was statistically significant (p<0.001)
Table 4: Transverse strength test results of the specimens according to the groups and forms


*The data have been shown in +/- standard deviation, the results were accepted as statistically significant for p<0.017 according to the comparisons made between paladent and meliodent groups within Forms, Student's t test and Bonferroni Correction, the results were accepted as statistically significant for p<0.025 according to the comparisons made between the groups within Paladent and Meliodent groups, between the test specimens to which cubic and tetragonal ZrO2 nanoparticle has been added, One-Way ANOVA and Bonferroni Correction, a: The difference between the test specimens to which control group and cubic form has been added is statistically significant (p=0.004).
Table 5: Modulus of elasticity values of the test specimens


*The data have been shown in median (interquartile range), the results were accepted as statistically significant for p<0.017 according to the comparisons made between paladent and meliodent groups within Forms, Mann Whitney U test, and Bonferroni Correction, the results were accepted as statistically significant for p<0.025 according to the comparisons made within Paladent and Meliodent groups, between cubic/tetragonal forms, Kruskal Wallis test and Bonferroni Correction, a: The difference between control and tetragonal was statistically significant (p=0.001), b: the difference between cubic and tetragonal was statistically significant (p<0.01), c: the difference between control and cubic form was statistically significant (p<0.001).
Table 6: Surface roughness results of the test specimens


*The data have been shown in average +/- standard deviation, the results were accepted as statistically significant for p<0.017 according to the comparisons made between paladent and meliodent groups within the test specimens with the addition of cubic/tetragonal ZrO2, Student's t test and Bonferroni Correction, the results were accepted as statistically significant for p<0.025 according to the comparisons made within Paladent and meliodent groups between the test specimens with the addition of cubic and tetragonal form ZrO2, One-Way ANOVA and Bonferroni Correction.
Table 7: Water sorption values of test specimens


*The data have been shown in median (interquartile range), the results were accepted as statistically significant for p<0.017 according to the comparisons made between paladent and meliodent groups within Forms, Mann Whitney U test, and Bonferroni Correction, the results were accepted as statistically significant for p<0.025 according to the comparisons made within Paladent and Meliodent groups between the test specimens to which tetragonal/cubic form ZrO2 nanoparticle has been added, Kruskal Wallis test and Bonferroni Correction.
Table 8: Water solubility values of the test specimens


*The data have been shown in median (interquartile range), the results were accepted as statistically significant for p<0.017 according to the comparisons made between paladent and meliodent groups within Forms, Mann Whitney U test, and Bonferroni Correction, the results were accepted as statistically significant for p<0.025 according to the comparisons made within Paladent and Meliodent groups between the test specimens to which tetragonal/cubic form ZrO2 nanoparticle was added, Kruskal Wallis test and Bonferroni Correction.
Table 9: Color measurements of the test specimens


Table 10: NBS units of the color differences of the test specimens to which control and different form (cubic/tetragonal) nano-ZrO2 have been added


Figure 1: FT-IR spectrum of (cubic/tetragonal) nano-ZrO2 and modified nano-ZrO2 with silane coupling agent


Figure 2: SEM images of Meliodent control group a: x1000 magnification b: x450 magnification


Figure 3: SEM images of Meliodent group to which cubic nano-ZrO2 was added a: x500 magnification 
b: x250 magnification 


Figure 4: SEM images of Meliodent group to which tetragonal nano-ZrO2 was added a: x500 magnification b: x50 magnification 


Figure 5: SEM images of Paladent control group a: x500 magnification b: x250 magnification 


Figure 6: SEM images of Paladent group to which cubic nano-ZrO2 was added a: x750 magnification 
b: x500 magnification 


Figure 7: SEM images of Paladent group to which tetragonal nano-ZrO2 was added a: x1500 magnification b: x330 magnification 
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