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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 16  |  Issue : 1  |  Page : 12-20

Synthesis of zirconia, organic and hybrid nanofibers for reinforcement of polymethyl methacrylate denture base: evaluation of flexural strength and modulus, fracture toughness and impact strength


1 Department of Dental Biomaterials, Faculty of Science, Tanta University, Tanta, Egypt
2 Department of Chemistry, Faculty of Science, Tanta University, Tanta, Egypt

Date of Submission21-Jun-2018
Date of Acceptance24-Aug-2018
Date of Web Publication13-Jun-2019

Correspondence Address:
Usama M Abdel-Karim
Department of Dental Biomaterials, Faculty of Dentistry, Tanta University, Tanta31527
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tdj.tdj_23_18

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  Abstract 

Objective
The objective of this study was to synthesize and characterize inorganic zirconium oxide (ZrO2), organic [bisphenol A diglycidyl ether dimethacrylate (Bis-GMA)+triethylene glycol dimethacrylate (TEGDMA)+polyethylene glycol dimethacrylate (PEGDMA)] and hybrid (ZrO2+ Bis-GMA + TEGDMA) nanofibers and the use of these nanofibers for improving flexural strength (FS) and flexural modulus (FM), fracture toughness (FT) and impact strength (IS) of polymethyl methacrylate (PMMA) resin.
Materials and methods
Inorganic, organic and hybrid nanofibers were synthesized by wet electrospinning technique. The study was divided into four groups according to the type of added nanofibers (6%) to the heat-curing PMMA denture base resin; control group: PMMA without nanofibers, inorganic group: PMMA with silanized ZrO2 nanofibers, organic group: PMMA with Bis-GMA/TEGDMA/PEGDMA nanofibers and hybrid group: PMMA with ZrO2/Bis-GMA/TEGDMA nanofibers. According to ISO FDIS 20795-1:2013, three-point bending test was used to measure FS, FM and FT for each group (n = 10). According to ISO 197-A1: 2005, impact tester was used to measure IS for each group (n = 10). One-way analysis of variance was used for statistical significance between groups and post-hoc (Tukey's test) was used for multiple comparisons. P value less than or equal to 0.01 was considered significantly different.
Results
The synthesized pure forms of three types of nanofibers were characterized by scanning electron microscope and Fourier transform infrared spectroscopy. Nanofibers-reinforced groups (ZrO2, hybrid and organic, respectively) recorded high means % than that of the control group as follows: 205, 184 and 170% for FS (MPa), 184, 149 and 120% for FM (GPa), 210, 189 and 177% for FT (MPa m1/2) and 151, 180 and 176% for IS (KJ/m2). In each of the four tested mechanical properties, one-way analysis of variance revealed a significant differences between the studied groups (P = 0.000) and post-hoc (Tukey's test) revealed that nanofibers-reinforced groups were markedly significantly higher than control group (P = 0.000).
Conclusion
ZrO2, Bis-GMA/TEGDMA/PTEGDMA and ZrO2/Bis-GM/TEGDMA hybrid nanofibers synthesized by electrospinning technique improved significantly FS, FM, FT and IS of PMMA resin (P = 000).

Keywords: electrospinning, mechanical properties, nanofibers, polymethyl methacrylate resin


How to cite this article:
Abdel-Karim UM, Kenawy ERS. Synthesis of zirconia, organic and hybrid nanofibers for reinforcement of polymethyl methacrylate denture base: evaluation of flexural strength and modulus, fracture toughness and impact strength. Tanta Dent J 2019;16:12-20

How to cite this URL:
Abdel-Karim UM, Kenawy ERS. Synthesis of zirconia, organic and hybrid nanofibers for reinforcement of polymethyl methacrylate denture base: evaluation of flexural strength and modulus, fracture toughness and impact strength. Tanta Dent J [serial online] 2019 [cited 2023 Feb 5];16:12-20. Available from: http://www.tmj.eg.net/text.asp?2019/16/1/12/260274


  Introduction Top


Polymethyl methacrylate (PMMA) material has more than 80 years clinically proven history for dentures fabrication since it exhibits natural appearance, low cost, ease of fabrication and repair, polishability, accurate fit and stability in the oral environment. However, PMMA is far from ideal for maintaining denture longevity. Low mechanical properties, low thermal conductivity and high coefficient of thermal expansion are its limitations[1]. A survey reported that 70% of dentures had broken within the first 3 years of their delivery[2]. Midline fractures in upper complete dentures are often a result of flexural fatigue and impact failures usually occur out of the mouth as a result of a sudden blow to the denture[3]. The design of denture bases is not regular and requires the presence of notches like anatomical structure such as labial and lingual frenums. These notches represent areas of stress concentration that cause denture fracture due to the low fracture toughness (FT)[4].

Fibers, rubber and microparticles have been tried to reinforce PMMA denture base material. Silanized glass fibers increased the flexural strength (FS) and impact strength (IS) of PMMA resin[4],[5]. With increasing aramid fibers concentration, the hardness of the resin decreased and yellowish color is a disadvantage[6]. Silanized polypropylene fiber improved IS, and FS, but its wear resistance was highly decreased[7]. Carbon fibers adversely affected denture esthetic because it's black color, while the Kevlar fibers showed polishing and esthetic problems[1]. Fibers increased surface roughness of dentures due to their size[8].

Addition of butadiene-styrene rubber improved IS at the expense of FS and flexural modulus (FM) producing too flexible denture[9]. A 10 wt% nitrile-butadiene rubber particles and 5 wt% [Al2O3+zirconium oxide (ZrO2)] fillers loading of PMMA reported improved IS and FT but the Vickers hardness was not affected[10].

Silica[11] or hydroxyapatite (HA)[12] microparticles did not produce a significant improvement in the FS or IS of PMMA resin. ZrO2 microparticles increased IS[13] and FS[14] of PMMA resin. However, a decrease in FS, IS and surface hardness were also reported[15].

Researchers have used nanoparticles (NPs) of ZrO2, SiO2, Al2O3, TiO2, Ag, and HA fillers for reinforcement of denture base resins[12]. A study reported that ZrO2 NPs enhanced FS[16] of repaired acrylic. Ahmed and Ebrahim[2] reported that 7 wt% ZrO2 NPs increased FS and FT by 44 and 66%, respectively, more than the control. Asopa et al.[17] reported that with 10 and 20 wt% ZrO2 NPs, there were 32 and 23% increase in FS and 10 and 6% decrease in IS, respectively. Other studies reported that the addition of 1 wt% ZrO2 NPs improved the IS by 33%, while with 2 and 3 wt% ZrO2 IS was decreased[6],[13],[18].

SiO2 NPs improved both IS and FS of PMMA[19]. On the contrary, a recent study reported that SiO2 NPs adversely affect the FS of PMMA[20]. The main disadvantage of Al2O3 NPs reinforced PMMA is opaqueness of the resin[21]. TiO2 [22] and Ag[23] NPs exhibited bactericidal and fungicidal activities. Controversial results have been reported about the influence of TiO2 [24,25] and Ag[23],[26] NPs on the mechanical properties of PMMA resin. In addition, brownish discoloration of the dentures was reported with Ag NPs[16]. Addition of 1.5%[27] of single-walled carbon nanotubes to PMMA increased FS and IS of PMMA, but surface hardness was decreased. Conversely, a study reported an insignificant effect of adding single-walled carbon nanotubes on FS of PMMA[28].

In majority of polymer's reinforcement, macroparticulate, microparticulate and nanoparticulate fillers were the most commonly used type. Few reports described the use of nanofibers in dental composites[29],[30]. The most commonly used method to prepare nanofibers is electrospinning method because of its convenience and simplicity[31].

To the authors' knowledge, there are no studies reported the reinforcing effect of nanofibers on heat-cured PMMA are available in the literature. The objective of this study was to synthesize and characterize inorganic (ZrO2), organic (Bis-GMA/TEGDMA/PEGDMA) and hybrid (ZrO2/Bis-GMA/TEGDMA) nanofibers and the use of these nanofibers for improving FS, FM, FT and IS of heat-cured PMMA denture base resin.

The null hypothesis of the study was that, the addition of inorganic, organic or hybrid nanofibers will not improve the mechanical properties of heat-cured PMMA resin.


  Materials and Methods Top


Materials

Materials of the study are presented in [Table 1].
Table 1 Materials of the study

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Methods

Nanofibers preparation

Needle electrospinning instrument (Nano-01A Electrospinning setup, MEC Co. Ltd, Tokyo, Japan) was used for nanofibers preparation. Zirconia nanofibers were prepared according to Shao et al.[32]. While organic and hybrid nanofibers were synthesized by the authors. Electrospinning instruments include a high-voltage supplier, capillary tube with small diameter needle, and a metal collecting screen. An electric field is applied to the end of the capillary tube that contains the polymer solution held by its surface tension. As the intensity of the electric field increases, the surface of the solution at the tip of the capillary tube elongates to form a conical shape known as the Taylor cone. Upon further increase in the electric field, the repulsive electrostatic force overcomes the surface tension and the solution is ejected from the tip of the Taylor cone to become very long and thin electrospun nanofibers[33].

Zirconia (ZrO2) nanofibers preparation polyvinyl alcohol (PVA) solution (12 wt%) were prepared by dissolving PVA granules (MW, 115 000) in distilled water at 85°C for 2 h in paraffin oil path. The resulting mixture was then stirred 24 h at room temperature by magnetic stirrer. Electrospun solution was brought in the following weight proportion 20: 1: 2 of PVA solution, zirconia oxychloride (ZrOCl), H2O, respectively. Twenty grams of PVA solution was dropped slowly into solution of 1 g ZrOCl and 2 g distilled water. The reaction was preceded in oil path at 60°C for 6 h with magnetic stirring (DAIHAN MaXtir 500 s Hi-Performance Digital Magnetic Stirrers, SRICO, South Korea). The solution was delivered to the capillary tube with a small diameter needle (18 G) that was connected to the positive terminal of a high-voltage supply (Spellman SL30) that able to generate 20 kV DC. 0.1 ml/h flow rate, 50 mm spinneret distance and 0.5 min cleaning time were chosen. The distance between the tip of the needle that eject nanofibers and the aluminum collector (diameter 10 cm) was fixed at 10 cm. Fibers were dried for 12 h to 70°C. Nanofibers were calcinated at 240°C/h until 500°C for 5 h then 1000°C for 2 h.

Organic (Bis-GMA/TEGDMA/PEGDMA) nanofibers preparation: electrospun solution was consisted of 60% of bisphenol A diglycidyl ether dimethacrylate (bis-GMA), 20% triethylene glycol dimethacrylate (TEGDMA) and 20% polyethylene glycol dimethacrylate (PEGDMA). Electrospun solution was stirred with the magnetic stir for about 2 h at 40°C in oil path then placed in the vacuum oven at 40°C for 24 h. The solution was delivered to the capillary tube with a small diameter needle (18 G) that was connected to the positive terminal of 24 kV DC high-voltage supply (Spellman SL30). 0.4 ml/h flow rate, 50 mm spinneret distance and 0.5 min cleaning time were chosen. The produced nanofibers were dried in vacuum oven for 48 h at room temperature.

Hybrid (ZrO2/Bis-GMA/TEGDMA) nanofibers preparation 50% Bis-GMA and 50% TEGDMA were mixed and stirred by magnetic stirrer for about 2 h at 40°C in oil path then placed in vacuum oven for 24 h at 40°C. The weight percent of ZrO2 to Bis-GMA/TEGDMA solution was 1–20. The mixture was stirred in magnetic stir for about 24 h at room temperature. The suspension was delivered to the needle that is connected to the positive terminal of 28 kV DC high-voltage supply (Spellman SL30). The distance between the tip of the needle and the aluminum collector (diameter 10 cm) was fixed at 10 cm. The following operative parameters were chosen: flow rate 0.2 ml/h, applied voltage. Electrospun mats were dried in vacuum oven (Vacuum Drying Chambers, Binder, Bohemia, North American) for 48 h at room temperature.

Characterization: scanning electron microscope

Nanofibers specimens were coated with gold coating (SPI-Modules Vac/Sputter Coater). Specimens were scanned by electron microscope (JEOL-JSM-5200LV, Tokyo, Japan) at 20 000 times magnifications. Fourier transform infrared spectroscopy (FTIR) (Model: EQUINO X55; Bruker, Germany): Nanofibers were added to potassium bromide (KBr) with 1: 80 ratio and pressed under hydraulic press to form a tablet. With FTIR, 10 scans between the wave numbers of 5000 to 200 cm−1 were recorded with resolution of 1 cm.

Ball milling of nanofibers

Nanofibers were ball-milled to obtain desired size (≤150 nm). Nanofibers ball milling was done by ball milling machine (Retsch–PM400, Haan, Germany) with ball size of 10 mm, at speed of 350 rpm and for 7 h.

Silanization of zirconium oxide nanofibers

ZrO2 nanofibers were dispersed in 200 ml of toluene and added to 10 wt% of silane [3-(trimethoxysilyl) propyl methacrylate (TMSPM)] at room temperature and was continuously stirred (Wise stir MSH-20A; Wisd Laboratory Instruments, DAIHAN Scientific Co., Korea) at 150 rpm for 15 h. The solution was then filtered in order to collect the modified nanofibers. Subsequently the nanofibers were washed with 300 ml of fresh toluene in Ulrasonicator (Power Sonic 405; Hwashin Technology Co., Korea) for 24 h. The final product was then dried in an oven at 110°C for 3 h under vacuum[34].

Grouping

Heat-curing acrylic denture-based material (Lucitone 199 Dentsply International Inc., Chicago, Illinois, USA) was used. 6 wt% of acrylic powder was replaced with each type of nanofibers, so viscosity of the mix will not be affected. Four groups were formed as follows:

  1. Control group: acrylic denture-base without nanofibers.
  2. Inorganic ZrO2 group: acrylic denture-base with 6% silanized ZrO2 nanofibers.
  3. Organic nanofibers group: acrylic denture-base with 6% Bis-GMA/TEGDMA/PEGDMA nanofibers.
  4. Hybrid nanofibers group: acrylic denture-base with 6% ZrO2/Bis-GMA/TEGDMA nanofibers.


Acrylic powder and nanofibers in each group were carefully mixed by mechanical stir (5040001 RW28; Atlanta, USA) with vertical blade at 50 rpm for 30 min to ensure uniform distribution of nanofibers through powders. According to manufacturer, the powder was mixed with monomer liquid by hand mixing with P/L ratio was set at 2.5: 1. When the mixture was reached the dough stage (12 min), the mixture was packed into the mould of the flask and pressed under 14 MPa using a hydraulic press for 30 min. The curing was carried out by placing the flask in a water bath at 78°C for 90 min. The flask was removed from the water bath and then left to cool slowly to room temperature and the acrylic plate was removed from the flask.

Mechanical testing

FS, FM, FT and IS were evaluated for the control and nanofibers-reinforced groups.

FS: according to ISO/FDIS 20795-1: 2012(E)[35], 10 specimens of each group were prepared to dimensions of 64 mm length, 10 mm width, and 3.3 mm in height. Specimens were stored in water at 37°C for 50 h prior to flexural testing. FS was measured in Universal Testing machine (Instron 3600 series, USA) at a cross-head speed of 5 mm/min. The FS was calculated as follows: FS= 3Fl/2bh2, where F = maximum force, l = distance (mm) between the two supports, b is width (mm) and h is the height (mm) of the specimen.

FM: after measuring FS, FM is calculated as follows: E = F1I3/4bh3d, where F1 is the load (N) at a point in the straight line portion (with the maximum slope) of the load/displacement curve, I is the distance (mm) between the supports, b is the width (mm) of the specimen, h is the height of the specimen (mm), d is the deflection (mm) at load F1.

FT: according to ISO/FDIS 20795-1:2013(E)[35]. After at least 24 h of curing, 10 specimens of each group were prepared with dimensions of 39 mm length (lt), 4 mm width (bt), and 8 mm in height (ht). 0.05 mm in diameter precrack was cut with a saw blade in the center of each specimen to a depth of 3 ± 0.2 mm. Sharp notch was cut on the bottom of the precrack with a scalpel to a depth of 100–400 μm. The notched specimens were stored in water at 37°C for 7 days. Prior to testing, the specimens were placed in water at 23°C for 60 min. The span length between two anchors was 32 mm. The specimen strip was placed with the notch facing opposite the load plunger. FT was measured in Universal Testing machine (Instron 3600 series) at a cross-head speed of 1 mm/min till fracture. After completion of the test, the depth of the precrack including the notch (a) next to the fracture surface was calculated with an optical microscope. The maximum stress intensity (Kmax) was calculated from the following equation:



where f (x)=3 ×1/2[1.99−(1 − x)(2.15 − 3.93×+2.7 x2]/[2 (1 + 2×)(1 − x)3/2, x = a/h, and Pmax= the maximum load exerted on the specimen.

IS: according to ISO 179-A1:2005[36] 10 specimens of each group were prepared with dimensions of 80 mm length, 10 mm width, and 4 mm height. For each specimen V-notch (bn) length 0.25 mm radius and 45° angle notch sensitivity (rn) was prepared and so the width under notch is 9.75 mm. The prepared specimens were kept at 37°C for 24 h prior to the IS test. Pendulum impact tester (Zwick Pendulum Impact Tester, Atlanta, USA) using IZOD method was used for IS measurement. The samples were tested with 18.76 cm pendulum arm length and 16.5633 N weight. The specimen was clamped at one end vertically, and the swinging pendulum was used to break the notched specimen. The fracturing impact loads of the specimens were recorded.

Statistical analysis

Statistical analyses were performed using an IBM compatible personal computer with SPSS statistical package, version 20 (SPSS Inc., IBM Corp., Armonk, New York). For each of the mechanical properties, one-way analysis of variance (ANOVA) was used for statistical significance between groups and post hoc (Tukey' test) was used for multiple comparisons. P value less than or equal to 0.01 was considered significantly different.


  Results Top


Nanofibers characterization: scanning electron microscope: zirconium oxide nanofibers

Scanning electron microscope (SEM) image at 20 000 [Figure 1]a shows ZrO2 nanofibers obtained after calcination at 1000°C. The fibers have rough surface owing to ZrO2 crystallization with cylindrical morphologies after the removal of PVA component. The fibers diameters are 50–150 nm range. Organic nanofibers: SEM image at 20 000 [Figure 1]b shows smooth surface of Bis-GMA/TEGDMA/PEGDMA organic nanofibers. The nanofibers diameters are 70–150 nm range. Hybrid (Bis-GMA/ZrO2) nanofibers:SEM image at 20 000 [Figure 1]c shows ZrO2/Bis-GMA/TEGDMA hybrid nanofibers have rough surface due to the attachment of ZrO2 NPs on the surface of the organic fibers. The fibers diameters are 50–150 nm range.FTIR: ZrO2 nanofibers: after calcination at 1000°C, pure ZrO2 was indicated by the IR spectra intense peak at 520 cm−1 and 750 cm−1 [Figure 1]A assigned as due to Zr-O stretching of ZrO2. In addition, the disappearance of the absorptions corresponding to the PVA molecule indicated the complete removal of PVA at this temperature and the fibers formed were consisting of only pure ZrO2. Organic nanofibers: the formation of mixed organic nanofibers (Bis-GMA/TEGDMA/PEGDMA) is indicated by the IR spectra of the aromatic (Bis-GMA) and aliphatic (TEGDMA/PEGDMA) compounds which displayed intense peaks: at 2965–2873 due to C-H stretching of CH2, at 1608 is due to C=C stretching, at 1509 is due to C-C stretching, at 1036 is due to C-O-C stretching and at 1450 due to C=O stretching. IR spectra displayed peak at 1600–1625 due to benzene ring stretching in Bis-GMA [Figure 1]B. Hybrid (Bis-GMA/TEGDMA/ZrO2) nanofibers: the formation of hybrid nanofibers is indicated by the IR spectra of both the organic (Bis-GMA/TEGDMA) and inorganic (ZrO2) component as discussed above [Figure 1]C.
Figure 1: (a) is SEM image and (A) is FTIR of ZrO2 nanofibers. (b) is SEM image and (B) is FTIR of Bis-GMA/TEGDMA/PEDGMA organic nanofibers. (c) is SEM image and (C) is FTIR of ZrO2/Bis-GMA/TEGDMA hybrid nanofibers. SEM: Scanning electron microscope. FTIR: Fourier transform infrared spectroscopy

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Mechanical properties

Mean values and SDs of FS, FM, FT and IS for the investigated groups are presented in [Table 2]. Nanofibers-reinforced groups (ZO2, hybrid and organic, respectively) recorded high means % than that of the control group as follows: 205, 184, and 170% for for FS (MPa), 184, 149 and 120% for FM (GPa), 210, 189 and 177% for FT (MPa m1/2), and 151, 180 and 176% for IS (KJ/m2). In each of the four tested mechanical properties, one-way ANOVA [Table 2] revealed a significant differences between the studied groups (P = 0.000) and post-hoc k's test revealed that nanofibers-reinforced groups were markedly significantly higher than control group (P = 0.000). Post-hoc (Tukey' test) for FS, FM and IS revealed a significant difference between all studied groups (P = 0.000). For FT, there was a significant difference between studied groups (P ≤ 0.001) except between hybrid and organic nanofibers-reinforced groups (P = 0.111)[Table 2].
Table 2 Statistical analysis of mechanical properties of studied groups

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  Discussions Top


PMMA material is far from being an ideal denture base material because of its inferior mechanical properties[1],[2]. There is a need to achieve stronger and more fracture-resistant PMMA dentures[37]. In the current study, 6 wt% of ZO2, Bis-GMA/TEGDMA/PEGDMA) and ZrO2/Bis-GMA/TEGDMA are the materials that were used for synthesis of three types of nanofibers to reinforce PMMA resin. ZrO2 is biocompatible and presents high FT, FS and FM with white color which less likely to alter esthetic[14]. A 6 wt% of ZrO2 nanofibers was selected because percentage above 7% of ZO2 NPs was reported to cause changes in the acrylic color[38]. Bis-GMA is extremely viscous oligomer and it is strong, rigid and elongated organic molecule when polymerized. So, TEGDMA diluent was added to form spinnable solution and PEGDMA was added as cross-linking agent to produce strong organic fibers[39]. Electrospinning method was used for nanofibers synthesis as it is simple and convenient that enables the synthesis of nanofibers with wanted composition, structure and morphology[31]. Zirconia nanofibers were prepared according to Shao et al.[32]., while organic and hybrid nanofibers were synthesized by the authors.

In this study, the synthesized nanofibers (50–150 nm) were characterized by SEM and FTIR. SEM is essential to show the morphology of nanofibers and their diameters[40]. FTIR detects the presence of different bonds of the material, concentration of the materials by strength of bond peaks and differentiate between different levels of relative crystallinity[41]. Flexure strength was evaluated, as this test is more closely representing the type of loading applied to a denture in the mouth. High modulus of elasticity of PMMA is required, so great rigidity be achieved in thin section. Many authors found that the low FT of PMMA is a major disadvantage[2]. FS, FM and FT testing were done according to ISO FDIS 20795-1:2012[35]. Fracture of dentures as a result of sudden blow is a common problem due to low IS of PMMA dentures[4]. IS was tested according to ISO 197-A1: 2005[36].

The null hypothesis of the study is rejected as the results of the study reported that the addition of nanofibers improved the FS, FM, FT and IS of heat-cured PMMA denture base material.

The results of the study showed that nanofibers-reinforced groups (ZrO2, hybrid and organic, respectively) recorded high means % than that of the control group as follows: 205, 184 and 170% for FS, 184, 149 and 120% for FM, 210, 189 and 177% for FT and 151, 180 and 176% for IS. In each of the four tested mechanical properties, one-way ANOVA revealed a significant differences between the studied groups (P = 0.000) and post-hoc (Tukey's test) revealed that nanofibers-reinforced groups were markedly significantly higher than control group (P = 0.000).

To the authors' knowledge, there are no studies reported the reinforcing effect of nanofibers on heat-cured PMMA are available in the literature. Many macroparticles, microparticles and NPs and fibers of different materials were tried to reinforce PMMA resin, but there is no material reached to the reinforcing effect of the nanofibers to PMMA resin reported for the four tested mechanical properties in this study.

Carbon[1] and aramid fibers[6] adversely affected denture esthetic. Wear resistance decreased with polypropylene fibers[7], while Kevlar fibers showed polishing and esthetic problems[1]. Macrofibers and microfibers increased surface roughness of dentures due to their size[8].

Many studies reported the reinforcing effect of microparticles to the PMMA resin. Asar et al.[14] tried five oxides microparticles of 1% TiO2 and 1% ZrO2, 2% Al2O3, 2% TiO2 and 2% ZrO2 by volume to reinforce PMMA resin. The best result was recorded with 2% ZO2 where there were 40 and 30% increase in IS and FT, respectively. Recent study by Hussain and Hashim[6] reported that, addition of 1 wt% ZrO2 microparticles improve IS by (33%), while 2 wt% and 3 wt% ZrO2 decreased IS. This result was similar to previous studies[13],[18]. Addition of butadiene-styrene rubber increased IS but at the expense of FS and FM producing too flexible denture[9].

Researchers have used NPs of ZrO2, SiO2, Al2O3, TiO2, Ag, and HA for reinforcement of denture base resins[12]. Asopa et al.[17] reported that with 10% ZrO2 NPs, there was 32% increase in FS and 10% decrease in IS, whereas with 20% ZrO2 NPs, there was only 23% increase in FS and 6% decrease in IS. Ahmed and Ebrahim[2] studied the effect of adding 1.5, 3, 5 and 7 wt% ZrO2 NPs to PMMA resin and the best result was recorded with 7 wt% ZrO2 NPs that increased FS and FT by 44 and 66%, respectively more than the control. It is interesting that the FS of control group in Ahmed and Ebrahim's study was similar to our study (85.5 MPa). Other studies reported that the addition of 1 wt% ZrO2 NPs improved IS by 33%, while with 2 and 3 wt% ZrO2, IS was decreased[6],[13],[18].

Controversial results have been reported the reinforcement of PMMA with NPs of SiO2 [19,20], TiO2 [24,25] and Ag[23],[26]. Addition of 1.5% of single-walled carbon nanotubes to PMMA increased IS and FS of PMMA, but surface hardness was decreased[27]. Conversely, a study reported an insignificant effect of adding single-walled carbon nanotubes on FS of PMMA[28].

In majority of PMMA reinforcement, macroparticulate, microparticulate and nanoparticulate fillers are the most commonly used type. Few reports described the use of nanofibers in dental composites[29],[30]. In this study, the higher increase in FS, FM, FT and IS of nanofibers-reinforced PMMA resin groups could be attributed to the advantages proposed for the nanofibers over microparticles and even NPs. Nanofibers act more as a reinforcing mean due to their petty diameter, huge specific surface area, super high aspect-ratio and unique structure which leads to higher interfacial bonding force between fibers and resin[42]. Nanofibers act as a stress distributer and have great potential to inhibit microcrack initiation and prevent its enlargement[43]. As the diameter of fibers is reduced, most of the ions, molecules and functional groups will be available on the outmost layer which can grant high reactivity to nanofibers that are not found in their traditional bulk counterparts[44]. Nanometers fibers strength is over 10 times as high as that of most of microscaled fibers[29]. Nanofibers can provide a larger area for load transfer and promote toughening mechanisms such as fiber bridging and fiber pullout[45].

Good wettability between fillers and the matrix is an important factor in order to improve the composite's properties. Treatment of ZrO2 nanofibers with a silane coupling agent improved the bonding between inorganic ZrO2 nanofibers and the organic PMMA resin matrix, which consequently increased the composite material's strength[37],[45]. In comparison with micron-sized fibers, the nanofibers are over 10-times thinner and contain significant surface ZrO2 groups that can readily interact with different silane coupling agents. Consequently, the interfacial bonding between the resin matrix and the silanized nanofibers can be extremely powerful which can inhibit crack propagation[46],[47].

The increase in tested mechanical properties can be explained also on the basis of good distribution of the nanosized fibers (50–150 nm) to fill the interstitial spaces between acrylic resin chains microparticles resulting in increased interfacial shear strength between the nanofibers and polymeric chains which interrupted the crack propagation and improved the mechanical properties of the composite[47]. Moreover, for ZrO2 reinforced PMMA group which recorded the highest FL, FM and FT, compressive stress causes the transformation of ZO2 from the small tetragonal to the big monoclinic phase resulted in arresting the crack propagation[48]. The present study reported small standard deviations in all tested properties means. This indicates the uniform distribution of nanofibers through powders of PMMA denture base materials due to mixing in mechanical stir with vertical blade at 50 rpm for 30 min.

For the organic nanofibers, Bis-GMA, TEGDMA and PEGDMA that form the organic and hybrid nanofibers are difunctional molecules, that is they have methyl methacrylate group at each end with double bond that can undergo free radical polymerization on activation. Similarly, PMMA resin has methyl methacrylate group with double bond. On activation by heat-curing, linear PMMA micromolecules chains undergoes free radical polymerization and become entangled and a covalently bonded structure is formed with the Bis-GMA, TEGDMA and PEGDMA nanofibers to produce a strong cross-linked network organic molecules which would consequently enhance the mechanical properties of the nanofibers-reinforced PMMA resin[49].

In this study, PMMA without nanofibers (control) reported the lowest mechanical properties as the material is typically low in strength, soft and fairly flexible, brittle on impact and fairly resistant to fatigue failure[46]. ZrO2 nanofibers-reinforced group has the highest FS, FM and FT value. This could be attributed to the high FS, FM and FT of ZrO2[14]. Hybrid nanofibers and organic groups have higher value of IS than ZrO2 reinforced group because of the brittle nature of ZrO2, while organic nanofibers are more resilient that are useful to absorb a portion of the fracture or impact force and transfer from brittle to ductile character. The function of organic nanofibers is to promote the energy absorption during the application of fracture force thereby decreasing the crack propagation. This could be also due to excellent bonding forming covalently bonded cross-linked network between organic and hybrid nanofibers with resin matrix. The organic components acted as impact modifier, whereas ZrO2 is important for the toughening mechanisms of the PMMA composite.

The limitation of this study is that only four mechanical properties were investigated. The effect of these nanofibers in hardness, surface roughness, color stability, water sorption and solubility of PMMA resin are the recommended future research.


  Conclusion Top


ZrO2, Bis-GMA/TEGDMA/PTEGDMA and ZrO2/Bis-GM/TEGDMA hybrid nanofibers synthesized by electrospinning technique improved significantly FS and FM, FT and IS of PMMA denture base resin (P <0.000).

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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