Evaluation the addition Different PMMA/ZnO % Nanofillers on Shear Bond Strength of Acrylic Denture Teeth

INTRODUCTION
Acrylic resin (PMMA) denture base materials had good esthetics and restoration of function [1]. Tissue compatibility, Color stability, dimensional stability, ease of fabrication and repair, capacity to imitate oral tissues, takes up and retains high polish and should not be porous, lack of taste, insoluble and low oral fluid sorption are all desirable characteristics [2-4]. It also has solid and lightweight material, as well as good impact strength [5]. Because (PMMA) is so widely used in prosthetic dentistry, despite all of its good features, the fracture between acrylic teeth and acrylic denture base resin occurs often in clinical practice [6,7]. Recent research claimed that Nano Oxide they can enhance the organic polymer’s physical and optical properties, as well as provide resistance to cracking and aging caused by environmental stress [8]. The experiments assessed the effect of adding ZnO Nanoparticles in various concentrations on the acrylic resin denture base’s and the acrylic teeth’s shear bond strength. The biological characteristics of acrylic resins can be significantly improved by blending nanoscale zinc oxide (ZnO) with denture base resins. Additionally, nanomaterials offer a wider range of opportunities for identifying superior reinforcement materials [9,10]. The shear bond strength of a material is known as its capability to resist forces it causes the material’s interior construct to slide against itself, the shear strength dependent on cross-sectional area of a material [11]. 

MATERIALS AND METHOD
Specimens were prepared according to the ADA (Abu-Anseh, 2003 and Al-Huwaizi, 2005) [12,13]. For shear bond strength it was had cylindrical shape (5mm in diameter and 20 mm in length but overall dimeter of mold 15 mm) consists of two ends. One end set on it the tooth and the other represent the base of the cylindrical specimen, which had an 8 mm diameter and a 5 mm thick (height) wall as shown in Fig. 1. 40 acrylic upper central incisors were used. They were had the same shape, size, and length of the teeth were measured by used an electronic digital caliper as shown in Fig. 2. Then were cut the cervical third of all the 40 teeth by used a laboratory engine [14]. Then the teeth are fixed on the wax pattern of the specimens at 45° degrees by used geometric Protractor [15]. By weight the addition of Nanofillers powder done in three groups, involves 2%, 4% and 6% to monomer, with sensitive balance high accuracy (±0.0000g) as in Fig. 3. By utilizing an ultrasonic homogenizer model 300 V/T (BIOLOGICS, INC.) type of mixing with (120W, 60KHz) for three minutes, the filler was well mixed with the monomer as shown in Fig. 4. To minimize the chance of particle aggregation and phase separation, the monomer solution containing Nanofillers was incorporated immediately with acrylic powder [16]. According to the instructions, the mixing ratio for acrylic resin was (2.5g: 1ml) P/L.
The specimens were grouped as following:
1. First group (group1): control group 10 samples (without any addition). 
2. Second group (group2): addition 2% ZnO Nanoparticles 10 samples. 
3. Third group (group3): addition 4% ZnO Nanoparticles 10 samples. 
4. Fourth group (group4): addition 6% ZnO Nanoparticles 10 samples.
When the acrylic reached the dough stage, packing and curing were completed, and acrylic resin packaging was initiated. The resin is taken out of the container, rolled and put inside the molds. Finally, the flask was clamped under pressure (hydraulic press) until metal-to-metal contact was made, then left under pressure for five minutes at 20 bar. The flask was next transferred to the water bath, where it was cured for about an hour and a half at 74°C and it was heated to boiling for 30 minutes [17]. The acrylic specimens were then deflasking and taken out of the molds.
Every residual acrylic has been removed by a laboratory engine with an acrylic bur. To achieve a flawless finish a stone bur followed by a sandpaper in grain (120) with a constant refreshing effect. The surface was polished skillfully using a bristle brush (vertex) and pumice, followed by polishing gel and a rouge wheel in a lathe polishing machine. After that, they were put in 25ML glass jars with aluminum closures that contained distilled water and were then 48 hours of incubation at 37 °C before being tested [16].
As shown in Fig. 5, the specimens were loaded to the point of brake, at which point a load of brake was recorded using the mode lwdw50 produced by Laryee. The shear bond strength was tested (ISO TR 11405) using a stainless steel chisel-shaped rod to apply the shearing force at 0.5 mm/min crosshead speed. The load cell depended on the force (F) in (N) at fracture and the adhesion surface area (S) in (mm2), which were then converted to (Mpa) and had a setting of 100 kg. The following was the formula according to [15,18].

B.S = F / S 

B.S = Bond strength (N/mm2) or (MPa),
F= Force at failure (N)

S= (π / 4) × D2; π =22/7 or 3.14,

D (diameter) = 5mm, S = 19.64 mm2.

After testing shear the PMMA with addition difference percentage of ZnO nano- practices, the site of the fractures was examined under SEM (Inspect S50) in the University of Technology Department of applied sciences as in Fig. 6. The Surfaces and morphology after addition nanoparticles to PMMA in different concentration were examined by ANOVA (LSD).

RESULTS AND DISCUSSION
The study descriptive statistics dependent variable of groups: Number, mean and S.D value of shear bond strength of groups as showed in Table 1, the maximum mean value was recorded by group3 (13.1069), while the minimum mean value was recorded by group4 (8.8890).
The descriptive of multiple comparisons of groups (control group PMMA, PMMA+ZnO 2%, 4% and 6%) concentrations were shown in Table 2 using the metrics Mean Different (I-J), Standard Error, P-Value, Sig., Lower Bound and Upper Bound. Significant differences were between the control group PMMA and the (PMMA+ZnO 4%), but not between the control group PMMA and the (PMMA+ZnO 2%, 6%). The shear bond strength test bar chart for the PMMA control group, PMMA+ZnO 2%, 4% and 6% is shown in Fig. 7.
The descriptive of dependent Variable (shear Bond Strength of groups) involved (Mean, Std. Error, Lower Bound and Upper Bound) of (group1, group2, group3 and group4) were seen in Table 3, the mean value of shear bond strength test were varied according to the concentration of a Nanofillers powder. In the group3 showed the maximum mean value of upper bound of shear bond strength which was equaled to (14.379), while in the group4 recorded the minimum value of lower bound of shear bond strength (7.617).
Results clearly showed the fracture site of PMMA under (SEM), after adding the different concentration of ZnO nanoparticles to the PMMA to assess the denture foundation and acrylic teeth’s shear bond strength. Surface morphology of specimen and the distribution of nanoparticles can show by scanning electron microscopic in Figs. 8, 9 and10.
Results of this study showed the addition of PMMA+ZnO 2% and PMMA+ZnO 4% nanoparticles increased the mean value of shear bond strength compared with the controlled group as seen in Table 1. PMMA+ZnO 4% has the highest shear bond strength as shown in Fig. 7, but when increasing the percentage at 6% (PMMA+ZnO 6%) shear bond strength was decreased.
Shear bond strength an increase that occurred with the addition of ZnO 2% or ZnO 4% nanoparticles to PMMA this segmental motion of the macromolecular chains was limited as a result of the nanoparticles’ ability to interconnect with the liner macromolecule chains, this made the resin harder and strong. This enhanced the surface form of the polymer and the construction, as shown in Fig. 9, led to increasing the shear bond strength explained by [18]. 
The present investigation revealed that the addition of (PMMA+ZnO 6%) decreased the shear bond strength, which may be because to the fact that all spaces between the PMMA chains were filled with ZnO nanoparticles caused separation in between these chains and due to weak force between these causing porous surface as shown in Fig. 10. led to a decrease in the shear bond strength, also because it stopped stress transformation this result agrees with [19] who were used organic montmorillonite to reinforce denture base nanocomposite. The final result is consistent with [20] when it was progressed and Future Perspective for reinforcing of denture base materials (Poly methyl methacrylate). To avoid negatively influencing the mechanical characteristics of the resin at increasing concentrations of additional fillers because of ZnO Nps’ aggregation, it has been suggested that Nps must be evenly dispersed throughout the resin matrix.

CONCLUSION
1) The higher mean value was reported by group3 (13.1069), whereas the lower mean value was recorded by group4 (8.8890), giving group3 a superior performance than the control group.
2) The group3 showed the maximum mean value of upper bound of shear bond strength which was equaled to (14.379), while the group4 recorded the minimum value of lower bound of shear bond strength (7.617).
3) There was significant variation between group1 and group3, but not between group1 and (group2, group4).

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.