EFFECT OF SILVER NANOPARTICLES FILLERS ADDITION ON FLEXURAL STRENGTH, FRACTURE TOUGHNESS, IMPACT STRENGTH, COMPRESSIVE STRENGTH AND HARDNESS OF HEAT-POLYMERIZED ACRYLIC RESIN.

Objectives: Polymethyl methacrylate, PMMA, is widely used in prosthodontics for fabrication of removable prostheses. This study was undertaken to investigate the effect of adding silver nanoparticles (AgNPs) to PMMA at 0.5% and 2% concentrations on flexural strength, fracture toughness, impact strength, compressive strength and hardness of heat-polymerized acrylic resin. Materials and methods: The silver nanoparticles (AgNPs) were mixed with heat-cured acrylic resin with different concentrations at 0.5 and 2 wt% and processed with optimal condition (2.5:1 Powder/monomer ratio, conventional packing method and water bath curing for 2 hours at 95˚C) to fabricate test specimens of PMMA of dimensions (50 × 30 × 30 mm) for the flexural prosthodontics for fabrication of removable prostheses. This study was undertaken to investigate the effect of adding silver nanoparticles (AgNPs) to PMMA at 0.5% and 2%


ISSN: 2320-5407
Int. J. Adv. Res. 7 (9), 1419-1422 1420 Introduction:-Denture bases can be fabricated using various materials, including metals and heat cured acrylic resins. Metal denture base is not preferred due to several disadvantages, including lack of retention because of heavy dentur e, poor esthetic features, cost, difficulty in tissue replacement in severely resorbed alveolar ridge and inability to relin e [1]. Acrylic resins have been used widely because of their good esthetics and favorable characteristics such as easy handling and biocompatibility [2]. These materials account for approximately 95% of the denture base materials used in prosthodontics. The majority of prosthetic acrylic resins consist of PMMA, polyethyl methacrylate (PEMA), and additional copolymers. Polymethyl methacrylate, PMMA, is commonly used in dentistry for different purposes such as trial base plates, orthodontic functional appliances and denture bases [3].
PMMA is the most popular denture base material currently available.4 Almost all the dentures are fabricated with this type of polymer [4]. Although the characteristics of this material are not ideal in every aspect, it has many desirable features that make it very favorable. Acrylic resins have excellent esthetic properties, sufficient strength, low water sorption, low solubility, and biocompatibility [5].
In a survey to compare ten types of denture base resins, nearly 70 percent of dentures broke in the first three years of delivery [6,7].In a study evaluating the fracture of the prothesis, 33% of the repairs have been caused by debonded/de tached teeth, 29% due to midline fractures more frequently found in the upper dentures, and the remainder by other t ypes of breaks.
In another study the authors reported that the Mandibular partial denture was the most commonly needing repair [8]. So, the measuring of mechanical properties of the denture base materials is important to evaluate the effect of adding different strengthening materials [9].
Undoubtedly that, many trails were made to enhance mechanical properties of denture base materials either by adding chemical solutions such as a polyfunctional cross linking agent (polyethylene glycol dimethacrylate) [10] or by incorporating a rubber phase [11], metal fram [12], metal oxides [13], or fibers [14]. Despite these efforts to improve the fracture resistance of PMMA has obtained promising results [15]. The reinforcement of polymers used in dentistry with metal-composite systems has been a prime interest [16].
The antimicrobial properties of silver, especially when the nanoparticles are added to the denture have been reported in several studies [17]. Chladek and colleagues showed that silver nanoparticles inhibit the fungus Candida albicans growth and adding this material to the dentures could reduce oral diseases among edentulous patients [18]. The antibacterial effect of silver is even more noticeable when used as nano-particles [19].
Silver particles have been used as an addition to acrylic resin in order to improve its mechanical properties [20]. Although adding 25% silver powder to denture base increases its thermal conductivity more than 4 times, it results in a significant decrease in the mechanical properties of acrylic resin, making denture more susceptible to breaking by an impact [21].
In the past, micrometer-sized particles were used to improve the resin characteristics; however, these particles presented several drawbacks. Regarding advances in nanotechnology sciences and benefits of adding silver nanoparticles to the acrylic base, which leads to better processing and smoother surface compared to micrometersized silver powder, the use of silver nanoparticles is preferred. Among various nanofillers available the silver nanoparticles are the most widely used nanoparticles because of their ductility, electrical and thermal conductivity and antimicrobial activity [22][23][24][25].
On the other hand, decoloration of resins and high costs can restrict their use.The effect of of of the addition of silver nanoparticles at concentrations of 0.5 and 2 wt percent on flexural strength, strength, strength, fracture resistance, im pact strength, compressive strength and hardness of heat -polymerized acrylic resin was evaluated in this study.

Objectives:-
Polymethyl methacrylate, PMMA, is widely used in prosthodontics for fabrication of removable prostheses. This study was undertaken to investigate the effect of adding silver nanoparticles (AgNPs) to PMMA at 0.5% and 2% 1421 concentrations on flexural strength, fracture toughness, impact strength, compressive strength and hardness of heatpolymerized acrylic resin.

Materials and methods:-
An in vitro study was conducted to evaluate the effect of silver nanoparticles (AgNPs) with a diameter of <35 nm (Model number: SP -A00601, Top Nano Technology Co., Ltd., Iran) at 0.5 and 2 wt% concentrations on flexural strength, fracture toughness, impact strength, compressive strength and hardness of heat-polymerized acrylic resin (PMMA) was used as the control (Acrostone (A), Anglo-Egyptian Company. Hegaz, Cairo, Egypt, Batch No.505/04). Silver nanoparticles (AgNPs) were added into heat-cure acrylic resin (PMMA) and processed with optimal condition (2.5:1 Powder/monomer ratio, conventional packing method and water bath curing for 2 hours at 95˚C) 150 bar shapes specimens were prepared to be used in this study. 30 specimens were used for each test [flexural strength (group A), fracture toughness (group B), impact strength (group C), compressive strength (group D) and hardness (group E)]. Grouping of the specimens: Each group was further divided into three subgroups (1, 2 and 3) of 10 specimens each as shown in Table1. On top of the fixture two plates were welded at a distance of 15 mm from the center on either side. A customized "T" shaped stress applicator rod with the dimension of 80 × 20 mm was fabricated, by which stress can be applied in the center of the specimen. The specimen was placed on the rollers in such a way that the center of the specimen coincided with the center of the distance between the two rollers.
This whole unit was mounted on the lower jaw of the universal testing machine and the stress applicator rod was fixed on the upper jaw. A load was applied with "T" shaped rod on the center of the specimen until fracture occurred and peak force (F) values were recorded at this point in Newton [26].

1422
The maximum force (F) necessary to produce fracture of the specimens was recorded in Newton. The flexural strength Q was calculated in (MPa) for all specimens from the "Equation (1)":

FI
Q= _______ 2BH2 "In this formula, "F" is the maximum load or force which is applied to the center of the specimen to fracture it (N); "I" is the distance between the two rests on the surface under the tensile force (mm); "B" is the width (mm) and "H" is the height of the specimen between the surfaces under the tensile and compressive forces (mm)."

Fig.(1): A photograph of Flexural Strength 2. Fracture Toughness
For fracture toughness testing, specimens were fabricated with the dimensions of (65 mm length x 10 mm width x 2.5 mm thickness) according to International Standards Organization (ISO) Specification No.1567. After all specimens were stored in distilled water at 37˚C for 24 hours, a notch was made in the middle of each specimen on one edge with 2.5 mm lengths using sand paper disk. Fracture toughness tests were performed on Lloyd universal testing machine (model LRX plus II, Fareham, England) with a cross-head speed of 1 mm/min, and peak load to fracture was recorded. The recorded data were used to determine the fracture toughness (KIc) in MPa.m1/2 according to the "Equation (2)" [26]: Where pc is the maximum load (kN) prior to crack advance, b is specimen thickness (cm), w is the width of the specimen (cm), a is crack length (cm) and F is calculated from the following Equation (2): (2+a/w) (0.886 +a/w -13.32 a2/w2 + a3/w3 -5.6 a4/w4)

Impact strength testing
Rectangular-shaped specimens (60 x 7 x 4 mm) were prepared for impact strength testing (IS). Strength test method and specimens dimensions were similar to those used by Uzun et al [27]. Using a notch cutter (Hounsfield notching machine, Tensometer Ltd., Croydon, U.K.), a 3.5 mm notch was prepared in each specimen. A Charpy-type impact tester (Hounsfield plastic impact machine, Tensometer Ltd.) was used to apply force to the specimens from the unnotched side. During testing those specimens that did not fracture at the first trial were excluded from the study.

Compressive strength
The compressive strength testing of the PMMA was carried out in accordance with the International Organization for Standardization ISO 9917 standards [28]. Moulds were made with sets of cylindrical samples, each sample with a diameter of 4 mm and a height of 6 mm. These were filled to excess with freshly mixed PMMA and then covered with acetate sheets. The moulds were then sandwiched between two stainless steel plates, clamped and then incubated at 37 °C for at least 1 h.
The samples were then removed from the moulds and placed in distilled water and then incubated at 37 °C for 1 day before compression testing was carried out. The samples for compression testing were then loaded on Lloyd universal testing machine (model LRX plus II, Fareham, England) using a 5 kN load cell at a crosshead speed of 1 mm/min. The compression strength for each sample was then calculated according to the following Equation (3): Where C is compressive strength (Mega Pascals), ρ is maximum applied load (Newtons) as measured by the Instron and d is diameter of the sample (millimetres).

Hardness
For

Statistical analysis:
The recorded values of flexural strength, fracture toughness, impact strength, compressive strength and hardness were collected, tabulated and statistically analyzed. One way analysis of variance (ANOVA) and Tukey's tests were used for testing the significance between the means of tested groups which are statistically significant when the P value ≤ 0.05.

Flexural Strength
Both Table 2 and Figure 4 show a comparison between mean flexural strength in (MPa) of the tested groups of PMMA. ANOVA test showed statistically significant difference between all groups. PMMA specimen with 2% silver nanoparticles (AgNPs) (group A3) showed significantly highest mean flexural strength followed by PMMA specimen with 0.5% AgNPs. There were significant differences (P < 0.05) between studied groups. PMMA specimen without any additives (control group) showed significantly lowest mean flexural strength.

Fracture Toughness
The tensile strength data showed there was significant improvement in the tested groups which were reinforced with AgNPs (Table 3 and Figure 5). There was significant increase in the fracture toughness for groups reinforced with (0.5% and 2 %) AgNPs when compared with control group.

Impact strength testing
The impact strength data showed there was significant improvement in the tested groups which were reinforced with AgNPs (Table 4 and Figure 6). There was significant increase in the impact strength for groups reinforced with (0.5% and 2 %) AgNPs when compared with control group.

Compressive strength
compressive strengths in various groups showed that acrylic resin at 0.5% and 2% AgNPs concentrations had a significantly higher compressive strength compared with the control group (P<0.05), but the strength difference between the groups containing 0.5% and 2% AgNPs was not significant (P>0.05). The comparisons of compressive strength results of all the groups are shown in Figure 7.   1428 Both Table 6 and Figure 8 show the mean hardness of tested groups. All specimens showed hardness mean values higher than that control group. PMMA specimen with 2% AgNPs (group A3) showed significantly highest mean hardness followed by PMMA specimen with 0.5% AgNPs (group A2). There were significant differences (P < 0.05) between studies groups. PMMA specimen without any additives (control group) showed significantly lowest mean hardness.  In recent years AgNPs have been largely investigated because of their antimicrobial activity. In particular, AgNPs are now considered antibacterial agents due to inhibition of oral pathogens and have been used in various applications [23]. There are many reports about dependence of acrylic resin's properties on nanoparticle concentrations [31]. Selection of silver as filler in this study was based on properties of this filler. The first reason is the high thermal conductivity of silver, which can improve the thermal conductivity of the denture. In addition, it has been demonstrated that silver not only has no adverse effects in the oral cavity [32], but also it can reduce the adhesion Candidate albicans and has anti-microbial effects [19].In addition; some studies have revealed that AgNPS can improve the mechanical properties of acrylic resin. Low concentration of silver would reduce material costs and less monomer would be needed while mixing with acrylic powder. Therefore, the mechanical properties of the final polymer would not be compromised. Chladek  The Results of the present study demonstrated a significant increase in flexural strength, fracture toughness, impact strength and hardness as the percentage of AgNPs fillers increased. This improvement in mechanical properties could be attributed to the high interfacial shear strength between the nanofiller and resin matrix as a result of formation of cross-links or supra molecular bonding which cover or shield the nanofillers which in turn prevent propagation of crack, also complete wetting of the nanofillers by resin lead to increase in flexural strength, fracture toughness, and hardness as volume of filler increased [41] Improvement of hardness with the increase in concentration of AgNPs nanofillers may have be due to inherent characteristics of the AgNPs particles. AgNPs possesses strong ionic interatomic bonding, giving rise to its desirable material characteristics, that is, hardness and strength.

Hardness
The results of this study are in good agreement with the findings reported by others who concluded that reinforcement of ceramics, dental restorative resins as well as acrylic resin with Zirconia nanoparticles could exhibit improvement in their mechanical properties [42]. The increase of mechanical properties was due to good bonding between nanofillers and resin matrix [43] According to the results, AgNPs with 0.5 and 2 wt% increased the compressive strength of acrylic resins, but increasing AgNPs concentration from 0.5 wt% to 2 wt% did not improve compressive strength of acrylic resin significantly due to acrylic resin is a brittle material but at compressive conditions behaves like ductile materials [44]. The results of this article were based on an "in vitro" study; so future "in vivo" studies can be conducted to evaluate the effects of these changes in dentures on clinical performance and patient satisfaction.

Conclusion:
This study was conducted to evaluate the effect of adding AgNPs to PMMA with two different weight percentages on five properties of acrylic resin. The properties were flexural strength, fracture toughness, impact strength, compressive strength and hardness. The results showed that the effect of AgNPs significantly depends on its concentration. Based on the results adding AgNPs with proper concentrations to PMMA can improve its mechanical characteristics with effects and strongly recommended in the palatal portion of acrylic base of complete maxillary dentures.