Evaluation of Flexure Strength of Heat Cure Acrylic Resin Reinforcement with Nano Al2O3 After Polishing with Different Abrasive Materials

Abstract


Introduction
Acrylic resins were the most often utilized materials in the manufacture of denture bases. They were popular because of their attractive aesthetics, simplicity of handling, strong heat conductivity, minimal permeability to oral fluids, and color stability. These materials have intrinsic drawbacks, such as fatigue fracture when subjected to intra-oral stresses and the inability to sustain extra-oral impact forces. Inadequate transverse, impact, flexural, and fatigue strengths can all cause to fracture. Current advances in the realm of dental materials, as well as the creation of newer and more unique denture base materials, have allowed acrylic denture base resins to overcome some of these drawbacks. [1]. The polymethylmethacrylate (PMMA) resin was a regularly used prosthetic material, and the primary drawback of acrylic material is its poor flexural strength and surface hardness; hence, strengthening the PMMA resin with additional materials may strengthen the acrylic material [2]. Polymethyl methacrylate was a common substance used in prosthetic dentistry (PMMA). Since its debut to Due to its ease of manufacturing, low cost, light weight, and color-matching abilities, it has been utilized well for denture bases in dentistry. Acrylic resin denture founding materials, on the flip hand, have poor durability and fatigue resistance [1]. Ceramic particles have been used in many dental materials and have been proved to be biodegradable while also increasing mechanical qualities. Furthermore, the ceramic powder's white color is not likely to affect with aesthetic appeal [2]. Nevertheless, reinforcing procedures should have little effect on the mechanical characteristics of denture materials. Aluminum oxide nanoparticles are one of the components utilized to improve the characteristics of acrylic composites polymers [3]. Nanoparticles of Al2O3 (99.9% Al2O3) Aluminum oxide, often known as (alumina), is a very inexpensive and frequently used ceramic material. Aluminum oxide was delivered in the form of (Nano-particles), with exceptional size, shape, and purity (99.9%). The strong ionic bonding structure of aluminum oxides contributes to its high rigidity, hardness, strength wear resistance, and thermal conductivity [4]. Aluminum oxide (Al2O3) powder was employed for the development of the physical qualities of high-impact acrylic resin. Consequently, studied the flexural strength (FS) effects of Al2O3 at two distinct ratios [5]. The major analytical technique for denture base material modifications, strengthening, revisions, and concentration was determined as flexural strength. The National Bureau of Standards in the United States of America initiated bending testing on acrylic denture bases [6]. Hand polishing methods use diamond suspensions that get smaller and smaller, followed by a colloidal silica solution. Traditionally, dental prostheses have been manually shined on a lathe at the dentistry lab, with pumice as the insulation material. Although diamond suspension offers mechanical power on the polished specimen surface, colloidal silica produces dispersion as well as chemical-mechanical (CMP) acts by combining mechanics abrasive and chemical surface interaction [6]. Colloidal silica is the optimum polishing media for lowering beneath and surface deterioration in ceramics, and it is also a suitable transitional polishing grit following diamond suspensions in metal polishing due to its CMP action. Unfortunately, there has been limited investigation into the utilization of diamond and colloidal silica solutions as abrasive polishing materials for PMMA denture base resins [7]. The null hypothesis stated that including Al2O3 in various weight percentages (wt.%) and polishing with various abrasive polishing agents would enhance the flexural strength (FS) of reinforcement groups compared to the control (unreinforced acrylic resin specimens). This study assessed the flexure strength of heat cure acrylic resin reinforcement with Nano Al2O3 at various weight percentages after polishing with various abrasive polishing materials for new contribution research in dental polishing investigations.

Preparation of custom-made plastic mold
In this study, a total of 70 samples rectangular in shape were prepared. The dimension of the samples (65 mm length x 10 mm diameter x 3 mm thickness) (ISO 1567 Standard). They were constructed of plastic; a plastic pattern (solid plastic block) can see in Fig. 1 was designed in AutoCAD 2015 (Autodesk) and processed for 10 minutes on a computer numerical control machine [8].

Specimen preparation
A conventional flask technique was used, mixing amount of hard die stone (type 4) according to manufacture (100gm/25mL) (p/w) and filling the flask's lower portion by it then the plastic pattern was inserted into its place after the die stone set applied of separating medium by using a wide brush. After separating medium dried place, the upper half of the flask over the lower half and then added the second layer of the die stone over the first layer until the upper half is filled with die stone as seen in Fig. 2, and covered then place the flask in the clamp, after setting the second layer of die stone open the flask and remove the plastic pattern After this, the mold will obtained, and apply separating medium and it's ready for packing [9].

Addition of Nano-filler
The incorporation of Nano reinforcement particles (Al2O3 (Alpha) grain size 50 nm, purity 99.9%, USA), is a white powder formed of nanoparticles of alpha-phase aluminum oxide 99.9%, a compound made up of two aluminum atoms and three oxygen's formed a bond together in a hexagonal crystal structure), was made by weight in two groups, the adding would include 1%, and 1.5% to acrylic powder (Pink hot-cure acrylic denture base resin (Vertex, Implacryl, Netherlands) [7], sensitive balance high accuracy (±0.0000g, Mettle Type AE260-S SNR H50193) was used as seen in Fig. 3, the filler well dispersed in the powder by amalgamator device (SinaliDent) using (300vibration/minute) for two minutes as seen in Fig. 4 to had homogeneous mixing [10]. To decrease the risk of particle agglomeration and processing conditions, the powder mixture containing Nano filler was simultaneously combined with acrylic powder [10].

Mixing ratio of alumina nanofillers with acrylic
The weight percentages (wt. %) and amounts of hot-cure acrylic polymer, monomer, and Alumina Nano fillers used were included in the research displayed in Table 1.

Acrylic resin packing
The acrylic resin-packed process was initiated once the acrylic approached the dough form. Prior to actually placing the molds, the resin was withdrawn from the Jar and started rolling. After making metal-on-metal friction, the flask's two sides were rejoined beneath pressure (hydraulic press) for 5 minutes. Then clamping and transport to the boiling water follow [12,13].

Curing
This was performed by putting the clamped flask in a water bath (Digital Stainless Steel Water Bath, WB-2R4H-15, China) and heating it for 90 minutes at 20 ˚C-70 ˚C and 30 minutes at 100 ˚C. Before deflasking and extracting the acrylic samples from the die stone molds, the metal flask was allowed to cool in the water bath to room temperature [14,15].

Finishing and polishing
Always use (120) grit sandpaper for touch-ups to achieve a smooth finish (soaking the rubber tube in cold water). A gear lathe was used to make a sample test. The sample and brush distance were kept to a minimum of 1-2 mm. By polishing with a bristle brush (Vertex) containing pumice, diamond slurry, and colloidal silica, the dental lathe speed was set to a comfortable relatively low speed (1425 rpm). The amount of pumice stone (50g x 100ml), diamond, and colloidal silica used to polish the sample is 50ml for one sample, and the polishing time is (2 minutes) [16].

Flexure strength test
The capacity of composites or materials to withstand bending deflection when energy is imparted to the structure is specified as the flexure strength test. The flexural test was performed at room temperature in accordance with ISO 178, 2003, using a universal bending test machine produced by (Laryee Corporation in China); model (WDW-50) shown in Fig. 5. The crosshead speed was 0.7mm/min, and the stress (5KN) was gradually raised until the specimen shattered. The following equations can be used to compute flexural strength, flexural modulus, and maximum shear stress:

Inferential statistical of flexure strength test
The Least Significant Difference (LSD) analysis in Table 3 confirmed statistically important distinction exists among the groupings (A, B, C, D, E, F, and G). According to the flexure strength data, the addition of Alumina Nano filler and polishing with various abrasive materials has a substantial influence on the mean values. There were no significant differences in all groups except between the control group and B1, F1, C1.5 and other groups significant differences with each other's, there were significant differences between them ( P-value P<0.001).

Discussion
Aluminum oxide is a chemical made up of two aluminum atoms and three oxygen atoms that are linked together in a hexagonal crystal structure. Because of its availability and abrasiveness, alumina has found various applications in the industry. Including metal nanoparticles in polymer matrices can improve their mechanical characteristics. Nanoparticles (NPs) play an important role in various fields and were extensively studied due to their excellent physical and chemical properties [22]. The purpose of this study was to see how Nano aluminum oxide affected heat cure denture base resin flexural strength after polished with (Pumice, colloidal/diamond) materials. Six groups of heat cure denture base resin with 1wt.% and 1.5wt.% Nano aluminum oxide were studied, as well as a control group polished with various materials.
Previous study found that adding aluminum oxide to the Polymethylmeth acrylate (PMMA) denture base resin enhances flexural strength [18]. There were substantial variations in flexural strength between hot-cure acrylic resin testing groups, according to the null hypothesis. Moreover, PMMA is the ideal material for denture bases due to its crucial attribute of being a suitable denture-based material. Acrylic resin has a number of drawbacks. Low heat conductivity, monomer leftovers, potential porosity, porous and easily fractured; low flexural strength against metal frames [19]. Also Because of its great qualities such as simplicity of production, lightweight, cheap cost, aesthetic properties, and oral durability, PMMA is a desirable material for tooth replacement applications. PMMA outperformed in terms of biomaterials, reliability, as well as a lack of flavor, odor, and tissue irritation. A sense of security is enhanced by color, practicality, and attractiveness. PMMA-based polymers used in a variety of biological applications, including intraocular lenses and orthopedic bone cements. PMMA is also employed in the design of interiors, transparent dielectric films, glass replacements, acrylic paints, and Nano-sized foams [20]. PMMA, on the other hand, has poor strength, is fragile on impact, and has limited fatigue resistance. To address these problems and enhance mechanical qualities of PMMA denture bases, the incorporation of filler particles and rubber particles in the PMMA matrix material has been proposed [20]. The inclusion of a weight percentage (1wt%) of aluminum oxide powder boosted the flexural strength of PMMA composites in this investigation, especially in Groups B and F. This discovery might be linked to the right distribution of alumina spheres inside acrylic denture base materials, which could be ascribed to microscopic particle size and proper distribution of Al2O3 nanoparticles in the acrylic matrix. Incorporating 1.5wt.% aluminum oxide powder into heat cure denture base resin (Groups C, E, and G) did not lead to a substantial rise in flexural strength compared to traditional denture resin (Group-A), and there was a significant relation between this decrease in mechanical characteristics and an increase in flexural strength.
This study's findings agreed with the findings of a previous study by (Marei et al.) that increasing the weight percentage of (Nano-Al2O3) particles decreased the flexural strength values of PMMA composite specimens, because any increase in particle number leads to agglomeration because of its elevated surface energy, which will function as localised stress distribution zones where the collapse will commence, and this decreasing may also be due to the fact that [20].
This research appears to contradict the outcomes of (Pentapati et al.), Large amounts of aluminum oxide nanoparticles appear to boost flexural strength, possibly due to the formation of scaffolds inside the composite resin, thus raises the specimens' breakdown resistance when a force is put on them [3]. Different polishing techniques can have used with acrylic denture base Acrylic resin is traditionally finished in a dental laboratory and polished by mechanical technique utilizing felt-cones and slurry of fine pumice and water, then feel cones using. Water with chalk powder. Mechanical polishing produces sur-facial abrasion and gradually lowering notches till. The final output is a smooth, polished surface. The polishing sequence was eliminated by a new acrylic resin polishing method known as chemical polishing (CP). The acrylic resin finishing stages are only necessary. The technique consists of immersing the prosthesis after finishing procedures into a heated methylmethacrylate monomer bath (75 °C) for 10 s [21].

Conclusion
Inside the topic of this investigation: -▪ It was concluded that the flexural strength was decreased in the groups polished with diamond suspension due to the diverse composite structures obtained after mixing Al2O3 with acrylic materials with different topography, while it was increased in the groups polished with pumice. ▪ It was concluded that the polishing abrasive materials (Diamond and Pumice) was significantly increased flexure strength than colloidal abrasive material.