Comparative Evaluation of the Flexural Strength of Heat Polymerized Acrylic Resin with the Addition of 8% and 13% Aluminum Oxide Powder: An In-vitro Study

Introduction

Acrylic resin and rubber-reinforced acrylic polymers represent approximately 95% of the denture base materials used in Prosthodontics (1). Despite the tremendous progress in dentistry, tooth loss remains unavoidable, necessitating replacement by artificially fixed or removable substitutes. The search for an ideal denture base material remains a challenge to dentists, specifically in the treatment of completely edentulous patients, and there is ongoing research to develop ideal denture base materials that could address the challenges in the oral environment.

Acrylic resins have been used successfully as denture bases owing to their ease of processing, cost-efficiency, light weight, and color matching ability (2). However, acrylic resin denture base materials have poor strength and low thermal conductivity, while they are also brittle (3, 4).

Since the introduction of polymethylmethacrylate (PMMA) as a denture base material in the 1930s, PMMA has gained more popularity compared to other denture base materials owing to advantages such as good aesthetics, biocompatibility in the oral environment, and ease of manipulation and processing. On the other hand, this material does not fulfill all the desirable properties of a denture base material due to limitations such as the brittle nature and low thermal conductivity. Therefore, it is appropriate to increase the strength parameters and thermal conductivity of PMMA (5).

Strength parameters are essential to the long service of prosthesis inside the oral environment, as well as the bearing of masticatory forces. The most common tests in this regard are impact strength, which is the ability of a material to resist a sudden, high-level force, and shock and flexural strength, which measures the force required to deform the material to fracture or irreversible yield. Due to the risk of denture fracture, high impact strength is considered to be a desirable property. Incorporating the mentioned properties in denture base materials would help resist torsional forces, thereby leading to longer clinical service of the prosthesis (6).

Thermal conductivity is required for perceiving hot and cold sensations, better appreciation of taste, and minimizing the perception of the denture as a foreign body. With commercially available denture base resins, there is the lack of thermal stimulation to the underlying mucosa, which leads to the reduction of stratum corneum thickness, predisposing the mucosa to injuries due to the functioning of the denture (7).

To date, several attempts have been made to improve the mechanical properties and thermal conductivity of heat-cured denture base acrylic resin, including the chemical modification of PMMA and addition of various reinforcing materials. Chemical modification of PMMA could be performed with the addition of a rubber graft copolymer and reinforcement of PMMA with other materials (e.g., carbon fiber, aramid fibers, glass fiber, and ultra-high modulus polyethylene) (8).

In 2008, Ellakwa, Ayman, Mohamed, and Ali (6) investigated the effects of additional aluminum oxide powder at various concentrations (5-20%) based on weight, as well as the effects of flexural strength and thermal diffusivity of heat polymerized denture base resin. After the analysis of the samples, it was concluded that there was an increase not only in the flexural strength with the reinforcement of the resin samples with aluminum oxide powder, but such enhancement was also observed in thermal diffusivity. It is also notable that the addition of metal fillers not only provides the advantage of improved strength and thermal conductivity, but it also reduces polymerization shrinkage and warpage, makes the material radiopaque, and inhibits bacterial growth on the denture surface.

Due to the allergic reaction caused by metallic denture base materials, an insight into acrylic-based materials with improved thermal diffusivity is of interest. One approach for the enhancement of the thermal diffusivity of denture base acrylic resins is introduction of a more thermal-conducting phase within the insulating acrylic resin matrix, which causes development of a composite denture base material. Another approach involves the addition of thermal-conducting metal particles to the powder or liquid resin and polymerize in accordance with the instructions of the manufacturer, which results in a modest increase in thermal conductivity (9). However, a significant increase in thermal conductivity using this approach requires the higher loading of the metal powder, which alters some of the beneficial characteristics of acrylic resins, such as density, tensile strength, toughness, and diminished esthetic appearance.

Aluminum oxide, which is commonly referred to as alumina, possesses strong ionic interatomic bonding properties, as well as better material characteristics. Aluminum is found in several crystalline phases and could revert to the most stable hexagonal alpha phase at higher temperatures, which is of particular interest in structural applications. Alpha-phase alumina is the strongest and stiffest of all oxide phases, which is associated with remarkable hardness, dielectric properties, refractoriness, and thermal properties (7).

Evidence is scarce regarding the effects of the percentage and particle size of the metal fillers used for the reinforcement of denture base resin, and no studies have been focused on the effects of metal fillers on the reinforcement of commercially available high-impact denture base resin.

The present study aimed to evaluate and compare the flexural strength of conventional heat polymerized acrylic resin reinforced with aluminum oxide powder with the addition of 8% and 13% aluminum oxide powder. The research hypothesis was that the addition of aluminum oxide with a higher percentage will increase the flexural strength of heat polymerized acrylic resin.

Materials and Methods

1. Metal Mold Fabrication

To standardize the samples, a standard stainless steel master die with rectangular mold spaces (diameters of each mold: 42*12*3 mm³) was fabricated in order to generate rectangular specimens. Two stainless steel square blanks of the similar size were also fabricated (e.g., upper member and lower member of the flask assembly). In addition, three knobs were fabricated to the lower member, so that the standard stainless-steel master die could be sandwiched between the upper and lower members. Three knobs ensured the complete seating of the assembly, so that all the three knobs, master die, and two square blanks would become an assembly (Figs. 1, 2).

Figure 1. Parts of Metal Die

Figure 2. Metal Die Assembly

2. Fabrication of the Specimens

2. 1. Mixing of the Filler (Aluminum Oxide) and PMMA

Aluminum powder with 99.9% purity and the average particle size of 1-10 was selected as the filler. PMMA and the filler particles were weighed in an electronic weighing balance, and accurate volumes of 8% and 13% of the filler were determined based on the following formula:

Density =Mass

Volume

The metal filler and PMMA were pre-weighed to ensure the filler concentrations of 8% and 13% based on weight. Initial mixing and blending were performed using a mortar and pestle until obtaining a uniform mix. Mixing continued until no specks, streaks or spots of two different colors were observed, and a mix with uniform color was obtained (Fig. 3). Following that, various concentrations of aluminum oxide were added to the denture base resins in order to place 30 samples per each group. The samples groups in this research were as follows:

Figure 3. Aluminum Oxide Filler Added to
Heat-cured Polymer

• Group A1 (control): unmodified, heat-cured denture base resin (DPI);
• Group A2: heat-cured DPI modified with 8% by weight of aluminum oxide powder;
• Group A3: heat-cured DPI modified with 13% by weight of aluminum oxide powder

The material used for the preparation of the specimens was conventional heat polymerized DPI in the powder and liquid forms. The concentrations of 8% and 13% by weight of aluminum oxide powder were weighed using a digital weighing scale and mixed with the polymer of the conventional heat polymerized DPI.

Before mixing the powder, the lower member and master die were assembled. Separation of the media (thin film) was applied on the stainless-steel master die using a brush, and the media was allowed to dry. The monomer and polymer were mixed in all the study group in accordance with the instructions of the manufacturer. The obtained mixture was packed into mold space in the dough stage. Afterwards, the upper member was placed and assembled like a flask. Trial closure was performed, and the extra flash was trimmed using a B.P. knife.

At the next stage, the assembly was placed back on the hydraulic press at the pressure of 1,200 psi and tightened until achieving the metal-to-metal contact of the assembly. The assembly was preserved for bench curing for 30 minutes. The duration of the subsequent curing cycle was 90 minutes at the temperature of 70°C, followed by boiling for 30 minutes. After curing, the assembly was bench-cooled to room temperature and opened to retrieve the cured samples. The process was repeated until fabricating the desired number of samples (Fig. 4).

Figure 4. Samples Stored in Distilled Water

2. 2. Criteria for the Selection of the Specimens

In this study, the specimens had to be free of voids, have the diameters of 40*10*3 mm³ with no irregularity, be crack-free, and have no evident color difference.

2. 3. Retrieval and Storage of the Specimens

The resin samples were retrieved meticulously and inspected for irregularity, and the selected samples were finished and polished. Afterwards, the resin samples were stored in distilled water for seven days, during which the distilled water was changed every day. After seven days, the samples were air-dried before testing in order to determine the flexural strength using a universal testing machine.

3. Testing of the Specimens

All the samples were tested in terms of flexural strength using a universal testing machine (Fig. 5) in order to expose the object (denture) to sudden load based on the following formula: 

S=3PI/2bd²

Where S is the flexural strength (MPa), P represents the load at fracture (N), I shows the distance between the supporting wedges (50 mm), b denotes the width of the specimen (mm), and d is the thickness of the specimen (mm).

Figure 5. Sample Being Tested

Results

In this study, each group contained 30 samples of conventional heat polymerized PMMA resin (DPI, Wallace Street, Mumbai, India), which were reinforced with 8% and 13% aluminum oxide powder by weight. After fabrication, the samples were subjected to the universal testing machine to assess the flexural strength (Table I-III).

The flexural strength of the samples was calculated based on the following formula:

Flexural Strength = S=3PI/2bd²

The obtained results were analyzed using one-way analysis of variance (ANOVA). Comparison of groups A1 and A2 (8%) indicated the highest flexural strength (85.94 MPa) in group A2, and the difference in the mean flexural strength was not considered significant. (Diagram 1) Furthermore, a highly significant increase was observed in the values of flexural strength with 13% alumina addition.

Comparison of groups A1 and A3 (13%) showed the highest flexural strength (86.41 MPa) in group A3, and the difference in the mean flexural strength was considered significant. In addition, comparison of groups A2 and A3 indicated the highest flexural strength in group A3, and the difference in the mean flexural strength was not considered significant.

Table I. Mean Flexural Strength (MPa) of Group A1, A2, A3

Table II. Results of One-way ANOVA regarding Mean Flexural Strength in Study Groups

              One-way ANOVA; *F test; P                              P

Table III. Inter-group Comparison of Mean Flexural Strength

                             NS: not significant; S: significant

Diagram 1. Mean Flexural Strength in Study Groups