PSI - Issue 3

G.M. Domínguez Almaraz et al. / Procedia Structural Integrity 3 (2017) 562–570 Author name / Structural Integrity Procedia 00 (2017) 000–000

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modern industry, including: biomedical, solar, optical, nanotechnology, polymer conductivity, sensors, viscosity, molecular separators, battery electrolytes and pneumatic actuators (Huang and Rempel, 2013, Frazer et al., 2005, Zhou et al., 2015, Nakazawa et al., 2006, Pham et al. 2012, Tan et al., 2010, Ali et al., 2015). The polymer PMMA (or its commercial names: plexiglas, acrylate or perspex), is one of most popular thermoplastics due to its physical and mechanical properties: low affectation by ultraviolet radiation, low elongation at break, highly scratch resistant, it exhibits low moisture and water absorbing capacity, good dimensional stability, high Young’s modulus and hardness, high volume/weight ratio. PMMA used for dentistry application needs to fit a large number of conditions, such as: strength enough to biting, chewing, resists impact forces and wear during mastication, stable under thermal and mechanical load (Bhola et al., 2010, Park et al., 2009). Recent studies have been carried out to investigate the mechanical properties of PMMA used on odontology applications: the effect of adding di-methyl itaconate (DMI) and di-n-butyl itaconate (DBI) to improve the mechanical properties of commercial PMMA denture base material (Spasojevic et al., 2015), the increase of mechanical and tribological properties of PMMA based denture composite by adding seashell nanopowder (Karthick et al., 2014), or the effect of fabrication procedures, moulding and thermoforming of denture base resins PMMA, on its mechanical properties (Baloš et al., 2015). In this work is presented the ultrasonic fatigue endurance of self-fabricated PMMA dental resins, immersed in a liquid with pH close to human saliva. No references have been found by the authors of this work concerning ultrasonic fatigue tests on this polymeric material. The polymer PMMA is used for dental applications due its high dentistry properties: low toxicity and high biocompatibility, ease shape forming and manipulation, good wear and impact resistance, versatility of colors close to human tissues, low absorption of visible and UV light, relatively low cost and high durability (Frazer et al., 2005, Ali et al., 2015). The fabrication process for the ultrasonic fatigue testing specimen PMMA was undertaken following the conventional method for the denture polymer materials: compression molding with heat activation in water bath for polymerization (Tanaka and Setcos, 1989, Ali Moussa et al., 2012); taking special attention to reduce the shrinkage and dimensional effect during the resin polymerization. Three process of polymerization are available for this dental material: heat curing, auto or chemical curing and the light or microwave curing (Bhola et al., 2010, Ritzenthaler et al., 2000, Rashid et al., 2015, Muhtaroǧullar et al., 1999). The polymerization process used in this work is the one widely used in dentistry: the heat curing method. The general steps are described as follows: a) Mixture at room temperature of dust polymer and liquid monomer in the proportion 3:1 in volume; then, rest during 10 minutes, b) The second step is the polymer draining inside the mold. The mold used for this work is shown in Figure 1a, allowing to fabricate 5 cylindrical specimens of PMMA polymer at same time, c) The third step was the immersion of the mold in a recipient with water at room temperature; then, heating to attain and maintain 60° C, all the process during 30 minutes, d) Afterwards, heating is increased to attain and maintain 75° C during another 30 minutes, e) Finally, heating is increased to attain 95° C and the material is maintained for 1 hour at this temperature. After the last step, the mold is retired and cooled with water and the PMMA specimens are obtained, as shown in Figure 1b. The PMMA fabricated specimens present good values for dentistry applications: minimum porosity and standard mechanical and physical properties: Vickers hardness number close to 20 (Merin et al., 2014), ultimate tensile stress around 65 MPa (Spasojevic et al, 2015), tensile modulus of 3.3 GPa (Du et al., 2003), Poisson ratio  = 0.4, density  =1180 Kg/m 3 and glass transition temperature Tg = 110° C. 2.2 Ultrasonic fatigue specimen for PMMA In order to carry out ultrasonic fatigue testing on the self-fabricated PMMA material, the specimen’s profile was determined to fit the resonance conditions which are required for this modality of fatigue testing. 2. Experimental procedures 2.1. Material and preparation of specimens