Direct restorative dental materials possess very special characteristics. These materials operate in a singular environment that is the mouth. The factors at play are numerous and their variation range is important, such as; humidity, load cycles, pH, temperature, bacteria, external agents, environment, material properties and contact geometry, etc, are some examples. Teeth and restorative
... [Show full abstract] materials are, therefore subjected to complex and numerous factors.
During the past decade clinicians and patients have developed an interest in to posterior composite resins. The demand for restorations with a natural appearance in the posterior region and the controversy as to the mercury action on amalgam, although it still has excellent mechanical properties, have attracted researchers to focus on the improvement of the resin composites in order to be applied in posterior teeth.
Resin composites have been by far the most successful in dental applications by meeting several stringent design requirements difficult to achieve with homogeneous materials such as ceramics and metal alloys. When the intention is to repair of cavities the product needs to be aesthetically matched in color and translucence with other teeth and retain its gloss. It must match the hardness of the opposing tooth and be resistant to wear or fatigue fracture. It must be dimensionally stable and withstand the largely varying thermal stresses in the mouth. “Packable” composites have been termed as alternative to amalgam, for usage in posterior restoration.
Criteria to select a composite in detriment of another are very complex and the few existent standards do not allow establishing a consensus to which test must be used to determine and compare composites properties.
Mechanical and tribological properties of direct restorative filling materials are crucial not only to serve and allow similarity with human enamel and dentine but also to compare composites between them and determine objective criteria for their selection.
This work is focused on the mechanical and tribological characterization of posterior direct commercial restorative materials. Due to their characteristics, commercial composites are materials which can not be changed (filler weight fraction, average particle dimension and matrix composition are pre-fixed parameters), which limited the objectives of the initial study. With the introduction of the production of a composite material which authorized variation of its constituents, the initial work was complemented.
The mechanical properties assessment was done with the following tests: Vickers hardness, Impulse excitation of vibration and four point bending tests. The mechanical properties assessed were: hardness, elastic modulus (static and dynamic), flexural resistance and work-of-fracture.
To determine the tribological behavior of composites reciprocating sliding tests was used. Wear and the mechanisms involved were evaluated on the composites as well as on the antagonist (human teeth and glass spheres). A classical and an energetic approach were used to understand the behavior of the composites and their antagonists. The use of glass sphere as antagonist material was subjected to validation, by comparison to the material pair composite natural tooth. Although the difference in wear volumes of the distinct pair does exist extrapolation of results is possible with similar and acceptable results.
Eight direct restorative commercial composites were tested against glass-spheres, to determine wear resistance under different wear mechanisms: abrasion and attrition. The evaluating of environment influence in these tests allow to understand composites behavior under reciprocating contact through measuring wear volumes and coefficients of friction of the contact pair and analyzing the removal mechanisms involved in the wear process. In this analysis of some composite-glass pair some materials showed some discrepancy regarding wear and the coefficient of friction, thus a complementary study involving, only, three commercial composite were done. A load-scanning test was used to understand and explain the wear regime transition of these. This test permitted to perceive the wear/fracture mechanisms induced by the different values of coefficient of friction.
The influence of external factors on the mechanical behavior of composites was also evaluated. Two types of environment in the reciprocating wear tests were introduced: artificial saliva/distillate water and abrasive slurry. The other external parameters introduced in the study were aging time and pH. Two batches of seven commercial composites were aged in 3, 7 and 9 pH buffer solutions during three periods 3, 6 and 22 months. The tribological and mechanical evaluation of these composites was done after each aging period. Generally and from the point of view of application, it was possible to draw conclusions relatively to commercial composite behavior mechanical and tribological. The effect of external factors on the behavior of these commercial materials was also evaluated and the properties variation quantified.
Due to the nature of the study, parameterized evaluation, of produced composites brought further progress, with clear conclusion relatively to weight content, filler dimension, matrix on the mechanical and tribological properties studied.
The tribological behavior was evaluated and removal mechanisms enclosed in the wear process are discussed, taking into account the systematic SEM observations to evaluate the failure modes.
Keywords: Dental composites, filler effect, pH and aging effect, mechanical properties, tribological characterization.
