Effects of 5 Nanosilica Concentrations and Humid Environments at 6 Different pH Levels on Fracture Toughness and Moisture Absorption of Dental Polymethyl Methacrylate Resin‎ Reinforced With Silica Nanoparticles: An Explorative Experimental Scanning Electron Microscopy Study

Abstract

Introduction

No study has assessed the effects of nanosilica within polymethyl methacrylate (PMMA) resin and environmental pH on resin's fracture resistance and moisture absorption.

Methods

A total of 90 specimens were divided into 30 subgroups of three, as per the ASTM D5045 standard: five groups of nanosilica percentages (0%/2%/5%/7%/10%), each ‎divided into six subgroups of pH levels (pH = 5/6/7/8/9, + “dry” control). The specimens were prepared by mixing silica nanoparticles with PMMA powder in a vacuum mixer. Then, the specimens were mixed with a diluent liquid (TEGDMA) according to the manufacturer's instructions. For each of the five weight percentages, 36 samples were produced. The 18 specimens in each group were randomly divided into six subgroups of pH levels. The specimens were kept in containers of liquid at different pH levels at room temperature for 1 week. Their before- and after-storage weights were recorded to calculate moisture absorption. The fracture resistance test was performed (ASTM D5045 standard) using the three-point bending method. Scanning electron microscopy was performed. Data were analyzed.

Results

Both nanosilica extents and pH levels significantly affected the fracture toughness with a significant interaction (p < ‎‎0.00001). All post hoc comparisons of different pH levels (except pH= 5 vs. 6) were significant (p < ‎‎0.0001). All post hoc comparisons of different nanosilica concentrations were significant (p < ‎‎0.0001). Both nanosilica extents and pH levels significantly influenced the fracture toughness with a significant interaction (p < ‎‎0.00001). All post hoc comparisons of different pH levels and also between different nanosilica concentrations were significant (p < ‎‎0.0001). The correlation between moisture absorption and fracture toughness was significant (R = −0.382, p = 0.0009).

Conclusions

Fracture toughness decreases when placed in humid and acidic environments. Also, the samples that were placed in a humid environment suffered a brittle fracture. Increasing silica nanoparticles improved fracture toughness (becoming optimal at 5 wt% nanosilica).

Objective and Materials

The objective of this study was to investigate the fracture toughness of dental samples made of PMMA reinforced with various percentages of nanosilica at various pH levels. For this purpose, dental resins with various amounts of nanosilica were placed in moist media at different pH values ranging from 5 to 9 (mimicking the normal pH range of the human mouth).