On the properties of nanosilicate-based filled dental adhesives: Synthesis, characterization, and optimized formulation

Abstract

Objective: In this study, we incorporated hybrid nanoparticles (poly (acrylic acid)-grafted nanoclay/nanosilica, respectively, with platelet and spherical morphologies, abbreviated as PAA-g-NC-Sil) in different concentrations (0, 0.2, 0.5, 1, 2 and 5 wt%) to an experimental dentin bonding system and investigated the physical properties of the filled adhesive and its shear bond strength (μ-SBS) to dentin. We subsequently compared the properties of the adhesives containing PAA-g-NC-Sil with previously studied adhesives containing poly (methacrylic acid)-g-nanoclay (PMA-g-NC) (Solhi et al., 2012a), poly (acrylic acid)-g-nanoclay (PAA-g-NC) (Solhi et al., 2012b), and the hybrid poly (methacrylic acid)-grafted-nanoclay-nanosilica (PMA-g-NC-Sil) (Solhi et al., 2020).

Materials and methods: In a set of previous publications and the present paper, we grafted poly (acrylic acid) (PAA) or poly (methacrylic acid) (PMA) onto the surface of pristine Na-MMT nanoclay (Cloisite® Na⁺) through free radical polymerization of monomer in an aqueous media in the presence or absence of nanosilica particles. We characterized the resulting modified nanoparticles (PMA-g-NC, PAA-g-NC, PMA-g-NC-Sil and PAA-g-NC-Sil) using GPC, FTIR, TGA, and XRD. We then incorporated the modified particles as functionalized fillers to experimental dentin adhesives in different concentrations and studied the stability of modified fillers dispersion by separation analysis. We also studied the properties of the photo-cured adhesive matrices using FTIR, TEM, SEM, EDXA, and XRD. We examined the shear bond strength of the adhesives (containing different contents of each modified filler, separately) to human premolar teeth. The results were analysed and compared statistically.

Results: The results confirmed that the polymers have been grafted onto the surface of nanoclay. An exfoliated structure for the nanoclay platelets in the photo-cured adhesive containing PAA-g-NC-Sil was observed. Addition of 0.5 wt% of PAA-g-NC-Sil to the experimental adhesive increased the shear bond strength and the dispersion stability in comparison to unfilled adhesive. The same trend was also observed for adhesives containing PMA-g-NC, PAA-g-NC, and PMA-g-NC-Sil. The adhesive containing PAA-g-NC-Sil showed the best dispersion stability and subsequently the highest shear bond strength in the optimal concentration among adhesives containing the four available fillers (PMA-g-NC, PAA-g-NC, PMA-g-NC-Sil and PAA-g-NC-Sil).

Significance: Addition of poly (acrylic acid) modified nanoparticles to the experimental dentin adhesives resulted in higher shear bond strength due to the potential interactions between the carboxylic acid functional groups on the surface of the modified particles and the dentin structure. Between the poly (acrylic acid) and poly (methacrylic acid), the former acid with higher PK_a performed better. Addition of the spherical nanosilica particles to the adhesives containing platelet nanoclay helped to better exfoliate the platelets resulting in improved μ-SBS and dispersion stability.

Keywords: Bond strength; Dental adhesives; Nanoclay; Nanoparticles; Nanosilica; Poly (acrylic acid).