# Cyclic Acidic Beverage Exposure Induces Formulation-Dependent Mechanical Softening and Tribological Alterations in Microhybrid and Nanohybrid Dental Resin Composites

## TL;DR

Acidic drinks can weaken dental fillings over time, with some materials degrading more than others depending on the drink and formulation.

## Contribution

This study reveals how acidic beverage exposure affects the mechanical and tribological properties of dental composites in a formulation-dependent manner.

## Key findings

- All composites showed significant microhardness reduction after acidic exposure, with Herculite Ultra being most affected.
- Surface roughness changes varied by formulation, with Herculite Ultra roughening and Filtek Z550 smoothing.
- Tribological behavior was mainly influenced by matrix softening, leading to unstable frictional responses.

## Abstract

Dental resin composites are routinely exposed to chemically aggressive beverages that may compromise long-term functional performance. This study investigated the structure–property–tribology relationships of four restorative composites (Filtek Z250, Filtek Z550, Herculite, and Herculite Ultra) subjected to cyclic immersion in beverages with different pH values. A total of 120 cylindrical specimens (7 mm diameter, 2 mm thickness; n = 5 per material per condition) were fabricated and exposed to mineral water, tea, coffee, Coca-Cola®, Cola Light®, and red wine for 28 days under cyclic conditions. Microhardness, surface roughness (Ra), steady-state coefficient of friction (COF), and mass variation were evaluated. All composites exhibited significant microhardness reduction after acidic exposure (p < 0.05), with the greatest decrease observed for Herculite Ultra in red wine (−47.4%) and Coca-Cola® (−35.3%). Filtek Z250 demonstrated the highest baseline hardness and the lowest degradation susceptibility. Surface roughness changes were formulation-dependent, with Herculite Ultra showing pronounced roughening (ΔRa up to +0.074 µm), whereas Filtek Z550 exhibited erosion-driven smoothing (ΔRa down to −0.068 µm). Tribological behaviour was primarily governed by matrix softening rather than roughness alterations, with softened systems displaying unstable frictional responses (COF range: 0.127–0.697; p < 0.05). The results indicate that polymer matrix stability plays a more critical role in long-term functional performance than surface roughness or mass variation alone. Clinically, frequent exposure to acidic and solvent-containing beverages may accelerate mechanical and tribological degradation of susceptible composite formulations.