Understanding the role of tribology in maintaining oral hygiene

Abstract

The importance of maintaining good oral hygiene is vital to a healthy body and aesthetically attractive smile. However, cleaning your teeth comes with a drawback. Toothpastes contain abrasive particles that have the potential to harm the tooth enamel resulting in wear. The aim of this project is to develop an understanding on the behaviour of abrasive particles in a toothpaste and how their impact and performance affect enamel wear. An integrated approach is employed in this study to investigate the delivery of abrasive particles to a tooth surface, using a Nylon toothbrush or ball, to understand how a polished surface with minimal tooth wear can be achieved.

To understand the interface of the tooth and the toothbrush, the microstructure, composition and mechanical properties of bovine enamel were visualised using Scanning Electron Microscopy (SEM) and nanoindentation. The filament properties such as tuft size and shape were analysed by metallographic preparation, SEM and optical microscopy respectively.

For a better understanding of the effects of tribology, tests on the TE77 high speed reciprocating tribometer were undertaken, using a load of 5N and a frequency of 4Hz. Wear depth, roughness data, groove width analysis and high speed friction was recorded using profilometric and measurement analysis. Tests were carried out with alumina, silica, spherical silica and control saliva slurries. The wear mechanism identified was 2-body grooving formed by microchipping, by abrasive particles embedded around the filament tips. Results showed that spherical silica produced the smoother surface (Ra 0.22µm) with the least amount of enamel wear. There was no measurable wear with the saliva test indicating the particles are causing damage. A higher coefficient of friction was reported for the saliva control slurry test compared to the particle tests. This could be due to the higher friction between hydrated enamel and the filaments as compared to hydrated enamel and the particles. The groove analysis of the 2-body wear scars showed grooves in the size range of the filaments and particles (12- 130µm). This indicated agglomeration on the filaments should be avoided to minimise wear on the enamel surface.

The relationship between the combined effects of particle shape/ size and load on the volume loss, wear rates and wear mechanisms of enamel on the TE66 microabrasion rig were reported. A nylon ball was used and volume concentrations of 5% – 20% alumina, silica and spherical silica abrasives. Post experimental surface analysis was performed to analyse the wear scar morphology and groove analysis. Mono-sized particles as compared to bimodal. appeared to give the best outcome and least wear of enamel. The synergistic effects of bimodal particles vs. mono sized particles in toothpastes were quantified. A positive synergy existed for the alumina and silica synergy tests, indicating the bimodal particles were causing more wear than the mono-sized particles and contributing to the increased volume loss of enamel. The synergistic mechanisms were a combination of crushing and fracture of the enamel rods, followed by micro-chipping and removal of enamel. The magnitude and the distribution of the particle sizes is the most dominant factor in determining the levels of wear, with smaller particles and narrow distributions of the particle size reducing the magnitude of wear. The shape of the particles is another factor influencing wear, with the spherical tests generating lower wear rates than the angular tests. Load is an important variable along with slurry concentration affecting the load per particle, with higher loads per particle resulting in greater wear. The factor which influences wear the least is the composition of the slurry.

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ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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