Effect of formulation on the rheological and tribological properties of toothpastes

An improved understanding of the impact of formulation of toothpaste tribology would be highly beneficial for tuning formulations to individual consumer preference which has the potential to improve adherence to a robust oral care regime. A prototype, 3-ball rheo-tribometry geometry and measurement protocol was trialled. This system was found to demonstrate excellent reproducibility and, when combined with rheology, improved understanding of toothpaste microstructure during tribological measurements. By varying solvent, binder, surfactant and solid composition of model toothpaste formulations, a negative, power law correlation between high shear viscosity (HSV) and coefficient of friction (CoF) of toothpaste slurries has been established. It was apparent that sodium carboxymethylcellulose, whilst not typically very beneficial in toothpastes for other commercially-relevant properties like shape retention and stringiness (Ahuja and Potanin, 2018), was excellent in reducing CoF/increasing HSV. Although not typically used in commercial toothpaste formulations, gellan and locust bean gum in combination with xanthan gum demonstrated excellent rheological performance on a w/w% basis, particularly in terms of shape retention (height loss on a brush). However, low binder concentration in slurries imparted high CoF values and so there is a trade-off involved between tribological and rheological properties. Humectant type and ratio and surfactant (SLS) content made little difference to neat paste/slurry rheology. However, CoF at low sliding speeds were observed but did reduce CoF at low sliding speeds. Hypothesized improvements in wetting, molecular/micellar hindrance and surface passivation have been related to humectant and surfactant presence. Relative dependence of these formulation effects on slurry HSV and CoF were established through multiple linear regression. A microstructural model of neat toothpastes and slurries during the tribological measurements performed in this work has been suggested, with some significant limitations that arise from the measurement protocol used. Overall, the impact of the findings of this work, combined with recent consumer panel studies on toothpastes (Teoman and Potanin, 2025) progresses understanding of how to potentially tune toothpaste formulations for consumer experiences.