Cosmetotecnia de los dentífricos.Relevancia del comportamiento reológico
Dentifrices are considered to be semi-solid products which are designed to clean the teeth and provide a fresh and pleasant breath. These products are a mix of abrasives (e.g. hydrated silica), humectants, water, surfactants, agents for preventing tooth decay (e.g. fluoride), thickeners (e.g. cellulose gum, xanthan gum and carrageenans) and flavor, among others. Furthermore, it should be easily extrudable from the package (“squeezing out”), keep enough tightness (“stand up”) over the toothbrush and have an acceptable flavor for the consumer. The dentrifices manufacturing implies complex industrial procedures, and the scientific literature about their composition, manufacturing or quality control, and rheological properties is very limited. The main objective of this PhD thesis was to improve the knowledge about cosmetotecnica of the dentifrices. In order to achieve this goal, three different studies have been carried out: First, characterisation of different dentifrices categories (“pastes”, “gels” and “liquid dentifrice” o “2 en 1”) of Korott manufactured products has been performed. The main physical (density, pH, viscosity), chemical (preservatives and fluoride) and microbiological indicators which guarantee stability have been analysed. The values of density and pH presented similar values in all categories, whereas the viscosity showed significant differences. The concentrations of the different preservatives and the active ingredient fluoride remained stable over time. Likewise, the dentifrices fulfilled with the microbiological limits established and in any case the presence of pathogenic microorganism were detected. Following, a rheological study (flow and viscoelastic behavior) of Korott® dentifrices was carried out in order not only to characterize them but also to compare them with some reference brands at Oral Care sector (Colgate®, Sensodyne®, Binaca®, etc.) . The Korott® products presented a highly shear thinning and viscoelastic behaviour, with a typical weak gel-like structure. The Korott® pastes and gels showed an intermediate rheological behavior, while the 2 in 1 had a lower shear thinning behavior than the reference brands. As a consequence of the different shear thinning behavior, the differences observed in visual consistency over the toothbrush (“stand up”) are reduced for brushing shear rates. On the other hand, the pastes and gels present greater yield stress values (“squeezing out”) than the ones corresponding to 2 in 1 dentifrices. Finally, in order to study the influence of the main rheological modifiers, some mixtures of increasing complexity have been studied. Presence of CMC modifies the Newtonian behavior of sorbitol/water mixtures, developing shear thinning systems. The addition of PEG-12 hardly modifies the rheological properties but increases the initial viscosity of the mixtures (sorbitol/water and CMC). The addition of a 10% of abrasive silica to the aqueous medium with sorbitol, CMC and PEG, does not significantly affect the flow behavior, whereas the same quantity of thickening silica increases initial viscosity and shear thinning characteristics. The combination of thicknening and abrasive silica produces systems with similar viscosity to the final product.