Rheology assessment of (cosmetic) lipophilic pigmentary pastes

Pigmentary pastes are used in the cosmetic industry as primary coloring agents to gain agility in cosmetic finished goods manufacturing. Indeed, pastes are much easier materials to handle and transport compared to powders of dry coloring agents. From a physico-chemical standpoint, pigmentary pastes consist of pigment particles dispersed in a continuous phase. The pigment content makes pigmentary paste suspensions that cover semi-diluted to concentrated regimes, which leads to pastes displaying “soft” to “hard” texture. The control of this texture is crucial to monitor the quality of pigmentary pastes or to improve the processing of these materials. In this study, we propose to characterize the texture of pigmentary pastes via rheology, more precisely thanks to elastic modulus and yield stress measurements. Results collected on a set of pigmentary pastes, manufactured from lab to plant scale, indicate that these two indicators allow to detect wide texture ranges, plant-to-plant and lab-to-plant variabilities, and texture evolution over time. Combining these indicators to plot yield stress–elastic modulus mapping identifies master curves corresponding to specific paste families. Such mapping also provides a direct characterization of the texture, which is valuable to sort quantitatively various types of complex pastes. Moreover, combining the mapping with physicochemical parameters helps to decode the paste texture behavior. Finally, the yield stress–elastic modulus mapping generalizes a previous approach on model silica suspensions [J. Fusier et al., J. Rheol. 62, 753–771 (2018)] to cosmetic pigmentary pastes, which exemplifies the transfer of academic knowledge to industrial applications.