Abstract
Soap bars offer a valuable alternative to liquid soaps and their market is flourishing in response to society's commitment to the Green Economy and sustainable products. The advent of synthetic detergent (syndet) “soap” formulations has opened markets for products such as shampoo, conditioner, and facial bars. However, their processability has been revealed to be less controllable than conventional fatty acid-based soaps. In this work, we present a rheological characterization of a set of syndet formulations as a function of both their moisture content and of a compressional stress applied perpendicularly to the shear deformation, as experienced by the materials within extruders during the production process. The main outcome of our investigation reveals that syndet shows a significant stiffening when subjected to compressional stress and a slight reduction of the yield stress as a function of the moisture content. In particular, we report that, within the instrumental limits of applicable normal stresses (i.e., from ∼1 to ∼300 kPa), both the linear viscoelastic moduli of syndets and their yield stress increase by two orders of magnitude; thus, potentially explaining the difficulties encountered during their production.
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