Soap bars offer a valuable alternative to liquid soaps and their market is flourishing in response to 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 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

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... s a function of both their moisture content and of a compressional axial 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 a compressional stress and a slight reduction of the yield stress as function of the moisture content. In particular, we report that, within the instrumental limits of applicable normal stresses (i.e. from ~1kPa to 300kPa), 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.