Plastic flow
A material exhibiting plastic flow requires a minimum force before significant flow begins.
This behavior is common in many cosmetic creams, ointments, pastes and highly structured gels.
Below the yield point, the material can maintain its shape. Once the applied stress exceeds the yield stress, it begins to deform and flow.
Cosmetic Importance
Plastic flow can be desirable in packaging and consumer use.
A cream with suitable yield stress can remain in a jar without flowing excessively but spread easily when the consumer applies pressure.
Formulation Factors
Polymers, waxes, emulsifier systems, fatty alcohols and dispersed particles can all contribute to yield-stress behavior.
The desired rheological profile depends on the product format and consumer experience.
References
Barnes, H. A. A Handbook of Elementary Rheology.
Barel, A. O., Paye, M. and Maibach, H. I. Handbook of Cosmetic Science and Technology.
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