Plastic Flow – Cosmetic Uses in Rheology and Product Formulation

What Is Plastic Flow?

Plastic flow describes the behaviour of materials that do not flow until an applied stress exceeds a certain minimum value, known as the yield stress. Below this stress, the material behaves as a structured or solid-like system. Once the yield stress is exceeded, the material begins to flow.

This behaviour is common in structured cosmetic formulations such as creams, gels, pastes and suspensions.

Plastic flow is different from Newtonian flow, where a liquid can begin flowing immediately when a shear stress is applied.

Plastic Flow in Cosmetic Formulations

Plastic flow can be desirable when a cosmetic product needs to remain stable in its container but become easy to dispense and apply when force is applied.

Examples include:

• Creams and emulsions
• Gels
• Pastes
• Foundations
• Masks
• Toothpastes
• Structured cleansing products
• Suspensions

For example, a cream with plastic-flow behaviour can remain in place in a jar without flowing under its own weight. When the consumer applies sufficient force with a finger, the structure is overcome and the cream spreads across the skin.

Plastic Flow and Yield Stress

Yield stress is a key characteristic of plastic-flow systems. It represents the minimum stress required to initiate measurable flow.

The desired yield stress depends on the intended product and packaging. If it is too high, a product may be difficult to pump, squeeze or spread. If it is too low, the product may flow excessively during storage or lose its desired structure.

Rheological measurements can therefore help determine whether a formulation has an appropriate balance between stability at rest and flow during application.

Plastic Flow and Cosmetic Stability

Plastic-flow behaviour can contribute to the physical stability of structured formulations. A sufficiently high yield stress can restrict the movement of suspended particles or dispersed droplets while the product is at rest.

However, yield stress alone does not determine overall formulation stability. Particle size, viscosity, emulsifier system, phase composition and the formulation's broader rheological profile also need to be considered.

During formulation development, plastic-flow behaviour can be evaluated using rheological measurements to understand how the product responds to increasing shear stress and how readily it begins to flow.

Source: Handbook of Cosmetic Science and Technology – André O. Barel, Marc Paye, Howard I. Maibach.