1. Paste products and mechanisms involved

A pasty or semi-solid product is solid at rest and can flow when subjected to mechanical stress. Such mechanical properties are obtained by adding solid-texturing ingredients to a liquid base. An elastic solid is obtained when the additives form a continuous solid network through the liquid medium, giving the material a macroscopic solid texture. The material is solid on a large scale, but remains liquid on a smaller scale, as the formulation base is nevertheless a liquid. Molecules move as in a liquid, and molecules can be dissolved in the liquid base, such as active ingredients. A pasty system can flow, because the macroscopic network that provides the solid texture can be broken by sufficient mechanical stress. The minimum stress that allows flow is called the "flow threshold".

The European Pharmacopoeia defines pasty systems as "semi-solid preparations intended...