Heat sensitivity is also a factor to consider, requiring tight control of process temperature, and keeping mixing times to a minimum. Rheology modifiers, such as gums and thickening agents, and emulsifiers have a strong tendency to form agglomerates when added to the liquid in the vessel. This is often exacerbated by operator error, for example by adding the powder too quickly. Agitators do not produce sufficient shear to break down these agglomerates, relying ona simple washing action to slowly dissolve or hydrate the outer layer and expose the dry material on the inside to the surrounding liquid. This not only results in long mixing times, but a totally lump-free and homogeneous product cannot be guaranteed.
Incomplete hydration can also lead to undesired changes in product viscosity during storage.
Maximised functionality can only be achieved if the powder is correctly dispersed and fully hydrated. This may require a degree of mechanical energy that agitators simply cannot deliver. The types of mixer available divide into either those which operate immersed in the vessel, and external or 'in-line' mixers. The workhead assembly consists of a centrifugal type rotor which rotates at high speed within a fixed stator. The size and shape of the openings in the stator and the clearance between the tips of the rotor blades and the inner wall of the stator determine the flow pattern and shear rates of the machine.