Dispersant formulation is an important part of achieving good dispersion. Optimizing a recipe can sometimes be more about how and when ingredients are added, as basic recipe changes may not be acceptable. Quickly adding about half of the total amount of powder to the liquid carrier is acceptable in most cases, but it is prudent to watch each initial recipe carefully to ensure that the powder does not float on top of the batch for more than a few seconds. Adding dry powder too quickly will "clog" the blade and may result in incomplete, erratic dispersion. The final half of the powder should be added gradually and slowly until the final percentage completes the recipe.
Blade speed should be adjustable from one-half the minimum final tip speed at the start of powder addition to a maximum final tip speed with batch thickness and slow flow. This procedure helps prevent splashing and excessive vortexing, which are inefficient for dispersion and can lead to excessive air entrapment in the dispersion. Assuming rules for tank size, horsepower, blade size, etc.,
After that, most dispersions are complete within 20 to 30 minutes of the last proper powder addition. Continuing the dispersion process beyond this time is generally ineffective and can actually cause damage to some components if the temperature continues to rise.
Generally, dispersers perform best when the flow pattern is toroidal and the blade tips are traveling at about 5000 ft/min in moderate viscosity (1500 to 5000 centipoise). Lower tip speeds may
It is acceptable at higher viscosities and higher tip speeds are acceptable at lower viscosities to obtain the same shear rate and stress. In other words, shear is a function of blade tip speed and product rheology.
