Coating

What does coating mean in the context of bulk solids blenders?

Coating of particles is used when the surface properties of raw products are to be changed. The question often arises as to the difference between painting and coating: While painting serves primarily decorative purposes, coating provides active protection against external, material-damaging influences.

Bulk particles can also be coated to change the surface properties of powders. A suitable powder mixing process using amixon^® mixers can provide an economically viable method.

• For example, small metal particles can be coated with a thin varnish to interrupt their electrical conductivity in the particle composite. This can be used to influence magnetism, for example.
• Ground sugar or ground salt can be coated with fat. Thus, they can dissolve in a humid environment only if the fat is melted by applying heat.
• Nanoparticulate materials can completely coat bulk particles, with the goal of producing a new material in a subsequent calcination process.
• Nanoparticles can serve as flow aids by occupying and sealing the existing porosity of the bulk material. Any moisture present is then either absorbed or encapsulated.
• Beaded agglomerated black pigments can be rounded and coated. In this way, the mixed powders are protected from moisture or oxygen from the ambient air.
•     Instant products can be produced in the mixer (in the form of the multi-stage process) by coating the particles layer by layer with liquid and powder. Dust is bound by the formation of larger particles. Such a layer build-up can be repeated several times if necessary. In this respect, the sensory properties of nutrients/pharmaceuticals and confectionery can also be controlled by layer build-up.

In short, coating processes optimize the properties of a bulk product in terms of shelf life, appearance, solubility, dust binding, flow behavior, durability, chemical reaction and much more.

How does the coating process work?

The coating process in powder processing is based on the principle of particle agglomeration as a companion process to powder blending or fluid bed processing. The adhesion effects between particles can be particularly large for small particles. They result from Van der Vals forces and electromagnetic forces. If the particles are only a few nanometers in size, they can coat drug particles particularly well. During coating, an attempt is made to enlarge the mixture particles in the shape of a shell. Ideally, they become round. Round particles are more stable and enable a multilayer structure. This works better the wider the particle size distribution of the coating product. The particle composites are deposited, rolled up and compacted (agglomerated) by mixing, rolling and pushing movements. By repeating this process several times, the particles grow layer by layer.

In practice, the coating process is favored if small quantities of a binder liquid are added. The mixing process first serves to ensure uniform wetting of all active ingredient particles. Then the coating product is added. The mixing process must be space-controlled and efficient. The energy input must be sufficient to separate unwanted agglomerates into primary particles. On the other hand, the resulting coatings should not be destroyed. This process is an accompanying process to the mixing process. It is based on the ideal random distribution during precise powder mixing.

Fields of application: Coating in the food and pharmaceutical industries

Whether pharmaceutical tablets or snack products: Many products only obtain their specific shapes and properties through the coating of particles. With the growing variety of food products, the demands on coating are therefore also increasing. Film formation on tablet masses is typical. Snack products are electrically charged for a short time, which favors effective sprinkling with powdered spice mixtures. Bears formed from wine gum are coated with a sugar acid mixture to keep them flowable. Sugar or salt particles are coated with a finely dispersed flavoring powder, hot caramel coats frozen chocolate pieces, and bacterial cultures are coated with a rapidly solidifying sweetener. Functional nutrients are coated with a starch suspension, etc. The list of examples could be continued endlessly. Tablets can be coated with a poorly soluble coating to slow the release of the active ingredient (retard effect). Medicines are often coated to make them easier to swallow. Like food, drugs must be protected from oxygen and moisture.

A coating is applied in the form of suspensions, emulsions, melts or finely dispersed powders. In addition to powder mixers, suitable equipment can include coating drums, fluid bed systems, high coaters, free fall mixers or continuous conveyors.