What is coating in powder processing?

In bulk material processing, coating is the process of altering the material characteristics of particles only on their outermost surface in order to protect the material characteristics underneath. Coating often takes place directly after or as a final stage in industrial mixing processes.

Coating differs from painting in that painting is performed mostly for decorative purposes. Coating, on the other hand, has the functional purpose of making dry, powdery particles less susceptible to damage or degradation through external or atmospheric conditions, such as moisture or oxidization.

Because it often involves the addition of ingredients such as fats, oils, or wax, coating alters the sensory and handling characteristics of particles. This gives the manufacturer more control over the resulting structure of the particles after mixing.

How does the coating process work?

Coating is a mechanofusion process. In mechanofusion, mechanical energy is applied to particles in order to induce a reaction that chemically changes the materials.

Within the mixing vessel, this mechanical energy is generated via agitating mixing blades, eliciting a chemical reaction on the surface of the mixing goods, encasing them in a layer of nanoscale particles.

In industrial mixing, the coating process can be broken down into three distinct steps:

    • Material fines are added to a coarser, more granular bulk material; the material fines will loosely cling to any agglomerates in their close proximity.
    • Mechanical energy is generated, causing the material fines to forcibly collide with and chemically fuse to the surface of the agglomerates.
    • The constant friction generated between particles causes the material fines to become evenly distributed over the surface of the agglomerates. In turn, the coated surface grows in size, the agglomerates disperse slightly from one another, and all particles become evenly coated.  

Coating with amixon® mixers

The duration of the coating process differs according to the volume of mixing goods, the amount of mechanical energy generated, and the mixing equipment. Industrial mixers by amixon® are capable of coating particles to achieve 100% coverage in short mixing cycles.

HM Twin Shaft Mixer is especially well-suited for applications requiring coating. In this highly precise powder mixing machine, additive liquids, fats, or material fines are evenly distributed through the bulk materials in a three-dimensional flow created by the high-shear rotation of a helical mixing blade. The resulting mixture is homogeneous, uniformly granulated, dust-free, and perfectly coated.

Applications of coating in the food and pharmaceutical industries

From prescription medications in tablet form, to some of our favorite snacks, coating ensures that these products retain their physical form and chemical characteristics.

With such a wide variety of food products on the market today, coating is used in a growing number of applications. Many snack foods, cereals, and granolas, for example, are coated with cacao powder, sugar, milk powder, salt, or fats. In addition to enhancing the taste of the food products, these coatings can make the product more shelf-stable and improve their texture.

In the pharmaceutical industry, coating plays an important role in the manufacture of delayed-release medications. In other applications, tablets are coated in order to make them more palatable and thus easier to administer. Furthermore, coating helps to make the active ingredients in medications less susceptible to degradation via exposure to light, moisture, and oxidation.

In some processes, food products and pharmaceuticals are coated using suspension, emulsion, melt, or spraying techniques. Coating in a mixing machine, on the other hand, uses the same mechanical energy generated during the mixing process to evenly distribute and harden the material fines upon the surface of the larger agglomerates.