The stages of the technological process of tablet dredging are as follows: enveloping, or priming; layering, or rolling – the most important stage, as it is here that the formation of the entire shell occurs mainly; smoothing, or polishing; glossing – the last stage of dredging, at which the tablets are given a glossy, marketable appearance. Prof. PD. Pashnev in 1975 developed a new method of coating tablets – suspension method of dragees, the dispersion medium in which is 70% sugar syrup. Polyvinylpyrrolidone (PVP) as a plasticizer, magnesium carbonate as a filler, titanium dioxide as a dye and aerosil to stabilize the suspension. PVP is a high molecular weight compound, the molecules of which, attaching to each other, form a spatial grid in solution. Sugar molecules dissolved in water are enclosed in the grid cells. During the drying process of the processed tablets, the water in the individual cells is removed. The sugar remaining in the cells, crystallizing, is not able to combine into agglomerates. Fine crystals with low brittleness and high plasticity are formed [2, 3]. BASF suggests the use of Collidon 25 and Collidon 30 in sugar dredging (see section 4.9). The physical characteristics of these substances, such as prevention of microcracking, adhesion of the sugar layer to the tablet core, uniform distribution of dye over the tablet coating, slower and more homogeneous crystallization of the sugar, favor the coating process and improve the quality of the coatings.

Sugar coatings are particularly susceptible to cracking when applied to large batches of tablet cores that dry quickly. Since most drug substances are hydrophobic, Collidon 25 and Collidon 30 are used as additives to prevent cracking-induced peeling of the tablet coating from its core during manufacture.

The table shows an example of a slurry composition for automatic sugar coating. The data are shown for 40 kg of 420 mg tablet nuclei in a conventional coating tank when 25 kg of the above-described slurry is atomized under the following conditions: duration of the atomization stage – 5 s; interval – 10 min; hot air drying time – 10 min; total coating time -16 hr. A more modern way of sugar draying process is offered by Glatt company, which uses for this purpose new generation drum units. Coating in drum-type units ensures gentle processing, especially of temperature-sensitive and brittle dosage forms. For sugar draping as well as for film coating, Glatt uses a method in which a coating solution is applied to the particle from above using a two-component nozzle.

The particles are moved and mixed in the GMPC II Glatt by rotating the drum. The supplied process air evaporates the liquid and dries the layer. Softening and stickiness of the particle surface is prevented by selecting the appropriate process parameters. The required efficient heat exchange is achieved by the continuous perforation of the cylindrical part of the drum, thus minimizing the process time.

Significant advantages of Glatjp’s sugar dredging process are gentle mixing and coating of the tablets due to the geometry of the drum and the company’s specially developed Fischer mixing paddles, which ensure absolutely uniform mixing with minimal mechanical impact on the product.

With this method of dredging it is possible to coat large, non-fluidizable particles (e.g. tablets, capsules and large pellets). In general, this plant design achieves a high coating quality with throughputs ranging from 500 g to 2000 kg. DGM (Switzerland) has developed a sugar coating plant with an efficient system for spraying the coating medium and drying.

The tablets in the rotating perforated drum of the plant make complex orbital movements, during which the coating medium is automatically sprayed over them according to a defined process. At the same time, hot air is fed into the drum, penetrating through the tablet layer, so that the coating medium applied to the*surface of the tablets dries quickly and evenly. In this way, a strong and smooth thin-layer coating is formed.