However, the vast majority of drugs are not able to spontaneously fill the matrix of the tablet machine due to the significant (more than 70%) content of fine fractions and uneven surface of the particles, causing strong interparticle friction. In these cases, auxiliary substances are added that improve the flow properties and are classified as sliding. In this way, tablets of vitamins, alkaloids, acetylsalicylic acid, phenobarbital, ascorbic acid, sodium bicarbonate, streptocide, phenacetin are obtained. These characteristics are very important for the control of substances used in direct compression technology, especially in large quantities, since the quality of the tablets in this case will directly depend on the technological parameters of the tablet mass, its flowability, compressibility and compactability. It was experimentally established that the lower the concentration in the tablet mass of the component, the smaller its particles should be. You cannot get a homogeneous tablet mass consisting of components with sharply different particle sizes. It is known that a system consisting of two fine powders forms more homogeneous and stable mixtures than a system in which particles of one component are larger than the other. To obtain the optimal composition of the mixture of multicomponent preparations, it is desirable to observe the following conditions: the particle sizes of the individual components must correspond to their concentration; the density of the substances of the individual components is desirable to choose close to each other; the particle shape should be closer to spherical. If the drug substance is suitable for the direct compression process, it is tableted using conventional excipients. If the medicinal substance when using conventional excipients is not suitable for direct compression, then use excipients that have a sufficient binding effect on the particles, or use granules of the drug substance with a binder suitable for direct compression. Currently, tabletting without granulation (direct compression) is carried out in the following ways: with the addition of auxiliary substances that improve the technological properties of the material; by forcing the tableted material from the feed hopper of the tablet machine into the die; preliminary directed crystallization of the pressed substance.

Preliminary directed crystallization is one of the most difficult methods for producing medicinal substances suitable for pressing, which consists in achieving a tableted substance in crystals of a given flowability, compressibility, and moisture by selecting certain crystallization conditions. The result is a crystalline drug substance with crystals of isodiametric shape, freely precipitating from the funnel and therefore easily subjected to volumetric dosing, which is an indispensable condition for direct compression. This method is used to obtain tablets of acetylsalicylic and ascorbic acids. To increase the compressibility of drugs during direct compression, dry binders are added to the composition of the powder mixture — most often microcrystalline cellulose (MCC) or polyethylene oxide (PEO). Due to its ability to absorb water and hydrate individual layers of tablets, MCC has a beneficial effect on the release of drugs. With MCC, you can make durable, but not always well-disintegrating tablets. To improve the disintegration of the tablets, together with the MCC, it is recommended to add ulipraamilopectin. Copovidone (a vinylpyrrolidone copolymer) has a small particle size, which leads to improved ductility and excellent dry binding ability. Analysis of the presented binders shows that Kollidon VA 64 fine has one of the best binding effects in the production of tablets by direct compression. For direct compression, it is recommended that modified starches be used as binders, which enter into chemical interaction with medicinal substances, significantly affecting their release and biological activity. Milk sugar is often used as a means of improving the flowability of powders, as well as granular calcium sulfate, which has good flowability and provides tablets with sufficient mechanical strength. Cyclodextrin is also used, which increases the mechanical strength of the tablets and their disintegration.
With direct compression, maltose is recommended as a substance that provides a uniform rate of backfill and has a slight hygroscopicity. A mixture of lactose and crosslinked polyvinylpyrrolidone is also used. Anhydrous lactose is capable of direct compression and has good flowability. It does not lose the properties of tableability even when grinding to a fine powder, although at the same time its fluidity decreases. Spray dried lactose consists of microcrystals - particles of an amorphous and glassy structure.

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