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Tablet coating technology – coating conditions and methods and coating process

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This article discusses pill coating technology, including coating condition requirements, coating methods, and coating processes.


Tablet coating is an operational unit in formulation technology, i.e. coating material of a certain thickness is evenly wrapped around the outer circumference of the tablet (core, vegetarian tablet). The purposes of coating are.
①Mask the bad odor of the tablet.
② To prevent moisture, shade, and isolate the air to increase the stability of the drug.
③Control the release or slow release of the drug in certain parts of the gastrointestinal tract. The drugs which are destroyed by acid or gastric enzymes in gastric juice, irritate to the stomach and can cause vomiting can be wrapped with an enteric-soluble film coating.
④ Separate drugs with contraindications. One drug can be pressed into a tablet core, and after the core is covered with an isolation layer, another drug can be added to the coating material, or the two drugs can be made into granules separately, and then mixed and pressed after coating.
⑤ Improve the appearance of tablets for easy identification.

The coating of tablets is usually divided into two categories: sugar coating and film coating according to the different coating materials, among which film coating can be divided into three categories: stomach-soluble, intestine-soluble and insoluble. Quality requirements for coating.
① The core of the tablet must have a suitable curvature in appearance, otherwise it is difficult to cover the coating layer at the edge, and even the coating layer will break at the edge. The cores should have a certain degree of hardness and the lowest absorption of the solvent used in the coating process. The brittleness of the core is required to be minimal, which is more important than hardness to avoid rupture due to collision.
②The coating layer should be uniform and firm, and not react with the core.
③The disintegration time limit of the coated tablets should meet the requirements.
④ by long time storage can still maintain the glossy, beautiful, consistent color, and no lobes, or discoloration.

coating conditions requirements

The angle of the coating plane is generally 45°, which is directly related to the exchange and impact of the tablet. If the angle is greater than 45 °, the tablets can not roll well in the pot, so the tablets are not exchanged well, the added powder and coating materials can not be evenly spread and adhere to the tablets, so the angular package is not flat and difficult to shape when wrapping tablets; if the angle is less than 45 °, the contents of the pot are reduced, while the tumbling is not sufficient, making the drying speed slower.

The rotational speed of the coating pot directly affects the coating efficiency. The pot is equipped with a speed regulating device, and the purpose of speed regulation is to control a certain centrifugal force, which should enable the tablets in the pot to turn to the highest point and fall in an arc motion to do uniform and effective turning so that the coating materials added are evenly distributed. If the rotational speed is too slow, the centrifugal force generated is small, and the tablets fall down before reaching a certain height, which does not achieve the effect of good exchange and rounding of tablets, and thus the coating material is not uniformly distributed; if the rotational speed is too fast, the centrifugal force generated is large, so the tablets cannot fall down against the wall of the pot and lose the role of rolling and turning, which also does not play the role of uniform coating.

In addition, when using a larger capacity coating pot, the centrifugal force on the tablet is large, and the rotational speed should be set slower; when using a smaller capacity coating pot, the rotational speed should be set faster. For round tablets, the rotational speed should be set faster. For tablets with large angles, the rotational speed should not be set too fast. A reasonably installed coating pot should ensure that no dead stagnant flow is formed during the turning motion of the tablets and that once too much coating material accumulates somewhere, it should be quickly and evenly distributed.

Different parts of the coating pot have different rolling speeds, i.e., the speed is slower at the bottom of the pot, the back, and the vortex section, etc. Therefore, the syrup and so on should be added to help to stir, and if this uneven movement condition is not changed in time, the weight and thickness of the coating layer may not be the same.

The coating method

Commonly used coating methods are the rolling coating method (pot coating method), fluidized coating method, and press coating method (dry pressure coating method). This article mainly introduces the rolling coating method and fluidized coating method.

Roll coating method

The roll coating method is a classic and widely used coating method, which can be used for sugar coating, film coating, and enteric coating, including the ordinary roll coating method and high-efficiency horizontal coating method.

(1) Ordinary rolling coating method. This method mainly adopts a tilting coating pot, which is generally made of stainless steel with good thermal conductivity. There are rosette-shaped and water chestnut-shaped coating pans. The angle between the axis of the coating pot and the horizontal line is 30°-45° so that the tablets can both roll with the direction of rotation of the pot and move in the direction of the axis during the coating process so that the mixing effect is better. The rotation speed of the coating pot is generally controlled at 20-40 r/min to enable the tablets to rise to a certain height in the pot with the rotation of the pot and then fall with the arc motion so that the coating materials can be evenly distributed on the surface of the tablets, and there is suitable friction between the tablets and the tablets.

The coating pot is heated by an electric stove or gas, and dry hot air is introduced to accelerate the evaporation of the solvent in the coating material. Exhaust devices and powder hoods are installed to accelerate the discharge of water vapor and absorb dust, which not only accelerates drying but also facilitates personnel protection.

The operation method of tablet coating is as follows: place the tablet core in the rotating coating pot, add the coating material solution, make it evenly dispersed and adhere to the surface of each tablet core, then heat and ventilate it to dry. This process is repeated until the specified requirements are met.

(2) High-efficiency horizontal coating method. High-efficiency coating machine is divided into two types according to the form of heat exchange: perforated coating machine and non-perforated coating machine.

Pore coating machine with high heat exchange efficiency, mainly used for tablets, larger pills, and other organic film coatings, water-soluble film coating and slow-release coating, controlled-release coating.

Non-porous coating machine has lower heat exchange efficiency and is commonly used for sugar coating of tiny pills, small pills, drip pills, pellets, organic film coating, water-soluble film coating and slow-release coating, controlled-release coating.

High-efficiency coating machine is divided into production-type high-efficiency coating machine and experimental-type high-efficiency coating machine according to production scale. The production type high-efficiency coating machine is a kind of high efficiency, energy saving, cross all, clean, and GMP compliant electromechanical integration equipment.

The working principle of the high-efficiency coating machine is that the coated tablet cores are placed inside the drum of the coating machine and controlled by the programmable control system to make complex trajectory movements continuously, continuously, and repeatedly. During the movement, the programmable control system is controlled by the control system, and the medium is sprayed on the surface of the core in the form of mist by a spray gun according to the requirements of the process sequence and parameters. At the same time, the hot air cabinet provides clean hot air with 100,000-grade filtration, which penetrates the core gap layer, and the sprayed media and hot air on the core surface are fully contracted and dried gradually. Thus, a solid and smooth film is formed on the surface of the core.

From the above process sequence, it can be seen that the working principle of the high-efficiency coating machine is mainly through the tablets added to the pot are driven up to a certain height with the movement of the rotating drum, and then sliding down while rotating on the sloping surface of the material layer due to the effect of gravity.

High efficient horizontal coating pot is a new type of coating pot developed to improve the shortcomings of the traditional inclined coating pot with poor drying ability, its drying speed is fast and the coating effect is good, and it has become the mainstream equipment of coating device. The wall of the rotating pot is equipped with a baffle plate that drives the particles upward, and the sprayer is installed on the upper part of the inclined surface of the particle layer to spray the coating solution to the surface of the material layer. The drying air enters from the air inlet in front of the rotating pot and is discharged from the sandwich of the pot through the particle layer. This device is suitable for film coating and sugar coating.

Fluidized coating method

The principle of the fluidized coating method is similar to that of fluidized spray granulation, with program control and a higher degree of automation. Due to the effect of heat flow, the surface dries rapidly, and a number of layers are wrapped in this way until they meet the specified requirements.

coating process

Sugar coating

Sugar coating automatic coating process and manual operation principle are the same and are determined by the experience of the amount of sugar syrup or mixed suspension slurry, sizing, and making it as soon as possible in all the tablets on the core evenly distributed after drying. When using the buried tube method for sugar coating, the conventional method of sugar coating can be changed to the continuous method of sugar coating, so that the coating time can be greatly shortened. In the buried tube method, the continuous sizing spraying, and drying are completed in one step, and the water is quickly vaporized without penetrating into the cores.

The individual coating layers of the buried tube method and the operation process are as follows.

(1) Isolation layer. The isolation layer refers to the surface of the core, a layer of the coating to play a role in isolation. The materials used as the isolation layer are a 10% ethanol solution of corn prion, 15%-20% ethanol solution of shellac, 10% cellulose acetate phthalate (CAP), ethanol solution, and 10%-15% gelatin slurry. The most commonly used of these is the isolation layer of corn Nguyen. Because the package isolation layer uses organic solvents, so should pay attention to the explosion-proof fire, using low-temperature drying (40 ℃ -50 ℃), each layer drying time of about 30 minutes, generally package 3-5 layers. For general tablets, most of them do not need to wrap the isolation layer.

(2) Powder coating layer. The powder coating layer refers to the coating layer after the isolation layer, in order to make the corners of the tablets flat, add the powder as a filler form when coating. Tablets that do not need to wrap the isolation layer can directly wrap the powder coating layer. The common material of the powder coating layer is talcum powder, which contains calcium carbonate and magnesium carbonate with a mass fraction of 10%-20%.

(3) Sugar coating layer. The sugar coating layer refers to the coating layer formed by moistening and drying the outside of the powder coating layer with simple sugar syrup (without adding talc), so that the core of the tablet is covered with a layer of sucrose powder crystals.

(4) Colored sugar coating layer. The colored sugar coating layer can make the tablets have certain characteristic colors, which is easy to identify each other and can make the surface of the tablets bright and beautiful. After the completion of the sugar coating layer, a fine white cream appears on the surface of the tablet, and the surface of the tablet appears glossy when it meets water vapor, at which time the operation of the colored sugar coating layer can be carried out. Each time the pigment color is gradually added from light to dark-colored syrup, the amount added should ensure that all the tablets are wet and the layers are dry, so as not to produce different shades of flower spots on the tablet surface. The last few times the amount of colored syrup should be less and the color should be lighter, and then slowly dry. In general, it is necessary to stop and close the mouth of the icing pot and turn it over once at intervals of a few moments, so as to avoid the powdered sugar falling or the coating layer being broken under continuous rolling and to prevent sticking together into a lump. Generally, 8-15 layers of sugar coating are wrapped.

In order to make the sugar-coated tablets more beautiful and shiny, a very thin layer of wax can be applied outside the sugar coating, generally using insect wax, and a small amount of silicone oil can be added to make the surface of the tablets shinier.

Notes on the coating process: Each time the liquid or powder coating material is added, it should be evenly distributed; after each time the liquid is added and evenly distributed, it should be thoroughly dried before the solution or powder coating material is added again. The solution viscosity should not be too large, otherwise, it is not easy to distribute evenly. In the production of the powder coating layer, etc. often use the mixed slurry method, that is, the powder coating material is mixed and suspended in the adhesive solution, added to the rotating tablets, this method can reduce dust and simplify the process.

Film coating process

The film coating process is to wrap a very thin polymer coating material outside the core of the tablet to form a film, compared with sugar-coated tablets, its advantages are.
① easy operation, material saving, and lower cost.
②Tablet weight only increases by 2%-4%; saving packaging materials.
③favorable for making gastric, enteric, or long-acting slow-release formulations.
④The logo pressed on the core of the tablet is still clearly visible after the film coating is applied.
⑤ facilitate the automation of the production process.

The film coating process is as follows.
(1) Fill the coating pot with a suitable amount of shaped baffles to facilitate the rotation and turning of the tablet cores.
(2) Put the cores sieved from fine powder into the coating pot and spray a certain amount of solution of film coating material to make the surface of the cores wet with a spoon.
(3) blow into the gentle hot air (the temperature should preferably not exceed 40 ℃), so that the solvent evaporates, so repeat the above operation several times (the amount decreases one by one), until a certain thickness is reached.
(4) Most of the film coatings need a fixed period, the length of time varies depending on the material, method, and thickness.
(5) Organic solvents with residues must be completely removed, and generally also dried at 50 ℃ for 12-24 hours.

Film coating materials need to be used to form the film, plasticizers, solvents, and other materials. The commonly used coating materials are as follows.

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