Coated Tablet Evaluation: - Evaluation of the quality of coating on tablet involves studying not only the film but also the film-tablet interactions. A number of test methods can be employed.
1. Adhesion test with tensile strength testers have been used to measure the force required to peel the film from the tablet surface.
2. Diameteral crushing strength of coated tablets can be determined with a tablet hardness tester.
3. The rate of coated tablet disintegration and dissolution must also be assessed.
4. Stability studies must be conducted on the coated tablets to determine if temperature and humidity changes will cause film defects.
5. Exposure to elevated humidity and measurement of tablet weight gain provide relative information on the protection provided by the film.
6. Some investigators have attempted to quantify film surface roughness, hardness, and color uniformity through instrumental means. A rubbing tablet on a white paper sheet if no color is transferred to the paper that means a good resistance to abrasion.    

Specialized coating techniques: -

A. Compression coating: - Compression coating is not widely used, but it has advantages in some cases in which the tablet core cannot tolerate organic solvents or water and yet needs to be coated for taste masking, or to provide delayed or enteric properties to the product. In addition incompatible ingredients can be conveniently separated by process. This type of coating requires a specialized tablet machine.

B. Electrostatic coating: - It is an efficient method of applying coating to conductive substrates. A strong electrostatic charge is applied to the substrate. The coating material containing conductive ionic species of opposite charge is sprayed on to the charge substrate. Complete and uniform coating of corners and intagliations on the substrate is achieved.

C. Dip coating: - Coating is applied by dipping them into coating liquid the wet tablets are dried in conventional coating pans. Alternate dipping and drying steps may be repeated several times to achieve the coating of desired one. The process lacks the speed, versatility, and the reliability of spray coating techniques.

D. Vacuum film coating: - it is new coating technique that employs specially designed baffled pan. The pan is hot and water jacketed and it can be sealed to achieve a vacuum system. The tablets are placed in pan and the air in the pan is displaced by nitrogen before the desired vacuum level is obtained. The coating solution is applied by airless spray system. The vapors of the evaporated solvents are removed by vacuum system. Organic solvents can be effectively used with this coating techniques and high environment safety is also there.

E. Magnetically assisted impaction coating (maic):
Several dry coating methods have been developed such as compression coating, plasticizer dry coating, heat dry coating and electrostatic dry coating.7, 8, 9 These methods generally allow for the application of high hearing stresses or high impaction forces or exposure to higher temperature to achieve coating. The strong mechanical forces and the accompanying heat generated can cause layering and even embedding of the guest particles onto the surface of the host particles. Many food and pharmaceutical ingredients, being organic and relatively soft, are very sensitive to heat and can quite easily be deformed by severe mechanical forces. Hence, soft coating methods that can attach the guest (coating material) particles onto the host (material to be coated) particles with aminimum degradation of particle size, shape and composition caused by the build up of heat are the better candidates for such applications. The magnetically assisted impaction coating (MAIC) devices can coat soft organic host and guest particles without causing major changes in the material shape and size. Although there is some heat generated on a microscale due to the collisions of particles during MAIC, it is negligible. This is an added advantage when dealing with temperature sensitive powders such as pharmaceuticals.

F. Electrostatic dry coating: The electrostatic coating process is very useful in paint technology, food technology, metal coatings, finishing industry and coating of living cells.15-19 It is also useful in the coating of tablets as well as capsules.20 The principle of electrostatic powder coating involves spraying of a mixture of finely grounded particles and polymers onto a substrate surface without using any solvent and then heating the substrate for curing on oven until the powder mixture is fused into film. There are two types of spraying units, according to the charging mechanism a) corona charging and b) tribo charging.

a) Corona charging: This is done Characterized by the electrical breakdown and then ionization of air by imposing high voltage on a sharp pointed needle like electrode (i.e. charging pin) at the outlet of the gun. The powder particles pick up the negative ions on their way from the gun to the substrate. The movement of particles between the charging gun and the substrate is mainly governed by the combination of electrical and mechanical forces. The mechanical forces produced by the air blows the powder towards the substrate from the spray gun. For the corona charging, the electrical forces are derived from the electrical field between the charging tip of the spray gun and the earthen substance, and from the repulsive forces between the charged particles. The electrical field can be adjusted to direct the powder's flow, control pattern size, shape, and powder density as it is released from the gun.

b) Tribo charging: -Unlike corona charging guns, the tribo charging makes the use of the principle of friction charging associated with the dielectric properties of solid materials and therefore no free ions and electrical field will be present between the spray gun the grounded substance. For tribo charging guns, the electrical forces are only regarded to the repulsive forces between the charged particles. After spraying when charged particles move into the space adjacent to the substrate, the attraction forces between the charged particles and the grounded substrate makes the particle to deposit on the substrate. Charged particles are uniformly sprayed onto the earthen substrate in virtue of mechanical forces and electrostatic attraction. Particles accumulate on the substrate before the repulsion force of the deposited particles against the coming particles increase and exceed the electrostatic attraction. Finally once the said repulsion becomes equivalent to the said attraction, particles cannot adhere to the substrate any more, and the coating thickness does not increase any more.

Defects of coated tablets: -
1. Picking and sticking. This is when the coating removes a piece of the tablet from the core. It is caused by over-wetting the tablets, by under-drying, or by poor tablet quality.
2. Bridging. This occurs when the coating fills in the lettering or logo on the tablet and is typically caused by improper application of the solution, poor design of the tablet embossing, high coating viscosity, high percentage of solids in the solution, or improper atomization pressure.
3. Capping. This is when the tablet separates in laminar fashion. The problem stems from improper tablet compression, but it may not reveal itself until you start coating. How you operate the coating system, however, can exacerbate the problem. Be careful not to over-dry the tablets in the preheating stage. That can make the tablets brittle and promote capping.
4. Erosion. This can be the result of soft tablets, an over-wetted tablet surface, inadequate drying, or lack of tablet surface strength.
5. Twinning. This is the term for two tablets that stick together, and it’s a common problem with capsule shaped tablets. Assuming you don’t wish to change the tablet shape, you can solve this problem by balancing the pan speed and spray rate. Try reducing the spray rate or increasing the pan speed. In some cases, it is necessary to modify the design of the tooling by very slightly changing the radius. The change is almost impossible to see, but it prevents the twinning problem.
6. Peeling and frosting. This is a defect where the coating peels away from the tablet surface in a sheet. Peeling indicates that the coating solution did not lock into the tablet surface. This could be due to a defect in the coating solution, over-wetting, or high moisture content in the tablet core.
7. Mottled color. This can happen when the coating solution is improperly prepared, the actual spray rate differs from the target rate, the tablet cores are cold, or the drying rate is out of spec.
8. Orange peel. This refers to a coating texture that resembles the surface of an orange. It is usually the result of high atomization pressure in combination with spray rates that are too high.

Tablet coating is the key step involved in the manufacturing of tablets having controlled release, delayed release profiles. The tablet coating have number of advantages like masking odor, taste, color of the drug, providing physical and chemical protection to drug, Protecting drug from the gastric environment. 3 primary components of tablet coating are tablet properties, coating process and coating composition. In recent decades the tablet coating process have undergone many remarkable developments
Magnetically assisted impaction coating and electrostatically dry coating avoids major disadvantages of solvent based coating. The techniques like compression coating are suitable for those tablet core which are not tolerate the organic solvents while electrostatic coating is suitable for conductive substrates. The defects of coated tablets are due to variations in formulation and processing conditions so it needs good process development.The techniques like pan pouring or ladling are mainly based on skill and techniques by the operator. The achievement of optimal manufacturing efficiency and high product quality is still remains a major challenge for the future research.

1. Lachman L., Lieberman, H. A., Joseph L. K. The Theory and Practice of Industrial Pharmacy; Varghese Publishing House; Mumbai; Third Edition; Pp .297-321.
2. Lachman L., Liberman H., and Kanig J. The Theory and Practice of Industrial Pharmacy; Third Edition: 293-345, 346-373.
3. Aulton M. Pharmaceutics: The Science of Dosage Form Design; International Student Edition: 304-321, 347-668.
4. Ansel H., Allen L., Jr. Popovich N. Ansel’s Pharmaceutical Dosage Forms and Drug Delivery Systems; Eighth Edition: 227-259.
5. Remington J. Remington: The Science and Practice of Pharmacy; Nineteenth Edition: Volume II 1615-1641.
6. American Pharmaceutical Reviews; 2001; 4(3): 28-35.
7. Vyas S., Khar R. Controlled Drug Delivery Concepts and Advances; First Edition: 219-256.



Subscribe to Pharmatutor Alerts by Email