Classification of tablet coating:
A) Sugar coating
B) Film coating
Major differences between sugar coating and film coating.
It is mainly used or applied for the following reasons:
To mask unpleasant taste & odor of the ingredient.
To protect ingredient from in vitro degradation as a result of exposure to moisture, oxygen & light.
To separate incompatible ingredients; by formulating on of the ingredient in the tablet coat, while the other is included in the tablet core or the original tablet.
For example iron salts & other divalent metal which enhance the oxidation of certain vitamins such as oil soluble vitamins (K, E, D and A) can be co-formulated by including the iron salts in the tablet coat and the vitamins in the tablet core.
To improve tablet appearance & elegance so as to be acceptable to the patient.
The process of sugar coating:
It involves the building up of layers of coating material on the original tablet core, as the tablets are rotated in a revolving coating pan. The sugarcoating of tablets may be divided into the following steps: (a) dedusting (b) waterproofing and sealing if needed, (c) subcoating, (d) smoothing and final rounding, (e) finishing and coloring if desired, and (f) polishing.
The process of sugar coating can be summarized in the following steps:
Before coating starts, the tablets are first dedusted (i.e. the removal of powder or fine particles from the tablet surface; in order to avoid the production of rough coat.
Waterproofing and Sealing Coat:
A sealing coat is then applied to protect the tablet core from moisture resulting from subsequent application of aqueous syrup layers of the coat. This is achieved by applying one or two layers of 40% alcoholic solution containing one of the following polymers:
Polyvinyl acetate phthalate.
Cellulose acetate phthalate.
All these are use for enteric coating but used as a sealing at very low concentration which is not sufficient to give an enteric action.
In this step a number of subcoat layer are made which will provide a good bonding between the main coat and sealing coat and also aid in tablet rounding by building a suitable coat thickness. Subcoating is a two step process which involves:
The application of warm subcoat syrup containing sucrose, acacia and gelatin, which evenly spread over the sealing coat, when this layer become partially dry, then
A subcoat powder containing talc or CaCO3 is sprayed over the partially dry syrup subcoat. This powder is added until the tablets move freely.
The subcoat powder will adsorb the syrup solution and assist in the building up of the syrup-subcoat .This process of subcoating is repeated (3-5) times.
4. Smoothing& coloring:
The subcoat produced in step-3 is rather rough and in order to obtain a very smooth coat, the tablet is then subjected to additional layers of coating (5-10 layers), which are made of thick syrup solution (with or without starch or calcium carbonate). To attain final smoothness and the appropriate color to the tablets, several coats of thin syrup containing the desired colorant are applied in the usual manner. This step is performed in a clean pan, free from previous coating materials. The imprinting of the tablets with special codes may follow.
The coated tablets are then transferred to rotating polishing pan containing a polishing solution of carnauba wax or beeswax in petroleum spirit. The polish solution is applied (2-3) times, very lightly, to give a shiny appearance to the tablet & finally tablets are left to dry & then packed in their containers.
B) Film coating:
The sugarcoating process, as described, is not only tedious, time-consuming, and specialized, requiring the expertise of highly skilled technicians but also results in coated tablets that may be twice the size and weight of the original uncoated tablets. Also, sugarcoated tablets may vary slightly in size from batch to batch and within a batch. All of these factors are important considerations for a manufacturer. From a patient's point of view, large tablets are not as easily swallowed as are small tablets. Film-coated tablets also are far more resistant to destruction by abrasion than are sugar coated tablets. However, like sugarcoated tablets, the coating may be colored to make the tablets attractive and distinctive.
Film coating is the most popular method of tablet coating nowadays. In this process polymeric solution are sprayed or applied to the tablets to form a thin film or coat.
Enteric film coating has advantages over other tablets coating. These are:
Enteric coating can be used to control the site of release of the drug, as it drug, dissolves in the small intestine it can release the drug in the small intestine not the stomach . e.g. anthelmintics.
2) Enteric Coating can also be used to protect the drug from the acidic pH of the stomach as it does not dissolve in the stomach, e.g. Erythromycin base.
Enteric Coating can also protect the stomach from irritant drugs
As plasticizers such as polyethylene glycol, propylene glycol and glycerin is added to film coating, it will produce a flexible and elastic film which resists splitting or chipping – unlike sugar coating.
Unlike sugar coating, film coating is quick, simple, & time saving.
Film coating also enhances invitro stability of drugs against hydrolysis, oxidation &and photolysis (i.e. degradation of drugs by light).
The film-coating solutions consist of:-
Polymeric materials which could be non-enteric or enteric.
Polymeric materials used in film coating are classified into two major groups:
Non-enteric polymeric material :
These are materials which readily dissolve in the acidic pH of the stomach, to release the drug. They are applied to protect the drug against in vitro degradation and to mask the unpleasant taste of drug or other excipients.
Example of some non- enteric polymers includes:
i) Cellulose derivatives:- Such as
A- Hydroxypropyl methylcellulose (HPMC; Hypromellose) an excellent non-enteric polymer which is readily soluble through out the G.I.T.
B- Hydroxypropyl cellulose.
E- Sodium carboxymethyl cellulose.
ii) Polyvinylpyrrolidone (PVP)
Enteric polymeric materials:
These are polymers which do not dissolve in the acidic pH of the stomach but dissolve in an alkaline pH of the intestine. They are esters of weak acids such as phthalic and methacrylic acids. Being acidic in nature these polymers remain undissociated in the acidic of pH stomach and once in the intestine, they dissolve, releasing the drug.
Examples of some enteric polymers include:
Phthalic acid esters:- Such as
Polyvinyl acetate phthalate (PVAP)
Hydroxypropyl methylcellulose phthalate.
Cellulose acetate phthalate (CAP)
Methalcrylate polymer as Methyl methacrylate-methacrylic acid copolymers
These are materials added to prevent the film coat from becoming brittle or fragile. They enhance the elasticity & flexibility of the film. Some plasticizers used in film coating include:-
1/ Polyethylene glycol (PEG)
2/ Propylene glycol (PG)
The polymer and the plasticizer (in addition to colourants, Opaquant-Extenders and others) used in film coating are dissolved in a common solvent and then sprayed onto the tablet surface as a thin film. In the past organic solvents such as ethanol & propanol have been used. But because they have some disadvantages such as they are inflammable and may cause explosion, their vapour is toxic & are expensive. These solvents are now replaced by water, which is used as a solvent to dissolve the polymer & the plasticizer.
B) Film coating:
Film coating can be done in the classical pans but since the exhaust of the vapour requires special attention, mostly perforated pans are used with the exhaust around the pan.
Perforated Pan Systems:
In general, all equipment of this type consists of a perforated or partially perforated drum that is rotated on its horizontal axis on in a closed housing. In the Accela-Cota and Hi-Coater system, drying air is directed into the drum, is passed through the tablet bed, and exhausted through perforations in the drum. The coating solution is applied to the surface of the rotating bed of tablets through spraying nozzles that are positioned inside the drum.
Perforated pan coaters are efficient drying systems with high coating capacity, and can be completely automated for both sugar coating and film coating processes.
Simplified diagram of Hi-coater system
Fluidized Bed (Air Suspension) Systems:
Diagram of fluidized bed coater
Highly efficient drying systems
Fluidization is achieved by the upward flow of drying air.
The air flow is controlled so that more air enters the center of the column, causing the tablets to rise in the center.
Coating solutions are continuously applied from a spray nozzle.
Tablets that are friable and prone to chipping and edge abrasion may be difficult to coat.