Tablet Coating1

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Tablet Coating:

Tablet Coating -- Dr J Raju

Objectives of tablet coating:

Objectives of tablet coating To mask the taste, odor, or color of the drug. To provide chemical and physical protection to the drug. To control the release of the drug from tablet i.e; controlled release. To protect drug from the gastric environment of the stomach with an acid-resistant enteric coating.


Contd.. To incorporate another drug or formulate adjuvant in the coating to avoid chemical incompatibility or to provide sequential drug release. To improve the pharmaceutical elegance by use of special colors and contrasting printing.

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Tablet properties Coating process Coating equipment Parameters of coating process Facility and ancillary equipment Automation in coating process 3. Coating composition Primary components involved in tablet coating

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For coating tablet must possess the proper physical characteristics. In coating process, the tablets roll in a coating pan or cascade in the air stream of an air suspension coater as coating composition is applied. Tablets must be resistant to abrasion and chipping (ie; to tolerate intense attrition of tablet – tablet and tablet and wall of equipment) Tablet Properties

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Film coatings adhere to all exposed surfaces, so that any surface imperfection is coated and not eliminated, Sugar coating with high solid content, fill many imperfections. Quality of thin film coating applied to compressed tablets usually depends much more on the quality of starting tablet. In addition to smooth surface, the physical shape of tablet is important. Tablets in coating pan, become covered with tacky polymeric films and before surface dries, the applied coating changes from a sticky liquid to a tacky semisolid, and eventually to non-sticky dry surfaces.

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So tablets must be in constant motion during the early drying phase or tablet agglomeration can occur. Coated tablets have rounded surfaces; the more convex the surface is, the fewer difficulties will be encountered with tablet agglomeration. Coating composition must wet the surface, to adhere.

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A very even application of the coating material is an important feature of the coating process. Coatings must be dense and without mechanical damage and cracks. Film coating is an effective process for the application of protective films for manipulating the product characteristics.

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Sugar coating is an effective process for the application of thick coating layers, primarily for masking taste. The syrup is sprayed onto the particles. The introduction of the process air evaporates the fluid and dries the sugar coating.

Film Formation:

Film Formation latex particles dispersed in aqueous phase formation of thin film with water evaporation through film continuous film

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Sugar coated pills Film coated pills

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Coating Equipment Stainless steel – 40 inches in diameter, with variable speed control, with 2 to 3 atomizing nozzles. Application of coating composition to a moving bed of tablets with the concurrent use of heated air to facilitate evaporation of solvent.

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Standard Coating pan

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Immersion Tube System Coating solution simultaneously with heated air from immersed tube Rapid processing time

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Accela-Cota System

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Hi-Coater System

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Hi-Coater System

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Driacoater pan

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Di-Coater System

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Fluidized bed (Air Suspension Coater) Tablet cores that are friable and prone to chipping and edge abrasion may be difficult to coat even under optimum conditions, owing to relatively rough tablet to tablet impact.

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Fluidized bed (Air Suspension Coater)

Spray Application systems:

Spray Application systems High pressure airless system Low pressure air atomized system Difference in the system is atomization of liquid. Airless system - Liquid pumped at high pressure (250-3000 psig) through small orifice, fine spray. Degree of atomization depends on - Fluid pressure - Orifice size - Viscosity of liquid.

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Air atomized - Liquid pumped through large orifice at low pressure (5-50 psig). Air contacts liquid stream at tip of atomizer and fine spray is produced. Controlling variables - Fluid pressure - Fluid cap orifice - Viscosity of liquid - Air pressure - Air cap design Choice depends on coating solution composition

Sugar Coating:

Sugar Coating The sugar coating process involves several steps, the duration of which ranges from a few hours to few days. A successful product depends on the skill of the coating operator. A successful sugar coating process yields elegant, highly glossed tablets. Basic sugar coating process involves the following steps Sealing Subcoating Syruping (smoothing) Finishing Polishing

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SEAL COATING Why seal coating is done ? Need of the seal coating ? To prevent the moisture penetration into the tablet core, a seal coat is applied. It’s needed in the pan-ladling process in which localized overwetting of a portion of the tablet bed occurs. Examples: Shellac Zein is an alcohol-soluble protein derivative from corn that has also been used as an effective sealant. Lengthening dissolution times have not been seen on aging of zein seal coated tablet.

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SUBCOATING - Why sub-coating is necessary? It’s is applied to round the edges and build up the tablet size. It increase the tablet weight by 50 to 100%. Steps of the sub-coating Alternatively applying a sticky binder solution to the tablets followed by a dusting of the sub-coating powders and then drying. The same procedure is done to increase the thickness of the tablet to the desired one. Sub-coating is also done by the spraying method. In both methods the drying rate is very critical. Subcoating solutions: Gelatin, Acacia in syrup & water Subcoating powders: Kaolin, Dextrin sugars etc.

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SYRUP SMOOTHING - What is the need of syruping ? It covers and fill the imperfection in the tablet surface caused by the subcoating step, and to impart the desired color to the tablet. Glossing syrup The first syrup coats which contain the suspended powders are called Glossing syrup. Colorants can be added to this syrup. No color can be added until the tablets are quite smooth. In the second step, syrup solution containing the dye are applied until the final size and color are achieved. In the final step, a few clear coats of syrup may be applied.

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POLISHING The tablets can be polished in clean standard coating pans, or canvas-lined polishing pans, by carefully application of the powdered wax (beeswax or carnauba) or warm solution of these waxes in naphtha or other suitable volatile solvents. Example : An infinite no of variation in the materials and processes are possible.

Film Coating:

Film Coating Coating materials may be a physical deposition of the materials on tablet surface or they may form a continuous film depending on composition of coating formulation. e.g. Sugar, Shellac, Wax coatings Different types of coating materials are Synthetic polymers Solvents Plasticizer Colorants Opaquant extenders Miscellaneous coating solutions

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Ideal characters of coating material Solubility in the coating solution Solubility required for intended use - Free water solubility, Slow water solubility, pH- dependent solubility Capacity to produce elegant looking product Stability in presence of water, heat, moisture, air, and substrate being coated and no change in properties with aging. Essentially no color, odor, or taste Compatibility with common coating solution additives Nontoxic and ease of application Resistance to cracking and should act as barrier No bridging or filling of the debossed tablet surfaces by the film former Ease of printing procedure on high-speed equipment Low cost & Ease of application without specialized equipment.

Film Formers (Non-enteric materials):

Film Formers (Non-enteric materials) Hydroxypropyl methylcellulose, USP : It is ideal coater and used as glossing solutions, and polymer has tendency to bridge or fill the debossed tablet surfaces. - HPMC+Plasticizers = eliminate bridge or filling problems. Methyl Hydroxyethyl Cellulose : Same properties as HPMC but not frequently used due to it’s fewer solvents. Ethyl cellulose, NF : Various viscosity grades are available depend on the ethoxy substitution and it’s water and GI-Fluid insoluble and so it’s added along with water soluble additives eg: HPMC.

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Hydroxypropylcellulose, FCC : It’s water and GI- Fluid soluble and organic solvents below 40ºC and insoluble above 45 ºC. Povidone, USP : Povidone (K-30) is mostly used as binder and a tablet coating, and forms clear, glossy, and hard film. Sodium carboxymethylcellulose, USP Polythene glycols & Acrylate polymers

Film Formers (Enteric materials):

Film Formers (Enteric materials) Cellulose Acetate Phthalate (CAP): Widely used in industry but has disadvantage of dissolving above pH 6, and thus delays absorption. - Latest CAP is available as Aquateric (composed of solid or semisolid polymer spheres of cellulose acetate phthalate , size - 0.05 to 3 microns) Acrylate polymers Hydroxypropyl Methylcellulose Phthalate Polyvinyl Acetate Phthalate

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Types of enteric coating materials Water-resistant pH-sensitive materials Digestible materials Emulsified by intestine Some slowly solvated Ideal enteric polymer should dissolve or become permeable near and above pH 5.0

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Reasons for enteric coating Protect acid-labile drugs. Prevent gastric distress or nausea due to irritation from a drug e.g. sodium salicylate. Deliver drugs intended for local action in the intestine. Deliver drugs that are absorbed in the small intestine to their primary absorption site in most concentrated form. Provide a delayed-release component.

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SOLVENTS Primarily functions as to dissolve or disperse the polymers and other additives to the substrate surface. Important considerations of the Ideal solvent system are Should dissolve or disperse the polymer system. Should easily disperse other coating solution components. Should not create the processing problems. Should have no color, taste, odor, inexpensive, nontoxic, inert, and nonflammable. Should be easily dried. Should have environmental friendly. Examples: water, ethanol, methanol, isopropranol, chloroform, acetone, methylethyl ketone, and methylene chloride.

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PLASTICIZERS Internal Materials : Chemical modification of the polymer that alters the physical properties. Degree of substitution Type of substitution Chain length. External Materials : They are non-volatile or the other polymer, which when include with primary polymeric film former, changes the Flexibility Tensile strength, or Adhesion properties of the resulting film.

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Choice of the plasticizer material depends upon The ability of plasticizer material to solvate the polymer, as well as they impart flexibility by relieving the molecular rigidity. Viscosity of the plasticizer; It’s influence on the final coating solution; It’s effect on film permeability, tackiness, flexibility, solubility, and taste; and It’s toxicity, compatibility with other coating solution components, and stability of the film and final coated product.

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Concentration Of Plasticizer Expressed As - The amount of polymer being plasticized. Recommended Level of Plasticizer : 1 to 50% by weight of the film former. EXAMPLES Castor oil; propylene glycol of 200 and 400 series; and surfactants eg; Tweens; Spans; and organic acid esters. Water- soluble plasticizer : PEG, propylene glycol. Organic- soluble plasticizer : castor-oil and Spans.

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COLORANTS Colorants may be soluble in the solvent system or suspended as insoluble powders. Used to provide distinctive color and elegance to a dosage form. To achieve proper distribution of suspended colorants in the coating solutions requires the use of fine-powdered colorants (< 10 microns ). Most of colorants are synthetic DYES or LAKES OF DYES approved by the FD&C and D&C.

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LAKES : derived from dyes by precipitating with carriers. Eg ; alumina or talc. Lakes contains 10 to 30 % of the pure dye content. For very light shade, concentration : less than 0.01 %. For dark shade, concentration : more than 2.0 % Examples Inorganic materials: iron oxides Natural coloring materials :Anthocyanins, caramel, carotenoids, chlorophyll, indigo, flavones, turmeric, and carminic acid. Various concentrates promoted as achieving less lot-to-lot color variation Opalux – Opaquant color concentrate for sugar coating. Opaspray- Opaque color concentrate for film coating. Opadry – Complete film coating concentrate

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OPAQUANT-EXTENDERS These are very fine inorganic powders used in the coating solution formulation to provide more pastel colors and increase film coverage. Opaquant provides a white coating or mask the color of the tablet core, and thus the less amount of the colorants are required. Examples: Silicates (talc, aluminium silicate); Carbonates (magnesium carbonate); Sulfates (calcium sulfate); Oxides (Mg oxides)

Press Coating:

Press Coating use of compression to form coat around a pre-formed core used mainly to separate chemically incompatible materials also dual release patterns possible

Film Defects:

Film Defects Sticking and picking Roughness Orange –peel effects Bridging and filling Blistering Hazing/Dull film Colour variation Cracking


STICKING & PICKING Over wetting or excessive film thickness causes tablets to stick each other or to the coating pan. On drying at the point of contact, a piece of film may remain adhere to pan or tablet, giving “picked” appearances to the tablet surface. Resulting in a small exposed area of the core. Remedies Reduction in liquid application rate. Increase in drying air temperature and air volume.


ROUGHNESS A rough or gritty surface observed when the coating is applied by spray. Some of the droplets may dry too rapidly before reaching the tablet bed and deposits on tablet surface. On tablet surface spray- dried particles of finely divided droplets of coating solution. Surface roughness also increases with pigment concentration and polymer concentration in the coating solution. Remedies Moving the nozzle closer to the tablet bed. Reducing the degree of atomization can decrease the roughness due to spray drying.


ORANGE PEEL EFFECT Inadequate spreading of coating solution before drying causes a bumpy or Orange –peel effects On the coating Causes Indicates that spreading is impaired by rapid rate of drying or by high solution viscosity. Remedies Thinning of coating solution with additional solvents may correct this problem. Adjustment of speed of coating pan


BRIDGING & FILLING During drying film may shrink and pull away from the sharp corners of bisect, resulting in a “Bridging” of surface dispersion. These defects can be so severe that the monogram or bisect is completely obscured. Remedies Increase in plasticizer contents Change in plasticizer


FILLING Applying too much solution, resulting in thick film, causes filling. Fills and narrows the monogram or bisects. In addition, if solution applied too fast, over wetting may cause the liquid to quickly fill and be retained in the monogram. Remedies Judicious monitoring of the fluid application rate. - Thorough mixing of tablets in the pan prevent filling.


BLISTERING Evaporation of solvents from the core in the oven. And effect of high temperature on the strength, elasticity and adhesion of the film may results in blistering. Remedies Controlled milder drying conditions.


HAZING/DULL FILM Also called as bloom. It can occur when too high a processing temperature is used for a particular formulation. Dulling is particularly evident when cellulosic polymers are applied out of aqueous media at high processing temperature. Also occur if the coated tablets are exposed to high humidity conditions and partial solvation of film results.


COLOR VARIATION Problem caused by process conditions or the formulation Improper mixing, uneven spray pattern and insufficient coating may results in color variation. The migration of soluble dyes, plasticizer and other additives give the coating a mottled or spotted appearance. Remedies Use of lake dyes eliminates dye migration. Reformulation with different plasticizer and additives is the best way to solve film instability.


CRACKING Cracking occurs if internal stresses in the film exceed the tensile strength of the film. The tensile strength of the film can be increased by using higher molecular weight polymers or polymer blends. Remedies Adjusting the plasticizer type and concentration can minimize internal stresses. Adjusting the pigment types and concentration can minimize internal stresses.

Specialized Coatings:

Specialized Coatings Compression Coating – Tablet within a tablet Electrostatic coating – Conductive substrates Dip coating Vacuum film coating

Solventless Coating Techniques:

Solventless Coating Techniques Most film coatings are applied as aqueous- or organic-based polymer solutions. Solventless coating technologies can overcome many of the disadvantages associated with the use of solvents (e.g., solvent exposure, solvent disposal, and residual solvent in product) in pharmaceutical coating. Solventless processing reduces the overall cost by eliminating the tedious and expensive processes of solvent disposal/treatment. In addition, it can significantly reduce the processing time because there is no drying/evaporation step. These environment-friendly processes are performed without any heat in most cases (except hot-melt coating) and thus can provide an alternative technology to coat temperature-sensitive drugs.

Dry Coating:

Dry Coating Dry coating avoids the use of water or, at least, allows it to be reduced to very small amounts with respect to the coating material, thus overcoming the need for time- and energy-consuming drying phases, as well as possible drug stability issues. In this technology, powdered coating materials are directly coated onto solid dosage forms without using any solvent, and then heated and cured to form a coat.

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Classification of Dry Coating Techniques

Current Dry Coating Technologies:

Current Dry Coating Technologies Plasticizer-dry-coating Electrostatic-dry-coating Heat-dry-coating Plasticizer-electrostatic-heat-dry-coating Low softening temperature ( T s) or glass transition temperature ( T g) of the polymers, allows improved flexibility and tensile strength of the coat.


PLASTICIZER DRY COATING Film formation in the plasticizer-dry-coating The film formation is the combined response of improved viscous flow and particle deformation resulted from plasticizer and heat. Applications - Both tablets and pellets can be coated. Limitations Coat thickness increases with increasing plasticizer concentration. However,surplus plasticizer leads to very soft or sticky films.


ELECTROSTATIC-DRY-COATING Schematic diagram of: (a) an electrostatic coating apparatus for solid dosage forms. (10) tablet feeding chute; (12, 12) rotary drum; (16, 16) electrostatic spraying gun; (18, 18) tray to hold particles; (20, 20) infrared ray heater; (22) tablet collection chute; (A) preconditioning station; (B) coating station; (C) fusing station.

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Applications This special design aims at making every tablet effectively grounded, and directing and restricting the charged particles onto the tablet surface without spraying onto the surrounding, by which the coating efficiency is greatly improved. Moreover, the two sides of a tablet may be coated with different color or different formulation. Drawbacks This apparatus was found unable to focus all charged powder to the tablets but the drum also received some powder. This is wasteful of powder and also makes cleaning of the apparatus time consuming.

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(1) rotating disk; (2) infrared lamp; (3) powder feeder; (4) temperature probe; (5) coating tablets; (6) glass cover Heat-Dry-Coating

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The advantages of heat-dry-coating include abandoning plasticizer for lower T g film-forming polymers or avoiding high concentrations of plasticizer because of pre-plasticization. However, it is still a challenge for heat-dry-coating technology to get a smooth, uniform and thick coating only by the help of the said heat-based adhesion.


Plasticizer-Electrostatic-Heat-Dry-Coating Coating process comprises the steps Positioning pre-heated solid dosage forms in a chamber of a rotatable, electrically grounded pan coater Spraying powdered coating materials and plasticizer on the solid dosage forms in the pan coater during rotation thereof for a pre-selected length of time using an electrostatic spray gun Curing the coated solid dosage forms to form continuous, uniform and flexile coats. It is an integration of five kinds of “forces”: - Softening or melting effects - Wetting - Electrostatic attraction forces - Hydrodynamic force - Mechanical force. These are combined to enhance the adhesion of powdered coating materials to solid dosage surface.

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Applications Conventional coating pharmaceutical polymers,such as Eudragit RS, Eudragit RL, Eudragit L, Eudragit EPO in combination with standard excipients were successfully coated onto tablets and beads. Produce smooth and uniform coating surfaces with controlled coating thickness on both larger dosage forms and smaller ones. Limitations Particularly applicable for pharmaceutical coating only with a single colour.


Conclusion The large number and diversity of dry coating approaches reviewed confirm the increasing interest towards avoiding the use of water within the coating of solid cores. Advantageous applications have been described with respect to process time, overall manufacturing costs and ability to overcome water-induced degradation of active ingredients. In-depth knowledge of the mechanisms of coating formation and, in some cases, availability of suitable coat-forming agents and industrial-scale equipment should be regarded as the main issues for the consolidation of dry coating technology in the pharmaceutical field.

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