1. 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.

2. 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.

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

4. Tablet Properties
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)

5. 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.

6. 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.

7. 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.

8. 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.

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

10. Sugar coated pills
Film coated pills

11. 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.

12. Standard Coating pan

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

14. Accela-Cota System

15. Hi-Coater System

16. Hi-Coater System

17. Driacoater pan

18. Di-Coater System

19. 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.

20. Fluidized bed (Air Suspension Coater)

21. 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 ( psig) through small orifice, fine spray.
Degree of atomization depends on
- Fluid pressure
- Orifice size
- Viscosity of liquid.

22. - Fluid cap orifice - Viscosity of liquid - Air pressure
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.
Orifice id inch to .020 inch.
Controlling variables
- Fluid pressure
- Fluid cap orifice
- Viscosity of liquid
- Air pressure
- Air cap design
Choice depends on coating solution composition

23. 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

24. 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.

25. - 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.

26. 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.

27. 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.

29. 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

30. 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.

31. 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.

32. 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

33. 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 to 3 microns)
Acrylate polymers
Hydroxypropyl Methylcellulose Phthalate
Polyvinyl Acetate Phthalate

34. pH-sensitive materials Digestible materials Emulsified by intestine
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

35. 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.

36. 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.

37. 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.

38. 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.

39. 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.

40. 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.

41. Lakes contains 10 to 30 % of the pure dye content.
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

42. 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)

43. 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

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

45. 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.

46. ROUGHNESS Remedies Moving the nozzle closer to the tablet bed.
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.

47. 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

48. 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

49. 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.

50. 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.

51. 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.

52. 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.

53. 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

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

55. 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.

56. 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.

57. Classification of Dry Coating Techniques

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

60. 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.

61. 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.

62. 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.

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

65. The advantages of heat-dry-coating include abandoning plasticizer for lower Tg 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.

66. 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.

67. 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.

68. 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.