Tablet Coating Overview

Definition of Tablet Coating:

Coating is the process of compressing a granulating layer around a precompressed core tablet. The coating is the process of application of a coating composition to moving bed of tablets with concurrent use of heated air to facilitate evaporation of solvent and with the intention of conferring benefits and properties to the dosage form over the uncoated variety.

Coated tablets are defined as “tablets covered with one or more granulating layers of mixture of various substances such as natural or synthetic resins ,gums ,inactive and insoluble filler, sugar, plasticizer, polyhydric alcohol ,waxes ,authorized colouring material and sometimes flavoring material.

You may read: Problems in Tablet Manufacturing

 Basic principle of tablet coating

The principle of tablet coating is relatively simple. Tablet coating is the application of coating composition to moving bed of tablets with concurrent use of heated air to facilitate evaporation of solvent. Basic principles involve:

1. Insulation which influences the release pattern as little as possible and does not markedly change the appearance.

2. Modified release with specific requirement and release mechanism adapted to body function in the digestive tract.

3. Colour coating which provides insulation or is combined with modified release coating.

What is the rationale for coating a solid dosage form? /aspects /importance/ advantages / reasons:

Tablet coating is to be done on three main purposes like

Therapy

Technology &

Marketing

A.     Therapy

i) To minimize irritation of oesophagus and stomach

ii) Avoid unpleasant taste.

iii) Avoid inactivation of drug in the stomach

iv) Improve drug effectiveness

v) Prolong dosing interval

vi) Improve dosing interval

vii) Improve patient compliance e.g. easier to swallow,

B.      Technology

i) Reduce influence of moisture, atmosphere.

ii) Minimize dust formation and contamination with respect to tablets.

iii) Facilitates their handling on high speed automated filling and packaging equipment.

iv) Improves drug stability e.g. Protection of active ingredient from environment such as sunlight, moisture.

v) Prolong shelf life

vi) Masks batch differences in the appearance of raw materials.

c. Marketing

i) Mask unpleasant taste.

ii) Improve product identity

iii) Aid sales appeal as improved appearance and acceptability with respect to gloss and

colouration.

Sugar coating process:

Sugar coating process involves five separate operations:

1.Sealing/Water proofing: Prior to applying any sugar/water syrup, the tablet cores must be sealed, thoroughly dried and free of all residual solvents. The seal coat provides a moisture barrier and hardness the surface of the tablet in order to minimize attritional effects. Core tablets having very rapid disintegration rates conceivably could start the disintegration process during the initial phase of sugar coating. The main purpose of the sealing layer application is to protect the cores from subsequent damage, especially for water-sensitive cores in which an additional protective film often is applied prior to subcoating, and for gastro-sensitive cores in which gastro-resistant layers are incorporated. The sealants are generally water-insoluble polymers/film formers applied from an organic solvent solution. Common materials used as a sealant include Shellac, Zine, Cellulose acetate phthalate (CAP), polyvinyl acetate phthalate, Hyroxy lpropyl cellulose, Hyroxy propyl methyl cellulose, zein, synthetic resins etc.

2. Subcoating /critical step: Subcoating is the actual start of the sugar coating process and the main purpose of applying the subcoating layer is to facilitate rounding of the sharp edges of the cores. It also acts as the foundation for the smoothing and colour coats. There are two techniques for the subcoating application:

a)      Liquid/powder (traditional method)

b)       Suspension

subcoating are done by subcoating syrup that contain sugar syrup and a larger proportion of glutinous binders such as polyvinylpyrrolidone (pvp), acacia gum, gelatin, sodium carboxymethylcellulose (nacmc), hydroxyethyl cellulose (hec), hydroxyethyl ethylcellulose (heec), hydroxyethyl hydroxypropylcellulose (hehpc), or polyvinyl acetate (pva) in levels up to 10% or suspention that consists of calcium carbonate, talc, acacia gum and titanium dioxide (white pigment) suspended in water.

3. Grossing/ smoothing/syrup coat: The grossing/smoothing process is specifically for smoothing and filing the irregularity on the surface generated during subcoating. It also increases the tablet size to a predetermined dimension. Smoothing usually can be accomplished by the application of a simple syrup solution (approximately 60-70 % sugar solid). This syrup generally contains pigments, starch, gelatin, acacia or opacifier if required. Small quantities of colour suspension can be applied to impart a tint of the desired colour when there are irregularities in coating. The syruping coat involves 3 basic phase: grossing syrup (a syrup solution with subcoating powers dispersed in it0, heavy syrup and regular syrup.

4. Colour coating: Colouring: gives the tablet its colour and finished size. Mainly soluble dyes were used in the sugar coating to achieve the desired colour, Pigments provide better covering power than dyes, therefore, and only a small amount of pigment is used recent.

5.Polishing: Sugar-coated tablets needs to be polished to obtain a transparent, glossy and reflective coating with no opalescence or cloudiness remaining from previous steps. Polishing is achieved by applying the mixture of waxes like beeswax, carnubawax, candelila wax or hard paraffin wax to tablets in polishing pan.

Advantages of Sugar Coating-

1. It utilizes inexpensive and readily available raw materials.
2. Constituent raw materials are widely accepted-no regulatory problems.
3. Modern simplified techniques have greatly reduced coating times over traditional sugar coating methods.
4. No complex equipment or services are required.
5. The process is capable of being controlled and documented to meet modern GMP standards.
6. Simplicity of equipment and readily availability of raw materials make sugar coating an ideal coating method for developing countries.
7. The process is generally not as critical as film coating; recovering and reworking procedures are usually possible.
8. For high humidity climates, it generally offers a stability advantage over film coating tablets.
9. Results are aesthetically pleasing and have wide consumer acceptability.
10. Tablet cores may generally be softer than those demanded by film coating especially those for aqueous film coating.

Disadvantages: from the comparison between film coating and sugar coating

advantages of film coating/ film coating is more favored over sugar coating/ comparison between film coating and sugar coating:

FEATURES	FILM COATING	SUGAR COATING
Tablet: Appearance Weight increase because of coating material Logo or ‘break lines’	Retain contour of original core.  Usually not as shiny as sugar coat type 2-3% Possible	Rounded with high degree of polish 30-50% Not possible
Process Operator training required Adaptability to GMP Process stages Functional coatings Time	Process tends itself to automation and easy training of operator High Usually single stage Easily adaptable for controlled release Less time required	Considerable Difficulty may arise Multistage process Not usually possible apart from enteric coating More time required

Key Factors/Critical Components/parameters of Pharmaceutical Film Coating:

1.      Coating Equipment

q  Pan size, shape, baffles and angle
q  Spray nozzle diameter, Air cap design
q  Drying air system
q  Exhaust air system
q  A slight negative pressure inside pan
q  Stirring mechanism for bulk suspension
q  Good air handling unit for the area
q  Flame and explosion proofing where ever necessary

2.      Tablets Core or Substrate

q  High hardness
q  Low friability
q  Good surface adhesion property

3.      Coating Suspension/Coating System Formulation

q  Stir 40 - 50 mins prior to spraying
q  Low particle size of suspended pigments & opacifiers
q  Low settling rates
q  Good flowability

4.      Process Variables

q	Tablet load size
q	Pan RPM
q	Quantity and temp. of drying air
q	Quantity and temp. of exhaust air
q	Tablet bed temp. Profile
q	Atomization pressure
q	The spray rate
q	The spray pattern
q	The spray distance

What is film coating? Mechanisms? /film coating is based on two distinct phenomena:

A film coating is defined as a thin, continuous, nonporous membrane and uniform polymer based coat of about 20 to 100 µm in thickness, which is applied to the surface of core substrates such as tablets, granules, powder, capsules, multiparticulates or pellets.

Mechanisms: Film coating is based on two distinct phenomena:

1. Formation of film over the surface of core tablet known as cohesion
2. Bonding between the polymeric film and core tablet surface known as adhesion.

n  It is Single stage process, which involves spraying a coating solution containing the following;

1. Polymer
2. Solvent
3. Plasticizer
4. Colourant

First prepare Solution or suspension for coating. The solution is sprayed onto a rotating tablet bed followed by drying, which facilitates the removal of the solvent leaving behind the deposition of thin film of coating materials around each tablet.

When spraying is started the following occurrence are occur:

        i.            Droplets formation by a spray gun that show in figure A. Coating solution are sprayed as finely droplets with spray gun with a continuous motion over the surface of the tablet ensure almost uniform coverage of the tablet.

      ii.            Impingement of droplets over the tablet surface that show in figure B

    iii.            Spreading of polymeric surface that show in figure C

     iv.            Wetting of tablet surface by the polymeric solution that show in figure D

       v.            Coalescence, adhesion, cohesion and autoadhesion of polymer film that show in figure E

Typical Film Coating Formulation Components/Materials used in film coating

I.         Film formers, which may be enteric or nonenteric

II.        Solvents

III.       Plasticizers

IV.      Colourants

V.       Opaquant-Extenders

VI.      Miscellaneous coating solution components

Film formers :Polymers are flexible linear macromolecules having a molecular weight range between 10,000 and several million daltons. Polymers consistof a number of repeating units in the structure.

Ideal requirements of film coating materials are summarized below:

1. Solubility in solvent of choice for coating preparation
2. Solubility requirement for the intended use e.g. free water-solubility, slow water-solubility or pH -dependent solubility
3. Capacity to produce an elegant looking product
4. High stability against heat, light, moisture, air and the substrate being coated
5. No inherent colour, taste or odor
6. High compatibility with other coating solution additives
7. Nontoxic with no pharmacological activity
8. High resistance to cracking
9. Film former should not give bridging or filling of the debossed tablet
10. Compatible to printing procedure
11. Solubility in a wide range of solvent systems in order to allow flexibility in formulations
12. Ability to produce films with excellent mechanical properties
13. Stability against light, oxygen, hydrolysis
14. Optimum dissolution in the gastrointestinal tract.

Coating polymers can be categorized into two types:

a)      Non-functional or conventional film coating polymers, which can be used as a coating to improve the appearance, improve the handling, and prevent dusting of dosage forms;

b)      Functional coating polymers, which can be used to modify the pharmaceutical function of the dosage forms, especially with enteric or modified release coatings.

Based on the method of preparation, polymer dispersions can be classified into two types: true latexes and pseudolatexes.

Type	True Latex	Pseudolatex
Description	Very fine dispersion of polymer in an aqueous phase	Fine dispersion of polymer in an aqueous phase
Particle size range (nm)	10-1000	10-1000
Method of preparation	Emulsion polymerization of monomer, initiator, and catalyst	Produced from the polymer, by mechanical means Free of residual monomer   and traces of initiators
Examples	Acrylate polymers (Eudragit® L100-55, and Eudragit®          NE30D, Röhm Pharma GmbH)	Ethylcellulose dispersion (Aquacoat® ECD, FMC         BioPolymer)

The MFT is the minimum temperature above which film formation will occur under specific conditions and is dependent on the temperature at which the polymer changes from a hard glassy amorphous state to a softer rubbery state. This change in state is defined as the glass transition temperature (Tg) of the polymer.

Commonly used film formers are as follow

i.Hydroxy Propyl Methyl Cellulose (HPMC)

It is available in different viscosity grades.

 It is a polymer of choice for air suspension and pan spray coating systems because of solubility characteristic in gastric fluid, organic and aqueous solvent system.

Advantages include:

i)it does not affect tablet disintegration and drug availability,

 ii)it is cheap, flexible,

iii)highly resistant to heat, light and moisture,

 iv)it has no taste and odor, colour and

 v)other additives can be easily incorporated.

Disadvantage includes: when it is used alone, the polymer has tendency to bridge or fill the debossed tablet surfaces. So mixture of HPMC and other polymers/ plasticizers is used.

ii.Methyl Hydroxy Ethyl Cellulose (MHEC)

It is available in wide variety of viscosity grades. It is not frequently used as HPMC because soluble in fewer organic solvents.

iii. Ethyl Cellulose (EC)

Depending on the degree of ethoxy substitution, different viscosity grades are available. It is completely insoluble in water and gastric fluids. Hence it is used in combination with water-soluble additives like HPMC and not alone. Unplasticized ethyl cellulose films are brittle and require film modifiers to obtain an acceptable film formulation. Aqua coat is aqueous polymeric dispersion utilizing ethyl cellulose. These pseudolatex systems contain high solids, low viscosity compositions that have coating properties quite different from regular ethyl cellulose solution.

iv.Hydroxy Propyl Cellulose (HPC)

It is soluble in water below 40oc (insoluble above 45 oC), gastric fluid and many polar organic solvents. HPC is extremely tacky as it dries from solution system. It is used for sub coat and not for colour or glass coat. It gives very flexible film.

v. Povidone

It is a synthetic polymer consisting of linear 1- vinyl-2-pyrrolidinone group.Degree of polymerization decides molecular weight of material. It is available in four viscosity grades i.e. K-15, K-30, K-60 and K-90. Average molecular weight of these grades is 10000, 40000, 160000 and 360000 respectively.

 K-30 is widely used as tablet binder and in tablet coating. It has excellent solubility in wide variety of organic solvents, water, gastric and intestinal fluids. Povidone can be cross-linked with other materials to produce films with enteric properties. It is used to improve dispersion of colourants in coating solution.

vi. Sodium carboxy methyl cellulose

It is available in medium, high and extra high viscosity grades. It is easily dispersed in water to form colloidal solutions but it is insoluble in most organic solvents and hence not a material of choice for coating solution based on organic solvents. Films prepared by it are brittle but adhere well to tablets. Partially dried films of are tacky. So coating compositions must be modified with additives.

viii. Polyethylene glycols (PEG)

Lower molecular weights PEG (200-600) are liquid at room temperature and are used as plasticizers. High molecular weights PEG (900-8000series) are white, waxy solids at room temperature. Combination of PEG waxes with CAP gives films that are soluble in gastric fluids.

ix. Acrylate polymers

E is cationic. Eudragit. It is marketed under the name of Eudragit E is freely soluble in gastric fluid up to pH 5 andco-polymer. Only Eudragit expandable and permeable above pH 5. This material is available as organic solution (12.5% in isopropanol/acetone), solid material or 30% aqueous RLdispersion. Eudragit & RS are co-polymers with low content of quaternary ammonium groups. These are available only as organic solutions and solid materials. They produce films for delayed action (pH dependent).

Solvents

Solvents are used to dissolve or disperse the polymers and other additives and convey them to substrate surface.

Ideal requirement are summarized below:

1.         Should be either dissolve/disperse polymer system

2.         Should easily disperse other additives into solvent system

3.         Small concentration of polymers (2-10%) should not in an extremely viscous solution system creating processing problems

4.         Should be colourless, tasteless, odorless, inexpensive, inert, nontoxic and nonflammable

5.         Rapid drying rate

6.         No environmental pollution

Mostly solvents are used either alone or in combination with water, ethanol, methanol, isopropanol, chloroform, acetone, methylene chloride, etc. Water is more used because no environmental and economic considerations. For drugs that readily hydrolyze in presence of water, non-aqueous solvents are used.

Plasticizers: are nonvolatile, organic solvent.it has low molecular weight.it is also known as dispersant additives. Commonly used plasticizers are castor oil, PG, glycerin, lower molecular weight (200-400 series), PEG, surfactants, etc. For aqueous coating PEG and PG are more used while castor oil and spans are primarily used for organic-solvent based coating solution. Plasticizers are normally used at concentrations between 15-35% based on polymer weight.

Why Plasticizers need for film coating? Importance?

1.      Plasticizers are low molecular weight organic solvents with high boiling points. They are used to alter the physical properties of a polymer (i.e. hard or brittle) and render it more flexible and softer to function as a film-coating material.

2.      Plasticizers also have a significant influence on mechanical properties of the film.

3.      Specifically, they can reduce cohesive intermolecular forces along the polymer chains and enhance flexibility by increasing strain or film elongation and decreasing tensile strength and elastic modulus of the polymer.

4.      Additionally, they influence the permeability characteristics of the film, especially to water vapor, as well as lowering the glass transition temperature of the polymer to allow a more feasible coating process.

5.      Plasticizers also possess solvent power to insure compatibility with the polymer.

6.      Plasticization time (i.e. mixing time of plasticizer with polymer) and plasticizer level influence the nature of the polymer films.

Classification of plasticizer:

Three types of plasticizers are commonly used in pharmaceutical coating processes:

a). Polyols: water miscible

Glycerol (glycerin)

Propylene glycol (PG)

Polyethylene glycol (PEG 200-6000 grades)

b). Organic esters:

Diethyl phthalate (DEP) - water insoluble

Dibutyl phthalate (DBP) - water insoluble

Dibutyl sebacate (DBS)- water insoluble

Triethyl citrate (TEC)-water miscible

Acetyltriethyl citrate (ATEC) - water insoluble

Acetyltributyl citrate (ATBC) - water insoluble

Tributyl citrate (TBC) - water insoluble

Triacetin (glyceryl triacetate; TA)-water miscible

c). Oils/glycerides: water insoluble

Castor oil

Distilled acetylated monoglycerides (AMG)

Fractionated coconut oil

Mechanism of plasticizers:

If we use only film former in coating solution it’s become brittle because most acceptable film coating polymers are brittle in nature. It is generally accepted that the mechanism by which plasticizers exert their effects by interposing themselves between the polymer molecules, thus increasing free volume and faciliting increased polymer chain motion within in the structure of the coating.

Coluorants: These are synthetic dyes or lakes. Lakes are choice for sugar or film coating as they give reproducible results. Colourants can be used in solution form or in suspension form. To achieve proper distribution of suspended colourants in the coating solution requires the use of the powdered colourants (<10 microns). Most common colourants in use are certified FD & C or D & C colourants.

 Concentration of colourants in the coating solutions depends on

-the colour shade desired,

- the type of dye, and

- the concentration of opaquant-extenders.

 If very light shade is desired, concentration of less than 0.01 % may be adequate on the other hand, if a dark colour is desired a concentration of more than 2.0 % may be required. The inorganic materials (e.g. iron oxide) and the natural colouring materials (e.g. anthrocyanins, carotenoids, etc) are also used to prepare coating solution. Magenta red dye is non absorbable in biologic system and resistant to degradation in the gastro  (opaque colour concentrate for film coating) and intestinal track. Opasray  (complete film coating concentrate) are promoted as achieving less.Opadry lot-to-lot colour variation.

Opaquant-Extenders

These are very fine inorganic powder used to provide more pastel colours and increase film coverage. These inorganic materials provide white coat or mask colour of the tablet core.

Colourants are very expensive and higher concentration is required. These inorganic materials are cheap. In presence of these inorganic materials, amount of colourants required decreases. Most commonly used materials are titanium dioxide, silicate (talc &aluminum silicates), carbonates (magnesium carbonates), oxides (magnesium oxide) & hydroxides (aluminum hydroxides). Pigments were investigated in the production of opaque films and it was found that they have good hiding power and film-coated tablets have highlighted intagliations.

Pigments or opacifiers are used in film coating to:

          Enable product identification

          Protect the active ingredient against light by optimizing the opacifying properties of pigments

          Modify the gas permeability of a film

          Decrease the risk of counterfeiting the product

However, the use of pigments and opacifiers could be omitted from the formulation if a clear coating is required.

Water insoluble pigments are more favourable than water soluble colours for the following reasons;

ü  Better chemically stability in light

ü  Optimised impermeability to water vapour

ü  Better opacity

ü  Better covering ability 

Miscellaneous coating solution component: Flavors, sweeteners, surfactants, antioxidants, antimicrobials, etc. may be incorporated into the coating solution.

Adhesion enhancers (e.g. saccharides such as polydextrose, maltodextrin, and lactose)

Surfactants or dissolution enhancers (i.e. xanthan gum with Eudragit® NE30D coated theophylline granules),

pore-forming agents (e.g. sucrose or sodium chloride with  ethylcellulose-coated salicylic acid tablets

Anti-tacking agents/glidants (e.g. talc, magnesium stearate, kaolin, glyceryl monostearate

preservative (i.e. sorbic acid), antifoaming agents (i.e. dimethylpolysiloxane), stabilizing agents, or waxes

Modified Release Film Coatings/Enteric coating

This type of film coating provides a delayed release action for a drug from a coated dosage form in the acidic environment of the stomach, but readily releases the drug once it passes into more basic pH environment of the upper intestine (i.e. duodenum). An enteric coating remains intact at a low pH but will undergo dissolution at a higher pH and allow the release of active ingredient from the dosage form.

Reason for enteric coating/significance

This type of coating is used for the following reasons:

1.         To prevent degradation of acid sensitive API in the acid environment of the stomach

2.         To prevent irritation of stomach by certain drugs like sodium salicylate

3.         Delivery of API into intestine

4.         To provide a delayed release component for repeat action tablet

5.          Avoid nausea, vomiting

6.          Release of active ingredient in specific target area within gastrointestinal tract.

7.             To mask taste or odour

Several kinds of enteric layer systems are now available/enteric coating method:

One layer system - The coating formulation is applied in one homogeneous layer, which can be whites-opaque or coloured. Benefit is only one application needed.

Two layer system - To prepare enteric tablets of high quality and pleasing appearance the enteric formulation is applied first, followed by coloured film. Both layers can be of enteric polymer or only the basic layer contains enteric polymer while top layer is fast disintegrating & water-soluble polymer.

An ideal enteric coated material should have the following properties:

1. Resistance to gastric fluids.

2. Ready susceptibility to or permeability to intestinal fluids.

3. Compatibility with most coating solution components and the drug substrates.

4. Stability alone and in coating solutions. The films should not change on aging.

5. Formation of a continuous (uninterrupted) film.

6. Nontoxicity.

7. Low cost.

8. Ease of application without specialized equipment.

9. Ability to be readily printed or to allow film to be applied to debossed tablets.

Polymers used for enteric coating are as follow

I .Cellulose acetate phthalate (CAP)

It is widely used in pharmaceutical industry .It dissolves above pH 6 only, delays absorption of drugs, it is hygroscopic and permeable to moisture in comparison with other enteric polymer, it is susceptible to hydrolytic removal of phthalic and acetic acid changing film properties. CAP films are brittle and usually used with other hydrophobic film forming materials.

II Acrylate polymers

Eudragit®L & Eudragit®S are two forms of commercially available enteric acrylic resins. Both of them produce films resistant to gastric fluid. Eudragit®L & S are soluble in intestinal fluid at pH 6 & 7 respectively. Eudragit®L is available as an organic solution (Isopropanol), solid or aqueous dispersion. Eudragit®S is available only as an organic solution (Isopropanol) and solid.

III.Hydroxy propyl methyl cellulose phthalate

HPMCP 50, 55 & 55-s (also called HP-50, HP-55 & HP-55-s) is widely used. HP-55 is recommended for general enteric preparation while HP-50 & HP-55-s for special cases.

 These polymers dissolve at a pH 5-5.5.

Insoluble in water

Soluble in aqueous alkaline media, acetone/water (95:5), acetone/methanol (1:1), acetone/-ethanol (1:1), or methylene chloride/ethanol (1:1)

IV Polyvinyl acetate phthalate

It is similar to HP-55 in stability and pH dependent solubility.

V. Shellac

This naturally occurring polymer is produced from a purified resinous secretion of the insect Laccifer lacca. It can be modified to meet certain specifications. Shellac is insoluble in water, but soluble in alkaline media, and moderately soluble in warm ethanol. Due to its many drawbacks, shellac is not often used in coating today

VII.Cellulose Acetate Trimellitate (CAT)

CAT polymer has similar properties to CAP polymer, especially solubility. In addition, CAT has an additional carboxylic acid group on the aromatic ring and dissolves at a pH of 5.5. To obtain the best enteric coating results from aqueous processing, ammoniated solutions of CAT in water are recommended.

Summary of Polymers used in pharmaceutical formulations as coating materials/different coating polymer with example and uses:

Polymer	Trade name	Application
Shellac	EmCoat 120 N  Marcoat 125	n   Enteric Coatings n   Taste/Odor  Masking
Cellulose acetate	Aquacoat CPD®   Sepifilm™ LP  Klucel®  Aquacoat® ECD  Metolose®	n   Enteric Coatings n   Taste masking n   Sustained release coating n   Sub coat moisture and  barrier sealant pellet coating
Polyvinylacetate phthalate	Sureteric®	n   Enteric Coatings
Methacrylate	Eudragit®	n   Enteric Coatings n   Sustained Release  Coatings n   Taste Masking n   Moisture protection n   Rapidly disintegrating Films

Classification of coating equipment: Most coating processes use one of the three general types of equipment:

1. Standard coating pan

2. Perforated coating pan

3. Fluidized bed (air suspension) coater

1.     Standard/ Conventional Pan System:

i) The standard coating pan consists of a circular metal pan mounted angularly on a stand.

ii) The pan is 8 to 60 inches in diameter and is rotated on its horizontal axis by a motor.

iii) Heated air is directed into the pan and onto the tablet bed surface and is exhausted by means of ducts positioned through the front of the pan.

Iv) Coating solutions are applied to the tablets by ladling or spraying the material on to the rotating tablet bed.

V)Use of atomizing system to spray the liquid coating material on to the tablets produce a faster, a more even distribution of the solution or suspension.

2.      Perforated Pan Systems-

All the equipment’s of this type consist of a perforated or partially perforated drum that is rotated on its horizontal axis in an enclosed housing.

3.     Fluidized Bed (Air Suspension) Systems-

Principle: Fluidized bed coaters are also highly efficient drying systems. Fluidized of the tablet mask is achieved in a columnar chamber 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. 

The Movement of the tablets is upward through the center of the chamber. They then fall towards the chamber wall and move downwards to reenter the air stream at the bottom of the chamber.  In some units, a smaller column(s) is used to direct the tablet movement within the main column. Coating solutions are continuously applied from a spray nozzle located at the bottom of the chamber or are sprayed  on to the top of the cascading tablet bed by nozzles located in the upper region of the chamber.

Lamination/disadvantage:

Tablet cores that are friable and prone to chipping and edge abrasion may be difficult to coat even under optimum conditions in the fluidized bed system, owing to the relatively rough table-to-tablet impact and tablet-chamber contact.

The consequences of non-uniform coating include visual defects such as variations in appearance as well as variations in functionality such as drug-release performance and stability.

Advantages:

The Effect of Core Design and Formulation on the Quality of Film Coated Tablets/ Importance of core design and formulation on the quality of a film coated tablet:

Tablet Design: The design of such a substrate has to be considered in terms of:

• The ability of the core to withstand the mechanical stress of the process.

• Maximized adhesion of the coating to the tablet surface, especially when a logo is present.

• A film coat with uniform thickness.

Tablet shape: Tablet shape is a very important factor for successful film coating. There are some types of tablet shape produce by pharmaceutical companies that given bellow:

                 Flat face:

Shallow concave

Standard concave

Deep concave

Extend deep

Ball shape

Caplet shape

But all of them are not preferable for film coating. During critical stages of the drying process, if tablets exhibit large areas of relative ‘flatness’ on their surfaces, it is possible for them to become bonded together. Flat faced, caplet shaped and shallow concave tablets have relatively high overall surface hardness, but tend to be brittle at the edges. The deep concave/extend deep concave and ball shaped tablets have good mixing characteristics, but offer the lowest levels of mechanical strength. Thus Flat-face, shallow/deep concave-, ball/caplet-shaped tablets are not the best choice for film coating. Therefore, standard concave is the preferred shape for film coating.

Tablet Formulation

The formulation of a robust tablet has to be considered in terms of:

• The ability of the core to withstand the mechanical stress of the process.

• Maximized adhesion of the coating to the tablet surface, especially when a logo is present.

• A smooth film coat with uniform thickness.

• The stability of the final coated dosage form on storage.

(Quantity of lubricant and disintegrates used in core tablet formulation has a great effect in film coating/ Quantity of mg stearate used in core tablet formulation should be minimized)

Lubricants are added to tablet formulations to minimize both die-wall friction and punch adhesion. Both of these requirements necessitate that the lubricants function at the tablet surface, precisely where they are counterproductive in the adhesion process considering the inherent hydrophobicity of lubricants, such as metal stearates (magnesium stearate). They can reduce the mechanical strength of the cores, decrease film adhesion and slow drug dissolution.

Magnesium stearate, although a very effective lubricant, can reduce the mechanical strength of the cores, decrease film adhesion and slow drug dissolution. Therefore, the quantity of magnesium stearate used in a tablet formulation should be minimized. Self-lubricating products such as Starch 1500 can also be used to reduce the need for significant lubricant addition.

Most pharmaceutical solid dosage formulations contain disintegrants. They are hygroscopic in nature. Modern disintegrants, often referred to as superdisintegrants, act by rapid uptake of water followed by rapid and, for some, enormous swelling up to 300 times excipient volume. Inclusion of a high level of superdisintegrants in tablet formulations can affect the physical appearance of the final coated dosage form, such as the smoothness of the film.

Superdisintegrant particles compressed into the surface of the tablet may get activated prematurely on contact with droplets of aqueous film coating solution resulting in very fast and excessive water penetration into the core and uneven surface of the coated product .Water penetration into the tablet core can lead to potential storage problems with formulations that contain moisture-sensitive materials.

Thus we should use very low amount of disintegrates.

Superdisintegrant (sodium starch glycolate or croscarmellose sodium)

Specialized coating

Compressed coating

This type of coating requires a specialization tablet machine. Compression coating is not widely used but it has advantages in some cases in which the tablet core cannot tolerate organic solvent or water and yet needs to be coated for taste masking or to provide delayed or enteric properties to the finished product and also to avoid incompatibility by separating incompatible ingredients.

Electrostatic coating

Electrostatic coating 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 onto the charged substrate. Complete and uniform coating of corners and adaptability of this method to such relatively nonconductive substrate as pharmaceutical is limited.

Dip coating

Coating is applied to the tablet cores by dipping them into the coating liquid. The wet tablets are dried in a conventional manner in coating pan. Alternative dipping and drying steps may be repeated several times to obtain the desired coating. This process lacks the speed, versatility, and reliability of spray-coating techniques. Specialized equipment has been developed to dip-coat tablets, but no commercial pharmaceutical application has been obtained.

Vacuum film coating

Vacuum film coating is a new coating procedure that employs a specially designed baffled pan. The pan is hot water jacketed, and it can be sealed to achieve a vacuum system. The tablets are placed in the sealed pan, and the air in the pan is displaced by nitrogen before the desired vacuum level is obtained. The coating solution is then applied with airless spray system. The evaporation is caused by the heated pan, and the vapour is removed by the vacuum system. Because there is no high-velocity heated air, the energy requirement is low and coating efficiency is high. Organic solvent can be effectively used with this coating system with minimum environmental or safety concerns.

Basic process requirements for film coating

The fundamental requirements are

1. independent of the actual type of equipment’s being used and include adequate means of atomizing the spray liquid for application to the tablet core,
2. adequate mixing and agitation of tablet bed,
3. Sufficient heat input in the form of drying air to provide the latent heat of evaporation of the solvent. This is particularly important with aqueous-based spraying and good exhaust facilities to remove dust and solvent laden air.

Coating formula optimization

Basic formula is obtained from past experience or from various sources in the literature. Modifications are required to improve adhesion of the coating to the core, to decrease bridging of installations, to increase coating hardness, etc. Usually concentration of colorant and opaquant are fixed to get predetermined shade. Common modification is to alter polymer-to-plasticizer ratio or addition of different plasticizer/ polymer. Experimentation of this type can be best achieved by fractional factorial study.

How does plasticizer influence on mechanical properties of polymeric film?

Answer: why plasticizer need for film coating ar 2-6 steps

Three types of sugar coating techniques are commonly used:

a) Plain sugar coating (application of syrup at room temperature): This coating technique includes 3 steps: application of coating formulation onto the cores, distribution of formulation on the core surfaces, and drying to increase the strength of each coating layer

b).Two-component coating or lamination process (application of a syrup or binder solution first in a slight excess amount, and then dusting with a powder to bind the excess solution):

c). Hot sugar coating (application of heated syrup):

For the hot sugar coating technique, syrup is heated above room temperature to reduce the viscosity of the syrup.

Problem and remedies       :

Blistering: It is local detachment of film from the substrate forming blister.

Sr. No.	CAUSE	REMEDY
1.	Effect of temperature on the strength, elasticity and adhesion of the film.	Use mild drying condition.

Chipping: It is defect where the film becomes chipped and dented, usually at the edges of the tablet.

Sr. No.	CAUSE	REMEDY
1.	High degree of attrition associated with the coating process.	Increase hardness of the film by increasing the molecular weight grade of polymer.

Cratering: It is defect of film coating whereby volcanic-like craters appears exposing the tablet surface.

Sr. No.	CAUSES	REMEDIES
1.	Inefficient drying.	Use efficient and optimum drying conditions.
2.	Higher rate of application of coating solution.	Increase viscosity of coating solution to decrease spray application rate.

Picking and sticking: It is defect where isolated areas of film are pulled away from the surface when the tablet sticks together and then part.

Sr. No.	CAUSE	REMEDY
1.	Inefficient drying.	Use optimum and efficient drying conditions
2.	Higher rate of application of coating solution	Decrease the rater of application of coating solution by increasing viscosity of coating solution.
3	Application rate too high	Reduce spray rate
4	Drying air volume too low	Increase air volume
5	Drying air temperature too low	Increase the inlet air temperature.
6	Pan speed too low	increase
7	Low atomization pressure	increase
8	Poor spray gun set up	Increase spray gun
9	Surface solubility	Increase gun to bed distance
10	Surface porosity high	Improve core characteristics

Pitting: It is defect whereby pits occur in the surface of a tablet core without any visible disruption of the film coating.

Sr. No.	CAUSE	REMEDY
1.	Inappropriate drying (inlet air ) temperature	Dispensing with preheating procedures at the initiation of coating and modifying the drying (inlet air) temperature such that the temperature of the tablet core is not greater than the melting point of the batch of additives used.

Blooming/hazing/dull film: It is defect where coating becomes dull immediately or after prolonged storage at high temperatures.

Sr. No.	CAUSE	REMEDY
1.	High concentration and low molecular weight of plasticizer.	Decrease plasticizer concentration and increase molecular weight of plasticizer.

Blushing: It is defect best described as whitish specks or haziness in the film.

Sr. No.	CAUSES	REMEDIES
1.	High coating temperature	Decrease the drying air temperature
2.	Use of sorbitol in formulation which causes largest fall in the thermal gelation temperature of the Hydroxy Propyl Cellulose, Hydroxy Propyl Methyl Cellulose, Methyl Cellulose and Cellulose ethers.	Avoid use of sorbitol with Hydroxy Propyl Cellulose, Hydroxy Propyl Methyl Cellulose, Methyl Cellulose and Cellulose ethers.

Colour variation/ a defect which involves variation in colour of the film.

Sr. No.	CAUSE	REMEDY
1.	Inadequate mixing of tablet	Go for geometric mixing,
2	Poor opacity of suspension	Increase opacity of suspension
3	migration of soluble dyes-plasticizers and other additives during drying	Reformulation with different plasticizers and additives or use mild drying conditions.
4	Poor spray gun set up	Correct gun set up-spacing, gun to bed distance fan width
5	Low/high pan load	Load the correct quantity of tablet
6	High core friability	Improve core friability
7	Insufficient number of spray guns	Increase number of spray guns
8	High coating suspension solid	Reduce suspension solid level
9	Low film weight gain	Increase quantity of film coat

Spotty tablet/ mottling: Unequal distribution of colour on the tablet surface with light and dark areas standing out in an otherwise uniform coloured surface.

cause	solution
1)Variation in the colours of ingredients (drug and other additives) 2)The improper size either large or small particles enhances color distribution	1)By using bright coloring agent that will mask all the color variations of the ingredients 2) Grinding to small particle size

3) pigment are poorly dispersed	3)make proper dispersed
4)liquid plasticizer migration	Chose a solid plasticizer system
Poor opacity of suspension	Increase opacity of suspension

Infilling: It is defect that renders the intagliations indistinctness.

Sr. No.	CAUSE	REMEDY
1.	Bubble or foam formation because of air spraying of a polymer solution	Add alcohol or use spray nozzle capable of finer atomization.

Orange peel/Roughness: It is surface defect resulting in the film being rough and nonglossy. Appearance is similar to that of an orange.

Sr. No.	CAUSES	REMEDIES
1.	Rapid Drying	Use mild drying conditions
2.	Coating suspension viscosity too high	Use additional solvents to decrease viscosity or reduce coating suspension solid
3	Atomizing air pressure too low	Increase Atomizing air pressure.
4	High spay rate	Decrease spray rate
5	Poor spray gun performance	Use better spray gun

    

   

Cracking/Splitting: It is defect in which the film either cracks across the crown of the tablet (cracking) or splits around the edges of the tablet (Splitting)

Sr. No.	CAUSE	REMEDY
1.	Use of higher molecular weight polymers or polymeric blends.	Use lower molecular weight polymers or polymeric blends.  Also adjust plasticizer type and concentration.
2	Core and coating have different thermal expansion properties	Avoid mineral type filler
3	expansion of core due to overheating	Do not overheat tablet core
4	Low mechanical strength of coating	Use proper mechanical strength
5	Inadequate coating formulation	Use proper Inadequate coating formulation

Twinning:

cause	solution
i)inappropriate tablet shape	Chose tablet design that eliminates flat areas, refer tablet core selection.
ii)pan speed too low	Increase pan speed
iii)spray droplet size too large	Increase atomizing air pressure
Iv)spray rate too high	Reduce spray rate
v)insufficient drying	Increase drying: -high inlet air temperature -Higher drying air flow rate

Splitting and peeling:

cause	solution
Low film mechanical strength	Select optimized coating: film strength
Poor adhesion of coating to core	Increase adhesion
Poorly plasticized film	Select optimized coating: plasticization
Core tablet erosion	Improve tablet core
Overheating of tablet core	Avoid overheating

Logo bridging:

cause	solution
Poorly plasticized coating system	Use optimally plasticizer coating
Low adhesion coating system	Use high adhesion coating
Low adhesion core ingredients	Use high adhesion core ingredients
Spray rate too high	reduce
Product temperature too low	increase
Organic to aqueous conversation
Poor logo design	Improve logo design

Spray drying or logo filling:

cause	remedy
High atomizing air pressure	reduce
High drying air temperature	reduce
High solid coating suspension	Use low viscosity coating system
Gun to bed distance too high	optimized Gun to bed distance
Poor spray gun design	Use proper spray gun design
Turbulent air flow	Minimize pan depression
Aeration of coating suspension	Use optimized suspension preparation to avoid aeration

Surface erosion:

cause	solution
Hygroscopic tablet core	Reduce hygroscopic core material(e.g. superdisintegrant)
Application rate too high or low	Select appropriate spray rate
Pan speed too fast	Select correct pan speed
Friable tablet core	Use less friable tablet
Low coating film strength	Select high film strength opadry
Poor punch design	Use High quality punch
punch wear	Use non-wear punch
Poor logo design or placement	Check punch when use
Low coating suspension solid	Raise suspension solid level

Edge erosion:

cause	solution
Low coating film strength	Select high film strength opadry
Pan speed too fast	Select correct pan speed
Worn tablet tooling	Replace tablet tooling
Sharp edge on tablet	Change tablet shape
High core attrition	Change core formulation
Low spray application rate	Select appropriate spray rate
Low coating suspension solid	Raise suspension solid level
Incorrect coating pan fill	Fill pan in correct volume

Release or disintegration issues:

cause	solution
Interaction between the core and the coating ingredients.	Select alternative core or coating ingredients
Coating formulation contains ingredients that hinder the solubility of the coating.	Use a sub coat
Core ingredient are sensitive to heat or moisture in the coating process.	Select parameters to minimized effect on core

Tablet breakage:

cause	solution
Tablet are too soft	Change core formulation
Tablet are too brittle	Change core compression parameters
Tablet cap/laminate
Poor tablet shape for coating	Change tablet shape
Poor baffle design	Change baffle design

Opaspray: (opaque colour concentrate for film coating)

Opadry: (complete film coating concentrate)

Opalux-opaquant colour concentrate for sugar coating

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