Tablet Coating Pharmaceutical Technology II. Tablet Coating It is the application of a coating material to the exterior of a tablet with the intention

Tablet Coating

Pharmaceutical Technology II

Tablet Coating

It is the application of a coating material to the exterior of a tablet with the intention of achieving benefits and properties to the dosage form over the uncoated ones.

It involves additional steps in the manufacturing process, therefore increasing the cost of the product.

Types of tablet coating

There are three main types of tablet coating: Sugar coating Film coating Press coating

Reasons for tablet coating

1. Extend shelf life by protection of the ingredients from the environment (light, moisture and oxidation).

2. Coating provides an efficient sealing of the tablet cores as a method of taste masking for drugs that have bitter or unpleasant taste.

3. Tablets that are coated are somewhat easier to swallow than uncoated tablets (significant for large tablets).

4. Coloured coatings mask any batch differences in the appearance of raw materials (patient concern).


5. Coloured coatings aid in the rapid identification of product by the manufacturer, dispensing pharmacist and patient.

6. Coating tablets facilitates their handling on high speed automatic filling and packaging equipment.

7. Cross contamination is also reduced in the manufacturing

plant, as 'dusting' from tablets is eliminated by coating.


8. Functional film coatings are used to impart enteric or controlled-release properties to the coated tablet.

9. Coatings may be optimized with respect to colouration and gloss to aid in their sales appeal or to reinforce a marketing brand identification.

10. Avoid chemical incompatibilities by incorporating another drug or adjuvant in the coat or applying separate granular coating.

Sugar Coating

The traditional method of coating tablets.

It involves the successive application of sucrose‑based solutions to tablet cores in suitable coating equipment.

Conventional panning equipment with manual application of syrup has been extensively used although more specialized equipment and automated methods are now making an impact on the sugar coating process.

Stages of Sugar Coating Process

1. Sealing of tablet cores2. Subcoating3. Smoothing4. Colouring5. Polishing 6. Printing

Sealing

Sugar coating is an aqueous process during which the tablet cores are thoroughly wetted by syrup applications.

A tablet sealant is therefore applied to protect the tablet

cores during this initial susceptible period from the action of water.

Tablet sealants are generally water‑insoluble polymers or film formers applied from an organic solvent solution.

Sealing

Examples of tablet sealants include: Shellac ‑ very commonly used and is best in combination

with Polyvinyl pyrrolidone (PVP) which prevents hardening of the polymer on ageing.

Cellulose acetate phthalate. Polyvinyl acetate phthalate. Acrylate polymers. Zein

over‑application of tablet sealants can lead to disintegration problems

Sealing

Subcoating

Sugar coated tablets have a completely smooth profile with no visible edges remaining from the original tablet core.

Subcoating is applied to build up the tablet size and round the edges.

To attain this shape the sealed tablet core must be built up to gain the desired profile. Tablet cores for sugar coating should have a small edge so as not to make the 'rounding' process more difficult than it need be.

Biconvex tablets are preferred over flat surfaced tablets.

Subcoating

The process of subcoating is usually performed by adding bulking agents such as calcium carbonate or talc to the applied sucrose solutions. A gum such as acacia is also added to the applied suspension.

Subcoating

Subcoating can be accomplished by two methods:

1. The application of a gum/sucrose solution followed by dusting with powder and then drying. This routine is repeated many times until the desired shape is achieved.

2. The application of a suspension of dry powder in the gum/sucrose solution followed by drying. As above the procedure is repeatedly performed until the correct shape is achieved.

Subcoating

The solution used in subcoating is sucrose based and contains a gum such as gelatin, acacia or starch derivatives.

The gum will aid in the adhesion of the powder fillers such as calcium carbonate or talc.

If a coating suspension is used then the solids content is made as high as possible in order to reduce the drying time between each application.

Smoothing

After the correct profile has been attained the subcoated tablets will have a rather rough surface.

The purpose of the smoothing step is to cover and fill in the imperfections in the tablet surface caused by subcoating.

They are made perfectly smooth by successive applications of dilute syrup.

The tablets are subjected to drying air after each application.

Colour coating

Nearly all sugar coated tablets are coloured. There are two groups of colouring substances generally

used in coloured tablet coatings: Water soluble dyes Water insoluble pigments

Pigments are much more commonly used

Polishing

After color coating the tablets will require a separate polishing step for them to achieve an attractive appearance.

The tablets receive one or two applications of a wax dissolved in an organic solvent.

Usually beeswax or carnauba wax is used.

Printing

The use of indented monograms on sugar coated tablet cores is not feasible because the considerable thickness of coating conceals any core markings.

Instead, if identification is required then this can be accomplished by printing with special edible printing inks.

Process

Typically tablets are sugar coated by a panning technique.

The most simple form would be a traditional sugar coating pan with: A circular metal pan mounted angularly on a stand.

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

a supply of drying air (preferably of variable temperature and thermostatically controlled).

a fan‑assisted extract to remove dust and moisture laden air.

Process

Standard coating pan

Process

Significant improvement in the drying efficiency of the standard coating pan is achieved by the immersion-sword system.

Drying air is introduced through a perforated metal sword device that is immersed in the tablet bed.

The drying air flows upward from the sword through the tablet bed.

Since the air is more intimately mixed with the wetted tablets, a more efficient drying environment is provided.

Process

Perforated pan systems: developed to further improve drying efficiency. They consist of a perforated or partially perforated drum

that is rotated on its horizontal axis in an enclosed housing. In the Accela-Cota and Hi-Coater systems, drying air is

directed into the drum, passed through the tablet bed, then exhausted through perforations in the drum.

The Driacoater introduces drying air through hollow perforated ribs located on the inside periphery of the drum. As the coating pan rotates , the ribs dip into the tablet bed, and drying air passes through and fluidizes the tablet bed. Exhaust is from the back of the pan.

Process

Baffles maybe used to improve the mixing efficiency of the tablets during coating and thus uniform coating is ensured (applied in Accela-Cota).

Perforated pan coaters are efficient drying systems with high coating capacity, and can be completely automated for both sugar coating and film coating processes.

Process

Accela-Cota Driacoater

Process

Process

Process

Methods of applying the coating syrup include:

manually using a ladle. automatic control (peristaltic pump). spraying (atomizing systems).

Process

A successful product greatly depends on the skill of the coating operator.

This is especially true in the pan-ladling method, in which the coating solution is poured over the tablet cores.

The operator determines the quantity of solution to add, the method and rate of pouring, when to apply drying air, how long and how fast the tablets should be tumbled in the pan.

Process

The coating solution may be also sprayed onto the tablet bed.

Use of atomizing systems to spray the liquid coating material onto the tablets produces a faster, more even distribution of solution or suspension.

Spraying can significantly reduce drying time between solution applications in sugar coating.

Process

In perforated pan systems (ex. Accela Cota, Hi-Coater, Driacoater) the coating solution is applied to the surface of the rotating bed of tablets through spraying nozzles that are positioned inside the drum.

Process

Ideal characteristics of sugar-coated tablets

Sugar-coated tablets should ideally be of a perfectly smooth rounded contour with even colour coverage.

They are usually polished to a high gloss

Any printing should be distinct, with no smudging or broken print.

Coating faults: splitting of the coat on storage, caused by inadequate drying during the coating application.

Film Coating

This is the more modern of the two major coating processes.

First introduced to the pharmaceutical industry in the 1950’s

Film Coating

Film coating can be defined as a process in which thin (in the range of 20 - 200μm) polymer – based coatings are applied on to appropriate drug-containing cores that can be either tablets, beads, granules, capsules or drug powders and crystals.

It is possible to utilize conventional panning equipment but more specialized equipment are usually used with advantages of fast coating times and high degree of automation involved.

Film Coating

The coating liquid (solution or suspension) contains a polymer in a suitable liquid medium in addition to pigments and plasticizers. This solution is sprayed on to a rotating, mixed tablet bed or fluid bed. The drying conditions permit the removal of the solvents so as to leave a thin deposition of coating material around each tablet core.

Film Formation

The film formation process is the most important step in the production of coated dosage forms since the performance of these dosage forms depends mainly on the quality of the films deposited on the surfaces of the pharmaceutical substrates.

Polymeric films can be produced from organic solutions of polymers or from aqueous pseudolatex colloidal dispersions of polymer particles.

Film Formation

The film formation sequence differs according to the nature of the applied system resulting in differences in the film structure and appearance, film permeability, the need for post coating thermal treatment and the coat performance in modifying drug release.

Film Formation

The formation of film coatings from an organic solvent-based polymer solution involves:

Spraying of the coating solution onto a suitable substrate.

The wetting and deformation of the sprayed droplets. The entanglement of the polymer chains. As the solvent evaporates, the polymer gels. With further loss of solvent a continuous film is

produced.

Film Formation

The formation of films from a pseudolatex system involves: A film-forming polymer latex being deposited from

an aqueous colloidal dispersion of discrete polymer spheres.

Individual submicron-size spheres (0.1-0.2 μm), each containing hundreds of polymer chains, coalesce into a continuous film as the aqueous phase evaporates.

The polymer spheres in the latex dispersion are suspended and separated by electrostatic repulsion.

Film Formation

As water evaporates, interfacial tension between water and polymer pushes particles into point contact in a close-packed ordered array.

A strong driving force is necessary to overcome repulsive forces, deform the particles, and cause the spheres to fuse, resulting in complete coalescence.

Capillarity caused by the high interfacial surface tension of water provides the driving force to fuse the particles.

Film Formation

Plasticizer inclusion in the dispersion swells and softens the polymer spheres, facilitating coalescence and reducing minimum film formation temperature.

The plasticizer reduces the glass transition temperature (Tg) of the polymer.

Film Formation

Film Formation

Film Formation

Glass transition temperature (Tg) maybe defined as the temperature at which a polymer undergoes a marked change in material properties.

Below Tg, the polymer is said to exist in the glassy state and is usually characterized by a somewhat ordered structure in which there is minimal polymer chain movement.

Above Tg, the polymer is in a rubbery state, which is characterized by amorphous portions or regions with increased polymer chain movement and polymer elasticity.

Film Formation

While a certain degree of coalescence occurs during the coating application process, an additional processing step is often recommended in order to affect greater polymer coalescence for aqueous – based coatings.

This step known as the “curing step” involves exposing the film coating to temperatures above Tg for an extended period of time so that the polymer chains and segments in adjacent polymer particles diffuse across particle boundaries, eventually leading to complete coalescence and disappearance of the individual particle contours.

Film Formers

A variety of polymers have been used in film coating for different purposes.

Some of these polymers may be applied to improve the product appearance, enhance stability, improve handling properties or for taste masking.

Such coatings are usually described as non – functional film coatings because they do not interfere with the drug release properties of the drug containing cores.

Film Formers

Different classes of polymers can be used for non-functional coating of solid dosage forms including: Cellulose derivatives (Hydroxypropyl Methylcellulose

HPMC, Hydroxypropylcellulose HPC, Hydroxyethylcellulose HEC, Methylcellulose MC, Ethylcellulose EC, Sodium carboxymethylcellulose NaCMC)

Vinyls (Polyvinyl pyrrolidone, PVP) Glycols (Polyethylene glycols, PEG’s) Acrylic polymers (Dimethylaminoethyl methacrylate –

methylacrylic acid ester copolymer, Ethylacrylate – methylmethacrylate copolymer).

Most of these polymers are water – soluble, however water insoluble polymers (EC and some acrylate derivatives) can be used at low polymer loading levels as non-functional coatings.

Film Formers

Other polymers can be used to modify the release of the active ingredient from the drug – containing cores. In such a case the coating is described as a functional coating.

Modified release dosage forms are classified by the USP into two types:

1. Extended release: One that permits at least a twofold reduction in the dosing frequency as compared to the situation in which the drug is presented as a conventional dosage form.

2. Delayed release: One that releases the active ingredient at some time other than promptly after administration (enteric coated products for example).

Film Formers

Different types of polymers can be used to produce either type of modified release coated dosage forms.

Enteric polymers can be either Natural polymers (Shellac) Cellulosic (Cellulose Acetate Phthalate CAP, Cellulose Acetate

Trimellitate CAT, Hydroxypropyl Methylcellulose Phthalate HPMP, Hydroxypropyl Methylcellulose Acetate Succinate HPMAS)

Acrylic (Poly (Methacrylate – Ethylacrylate) 1:1, Poly (Methacrylic acid – methyl methacrylate) 1:1)

Polymers used in extended release film coating include Natural polymers (Zein), Cellulosic polymers (Ethylcellulose) Silicone elastomers Acrylic esters.

Film Formers

Aqueous dispersions have been developed for almost all polymers used for extended release film coating.

This switch from the traditional organic solvent-based systems occurred because of the many advantages associated with aqueous dispersions over organic based polymeric solutions including: Low viscosity (even at a high solids content). Low tackiness. The reduction of costs associated with explosion proof

equipment and solvent handling technology. Solvent residues (the amount of residual organic solvent in

the film must be investigated). Toxicity hazards, and concerns over environmental

pollution.

Formulation Components

In addition to the film – forming polymers, other components may be included in the coating formulation for different purposes. Some of these components may be added to improve the

film quality in terms of coalescence and/or facilitate large-scale production (e.g. plasticizers, surfactants and anti-tack agents).

Others are added to modify the film permeability properties to achieve a target release profile from the dosage form (e.g. pore formers or channeling agents).

Plasticizers

Plasticizers are usually high – boiling point organic solvents that are used to impart flexibility to the otherwise hard or brittle polymeric materials.

Plasticizers generally cause a reduction in the cohesive intermolecular forces along the polymer chains resulting in various changes in polymer properties, such as reduction in tensile strength, increase in flexibility and reduction of polymer Tg therefore enhancing the coalescence process and improving the integrity of the coat.

Plasticizers

With aqueous colloidal polymer dispersions, the addition of plasticizers is required for polymer dispersions having a minimum film formation temperature (MFT) above the coating temperature.

During plasticization, the plasticizer will diffuse into and soften the polymeric particles thus promoting particle deformation and coalescence into a homogeneous film.

Plasticizers

Water soluble: Glycerin, propylene glycol, low molecular weight polyethylene glycols (PEG 200 and 400). Triethyl citrate (Citroflex®) and surfactants such as Tweens.

Water insoluble: Acetyl triethyl citrate (ATEC), Acetyl tributyl citrate (ATBC), Dibutyl phthalate (DBP), Dibutyl sebacate (DBS), Diethyl phthalate (DEP), Tributyl citrate (TBC).

Plasticizers

Water insoluble: Oils such as oleic acid, caster oil and coconut oil. Surfactants such as Spans and Myvacet (acetylated mono glycerides).

With aqueous polymer dispersions, water-soluble plasticizers dissolve whereas water-insoluble plasticizers have to be emulsified in the aqueous phase of the dispersion.

Surfactants

Surfactants can be “endogenous” to the polymeric aqueous dispersion since they are used in the synthesis of some coating polymers (e.g. Nonoxynol 100 in Eudragit® NE 30D) by emulsion polymerization.

“Exogenous” surfactants may be added to the coating formulation to facilitate the spreading of the coating droplets on the surface of the substrate. They lower interfacial tension between organic polymer solution and the aqueous phase during pseudolatex formation.

Surfactants

Surfactants prevent agglomeration and coalescence of dispersed polymer particles during shelf life.

They wet and homogenize the coating mixtures and even act as plasticizers.

Examples: Tweens, Cetyl alcohol, SLS, Myvacet, Pluronic.

In case of controlled release coatings different

surfactants were found to increase the drug release rate of film coated granules by leaching into the release media leaving a porous membrane.

Pore Formers

Film coats made of impermeable or semi-permeable polymers may be combined with water-soluble pore formers such as micronized sucrose, sorbitol, lactose, NaCl and Calcium phosphate.

Upon contact with dissolution fluids, the pore former leaches out rapidly to form a multiporous rate-controlling membrane through which the drug diffuses in a zero order fashion.

pH dependent pore forming agents are used in enteric coated substrates.

Anti-tacking Agents

Tack is the ability of a polymer to adhere to a substrate with little contact pressure.

During the application of the coating material in a coating pan, pressure is induced by the movement of tablets.

Tumbling action of the coating pan: Enhances the film adhesion to the tablets surfaces but Causes some adherence and sticking between tablets and

adherence and sticking to the coating pan surface.

An unwanted and sometimes irreversible effect of tackiness is the agglomeration of several coated units or, in the worst case, the whole batch which can occur at higher product temperatures and higher plasticizer content.

Anti-tacking Agents

In addition, the tackiness of polymeric films is important for subsequent curing step (post coating thermal treatment) where the coated product is exposed to temperatures above the coating polymer’s Tg.

Insoluble additives including talc, kaolin, Mg stearate and colloidal silica have been used as anti-tack agents or anti-adherents to help reduce agglomeration or sticking of coated substrates during and after coating.

Anti-tacking Agents

Anti-tack agents are usually added directly to the coating dispersion formula and applied with the coating polymer, or can be mixed with the coated pellets immediately after coating and before drying and curing.

Anti-tack agents comprise an important part of the coating formula’s solids content, talc for example has been used at levels up to 100% w/w based on the dry polymer weight.

Anti-tacking Agents

The effect of anti-tack agents on drug release is basically a stabilization effect by preventing damage to the film coats through sticking during the coating and curing steps.

Colourants

Usually water-insoluble colours (pigments). Pigments have certain advantages over water-soluble colours:

they tend to be more chemically stable towards light, provide better opacity and covering power, and optimize the impermeability of a given film to water vapour.

Examples of colourants: Iron oxide pigments. Titanium dioxide. Aluminium Lakes.

Process Parameters:

Different process parameters are expected to affect film formation by affecting the rate of evaporation of the vehicle from the substrate surface thus affecting the nature of the coating deposited onto the substrate and the performance of the final dosage form.

Such parameters include product bed temperature, spraying rate, spraying pressure, pan rotation speed and curing temperature.

Process Parameters:

The effect of the process parameters on the polymeric coatings and dosage form performance is more pronounced if aqueous dispersions are used in comparison to organic solutions.

This can be explained by considering the complex nature of film formation from latex particles which (in comparison to film formation from organic solutions): Undergoes more steps. Takes more time to achieve a coalescent film.

Product Bed Temperature:

Sufficient thermal energy input is critical for optimal film formation, adjusting the levels of the bed

temperature can control its magnitude and consequently affect the coating quality.


A low product bed temperature will result in a defective film and higher rates of drug release

because of: Drug migration into the film layer during the coating

process. Incomplete film formation due to hardening of the

uncoalesced polymer spheres.


High product bed temperatures results in: Faster evaporation of the dispersion medium of

the coating droplets. Improper spreading of the polymer on the surface

of the substrate affecting the integrity of the film and its ability to control the drug release.

Increased susceptibility to sticking proplems.


In general, a product bed temperature that is 10-20°C above the Tg is usually recommended for optimal film formation.

Spray Rate:

Proper film formation requires a uniform application of the polymeric dispersion onto the substrate surface as well as a well-controlled evaporation of water from that surface.

The rate of spraying depends on the mixing and drying efficiency of the system.

Good mixing and efficient drying leads to higher feed rate. Good mixing is achieved by the use of baffles. Tablet shape affects mixing, flat shapes lead to poor

mixing, while as the shape is more spherical more optimum mixing is achieved as with biconvex tablets.

Spray Rate:

In a study to examine the effect of the spray rate in a Wurster-insert fluidized bed processor on the release of ibuprofen from ethylcellulose-coated tablets. Faster release rates from film-coated tablets were found to

be associated with higher spray rates. High spray rates can result in delayed coalescence of

polymer spheres due to reducing bed temperatures resulting in poorly coalesced films.

The reduction in bed temperatures is due to the depletion of the applied thermal energy by relatively large volumes of

the aqueous vehicle for evaporation.

Spray Pressure:

In order to obtain a uniform film on substrate surface, the polymeric dispersion must contact all

surfaces evenly and evaporate quickly.

The best way for this to be accomplished is by breaking the liquid into small droplets using the

atomizing air nozzle.

Spray Pressure:

The droplets are formed by the shearing action of pressurized air on an emerging column of polymer

liquid.

These droplets are then sprayed from the atomizing nozzle onto the substrate surface where they

spread, and evaporate to their solid constituents.

Increasing spray nozzle pressure has been shown to reduce mean droplet size of polymeric solutions.

Spray Pressure:

Optimum atomization pressure and droplet size are necessary to achieve a uniform film coating. At high atomization pressures, very small droplets may dry

before contacting the substrate, a phenomenon termed spray drying, others may contact the surface and dry before complete spreading of the polymer spheres or coalescence occurs resulting in incomplete film formation.

At low atomization pressures, large droplets can overwhelm the evaporative capacity of the system, causing over wetting. This may lead to sticking of tablets in pans, or agglomeration and loss of fluidization in air suspension coaters.

Coating Pan Speed:

Coating pan speed has been reported to have a major influence on the intra-tablet coating uniformity in aqueous-based film coating.

Spherical substrates, such as pellets, have the best coating uniformity because there is no spatial orientation due to which all parts of the surface are equally exposed to the spray coating over time.

Coating Pan Speed:

However, as the substrate shape diverges from spherical to a flatter shape, as in tablets, it is more likely to have a preferred spatial orientation. The flatter and broader the tablet, the more likely it is to pass through the spray zone flat with the face exposed.

The rotational exposure of all tablets required for

uniform coating can be enhanced by increasing the motion of the tablet bed by increasing the pan speed.

Coating Pan Speed:

Pan speed affects not only mixing but also the velocity at which the tablets pass under the spray.

Too slow speed results in localized overwetting and sticking.

Too high speed may not allow for enough time for drying before the same tablet is reintroduced to the spray.

Pan speeds of 10-15 rpm are commonly used in large pan coaters for non-aqueous coating, while slower pan speeds 3-10 rpm for aqueous coating.

Curing:

The coalescence of the colloidal polymer particles into a homogenous film is often incomplete after coating with aqueous polymer dispersions. As a consequence, changes in the drug release from the coated dosage form caused by further coalescence during storage have been observed as a function of storage temperature and time.

A curing step or thermal treatment (storage of the coated dosage forms at elevated temperatures for short periods) is often recommended to accelerate the coalescence of the polymer particles prior to long-term storage.

Curing:

During the curing step, the coated dosage forms are subjected to a heat treatment above the Tg of the polymer.

This is achieved either by storing the coated dosage forms in an oven or through further fluidization in the heated fluidized bed coater immediately after the completion of the coating process for a short time.

The storage temperature should be about 10°C above the minimum film formation temperature (MFT).

Curing:

Higher curing temperatures could cause excessive tackiness and agglomeration.

The effect of curing on the release of chlorpheniramine maleate from ethylcellulose coated beads (coating temperature 40°C) indicates that the limiting drug release pattern was approached after curing the beads for 1 hour at 60°C.

Curing may not be necessary for organic solution – based coatings.

Curing:

Effect of curing conditions

on chlorpheniramine maleate

release in 0.1M pH 7.4

phosphate buffer.

Curing:

An increase in the curing time and temperature results in slower release rates.

However, curing may not always slow down drug release rates. In drug layered pellets where the drug is applied as a solution or a dispersion in a water soluble polymer onto a placebo pellet core and then coated with a water insoluble polymer, curing was found to accelerate the drug release by increasing the rate of drug diffusion through the polymer layers towards the bead surface.

Curing:

The following figure shows the effect of curing time on ibuprofen release from beads cured at 50°C.

The release was initially rapid with the beads cured for 15min, then decreased with increasing curing time up to a period of 4h.

Curing times in excess of 4h resulted in an increase in drug release.

The initial decrease in drug release was due to further coalescence of polymer particles in the ethylcellulose film.

Curing:

Effect of curing time on

ibuprofen release in 0.1 M

pH 7.4 phosphate buffer.

Curing:

The increase in drug release (curing periods in excess of 4h) could be explained with the migration of ibuprofen from the bead interior to the bead surface through ethylcellulose coating during the curing step.

After the application of Aquacoat layer (Ethylcellulose 87%, Cetyl alcohol 8.7%, SLS 4.2%) onto the drug beads, the surface of uncured beads was uniform and smooth. However, after the coated beads were subjected to the curing step, large drug crystals could be observed throughout the coated surface by scanning electron microscopy.

Curing:

This phenomena of drug migration was not observed in Chlorpheniramine maleate (melting point 130-135°C).

Ibuprofen has a much lower melting point (75-77°C) and higher drug polymer affinity, which may explain the phenomena of drug migration that was accelerated at elevated temperatures.

In order to retard or avoid drug migration during the curing step, the drug beads could be seal-coated with a polymer having low affinity for the drug.

Fluidized bed coaters

Fluidized bed coaters function by suspending a bed or a column of solid particles in a moving gas stream.

They are considered as highly efficient drying systems. Fluidization of the tablet mass 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 tablets is upward through the center of the chamber.

They then fall toward the chamber wall and move downward to re-enter the air stream at the bottom of the chamber.


A liquid coating formulation is sprayed onto the individual particles and the freshly coated particles are cycled into a zone where the coating formulation is dried either by solvent evaporation or by cooling.

The coating and drying sequence is repeated until the desired coating thickness has been applied.


A major advantage of fluidized bed coaters is their ability to handle an extremely wide range of coating formulations including: Hot melts (waxes and fats). Aqueous latex dispersions (polymers). Organic solvent solutions (polymers). Aqueous solutions (polymers).


According to the spraying patterns fluid bed equipment can be classified to: Top spray. Tangential spray. Bottom spray.

Spray application systems: two basic systems used to apply a

finely divided (atomized) spray of coating solution or suspension.

High pressure, airless. Low pressure, air atomized.


Top spray fluidized bed coaters

The spraying nozzle is located on top of the product chamber. The liquid is sprayed in the opposite direction to material motion.

If the spray formulation contains a volatile solvent, evaporation of this solvent from the spray droplets occur, thereby increasing the solid content and reducing the ability of the sprayed droplets to spread on the particles being coated.


It yields coated particles with significant void volume and porosity.

They are simple to operate and have higher production capacities than other types of fluidized bed units.


Tangential spray fluidized bed coaters

The spray nozzle is located on the side of the product bed and liquid is sprayed in a tangential pattern while the materials are in motion.

A rotary disc replaces the air distribution plate whose motion together with that introduced by the fluidizing air and the gravitational pull cause the materials to move in rope-like motion.

Tangential spray fluidized bed coaters

Bottom spray fluidized bed coaters

The spraying nozzle is located at the bottom of the product chamber.

The liquid is sprayed in the same direction as the material motion in the partition chamber.

Such layout allows more contact between the liquid and the solid.


A B C


A: Granulator Top-Spray Process, preferred when taste masking coating is being applied, also for application of hot melt coatings.

B: Wurster, Bottom-Spray Process, preferred for the application of modified-release coatings to a variety of multiparticulates, also suitable for drug layering when drug dose is in the low-to-medium range.

C: Rotor, Tangential-Spray Process, suitable for the application of modified-release coatings to a variety of multiparticulates. Ideal for drug layering when the dose is medium-to-high. Also useful as a spheronizing process for producing spheres from powders.


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 systems, owing to the relatively rough tablet-to-tablet impact and tablet-chamber contact.

Coating Defects

Variations in formulation and processing conditions may result in unacceptable quality defects in the film coating.

Sticking and Picking

Large droplets on tablet surface are difficult to dry which may lead to localized overwetting and sticking.

Overwetting or excessive film tackiness causes tablets to stick to each other or to the coating pan.

On drying, at the point of contact, a piece of the film may remain adhered to the pan or to another tablet giving a "picked" appearance to the tablet surface and resulting in a small exposed area of the core.

A reduction in the liquid application rate or increase in the drying air temperature and air volume usually solve this problem.

Excessive tackiness may be an indication of a poor formulation.

Sticking and Picking

Roughness

A rough or gritty surface is a defect often observed when the coating is applied by a spray.

Some of the droplets may dry too rapidly before reaching the tablet bed resulting in deposits on the tablet surface of "spray dried" particles instead of finely divided droplets of coating solution or suspension.

Moving the nozzle closer to the tablet bed or reducing the degree of atomization (reduce the atomization air pressure to achieve larger droplets) can decrease the roughness due to "spray drying."

Surface roughness also increases with pigment concentration and polymer concentration in the coating solution.

Roughness

Orange-Peel Effect

Inadequate spreading of the coating solution before drying causes a bumpy or “orange-peel” effect on the coating.

This indicates that spreading is impeded by too rapid drying or by high solution viscosity.

Large droplet size of highly viscous solutions (ex. HPMC in water) leads to poor spreading on tablet surface and hence orange peeling.

In this case higher atomizing air pressure is required to obtain smaller droplets. Also thinning the solution with additional solvent may correct the problem.

Orange peeling and roughness affect the visual perception of a coloured tablet (a rough film will appear lighter and less saturated in colour compared to a smooth glossy film).

Bridging and Filling

During drying, the film may shrink and pull away from the sharp corners of a bisect, resulting in a "bridging" of the surface depression.

This defect can be so severe that the monogram or bisect is completely obscured. This mainly represents a problem the formulation.

Increasing the plasticizer content or changing the plasticizer can decrease the incidence of bridging.

Bridging and Filling

Filling is caused by applying too much solution resulting in a thick film, that fills and narrows the monogram or bisect. In addition. if the solution is applied too fast overwetting may cause the liquid to quickly fill and be retained in the monogram.

Judicious monitoring of the fluid application rate and thorough mixing of the tablets in the pan prevent filling.

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.

Internal stresses in the film can be minimized by adjusting the plasticizer type and concentration, and the pigment type and concentration.

Cratering

The coating liquid penetrates the surface of the tablet causing localized disintegration of the core and disruption of coat, which would appear such as volcanic like craters.

Generally occurs in the initial stages of the coating process and becomes partially obscured as more film is deposited during coating.

Hazing (Dull film)

It can occur when too high processing temperature is used for a particular formulation.

It is particularly evident when cellulosic polymers are applied from an aqueous media at high temperature.

It can also occur if the coated tablets are exposed to high humidity conditions and partial solvation of film occurs.

Colour Variations

Caused by processing conditions or the formulation. Improper mixing, uneven spray pattern, and insufficient coating

may result in colour variations. The migration of soluble dyes, plasticizers, and other additives

during drying may give the coating a mottled or spotted appearance.

The use of lake dyes eliminates dye migration. A re-formulation with different plasticizers and additives is the

best way to solve film instabilities caused by the ingredients.

Sugar and film coating

Press Coating

Press coating involves the compaction of granular material around an already preformed core using compressing equipment similar to that used for the core itself.

The process was developed originally to serve as alternative for sugar and film coating in case of a very water-sensitive drugs.

The granular coating material usually contains a high proportion of sugar to mimic the more conventional sugar coated tablets.

Press Coating

Press coating is also used in the separation of chemically incompatible materials in tablets (combination tablets).

It is even possible to apply two press coatings where an inert middle layer separates the active core from the coating.

The press coating also offers potential for having a dual release pattern.

The use of the process is limited by the relative complexity of the mechanism used in the compression equipment.

Press Coating

The success of the process depends on the use of: Core material that develops reasonable strength at low

compressional loads. Coating material in the form of fine free-flowing granules

with good binding qualities.

The coating material is supposed to have a good particle size distribution with no agglomerates to produce a stable powder bed that prevents the core from tilting through the coating material below it.

Reading

http://www.eudragit.com/pharmapolymers/en/downloads/

http://www.fmcbiopolymer.com/Pharmaceuticals/AestheticCoatings/tabid/501/Default.aspx

http://www.fmcbiopolymer.com/Pharmaceuticals/FunctionalCoatings/tabid/504/Default.aspx

http://www.basf-pharma.com/(p5gu5naw15hgryqx4f41sn55)/excipients.aspx

http://www.colorcon.com/pharma/mod_rel/index.html

Documents

Tablet Coating Pharmaceutical Technology II. Tablet Coating It is the application of a coating material to the exterior of a tablet with the intention