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Polymer Use In Pharmaceutical Application And Polymer Coating

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Shridhar J PandyaTablets are by far the most popular dosage form when

administering drugs to patients, and a large proportion of the

tablets produced around the world are film coated. With the

current awareness of health, safety and environmental problems,

film coating is a process that is routinely employed in the

preparation of pharmaceutical solid dosage forms.

The success of coating process is determined by three factors:

formulation of the coating system, coating process parameters

and tablet core. During the last 20 years, there has been

significant research into coating formulations and processes.

It has been used extensively in the pharmaceutical industry, e.g.

for the application of non-functional or functional coats (aesthetic,

protective or rate controlling polymer films) and for the deposition

of Active Pharmaceutical Ingredients (APIs) onto nonpareils (multi-

particulate dosage forms).

An accurate method of coating objects 3 to 30 mm in length with

APIs is also desired in the pharmaceutical industry as this is the

size range of most single-unit solid dosage forms. These include

tablets for oral administration and forms for other methods of

delivery including human implantation.

Existing methods of coating objects in this size range have

limitations, e.g. in terms of coating speed and accuracy /

uniformity, particularly for the deposition of low dose API onto

single unit tablet dosage forms which requires a greater degree of

accuracy than can be achieved using current tablet coating

techniques .

A novel method of coating small objects has been developed that

has demonstrated the ability to uniformly coat inert objects of

sizes between 3 and 30 mm in length with a high degree of

accuracy. Using the coating method, Relative Standard Deviations

(RSDs) below 2% have been achieved for total coating contents as

low as 200 micrograms per object. However, it is unknown how

accurately the deposition will be on conventional pharmaceutical

tablets (which have a higher degree of friability and more irregular

surface compared to the inert objects used).

1




3/23

Tablet Coating Applications:The application of coating of tablets, which is an additional step in

the manufacturing process, increases the cost of the product.

Therefore, the decision to coat a tablet is usually based on one or

more of the following objectives:

To mask the taste, odor, color of the drug.1.

To provide physical and chemical protection for the drug.2.

To control the release of the drug from the tablet.3.

To protect the drug from the gastric environment of the

stomach with an acid resistant enteric coating.

4.

To incorporate another drug or formula adjuvant in the

coating to avoid chemical incompatibilities or to provide

sequential drug release.

5.

To improve the pharmaceutical elegance by use of special

colors and contrasting

6.

Primary Compo nents Involved In Tablet CoatingThere are three primary components involved in tablet coating:

1.Tablet properties

2.Coating process

Coating equipment

Parameters of the coating process

Facility and ancillary equipment

Automation in coating process

3.Coating compositions

1. Tablet Properties The benefits of film coating more than justify the exposure of the

product to the rigor of the coating process, during which the

tablets (and the applied coating) are constantly subjected to

mechanical stress along with conditions of elevated temperature

and humidity. Therefore, cores must be designed using more

stringent criteria compared with uncoated dosage forms to




4/23

guarantee a product robust enough to withstand the additional

stress imparted by the film coating process.

The design (properties) of such a substrate (tablet) should be:

The tablets must be resistant to abrasion andchipping, which

is caused by the intense attrition of the tablets striking othertablets or walls of the coating equipment during the coating

process.

1.

Tablet surfaces that are brittle, that soften in the presence of

heat, or that are affected by the coating composition tend to

become rough in early phase of the coating process and are

unacceptable for film coating.

2.

Films coatings adhere to all exposed surfaces, so that any

surface imperfection is coated is not eliminated. The quality

of thin film coatings applied to compressed tablets usually

depends much more on the quality of the starting tablet than

on the quality of the time at which sugar coatings are

applied.

3.

In addition to a smooth surface, the physical shape of the

tabletis important. The ideal shape for coating is a sphere,

which allows tablets to roll freely in the coating pan, with

minimal tablet-to-tablet contact. The worst shape is a square

flat-faced tablet, in which case coating materials would

collect between the surfaces to glue them together. For this

reason, coated tablets have rounded surfaces, the more

convex the surface is, and the fewer difficulties will be

encountered with tablet agglomeration.

4.

For the coating to adhere to the tablet, the coating

composition must wet the surface. The surface properties of

the tablet depend on the chemical nature of the ingredients

utilized in the formulation. Hydrophobic tablet surfacesare

difficult to coat with aqueous-based coatingsthat do not wet

the surface.

5.

Figure1: Indentation hardness profiles for tablets of differentshape.




5/23

Figure 1 shows the surface hardness across the crown of

differently shaped tablets . Flat faced and shallow concave tablets

have relatively high overall surface hardness, but tend to be brittle

at the edges. The deep concave and ball shaped tablets have good

mixing characteristics, but offer the lowest levels of mechanical

strength, particularly at the crown. Flat, shallow/deep concave,

ball/caplet-shaped tablets are not the best choice for film coating.

A B

Fig:1 Areas prone to surface erosion for (A) flat, shallow concave,caplet shaped and (B) deep concave tablets.

1




6/23

Figure 2. Illustrates areas on the tablet that have the highesterosion potential. Therefore, normal concave is the preferred shapefor film coating.During critical stages of the drying process, usually immediately

after deposition of the coating onto the tablet surface, coating

systems become extremely viscous and adhesive. As a result, if

tablets exhibit large areas of relative flatness on their surfaces, it

is possible for them to become bonded together. This situation is

prevalent when attempting to coat flat-faced or caplet-shaped

tablets (Figure 3).

Remedy: Placing even a very subtle amount of curvature on an

otherwise flat surface can minimize twinning problems.

Figure3: Twining during the coating process for (a) flat-faced and (b)caplet-shaped tablets. Twinning occurs twinning less likely

Flat sideadhere slight curvature More readilylimits contact.

2. Coating Process-The principles of tablet coating are relatively simple.

Tablet coating is the application of a coating composition to a

moving bed of tablets with the concurrent use of heated air to

facilitate evaporation of the solvent.




7/23

The distribution of the coating is accomplished by the movement

of the tablets either perpendicular (coating pan) or vertical (air

suspension) to the application of the coating composition.

3. Equipments Various techniques can be used to improve coating uniformity. A

study suggested that shallow bed depths, a large number of spray

guns, and fully optimized baffle systems produce the best coating.

There are generally four types of uniformity in manufacturing

uniformity within the products life cycle

uniformity within a batch

uniformity at various dose levels

uniformity of individual pellets, 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

Generally, more energy efficient, automated systems arepreferred, to shorten the total coating time and reduce operator

participation in the coating process.

Standard/ Conventional Pan System:1.The standard coating pan consists of 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.

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.

Coating solutions are applied to the tablets by ladling or

spraying the material on to the rotating tablet bed.




8/23

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.

Perforated Pan Systems-1. All the equipments of this type consist of a perforated or

partially perforated drum that is rotated on its horizontal axis in

an enclosed housing.

Fluidized Bed (Air Suspension) Systems-1.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.

Disadvantages: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.




9/23

Surface Effects In Film Coating:This includes the process (wetting) which occurs at the interface

between the droplets of coating liquid and the surface of the

substrate cores-

Wetting-True wetting is defined as the replacement of a solid-air interface

with a solid-liquid interface. During this process, individual gas

and vapor molecules must be removed from the surface of the

solid and replaced by solvent molecules. This process is influenced

by the two properties of wetting power and wettability.

Wetting power can be defined as the ability of the atomized

droplet to wet the substrate and wettability can be defined as

the ability of the substrate to be wetted by the atomized droplets.

Coating Variables-The processing parameters for coating can be divided into two

groups-

Independent variables

Dependent variables

Independent V ariables-Four independent variables have a direct effect on the quality of

the coated tablet-

Spray rate

Inlet drying air volume

Inlet air temperature

Spray atomizing temperature

The objective must be to obtain a satisfactorily coated tablet with

the minimum coating time.

Dependent Variables-These are the variables that result from the value of the settings

for the independent variables.




10/23

Dew point of exhaust air

Outlet air temperature

Tablet bed temperature

Coat quality

Types Of C oating:Sugar Coating:-1.

The pharmaceutical process of sugar coating remains a widely

practices technology.Unlike film coating, sugar coating is still a

multistep process. Its use of labour is more intensive than in film

coating and process operators require a fair degree of skill.

In suitable sugar- coating equipment, the tablet cores are

successively treated with aqueous sucrose solutions which,

depending on the stage of the coating reached, may contain other

functional ingredients e.g. fillers, colors etc. The build up of

coating material is due to transference of coating medium from

one tablet to another. Typically a single liquid application will be

made which will be allowed to spread over the entire tablet bed

utilizing the mixing capability of the particular equipment. At this

point, drying, usually in the form of heated air will be used t o dry

the application. The whole cycle will then be successively

repeated.

In this respect, sugar coating differs from film coating as in this

process each tablet passes through a zone of application which is

subjected to rapid and continuous drying.

Advantages Of Sugar Coating-It utilizes inexpensive and readily available raw materials.1.

Constituent raw materials are widely accepted-no regulatoryproblems.

2.

Modern simplified techniques have greatly reduced coating

times over traditional sugar coating methods.

3.

No complex equipment or services are required.4.

The process is capable of being controlled and documented

to meet modern GMP standards.

5.

Page 10 of 23Various Polymer Use In Pharmaceutical Application And Polymer Coating | Pharmainf...



11/23

Simplicity of equipment and readily availability of raw

materials make sugar coating an ideal coating method for

developing countries.

6.

The process is generally not as critical as film coating;

recovering and reworking procedures are usually possible.

7.

For high humidity climates, it generally offers a stability

advantage over film coating tablets.

8.

Results are aesthetically pleasing and have wide consumer

acceptability.

9.

Tablet cores may generally be softer than those demanded

by film coating especially those for aqueous film coating.

10.

Stages Of Sugar Coating-Sealing1.

Subcoating2.

Sealing -This is an organic solvent-dependent step.

It is necessary to protect the tablet core from the aqueous

nature of sucrose applications to follow.

It also prevents certain types of materials from migrating to the

tablet surface and spoiling the appearance, e.g. oils, acids etc.

A film of water-impervious polymer is built up using materials

such as:

Shellac

CAP

PVAP

Zein

Shellac has all the advantages of a natural material, the other

polymers used tend to be those which have an additional use as

enteric-coating materials so that they should be applied only in

sufficient quantity to form an efficient seal.

A lamination process, whereby an application of dusting power,

e.g. talc, is nearly always used.




12/23

Subcoating -During the sugar coating process the increase in weight achieved

can be 30-50% of the weight of the original tablet core. Much of

the added weight is appliedat the subcoating stage. Subcoating

serves to confer the tablet core a perfectly rounded aspect.

The ideal shape for sugar coating is a deeply convex core with

minimal edges. This condition will obviously require less coating

material than where the tablet edge is comparatively thick.

Defects Of Sugar Coating:As the sugar coating itself is deliberately isolated from the tablet

core there is the possibility of much more standardization than in

the area of film coating formulae.

1. Cracking And Splitting Of The Sugar Coat-CAUSES: Excess residual moisture from the processing.

REMEDY: To allow sufficient time between individual applications

of syrup.

2. Inversion And Stickiness-CAUSE: Presence of inverted sugar which is difficult to dry

adequately. It can be encountered if slightly acidic color coating

suspensions are maintained at too high a temperature for too

long.

REMEDY: Low temperature should be maintained.

Sugar coatings are brittle and are prone to chipping if

subjected to an inappropriate mechanical stress.

1.

Wax collects in the depressions on the rough tablet surfaces

on polishing and remains as tiny white spots at the end of

the process.

2.

REMEDY:Ensure smooth tablet surface

Facility And Ancillary Processes:The facility required for any coating operation should be designed

to meet the requirements of current Good Manufacturing

Practices (GMPs). Adequate space is required not only for the




13/23

coating equipment, but also for the coating equipment, but also

for solution preparation and inprocess storage. The specific

safety requirement for coating areas depends on the nature of the

solvent. Where explosive or toxic concentrations of organic

solvent could occur, during either solution preparation or coating

operation, electrical explosion-proofing and specialized ventilation

are required.

Treatment of the exhaust air from the coating operation may be

desired to recover expensive organic solvents or to prevent

solvents and particulate allowed in the limits of organic solvent

and particulate allowed in the atmosphere. Compliance with the

regulations can be extremely expensive and this cost factor

should be considered in developing a new coating. A major

advantage of totally aqueous based film coating is that all direct

and indirect expenses relating to the purchase, handling and

envioronmentally acceptable removal of the organic solvent are

circumvented.

Other equipment needed is to support the coating operation.

Solution preparation requires tanks, filters and mixers. A colloid

mill or ball mill may be needed for the homogeneous dispersion

of insoluble solids in the liquid coating mixture. Jacketed tanks

may be needed for keeping the some solutions at an elevated

temperature.

Automation-Within last 6 to 8 years, automation has been achieved in sugar

coating and film coating systems. Through a series of sensors and

regulating devices for temperature, airflow, spray rate and pan

speed, a feedback control of the process is maintained. For

process automation, the perforated pans are preferred over the

old conventional coating pans because of their better efficiency .

Materials Used In Film Coating:The coating materials may be a physical deposition of the material

in the tablet substrate, or they may form a continuous film with a

wide variety of properties depending upon the composition of the

coating formulations.

During the last 40 years, wide varieties of polymers have been

evaluated and are being used commercially for tablet coating.

8




14/23

Coating materials generally used are-

Synthetic polymers

Solvents

Plasticizers

Colorants

Opaquant-extenders

Miscellaneous coating solution components

Film Formers:Introduction:Mechanism Of Film Formation:Film formation from an aqueous polymeric dispersion is a

complex matter (Bindschaedler et al., 1983: Zhang et al., 1988,

1989). In the wet state, the polymer is present as a number of

discrete particles, and these have to come together in close

contact, deform, coalescence and ultimately fuse together to form

a discrete film. During processing, the substrate surface will be

wetted with the diluted dispersion. Water will be lost under the

prevailing conditions as water vapor and the polymer particles will

increase in proximity to each other. Complete coalescence occurs

when the adjacent particles are able to mutually diffuse into one

another.

Minimum Film Forming Temperature-This is the minimum temperature above which formation will take

place using individual defined conditions. it is largely dependent

on the glass transition temperature (T ) of the polymer. ( Lechman1992)The available film formers are classified into -

1.Enteric materials

2.Non-enteric materials

g




15/23

1. Enteric M aterials-Enteric coating of pills and compressed tablets has existed for

more than a century. These coatings form a subgroup of modified

release coatings and are defined as the coatings that resist the

action of stomach acids but rapidly breaks down to release its

contents once it has passed into the duodenum.

Some of the most important reasons for the enteric coating are as

follows:

To protect the acid-liable drugs from the gastric fluid.

Eg: enzymes and certain antibiotics

To prevent gastric distress or nausea due to irritation from a

drug.

Eg: sodium salicylate

To deliver drugs intended for local action in the intestines.

Eg: intestinal antiseptics could be delivered to their site of action

in a concentrated form and bypass systemic absorption in the

stomach.

To deliver drugs that are optimally absorbed in the small

intestine to their primary absorption site in their most

concentrated form.

To provide a delayed-release component for repeat action

tablets.

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.




16/23

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.

MECHANISM: The mechanism by which these polymers function is

by a variable pH solubility profile where polymer remains intact at

low pH but at a higher pH will undergo dissolution to permit the

release of the contents of the dosage form. (SCHROETER 1965)

Some of the commercially available enteric polymers:

Cellulose acetate phthalate (CAP)1.

Acrylate Polymers2.

Hydroxypropyl methylcellulose phthalate3.

Polyvinyl Acetate Phthalate (PVAP)4.

2. Non-Enteric M aterialsSome of the most commonly used materials by the

pharmaceutical industry:

Hydroxypropyl Methylcellulose, USP

Methyl Hydroxyethylcellulose

Ethyl Cellulose, NF

Hydroxypropylcellulose, FCC

Povidone, USP

Sodium Carboxymethylcellulose, USP

Polyethylene Glycols

Acrylate polymers




17/23

Film Defect:Variation in formulation and processing conditions may result in

unacceptable quality defects in the film coating. The source of

these defects and some of their probable causes are described in

the following sections.

1.Sticking And Picking-Overwetting or examples 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.

REMEDY: A reduction in the liquid application rate or increase in

the drying air temperature and air volume usually solves this

problem. Excessive tackiness may be an indication of a poor

formulation.

2. Roughness-A rough or gritty surface is a defect often observed when coating

is applied by a spray.

Some of the droplets may dry too rapidly before reaching the

tablet bed, resulting in the deposits on the tablet surface of spray

dried particles instead of finely divided droplets of coating

solution.




18/23

Surface roughness also increases with pigment concentration

and polymer concentration in the coating solution.

REMEDY: Moving the nozzle closer to the tablet bed and

reducing the degree of atomization can decrease the roughness

due to spray drying.

3. Orange-Peel Effects-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.

REMEDY: Thinning the solution with additional solvent may

correct this problem.

4. Bridging And Filling-(i) During drying, the film may shrink and pull away from the

sharp corners of an intagliation or bisect, resulting in a bridging

of the surface. This defect can be so severe that the monogram or

bisect is completely obscured. This mainly represents a problem

with the formulation.

REMEDY: Increasing the plasticizer content or changing the

plasticizer can decrease the incidence of bridging.

(ii) 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.

REMEDY: Judicious monitoring of the fluid application rate and

thorough mixing of the tablets in the pan can prevent filling.

5. Blistering-When coated tablets require further drying in ovens, too rapid

evaporation of the solvent from the core and the effect of high

temperature on the strength, elasticity and adhesion of the film

may result in blistering.

REMEDY: Milder drying conditions are warranted in this case.




19/23

6. Hazing / Dull Film-This is sometimes called 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 temperatures.

It can also occur if the coated tablets are exposed to high

humidity conditions and partial salvation of film results.

7.Color Variation-This problem can be caused by processing conditions or the

formulation.

Improper mixing, uneven spray pattern and insufficient coating

may result in color variation.

The migration of soluble dyes, plasticizers and other additives

during drying may give the coating a mottled or spotted

appearance.

REMEDY: (i) The use of lake dyes eliminates dye migration.

(ii) A reformulation with different plasticizers and additives is thebest way to solve film instabilities caused by the ingredients.

8. Cracking-It occurs if internal stresses in the film exceed the tensile

strength of the film.

REMEDY: tensile strength of the film can be increased by Using

higher molecular weight polymers or polymer blends.

References:1. M.E.Aulton, Pharm. Acta. Helv. 56(45), 133136 (1981).

2. K.E.Wilson and E. Crossman, Drug Dev. Ind. Pharm. 23(12), 1239

1243 (1997).




20/23

3. L.L.Young (Ed.),Tableting Specification, Manual (American

Pharmaceutical, Association, Washington DC,USA, 2003), pp 54

61.

4. M.P. Jordan, J.Taylor and P.J. Hadfield,, Contributed paper, AAPS

National, Meeting (Denever, CO, USA) 2001.

5. G.R.B. Down et al., Drug Dev.Ind., Pharm. 19(20), 27432749

(1993).

6. C.R. Cunningham, B. Korchok and, F. Nuneviller,Contributed

paper, CRS , Annual Meeting (Newport Beach, CA, USA) 2004.

7. E. Okutgen et al., Drug Dev. Ind.Pharm. 17(14), 20052016

(1991).

8. N. Poukavoos and G.E. Peck, Pharm. Res. 10(9), 13631370(1993).

9. D. Faroongsang and G.E. Peck, Drug Dev .Ind. Pharm. 20(10),

17771794 (1994).

10.A.M.Twitchell, J.E. Hogan and M.E.Aulton, STP Pharma. Sci. 5,

190195 (1995).

11. R.Thibert and B.C. Hancock,J. Pharm. Sci. 85(Nov), 12551258

(1996).

12. A.H. Kibbe (Ed.), Handbook of Pharmaceutical Excipients

(Pharmaceutical Press, London, UK, 2000) pp 501504.

13.C.R. Cunningham, B.K. Kinsey and L.K.Scattergood,

Pharm.Technol. Eur. 13(5),4453 (2001).

About Author:

Shridhar J. Pandya




21/23

Shridhar pandya. completed B.Pharm form TVES College of

Pharmacy, faizpur, Jalgaon. Presently he is pursing M.Pharmacy at

AISSMS College of Pharmacy, Pune, he is working on Taste

masked formulations of various bitter taste drugs. He also worked

on orodispersible tablets of bitter drugs.

Amit PatelAmit Patel completed B.Pharm form LM College of Pharmacy,

ABD. Presently he is pursing M.Pharm at MS Uni. Baroda, he is

working on Nano Suspension

Golakiya Nikul. VDixit K. Shah

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