Film Coating Technology

Tablet coating is perhaps one of the oldest pharmaceutical processes still in existence. The sugar-coating process was a skilled manipulative operation and could last for even five days. The operator must be highly skilled for such coating. In the last 25 years tablet coating has undergone several fundamental changes. Many modifications were advocated to improve the basic process and film coating chosen in place of sugar coating. Coating solution composition may affect the quality of final coated tablets. Some coating process parameters are affect the final quality of coated tablets so it is necessary to optimize the coating process parameters for particular equipment and particular film former. Optimization of composition of film coating solution is also required.
Keywords : Film coating, Coating Process parameters, Coating composition

Introduction

All drugs have their own characteristic, like some drugs are bitter in taste or has an unpleasant odor, some are sensitive to light or oxides, some are hygroscopic in nature.^1,2,3 Because of this reason tablet coating is the choice of option to solve such problems in conventional dosage form.
In the past sugar coating was mostly borrowed from the confectionary industry. But now a days it is replaced with film coating, because the sugar coating process was a skilled manipulative process and could last for even five days. The operator must be highly skilled for such coating. Hence film coating is preferred over sugar coating.
Tablet film coating is performed by two types, one is aqueous film coating (generally water is used as a solvent) and non aqueous film coating (generally organic solvent are used.) Some problems are associated with the non aqueous film coating like employee safety (it’s dangerous, it smells, and it’s not good to breathe.) atmosphere pollution etc.  But key problem is with the approval of the regulatory authority⁴.  High quality  aqueous  film  coating  must  be  smooth,  uniform  and  adhere satisfactorily to the tablet surface and ensure chemical stability of a drug.

“ Why Tablet Coating is Required ? ” ^{1, 2, 3}

A number of reasons can be suggested:
§ The core contains a material which has a bitter taste in the mouth or has an unpleasant odour.
§ Coating will protect the drug from the surroundings with a view to improve its stability.
§ Coating will increase the ease by which a tablet can be ingested by the patient.
§ Coating will develop the mechanical integrity, means coated products are more resistant to mishandling (abrasion, attrition etc.)
§ The core contains a substance which is incompatible in the presence of light and subject to atmospheric oxidation, i.e. a coating is added to improve stability.
§ The core alone is inelegant.
§ The active substance is coloured and migrates easily to stain hands and clothes.
§ The coated tablets is packed on high-speed packaging machine. Coating reduces friction and increases packaging rate.
§ Coating can modify the drug release profile, e.g., enteric coating, osmotic pump, pulsatile delivery.

Introduction to Film Coating Materials

A  film  coating is  a  thin  polymer-based  coat  applied to  a  solid dosage  form  such as  a tablet. The thickness of such a coating is usually between 20-100 µm.  Under  close  inspection the  film  structure can  be  seen to  be  relatively non- homogenous  and  quite distinct  in  appearance, from  a  film  forming,  from  casting a polymer solution on a flat surface.^{5, 6}

Film coating formulations usually contain the following components

Polymer,
Plasticizer,
Colourants / Opacifiers,
Solvent / Vehicle.

Polymers

Amongst the vast majority of the polymers used in film coating are cellulose derivatives or acrylic polymers and copolymers.^{5, 6, 7}

 Non-enteric polymers⁸

·Hypromellose
·Hydroxyethyl cellulose
·Hydroxyethylmethyl cellulose
·Carboxymethylcellulose sodium
·Hydroxypropyl cellulose
·Polyethylene glycol
·Ethylcellulose

Enteric polymers

Some examples of enteric coating polymers

·Hypromellose phthalate
·Polyvinyl acetate phthalate
·Cellulose acetate phthalate
·Polymethacrylates
·Shellac

Plasticizers

Plasticizers are simply relatively low molecular weight materials which have the capacity to alter the physical properties of the polymer to render it more useful in performing its function as a film-coating material.^{7, 8, 9} It  is   generally   considered  to   be   mechanism   of   plasticizer   molecules  to   interpose themselves  between  individual  polymer  strands thus  breaking  down  polymer-polymer interactions. Thus polymer is converted in to more pliable materials. Plastisizers are classify in three groups. Polyos type contain glycerol, propylene glycol, PEG( Polyethylene glycol ). Organic esters contain phthalate esters, dibutyl sebacete, citrate esters, triacetin.  Oils/glycerides contain castor oil, acetylated, monoglycerides, fractionated coconut oil.

Solvents/Vehicles

The  key  function  of  a solvent  system  is to  dissolve  or disperse  the  polymers and other   additives.   All   major   manufactures   of  polymers   for   coating  give   basic physicochemical data on their polymers. These data are usually helpful to a formulator. Some important considerations for solvent are as follows:⁶

The major classes of solvents being used are

·Water
·Alcohols
·Ketones
·Esters
·Chlorinated hydrocarbons
Because of environmental and economic considerations, water is the solvent of choice; however  organic  coating  is  totally  cannot  be avoided. 

Colourants / opacquants

The sematerials are generally used as ingredients in film-coating formulae to contribute to the visual appeal of the product, but they also improve the product in other ways^7,8,9:
Identification of the product by the manufacturer and therefore act as an aid for existing GMP procedures.
-  Reinforcement of brand imaging and reduction in product counterfeiting.
-  Identification of the product by patients by using colourants.
Colourants  for film  coating  are having,  in  more or  less  amount, property  of opacifier.  So  they  would  give protection  to  active ingredients  in  presence of light. Colourants are mainly classified in to three part. Sunset yellow, tartrazine, erythrosine are examples of Organic dyes and their lakes. Iron oxide yellow, red and black, titanium dioxide, talc are the examples of Inorganic colours. Anthrocyanins, ribofloavine and carmine are the examples of natural colours.

Miscellaneous coating solution components

To  provide  a dosage  form  with a  single  characteristic,  special  materials  may  be incorporated into a solution⁶.
Flavours and sweeteners are added to mask unpleasant odours or to develop the desired taste. For example, aspartame, various fruit spirits (organic solvent), water soluble pineapple flavour (aqueous solvent) etc.
Surfactants  are  supplementary  to  solubilize  immiscible  or  insoluble  ingredients  in  the coating. For example, Spans,Tweens etc.
Antioxidants are incorporated to  stabilize  a dye  system to oxidation  and colour change. For example oximes, phenols etc.
Antimicrobials are added to put off microbial growth in the coating composition. Some  aqueous  cellulosic  coating  solutions  are  mainly  prone  to  microbial growth, and long-lasting storage of the coating composition should be avoided. For example alkylisothiazloinone, carbamates, benzothiazoles etc.

Coating Process

Film-coating of tablets is a multivariate process, with many different factors, such as coating equipment, coating liquid, and process parameters which affect the pharmaceutical quality of the final product^10-13.

Coating equipment¹⁴

Before few years different types of coating pans are used for coating like conventional coating pans, manesty accelacota, driam ( driacoater ), butterfly coater etc. Now a days the side-vented, perforated pan-coater is the most commonly used coating device of tablets. In equipment spray nozzle, number of spray nozzle, pan size, etc may also affect the quality of final product.  Its air flow system through a perforated pan ensures rapid and continuous drying conditions. The low evaporation capacity of water requires high drying efficiency of aqueous film-coating equipment.

Coating liquid

Coating liquid may affect the final quality of the tablets. Different film former have different chemical nature and different charesteristic. Viscosity may affect the spreading of coating liquid across surface of substrate.  Surface tension may affect in wetting of surface. % Solid content generally affect the tablet surface and coating efficiency.¹⁵

Process parameters

Spray rate

The spray rate is an significant parameter since it impacts the moisture content of the formed coating and, subsequently, the quality and uniformity of the film.^16,17,18 A low coating liquid spray rate causes incomplete coalescence of polymer due to insufficient wetting, which could effect in brittle films¹⁶. A high coating liquid spray rate may result in over wetting of the tablet surface and subsequent problems such as picking and sticking.^16,17 If the spray rate is high and the tablet surface temperature is low, films are not formed during the spraying but the post drying phase, and rapid drying often produces cracks in the films¹⁶.

Atomizing air pressure

In general, increasing the spraying air pressure decreases the surface roughness of coated tablets and produces denser and thinner films.^19,20,21  If spraying air pressure is excessive, the spray loss is great, the formed droplets are very fine and could spray-dry before reaching the tablet bed, resulting in inadequate droplet spreading and coalescence²¹. If spraying air pressure is inadequate, the film thickness and thickness variation are greater possibly due to change in the film density and smaller spray loss. In addition, with low spraying air pressure big droplets could locally over wet the tablet surface and cause tablets to stick to each other.

Inlet air temperature

The inlet air temperature affects the drying efficiency (i.e. water evaporation) of the coating pan and the uniformity of coatings¹⁹. High inlet air temperature increases the drying efficiency of the aqueous film coating process and a decrease in the water penetration into the tablet core decreases the core tablet porosity, tensile strength and residual moisture content of coated tablets.^19,22 Too much air temperature increases the premature drying of the spray during application and, subsequently, decreases the coating efficiency.^18,23 Measuring the pan air temperature helps to manage the optimum conditions during the coating process and, consequently, enables predicting possible drying or over wetting problems which may result in poor appearance of the film or may have unfavorable effects on the moisture and heat sensitive tablet cores.²⁴

Rotating speed of pan

It is well documented that increasing the rotating speed of the pan improves the mixing of tablets.^23-27 The pan speed affects the time the tablets spend on the spraying zone and, subsequently, the homogeneous distribution of the coating solution on the surface of each tablet throughout the batch. Increasing the pan speed decreases the thickness variation and increase the uniformity of coatings.^16,23,26  Too much  rotating speed of the pan will cause the tablet to undergo unnecessary attrition and breakage.

Conclusion

Film coating technology is now a days very important in the field of pharmacy particularly in formulation development. Process parameters and coating composition play an important in coating of tablets. So for getting good final quality of coated tablet it would be necessary to optimize the parameters.

References

1. Cole G. Pharmaceutical Coating Technology,Taylor and  Francis Ltd, 1998;1-5.
2.Porter C. Coating of Pharmaceutical Solid-dosage forms, Pharm. Tech., 1980,4(3), 66.
3.Libermen H, Lachman L, Pharmaceutical Dosage Forms: Tablets, Vol. I to III, Marcel Dekker Inc., N.Y, 85-143.
4.Hinkes T, Solvent  film  coating:  aqueous  vs.  Organic, Wisconsin  Alumni Research Foundation Madison, Wisconsin.
5.Nyamweya N, Mehta K. Film Coating with Aqueous  Latex Dispersions, Pharmaceutical Technology Yearbook 2001.
6.Hogan J. Pharmaceutical Coating Technology, Taylor  and  Francis Ltd 1998; 6-52.
7.Rowe R, Sheskey P, Owen S. Pharmaceutical Excipients.
8.Martini  L, Ford J, Roberts  M. The  use of hypromellose in oral drug delivery, J Pharm Pharmacol. 2005,57,533-546.
9. Porter  S. Coating  of  Pharmaceutical  Dosage  forms, Remington’s book  of science,volume 1, Ch 46; 894-902.
10.Rowe R. The effect of some formulation and process variables on the surface roughness of film-coated tablets. J. Pharm. Pharmacol 1978a,30,669-672.
11. Elaine S, Celine V, Dawn Z, Xiaohua L,  Anthony J, Paul W. Study of Coat Quality of Tablets Coated by an On-line Supercell Coater, AAPS PharmSciTech 2007,8(3), Article 63.
12. Tobiska S, Peter K. Coating uniformity and coating efficiency in a Bohle Lab-Coater using oval tablets European Journal of Pharmaceutics and Biopharmaceutics. 2003,56,3-9.
13. Philip A,  Rowe R, York P, Doherty C. The effect of experimental design on  the  modeling  of a tablet coating formulation  using  artificial neural  networks;European  Journal  of  Pharmaceutical  Sciences. 2002,16,281-288.
14.Heinamaki. J,  Ruotsalainen M, Lehtola V, Antikainen O, Yliruusi J. Optimization of Aqueous-Based Film Coating of Tablets Performed by a Side-Vented Pan- Coating System. Pharmaceutical  Development and Technology. 1997,2(4),357-364.
15.Optimal Coating Process Parameters for a New, Fully-Formulated, Acrylic-based, Enteric, Film-Coating System,Poster ReprintAmerican Association ofPharmaceutical Scientists,November 2000.
16. Obara S, McGinity J. Influence of processing variables on the properties of free films prepared from aqueous polymeric dispersions by a spray technique. Int. J. Pharm.1995,126,1-10.
17. Franz R, Doonan G. Measuring the surface temperature of tablet beds using infrared thermometry. Pharm Technol. 1983,7,55-67.
18.Porter S, Verseput R, Cunningham C. Process optimization  using design of experiments. Pharm. Technol. 1997,21,60-70.
19. Twitchell A, Hogan J, Aulton M. The behaviour of film coating droplets on the impingement onto uncoated and coated tablet. S.T.P. Pharm. Sci. 1995a,5,190-195.
20. Twitchell A, Hogan J, Aulton M. Assessment of the thickness variation and surface roughness of aqueous film coated tablets using a light-section microscope. Drug Dev Ind Pharm 1995b,21,1611-1619.
21.Tobiska S, Kleinbudde P. Coating Uniformity: Influence of atomizing air pressure. Pharm. Dev. Tech. 2003,8,39-46.
22. Poukavoos N, Peck G. Effect of aqueous film coating conditions on water removal efficiency and physical properties of coated tablet cores containing superdisintegrants. Drug Dev. Ind. Pharm. 1994,20,1535-1554.
23. Rege B,  Gawel J, Kou H. Identification  of  critical  process  variables  for  coating  actives onto tablets  via  statistically  designed  experiments, International  Journal  of  Pharmaceutics. 2002,237,87-94.
24. Okutgen E, Jordan M, Hogan J, Aulton, M. Effects of tablet core dimensional instability of the generation of internal stresses within film coats. Part II: Temperature and relative humidity variation within a tablet bed during aqueous film coating in an Accela-Cota. Drug. Dev. Ind. Pharm. 1991b,17,1191-1199.
25. Tobiska S, Kleinbudde P. A simple method for evaluating the mixing efficiency of a new type of pan coater. Int. J. Pharm. 2001,224,141-149.
26. Wilson K, Crossman E. The influence of tablet shape and pan speed on intra-tablet film coating uniformity. Drug Dev Ind Pharm 1997,23,1239-1243.
27.Skultety P, Rivera D, Dunleavy J, Lin C. Quantification of the amount and uniformity of aqueous film coating applied to tablets in a 24" Accela-Cota. Drug Dev.Ind. Pharm. 1988,14,617-631.