AQUEOUS FILM COATING

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Seminar on AQUEOUS FILM COATING BY N.RAJU

CONTENTS:

CONTENTS INTRODUCTION AQUEOUS FILM COATING OF DOSAGE FORM Film formation mechanism Film formers Plasticizers and colours PROCESS PARAMETERS HOW THE COATING PROCESS WORKS COATING EQUIPMENTS COATING DEFECTS SOLVENT FILM COATING

Introduction:

Introduction COATING : I T is the application of coating composition on to the moving bed of tablets with concurrent use of heated air to facilitate evaporation of the solvent.

Water Soluble Polymers . :

Water Soluble Polymers . Hydroxypropyl methylcellulose (HPMC) Methylcellulose (MC) Hydroxypropyl cellulose (HPC) Polyvinylpyrrolidone (PVP) Food starch (modified) Water-soluble coating materials dissolve completely in the gastrointestinal tract and do not modify the drug release characteristics of the dosage. These polymers are applied as aqueous solutions.

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Hydroxypropyl methylcellulose ADVANTAGES: Solubility characteristics of the polymer in gastrointestinal fluid, and in organic and aqueous solvent systems. Noninterference with tablet disintegration and drug availability. Flexibility, chip resistance, and absence of taste or odor. Stability in the presence of heat, light, air, or reasonable levels of moisture. Ability to incorporate color and other additives into the film without difficulty. The interaction of polymer with colorants is rare. It forms a transparent, tough and flexible film that protects fragile tablets, masks the unpleasant taste of a drug and improves the appearance.

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COMMON FILM FORMING MATERIALS IN SUSTAINED RELEASE PRODUCTS Sustained Release: Ethyl cellulose (EC) Methacrylic acid copolymers ( Eudragit types) Enteric Release: Cellulose acetate phthalate (CAP) Hydroxypropyl methylcellulose phthalate (HPMCP) Polyvinyl acetate phthalate (PVAP)

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Ethyl cellulose This material is completely insoluble in water and gastrointestinal fluids. A combination of ethyl cellulose with water soluble additives has been widely used in preparing sustained release coatings of fine particles.

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Effect of polymer molecular weight on coating properties Property Effect of increasing polymer molecular weight Tensile strength Increases. Elastic modulus Increases. Film adhesion Decreases. Solution viscosity Significantly increases.

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WATER SOLUBLE: Polyethylene glycol (PEG) Propylene glycol (PG) WATER INSOLUBLE: Tributyl citrate (TBC) Acetylated monoglyceride (AMG) Castor oil PLASTICIZERS Film coatings prepared from pure polymers tend to be brittle and crack upon drying. The addition of plasticizers to the coating liquid decreases the intermolecular forces along the polymer chains by relieving molecular rigidity. The plasticizer improves the flexibility and reduces the brittleness of the film coating and makes it more resistant to mechanical stress during the coating process.

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Effects of Plasticizers on the properties of film coatings Property Effect of increasing plasticizer concentration Tensile strength Decreased. Elastic modulus Decreased. Film adhesion May be increased, but results often variable. Solution viscosity Increased, and magnitude of effect dependent on molecular weight of plasticizer. Film permeability can be increased or decreased, depending on chemical nature of plasticizer. Glass-transition temperature Decreased, but magnitude of effect dependent on compatibility with polymer.

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COLOURS Primarily used for tablet identification. Aluminum lakes Iron oxides or natural colours such as riboflavin and carotenoids .

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Effects of Pigments on the properties of film coatings Property Effect of increasing pigment concentration Tensile strength Decreases Elastic modulus Increases Film adhesion Little effect. Solution viscosity Increases, but not substantially. Film permeability Decreases, unless critical pigment volume concentration is exceeded.

Formation of films from aqueous polymeric dispersions:

This requires the coalescence of polymer particles into a continuous film. This process involves: Rapid evaporation of water, causing the particles of dispersed polymer to be brought into close contact with one polymer. Development of pressures (associated with capillary forces within the structure) that overcome repulsive forces between particles and cause deformation of the polymer particles. Gradual coalescence of the polymer particles as a result of viscous flow and movement of polymer molecules across the interfaces between particles. Aqueous polymeric dispersions must be processed at temperatures in excess of the glass-transition temperature of the polymer. Formation of films from aqueous polymeric dispersions

Aqueous Film Coating Formulation :

Aqueous Film Coating Formulation Film Coating Composition Film-forming polymer 7.0-18.0% Plasticizer 2.5-8.0% Pigment/Colorant 0.5-2.0%

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Process Parameters Many quality aspects of the final coated product are greatly influenced by the combined effect of process parameter values used in aqueous film coating. Coating process parameters affect the spreading, penetration and drying (i.e. evaporation of water) of the coating liquid on the tablet surface and, subsequently, the surface roughness and the residual moisture of the coated tablets.

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Process Parameters Spraying air pressure : The spraying air pressure disperse the coating liquid into droplets and effects the droplet size distribution and droplet spreading and penetration on the tablet surface. The formation of adequate and adhesive film coat, the atomized droplets have to spread completely over the surface of the tablet. Increasing the spraying air pressure decreases the surface roughness of coated tablets and produces denser and thinner films. 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 coalescence. Spraying air pressure is insufficient, the film thickness and thickness variation greater possibly due to change in film density and smaller spray loss.

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Flow rate of coating solution Successful aqueous coating process, the flow rate of the coating liquid is equal to the rate of water evaporation from the coated tablet surface. Increasing the flow rate allows greater number of droplets to be spread on to the tablet bed per time and increases droplet size. The flow rate is important parameter since it impacts the moisture content and the quality and uniformity of the film. Low coating liquid flow rate causes incomplete coalescence of polymer due to insufficient wetting, which result in brittle films. High coating liquid flow rate may result in over wetting of the tablet surface and subsequent problems such as picking and sticking.

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Pan air temperature The pan air temperature effects drying efficiency of the coating pan and the uniformity of the coatings. High inlet air temperature increases drying efficiency of aqueous film coating process and decreases in water penetration into the tablet coating. Excessive air temperature increases premature drying of the spray during application and subsequently decreases the coating efficiency.

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Rotating speed of the pan Increasing rotating speed of the pan improves mixing of the tablets. The pan speed effects the time the tablet spend on the spraying zone and subsequently, the homogenous distribution of the coating solution on the surface of each tablet throughout the batch. Increasing the pan speed decreases thickness variation and improves the uniformity of the coating. Too rapid rotating speed of the pan will cause the tablet to undergo excessive attrition and breakage.

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Application of the film Aqueous film coating requires equipment with high air flow and excellent control over the coating process. Aqueous film coating needs tighter control of the coating process than organic solvent-based coating systems. The tablets are normally designed to disintegrate in water, so the rate of application of the coating composition is critical; Too slow or too rapid application of the coating will cause the tablet to undergo excessive erosion and breakage. The following coating pans can provide adequate air flow and control to be used for aqueous film coating: Accela Cota. Hi-Coater. Dria Coater. .

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Accela Cota This is an angular pan operating on a horizontal axis. Drying air is directed into the pan, through the tablet bed, and exhausted out the perforations in the periphery of the pan.

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Hi-Coater This is similar to Accela Cota, but only a portion of the pan periphery has perforations. Like the Accela Cota, continuous venting of the exhaust air from the pan is still attained.

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The aqueous coating liqid is commonly applied by pnemutic (air) spray systems where the pressure of the spraying air disperses the coating liqid as appropritely sized droplets The coating of tablets in a coating pan involves spraying the coating compositions through one or more spray guns onto rotating bed of tablets. Coating process consists of the continuous application of coating liquids to a small portion of the tablets in the pan. The applied coating must dry before it touches the coating pan or receives its next application. To attain a continuous coating operation, the rate of water evaporation from the coated tablets must equal the rate of water applied in the coating liquid. The coating composition is also significant factor in establishing the tablet coating rate. Coating compositions that are quite tacky during the drying phase must be applied at a slower rate to avoid tablets sticking to pan surface or other tablets. General procedure

Common Coating Defects & Causes :

Common Coating Defects & Causes Rough/Orange Peel Appearance Causes : Not enough vehicle. High atomization air. Moving the nozzle closer to the tablet bed or reducing the degree of atomization remedy

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“ Picking ” Problem : Tablets are too wet. Possible Causes : Spray rate too high. Guns too close together. Insufficient atomizing air. Pan speed too low. REMEDY:Reduction in the liquid application rate or increases in air dry temparature

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Film Cracking Problem : Small cracks appear in the coating. Causes : Wrong plasticizer. Insufficient plasticizer. Solution too concentrated (Thick). Insufficient atomizing air.

Film Chipping :

Film Chipping Problem : Coating gone from the tablet edge. Causes : High pan rpm. Low spray rate. Both together. Sharp tablet edges.

Bridging of the Logo :

Bridging of the Logo Problem : The letters and numbers fill in with dried suspension. Causes : High spray rate Inadequate atomizing air. Poor tooling design.

Twins or Twinning :

Twins or Twinning Problem : Tablets stick together. Causes : High spray rate. Inadequate drying capacity. Tablet shape/design. Tablet too long.

Logo Erosion :

Logo Erosion Problem : Tablet erodes before coating can adhere to the surface. Causes : Spray rate too slow. Pan speed too fast. Both.

CRACKING:

CRACKING Cracking occurs if internal stress in the film exceed the tensile strength of the film can be increased by using higher-molecular –weight polymers or polymer blends .

HAZING/DULL FILM:

HAZING/DULL FILM It can occur when to high processing temperature used for a particular formulation. Dulling is particular evident when cellulosic polymers are applied out of aqueous media at high processing temperature.

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Aqueous coatings are in many ways less expensive than organic solvent coatings. The removal of water from water soluble films such as hydroxypropylmethyl cellulose, methylcellulose, gelatin, or starch requires the use of higher drying temperatures than does the removal of methanol from a film. This is not surprising when one considers their heats of evaporation. Another comparison between aqueous and organic film coating systems has to relate to time. The processing time for any film coating application is extremely important to meet production goals. Aqueous coatings require longer processing time than organic solvent systems. SOLVENT FILM COATING: AQUEOUS VS. ORGANIC Water and organic solvents differ in their support facilities. Because of their volatile and toxic nature, organic solvent must be handled and stored in areas designed for this purpose.

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Stability and bioavailability are greatly influenced by the selection of coating and the solvent system from which it is applied. Many drugs react in the presence of water; thus, residual moisture in the film coated product may present more of a problem than residual organic solvents. Although moisture can be eliminated by heat, some products are unstable at higher temperatures or the combination of heat and moisture. Aqueous films also tend to have a lower gloss level than the same polymers applied from organic solvents.

CONCLUSION :

CONCLUSION Cellulosic and acrylic polymeric films have been used to coat pharmaceutical drug products for decorative, protective and purposes. These materials are generally applied using a spray-atomization technique. Additional excipients, including plasticizers, pigments, anti adherents, and surfactants, may be incorporated into the coating formulation to aid in processing or to improve the esthetic appearance of the coated solid. The addition of these excipients, however, may alter polymer properties and affect drug release rates. In the development of a film coating formulation, various polymer properties are commonly evaluated and these data are used to make predictions regarding the dissolution characteristics and long-term stability of the final product. Adhesion of the polymer is critical to the performance of the film and may be affected by the physicochemical properties of the substrate, additives in the coating formulation, and processing conditions. Sub coats have been used to improve polymer adhesion as well as to separate reactive components in the film and substrate. Defects in the polymeric film may be eliminated by adjusting processing parameters or by reformulating the coating and/or the substrate. Aging of coated solids may cause changes in the drug release characteristics and polymer films should be completely coalesced to minimize aging problems.

REFERENCES:

REFERENCES Encyclopedia of pharmaceutical technology by James Swarbrick, James C.Boyane volume – 1 The Theory And practice of Industrial Pharmacy by Leon Lachmann and Herbert . A. Liebermann. Pharmaceutical dosage forms : Tablets volume-3 –Liebermann series. www.pubmed.gov www.fda.gov

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