PHR 221: Pharmaceutical Technology II

Course Teacher: Mohammad Nasir Uddin

Lecturer, Department of Pharmacy, NSU

Tablet Coating

Nonenteric Materials

Hydroxypropyl methyl cellulose (HPMC)

Methyl hydroxyethylcellulose

Ethylcellulose (EC)

Hydroxypropylcellulose (HPC)

Povidone

Sodium carboxymethylcellulose

Types of Film Former or Polymer

Hydroxypropyl Methylcellulose (HPMC)

HPMC closely approaches the desired attributes of an ideal polymer for film coating

The reasons for widespread use of HPMC include:

1. Solubility characteristics of the polymer in gastrointestinal fluid, and in organic and aqueous systems.

2. Noninterference with tablet disintegration and drug availability.

3. Flexibility, chip resistance and absence of taste or odor.

4. Stability in the presence of heat, light, air or reasonable levels of moistures.

5. Ability to incorporate color and other additives into the film without difficulty.

Hydroxypropyl Methylcellulose (HPMC)

Material of choice for air suspension and pan spray coating

systems

The interaction of this polymer with colorants is rare

This polymer is also used considerably in glossing solution

Limitation:

When used alone, the polymer has the tendency to bridge or fill the debossed tablet surfaces. A mixture of HPMC and other polymers or plasticizers are is used to eliminate bridging or filling problems.

Povidone

Available in four viscosity grades which are K-15, K-30, K-60

and K-90. The K number refers to the mean molecular weight of

povidone.

Povidone (K-30) is widely used in pharmaceutical industry as a

tablet binder and in tablet coating.

Povidone has excellent solubility in a wide variety of organic

solvents, in water and in gastrointestinal fluids.

Although povidone is soluble in both acidic and basic fluids, it

can be cross-linked with other materials to produce films with

enteric properties.

Povidone has been used to improve the dispersion of colorants

in coating solution to obtain a more uniformly colored film.

Polyethylene Glycol (PEG)

PEG is available in a wide variety of molecular weights ranging

from low (200 to 600 series) to high (900 to 8,000 series).

PEG with low molecular weights are liquid at room temperature

and used as plasticizers for coating solution films.

PEG with high molecular weights are white, waxy solids at room

temperature and used in combination with other polymers to

modify film properties.

Coats with high molecular weight PEG are hard, smooth, tasteless

and non toxic but somewhat sensitive to elevated temperature.

Enteric Materials

Cellulose acetate phthalate (CAP)

Hydroxypropyl methylcellulose phthalate (HPMCP)

Polyvinyl acetate phthalate (PVAP)

Acrylate polymer

Types of Film Former or Polymer

Hydroxypropyl Methylcellulose Phthalate (HPMCP)

Three grades are commercially available which are HPMC 50,

55 and 55S.

These polymers dissolve at lower pH (5 to 5.5) than CAP or

acrylate copolymers and thus solubility characteristics may

result in higher bioavailability of some specific drugs.

For general enteric preparation, HP-55 is recommended

These polymers are quite stable compared with CAP.

Cellulose Acetate Phthalate (CAP)

CAP dissolves at higher pH (˃ 6) and thus delay the absorption

of drugs.

It is hygroscopic and relatively permeable to moisture and

gastric fluids in comparison with other enteric polymers.

CAP films are susceptible to hydrolytic removal of phthalic and

acetic acids, resulting in a change of film properties.

CAP films are brittle and usually formulated with hydrophobic

film forming materials to achieve a better enteric film.

Plasticizers are generally added to film coating formulations to modify the physical properties of the polymer to make it more usable.

When incorporated with film former or polymer, plasticizer increase film flexibility and film tensile strength.

To choose an ideal plasticizer following factors should be considered :

Viscosity of the plasticizer It’s influence on the final coating solution It’s effect on film permeability, tackiness, flexibility, solubility

and taste It’s toxicity, compatibility with other coating solution

components

Plasticizers

Examples of commonly used plasticizers are:

Polyols such as polyethylene glycol and propylene glycol

Organic esters, such as diethyl phthalate and triethyl citrate

Oils/glycerides, such as fractionated coconut oil

Surfactants such as polysorbates (Tweens), sorbitan esters

(Spans)

Plasticizers

The primary function of a solvent system is to dissolve or disperse polymers and other additives and convey them to the substrate surface.

Characteristics of an ideal solvent system:

1. It should either dissolve or disperse the polymer system.2. It should easily disperse other coating components into the

solvent system.3. Small concentration of polymers (2-10%) should not result in

an extremely viscous solution system (>300cps), creating processing problems.

4. It should be colorless, tasteless, inert and non-inflammable.5. It should have a rapid drying rate (the ability to coat a 300

kg/load in 3 to 5 hours)6. It should have no environmental impact.

Solvents

The most widely used solvents either alone or in combination

are water, ethanol, methanol, isopropanol, chloroform, acetone

etc.

Water is the solvent of choice because of environmental and

economic considerations.

Drugs that readily hydrolyze in the presence of water can be

more effectively coated with non-aqueous solvent based

coatings. Such a process might require applying an initial

sealing coat from organic based sub coating followed by

aqueous color and gloss coating.

Solvents

Pharmaceutically acceptable colorants are available in both

water soluble form (known as dye) and water insoluble form

(known as pigments).

The water insoluble form or pigment is preferred for film

coating formulations due to following reasons:

Pigments tend to be more chemically stable towards

light

Provide better opacity and covering power

Provide a means of optimizing the permeability

properties of the applied film coating

Colorants

Examples of colorants are:

Iron oxide pigments

Titanium dioxoide

Aluminium lakes

Colorants

These are very fine inorganic powders used in coating solution

formulations to provide more pastel colors and increase film

coverage.

These opaquants can provide a white coating or mask the color

of the tablet core.

Less colorants need to be used when opaquants are used.

Most commonly used opaquant is titanium dioxide. Some other

materials are silicates (talc, aluminium silicate), carbonates

(magnesium carbonate), sulfates (calcium sulfates), oxide

(magnesium oxide) and hydroxides (aluminium hydroxide).

Opaquant-Extenders

Flavors or sweeteners can be added to mask objectionable odor

or to enhance a desired taste.

Surfactants can be used to solubilize insoluble ingredients or to

facilitate faster dissolution of the coating.

Antioxidants can be added to stabilize a dye system to

oxidation and color change.

Antimicrobials can be added to prevent microbial growth in the

coating composition during its preparation and storage and on

the coated tablets.

Miscellaneous Coating Solution Components

Two methods are used: 1. Pan-Pour and 2. Pan-Spray Methods

1. Pan-Pour Methods:

The coating solutions are poured over the tablet cores. The operator determines the quantity of solution to add, the

method and rate of the pouring, when to apply drying air and how long or how fast the tablets should be tumbled in the pan.

Tablets coated by pan pour methods are subjected to alternate solution application, mixing and drying steps.

This method is relatively slow and relies heavily on the skills and technique of the operator to balance the steps to produce an acceptable product.

Tablets that are coated by pan pour method almost always require additional drying steps.

Film Coating Process

1. Pan-Spray Methods:

The introduction of spraying equipment has improved the

coating efficiency process.

Spraying allows for automated control of liquid application.

Spray patterns are selected to provide a continuous and across

the tablet bed surface.

Broad, flat spray patterns are usually chosen by selection of

appropriate nozzle systems so that the entire width of the tablet

bed can be covered by the spray from 1 to 5 nozzles.

Film Coating Process

Process Variables

The variables to be controlled in pan-spray film coating processes are:

1. Pan variables• Pan design/baffling• Speed• Pan load

2. Process air• Air quality• Temperature• Airflow rate/volume/balance

3. Spray variables• Spray rate• Degree of atomization• Spray pattern• Nozzle-to-bed distance

Film Coating Process