**2. Plasticized polymers used as coatings of the dosage forms**

It is common practice to coat oral solid dosage forms with polymeric materials for several purposes, such as modifying drug release, affording gastroprotection, protecting from environmental agents, masking unpleasant taste or just enhancing product aesthetics. A film coating is defined as a thin and uniform polymer based-layer of about 20 to 100 µm in thickness, which is applied to the surface of substrates such as tablets, granules, powders, capsules, particles or pellets (Porter, 1995).

Polymers used in coating can be categorized into two types: (i) non-functional or conventional film coating polymers for immediate release coatings; which improve the appearance, the handling, prevent dusting or (ii) functional coating polymers, which can be used to modify the pharmaceutical function of the dosage forms, and include the delayed release dosage forms and sustained release (or extended or prolonged) dosage forms.

Coating films can be usually prepared from organic solvent-based polymers or aqueous solvent-based latex dispersions. However, the aqueous film coating have been favoured in coating technology because of its wide applications, low environmental concerns, efficiency of the process, and wide commercial availability of coating materials. The cellulose ethers and esters are the major group of polymers used in the film coating process. The

Pharmaceutical Applications of Plasticized Polymers 71

A sufficient amount of time should be allowed for the plasticizer uptake to ensure a homogeneous distribution of the plasticizer in the coating film. The time between the addition of the plasticizer to the polymer dispersion and the coating step is usually called as the plasticization time and presents a critical process variable, especially when waterinsoluble plasticizers are used. Water insoluble plasticizers have to be emulsified in the aqueous phase of the polymer dispersions. During plasticization of the polymer dispersions, the plasticizer partitions into the colloidal polymer particles and softens them thus promoting particle deformation and coalescence into a film upon drying. Commercial suppliers of polymer dispersions often recommend a relatively short plasticization time of 30 - 60 minutes irrespective of the solubility of the plasticizer. Plasticizers are incorporated in the amorphous parts of polymers while the structure and size of any crystalline part remains unaffected (Fedorko et al., 2003). The thermal treatment following the application of the coat is known as curing. During this stage of coating, the coated dosage form is subjected to temperatures higher than the Tg, which facilitates uniform distribution of plasticizers. During curing of the coatings, plasticizers could also redistribute in the

Controlled released pharmaceutical formulations comprising pseudoephedrine embedded in core and coating shell made from mixture of two different polymers were approved. One of the used polymers was water soluble, the second one water insoluble. The water insoluble

Good flexibility of the coating is essential for insuring the resistance of the coating to the mechanical impact force expected during the coating process, and in the application place of the coated dosage form (particularly in the gastrointestinal tract). However, pure polymer coatings are often brittle. An addition of an external plasticizer to the polymeric networks

The affirmative effect of the plasticizers on mechanical properties of the coating is caused by reducing of the cohesive intermolecular forces along the polymer chains. The flexibility is enhanced by increasing strain elongation and decreasing tensile strength and elastic

For example, triethyl citrate, triacetin and acetyltriethyl citrate used as a water-soluble plasticizer of the hydroxypropylmethylcellulose acetate succinate in press-coated tablets for colon targeting formulation ensure the plastic deformation property of the outer shell due to some interaction between hydroxypropylmethylcellulose acetate succinate and triethyl citrate (Fukui et al., 2001). Plasticized polymeric coating with dispersed bioactive antiinflammatory and anti-thrombogenic drugs was applied for stents (Udipi & Cheng, 2004).

Good adhesion between a polymer and the surface of a solid is a major prerequisite for the film coating of pharmaceutical dosage forms. Loss of adhesion may reduce the functions that the film-coating provides to the solid substrate, particularly the mechanical protection that the coating ensures to the solid substrate and the drug release modified by coating.

plasticizer ranged from 0.1 to 15.0 % by weight in this composition (Nair et al., 2010).

**2.1.2 Mechanical properties of the coating improved by plasticizer** 

will increase the flexibility of the coatings (Siepmann et al., 1999).

modulus of the polymer (Gutierrez-Rocca & McGinity, 1994).

**2.1.3 Adhesion of coating influenced by plasticizer** 

polymeric film.

pharmaceutical compositions of two kinds of coated beads were patented. The polymer coatings of the beads differed in water solubility. This pharmaceutical composition comprised phtalates, trimellitates, adipates, sebacates, glycols, polyethers and alkyl citrates as the plasticizers (Paborji & Flugel 2011).

Cellulose acetate phthalate (CAP), widely used in enteric film coating, is available as a white powder or also as a 30 % solid nanodispersion (pseudolatex) (Aquacoat® CPD). Polyvinyl acetate phthalate is less susceptible to hydrolysis, which minimizes or limits the content of free phthalic acid and other free acids.

Cellulose acetate trimellitate (CAT) has three carboxylic groups on the aromatic ring and dissolves at a pH of 5.5. The plasticizers as triacetin, acetylated monoglyceride, diethyl phthalate are recommended, and to obtain the best enteric coating results from aqueous processing, ammoniated solutions of CAT in water are the best choice.

Hydroxypropyl methylcellulose phthalate (HPMCP) characteristics, particularly at the pH where dissolution occurs, are determined by the degree of substitution of the three kinds of substituent groups (i.e. methoxy, hydroxypropoxy, and carboxybenzoyl). Valuable plasticizers include triacetin, acetylated monoglyceride and diethyl phthalate.

Acrylic polymers as a group of various synthetic polymers, methacrylic amino ester copolymers, have diverse functionality in film coating. They can be used for rapidly disintegrating coatings, taste and odour masking, coloured or transparent coatings, etc., and are available mainly as the Eudragit® products.

Methacrylic acid copolymers contain free carboxylic acid groups, and therefore can be used for enteric coating purposes by forming salts with alkalis. Methacrylic acid copolymers are soluble at pH values higher than 5.5. The addition of a plasticizer in these polymers is necessary. Methacrylic acid copolymers are available as various grades of Eudragit® products.
