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Jun 18, 2018


Despite the considerable difference in application between a polymer intended for a simple conventional (non-functional) coating and one intended to confer a modified release performance on the dosage form, the categorizing of the polymers themselves into these divisions is not such an exact process. Several
examples exist of polymers fulfilling both needs, hence there is a considerable overlap of use. However, the divisions used here represent perhaps the majority practice.

1. Methacrylate ester copolymers

Structurally these polymers bear a resemblance to the methacrylic acid copolymers but are totally esterified with no free carboxylic acid groups. Thus these materals are neutral in character and are
insoluble over the entire physiological pH range. However they do possess the ability to swell and become permeable to water and dissolved substances so that they find application in the coating of
modified release dosage forms. The two polymers Eudragit RS and RL, can be mixed and blended to achieve a desired release profile. The addition of hydrophilic materials such as the soluble cellulose
ethers, polyethylene glycol (PEG), etc., will also enable modifications to be achieved with the final formulation. The polymer Eudragit RL is strongly permeable and thus only slightly retardant. Its films are therefore also indicated for use in quickly disintegrating coatings. The polymers themselves have solubility characteristics similar to the methacrylic acid copolymers. For aqueous spraying a latex form of each polymer is available. In addition the polymer Eudragit NE30D has been made for this purpose. This materal is also used as an immediate-release nonfunctional coating in film coat formulations where relatively large quantities of water-soluble materials are added to ensure efficient disruption of the coat.

2. Ethylcellulose (EC)

Substituent group (Fig. 2.2): —CH2—CH3
Ethylcellulose is a cellulose ether produced by the reaction of ethyl chloride with the appropriate alkaline solution of cellulose. Apart from its extensive use in controlled release coatings, ethylcellulose
has found a use in organic solvent-based coatings in a mixture with other cellulosic polymers, notably HPMC. The ethylcellulose component optimizes film toughness in that surface marking due to handling is minimized. Ethylcellulose also conveys additional gloss and shine to the tablet surface. In many ways ethylcellulose is an ideal polymer for modified release coatings. It is odourless,
tasteless and it exhibits a high degree of stability not only under physiological conditions but also under normal storage conditions, being stable to light and heat at least up to its softening point of c. 135°C (Rowe, 1985). Commercially, ethylcellulose is available in a wide range of viscosity and substitution types giving a good range of possibilities for the formulator. It also possesses good solubility in common solvents used for film coating but this feature is nowadays of lesser importance with the advent of water-dispersible presentations of ethylcellulose which have been especially designed for modified release coatings. The polymer is not usually used on its own but normally in combination with secondary polymers such as HPMC or polyethylene glycols which convey a more hydrophilic nature to the film by altering its structure by virtue of pores and channels through which drug solution can more
easily diffuse. Only the USNF contains a monograph, an ethoxy group content of between 44.0 and 51.0% is specified. The USNF also contains a monograph ‘Ethylcellulose Aqueous Dispersion’ which defines one type of such material which finds a use in aqueous processing. The monograph permits the presence of cetyl alcohol and sodium lauryl sulphate which are necessary to stabilize the dispersion.


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