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POLYMERS FOR CONVENTIONAL FILM COATING
Jun 16, 2018

POLYMERS FOR CONVENTIONAL FILM COATING

The term conventional film coating has been used here to describe film coatings applied for reasons of improved product appearance, improved handling, and prevention of dusting, etc. This is to make a distinction with functional film coats, which will be described in a later section, and where the purpose of the coating is to confer a modified release aspect on the dosage form. An alternative term for conventional film coating, therefore, would be non-functional film coating.


Cellulose ethers
The majority of the cellulose derivatives used in film coating are in fact ethers of cellulose. Broadly they are manufactured by reacting cellulose in alkaline solution with, for example, methyl chloride, to obtain methylcellulose. Hydroxypropoxyl substitution is obtained by similar reaction with propylene oxide.
The product is thoroughly washed with hot water to remove impurities, dried and finally milled prior to packaging.
The structure of cellulose permits three hydroxyl groups per repeating anhydroglucose unit to be replaced, in such a fashion. If all three hydroxyl groups are replaced the degree of substitution (DS) is designated as 3, and so on for lower degrees of substitution. The term molar substitution (MS) covers the situation where a side chain carries hydroxyl groups capable of substitution and takes into account the total moles of a group whether on the backbone or side chain. Both DS and MS profoundly affect the polymer properties with respect to solubility and thermal gel point. The polymer chain length, together with the size and extent of branching, will of course determine the viscosity of the polymer in solution. As a generality, film coating demands polymers at the lower end of the viscosity scale.This polymer provides the mainstay of coating with the cellulose ethers and its usage dates back to the early days of film coating. It is soluble in both aqueous media and the organic solvent systems normally used for film coating. HPMC provides aqueously soluble films which can be coloured by the use of pigments or used in the absence of pigments to form clear films. The polymer affords relatively easy processing due to its non-tacky nature. A typical low-viscosity polymer can be sprayed from an aqueous solution containing around 10–15%w/w polymer solids. From the regulatory aspect, in addition to its use in pharmaceutical products, HPMC has a long history of safe use as a thickener and emulsifier in the food industry. USP and JP recognize definite substitution types in separate monographs.
The first two digits of the four-digit designation specify the nominal percentage of methoxyl groups while the final two specify the nominal percentage of hydroxypropoxyl groups. The EP has no specified ranges for substitution. Significant differences exist between the USP and EP monographs. These relate to tighter requirements for ash, chloride for the EP which also possesses tests on solution colour, clarity and pH. Methodology differences also exist, particularly with regard to solution viscosity. The JP has a very low limit on chloride content.


1.Methylcellulose (MC)
Substituent group: —CH3
This polymer is used rarely in film coating possibly because of the lack of commercial availability of low viscosity material meeting the appropriate compendial requirements. As a distinction from the USP and the JP the EP has no required limits on the content of methoxyl substitution. However, the USP and JP have slightly different limits, which are 27.5–31.5% against 26.0–33.0% respectively.


2.Hydroxyethyl cellulose (HEC)
Substituent group: —CH(OH)—CH3
This water-soluble cellulose ether is generally insoluble in organic solvents. The USNF is the sole pharmacopoeial specification; there is no requirement on the quantity of hydroxyethyl groups to be present. The USNF allows the presence of additives to promote dispersion of the powder in water and to prevent caking on storage.


3.Hydroxypropyl cellulose (HPC)
Substituent group: —CH2 —CH(OH)—CH3
HPC has the property of being soluble in both aqueous and alcoholic media. Its films unfortunately tend to be rather tacky, which possess restraints on rapid coating; HPC films also suffer from being weak. Currently this polymer is very often used in combination with other polymers to provide additional adhesion to the substrate. The EB/BP has no requirements on hydroxypropoxyl content. The USNF states this must be less than 80.5% while the JP has two monographs differing in substitution requirements. The monograph most closely corresponding to the USNF material has a substitution
specification of 53.4–77.5%. The other monograph relates to material of much lower substitution
content and is used for purposes other than film coating, e.g. direct compression.








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