Issue link: https://www.e-digitaleditions.com/i/292644
14 April 2014 Tablets & Capsules Coating constituents Basic coating formulations comprise a polymer, plasti- cizer, pigment (optional), and solvent vehicle, usually water but sometimes an organic. To this can be added secondary film formers, surfactants, lubricants, flavors, sweeteners, antioxidants, and preservatives. Cellulosic derivatives are the typical polymer of choice for immediate-release coating systems because they are inert (non-reactive) and approved worldwide because cellulose is derived from a natural source, wood fiber. These non-enteric polymers include hypromel- lose (hydroxypropyl methyl- cellulose, or HPMC), hydroxyethyl cellulose, hydroxymethyl cellulose, and sodium carboxymethylcellu- lose. These cellulosic polymers hydrate easily and provide other desirable qualities, including excellent flowability that allows droplets to spread quickly over the tablet cores and shorten drying times. They also produce a smooth surface and offer some barrier functionality but disinte- grate quickly, making them well suited for immediate- release formulations. Cellulosic polymers comprise a network of hydrogen bonds, and during tablet coating, the more hydrophobic side chains stack and form a flat intra-molecular matrix that gives a smooth surface. Yet different cellulosic viscos- ity-graded polymers impart different mechanical proper- ties, such as adhesion and film strength; they can also be selected to limit plasticizer migration and thus provide sharp logo definition. These different functional mechani- cal properties of cellulosic polymers allow formulators to adjust film coatings to meet the customer's desired CQAs of a strong film that resists edge wear and chipping and that provides a smooth surface and well-defined logo. In fact, logo definition is a key CQA because it allows the manufacturer, pharmacist, and end-consumer to recognize the brand. It's also a common area for defects. Assessing the core Choosing the right coating formulation begins with an assessment of the core. The first step is to identify the shape of the tablet: round, oval, capsule-shaped, or some modification of those. The next step is to identify the tablet profile: convex, flat-faced, and compound-cup are a few of the common ones. The tablet's profile is important because it affects how the coating is applied. Flat-faced tablets, for example, are typically at risk of "twinning," or sticking to each other, and to prevent that, the coating must dry rapidly after application and the pan speed must usually be increased. Tablets bearing a logo with narrow areas be tween letters or numbers can erode during the coating pro cess. To prevent that and maintain logo definition, formulators will seek a high-strength film coating that has a low viscosity and high solids content and that dries quickly. To further improve the mechanical properties of cellu- losic polymers and resolve tablet coating defects, formula- tors add plasticizers to film formulations. Plasticizers increase film flexibility, reduce tackiness, and decrease both glass transition temperatures and minimum film tempera- tures. These all enhance the coating process and the film's functional properties. Like polymers, plasticizers must be chosen according to how their physical characteristics will improve the func- tional characteristics of the film. Plasticizers are typically low-molecular-weight com- pounds that can alter the physical properties of the polymer in the film coating system. They do this mecha- nistically, with the plasticizer's molecules intercalating between individual polymer chains and thus modifying the polymer-to-polymer inter- action. In some cases, they form hydrogen bonds with particular polymer sites to create a more elastic film. Plasticizers can be grouped into three classes: polyols (both monomeric and polymeric), organic esters, and oils/glycerides. Common plasticizers include glycerol, propylene glycol, polyethylene glycol (PEG), triethyl cit- rate, triacetin, castor oil, and medium-chain triglycerides, but there are many others. Understanding the mechanism and physical characteristics of plasticizers enables formu- lators to envision how plasticizers will affect the various components of the tablet core and the coating. Case study: Plasticizer choice In one study, an HPMC-based film was to be used to coat a highly friable, lipophilic, and dark vitamin core. The coating was supposed to provide a strong adhesive base coat, after which a pastel top coating would be applied. To see how the top coating would perform, two white film coatings—each with similar components and identical viscosities—were selected for study. Both were high-solids coatings and both were properly prepared. Only the plasti- cizers were markedly different: glycerin versus a medium- chain triglyceride. The coating solution containing the medium-chain triglyceride exhibited no major coating defects (chipping, roughness, cracking, blistering, or pick- ing). The glycerin formulation, however, showed poor adhesion, chipping, edge wear, cracking, roughness, and slight picking. In fact, tablets coated with the formulation containing glycerin peeled at the edges, while the one con- taining the medium-chain triglyceride provided a very smooth and adhesive film. The more elegant film was the result of using a more hydrophobic-lipophilic plasticizer that was more compat- ible with the core's lipophilic components. The glycerin, on the other hand, provided little to no penetration of the base coat, which led to poor adhesion. As this case study illustrates, it is critical that formulators understand the core's components in order to arrive at a zero-defect film coating, which minimizes costs and safeguards over- Plasticizers increase film flexibility, reduce tackiness, and decrease both glass transition temperatures and minimum film temperatures. c-Schadart_12-19_Masters 4/3/14 2:34 PM Page 14