Tablet Coating Techniques
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Direct answer
Coating applies a thin, functional layer to a tablet core for taste masking, moisture protection, elegance or site-specific release, by three broad routes: sugar coating, film coating and press coating. Sugar coating builds a shell of sucrose layers through sealing, subcoating, syrup smoothing, colouring, polishing and printing, and can raise tablet weight by 30 to 50 per cent; film coating deposits a 2 to 5 per cent polymer layer — usually hypromellose in an aqueous dispersion — in a perforated pan or fluidised bed. Enteric coating swaps the soluble hypromellose for pH-triggered polymers such as cellulose acetate phthalate or methacrylate copolymers (Eudragit L and S) that resist gastric acid and dissolve at intestinal pH, protecting acid-labile drugs or the stomach itself. Each process is then judged by the defects it produces — a diagnostic list every practical examiner loves.
What you must remember
- Sugar stages in order: seal coat (shellac), subcoat (syrup plus talc to build edges), smoothing syrup, colour coat, polishing with carnauba or beeswax, printing.
- Film coating: hypromellose (HPMC) is the workhorse polymer, with plasticisers (polyethylene glycol) and pigments at typically 5 to 15 per cent solids; ethylcellulose serves for moisture barriers.
- Enteric triggers: cellulose acetate phthalate and Eudragit L 30 D-55 dissolve above about pH 5.5 (duodenal release), Eudragit L above pH 6, Eudragit S above pH 7 (colonic targeting).
- Equipment: conventional pans for sugar work; perforated pans (Accela-Cota type) with heated airflow for film coating; fluidised bed (Wurster) columns for pellets and multiparticulates.
- Process balance: inlet temperature typically 40 to 50 °C for aqueous film coating; spray rate and pan speed tuned so drying keeps pace with application — wet surfaces pick, hot surfaces spray-dry.
- Defect dictionary: orange peel (poor spreading), picking and sticking (over-wetting), bridging of monograms (weak viscoelastic film), mottling (uneven pigment), cracking and twinning.
- Speciality coats: sustained release by ethylcellulose or Eudragit RL/RS membranes; press coating for separating incompatible drugs into core and shell.
Walking a batch through a perforated pan
Seal a batch of bitter cores first with thin shellac to survive the aqueous sprays. Warm to about 40 °C, start the hypromellose dispersion slowly; if the bed climbs past roughly 45 °C the droplets dry in flight and orange peel appears, while too cold leaves tablets tacky and stuck in pairs. The film grows to about 3 per cent of core weight in an hour or two — the elegance of film coating is that a 60 mg layer does the work of a sugar shell ten times heavier. For an acid-labile proton-pump inhibitor the operator switches to a methacrylate dispersion with ample plasticiser, because enteric films are brittle and crack on tablet edges if coalescence is rushed; tablets are then held warm to complete film curing. Final checks are appearance, weight gain, and dissolution — almost no release in acid for two hours, prompt release in pH 6.8 buffer. The coating earns its place only in the dissolution bath.
How examiners frame coating questions
Two inversions dominate. First, sugar versus film coating: candidates recite stages but forget the quantitative discriminator — 30 to 50 per cent weight gain for sugar against 2 to 5 per cent for film, which halves bulk and packing cost, the real reason the industry abandoned sugar. Second, "enteric coat is for taste masking" is a standing error; taste masking needs only a fast-dissolving barrier, whereas enteric coating is defined by acid resistance followed by intestinal dissolution, verified in the disintegration apparatus, not on the tongue. A practical favourite matches defect to cause on a photograph: orange peel with insufficient spreading, picking with over-wetting, bridging with a weak film. Naming the process variable that fixes each — solvent, spray rate, plasticiser, bed temperature — separates a pharmacist from a memorised list.
Frequently asked questions
List the stages of sugar coating in order.
Seal coating, subcoating, smoothing (syrup) coating, colouring, polishing and printing — each stage fixes the faults the previous stage could not.
Why has film coating largely replaced sugar coating?
A 2 to 5 per cent weight gain masks taste and protects the core as effectively as a 30 to 50 per cent sugar shell, while cutting process time and avoiding the sugar load.
At what pH do the main enteric polymers dissolve?
Cellulose acetate phthalate and Eudragit L 30 D-55 above about pH 5.5, Eudragit L above pH 6, Eudragit S above pH 7 — the polymer is chosen by the release site required.
What causes orange peel and picking during film coating?
Orange peel arises when droplets dry before spreading (insufficient solvent, hot bed); picking when tablets stick and film tears (over-wetting, slow drying) — opposite faults of one drying balance.
Which equipment coats pellets and multiparticulates?
Fluidised bed coaters with a Wurster insert spray particles riding an air column, giving uniform coats to spheres that a rotating pan would agglomerate.