Aqueous Film Coating.

Hypromellose is the usual film-forming polymer dispersed in water-based coating systems, so the coating and the polymer are the same functional unit. It sets into a continuous, water-permeable film once the water evaporates.

HPMC is the cellulose ether that gives an aqueous coating its film strength and its fast wetting on contact with gastric fluid. Coating suspensions are built around it at a few percent solids.

Triacetin acts as a plasticiser that lowers the glass transition temperature of the polymer film so it coalesces smoothly rather than cracking. Without a plasticiser the dried film is brittle and flakes at the tablet edges.

Glycerin plasticises and humectifies water-based coating films, keeping them flexible as they dry. It is a standard alternative to triacetin in hydrophilic coating systems.

Talc is the classic anti-tacking solid in coating suspensions, keeping tablets from sticking to each other in the coating pan. It also reduces the gloss and moisture permeability of the finished film.

Ethylcellulose is applied from an aqueous dispersion to build a water-insoluble film that slows the rate at which the core dissolves. Blending it with a soluble pore former sets how quickly the contents leave the tablet.

Methylcellulose forms water-soluble films from the same class of cellulose ethers used in aqueous coatings. It is used where a slightly different viscosity or wetting profile than hypromellose is wanted.

Pullulan makes clear, low-oxygen-permeability films from water, so it is used as a plant-derived film former in coatings and capsule shells. Its oxygen barrier suits oxidation-sensitive contents.

Riboflavin degrades quickly under visible and ultraviolet light, which is why riboflavin-containing tablets are usually opacified rather than left clear. An opacified aqueous film coat blocks a large part of that incident light before it reaches the core. Riboflavin also appears on the other side of the relationship, as a yellow colourant within the coating itself.

Folic acid photodegrades to pterin fragments and para-aminobenzoylglutamate under light exposure. An opacified coating is the routine formulation answer to that in a tablet. The protection is physical shielding, nothing to do with how the folate is absorbed.

Retinyl esters oxidise and isomerise when exposed to oxygen and light, and the conjugated polyene chain is the vulnerable part. A film coat slows oxygen ingress at the tablet surface and blocks light. It supplements rather than replaces the beadlet encapsulation retinol usually already carries.

Ascorbate oxidises to dehydroascorbate in the presence of moisture, oxygen and trace metals, and a hygroscopic tablet core accelerates that. A continuous polymer film raises the water vapour barrier of the finished tablet. The barrier is partial, so it works alongside packaging desiccation rather than instead of it.

N-acetylcysteine has a strong sulfurous odour and taste from its thiol group, which persists on an uncoated tablet surface. A film coat physically separates that surface from the tongue and from the headspace of the bottle. Odour masking is one of the two main reasons a coat is applied at all.

Valerian's isovaleric acid content gives its powder a pungent smell that carries through an uncoated tablet. Coating contains it at the surface. The same logic applies to garlic, fish oil powders and most sulfur-bearing botanicals.

Microencapsulated fish oil powders carry oxidised lipid odour that becomes noticeable in a compressed tablet. A film coat plus an oxygen-barrier polymer limits how much of that reaches the user and slows further oxidation at the surface. Softgels solve the same problem a different way.

Iron salts stain, taste metallic and can discolour a tablet surface over time through reaction with excipients. A pigmented film coat covers both the taste and the mottling. It does not change how the iron is absorbed once the film dissolves.

Calcium carbonate is used as a white opacifying pigment inside coating suspensions where titanium dioxide is being avoided. It scatters light and raises film opacity, though it needs a higher loading than titanium dioxide for the same hiding power. It also appears as a tablet core ingredient, where the coating is protecting it rather than being made from it.

Lecithin acts as a wetting and dispersing aid in an aqueous coating suspension, keeping pigment particles from flocculating during the spray run. Better dispersion gives a more even film. Its function is entirely process-side.

Phycocyanin from spirulina is one of the few blue colourants permitted in supplement coatings, and it is dispersed into the aqueous suspension like any other pigment. It is heat-sensitive and light-sensitive, which constrains the drying temperature of the coating run. The role is appearance only.

Beta-carotene supplies orange to yellow shades in a coating suspension, usually as a water-dispersible beadlet rather than the neat carotenoid. It is oxidation-sensitive, so coated tablets using it can shift shade during storage. Colour choice is cosmetic and does not touch the core formulation.

Curcumin is used as a yellow colourant in coating suspensions at levels far below any nutritional dose. It is pH-sensitive and shifts from yellow toward red in alkaline conditions, which formulators account for. Its presence in a coating should not be read as a curcumin dose in the product.

Enzyme actives lose activity in stomach acid, so they are commonly given an enteric aqueous coating built from a methacrylic acid copolymer dispersion rather than a standard hypromellose film. That polymer stays intact below about pH 5.5 and dissolves as the pH rises further along. The coating choice is what determines where the tablet opens.

Live organisms are damaged by the water and warm drying air an aqueous coating run involves, so tableted probiotics need low-moisture coating conditions or a different protection strategy entirely. The same coat that helps an enteric enzyme can cost viable count if the process is run wet. This is a genuine constraint rather than a synergy.

Betaine hydrochloride is hygroscopic and acidic at the tablet surface, which can pit and discolour an uncoated tablet and irritate on contact with mucosa. A film coat both keeps moisture out and keeps the acidic surface away from the mouth and oesophagus. The barrier is partial and packaging still carries most of the moisture load.