binder

A binder is the glue of a tablet. Without something to make particles stick, a pressed powder would crumble back to dust the moment it left the press. The binder bridges the particles together so the granules form and the final tablet holds its shape during coating, packing, shipping and handling.

Binders are usually water-soluble polymers or sugars such as povidone, hydroxypropyl methylcellulose, pregelatinised starch or gelatin. In wet granulation the binder is dissolved in the liquid and sprayed onto the powder, where it dries into solid bridges. In direct compression a dry binder is mixed straight into the blend. The amount is a balancing act: too little and the tablet is weak and friable; too much and it becomes so hard it will not disintegrate, slowing drug release.

Note that binder and diluent roles can overlap — some materials, like microcrystalline cellulose, act as both — but the binder's defining job is mechanical strength, not bulk.