How phosphorus flame retardancy works

Phosphorus compounds decompose on heating to PO· and HPO· radicals, which scavenge H· and OH· in the flame and interrupt the radical chain — the main reason phosphorus is efficient.

At the same time, phosphorus acids catalyse dehydration and crosslinking, forming a dense carbonaceous char that blocks heat and oxygen and suppresses fuel release.

With nitrogen present, decomposition gases expand the char into an intumescent structure — the classic acid source, carbon source and gas source system.

Reactive phosphate esters bind phosphorus covalently into the network, so efficiency does not drift over time the way additive systems do.

Do not judge by LOI alone: UL 94 vertical burn, TGA char residue and smoke density matter. Many formulations pass LOI but drip and ignite cotton in UL 94.