Study of the mechanism of intumescence in fire retardant polymers: Part III—Effect of urea on the ammonium polyphosphate-pentaerythritol system

Urea, which is commonly used as a ‘blowing’ co-additive in intumescent coatings, is shown to depress intumescence when it is added to ammonium polyphosphate-pentaerythritol mixtures incorporated into the bulk of polypropylene. Concurrently, the fire retardant properties of the intumescent additive are depressed in the presence of urea although, in this case, a smaller amount of flammable hydrocarbons is evolved in the thermal degradation of the polymer.