Blends and Additives - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 152

To understand how flame retardants work, it is useful to think about the
fire triangle. The fire triangle is a visual aid to show that heat, fuel, and oxy-
gen are required for combustion. If any one of these is removed, combustion
is not maintained. Consider an unlit candle in a jar. Fuel (the candle wax)
and oxygen (in the air) are present but there is no combustion unless heat is
supplied, initially in the form of a match and subsequently from the burning
vapor of the degraded candle wax. Combustion is maintained until one of the
three elements of the fire triangle is removed. When all the candle wax has
been consumed, there is no more fuel and the flame goes out. If, while the
candle is burning, a cover is put over the jar, then no more oxygen is supplied
and the flame goes out.
Heat
Fire triangle
Many plastic articles require flame retardants so that adequate safety is
provided. Some areas where they are used include the manufacturing of mat-
tresses and upholstery, in electrical applications such as wires and cables, in
building construction, and in transportation such as aircraft and automotive
interiors. There are three different mechanisms that can explain how most
flame retardants work. One is by absorbing heat by the release of water. A
second type of flame retardants is thought to work by providing an insulating
char on the burning plastic. This char prevents more fuel (plastic) from reach-
ing the flame and starves the fire. A third method is by interfering with the
chemical reactions that maintain the fire and promote the spread of the flame.
The largest volume flame retardant is alumina trihydrate, Al 2 O 3 . 3H 2 O.
In the flame, water is released and this cools the plastic, preventing further
combustion. Alumina trihydrate is used in polymers such as polyolefins,
PVC, polyacrylates, and thermoset polyesters. It cannot be used in polymers
such as polycarbonate, nylon 6,6, or polyethylene terephthalate because
these are processed at temperatures that will cause the evolution of water
during processing.
Phosphorus-based flame retardants such as ammonium polyphosphate or
phosphate esters are thought to form phosphoric acid in the flame and the acid
is thought to promote char forming crosslinking reactions. They are often used