Ammonium polyphosphate

Overview

Structural formula of ammonium polyphosphate

Ammonium polyphosphate (APP) is a type of polyphosphate that is commonly used as a flame retardant. It is a polymeric compound composed of ammonium and phosphate units. The general formula for ammonium polyphosphate is (NH_PO_)_n, where n indicates the degree of polymerization.

Chemical Structure

Ammonium polyphosphate consists of chains of phosphate units linked together by sharing oxygen atoms. The ammonium ions (NH__) are associated with the negatively charged phosphate chains to maintain electrical neutrality. The degree of polymerization can vary, leading to different forms of APP with varying properties.

Properties

Ammonium polyphosphate is a white, crystalline powder that is insoluble in organic solvents but soluble in water. It is stable under normal conditions but decomposes upon heating, releasing ammonia and phosphoric acid. This decomposition is the basis for its use as a flame retardant, as it helps to form a protective char layer on materials, reducing flammability.

Applications

APP is widely used in the production of flame-retardant materials. It is commonly applied in:

• Textiles: To reduce the flammability of fabrics.
• Plastics: Used in polyurethane foams, polypropylene, and other polymers.
• Coatings: As an additive in intumescent coatings that swell when exposed to heat, providing a barrier to fire.

Mechanism of Action

The flame-retardant action of ammonium polyphosphate is primarily due to its ability to promote char formation. When exposed to heat, APP decomposes to release ammonia and phosphoric acid. The phosphoric acid acts as a catalyst for the dehydration of the substrate, leading to the formation of a carbonaceous char layer. This char layer acts as a barrier, protecting the underlying material from heat and oxygen, thus slowing down the combustion process.

Safety and Handling

While ammonium polyphosphate is generally considered safe, it should be handled with care to avoid inhalation or ingestion. Appropriate personal protective equipment, such as gloves and masks, should be used when handling the powder.

Related Pages

• Flame retardant
• Polyphosphate
• Ammonium
• Phosphate