Aluminum hypophosphite has a high phosphorus content, reaching up to 40%, and is a high-phosphorus halogen-free flame retardant. It can generally be compounded with MHB, MCA, aluminum hydroxide, etc., to achieve a synergistic flame retardant effect. Ammonium polyphosphate (APP) is a phosphorus and nitrogen-containing inorganic flame retardant. AHP and APP are generally not used alone, otherwise, the large amount added will have a significant impact on the mechanical properties of the polymer. If aluminum hypophosphite and ammonium polyphosphate are used as the main flame retardants, the compounded product has a good flame retardant effect on ABS. The flame retardant mechanism is as follows:

I. Flame Retardant Mechanism

1. The combined action of aluminum hypophosphite (AHP) flame retardant and carbon source significantly increases the char residue rate. Under high temperatures, aluminum hypophosphite and ammonium polyphosphate react chemically to form a dense char layer, similar to the glassy structure of inorganic phosphates, covering the surface of the burning material and isolating it from oxygen and heat.

2. During combustion, the polyphosphoric acid produced by the decomposition of aluminum hypophosphite (AHP) flame retardant can remove water molecules from the system, causing charring of the burning surface and playing a solid-phase flame-retardant role.

II. Flame Retardant Effect and Formulation

Based on an AHP: 2.5%, APP: 17.5%, and an appropriate amount of charring agent, this formulation for flame-retardant ABS showed a char residue rate of 6.9% and an LOI: 30% at 700℃, achieving a flame retardant rating of V-0.

The flame-retardant mechanism of aluminum hypophosphite and ammonium polyphosphate on ABS may involve more than just the two aspects mentioned above. Different carbon source types will result in completely different charring mechanisms, effects, and charring chemical reaction processes. If AHP is used alone to flame-retard ABS, there is no problem with flame retardancy, but the addition of flame retardants makes ABS more brittle, significantly affecting its physical properties. If modified or coated aluminum hypophosphite is used, the amount added can be reduced, thus minimizing the impact on the mechanical properties of ABS.