Ammonium polyphosphate (APP) is a low-cost halogen-free flame retardant. Like aluminum hypophosphite and aluminum diethylphosphite, it achieves its halogen-free flame retardant effect through a synergistic effect of phosphorus and nitrogen. APP can be compounded with various other flame retardants and charring agents to form high-efficiency flame retardants for polypropylene. However, APP's hygroscopicity, hydrolysis, solubility, and thermal stability are not as good as other phosphoric acid-based flame retardants. We will discuss this from the following aspects:

1. Thermal Stability

An excellent characteristic of APP as a flame retardant is its good thermal stability, with a decomposition temperature greater than 250℃. APP in intumescent fire-retardant coatings and intumescent flame-retardant systems places special requirements on thermal stability. Firstly, a high initial thermal decomposition temperature is required, and secondly, rapid decomposition is required, with the formed polyphosphoric acid immediately catalyzing dehydration and carbonization. The initial decomposition temperature of Type I APP is lower than that of Type II APP.

2. Solubility

The solubility of ammonium polyphosphate (APP) is affected by several factors, such as the water ratio, temperature, crystal structure type, degree of polymerization, particle size, dissolution time, and stirring conditions. Generally, smaller particle size increases solubility; longer dissolution time also increases solubility; and a higher degree of polymerization decreases water solubility. Type I APP has a higher solubility than Type II APP.

3. Hygroscopicity and Hydrolysis

As the particle size of ammonium polyphosphate decreases, the specific surface area increases, and thus the hygroscopicity increases. Furthermore, different crystal structures of APP exhibit different hygroscopicities due to variations in surface structure. Type I APP typically has a rough surface, making it easier to absorb moisture; Type II APP has a smooth surface, thus the former has a higher hygroscopicity than the latter. Type I APP is a linear condensation polymer with exposed oxygen bonds, which is advantageous for moisture absorption. Type II APP is a branched condensation polymer with several cross-linked structures; the branches surround the oxygen bonds, making it difficult to absorb moisture. Hygroscopicity decreases with increasing degree of polymerization of ammonium polyphosphate (APP). As the degree of polymerization increases, chain length increases, interchain forces strengthen, and the aggregate becomes more compact, thus reducing hygroscopicity. During hygroscopic processes, some surface-mounted APP dissolves and gradually undergoes hydrolysis, causing the flame retardancy of APP flame-retardant materials to gradually disappear. This should be noted when using APP in solution as a flame retardant.

The degree of polymerization(n) of APP significantly affects hygroscopicity, hydrolysis, and solubility. Generally, a higher n value results in better flame retardancy, hydrolysis resistance, and resistance to precipitation.