Types and Selection of Flame Retardants

Sponge flame retardants must possess one or more of the following functions: absorbing heat and decomposing into non-combustible substances at or near ignition temperature; reacting with foam combustion products to form non-flammable substances; or releasing substances that can terminate the free radical oxidation reactions during foam combustion.

Compounds containing elements such as phosphorus, nitrogen, halogens, antimony, lead, and boron are found to exhibit good flame-retardant properties. Sponge flame retardants come in various types, including common inorganic sponge flame retardants like aluminum hydroxide, antimony oxide, and ammonium polyphosphate, as well as organic sponge flame retardants like phosphate esters, halogenated organic compounds, and halogenated phosphorus (phosphine) esters. Many sponge flame retardants have shown effective flame-retardant performance in polyurethane formulations.

In combustion systems, phosphorus-containing sponge flame retardants primarily act in the condensed phase. Phosphorus compounds consume combustible gases released during foam combustion, converting them into less flammable char, a process known as catalytic charring technology. A phosphorus content of approximately 1.5% in foam can achieve optimal flame-retardant effects.

Halogen-containing sponge flame retardants primarily function in the gas phase, with halogens serving as chain terminators for foam combustion reactions. During combustion, halogen-containing sponge flame retardants produce hydrogen halide, which inhibits the combustion reaction.

Polyurethane foam is a reactive polymer, and its sponge flame retardants fall into two categories: reactive and non-reactive types. Reactive sponge flame retardants contain flame-retardant elements in polyols or polyisocyanates. Currently, reactive sponge flame retardants are typically hydroxyl-containing compounds. Compared to reactive sponge flame retardants, non-reactive sponge flame retardants are more widely used. These additive-type sponge flame retardants do not participate in chemical reactions and remain in the foam after forming, providing flame-retardant effects.

When selecting sponge flame retardants for polyurethane foam formulations, the following factors must be considered in addition to ensuring good flame-retardant performance and cost-effectiveness:

1. Compatibility: The sponge flame retardant must have good compatibility with raw material systems such as polyols and isocyanates, without causing layering or precipitation in the mixture. During storage and use, the sponge flame retardant should not volatilize or migrate, ensuring sustained flame-retardant performance.

2. Stability: The sponge flame retardant should exhibit storage stability in polyether blends without significantly affecting the reaction activity.

3. Processability: The addition of sponge flame retardants should not interfere with the foaming process. Solid sponge flame retardants are less commonly used due to their potential for precipitation and risks of clogging foam machine nozzles or causing wear to mixing heads.

4. Thermal Stability: The sponge flame retardant should demonstrate good thermal stability and durability. In the production of polyurethane foams, especially large blocks, high internal temperatures are generated, requiring the sponge flame retardant to remain stable without decomposition under such conditions.

5. Physical Properties: The sponge flame retardant should have minimal impact on the original physical properties of polyurethane products. High flame-retardant efficiency and low dosage are preferred.

6. Toxicity and Smoke Production: sponge flame retardants should have low toxicity and produce minimal smoke during combustion.