Application of Phosphorus-Based Flame Retardants in PP

Phosphorus-based flame retardants are a type of high-efficiency, reliable, and widely used flame retardants that have garnered significant attention from researchers. Remarkable achievements have been made in their synthesis and application.

1. Application of Phosphorus-Based Flame Retardants in PP

The physical properties of polypropylene (PP) play a crucial role in its industrial applications. However, its limited oxygen index (LOI) is only around 17.5%, making it highly flammable with a rapid burning rate. The value of PP materials in industrial applications is influenced by both their flame retardancy and physical properties. In recent years, microencapsulation and surface modification have become the primary trends in flame-retardant PP materials.

Example 1: Ammonium polyphosphate (APP) modified with a silane coupling agent (KH-550) and a silicone resin ethanol solution was applied to PP materials. When the mass fraction of modified APP reached 22%, the LOI of the material increased to 30.5%, while its mechanical properties also met requirements and outperformed PP materials flame-retarded with unmodified APP.

Example 2: APP was encapsulated in a shell composed of melamine (MEL), hydroxyl silicone oil, and formaldehyde resin via in-situ polymerization. The microcapsules were then combined with pentaerythritol and applied to PP materials for flame retardancy. The material exhibited excellent flame retardancy, with an LOI of 32% and a vertical burning test rating of UL94 V-0. Even after hot water immersion treatment, the composite retained good flame retardancy and mechanical properties.

Example 3: APP was modified by coating it with aluminum hydroxide (ATH), and the modified APP was combined with dipentaerythritol at a mass ratio of 2.5:1 for use in PP materials. When the total mass fraction of the flame retardant was 25%, the LOI reached 31.8%, the flame retardancy rating achieved V-0, and the peak heat release rate was significantly reduced.

2. Application of Phosphorus-Based Flame Retardants in PS

Polystyrene (PS) is highly flammable and continues to burn after the ignition source is removed. To address issues such as high heat release and fast flame spread, halogen-free phosphorus-based flame retardants play a critical role in PS flame retardancy. Common flame-retardant methods for PS include coating, impregnation, brushing, and polymerization-stage flame retardancy.

Example 1: A phosphorus-containing flame-retardant adhesive for expandable PS was synthesized via the sol-gel method using N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane and phosphoric acid. Flame-retardant PS foam was prepared using a coating method. When the temperature exceeded 700°C, the PS foam treated with the adhesive formed a char layer exceeding 49%.

Researchers worldwide have introduced phosphorus-containing flame-retardant structures into vinyl or acrylic compounds, which are then copolymerized with styrene to produce novel phosphorus-containing styrene copolymers. Studies show that compared to pure PS, phosphorus-containing styrene copolymers exhibit significantly improved LOI and char residue, indicating superior thermal stability and flame retardancy.

Example 2: A vinyl-terminated oligomeric phosphate hybrid macromonomer (VOPP) was grafted onto the main chain of PS via graft copolymerization. The graft copolymer exhibited flame retardancy through a solid-phase mechanism. As the VOPP content increased, the LOI rose, the peak heat release rate and total heat release decreased, and melt dripping disappeared, demonstrating significant flame-retardant effects.

Additionally, inorganic phosphorus-based flame retardants can be chemically linked with graphite or nitrogen-based flame retardants for use in PS flame retardancy. Coating or brushing methods can also be employed to apply phosphorus-based flame retardants to PS, significantly improving the material’s LOI and char residue.

3. Application of Phosphorus-Based Flame Retardants in PA

Polyamide (PA) is highly flammable and produces substantial smoke during combustion. Since PA is widely used in electronic components and equipment, the risk of fire hazards is particularly severe. Due to the amide structure in its main chain, PA can be flame-retarded using various methods, with both additive and reactive flame retardants proving highly effective. Among flame-retardant PAs, alkyl phosphinate salts are the most widely used.

Example 1: Aluminum isobutylphosphinate (A-MBPa) was added to a PA6 matrix to prepare a composite material. During flame retardancy testing, A-MBPa decomposed before PA6, forming a dense and stable char layer that protected the PA6. The material achieved an LOI of 26.4% and a flame retardancy rating of V-0.

Example 2: During the polymerization of hexamethylenediamine and adipic acid, 3 wt% of the flame retardant bis(2-carboxyethyl)methylphosphine oxide (CEMPO) was added to produce flame-retardant PA66. Studies showed that flame-retardant PA66 exhibited superior flame retardancy compared to conventional PA66, with a significantly higher LOI. Analysis of the char layer revealed that the dense char surface of flame-retardant PA66 contained pores of varying sizes, which helped isolate heat and gas transfer, demonstrating notable flame-retardant performance.

More info., pls contact lucy@taifeng-fr.com