Polypropylene (PP) is widely used across various industries due to its low density, excellent mechanical properties, chemical resistance, ease of processing, good heat resistance, low cost, and simple production process. Flame-retardant PP has become one of the fastest-growing and most commonly used polymer flame retardants. It finds applications in automotive plastics, electrical components, electronic products, housing, home appliances, electrical equipment, and many other sectors.
PP modification techniques include copolymerization, grafting, chlorination, cross-linking, and blending, which are commonly used for filling, toughening, strengthening, flame retardancy, and weather resistance. In recent years, flame-retardant PP modification has gained significant momentum.
Flame-retardant PP is typically produced by melt-blending a flame retardant with polypropylene using a twin-screw extruder. Commonly used flame retardants include organic brominated and chlorinated compounds such as octabromodiphenyl ether (BDE-209), decabromodiphenyl ether (BDE-207), decabromodiphenylethane, and chlorinated paraffins. Inorganic flame retardants like magnesium hydroxide, aluminum hydroxide, zinc borate, and intumescent agents containing nitrogen and phosphorus are also widely used.
Brominated flame retardants exhibit excellent compatibility with plastics, have a mature production process, and offer good cost-performance. They are often preferred in the production of flame-retardant PP. However, their use can lead to issues such as reduced heat distortion temperature and poor dispersion, especially in co-polymerized or weather-resistant PP.
Octabromoether (BDDP) is a halogen-based flame retardant with a molecular formula of C21H20Br8O2 and a molecular weight of 943.62. It appears as a white powder with a melting point of 107–120°C and a bromine content of at least 67%. Its flame-retardant mechanism involves controlling the decomposition of polymers during combustion, acting both in the gas and condensed phases. To achieve optimal performance, it should be combined with at least 1/3 of its own weight in antimony trioxide. However, in practical applications, BDDP performs better on homopolymerized PP than on co-polymerized or toughened PP. Additionally, it may cause issues like flame retardant precipitation, especially in pigmented PP products, which can reduce product quality and flame-retardant effectiveness.
Decabromodiphenyl ether (TDE) and decabromodiphenylethane are also popular flame retardants. TDE has a molecular formula of C14Br10H4, a molecular weight of 971.27, and a melting point of 350°C. It is effective for materials like ABS and HIPS but may negatively impact the mechanical properties of PP and PE when used in high concentrations. Despite this, it remains a popular choice due to its ease of use and familiarity among engineers.
Inorganic flame retardants like magnesium hydroxide are considered environmentally friendly and halogen-free. While they offer higher decomposition temperatures compared to aluminum hydroxide, they often require coupling agents to improve compatibility with PP. Their flame-retardant efficiency is generally lower than that of brominated flame retardants, leading to higher addition ratios and potential loss of mechanical properties.
Expanded graphite and red phosphorus are alternative flame retardants. Expanded graphite is highly efficient in achieving V0 flame ratings when combined with phosphorus-based agents, but its appearance and dispersibility can be challenging. Red phosphorus, known as an environmentally friendly option, works well with polyethylene and nylon but may not perform as effectively with PP due to its side-chain structure and color sensitivity.
When using flame retardants, it’s essential to consider processing conditions, the interaction with other additives, and the uniformity of dispersion. Poor dispersion can lead to ineffective flame retardancy, while incompatible materials may reduce the overall performance.
Flame retardant masterbatches were developed to address common issues such as uneven dispersion, dust pollution, and handling difficulties. Initially introduced in the 1960s, they have evolved into advanced formulations that significantly enhance flame-retardant efficiency, dispersion, and compatibility. As the demand for safer and more sustainable materials grows, flame retardant masterbatches are becoming a preferred solution in the industry.
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