Flame Retardants for Polyethylene

1. Introduction

Polyethylene (PE) is like other polyolefins and in comparison to other standard plastics (PS, PMMA, PC) a polymer with high heat of combustion, high rate of heat release with medium time to ignition, and low smoke release. When taking the limiting oxygen index (LOI, ISO 4589) values as the measure for flammability of polymers, PE ranks with a value of 18% among the most flammable commodity and engineering plastics. Surprisingly low molecular weight PE is experimentally less flammable than high molecular weight grades. Thermal degradation of PE takes place via random and chain-end scission and results in the formation of diolefins, olefin and paraffin fragments, and lower molecular weight products at higher temperatures of pyrolysis.

Combustion of PE yields mainly CO₂, CO, and hydrocarbons, however, aromatic compounds are detected with increasing temperatures through pyrosynthesis reactions. To reduce the fire risk, many PE applications, such as wire and cable, pipes, construction films, and others, require fire or flame retardant PE. There are 3 strategies to achieve flame retardancy in polymers: a) incorporation of flame retardants as additives, b) use of flame retardant coatings to protect the substrate, or c) modification of the PE backbone during synthesis with flame retardant comonomers. The latter is possible via the copolymerization of ethylene with brominated comonomers in the presence of coordination catalysts, however, it is commercially of no importance.

2. Flame Retardants with Endothermic Decomposition

Representatives of the group of flame retardants, which act mainly through endothermic decomposition, are inorganic or mineral fillers and mainly aluminum hydroxide (ATH, Al(OH)₃), Boehmite (AlO(OH)), and magnesium hydroxide (MDH, Mg(OH)₂). Other mineral flame retardants include huntite (Mg₃Ca(CO₃)₄) and hydromagnesite (Mg₅(CO₃)4(OH)₂·4H₂O), or a natural mixture of both. ATH has by far the largest market share in this class with more than 90%, which corresponds to a global consumption of 675 kt/yr. In the case of fire, the mineral fillers decompose endothermically, releasing water (huntite releases CO₂), forming stable metal oxides and thus cooling the material and diluting the burnable gases and fire effluents. The loss on ignition is 34.6% for ATH, 31% for Mg(OH)₂, and 17% for Boehmite.

The main application area of PE flame retarded through inorganics is wire and cable, often based on blends of LLDPE or LDPE and EVA. About 60 to 65% ATH or up to 80% of MDH are required to provide flame retardancy and to pass standards such as UL-1072 and UL-1277. For construction applications in the form of pipes, 67 to 80% of mineral fillers are mentioned to pass the relevant standards (e.g., DIN 4102 B1). An exemplary flame retardant cable formulation based on ATH consists of LDPE 15.8 wt%, EVA 19.0%, a coupling agent (PE-graft-maleic anhydride) at 5%, a stabilizer/antioxidant at 0.2%, and 60% ATH.

3. Flame Retardants with Gas Phase Mechanisms

Traditional flame retardants are active in the gas phase, interrupting the radical decomposition process of the polymer in the case of fire, through reactions with halogen radicals forming low energy intermediates. Chlorinated and brominated components are often used in combination with antimony trioxide (ATO, Sb₂O₃) as an efficient synergist. Examples of chlorinated compounds as flame retardants are polychlorinated paraffins. The PE formulations that pass the standard UL-94 V-0 contain 24% chlorinated paraffins and 10% ATO.

The choice of brominated flame retardants is much larger than for chlorinated flame retardants. As replacements for the phased out decabromodiphenylether (Figure 35.1, formula (I)), the preferred brominated flame retardants for PE are aromatic bromine derivatives such as decabromodiphenylethane (II), ethylene(bis(tetrabromophthali mide)) (III), aliphatic bromine derivatives such as tris(tribromoneopentyl)phosphate (IV), 1,3,5-tris(2,3-dibromopropyl)isocyanurate (V), or mixed aliphatic-aromatic derivatives such as tetrabromobisphenol-A-bis(2,3-dibromopropylether) (VI). More recently, polymeric structures with aromatic and or aliphatic bromine groups were introduced such as poly(pentabromobenzyl) acrylate (VII), polystyrene-brominated butadiene-polystyrene block copolymers (VIII), or brominated polyphenylene ether (IX). The structures of the materials are provided in Figure 35.1.