We previously published articles on an introduction to flame retardants, polymer combustion mechanisms, and halogenated flame retardants. To complement these articles, we now discuss halogen-free flame retardants, which have attracted considerable attention in recent years.
Halogenated flame retardants were the first flame retardants to gain widespread use, offering high efficiency at relatively low loadings. However, their toxicity and environmental impact have led to strict regulations on their use. Under the Restriction of Hazardous Substances (RoHS) Directive, most halogenated flame retardants have been banned in Europe and America. Concerns include pollution from flame-retardant production, toxic smoke released during combustion, and pollution from plastic waste. Efforts to introduce new flame retardants have been underway for years. The following sections examine two categories: phosphorus-containing flame retardants and mineral flame retardants.
Part 1: Phosphorus-Containing Flame Retardants
Numerous flame-retardant compounds containing phosphorus or phosphorus-bromine combinations have been developed. They are primarily used in polymers that form char during combustion and are particularly effective in oxygen-containing polymers. Good compatibility between phosphorus compounds and polyurethane results in good processability, favorable flammability characteristics, and suitable physical and mechanical properties. Phosphorus-containing compounds enable compliance with various fire-performance standards in polyurethane, epoxy resins, phenolic resins, polycarbonate and its blends, polyvinyl chloride, cellulosic products such as paper and wood-plastic composites, and other coatings and paints. However, using phosphorus-containing flame retardants in polyolefins requiring high fire resistance is generally challenging.
One major application of phosphorus-based flame retardants is replacing conventional plasticizers with phosphorus-containing plasticizers in poly(vinyl chloride) production. Chlorine accounts for nearly 57% of PVC, increasing its LOI and giving it a V-0 classification under UL 94. However, adding conventional plasticizers makes PVC flammable for two reasons: it reduces the chlorine content, and the plasticizers themselves are flammable. Adding phosphate esters to PVC preserves its self-extinguishing properties.
Mechanisms of Phosphorus-Containing Flame Retardants
Phosphorus compounds fall into three broad categories: organic compounds such as resorcinol diphenyl phosphate, inorganic compounds such as red phosphorus, and halogen-containing compounds such as chlorinated phosphates. Some are “reactive,” meaning they chemically bond to the polymer, while others are incorporated as additives. The compound should be selected according to the polymer’s chemistry, its decomposition temperature, and the fire classification required for the finished product.
No single mechanism explains the action of phosphorus compounds. Their mechanisms depend on both the phosphorus compound and the base polymer and involve chemical, physical, thermal, and other effects. Their gas-phase action is comparable to that of halogenated compounds, involving chemical mechanisms and radical trapping. For example, triphenylphosphine acts in the gas phase to reduce the flammability of polycarbonate. Phosphoric acid acts in the gas phase after decomposing into HPO₂, HPO, PO, and other possible radical species. However, phosphorus-containing compounds primarily act in the solid phase by forming a char layer on the polymer surface through physical and barrier mechanisms. They alter the polymer’s decomposition pathway to promote greater formation of a carbonaceous char layer. This reduces the release of decomposition products and heat transfer to the polymer surface.
In another mechanism, some phosphorus compounds accelerate polymer decomposition and softening during a fire, producing a flow of molten polymer droplets. Dripping burning polymer carries fire and heat away from the polymer surface. Through this mechanism, triphenylphosphine enables polystyrene to achieve a V-2 classification.
Intumescent Flame Retardants
Polyolefins such as polypropylene produce very little char during combustion. Conventional phosphorus-containing compounds also have limited ability to form a carbonaceous layer on polyolefin surfaces. A special class known as intumescent flame retardants can overcome this limitation. Their primary mechanism occurs in the solid phase through the formation of an expanded char layer.
Intumescent flame retardants mainly consist of phosphorus- and nitrogen-containing compounds. They comprise three components: a char-forming agent, a blowing agent, and a catalyst. The char-forming agent produces a surface char layer with the aid of the catalyst. The blowing agent, generally a nitrogen-containing compound, expands the char layer, forming a thick barrier against heat and oxygen penetration while reducing smoke. Ammonium polyphosphate combined with a char-forming agent is one of the earliest systems used. Ammonium polyphosphate decomposes to release ammonia gas, water, and phosphoric acid. Phosphoric acid acts as a catalyst, while ammonia and water vapor act as blowing agents.
Early intumescent systems faced several problems, including incompatibility with polyolefins, water solubility, migration to the surface, and increased thermal and electrical conductivity. Their incompatibility significantly reduces certain mechanical properties, such as impact resistance. This is particularly important in electrical components and equipment. Approaches to improving intumescent systems include developing phosphorus compounds suited to the polymer’s chemical structure, improving compatibility between the ingredients and the base polymer through methods such as encapsulating ammonium polyphosphate, and reacting the ingredients to form a single macromolecule. Nanoparticles have also attracted interest for improving the mechanical properties of components and enhancing flame-retardant efficiency. Intumescent compounds are also used in coatings for non-polymeric surfaces such as metals and concrete.
The graphs in Figure 2 show total heat release, heat release rate, and CO emissions during cone calorimetry testing of a sample containing an intumescent flame retardant. Adding an intumescent system significantly reduces total heat release (THR) and heat release rate (HRR) per unit area, extends burning time, and increases the material’s resistance to ignition. Carbon monoxide (CO) emissions are also markedly reduced. Inhalation of toxic gases is among the most common causes of death in fires. These improvements result from the formation of an insulating carbon layer on the polymer, which reduces oxidation and, consequently, heat release.
Advantages and Disadvantages of Intumescent Flame Retardants
Intumescent flame retardants have lower densities than halogenated compounds. However, in some polymers, higher loadings are required to achieve equivalent performance. Achieving a desired fire-performance classification such as V-0 requires more than 20% of these compounds in the final product. The ingredients used in intumescent systems are solid at processing temperatures and therefore act as fillers, making particle size important. Phosphorus compounds have better light stability, and some types exhibit biodegradability.
Compounding and processing materials containing intumescent compounds into finished parts must be carried out under mild conditions. These materials are sensitive to high temperatures and stresses, which can cause degradation. A suitable processing temperature is 180–210°C, and residence time should be kept as short as possible. These factors affect the physical and mechanical properties of the finished product. Therefore, an intumescent compound may be unsuitable depending on the base polymer, processing conditions such as temperature and extrusion or injection speed, and the intended application of the finished part.
Part 2: Inorganic Metal-Based Particles
Mineral flame retardants hold the largest share of the flame-retardant market. They are used in various household and industrial applications and generally consist of hydrated metal compounds. They are halogen-free and can be reprocessed. They cause no environmental pollution and have little damaging effect on production equipment. They also contribute to char formation and reduce smoke and dripping.
Magnesium dihydroxide (MDH) and aluminum trihydroxide (ATH) powders are among the most widely used mineral flame retardants. They are white and produced in various particle sizes. Particle size is selected according to the application and the desired thermal and mechanical properties. However, smaller particles provide better flame-retardant performance. In addition to particle size, uniform dispersion strongly affects their effectiveness. Surface modification of MDH or ATH for use in polyolefins has a major influence on final performance. Uniform dispersion reduces the required powder loading and improves consistency in the final product’s properties. Despite these advantages, high loadings of mineral flame retardants significantly reduce mechanical properties such as impact resistance and tensile strength, especially at low temperatures. Some other mineral flame retardants are used as auxiliary additives alongside other flame retardants.
Mechanisms of Mineral Flame Retardants
Mineral flame retardants act through thermal, gas-phase, and barrier mechanisms. These particles decompose endothermically, releasing water. This cools the polymer and delays ignition. The released water dilutes the gas phase and forms a protective gaseous layer over the solid phase, reducing the oxygen content and the concentration of oxidizable substances. Mineral particles also reduce the amount of polymer in the finished product, further reducing combustion. Achieving the desired fire-performance classification requires high loadings of these materials in the polymer matrix.
Applications
Mineral flame retardants are used in a wide range of products, including wire and cable sheathing, pipes, flooring, roofing, wall coverings, injection-molded parts, and films. Their applications cover most polymer types, including polyolefins and PVC.
Source of the heat-release graphs:
- Zhou, Shun, et al. Polymer Degradation and Stability, 93 (2008), 1799–1806.
