Flame Retardants: Halogenated vs. Halogen-Free Systems and Their Environmental Impact

Photo: Matt C / Unsplash

Flame retardants play a crucial role in protecting materials from fires, whether it involves plastics, textiles, coatings, or construction materials. However, their selection is not just a matter of effectiveness—safety for health and the environment must also be considered. Halogenated systems, long regarded as the most effective, face increasing pressure due to toxic combustion byproducts. Halogen-free alternatives, on the other hand, offer a more environmentally friendly solution, but often at the cost of lower performance or higher expenses. How can one navigate these parameters, and what should be considered when selecting a solution for a specific application?

Introduction to Flame Retardants and Their Role in Industry

Flame retardants, also known as fire retardants, are key additives used in a wide range of industrial applications, from construction materials and electronics to textiles and plastics. Their primary function is to slow down or prevent the spread of flame, thereby enhancing the fire safety of products and structures. In practice, this means that materials with these additives resist ignition longer, produce less smoke, and reduce the rate of fire spread. This is crucial, especially in environments with high safety requirements, such as public buildings, transportation vehicles, or electrical equipment.

Selecting the right type of flame retardant depends on several factors, including the desired physical and chemical properties of the material, legislative requirements, and environmental impacts. The market offers two main categories: halogenated and halogen-free systems. While halogenated flame retardants are historically proven and highly effective, their use has been restricted in recent years due to potential risks to health and the environment. Halogen-free alternatives, on the other hand, offer a more eco-friendly solution, often at the cost of lower efficiency or higher expenses.

Halogenated Flame Retardants: Advantages and Risks

Halogenated flame retardants, particularly those based on bromine or chlorine, are among the most effective substances for slowing combustion. Their mechanism of action involves a chemical reaction with radicals formed during burning, thereby interrupting the chain reaction and reducing the flame temperature. As a result, they can significantly extend the time to material ignition and limit the spread of fire. Typical applications include electronic components, cables, insulating foams, or textiles, where high fire resistance is required with a relatively low amount of added additive.

However, halogenated systems face growing criticism due to their environmental and health impacts. When burned, they can release corrosive and toxic gases, such as hydrogen halides or dioxins, which pose risks to human health and ecosystems. Many of these substances are also persistent, bioaccumulative, and toxic (PBT), leading to their regulation under REACH and other legislative measures. Manufacturers are therefore seeking alternatives that maintain high efficacy while eliminating negative environmental impacts.

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Halogen-free flame retardants: Eco-friendly alternatives with challenges

Halogen-free flame retardants represent a modern and environmentally friendly alternative to traditional halogenated systems. The most commonly used include compounds based on phosphorus, nitrogen, silicon, or metal hydroxides, such as aluminium hydroxide or magnesium hydroxide. These substances work through various mechanisms – for example, by forming a protective layer on the material's surface, releasing non-flammable gases, or absorbing heat. Their advantage is minimal formation of toxic combustion products and lower environmental impact throughout the product's life cycle.

However, halogen-free systems are not without challenges. They often require higher dosages to achieve comparable effectiveness to halogenated alternatives, which can affect the material's mechanical properties, such as strength, flexibility, or processability. For instance, metal hydroxides must be added in quantities of up to 60% of the polymer's weight to effectively slow combustion, which may lead to deterioration of its mechanical properties. Another challenge is the higher cost of some halogen-free additives, which can be a limiting factor for manufacturers, particularly in price-sensitive segments.

Impact on the Environment and the Future of Flame Retardants

The environmental impacts of flame retardants are a key factor in their selection and regulation. Halogenated systems, particularly brominated flame retardants, are under scrutiny due to their persistence in the environment and ability to accumulate in food chains. Studies have detected their presence in soil, water, and living organisms, including humans, leading to gradual restrictions on their use in many countries. In contrast, halogen-free alternatives are generally considered less harmful, although their overall life cycle, including energy intensity of production and material recyclability, must still be assessed.

The future of flame retardants lies in innovations that combine high efficiency with minimal environmental impact. Research is focusing on the development of new additives, such as nanoparticles or bio-based flame retardants derived from renewable sources. The circular economy also plays a key role, with an emphasis on recyclability and the reuse of materials. For manufacturers and buyers, it is crucial to monitor current legislative requirements, such as REACH or CLP regulations, and choose solutions that meet not only technical but also ecological standards.

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Mechanisms of action: How flame retardants prevent flame spread

Flame retardants act on several levels, with their effectiveness depending on the chemical structure and interaction with the material. Halogenated systems, particularly those based on bromine or chlorine, primarily function in the gas phase. When heated, they release halogen radicals that react with highly reactive hydroxyl radicals formed during combustion. This interrupts the combustion chain reaction and reduces the flame temperature. This mechanism is exceptionally effective—just 5–10% by weight of a halogenated additive can significantly extend the time to ignition of the material. The disadvantage is the formation of corrosive and toxic by-products, such as hydrogen halides or dioxins, which complicate the disposal of fire-damaged structures.

Halogen-free systems, on the other hand, act predominantly in the solid phase. The most widespread include phosphorus-nitrogen compounds, metal hydroxides (e.g., aluminium hydroxide or magnesium hydroxide), and intumescent systems. Phosphorus compounds form a glassy layer when heated, which insulates the material from oxygen and heat. Metal hydroxides release water vapour at temperatures above 200 °C, thereby cooling the surface and diluting flammable gases. Intumescent systems expand during a fire and form a foamy barrier. These mechanisms are less aggressive towards the environment but often require higher dosages (up to 30–50% by weight) and may affect the mechanical properties of the material.

Practical applications: Where different types of flame retardants are used

The choice between halogenated and halogen-free flame retardants depends on the specific application, fire resistance requirements, and environmental limits. Halogenated systems dominate in electrical engineering and electronics, where high efficiency at low dosages is key. Typical applications include printed circuit boards, cables, electronic device housings, or wire insulation. Their advantage is compatibility with common polymers (e.g., polyethylene, polypropylene, or epoxy resins) and the ability to maintain electrical insulating properties. In these applications, adding just 3–7% of brominated compounds is often sufficient for the material to meet strict fire safety standards, such as UL 94 V-0.

Halogen-free alternatives are used in areas with higher demands for ecology and health safety. Typical examples include construction materials (insulation, flooring, façade panels), transportation vehicles (car interiors, trains, aircraft) or textiles for public spaces. In these segments, metal hydroxides or phosphorus-nitrogen systems are often used, which, although requiring higher dosages, minimize the risk of toxic fumes. Intumescent coatings also play a significant role, applied to steel structures and wooden elements, where they form a protective layer during a fire, preventing the collapse of load-bearing parts.

Challenges in Processing and Formulation Optimization

The integration of flame retardants into polymer matrices presents several technological challenges that affect processability, mechanical properties, and production economics. Halogenated systems, particularly brominated compounds, may degrade at high processing temperatures (above 250 °C) and release corrosive gases, placing demands on the material of processing machinery. Additionally, compatibility with other additives such as stabilizers, plasticizers, or fillers must be addressed to prevent undesirable reactions that reduce the effectiveness of the flame retardant. In practice, synergistic combinations with antimony trioxide are often used, which can increase the efficiency of brominated compounds by up to 50 %.

Halogen-free systems require a different approach to formulation optimization. High loadings of metal hydroxides (up to 60 % by weight) can lead to reduced mechanical strength, increased melt viscosity, or dispersion issues. Solutions include the use of surface-treated fillers or combinations with other types of flame retardants, such as phosphorus compounds. Intumescent systems, in turn, require precise control of particle distribution in the matrix to ensure uniform expansion during a fire. In both cases, testing the rheological properties of the melt and the mechanical characteristics of the final product is key to avoiding compromises in performance.

Need advice on selecting flame retardants?

Every application requires an individual approach – whether it’s demands for temperature resistance, material compatibility, or environmental certifications. GCG Group provides complete technical documentation for supplied raw materials and advises on selecting the optimal solution for your production. Contact us – or browse our catalog of over 1,300 products.