Types of flame retardant additives

flame retardants

Flame retardants are additives used in plastics and composites to reduce their flammability and slow down the spread of fire. 

Plastics and composites are widely used in various industries due to their lightweight, durable, and versatile nature. However, they can be highly flammable, posing significant risks in terms of fire safety. 

For this reason, flame retardants are an important tool for reducing the risk of fire and improving public safety, but there is growing concern about the potential health and environmental impacts of some flame retardants.

That being said, not all flame retardant additives are created equally and there’s no one-size-fits-all formulation. In this blog, we will discuss the different types of flame retardants and their effectiveness along with their potential environmental impacts.

Understanding the functions of flame retardants 

Flame retardants can either be chemically bound to the polymer by grafting or dispersed in the polymer matrix as additives in the production of customised formulations. If these additives are chemically compatible with the plastic, they act as plasticizers otherwise, they are considered as fillers.

Flame retardants work by a combination of physical and chemical actions. These can be broadly divided into three modes; 

  • Cooling by triggering an endothermic process that either cools the substrate or the gaseous phase. The system cools down and the supply of flammable gases is suppressed so that the combustion process cannot be sustained. 
  • Formation of protective layer or char: The polymer is shielded and insulated from the combustion process. The oxygen required for combustion is excluded and heat transfer is impeded, preventing further decomposition of the material.
  • Dilution of the polymer and any gases produced during burning so that the lower ignition limit of the gas mixture is not exceeded. 
application of flame retardant

The most common flame retardants

Flame retardants work through several mechanisms to inhibit or delay the ignition, combustion, and spread of fire. 

Here are some common types of flame retardants used in plastics and composites:

  1. Halogenated flame retardants: This category includes compounds containing halogens such as bromine, chlorine, or both. They act by releasing halogen radicals during combustion, which interfere with the chemical reactions involved in the fire. These flame retardants are highly effective at preventing fires and have been widely used, but there are concerns about their potential environmental and health impacts.
  2. Phosphorus-based flame retardants: Phosphorus compounds are commonly used as flame retardants in plastics and composites. They can function through a combination of mechanisms, including forming a protective layer on the material’s surface, diluting the flammable gases, or promoting char formation, which acts as a barrier against heat and oxygen. They are less effective than halogenated flame retardants but are considered to be safer and more environmentally friendly.
  3. Nitrogen-based flame retardants: These additives release nitrogen gases when exposed to high temperatures, which dilute the oxygen concentration and inhibit the combustion process. They can also promote the formation of a protective char layer.
  4. Mineral fillers: Inorganic materials such as alumina trihydrate (ATH) and magnesium hydroxide (MDH) are used as flame retardants. When heated, these fillers release water vapour, which cools down the material and reduces flammability. They also act as physical barriers that inhibit the transfer of heat and flame.
  5. Intumescent flame retardants: Intumescent systems are a combination of several components, including acid sources, carbon sources, and blowing agents. When exposed to heat, these systems undergo a chemical reaction, forming a protective char layer that insulates the material and prevents further combustion.
flame retardant additives

Flame retardant products at RBH

RBH offers a range of synergistically functioning fire and smoke-suppressing chemicals, developed in conjunction with industry leaders and global producers. We pride ourselves on our commitment to quality and excellence, providing a diverse portfolio of products that can be tailored to specific requirements and manufacturing processes.

Ammonium polyphosphate (APP) liquid and powders 

APP is a commonly used flame retardant in various industries, including plastics and composites. They are particularly effective in materials having a high oxygen content, such as cellulose and some oxygen-containing plastics. 

How it works

APP functions through a combination of chemical and physical mechanisms to reduce the flammability of materials. When exposed to heat or flames, APP undergoes a condensation reaction, where it breaks down into phosphoric acid and ammonia. This reaction releases non-combustible gases, primarily water vapour and ammonia, which dilute the oxygen concentration in the surrounding environment. By reducing the availability of oxygen, a crucial component for combustion, the primary flame retardant action of APP is achieved. 

In addition, the phosphoric acid produced during the condensation reaction reacts with the material’s surface, forming a protective layer of char. This char layer offers two functions:

  1. Acts as a physical barrier, preventing the transfer of heat to the underlying material
  2. Helps reduce the release of combustible volatiles and slows down the spread of fire

The formation of this char layer depends on the composition of the material and the specific conditions during combustion. Some materials may have inherent charring properties, while others may require additional additives or synergists to enhance the char formation. 

The effectiveness of APP as a flame retardant can be influenced by factors such as:

  • The concentration of the additive
  • The processing conditions during the manufacturing of the material
  • The overall formulation of the composite or plastic. 

It is essential to optimise these parameters to achieve the desired fire safety performance. The RBH group can provide custom formulations tailored to your specific requirements to ensure suitability.

Environmental impacts

It’s worth noting that APP is considered relatively safe compared to some other flame retardants, such as halogenated compounds. It has low toxicity and is not associated with significant environmental concerns. However, as with any chemical additive, it is important to follow appropriate handling, usage, and disposal guidelines to ensure safety and minimise potential environmental impacts.

RBH’s ammonium polyphosphate products range from 20 – 35 microns, with Silane, Melamine and epoxy-coated additions also available.

ammonium polyphosphate flame retardant

Aluminium Trihydrate (ATH) and Magnesium Hydroxide

ATH is the most widely used fire retardant and smoke suppressive additive in the industry and is compatible with a wide range of polymer types including thermoplastics, thermosets and elastomers. 

How they work

The flame retardant mechanism of ATH is physical and combines cooling with the formation of a protective layer and gaseous phase dilution. The ATH breaks down around 200°C to aluminium oxide in an endothermic reaction with the release of water vapour. The resulting endotherm cools the polymer so fewer pyrolysis products are formed. 

The aluminium oxide, together with the charring products formed on the substrate acts as an insulating protective layer and the water vapour liberated has a diluting effect in the gas phase and forms an oxygen-displacing protective layer. 

Magnesium hydroxide (MDH) decomposes in the same manner as ATH and its action is identical. 

The two main differences between ATH and magnesium hydroxide are:

  1. Magnesium hydroxide has a decomposition temperature 100°C higher than that of ATH, allowing a higher processing temperature in compounding and extruding the plastic. 
  2. Magnesium hydroxide has a higher Enthalpy of Decomposition and adsorbs more energy during the decomposition process.

Environmental impacts

While ATH and MDH as additives are not known to pose significant environmental risks, the production, use, and disposal of products containing these chemicals may have some environmental implications.

RBH’s ATH product range includes both precipitated and ground grades in various colours and particle sizes. For products processed at temperatures exceeding 200°C, we can offer both fine natural MDH and new coated variants.

magnesium hydroxide flame retardant

Zinc Borate

Zinc Borate is a multifunctional flame retardant compatible with many polymeric matrices. It is effective both in the solid phase and in the gas phase and is commonly used in combination with other flame retardants where it has a synergistic effect. 

How it works

It promotes the formation of a strong glass-like char layer and releases its water of hydration at temperatures above 290°C, cooling the front of the flames and removing energy from the fire. Zinc Borate can also be used as an afterglow suppressant.

Environmental impacts

Zinc borate is generally considered to have a low environmental impact, but its environmental friendliness depends on specific applications and manufacturing practices.

RBH offers high-purity Zinc Borate products in a range of particle sizes and surface treatments.

zinc borate used as a flame retardant

Dicyandiamide (DICY) 

Dicyandiamide (DICY) is a nitrogen-based flame retardant with various desirable performance and environmental protection characteristics, and is commonly used alongside Ammonium Polyphosphate. 

How it works

It can interact with other flame retardants and is used as a synergist to help in char formation. Nitrogen gas is released at elevated temperatures diluting the available oxygen and flammable gases and suppressing combustion.

Environmental impacts

Dicyandiamide is generally considered to be a low-risk compound with a low toxicity profile and doesn’t pose any significant environmental concerns.

RBH supplies high-purity dicyandiamide for intumescent systems, free from impurities and contaminants.

Developments in flame retardant regulations

In recent years, there has been a push to regulate the use of flame retardants and phase out the use of certain chemicals that are known to be harmful. 

The European Union has implemented regulations that restrict the use of several halogenated flame retardants, while the United States has introduced regulations that require the testing and labelling of certain products containing flame retardants.


Flame retardants are an important tool for reducing the risk of fire and improving public safety. The selection of flame retardants depends on the specific requirements of the application, considering factors such as desired fire safety performance, regulatory compliance, material compatibility, and environmental considerations.

It is essential to balance fire safety with growing concerns about the potential health and environmental impacts of some flame retardants, particularly halogenated flame retardants. While regulations are being implemented and research is ongoing to fully understand the risks and benefits of different types of flame retardants and to address these concerns, more research is needed. 

Simultaneously, ongoing research and development are focused on developing more environmentally friendly flame retardant technologies, such as using bio-based additives or optimising the formulation of existing flame retardants to minimise their potential negative impacts.

RBH’s halogen-free endothermic materials have mechanisms to control ignition, fire spread, containment and after-burn in resin, polymer and elastomer products, developed using environmentally friendly technologies. Our team of experts can help to determine the most suitable flame retardant additive for your application, call us or contact us online to find out more.