What Are the Types of Inorganic Flame Retardants?

Inorganic flame retardants are a class of flame retardants that contain minerals that have been added to materials such as plastics, rubber, paints, and cables. These types of flame retardants have emerged as a replacement for halogen-containing chemicals that are traditionally used by industry.

The most prominent aspect of these compounds has been the environmentally friendly nature of their reactions when in contact with fire. Halogen-containing flame retardants react with fire and release very harmful substances, while inorganic flame retardants produce minimal amounts of smoke and release only harmless substances such as water. They are also extremely affordable and thus widely used on an industrial scale due to modern safety requirements, which regulate the amount of smoke and toxicity level indoors.

This article describes in detail various inorganic flame retardants, explains how they operate, and provides advice on compounding them.

Among all the types of inorganic flame retardants, metal hydroxides are the most used. These include aluminum hydroxide, also known as aluminum trihydrate (ATH) in the industrial sector, and magnesium hydroxide (MDH). Both of them work by cooling but can be applied in different manufacturing processes due to their inability to withstand high temperatures.

Aluminum hydroxide is relatively cheap and comes in the form of white crystalline powder. Once heated up to 200°C-220°C, it causes an endothermic reaction, meaning that it takes a lot of energy from the burning fire. In the course of the process, it releases steam. This steam helps to reduce the flammability of gases, while aluminum oxide protects the material from catching fire, creating a thin film on its surface. Due to the low decomposition temperature, it is appropriate for polymers with low processing temperatures, like PVC, PE, and rubber.

MDH is based on the same concept, although MDH exhibits very high thermal stability. The compound does not start giving up its bound water content until a temperature of 340°C is attained. Therefore, it is ideal for use in engineering thermoplastics such as PP or nylon due to their high thermal stability.

Both ATH and MDH are extensively used in wire and cable insulation, building materials, and electronics due to their excellent smoke suppression. However, because they rely on physical water release to fight fire, they require high loading levels—often making up 50% to 65% of the total compound weight.

When using these materials in high amounts, the uncoated minerals will create poor compatibility, resulting in fragile plastics, as well as a significant reduction in their tensile strength. In order to deal with this problem, manufacturers use only surface-coated minerals. The application of silanes or fatty acids as coating agents helps to ensure good compatibility of the filler with the plasticizer.

When simple metal hydroxides fail to satisfy severe fire standards, or when the high use of these additives compromises the material’s performance, chemists have turned to other inorganic retardants. Such materials can sometimes operate according to chemical mechanisms, like charring, or serve as performance enhancers.

In contrast to the cooling effect of hydroxides, red phosphorus and APP exert their influence through condensing reactions and create an insulation layer by acting physically. Once a fire develops, these phosphorus materials interact with the degraded polymer material and create a thick carbonaceous char coating on its surface. The layer serves as a thermal barrier, preventing access of oxygen to the burning plastic and the escape of vapor fuels to the flame zone. APP is often employed as an ingredient for intumescent materials (those that expand in heat), whereas micro-encapsulated red phosphorus is a superb fire retardant for electronic applications.

Zinc borate is a multifunctional additive typically used as a supporting player rather than a standalone retardant. When heated to around 290°C, it releases water of hydration, but its main power is promoting a glassy, boron-rich layer over the burning surface. This glass layer stabilizes the char, stops dripping, and acts as an exceptional smoke suppressant. It is frequently paired with metal hydroxides to improve afterglow control—meaning it stops the material from smoldering after the open flame is put out.

Antimony trioxide does not possess strong flame-retardant properties on its own. Instead, it functions as a synergist, meaning it acts as an amplifier for other additives. When paired with halogenated compounds or specific inorganic systems, it undergoes gas-phase reactions that aggressively extinguish free radicals in the flame. It allows manufacturers to achieve high fire-safety ratings while using a lower overall concentration of additives.

When working with these secondary additives, particle size distribution is a critical variable. Fine-particle grades (typically below 2.5 microns) provide a higher surface area, which dramatically increases fire performance and ensures a smooth surface finish on extruded parts. However, very fine powders have a natural tendency to clump together during mixing. Utilizing high-shear compounding equipment is necessary to ensure uniform dispersion through the polymer matrix.

Successfully integrating inorganic flame retardants into a production line requires balancing fire safety standards, material integrity, and cost.

The absolute first step is matching the thermal stability of the flame retardant to the melt-processing temperature of your polymer. Attempting to extrude a Polypropylene profile at 240°C using standard ATH will cause the additive to decompose inside the extruder barrel, ruining the batch with trapped steam bubbles. For high-heat applications, MDH or zinc borate grades rated up to 300°C must be specified. Conversely, for low-temperature PVC cable jackets, cost-effective ATH is highly efficient.

The major limitation in using mineral flame retardants is the large amount that must be added to meet tough criteria such as the UL 94 V-0 test standards. Adding 60% of mineral powder in its raw form to a polymer compound results in high melt viscosity, leading to high pressure requirements during production and low elongation at break in the product. This challenge can be overcome by utilizing one of the following three methods:

Practical Tip: When using high-efficiency synergists like SF-600, ensure the additive is thoroughly pre-mixed with other auxiliaries before blending with the resin. This ensures uniform dispersion and stable flame retardancy across the entire batch.

In spite of all the mentioned difficulties, inorganic compounds are very valuable when used for the mass production of various materials such as building panels, automobile parts, or transit system cables. They make material costs quite stable and help prevent finished products from possible bans related to halogenated substances. It is very important always to test your formulas in small batches and tune your extrusion torque and mechanical properties in advance.

Inorganic flame retardants are a very safe and eco-friendly means of making products more fire-resistant. With the help of various natural phenomena such as endothermic heat absorption, steam dilution, and the creation of protective layers, these materials provide protection for products without using halogens. Manufacturers of polymers should choose their ingredients very carefully to produce safe yet competitive products.

Every matrix and production process has different characteristics. Feel free to contact our technical team todayfor product data sheets, sample orders, or special formula development for your products.