Below I list the main inorganic flame retardants one by one!
1. Aluminum hydroxide (ATH) - the most commonly used inorganic flame retardant
It accounts for more than 80% of global inorganic flame retardant consumption and works by endothermic dehydration (releasing water vapor) and diluting oxygen.
2. Magnesium hydroxide (MH) - the "tough guy" in high-temperature processing
The decomposition temperature is as high as 340~490℃, which is about 140℃ higher than ATH, and is suitable for processing engineering plastics with higher temperatures.
3. Red Phosphorus (RP) – the “radical” with the highest phosphorus content
Red phosphorus is the only flame retardant used in the form of elemental elements. The phosphorus content is as high as more than 70%. A small amount of addition can make oxygen-containing polymers (such as nylon and PBT) flame retardant. However, it easily absorbs moisture and releases highly toxic PH₃, so it must be microencapsulated.
4. Ammonium polyphosphate (APP) - the "soul" of the intumescent flame retardant system
APP is the core acid source of intumescent flame retardants. It decomposes when heated to generate polyphosphoric acid, which promotes the formation of char in the polymer. According to the degree of polymerization, it is divided into type I and type II. Type II has better thermal stability (>300℃).
5. Antimony trioxide (Sb₂O₃) - the "golden partner" of halogen flame retardants
Its own flame retardant effect is average, but when combined with a halogen flame retardant, it can generate antimony trihalide, capture free radicals in the gas phase and isolate oxygen. This is the famous "halogen-antimony synergistic effect".
6. Zinc Borate - a multi-functional "all-rounder"
It can be used alone or as a substitute for part of antimony oxide. It can suppress smoke, promote char formation, and inhibit droplets. It has a better effect when combined with aluminum hydroxide or red phosphorus.
7. Hydrotalcite (LDH) – the “new star” of layered structures
Layered double metal hydroxide. When heated, the layers decompose and absorb heat, releasing water and CO₂. At the same time, the remaining metal oxides can catalyze the formation of carbon. Often synergized with intumescent flame retardant systems.
8. Montmorillonite (MMT) – a “physical barrier” at the nanoscale
Organically modified montmorillonite (OMMT) can form a peeling structure in the polymer, which migrates to the surface to form a dense protective layer during combustion, slowing down heat and mass transfer.
9. Expandable graphite (EG) – a “magic worm” that expands when exposed to fire
When heated, the intercalations between the graphite layers decompose, causing the graphite to rapidly expand dozens to hundreds of times to form a worm-like carbon layer, which insulates heat and oxygen.
10. Attapulgite——One-dimensional nano “skeleton”
It has a fibrous crystal structure and can be used as a flame retardant synergist to enhance the strength of the carbon layer.
11. Nano calcium carbonate - low-cost "filling expert"
It can be used as filler in polyolefins, and can promote char formation and reduce the combustion heat release rate.
12. Aluminum hypophosphite (AHP) - a "potential stock" among inorganic salts
With high phosphorus content, the flame retardant efficiency is better than that of ordinary inorganic salts, and it is suitable for PBT, nylon, etc. However, the thermal stability is slightly poor, so care must be taken during processing.
13. Melamine polyphosphate (MPP) - a high-temperature-resistant "nitrogen and phosphorus duo"
Compared with melamine phosphate (MP), MPP has a higher thermal decomposition temperature (>310°C) and is suitable for flame retardant glass fiber reinforced nylon.
