Magnesium hydroxide's endothermic decomposition, high heat resistance and cost advantage make it an ideal choice for flame retardant additives.
Flame retardants are essential additives in the development of safe, high-performance materials across a wide range of industries, from construction to electronics. Among the various flame retardants available, contact us (Mg(OH)â‚‚) and zinc borate (ZnB) stand out as effective inorganic, halogen-free options. While both play crucial roles in fire prevention and suppression, their mechanisms of action, decomposition behavior, efficiency, and applications differ significantly.
In this blog, we explore the differences between magnesium hydroxide and zinc borate, comparing their flame retardant mechanisms, usage scenarios, synergistic effects, environmental impact, and cost-effectiveness.
One of the most fundamental differences between magnesium hydroxide and zinc borate lies in how they suppress fire.
Magnesium hydroxide functions primarily through a physical, endothermic reaction. When exposed to high temperatures, it decomposes around 340°C, releasing water vapor:
Mg(OH)â‚‚ → MgO + Hâ‚‚O (↑)
This decomposition:
Absorbs large amounts of heat, effectively reducing the surrounding temperature,
Dilutes flammable gases with steam,
Forms magnesium oxide (MgO), a stable, heat-resistant residue that acts as a barrier against further combustion.
This makes magnesium hydroxide highly effective in delaying ignition and reducing heat release during a fire.
Zinc borate works through chemical and physical mechanisms:
Upon heating (typically decomposing below 300°C), zinc borate releases water and forms a glassy, boron-rich layer that:
Coats the material surface,
Prevents oxygen from reaching the substrate,
Suppresses the release of flammable volatiles.
It also promotes char formation, especially in polymeric materials, which enhances the fire barrier effect.
Zinc borate is often used as a synergist, enhancing the flame retardant effect when combined with other flame retardants, especially halogenated types.
The thermal decomposition temperature of magnesium hydroxide is approximately 340°C, making it suitable for materials that undergo high-temperature extrusion or molding, such as:
Polypropylene (PP),
Polyethylene (PE),
Rubbers and elastomers,
Engineering plastics.
Its high decomposition point ensures that it remains stable during processing, releasing its flame-retardant benefits only when the material is exposed to fire or excessive heat.
In contrast, zinc borate begins to decompose at lower temperatures, generally between 290°C and 300°C. This makes it suitable for lower-temperature applications, such as:
PVC (polyvinyl chloride),
Wire and cable insulation,
Textiles,
Coatings and adhesives.
Its ability to form a protective, glassy surface even at moderate temperatures is valuable in situations where thermal sensitivity is a concern.
One of the challenges of using magnesium hydroxide is its relatively low flame retardant efficiency per unit weight. As a result, it often requires high loadings (up to 60%) in formulations to achieve desired fire performance levels. This high filler content can:
Negatively impact mechanical properties,
Lead to brittleness or reduced flexibility,
Increase processing difficulty.
To mitigate these effects, surface treatments or coupling agents may be used to improve dispersion and compatibility with polymers.
Zinc borate, on the other hand, is typically used at much lower loadings (5–15%), especially as a synergist with other flame retardants. Its benefits include:
Minimal impact on mechanical properties,
Maintained flexibility and toughness of base materials,
Enhanced electrical insulation properties—especially valuable in cable and electronics industries.
This efficiency at low doses makes zinc borate an attractive option for formulations requiring high performance with minimal trade-offs.
Magnesium hydroxide can work in combination with other flame retardants, but it typically does not exhibit strong synergistic behavior. Its effect is more independent, relying on its own heat absorption and smoke suppression capabilities.
Zinc borate is renowned for its synergistic capabilities, particularly in:
Halogen-containing systems (e.g., with antimony trioxide or brominated flame retardants),
Halogen-free systems (e.g., with ammonium polyphosphate or aluminum hydroxide).
It can:
Suppress afterglow,
Promote char formation,
Reduce smoke generation,
Lower the required concentration of other additives.
This makes it a powerful tool for optimizing flame retardant formulations in complex systems.
Both magnesium hydroxide and zinc borate are considered environmentally friendly flame retardants, but they differ in availability and cost.
Magnesium hydroxide is:
Widely available,
Low in toxicity,
Cost-effective,
Often derived from natural mineral sources like brucite or seawater.
It is a preferred choice for manufacturers prioritizing low-cost, halogen-free solutions for high-temperature materials.
Zinc borate:
Has a higher unit cost due to its complex synthesis process,
Is typically used in smaller amounts, so the overall cost impact may be moderate,
Offers greater performance per unit weight, especially when used synergistically.
Its value lies in its multifunctionality—flame retardant, smoke suppressant, anti-drip agent, and synergist.
| Property / Application | Magnesium Hydroxide | Zinc Borate |
|---|---|---|
| Decomposition Temp | ~340°C | ~290–300°C |
| Flame Retardant Mechanism | Heat absorption, water release, MgO barrier | Glassy coating, carbonization promotion |
| Processing Suitability | High-temp polymers (e.g., PP, PE, EVA) | Low-temp systems (e.g., PVC, cables) |
| Addition Amount | High (30–60%) | Low (5–15%) |
| Mechanical Impact | May reduce strength or flexibility | Minimal impact |
| Synergistic Effects | Limited | Strong, especially with halogenated systems |
| Environmental Safety | Halogen-free, non-toxic | Halogen-free, low toxicity |
| Cost | Low | Moderate to high |
| Typical Applications | Polyolefins, rubber, thermoplastics | PVC, wire and cable insulation, coatings, adhesives |
Magnesium hydroxide and zinc borate represent two powerful and unique inorganic flame retardant approaches. Magnesium hydroxide’s endothermic decomposition, high heat resistance and cost advantages make it an ideal choice for high temperature plastics and cost-sensitive applications.
Magnesium hydroxide is widely used in halogen-free systems due to its non-toxic nature and ability to suppress smoke and heat. However, zinc borate also performs well in halogen-free systems and can act as a synergist with other additives. The best choice depends on the material and processing temperature. For high-temperature applications, magnesium hydroxide is often preferred.
Yes. Magnesium hydroxide and zinc borate can be combined in formulations to enhance flame retardant performance. Zinc borate can complement magnesium hydroxide by promoting char formation and reducing smoke, while magnesium hydroxide provides heat absorption and thermal barrier effects. This combination is particularly effective in halogen-free, environmentally friendly flame retardant systems.
The main limitation is that magnesium hydroxide requires a high loading level (often 40–60%) to be effective, which can negatively impact mechanical properties, such as flexibility and impact strength. Manufacturers may need to balance flame retardancy with performance by using compatibilizers or plasticizers, or by combining magnesium hydroxide with other synergistic additives like zinc borate to reduce the required filler amount.