Flame Retardant Modification of TPU with Inorganic Flame Retardants

Thermoplastic polyurethane (TPU) is widely used in electronics, automotive, cable, footwear, and industrial applications, but its flammability limits its performance in fire-sensitive fields. To improve flame resistance, TPU can be modified with inorganic flame retardants such as aluminum hydroxide (ATH), magnesium hydroxide, nanoclays, and expandable graphite (EG). This article introduces the commonly used inorganic flame retardants for TPU, their effects, and synergistic strategies for improving performance.

1. Common Inorganic Flame Retardants for TPU

Aluminum Hydroxide (ATH)

Aluminum hydroxide is the most widely used mineral flame retardant, accounting for about 45% of the global market. In TPU compounding, ATH provides excellent flame retardancy, but requires high loading levels, which can reduce the material’s tensile strength, hardness, and thermal stability. To improve compatibility, surface treatment of ATH is often necessary.

Magnesium Hydroxide

Magnesium hydroxide (brucite) is another important inorganic flame retardant. Similar to ATH, it requires high dosage to achieve effective flame resistance. While it improves fire safety, excessive addition negatively impacts the mechanical performance of TPU unless proper dispersion and surface modification are applied.

ATH + Mica Combination

When ATH is combined with mica in TPU formulations, flame retardancy is enhanced, while mica helps counteract the negative effect of ATH on mechanical properties. Mica provides thermal insulation and strengthens the overall composite, making it an effective supplementary filler.

2. Nanoclay Flame Retardants

Nanoclay minerals, when dispersed at the nanoscale in TPU, significantly improve barrier effects during combustion. Their layered structure:

• Blocks heat and flammable gases from spreading,

• Reduces thermal feedback from the gas phase to the condensed phase,

• Restricts the mobility of TPU molecular chains, increasing decomposition temperature.

When used with nitrogen-phosphorus flame retardants (FRs), nanoclays enhance performance. For example, TPU with 18% FRs can achieve UL94 V-0 rating and a LOI (Limiting Oxygen Index) of 32.8%. Adding 1% nanoclay further improves char formation and reduces mechanical losses compared to pure FR formulations.

3. Expandable Graphite (EG)

Expandable graphite is produced by chemically treating natural flake graphite to form an intercalation compound. Upon heating:

• It expands up to 280 times its volume,

• Transforms into a “worm-like” structure,

• Creates a stable char layer on the TPU surface,

• Provides thermal insulation and oxygen barrier,

• Prevents the release of flammable gases.

EG does not chemically react with TPU but forms a protective barrier layer at 200–300 °C. This improves the thermal stability of TPU and blocks further decomposition, effectively breaking the combustion chain reaction.

4. Conclusion

Inorganic flame retardants such as ATH, magnesium hydroxide, nanoclays, and expandable graphite play vital roles in improving the flame retardancy of TPU. While high loadings may reduce mechanical performance, surface modification and synergistic formulations (e.g., ATH with mica or nanoclay with phosphorus-nitrogen FRs) help balance fire safety and mechanical strength. Expandable graphite, in particular, offers a highly effective physical barrier against heat and oxygen, making it a strong candidate for flame-retardant TPU applications.

By combining different inorganic flame retardants and optimizing their dispersion, manufacturers can develop flame-retardant TPU materials that meet strict fire safety standards such as UL94 V-0 and LOI ≥ 32%, expanding their use in high-performance and safety-critical industries.

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