Halogen-Free Flame Retardant Application Solution Part 2: Selection, Application, And Challenges of Flame Retardants for Thermoplastic Elastomers

Halogen-Free Flame Retardant Application Solution Part 2: Selection, Application, and Challenges of Flame Retardants for Thermoplastic Elastomers

Thermoplastic Elastomers combine the elasticity of rubber with the processing convenience of plastics, finding wide application in automotive, electronics, electrical, medical, and other fields. However, most TPE materials are highly flammable, making flame retardant modification a key step in their performance enhancement. With increasingly stringent environmental regulations, the development of efficient, low-toxicity, halogen-free flame retardant systems, particularly the application of phosphorus-nitrogen flame retardants, has become a major research focus in the industry.

I. Challenges and Design Considerations for Flame Retarding Thermoplastic Elastomers

There are numerous types of TPEs, mainly including styrenic block copolymers (SBS, SEBS), polyolefin-based (TPO, TPV), thermoplastic polyurethane (TPU), and thermoplastic polyester elastomer (TPEE). Their flame retardant modification faces several inherent difficulties:

1. Complexity of Multi-Phase Systems: Many TPEs are multi-component blend systems, such as SEBS/PP/paraffin oil blends. The different combustion behaviors of each component make it difficult for a single flame retardant mechanism to be effective.

2. Impact of Plasticizers: Plasticizers like mineral oils, added to adjust hardness, are often flammable. They significantly reduce the Limiting Oxygen Index of the system and increase the difficulty of achieving flame retardancy.

3. Performance Balance Challenge: The addition of flame retardants often negatively impacts the material's mechanical properties, processability, aging resistance, and appearance.

Therefore, a successful flame retardant solution requires systematic consideration:

1. Define Requirements: Determine the required flame retardancy rating, oxygen index, and environmental requirements based on the specific application field.

2. Select the Flame Retardant System: Halogen-free is the current mainstream trend. Choose flame retardants based on the TPE type, and prioritize synergistic systems to improve efficiency and reduce loading.

3. Optimize Processing: Use high-shear twin-screw extruders to ensure uniform dispersion of the flame retardant in the matrix. This is crucial for preventing migration, improving surface appearance, and enhancing flame retardant efficiency.

4. Evaluate and Balance Performance: Through formulation adjustments and process optimization, strive to maintain the TPE's mechanical and processing properties while meeting flame retardancy requirements.

II. Selection of Halogen-Free Flame Retardants: The Advantages of Phosphorus-Nitrogen Systems

There are various types of halogen-free flame retardants, including metal hydroxides, intumescent flame retardants, phosphorus-based, nitrogen-based, and silicon-based systems. Among these, phosphorus-nitrogen flame retardants are highly favored for their efficiency and eco-friendliness.

Phosphorus-nitrogen flame retardants primarily work through synergistic mechanisms in the condensed and gas phases:

1. Condensed Phase: The phosphorus component promotes dehydration and char formation on the polymer surface, creating a dense, intumescent char layer. This effectively blocks the inward transfer of heat and oxygen and inhibits the outward release of flammable volatiles.

2. Gas Phase: Nitrogen compounds decompose upon heating to release non-flammable gases, diluting the concentration of combustible gases and oxygen, thereby suppressing the combustion chain reaction.

Their synergistic effects are often significant, achieving a "1+1>2" result, which allows for a reduction in total additive loading and minimizes the impact on the material's physical and mechanical properties.

III. Application Practices of Phosphorus-Nitrogen Flame Retardants in Different TPEs

1. Application in Thermoplastic Polyester Elastomer (TPEE)

TPEE is known for its excellent mechanical properties and fatigue resistance, commonly used in automotive parts and cable sheathing. However, its LOI is only about 19%, making it flammable with severe melt dripping. Studies show that using compound flame retardants like aluminum diethylphosphinate can effectively address this. With appropriate mass ratios and a total loading of 22-25%, LOI can be increased to 31.5%, achieving a UL94 V-0 rating. A dense char layer forms during combustion, effectively suppressing melt dripping.

2. Application in Styrenic Thermoplastic Elastomers (TPS, e.g., SEBS)

SEBS is often used in light-colored products like data cables and earphone wires, but faces issues of easy yellowing and flammability after oil extension. Targeted solutions include:

• Flame Retardant System: Using a compound of phosphorus-nitrogen flame retardants with flame retardant silicone masterbatches. Silicone masterbatches can promote char formation and offer good compatibility with the matrix, reducing impact on mechanical properties. This system can achieve UL94 V-0 while maintaining good tensile strength and flexibility.

• Yellowing Resistance: Adding yellowing inhibitors like benzophenone-based UV absorbers can effectively suppress yellowing during use and storage.

3. Application in Polyolefin-based Elastomers (TPO/TPV)

TPO/TPV are widely used in automotive interiors and seals. Their flame retardancy often relies on magnesium/aluminum hydroxide, but these require high loadings. Phosphorus-nitrogen systems, such as ammonium polyphosphate-based intumescent systems, offer a new option. APP compounded with charring agents can form an intumescent char layer upon heating. Non-APP based IFRs, with their high-temperature resistance and effective char-forming properties, are also suitable for TPE systems and can be used for white products.

4. Application in Thermoplastic Polyurethane (TPU)

TPU is known for its abrasion resistance and transparency but is also flammable. Its flame retardant design requires attention to type differences. Polyester-based TPU typically requires 8-12 phr of halogen-free flame retardant to meet requirements, while polyether-based TPU may need 30-35 phr, and attention must be paid to potential blooming of the flame retardant. Phosphorus-nitrogen flame retardants like phosphinates and MCA are common choices.

IV. Application Recommendations and Future Outlook

When selecting phosphorus-nitrogen flame retardants, consider the following:

1. Targeted Selection: Choose flame retardants that match the specific TPE base polymer type.

2. Surface Treatment: Surface modification of flame retardants can improve compatibility with the polymer matrix, reduce the impact on mechanical properties, and potentially enhance flame retardant efficiency.

3. Processing: Strictly control processing temperature and shear force to prevent decomposition or deactivation of certain phosphorus-nitrogen flame retardants under high heat and shear.

4. Synergistic Effects: Explore the compounding of phosphorus-nitrogen flame retardants with other halogen-free systems to further reduce loading and optimize overall performance.

In the future, as environmental regulations tighten and demand for high-performance materials grows, halogen-free flame retardant technology for TPEs will continue to develop towards higher efficiency, multifunctionality, and eco-friendliness. The molecular design of new phosphorus-nitrogen flame retardants, the application of nanocomposite technology, and the development of bio-based flame retardants will inject more momentum into the green flame retardant innovation for TPEs.