New Breakthrough in HFFR: Advanced Applications and Challenge of Flame Retardants in Thermoplastic Elastomers

New Breakthrough in HFFR: Advanced Applications and Challenge of Flame Retardants in Thermoplastic Elastomers

Thermoplastic elastomers (TPE) are widely used in various fields due to their combining rubber elasticity with plastic processability. However, most TPEs are highly flammable, making flame retardant modification crucial. With environmental regulations tightening, developing efficient, low-toxicity, halogen-free flame retardant systems, especially phosphorus-nitrogen flame retardants, has become a research hotspot.

I. Flame Retardant Challenges and Design Concepts for TPEs

TPEs, including styrene-based (SBS, SEBS), polyolefin-based (TPO, TPV), polyurethane-based (TPU), and polyester-based (TPEE) types, face challenges in flame retardant modification:

1. Complex multiphase systems: Multicomponent blends like SEBS/PP/mineral oil systems have varying combustion behaviors, complicating flame retardant mechanisms.

2. Impact of plasticizers: Added mineral oils and other plasticizers are flammable, reducing the system's limiting oxygen index (LOI) and increasing flame retardancy challenges.

3. Performance balance dilemma: Flame retardants often negatively affect mechanical properties, processability, aging resistance, and appearance.

A successful flame retardant solution must consider:

1. Define requirements: Determine flame retardant grade (e.g., UL94 V-0), oxygen index (LOI), and environmental requirements based on application fields.

2. Select flame retardant systems: Prioritize halogen-free systems, choosing flame retardants based on TPE type and synergistic systems (e.g., phosphorus-nitrogen synergy, intumescent systems) to improve efficiency and reduce additive amounts.

3. Optimize processing technology: Use high-shear twin-screw extruders to ensure uniform flame retardant dispersion in the matrix, preventing phase separation, improving surface appearance, and enhancing flame retardant efficiency.

4. Performance evaluation and balance: Adjust formulas (e.g., using compatibilizers, lubricants) and optimize processes to meet flame retardant requirements while maintaining TPE’s mechanical properties and processability.

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

Halogen-free flame retardants include metal hydroxides, intumescent flame retardants (IFR), phosphorus-based, nitrogen-based, silicon-based, and others. Phosphorus-nitrogen flame retardants are favored for their high efficiency and eco-friendliness, functioning through synergistic mechanisms in the condensed and gas phases:

1. Condensed phase: Phosphorus components promote polymer surface dehydration and carbonization, forming a dense, intumescent carbon layer to isolate heat and oxygen transfer and suppress flammable substance volatilization.

2. Gas phase: Nitrogen compounds decompose upon heating to release non-flammable gases (e.g., NH₃, N₂), diluting flammable gas and oxygen concentrations and inhibiting combustion chain reactions.

3. Their synergistic effects achieve "1+1>2" flame retardant results, reducing total additive amounts and minimizing impacts on material properties.

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

1. Application in Polyester Elastomers (TPEE): TPEE, used in automotive components and cable sheaths, has a low LOI of about 19%, making it highly flammable with severe melt dripping. Using flame retardant combinations like aluminum diethyl phosphinate (AlPi) can increase LOI to 31.5%, passing UL94 V-0, with a dense carbon layer forming during combustion to suppress melt dripping.

2. Application in Styrene-Based Elastomers (TPS, such as SEBS): SEBS, used in data cables and earphone wires, suffers from yellowing and flammability after oil addition. Solutions include combining phosphorus-nitrogen flame retardants (e.g., aluminum hypophosphite, MCA) with flame retardant silicone masterbatches to achieve UL94 V-0 while maintaining tensile strength and flexibility, and adding benzophenone-type UV absorbers to inhibit yellowing.

3. Application in Polyolefin-Based Elastomers (TPO/TPV): TPO/TPV, used in automotive interiors and sealing strips, often employs magnesium/aluminum hydroxides for flame retardancy but requires large additive amounts. Phosphorus-nitrogen flame retardants like ammonium polyphosphate (APP)-based intumescent systems offer a new alternative, forming an intumescent carbon layer upon heating. Non-APP-based intumescent flame retardants, with high-temperature resistance and excellent carbonization effects, are also suitable for TPE systems and can produce white products.

4. Application in Polyurethane-Based Elastomers (TPU): TPU, known for wear resistance and transparency, is highly flammable. Flame retardant design must consider differences between TPU types: Polyester-based TPUs typically require 8-12 parts of halogen-free flame retardants, while polyether-based TPUs may need 30-35 parts, with attention to potential additive exudation. Phosphorus-nitrogen flame retardants like hypophosphite salts and MCA are common choices.

IV. Application Recommendations and Future Outlook

When selecting phosphorus-nitrogen flame retardants:

1. Targeted selection: Choose flame retardants matching the TPE matrix type.

2. Surface modification: Modify flame retardants (e.g., with silane coupling agents) to improve compatibility with the polymer matrix, reduce impacts on mechanical properties, and enhance flame retardant efficiency.

3. Processing technology: Strictly control processing temperature and shear force to prevent decomposition or deactivation of flame retardants under high temperature and high shear conditions.

4. Synergistic enhancement: Explore combinations of phosphorus-nitrogen flame retardants with other halogen-free flame retardants (e.g., metal hydroxides, silicon-based, nanofillers) to reduce additive amounts and optimize overall performance.

In the future, as environmental regulations become stricter and demand for high-performance materials grows, halogen-free flame retardant technology for thermoplastic elastomers will evolve toward higher efficiency, multifunctionality, and eco-friendliness. The molecular design of novel phosphorus-nitrogen flame retardants, the application of nanocomposite technologies, and the development of bio-based flame retardants will drive green flame retardant innovation for TPEs.

In flame retardant modification research for thermoplastic elastomers, Yinsu, as an industry participant, will increase R&D investment to develop and innovate novel flame retardants. Yinsu will focus on highly efficient, low-toxicity, halogen-free flame retardant systems, particularly the molecular design and optimization of phosphorus-nitrogen flame retardants, and explore their application potential in different TPE materials. Through continuous technological innovation and industrial upgrades, Yinsu will contribute to the green flame retardant development of thermoplastic elastomers and help the industry move toward higher efficiency, multifunctionality, and eco-friendliness.