Why Is TPE Flame Retardancy Difficult? What Are The Processing Challenges?

Why is TPE Flame Retardancy Difficult? What are the Processing Challenges?

Thermoplastic Elastomer TPE, also known as artificial rubber or synthetic rubber. Its products not only possess the high elasticity, aging resistance, and oil resistance of traditional cross-linked vulcanized rubber but also feature the processing convenience and wide range of processing methods of ordinary plastics.

TPE is mainly composed of two parts: one is plastic, which serves as the continuous phase, and the other is rubber, which serves as the dispersed phase. Usually, the rubber needs to be combined with softening oil or plasticizer. Vulcanizing agents and some auxiliary additives are also essential. Furthermore, to reduce costs or improve certain properties, some inorganic fillers are also added.

Based on material composition, it can be divided into: Styrenic (SBS, SIS, SEBS, SEPS), Olefinic (TPO, TPV), Diene (TPB, TPI), Chlorinated (TPVC, TCPE), Urethane (TPU), Ester (TPEE), Amide (TPAE), Fluorinated (TPF), Silicone and Vinyl classes, etc., covering almost all areas of modern synthetic rubber and synthetic resin.

The following lists the composition of several common TPE.

However, the TPE thermoplastic elastomer formulation system has numerous components. Due to the differences in the structure and properties of each component and unclear mechanisms, flame retardant treatment brings considerable difficulty. There are also significant potential safety hazards during use, which imposes certain limitations on the application of TPE products. So, what are the basic requirements for TPE flame retardant treatment, and what difficulties might be encountered?

I. Basic Requirements for Flame Retardants in TPE
When selecting flame retardants, it is mostly about adopting a compromise method under the condition of meeting the following basic requirements, based on the principle of value engineering, mastering a "degree" to find an optimal balance:

• High flame retardant efficiency, requiring less dosage per unit of flame retardant efficiency, meaning high efficacy/price ratio.

• Should possess ecological and environmental protection requirements and characteristics.

• Good compatibility with the substrate to be flame-retarded, not prone to blooming, and meeting plastic processing conditions.

• Possess sufficient thermal stability.

• Does not excessively deteriorate the processing performance of the flame-retarded material and the physical mechanical properties and electrical properties of the final product, achieving a harmonious unity between material flame retardancy and practicality.

• Possess acceptable light stability.

• Raw material sources are sufficient, manufacturing process is simple, price is moderate, and acceptable to users.

II. Potential Problems Encountered in TPE Flame Retardancy

1. Difficulty in Selecting Flame Retardants
Due to the low flame retardant efficiency of halogen-free flame retardants, and another important reason: usually halogen-free flame retardants have a relatively low density. If the same weight of flame retardant is added to the material, the volume proportion of the halogen-free flame retardant in the material is larger.
Therefore, excessive flame retardant leads to poor compatibility with the substrate, a significant decrease in the elastic modulus, flexibility, mechanical properties, and wear resistance of the thermoplastic elastomer TPE-S, poorer melt fluidity, difficulty in molding the elastomer, and the surface is prone to flame retardant migration, polluting the environment.

2. Unclear Flame Retardant Mechanism
The TPE-S system is mainly a multi-phase mixed structure composed of styrenic block copolymer elastomers, polyolefin resins, plasticizers, and fillers.
Although there are relatively mature formulations and examples for halogen-free flame retardancy of polyolefin resins, the flame retardant mechanisms of SEBS and polyolefins like PP and PE are actually not the same. It is difficult for one flame retardant to simultaneously flame retard multiple plastic components effectively.

3. Plasticizers Reduce Flame Retardancy
To reduce hardness and make the handfeel softer, TPE plastics often require the addition of plasticizers (white oil), primarily including #26, #32, and #70 white oils.
Due to differences in the limiting oxygen index and fluidity of these four oil types, and the inherent flammability of oil substances which greatly reduces the flame retardancy of the elastomer, how to resolve the relationship between the plasticizer and flame retardancy when producing low-hardness products, and how to select the type of white oil, is crucial.

III. Problems Possibly Encountered During Processing of Flame-Retardant TPE

1. Unsmooth Wire Surface
This often occurs in TPE wire processing. It is generally caused by improper temperature adjustment and uneven mixing of the flame retardant, or possibly by incorrect mold selection. Of course, it is also greatly related to the material formulation.
Experienced processors will rule out equipment problems and can find the suitable temperature through repeated debugging. However, selecting the appropriate material and specific formulation requires careful consideration by engineers. It is recommended to communicate more with raw material suppliers to understand the material properties and processing precautions. Choosing the right material will achieve twice the result with half the effort. TPU wires generally have better surface processing compared to TPE wires.

2. Exudation on the Wire Surface
Specific exudation manifests as: surface whitening, oil seepage, or a fog-like appearance. It is recommended that customers preferably choose materials with slight surface whitening; this type of exudation will not affect its physical and electrical properties. Do not use materials with surface oil seepage, as they are prone to peeling and shedding during use.
Additionally, exudation varies in severity. Good materials have slight exudation, which is not prone to occur in the short term, and even if it occurs, it can be wiped off gently by hand.

3. Surface Scraping Whitening, Peeling, and Degumming
Scraping whitening is caused by the addition of PP and PE components in the material and can be improved. Some manufacturers have achieved products that do not scrape white, but it is more difficult for TPE than TPU. If peeling or degumming occurs, the level is quite basic. What needs to be emphasized here are still the formulation and process!

4. Processing Difficulty
Elastomers manifest in relatively slow processing speeds, about 50% slower than PVC. Temperature is difficult to adjust, a lot of material is wasted during machine adjustment, and the machine must be cleaned before production, resulting in significant waste. These are unavoidable. Sometimes problems occur even when the initial stage of production is completely normal, and issues arise in the later stages. It needs to be stated here: using off-grade materials carries risks and requires caution.

IV. Future Trends and Solution Prospects
In the future, flame-retardant TPE will continuously pursue higher flame retardant ratings and better flame retardant effects. R&D personnel will dedicate efforts to developing new flame retardant systems or improving existing flame retardant formulations to enhance the flame retardant performance of TPE, enabling it to meet stricter fire safety standards.
Against this backdrop, Yinsu's organic phosphorus flame retardant WADP-10 provides an excellent solution to the aforementioned challenges. WADP-10, as an efficient and environmentally friendly organic phosphorus flame retardant, has characteristics that precisely address the pain points of TPE flame retardancy:

1. Efficient Flame Retardancy: High phosphorus content, significant flame retardant efficiency, helps reduce the amount added.

2. Excellent Compatibility: Good compatibility with main TPE components such as SEBS and PP, can effectively reduce the risk of surface exudation caused by adding flame retardant, and better maintain the mechanical properties and handfeel of TPE.

3. Environmentally Friendly and Halogen-Free: Complies with the halogen-free environmental trend, suitable for applications with strict environmental requirements such as electronic appliances and wires/cables.

4. Good Thermal Stability: Can meet the requirements of conventional TPE processing temperatures, ensuring stability in the production process.

Conclusion
The flame retardancy of TPE is a systematic project that requires attention to three aspects: flame retardant selection, formulation design, and processing technology. Although the challenges are significant, with the continuous emergence of high-performance specialized flame retardants like Yinsu's WADP-10, TPE materials are destined to play a key role in broader safety-critical applications.