Aspects of ABS Flame Retardant Modification, What Are The Applicable Flame Retardants ?

Aspects of ABS Flame Retardant Modification, What are the Applicable Flame Retardants ?

ABS resin is a copolymer composed of three monomers: acrylonitrile (A), butadiene (B), and styrene (S). It is a thermoplastic polymer that lies between general-purpose plastics and engineering plastics.

ABS resin is known for its excellent impact resistance, heat resistance, low-temperature resistance, chemical resistance, ease of processing, and good surface gloss. Due to its wide range of applications in automotive, electronic, electrical, textile, household appliances, and construction industries, the demand for flame-retardant properties in ABS has been increasing.

Let's take a closer look at the importance of ABS flame retardants, methods to improve the flame-retardant properties of ABS, and the main types of flame retardants used for ABS.

I. The Importance of ABS Flame Retardancy

With the widespread application of polymers in modern life and various industries, the requirements for flame retardancy and other properties of materials are increasing. In China, the demand for flame-retardant polymers is becoming more urgent.

Like most polymers, ABS is a combustible material (with an oxygen index of only 22%). It burns rapidly and generates a large amount of smoke when ignited. Therefore, flame retardancy of ABS is an important research topic.

When used alone, ABS has poor flame-retardant properties. In practical applications, flame retardants must be added to improve its flame-retardant performance. However, the addition of flame retardants can reduce the physical and mechanical properties of ABS, especially the impact strength of the products, which can decrease significantly. Moreover, flame retardants are typically 2 to 3 times more expensive than ABS, which increases the cost of the products.

Most domestic manufacturers use decabromodiphenyl ether (DEBDE) as a flame retardant. This flame retardant has a high bromine content, excellent thermal stability, low addition amount, and good flame-retardant effect.

However, with the development of modern high-tech applications, traditional flame retardants can no longer meet the market's requirements for performance. Future flame retardants will inevitably be halogen-free, efficient, low-smoke, low-toxicity, and multifunctional composite flame retardants.

II. Main Methods to Improve the Flame-Retardant Performance of ABS

Modifying the Composition of ABS Copolymer: For example, adding maleic anhydride or tribromostyrene as a fourth monomer to copolymerize with styrene, butadiene, and acrylonitrile to produce a four-component flame-retardant copolymer. This method provides good long-term flame retardancy, but it needs to be added during the ABS polymerization process, which is complex and costly, so it is rarely used.

Blending with High Flame-Retardant Resins (e.g., PVC, CPE, etc.): This method requires a large amount of high flame-retardant resin to be effective, which can significantly affect the inherent properties of ABS.

Adding Inorganic Flame Retardants (e.g., Al(OH)₃, Mg(OH)₂, MoO₃, etc.): This method requires a large amount of flame retardant (usually more than 60 parts) to achieve a noticeable flame-retardant effect, which can significantly reduce the mechanical and processing properties of the polymer, making it less valuable for use.

Adding Organic Flame Retardants (e.g., halogenated compounds, phosphorus-based flame retardants, etc.): This method requires less flame retardant and provides good flame-retardant performance, but it has drawbacks such as poor weather resistance, high cost, and black smoke generation during combustion.

Currently, most methods involve a combination of the last three additive-based flame-retardant approaches to prepare a low-smoke flame-retardant ABS system with optimal overall properties.

III. Types of ABS Flame Retardants

Halogenated Flame Retardants

1. Brominated Flame Retardants: Halogenated flame retardants mainly refer to brominated flame retardants, which are divided into the following categories:

• Polybrominated Diphenyl Ethers: These include octabromodiphenyl ether and decabromodiphenyl ether. These flame retardants are efficient, require low usage levels, and provide good mechanical properties at a moderate cost. However, they do not comply with the RoHS directive and are considered non-environmentally friendly products that are now banned.

• Decabromodiphenyl Ethane (DBDPE): Since it contains non-free bromine, it does not belong to the polybrominated diphenyl ether category and does not produce substances such as polybrominated biphenyls and polybrominated diphenyl ethers, which are strictly prohibited by the RoHS directive, during combustion. Its cost is comparable to decabromodiphenyl ether, and safety assessments have shown that DBDPE is a low-toxicity, non-irritating flame retardant with negative effects on genetic genes and low repeated-dose toxicity. It can replace polybrominated diphenyl ethers as a flame retardant for ABS.

• Brominated Epoxy Resin: Brominated epoxy resin refers to epoxy resin synthesized from tetrabromobisphenol A. It has excellent melt flowability, high flame-retardant efficiency, superior thermal and light stability, and good physical and mechanical properties, making it an ideal flame retardant for ABS.

2. Chlorinated Polyethylene (CPE) Flame Retardant: CPE appears as a white powder, is non-toxic, soluble in aromatic and halogenated hydrocarbons, and insoluble in aliphatic hydrocarbons. It decomposes above 170°C, releasing hydrogen chloride gas. Due to the absence of unsaturated double bonds in its molecular structure and the presence of chlorine groups, it has a stable chemical structure, excellent heat resistance, cold resistance, weather resistance, chemical resistance, ozone resistance, and electrical insulation properties. As an elastomeric polymer, CPE is compatible with various polymers and can be used as a modifier for ABS, PVC, PP, PE, PS, etc. Since CPE contains halogens, it has flame-retardant properties and can be used as a secondary flame retardant. Moreover, CPE is cheaper than ABS, making it an ideal choice for flame-retardant ABS modification.

3. Synergistic Flame-Retardant Effect of CPE and Sb₂O₃: When CPE is added to ABS materials with the composite flame retardant DBDPE/Sb₂O₃, the oxygen index of the composite material increases, while the heat release rate, effective heat of combustion, and mass loss rate decrease. This indicates that CPE has flame-retardant properties and can be used as a secondary flame retardant. After adding CPE, the impact strength of the composite material significantly increases, suggesting a synergistic effect between CPE and Sb₂O₃. Additionally, the addition of CPE/Sb₂O₃ significantly reduces the smoke generation rate of the composite material, indicating that CPE/Sb₂O₃ also has a good smoke-suppressing effect on ABS.

Halogen-Free Flame Retardants

1. Aluminum Hydroxide and Magnesium Hydroxide: Aluminum hydroxide and magnesium hydroxide are two common inorganic filler-type flame retardants. They are characterized by being halogen-free, non-toxic, smoke-suppressing, and cost-effective.

Their flame-retardant mechanisms are essentially the same, involving both the condensed-phase mechanism (i.e., at high temperatures, aluminum hydroxide forms a condensed phase on the surface of ABS, isolating air, preventing heat transfer, and reducing the release of combustible gases; magnesium hydroxide/ABS composites form a dense char layer when burned) and the cooling mechanism (i.e., the flame retardant undergoes endothermic dehydration, phase transition, decomposition, or other endothermic reactions, lowering the temperature of the polymer surface and combustion zone, preventing thermal degradation, and thereby reducing the volatilization of combustible gases, ultimately disrupting the conditions that sustain polymer combustion).

However, when used alone, they require large amounts and significantly alter the mechanical properties of the resin, so they are generally not used as primary flame retardants.

2. Encapsulated Red Phosphorus: Encapsulated red phosphorus contains only the flame-retardant element phosphorus and has a higher flame-retardant efficiency than other phosphorus-based flame retardants. Its flame-retardant mechanism is primarily based on condensed-phase flame retardancy, i.e., at high temperatures, encapsulated red phosphorus forms a condensed phase on the surface of ABS, isolating air, preventing heat transfer, and reducing the release of combustible gases to achieve flame retardancy.

However, using encapsulated red phosphorus alone does not provide significant flame-retardant effects on ABS, but combining it with other synergistic agents can achieve better flame-retardant results. Research has found that when the amount of encapsulated red phosphorus is 9% and the amount of aluminum hydroxide is 20%, an ideal synergistic, environmentally friendly, flame-retardant ABS with both good mechanical and flame-retardant properties can be obtained.

3. Phosphorus-Nitrogen Composite Flame Retardants: Nitrogen-containing flame retardants mainly work by forming non-combustible gases such as nitrogen during decomposition, which dilute and displace combustible gases or cover the material surface to achieve flame retardancy. Common examples include melamine, melamine cyanurate (MCA), and melamine pyrophosphate.

Phosphorus-nitrogen flame retardants, which primarily consist of phosphorus and nitrogen, have become popular in halogen-free flame retardancy for ABS. When phosphorus compounds are combined with nitrogen, they form phosphorus-nitrogen flame retardants. The nitrogen compounds release gases such as N₂, CO₂, NH₃, and H₂O upon heating, which block the supply of oxygen, achieving synergistic and enhanced flame-retardant effects.

IV. Development Trends of ABS Flame Retardants

Looking at the research, development, and progress of flame retardants in recent years, the following trends can be observed:

1. Environmental-Friendliness and Low Toxicity: Although halogenated flame retardants will remain a major category, their combustion generates toxic and corrosive substances. Therefore, the demand for halogen-free and environmentally friendly flame retardants is increasing.

2. High Efficiency and Multi-Functionality: Developing flame retardants with high efficiency and multiple functions can not only reduce the impact on the physical and mechanical properties of the base material but also help in reducing pollution and lowering costs.

3. Nano- and Microcapsule Technologies: With the continuous development of nano and microcapsule technologies, surface modification and ultra-finement of inorganic flame retardants are becoming the direction of development. Since inorganic flame retardants have poor flame-retardant effects and require large additions, new technologies such as ultra-finement, surface modification, and macromolecular bonding are needed to improve their performance.

4. Application of Compound Formulation Technology: Some products use phosphorus elements to replace halogens, achieving flame retardancy through volume expansion. When added to composite materials, these flame retardants foam and crosslink, forming a stable char layer on the material surface. This char layer can block heat, reduce oxygen ingress, prevent dripping of molten polymer, and lower smoke concentration and organic emissions, providing good protection for the material. Moreover, this masterbatch has good high-temperature stability, is easy to process, and has excellent coloration properties.

V. Conclusion

With the rapid development of the home appliance and automotive industries, the consumption of ABS resin in China is growing swiftly, and higher requirements are being placed on the environmental friendliness and flame retardancy of ABS. Developing new types of flame retardants with excellent performance is not only an important research topic in flame-retardant materials but also a key direction for the development of ABS.

Traditional halogenated flame retardants, such as decabromodiphenyl ether (DEBDE), are effective but face environmental challenges. In response, the industry is shifting towards halogen-free, high-efficiency, and multifunctional flame retardants. Methods like modifying the ABS copolymer composition, blending with high flame-retardant resins, and adding inorganic or organic flame retardants are discussed, with a focus on achieving low-smoke and improved mechanical properties.

YINSU Flame Retardant is at the forefront of this innovation, offering a range of products tailored for ABS. Their red phosphorus flame retardant ABS-P-20M provides excellent flame retardancy with minimal impact on mechanical properties. The antimony bromide masterbatch for ABS YS-BRT and the composite antimony flame retardant T series are designed to enhance flame-retardant performance while maintaining high thermal stability and ease of processing. These products exemplify the industry's trend towards environmentally friendly and high-performance flame retardants, aligning with the growing demands of modern applications.