​Flame Retardant Solutions for Modified Plastics (Part 1): Product Characteristics + Application Scenarios, Explained in One Article

Flame Retardant Solutions for Modified Plastics (Part 1): Product Characteristics + Application Scenarios, Explained in One Article

Flame retardants are compounds added to plastics to slow down the spread of fire. They are widely used in electronic and electrical components, wires and cables, construction, home appliances, and other fields.
To prevent combustion, polymers should possess thermal stability, meaning they should have a low probability of decomposing into flammable gases under thermal stress. However, polymers with high thermal stability often have performance limitations, being either too costly or difficult to process.

Flame retardancy can be imparted to plastics by adding various flame retardants. Currently, there are multiple commercial flame retardants available on the market. Selecting the best technical solution for a specific application can be quite challenging.
This guide will help you understand how to choose the right flame retardant for your target application and introduces the criteria to keep in mind when making the correct choice.

I. The Role of Flame Retardants in Plastics
Due to their organic nature, polymers often act as fuel in fires, decomposing into combustible products when heated. Despite their many advantages, the flammability of polymers limits their applications in many fields such as electrical and electronics, transportation, and construction.

II. Types of Flame Retardants (Part 1)
There are several major chemical categories of flame retardants used with polymers, for example:

• Halogenated flame retardants (including brominated and chlorinated compounds)

• Phosphorus-based flame retardants (including organic phosphorus compounds and red phosphorus)

• Metal hydroxide flame retardants (including aluminum hydroxide and magnesium hydroxide)

• Melamine-based flame retardants

• Silicon-based flame retardants

Apart from these chemical categories, there are other flame retardants that can be incorporated into polymers. They can be classified as additive and reactive flame retardants. These two types may affect similar properties (such as viscosity, flexibility, density, etc.) in different polymers. The table below lists some characteristics of additive and reactive flame retardants.

1. Brominated Flame Retardants
Brominated flame retardants (BFRs) are the most commonly used class of flame retardants today. This type of flame retardant is very versatile, offering the best balance between flame retardant performance, mechanical properties, processability, and cost.
Industrial brominated flame retardants are produced by brominating bisphenol A. This reaction is carried out in the presence of a solvent, for example:

• Methanol or halogenated hydrocarbons

• 50% hydrobromic acid

• Aqueous alkyl monoethers

Combining brominated flame retardants with minerals helps improve mechanical properties, reduce the opacity and corrosiveness of smoke generated during combustion, and helps mitigate environmental hazards from incineration smoke. They provide effective solutions that meet regulatory requirements and offer excellent performance for products.

2. Chlorinated Flame Retardants
Chlorinated compounds are molecules containing a high concentration of chlorine, acting primarily chemically in the gas phase. They are usually used in combination with antimony trioxide as a synergist.
Chlorinated compounds can be broadly divided into two main categories:

• Chlorinated paraffins

• Chlorinated alkyl phosphates

Several parameters need to be considered when selecting chlorinated compounds, including chlorine content, thermal stability, volatility, and physical form.

Chlorinated Paraffins:
The general structure of chlorinated resins is:

C_xH(2x+2-y) Cl_y

Depending on the paraffin chain length, various chlorinated resin products are available. Liquid grades are produced from short-chain paraffins, while solid grades with 70-72% chlorine content are produced from high molecular weight waxes.
A key application of chlorinated paraffins is as a plasticizer for flexible PVC in combination with DOP or DINP, used in flooring, cables, etc., to improve flame retardant properties.
Solid grades with high chlorine content are used in thermoplastics, for example, combined with antimony trioxide for CTI cable sheathing in LDPE.

Chlorinated Alkyl Phosphates:
The most common molecule is:

These products are mainly used in rigid and flexible polyurethane foams, with typical addition concentrations ranging from 5% to 15%, depending on foam density and test severity. Examples of flame retardancy standards achievable using chlorinated phosphorus products: flexible foam BS4735, rigid foam BS 476, NFP92-501, DIN 4102.

Chlorinated Alicyclic Compounds:

Dodecachlorodimethanodibenzo cyclooctane is a commercially available molecule. Its main advantages include:

• High-temperature resistance (up to 320°C)

• Good UV aging resistance

• Non-plasticizing product

• Acts as an insoluble filler and does not bloom

• CTI value greater than 400°C in flame-retardant nylon

• Low smoke generation

• Low density and cost-effective

This product can be used in various polymers, including polyamides, polyolefins, and polypropylene, and can be combined with various synergists such as antimony trioxide and zinc borate.

3. Organic Phosphorus-Based Flame Retardants
Organic phosphorus compounds are one of the main flame retardant categories for thermoplastics and polyurethane foams, mainly including phosphates and phosphonates. They may also include phosphorus-halogen compounds and mixtures of phosphorus with halogenated flame retardants (brominated flame retardants). Thermoplastic alloys such as PC/ABS and PPO/HIPS often need to meet stringent flame retardancy standards like UL94 V0. Phosphate-based flame retardants perform well in these resins and offer good physical properties and UV stability.
In many applications, rigid and flexible PU foams need to exhibit a certain degree of flame retardancy to pass specific national flammability tests. Phosphorus-based flame retardants (both halogenated (chlorinated phosphates) and non-halogenated) are widely used in these applications and are considered ideal choices offering a balance of good processability, flame retardancy, and physical properties. In some cases, particularly where low scorch is required, phosphorus-bromine mixtures are used.
Polyurethane foam producers can choose between reactive additives, halogenated and non-halogenated phosphorus-based flame retardants based on the final application, key requirements, and the flame retardancy standards that must be met. These options provide flexibility to meet market demands for performance, compatibility, efficiency, physical properties, processability, and cost.
There are various products on the market for flame retardancy based on phosphate molecules. Some common products based on phosphate molecules include:

• Triphenyl phosphate - Can be used in ABS/PC blends, PPO and other engineering plastics, and phenolic resins.

• Tricresyl phosphate - Mainly used as a flame retardant plasticizer in PVC and styrenic compositions. Commercial products are mixtures of ortho, meta, and para isomers, but the ortho isomer is highly toxic and should be excluded as much as possible.

Diaryl Phosphates
Commercialized diaryl phosphates include resorcinol bis(diphenyl phosphate) and bisphenol A bis(diphenyl phosphate).

• RDP - Typically a colorless liquid, used in ABS/PC, PBT, and PPO. Compared to aryl phosphates or alkyl phosphates, these products exhibit lower volatility, higher heat resistance, and lower plasticizing effect. Usually, 10-15 phr is required to pass traditional flame retardancy tests. At lower addition levels, RDP can improve the processability of ABS and styrenics for thin-wall injection molding.

• BDP - Similar to RDP, used in the same applications with an addition level of about 20 phr. Compared to RDP, BDP provides better melt stability and lower volatility to the polymer. This product also has good hydrolytic stability, which is beneficial for polymers like polycarbonate.

Alkyl Phosphonates
The general structure of phosphonates is:

Dimethyl methylphosphonate is a very effective flame retardant due to its high phosphorus content. However, high volatility limits its use in rigid PU and highly filled polyesters.

Dimeric or Oligomeric Cyclic Phosphates
They are usually high-viscosity liquids, making them difficult to handle. Some manufacturers offer masterbatch solutions. Dimeric cyclic phosphonates can be used in PET at about 6 wt% for flame-retardant PET fibers and also in rigid polyurethane without the volatility drawbacks.

4. Red Phosphorus Flame Retardants
The term "red phosphorus" is used to describe one of the allotropes of phosphorus, obtained by heating white phosphorus near 300°C under oxygen-free conditions. The color varies from orange to deep purple, depending on molecular weight, particle size, and impurities.
Red phosphorus is an amorphous inorganic polymer. However, X-rays have identified the presence of several crystalline forms (usually limited content, <10% w). It is known that red phosphorus can act as a single additive in nitrogen- and/or oxygen-containing polymers, for example:

Red phosphorus flame retardants must be used in polyolefins, styrenics, rubbers, etc., in combination with blowing agents, char formers, and/or inorganic hydroxides.
Red phosphorus flame retardants are typically used to meet high flame retardancy requirements. They do not produce toxic fumes and have good electrical properties (i.e., high CTI values) and mechanical properties. Due to color considerations, it is not suitable for white or very light-colored finished products but is suitable for black to medium gray finished products.
Red phosphorus-based flame retardants have high thermal stability, allowing products to withstand harsh extrusion temperatures up to 320°C without decomposing, releasing hazardous substances, generating carbonaceous residue, or causing corrosion to extrusion equipment.

Above, we have outlined the basics of flame retardants and detailed the most widely used halogenated (bromine, chlorine) and phosphorus-based (organic phosphorus, red phosphorus) flame retardants on the market. Each has its own strengths and weaknesses, and selection requires comprehensive consideration of polymer type, processing conditions, final product performance, and regulatory requirements.
In the next part, we will continue to delve into several other important categories of flame retardants:

• Metal hydroxide flame retardants (aluminum/magnesium hydroxide): Representatives of the halogen-free eco-friendly route, how to balance performance under high filler loading?

• Nitrogen-based flame retardants (melamine): How do they achieve efficient flame retardancy through multiple mechanisms?

• Silicon-based and nano flame retardants: How do these emerging technologies bring performance breakthroughs with low addition levels?