​Flame Retardant Solutions for Modified Plastics (Part 2): Mainstream Halogen-Free Categories + Emerging Nanotechnology

Flame Retardant Solutions for Modified Plastics (Part 2): Mainstream Halogen-Free Categories + Emerging Nanotechnology

In Flame Retardant Solutions for Modified Plastics (Part 1): Product Features + Application Scenarios, Explained in One Article, we introduced the core functions and main classifications of flame retardants, and provided an in-depth analysis of mainstream halogen-based (bromine, chlorine) and phosphorus-based (organic phosphorus, red phosphorus) flame retardants. These traditional flame retardant technologies are mature and highly efficient, making them the preferred choice for many high-performance applications.

However, as environmental regulations tighten and the market increasingly demands halogen-free, low-toxicity, and low-smoke materials, halogen-free flame retardants represented by metal hydroxides and silicon-based systems, along with emerging nano flame retardant technologies, are playing an increasingly important role.

Today, we will continue the journey, focusing on distinctive flame retardant types such as metal hydroxides, melamine-based systems, silicon-based flame retardants, and expandable graphite. We will delve into their unique mechanisms of action, performance advantages, and innovative application directions, providing you with more comprehensive reference for your material selection.

Types of Flame Retardants (Part 2)

1. Metal Hydroxide Flame Retardants
Metal hydroxides are the most commonly used family of halogen-free flame retardants. These mineral compounds are used in:

• Polyolefins

• Thermoplastic elastomers

• Polyvinyl chloride (PVC)

• Rubbers

• Thermosetting plastics

• Some engineering polymers (e.g., polyamides)

They provide flame retardant formulations that meet appropriate standards for many applications. The combustion products from such formulations have low opacity, low toxicity, and minimal corrosivity. Compounding with inorganic hydroxides offers a cost-effective way to achieve low-smoke flame retardant formulations.

Furthermore, inorganic hydroxides are easy to handle and relatively non-toxic. Due to considerations regarding long-term environmental impact, several inorganic hydroxides, such as aluminum hydroxide and magnesium hydroxide, are replacing halogenated and phosphorus-containing flame retardants.

Aluminum Hydroxide (ATH)
Aluminum hydroxide is the highest volume inorganic hydroxide sold as a flame retardant. ATH is processed at temperatures below its decomposition point (190-230°C, depending on particle size). It is used as a flame retardant for elastomers, thermosetting resins, and thermoplastics processed below 200°C.

ATH obtained via the Bayer process is a gibbsite with a particle size exceeding 50 µm, which can be redissolved and reprecipitated to produce higher purity ATH. Modifications to this process can reduce iron, silica, or residual solid impurities.

It can be categorized into ground hydrate (beige to white, containing sodium silicate, iron impurities, 1.5 to 35 µm) and fine precipitated hydrate (white, bright, pure, 0.28 to 3 µm). The main differences between various grades of ATH are essentially in particle size and surface treatment.

The purpose of surface treatment is to enhance one or more specific mechanical properties, such as elongation at break. Fatty acids or metal stearates are commonly used as surface treatments for ATH or MH to limit additive agglomeration and improve EB performance in wire and cable applications. There are also surface treatments based on silanes, with non-reactive (alkyl) and reactive (vinyl, amino, epoxy, methacryloyl) substituents. The type of reactive substituent depends on the polymer in which the flame retardant is used. Silane surface treatments, due to their higher cost compared to fatty acids, are typically developed for specific applications. Other surface treatments include those centered on phosphorus, titanium, and zirconium instead of silicon. Titanates and zirconates have specific applications and are generally more expensive than silanes.

Magnesium Hydroxide (MDH)
Magnesium hydroxide is a more thermally stable inorganic flame retardant, stable above 300°C, and is used in many elastomers and resins, including engineering plastics and other materials processed at higher temperatures.

It is produced using different processes from magnesium-bearing ores (such as magnesite, dolomite, or serpentine) as well as from brines and seawater. Some ores like brucite, hydromagnesite, and artinite can be used directly as flame retardants or converted to MDH.

The three different processes for producing MDH are: seawater and brine process, Aman process, and Magnifin® process.

MDH used as a flame retardant is typically of high purity (>98.5%), most often derived from seawater or brine. Although it is an ore-derived product, high purity can also be achieved.

Most flame retardant grades of MDH are white powders with a median particle size between 0.5 and 5 µm. The specific surface area ranges between 7 and 15 m²/g, depending on particle shape and size. Similar to ATH, MDH is used at high loadings, typically between 50% and 70%. The smaller usage volume of MDH as a flame retardant stems from its higher price compared to precipitated grade ATH.

Melamine-Based Flame Retardants
Melamine-based flame retardants represent a small but fast-growing segment of the flame retardant market. These products offer particular advantages over existing flame retardants: cost-effectiveness, low smoke density and toxicity, low corrosivity, safe handling, and environmental friendliness.

Within this family of halogen-free flame retardants, three chemical groups can be distinguished:

• Pure melamine.

• Melamine derivatives (i.e., salts formed with organic or inorganic acids, such as boric acid, cyanuric acid, phosphoric acid, or pyro/polyphosphoric acids).

• Melamine homologs, such as melem, melam, and melon.

Melamine-based flame retardants exhibit excellent flame retardant performance and versatility of use due to their ability to employ multiple modes of flame retardant action.

Currently, the main application areas for melamine-based flame retardants are:

• Flexible polyurethane foam.

• Intumescent coatings.

• Polyamides.

• Thermoplastic polyurethanes.

Through ongoing research and application development, the market for melamine-based flame retardants will expand, moving towards polyolefins and thermoplastic polyesters in the near future.

2. Silicon-Based Flame Retardants
Silicon-based flame retardants can generate a protective surface coating during a fire, thanks to their low heat release rate. It is reported that low content of silicon in certain organic polymer systems can improve their LOI and UL-94 performance.

Some composite silicas (polydimethylsiloxane type) contain dry powders for various organic plastics. In polystyrene, an addition as low as 1% to 3% can reduce the heat release rate by 30% to 50%. Similar improvements have been reported in HIPS, PS blends, PP, and EVA.

Research on silicon-modified polyurethanes indicates reduced heat release rates in these materials compared to unmodified PU. The proposed mechanism is as follows:

• Upon combustion, a silica layer forms on the material surface, which acts as a thermal insulator, preventing energy feedback into the substrate by re-radiating external heat flux.

Novel silicon-based flame retardants for polycarbonate and PC/ABS resins offer good mechanical properties and high flame retardant performance, including strength, moldability, and UL-94 1/16 inch V-0 (at 10 phr loading). Both linear and branched types of silicones with (hydroxy or methoxy) or without (saturated hydrocarbon) functional reactive groups have been evaluated. Silicones with branched structures, aromatic groups in the chain, and non-reactive end groups are highly effective. In this case, the silicone disperses in the PC resin and may migrate to the surface during combustion, forming a highly flame-retardant barrier.

3. Expandable Graphite
Expandable graphite provides good flame retardancy at low loading levels and can be used in both thermoplastic and thermoset resins. Expandable graphite can be used alone in naturally char-forming polymers (PA, PU, PVC), but is often combined with other flame retardants such as phosphates, boron compounds, antimony trioxide, or magnesium hydroxide to form a substrate strong enough to support the expandable graphite barrier. Graphite can also be used in nanocomposite PP.

4. Nanoclays
Nanoclays can reduce the relative heat release, promote surface char formation, create an anti-dripping effect, and reduce smoke generation. In halogen-free formulations, nanoclays allow for reduced loadings of mineral flame retardants. In halogenated systems, they reduce the amount of brominated flame retardant or ATO required, offering lower density, less blooming, and better mechanical properties.

5. Carbon Nanotubes (CNTs)
Multi-walled carbon nanotubes are commercially used for their antistatic, strength-enhancing, and flame retardant properties. Their key characteristics include: effective char formation, delaying ignition through endothermy, increasing viscosity helping to prevent dripping, not promoting depolymerization. CNTs are expected to find application in electronics, where they can provide both antistatic and flame retardant properties simultaneously.

6. Polyhedral Oligomeric Silsesquioxanes (POSS)
POSS-based hybrid polymers are molecularly well-defined at the nanoscale size and can be functionalized with reactive groups suitable for synthesizing new organic-inorganic hybrids. POSS has been successfully incorporated into common polymers via copolymerization, grafting, or blending.

Synthesizing POSS cages, POSS cage-containing monomers, POSS-dendrimer cores, POSS-containing polymers, and POSS nanocomposites can bring specific properties, such as:

• Mechanical.

• Thermal.

• Flame retardant.

• Viscoelastic.

They are used commercially as flame retardant synergists in phenolic resins, PPE, and COC. A key advantage of POSS is its role as an intumescent synergist and dispersing aid for halogen-free flame retardants, which may allow higher HFFR loadings by improving flowability.

7. Polymer-Clay Nanocomposites
Polymer-clay nanocomposites are hybrid organic polymer-inorganic layered materials with unique flammability characteristics compared to conventionally filled polymers. Polyamide-6, polystyrene, and polypropylene are some polymers used in combination with clays.

8. Hyperbranched Polymers (HBPs)
Hyperbranched polymers, with their numerous ordered branches, open avenues for hyper-functionalized additives. They can be prepared from AxBy type "branching" monomers, where A and B represent functional groups that can react with each other but not with themselves. For the simplest AB2 monomer, the resulting polymer structure is as shown. Considering the advantages of both HBPs and flame retardant additives, the figure suggests some possible pathways for developing novel flame retardant additives.

Dendrimers are a special form of HBP, offering additional possibilities. They start from a single focal point or core, each branch dividing into two (or more) other branches down to the functionalized ends. They can host metal atoms to form metallodendrimers. The metal can be located in the repeating units, the core, or the end groups. Metal hosting expands the properties of metallodendrimers.

Due to their structure, dendrimers can be active even at low loadings. The following diagram compares HBP and dendrimer structures with a conventional linear macromolecule having some short branches.

One product is produced using a polyol core, hydroxy acids, and a technology based on proprietary materials. The dendritic structure is formed via the polymerization of a specific core with 2,2-bis(hydroxymethyl)propionic acid. The obtained base is a hydroxyl-functionalized dendritic polyester, fully aliphatic and composed solely of tertiary ester bonds, claimed to have excellent thermal and chemical stability.

A wide variety of different HBPs include polyamides, poly(amidoamine)s, polyureas, polyurethanes, polyesters, polycarbosilanes, polycarbosiloxanes, polycarbosilazanes, perfluorinated derivatives of many of the aforementioned polymers, etc. These polymers are suitable for specialty coating applications, including plastic additives.

Conclusion
The iteration of flame retardants towards halogen-free and the advancement of flame retardant technology down to the nanoscale mean flame-retardant materials can now uphold safety standards while shedding the "high-pollution" label. As both environmental regulations and performance requirements are upgraded, those who master the keys to halogen-free formulations and nano-dispersion hold the fast pass for modified plastics to enter high-end applications. Before the next flame test arrives, the industry has already written the answer into lighter, thinner, and more sustainable flame-retardant materials.