Recent Advances in Global Flame Retardant Technology

Recent Advances in Global Flame Retardant Technology

Polymer materials, alongside ceramic and metal materials, are one of the three major material categories, with their applications covering almost every aspect of people's work and daily lives. However, the vast majority of polymer materials are polymerized with a carbon-based skeletal structure, making them prone to combustion when exposed to an open flame during use, posing a serious threat to human life and property safety.

Therefore, research on the flame retardant properties of polymers is extremely necessary. Based on the combustion characteristics of polymer materials, various methods can be employed to interrupt their combustion process, thereby achieving flame retardancy.

In 1930, the antimony oxide-chlorinated paraffin synergistic flame retardant system was discovered and soon successfully applied in some polymer materials. In the 1950s, Hooker Company developed flame-retardant unsaturated polyester using the reactive monomer chlorendic acid. Subsequently, new bromine-containing and phosphorus-containing reactive flame-retardant monomers continuously emerged. In the 1980s, discussions on toxicity and environmental issues began in the flame retardant field, making halogen-free, smoke-suppressing, and toxicity-reducing properties new goals for flame retardant development. With the rapid advancement of polymer materials, research on flame retardant technologies and mechanisms has also become increasingly extensive and in-depth.

Research and discussions have been conducted on various flame retardant technologies based on the mechanisms of gas-phase flame retardancy, condensed-phase flame retardancy, and heat interruption. In recent years, flame retardant technologies such as composite flame retardancy, synergistic flame retardancy, and macromolecular flame retardancy have attracted attention. Based on current research on polymer flame retardant technologies, this article provides an overview of the progress in these types of flame retardant technologies both domestically and internationally in recent years.

I. Composite Flame Retardant Technology

1.Layered Double Hydroxides (LDH)

Layered double hydroxides (LDHs) are layered inorganic nanomaterials. They have similar composition and structure to aluminum hydroxide (Al(OH)₃, ATH) and magnesium hydroxide (Mg(OH)₂, MH), combining the advantages of both. Moreover, as they do not contain any toxic substances themselves, they represent an ideal green flame retardant with smoke-suppressing properties. The flame retardant mechanism of LDHs involves their decomposition upon combustion into CO₂, H₂O, metal oxides, etc. On one hand, CO₂ and H₂O can dilute combustible gases and O₂, lowering the combustion temperature. On the other hand, the metal oxides facilitate the formation of a char layer, which acts to insulate against O₂ and heat, further reducing the degradation rate of the substrate.

2.Nanoscale Metal-Organic Frameworks (MOFs)

MOFs are organic-inorganic hybrid porous materials with a network structure, formed through the self-assembly of organic ligands with metal ions or clusters. Their structure is illustrated in the figure.

The design of MOFs offers flexibility, and their structure is tunable. Whether modifying the organic ligand or the metal coordination site, MOFs with specific desired properties can be obtained through rational design modifications, indicating their broad application prospects. Zeolitic Imidazolate Frameworks (ZIFs) combine the advantages of traditional MOFs and zeolites, exhibiting excellent performance. ZIFs are formed by the self-assembly of transition elements with organic ligands containing imidazole rings. They are easy to synthesize, possess good stability, have regular pore channels, diverse structures, and high catalytic activity.

3.Polyhedral Oligomeric Silsesquioxane (POSS)

POSS is a novel type of silicon-based flame retardant with an organic-inorganic hybrid structure, as shown in the figure below.

While improving the flame retardancy of materials, POSS can also effectively enhance the mechanical properties, processability, and heat resistance of polymers. Characterized by its organic-inorganic hybrid, cage-like, and nanostructured features, incorporating POSS into polymers can improve their heat resistance, flame retardancy, and mechanical properties, while lowering their dielectric constant. POSS and its derivatives, as a class of novel halogen-free flame retardants for polymers, have been widely applied.

4.Graphene (GNS)

GNS is a two-dimensional nanoscale sheet material composed of a single layer of carbon atoms. The schematic structures of GNS and graphene oxide (GO) are shown below.

GNS and its derivatives exhibit good flame retardancy due to nanoscale effects. Particularly, when combined with inorganic nanomaterials as flame retardant additives, GNS can form versatile flame-retardant materials. Compared to traditional carbon-based flame retardants like graphite, expandable graphite, and graphite oxide, GNS's unique two-dimensional nanoscale sheet structure offers higher flame retardant efficiency. Furthermore, compared to carbon nanotubes, GNS is relatively inexpensive, making it more suitable for industrial applications.

The flame retardant mechanism of GNS and its derivatives involves:

(1) GNS and GO both possess a unique two-dimensional layered structure. During combustion, GNS can promote the formation of a dense and continuous char layer, acting as a physical insulating barrier.

(2) The layered structure endows GNS and its derivatives with a large specific surface area, enabling them to more effectively adsorb flammable volatile organic compounds or hinder the release and diffusion of these organic volatiles.

(3) GNS, especially GO materials, have abundant reactive groups on their surfaces. At low temperatures, the oxygen-containing groups on GO decompose and undergo dehydration reactions. During combustion, the gases produced by these reactions can absorb a significant amount of heat, lowering the temperature of the polymer matrix. Simultaneously, the dehydrated gases can dilute the O₂ concentration around the flame, achieving a flame-retardant effect.

(4) GNS and GO can interact with the molecular chains of the polymer material, forming a three-dimensional network structure. During combustion, this three-dimensional network structure prevents melt dripping, thereby enhancing the flame retardancy of the composite material.

II. Synergistic Flame Retardant Technology

One of the latest research and development hotspots for halogen-free flame retardants internationally is the use of multiple flame-retardant elements synergistically to compensate for the shortcomings of single-element flame retardants. This approach helps balance the relationship between flame retardant dosage, performance, and cost, while meeting increasingly stringent environmental and safety requirements. Researchers have conducted extensive studies on the synergistic effects of various flame retardant systems, such as synergistic flame retardancy with metal hydroxides, phosphorus-nitrogen synergy, phosphorus-silicon synergy, and compounded nanoparticle systems.

1.Synergistic Flame Retardancy with Metal Hydroxides

Metal hydroxides include ATH and MH. Hydroxide flame retardants produce no harmful substances throughout the entire flame-retardant process. Moreover, their decomposition products can also absorb harmful gases and smoke generated during the combustion of polymer materials while providing flame retardancy, making them one of the most environmentally friendly flame retardants. Currently, common halogen-free compounded flame retardants are mainly categorized into layered compounds, phosphorus-containing compounds, rare earth oxides, etc.

2.Phosphorus-Nitrogen Synergistic Flame Retardancy

Phosphorus-nitrogen synergy has become the latest direction in the R&D of phosphorus-based and nitrogen-based flame retardants. It has emerged as one of the most practical choices for environmentally friendly halogen-free flame retardancy in many fields. Its flame retardant mechanism involves a combination of condensed-phase and gas-phase actions. Upon heating during combustion, the flame retardant decomposes to form inorganic acids like phosphoric acid and polyphosphoric acid, which can create a protective film on the substrate surface, insulating it from air. Simultaneously, upon heating, it readily releases non-combustible gases such as ammonia, nitrogen, water vapor, and nitrogen oxides. These gases block the oxygen supply, achieving the flame-retardant goal. For example, a water-soluble phosphorus-nitrogen synergistic intumescent flame retardant (PEIPO) for water-blown flexible polyurethane foam was prepared using polyethyleneimine and a phosphorus-containing intermediate precursor as raw materials.

3.Other Synergistic Systems

Alkylphosphinate flame retardants are favored for their good thermal stability, high phosphorus content, and excellent flame-retardant effect. However, they are relatively expensive, and when used alone as flame retardants, the mechanical properties of the material are often not ideal. Therefore, they usually need to be compounded with nanoparticles or inorganic flame retardants. Linear or branched alkylphosphinates are the most widely used type among alkylphosphinate flame retardants. Aluminum diethylphosphinate (AlPi) has been applied in polymer materials such as polyester and polyamide (PA).

For instance, synergistic flame retardancy of polyamide 6 (PA6) using AlPi and triglycidyl isocyanurate (TGIC) showed that when the total added mass fraction of AlPi and TGIC was 11%, with a mass ratio of AlPi to TGIC at 97:3, the material achieved a UL-94 V-0 rating and a Limiting Oxygen Index (LOI) of 30.3%.

In another example, blending ammonium polyphosphate (APP), aluminum hypophosphite (AHP), and melamine cyanurate (MCA) as a compounded flame retardant additive with polypropylene (PP) demonstrated that an ideal flame-retardant effect was achieved with a total flame retardant mass of 30% and a mass ratio of m(APP):m(AHP):m(MCA) = 4:1:1. Under these conditions, the flame-retardant PP had an LOI of 33% and reached a UL-94 V-0 rating.

III. Macromolecular Flame Retardant Technology

Macromolecular flame retardants are produced by linking small-molecule flame retardant structures through chemical reactions, resulting in macromolecular compounds rich in flame-retardant elements and exhibiting good interaction with polymers.

For example, an inorganic-organic hybrid macromolecular flame retardant, hexa-[4-(N-phenylamino-DOPO-methylene)phenoxy]cyclotriphosphazene (DOPO-PCP), was synthesized through substitution, condensation, and addition reactions. DOPO-PCP was then used for flame retardancy of DGEBA. In another instance, hindered amine groups with free radical quenching functionality were introduced into an intumescent flame retardant (IFR) to prepare a novel macromolecular intumescent flame retardant (HAPN) with free radical quenching capability. This was then compounded with APP to flame-retard PP.

Flame retardants have significantly developed alongside the widespread application of polymer materials. As environmental awareness increases, there is a continuous pursuit of environmentally friendly flame retardants that offer high efficiency, are non-toxic and harmless, have good compatibility with polymers, low migration, and low cost. New types of flame retardants are constantly emerging, and some emerging technologies are also being continuously applied to the research and production of flame retardants. The development of synergistic flame retardancy and macromolecular flame retardant technologies is progressing towards higher efficiency, lower toxicity, and greater environmental friendliness.

Future research directions in flame retardant technology include studies on ultra-fine design, microencapsulation design, compounding and synergy of different flame retardants, structural design of macromolecular flame retardants, and surface modification techniques for flame-retardant polymer materials. With the progress of research on flame retardant technology both domestically and internationally, the flame retardant industry is poised for vigorous development.