Research Progress In Fire Retardancy Of Building Foam Materials

I. Introduction

Foam materials are predominantly used in the construction field as thermal insulation materials, helping buildings save energy and enhance living comfort. Examples include polyurethane foam and polystyrene foam. In addition to thermal insulation, foam materials can also reduce the propagation of building noise, providing good sound insulation and creating a quiet living environment for users. Beyond insulation and soundproofing, foam materials also play a role in supporting and cushioning building structures. For instance, during natural disasters such as earthquakes, foam materials can absorb some of the energy and mitigate the impact of vibrations on buildings. Moreover, in architectural decoration, foam materials are suitable for various shaping and design requirements, such as sculptures and decorative panels. Due to their extensive applications and outstanding performance, foam materials have become irreplaceable in the construction field and have injected more innovative elements into the development of the construction industry. Therefore, foam materials are an essential component in the construction field.

However, due to the inherent carbon-hydrogen organic structure of polymeric foam materials, most of these materials are flammable and combustible. They release a large amount of heat, have high heat values, and exhibit rapid flame spread during combustion, making them difficult to extinguish. For example, polyurethane foam contains a large number of combustible carbon-hydrogen chains and amide bonds. Its high specific surface area and the presence of flammable gases in the pores make it burn very intensely once ignited. The flame spreads quickly, with many combustible components, and the material cannot self-extinguish due to high air permeability during combustion. As shown in Table 1, which lists the performance indicators of different types of insulation core materials, it is evident that the flame retardancy of current organic insulation foam materials on the market needs to be improved. By reasonably adding and selecting flame retardants, the fire resistance of building foam insulation materials can be significantly enhanced, reducing the risks and losses associated with fires. In recent years, many researchers have been committed to experimental exploration of flame-retardant foam materials and have made significant progress in the development of flame-retardant foam materials for construction use. This article aims to review the research progress of flame-retardant foam materials for construction over the past five years, analyze the existing problems in flame-retardant foams, and point out future development directions.

I. Research Progress on Flame Retardancy of Insulation Foam Materials

1. Traditional Polyurethane Foam

The high porosity of traditional polyurethane foam leads to air permeation within the structure, making polyurethane materials flammable. Additionally, the blowing agents used in the production of polyurethane foam are mainly hydrocarbons, which also pose a risk of fire. Based on these factors, it is necessary to enhance the flame retardancy of polyurethane foam without compromising its inherent properties.

Flame retardants are typically added to the formulation in powder form or combined with polyurethane during the synthesis process to improve its flame-retardant properties. Some common flame retardants include tris(2-chloropropyl) phosphate (TCPP), dimethyl methylphosphonate (DMMP), and ammonium polyphosphate (APP), all of which can enhance the flame retardancy of polyurethane. Flame retardants for polyurethane foam are broadly categorized into halogen-free and halogenated flame retardants. Halogen-free flame retardants are mostly phosphorus-based, such as dimethyl propyl phosphate and triethyl phosphate. While halogenated flame retardants offer excellent flame retardancy and lower costs, they produce toxic or carcinogenic smoke and large amounts of fumes during combustion, posing potential hazards. Therefore, their use is restricted, and efforts are being made to develop non-halogenated flame retardants.

• Halogenated Flame Retardants: Halogenated flame retardants primarily function through the cleavage of carbon-halogen bonds. The mechanism involves the generation of halogen radicals that capture hydroxyl (OH?) and hydrogen (H?) radicals, a process known as radical scavenging or trapping. Simply put, halogenated flame retardants produce low-energy radicals that act as chain terminators. Similar to chlorinated flame retardants, brominated flame retardants also exhibit good flame retardancy in polyurethane foam and other polymer materials. Combustion of brominated flame retardants generates HBr, which captures flame-propagating radicals, reducing heat generation and extinguishing the flame. However, these flame retardants also produce toxic, corrosive, and potentially carcinogenic smoke during combustion. Despite their significant flame retardancy, the high toxicity of halogenated flame retardants limits their widespread use, driving the development of non-halogenated alternatives.
• Phosphorus-Based Flame Retardants: Phosphorus-based flame retardants are considered the most promising alternatives to halogenated flame retardants. In the condensed phase, phosphorus-based flame retardants generate strong acidic substances such as phosphoric acid, polyphosphoric acid, and pyrophosphoric acid upon heating. These substances catalyze the formation of char in the substrate, preventing the transfer of oxygen, combustible gases, and heat. In the vapor phase, phosphorus-based flame retardants can produce phosphorus or phosphorus-oxygen radicals that quench reactive hydrogen or hydroxyl radicals. Researchers have developed a series of halogen-free flame-retardant rigid polyurethane foams using ethylene glycol-modified melamine-formaldehyde resin and phosphorus flame retardants. Another study successfully synthesized a bio-based phosphorus flame retardant, FPASO-DOPO, from rosin. The introduction of FPASO-DOPO significantly enhanced the flame retardancy of rigid polyurethane foam (RPUF) (mechanism shown in Figure 3). Additionally, the incorporation of rigid rosin structures improved the mechanical properties of the RPUF.
• Intrinsic Flame Retardancy: Intrinsic flame retardancy is a popular research area, offering advantages such as low flame retardant additive content and minimal impact on the mechanical properties of the substrate. Researchers designed and synthesized a reactive flame retardant containing dual phosphorus groups for RPUF. Another study developed a novel reactive phenyl phosphonate ethylene glycol ether oligomer through condensation reaction. Incorporating phenyl phosphonate ethylene glycol ether (PPGE) segments and expandable graphite (EG) into the RPUF chain enhanced the compression modulus of the polyurethane foam and achieved good synergistic flame retardancy. Reactive phosphorus-containing oligomers have shown broad prospects in developing high-performance flame-retardant insulating RPUF materials. Researchers also synthesized a reactive flame-retardant diol (BEOPMS) through the esterification of [(6-oxo-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methyl] succinic acid (DDP) with diethylene glycol (DG). The flame-retardant model in rigid polyisocyanurate-polyurethane foam (PIR) manufactured via a one-step process is shown in Figure 4. BEOPMS forms condensed-phase phosphates and releases PO and PO? radicals during thermal degradation, inhibiting radical chain reactions in the vapor phase. Therefore, PIR exhibits excellent flame retardancy.

1. Bio-Based Polyurethane Foam Flame Retardancy

Similarly, flame retardant modifications of bio-based polyurethane foams are divided into additive and reactive types. Polyurethane materials based on soybean oil and castor oil have been successfully applied in building energy-saving and insulation materials. In one study, researchers modified rigid polyurethane foam based on castor oil with expandable graphite and graphene oxide as flame retardants. In another study, researchers produced open-cell bio-based polyurethane foam using bio-polyols extracted from used rapeseed cooking oil, with the cooking oil completely replacing petrochemical polyols (100% substitution). Generally, compared to additive flame retardants, reactive flame retardants offer higher thermal stability. Most research on flame retardant bio-based polyurethane foams focuses on phosphorus-based reactive flame retardants. Researchers synthesized a phosphorus bio-polyol by reacting allyl phosphate with thioglycerol and mixed it with different bio-polyols (including soybean, orange peel, and castor oil-based polyols), significantly improving the flame retardancy of the bio-based polyurethane foam. In another study, researchers synthesized phenyl phosphate and epoxy propyl reactive flame-retardant polyols and used them with limonene-based polyols to prepare flame-retardant polyurethane foam, enhancing its flame retardancy.

2. Polystyrene Foam

Polystyrene foam, known for its excellent insulation properties, is the most widely used insulation material in the construction field. With an oxygen index of only around 20.0, polystyrene foam produces large amounts of toxic gases and dense smoke during combustion. It also exhibits dripping and melting, which can easily lead to fire spread and secondary damage. Therefore, flame retardant modifications are essential for polystyrene foam used in building insulation.

• Brominated Flame Retardants: Commonly used brominated flame retardants in China include tetrabromobisphenol A, octabromodiphenyl ether, and decabromodiphenyl ethane. In recent years, the use of brominated flame retardants in polystyrene foam has decreased due to growing concerns about environmental protection and health safety. As a result, environmentally friendly alternatives have emerged.
• Expandable Flame Retardants: The structure of char formation in expandable flame retardant systems is complex and influenced by numerous factors. Researchers developed a novel "ternary integrated" expandable flame retardant (MAP, Figure 5) using melamine (MEL), acrylonitrile-styrene-acrylate (ASA), and phytic acid (PA) as raw materials through electrostatic self-assembly. Another study prepared a new expandable flame retardant—phosphorylated histidine-aminotriazine-diaminopropane (PHTD)—and used it as a flame retardant and adhesive for polystyrene. The flame-retardant mechanism is shown in Figure 6.
• Inorganic Mineral Flame Retardants: Mineral materials used for flame retardancy in polystyrene foam include aluminum hydroxide, magnesium hydroxide, expandable graphite, and their composites. Researchers modified magnesium hydroxide (MH) with sodium dodecylbenzenesulfonate (SDBS) to prepare a flame retardant (MMH) and blended it with polystyrene (PS) to produce flame-retardant PS composite foam insulation boards (PS-MMH-3). These boards exhibited good corrosion resistance and aging resistance, making them effective for use in building flame-retardant insulation materials. Another study used a phenolic resin/Al(OH)? flame-retardant solution to encapsulate PS foam, transforming its combustion properties from combustible thermoplastic to non-combustible thermosetting, significantly improving the flame retardancy of traditional PS insulation boards.

3. Traditional Phenolic Foam

Phenolic foam is one of the unique organic polymer foam categories with distinct applications. While phenolic foam inherently possesses excellent flame retardancy, it has poor toughness. Although various toughening agents can be added to enhance the foam's toughness, the resulting flammability caused by these additives compromises its flame retardancy, creating a balance issue between mechanical and flame-retardant properties.

• Halogen-Free Flame Retardants: Phenolic foam is increasingly moving towards halogen-free, phosphorus-free, and more environmentally friendly directions. Researchers designed a novel environmentally friendly halogen-free hard stearate-based flame retardant (PSNCFR) and incorporated it into phenolic foam (PFs). In another study, a new phenolic-based siloxane (SAECD) with silane groups and reactive epoxy groups was prepared. PFs modified with different amounts of SAECD were then fabricated. Experimental results showed that the addition of SAECD also enhanced the flame retardancy of PF.
• Boron-Based Flame Retardants: In addition to halogen-free flame retardant systems, boron-based flame retardancy is another approach for PFs. Researchers used melamine phosphate borate in phenolic foam, with the maximum addition amount reaching 5% (by mass). All samples achieved UL 94 V-0 ratings. Studies indicated that melamine phosphate borate primarily exhibits flame retardancy in the condensed phase by forming a dense char structure. Researchers used varying amounts of boric acid in PFs, up to 6%. They found that as the boric acid content increased, the oxygen index (LOI) value increased, while the peak heat release rate and total heat release decreased, significantly enhancing flame retardancy. Researchers used boron-containing phenolic resin as a curing and char-forming agent for epoxy resins to produce thermosetting materials with high flame retardancy.

4. Bio-Based Phenolic Resin (BPF)

BPF is a polymer formed through phenol-formaldehyde addition-condensation reactions using natural phenols, aldehydes, or their derivatives. The primary raw materials for BPF include natural phenolic substances, phenols derived from biomass conversion, and aldehydes. Researchers introduced bio-oil and montmorillonite (MMT) into PF foam to enhance its toughness and flame retardancy. MMT can mix well with bio-oil, effectively improving the flame retardancy and toughness of PF. Researchers used larch tannin to produce closed-cell BPF foam for insulation purposes. Experiments proved that this BPF foam has excellent flame retardancy and can be used as an insulating material in the construction field. In another study, the flame-retardant characteristics of open-cell tannin-based PF foam were investigated. Bio-based PF foam has long ignition times and low heat release, making it an excellent insulating foam material for construction. Researchers successfully synthesized a bio-based phenolic foam based on lignin alkaline liquid and tannic acid, studying the effects of various curing catalysts (acids, bases, and heat) and curing temperatures on the production of BPF foam (as shown in Figure 7). During the combustion of BPF foam, no smoking, dripping, or fire spread was observed. Experimental results indicated that they can be used as flame-retardant foam materials for construction.

5. Cellulose Nanofiber Foam

Cellulose nanofiber (CNF)-based insulating foams, also known as nanocellulose and cellulose nanofibers, are currently in the early stages of development, with research primarily confined to academia. Our literature search indicates that CNF-based foams have not yet been commercialized. The production of CNF may involve a series of different operations, resulting in numerous variants of CNF. The general processing of cellulose to CNF includes raw material procurement, purification, mechanical pretreatment, biological/chemical pretreatment, principal mechanical processing, and post-processing. By using ice casting followed by freezing, supercritical, or evaporative drying strategies (Figure 9), foams composed of CNFs can be manufactured in laboratories. Hydroxyapatite (HAP) is a non-toxic calcium phosphate with a high phosphorus content (higher than typical commercial phosphorus-based flame retardants), making it highly flame retardant. Researchers combined renewable cellulose nanofibers with non-flammable hydroxyapatite (HAP) to produce organic-inorganic composite foams via freeze-drying (without ice casting). The CNF/HAP foam composites achieved excellent flame retardancy. Researchers used sodium alginate (a low-cost, non-toxic biopolymer commonly used in food and biomedical fields) along with boric acid and borates (also low-cost, non-toxic materials) as flame retardants to prepare crosslinked CNF foam composites. This environmentally friendly foam has low thermal conductivity, good flexibility, and non-flammability.

6. Aerogel Materials

Aerogels are a solid material form obtained by drying processes that maintain the three-dimensional network structure of gels while removing the liquid solvent. They are characterized by low density, low thermal conductivity, high porosity, and high-temperature resistance. Due to their extremely low thermal conductivity, aerogels enhance insulation performance. The most common types of aerogels include silica, carbon, and metal oxide-based aerogels. Among them, SiO? aerogels, as a novel nano-lightweight, multifunctional, and environmentally friendly material, are increasingly drawing public attention as an efficient insulating material. Researchers prepared a new type of phenolic resin/silica composite aerogel (synthesis process shown in Figure 10) through direct copolymerization and nano-phase separation methods. The composite aerogel with 70% silica content exhibited outstanding flame retardancy, withstanding flames of approximately 1300°C without decomposition. Researchers used konjac glucomannan and tetraethyl orthosilicate to prepare two different structural aerogels through physical blending (KTB) and co-precursor methods (KTC). Compared to the KTB aerogel with simple physical blending, the KTC aerogel with crosslinked interpenetrating networks demonstrated better mechanical properties, insulation, and flame retardancy.

III. Flame Retardancy Research on Sound-Insulating Foam Materials

In recent years, noise pollution has become one of the most severe environmental issues facing humanity, negatively impacting health and work efficiency. As a result, porous foam materials, known for their excellent sound absorption properties, low density, and high specific strength, have garnered significant attention. In the construction field, organic foams are widely used as noise reduction and sound insulation materials due to their controllable microstructures and abundant production. The pore structure of foams is closely related to their sound absorption performance, as the distribution of pathways in the foam greatly affects the dissipation of sound energy. Figure 11(a) shows the typical morphology of foam, which contains cavities and various structured pores (closed, partially open, and open pores). Taking polyurethane foam as an example, cavities and pore structures are formed during the polymerization process. The cell size is determined by the gelation and blowing reactions. If the cavity pressure is much greater than the wall strength, foam with an open pore structure can be obtained. Thicker cavity walls tend to solidify at low drainage flow rates, and if the solidification process occurs before the formation of fully open pores, partially open pores will be manufactured. If the cavity walls solidify completely before the walls rupture, closed pores will remain (Figure 11).

1. Polyurethane Foam for Sound Insulation

Polyurethane foam can be used not only as an insulating material but also as a sound insulation material. Various types of nanoparticles and fibers can be used to enhance the acoustic performance of polyurethane foam. The addition of nanoparticles and fibers affects the cell size and open porosity of the foam, thereby improving the sound absorption performance of polyurethane foam. The sound absorption coefficient of polyurethane foam is also affected by the number of pores. The more pores in the foam, the better the sound absorption performance.

• Boron-Based Flame Retardants: Inorganic boron-based flame retardants, such as boric acid, borax, and borate, can significantly improve the fire resistance, flame retardancy, and smoke suppression properties of materials, reducing the emission of toxic and harmful gases during combustion. Researchers prepared a composite material of rigid polyurethane foam with triphenyl phosphate, aluminum hydroxide, and zinc borate, as well as their binary mixtures, through a one-step molding process. The flame spread rate of the rigid polyurethane foam also decreased significantly, and in some cases, the flame was observed to self-extinguish. The flame retardant additives improved the flame retardancy of the rigid polyurethane foam.
• Nitrogen-Phosphorus Synergistic Flame Retardancy: The combination of flame retardants is based on the interaction between them to enhance flame retardancy, known as a synergistic effect. During the use of special types of flame retardants, one performance may be enhanced while another is weakened. At this point, a synergistic flame retardant system is crucial for foam materials to achieve optimal performance. Adding a flame retardant composed of phosphorus and nitrogen, such as toluene amine spirocyclic pentaerythritol diphosphate (TSPB) (synthesis process shown in Figure 12), to rigid polyurethane foam improves its flame retardancy because TSPB undergoes thermal degradation earlier than the rigid polyurethane foam. Researchers studied the effect of a new phosphorus-nitrogen flame retardant (DOPO-NIBAM, synthesis diagram shown in Figure 13) on the flame retardancy of polyurethane foam. The presence of nitrogen in the flame retardant dilutes the combustible gases formed during combustion, thereby improving the flame retardancy of polyurethane foam.

• Carbon-Based Flame Retardants: Carbon-based materials, including graphene, expandable graphite, reduced graphene oxide, and carbon nanotubes, have gained significant attention as sustainable green flame retardants for polymers, including polyurethane foam. Carbon-based flame retardants enhance flame retardancy by promoting char formation. Researchers used sunflower oil as an alternative and converted it into an active form through epoxidation and epoxy ring-opening to produce rigid polyurethane foam. Different concentrations of expandable graphite (EG) and methyl phosphonate dimethyl ester (DMMP) were used as non-halogenated flame retardants to prepare rigid polyurethane foam. The results show that this polyurethane can achieve efficient flame retardancy through DMMP or EG and has the potential for large-scale production.

2. Aerogel Materials

Aerogels are amorphous materials composed of a robust macromolecular network. Due to their porous structure, which effectively hinders the propagation of sound waves, they exhibit excellent sound insulation. Additionally, aerogels have a very high porosity, ranging from 88% to 99.8%. The higher the porosity, the greater the probability and frequency of collisions when sound waves enter the porous material, leading to faster energy dissipation and better sound absorption. The sound insulation of aerogel materials is quite remarkable, with sound reduction ranging from 30 to 50 decibels, significantly reducing noise interference.

• Inorganic Flame Retardants: Inorganic flame retardants include aluminum hydroxide, magnesium hydroxide, and expandable graphite-based flame retardants. Aluminum hydroxide and magnesium hydroxide are the main varieties of inorganic flame retardants, characterized by non-toxicity and low smoke. Researchers found that Al(OH)? and Mg(OH)? can enhance the flame retardancy of silica aerogels. Compared to the original aerogel, the aerogel with added Mg(OH)? exhibited better flame retardancy, with lower peak heat release rates and total heat release. The addition of hydroxide particles played a more significant role in reducing total heat release and heat release rates by diluting the combustible gases and removing heat from the fire through the evaporation of water. In particular, it effectively captures free radicals, preventing further combustion of the silica aerogel. The main role of inorganic flame retardants during combustion is to undergo chemical changes. The aluminum hydroxide in the aerogel composite decomposes under fire conditions, producing water that dilutes the generated combustible gases and carries away heat from the fire.
• Phosphorus-Based Flame Retardants: In addition to acting as flame retardants in the vapor phase, phosphorus-based compounds can also be introduced into aerogels to promote the formation of a char layer, acting as a physical barrier. The combination of silicon and phosphorus elements can exert a synergistic flame retardant and smoke suppression effect. Researchers prepared silica aerogels using sodium silicate and tetraethyl orthosilicate as precursors and phosphoric acid as an acid catalyst, then modified them with 10% trimethylchlorosilane. The hydrophobic trimethylsilyl groups [TMS, Si-(CH?)?] on the aerogel react to produce Si-OH groups. The physical properties of the silica aerogel are reduced due to the sintering and aggregation of the nan particles. The use of inorganic silicon sources (such as sodium silicate) can reduce fire hazards. The introduction of phosphorus elements further reduces the flammability of the silica aerogel.
• Expandable Flame Retardants: Expandable flame retardants mainly consist of three parts: carbon source (char former), acid source (char catalyst), and gas source (blowing agent). Researchers developed a new type of aerogel melamine foam-reinforced phenolic aerogel (MFPA). Due to the interconnected particle network of the aerogel, MFPA has a stronger viscosity, which can withstand the additional stretching and bending of the foam during sound energy impact, thereby increasing sound energy dissipation. The aerogel network also has a more "planar" shape, which is conducive to the reflection of sound waves. Combining these advantages, MFPA has excellent sound insulation. At the same time, MFPA has low density, high flexibility, low thermal conductivity, excellent sound insulation, and efficient flame retardancy. These excellent properties make MFPA a promising flame-retardant material for low-temperature applications and have broad prospects in construction applications.

IV. Conclusion

Although many researchers have been committed to exploring flame-retardant foam materials and have made meaningful progress, there are still many issues that need further research. These can be summarized as follows:

1. Foam materials use green flame retardants. Based on market demand and development prospects, developing recyclable flame retardants and environmentally friendly flame-retardant foam materials is a future research direction for construction foam materials.

2. Foam materials use compounded flame retardants. In-depth research on the formula composition, synergistic mechanism, and cost issues of two or more flame retardants is needed.

3. The multifunctionalization of foam materials. Developing high added-value flame-retardant foam materials with multiple functions is a future research direction. In summary, by researching and applying flame retardants, adjusting formulas, and improving production processes, the flame retardancy of construction foam materials can be significantly enhanced, reducing the risks and losses associated with fires. At the same time, it is necessary to consider other factors comprehensively to develop construction flame-retardant foam materials that meet comprehensive requirements, ensuring building safety and sustainable development.

In the flame retardancy research of construction foam materials, Guangzhou YINSU Flame Retardant Company has developed a variety of efficient flame retardant products tailored to the specific needs of building materials. These products include an antimony trioxide replacement, which effectively reduces flame retardant costs while maintaining excellent flame retardant performance. The company has also launched XPS Red Phosphorus Paste RP-TP46, a high-phosphorus content paste flame retardant with superior flame retardant efficiency. Additionally, the company offers XPS Insulation Board Flame Retardant FRP-950X, a microencapsulated red phosphorus flame retardant suitable for wire and cable materials, featuring low smoke, halogen-free, and high-efficiency flame retardancy. These innovative products not only enhance the flame retardant performance of building materials but also meet environmental and safety requirements, providing reliable flame retardant solutions for the construction industry.

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