Polyurethane Materials for Fire Retardancy: Synthesis, Structure, Properties, and Applications

Abstract

As the demand for high-performance polymers broadens, polyurethane (PU) polymers with various chemical modifications have attracted attention. This review explores the chemical structure and functional variations of PUs. PUs are used in a variety of fields, ranging from aerospace engineering to daily necessities, and show remarkable safety adaptability through designable synthesis processes. This study is divided into four main parts: (1) synthesis and structure, covering the synthesis of PU base and modification of additive compounds; (2) performance, studying physical properties and thermal degradation processes; (3) application, evaluating the commercial potential of PU polymers; and (4) flame retardancy, analyzing five established flame-retardant mechanisms. The last part discusses how PUs can meet sustainable development goals by replacing petroleum-based materials with green materials. By emphasizing non-petroleum resources and novel, sustainable modification strategies, this review conducts guidance for the safe and environmentally friendly application of PUs in the future.

1. Introduction

Polyurethane (PU) is a highly versatile polymer formed through the reaction of polyols and isocyanates. Its ability to adapt its chemical and physical properties makes it indispensable in a variety of industries, including construction, automotive, electronics, and healthcare. With its wide-ranging applications, PU has become a critical material in modern manufacturing, particularly for products requiring lightweight, durable, and adaptable properties [1]. This review explores the synthesis, structural characteristics, performance attributes, and applications of PU, with a strong focus on advancements in flame retardancy and sustainable development.

The inherent flexibility of PU’s chemical structure is its defining characteristic. By modifying the raw materials and adjusting the polymer composition, PU can exhibit properties ranging from soft foams, widely used in cushioning and insulation, to rigid forms, applied in structural and load-bearing components. This adaptability has made PU an industry leader in material innovation. However, its flammability presents a significant challenge, necessitating the development of fire retardant (FR) technologies that ensure safety while maintaining material integrity. This review evaluates PU’s evolution from traditional, halogen-based FRs to environmentally friendly alternatives, including nanotechnology-enhanced and bio-based FR systems, which are aligned with modern sustainability priorities [2].

The diversity of PU’s applications stems from the versatility of its synthesis and structural composition. PU is synthesized through the reaction of polyols (compounds with multiple hydroxyl groups) and isocyanates. Adjusting the type and ratio of these precursors allows manufacturers to fine-tune the polymer’s properties, making it soft, semi-rigid, or fully rigid depending on the intended application [3]. Additionally, structural modifications, such as incorporating bio-based polyols or introducing novel cross-linking agents [4], have expanded PU’s functionality while reducing its environmental impact. Innovations in PU chemistry, including the use of non-isocyanate Pus [5], further demonstrate the industry’s commitment to reducing toxicological risks and improving sustainability.

PU exhibits a wide range of mechanical and physical properties, such as elasticity, tensile strength, thermal stability, and chemical resistance. These properties can be tailored to meet specific application needs by selecting appropriate raw materials and processing methods [6]. For example, the mechanical flexibility and low thermal conductivity of soft PU foams make them ideal for insulation and cushioning applications, while rigid PU foams are valued for their strength and durability in construction and automotive components [7]. Advances in materials science have further enhanced PU’s resistance to abrasion, impact, and environmental degradation, broadening its potential uses. PU covers several key areas in commercial and industrial applications. In the construction sector, PU is used in insulation panels, sealants, and adhesives. Its lightweight and durable properties make it an important component of automotive interiors, coatings, and tires. In the healthcare sector, PU is used in medical devices, wound dressings, and biocompatible implants due to its flexibility and biocompatibility [8]. The growing demand for sustainable materials has also driven research into bio-based PUs, which utilize renewable feedstocks such as vegetable oils to replace fossil fuel-based polyols. This shift toward sustainable PU production is in line with global environmental goals and reduces the material’s ecological footprint.

Traditional FRs, such as halogen-based compounds, have proven effective in reducing flammability, but have raised concerns due to their toxicity, environmental persistence, and the potential for the release of harmful gases during combustion. These drawbacks have led to a shift toward halogen-free and more sustainable FRs. By comparing the FR mechanisms of reactions during PU preparation and those of post-treatment, future FR approaches are more likely to be combined with nanoengineering. Nanotechnology, including montmorillonite clay [9], zinc stannate [10], and colloidal silica [11], is added to PU to form nanocomposites. The protective properties of nanotechnology form a strong char-like barrier on the polymer surface when exposed to heat or flame, isolating the material and preventing further combustion [12]. Another advancement in flame retardancy involves the synergistic effects of combining phosphorus-based compounds with nanofillers. Studies have shown that these combinations significantly enhance both gas-phase flame suppression and solid-phase char formation processes. This dual-action mechanism not only improves fire resistance but also reduces smoke and toxic gas emissions, making the material safer to use in confined spaces such as vehicles and buildings.

Sustainability has become a guiding principle in the development of polyurethanes and their related FR systems. The integration of bio-based polyols, such as those derived from vegetable oils or lignocellulosic biomass, reflects the industry’s commitment to reducing its reliance on fossil fuels. Green blowing agents, such as carbon dioxide, have replaced traditional blowing agents with high ozone depletion potential (ODP), further reducing the environmental impact of polyurethane manufacturing [13]. Sustainable alternatives are also gaining traction in flame retardancy. For example, bio-based phosphorus compounds and natural minerals are being explored as environmentally friendly FR [14,15,16]. These materials not only meet fire safety standards but also meet global efforts to mitigate climate change by reducing greenhouse gas emissions and promoting recyclability.

This review explores the chemical structure and functional properties of polyurethanes and comprehensively analyzes the development trends of polyurethanes in terms of sustainability and safety FR mechanisms. Therefore, this review focuses on the following areas:

• Synthesis and structure: covering the synthesis of polyurethane bases and modification of additive compounds.
• Performance: studying physical properties and thermal degradation processes.
• Application: evaluating the commercial potential of polyurethane polymers.
• Flame retardancy: analyzing five established FR mechanisms.

By emphasizing non-petroleum resources and novel, sustainable modification strategies, this review provides guidance for the future safe and environmentally friendly applications of PU.

2. Synthesis and Structure

2.1. Chemical Components

PU is a polymer composed mainly of urethane units, which are formed by the polycondensation reaction of diisocyanates and polyols. Functional PUs can be linear or cross-linked depending on the different chemical structures. The various types of raw materials give different physical and chemical properties, depending on the specific type and proportion of their components, as shown in Figure 1. Understanding these classifications and the impact of each component can help design PU materials suitable for specific industrial and consumer needs. The main materials used to synthesize PU polymers are polyols, isocyanates, catalysts, surfactants, blowing agents, and other functional additives [2].

In the synthesis of PU, the polyols usually selected are oligomer polyols with hydroxyl groups at the end, including polyether polyols and polyester polyols. Isocyanates are linked to polyols to form PUs, which are aliphatic, cyclophanic, and aromatic. Chain extenders, i.e., cross-linkers, include amine chain extenders and alcohol chain extenders. The main reaction of isocyanates and polyols in the formation of PU requires catalyst acceleration, and tertiary amine catalysts and organometallic compounds are considered. Other compounding agents are based on ordinary PU matrices and are used to add other functional properties. Since then, polymer experts have learned more about the chemical reactions of hydroxyl and isocyanate groups to optimize and control the properties of the resultant.

2.1.1. Polyols

Polyols are organic compounds containing hydroxyl groups (-OH) and are widely used in food science and polymer chemistry. Polyols are generally soluble in water. Most polyols are viscous liquids or crystalline solids. They have the characteristics of high boiling point, strong solubility in polar substances, low toxicity, and low volatility. Their boiling point, viscosity, relative density, and melting point all increase with the increase in molecular weight. As the main raw material for the preparation of PU adhesives, their specific molecular structure determines the performance of the synthetic product, and their functionality is diverse [17]. Applied oligomer polyols can be divided into those three types according to the different chemical bonds (Figure 2). Polyols are also classified into low molecular weight and high molecular weight polyols according to the number of hydroxyl groups. The number of hydroxyl groups significantly affects the number of physical cross-linking points in the polymer [6]. High molecular weight polyols give PUs longer polymer chains. The transmission rate of shear stress in long chains is slowed down, resulting in the elasticity of the polymer body.

Polyester polyols with various functional types have different chemical structures. The main distinguishing indicators are physical properties such as hydroxyl value, acid ester, hygroscopicity, and viscosity. The ester and ether bonds contained in the structures of polyester and polyether polyols are different, resulting in mechanical properties [18]. Ester bonds are rigid and not resistant to hydrolysis, while ether bonds are flexible and resistant to hydrolysis [12]. Therefore, polyester polyols are often used as the main raw material, and products made with polyether as the main raw material have poor strength [19].

Currently, research has found that the combination of polyester and polyether polyols has the opportunity to improve polymer properties and reduce the degree of polymer microphase separation [12]. There is still a lot of room for development in the performance of polyols in catalysts, and even the structure of hyperbranched phases will change. Special polyols have unique properties compared to ordinary polyols. For example, polycarbonate polyols have unique chemical resistance, and polycaprolactone polyols have flexibility [20,21]. However, the innovation of these raw materials is in line with the global strategic goal of green and sustainable development.

2.1.2. Isocyanates

Isocyanate plays an absolute guiding role in the preparation process, reacting with various ligands such as polyether, polyester, polyamide, polyacid, etc., and polymerizing to form the main chain structure of PU [22]. Specifically, isocyanate reacts with polyether or polyester at a certain temperature and reaction time to form aminoisocyanate. Aminoisocyanate is an important intermediate in the preparation process of PU. It participates in the polymerization reaction and condenses with alcohol compounds to form PU. In addition, isocyanate can also change the physical properties of PU by adjusting the ratio and reaction conditions. For example, adding an appropriate amount of cross-linking agent can improve the thermal stability, aging resistance, and wear resistance of PU.

Isocyanate is a general term for various esters of isocyanic acid, identified in Figure 3. If classified by the number of -NCO groups, it can be divided into monoisocyanates, diisocyanates, esters, and polyisocyanates. Common diisocyanates include toluene diisocyanate (TDI), isophorone diisocyanate, diphenylmethane diisocyanate (MDI), dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, etc.

From a structural perspective, these isocyanates can also be divided into aliphatic isocyanates and aromatic isocyanates. Currently, the largest quantity of isocyanates are aromatic isocyanates, such as MDI and TDI, which are two important isocyanate varieties. The reaction characteristics of isocyanates with active hydrogen compounds are related to environmental conditions. They have high reactivity with alcohols, amines, water, phenols, and carboxylic acids, and the reaction temperature is low. On the contrary, they have low reactivity with amides, ureas, carbamates, etc., and the reaction temperature is high. In general, isocyanates react with amines, water, and carboxyl groups to form ureas.

The reaction characteristics of isocyanate with active hydrogen compounds are related to environmental conditions. It has high reactivity with alcohols, amines, water, phenols, and carboxylic acids, and the reaction temperature is low. On the contrary, it has low reactivity with amides, ureas, and carbamates, and the reaction temperature is high. Commonly, isocyanate reacts with amines, water, and carboxyl groups to form urea. High temperature promotes the further reaction of urea groups with isocyanate to form branched or cross-linked structure products [23]. Only when there is an excess of isocyanate groups, and the temperature is high can the polymer form branched or cross-linked structure products.

2.1.3. Other Additives

In the production of PU, various additives play a key role in optimizing the material’s structure, mechanics, and durability, enabling its wide application in various industries. Each class of additives and their common agents make unique contributions to enhancing the characteristics and performance of PU materials, summarized in

Table 1. Other additives in PU chemical components.

Additives	Common Selections	Ref.
Chain extenders	Glycerol, Trimethylolpropane, Pentaerythritol	[24]
Catalyst	Amines: triethylenediamine, N-alkyl morphine	[25,26,27]
	Organotin: dibutyltin diosilicate	[28]
Interface agent	Coupling agents, surfactants	[29,30,31,32]
Foaming agent	Water, liquid carbon dioxide, chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, pentane, cyclopentane	[33,34]
Foam stabilizer	Water-soluble polyether siloxane	[35,36,37]
Stabilizers	Antioxidants, UV absorbers, and polystyrene peroxide	[38,39]
Enforced fillers	Cellulose, glass fiber, carbon black, silica	[40]

. Common chain extenders help form cross-links between PU molecules. This cross-linking enhances the structural integrity and toughness of the polymer, resulting in a stronger, more durable PU material suitable for applications requiring high elasticity. Catalysts are essential for promoting the reaction rate between isocyanates (-NCO) and polyols (-OH), reducing reaction temperatures and improving polymerization efficiency. Organotin compounds interact directly with NCO and OH groups, while amine catalysts help promote reactions involving NCO and water molecules. These catalysts not only accelerate the polymerization process but also increase the molecular weight and durability of the final PU product. Generally, the two types of catalysts are used in combination.

Other non-essential additives participate in the physical modification of PU products. During the PU production process, interfacial agents such as coupling agents and surfactants are added to improve the compatibility between PU and other fillers or toughening agents. These agents enhance bond strength and dispersion, ensuring that fillers are evenly distributed and well-bonded within the PU matrix. This contributes to the stability and durability of PU in applications where enhanced strength is required. When manufacturing PU foam materials, blowing agents and foam stabilizers work together to create bubbles during the synthesis process to form the characteristic foam structure. Foam stabilizers help maintain the uniformity and stability of the foam and prevent foam collapse during the production process. These additives work together to form stable, lightweight foams with consistent properties, which are commonly used in applications such as cushioning and insulation. In addition, reinforcing fillers improve the mechanical properties and flame retardancy of PU. Some stabilizers mitigate oxidation and aging of PU due to exposure to high temperatures or ultraviolet rays to extend service life and durability. Each additive plays a specific and critical role in tailoring the properties of PU, allowing solutions to be customized to meet the requirements of a variety of industrial applications.

2.2. Polymerization

2.2.1. Reaction with Active Hydrogen

Because of the presence of two cumulative double bonds, the isocyanate group is very active. In the first stage of the urethane formation reaction, the nucleophilic center in the hydroxyl reagent will add to the charge affinity carbon atom in the carbonyl group of the isocyanate, and the proton will be transferred to the nitrogen atom at the same time. The reaction generates chemical structures such as carbamate, substituted urea, allophanate, biuret, etc. PU (urea) materials with diverse structures and properties are synthesized based on many active hydrogen compounds [41].

(1)

The resonance structure of the charge distribution and electron cloud density of the isocyanate group is displayed. It is a hybrid accumulation double bond. The electrons in the chemical reaction are electrophilic, and the isocyanate group is an electrophilic reagent. This is why it reacts extremely actively with active hydrogen [42].

Substances containing active hydrogen BH can undergo nucleophilic addition reactions with isocyanate groups. The nucleophilic center B attacks the positive carbon ion, and the active hydrogen H is transferred to the N atom. When R is an electron-withdrawing group and B is an electron-donating group, it is conducive to the nucleophilic addition reaction [43].

(2)

The order of reactivity of active hydrogen-containing compounds that undergo nucleophilic addition reactions to isocyanate groups is as follows:

$$R_{2}NH>RN{H}_2>N{H}_3>C_6{H}_{5}N{H}_2>ROH>H_{2}O>C_6{H}_{5}OH>RSH>RCOOH$$

Among that, aromatic isocyanates are more reactive than aliphatic isocyanates [44]. The reactions of isocyanates with different chemical groups are summarized in the table. It is worth noting that the reaction of hydroxyl groups with isocyanate is the most basic chemical reaction for synthesizing PUs. The reaction is characterized by the absence of a catalyst, an ambient temperature range of room temperature to 100 degrees Celsius, and when alcohol is in excess, the terminal group is an alcohol hydroxyl group. When urea participates in the addition reaction, the product biuret is characterized by a branched or cross-linked structure.

Isocyanates have high reactivity with alcohols, amines, water, phenols, and carboxylic acids at low temperatures. The chemical reaction equations are in

Table 2. Chemical reaction equations for different reactants.

Reactants	Reaction Formulas
Alcohol
Amines
Water
Phenol
Amide
Carboxylic acid
Urea
Carbamate

. However, they are less reactive with amides, ureas, and carbamates at high temperatures. When isocyanates react with amines, water, and carboxyl groups to form ureas, branched or cross-linked products are formed, and then the urea reacts with the isocyanates at high temperatures. This multi-step reaction is based on excess isocyanate groups and high temperatures.

$$-{\displaystyle \frac{d\left[R_{1}NCO\right]}{d\left[t\right]}}=k\left[R_{2}OH\right]\left[R_{1}NCO\right]$$

(3)

$$-{\displaystyle \frac{d\left[NCO\right]}{d\left[t\right]}}=k{\left[R_{1}NCO\right]}^2$$

(4)

The formation of PUs generally follows second-order reaction kinetics, where the reaction rate depends on the concentrations of the isocyanate ($R_{1}NCO$) and hydroxyl ($R_{2}OH$) components [45]. In the studies, the initial ratios of polyol (hydroxyl) and isocyanate were equal, and the concentrations of both reactants remained equal throughout the reaction, assuming no major side reactions involving the isocyanate [46].

The PU formation process is a stepwise, irreversible polymerization, where each stage builds on the previous one, rapidly forming a prepolymer structure. This structure allows for the rapid addition of additional monomer molecules, resulting in rapid growth. In addition, isocyanates can undergo self-polymerization, producing byproducts such as dimers and trimers, which complicates the separation of the initial reaction products.

2.2.2. Self-Polymerization

The second stage kinetic law is due to the autocatalytic effect of hydroxyl and -NH-COO groups or the influence of solvents [47].

(5)

(6)

The autocatalytic process forms an active complex in both cases, which then reacts with the hydroxyl group and decomposes to produce urea and hydroxyl compounds or urea compounds. The stoichiometric reaction rate of isocyanate and hydroxyl compounds can be described by a third-order kinetic equation, without an added catalyst but with autocatalysis. In the reaction in solvent, the autocatalysis is significant, although there is an accompanying effect, and it is enhanced in polar solvents.

As the polymerization proceeds, the viscosity of the reaction medium increases and the reaction rate decreases, because diffusion effects restrict the access of reactants to the active sites. In addition, a correct interpretation of the kinematic findings requires data on the catalytic activity of the external catalyst and the autocatalytic urea group. In general, it can be assumed that the rate of addition polymerization, measured by the disappearance of free -NCO groups, depends on the concentration of the acidic and basic catalysts. This is because the catalyst affects the rate of formation of the polar complex.

Essentially, it is affected by factors such as the structure of the complex formed by -NCO and -OH groups and the reaction of the complex with diols. The overall reaction rate constant of COOH depends on factors such as the structure of the complex formed by protons and -NCO and the reaction of the complex with diols. The overall reaction rate constant of C is affected by factors such as the structure of the complex formed by -NCO and the catalyst and the reaction of the complex with diols.

2.3. Phase Structures

PU is a segmented copolymer that exhibits a microphase-separated morphology due to its alternating soft and hard segments. This phase separation is driven by hydrogen bonding interactions and chemical incompatibility, significantly affecting the mechanical and thermal properties of PU materials. The interaction between the urethane and urea bonds in the hard segments produces physical cross-links, which enhance mechanical strength and thermal stability [7].

Foam is a typical continuous-phase polymer, and its discontinuous phase is the gas phase. PU, a very widely used polymer, has a variety of densities, cross-linking densities, and stiffnesses, from very soft structures to very hard structures. Different phase structures influence the properties significantly; those segments are presented, respectively, in Figure 4. Combinations of the hard segment phase and soft segment phase can construct the network design of PU [48].

Soft segments are usually derived from polyether or polyester polyols and exhibit low glass transition temperatures. Soft segments impart flexibility, elasticity, and low-temperature properties to the polymer. Hard segments consist of isocyanates and chain extenders and are highly polar and rigid. They form hydrogen-bonded domains that act as physical cross-linking points that reinforce the polymer matrix. The degree of microphase separation depends on the molecular weight of the soft segments and the content of the hard segments. Increasing the hard segment content enhances the strength of hydrogen bonds between hard domains, leading to a higher degree of phase separation.

2.3.1. Linear

Linear PU is a polymer composed of linear polyols, linear poly-isocyanates, and chain extenders, and its main chain is a linear structure. Linear PU exhibits high softness, elasticity, wear resistance, and weather resistance in terms of physical properties. In addition, linear PU also has good processing and molding properties and is widely used in coatings, inks, adhesives, cellulose products, footwear, car seats, foam materials, and other fields [49].

Linear PUs are polymers synthesized primarily via polyaddition reactions between diisocyanates and polyols. The backbone structure consists of aliphatic or aromatic segments connected by polar urethane bonds. The polyaddition process and chain formation can be carried out in a single stage or via a prepolymer approach, where an isocyanate-rich intermediate is first synthesized and then extended with low molecular weight diols such as 1,4-butanediol. While the backbone of linear PUs consists primarily of urethane groups, the secondary structure can also include ether or ester groups from the polyol component. The chemical reactions are correlated with urea groups, biuret, allophanate or isocyanurate bonds, and ionic groups. These chemical variations affect the phase separation, polarity, and intermolecular interactions of the material, directly affecting the mechanical, thermal, and surface properties.

2.3.2. Star-Shaped

Star PUs (SPUs) have a unique molecular structure characterized by a central core with multiple branching arms. This structure has enhanced molecular interactions and physical properties compared to linear structures, making SPUs of interest for advanced materials applications.

SPUs are synthesized by combining star poly(ε-caprolactone) (SPCL) with diisocyanates such as MDI and chain extenders such as 1,4-butanediol (BDO). The number of arms in the star structure (usually three, four, or six) significantly affects the physical and thermal properties of the resulting polymer [50]. The star structure enhances functionality by increasing the number of available reaction sites while maintaining molecular compactness.

The soft segment acts as a reversible phase, promoting elasticity and shape memory properties. The crystallinity of the SPCL decreases with increasing arm number, resulting in a wider melting temperature range. The hard segment acts as a physical cross-linking point through strong hydrogen bonding interactions with the star structure, enhancing mechanical properties [51].

The SPU exhibits a triple shape memory effect that enables the SPU to remember two intermediate shapes in addition to its original shape. Partial crystallization fixes the temporary shape at a moderate temperature, while segment melting promotes recovery to the original shape at a higher temperature. The mechanical properties of the SPU, including tensile stress and Young’s modulus, improve with increasing arm number. This enhancement is based on higher crystallinity molecular twinning that restricts segment motion, increasing stiffness and elasticity.

2.3.3. Cross-Linked

Cross-linked PU is a network structure polymer formed by the isomerization reaction of linear polyisocyanates or linear polyamines with polyisocyanates, and its main chain is composed of the main chain and side chain structure. Due to its different structure from linear PU, cross-linked PU has high hardness, strength, heat resistance, corrosion resistance, and high-pressure resistance. The different types and proportions of cross-linking agents used in the manufacturing process of cross-linked PU will greatly impact the performance of PU materials.

The construction of the cross-linked structure depends on the polyol. Polyether (ester) polyols with relatively large hydroxyl values can be selected. In addition, polyaryl polymethylene isocyanate (PAPI) with higher functionality can also be used as an auxiliary agent [52]. For example, some cross-linking agents contain small molecular weight polyols and polyol amines, propylene glycol, triethanolamine, etc. In the synthesis process, the index is increased, and some trimerization catalysts are added to allow MDI to undergo a trimerization reaction to achieve a cross-linked structure.

A novel cross-linked PU synthesized from bio-based polyols (castor oil) and industrial monomers (bisphenol and isophorone diisocyanate) with phenol-urethane bonds [4]. It has excellent dynamic network rearrangement properties. These bonds are dissociative dynamic covalent bonds, which can reversibly dissociate and associate under external stimuli such as heat. The dynamic behavior is highly dependent on the electronic effects of the substituents on the bisphenol backbone, including electron-withdrawing groups that reduce the initial dissociation temperature to promote faster network rearrangement and electron-donating groups that increase the dissociation temperature to provide higher thermal stability. The design allows for precise control of the network rearrangement kinetics by adjusting the type and ratio of bisphenols used during the synthesis.

The degree of cross-linking and chemical composition has an impact on the thermal and mechanical properties of cross-linked PU [53]. The initial dissociation temperature can be adjusted by changing the bisphenol structure or the cross-linking density. The storage modulus and flow temperature are also affected by the degree of cross-linking. A double glass transition temperature is observed in highly cross-linked samples due to distinct soft and hard segment transitions.

This innovative approach to cross-linked PU design has made significant progress in creating adaptable and sustainable materials. This phase structure opens up avenues for designing recyclable polymers with tailored thermal, mechanical, and dynamic properties.

2.4. Structural Modification

The properties of PU polymers can be adjusted through structural modification. In structural engineering, isocyanates (component A) react with amino compounds (component R) to form an elastomeric substance. Isocyanates can be either aromatic or aliphatic. Component A can be a monomer, a polymer, a derivative of isocyanate, a prepolymer, and a semi-prepolymer. Component R must be composed of an amino-terminated resin and an amino-terminated chain extender. The amino-terminated resin must not contain any hydroxyl components and catalysts but may contain additives that facilitate pigment dispersion [54].

Different synthesis processes have a direct impact on the microstructure. The main synthesis processes of the current production line are a one-step method and a pre-polymerization method. The one-step process involves combining all reactants (diisocyanate, polyol, and chain extender) in a single reaction step, typically at elevated temperatures. The process is well suited for the synthesis of PU elastomers and dispersions due to its simplicity and short processing time. The reactants are mixed in a controlled environment, and the diisocyanate reacts directly with the polyol and chain extender to form polymer chains. The reaction occurs rapidly and is typically catalyzed by compounds such as dibutyltin dilaurate [1]. The prepolymer process is a two-step process that first forms an isocyanate-terminated prepolymer, which is then chain extended using a diol, diamine, or water. The polyol reacts with an excess of diisocyanate to produce an NCO-terminated prepolymer. This intermediate step allows for better control of molecular weight and structure. The prepolymer is subsequently extended with chain extenders or emulsified water to produce the final PU [55].

Both synthesis methods have distinct advantages and trade-offs that are contrasted in

Table 3. Comparison of one-step method and prepolymer method.

Features	One-Step	Prepolymer
Efficiency	High	Moderate
Control Over Properties	Limited (less precise phase control)	High (allow tailored properties)
Morphology	Less defined	Better-defined phase separation
Application Suitability	Simple applications	High-performance materials

. and are suitable for different applications. The one-step method is ideal for fast production and simpler applications, while the prepolymer method offers greater control over properties and is preferred for high-performance PU systems. As the industry demands more customized materials, the ability of the prepolymer method to fine-tune properties may become more prominent.

Surfactants

Surfactants, composed of two molecular parts with opposite polarity, play a crucial role in modifying polymer molecular structures and facilitating the synthesis of nanostructures. Their hydrophobic tails experience repulsion with polar media, rendering the solution thermodynamically unstable [56]. This instability drives surfactants to adsorb onto nanoparticle surfaces, forming micelles that stabilize the system by reducing free energy. Through electrostatic or van der Waals forces, surfactant monomers orient their hydrophobic tails toward the particle surface and their hydrophilic heads toward the solvent, effectively lowering interfacial tension. Beyond their role in nanostructure synthesis, surfactants serve as dispersants, wetting agents, emulsifiers, and detergents. By reducing surface tension at solid, liquid, and gas interfaces, they stabilize dispersed phases through interfacial interactions and regulate energy and material exchanges in natural and synthetic processes. Their ability to mediate interactions between incompatible phases underscores their significance in diverse industrial and research applications.

Surfactants are important in the structural modification and performance optimization of PUs. They ensure the improvement of PU product performance by affecting the interaction between polyols and isocyanates, stabilizing the foam structure, or regulating the surface properties of coatings [2]. Different surfactants (Figure 5) can promote the uniform mixing of polyols and isocyanates, which have poor interactions during the reaction process. Surfactants form a stable emulsion, reduce the interfacial tension between the two phases, and make the reaction components more evenly dispersed, thereby building a more uniform PU structure [57]. Secondly, they regulate surface tension, directly affecting the size and distribution of cells in the foam, thereby avoiding defects such as foam rupture or holes and improving the mechanical strength and thermal insulation properties of the foam.

In the study of regulating bioresources, anionic surfactants, and cationic surfactants promote the extraction of different elements, respectively [58]. Biomass composite PU prepared using modified surfactants has significantly improved the tensile and peel strength of the coating, demonstrating its potential for application in the field of sustainable materials. On a microscopic level, interfacial polymerization is assisted in the synthesis, and side reactions with the mini-emulsion polymerization technique are minimized [59]. Nonionic surfactants, in particular, reduce the surface tension between the polyol and isocyanate phases. The lower surface tension caused by the physicochemical electronic adsorption allows for more efficient spreading and leveling of the coating material, ensuring uniform coverage and preventing problems such as delamination [60].

3. Properties

3.1. Molecular Weight in Polyols

Polyols containing two hydroxyl groups are used as chain extenders, and when they contain more than two hydroxyl groups, they are used as cross-linking agents [17]. The molecules of tetrahydrofuran and ethylene oxide are random. Low molecular weight polyols with higher hydroxyl numbers tend to have more cross-linking points and stronger intermolecular interactions in the soft segment.

Comprehensive analysis shows that the flexibility of high molecular weight polyols and the strength of low molecular weight polyols compensate for each other’s performance defects in polymer manufacturing, and other reinforcing agents can even be mixed to balance the chemical reaction. The high fluidity of high molecular weight polyol chains ensures the high elasticity of the resulting product. The reaction of diisocyanates with high molecular weight diols (MW: 2000–4000 g/mol) produces highly elastic linear PUs. Due to the possibility of hydrogen bonding, urethane bonds produce the “hard area” of PU elastomers. In addition, the difference in crystallinity between high molecular weight and low molecular weight polyols affects phase separation, and the chemical structure and application of memory foam are the keys to the modification of this polyol (MW: 6000 g/mol) in PU foam [61]. The physical cross-linking brought about by the synergistic application of polyols with different molecular weights exhibits better thermal and tensile properties.

3.2. Strength

Wide Range of Hardness: The classification of hardness for PU relies on the prepolymer’s molecular structure.

High Load Bearing Capacity: PU has a high load capacity in both tension and compression. PU may undergo a change in shape under a heavy load but will return to its original shape once the load is removed with little compression set in the material when designed properly for a given application.

Flexibility: PU performs very well when used in high flex fatigue applications. Flexural properties can be isolated allowing for very good elongation and recovery properties.

Abrasion and Impact Resistance: For applications where severe wear proves challenging, PU is an ideal solution, even at low temperatures.

Tear Resistance: PU possesses high tear resistance along with high tensile properties.

PU elastomers are used extensively around the world, often as replacements for traditional materials such as metal, plastic, and rubber. PU is often compared to these traditional materials due to their versatile mechanical properties. By leveraging innovative chemistry and structural manipulation, PU offers unique solutions to a variety of challenges, demonstrating performance characteristics that exceed those of other materials.

Table 4. Properties of PU compared to conventional materials.

Property	Vs. Rubber [62]	Vs. Metal [63]	Vs. Plastic [64]
Abrasion Resistance	High	High	High
Impact Resistance		High	High
Cut and Tear Resistance	High	-	-
Load Bearing Capacity	Superior
Elastic Memory			Present
Noise Reduction		High	High
Corrosion Resistance		High
Resilience		High	High
Flexibility		High
Surface Coating Durability			Frictions control
Temperature Resistance			Low
Radiation Resistance	High		High
Ozone Resistance	High

highlights the key properties of PU, comparing their advantages over rubber, metal, and plastic on a variety of performance criteria.

3.3. Mechanism of Thermal Degradation

3.3.1. Initial Decomposition

The urethane bonds in PUs undergo hydrolysis, especially in the presence of moisture. This reaction breaks down the urethane bonds into amines and alcohols. Hydrolysis weakens the polymer matrix and can lead to further degradation [65].

R-NH-CO-O-R’ + H₂O → R-NH₂ + R’-OH + CO₂

(7)

Early decomposition produces volatile compounds such as carbon dioxide (CO₂), carbon monoxide (CO), and various organic volatiles. The release of these gases can lead to the breakdown of the polymer backbone and may affect the structural integrity of the material.

(R-NH-CO-O-R’)n → CO₂ + CO + volatile fragments

(8)

3.3.2. Depolymerization and Fragmentation

The PU bonds break, resulting in the formation of smaller fragments or monomers. This process is usually accelerated by high temperatures. This breakage reduces the molecular weight of the polymer, resulting in reduced mechanical properties and the formation of more reactive intermediates [66].

(R-NH-CO-O-R’)n → (R-NH-CO-O-R’)x + small fragments

(9)

The urethane bond breaks down into isocyanate and alcohol. Reactions with other degradation products may further complicate the situation. Isocyanates are extremely reactive and may participate in further degradation reactions or generate toxic fumes.

R-NH-CO-O-R’ → R-NCO + R’-OH

(10)

3.3.3. Cross-Linking and Char Formation

As the material decomposes, some fragments may form cross-links, resulting in a more durable charred layer. This is usually the result of a reaction between the remaining polyols and isocyanates [67,68,69,70].

R-NCO + R’-OH → R-NH-CO-O-R’ (cross-linked structure)

(11)

Cross-linking can form a protective char layer, slowing down further degradation.

However, it also helps to form a dense carbon-rich residue. Carbonization occurs at high temperatures and the polymer forms a carbon-rich char residue.

(R-NH-CO-O-R’)n → Char + CO₂ + other gases

(12)

The formation of char provides some thermal insulation, but it can also affect the performance and safety of the material.

3.3.4. Further Decomposition and Residue Formation

The charred residue can further decompose into simpler compounds at extremely high temperatures. The residual char can continue to degrade, possibly leading to the formation of more harmful gases [71].

Char → CO + CO₂ + other volatile residues

(13)

If additives or fillers are present, they may form inorganic residues such as metal oxides or salts. These inorganic residues influence the final composition of the degradation products and may affect the post-degradation properties of the material [72,73,74,75,76].

Additives → Metal Oxides + Other Residues

(14)

3.4. Biodegradation

PU is not a completely degradable material, but it has a certain degree of biodegradability. Biodegradation is primarily a mechanism of polymer chain rupture, which can be defined as oxidative cleavage under pressure. The biodegradation mechanism of PU is a complex process [8]. The structural characteristics of PU, especially the composition ratio of hard segment and soft segment, directly affect its degradation behavior in the biological environment. The hard segment forms a stable microdomain in the material, which can effectively prevent the erosion of enzymes and delay the biodegradation process through strong hydrogen bonding and cohesion. However, with the increase in hard segment content, the mechanical stability of the material increases, and the biodegradation rate decreases accordingly. The aggregation structure of the hard segment plays a key role in protecting the material from enzymatic hydrolysis.

3.4.1. Fungal Biodegradation

The degradation enzyme system is determined by the microscopic physicochemical properties of polymer molecules, such as molecular orientation, crystallinity, cross-linking, and the chemical groups present in the molecular chains. Differences in degradation patterns are mainly attributed to topological structure and chemical composition [77]. PU degradation occurs in a selective manner, with amorphous regions degrading before crystalline regions. The regularity of synthetic polymers allows polymer chains to easily pack, thereby forming crystalline regions. This limits the accessibility of polymer chains for degradation, and amorphous regions on PUs are more susceptible to degradation [78].

3.4.2. Bacterial Biodegradation

Bacterial biodegradation is of great significance in the treatment and resource recovery of PU waste. Studies have shown that a single bacterial isolate cannot grow on PU independently, but different isolates can effectively degrade polyester-based PU through synergistic action. Physical tests showed that bacterial action significantly reduced the tensile strength and elongation of PU, indicating that its structure was disrupted. The degradation mechanism involves the binding of bacteria to the PU surface, forming flocs and further decomposing the substrate. However, the addition of carbon sources such as glucose inhibits the production of certain degrading enzymes (such as esterases), thereby affecting the degradation efficiency [79].

In anaerobic environments, the degradation ability of bacteria is particularly prominent, which is attractive for the treatment and conversion of solid waste (such as PU) into potentially useful products. By screening strains with proteolytic and lipolytic activities, it was found that a variety of anaerobic bacteria (such as Clostridium perfringens, Butyrivibrio fibrinolyticus, Ruminal Prevotella, anaerobic Vibrio, etc.) can effectively degrade PU [80].

Among them, anaerobic Vibrio lipolytica showed significant degradation ability, forming a transparent area on the PU culture medium, which indicated that PU was successfully decomposed. Bacterial biodegradation effectively destroys the molecular structure of PU through steps such as cell binding to PU, the action of degrading enzymes, and substrate decomposition. In the future, more anaerobic bacteria with PU degradation activity may be discovered, providing more efficient microbial solutions for solid waste management and resource utilization.

3.4.3. Degradation of PU by Polyurethanes Enzymes

The mechanism and characteristics of the PU degradation process make it an important tool for treating PU waste [81]. Compared with water-soluble substrates, the degradation of insoluble substrates such as PU faces the challenge of low contact efficiency. To overcome this obstacle, enzymes that degrade insoluble substrates usually have unique structural and functional properties that enable them to adsorb on the PU surface and perform catalytic reactions [82].

Studies have shown that polyurethane degrades PU through a two-step reaction. Firstly, the enzyme adsorbs on the hydrophobic surface of PU through its hydrophobic surface binding domain (SBD). Secondly, the catalytic domain mediates the hydrolysis of ester bonds, breaking down PU into small molecular products. The presence of SBD is crucial for the binding and degradation of enzymes to PU. This feature has been similarly observed in other enzymes that degrade insoluble polyesters, such as PHA depolymerase, showing the extensive role of SBD in substrate binding and catalysis [83].

Two types of PU esterase have been isolated: cell-associated membrane-bound PU esterase and soluble extracellular PU esterase. Membrane-bound PU esterase is attached to the cell membrane, allowing bacterial cells to directly contact insoluble PU substrates [84]. This cell-mediated contact mechanism reduces the competitive pressure with other cells and improves metabolic efficiency. The soluble extracellular PU esterase is able to diffuse freely and adhere to the PU surface and then decompose the polymer into smaller units by hydrolysis [85,86]. The soluble products produced by the decomposition facilitate further metabolism while enabling the enzyme to act more efficiently on the partially degraded PU.

Through this synergistic effect, PU not only promotes the degradation of PU but also improves the efficiency of bacterial substrate utilization. This mechanism provides an important microbial solution for the treatment of PU waste and also demonstrates the potential for improving PU through enzyme engineering to further optimize PU degradation efficiency.

4. Applications

4.1. Global PU Market

With the global emphasis on sustainable development, green products are increasingly replacing traditional plastics, and the use of environmentally friendly materials continues to expand. The rapid growth of bio-based PU is driven by its exceptional performance, particularly its ultra-high functionality and lightweight properties, which stand out when compared to inorganic materials. Unlike traditional construction materials such as cement, which contribute to significant carbon dioxide emissions, bio-based PU offers a more sustainable alternative. Over the next decade, the PU market is projected to grow by at least 6% [87]. To address this global shift, research on bio-based and sustainable PU focuses on advancements in production processes and performance evaluations relative to conventional materials. Key application areas include PU foams, coatings, sealants, concrete systems, adhesives, and road construction (as illustrated in Figure 6) [3].

The design of functional PU chemical structures promotes the development trend for more application scenarios. Different molecular structures and functional-induced designs are promising. Self-repairing PU, antifouling PU, antibacterial PU, shape memory PU, and electromagnetic shielding PU all provide more possibilities for future human life [88].

In the global industrial use survey, automobiles and consumer goods are another major consumer application. This is due to the sustainability of PU’s life cycle, involving synthesis, processing, and waste treatment. The environmental issues faced by PU plastics with targeted chemical and mechanical properties have been resolved, which will be an opportunity for wider global use of PU [89].

4.2. Commercial Application

4.2.1. Construction Industry

Generally, polymers are affected by humidity, temperature, and other conditions in different construction environments and curing environments. However, the excellent performance of PU elastomers will give cushioning performance and energy absorption status during application [90]. The construction industry has used PU materials in different forms, mainly in waterproof projects, such as waterproof coatings and sheet waterproof coatings. PU polymer materials are more widely used in frame construction, such as parking lot floors, building exterior wall insulation materials, insulation materials for hot and cold-water pipes, and landscape vegetation protection materials [91,92]. WPU floor coatings can even be combined with the mechanical properties of nanofibers to maintain the decorative, practical, and solvent-resistant properties of the coating [93].

4.2.2. Automotive Industry

In addition to the innovation of PU products in daily life, PU coatings can also be used in the high-end aerospace industry and automobile. By adding nanocomposites, the anti-corrosion, FR, and mechanical properties of PU coatings can be greatly improved [94]. In addition, advanced polymers have been used in both internal and external systems of automobiles, including engine components, exhaust systems, and decorative accessories. New bio-based materials of PU have successfully replaced petroleum-based polymers and have the potential to perform even better under the action of modifiers [95].

4.2.3. Artificial Leather

The main raw material of artificial leather and leather industry is PU, which is used in various types of velvet-free and imitation leather [96]. PU with elastomer or soft segment structure has the advantages of water resistance and aging resistance [97]. The innovative synthesis process uses a combination of mechanical foaming, scraping, and baking processes to achieve soft and comfortable leather [98]. The potential for PU in a wide range of industrial and daily applications is achieved by adjusting its chemical structure. Systems based on carbonate polyols and chain extenders have particularly demonstrated excellent mechanical properties, making them ideal materials for the preparation of wet artificial leather. This artificial leather has good uniformity and high tensile strength, making it suitable for high-end textile and decorative purposes [99]. In addition, the application of waterborne PU (WPU) coatings in leather processing has also gradually increased. WPU coatings can not only be used for the finishing of leather products but also achieve the flame retardancy of PU, making it perform well in special application scenarios [100]. Compared with traditional solvent-based coatings, WPU’s environmental friendliness and low volatile organic compound emission characteristics make it a greener choice.

4.2.4. Industrial Manufacture

Among the polymers used in sealants, adhesives, and coatings, PU is preferred because of its good adhesion and resistance to abrasion and shear [101]. Traditional sealants are based on polyisocyanate cross-linking systems. At the same time, the modification of PU sealants gives them more functions and superior performance, which enables them to unleash great potential in application scenarios such as civil engineering, transportation, and manufacturing [102]. Structural PU adhesives are widely used for bonding industrial products, forming a three-dimensional polymer network by combining chemical cross-linking through covalent bonds and physical cross-linking through intermolecular interactions. In addition to the chemical properties of isocyanates and isocyanate-reactive ingredients, the scientific concepts of bio-based raw materials and thermoreversible systems are introduced to focus on aging and reliability product design [103].

4.2.5. Medical Industry

Lignin-based PU elastomers have self-healing capabilities due to the combination of Diels–Alder bonds and hydrogen bonds and have good stability and application prospects in biosensors and smart skin [104]. PU with a block copolymer structure that is biocompatible and blood-compatible can be used as a carrier to control the release of drugs in the human body [105]. Low-viscosity WPU has also been developed as high-performance strain sensors, which show a strain factor of more than 960, a sensing range of 90%, and a detection limit of 0.5% strain. This strain sensor has a wide range of applications and has good durability and human motion detection capabilities [106].

5. Fire Retardancy

5.1. Material Fabrication

PU materials mainly include liquid PU and granular PU. Liquid PU is used in foaming technology and casting technology, such as the manufacture of PU foam and one-piece products. Solid PU is produced by extrusion, calendaring, etc., under specific environmental conditions and is suitable for products that require high elasticity and toughness. More types of application forms in Figure 7 are based on functional improvements of liquid and solid states [107].

Among them, adding FRs is a common fire prevention method. FRs can form a protective film at high temperatures, slow down the spread of flames, and improve the fire resistance of materials. Another method is used for most plastic products, foam materials, or surface materials, that is, using FR coatings. The fireproof layer can isolate oxygen and flames to achieve the purpose of fire prevention.

5.2. Fire Retardants

Flame-retardant properties are enhanced by flame retardants in various ways, and functional additives make flammable polymers flame retardant at different levels. The FR mechanism can be simply explained as heat absorption, covering, chain reaction inhibition, asphyxiation of non-flammable gases, etc. FRs have been developed for many generations from the last century to the current industrial development of PU materials. FRs can be divided into halogen, phosphorus, nitrogen, phosphorus–halogen, and phosphorus–nitrogen according to the different FR elements contained. FRs can be divided into three categories considering the different components contained: inorganic salts, organic, and organic–inorganic mixed. In the most important polymer synthesis process, FRs can be divided into additive, reactive, and synergistic types according to different methods of use. Conventional and innovative FR classifications are summarized with some typical examples in

Table 5. Classifications of fire retardants and mechanisms in PU.

Classifications	Representative Materials	Protection Mechanism	Ref.
Reactive FR	Phosphorus and nitrogen FR	Covalently bonded to the substrate to form a FR structure.	[14,15,16]
	Ammonium polyphosphate (APP) Pentaerythritol phosphate	Introduction of phosphorus/nitrogen monomers during polymerization.	[108,109,110]
Non-Reactive FR	Aluminum hydroxide (ATH) Magnesium hydroxide (MH)	The FR is physically mixed into the PU system and decomposes to produce water vapor when heated.	[111,112]
	Pentabromodiphenyl ether (PBDE) Antimony trioxide (Sb2O3)	Halogenated compounds decompose endothermally, releasing harmful gases to suppress flames.	[113]
Encapsulation and Coating	Phosphate coating Siloxane coating	Coating insulation and oxygen insulation.	[114,115]
	Nano-silicon dioxide (SiO2) Nano-montmorillonite (MMT)	Nanomaterials improve surface thermal stability.	[116,117]
FR Structural Units	Aromatic polyester Epoxy resin	Non-flammable structural units affect the thermal decomposition path of materials.	[118]
Nanomaterial Modification	Nanographene Nano-clay Carbon nanotubes	Nanomaterials in matrix improve heat resistance and oxygen isolation.	[119,120]
	APP/nano-silicon dioxide composite material	The additional synergistic effects brought by nanoparticles produce multiple flame-retardant mechanisms.	[121]

.

Intrinsic flame retardancy is a technology that directly integrates flame-retardant functions into polymer molecular chains through chemical structure design. Compared with traditional physical blending methods, it has higher stability and long-lasting flame-retardant effects. FRs are divided into non-reactive flame retardants and reactive flame retardants according to the method of use. Reactive FRs participate in the polymerization reaction of polymers as a monomer so that the material matrix itself contains FR ingredients. Its advantages are that it has little effect on the performance of polymer materials and long-lasting flame retardancy. Additive FRs are added to polymers by mechanical mixing to make them FR. They mainly include organic flame retardants and inorganic flame retardants, halogen FRs (organic chlorides and organic bromides), and non-halogen [122,123]. Organic FRs are represented by bromine, phosphorus–nitrogen [124], nitrogen, red phosphorus, and compounds, while inorganic FRs are mainly antimony trioxide, magnesium hydroxide, aluminum hydroxide, silicon, and other FR systems.

FR technology encompasses a wide range of approaches designed to enhance the fire resistance of materials. In addition to the invention of specific flame retardants, the methods of applying flame retardants are also diverse. Encapsulation and coating methods involve applying flame retardants in a protective layer around the material, effectively isolating it from heat and oxygen sources to prevent or delay combustion [125,126,127]. This approach not only protects the material but also changes its surface properties to inhibit ignition and flame spread. FR building blocks are components embedded in the polymer matrix and are designed to disrupt the combustion process at the molecular level. These units can chemically react in a fire to release non-flammable gases, dilute combustible gases, and interrupt the chain reaction of combustion [128]. By changing the chemical pathways for ignition and combustion, FR building units enhance the overall fire resistance of the material [129]. Nanomaterial modification represents a cutting-edge approach in flame-retardant technology, using the unique properties of nanoparticles to impart flame retardancy [130]. Nanoparticles such as metal oxides or carbon nanotubes are dispersed in a polymer matrix, where their high surface area-to-volume ratio and reactive surface interact with combustion byproducts [131,132]. This interaction can catalyze chemical reactions that inhibit flame propagation and reduce smoke emissions, significantly improving the fire resistance of the material.

5.2.1. Halogen-Free FR

The flame-retardant mechanism of organic flame retardants varies with different components. Halides can inhibit the basic reaction of polymer combustion and dilute the combustible gas to achieve the purpose of flame retardancy. The flame-retardant mechanism of phosphides is to consume the decomposition gas during polymer combustion and promote the formation of non-combustible carbides. This process prevents the oxidation reaction from proceeding to inhibit combustion. With the introduction of the concept of environmental friendliness, the development of polymers has paid more attention to the research on phosphorus halogen flame retardants.

Phosphorus-based FRs have emerged as a widely accepted, environmentally friendly alternative to traditional halogenated FRs due to growing concerns about toxicity and environmental impact. These halogen-free FRs not only enhance material fire resistance but also offer additional benefits, including reduced smoke production and the absence of toxic combustion byproducts [133]. Key phosphorus-based compounds used as FRs include phosphates, red phosphorus, phosphonates, phosphinates, and alkyl or hydroxyalkyl phosphonamides [134].

One of the main mechanisms of phosphorus-based FRs is their dual action in the condensed and gas phases. During thermal decomposition, these compounds release phosphoric acid or polyphosphoric acid, which promotes the formation of a protective char layer on the material surface. In the gas phase, phosphorus-based compounds inhibit flame propagation by interacting with free radicals, making them a highly effective alternative to halogenated systems.

Phosphorus–nitrogen (P-N) FRs are particularly attractive due to their synergistic effect, which can significantly improve fire resistance. Polymers can be endowed with unique polymer cross-linking structures by phosphorus-containing curing agents, forming unique flame retardancy [135,136]. The most commonly used phosphorus-based FR APP, which decomposes when heated to form a foam-like, heat-stable barrier that further protects the material by preventing the spread of fire [137]. An innovative example of a halogen-free FR 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO), a phosphorus-based compound [138]. Unlike halogenated systems, DOPO effectively traps phosphorus in a char layer, thereby reducing toxic gas emissions during combustion. However, its addition to certain polymers can reduce key mechanical properties such as glass transition temperature. Studies have shown that DOPO-derived groups contribute to the condensed phase flame-retardant effect [139]. Therefore, the design strategy of flame retardants with various elements such as phosphorus groups has enriched the design strategy of flame retardants towards safe and high-strength polymers.

Halogen-free phosphorus-based FRs represent an environmentally sustainable and highly efficient solution for improving fire resistance in materials. By acting in both the condensed and gas phases, these compounds offer strong flame-retardant properties while addressing environmental and safety concerns, making them a critical focus in the advancement of next-generation fire-retardant technologies.

5.2.2. Inorganic Metal Hydroxides

Inorganic metal hydroxides, such as ATH and MH, are widely used as FR fillers in polymers due to their non-toxic nature and ability to reduce flammability and smoke generation [140]. These FRs work through an endothermic decomposition process, where heat causes them to release water vapor [141]. Simultaneously, this vapor dilutes the combustible gases and cools the material, slowing combustion. In addition, the decomposition process forms metal oxides, such as aluminum oxide or magnesium oxide, which form a protective barrier on the polymer surface and inhibit the spread of flame. Due to its low decomposition temperature, ATH is often applied to flame retard elastomers, thermosets, and thermoplastics. MH is more suitable for high-temperature polymers, such as polyphenylene ether (PPO) and polypropylene–polyamide (PP-PA) blends [142,143].

Although inorganic metal hydroxides are very effective, they also have some limitations [144]. ATH has a low decomposition temperature, which limits its application to materials processed at lower temperatures [145]. In addition, higher loading levels are usually required to achieve the desired FR properties. This can have an adverse effect on the mechanical properties and processing of polymeric materials [146].

Recent advances have introduced tin–zinc compounds such as zinc stannate (ZS) and zinc hydroxystannate (ZHS) that have enhanced flame retardancy and smoke suppression effects [10]. These compounds form an effective surface barrier that prevents heat transfer and further enhances the flame retardancy of the material. In addition, coating traditional fillers such as ATH and MH with ultrafine particles of ZHS or ZS has been shown to improve their synergistic effects. This approach not only reduces smoke generation but also reduces the amount of filler required, helping to maintain the mechanical properties of the polymer.

Inorganic metal hydroxides are environmentally friendly and effective FRs that address the safety issues associated with halogen-based systems. Their dual functionality in reducing flammability and reducing smoke generation makes them valuable across industries. Advances in novel additives ensure the continued relevance of inorganic metal hydroxides in modern technology.

5.2.3. Nanoparticles

Incorporating nanoparticles into FRs is an efficient and environmentally friendly way to improve the fire resistance of polymeric materials. Nanofillers, such as organoclays, zinc stannates, borates, colloidal silica, and zeolites, are more effective at enhancing thermal stability and fire resistance than traditional halogen or phosphate-based systems [11]. Polymer nanocomposites containing aluminosilicate nanofillers are environmentally friendly and produce minimal carbon monoxide and soot during combustion [141]. Even at low concentrations (3–5% by weight), these nanofillers improve material properties such as tear strength, compressive strength, thermal stability, and chemical resistance without causing discoloration or degradation of mechanical properties [10]. They can also produce transparent materials.

The superior fire resistance of nanocomposites comes from the formation of a durable, crack-free char layer when exposed to heat or flame. This layer isolates the polymer, limits the release of flammable gases, and delays ignition [147]. Nanocomposites also maintain structural integrity at high temperatures, further enhancing protective properties. Layered silicates, such as montmorillonite clays, are highly effective nanofillers, along with hectorite, talc, and synthetic fluorinated mica. These materials are stable at high temperatures and compatible with polymer matrices. Other promising fillers such as zinc stannates, borates, colloidal silica, and zeolites promote carbonization and form heat-resistant barriers [9].

Nanocomposites are easy to manufacture using standard methods such as calendaring and extrusion. They require less filler, simplify processing, and avoid the disadvantages of traditional FRs such as weight and complexity. FRs based on nanoparticles have made significant advances in fire safety. By combining low filler requirements, enhanced thermal properties, and environmental benefits, these materials offer versatile and efficient solutions for industrial applications.

5.2.4. Synergistic Effect of FR

The so-called “synergistic effect” can reduce the number of FR compounds required. This effect refers to the phenomenon that the combined effect of a mixture of two or more components on delaying combustion is greater than the sum of their individual effects. Achieving a synergistic system requires careful selection of the components [141]. It is important to remember that synergistic factors contribute to the activation of substances in specific action zones, which may act alone or together, and usually at different temperatures [148].

The synergistic effect of phosphorus-containing polyols (BHPP) and nitrogen-containing polyols on the FR properties of EG/rigid PU foams was investigated [149]. Melamine-derived polyol (MADP) is a nitrogen-containing polyol. BHPP and MADP were synthesized by dehydrochlorination and the Mannich reaction, respectively. Phosphoric acid or phosphoric acid produced by the decomposition of BHPP is catalytically converted into char under the catalytic action of phosphoric acid. At the same time, the chemical cross-linking reaction between BHPP/MADP and PAPI leads to the formation of $O=P-O-$ and triazine ring groups. In addition, $N{H}_3$ gas released by MADP can accelerate the expansion of the char layer in this process. This synergistic work protects the substrate from further thermal degradation during combustion.

Silicone-based FRs have become increasingly popular in recent years. Among them, organosilicon compounds are the most favored. These compounds contain, in addition to the methyl group, irregularly arranged long alkyl chains, cyanoalkyl groups or aromatic rings, as well as silica, organosilanes, and aluminosilicates [150]. These materials are useful as components or modifiers of polymers, copolymers, and polymer blends.

6. Green and Sustainable Development

During the production process, PU resin produces harmful substances that pollute the environment. At the same time, it consumes a lot of energy, such as water resources and non-renewable raw materials. PU resin is difficult to degrade after being discarded, which has a long-term impact on the environment [91]. Environmentally friendly PU polymers are being considered for development based on lignin-based polyols [100]. In addition, more and more environmentally friendly PU groups are being synthesized and applied, such as alicyclic polyols [151]. During the combustion process, PU resin produces toxic gases, which pose a threat to human health.

6.1. Non-Isocyanate PU

There are multiple drawbacks to the use of isocyanates in the PU industry. First, the reaction of isocyanates with water releases carbon dioxide, which increases greenhouse gas emissions. In addition, the synthesis process of isocyanates involves toxic phosgene, which poses serious health and safety risks. Long-term exposure to isocyanates (such as TDI and MDI) can cause skin and respiratory irritation and may even cause cancer, which makes isocyanates classified as “very harmful” chemicals. Therefore, in the context of the global promotion of sustainable development, the use of isocyanates faces more and more challenges [152].

To address these issues, non-isocyanate PU (NIPU) has received widespread attention as an environmentally friendly alternative. The synthesis of NIPU does not use isocyanates, avoids the generation of toxic by-products, and significantly reduces carbon dioxide emissions during the production process [5]. This green synthesis process is not only safer and more environmentally friendly but also greatly reduces the threat to workers’ health, which is in line with the development trend of global green chemistry. The synthesis route of NIPU is mainly completed through the ring-opening addition reaction of cyclic carbonates and polyamines. The process is mild and avoids the use of highly toxic substances.

Schematic diagram of the most common PU production routes: 1. Dialkyl (aryl) carbonate route. 2. Chloroformate route. 3. Cyclic carbonate route (addition polymerization). 4. Ring-opening polymerization. 5. Traditional isocyanate route. 6. Rearrangement. 7. Carbamoyl chloride route. 8. Polyurethanization (condensation polymerization). Non-isocyanate is synthesized through a non-toxic route (Figure 8), avoiding the high toxicity of isocyanate vapors, and is an environmentally friendly chemical reaction [13].

The environmental advantages of NIPU have given it broad application prospects in many fields. Its performance is comparable to that of traditional PU and even better in some respects. Especially in applications such as coatings, foams, and elastomers, NIPU not only improves the environmental sustainability of the material but also enhances its chemical and physical properties. With the continuous advancement of technology, NIPU is expected to become an ideal substitute for PU materials and play an important role in the future of green chemistry.

6.2. Foaming Process

The synthesis conditions of bio-based polyols have a significant impact on their physicochemical properties and molecular weight, which directly affects the performance and processing characteristics of future polyurethane materials. In this study, a bio-based polyol that can be used in the preparation of thermoplastic polyurethane elastomers (TPU) was successfully developed by synthesizing fully bio-based poly (propylene succinate) under different temperature conditions. Further combined with natural chain extenders (1,4-butanediol or 1,3-propylene glycol) and diisocyanates, a new type of fully bio-based TPU material was prepared [153]. The choice of chain extender has a greater impact on material properties than the synthesis conditions of bio-based polyesters. Bio-based polyols have great potential in the development of green materials, not only reducing dependence on fossil resources but also promoting the realization of sustainable development goals through high-performance renewable materials [154].

The production of flexible PU foams has traditionally relied on chemical and physical blowing agents to achieve their lightweight and flexible structures. Historically, chlorofluorocarbon (CFC)-11 (F11) was widely used as a physical blowing agent, but it has been largely replaced due to its high ODP [155]. Currently, methylene chloride is the main alternative [156]. While it has a lower ODP compared to F11, it is not completely ozone-safe (ODP ≠ 0). In recent years, the use of carbon dioxide as a physical blowing agent has become increasingly popular due to its zero ODP. In addition, the carbon dioxide used in this process is often a byproduct of other industrial operations, which promotes recycling and sustainability [157]. Carbon dioxide is also preferred because of its low global warming potential (GWP), which is an important factor when assessing environmental impact. GWP takes into account direct and indirect emissions of greenhouse gases, primarily carbon dioxide. As a sustainable alternative, carbon dioxide is environmentally friendly.

However, advanced processes such as variable pressure foaming and forced cooling foaming have also been developed to further improve production efficiency and reduce environmental impact [158]. These technologies are gradually being adopted by the environmental initiatives of the flexible PU foam industry.

The transition from rigid PU foam to sustainable alternatives is a gradual process. Initially, HCFC-141b replaced CFC-11 as a transitional blowing agent with a significantly lower ODP [159,160]. Cyclopentane, a hydrocarbon-based blowing agent, has emerged as an affordable and readily available alternative [161]. Cyclopentane has zero ODP and a much lower GWP compared to HCFC-141b, making it an environmentally friendly choice. However, cyclopentane also has some challenges, such as higher thermal conductivity of the final product and a higher risk of flammability, so strict safety measures are required during use [162].

Cyclopentane is more strictly regulated because it is classified as a volatile organic compound (VOC) [163]. This has prompted careful consideration of its application in foam production. Hydrofluorocarbons (HFCs) are considered ideal long-term alternatives because of their zero ODP and low GWP [164,165]. Among HFC alternatives, HFC-245fa and HFC-365mfc are particularly promising [166]. These substances, together with blends of other HFC compounds (e.g., HFC-227ea), offer optimized performance in insulation and structural applications [167]. However, their development process, including prediction, synthesis, toxicity testing, and industrial-scale production, is complex and time-consuming.

The flexible and rigid PU foam industries have advanced significantly in promoting sustainability through the implementation of eco-friendly blowing agents and innovative manufacturing techniques. The adoption of CO₂-based alternatives for flexible foams and HFC-based solutions for rigid foams underscores a shift toward environmentally conscious practices while maintaining high performance standards. This transition highlights the industry’s dedication to sustainable development and adherence to global environmental regulations.

7. Summary and Outlook

7.1. Summary

This review comprehensively explores polyurethanes, focusing on their development, FR properties, and contribution to sustainable development. As a versatile polymer, its adaptability is widely recognized. By selecting different raw materials and adjusting the formulation, polyurethane products can exhibit a range of properties, including different hardness, flexibility, tear resistance, and compressive strength, making them valuable in multiple industries. However, its inherent flammability remains a major challenge, requiring the development of advanced FR technologies to improve safety while retaining the polymer’s desirable properties.

This study combines material considerations with FR strategies, drawing on approaches applied to similar polymer systems. The potential of this high-value platform material for FR applications lies in its modifiable chemical structure. Adjustment through cross-linking, additive incorporation, and graft copolymerization can achieve customized properties to meet specific requirements. This adaptability, coupled with global efforts to reduce dependence on petroleum-based feedstocks and mitigate environmental impacts, makes polyurethanes a sustainable alternative in materials science.

FR mechanisms are key to addressing the flammability issues of polyurethanes. This review identifies key degradation processes involved to dehydration, dehydroxylation, polymer oxidation and residual oxidation, which affect polymer thermal stability and char formation. Chars are protective barriers to heat and gases and are essential for enhancing flame retardancy. Mechanisms that promote char formation, such as the integration of halogen-free FRs, phosphorus-based compounds, and nanocomposites, represent important advances in the flame retardancy of polyurethanes. The addition of metals such as calcium and nickel during the cross-linking process enhances thermal stability and catalyzes char formation, providing strong ionic cross-linked FR protection. Cross-linking with transition and rare earth metals is considered a key strategy to improve the flame retardancy of polyurethanes. The synergistic use of nanocomposites, including montmorillonite clay and zinc stannate, further enhances these properties by forming a protective layer that reduces heat transfer and gas release.

7.2. Outlook

This review explores the decomposition and remodeling of organic components of PU chemistry. The performance of PU is revealed by analyzing the bond types and elements in the monomer units. Sustainability is the main direction for the future development of PU. The industry’s shift from halogen FRs to more environmentally friendly alternatives is in line with global environmental goals. Sustainable FRs, such as bio-based phosphorus compounds and nanotechnology-driven systems, provide effective fire protection while reducing ecological impact. In addition, the adoption of green blowing agents such as carbon dioxide and HFCs further reduce the environmental impact of polyurethane production while maintaining performance standards.

In the future, the development of advanced bio-based flame-retardant systems, especially those derived from renewable resources such as lignin, tannins and bio-phosphorus compounds, can provide sustainable alternatives to traditional flame retardants. Studying their molecular design, thermal stability, and compatibility with polyurethane chemistry is essential to optimize performance while maintaining ecological benefits. Nano-engineering fusion polyurethane composites is an extremely innovative way of exploration, that is, tailoring the interface between nanomaterials and polyurethane to achieve uniform dispersion and maximize synergistic effects. Similarly, no matter which innovation method is inseparable from the theme of green global development. The development of environmentally friendly and energy-saving synthetic routes, including the use of non-toxic, bio-derived isocyanates and polyols, is essential to minimize the environmental impact of polyurethane production. On this basis, the systematic study of multifunctional flame retardants that not only provide flame retardancy but also impart additional functions (such as antibacterial properties, UV resistance or self-healing capabilities) will expand the applicability of polyurethane in advanced fields such as healthcare, electronics, and aerospace. By addressing these future research directions, PU materials can continue to develop to meet the growing demand for high performance, sustainability and environmental responsibility in various industrial and consumer applications.

In summary, this review highlights the remarkable versatility and adaptability of polyurethane as a material. By integrating advanced FR technologies such as synergistic systems and nanotechnology, polyurethane can achieve higher safety and environmental performance. As the industry pays more attention to sustainability and safety, polyurethane becomes a promising material for high-performance, environmentally friendly applications. The continued exploration of innovative FR systems and green alternatives ensures that polyurethane will continue to be a cornerstone of material science to meet changing market demands and environmental responsibilities.