Low Ecological Impact Lignin-Based Flame Retardants for Polyurethane Foams

Abstract

The flammability of polyurethane is a great safety hazard, threatening both lives and goods. Recognizing this, efforts to enhance the fire resistance of polyurethanes can be pursued through various routes. Depending on the classes and applications of polyurethanes, fire retardation can be achieved by incorporating flame retardants or modifying the polymer structure. In response to growing environmental concerns, lignin is an abundant and renewable resource, which has been employed to develop effective flame-retardant polyurethanes, with a simultaneous focus on reducing their ecological impact. Lignin, characterized by its aromatic and phenolic structure, naturally can act as a reactive fire retardant for polyurethanes. Nevertheless, diverse chemical modifications of lignin have been explored to further enhance its fire resistance. This review highlights advancements in the design of phosphorus- and/or nitrogen-containing lignin-based reactive flame retardants tailored for bio-based polyurethanes.

1. Introduction

Polyurethanes (PU) are polymers that consist of aliphatic and aromatic portions linked through carbamates in their backbone. The predominant method for producing PU involves the reaction of polyols with a diisocyanate. PU materials are utilized in various forms such as elastomers, coatings, adhesives, and foams, with extensive applications in construction, clothing, medical devices, and electronic equipment [1]. However, PU materials in their original state are generally highly flammable and easily ignited in the presence of a heat source and oxygen [2]. This susceptibility can lead to severe material and equipment damage, compromising their utility in fields where high flame retardancy and thermal resistance are essential [3]. The fire hazards associated with PU have prompted the introduction of legislation and safety standards concerning flammability, and extensive research has been conducted on flame retardants for PU materials [4].

The flammability of PU poses a significant safety hazard, endangering lives and property [5]. In light of this, the fire resistance of PU can be enhanced through various methods. Depending on the classes and application fields of polymers, fire retardation can be achieved using flame retardants or by altering the polymer structure. Currently, the primary methods for flame retardancy of PU include blending and addition [6], nanocomposites [7], and reaction grafting [8]. Many flame-retardant additive molecules, such as halogen-based compounds, have been banned due to their toxic effects and environmental unfriendliness [9, 10]. These halogen-based flame retardant compounds release toxic gases upon incorporation into the polymer matrix, posing risks to both human health and the environment when the materials burn. Consequently, flame retardants based on phosphorus, nitrogen, silicon, and boron elements have gradually replaced halogenated flame retardants [11].

Among environmentally friendly flame retardants (Figure 1), those derived from nitrogen and phosphorus have garnered much attention in industrial and academic communities [12, 13]. Some studies have reported that both fire performance and mechanical properties can be improved by incorporating halogen-free flame retardant nanoparticles, such as graphene [14], graphite nanoplatelets [15], boron nitride [16], and MXenes [17], into the PU system. The effectiveness of these additives is attributed to their higher surface area, reactivity, and better bonding with the PU matrix. With the growing emphasis on environmental and sustainability concerns, there is a push to design safer biobased flame retardants from natural compounds such as lignin and its derivatives, presenting a valuable challenge in PU manufacturing [18].

Figure 1.

Classification of ecologically friendly flame retardant systems.

Lignin, a multifunctional natural polymer, is a component of biomass isolated from hemicellulose and cellulose through industrial pulping processes. For a long time, it has been considered a waste by-product of the paper industry and biorefineries [19]. Lignin possesses a three-dimensional network structure and has become an excellent sustainable chemical raw material due to its biodegradability, renewability, environmental friendliness, and low cost [20].

The utilization of lignin as a biobased flame retardant for PU materials is typically achieved by adding it to polymeric systems through physical blending. This method consists of incorporating lignin into the PU resin at any stage of production and processing. The aromatic structure of lignin imparts impressive properties as a char promoter agent, which acts in the condensed phase and reduces the combustion rate in fire scenarios [21]. However, the addition of a small amount could significantly influence the mechanical properties and the nature of PU materials [20, 21]. The presence of various functional groups, including carboxylic acid, carbonyl, methoxy, and hydroxyl groups [22], makes lignin attractive for incorporating phosphorous and/or nitrogen functionalities through chemical functionalization methods [23, 24]. This chapter provides a comprehensive review of the chemical modifications of lignin for effective halogen-free flame retardants, aiming to highlight advancements in the design of phosphorus- and/or nitrogen-containing lignin-based reactive flame retardants tailored for biobased PUs.

2. Lignin-based reactive flame-retardants

Lignin can be valorized as a biobased flame retardant with or without preliminary structural chemical modifications. However, preliminary chemical modifications assist lignin in incorporating more active sites that undergo additional reactions to produce value-added biobased polymers. For instance, chemical, biological, and thermo-catalytic routes are capable of achieving the demethylation of lignin (Figure 2) by cleaving aryl-ether linkages [25, 26]. The natural methoxyl groups are replaced with hydroxyl groups in the lignin macromolecule, rendering it more hydroxylated and reactive with a lower molecular mass distribution. Hydroxyl groups in lignin play a pivotal role in the solubility and reactivity of lignin [27]. Additionally, they are easily transformable into new chemical functional groups, expanding the potential of lignin in the continuous production of bio-based polyurethanes [25, 28].

Figure 2.

Scheme illustration of lignin demethylation.

Reactive flame retardancy, also known as intrinsic flame retardancy, can be achieved through various approaches. These approaches primarily involve modifying existing lignin macromolecules by chemically grafting active functional segments onto the lignin backbone or adjusting the structure with potentially active fire retardant functions before the foaming process. The flame retardancy of PU via lignin-based reactive flame retardants involves introducing lignin-containing flame retardant elements, such as phosphorus or nitrogen, into the polyurethane molecular chain through chemical bonding [29]. The development of lignin-based flame retardants presents a sustainable pathway to broaden the application of flame-retardant PU.

One advantage of reactive flame retardants is their ability to be permanently bonded to the main chain. In fact, a relatively low amount of lignin-based reactive flame retardants can achieve a similar effect to that achieved with relatively high loads of additive flame retardants [3]. Conversely, when the preparation of lignin-based reactive systems is not achieved in situ, it could be more expensive and time-consuming. This is because it requires the development of a novel polymer with specific chemical and physical properties.

Based on the differences in flame retardant elements, sustainable lignin-based flame retardant polyurethane can be categorized into phosphorus-containing, nitrogen-containing, and phosphorus- and nitrogen-containing flame retardant PU.

3. Lignin-based phosphorus-containing flame retardants

3.1 Phosphorus-containing flame retardants reactivity

Phosphorus-based flame retardants are the most widely used alternatives to halogen-based flame retardants [9, 12]. Their reactivity in the condensed phase leads to the production of phosphoric acid during the thermal dissociation and decomposition process of the flame retardants. This process, through the rejection of water, produces phosphate compounds. The release of water dilutes the combustible gases. Additionally, they can further act in the condensed phase by catalyzing char formation [10]. This solid residue protects the PU by isolating the non-burned material from the heat, oxygen, and flames. It hinders the volatile products from reaching the flame and feeding it. Phosphorus can take on many oxidation states, as observed in phosphine oxides, phosphines, phosphonium compounds, phosphonates, phosphates, phosphites, and elemental red phosphorus, demonstrating the diverse modes of action of lignin-based phosphorus-containing flame retardants. There is no doubt that the best flame retardants work in more than one way and in more than one phase.

Phosphorus-containing flame retardants from lignin can form volatile active radicals in the gas phase, such as PO2*, PO*, and HPO*, which readily scavenge hydroxyl radicals (OH*) or hydrogen radicals (H*) from the gas phase to inhibit combustion. The mechanism of radical scavenging by phosphorus can be summarized in Eqs. (1)–(3) [30]. The most abundant phosphorus radicals and compounds in the flame are HPO2*, HPO*, PO2, and PO.

$$\mathrm{H}\mathrm{P}{\mathrm{O}}_2{}^{\ast }+{\text{H}}^{\ast }\rightleftarrows \mathrm{P}\mathrm{O}+{\text{H}}_2\text{O}$$E1

$$\mathrm{H}\mathrm{P}{\mathrm{O}}_2{}^{\ast }+{\text{H}}^{\ast }\rightleftarrows \mathrm{P}{\mathrm{O}}_2+{\text{H}}_2$$E2

$$\mathrm{H}\mathrm{P}{\mathrm{O}}_2{}^{\ast }+\mathrm{H}{\mathrm{O}}^{\ast }\rightleftarrows \mathrm{P}{\mathrm{O}}_2+{\text{H}}_2\text{O}$$E3

3.2 Functionalized lignin as phosphorus-containing flame retardants

The phosphorylation reaction of aliphatic and phenolic hydroxyl groups is a common method for introducing phosphorus into lignin (Figure 3). This process enhances the thermal stability of lignin and facilitates the production of renewable flame retardants for PUs [31]. The thermal decomposition of phosphorylated lignin yields a high residual char through the rearrangement of unstable radicals. The phosphorylation mechanism was elucidated by Qin et al., who investigated the structural sensitivity of softwood lignin during phosphoric acid treatment [32]. They demonstrated that α-O-4 and β-O-4 linkages are prone to breaking during the phosphoric acid–acetone pretreatment process. Phosphorylation of α-O-4 and β-O-4 occurs through a two-step reaction mechanism during the acid treatment. Furthermore, Basso et al. enhanced the hydrophobicity of lignin by employing the phosphorylation reaction on desulfurized kraft lignin using triethyl phosphate at elevated temperatures (90–220°C) [23]. The products resulting from the phosphorylation reaction served as excellent raw materials for the production of lignin-based heat-resistant thermosetting resins.

Figure 3.

Scheme illustration of lignin phosphorylation.

In a separate study, Gao et al. systematically synthesized phosphorylated kraft lignin using an eco-friendly process that utilized ammonium dihydrogen phosphate (NH₄H₂PO₄) as a green and sustainable phosphating reagent [31]. The reactive aliphatic hydroxyl and phenolic groups of lignin underwent a nucleophilic substitution reaction with the phosphate group. The introduction of phosphorus groups into lignin molecules increased the maximum decomposition temperature to 620°C. Prieur et al. developed a straightforward synthetic route for lignin phosphorylation [33], achieving the phosphorylation process using phosphorus pentoxide (P2O5) as the phosphating agent. Phosphate groups were covalently bonded to the complex structure of lignin, promoting dehydration and decarboxylation reactions. Consequently, this significantly increased the amount of residue char.

Zhang et al. devised a pathway for incorporating phosphorus into lignin macromolecules. They synthesized flame retardants containing phosphorus through the chemical reaction of lignin, hexamethylene diisocyanate (HDI), and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) [34]. HDI serves as a bridge to connect lignin with DOPO, as illustrated in Figure 4. The resulting flame retardant, based on lignin, features a more O∙C∙P∙O conjugated structure, contributing to enhanced flame retardancy and thermal stability. Thermogravimetric analyses (TGA) revealed that the functionalized lignin acts as a proficient char-forming agent, increasing the residual carbon content to 16.55% at 600°C. When these lignin-based flame retardants are employed in the production of flame-retardant PU, the limiting oxygen index (LOI) value of the polyurethane reaches 30.2% with the addition of 15% phosphorus-containing lignin.

Figure 4.

Schematic illustration of lignin-HDI-DOPO synthesis.

In a similar study, Wang et al. synthesized a phosphorus-containing lignin-based flame retardant through in-situ reactions involving lignin, diphenylmethane diisocyanate, and DOPO [29]. The PU produced in a two-step process exhibited excellent flame-retardant properties, evidenced by a gradually increasing LOI value that reached 29.9% with a phosphorus-containing lignin content of 30%. Furthermore, the residual carbon content significantly increased with the phosphorus-containing lignin, reaching up to 34.9% at 700°C.

4. Lignin-based nitrogen-containing flame retardants

4.1 Nitrogen-containing flame retardants reactivity

Nitrogen-containing flame retardants encompass a diverse array of condensed-phase and vapor-phase mechanisms that determine flame retardant reactivity. This reactivity is contingent upon both the flame retardant structure and the polymer chain. When PU materials undergo combustion, various species capable of reacting with atmospheric oxygen are formed. Consequently, the propagation of combustion primarily occurs through the reactions outlined in Eqs. (4)–(6).

Nitrogen-containing flame retardants act chemically in the gas phase to scavenge free radicals generated as a byproduct of the thermo-oxidative decomposition of polymer. However, the reaction linked to Eq. (6) is the primary exothermic reaction and provides the most energy to sustain the flame. Hindering the chain-branching reactions related in Eqs. (4) and (5) can slow down or completely stop combustion. Some nitrogen-containing flame retardants can also demonstrate a physical mode of action in the gas phase that either generates a significant amount of non-combustible gases, diluting the flammable gases, or undergo endothermic dissociation, reducing the flame temperature [35]. For instance, nitrogen-containing flame retardants endothermically decompose into incombustible gases, such as nitrogen oxide or ammonia, without smoke. They act by providing both a heat sink and large amounts of inert diluents for the burnable gases in the flame.

$${\text{H}}^{\ast }+{\text{O}}_2\rightleftarrows \mathrm{H}{\mathrm{O}}^{\ast }+{\text{O}}^{\ast }$$E4

$${\text{O}}^{\ast }+{\text{H}}_2\rightleftarrows \mathrm{H}{\mathrm{O}}^{\ast }+{\text{H}}^{\ast }$$E5

$$\mathrm{H}{\mathrm{O}}^{\ast }+\mathrm{C}\mathrm{O}\rightleftarrows \mathrm{C}{\mathrm{O}}_2+{\text{H}}^{\ast }$$E6

4.2 Functionalized lignin as nitrogen-containing flame retardants

Elemental analysis reported in the research work of Ge et al. showed that nitrogen is not an abundant element in the composition of lignin [36]. The most common chemical modifications used to incorporate nitrogen into lignin include nitration and amination.

4.2.1 Nitration of lignin

The nitration reaction of lignin is a straightforward chemical modification process typically conducted in non-aqueous solvents utilizing nitrating agents (see Figure 5). Well-established nitrating agents encompass nitric acid in concentrated acetic acid or sulfuric acid [37]. The resulting nitrated lignin is recognized as an amorphous powder with a color spectrum spanning from yellow to brown. Nitration results in the incorporation of approximately 7 wt% of nitrogen into the lignin structure [38].

Figure 5.

Scheme illustration of lignin nitration.

Veshnyakov et al. optimized lignin nitration using nitric acid in binary mixtures of water and aprotic solvents, such as acetonitrile, dimethyl sulfoxide, 1,4-dioxane, tetrahydrofuran, and dimethylformamide. They employed a nitrating agent formed by mixing acetic acid and the anhydride of nitric acid to produce nitrolignin with a nitrogen content of 4.7% [37]. In another study, Zhang and Huang investigated the thermal stability of lignin-based PU by incorporating nitrolignin into an interconnected network, forming a star-like PU from castor oil and diisocyanate [38]. They observed a slight improvement in the properties of the nitrolignin-incorporated systems. Additionally, nitrolignin was employed as additives or chain extenders after the emulsification stage for the preparation of waterborne PU materials [39]. The reaction between isocyanate groups and hydroxyl groups in nitrolignin produced a crosslinked network in the polyurethane, resulting in a strength of 71.3 MPa.

4.2.2 Amination of lignin

Amination of lignin has been investigated through the Mannich reaction (see Figure 6), typically conducted using diethylamine and formaldehyde. The amination of lignin can be integrated into PU synthesis as a reactive precursor or filler to enhance the properties of the composites. Zhou et al. synthesized polyhydric aminated lignin through hydroxylation and amination [40]. The resulting lignin possessed primary amine groups, with a nitrogen content ranging from 0.64% to 4.58%. Polyurethane foams (PUF) prepared from aminated lignin polyol exhibited improved thermal stability and performance compared to those derived from PU using native soda lignin.

Figure 6.

Scheme illustration of lignin amination through Mannich reaction.

On the other hand, Huo et al. generated PUF by blending aminated lignin-based polyol, polyethylene glycol, and diphenylmethane diisocyanates in the presence of water as the blowing agent [41].

The utilization of amines to incorporate nitrogen into the lignin chain presents an intriguing avenue for enhancing the reactivity of lignin. Upon the introduction of amino groups into the lignin structure, its water solubility experiences a direct enhancement facilitated by improved hydrogen bonding and reactivity with isocyanate [42]. Furthermore, aminated lignin can serve as intermediates during the functionalization of lignin. In acidic environments, amino groups readily ionize, acquiring a positive charge, thereby rendering lignin more reactive in aqueous solutions [43].

Amino groups possess the capability to convert hydrophobic kraft lignin into a highly hydrophilic polymer, thereby augmenting certain properties such as foamability, emulsifying properties, aging resistance, flame-retardant properties, and mechanical strength [44].

5. Lignin-based phosphorus-and nitrogen-containing flame retardants

Most high-efficiency lignin-based flame retardant can be prepared by integrating multiple elements, such as nitrogen, phosphorus, and silicon, in a single flame retardant system [45]. Nitrogen-based flame retardants are very often used as synergists for phosphorus-based flame retardants. They also form an effective class of flame retardants but are treated separately. Some studies have reported that PUs prepared from phosphorus- and nitrogen-containing lignin-based flame retardants offer the most benefits for protecting the environment, as well as improving thermal stability and mechanical properties [9, 13].

Zhu et al. incorporated both phosphorus- and nitrogen-containing functional groups into lignin using a three-stage process, including liquefaction, esterification, and salt formation. Lignin-based flame retardants were prepared through the chemical grafting of phosphate groups and melamine. The lignin-based phosphate-melamine product was then employed to prepare a lignin-based flame retardant for the modification of PUF [46]. After copolymerization between the remaining hydroxyl groups of functionalized lignin and isocyanate, the resulting lignin-modified PUF exhibited excellent flame retardancy in both gas and condensed phases through the escalation of smoke suppression and the increment of thermal properties. The flame-retardant PUF had a LOI of 26.7% and a V-1 rating, as measured by the UL-94 vertical burning test. The presence of rigid aromatic rings in the lignin structure and the covalent bonds between modified lignin and the PU matrix are responsible for self-extinguishment, char residue formation, and inhibition from melt-dripping.

He et al. developed another process by combining mild grafting reactions with the ultrasound method to produce lignin nanoparticles containing phosphorus and nitrogen moieties [21]. The aim was to incorporate functionalized lignin nanoparticles into the formulation of PU so that they could act as both flame retardants and crosslinking agents. Good dispersibility and compatibility in the PU matrix lead to strong tensile properties. The LOI of the resulting PU reached a value of 29.8%, and it received a V-0 rating by adding 10 wt% of lignin-based phosphorus- and nitrogen-containing nanoparticles. Moreover, cone calorimetry tests on PU materials showed that the addition of nanolignin significantly reduced some important parameters during combustion, such as heat release (HR) rate, total HR, and total smoke production rate. The char residues of nanolignin-incorporated PU exhibit high oxidation resistance. The quenching and barrier effects of the char layer improved the flame retardancy performance of lignin-based PU.

Lu et al. investigated the flame retardancy of rigid polyurethane foams (RIPUF) from the lignosulfonate/ammonium polyphosphate intumescent flame-retardant system [47]. The preparation of lignosulfonate-based RIPUF was carried out by copolymerization between lignosulfonate macropolyols and isocyanate. Foams containing 15 wt% of lignosulfonate gave rise to the best thermal stability. They exhibited a lower HR rate (21 kW.m⁻²) and a total HR value of 13 MJ.m⁻². The incorporation of lignin/ammonium polyphosphate postponed the time-to-peak carbon monoxide production by 96 s compared with virgin RIPUF. The highest LOI over 30% was attained with a lignin/ammonium polyphosphate ratio of 1:5. In addition, lignosulfonate-based RIPUF showed a low-effect heat combustion value, less smoke, and carbon monoxide production.

6. Conclusion

The flammability of polyurethane (PU) materials poses a significant safety hazard, necessitating effective flame retardancy strategies. Current methods for enhancing the fire resistance of PU include blending and addition, nanocomposites, and reaction grafting. The transition from halogenated to environmentally friendly flame retardants, particularly those derived from nitrogen and phosphorus, reflects a growing emphasis on sustainability. This review has focused on lignin, a by-product of the paper industry and biorefineries, as a promising biobased flame retardant for PU materials. Aromatic structure of lignin makes it an effective char promoter agent, reducing combustion rates in fire scenarios. The incorporation of lignin into PU can be achieved through physical blending or chemical modifications. Chemical modifications, such as demethylation, phosphorylation, and amination, enhance lignin reactivity and its ability to serve as an effective flame retardant. The use of phosphorus-containing lignin-based flame retardants involves introducing phosphate compounds that act in the condensed phase, catalyzing char formation and isolating the PU from heat and flames. Nitrogen-containing lignin-based flame retardants, on the other hand, demonstrate diverse mechanisms, including scavenging free radicals and endothermic decomposition, effectively hindering combustion. Combining phosphorus and nitrogen elements in lignin-based flame retardants provides synergistic benefits, enhancing flame retardancy while also improving thermal stability and mechanical properties. Various studies have demonstrated successful synthesis and application of lignin-based flame retardants, showcasing their potential in PU manufacturing. The development of sustainable and environmentally friendly flame retardants aligns with the global shift toward green technologies. As research in this field progresses, lignin-based flame retardants tailored for biobased PU present a promising avenue for addressing fire safety concerns while promoting sustainability in material design and production.

Acknowledgments

The authors acknowledge Prof. Ndikontar Maurice Kor and Prof. Cheumani Yona Arnaud from the University of Yaoundé I, Faculty of Science, Research Unit for Macromolecular Chemistry, for their assistance and support.

Abbreviations

DOPO	9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide ()
HDI	Hexamethylene diisocyanate
LOI	Limiting oxygen index
PUF	Polyurethane foams
PUs	Polyurethanes
RIPUF	Rigid polyurethane foam
TGA	Thermogravimetric analyses
UL-94	Standard established by Underwriters Laboratories
HR	Heat release