1 Introduction

Fire has been and is still a big issue for most of the applications of polymeric materials: in fact, the latter, when exposed to an irradiative heat flux or to a flame, easily burn and may generate toxic smokes [1]. To limit this severe and stringent problem, specific additives, called flame retardants (FRs), have been designed, synthesized, and applied to different polymeric materials (i.e., bulk polymers, fibers and fabrics, and foams), starting from the early 1950s. Upon the application of a flame or an irradiative heat flux, according to their chemical structure and composition, they are able to favor the formation of a stable carbonaceous residue (i.e., the so-called char) in the condensed phase and/or to act in the gas phase, entrap** the H• and •OH radicals that are responsible for the propagation of the combustion reactions [2,3,4,5]. These phenomena are well depicted by the so-called “Emman’s fire triangle” (Fig. 1), i.e., a self-sustaining flaming combustion. In brief, any flaming combustion is the result of an oxidation, which takes place in the gas phase and involves the oxygen (or air) provided by the atmosphere that surrounds the burning polymer, as well as the gaseous products originating from the decomposition of the polymer itself. The inherent exothermicity of the reaction self-sustains the flaming combustion [6].

Fig. 1
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Emman’s fire triangle. Reprinted from [7] under the Creative Commons Attribution (CC BY) license

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So far, several different types of FRs have been conceived, synthesized, and applied to different polymer systems. Regardless of the final use (i.e., for scientific/academic and/or for commercial purposes), all FRs contain specific key elements in their chemical structure such as halogens (mainly chlorine and bromine), metals, phosphorus, nitrogen, or boron, alone or even in combination, as occurs in FR formulations containing nitrogen and phosphorus [7, 8]. Different approaches can be employed for preparing flame retardant polymer systems, namely: physical incorporation of the FR into the polymer matrix (either thermoplastic or thermosetting); melt compounding (in particular spinning, when synthetic flame retarded fibers/fabrics are involved); and grafting or copolymerization reactions (which can be successfully exploited for establishing covalent linkages between the macromolecular chains and the flame retardant, hence providing more stability and durability).

Around 15 years ago, the formation of full inorganic or hybrid organic-inorganic ceramic phases by sol-gel in situ synthetic routes started to be explored as an effective flame retardant strategy for different polymer systems (bulk thermoplastic or thermosetting polymers, fibers and fabrics, foams) [9, 10]. It has clearly been demonstrated that the formation of sol-gel derived phases within a polymer matrix causes both the enhancement of the mechanical and thermal behavior and the improvement of the resistance to a flame spread or to the exposure to an irradiative heat flux. Indeed, the obtained ceramic or hybrid moieties are able to protect the underlying material, slowing down the heat, oxygen and mass transfer phenomena between the degrading material to the surroundings or vice versa. In particular, they usually work in the condensed phase, providing a thermal shielding effect to the polymer matrix, favoring, at the same time, the formation of a stable carbonaceous residue (i.e., the so-called char) [11, 12].

The present work will review the most recent advances in the use of sol-gel processes for conferring flame retardant features to bulk polymers (either thermoplastics or thermosets), textiles (particularly regarding cotton fibers and fabrics) and foams. In addition, the current limitations and potential new developments of these approaches will be summarized.

2 Assessment of flame retardance through flammability and forced-combustion tests

Consistent test methods are now available that allow the behavior of polymer systems in relation to fire risk to be evaluated, which typically involve the evaluation of such important parameters as ease of ignition, flame propagation rate, heat release rate, generation of toxic and darkening smokes, among others. These tests must be performed under rigorously defined conditions, to provide a comparative classification of the behavior of different polymeric materials. This section will briefly summarize the most important tests that allow the flame retardance of polymer systems to be assessed.

2.1 Flammability tests

UL94 and Limiting Oxygen Index (LOI) measurements are the most common tests carried out to evaluate the flammability of any polymeric material.

UL94 is a flame spread test designed by the Underwriters Laboratories (also described in the ISO EN 11925-2 standard), which assesses the ability of the polymeric material to self-extinguish after flaming. The flame can propagate through the specimen according to the adopted configuration of the specimen, i.e., whether the latter is vertically or horizontally positioned. A rating is made according to the test results that comprise the burning time and rate, the time required for extinction, the possible drip** phenomena (i.e., drip** of incandescent drops that may propagate the flame to other ignitable materials), and the duration of afterglow. Undoubtedly, the rating in UL94 tests is strictly correlated with the wall thickness of the specimens.

In LOI measurements (standardized in ASTM D2863), a bar specimen is vertically positioned in a glass cylindrical tube and burns from the top to the bottom (i.e., according to a candle-like burning) within an ascending flow of a N2/O2 mixture. First, the bar specimen is ignited by a flame that is then removed; by varying the composition of the gas mixture, the minimum oxygen volume percentage, at which self-sustained combustion occurs for 3 min corresponds to the LOI of the polymer system. Usually, though it is not a strict general rule, polymers are considered flame retarded when the LOI exceeds 25%.

2.2 Forced-combustion tests

Forced-combustion tests refer to cone calorimetry measurements that take advantage of the empirical observation that the heat released by burning polymer systems is directly related to the amount of oxygen consumed during the combustion process. In this approach, described in the ISO 5660 standard, a square specimen (usually 10 × 10 cm2) is placed on a balance and exposed to the irradiative heat flux provided by a conical heater; an electrical spark is employed for igniting the flammable degradation gases that are produced by the degradation of the specimen under the heat flux. The combustion products (i.e., the smokes) are then collected, where the concentrations of the unconsumed oxygen and of the evolved gases (namely, CO and CO2) are measured. An optical system also assesses the obscuring power of the evolved smokes. This apparatus delivers a detailed overview of the combustion process as a function of time, by assessing the time to ignition, the heat release (estimated on the basis of the oxygen consumption), the sample weight loss, as well as opacity and composition of the produced smokes.

In addition, according to the ASTM 7309 standard, there exists a pyrolysis-combustion flow calorimetry test, known as micro-scale combustion calorimetry (MCC), able to measure the heat release rate and other related parameters using samples of a few milligrams. In MCC tests, the sample is first quickly heated up to a controlled pyrolysis state under an inert atmosphere; the gaseous pyrolyzed products that are formed during this stage are then rapidly oxidized at high temperature in excess oxygen, hence simulating a combustion process. Heat release rate vs. time curves are then plotted, which can be exploited for comparing the behavior of different polymeric systems.

3 Sol-gel derived systems for bulk polymers

3.1 Sol-gel chemistry and phosphorus-based compounds as flame retardant strategy

Phosphorus-based flame retardants (P-FRs) have been largely used to replace halogen-containing additives, due to their lower toxicity and high effectiveness. The decomposition of phosphorus-based flame retardants releases phosphoric anhydrides and acidic compounds, which mainly work as charring promoters in the condensed phase [13, 14]. The generation of phosphorus-nitrogen-silicon containing species in organic-inorganic composites is highly desirable because it results in a hybrid flame retardant activity in the gas and condensed phases. The production of nitrogen during the decomposition process can lead to the formation of phosphaphenanthrene groups in the gas phase through the reaction with acidic phosphorus compounds lying in the boundary phase. Also, phosphaphenanthrene and silicon groups can promote char formation in the condensed phase and exert a thermal shielding effect during the combustion. Luo et al. [15] synthesized a phosphorus-nitrogen-silicon flame retardant (DPHK) via a Kabachnik-Fields reaction and sol-gel method. DGEBA composites cured with 4,4′-diaminodiphenylmethane (DDM) containing 3 wt.% of DPHK achieved a V-0 rating in vertical flame spread tests (UL94) and LOI value of 29%, together with a strong suppression of released toxic fuel (carbon monoxide, CO), due to the production of a continuous and protective char promoting a decrease of both peak of Heat Release Rate, pkHRR, (by about 36%) and total heat release, THR, (by about 30%).

9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO) is one of the most used P-FRs in combination with the sol-gel approach. DOPO is characterized by a single reactive hydrogen functionality, therefore it can react with polymer (e.g., epoxy resin) or other functional groups present on the surface of materials. The reaction of DOPO with oxirane rings of epoxy may reduce the glass transition temperatures and consequently the mechanical performances of the composite material. This problem can be overcome by its derivatization to unreactive compounds [16,17,18]. However, the decomposition of DOPO produces a huge number of phosphorous radicals acting as flame inhibitors in the gas phase. This peculiar flame retardant mechanism can be combined with a sol-gel strategy to prepare polymer-based composites with enhanced flame retardance and good mechanical behavior. More specifically, the sol-gel strategy allows for the synthesis of silica nanoparticles using mild operative conditions and the use of several precursors (e.g., tetraethoxysilane - TEOS -, sodium metasilicate, etc). The sol-gel in situ generation of silica nanoparticles in the epoxy resin can improve the fire behavior of the latter, especially in combination with phosphorus-based flame retardants. Recently, Bifulco et al. [19,20,21] synthesized in situ sol-gel silica nanoparticles (NPs) in a DGEBA resin cured with isophorone diamine (IDA) as cycloaliphatic hardener. The presence of silica NPs at a very low loading (2 wt.%) prevented the occurrence of drip** phenomena during vertical flame spread tests [22]. An analogous sol-gel based methodology was applied to flame retard 2,5-bis[(oxyran-2-ylmethoxy)methyl]furan (BOMF) resin cured with methyl nadic anhydride (MNA) [23]. The generation of silica NPs conferred a high char-forming character and prevented drip** phenomena as well as decreasing the heat release rate up to 35%; this value is much higher than that (25%) obtained for DGEBA-based systems cured with an aliphatic polyamine. This flame retardant strategy was used by the same research group in DGEBA systems cured with IDA to achieve V-0 rating in UL94 tests by exploiting, together with melamine, 3-(6-oxidodibenzo[c,e][1,2]oxaphosphinin-6-yl) propenamide (DA) and 6H-dibenz[c,e][1,2]oxaphosphorin,6-[(1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-yl)methoxy]-, 6-oxide (DP) as flame retardants at 2 wt.% of P-loading [24]. No drip** self-extinguishing composites were obtained with the addition of 17.3 wt.% of DP and 11.5 wt.% of melamine, even using an aliphatic hardener for the curing process. Conversely, Zhang et al. [25] prepared epoxy composites showing V-0 rating in UL94 tests only with the use of DDM and 9.1 wt.% of DP. The combined presence of DA and in situ silica NPs guaranteed V-0 rating in UL 94 tests in the presence of just 2 wt.% of P. DA and DP also allowed for the formation of an abundant char during the combustion, playing an active role both in condensed and gas phase starting from the early stage of decomposition process [24, 26].

The same phosphaphenanthrene derivative (DOPO-HQ) was adopted by Shen et al. [27], who synthesized a hyperbranched organic-inorganic hybrid material containing Si, N and P, aimed at improving the flame retardancy of a polyol-based polyurethane. The sol-gel procedure employed DOPO-HQ and 3-isocyanatopropyltriethoxysilane (IPTS) through two polycondensation steps, the second following the addition of triglycidyl isocyanurate (TGIC). The hybrid flame retardant appeared uniformly dispersed in the polyurethane matrix, however the best classification achieved in the UL-94 test was V-2, at 40 wt.% loading of the FR. The chemical analysis of char showed that the siloxane and phosphide species produced during burning formed a protective layer improving the oxidation resistance of the polymer.

Ammonium polyphosphate (APP), a widely applied flame retardant, has been coupled with silicon-containing phases in the search for synergistic effects. Its microencapsulation in a silica gel shell by hydrolytic sol-gel has been proposed. However, the substantial hydroxylation of silica surface makes the modified APP highly hydrophilic and poorly compatible with polymers such as polyurethane.

The introduction of a siloxane component was therefore explored, for example by reacting APP with polydimethyl siloxane (PDMS), alkylphenol polyoxyethylene and then with TEOS, in the presence of ammonia [28]. The so-obtained APP microcapsules with hybrid silicone-organic shells were added to a thermoplastic polyurethane. Compared with simple APP-silica, 10 wt.% of this hybrid additive offered a greater improvement in the fire behavior of the composites, with 31.4% decrease of THR, though both allowed a V-0 rating in vertical flame spread tests. Moreover, it slightly reduced the negative impact of the filler on the tensile strength and elongation at break.

Zeng et al. [29] adopted a bottom-up sol-gel approach with the same aim. In particular, two different silanes, TEOS and vinyltriethoxysilane (VTES), were added sequentially to APP, controlling the pH (from acid to basic) and temperature to promote their hydrolysis and condensation. The resulting APP particles, wrapped by a silica gel-polysiloxane hybrid layer which provided hydrophobicity and reduced water solubility, were successfully applied to improve the fire resistance of polyurethane. The composite containing 20 wt.% of modified APP showed a LOI of 30% (+54% compared to pure polyurethane) and reached V-0 rating in vertical burning tests, showing no melt drip**, while the use of bare APP afforded only a V-2 rating and did not avoid drip**. Moreover, the smoke density upon combustion decreased by about 29%. Studying the flame retardant mechanism, the authors proposed that SiO2 and silicon pyrophosphate produced by the thermal degradation of the hybrid additive enhanced the density and uniformity of the char layer, and eventually its thermal stability, preventing the heat transfer and diffusion. A different strategy to reduce the fire hazard of thermoplastic polyurethane was explored by Qiu et al. [30], who developed a novel ternary nanostructure containing P, N, Si and transition metals. It was based on polyphosphazenes (PZS), hybrid polymers with a P-N-P-N backbone. PZS nanotubes were prepared, covered with a mesoporous silica (M-SiO2) layer and decorated with cobalt-copper bimetallic phosphide (CoCuP) nanocrystals (Fig. 2). In detail, polystyrene sulfonate and cetyltrimethylammonium bromide (CTAB) were anchored on the PZS nanotubes through layer-by-layer assembly, then M-SiO2 was deposited by adding TEOS in basic solution, and finally the CoCuP crystalline phase was obtained by hydrothermal treatment. The incorporation of 3 wt.% nanostructured PZS@M-SiO2@CoCuP into polyurethane led to a significant decrease in pkHRR (−58.2%) and THR (−19.4%), lowering the generation of toxic HCN and CO during combustion, as well as promoting the formation of a stable char. The enhanced fire behavior was attributed to the cooperative action of the components, i.e., the catalytic activity of the CoCuP/C system, combined with the charring effect of M-SiO2 and barrier effect of well-distributed PZS-based network.

Fig. 2
figure 2

Schematic illustration of flame retardancy mechanism of polyurethane composites with PZS@M-SiO2@CoCuP. Reprinted with permission from [30]. Copyright (2017) American Chemical Society

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A further example of the introduction of P and N together with Si by linking specifically designed organic molecules with silane compounds was reported by Satdive et al. [31], who designed an amine curing agent obtained by reaction of ethylenediamine (EDA) with phenylphosphonic dichloride (PPDC), followed by reaction with 3-glycidoxypropyltrimethoxysilane (GPTMS), and final hydrolysis and self-condensation in acidic solution. The product was used in different proportions with a commercial polyamide hardener for the deposition of epoxy resin coatings. The P- and Si-containing curing agent allowed self-extinction to be achieved in vertical flame spread tests already at 20 wt.% loading, while LOI values increased from 20 to 29% in the sample containing 40 wt.% of FR.

The most significant recent results on flame retardant systems based on sol-gel chemistry discussed in these sections are reported in Table 1.

Table 1 Recent outcomes in sol-gel flame retardant treatments applied to bulk polymer matrices
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3.2 Sol-gel chemistry and polysiloxane molecules as flame retardant strategy

Silicon-based compounds has been widely used for preparing several functional materials with enhanced thermal and fire resistance. Silicon-containing molecules can be found as precursors and/or coupling agents both for the sol-gel synthesis of inorganic nanoparticles and hybrid organic-inorganic materials. Such element can work in synergism with phosphorus and nitrogen to modify the chemical structure of polymer matrices and inorganic fillers, resulting in composites showing improved overall performances, as described in some abovementioned reports (Section 3.1). Silicon-based materials are potential flame retardants as they produce a char able to act as thermal shield and oxygen barrier during a fire, slowing down the heat release rate. Indeed, silicon oxide can be considered inherently a flame retardant material as, when exposed to heat and flames, it generates ceramics/glasses. Small amounts of silicon-containing species/moieties in certain organic polymer systems (e.g., polydimethylsiloxane-type) have been reported to improve their LOI and UL94 performance. In this Section, the role of polysiloxanes is discussed, while in the following Sections several examples of functionalization of fillers and polymers with silane compounds are reported, along with the use of different silica-based nanostructures in flame retardance (Section 3.4).

Polyhedral oligomeric silsesquioxane (POSS) molecules are nanosized polyhedral or cage-type structures and can be synthesized through sol-gel methodologies [32]. These structures are used as nanofillers in many polymer matrices and positively affect the overall performances of the resulting composites. The literature clearly demonstrates that POSS molecules can hinder drip** phenomena and boost the fire resistance of polymers [33]. Song et al. proved that the use of POSS structures led to the occurrence of a blowing-out effect during combustion, which involved the formation of some char able to trap flammable gases in the condensed phase [34]. This effect strongly reduces the flammability of epoxy composites. The synergistic activity of silicon-nitrogen significantly affects the flammability and pyrolysis process of hybrid nanocomposites and of high-performance polymers, enhancing their flame retardance. Hsieh et al. [35] synthesized branched benzoxazine-containing polysiloxane (PBz-PSO) polymers showing a high reactivity due to the presence of benzoxazine groups that could crosslink by thermal treatment. PBz-PSO samples showed high thermal stability and LOI values (>45%); furthermore, the addition of 5 wt.% of polysiloxane-based additive into a commercial benzoxazine resin allowed it to achieve V-0 rating in vertical flame spread tests and to increase its glass transition temperature up to ~41%. Therefore, the combined use of FR elements such as silicon and nitrogen and the flexibility of sol-gel routes can facilitate the synthesis of hybrid structures containing well dispersed inorganic phases, which could replace halogen-based flame retardants.

It is well known that silicone resins are widely used in industry because of their high thermal resistance and chemical stability. These materials are polymers formed by branched or cage-like oligosiloxanes and are mainly applied as silicone rubbers, coatings and additives [36]. Several sol-gel strategies have been developed to prepare silicone resins cured via traditional processes aimed at avoiding large-scale shrinkages and the formation of holes, which have a detrimental impact on the properties of the final product. Chen et al. [37] developed a sol-gel technology coupled with γ-radiation curing to overcome such limitations. The research group synthesized an organic-inorganic hyperbranched polysiloxane (H-PMAPS) through the fine control of the hydrolysis of 3-methacryloxypropyltrimethoxysilane. The thermally cured hybrid product, RC-T/PMAPS, was characterized by a riverlike morphology, optical transparency, and high thermal stability. Further, it exhibited dielectric stability from 10 Hz to 106 Hz and low dielectric loss at higher frequencies; therefore, this high-performance material is very promising for cutting-edge industries. Cage-like oligosiloxanes can be also functionalized with DOPO and several approaches were developed, employing sol-gel chemistry as a useful tool to achieve well-tailored interphases and generate multifunctional additives. Zhang et al. [38, 39] used DOPO-modified POSS (DOPO-POSS) molecules as flame retardant in a DGEBA resin cured with m-phenylenediamine (m-PDA), where the blowing-out effect in combination with the flame inhibition mechanism of DOPO lowered the concentration of pyrolytic gases in the flame zone, leading to V-2 rating during vertical flame spread tests, even with a low concentration (2.5 wt.%) of additive. To enhance the flame retardance of epoxy composites containing DOPO-modified POSS molecules, Liu et al. [40] functionalized POSS structures with two DOPO moieties (POSS-bisDOPO) by using aminopropyl substituent groups. POSS-bisDOPO showed the capability to self-assemble into nanoparticles and exhibited a very good dispersion in epoxy matrix. POSS-bisDOPO was used as flame retardant in DGEBA resin cured with DDM, and 20 wt.% loading of this DOPO derivative allowed a LOI value of 34.5% to be achieved. Wu et al. [41] prepared a titanium-POSS compound, namely N,N′-bis(methylene)-bis(dibenz[C,E][1,2]-oxaphosphorin6-oxide)-propylisobutyl-amino-titanium-polyhedral oligomeric silsesquioxanes (Ti-POSSbisDOPO), to combine the catalytic effect of metal (i.e. titanium) on the charring process with the flame retardant mechanism of POSS-bisDOPO. Ti-POSSbisDOPO could still self-assemble and was used to flame retard a DGEBA resin cured with DDM hardener. Conversely to POSS-bisDOPO, the incorporation of 3.0 wt.% of titanium-modified DOPO derivative in the epoxy matrix allowed self-extinction to be achieved and the LOI value to be increased up to 50%. The addition of Ti-POSSbisDOPO into the resin reduced pkHRR and THR values, compared to the pristine sample, and led to the formation of a residual char with honeycomb-like multilayer structure, acting as a barrier for heat and oxygen diffusion. The presence of Ti in Ti-POSSbisDOPO cages catalysed the charring process and promoted the production of stable TiO2 in a ceramic layer on the char surface. In addition, the DOPO moieties in the additive were responsible for the release of flame inhibitors PO2• and HPO2• radicals in the gas phase.

3.3 Sol-gel functionalized graphene oxide as flame retardant

Flame retardants with silicon-char forming capability can be obtained by adding graphene oxide (GO) to silica-containing systems. This char shows high thermal stability and can exert enhanced thermal shielding effects in the condensed phase because of its low thermal conductivity. To improve the mechanical properties of these polymer-based composites, their surface may be tailored by suitable coupling agents (Table 1). Thus, Mousavi et al. [45] prepared sol-gel functionalized GO with furfuryl alcohol (GOFA) and silica nanoparticles (SGOFA) by using (3-aminopropyl)triethoxysilane (APTES) as coupling agent. SGOFA was added to a novolac resin matrix cured with N,N′-dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP); the composites containing 8 wt.% of SGOFA showed a substantial amount of char residue, which was about 65.4% higher than that of pristine resin. The presence of silica nanoparticles increased the thermal degradation temperature of novolac resin, as the silica-based filler was very well dispersed in the matrix and grafted on the graphene layers’ surface (Fig. 3).

Fig. 3
figure 3

SEM images of (A) graphene oxide (GO), (B) silica nanoparticles and (C and D) sol-gel functionalized graphene oxide with furfuryl alcohol and silica nanoparticles (SGOFA) at different magnifications. Reprinted with permission from [45], Copyright 2018 Elsevier

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Along this research line, Ebrahimi et al. prepared silane-modified GO by grafting either furfuryl alcohol and (3-aminopropyl)trimethoxysilane (APTMS) [48] or (3-glycidyloxypropyl) trimethoxysilane (GPTMS) [47], and incorporated these hybrid fillers in composites of typical epoxy and novolac resins. In the first case, different samples were obtained by mixing epoxidized novolac, silane-modified novolac or epoxy resin, and functionalized GO. Another composite material was prepared from two silane-modified components of GO and DGEBA, cured with novolac resin in the presence of triphenylphosphine (TPP). Comparable results on the thermal properties were obtained for the two types of filler: for example, 1 wt.% of GPTMS-modified GO in the epoxy/novolac composite gave an increase of char residue from 26.7 to 34.3% and a slightly raised decomposition temperature (Tmax shifted from 417 to 422 °C). On the other hand, the authors did not perform a deep investigation of the fire behavior of the composites.

Also, Huang et al. [42] functionalized graphene oxide through sol-gel methodology via modification with a hyperbranched polysiloxane (P-HBPSi) (Fig. 4). The P-HBPSi@GO hybrids were used as flame retardants for thermoplastic polyurethane (TPU) by melt compounding. SEM measurements revealed a good dispersion of the additives with well-tailored interphases in the TPU matrix. The presence of P-HBPSi in the TPU nanocomposites led to a strong decrease of pkHRR (−63.5%) and peak of smoke production rate (pSPR, −58.3%), even at a very low loading of 2 wt.%. The decomposition of P-HBPSi released trap** phosphorous radicals (e.g. PO•) in the gas phase and formed a silicon-rich char through a catalytic charring effect of GO sheets in the condensed phase.

Fig. 4
figure 4

Synthesis route of P-HBPSi@GO hybrid by sol-gel methodology. Reprinted with permission from [42], Copyright 2021 Elsevier

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As reported above, the flame retardant ability of DOPO is mainly related to a gas phase mechanism, therefore the coupling with a charring agent may result in an improved thermal stability together with a strong inhibition effect. Zhi et al. [54] grafted DOPO on the surface of graphene oxide (GO) by a sol-gel route, then the so-obtained nanofiller was incorporated into a DGEBA resin cured with DDM hardener. Epoxy composite with a DOPO-GO loading of 3.75 wt.% showed an increase of LOI value up to 39% and a remarkable reduction of pkHRR (−45%) as well as of THR (−41%) compared to the pristine sample. The thermal degradation can involve both the formation a highly graphitized silica-rich char (i.e. Si−O − Si bridged structures) acting as heat barrier, and the partial decomposition of DOPO in the gas phase by a flame inhibition mechanism [63]. Luo et al. [57] functionalized reduced graphene oxide (DOPO-rGO) to be used as flame retardant for mesogenic epoxy resin (EO). The addition of 15 wt.% of DOPO-rGO to EO (DOPO-rGO/EO-15) increased the Young’s modulus by around 26.7% compared to pristine resin, which supported a reinforcing effect of the filler on the polymer matrix. DOPO-rGO/EO-15 composites exhibited higher thermal stability and char yield with respect to the unmodified resin, together with a decrease of THR value up to ~19%. Finally, DOPO-rGO/EO-15 was V-0 ranked in vertical flame spread tests.

3.4 Metal hydroxides, oxides, and sol-gel architectures as flame retardants

Metal hydroxides are considered green flame retardants because of their low toxicity and high effectiveness. Their flame retardant mechanism is based on the release of water diluting the flame zone and the formation of refractive oxides exerting thermal shielding and oxygen barrier effects. This mechanism in known in the literature as the “sink effect” and shows eco-friendly and low-cost characteristics. Metal hydroxides such as aluminum hydroxide and magnesium hydroxide are widely used for enhancing the fire protection and thermal stability of polymer-based composites.

Silicon-based structures act as versatile grafting agents, allowing for excellent interphases and tailored hybrid phases; moreover, they can play a relevant role in flame retardance by themselves. Silica has been incorporated in polymer matrices by either in situ synthesis or separately prepared nanoparticles, as summarized in Table 1. The former approach, relying on sol-gel reactions of different silane precursors, offers a vast range of possibilities.

Epoxy and novolac resins conjugated with silane compounds were synthesized by Abdollahi et al. [46, 64] through sol-gel processing, using TEOS, APTES, and (3-glycidyloxypropyl) trimethoxysilane (GPTMS). Specifically, silane-modified epoxy resin (MER) was prepared by grafting TEOS oligomers onto the hydroxyl groups of epoxy resin, forming silica-epoxy hybrids (SEH). Silane-modified novolac resin (MNR) was obtained with GPTMS through a ring-opening reaction, then it was mixed with MER to form silica-novolac-epoxy hybrids (SNEH). Both samples were prepared with 4 and 8 wt.% of SiO2. Moreover, hybrids with epoxidized novolac resin (ENR) were studied. The composites were cured with ethylenediamine in 4 steps, within 90 and 160 °C. The hydrolysis and condensation processes led to the growth of spherical silica nanoparticles anchored in the polymer matrix. Thermal analysis evidenced a char yield of about 22% in the silica-epoxy resin composite, and much larger values in the presence of novolac components (around 50–60%), confirming the significant thermal stability of novolac resins and the role of the SiO2 phase in its enhancement. In a further research effort [46], in addition to TEOS-modified epoxy resin (MER), GPTMS-modified novolac resin (MNR) and epoxidized novolac resin (ENR), silica-grafted carbon nanotubes (SFCNT) were synthesized and used to obtain the hybrid systems. Carbon nanotubes (CNT) were functionalized with APTES-modified hydroxylated silica nanoparticles. The formulation containing 4 wt.% of SFCNT, MNR, and ENR showed the highest char yield (58.4%), while that containing 4 wt.% of SFCNT and MER exhibited the highest degradation temperature (443 °C). The combined presence of 4 wt.% of SFCNT, MER, and MNR guaranteed a degradation temperature of 439 °C and char yield as high as 47.2%. Microscopy analysis and X-ray diffraction revealed that the silica nanoparticles were finely distributed on the surface of CNTs by chemical bonding (Fig. 5). The use of functionalized carbon-based nanostructures can be highly effective in boosting the char formation, and a strong decrease of flammability can be obtained by a synergistic effect with nitrogen or phosphorus. Carbon-based nanostructures can promote the charring process, thus giving a predominant role in the condensed phase during the combustion.

Fig. 5
figure 5

A SEM image of SFCNT and TEM images of (B) SiO2 and (C) SFCNT. Reprinted with permission from [46], Copyright 2018 Elsevier

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In situ methodology allows for a variety of self-extinguishing strategies, including the production of organic-inorganic hybrid composites. Among these materials, aerogels made of polymer-ceramic networks, characterized by very low density and thermal conductivity, attract large interest in thermal insulation and fire protection [

Fig. 8
figure 8

Preparation of magnesium hydroxide modified with TEOS and polymethyl–vinyl silicone rubber (a); photographs of a water droplet on bare (b) and modified Mg(OH)2 (c). Reprinted with permission from [62], Copyright (2019) Wiley

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3.5 Sol-gel chemistry and biowaste materials for the development of greener flame retardant strategies

The exploration of new greener methodologies to flame retard polymer-based composites is gaining attention. To reduce the environmental issues related to the growing pollution, the use of sol-gel strategies involving biowastes as flame retardant additives is considered highly promising. Traditional methodologies are based on the exploitation of halogen-based compounds to improve the fire behavior, although these species are responsible for the release of toxic gases, especially during the disposal of composites. In this context, Venezia et al. [55] investigated the use of humic acid (HA) as a biowaste flame retardant in epoxy resins. Among the bio-wastes, HA is one of the most interesting being explored as a flame retardant. Humic matter is the result of a chemical extraction from the soil organic matter and represents the humic molecules distributed in the soil or water. From a chemical point of view, it is composed of a mixture of many different acids containing carboxyl and phenolate groups [77]. To effectively use humic acid as a bio-waste flame retardant, the authors modified a DGEBA resin, cured with IDA, with APTES to obtain hybrid moieties with more polar groups (Fig. 9). This procedure allowed for a good dispersion of humic acid throughout the APTES-modified epoxy matrix and thus the final composites showed uniform composition and properties.

Fig. 9
figure 9

a Silanized epoxy species; (b) hybrid polymer moiety (reprinted with permission from [55]). Copyright (2021) American Chemical Society

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HA was used alone and in combination with urea and ammonium polyphosphate to flame retard DGEBA resin. Silicon-modified epoxy chains improved the dispersion of HA, which alone prevented melt drip** during combustion. Epoxy composites containing 6 wt.% of HA and 1 wt.% of APP achieved V-0 rating in vertical flame spread tests. Furthermore, the combination of HA with APP and urea was found to reduce the pkHRR up to 52% and increase the time to flashover up to 1.8 min, without any negative effect on the mechanical properties. The self-extinguishing capability of the hybrid epoxy resin was explained through microscopy and gas evolution analysis. HA was responsible for a strong condensed phase activity. The synergism of HA with urea and APP could produce N−P−O−Si chain containing intumescent char, acting as protective barrier for heat and oxygen, whereas ammonium polyphosphate mainly exerted a gas phase activity by a flame inhibition mechanism.

However, the conversion of bio-waste into flame retardant additives for polymers is allowing for a reduction of primary sources depletion (e.g., phosphorus) and the development of new products through a waste-to-wealth approach. Kaya et al. [43] prepared a silica xerogel from groundnut hull ash, which was sol-gel functionalized with tetrabutylammonium bromide to improve its dispersion in DGEBA epoxy. In addition, methyl nadic anhydride (MNA) and 2,4,6-tris(dimethylaminomethyl)phenol (TDP) were used as accelerator and curing agent, respectively. Amorphous silica xerogel was characterized by low thermal conductivity and high thermal stability and the incorporation of 2 wt.% of this additive into the resin resulted in an increase of char residue from 15.3 to 18.0% and self-extinguishing capability (UL 94-V-0 rating). In a context of green chemistry, the use of tetrabutylammonium bromide should be avoided, as it contains halogen species, although the sol-gel methodology allows for mild operative conditions and thus cost and energy saving.

Concerning thermosetting polymers, epoxy resins are widely used in several industrial applications and it is well known that these polymers can be cured by using aromatic or aliphatic hardeners. Indeed, the chemistry of curing agents strongly influences the final properties of epoxy-based composites. Though the use of aromatic hardeners guarantees higher mechanical performances and thermal stability, the aliphatic alternatives are preferred because of their lower environmental impact. At variance, these latter compounds are also highly flammable and require longer curing processes [78]. In light of this, the sol-gel methodology can also be applied to modify curing agents of epoxy-based materials, allowing for the production of more sustainable hardeners. The introduction of metals or phosphorus groups in the chemical structure of amino curing agents can improve the fire behavior without any addition of additive in the polymer matrix. This strategy is usually exploited to avoid any negative effect on the glass transition temperature and mechanical properties. In such perspectives, Cheng et al. [56] synthesized an amino curing agent composed of silicon/titanium flame-retardant elements (STCA) and based on APTES and tetrabutyl titanate. The incorporation of 4 wt.% of STCA into DGEBA resin led to composites showing high strength and glass transition temperature, with relatively good toughness. Besides, the composites containing 4 wt.% of STCA exhibited V-0 rating in vertical flammability tests and increased char yield (+52.7%) compared to the pristine system. The presence of silicon/titanium species (i.e., Si−O−Ti, Ti−O − Ti bonds) in the amino curing agent promoted the formation of a compact and intumescent char layer during the degradation process, slowing down the diffusion of heat and oxygen into the interior polymer structure.