**1. Introduction**

Today, materials are as diverse as the components and methods used to assemble them. Fire safety concerns and regulatory issues have fuelled past efforts to use formulated chemical technologies to help protect against unpredictable fire hazards. Flame retardant technology was introduced fifty years ago as bulk 'additive' materials to effectively reduce flammability of naturally combustible, ignitable or inflammable constituents of products used mainly in household products. An example of such materials include polymers which may be easily ignitable. As synthetic materials, their inclusion in industrial and domestic products has generally been considered essential to the safety of end-users either as chemically uncoordinated additives to the target materials or as chemically bonded with textiles [1], coatings [2] or plastics [3] for example during manufacturing. These additives primarily operate by 'retarding' or resisting the ignition phase of flammables thereby delaying their reactive pyrolytic nature to catch fire by insulating against its spread. However, to be effective in retarding flame progression, high quantities of additives

are required leading to enhanced toxic gas release. They also operate by diminishing flammability when chemically activated or use endothermic processes to drive the suppression of flames. The availability of flame retardants in self-applicable formats such as coatings and sprays has raised concerns on the uncontained exposure of chemicals through inhalation of vapor or through skin contact and other forms of possible contamination. The impact of anti-flame chemicals on the mortality rates [4] has been considerably significant to warrant their widespread use in delaying fire hazards. However in more recent times, health concerns have intensified over the safe use of flame retardants have surfaced— a concern that has emerged more intensified in the era of 'green and environmental chemistry' and authority driven demands for chemical risk to human-health after toxicity related irregularities based material class [5] were highlighted some thirty years ago [6].

There are numerous flame retardant types [7], however halogenated or organohalogen flame retardants containing carbon bonded chlorine (CFRs) or bromine (BFRs) as the major halogen-based constituents are assembled or integrated with polymers forming organic halogenated blends. For example, several flame retardant classes exist [8] as chlorine and bromine polymers while others are composed of phosphorous, nitrogen and sulfur or synergistically made. Others include antimony, aluminum and tin. Since 1992, halogenated flame retardant production has exceeded 20% reaching 25% of the global market and approximately doubling by 1998. The growing demand suggests that flame retardants are a valuable commodity in fire safety and can regulate the combustion process at the heating or decomposition stage, ignition phase and at the level of flame interference curbing its spread by broadly preventing oxygen consumption, heat production and fuel for flame production.

The chemical mode of action occurs at the level of combustion in the gaseous phase through a cooling process that entails curbing the evolution of flammable gases. An alternative action of flame retardants is the deposition of barriers as chemical layers compartmentalizing the supply of oxygen away from the material. Other mechanisms involve additive materials with endothermic properties lowering temperatures below combustible levels or mechanisms that allow the build-up of protective layers cutting off oxygen as essential part of the fueling process. The use of inert materials forming non-combustible gas products also reduce the space for flammable gases entering the ignitable phase. Compositionally, flame retardants are classed by their elementally important counterparts namely bromine, chlorine, phosphorous and nitrogen. The importance of the point of action of flame retardants is shown in **Figure 1** which often depends on the combustion profile of a material and how well it 'fits' with the flame retardant properties of choice [10]. The release of fire hazards are shown in **Figure 2**. In terms of the molecular mechanism of action which are largely unknown, signify only a broad but general based understanding at the bulk scale [11]. It therefore becomes important to re-visit the flame structure partitioned as the outer, middle and inner zones differentiated by color and temperature. Temperatures vary considerably by orders of magnitude in a single flame and temperature variability affects combustibility of materials generating different particle types that originate from different regions in the flame. These regions are further separated by the variable degree of combustibility of vapors dictated by the availability of oxygenated air and critical temperatures needed to ignite unburnt fuels generating high energy propagating particles during flame propagation [12]. It has also been suggested that modeling of flow configuration of flames if adequately understood can help direct particle-particle interactions with multi-scale implications that reside at the gas–solid phase [13]. The contribution of nano- and sub-micrometer particles to flame retardancy is not only dependent on low migration resulting in the surface decomposition of condensed material (char) and the vaporization of particles via surface diffusion at higher temperatures

**11**

*Nanoscale Configuration of Clay-Interlayer Chemistry: A Precursor to Enhancing Flame…*

phase but also surrounding combustion gaseous conditions. Both particle diameter (geometric mean size) and mass was affected by altered O2:CO2 and O2:N2:CO2 ratios during the vapor to particle conversion [14] (**Figure 3**). Given that the pattern of nanoparticle evolution is connected to how a flame is structured, motioned and governed, modeling the underlying features could have important ramifications in reducing or silencing the more turbulent aspects of flames. For example, the size increase of nucleating particles whilst traversing across the flame front rapidly aggregate and particle progression and accumulation can be suppressed by tem-

*Flame retardant property and the importance of chemical synergy in the behavior of phosphaphenanthrene* 

*retardant chemistry in suppression of flame progression. Reproduced with permission from [9].*

The basis for the use halogens as flame retardants originates from the production

High energy radicals combine with hydrocarbons such as methane to increase radical formation fueling green-house emissions like CO2 and toxic CO (**Figure 5**) thus considerably contributing to exothermic reactions — a process that is repeated by increasing the chain of radicals during combustion. In particular, hydrogen bromide

such as methane neutralizing radical hydrocarbon production and instead forming

Some studies have suggested however that radical recombination although important, constituent CO-H2 may propagate with differing burning velocities (flame speed progression relative to the unburned state) under conditions of minimal heat loss, particle formation is influenced by the residence time in pre-mixed

These chemical events work synergistically in reducing the propagation of flames through flame suppression and delaying flame ignition via chain termination 'type' reactions. The diverse use of combustible materials and their flammable nature has been at the core of safety and protective concerns to humans and the environment. At the forefront of chemical exposure are fire fighters, a risk assessment case study of this vulnerable group has highlighted chemical exposure through combustion as a leading cause for cancer and thyroid complications [17] among the firefighting population from direct exposure and direct inhalation of chemical constituents of flame retardant materials. Tens of thousands of deaths

, HO•

(bromine radical) and behaves as a

through protonation forming water and hydrogen respec-

and H•

and O•

from hydrocarbons

(**Figure 4**).

of radicals from the combustion of hydrocarbons such as H•

(HBr) at the ignition phase (decomposes to Br•

tively. Bromine radicals are able to compete with HO•

ground state hydrocarbons shown below in **Figure 4**.

and H•

*DOI: http://dx.doi.org/10.5772/intechopen.95788*

perature reduction [15].

**Figure 1.**

quenchers of HO•

flames [16].

*Nanoscale Configuration of Clay-Interlayer Chemistry: A Precursor to Enhancing Flame… DOI: http://dx.doi.org/10.5772/intechopen.95788*

#### **Figure 1.**

*Flame Retardant and Thermally Insulating Polymers*

are required leading to enhanced toxic gas release. They also operate by diminishing flammability when chemically activated or use endothermic processes to drive the suppression of flames. The availability of flame retardants in self-applicable formats such as coatings and sprays has raised concerns on the uncontained exposure of chemicals through inhalation of vapor or through skin contact and other forms of possible contamination. The impact of anti-flame chemicals on the mortality rates [4] has been considerably significant to warrant their widespread use in delaying fire hazards. However in more recent times, health concerns have intensified over the safe use of flame retardants have surfaced— a concern that has emerged more intensified in the era of 'green and environmental chemistry' and authority driven demands for chemical risk to human-health after toxicity related irregularities

There are numerous flame retardant types [7], however halogenated or organohalogen flame retardants containing carbon bonded chlorine (CFRs) or bromine (BFRs) as the major halogen-based constituents are assembled or integrated with polymers forming organic halogenated blends. For example, several flame retardant classes exist [8] as chlorine and bromine polymers while others are composed of phosphorous, nitrogen and sulfur or synergistically made. Others include antimony, aluminum and tin. Since 1992, halogenated flame retardant production has exceeded 20% reaching 25% of the global market and approximately doubling by 1998. The growing demand suggests that flame retardants are a valuable commodity in fire safety and can regulate the combustion process at the heating or decomposition stage, ignition phase and at the level of flame interference curbing its spread by broadly preventing

The chemical mode of action occurs at the level of combustion in the gaseous phase through a cooling process that entails curbing the evolution of flammable gases. An alternative action of flame retardants is the deposition of barriers as chemical layers compartmentalizing the supply of oxygen away from the material. Other mechanisms involve additive materials with endothermic properties lowering temperatures below combustible levels or mechanisms that allow the build-up of protective layers cutting off oxygen as essential part of the fueling process. The use of inert materials forming non-combustible gas products also reduce the space for flammable gases entering the ignitable phase. Compositionally, flame retardants are classed by their elementally important counterparts namely bromine, chlorine, phosphorous and nitrogen. The importance of the point of action of flame retardants is shown in **Figure 1** which often depends on the combustion profile of a material and how well it 'fits' with the flame retardant properties of choice [10]. The release of fire hazards are shown in **Figure 2**. In terms of the molecular mechanism of action which are largely unknown, signify only a broad but general based understanding at the bulk scale [11]. It therefore becomes important to re-visit the flame structure partitioned as the outer, middle and inner zones differentiated by color and temperature. Temperatures vary considerably by orders of magnitude in a single flame and temperature variability affects combustibility of materials generating different particle types that originate from different regions in the flame. These regions are further separated by the variable degree of combustibility of vapors dictated by the availability of oxygenated air and critical temperatures needed to ignite unburnt fuels generating high energy propagating particles during flame propagation [12]. It has also been suggested that modeling of flow configuration of flames if adequately understood can help direct particle-particle interactions with multi-scale implications that reside at the gas–solid phase [13]. The contribution of nano- and sub-micrometer particles to flame retardancy is not only dependent on low migration resulting in the surface decomposition of condensed material (char) and the vaporization of particles via surface diffusion at higher temperatures

based material class [5] were highlighted some thirty years ago [6].

oxygen consumption, heat production and fuel for flame production.

**10**

*Flame retardant property and the importance of chemical synergy in the behavior of phosphaphenanthrene retardant chemistry in suppression of flame progression. Reproduced with permission from [9].*

phase but also surrounding combustion gaseous conditions. Both particle diameter (geometric mean size) and mass was affected by altered O2:CO2 and O2:N2:CO2 ratios during the vapor to particle conversion [14] (**Figure 3**). Given that the pattern of nanoparticle evolution is connected to how a flame is structured, motioned and governed, modeling the underlying features could have important ramifications in reducing or silencing the more turbulent aspects of flames. For example, the size increase of nucleating particles whilst traversing across the flame front rapidly aggregate and particle progression and accumulation can be suppressed by temperature reduction [15].

The basis for the use halogens as flame retardants originates from the production of radicals from the combustion of hydrocarbons such as H• , HO• and O• (**Figure 4**). High energy radicals combine with hydrocarbons such as methane to increase radical formation fueling green-house emissions like CO2 and toxic CO (**Figure 5**) thus considerably contributing to exothermic reactions — a process that is repeated by increasing the chain of radicals during combustion. In particular, hydrogen bromide (HBr) at the ignition phase (decomposes to Br• (bromine radical) and behaves as a quenchers of HO• and H• through protonation forming water and hydrogen respectively. Bromine radicals are able to compete with HO• and H• from hydrocarbons such as methane neutralizing radical hydrocarbon production and instead forming ground state hydrocarbons shown below in **Figure 4**.

Some studies have suggested however that radical recombination although important, constituent CO-H2 may propagate with differing burning velocities (flame speed progression relative to the unburned state) under conditions of minimal heat loss, particle formation is influenced by the residence time in pre-mixed flames [16].

These chemical events work synergistically in reducing the propagation of flames through flame suppression and delaying flame ignition via chain termination 'type' reactions. The diverse use of combustible materials and their flammable nature has been at the core of safety and protective concerns to humans and the environment. At the forefront of chemical exposure are fire fighters, a risk assessment case study of this vulnerable group has highlighted chemical exposure through combustion as a leading cause for cancer and thyroid complications [17] among the firefighting population from direct exposure and direct inhalation of chemical constituents of flame retardant materials. Tens of thousands of deaths

**Figure 2.** *Halogen derived flame retardants in use.*

per year occur through the inhalation of toxic fumes such as carbon monoxide, hydrogen chloride, hydrogen cyanide [18] and dioxins [19] making the effects of smoke toxicity a leading reason for mortality during fire related incidents. The fact that gaseous toxicity release surpasses the ability of flame retardants to suppress fires is a major concern [4] and has shown to persist inside teaching classrooms [20] and college dormitories [21]. Poly-brominated and other halogen-based retardants are shown to be linked to bioaccumulation [22] with child developmental problems effecting child IQ performance, development and intelligence impacting neurologic function in children [23], weakening immune systems through Immunotoxicity [24], reproducibility [25], metabolic [26] and respiratory [27] interference.

**13**

**Figure 3.**

**Figure 4.**

*permission from [14].*

*Nanoscale Configuration of Clay-Interlayer Chemistry: A Precursor to Enhancing Flame…*

Such health risks also include harm to wildlife [28]. These chemicals discreetly find their way into the surrounding environments spreadable through the air [29] and water or become attached to dirt, soil, sand and powder particles and subsequently into human contact directly through inhalation or through food and water-intake. These health risks shown in **Figure 6** together with unknown and unsubstantiated toxic effects of flame retardants has caused considerable concern in recent risk assessments [31] fuelling demands for the disuse of constituent chemicals by National Oceanic and Atmospheric Administration and the EU driven Restriction of Hazardous Substances Directive [32]. Environmentalists and health risk advisers have particularly focused on finding alternatives for halogenated flame retardants [33] with a view to replacing them with non-halogenic constituents. However, there exists an 'unmet need [34], to make flame retardants toxic free as new technologies emerge. This situation is being addressed by a drive towards policy changes focused on a class of flame retardant types and steps for their replacement [5]. The current

*, Cl−*

 *etc.) in* 

*The global equation of combustion which propagates by a set of chain reactions mediated by high energy radicals that originate from interaction of hydrocarbons with flame retardant halogen anions (Br−*

*a mixture composed of oxygen, carbon dioxide, water and combustion fuels e.g. hydrocarbons.*

*Condensed to vapor phase particle conversion and associated processes during the combustion. Reproduced with* 

*DOI: http://dx.doi.org/10.5772/intechopen.95788*

*Nanoscale Configuration of Clay-Interlayer Chemistry: A Precursor to Enhancing Flame… DOI: http://dx.doi.org/10.5772/intechopen.95788*

#### **Figure 3.**

*Flame Retardant and Thermally Insulating Polymers*

per year occur through the inhalation of toxic fumes such as carbon monoxide, hydrogen chloride, hydrogen cyanide [18] and dioxins [19] making the effects of smoke toxicity a leading reason for mortality during fire related incidents. The fact that gaseous toxicity release surpasses the ability of flame retardants to suppress fires is a major concern [4] and has shown to persist inside teaching classrooms [20] and college dormitories [21]. Poly-brominated and other halogen-based retardants are shown to be linked to bioaccumulation [22] with child developmental problems effecting child IQ performance, development and intelligence impacting neurologic function in children [23], weakening immune systems through Immunotoxicity [24], reproducibility [25], metabolic [26] and respiratory [27] interference.

**12**

**Figure 2.**

*Halogen derived flame retardants in use.*

*Condensed to vapor phase particle conversion and associated processes during the combustion. Reproduced with permission from [14].*

#### **Figure 4.**

*The global equation of combustion which propagates by a set of chain reactions mediated by high energy radicals that originate from interaction of hydrocarbons with flame retardant halogen anions (Br− , Cl− etc.) in a mixture composed of oxygen, carbon dioxide, water and combustion fuels e.g. hydrocarbons.*

Such health risks also include harm to wildlife [28]. These chemicals discreetly find their way into the surrounding environments spreadable through the air [29] and water or become attached to dirt, soil, sand and powder particles and subsequently into human contact directly through inhalation or through food and water-intake. These health risks shown in **Figure 6** together with unknown and unsubstantiated toxic effects of flame retardants has caused considerable concern in recent risk assessments [31] fuelling demands for the disuse of constituent chemicals by National Oceanic and Atmospheric Administration and the EU driven Restriction of Hazardous Substances Directive [32]. Environmentalists and health risk advisers have particularly focused on finding alternatives for halogenated flame retardants [33] with a view to replacing them with non-halogenic constituents. However, there exists an 'unmet need [34], to make flame retardants toxic free as new technologies emerge. This situation is being addressed by a drive towards policy changes focused on a class of flame retardant types and steps for their replacement [5]. The current

#### **Figure 5.**

*The release of toxic gases, hydrocarbons and chemicals from polyamide based flame retardants.*

#### **Figure 6.**

*Prevalence and potential for toxicity of flame retardants (a) metabolic fate of flame retardants affecting gene expression and inducing toxicity (b) accumulation in finger nails and hair follicles (c) hormonal suppression/ activation effects of phosphorous flame retardants modeled through molecule-ligand binding (d) detection of organophosphate esters across tropical and subtropical Atlantic, Pacific, and Indian oceans. The release of toxic gases from polyamide based flame retardants. Reproduced with permission from [30].*

challenges weigh on the benefits of flame retardant use in saving lives against death due to toxic inhalation.

In this review, we focus on nanoclay materials and the potential to exploit both their intrinsic and modifiable properties adaptable as flame retardants with a view to make anti-fire materials safer by reducing their potential for toxicity and harm to humans and animals alike. Insight into the technological challenges that confront the flame retardant industry in securing safe and usable chemicals made acceptable by current environmental and health standards is highlighted. The low-dimensional characteristics of nanomaterials is discussed in the context of introducing and

**15**

*Nanoscale Configuration of Clay-Interlayer Chemistry: A Precursor to Enhancing Flame…*

exploiting new flame retardant properties which are absent at bulk scales. Particular attention is directed to polymers as high polymer content and their associated toxicities above ignition temperatures are often the cause of fire related fatalities. In particular, the synergistic role of nanoclays with polymer chemistry offer substantial improvements and the potential of these advancements is presented in the context of the underlying challenges that require new explorations to their development.

**2. Igniting low dimensional chemistry to extinguish flame propagation**

Increasingly, researchers and regulators of flame retardant use are looking towards the materials used by nanotechnology to improve the risks associated with anti-fire materials. Nanomaterial architectures offer considerable structural and functional control as host modifiers in parent structures. Particles of diminished size at nanoscales represent an important class of materials of technological value in controlling chemical and physical complexity. They hold the potential to offer considerable pyrolytic control as a new class of flame retardants through reinforcement of intercalated low dimensional materials. The effects of increased nanoparticle surface area and surface energy at the nanoparticle-polymer interface [35] caters for diverse orientations at nanoscales that impose changes to the overall microscale structure. Favorable polymer-particle associations can result in interactions that the affect physical and chemical properties of flame retardant behavior. Contributions in this respect may arise from surface interactions (in micro or nano confined spaces), in shape formations affecting polymer spacing and those that result from interfacial compatibility at polymer-filler interfaces. The larger elements of the effects that occur at the micro/nano level translate to globalized effects influencing mechanical properties, permeability and flow behavior of gases through interspaces and structures, thermal property and conductivity. Further, nano based flame retardants utilize less chemicals compared to their bulk counterparts diminishing

The search for new flame retardants has been intense mainly to make their use safer and more effective. The challenge has been to understand more clearly the underlying mechanisms of pyrolysis particularly in the context of polymers to effectively introduce and to deliver new chemistries and modes of action that manifest at the nanoscale. It has been well-known since the 1960's [36] that the liberation of high energy free radicals play an active role in fire propagation and the broad task of 'chemistry' has been to control heat combustion by limiting radical formation. The emphasis has shifted in controlling the decomposition of flame retardants [37] to limit toxic inhalation and the thermal degradation of polymers [38] and to gain a knowledge-based appreciation of mechanisms prevalent in flame propagation. The broader picture here is complicated by flame retardant mechanistic modes and the overlying chemical synergy with material decomposition. This aspect of flame control has been challenging since polymer chemistry is considerably more diverse than flame retardant chemistry making the alliance of synergistic control intellectually and technologically demanding. Vulnerability to fire-spreading scenarios lies within the combustion process itself aided by a number of processes that lead to the ignition phase. Flame retardants while designed to delay combustion and pyrolysis propmote secondary effects of toxic fume emissions and free radical formation from burning materials such as polymers which may override the ability of retardants to

*DOI: http://dx.doi.org/10.5772/intechopen.95788*

the production of toxic vapors.

**3. Current challenges in flame retardant design**

*Nanoscale Configuration of Clay-Interlayer Chemistry: A Precursor to Enhancing Flame… DOI: http://dx.doi.org/10.5772/intechopen.95788*

exploiting new flame retardant properties which are absent at bulk scales. Particular attention is directed to polymers as high polymer content and their associated toxicities above ignition temperatures are often the cause of fire related fatalities. In particular, the synergistic role of nanoclays with polymer chemistry offer substantial improvements and the potential of these advancements is presented in the context of the underlying challenges that require new explorations to their development.
