**5. Nanoclays composites: challenging the role of conventional flame retardant mode of action**

Nanoclays form a class of inorganic clay based nanomaterials (**Figure 11**) with chemical and structural attributes that enable their integration with diverse materials as clay nanocomposites including polymers [59]. They exist as silicate/ aluminum-silicate structures in the form of montmorillonite, bentonite, kaolinite, hectorite, and halloysite. Nanoclays comprise layers of 1 nm thickness separated by interlayer distances between 70 to 150 nm modifiable as nanocomposites through intercalation with guest structures. While compositional mergers of nanoclay result in superior mechanical and tensile strength, properties aligned to reduce gas permeability [60] is achieved through the deposition of thin coated layers, alterations in glass temperature (temperature distortion) of nanocomposites and changes in modulus occur proportionally with increasing amounts of nanoclays and significantly alter material characteristics. Further nanoclays could be attractive additives as anti- combustion materials due to the differential permeability

**Figure 11.**

*(a) Structure of nanoclay exemplified by the (b) TEM image of nontronite. Reproduced with permission from [58].*

behavior to oxygen and carbon dioxide. Nanoclay exhibits better gas barrier properties against oxygen then carbon dioxide and thus has the potential to limit oxygen as a fuel source to flames [61]. The possible tuning of the molecular spacing of silicon tetrahedral units suggests that gas permeability can be altered to a particular gas through morphological changes to the filler as in the case of nitrogen [60]. With special interest to polymer based flame retardants, the dispersion behavior and orientation properties of nanoclay particles can find significant use in generating intercalated layers with polymers that largely depend on their molecular assembly and ability to interact [62]. Also in the interest of minimizing the release of toxic fumes, nanoclays can be used to generate effective flame resistant barriers by making use of nanometer scale surface adhesion properties that extend across micrometer lengths with aspect ratios between 200 ∼ 1000 or indeed higher [63]. Hence a strong correlation of barrier properties to geometric considerations such as orientation, filler morphology and aspect ratios are critical to their functional use as additives in flame retardants to control the spread of fire [64].

To advance fire retardant chemistry however, there exits an underlying need to understand better the molecular transition from the homogenous nanoclay and polymer architectural state to a structurally unified polymer-nanoclay nanocomposite state at the nanometer level. An immiscible state between nanoclays and the surrounding polymer environment lacking a chemical union of bonds as a co-mixture rather than a self-ordered intermixture does not allow ascendency to a superior consolidated mechanically enhanced state. In other words, the filler chemistry must adopt bond, shape, physical and chemical structural characteristics unique to the assembly not shared by the components separately or as a co-mixture. In order to allow the next generation of fire retardants to be of greater general applicability and of broader scope to different polymer populations, the exfoliation of nanoclay inter-layers requires the use of interactants (modifiers such as inorganic minerals, synthetics, hydrophilic or hydrophobic solvents, dispersants, reductants/oxidants and other chemical agents under conducive physical conditions) so that the chemistry and spacing between interlayers is compatible with the polymer environment.

## **6. Synergistic effects of nanoclays**

A well-recognized route for the suppression of flames and probably the best studied process has been described by carbonaceous char formation exhibiting

**21**

properties of complexes.

**Figure 12.**

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

barrier properties [65] that effectively 'cut-off' gaseous fuel supply reducing flame capability. Decade old studies on nanoclay composites have shown to provide 'multistage' insulation during fire progression, however the dependence of fire performance on a measurable quantity of nanoclay was prominent ranging from optimal to infective in delaying and an advancing fire hazard [66]. Such studies have highlighted the complex chemical nature of nanoparticle interpenetration between layers (**Figure 12**) at nanoclay interfaces and the limited knowledge in selecting and activating chemically useful routes that diminish flame growth against accelerating factors. The ambiguity in the nature of these mechanisms is complicated by molecular changes that occur with nanoclays at the surface of different polymers pointing to the nature of interacting species that may be consumed or liberated during different stages of thermal reaction. Char formation is mediated via the catalytic crosslinking of polymers with properties that are identifiably different to their constituent reactants both by their physical and chemical nature. Nitrogen metal complexes have been suggested to counteract barrier char formation by catalytically weakening polymer stability through site specific decomposition by virtue of the metallic catalytic sites accommodated within the clay region [68]. The extent to which these processes operate play a central role in determining polymer stability and the dominant mechanisms that ultimately prevail However, dispersion becomes an important criteria at the nanoscale level in determining effective flame retardant properties [69] while a more informed selection of polymer types forming the nanoclay-composite chemistry can used to increase the carbon content of char [70] likely favoring crosslinking ability and subsequent barrier properties. The ionic nature of nanoclays also plays a significant role in governing the flame retardant

*Layered geometry visualization of nanoclay by atomic force microscopy showing the importance of ionic interpenetration at the nanoscale with particle diameter changes from 70–130 nm. Reproduced from [67].*

Polymer variability in terms of composition, structure and intrinsic properties are defining features that determine the degree of material inflammability. Understanding the mechanistic role of nanoclay particles in subduing polymer vulnerability to thermal heating and volatility becomes a challenging problem in the field of flame retardancy. **Figure 13** shows the chemical integration of polymernanoclay surface where the interface chemistry is a key factor in deciding flame retardant properties of the resulting nanocomposite. Some generalized insights into possible mechanisms have emerged from fire retardant investigations. As mentioned, much effort has been directed in uncovering the complexities surrounding char deposition pathways to improve mechanisms that have an obvious advantage in blockading the mass transport of fire enhancers (e.g. free radicals, gases,

*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 12.**

*Flame Retardant and Thermally Insulating Polymers*

**Figure 11.**

*from [58].*

behavior to oxygen and carbon dioxide. Nanoclay exhibits better gas barrier properties against oxygen then carbon dioxide and thus has the potential to limit oxygen as a fuel source to flames [61]. The possible tuning of the molecular spacing of silicon tetrahedral units suggests that gas permeability can be altered to a particular gas through morphological changes to the filler as in the case of nitrogen [60]. With special interest to polymer based flame retardants, the dispersion behavior and orientation properties of nanoclay particles can find significant use in generating intercalated layers with polymers that largely depend on their molecular assembly and ability to interact [62]. Also in the interest of minimizing the release of toxic fumes, nanoclays can be used to generate effective flame resistant barriers by making use of nanometer scale surface adhesion properties that extend across micrometer lengths with aspect ratios between 200 ∼ 1000 or indeed higher [63]. Hence a strong correlation of barrier properties to geometric considerations such as orientation, filler morphology and aspect ratios are critical to their functional use as

*(a) Structure of nanoclay exemplified by the (b) TEM image of nontronite. Reproduced with permission* 

To advance fire retardant chemistry however, there exits an underlying need to understand better the molecular transition from the homogenous nanoclay and polymer architectural state to a structurally unified polymer-nanoclay nanocomposite state at the nanometer level. An immiscible state between nanoclays and the surrounding polymer environment lacking a chemical union of bonds as a co-mixture rather than a self-ordered intermixture does not allow ascendency to a superior consolidated mechanically enhanced state. In other words, the filler chemistry must adopt bond, shape, physical and chemical structural characteristics unique to the assembly not shared by the components separately or as a co-mixture. In order to allow the next generation of fire retardants to be of greater general applicability and of broader scope to different polymer populations, the exfoliation of nanoclay inter-layers requires the use of interactants (modifiers such as inorganic minerals, synthetics, hydrophilic or hydrophobic solvents, dispersants, reductants/oxidants and other chemical agents under conducive physical conditions) so that the chemistry and spacing between interlayers is

A well-recognized route for the suppression of flames and probably the best studied process has been described by carbonaceous char formation exhibiting

additives in flame retardants to control the spread of fire [64].

compatible with the polymer environment.

**6. Synergistic effects of nanoclays**

**20**

*Layered geometry visualization of nanoclay by atomic force microscopy showing the importance of ionic interpenetration at the nanoscale with particle diameter changes from 70–130 nm. Reproduced from [67].*

barrier properties [65] that effectively 'cut-off' gaseous fuel supply reducing flame capability. Decade old studies on nanoclay composites have shown to provide 'multistage' insulation during fire progression, however the dependence of fire performance on a measurable quantity of nanoclay was prominent ranging from optimal to infective in delaying and an advancing fire hazard [66]. Such studies have highlighted the complex chemical nature of nanoparticle interpenetration between layers (**Figure 12**) at nanoclay interfaces and the limited knowledge in selecting and activating chemically useful routes that diminish flame growth against accelerating factors. The ambiguity in the nature of these mechanisms is complicated by molecular changes that occur with nanoclays at the surface of different polymers pointing to the nature of interacting species that may be consumed or liberated during different stages of thermal reaction. Char formation is mediated via the catalytic crosslinking of polymers with properties that are identifiably different to their constituent reactants both by their physical and chemical nature. Nitrogen metal complexes have been suggested to counteract barrier char formation by catalytically weakening polymer stability through site specific decomposition by virtue of the metallic catalytic sites accommodated within the clay region [68]. The extent to which these processes operate play a central role in determining polymer stability and the dominant mechanisms that ultimately prevail However, dispersion becomes an important criteria at the nanoscale level in determining effective flame retardant properties [69] while a more informed selection of polymer types forming the nanoclay-composite chemistry can used to increase the carbon content of char [70] likely favoring crosslinking ability and subsequent barrier properties. The ionic nature of nanoclays also plays a significant role in governing the flame retardant properties of complexes.

Polymer variability in terms of composition, structure and intrinsic properties are defining features that determine the degree of material inflammability. Understanding the mechanistic role of nanoclay particles in subduing polymer vulnerability to thermal heating and volatility becomes a challenging problem in the field of flame retardancy. **Figure 13** shows the chemical integration of polymernanoclay surface where the interface chemistry is a key factor in deciding flame retardant properties of the resulting nanocomposite. Some generalized insights into possible mechanisms have emerged from fire retardant investigations. As mentioned, much effort has been directed in uncovering the complexities surrounding char deposition pathways to improve mechanisms that have an obvious advantage in blockading the mass transport of fire enhancers (e.g. free radicals, gases,

**Figure 13.**

*Clay-polymer assembly processes under thermally elevated conditions determine fire retardant potency driven by nanoscale chemistry.*

hydrocarbons etc.) that can be released from clay surfaces via an initial pyrolytic state. Barrier properties become progressively more effective during the condensed phase as more material is retained and cross-linked leading to enhancement with subsequent layers. The synergistic cooperation between nanoclay layers and mineral addictive's has attracted much attention in recent years and has rapidly become the most favored approach in the field of flame retardancy.

While the immobilization of decomposed components aided by low migration at the surface of silicate layers is an important mechanistic approach to barrier creation and polymer modification, the unrestricted migration of the more volatile degradation products can release free radicals with the ability to passivate halogen radicals through the chemical association with hydrocarbon radicals. Such radicals may take the form of metals embedded or chemically attached to the nanoclay polymer matrix. The intrinsic and selective nature of the polymer materials modifiable by thermal processes potentially embody exploitable synergistic characteristics particularly at the nanoscale. For example, improvement in interfacial adhesion has been correlated to a change in thermal decomposition to elevated temperatures during the merger of nanoclays with flame retardant supporting the synergism of nanoclays to char layer enhancement [71]. Hence, the chemical synergy between the reactive nature of starting materials (nanoclay, polymers and minerals), the initiation temperature and pathway for thermal activation of condensed and volatile degradation products and the knowledge of key mechanistic events in terms of mapping structure to flame retardant function and performance still need to be resolved. Certainly, the correlation between clay composition and decompsition kinetics ohas been identified to be a key factor in charring [72]. Despite the hidden challenges which persist in the field, the effects of scale are slowly emerging using a number of experimental approaches. Some observations that form the basis of well-accepted outcomes coupled to unresolved mechanistic ideas are generalized in **Figure 14**.

The potential for polymer-layered nanoclay composites for applications as flame retardants has been recognized in earlier studies [73]. A common objective in nanocomposite design has been the search for effective interacting agents for the alignment of polymers along the corridor-like arrangement in stacked layers of nanoclays to improve mechanical strength, thermal stability and gas barrier properties. Diagrammatically, the stacking is described as intercalated in which polymers lie between inorganic layers in well-ordered arrangements or adopt exfoliated patterns represented by disordered arrangements within the layers exemplified by poly(ethylene oxide) adsorption on nanoclay surfaces (**Figure 15**). In similar studies [75], the use of polypropylene-graft-maleic anhydride and polystyrene-layeredsilicate nanocomposites in their ability to suppress flames was mechanistically similar and it was established that the behavioral properties depended on the degree

**23**

**Figure 14.**

**Figure 15.**

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

of dispersibility and type of silicate material. The effects of flammability reduction were inferred through a protective insulation surface slowing down the decomposition rate of the underlying material and thus preventing exposure to the decomposition products. Mechanistic insight has been better aided by knowledge of the molecular orientation of nanoclays that have resulted in establishing three types of nanoclay geometries. Understanding the geometrical orientations of nanoclay shown in **Figure 16** may help in our decipher the barrier properties exhibited by these class of materials which exist as (a) phase separated (immiscible), (b) intercalated or (c)

*Schematic showing two types of silicate-polymer composite layering either as (a) ordered (intercalated) or (b)* 

*disordered (delaminated) arrangements. Reproduced with permission from [74].*

*General mechanistic routes to fame retardancy by char barrier formation and free radical suppression mediated by catalytically induced sites at the nanoclay polymer interfaces. Migration of particles formed by degradation products originating from pyrolytic chemical processes are assisted by air bubble movement pushing particles* 

*upwards and low migration of condensed phased char particles.*

*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 14.**

*Flame Retardant and Thermally Insulating Polymers*

hydrocarbons etc.) that can be released from clay surfaces via an initial pyrolytic state. Barrier properties become progressively more effective during the condensed phase as more material is retained and cross-linked leading to enhancement with subsequent layers. The synergistic cooperation between nanoclay layers and mineral addictive's has attracted much attention in recent years and has rapidly become the

*Clay-polymer assembly processes under thermally elevated conditions determine fire retardant potency driven* 

While the immobilization of decomposed components aided by low migration at the surface of silicate layers is an important mechanistic approach to barrier creation and polymer modification, the unrestricted migration of the more volatile degradation products can release free radicals with the ability to passivate halogen radicals through the chemical association with hydrocarbon radicals. Such radicals may take the form of metals embedded or chemically attached to the nanoclay polymer matrix. The intrinsic and selective nature of the polymer materials modifiable by thermal processes potentially embody exploitable synergistic characteristics particularly at the nanoscale. For example, improvement in interfacial adhesion has been correlated to a change in thermal decomposition to elevated temperatures during the merger of nanoclays with flame retardant supporting the synergism of nanoclays to char layer enhancement [71]. Hence, the chemical synergy between the reactive nature of starting materials (nanoclay, polymers and minerals), the initiation temperature and pathway for thermal activation of condensed and volatile degradation products and the knowledge of key mechanistic events in terms of mapping structure to flame retardant function and performance still need to be resolved. Certainly, the correlation between clay composition and decompsition kinetics ohas been identified to be a key factor in charring [72]. Despite the hidden challenges which persist in the field, the effects of scale are slowly emerging using a number of experimental approaches. Some observations that form the basis of well-accepted outcomes coupled to unresolved mechanistic ideas are generalized in

The potential for polymer-layered nanoclay composites for applications as flame retardants has been recognized in earlier studies [73]. A common objective in nanocomposite design has been the search for effective interacting agents for the alignment of polymers along the corridor-like arrangement in stacked layers of nanoclays to improve mechanical strength, thermal stability and gas barrier properties. Diagrammatically, the stacking is described as intercalated in which polymers lie between inorganic layers in well-ordered arrangements or adopt exfoliated patterns represented by disordered arrangements within the layers exemplified by poly(ethylene oxide) adsorption on nanoclay surfaces (**Figure 15**). In similar studies [75], the use of polypropylene-graft-maleic anhydride and polystyrene-layeredsilicate nanocomposites in their ability to suppress flames was mechanistically similar and it was established that the behavioral properties depended on the degree

most favored approach in the field of flame retardancy.

**22**

**Figure 14**.

**Figure 13.**

*by nanoscale chemistry.*

*General mechanistic routes to fame retardancy by char barrier formation and free radical suppression mediated by catalytically induced sites at the nanoclay polymer interfaces. Migration of particles formed by degradation products originating from pyrolytic chemical processes are assisted by air bubble movement pushing particles upwards and low migration of condensed phased char particles.*

#### **Figure 15.**

*Schematic showing two types of silicate-polymer composite layering either as (a) ordered (intercalated) or (b) disordered (delaminated) arrangements. Reproduced with permission from [74].*

of dispersibility and type of silicate material. The effects of flammability reduction were inferred through a protective insulation surface slowing down the decomposition rate of the underlying material and thus preventing exposure to the decomposition products. Mechanistic insight has been better aided by knowledge of the molecular orientation of nanoclays that have resulted in establishing three types of nanoclay geometries. Understanding the geometrical orientations of nanoclay shown in **Figure 16** may help in our decipher the barrier properties exhibited by these class of materials which exist as (a) phase separated (immiscible), (b) intercalated or (c)

#### **Figure 16.**

*Differing morphological states of polymer–nanoclay structure of nanoclay. Reproduced with permission from [76].*

exfoliated as polymer nanocomposites. In (a), polymers do not reside inside the inorganic layers that may exhibit conventional silicate layer properties as layered blocks while (b) shows interlayer distancing of a few nm between silicate layers which is a result of polymer intervention within the layers by the surrounding polymer. Finally (c), shows an exfoliated structure where clay layers are singularly separated by the polymer. Polymer matrix and nanofiller interactions play an important role for atomic-scale nanoclay dynamics. The interaction of the cationic surface of nanoclays with organic layers of polymers is identifiably important in the advancement of flame retardant nanotechnology. There are many reported studies that have pursued the effects of polymer matrix/nanofiller (nanoclay) associations and while there are benefits to the formulations, the precise role of nanoparticle ambulation during surface modification remains unknown. Current knowledge however, stipulates that organic components of polymers can sufficiently reduce excess energy (surface energy) [77] by minimizing cohesive and adhesive forces at the solid-wet interface between cationic and polymer states of matter respectively while supporting the stabilization of interlayer spacing. Spacing serves a useful purpose in reducing surface energy and preventing nanoparticle aggregation generating enhanced dispersion and the level of ordered intercalation largely dictate the nanocomposite structure. In context to **Figure 12**, it might be expected that separation achieved in exfoliated nanocomposites generate maximum space and hence maximum surface area allowing polymer migration to occur and spread along interfaces unperturbed between silicate layers. Schartel et al. [78] demonstrated differing morphologies of phosphonium-modified layered silicate epoxy resin nanocomposites but structures closer to exfoliated forms (**Figure 16**) with silicate as a nanofiller revealed a barrier effect but had little effect on the suppression of decomposition products and combustion of volatiles. The current perception of exfoliated surfaces yielding superior surface and mechanical properties is a phenomenon that is now less clear and conflicting evidenced by experimental interpretations. By its nature, exfoliation generates better dispersity between the nanoclay surface in an uninterrupted polymer matrix. Variability in experimental observations often uncovered inconsistency from glass transition values and suggest that a state of higher thermal stability depends on other factors and not solely on the visible state of a structure. Hence, the level

**25**

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

of unpredictability associated with well-separated exfoliated layers of separation could reside with the degree of variability in interfacial patterns that may ultimately determine polymer mechanics and further complexities introduced by compatibilizers as components for in epoxy adhesives [79] for example, or as thermally reactive additives. The lack of understanding of these factors could be contributing factors to discrepancies among related samples and more insightful explanations are needed to

The use of non-halogenated flame retardant materials have recently come into effect like aluminum diethylphosphinate (ADP) [80] improving char yield. It has been reported that the multi-phase use of ADP in the vaporized and condensed phase [81] significantly contributes to flame inhibition. Investigations by Kaynak and Polat [82] further established the role of path driven intercalated/exfoliated NC layers that contributed to the emergence of insulative barrier properties as judged by the increased limiting oxygen index. Chemical synergy between nanoclay layers and ADP likely favored the retainment of aluminum phosphinate at the clay surface forging the incorporation of the mineral within the layers of the carbonized char residues within the mechanically strengthened polymer structure of diminished chain flexibility. Thermally activated vapourization of ADP triggered the oxidation of surface unbound minerals in the gaseous phase to phosphinate radicals HPO2•, PO•, PO2• creating opportunities to neutralize the toxic effects of H• and OH• curbing flame growth. Here, the use of 5% silicate layers was effective in combination

In an attempt to better elucidate the mechanistic impact of nanoparticles during pyrolysis, two-dimensional transient-state models based on nanoclay layering using carbon nanotubes as potential polymer reinforcement additives. The study revealed a number of nanoscale effects between charred and uncharred regions that span the clay network serving as the major interface in regulating the transportation of surface degraded and diffusive particles that affect thermal transfer [83]. However, the synergistic shift in surface chemistry at the clay-nanoparticle that permits mass transfer loss via polymer degradation by addition of carbon nanotubes during the initial stages of pyrolysis suggests that improved stability around structured nanoclay layers will be pivotal to controlling flame retardant properties in future nanocomposite designs. Tuning the thermal stability of nanoclays with compatibilizers can provide access to the incorporation of smarter materials such nanopolymer confined quantum sized TiO2 [84] that demands better exfoliation of silicate layers

Suter et al. [86] have more recently used a novel multiscale modeling to shed light on the mechanisms driving exfoliation behavioral properties of clay-polymer nanocomposites. The results show how molecular simulation techniques targeted around clay interfaces using free energy profiles and structure based coarse-grained iteration processes to understand clay layer exfoliation and how such interactions lead to experimentally observed changes. This was achieved by simulating the interactions of montmorillonite clay, a polymer (PEG) and a quaternary ammonium dimethyldioctadecylammonium ionic surfactant. **Figure 17(a)** shows molecular dynamics simulations of models predicted I, II and III after a simulation of 3 μs with model II and III forming partially and fully exfoliated single layers respectively. **Figure 17(b)** shows the radial distribution as a function of sheet distance and provides evidence supporting a completely dispersed system. In the bottom panel of **Figure 13**, different stages of the molecular dynamic simulation of model II is presented illustrating (a) the clay structure prior to the interactive state with PEG (b) intercalated nanoclay-PEG formation resulting in increased gallery separation and the (c) departing motion of neighboring of layers favoring an exfoliated increases in Young's modulus derived from the stress–strain behavior models of II and III. The

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

develop favorable material properties.

improving immiscibility of layers with TiO2 [85].

with 15% ADP.

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

of unpredictability associated with well-separated exfoliated layers of separation could reside with the degree of variability in interfacial patterns that may ultimately determine polymer mechanics and further complexities introduced by compatibilizers as components for in epoxy adhesives [79] for example, or as thermally reactive additives. The lack of understanding of these factors could be contributing factors to discrepancies among related samples and more insightful explanations are needed to develop favorable material properties.

The use of non-halogenated flame retardant materials have recently come into effect like aluminum diethylphosphinate (ADP) [80] improving char yield. It has been reported that the multi-phase use of ADP in the vaporized and condensed phase [81] significantly contributes to flame inhibition. Investigations by Kaynak and Polat [82] further established the role of path driven intercalated/exfoliated NC layers that contributed to the emergence of insulative barrier properties as judged by the increased limiting oxygen index. Chemical synergy between nanoclay layers and ADP likely favored the retainment of aluminum phosphinate at the clay surface forging the incorporation of the mineral within the layers of the carbonized char residues within the mechanically strengthened polymer structure of diminished chain flexibility. Thermally activated vapourization of ADP triggered the oxidation of surface unbound minerals in the gaseous phase to phosphinate radicals HPO2•, PO•, PO2• creating opportunities to neutralize the toxic effects of H• and OH• curbing flame growth. Here, the use of 5% silicate layers was effective in combination with 15% ADP.

In an attempt to better elucidate the mechanistic impact of nanoparticles during pyrolysis, two-dimensional transient-state models based on nanoclay layering using carbon nanotubes as potential polymer reinforcement additives. The study revealed a number of nanoscale effects between charred and uncharred regions that span the clay network serving as the major interface in regulating the transportation of surface degraded and diffusive particles that affect thermal transfer [83]. However, the synergistic shift in surface chemistry at the clay-nanoparticle that permits mass transfer loss via polymer degradation by addition of carbon nanotubes during the initial stages of pyrolysis suggests that improved stability around structured nanoclay layers will be pivotal to controlling flame retardant properties in future nanocomposite designs. Tuning the thermal stability of nanoclays with compatibilizers can provide access to the incorporation of smarter materials such nanopolymer confined quantum sized TiO2 [84] that demands better exfoliation of silicate layers improving immiscibility of layers with TiO2 [85].

Suter et al. [86] have more recently used a novel multiscale modeling to shed light on the mechanisms driving exfoliation behavioral properties of clay-polymer nanocomposites. The results show how molecular simulation techniques targeted around clay interfaces using free energy profiles and structure based coarse-grained iteration processes to understand clay layer exfoliation and how such interactions lead to experimentally observed changes. This was achieved by simulating the interactions of montmorillonite clay, a polymer (PEG) and a quaternary ammonium dimethyldioctadecylammonium ionic surfactant. **Figure 17(a)** shows molecular dynamics simulations of models predicted I, II and III after a simulation of 3 μs with model II and III forming partially and fully exfoliated single layers respectively. **Figure 17(b)** shows the radial distribution as a function of sheet distance and provides evidence supporting a completely dispersed system. In the bottom panel of **Figure 13**, different stages of the molecular dynamic simulation of model II is presented illustrating (a) the clay structure prior to the interactive state with PEG (b) intercalated nanoclay-PEG formation resulting in increased gallery separation and the (c) departing motion of neighboring of layers favoring an exfoliated increases in Young's modulus derived from the stress–strain behavior models of II and III. The

*Flame Retardant and Thermally Insulating Polymers*

exfoliated as polymer nanocomposites. In (a), polymers do not reside inside the inorganic layers that may exhibit conventional silicate layer properties as layered blocks while (b) shows interlayer distancing of a few nm between silicate layers which is a result of polymer intervention within the layers by the surrounding polymer. Finally (c), shows an exfoliated structure where clay layers are singularly separated by the polymer. Polymer matrix and nanofiller interactions play an important role for atomic-scale nanoclay dynamics. The interaction of the cationic surface of nanoclays with organic layers of polymers is identifiably important in the advancement of flame retardant nanotechnology. There are many reported studies that have pursued the effects of polymer matrix/nanofiller (nanoclay) associations and while there are benefits to the formulations, the precise role of nanoparticle ambulation during surface modification remains unknown. Current knowledge however, stipulates that organic components of polymers can sufficiently reduce excess energy (surface energy) [77] by minimizing cohesive and adhesive forces at the solid-wet interface between cationic and polymer states of matter respectively while supporting the stabilization of interlayer spacing. Spacing serves a useful purpose in reducing surface energy and preventing nanoparticle aggregation generating enhanced dispersion and the level of ordered intercalation largely dictate the nanocomposite structure. In context to **Figure 12**, it might be expected that separation achieved in exfoliated nanocomposites generate maximum space and hence maximum surface area allowing polymer migration to occur and spread along interfaces unperturbed between silicate layers. Schartel et al. [78] demonstrated differing morphologies of phosphonium-modified layered silicate epoxy resin nanocomposites but structures closer to exfoliated forms (**Figure 16**) with silicate as a nanofiller revealed a barrier effect but had little effect on the suppression of decomposition products and combustion of volatiles. The current perception of exfoliated surfaces yielding superior surface and mechanical properties is a phenomenon that is now less clear and conflicting evidenced by experimental interpretations. By its nature, exfoliation generates better dispersity between the nanoclay surface in an uninterrupted polymer matrix. Variability in experimental observations often uncovered inconsistency from glass transition values and suggest that a state of higher thermal stability depends on other factors and not solely on the visible state of a structure. Hence, the level

*Differing morphological states of polymer–nanoclay structure of nanoclay. Reproduced with permission* 

**24**

**Figure 16.**

*from [76].*

**Figure 17.**

*Dynamic simulation models of (a and b) of (c) partially and fully exfoliated nanocomposite state. The exfoliated models are found to exhibit considerable elastic properties with substantial structures based on model II as described in the text. Modified with permission from [86].*

density charge increase around the polymers attributed to the surfactant molecules rendering the nanoclay structure to an exfoliated state by inhibiting a diffusive state which is assisted by a transversally sliding motion of clay. Such simulations provide plausible clarity on the dynamics of nanoclay sheets and important mechanistic clues that can be evidenced and supported by experimental findings with additives. As simulation tools become more powerful, it will become easier to assign definitive structure–function relationships to earlier observations that have been more phenomenological in their explanatory findings. The re-organization of intercalated or exfoliated states in nanoclay based nanocomposites is a promising way to better regulate not only polymer behavior but also the behavior of flame retardants under nanoconfinement signifying the functional role of clay particle size [87].

Nanoconfinement of polymer chains in nanoclays can influence polymer behavior affecting the chemical and physical behavior with additives in the melt, altering decomposition, thermal degradation and intermolecular interactions kinetically of components globally determined by the local morphological states that adapt to intercalated or exfoliated or indeed partial intermediate states. Over the last two-to-three decades, polymer research has intensified the use of nanofiller materials to form polymer bends into tunable nanocomposites. Nanofillers in the form of layered silicates have enabled a considerable reduction in the use of conventional load bearing modifiers in comparison to their low dimensional counterparts by as much as 3 ∼ 8 fold of the total content normally used by conventional processes. A significant trend that continues to be a promising pathway for the future use of flame retardants is the chemical synergy driving nanocomposite assembly while extinguishing the properties of its individual components in the blend. Some of

**27**

**Figure 18.**

*as the polymer matrix.*

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

ing heat release rate through the formation of dense char layer [88].

changes affecting interactions at the molecular level.

these examples include the synergistic effects of clay-organic intumescent hybrid systems. Distances among silicate layers in nanoclay modified with organic surfactants show functional significance in the absence and presence of intumescent agents. Irrespective of whether nanoclays adopt an exfoliated or intercalated state of matter, increase in the layer distance from 9.8 to 13.8 Å revealed that the stacking morphological architecture of the nanocomposite was important to its role in reduc-

Here, we provide a summary of some of the more recent approaches and strategies in flame retardant design used for targeting flammable plasticizers widely used industrial polymers with reference to some key studies on nanoclay-polymer nanocomposites from pervious works. In earlier work, quin et al. [89] in their investigation with nanoclay-polypropylene aimed to clarify some unresolved aspects of key mechanisms of flame retardancy. The fundamental problem related to the dispersion state of the nanocomposite and addressed its influence on flammability which tied in with the underlying role of nanoclay in thermal oxidative degradation and combustion. It was concluded the delay to combustible ignition of volatiles was in fact due to the char barrier formation and the route to char deposition occurred via catalytic dehydrogenation and crosslinking of the nanocomposite which was mainly assisted by acid sites of the silicate. The process is summarized in **Figure 18**. An important component in this mechanism that is most relevant to the liberation and availability of silicate catalytic sites was the addition of the alkyammonium salt which in itself decomposes leading to the pyrolytic release of ammonia and olefin hydrocarbons and the accompaniment of rich acidic cations on silicate surfaces. While this mechanism was effective in suppressing flammability, evidence continues to accumulate and support increased mechanical strength and hardness capacity of nanoclay modified polymers enhanced by significant increases in tensile strength and stiffness using tetramethylammonium chloride but may decline with suboptimal quantities [90]. For example, the addition of nanoclays have shown to disadvantage the working performance of flame retardants as noted for a phosphorous-based retardant [91] comprising a Diglycidyl Ether of Bisphenol A (DGEBA) resin composite. This emphasizes the importance of tailoring the cooperativity between the filler and polymer space and dispersion which facilitates the nanoscale chemistry of the retardant components in the composite in response to parametric

A future direction in the controlled degradation of polymers using regulated by filler amounts is the prediction of degradation products. In a recent investigation,

*Active-site catalytically induced char formation during combustion on clay surfaces supported by polypropylene* 

*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*

these examples include the synergistic effects of clay-organic intumescent hybrid systems. Distances among silicate layers in nanoclay modified with organic surfactants show functional significance in the absence and presence of intumescent agents. Irrespective of whether nanoclays adopt an exfoliated or intercalated state of matter, increase in the layer distance from 9.8 to 13.8 Å revealed that the stacking morphological architecture of the nanocomposite was important to its role in reducing heat release rate through the formation of dense char layer [88].

Here, we provide a summary of some of the more recent approaches and strategies in flame retardant design used for targeting flammable plasticizers widely used industrial polymers with reference to some key studies on nanoclay-polymer nanocomposites from pervious works. In earlier work, quin et al. [89] in their investigation with nanoclay-polypropylene aimed to clarify some unresolved aspects of key mechanisms of flame retardancy. The fundamental problem related to the dispersion state of the nanocomposite and addressed its influence on flammability which tied in with the underlying role of nanoclay in thermal oxidative degradation and combustion. It was concluded the delay to combustible ignition of volatiles was in fact due to the char barrier formation and the route to char deposition occurred via catalytic dehydrogenation and crosslinking of the nanocomposite which was mainly assisted by acid sites of the silicate. The process is summarized in **Figure 18**. An important component in this mechanism that is most relevant to the liberation and availability of silicate catalytic sites was the addition of the alkyammonium salt which in itself decomposes leading to the pyrolytic release of ammonia and olefin hydrocarbons and the accompaniment of rich acidic cations on silicate surfaces. While this mechanism was effective in suppressing flammability, evidence continues to accumulate and support increased mechanical strength and hardness capacity of nanoclay modified polymers enhanced by significant increases in tensile strength and stiffness using tetramethylammonium chloride but may decline with suboptimal quantities [90]. For example, the addition of nanoclays have shown to disadvantage the working performance of flame retardants as noted for a phosphorous-based retardant [91] comprising a Diglycidyl Ether of Bisphenol A (DGEBA) resin composite. This emphasizes the importance of tailoring the cooperativity between the filler and polymer space and dispersion which facilitates the nanoscale chemistry of the retardant components in the composite in response to parametric changes affecting interactions at the molecular level.

A future direction in the controlled degradation of polymers using regulated by filler amounts is the prediction of degradation products. In a recent investigation,

#### **Figure 18.**

*Active-site catalytically induced char formation during combustion on clay surfaces supported by polypropylene as the polymer matrix.*

*Flame Retardant and Thermally Insulating Polymers*

density charge increase around the polymers attributed to the surfactant molecules rendering the nanoclay structure to an exfoliated state by inhibiting a diffusive state which is assisted by a transversally sliding motion of clay. Such simulations provide plausible clarity on the dynamics of nanoclay sheets and important mechanistic clues that can be evidenced and supported by experimental findings with additives. As simulation tools become more powerful, it will become easier to assign definitive structure–function relationships to earlier observations that have been more phenomenological in their explanatory findings. The re-organization of intercalated or exfoliated states in nanoclay based nanocomposites is a promising way to better regulate not only polymer behavior but also the behavior of flame retardants under

*Dynamic simulation models of (a and b) of (c) partially and fully exfoliated nanocomposite state. The exfoliated models are found to exhibit considerable elastic properties with substantial structures based on model* 

*II as described in the text. Modified with permission from [86].*

nanoconfinement signifying the functional role of clay particle size [87].

Nanoconfinement of polymer chains in nanoclays can influence polymer behavior affecting the chemical and physical behavior with additives in the melt, altering decomposition, thermal degradation and intermolecular interactions kinetically of components globally determined by the local morphological states that adapt to intercalated or exfoliated or indeed partial intermediate states. Over the last two-to-three decades, polymer research has intensified the use of nanofiller materials to form polymer bends into tunable nanocomposites. Nanofillers in the form of layered silicates have enabled a considerable reduction in the use of conventional load bearing modifiers in comparison to their low dimensional counterparts by as much as 3 ∼ 8 fold of the total content normally used by conventional processes. A significant trend that continues to be a promising pathway for the future use of flame retardants is the chemical synergy driving nanocomposite assembly while extinguishing the properties of its individual components in the blend. Some of

**26**

**Figure 17.**

Saha et al. [92] applied reactive force field molecular dynamics simulation to polyacrylester in which clay nanofillers in conjunction with graphene was used to modify the elastomer polymer characteristics. The model developed using this approach very reliably predicted the effect of adding graphene oxide (GO) in relation to the evolution of degradation products which agreed well with the experimental outcome. Improving the knowledge base of volatile materials originating from different polymer compositions computationally will certainly have meritable consequences in implementing better control strategies against flame enhancing volatile materials [93] and predicting combustion behavior such as ignition times [94], char oxidation and particle sizes [95]. The differences in melting and shrinking behavior that arise from physical conditions affecting ignition performance for example and other important parameters may not be easily accurately measured by micro-scale tests [96] will significantly shift the burden for reliability more towards computation methods.
