Nanostructure of montmorillonite barrier layers: A new insight into the mechanism of flammability reduction in polymer nanocomposites

doi:10.1016/j.polymdegradstab.2011.09.021

Polymer Degradation and Stability

Volume 96, Issue 12, December 2011, Pages 2284-2289

https://doi.org/10.1016/j.polymdegradstab.2011.09.021 Get rights and content

Abstract

This study describes the mechanism of flammability reduction in flame-retarded polymer matrix organo-montmorillonite reinforced nanocomposites. Morphologies of untested polymer nanocomposites and char residues formed by combustion in the mass loss calorimeter are characterized by XRD and TEM techniques. It is postulated that a combination of well-dispersed montmorillonite platelets and flame retardants in the polymer matrix provides nano-structured char formation. Initial montmorillonite dispersion in flame-retarded nanocomposites is found to be a major controlling factor on formed char nanostructures. An initially intercalated structure is invariantly converted to complete montmorillonite collapse whereas an initially exfoliated structure transforms to nano-structured chars demonstrating retained exfoliation or a new state of intercalation via incomplete collapse of montmorillonite layers. It is proposed that nano-structured char formation is the effective mechanism of flammability reduction, i.e. reduction in rate of heat release during combustion, in flame-retarded polymer nanocomposites.

Introduction

Incorporation of nanoparticles into polymer matrices to form polymer nanocomposites (PNCs) is a versatile approach to effectively reduce the flammability of polymers. It is generally accepted that nanoparticles function through an ascribed mechanism of formation of a surface layer that shields the underlying polymer against external heat exposure and impedes diffusion of volatiles during pyrolysis and combustion.

The formation of a superficial barrier layer during pyrolysis/combustion via migration and accumulation of nanoparticles at the exposed surface is driven in principle by (i) thermodynamic effects, (ii) temperature and viscosity gradients, and (iii) rising gas bubbles [1]. In the case of polymer/organo-montmorillonite nanocomposites, with the decomposition and escape of organic modifier at high temperatures, montmorillonite platelets collapse and at the same time become incompatible with the host polymer. Thus, rapid migration takes place towards the exposed surface [2], [3], [4], [5], [6] where montmorillonite platelets accumulate to form the barrier layer.

In the particular case of flame-retarded polymer nanocomposites (FR-PNCs), i.e. materials that combine nanoparticles and conventional flame retardant additives, well-dispersed montmorillonite platelets in the matrix were found to play a physical role in reinforcing the char residue formed during combustion [7], [8]. By this means, the residue is rendered more consolidated, crack-free and mechanically strong so that it demonstrates better transport barrier properties. Although there appeared a number of publications on the structure of carbonaceous montmorillonite barrier layers formed by thermal degradation or combustion of PNCs without flame retardants [6], [9], [10], [11], [12], [13], to the best of our knowledge, there exists no reports discussing the barrier nanostructure formed by combustion of flame-retarded PNCs. Therefore, in this study, nanostructure of char residues formed under forced flaming conditions in the mass loss calorimeter was investigated ex-situ using X-ray diffraction (XRD) and transmission electron microscopy (TEM) in order to understand the mechanism of flammability reduction in flame-retarded polymer nanocomposites.

Section snippets

Materials and processing

Polyamide-6 (PA6; DSM Akulon K125), polylactide (PLA; Cargill Dow) and poly(methyl methacrylate) (PMMA; Altuglas V825 T) were obtained from commercial sources. One tailed quaternary ammonium cation modified montmorillonite (Cloisite 30B) was supplied by Southern Clay Products, Inc (USA). Flame retardant additive, based on metal phosphinate and synergists was kindly provided by Clariant (Germany). Table 1 lists the designations and compositions of the studied materials. Polymer nanocomposites

Results and discussion

Polymer nanocomposites with a variety of montmorillonite dispersion states, i.e. initial nanostructures, were obtained by utilizing different polymer matrix materials in combination with an organo-montmorillonite and phosphorus-based flame retardant additive. It can be inferred from bright-field TEM micrographs (Fig. 1) and corresponding XRD patterns (Fig. 2) that FR-PNC1 contains well-dispersed, exfoliated montmorillonite layers with almost no layer stacking resulting in the absence of (001)

Conclusion

In this study, a completely different mechanism is proposed for flammability reduction in flame-retarded polymer/organo-montmorillonite nanocomposites compared to the one operative in traditional nanocomposites without flame retardant additives. It was first confirmed for nanocomposites with different states of initial montmorillonite dispersion that complete collapse of montmorillonite occurs leading to final interlayer spacing of d₀₀₁ ∼ 1.3 nm in the absence of flame retardants.

Unlike

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The reason may be that the big molecules in nanocomposite material entered the interlayer of EA-Mt in the reaction process. No basal reflections in the measurable range could be detected which would mean that Mt layers stacking were hindered due to rapid charring and a char nanostructure with retained exfoliation was formed (Nihat and Cevdet, 2011). The crystallinities of EA-Mt/MRO nanocomposite materials were obtained by X-ray diffraction with the help of MDI/JADE5.0 software in Fig. 3(b).
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Short noteNanostructure of montmorillonite barrier layers: A new insight into the mechanism of flammability reduction in polymer nanocomposites

Abstract

Introduction

Section snippets

Materials and processing

Results and discussion

Conclusion

Polymer

Polym Degrad Stab

Polym Degrad Stab

Appl Clay Sci

Appl Clay Sci

Polym Degrad Stab

Polymer

Appl Clay Sci

Short note
Nanostructure of montmorillonite barrier layers: A new insight into the mechanism of flammability reduction in polymer nanocomposites