Study of hydromagnesite and magnesium hydroxide based fire retardant systems for ethylene–vinyl acetate containing organo-modified montmorillonite

doi:10.1016/j.polymdegradstab.2006.08.011

Polymer Degradation and Stability

Volume 91, Issue 12, December 2006, Pages 3074-3082

https://doi.org/10.1016/j.polymdegradstab.2006.08.011 Get rights and content

Abstract

A new flame retardant (FR) system for ethylene–vinyl acetate, mainly based on the combination of hydromagnesite (HM, obtained from an industrial by-product) and organo-modified montmorillonite (oMMT) has been compared with a magnesium hydroxide (MDH) and oMMT flame retardant system. The presence of oMMT in association with both hydrated minerals gave a strong decrease of heat release rate in cone calorimeter tests. Moreover, the HM/oMMT combination leads to a better improvement of resistance to ignition and self-extinguishability in comparison with the MDH/oMMT one. The study of residues formed during thermal decomposition revealed the formation of forsterite (Mg₂SiO₄) when either MDH or HM was used in combination with oMMT. SEM observations of residues showed sintering of the mineral particles at high temperature particularly in the case of HM/oMMT composition.

Introduction

Fire retardancy of EVA is currently achieved using hydrated mineral fillers such as alumina trihydrate (ATH) or magnesium hydroxide (MDH). However, high filler contents (possibly higher than 60% w/w) are required to obtain satisfactory fire properties [1], [2]. This high mineral loading results in a decrease of the mechanical performance of the materials. Therefore, in order to obtain a set of competitive properties, it becomes interesting to enhance the efficiency of the hydrated minerals by partially substituting them with synergistic additives.

Recently, a great interest has been found in the partial substitution of ATH or MDH by organo-modified montmorillonite (oMMT) nanoparticles. These organoclays had been previously shown [3] to slow down the thermal degradation of the deacetylated polymer in EVA compounds, due to the formation of a charred barrier at the surface of the materials. Therefore, it has been demonstrated that modified nanoclays combined with ATH allow an improvement of the fire behaviour by a charring effect [4]. In a recent paper [5], it was also shown that for similar formulations of EVA filled with magnesium hydroxide (MDH) and organoclays, the global improvement of fire performance could be ascribed to the formation of expanded structures by a foaming process. This intumescence phenomenon, which occurs during the pre-ignition period in cone calorimeter tests, results from the trapping of gas bubbles formed by the decomposition products of EVA, magnesium hydroxide and organoclay. This process seems to be governed by bubble heterogeneous nucleation and viscosity gradients. It is also noticed that, according to X-ray diffraction results, organoclay platelets were apparently not completely exfoliated in the material [5].

In this work, we have studied the thermal decomposition and the fire properties of EVA containing hydromagnesite/organo-modified montmorillonite blend in comparison with magnesium hydroxide/organo-modified montmorillonite. Particular attention has been focussed on the structure of combustion residues in order to investigate expansion phenomena and also possible chemical interactions between the mineral components.

Basic magnesium carbonates have been previously reported as suitable inorganic flame retardants for thermoplastics [6], [7]. Hydromagnesite, a basic magnesium carbonate used in this study was obtained by Magnesitas Navarras S.A from an industrial by-product rich in magnesium oxide [8], [9], with the following chemical formula 4MgCO₃·Mg(OH)₂·4H₂O. In the temperature range of 200–550 °C, it undergoes an endothermic decomposition with an associated heat of 800 J/g and a final weight loss of 54%. The steps of decomposition reactions are shown below [10] and illustrated by the TGA curve in Fig. 1:4MgCO₃·Mg(OH)₂·4H₂O → 4MgCO₃·Mg(OH)₂ + 4H₂O4MgCO₃·Mg(OH)₂ → 4MgCO₃·MgO + H₂O4MgCO₃·MgO → 5MgO + 4CO₂

This endothermic release, as well as the secondary effects like dilution and cooling of the gas phase by water vapour and carbon dioxide released and the formation of a MgO ceramic substrate, makes hydromagnesite a potential flame retardant, acting with a mechanism similar to those of metal hydroxides like aluminium or magnesium hydroxide [6], [7], [8], [9], [10], [11].

Nevertheless, the amount of water release and the decomposition enthalpies corresponding to the above three endothermic phenomena are smaller than for both metallic hydroxides, as shown in Table 1.

Section snippets

Materials

Copolymers used in this study were ethylene–vinyl acetate (EVA) with, 28 wt% vinyl acetate (Elvax 260 from Du Pont).

Nanoclay (Nanofil 5, alkylammonium modified montmorillonite type) was supplied by Sud Chemie. Magnesium hydroxide was produced by Martinswerk (Magnifin H10, d₅₀ = 0.85 μm, specific surface area = 10 m²/g). Hydromagnesite was synthesised by Magnesitas Navarras S.A and ground at the laboratory (CMGD) to obtain a median particle size of 14 μm.

Processing

Blending of fillers with molten EVA pellets was

Thermal analysis

Fig. 1, Fig. 2, respectively, show the mass loss and heat flow curves of HM, MDH and studied EVA compositions. One can observe that the EVA/MDH composition is thermally more stable than EVA/HM. This can be ascribed to the mass loss features of both hydrated minerals. MDH starts to lose its water content (31.5 wt% of the hydrated mineral) at around 300 °C, whereas in the case of HM, the mass loss is more progressive and the decomposition enthalpies are lesser than for MDH (cf Table 1). This

Conclusions

The partial substitution of both magnesium hydroxide (MDH) and hydromagnesite (HM) by oMMT in EVA leads to an improvement of the fire properties (peak of HRR and auto-extinguishability). Moreover, this substitution imparts cohesion to the residues formed during the thermal decomposition of EVA in comparison with the behaviour of EVA/MDH or EVA/HM compositions at the same global loading (60 wt%).

In comparison with EVA/MDH/oMMT composition, EVA/HM/oMMT shows better results in terms of

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Study of hydromagnesite and magnesium hydroxide based fire retardant systems for ethylene–vinyl acetate containing organo-modified montmorillonite

Abstract

Introduction

Section snippets

Materials

Processing

Thermal analysis

Conclusions

Eur Polym J

Polymer (Guildford)

Polym Degrad Stab

Polym Degrad Stab

Polym Degrad Stab

Polymer

Fire Mater