Effect of hydroxides and hydroxycarbonate structure on fire retardant effectiveness and mechanical properties in ethylene-vinyl acetate copolymer

doi:10.1016/S0141-3910(01)00167-7

Polymer Degradation and Stability

Volume 74, Issue 3, 2001, Pages 457-464

https://doi.org/10.1016/S0141-3910(01)00167-7 Get rights and content

Abstract

The flame retardant effect of inorganic hydroxides, like aluminum hydroxide and magnesium hydroxide in ethylene vinyl acetate copolymer, was evaluated and compared with that of other inorganic fillers such as boehmite and hydrotalcite. The thermal decomposition of the fillers and the mixtures with EVA polymer were analysed by differential scanning calorimetry (DSC), Thermogravimetry (TG) and X-ray diffraction (XRD). The most significant flame retardant effects, observed using a mass loss calorimeter, indicated that the EVA polymer filled with 50 wt.% of hydrotalcite has the slowest heat release rate and the lowest evolved gas temperature. XRD data, combined with thermal analysis results, indicated that the layered structure of hydrotalcite may play a role in the degradation mechanism. The improved fire resistance of EVA filled with hydrotalcite also results from its intumescent behaviour.

Introduction

In recent years inorganic hydroxides fillers have replaced the halogen based fire retardants in many applications because of their better environmental compatibility. For example they are commonly used in ethylene vinyl acetate copolymer (EVA) for manufacturing halogen-free, low smoke cables. These compounds act both in the condensed and gas phase, decompose following an endothermic reaction, reducing the temperature of the polymer and releasing water to the gas phase, which dilutes the flame. They also act as catalysts for the oxidation of the carbonaceous residues reducing the CO/CO₂ ratio [1], [2]. The oxides, produced by the decomposition, can contribute to the formation of an insulative charred layer acting as a further protection for the polymer [3].

A comparison of the fire retardant effectiveness and mechanical properties of EVA polymer filled with inorganic hydroxides (Mg(OH)₂, Al(OH)₃) already used in the cable manufacturing, with other fillers like boehmite and hydrotalcite is the objective of this work.

Section snippets

Materials

The raw materials used for the experiments are:

•
poly(ethylene-co-vinyl acetate) EVA ESCORENE UL0019, EXXON, Melt Flow Index=0.65 containing 19 wt.% of vinyl acetate,
•
Mg(OH)₂ 99.8% purity Magnifin® H5, H7 and H5GV. Magnifin H5 and H5 GV have the same sizes 1.25–1.45 μm and surface area between 4 and 6 m²/g but Magnifin® H5GV only was treated with a proprietary surface treatment; Magnifin H7 (0.65–0.95 μm) has a surface area between 7–9 m²/g.
•
Al(OH)₃ 99.5% purity Nabaltec Apyral® 40D and 120 which

Thermal characterization

The results of DSC measurements performed on all the inorganic fillers studied are reported in Fig. 1.

The DSC thermogram of Al(OH)₃ shows a first weak endothermic peak which occurs at about 231 °C that is due to a partial decomposition to boehmite, the second one, at 312 °C, is related to the complete conversion of hydroxide to oxide, whereas the third one at 512 °C is due to the decomposition of boehmite [8]. This result is confirmed comparing the thermograms of boehmite and Al(OH)₃: the

Conclusions

In this work the flame retardant performances of aluminum and magnesium hydroxides traditionally used as filler in EVA copolymer, are compared with those of less common inorganic fillers such as boehmite and hydrotalcite. In order to achieve useful flame-retarding properties in EVA polymer, the overall filler loading needed must be higher than 50 wt.%.

The lower water production and absorbed heat by the decomposition reaction of boehmite is not compensated by its potentially improved char

Acknowledgments

The authors would like to thank TELECOMITALIA for financial support and Mr. M. Degli Effetti and Mr. F. Del Priore for the encouragement and support given.

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