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Synergies of Metal Hydroxides and Metal Molybdates in Low-Smoke Flexible PVC


Various flame retardant additives have been used to formulate low-smoke flexible PVC for cable applications. Metal hydroxides, which primarily consist of aluminum trihydrate (ATH) and magnesium dihydroxide (MDH), are most commonly used. Typically used at high loading levels, these provide fire performance via endothermic cooling while also contributing to char formation. Molybdates are known to promote char formation during combustion by the Friedel-Crafts alkylation of alkene linkages that are formed during PVC thermolysis.

The most rigorous standards for flame and smoke propagation have been established for plenum rated communication cable. To meet the NFPA 262 standard, PVC jacket compound is typically compounded with combinations of additives. While the mechanism for individual additives in simple polymer systems may be well understood, interactions between additives have been much less studied. These interactions can result in enhanced performance through synergies, or in some cases, diminished performance through antagonistic effects.

In this work we examine the potential synergies between metal hydroxides (both ATH and MDH) and metal molybdates in several different flexible PVC formulas. Performance testing was conducted using the cone calorimeter and NBS smoke chamber. Thermal techniques were also used to gain insight into chemical interactions. The data generated provide strategies for the formulation of cost effective, low smoke PVC jacket compound.

Keywords: Magnesium hydroxide, aluminum trihydroxide, zinc molybdate, calcium molybdate, flexible PVC, low smoke, jacket compound, char formation.

1. Introduction

Wire and cable applications are increasingly faced with meeting both flame spread and smoke standards. The most stringent standards have been established for cable applications in ceiling cavity plenums. The standard for plenum cable was established by the National Fire Protection Association (NFPA) and is based on NFPA 262, a modified Steiner Tunnel Test. To qualify for use, flame spread must not exceed 5 feet. In addition, peak smoke must not exceed 0.5 units and the average smoke throughout the test must be below 0.15 units.

Because of the NFPA requirements, the preferred materials for plenum rated communication jacketing are fluorinated ethylene propylene (FEP) and plasticized polyvinyl chloride (PVC). PVC enjoys a significant cost advantage as well as processing advantages. However, a number of additives are required in order to meet the various technical requirements. Plenum grade PVC jacket compound generally contains several flame retardant additives. Antimony trioxide, Sb2O3, is often added to PVC compounds and has been shown to enhance flame retardancy via a gas phase mechanism involving the scavenging of free radicals. In addition, flame retardancy can be improved by the use of halogenated and/or phosphate plasticizers. A deleterious consequence of the use of antimony and halogen flame retardants is the increase in smoke.

The first step in the thermal decomposition of PVC is dehydrochlorination of PVC accompanied by the release of HCl and the formation of a polyene. This process is autocatalytic since the reaction product HCl accelerates the rate of thermal decomposition.

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