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Molybdate and Borate Complexes for Enhanced Cable Compound Fire Performance Technical Paper

Abstract

Demand for low smoke wire and cable compounds including low smoke zero halogen (LSZH) compounds has been steadily growing. Such compounds often contain special additives promoting char formation thus lowering amount of smoke generated when burned. Molybdenum compounds (molybdenum oxide, ammonium octamolybdate, zinc molybdate, etc.) are widely used in flexible PVC-based wire and cable formulations as FR synergists and smoke suppressants. It was also recently demonstrated that zinc molybdate can promote char formation in certain EVA compound systems. Precipitation of molybdates onto the surface of inert minerals can significantly increase active surface area resulting in enhanced FR efficacy. Further improvement of FR performance can be achieved by precipitation of a molybdate phase on the surface of a select group of flame retardant materials such as metal hydroxides. This paper discusses the fire performance of a molybdate/borate complex consisting of a zinc molybdate shell and a zinc borate core, in two cable compound systems – flexible PVC and EVA. The effect of zinc molybdate/borate complex structure and composition on FR efficacy is discussed.

Keywords: Low-Smoke, Cable Compounds, Flexible PVC, EVA. Magnesium Hydroxide, Aluminum Hydroxide, Zinc Molybdate, Zinc Borate, Char Formation.

1. Introduction

The development of polymer compounds for wire and cable applications is a challenge that must address demanding specifications and regulations with well defined end-use requirements. For some applications, the National Fire Protection Association (NFPA) has established particularly rigorous flame and smoke standards. For example, plenum rated communication cable must pass NFPA 262, the 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 PVC is an inherently flame retardant polymer, it remains a material of choice in many wire and cable applications. However, PVC, particularly when flame retarded with antimony oxide, is known to generate high levels of smoke during combustion. Also new directives such as the construction products directive (CPD) in Europe appear to promote the use of more environmentally friendly zero-halogen wire and cable materials.

The trend toward low smoke zero halogen (LSZH) materials is expected to continue in the coming years and in recent years a number of new wire and cable formulations have emerged. Ethylene vinyl acetate copolymer (EVA) is commonly used in wire and cable because of its chemical and physical properties. Similarly, polyolefins can offer desirable properties for wire and cable insulation. However, to meet fire retardant standards, effective environmentally friendly FR additives are needed. Metal hydroxides such as aluminum trihydrate (ATH) and magnesium dihydroxide (MDH) are widely used and provide fire performance by heat reduction and smoke suppression. However loading levels of greater than 60% are often required to achieve necessary FR performance leading to deterioration of mechanical properties and high processing viscosity that can diminish compounding or extrusion efficiency. In some cases, loading requirements of metal hydroxides can be lowered by controlled particle size or surface modification. From a practical standpoint, some optimized balance among FR properties, mechanical properties, processing characteristics and compound cost is desired.

One approach to improving FR performance and physical properties in metal hydroxide filled LSZH compounds is with coadditives. Metal molybdates are environmentally friendly char forming catalysts that have been used for years in low-smoke PVC and other polymer systems. It was also recently demonstrated that zinc molybdate can promote char formation in certain EVA compound systems [1]. Despite recent advancements, the search for improved FR co-additives and FR synergies continues to generate interest in the wire and cable industry.

In this work we report on the development of a new engineered flame retardant/smoke suppressant technology based on the surface treatment of zinc borate with zinc molybdate. In flexible PVC, this zinc borate/molybdate complex provides smoke suppression superior to that of a physical blend of the two components [2]. This material also provides FR and smoke suppression in LSZH systems. In EVA compound, the combination of zinc borate and zinc molybdate results in a significant reduction in the rate of heat release as determined by cone calorimetry.
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