Thank You For Not Smoking
September 1997

In large buildings, the space above suspended ceilings or below raised floors, known as the plenum, plays host to dozens, in some cases hundreds, of communication cables. Their copper wiring transmits data between computers, powers heating and cooling installations, facilitates telephones, faxes, and video communications, controls security systems. Cabling is one of the main arteries of modern life - but potentially can also be an artery of death. If fire breaks out, the cabling can act as a conduit to spread flame quickly from floor to floor.

Stricter and stricter safety measures are being enforced in most countries and most industries. In many regions, there are significantly fewer fires as a result of growing safety awareness, but one of the penalties of this success is that when fires do occur they are often significantly worse - hotter, more noxious, more destructive - than in the bad old days. Zinc borate is becoming a major player in a new generation of complex fire retardant cocktails designed to fight these threats.

In the United States, which enforces some of the world's strictest fire regulations, plenum cabling has to meet the requirements of Underwriters Laboratory Standard UL-910. This calls for materials that on combustion provide both low smoke evolution and a low degree of flame spread. A number of fluoropolymers, in particular DuPont's fluorinated ethylene-polypropylene, meet the UL-910 specification, and have been used safely and successfully for several years. The cable industry, however, has always considered them expensive, and found that some are physically too stiff to provide the flexibility ideally required in plenum installations. Accordingly it has long been investigating alternative polymer formulations.

Early on PVC (polyvinyl chloride) was identified as an ideal material for sheathing and perhaps for insulating plenum cables - flexible, durable, easy to use, and above all cheap. It could quite easily be formulated to meet the flame retardant standards of UL-910, but it failed on the low smoke evolution requirement. In fact, fumes and smoke from plastic materials are known to have exacerbated the harmful effects of some major hotel, airport, and public transport fires in recent years.

A major research effort, in which smoke suppressants played a significant part, was undertaken to find low smoke systems which would bring PVC up to the plenum cable standard.

In many cases, the electrical properties of fluoro-polymers still make them the preferred material for plenum cable insulation despite the fact that they cost six or seven times as much as smoke-suppressed PVC. FirebrakeŽ ZB zinc borate was developed by Borax as a smoke suppressant for PVC, and is now being used increasingly in the new UL-910 formulations. Highly effective in reducing smoke and fumes, it simultaneously promotes fire-quenching char (see below). Recent research shows that the beneficial effects of zinc borate are enhanced when alumina trihydrate is also present in the formulation. Combinations of zinc borate and certain molybdenum compounds can provide further smoke reduction effects. To meet growing demand, zinc borate capacity has recently been doubled by Borax at its specialty products refinery.

Firebrake ZB can be used in conjunction with - and as a partial replacement for - antimony oxide. In addition to smoke suppression and char formation it inhibits afterglow, electrical tracking, heat transfer, and molten plastic dripping.

Borax Technology has developed other zinc borates with special characteristics: FirebrakeŽ 500 for engineering polymers at temperatures above 290°C and for enhancing the smoke suppressant qualities of fluoropolymers; and FirebrakeŽ 415 which has a uniquely high dehydration temperature, and improves the thermal stability of fire retardant systems, especially in high temperature nylon. Like Firebrake ZB, both these zinc borates reduce the rate of heat release from flame to plastic during fires - and dramatically cut carbon monoxide production.

Char?

The hard, intumescent (i.e. expanded) residue produced by some polymers during combustion is known as char. In effect, part of the polymer is transformed into a solid, carbonaceous mass rather than smoke or flame. Thus char will reduce the duration of a fire and inhibit its spread. It also provides electrical insulation to protect wiring even after it is burnt, and prevents short circuiting and sparking. Zinc borate in a polymer is known to promote the formation of char.