Low smoke zero halogen

Low smoke zero halogen (also known as low smoke free of halogen, LSZH, LSOH, LS0H, LSFH, OHLS, or ZHFR) refers to a classification for materials primarily used in cable jacketing within the wire and cable sector. LSZH cable jacketing is derived from thermoplastic or thermoset compounds designed to emit minimal smoke and absolutely no halogens when subjected to high heat sources.

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Description

The origins of commercial thermoplastic LSZH materials trace back to an invention by Richard Skipper, patent-holder of Raychem Corporation. This pioneering invention adeptly combined the necessary inorganic filler, aluminum trihydrate (ALTH), with an optimal thermoplastic matrix to suppress flames while forming a char. This process minimizes the emission of toxic gases, smoke, and carbon particles, all while preserving the necessary electrical insulation and physical durability for final products. The prominent inorganic filler utilized for flame resistance remains aluminum trihydrate (ALTH). Upon exposure to fire, this material engages in a unique endothermic reaction:

2Al(OH)3 ' Al2O3 + 3H2O (180 °C)

This reaction absorbs thermal energy and releases steam when the compound reaches the requisite temperature. It is essential that the polymer decomposition occurs around the same temperature. The steam produced disrupts combustion while contributing to a char layer that protects adjacent materials by trapping particulates. The substantial filler content (≥ 60%) also substitutes for the base polymer, significantly reducing combustible material available for fire.

Low smoke zero halogen cables greatly diminish the toxic and corrosive gas emissions during combustion. In case of a fire, these cables emit less optically dense smoke at a reduced release rate. Consequently, this feature is critical for occupant safety, allowing easier ventilation and enhancing firefighting operations. Industries frequently utilizing this material include aircraft, rail carriages, military submarines, offshore installations, and tunnels, especially where high voltage or signaling cables run. The nuclear sector has seen extensive LSZH cable use since the early years, with future projects likely to rely heavily on such cables, minimizing the risk of toxic gas accumulation and reducing potential fire hazards to sensitive systems.

Since the 1980s, low-smoke, low-halogen materials have found varied applications in the wire and cable field. The advent of thermoplastic LSZH broadened its use to accessories like heat shrink tubing and labeling. This development aimed to create cable jacketing systems capable of both flame resistance and minimal smoke generation, along with less toxic gases. Significant incidents, such as the HMS Sheffield smoking post-Exocet missile strike during the Falklands War, as well as the King's Cross fire—which tragically resulted in 31 fatalities in 1987—heightened awareness of the risks posed by conventional wire and cable materials in fire scenarios, further propelling LSZH adoption.

Multiple standards detail processes for measuring smoke outputs during combustion, such as Def Stan 02-711 in the UK and ASTM E662 in the US, both derived from an ASTM STP No. 422 protocol. During standardized tests, a specified material sample is exposed to radiant heat, and the smoke's optical density is photometrically gauged. Although these tests are conducted under controlled laboratory conditions and may not replicate real fire environments, they effectively assess potential smoke emissions and detect hazardous materials before further testing is pursued if necessary.

References

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