EoW March 2009

technical article

PVC improvement: a new range of eco-compounds By Claudia Attanasio and Laura Colloca, B&B Compounds, Italy

Abstract: This paper covers a new range of compounds using nanofiller that exhibit lower environment impact both in their production and throughout their life cycle. The compounds show resistance to flame, reduction in smoke density and HCl emission, optimal electrical properties (as high-volume resistivity values), and a high thermal stability with a significant reduction in density and therefore weight. The products are free from dangerous substances such as phosphoric plasticisers, DEHP or heavy metals. 1 Introduction With such a large variety of end uses, cables have to fulfil very special requirements. Many different polymers have been developed during the last few decades to meet the needs of various applications. These polymers can be classified roughly into thermoplastics, thermoplastic elastomers, elastomers, cross-linked thermoplastics and cross-linked elastomers. The choice of the appropriate polymer depends on the physical and chemical compound properties defined in the cable standard. The excellent electrical and mechanical properties of PVC make it an ideal material for sheathing, insulation and protection of cables. PVC-covered cables have a service life of decades, much longer than can be guaranteed by any other type of material. The mechanical resistance and the robustness of the material are important for any installation, whether underground, within buildings or under pavements. The electrical characteristics of PVC make it the ideal material for cables for low and medium voltage up to 5kV. The normal operating temperature range is up 70°C, but can be increased to 105°C using specialised formulations. The PVC remains stable down to –40°C and is impermeable to humidity.

Ignition : PVC is resistant to ignition. The temperature required to ignite rigid PVC is more than 150°C higher than that required to ignite wood. The ignition resistance of common flexible PVC formulations is lower, but with specialised formulations it may be significantly increased. Flammability : Once a material has been ignited, the associated hazard will be related directly to its flammability. One of the most reliable quantitative small-scale flammability tests is the Limiting Oxygen Index test, which measures the limiting concentration of oxygen in an oxygen/ nitrogen mixture necessary for sustained combustion. A material with a LOI value above 21 (air contains 21% oxygen) should not burn in air at room temperature, and a value above 25-27 means that the material will only burn under conditions where very high heat is applied to it. Rigid PVC has an oxygen index of 45-50, compared to 21-22 for wood and 17-18 for most thermoplastics. Oxygen index values above 27 can easily be attained with flexible PVC. The significance of this is that most rigid and flexible PVC will not burn alone without the application of heat from another source. Smoke density : Decreased visibility is a serious concern in a fire, because both escape from the fire and rescue by fire fighters is more difficult. The main way in which a fire decreases visibility is by the release of smoke. However, decreased visibility is the result of a combination of two factors: how much material is burned in the fire (which will be less if the material has better fire performance) and how much smoke is released per unit of material burned. Several empirical parameters have been proposed to compensate for incomplete sample consumption under testing conditions. One of them – known as the smoke factor – recently has been used with small-scale rate of heat release calorimeters. It combines the two aspects mentioned above: light obscuration and rate of heat release.

The cables used in industrial plants, power stations, multi-store buildings, hotels, subway tunnels, road tunnels or in vehicle construction must comply not only with the electrical and mechanical standards corresponding to the characteristic of the material, but also to exacting standards of flame retardancy. In case of fire, the materials used must also demonstrate a reduction in density, toxicity and corrosiveness of combustion smokes. Many studies have shown that the initiation and development of accidental fires are complex matters. A number of factors must be taken into account in assessing the contribution of any one material to a fire situation. The several plastics materials used in the building and construction industries have differing reactions to fire. The high chlorine content of PVC polymer reduces its ignitability and also the heat it contributes to a fire, in comparison with other plastics. As the basic polymer is diluted with additives, the fire performance changes. High concentrations of organic materials will increase flammability; high con- centrations of inorganic materials will reduce it. PVC formulations, like other natural and synthetic materials, give rise to smoke and to toxic gases when they burn. Significant reductions in the emission of smoke and hydrogen chloride may be achieved by the use of special additives. Independent studies have concluded that PVC fire gases are not significantly more toxic than those from other common building materials. It has been recognised in a number of studies that the substitution of traditional building materials by PVC brings no significant change to the hazards of accidental fires in buildings. In a detailed assessment of the overall fire-performance of a material many factors must be taken into account:

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EuroWire – March 2009 Euro ire – March 20 9