Electric cables are used for the transmission of electrical power (low, medium and high voltage cables) or signals (instrumentation cables, low to medium voltage). Very simply put a cable is an assembly of conducting material (copper, aluminium) to facilitate the transmission and other materials (metal, paper, polymeric) to ensure optimal transmission and protection against the environment. The polymeric material needs to be, flexible, electrical insulating and flame-retardant (preventing a rapid spreading of a fire throughout a building via the cable conduits) in order to be effectively used as:
- Insulator-compound, material in direct contact with the conductor, preventing any losses of power or signal
- Bedding compound (also called inner-sheath and sometimes made of same constituents as the filler compound), material around the insulated conductors, keeping these in place and the bedding for the protective armour
- Outer sheathing/ jacketing compound, the material applied onto the armour and in direct contact with the environment and mechanical damage
Traditionally these polymeric materials were mostly PVC based as PVC has good electrical insulating properties and containing chlorine has intrinsic flame-retardant properties. However in case of a fire PVC based compounds release hydrochloric acid, a toxic and corrosive gas. In order to protect the public the use of these PVC-based compounds has been restricted in Europe and HFFR (Halogen Free Flame Retardant) compounds based on PE and PE/EVA have become more and more common. However plasticised PVC is still very much used in the production of cable-compounds.
While PVC has intrinsic flame-retardant properties, cable compounds based on PVC do contain mineral fillers to improve the flame-retardancy and to reduce smoke development. PE and PE/EVA not have the same intrinsic flame-retardant properties as PVC has; compounds based on these polymers contain a lot more flame retardant fillers (up to 70%), often ATH or MDH (depending on processing conditions).
Sibelco offers a range of functional minerals (ATH, Colemanite, Hydromagnesite) to make PVC based compounds :
- Flame retardant
- Improved char formations – low smoke emissions
These flame-retardant mineral fillers, partially or completing substituting calcium carbonate, can either be used by themselves or in combinations with one another. With the coarser grades high loading levels of the flame retardant fillers can be achieved. For the jacketing compounds good char formation is a critical property (reduction of smoke emissions).
Other Sibelco minerals (Talc and Kaolin) with their platy structure help improve the rheology of the compounds, the mechanical properties (more important for the jacketing/outer sheathing compound) and the moisture resistance (talc is hydrophobic mineral), which is of course also critically important for compounds used for the outside jacketing. Talc and Kaolin also help to improve the surface smoothness of the outside jacket of the cable.