The behaviour in case of a fire is another important aspect for cables. Flame-retardant insulation material is required wherever there is a risk of fire, in order to meet the fire protection classes of the European Construction Products Regulation (CPR). The easiest way to achieve good fire protection is by mixing the plastic with substances containing halogens. These are elements of the seventh group in the periodic table, often brominated compounds. This fire protection strategy is highly efficient as it requires few additives, thus maintaining the mechanical properties of the material. These cables are therefore often used by car manufacturers in the engine compartment. In public areas like buses, however, halogen has a major disadvantage. In the event of a fire, halogen creates noxious smoke that mixes with extinguishing water to form corrosive vapours.
HFFR plastics (halogen free flame retardant) are a non-toxic alternative, but they require a filling level of sometimes over 60 percent, which can significantly affect the mechanical properties of the plastic. One new trend is the use of so-called synergistic systems. These combine two substances that together provide better flame protection than either individual material would alone. One possible combination is halogen-free aluminium trihydrate and silane compounds. When it comes into contact with fire, aluminium trihydrate reacts to form aluminium oxide and water. This endothermic reaction draws energy from the fire. A crust of combusted material also forms and acts as a protective barrier.
Another factor to bear in mind is that flame retardants are hydrophilic, meaning they have the unwanted characteristic of attracting water. In a worst-case scenario, this can cause an electric breakdown. This is not a problem for cables that do not come into contact with water. But in some sectors like the food industry, where hot water is used for cleaning, different materials are needed. Flame retardants coated with silane compounds are more suitable here. These make the plastic hydrophobic, i.e. water repellent.