Why are cables with black outer sheaths better suited to outdoor use than cables of different sheath colours?
In outdoor use, cables are subjected to higher levels of UV radiation than when used indoors. They are also exposed to ozone effects and other atmospheric influences. In principle, all plastics are susceptible to oxidation. However, different types of plastic possess varying degrees of UV stability. Light and oxygen cause premature ageing of plastics as a result of photo-oxidation. The UV rays contained in sunlight penetrate the molecular chains of the cable jacket. This causes the chains to split and results in the formation of highly reactive radicals, which continue to attack the molecular structure of the plastic. The ultimate consequence of this process is that plastics age and embrittle far more quickly in outdoor use. PVC cables in particular are subject to increased wear as the added volatile plasticisers or softeners in the thermoplastic polymer vaporise more quickly.
There are a number of ways of protecting plastics against the effects of UV radiation. The material can be shaded from the light or special UV absorbers can be added to filter out the UV rays. The simplest way of making a cable UV-resistant is to add carbon black to the polymer used for the outer sheath, thus colouring it black. This ensures full shading of the sheath material, which results in complete light absorption. The harmful UV rays are absorbed by the carbon particles in the outer sheath and transformed into far less damaging thermal energy. This also prevents the formation of free radicals as well as the occurrence of photooxidation.
As mentioned above, the different materials possess very different degrees of UV resistance. Some sheath materials display a good level of resistance to ultraviolet rays without the need for black colouration. Most of these substances are not thermoplastic polymers, which often require the addition of plasticisers, but belong to the group of cross-linked elastomers or TPE types.
Non-black-coloured PUR cables, for example, may fade or whiten in outdoor use, but usually maintain their flexibility and mechanical stability. Silicone cables without black-coloured sheaths, like the ÖLFLEX® HEAT 180, also possess a good level of UV resistance. However, such cables are only suitable for temperate climates. If used in locations with persistently high levels of UV radiation (coastal areas, deserts, oceans, high mountains, polar regions and areas with very high UV radiation such as South Australia or New Zealand), these cables should also be encased in a black, carbonised outer sheath.