Armed against the elements
The sheath is subject to even tougher demands than the insulation, as it is directly exposed to environmental influences. It must withstand abrasion, chemicals, cleaning agents, UV light, temperature and much more. Unfortunately, there is no one material that meets all requirements. The material must be tailored precisely to the respective purpose. Depending on the application, cables must withstand lubrication oil, greases and cleaning agents to name just a few. The mechanical engineering sector uses tried-and-tested cables with sheaths made from polyvinyl chloride or polyurethane (PUR). PUR is the workhorse of sheath materials. It offers some of the strongest chemical bonds available. It is difficult to process, however, when producing both the cable and assemblies, as the sheath does not cut easily. PUR is also flammable and expensive. The cable types ÖLFLEX® 408P and ÖLFLEX® 409P find a compromise that combines the toughness of PUR with the easy processing of PVC. These cables feature a PUR outer sheath and a interstice filler functional layer made from PVC.
Cables used outdoors are exposed to the sun, and require a different combination of materials. In this case, the sheath must contain UV stabilisers. In solar cables (such as ÖLFLEX® SOLAR), soot is added to the mix to block sunlight, hence the fact that these cables are usually black. The ideal solution for outdoor cables is radiation crosslinking. Here, the cable is bombarded with electron beams. The plastic molecules absorb the energy from the electrons and become interlaced, making the material much more resistant. This allows the cables to withstand extreme temperature swings from minus 40 to 120 degrees Celsius, as well as high mechanical loads. This mechanical resistance is also the reason why crosslinking is also used for cables in the rail industry, such as ÖLFLEX® TRAIN. Several materials are suitable for crosslinking, including polyethylene (PE), polyolefin elastomers (POE), ethylene vinyl acetate (EVA) or ethylene ethyl acrylate (EEA). Additives are generally also added in crosslinking, usually around one percent, in order to improve the bonds between the molecule chains. They also reduce the amount of energy required in the crosslinking process.
Unlike unlinked material, which eventually softens, crosslinked material has no melting point. When heated, it oxidises and becomes brittle, which makes it necessary to add antioxidants and stabilisers. Crosslinking offers no advantage at very low temperatures, as the material will inevitably become brittle at some point. This makes it all the more important to select a base polymer that is suitable for low temperatures. Possible materials include polyolefin elastomers (POE), linear low-density polyethylene (LLDPE), certain kinds of ethylene vinyl acetate copolymers (EVA) or thermoplastic elastomers (TPE). If the application calls for tougher mechanical properties, suitable materials include high-density polyethylene (HDPE) or polypropylene (PP, for greater strength), as well as polyolefin elastomers (for greater elasticity).