Optimization of PVC-free materials in cables

2. Summary

During an earlier project concerning the possibilities of substituting PVC compounds in cables it was concluded that it is technically possible to substitute a significant amount of the current consumption of PVC for cables by Halogen Free Flame Retarded (HFFR) materials.

It was also concluded that there are important barriers towards the increased use of these types of compounds. These barriers are partly due to economical and partly technical reasons but the above mentioned project also stressed the fact that important material properties were not yet satisfactory to justify widespread use of the HFFR materials with the exception of indoor use without the influence of agressive environmental factors. Furthermore the processing properties of these compounds were considered problematic.

The scope of this project has been to pave the way for increasing the substitution of PVC compounds by optimizing HFFR materials for cables. This was done by carrying out a material test programme aiming to identify the best possible candidates of HFFR compounds for cables and at the same time optimizing the extrusion process with the goal to increase the cable production line speed with these high viscosity materials.

The material test programme included 16 HFFR compounds and PVC compound which were all subjected to accelerated ageing in different environments at different temperatures in order to assess their use as candidates for the substitution of PVC compounds.

The outcome of the test programme was that compounds do exist which can substitute PVC compounds in several cable applications e.g. in buried cables at high humidity, cables in contact with oil at moderate temperatures, under the influence of sunlight etc. It also became evident that for applications in harsh environments special compounds which may have other limitations (e.g. flame retardancy properties) must be selected or developed, as is the case for PVC compounds.

It turned out that the HFFR compounds with the best overall properties were not among the easiest to process. This emphasizes the need for improving the processing equipment for HFFR materials i.e. design of screw and crosshead for extruder lines.

During the programme it was decided to use computer simulation as the method for optimizing the extrusion process. The result of the simulations was a new extruder screw design. Further the die geometry was adjusted which turned out to give significant process improvements compared to traditional machinery.

With the new production set up, optical test cables were produced with the compounds which had the best overall properties found during the material test programme.

The produced optical cable had good mechanical properties and also the flame retardancy behaviour was satisfactory.

Only the highly filled and due to that the HFFR compounds of the high viscosity type were able to fulfil the requirements for mechanical properties when used in power cable constructions.

The attempt to scale up the screw design which were used succesfully during the production of optical cables on an extruder with an internal diameter of 63 mm to an extruder with an internal diameter of 150 mm and the length 18 times the diameter was definetely not succesful.

Further a single flight screw for a 120 mm diameter extruder was tested within the programme without succes.

With both screw designs the developed heat due to friction increased the mass temperature of the tested compound and caused thermal break-down of the HFFR compound within the extruders.

Screw designs for HFFR compounds have now become commercially available for extruders in the range of 100 - 150 mm diameter.