TECHNOLOGY | WIRE AND CABLE
Right: A Buss Compeo 55 kneader extruder configured in an HFFR compound version with single screw discharge
flammability tests,” he says. “Traditional HFFR compounds perform
better during flammability tests but are typically characterised by lower oil and wear resistance and mechanical performance in general. These limita- tions can be overcome by replacing traditional crosslinkable or HFFR cable compounds by crosslinkable flame retarded alternatives,” according to Loviat. Production of crosslinkable flame retard- ant cable materials has traditionally been a two-step process. In the first step the base polyolefin polymer is grafted with a reactive silane, then in the second step the grafted material is compounded with a flame retardant additive/filler. This well-established process works, but Buss says it is inefficient and cost-intensive because the material has to be melted and pelletised twice. This either requires two compounding machines or the use of alternate production cycles on a single machine. Buss has developed what it believes is a more
efficient process allowing crosslinkable HFFR compounds to be produced in one step and this is already being used on an industrial scale at some of the main global producers of crosslinked HFFR compounds. The company claims it pro- vides a significant competitive advantage in terms of productivity, product quality and energy consumption. The one-step Buss production process requires fast mixing of the grafting chemicals and good temperature control of the process. “From this point of view, the Buss kneader is tailor-made for this application,” Loviat claims. “A minimum temperature of 170-190˚C must be reached in the first section of the machine to perform the grafting successfully. The flame retardant material – typically aluminium or magnesium hydroxide – is then added and mixed with the grafted compound.
Right: Evonik’s cone calorim- eter fire testing equipment helps speed development of wire and cable compound formulations
During this operation, an excellent dispersion of the flame retardant and
accurate temperature control are crucial in order to achieve good mechanical properties while not exceeding the degradation temperature of the flame retardant, which is around 200˚C.” Loviat claims the flexible modular design of the
latest Buss Compeo allows it to be fine-tuned for this application. “The Compeo – the latest genera- tion of Buss kneader – combines three and four flights kneading elements within the same machine and allows further optimisation of the production process of xl-HFFR compounds. The four flight kneading elements in the first section of the machine melt and heat-up the resin quickly and at a high throughput, while the three flight kneading elements in the second section of the machine provide an ideal combination of good flame retardant dispersion and temperature control.”
Additive innovation Wire and cable is also a key market for Evonik, which offers a number of additive products designed for the development of new cable compounds that meet the highest standards. “We have invested in new laboratory capabilities, such as cone calorim- etry, which allow us to evaluate the heat release and smoke density of compounds using our additives during burning that ultimately helps us to develop safer additives,” says Dr Ido Offenbach, Americas Segment Manager, Polymer Specialties. “There are a number of new regulations that are the main driver for growth in wire and cable compounds. For example, the new EU Construc- tion Products Regulation (CPR) – EN 45545 – for fire protection of railway vehicles requires elevated flame retardancy performance with low smoke and heat release,” says Offenbach. He also sees a variety of different development
trends emerging in wire and cable compounding, explaining that these vary depending on the type of compound. For example, thermoplastic com- pounds need improved flame retardance perfor- mance with minimum impact on mechanical
16 COMPOUNDING WORLD | May 2021
www.compoundingworld.com
IMAGE: EVONIK
IMAGE: BUSS
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