WCA March 2021

Technology news

Ethylene/butyl acrylate copolymer as a new polymer base for HFFR cable compounds

Classic HFFR cable compounds were developed more than 20 years ago. They are based on a combination of inorganic flame retardant filler – aluminium hydroxide (ATH) – and ethylene/vinyl acetate copolymer (EVA). Primarily fine synthetic grades of ATH (D50 ~ 1-2µm) are used to obtain high mechanical properties. With the close thermal decomposition points of ATH (~180°C) and EVA (~160°C), this combination provides high LOI, which is widely used as a flame retardancy indicator. However, despite being appropriate for evaluating a compound’s flame retardancy, LOI does not reflect cable fire performance. Even cables with high LOI values can rapidly spread flame and form burning drops. Together with continuous tightening of fire performance and ecological requirements over the last 10-15 years, this has led to more frequent attempts to use magnesium hydroxide (MDH) in this field of application. Synthetic MDH is more expensive than ATH, so its use is still limited to automotive and telecommunication cables. Natural MDH (milled brucite mineral) was successfully introduced into HFFR formulations, but its use is limited by issues with cable surface, which appear when the dosage is higher than 50 per cent due to wide particle size distribution – a result of the milling process. As a result, synthetic ATH remains the main FR filler for HFFR compounds. Attempts to develop ATH-free HFFR formulations based on brucite have continued. Classic polymer combination EVA- ULDPE provides flame retardancy with brucite, but the cable surface remains rough. Full replacement of EVA by ULDPE can provide a smooth surface, but leads to worse fire performance at the same filling level, so is only suitable for a bedding layer, where the amount of filler is as high as possible. As conventional polymers failed to provide the desired properties, the search for new polymers began. The polymers of the acrylic group, in particular EBA (ethylene/

❍ Table 2

butyl acrylate) are considered promising. Similar to ATH-EVA, EBA and MDH have close thermal decomposition points (300-330°C for MDH and >350°C for EBA). Studies of EBA fire performance showed the possibility of achieving 31 per cent LOI using calcium carbonate as the only filler (35 per cent). Europiren polymer laboratory studied a series of HFFR formulations based on EBA filled with EcoPiren ® . The first step included full replacement of EVA by EBA in classic cable sheathing. EcoPiren 3,5 treated with stearic acid (C), alkylsilane (NP) and without surface treatment was used as the filler. Mechanical properties, melt flow rate and LOI were tested. Table 1 shows that such formulations do not provide appropriate mechanical properties. Stearic acid, as expected, lowers both the LOI value and mechanical properties. However, despite the low mechanical properties, a very high LOI level for HFFR sheathing was achieved. This was also expected due to the close thermal decomposition points of MDH and EBA.

❍ Table 1

An attempt to improve the mechanical properties by using ULDPE was made ( Table 2 ). ULDPE introduction allowed an improvement in mechanical properties without losing too much LOI; however, the desired level of mechanical properties was achieved only by reducing the total filler content. Despite thework to introduce EBA toHFFRcable formulations and development of EBA-brucite compounds being only at the starting point, promising results have been obtained. Similar to the ATH-EVA combination, the synergetic behaviour is observed in the combination of MDH-EBA, and natural MDH (hexagonal particles) provides better fire performance. RMCC LLC www.brucite.plus

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Wire & Cable ASIA – March 2021