EuroWire September 2021

Technical Article

New ethylene vinyl acetate copolymers for highly filled wire and cable compounds By Chelsea Ringham, Nagarjuna Palyam and Jeffrey C Haley, Celanese EVA Polymers, USA

Abstract Ethylene vinyl acetate (EVA) copolymers play an important role in a variety of cable applications, serving as a polymeric base material that is well suited to disperse filler. This paper describes a new EVA copolymer designed for use in non-halogen flame-retardant compounds that has been tailored to simultaneously reduce water absorption and increase tensile elongation when highly filled with a mineral flame retardant. This increase in tensile elongation shows potential for reducing the need for maleic anhydride grafted polyolefins within these highly filled formulations. 1 Introduction Ethylene vinyl acetate (EVA) copolymers are flexible, semi- crystalline thermoplastic materials where the modulus andmelting temperature of the polymer can be tuned by adjusting comonomer ratio [1] . The free radical polymerisation of ethylene results in low density polyethylene (LDPE), a material with a nominal modulus in the range of approximately 100 to 300 MPa, and a melting temperature between 100 and 120°C. Introducing vinyl acetate during this polymerisation results in a random copolymer of ethylene and vinyl acetate. In the solid form, incorporated vinyl acetate serves to disrupt crystallinity, reducing both the modulus and melting temperature of the material. These reductions are substantial: an EVA copolymer with 40 per cent by weight of incorporated vinyl acetate has a modulus of approximately 2 MPa and exhibits a broad melting transition centred at approximately 50°C. EVA possesses a mix of characteristics that make it useful in filled compounds. The low modulus of EVAs that contain moderate levels of vinyl acetate (vinyl acetate levels of 12 to 40 per cent by weight) combined with a moderate level of polarity enables several applications where EVA is highly loaded with fillers. Highly filled compounds of EVA (with inorganic content sometimes as high as 80 per cent by weight) show up in applications such as thermoformable automotive sound barrier layers, colour concentrates, and wire and cable compounds. 2 EVA in wire and cable compounds While EVA can play a role in a variety of cable compounds, its two major applications in wire and cable are semi-conducting layers for medium voltage cables and as a matrix material for low-smoke, zero-halogen (LSZH) jacketing compounds. Both applications play on EVA’s ability to maintain flexibility at high filler loadings.

EVA polymers are widely used to produce LSZH jacketing compounds [2] . These compounds are made by combining EVA with certain inorganic components, as well as other ingredients such as other polymers, stabilisers and crosslinking agents. The inorganic components are generally mineral fillers that endothermically decompose at elevated temperatures that would be encountered during a fire. Common flame retardants include aluminium hydroxide, magnesium hydroxide and magnesium carbonate [3] . The endothermic decompositionof thesematerials inhibits combustion by both absorbing heat and releasing an inert gas, such as water vapour. To achieve the targeted level of flame retardancy, mineral-based LSZH compounds typically contain filler levels of 50-60 per cent by weight. With such high filler loadings, compound formulators have to grapple with a variety of issues. Highly filled compounds can become stiff and brittle, which makes meeting target mechanical properties difficult. Additionally, whileEVA is hydrophobic, flame-retardingcompounds consisting of 60 per cent by weight of a hydroscopic mineral can absorb substantial quantities of water. This water absorption in turn can further compromise mechanical properties [2] . Maleic anhydride grafted polyolefins are often used to improve the compatibility of these fillers in the EVA polymer matrix, which can improve the mechanical properties of the compounds [6] . However, these maleic anhydride grafted polyolefins are typically expensive when compared to the rest of the compound. Motivated by these concerns, we undertook the development of new EVA materials that exhibit less water absorption when combined with hydroscopic fillers, and demonstrated that substantial reductions in water absorption were possible with both thermoplastic and thermosetting LSZH formulations [4] . We have also demonstrated that substantial improvements in mechanical properties were possible in LSZH formulations containing EVA and MDH [5] . This paper extends these results to model fully formulated compounds, and also explores the possibility of using our new material to reduce the use of maleic anhydride grafted polyolefins.

3 Experimental methods 3.1 Materials

Two EVA copolymers were used in this study. 28 per cent VA copolymer with melt index of 3g/10min was used as a control sample (Cont-28), and was compared with a second 28 per cent VA copolymer with a melt index of 3g/10min possessing a proprietary modification intended to improve filler acceptance (Exp-28).

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September 2021