EoW May 2008

english Olefinic Elastomers for Wire and Cable Applications By Day-Chyuan Lee, Ray Laakso, Larry Gross and Jack Muskopf, the Dow Chemical Company

Abstract Polymeric insulations and jacketing materials provide a reliable, cost-effective means of protecting cables, wires, and fibres used in today’s power and data transmission applications. The choice of polymeric coating for a particular application is dictated by numerous factors, including the electrical, environmental, and physical property requirements of the cable. Plastic and elastomeric compounds are two of the most common classes of materials used in the majority of wire and cable constructions. Polyethylene, one of the most popular plastics being used in the wire and cable industry due to its good performance and relatively low cost, maintains a large market share. Complementary to plastics, cables made using elastomeric compounds have their own distinct performance attributes due to their particular polymeric architecture. Elastomers are polymers that exhibit extreme elastic extensibility and flexibility when subjected to relatively low mechanical stress. Comparison of the polymeric architecture of polyethylene with three common commercially available elastomers (ethylene-octene (EO) (or–butene (EB)), chlorinated polyethylene (CM), and EPDM (ethylene propylene diene terpolymer) reveals the similarities and differences of these various classes of materials and provides an insight into their performance characteristics. All of these polymers have a saturated backbone structure, but the flexibility, tactile nature, and performance of the compounds are very different. The more amorphous elastomeric EO/EB, CM, or EPDM materials tend to produce compounds that are more flexible than polyethylene compounds. This paper will provide an overview of the role of these elastomeric polymers in wire and cable applications and discuss the similarities and differences of the materials due to their polymer architecture. Emphasis will be on the elastomer resins. Structure-property relationships will be highlighted to help explain the major benefits and deficiencies that each polymer type offers for the wire and cable industry. Elastomers can participate in a broad range of applications, including thermoplastic or thermoset systems, such as jacketing, insulation, bedding, low-smoke zero halogen, and low-voltage insulation. Technical data on the use of these polymers in typical wire and cable applications will be presented for illustrative purposes.

1. Elastomer structure Commercial polyolefin elastomers are co-polymers of ethylene with one or more higher α-olefins such as ethylene-octene (EO), ethylene-hexene (EH), or ethylene-butene (EB) as illustrated in Figure 1 . are co-polymers of ethylene and propylene arranged in a random manner to produce rubbery and stable polymers. A third, non-conjugated diene monomer can be terpolymerised in a controlled manner to maintain a saturated backbone and place the reactive unsaturation in a side chain available for vulcanisation or polymer modification chemistry. The ASTM designation for EPR is EPM for the co-polymers and EPDM for the terpolymers where ‘E’ denotes ‘ethylene’, ‘P’ denotes ‘propylene’, D denotes ‘diene’ and ‘M’ denotes a saturated chain of the polymethylene type. An EPDM polymer structure with ethylidene norbornene (ENB) as the diene monomer is illustrated in Figure 2. Chlorinated polyethylene (CPE) is a synthetic elastomer produced by the controlled chlorination of polyethylene feedstock [1] with chlorine atoms randomly distributed on the polymer backbone. A generalised chemical structure for CPE is shown in Figure 3 . The ASTM designation for CPE is CM or chloro-polyethylene where ‘C’ denotes ‘chloro’ and ‘M’ denotes a saturated chain of the polymethylene type. Due to their stable, saturated polymer backbone structure, compounds made from ethylene containing elastomers are valuable for their excellent combination of heat and oil resistance as indicated and classified by the ASTM D2000/ SAE J200 specification [2,3] as shown in Figure 4 . In addition to their durability in harsh environments, their flexibility enables ease of cable installation and helps provide reliable splices and terminations, especially in cold weather. This makes them attractive candidates for cable insulation and jacketing. The flexibility ranges of ethylene containing elastomers relative to other common plastics are shown in Figure 5 . 2. Structure-property relationship Crystallinity, molecular weight (MW), (MWD), branching, copolymer/diene type/level, and Ethylene-propylene rubbers (EPR) molecular weight distribution

Figure 1 ▲ ▲ : Structure of ethylene-butene elastomer

Figure 2 ▲ ▲ : Structure of EPDM containing ENB

Figure 3 ▲ ▲ : Structure of CPE

chlorine contents are some of the major chain architecture variables that can be engineered to optimise the performance of the variety of commercial ethylene containing elastomers. 2.1 Crystallinity and molecular weight Ordered and regularly repeating arrange- ments of atoms or groups of atoms can result in crystallinity. Crystallinity and crystalline melting point are functions of the ethylene segment block length and crystal imperfections of ethylene containing elastomers. Compared to ethylene-higher α-olefins copolymers, ethylene propylene co-polymer typically has shorter ethylene blocks with more defects in the crystalline phases. The crystallinity of CPE can be adjusted by the chlorination process to yield amorphous or semi-crystalline product. The level of residual polyethylene crystallinity also plays a role in controlling the overall physical properties of CPE. Ethylene blocks are too short in a random co-polymer to give significant crystallinity and an amorphous polymer results. Amorphous ethylene co-polymer is soft and easy to process and generally has lower physical properties than a higher crystallinity elastomer. At around 60 wt% ethylene, an olefin elastomer has sufficient crystallinity and is hard enough to be pelletised.

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EuroWire – May 2008