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Compounding of Ethylene-Propylene Polymers for Electrical Applications

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Compounding of Ethylene-Propylene Polymers for Electrical Applications Compounding of Ethylene-Propylene Polymers for Electrical Applications Morton Brown, A. Schulman Inc., Akron, Ohio Background Frequently, the considerations given to the arlier articles in this magazine have reviewed selection of compound ingredients may crosslinked polyethylene (XLPE) insulation and require compromises in one characteristic in Eethylene-propylene rubber (EPR). This article is intended as a sequel to the latter order to improve another. Because of its combination of superior electrical properties, its flexibility over a wide temperature range and its resistance to moisture and weather, EPR is used Proper selection of the ingredientk) in each category in a diverse range of electrical applications: requires that consideration be given to the desired 0 Power cables physical, electrical and environmental properties, as 0 Flexible cords well as cost, ease of mixing, chemical stability and ease 0 Control and instrument wire of processing. Frequently these considerations may re- Automotive ignition wire quire compromises in one characteristic in order to 0 Appliance wire improve another. In the following discussion, factors to 0 Motor lead wire be considered in the proper choice of each ingredient 0 Mining cable will be explored. Molded electrical accessories Compounding is the technology of converting the raw rubber resin into useful materials through the ad- Curatives and Curing Coagents dition of fillers, reinforcers, stabilizers, process aids, curatives, flame retardants, pigments, etc. The resulting composition is called a compound. In Although these components comprise only a small this paper, additive levels will be expressed in parts per percentage of the total compound, the development of hundred of resin (or rubber), commonly called by com- a practical compound and the selection of the ingredi- pounders "phr." This is a more convenient form than ents cannot be carried out on a rational basis without weight percentages, some of which are optional, de- considering the crosslinking chemistry to be employed. pending on the intended application and on processing A sulfur cure, possible with an EPDM but not with requirements. A typical compound usually has the fol- an EPM, is used only in low voltage insulations, cable lowing ingredients: jackets, or in some molded electrical components. Its EP rubber principal deficiencies in electrical applications are its Filler/Reinforcing Agent(s) electrical stability under wet and high temperature con- Plasticizer ditions and its high temperature ageing characteristics. Antioxidants/Stabilizers For most electrical applications, peroxide cures, pos- Flame Retardants sible with either an EPM or EPDM, are the most com- Process Aids mon curing method. Compared with sulfur cures they Ion Scavenger are more expensive both in terms of the selection of Coupling Agent other compatible compounding ingredients and in the Curing Coagent temperatures and pressures required to achieve void- Curative free vulcanizates. 0883-7554/94/$1.0001994 16 January/February 1994-Vol.10, No.1 IEEE Electrical Insulation Magazine Table I1 Effect of Ingredients on Peroxide-Cured EPM/EPDM Type of Cure Benefits Disadvantages Peroxide Electrical stability Requires higher temperatures and pressure Heat stability vs. sulfur Requires coagent if used with EP copolymer Inhibited by oxygen or acidic compounding ingredients Sulfur High tensile and tear strength Wet electrical stability Can be carried out at lower Heat stability pressures and temperatures L in air Odors Peroxide cures are inhibited by oxygen, a potent scavenger of free radicals, and by acidic components in the mixture, which cause the peroxide to decompose by an ionic (no free radicals are generated) mechanism. A little or no effect on the rate but will have significant comparison of sulfur and peroxide cures is summarized effects on the cure state. in Table I. Coagents are polyfunctional, unsaturated organic The most commonly-used peroxide is dicumyl per- compounds used in conjunction with peroxides to oxide, available in several substantially pure grades achieve a more versatile and efficient cure system. They (90% and above) or as 40% dispersions on inert particu- are used to increase both the rate and the state of cure. late carriers such as treated calcined clay diatomaceous Coagents may be grouped into two classes. Class I earth or calcium carbonate. It can be used in all cures includes acrylates, methacrylates, vinyl esters and bis- above 150°C, and produces a characteristic odor in the maleimides, which function both by addition to a free product due to a byproduct of its thermal decomposi- radical and by hydrogen abstraction to increase both the tion, acetophenone. rate and state of cure. Class I1 includes allylic com- Another commonly-used peroxide is bis-(tertiary pounds and low molecular weight polymers with a buty1peroxy)-diisopropyl benzene. Like dicumyl per- high vinyl content and affects only the cure state, not oxide it is available in a concentrated or a dispersed the cure rate. A commonly used example of Class I is form and has two advantages: It is free of odorous trimethylolpropane trimethacrylate, and an example of byproducts and, because of its multifunctionality Class I1 is triallyl cyanurate (TAC). Another more spe- (higher peroxide content), it is used at levels much cialized coagent is elemental sulfur. Used at very low lower than those used with dicumyl peroxide. It is levels, it imparts higher tensile and tear strengths to the slightly less reactive than dicumyl peroxide and is fre- vulcanizate. It is most frequently used in molded cable quently used in thin-walled low voltage applications or accessories in which the improved hot tear strength in semiconductive insulation shields. In both cases the facilitates demolding. Unfortunately, it also reduces di- lower reactivity is acceptable because these products electric breakdown strength because of the presence of reach their cure temperatures more rapidly in a con- sulfur crosslinks and also creates persistent disagree- tinuous vulcanization tube than does a heavy-walled able odors in the vulcanizate. medium voltage insulation, for example. The effects of these various compounding ingredi- ents on the cure rate or the cure state are indicated in Cure rate and cure state, the latter often called Table 11. Discussions of the proper selection of each of crosslink density, are terms that are frequently misinter- these classes of ingredients will be considered in more preted or misunderstood as applied to peroxide cures. detail below. Between EPM and EPDM elastomers in general, there is essentially no difference in the cure rate with a given peroxide, but there may be a large difference in the cure EPM and EPDM Selection state at a given peroxide level. The primary factor af- fecting the cure rate for a given peroxide is temperature, As indicated above, both polymer types can be although the filler type and amount may also have an crosslinked with a peroxide and, if properly com- effect. The polymer type, plasticizer type and level, pounded, will have equivalent physical, electrical and antioxidant type and level, and peroxide level have rheological properties. But the selection of an IEEE Electrical Insulation Magazine January/February 1994-Vol.10, No. 1 17 ~ -___ -~ ~ - fillers and/or plasticizers as well as ”green” (uncured) strength. Since increasing levels of filler and plasticizer increase electrical losses and reduce breakdown strength (see Table I of [21), it is immediately apparent that com- pounds suitable for low voltages will be based on poly- mers found in the upper right quadrant of Fig. 1. Polymers suitable for higher voltages, hence lower filler levels, will be found in the lower left quadrant. Pelletizable com- pounds will be based on polymers of increased crystal- linity Polymers found in the lower left quadrant will be most suited for highly flexible, low viscosity compounds I CrysfaXiniiy with low filler levels-a molded accessory perhaps. The analysis based on Fig. 1 ignores a very important Fig. 1. Effect of molecular weight and crystallinity on poly- factor in polymer selection, namely the question of mo- mer and compound properties. lecular weight distribution (MWD).Variations in MWD usually are manifested in processing characteristics, but Table I11 at equal filler loading, narrow MWD polymers are likely Effect of EPMlEPDM Polymer Variables on to have higher tensile strengths and moduli than broad Processing and Properties of Electrical Compounds MWD types. Expressed another way, narrow MWD At the Expense of polymers can accept higher levels of filler and plasticizer Difficult mixing and than the wide MWD types for equivalent properties-a dispersion possible factor in lower cost. Indeed, narrow MWD weight types may require higher levels of filler and plasticizer to Filler and plasticizer can be More difficult to extrude increased for lower cost attain the extrusion rates and degrees of smoothness Higher filler/plasticizer characteristicof the broad MWD types. These considera- Distortion or “sag resistance” reduces electrical properties tions of MWD are summarized in Table 111. during curing improves Increasing Tensile strength increases Higher hardness ethylene Fillers content Accepts higher Poorer permanent set filler/plasticizer levels Poorer elastic recovery Although the compounding of El‘ rubber for general Easier to pelletize polymer and compounds Poorer low temperature industrial applications employs a wide variety of fillers, flexibility
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