Elastomer Basics

Polymers themselves can be classified as rubbers, plastics, resins, or fibers. Elastomers are polymers that are inherently rubbery and flexible by nature. Elastomers are produced by joining (polymerizing) small molecules (monomers), and converting them into large, long-chain

molecules. The process leads to elastomers with high molecular weight. When many of the same monomers (such as ethylene) are joined to others, a homopolymer is formed. If two different monomers such as ethylene and propylene are used, a copolymer such as EPR results. These polymers are called chains—the longer the chain, the higher the molecular weight and the better the properties. Elastomeric chains are not entirely linear and will possess branches. Elastomers, being inherently soft, require inorganic mineral fillers to be useful as insulation. The inorganic fillers improve their strength and make them firmer. Additional details on fundamentals of elastomer technology are provided in Appendix D.1.

Fundamentals of Cable Insulation Systems

6.2.1 Cross Linking

The long elastomer molecules are mixed together, as in a bowl of spaghetti. The “noodles” may have branches, but they are not connected to each other. When the different chains are joined together, this linkage is called the cross link (see Appendix D.2).

Cross-linking has numerous beneficial effects. The resulting polymer insulation becomes tougher, resists softening at elevated temperatures, and maintains form stability at elevated temperatures. These changes are particularly important for elastomers such as ethylene copolymers, as compared to homopolymers such as PE because elastomers lack crystallinity, making them soft. Regardless, property improvements induced by cross-linking of the elastomer alone are not adequate for a butyl or EPR cable, and additional additives are needed to create a useful insulation. Cross linking does not improve the electrical properties (dielectric constant or dissipation factor). It only improves strength and form stability of the polymer.

6.2.2 Fillers Used in Rubber Insulations

Fillers (inorganic chemicals) are needed in elastomeric insulations to provide structural strength and stability. The nature of the filler additives used in the elastomer blend varies with the elastomer type. A general overview is provided here; specific additives are described with the different polymer types.

Typical additives used in an elastomer for wire and cable insulation are the following:

 Inorganic fillers such as clays that have undergone various treatments. This type of filler improves structural integrity of the polymer. Fillers influence stiffness, which in turn influences abrasion [19, 20].

 Plasticizers that are used to modify the physical characteristics of the wire coating or the viscosity of the compounded rubber (before extrusion)—often called softeners. Plasticizers cause polymers that are normally rigid to become flexible and stretchable.

 Metal oxides that serve as heat and/or moisture stabilizers.

 Curing or vulcanization agents. These agents promote the cross linking desired during the curing process.

 Co-curing agents—chemicals that facilitate curing.

 Antioxidants or antiozonants—chemicals that retard aging.

The inorganic clay component (also referred to as kaolin) is significant, and its nature requires description [21]. Clay is an inorganic mineral composed primarily of aluminum silicate, with trace amounts of other metal oxides and impurities. After mining, it is washed, ground, and cleaned to remove impurities and adjust particle size. Water of hydration is present, and clays are then heat treated (calcined) to reduce the water content from ~14% to ~1%. This thermal

treatment takes place at more than 1500°F (815°C). Calcined clays were used in the past to reinforce (strengthen) butyl rubber.

Clays can also be treated with functional silane chemicals, which serve to bind the calcined clay with the elastomer. This technology is used with EPR. Experience has demonstrated that as one moves from using hydrous clay to calcined clay to silane-treated clay, the properties of the

Fundamentals of Cable Insulation Systems

elastomer improve; the tensile strength, modulus, volume resistivity, and breakdown strength all increase. The silane also serves to remove remaining water and provides the capability to link between the polymer and the clay particles. Clay minerals have the ability to adsorb cations on their surfaces [22]; therefore, silane treatments prevent the ions from participating in water treeing. Several different types of silanes can be used. The modification of the clay, a major component of EPR formulation, is a complex process.

The term compounding is used to describe the mixing of all the materials in an EPR formulation, and the EPR that is blended in the soon-to-be cable insulation is called a compound.

6.2.3 Crystallinity

Crystallinity refers to chain alignment, a tendency of unbranched polyolefins to form structures that are impermeable and exclude impurities [23]. In general, elastomers, including butyl rubber, are not crystalline, but some types of EPR used for wire and cable can possess a low level of crystalline regions as a result of high ethylene-to-propylene comonomer ratios. Branches inhibit crystallinity, as they hinder the tendency of chains to align. To place the subject of crystallinity in perspective, the highest level of crystallinity in an EPR wire or cable insulation material is perhaps 6% to 8%, whereas the level of crystallinity for low-density PE is approximately 50%.

In general, the level of crystallinity is low in the EPRs used in medium-voltage cable.

Crystallinity plays a bigger role in XLPE insulation (see Section 6.6, Cross-Linked Polyethylene).

6.2.4 Cable Conductor and Insulation Shields

Although 5-kV cables might or might not use insulation shields, 8-kV rated cables are generally shielded, and all 15-kV cables are shielded. Modern standards require insulation shields at 8 kV and are tending to require shields at 5 kV. All the medium-voltage cables in use in nuclear plants have conductor shields (shields at the interface between the conductor and the inner wall of the insulation).

Different generations of medium-voltage cables have different types of shields. Early generations of nuclear plant cables had cotton tape conductor and insulation shields. Later conductor shields were formed of extruded polymer. Cotton tape insulation shields were supplanted by polymer tape shields, which in turn were supplanted by extruded shields.

In a modern cable, an insulation shield consists of an elastomeric (an ethylene copolymer) combined with semiconducting carbon black filler that imparts the semiconducting properties.

The role of the cable shield is to provide a controlled stress gradient between the conductor and the insulation. The nature and amount of carbon black influences the conductivity of the shield layer; the carbon black must possess the ability to aggregate into small clusters (termed

structure) that provide the conducting path within the elastomeric matrix. The shield must also facilitate a conductor–insulation intermediate interface that is smooth and defect free. The semiconducting shield materials are processed in the same manner as the insulation [23]. The semiconducting insulation shield connects the surface of the insulation to the metallic portion of the shield, so that air gaps do not exist that could ionize at operating voltage and cause PDs that would lead to early insulation failure.

Fundamentals of Cable Insulation Systems

Elastomeric cables with cable shields are intended to be discharge free, in the sense that they do not exhibit discharges that exceed a specific allowable level on testing—the exact maximum is defined in industry specifications. The requirements have forced lower and lower maximum allowable discharge levels as acceptance criteria became more stringent over the years.