Guidelines for specifying bedding compounds.

Bedding compounds, used typically to fill voids in or literally "round out" cable assemblies, provide a number of benefits to the wire and cable manufacturer. The primary use of bed ding compounds is in power cables, with an important secondary use in telecommunications/data communications. In their simplest forms, bedding compounds are used in multi-conductor cables, filling spaces between conductors and the cable jacket, insulation or sheathing (figure 1). Simple cables may use a single bedding compound layer, while more complex cable assemblies might use two or more bedding layers. Bedding compounds can be conductive, semi-conductive or non-conductive, and they may be non-flame retardant or flame retardant. Generally, bedding compounds are elastomer-based, comprised of a variety of polymer blends or polymers. The exact formulation depends on cable construction and application.

[FIGURE 1 OMITTED]

Bedding compound benefits

In typical applications, bedding compounds make cables round for easier pulling by installers. In addition, by protecting conductors from mechanical and/or water damage, bedding compounds also can add to field life. Because bedding compounds show lower extrusion temperatures compared to PVC, they contribute to lower cost of cable fabrication by reducing energy costs. In addition, with typically 20 or less L/D, they reduce costs for extrusion equipment. Because of their ease of peeling, or "banana effect," ethylene-propylene robber (EPR)-based bedding compounds have advantages for installation and they are appropriate for tandem production. Finally, for non-halogenated bedding compound formulations, there is an environmental advantage over compounds based on PVC.

Selection of bedding compounds

Table 1 shows select criteria for 10 representative bedding compounds. Five criteria go into the selection of the right type of bedding compound for a given application:

* Conductivity;

* flame retardancy;

* processing;

* type of cable construction; and

* polymer type (in contact with bedding).

Conductivity and flame retardancy are simple yes or no decisions. Specific limiting oxygen index (LOI) requirements need to be known to specify flame retardancy fully. As LOI requirements go up, the bedding compound needs to be more flame retardant.

Processing, type of cable and polymer type are each more complex, and require a bit more discussion.

Processing

Selection of the right type of bedding compound depends on key processing decision factors. These include temperature profile control as well as such extruder parameters as screw dimension and die construction. Equally important is the extrusion type used in the manufacturing of cable. Often, the choice of bedding compound is dictated by the equipment a processor owns. For example, a compound of relatively low viscosity would be suggested for underpowered extruders.

Processing parameters

Bedding compounds can be introduced to the cable at any point during manufacture. Exactly where bedding compound is added or extruded will depend on the types of layers in the cable being produced. Precise placement is dependent on the construction, and the bedding compound supplier is in the best position to advise manufacturers on the best possible point(s) of introduction.

How cable is manufactured determines where and when bedding compounds become part of the cable assembly. Perfect for tandem extrusion, the bedding compound's extruder is typically upstream of the jacketing compound extruder (figure 2). As the insulated wires pass through the crosshead of the bedding compound extruder, the bedding compound surrounds them with the required thickness/diameter. This composite passes through the jacketing extruder crosshead after a very short length for cooling, and the jacket layer is applied.

[FIGURE 2 OMITTED]

Although discontinuous production is an extra manufacturing step, it is sometimes the only way to produce cable with multiple-component layers, some of which are wound around assemblies (such as metal braiding, armoring or tape), and some of which are extruded, such as insulation or bedding. The bedding compound must be designed to be coilable if a discontinuous process is used. Coilable bedding compounds are designed to withstand radius bending and the frictional pull experienced during the take-up process, as when process cable passes over sheaves and capstans.

Type of cable construction

The type of bedding compound depends on the type of cable, and there is a broad range of complexity in cable construction. One extreme would be common 110/220V three-wire household cable, half a centimeter in diameter, with one uninsulated and two insulated solid copper conductors, plus an outer jacket. The other extreme might be a 60kV submarine cable, more than 15 cm in diameter, and made up of 12 or more layers of jacket, armoring, bedding compound, sheathing, tape, insulation, semi-conductive layers and conductors.

Also important for selecting bedding compounds is a detailed study of the application requirements of the cable, its voltage, environmental issues, temperature ratings and polymer base. For example, when bedding compounds sit between steel armoring and other components, the compound will need to be mechanically stronger and more abrasion resistant than compounds for less exacting needs. Compounds need to be selected with maximum operating temperatures in mind as well.

Insulation and jacket polymer type

Bedding compound selection must fit with the dielectric properties of the jacketing or insulating material. These jacketing /insulating materials can be based, among others, on PVC, PE, XLPE and HFFR. EPR-based beddings should be selected for PVC/PVC constructions containing plasticizer. This is because the EPR compound can withstand and/or resist plasticizer migration. If the wrong bedding compound is chosen, there is a chance that the plasticizer in the PVC jacketing will migrate into the non-plasticized bedding or into the XLPE insulation, potentially changing their electrical or physical properties. The migration can be two-way: through the bedding and into the jacket from the plasticized PVC insulation. Aesthetic or environmental issues can arise in severe cases, when the migration blooms to the surface.

In cables with XLPE construction, the right bedding compound will be the one that does not stick to the insulated core, that is, unless the product requires adhesion to counteract the typical shrinking of PE.

Testing

Four common tests for mechanical properties of bedding compounds are as follows:

* Viscosity;

* hardness:

* limiting oxygen index (LOI); and

* density

Viscosity (Mooney viscosity)

Viscosity is the resistance to flow of a polymer. Most bedding compounds have a relatively low viscosity, which is a good predictor of processing efficiency. Measured at 100[degrees]C, typical values are from 20 to 50.

Hardness

Hardness is a good indicator of mechanical strength, and is defined as the resistance to penetration by a round or pointed object under a given load. Coilability and flexibility are reflected in hardness as well. Typical hardness for compounds range from 50 to 80 durometer A hardness.

Limiting oxygen index (LOI)

LOI describes the minimum oxygen concentration of an oxygen /nitrogen atmosphere needed to sustain a "candle light" flame burning on a sample. It also indicates the degree of flame retardancy, with typical values ranging from 20 to 80% oxygen.

Density/specific gravity

Density is defined as mass divided by volume, expressed as g/[cm.sup.3]. Density predicts overall weight or mass of a bedding application, and additionally, can indicate the concentration of filler, helping to predict how the bedding compound will behave under mechanical shock. The denser the material, the more protection it provides. Finally, density is required to calculate the price per unit volume. Typical values are 1.5 to 2 g/[cm.sup.3].

Testing as a value added activity

Two additional tests, for plasticizer migration and for sticking, can add value to the engineering process. For complex or difficult installations, both of these additional tests provide valuable information.

Plasticizer migration

Plasticizer migration is tested via a sandwich that measures the degree to which the polymers or substances adjacent to the bedding compound will interact with the bedding compound. Long-term field reliability of a cable assembly can be tested this way, and the test provides much-needed indicators for both the bedding compound and insulation selection. Environmental parameters that match the cable's application should be included in the test protocol.

Sticking/adhesion

In applications where the bedding compound should be easy to peel, testing for the degree to which a bedding layer sticks to the insulated core will be important.

Applications

Bedding compounds are finding increased application throughout the electrical generation, transmission and distribution grid. Cable design varies widely, depending on the number of components under the outer jacket and the engineering functionality required by the cable's application. What is important for underground cabling will make little sense for cables meant for tower transmission, subject to heavy ice and wind loads. Cables meant for residential interior electrical runs will differ greatly from underwater cables, which must protect against water encroachment, sometimes under great pressures. Figures 3-8 illustrate a range of cable assemblies.

Figure 3--building cable sheathed with copper,
designed to minimized F/EMI interference--this is
a type increasingly necessary around densely
networked computers or controls

1 - PVC/HDPE/HFFR sheathing; 2 - copper screen;
3 - bedding compound; 4 - PVC/XLPE/PP/PE insulation;
5 - common conductor

[ILLUSTRATION OMITTED]
Figure 4 - standard building cable

1 - PVC/HDPE/HFFR sheathing; 2 - bedding compound;
3 - PVC/XLPE/PP/PE insulation; 4 - copper conductor

[ILLUSTRATION OMITTED]
Figure 5 - 600/1,000V low voltage cable

1 - copper conductor; 2 - PVC/XLPE/PP/PE insulation;
3 - bedding compound; 4 - PVC/HDPE/HFFR sheathing;
5 - steel wire armoring

[ILLUSTRATION OMITTED]
Figure 6 - 12/20 kV high current cable with
internal semi-conductive layers to relieve and
smooth out voltage stresses

1 - Copper conductor
2 - Inner semi-conductive layer
3 - XLPE insulation
4 - Outer semi-conductive layer
5 - Semi-conductive tape
6 - Copper screen
7 - Bedding compound
8 - Steel wire armoring
9 - PVC sheathing

[ILLUSTRATION OMITTED]
Figure 7 - 60 kV submarine cable, single
conductor with semi-conductive bedding
compound and steel and aluminum armoring

1 - PE sheathing
2 - Semi-conductive tape
3 - Steel wire armoring
4 - Semi-conductive tape
5 - Semi-conductive bedding compound
6 - Semi-conductive tape
7 - Aluminum sheathing
8 - Conductive tape
9 - Outer semi-conductive layer
10 - Inner semi-conductive layers
11 - XLPE insulation
12 - Copper conductor

[ILLUSTRATION OMITTED]
Figure 8 - halogen-free cable

1 - Halogen-free, flame retardant sheathing
2 - Copper screen (bottom cable only)
3 - Halogen free bedding compound
4 - Halogen free insulation
5 - Copper conductor

[ILLUSTRATION OMITTED]

Conclusion

By choosing the right bedding compound, cable manufacturers can consistently increase service life, ease installation and help ensure full functionality in the face of difficult environmental or ambient conditions. Bedding compounds thus add value for both protection and installation. Available in a variety of densities and polymer bases, bedding compounds can be tuned for a broad range of uses. Through careful testing, as well as by thorough specification of mechanical properties and insulation types, the right candidates for a given application become clear.

Table 1 - summary of select physical characteristics of 10 types of
bedding compounds, as well as their polymer base materials

Type    Polymer      Flame        Density        Hardness
         base      retardant   (g/[cm.sup.3])   durometer A

 1       Butyl         N            1.84           1.84
 2     Butyl/EPR       N            1.90           1.90
 3 *     EPDM          N            1.52           1.52
 4       EPDM          Y            1.84           1.84
 5        EPR          N            1.91           1.91
 6        EPR          N            1.91           1.91
 7        EPR          Y            1.72           1.72
 8        EVA          Y            1.83           1.83
 9        POE          N            2.00           2.00
10        POE          Y            1.94           1.94

Type      Mooney viscosity                     Volume-
       (ML(1+4) 100[degrees]C                resistivity
                                               (Q X cm)

 1               45             [greater than or equal to][10.sup.12]
 2               22             [greater than or equal to][10.sup.14]
 3 *             35             [greater than or equal to][10.sup.3]
 4               24             [greater than or equal to][10.sup.10]
 5               24             [greater than or equal to][10.sup.14]
 6               23             [greater than or equal to][10.sup.14]
 7               25             [greater than or equal to][10.sup.10]
 8               42             [greater than or equal to][10.sup.10]
 9               30             [greater than or equal to][10.sup.14]
10               30             [greater than or equal to][10.sup.10]

Type      Insulation/sheathing

 1             XLPE, PE/PE
 2               PVC/PVC
 3 *             PVC/PVC
       (* semiconductive compound)
 4           PP/HFFR-LOI=45
 5               PVC/PVC
 6               PVC/PVC
 7          PVC/PVC-FR-LOI=45
 8          HFFR/HFFR-LOI=80
 9               PVC/PVC
10     XLPE, PE, PP/HFFR/PE-LOI=45

Acknowledgements

"Guidelines for specifying bedding compounds" is based on a paper given at the April 2001 Rubber Division meeting.

"Compounding and extrusion of TPVs; new developments in equipment" is based on a paper given at the October, 2001 Rubber Division meeting.

"EPDM-metallocene elastomer blends for W&C" is based on a paper given at the April, 2001 Rubber Division meeting.