Insulation/Jacket

• Voltage dielectric for higher voltage charge at the conductor surface.
• Low loss material for higher frequency signal cables.
• Heat resistance in high temperature environments.
• Low temperature flexibility.
• Toughness for cut-through, abrasion and crush resistance.

Insulation compounds serve an electrical function first. Secondary properties consider the environmental factors.

Polyvinyl Chloride (PVC)
This material is available in many formulations tailored to meet specific needs. Madison provides two (2) basic types

(1) Plasticized flexible materials for 80°, 90°, and 105°C applications.

(2) Semi-rigid compounds rated at 80°C that can be made as thin wall products (8-9 mils).

PVC compounds are moderately good dielectric materials. Depending on the formulation, the dielectric constant can vary from 3 to 6. Formulations typically include the PVC resin, plasticizer, stabilizer, flame retardants, fillers, and specialty additives.

PVC compounds are limited to 105°C temperature applications and a cold environment of -40°C. Plasticizers can migrate from the compound causing the material to become brittle, especially at lower temperatures.

Typical Properties of Madison PVC Insulations

Property	Flexible*	Semi-Rigid
Physical
Specific Gravity	1.30-1.40	1.5
Durometer (Hardness)	90 Shore A	63 Shore D
Tensile Strength (psi)	1500	3500
Elongation (%)	150-300	200
Max. Opr. Temperature (C)	60-105	80
Oxygen Index	25-30	30
Solder Iron Resistance	Poor	Poor to Fair
Cut-through	Poor to Fair	Good
Electrical
Dielectric Constant	4-6	3.0-3.5
Volume Resistivity (ohms-cm)	10"-10"	10"
Dielectric Strength (Volts/Mil)	300-600	700
Insulation Resistance (Megaohm - 1000 ft.)	500-2000	5000

Electrical performance of polyethylene is excellent. Dielectric quality is known by a high dielectric strength (volts per mil), low dielectric constant, low dissipation factor and high insulation resistance. These properties are stable over a broad range of frequencies and temperature.

Physical properties of polyethylene are generally considered good except for fire resistance and ultra-violet resistance (weatherability). Modifiers are used to tailor specific improvements in these areas.

Polypropylene
This polyolefin material is characteristic in many ways to high density polyethylene; electrical and chemical resistance are similar. It has superior physical properties such as abrasion, cut through, and heat resistance; however, it has a lower density. It is flammable, but flame retardant grades can be made available. It is preferred to polyethylene for stress crack resistance applications. Much of polypropylene is used in telecommunication cables for physical and dielectric quality.

Cellular Polyolefin
Dielectric improvements in capacitance within insulations are provided by production of a cellular structure in the finished insulation. Processes of producing an inert gas in the polymer melt are controlled in the extruder and the resulting extrudate can be provided with a variation in the amount of voids (air to solid regions). This allows control over the dielectric constant and dissipation factor. Polyolefin dielectric constant (typically 2.27) can be lowered to 1.55 by expansion.

Flame Retardant Polyethylene
Compounds of polyethylene employing fire retardant additives are available, but there is some sacrificing of properties to consider when designing these materials into electrical wire applications.

Non Halogen Compounds

Over the past few years, non halogen, flame retardant, reduced emissions compounds have been developed in response to a growing demand for products which offer greater protection against fatalities, injuries and property damage from fire. When burned, cables made with non-halogen flame retardant compounds give off as little as one-quarter the smoke and fumes of conventional cable materials. These compounds have good crush and deformation resistance, good flexibility, excellent long term ageing properties plus physical integrity at low temperatures.

There are a number of fluorocarbon resins available as insulating materials. Each fluorocarbon type is distinctly different, however they all can be classified as highly fire resistant and physically and electrically stable at elevated temperature.

FEP
FEP has a service temperature of 200°C with excellent electrical properties - dielectric constant (2.1) and dissipation factor (.001) that is consistent through its maximum operating temperature and frequency range.

Low temperature properties of FEP are similar to those of TFE resulting in a -65°C rating. FEP insulated wire can be supplied in long continuous lengths allowing it to service a wider range of applications. FEP cannot be used in applications where thermosetting quantities are required (solder iron or short term overload). Along with the inherent flame resistance, this material is widely used in plenum cable applications because it produces low smoke in fire events.

PFA
PFA has a 260°C temperature rating, therefore it is an excellent choice for wiring requiring TFE properties and long lengths.

ETFE (Tefzel^®)
For application where properties of FEP are needed, with better chemical resistance.

ECTFE (Halar^®)
This material is slightly different from ETFE in chemical resistance, cross-linking ability, electrical, physical and thermal properties.

Like FEP and TFE, ECTFE is not useful where corona conditions prevail as in high voltage applications. As with other resins, irradiation cross-linking improves stress crack resistance. ECTFE ranks among the most radiation resistant polymers comparing with ETFE and polyethylene in this property.

PVDF (Kynar^®)
This material is rated for continuous use over a temperature range of -65° to 125°C. It has good resistance to corrosive chemical and organic solvents. Although this material is very hard with high tensile strength, abrasion resistance and excellent cut-through, limitations of flexibility are evident. It is resistant to creep and fatigue. It can be used in exterior applications because it is stable in sunlight and other sources of UV radiation.

Electrical properties of PVDF are not as good as other fluoropolymers. Most common use of this material is for jackets and back panel wire where electrical performance
is not critical. PVDF is highly flame resistant and low smoke producing finding wide use as plenum cable jackets.

Foam Fluorocarbons
To further improve on the superb properties of Teflon, processes have been developed to foam the FEP, resulting in lower dielectric material. These materials are increasingly used in plenum applications. They produce little smoke and minimize dripping and fire propagation.

Jacket Compounds

Jacket or sheaths over multicomponent cable or single components act as a protective covering as well as contain the component elements and shields. Jackets can be made semiconductive, depending on the application. Jacket materials are called upon to be flame resistant, physically tough, flexible, chemically resistant and to have a good appearance.

Types:

PVC
Is the most widely used non-plenum jacket. A variety of compounds are available to serve a wide range of applications. Fire safety is an important role served by PVC jackets.

Polyurethane
A material used for severe service of abrasion and cut-through with flexibility. A range of grades are available to meet various applications, such as extreme low temperatures.

Polyethylene
Inherent properties make it ideal for direct burial applications.

Thermoplastic Elastomer (TPE)
A suitable replacement to rubber where the thermosetting properties of rubber are not critical.

Fluorocarbon
Physical toughness and fire resistant characteristics override the slight increase in cost. See description of benefits in the section on dielectric material.

Shields
The increasing number of high frequency interference sources has emphasized the necessity for shielding in electronic equipment. Shields are used for EMI and RFI protection.

If a shield is required, the end user has a choice among several options - braided copper wire; spiral (served) copper wire; copper and aluminum tapes; laminates of aluminum/polyester and aluminum/polyester/aluminum with spiral drain wires for ease of termination; semi-conductive plastics.

The most effective for high frequency applications is a braided copper shield. For the majority of audio frequency applications (20 to 20,000 Hz) a coverage of 75% to 85% will prove effective, but for the high frequency range (3 to 30 MHz) a coverage of 85% to 95% will be necessary to give adequate protection.

The most economical shield is an aluminum polyester laminated tape used in conjunction with a drain wire applied either spirally or longitudinally, directly adjacent to the aluminum side of the tape. For frequencies up to 400 MHz it is as effective as a braid copper shield since it provides 100% coverage.

Cabling of individual layers may be either concentric or bunched. The concentric lay-up consists of a central wire or filler surrounded by one or more layers of helically laid wires, with the direction of lay reversed for successive layers and with the length of lay increasing for each successive layer. The direction of lay of the outer layer is generally left-hand. This construction assures cable roundness and greater mechanical strength. A bunched or unilay cable lay-up consists of any number of insulated wires cabled together in the same direction. It results in a smaller overall cable diameter, lighter weight, and has greater flexibility than concentric lay-ups.

Flexibility of a cable is directly related to the lay length of the individual layers. Usually this is 8 to 16 times the pitch diameter of each layer; the smaller the lay length, the greater the flexibility of the cable.

Fillers are used to round out a cable and obtain symmetry.

Binders and Servers are sometimes needed (depending on construction) to prevent flaring or untwisting of components.

Tapes are frequently placed under the outer jacket as an added protection against mechanical abuse, and between overall shields and underlying conductors to prevent physical damage to the insulation.