New Roles for Conductive Rubbers and

Plastics

Keith Evans Consultant

Growing interest is taken by materials engineers today in the seemingly anolalous characteristic o f conductivity in rubbers and plastics. The intrinsically conductive polymers such as polyacetylene, for example, are o f great potential for all-plastic batteries, and conductive and anti-static rub- bers are essential for tyres, and for conveyors and drive belts in hazardous environments. Consultant R. K. Evans looks at the characteristics and some applications o f these important materials.

Much work has been carried out in recent years on the manufacture and use of conductive rubbers and plastics, both by Britain's Rubber and Plastics Research Association (RAPRA) at Shawbury, and by companies such as Dunlop in Britain and Acushnet in the United States.

Major uses of such products are for antistatic purposes and, in the case of plastics, for shielding such equipment as computers and business machines from electro-magnetic interference; means of EMI shielding include, of course, metallic plating, spraying or vacuum deposition as well as the use of metal fibre- or flake-filled plastics. Addition of metallic fibres, or metallised glass fibres, to form metallo- plastics with electrical resistivity as low as 0.01 ohm-cm opens up a wide range of new uses for plastics combining electrical conductivity and electromagnetic shielding cap- ability with the properties of mouldability and light weight.

An early application for electrically conductive silicone elastomers was that of conductive gaskets; since then, the past 15-20 years have seen the advent of static charge dis- sipating devices, printed circuit board connectors, low-profile switches and digital displays.

A recent commercial application is in a food-warming tray, in which electrically conductive, copper-filled epoxy resin is used for bonding positive-temperature- coefficient ceramic discs to aluminium inserts. The copper-filled adhe- sive outperforms silver-filled epoxies under severe conditions of moisture and impact, at no more than one-third of the cost. In another widely-used application, antistatic floor mats for computer and radio-equipment rooms are manu- factured from carbon-filament filled, high-impact polyethy- lene.

In this context of antistatic protection, the U.S. Electro- nics Industries Association is establishing new standards for the protection of electronic devices; to meet these, manu- facturers are looking at a range of thermoplastic compounds using carbon blacks and ethoxylated amines to provide electrical conductivity and antistatic properties.

Heat-transfer properties, as well as electrical conductivity, are an important characteristic of the use of aluminium flakes or fibres in many plastics - polyesters, polycarbon- ates and nylons - in applications for heat conduction and resistive heating; the polyacetylenes, too, are seen as poten- tial materials for the manufacture of all-plastic batteries, in which polyacetylene film 'plates' are immersed in electro- lytes of polyethylene oxide/sodium/iodine complexes. Con- siderable research is being undertaken to establish the com- mercial value of such batteries.

Two other areas of commercial application are the use of plastics with a high negative temperature coefficient of res- istance to prevent overheating in, for example, domestic electric blankets; and ceiling-mounted electric heating ele- ments comprising flexible sheets of woven glass substrate coated with electrically-conductive silicone polymer.

Conducting rubbers f'md applications throughout industry No other material can match the range of electrical re-

sistivity of rubbers. That of a normal insulating rubber is 1013 to 1015 ohm-cm. A moderate reduction in resistivity is provided by the addition of such antistatic agents as quaternary ammonium compounds. Greater conductivity (though at the expense of some flexibility) requires the addition of particulate materials: carbon black - subject of much research at RAPRA - provides resistivity as low as 1 ohm-cm, while metal powders make possible a figure of 0.01 ohm-cm.

In many applications, however, where antistatic proper- ties alone are required, such a highly conductive material could be hazardous to the casual user, and for antistatic products BS 2050 stipulates low-resistance limits of 104 to 105 ohms. Upper limits, to ensure adequate leakage of static electrical charges, are between 106 and 108 ohms.

MATERIALS & DESIGN Vol. 5 FEBRUARY/MARCH 1984 43

Generally, products with such a specified lower resist- ance limit are termed antistatic, while those without are termed conductive. In some cases, such as footwear, items are manufactured in both grades and are so designated.

Much of the antistatic rubber produced today finds ap- plication in hospital products (anaesthetic breathing bags, for example), in petrochemical processing, conveyor belts and hoses used in handling explosives or inflammable mat- erials in powdered form, and in electrical cables and their terminations. Antistatic components are used to short- circuit the pins of solid-state circuits to prevent break- down of thin layers by static charges developed in handling; in cables, which must be as flexible as possible for ease of installation, high conductivity may be required to return fault currents to a leakage trip, or for stress equalisation.

Most vehicle tyres currently produced are both flexible, and sufficiently antistatic for general use. However, tyres for vehicles handling explosives and certain inflammable products call for even lower resistivity from a safety aspect, while still retaining adequate flexibility.

As mentioned, extensive research into carbon-black additives to natural or synthetic rubbers has been under- taken at RAPRA, including studies into electrical resis- tivity variation against hardness of different rubber mixes; though conductive mixes contain cross-linking (curing or vulcanising) systems, and often antioxidants and softeners or plasticisers, greatest influence on resistivity and hardness

is the nature of the rubber and the carbon black. Ability of a softener to depress resistivity may vary with the amount of carbon black in a given mix, as well as with the type of softener.

Blending natural and polychloroprene rubbers, again, offers a range of hardness and resistivity characteristics. As illustrated in Figure 1, a natural rubber compound with a carbon black loading of D% has the same resistivity as a 50/50 natural/chloroprene blend with a lower black loading of C%, with the advantage of considerably reduced hardness.

Hardness and, particularly, resistivity are affected by the properties of different types of carbon black. Table t shows these characteristics for four types of black added to poly- chloroprene ano natural rubbers, and to 50/50 mixes of these with the black added, respectively, to the natural rub- ber before blending, and to the blended mix. Research indicates that a given combination of hardness and resistivity may be obtained in a number of ways: technological con- siderations permitting, there may well be economic advan- tages in blending different blacks so as to use a minimum quantity of the more costly ingredients.

From the manufacturing viewpoint, consistency of para- meters between successive batches of materials is vital. Theo- retical testing of conductive rubber materials and compo- nents includes the measurement of tracking and erosion resistance, corona, dielectric constant and volume resist- ivity.

8

r r

CR

NR

1014 I I A to F represent increasing black ~ A content from zero

10 lo ~ ~ -

10 6

102

100 50 0

0 50 100

Figure 1 Likely effect of increasing black loading in compounds based on pre-blends of natural rubber and polycholoroprene

44 MATERIALS & DESIGN Vol. 5 FEBRUARY/MARCH 1984

Poly- ch 1 oroprene

Vulcan P

Vulcan XXX

Cabot XC72

Ketj en EC*

R H

9.4×1010 54

7.8xi0 I0 60

3.8xi09 56

9.2×101 59

Natural

R H

1.7xlO 12 43

l.OxlO 7 45

2.1xlO 6 46

1.6xlO 2 47

50150 CR/NR all black

in NR

R H

9.8xi03 44

1.7xl04 45

2.1xi03 46

3.3xi0 i 49

50/50 CR/NR blended polymers

R H

1.9x103 45

9.2x102 46

3.0x102 48

2.1xl01 49

Table 1 Resistivity and hardness values for rubbers containing either 10 parts of Ketjen EC or 20 parts of other blacks. (Resistivity R in Ohm-cm, Hardness H in IRHD)

New applications expand product range What potential applications are in view for the newer

conducting rubbers'? One recent development has been that of a conductive rubber transmitting/receiving aerial for CB radios. Flexible, non-corrosive and low in cost, such an aerial works effectively at frequencies- up to 500 MHz, and reduces the usual variations in transmission efficiency as the unit is handled by its operator.

Conductive rubber electrodes, similarly, can be moulded in a variety of shapes, and promise to be low-cost substitutes for platinum electrodes measuring fluid dissocation or con- ductivity.

In another field, that of temperature sensing, the addition of wax to a low-resistivity rubber mix makes possible the manufacture of a rubber transducer: as temperature rises, and the wax reaches its melting point, thermal expansion causes a sudden increase in volume resistivity. The process is repeatable as temperature rises and falls, and the device could thus form a reliable, low-cost temperature alarm.

There is a little doubt that materials engineers and com- ponent designers are today finding a variety of applications for the conducting rubbers and plastics, applications that will increase in range and number as research continues into their characteristics and consistency.

Fig. 2 High-voltage cable splice housing. Fig. 3 Cable stress-relief termination.

MATERIALS & DESIGN VoL 5 FEBRUARY/MARCH 1984 45