carbon black on electrical resistivity of vulcanised rubber

Rubber material is of course commonly regarded and indeed widely used as an electrical insulating material and if insulation is what is required then rubber can be ideal material.It must be note however that by mixing the rubber material with certain types of carbon black ,this electrical resistivity can be enormously reduce ; Table A illustates the magnitude of this effect.

Table A Effect of various type of carbon black on electrical resistivity of vulcanised natural rubber : Data from Norman (1957)

Amount of Carbon black(phr)	log	volume	resistivity	,ohm-cm
	Channel black	Acetylene black	Oil-based furnance black	Gas-base furnace black	Thermal Black
0
10	15.5-15.8	10.3	14.5-16.0	15.0-15.5	-
20	9.5-13.5	6.5	3.5-15.0	14.5-15.2	15.0-15.5
30	7.2-8.8	3.4	3.0-14.0	10.5-15.0	13.0-15.0
50	5.5-7.2	0.4	2.3-5.0	4.0-8.0	10.0-15.0
85	5.2-6.2	0.2	0.5-3.0	2.6-4.3	8.0-14.5
140	-	-	-	1.8-2.8	5.5-7.0

When the resistivity is below about 10e7 – 10-e9 ohm-cm the rubber material becomes what is now called “anti static” , since it is able to conduct away static charges sufficiently rapidly to prevent any dangerous accumulation of charge.A further stage beyond this is “conductive” rubber with resistivity below 10e5 – 10e7 ohm-cm ,which will carry quite considerable currents.

It is important to note that the resistivity of an anti-static or conductive rubber is effected to a greater or less extent by deforming the rubber material and under unfavourable circumstances the resistivity may be so much increased that the anti-static properties are lost, an effect which is only slowly and pershap never completely recovable.

Another important point to bear in mind is that carbon blacks are commonly incorporated into rubber material in order to increase its strength and resistance to tearing and abrasion; indeed many of the rubber material suitable for engineering uses would be compounded in this way, with and inevitable reduction in their electrical resistivity. If therefore a really hight resistance is required the user should again make this point clear to the rubber manufacturer .