Conductive Silicone: Fillers, Resistivity and Compression Set

What conductive means, and how conductive

Silicone is an insulator: a volume resistivity of 10 to the 14 or higher, in ohm centimetres. Conductive silicone is the same polymer with enough filler to build a continuous path through it, and the resistivity that results spans an enormous range.

The application decides the target. An electrostatic discharge part needs 10 to the 6 to 10 to the 9. An EMI gasket needs 10 to the minus 1 or lower. A grounding pad needs something in between, and a thermally conductive but electrically insulating part is a different product altogether.

Table 1 - The conductivity classes and what it takes to reach each one. Each step down costs hardness, tear and compression set.

Volume resistivity is the number that matters for a gasket. Surface resistivity is easier to measure and is often quoted for a film or a mat, but a gasket conducts through its thickness, so the volume number is the one that predicts performance.

Percolation: the step change

Conductivity does not arrive gradually. As filler is added, the particles are isolated from each other and the material is still an insulator. At a critical loading they touch, a continuous path forms, and the resistivity falls by orders of magnitude over a narrow range of loading.

For carbon black in silicone, the percolation window is around 12 to 18 parts per hundred. Across it, the resistivity falls from 10 to the 12 to 10 to the 2, ten orders of magnitude, for six parts of filler. Below the window the material is an insulator; above it, the conductivity improves only slowly with more filler.

That shape is what makes conductive silicone a formulation problem rather than a mixing problem. A compound at 16 parts is on the steepest part of the curve, and a two part variation in loading changes the resistivity by two orders of magnitude. A compound at 25 parts is on the flat part, where the same variation changes almost nothing.

The fillers, and what each one costs

• Carbon black: cheap, effective at 15 to 25 parts, gives 10 to 1 to 10 to 3. It raises hardness and destroys compression set, and it only comes in black.
• Graphite: better conductivity per part than carbon black at high loadings, softer on set, and it lubricates the compound so it is harder to mould.
• Silver: 1 to 10 to 1 ohm centimetre at 40 to 70 parts, the best conductivity, and the price of the part is the price of the silver.
• Nickel-coated graphite: a compromise, 0.1 to 1 ohm centimetre, cheaper than silver, and it oxidises in humid conditions.
• Silver-plated glass or copper: good conductivity at lower density, and the plating can be damaged during mixing.
• Carbon fibre: anisotropic, conductive along the fibre, and it reinforces at the same time.

Table 2 - Fillers for conductive silicone. The cost index is relative to a carbon black compound at the same hardness, and it is the number that decides most projects.

The choice is driven by the resistivity target and by what else the part has to do. A silver-filled gasket at 60 parts is barely a rubber any more: the hardness is above 80 Shore A, the tear is poor, and the compression set is worse than a carbon-filled compound at the same conductivity.

Compression set: what conductivity costs

Every part of filler takes recovery away. A 30 Shore A silicone takes 12 percent set at 175 degrees; at 30 parts of carbon black it takes 33, and the hardness has gone up by 40 points at the same time.

That combination is the central design problem of a conductive gasket. The filler makes the material stiffer, so it needs more force to compress, and it makes it take a worse set, so it holds less force over time. The sealing force at end of life is the product of the two effects, and it falls faster than either.

The design response is to use a hollow or a shaped profile rather than a solid O-ring, so that the same force produces more deflection, and to specify the conductivity at the loading that meets the requirement rather than at the highest loading available.

Manufacturing: mixing, anisotropy and cure

Conductive compounds are difficult to mix. The filler loading is high, the viscosity is high, and the filler has to be dispersed without being broken. Silver-plated particles are fragile, and over-mixing removes the plating and destroys the conductivity.

• Mix at low shear and short time. A dispersion that is good enough is better than a dispersion that is perfect and damaged.
• Watch the cure: carbon black and graphite are usually benign, but some metal fillers and their surface treatments inhibit a platinum cure.
• Expect anisotropy. Fillers align during moulding, so the resistivity through the thickness differs from the resistivity along the flow. Measure in the direction the current will flow.
• Compression moulding is the usual process for high loadings, because the viscosity is too high for injection. Liquid silicone with conductive filler exists but the loading is limited.
• Post-cure carefully: a conductive part that is under-cured has both a worse set and a higher resistivity, because the network is not finished.

The anisotropy point is the one that catches people out. A gasket measured on a flat plaque can read 0.05 ohm centimetre, and the same compound moulded into a tall rib reads 0.3 through the thickness because the filler aligned with the flow. The test has to be on the geometry that will be used.

Testing a conductive part

• Volume resistivity by a four-point probe or a guarded two-point method, on a sample of the production geometry, in the direction of current flow.
• Contact resistance on the assembled gasket, which is what the enclosure actually sees and which includes the surface condition of both mating parts.
• Shielding effectiveness for an EMI gasket, measured on a test enclosure, because a material number does not predict a system number.
• Compression set at the service temperature, because the conductivity is irrelevant if the gasket stops sealing.
• Ageing: 85 degrees and 85 percent relative humidity for 1,000 hours, then resistivity and set again. Nickel and silver-plated fillers oxidise, and the resistance rises.

The ageing test is the one that separates a gasket that works from one that works in the datasheet. A silver-plated glass filler in a humid environment can lose two orders of magnitude of conductivity in a year, and the sealing force falls at the same time.