Silicone Gel: A Liquid That Has Agreed to Hold Still

What a gel actually is

A silicone gel is polydimethylsiloxane with just enough crosslinks to stop it flowing. There is no reinforcing filler, which is what distinguishes it from a silicone elastomer, and there are very few crosslinks, which is what distinguishes it from a rubber. The result is a material with the consistency of a soft solid that will slowly recover its shape after being deformed, and that will not flow out of an open container under its own weight.

The mechanical description is that the network is below the percolation threshold for load bearing. In a filled elastomer, the silica filler forms a continuous network that carries stress and gives the material its strength. In a gel there is no filler, so the only load path is the polymer chain network itself, and that network is sparse. The material deforms by chain rearrangement rather than by stretching chains to their limit.

That is why a gel feels the way it does. Press a finger into it and it yields almost without resistance, then pushes back slowly. Cut it and it will not hold a sharp edge. Drop it and it will not bounce. It is a liquid in the rheological sense and a solid in the practical sense, and both statements are true at once.

Crosslink density and penetration

The standard measurement is penetration, using a cone of specified mass and geometry, reported in tenths of a millimetre. A soft gel might read 350; a firm one 60. The measurement is essentially a hardness test for materials that are too soft for a Shore durometer, and the scale runs in the opposite direction to Shore A: a higher penetration number means a softer gel.

Penetration falls steeply with crosslink density. At an index of 0.2 the cone sinks 350 tenths of a millimetre; at 0.4 it is 250; at 0.6 it is 170; at 0.8 it is 110; at 1.0 it is 60. The relationship is not linear, and the practical consequence is that the soft end of the range is very sensitive to formulation. A small increase in crosslinker at the soft end changes the feel of the gel noticeably, while the same increase at the firm end barely registers.

This is why gel suppliers quote penetration with a tolerance of plus or minus 20 or 30 tenths rather than a tight number. The measurement itself has a spread, the cure has a spread, and the application almost never needs better than that. If an application needs a tighter specification than the penetration scale can hold, it needs a different material.

The strength that comes with it

Tensile strength rises with crosslink density in the same way penetration falls, and it rises from 0.05 MPa at the soft end to 0.80 MPa at the firm end. Even at the top of the range a gel is an order of magnitude weaker than a soft silicone elastomer, which would be 5 to 8 MPa.

That weakness is not a defect, it is the specification. A gel is chosen because it transmits pressure without transmitting shear, because it conforms to a surface without imposing a load, and because it can be cut or torn away for rework. A material with the strength of an elastomer cannot do those things, because strength and conformability are bought with the same crosslinks.

Table 1 — Penetration and tensile strength across the gel range, and what each grade is used for.

Gel, foam and elastomer

Three materials sit close together in the soft-materials catalogue and are constantly confused: gel, sponge, and soft elastomer. They are distinguished by what is inside them. A gel has no gas and no filler, only a sparse polymer network. A sponge has gas but no filler, and its properties come from the cell structure. An elastomer has reinforcing filler and a dense crosslink network.

Table 2 — Gel, sponge and elastomer, distinguished by what is inside the material.

The compression set row is the one that decides a lot of applications. A gel has no measurable compression set because it is not a solid in the engineering sense; it relaxes and recovers by flow. That makes it excellent for a cushion that has to stay soft for a decade and useless for a gasket that has to hold a load.

Curing: platinum, inhibition and the things that stop it

Almost all silicone gels are addition cured, with a platinum catalyst and a hydride crosslinker. The reaction is fast at temperature and can be inhibited at room temperature so the material has a usable pot life. The inhibitor is the whole trick: it blocks the catalyst until the temperature rises, and then it releases and the gel cures in minutes.

The system is sensitive to poisons in a way that is worth knowing about, because the failure looks like a cure problem and the cause is usually a contamination problem.

Table 3 — The common cure inhibitors, where they come from, and how the failure presents.

The most common of these in practice is the glove. An operator wearing vulcanised rubber gloves who touches a mixed gel before it goes into the mould will leave enough sulphur to inhibit the surface, and the part will come out with a tacky patch exactly where the fingers were. Nitrile gloves, changed often, are the standard answer.

Where a gel wins

Gel wins wherever the requirement is to transmit pressure, or light, or heat, while transmitting as little shear as possible. That combination is unusual and it is why gels occupy a niche that neither elastomers nor foams can fill.

• Potting and encapsulation of electronics, where the gel protects against moisture and vibration without imposing stress on solder joints or wire bonds as it cures and as it thermally expands.
• Optical coupling between an LED and a lens or a light guide, where the gel's refractive index reduces the Fresnel loss at the interface and its softness accommodates thermal movement.
• Cushioning and pressure redistribution in insoles, seat cushions and prosthetic liners, where the gel flows to equalise pressure and then recovers.
• Wound contact layers and scar therapy sheets, where a soft, non-adherent, breathable interface is needed against skin.
• Thermal interface in some low-power assemblies, where a gel pad conforms to two imperfect surfaces and can be replaced without cleaning.
• Vibration isolation for very light components, where an elastomer would be too stiff and a foam would bottom out.

The potting application is the largest by volume and the one where the material properties matter most. A potting gel has to cure in place around components with different thermal expansion coefficients, and it has to do so without generating the shrinkage stress that an epoxy would. A gel shrinks very little on cure and has a low modulus, so the stress it imposes is small even when the thermal excursion is large.

Handling and specifying

A gel specification needs four numbers, and most purchase orders carry one. The penetration, with a tolerance. The cure schedule, including the temperature the part will actually see rather than the one in the data sheet. The adhesion, meaning whether the gel is self-adhesive or requires a primer, and to what substrate. And the extractables or the volatility, if the part goes anywhere near a patient or an optical surface.

Two practical points about handling. Gels are usually supplied as two components that have to be mixed in the correct ratio, and the ratio is not forgiving: a 10:1 system mixed at 9.5:1 will cure soft and stay soft, and the error is invisible until the part is tested. And the pot life after mixing is short, often a few hours, so the batch size has to match the shift rather than the day.

The last thing worth saying is that a gel cannot be reworked. A part that cures with an inhibited patch, a bubble or the wrong hardness cannot be repaired, only discarded, because there is no way to add material that will bond to a cured gel without a visible interface. That makes the mixing and dispensing discipline the whole quality system for a gel part, and it is why gel moulding is a process that rewards a good dispenser more than a good operator.