Degassing Silicone: Every Minute Under Vacuum Costs Pot Life

Void is a pressure problem with a time cost

Air gets into silicone in two ways. It is entrained during mixing, as bubbles that are large enough to see, and it is dissolved in the components, as gas in solution that is invisible. Vacuum removes the first quickly and the second slowly, and most degassing schedules are set for the first while being judged on the second.

A bubble rises because it is buoyant and it is resisted by the viscosity of the surrounding liquid. For a bubble of radius r in a liquid of viscosity eta, the rise velocity scales with r squared. Halve the bubble diameter and it rises four times slower. This is why vacuum works well on visible bubbles and poorly on the ones that cause the fine surface porosity nobody can explain.

The other half of the problem is that the cure does not pause. Platinum-catalysed silicone starts reacting the moment the two components meet, and every minute spent under vacuum is a minute of pot life that has been spent without producing a part. Degassing is therefore a trade between void content and working time, and the trade has a clear optimum.

Most of the void leaves in the first decade

Measured void content against vacuum level: 2.4 percent at 100 mbar, 1.1 at 50, 0.4 at 20, 0.15 at 10 and 0.08 at 5. The first two steps remove 83 percent of the void. The last two remove 0.32 percentage points between them, which is the difference between a part with visible porosity and a part without it only in the most demanding optical applications.

The shape is not an accident. Void removal is limited by how fast a bubble can travel through the liquid to the surface, and that travel time is set by viscosity and by the distance to the surface. Once the large bubbles are gone, the remaining ones are small, and small bubbles take disproportionately long. Below about 20 mbar you are no longer removing the bubbles that matter, you are waiting for the ones that do not.

There is a second limit that no vacuum level overcomes: the vapour pressure of the components. Pull too hard and the volatile fractions in the compound start to evaporate, which changes the formulation at the surface and can leave a skin that traps the bubbles underneath. That is why very low vacuum levels sometimes make porosity worse rather than better.

The clock does not stop while you degas

Pot life consumed against degassing time: 0 percent at zero minutes, 4 percent at five, 9 percent at ten, 20 percent at twenty and 34 percent at thirty. The relationship is close to linear at first and steepens as the material warms, because the reaction rate rises with temperature and a vacuum pump is a heater in disguise.

That last point is the one that catches people. A vacuum chamber at 5 mbar is a chamber where convection has been removed, so heat from the pump, the chamber walls and the exothermic reaction accumulates in the silicone instead of leaving. A 30 minute degas can raise the mass by 8 to 12 degrees, which roughly doubles the reaction rate for the rest of the pot life.

The practical consequence is that a degassing schedule optimised in a cool laboratory fails in a warm shop. The material arrives at the mould already partly cured, the flow is shorter, and the parts show knit lines that were not there in the trial.

What vacuum removes, and what it cannot

Not all bubbles respond to vacuum in the same way, and the size dependence is severe enough to be worth a table.

Table 1 — Bubble size against rise time in a 30 Pa.s silicone, and whether vacuum can clear it

The 0.05 mm bubbles are the ones that cause the fine surface haze on an optically clear part. They do not rise out in any schedule a production line can afford, and the only real solutions are to prevent them during mixing, to degas the components separately before combining, or to pressurise the mould so that the bubble collapses instead of expanding.

That is the counter-intuitive part of vacuum degassing. Vacuum expands a bubble to make it buoyant, which works for large bubbles. Pressure shrinks a bubble until it dissolves, which works for small ones. A process that has both a vacuum step and a pressurised cure is doing two different jobs, and neither one substitutes for the other.

Temperature is the lever that costs twice

Warming the silicone before degassing lowers its viscosity, which speeds bubble rise, and raises the vapour pressure, which makes the vacuum more effective at removing dissolved gas. It also doubles the cure rate for every ten degrees, which is the cost.

Table 2 — The temperature trade: faster degassing against a shorter working window

The optimum is usually 25 C, where the degas takes eleven minutes and the pot life is still two hours. Warming to 40 C cuts the degas to four minutes and leaves 25 minutes to do everything else, which is fine for a metered dispensing machine and impossible for a hand-pour operation.

What to specify

A degassing specification has to name the four variables together, because changing one changes the others.

• Vacuum level in mbar, not in percent of vacuum, because the interesting range is all below 100 mbar.
• Hold time at that level, measured from when the level is reached rather than from when the pump starts.
• Material temperature during the degas, with a tolerance.
• The pot life budget the degas is allowed to consume, which is usually 10 percent or less.

Then check the result on a cured section, not on the surface of the liquid. A degassed cup looks perfect and a cut section shows whether the small bubbles are still there.