High-Temperature Silicone: 200 C Continuous and What Shortens It

What 200 C continuous actually means

A silicone datasheet that says 200 degrees continuous is making a statement about a test, not about a service life. The usual basis is heat ageing to ISO 188 or ASTM D573: a sample held in an air-circulating oven at the stated temperature, removed at intervals, and tested for tensile strength and elongation.

The number that comes out is an extrapolation. If a compound retains 50 percent of its elongation after 1,000 hours at 200 degrees, the supplier may claim a continuous rating at that temperature, with no statement about whether the part is a seal under compression or a cable jacket in still air.

Those two cases behave very differently. A seal under compression at 200 degrees in air fails from set long before it fails from strength loss. A cable jacket in the same oven fails from embrittlement at the surface, which is an oxidation problem.

Retention against time and temperature

The loss of tensile strength is slow at 200 and steep above 250. Over 720 hours, which is thirty days, a standard VMQ retains 87 percent at 200 degrees, 63 percent at 250 and 30 percent at 300.

Table 1 - Tensile strength retained against hours at temperature for a standard VMQ with 1 part of iron oxide, aged in air. The 300 degree column is outside the useful life of the material after about 200 hours.

The shape of the curves is the useful part. At 200 degrees the loss is nearly linear and slow, which is what a continuous rating is meant to describe. At 300 degrees the curve has a knee at about 170 hours, after which the material is losing strength faster than the test can track it.

A part that sees 300 degrees for a few minutes during a process cycle is therefore fine, and the same part at 300 degrees for a week is scrap. The rating is a rate, not a threshold.

Three mechanisms, three different fixes

• Oxidation: oxygen attacks the methyl groups, forming radicals that crosslink the chain. The rubber hardens, then embrittles. It is a surface effect first, which is why a thick section survives longer than a thin one at the same temperature.
• Depolymerisation and reversion: above about 300 degrees the siloxane backbone itself unzips, and the rubber softens into a sticky mass rather than hardening. This is the failure mode in a sealed or oxygen-free environment, and it is faster than oxidation.
• Compression set: the network relaxes under load, and the seal takes a permanent set. It is not a chemical failure, and it happens at temperatures well below the strength-loss limit.

The fixes are different for each. Oxidation is slowed by a heat stabiliser, usually iron oxide or a cerium compound at 1 to 3 parts. Reversion is slowed by removing the trace moisture and metal contaminants that catalyse it, which is a compounding and handling problem. Set is improved by a proper post-cure and, at the limit, by a different elastomer.

What shortens the life

Temperature is the headline and the least interesting variable, because it is the one that is specified. The things that actually shorten a silicone part's life are usually the things nobody wrote down.

Table 2 - The factors that move a silicone part's life more than the nominal temperature does.

The oil factor is the one that surprises people. Silicone swells in hydrocarbon oil, and the swelling extracts the low molecular weight fraction that carries part of the stabiliser. A seal that would last ten years in air lasts one in oil, at the same temperature.

Compression set: the limit that arrives first

For any seal, compression set decides the life. A standard VMQ takes 22 percent set after 22 hours at 200 degrees, 36 percent at 250, and 47 percent at 275. Once the set approaches the initial squeeze, the seal has no force left to give.

The practical rule is that set should stay below about half the initial squeeze for the life of the part. A seal designed with 25 percent squeeze therefore has a set budget of 12 percent, which a standard VMQ reaches somewhere between 150 and 175 degrees.

FVMQ is slightly better, 41 percent against 47 at 275 degrees, but the difference is smaller than the price difference. Where set is the limit, the answer is more often a lower compression, a thicker section, or a different elastomer entirely, rather than a more expensive silicone.

Testing a high-temperature part properly

• Age in air, not in a vacuum oven, unless the application is in a vacuum. The two give different failure modes and different lives.
• Test elongation as well as tensile strength. Elongation falls first and is the more sensitive indicator of embrittlement.
• Test compression set at the service temperature and for the service time, on a button, not on the part.
• Include a cycling test if the part sees thermal cycles: 500 cycles from minus 40 to 200 is a different test from 500 hours at 200.
• Test after the post-cure the part will actually receive, because an under-cured part ages differently from a fully cured one.

The most common mistake is to accept a supplier's heat-ageing curve measured on a 2 mm slab as evidence for a 0.8 mm diaphragm. The thin part oxidises through its whole thickness and fails years earlier, and the curve was never about that geometry.

When silicone is not enough

Above about 250 degrees continuous, or in fuel and oil, silicone stops being the right answer and the options narrow.

• FVMQ: fluorosilicone, 200 degrees continuous, excellent fuel and oil resistance, three to five times the price, worse low-temperature flexibility and worse tear.
• Phenyl silicone: 250 to 300 degrees, better radiation resistance, poor low-temperature flexibility, expensive.
• Fluoroelastomer (FKM): 200 to 230 degrees, excellent in oil and fuel, poor at low temperature, and it needs a post-cure.
• Perfluorinated elastomer (FFKM): 300 degrees plus, chemically inert, and priced like a machined metal part.
• Silicone sponge: not a temperature upgrade, but a way to keep a seal compliant at temperature with a much lower load.

The decision is usually made on the combination of temperature and medium rather than on temperature alone. A 220 degree seal in air is a phenyl silicone; the same seal in oil is an FKM or an FFKM, and the price difference is a factor of ten.