how much does the grease actually matter to silicone rubber in cable accessories under heat aging?

If you make HV cable accessories, you know silicone rubber and silicone grease inside out. Silicone rubber has been the core insulation in cable joints and terminations for ages, thanks to steady insulation performance, tough environmental resistance, and the fact that it doesn’t hog much space. Smearing grease on the interface between silicone rubber and XLPE to fill gaps, improve sealing, and boost interface insulation has also been standard practice for years.

But honestly, not many people have sat down and actually broken down the numbers: under long-term high-temperature aging, how big a difference do different greases really make to silicone rubber insulation? Think about it. In normal operation, the area near the conductor can already sit at around 70°C. When overload or a short circuit hits, it can shoot up to 250°C in no time. Throw in the swelling effect from the grease sitting against the rubber, and you have to wonder whether the insulation quietly starts to fail over the years.

A team from the Zibo branch of State Grid Shandong recently ran a full comparison test on exactly this, and the results were published in issue 5, 2026 of Plastics Science and Technology. Let’s pull out the useful parts, skip the heavy academic wording, and give people doing material selection and process design something they can actually use.

The setup, quick and simple

Here’s the test basis so the results make sense. The substrate was Wacker’s methyl vinyl silicone rubber, CENUSIL M810A/B. Two commercially common greases were compared: Dow Corning’s dimethyl silicone fluid XIAMETER PMX-200, and Wacker’s methyl phenyl silicone grease Poweisil Paste AP. There was also a blank sample with no grease at all as the control.

The samples were pressed into 1 mm sheets, coated evenly with the corresponding grease, held under constant interface pressure, then heat-aged at 175°C and 200°C. Aging periods were 3, 10, and 30 days. After each stage, they measured volume conductivity, relative permittivity, dissipation factor, and breakdown strength, which are the core insulation indicators.

Conductivity: temperature and aging stack up, and the grease gap shows clearly

Volume conductivity is the most direct one, no need to explain much: the higher the number, the worse the insulation. The basic pattern matches common sense. With or without grease, and no matter which grease, conductivity climbs as temperature rises. Higher temperature wakes up the charge carriers, molecular motion gets more intense, and conductance naturally goes up.

As aging time and temperature increased, conductivity crept up across all samples, and the spread really opened up at the 30-day mark. In plain terms, heat aging slowly breaks apart the crosslinked structure of the silicone rubber. Molecular chains snap, internal free volume grows, carriers have an easier path, and the broken-off free radicals can carry charge too. No wonder conductivity goes up.

Where the real difference shows is between greased and ungreased samples, and between the two greases. The greased samples saw conductivity climb much harder than the blank, and the dimethyl grease group rose the most, while the methyl phenyl grease group was a lot more moderate. You might ask, they’re both greases, so what’s the difference? It comes down to that swelling effect everyone talks about. Grease gradually diffuses into the silicone rubber and drags short molecular chains in with it, which react with the main chains and break down the crosslinked structure further. Basically, it pokes holes in the insulation. Methyl phenyl grease is more polar, more stable, and more viscous, so it doesn’t sneak into the rubber as easily, swells less, and does much less damage to conductivity.

Dielectric performance: same trend, different severity

Permittivity and dissipation factor are the other two key checks for insulation materials. The overall trend lines up with conductivity. As temperature goes up, permittivity drops a bit while dissipation rises, and the more severe the aging, the more both permittivity and dissipation climb.

Same logic applies: greased samples degraded more visibly than the blank, and the dimethyl grease group again performed the worst. The mechanism is the same one as above. Swelling brings short chains and polar groups into the rubber, which strengthens internal polarization, so permittivity and losses both go up. Since methyl phenyl grease barely swells, its impact on dielectric performance is much smaller.

Breakdown strength: the toughest metric, and the gap is just as clear

Breakdown strength decides the voltage limit of the insulation, so it’s the hard-core metric that shows performance differences best. The team used Weibull distribution for the statistics, and the conclusion is clean: the higher the aging temperature and the longer the aging time, the lower the breakdown strength. Greased samples lost breakdown strength faster than ungreased ones, and the dimethyl grease group lost the most.

The reasoning is easy to follow. Heat aging plus swelling hit the crosslinked structure from both sides, free volume grows, free electrons multiply, and electrons can build up enough energy in the electric field to punch through the material more easily. That drags down the voltage limit. Because methyl phenyl grease swells less, its breakdown strength stayed clearly above the dimethyl grease group the whole way through.

What you can take away for real-world use

Enough numbers. Here’s the practical part, so you don’t have to dig through the original paper:

1.Heat aging keeps degrading silicone rubber insulation, and the effect gets stronger with higher temperature and longer time. That’s a built-in property of the material, so products need to leave room for long-term aging in the design.

2.Grease at the interface solves the immediate sealing and insulation problem, but at sustained high temperatures, the swelling effect speeds up degradation of the silicone rubber. That downside shouldn’t be ignored just because the short-term benefit is real.

3.In the same comparison, methyl phenyl grease swells silicone rubber far less than dimethyl grease, holds up electrical performance better after long-term heat aging, and is the stronger choice for cable accessory applications where reliability matters most.

Reference: “Study on Electrical Properties of Silicone Rubber for HV Cable Accessories,” Plastics Science and Technology, 2026, Issue 5. Authors: Ma Tao, Wang Yikui, Kong Xuecheng, Gong Yubin, Zhao Renyong, Zhang Xiao, Wang Jingran (Zibo Branch, State Grid Shandong Electric Power Company). Project support: State Grid Shandong Electric Power Company Science and Technology Project (520603230008).