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EPR (Part 2 of 3)

December 13, 2011

EPR (Part 2 of 3)

In yesterday’s post, EPR (Part 1 of 3), I provided a response to an inquiry on whether it made sense to rejuvenate aging EPR cables. The short answer was yes and I dispelled some EPR myths along the way. This post explores the chemistry of EPR and dispels another myth suggested by a physicist at the ICC meeting. The gentlemen suggested that the silanes in rejuvenation fluid might interact in some nefarious way with the silane surface treatments employed in the manufacture of EPR compounds.

First some background

EPR or ethylene propylene rubber comes in several flavors—often designated with colors. All EPRs are roughly half rubber—the balance is filler. Modern EPR materials use treated clays as the filler and their color varies from gray to brown to pink. Some early EPR compounds were filled with carbon black and these are referred to as Black EPR.  Today we are going to limit our discussion to clay-filled EPR. Those who manufacture EPR hold the details of their clay formulations close to their vests. There are several clays including hectorite, beidellite, montmorillonite, and kaolinite. All share several chemical properties. All include silicon as their most prevalent low electronegative atomic constituent and all include hydroxyl groups (oxygen bonded to hydrogen represented as “-OH”). The hydroxyl groups are connected most commonly to silicon, and less commonly to aluminum, magnesium, or lithium. These hydroxyl groups, illustrated nearby are polar in nature and incompatible with the organic ethylene-propylene polymer. To improve the compatibility of the clay with the rubber, compounders treat the surface of the clay with silanes very similar to rejuvenation compounds. These silanes form oxane bonds, largely eliminating the hydroxyl groups. For all practical purposes these reaction are permanent—that is the silanes are permanently bonded to the clay surface. One purpose of this surface treatment is to ensure uniform dispersion of the clay in the polymer during processing. Another advantage of silane treatment of the clay filler is a reduction in the effective solubility of water in the EPR compound, because of the replacement of the hydrophilic hydroxyl groups with hydrophobic silicone.

Now the facts … 

There is no evidence that anything in any treatment fluid interferes in any way with EPR. In fact, the opposite is true.

1.    Hundreds of thousands of feet of EPR and butyl rubber cables have been rejuvenated with modern treatment fluids. There are no indications of systemic reliability issues. At Novinium, not a single EPR cable treated has ever failed dielectrically.

2.    EPDM (ethylene-propylene-diene with “M” referring to the saturated backbone structure) rubber is very similar to EPR and millions of EPDM injection elbows and splices have been exposed to silane treatment fluids. As long as the concentration of fluid is not allowed to get too high, there are no compatibility issues. Click here to learn more about high temperature issues with first generation injection technology in “Improving Post-treatment Reliability: Eliminating Fluid-Component Compatibility Issues.” In “EPR (3 of 3)” we will examine the high temperature oversaturation issue in more detail.

3.    If there were any unreacted hydroxyl ligands on the clay surface. They would be there either because they were not originally treated or because they were newly formed from oxidation processes associated with aging. Treatment with alkoxysilanes (e.g. rejuvenation fluids) is precisely what one would do to rejuvenate the clay-polymer interface.

In “EPR Part 3 of 3,” I will also provide guidance on how one should choose the right rejuvenation fluid for treating EPR cables.