Why do the copper conductors (bunched /stranded) get tarnished after insulation/sheathing with PVC or XLPE? Whereas the bare conductors lying in open air remain shiny even after 10 days?

Hello there SumitAgarwal,

My first response is that I suspect your company uses a small amount of lubricant on the wire when or before it is bunched or stranded. It could also be a residual amount of drawing lubricant. At sheathing or insulating, I suspect that this trace lubricant is burned off when preheating the conductor just prior to extrusion. This would certainly explain the differences you have noted.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212

Dear Mr. Peter J. Stewart-Hay,

Thanks for the prompt reply. You are right, we may have lubricant on the wire, and have lots of bunched conductor which may have lubricant traces. Could you let me know what to do to use the bunched conductor and not get tarnished copper after PVC insulation? We have tried even without using preheating.

Thanks

Hello again,

Please understand that a very small amount of Copper(1) Oxide, Cuprous Oxide, Cuprite or Red Copper Oxide will coat the conductor. It is a brownish-red solid. The chemical formula is CuO2 meaning two oxygen atoms combine with one copper atom. (Cu + O2 = CuO2). On copper wire, you can see the cuprous oxide layer in full color as various shades of reds, oranges, pinks, and purples if the copper has been heated too high (Above about 200 degrees Celsius (392 Fahrenheit) and exposed to oxygen. Moreover keep in mind that when copper is hot, it quickly oxidizes (corrodes or rusts) and this is nothing more than a simple chemical reaction with oxygen.

Please try to quantify the type of tarnishing you have after extrusion and advise so that we can understand your situation more precisely.

- Now, if you manufacture your own PVC, you must talk with your chemists to make sure there is no copper reactive trace component(s) in your mixture.

- Likewise if you are your making silane grafted XLPE with gravimetric mixers, we suggest you immediately recalibrate your mixers, review your operator training procedures and retrain all your operators accordingly.

- Finally, if you are using an open flame to preheat the conductor with propane or natural gas, please make sure that you are passing the conductor through the blue (reducing) part of the flame so that you are not exposing the conductor to an oxidizing flame.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212