[cetoole]

I got this rather larger spool of 500' of 26awg teflon coated (white) stranded wire off ebay a while back, and just assumed that since I got it for $5, that it was probably SPC. Well, a couple weeks ago, I took a look again, and couldnt see the copper at the ends of the wires where it had been cut like I can on my 22awg SPC. Now, granted, the individual strands are much smaller, but I would expect to be able to see something if it was truly just a silver plating, especially since I also took a look at it with my jewlers loupe. I have also tried to scratch the plating off with a sharp knife to no avail, and this hasnt been successful either. The ebay description was rather garbled, but it mentioned silver and teflon, but no metnion of copper anywhere, but I dont really trust what it said. Does anyone know of any chemical tests or other ways I could go about identifying this, but I would prefer not to have to track down and pay a jewler. I am pretty sure the coating is teflon, I have had my soldering iron on it at times for a while, and it didnt so much as discolor, even with the temperature cranked way up, high above where I solder. The conductor looks silvery to me, but we all know how good of a test that is. Thanks for any suggestions you can give me, if this is what I think/hope it is, I will be using it in several new cables, but mainly a Senn rewire.

 

[diablo9]

inaccurate way: use a needle to scratch it, silver easily get scratched, copper don't
accurate way: add a drop of HNO3 acid to it, silver gets light green color.

 

[robzy]

Quote:

Originally Posted by diablo9 accurate way: add a drop of HNO3 acid to it, silver gets light green color.

That wont nesacerily work. Because if its either SPC or pure silver it will go green either way...

Ill ask my high school chem teacher tomorrow for ya

Rob.

[edit]: Just read diablo's sig... he obviously knows his chem

 

[Francis_Vaughan]

If it is teflon (which sounds reasonable) it will at least be silver plated - it is necessary to use silver under teflon in any cable. As to whether it is plated over copper or not is the real question. Cutting may simply be flowing the plating over the cut end making it hard to see the the copper.

Clearly what you are not looking for is the presence of silver - you already know that there is some there. What you are looking for is copper. This is reasonably easy since it has such a charateristic set of colours in its various oxidation states. Add acid - bright green or blue.

Assuming it is exactly 26 AWG the most trivial test is to simply measure its resistance.

26 AWG Copper has a resistivity of 0.0480 Ohms per foot. Silver of the same guage 0.045. So for 500 feet of silver 22.67 Ohms, whilst 500 feet of copper would be 24 Ohms.

There is a clue in the above.

 

[bg4533]

Quote:

Originally Posted by Francis_Vaughan 26 AWG Copper has a resistivity of 0.0480 Ohms per foot. Silver of the same guage 0.045. So for 500 feet of silver 22.67 Ohms, whilst 500 feet of copper would be 24 Ohms.

What are the odds he has exactly 500 ft?

 

[DaKi][er]

Quote:

Originally Posted by bg4533 What are the odds he has exactly 500 ft?

and it measuring 23.3 or so ohms

 

[Francis_Vaughan]

I said there was a clue here

 

[cetoole]

Just did some testing, its actually 28awg wire, and the conductivity for 10' is between .6-.7 ohms.

 

[tangent]

In addition to the color test diablo9 proposed, you could just let the wire soak in the nitric acid for a while, and pull it out periodically to see if the silver dissolves away leaving copper (which will also dissolve, eventually) or if it just seems to dissolve away entirely before you see copper.

You can do the same thing with sulfuric acid, but it must be concentrated and heated for this to work. Be careful: I believe this will give off hydrogen gas. You need to get rid of it quickly so your heat source doesn't ignite it.

 

[cetoole]

Quote:

Originally Posted by tangent In addition to the color test diablo9 proposed, you could just let the wire soak in the nitric acid for a while, and pull it out periodically to see if the silver dissolves away leaving copper (which will also dissolve, eventually) or if it just seems to dissolve away entirely before you see copper. You can do the same thing with sulfuric acid, but it must be concentrated and heated for this to work. Be careful: I believe this will give off hydrogen gas. You need to get rid of it quickly so your heat source doesn't ignite it.

Where do I get these acids?

 

[diablo9]

ask the chem lab of your local high school or college, they have it.

 

[ericj]

Quote:

Originally Posted by Francis_Vaughan If it is teflon (which sounds reasonable) it will at least be silver plated - it is necessary to use silver under teflon in any cable.

Say what? How could that be?

A lot of mil-spec high-temp wire is polyvinylidene fluoride insulated, or double insulated with Polyalkene bonded to PVF. For example, MIL-W-81044/8 acceptance criteria includes silver plated copper with polyalkene insulation under a bonded PVF jacket.

MIL-W-16878/6 usually implies silver plated copper under extruded PTFE but some wire meeting MIL-W-16878/6 acceptance criteria is tin plated copper - for example JCH part number 12-CE-1414.

If there is a MIL-W- or M- number on the spool or the wire itself, google around - try some wildcards and some variations of the prefix - and you will probably be able to determine exactly what it is. Could also be an Alpha or Teledyne/Thermax part number on it, for example.

I have 600 feet of some 26awg stranded wire that was sold to me as mil-spec silver-plate w/ teflon insulation. It's really nickel plate w/ PVA insulation, and it's awful for soldering as nickel doesn't really like to be soldered. It's intended for wire-wrap use. The insulation does not melt but easily deforms when abraded, as necessary for wire-wrap.

I think plenty of people selling government surplus wire on ebay really have no idea what they're selling.

 

[Francis_Vaughan]

Yes, a bit more research, and indeed I have been labouring under a bit of a misapprehension.

There are a couple of points. Teflon isn't a generic, it is a trade name that refers only to PTFE. Other fluorinated resins are not Teflon.

Most wire that is Teflon insulated gets a silver plating, but by no means all, and indeed there are nickel plated versions. Silver is used because often the wire is intended for radio frequency use.

If teflon is hot extruded it is too hot to push over tinplate, so silver and nickel are preferred, but cold extrusion processes are often used where the temperature limitations do not apply.

 

[tangent]

Quote:

Say what? How could that be?

The melting point of pure tin is just under the melting point of the various forms of Teflon. And most "tinned" wire is in fact plated with solder, which has a far lower melting point than Teflon. With the possible exception of cold extrusion processes, the only way you could have tinned copper with Teflon insulation is if it's solid-core wire, because the heat of applying the Teflon will fuse stranded wire together.

Quote:

A lot of mil-spec high-temp wire is polyvinylidene fluoride insulated

Okay. But PVF is not Teflon.

EDIT: PVF has a much lower melting point than the various forms of Teflon.

Quote:

Teflon isn't a generic, it is a trade name that refers only to PTFE.

No. There are three different polymers sold under the trade name of Teflon: PTFE, FEP and PFA.

Trademarks of this sort do not have to refer to exact chemical compounds in order to be valid. The most exact language I could find among duPont's many applications is in US Trademark 1592650: "FLUORINE-CONTAINING POLYMERS IN RESIN AND DISPERSION FORM" That's pretty broad.

 

[ericj]

Quote:

Originally Posted by tangent Okay. But PVF is not Teflon.

I should have been more clear. I was questioning the conclusion that it is surely PTFE insulated.


Quote:

Originally Posted by tangent EDIT: PVF has a much lower melting point than the various forms of Teflon.

Yes, but it's still heat-resistant enough that it's often referred to as a high-temperature insulation.

In my experience, it doesn't melt off the wire like the cheaper insulation does, but it's easier to damage when hot than PTFE.

I went looking for high-temp insulated wire when i got tired of burning the insulation off hookup wire. It's a real drag. Silver or tin plate is just icing on the cake.

Nickel plate is . . . . like cement on the cake. I need to resell the spool of nickel plated wire to someone who wants to use it for it's intended purpose. It'd also be great for crimped connectors.