Silicone wiring degradation in sealed applications?

[Uncle Bulgaria]

I just had a thought about the degredation of silicone wiring. I'm sure I recall a thread where there was some decomposition into silane and the subsequent deposition of silica onto contacts in a sealed relay which caused a machine to stop working.

I've repaired the wiring in a hermetically sealed civil defence geiger counter with new silicone. Ought I to redo it with PTFE, some more exotic insulation or is the silicone fine?

[kalee20]

I've had no problems with silicone-insulated wire.

Silicone resins - yes they can be prone to cure inhibition, partial curing, or reversion where the stuff depolymerises, and yes they're notorious for creeping where they're not wanted. But my experience to date with silicone wire is that it's has all the desirable properties of traditional rubber, together with high-temperature capability and zero time-based degredation.

[Nickthedentist]

My only bad experience has been the silicone elastomer rubber keypad mats in remote controls. Often, the R/C stops working, and on opening, is full of oily liquid. The first thought is to blame the owner for spilling baby oil or similar into it. But the actual cause often seems to be degradation of the keypad mat, presumably the "reversion" which kalee20 talks about.

[emeritus]

If you would like some ptfe- insulatedwire, I can put a couple of metres in the post foc if you pm me your details. I have a couple of drums in different colours, saved from the skip when the Marconi site I was at was closed. We always used ptfe wire in hermetically sealed avionics equipment at Plessey in the 1970's. The chlorine compounds evolved from pvc were good at dissolving polystyrene capacitors in hermetically-sealed units!

[G6Tanuki]

A lot of the military Larkspur and Clansman sealed kit includes wiring with the pink Hellermann sleeves, which I have always assumed to be a form of silicone rubber.

[kalee20]

I think the pink Hellerman sleeves are synthetic (Chloroprene) rubber - I don't know the chemistry, but I'm fairly sure it's not silicone.

I've used the Hellermann sleeves for the tip of a solder sucker - they're great, especially for making a seal to suck solder out of plated-through circuit boards, but they don't have a long life at this sort of game, confirming my thought that it's not silicone.

[markgengine]

Apologies fpr being only slightly on-topic, but I have had a mastic gun cartridge of clear rtv silicone that once applied on ceramic tiles to glass refused to "go off" and just remained a sticky goo for over a week until I painstakingly removed it. This was Dow-Corning so not cheap stuff, and had rhe familier acetic acid odour. Was a few years ago but just an example of the vaguaries of silicones, they are not as chemically stable as we at first assume. never did find out what was wrong, but as the offending tube was "aquired" from work I could not complain and got what I deserved !

[The Philpott]

Yes, never underestimate the 'bad batch' phenomenon.

The silicon oil problem issue with remote control keypads is reportedly due to underbaking at manufacture- and one would hope the quality (and quality control) for those is inferior to that for silicon sleeving or cabling.

Dave

[Uncle Bulgaria]

Is that a silicon or silicone oil problem? I didn't know either was able to get oily - doesn't sound pleasant! I believe there are Hellermann sleeves on the rubbery wiring in the Geiger counter that I had to replace. The sleeves were fine, but the insulation had degraded somewhat. They don't feel like the silicone insulation on the cabling I have (from whichever member on here supplies it).

Thanks for the PTFE offer, emeritus. I'll send you a PM as it sounds worth experimenting with.

[ScottBouch]

Originally Posted by markgengine

Apologies fpr being only slightly on-topic, but I have had a mastic gun cartridge of clear rtv silicone that once applied on ceramic tiles to glass refused to "go off" and just remained a sticky goo for over a week until I painstakingly removed it. This was Dow-Corning so not cheap stuff, and had rhe familier acetic acid odour. Was a few years ago but just an example of the vaguaries of silicones, they are not as chemically stable as we at first assume. never did find out what was wrong, but as the offending tube was "aquired" from work I could not complain and got what I deserved !

Could this be a type requiring an 'activator' to be mixed in before applying? Just thinking if its from an engineering company, it could be a bit special, there are a few types of Dow Corning RTV. I've used a high temperature Dow Corning RTV (dark red) on industrial turbine casings, this did not need an activator. Could some be heat cured too?

I've also done some silicone moulding for resin casting, this type requires an activator to go off, the downside is that mixing it introduces air bubbles. One needs a vacuum pump and sealed chamber to quickly pull them out before it sets!

Cheers, Scott

[ScottBouch]

Originally Posted by kalee20

I think the pink Hellerman sleeves are synthetic (Chloroprene) rubber - I don't know the chemistry, but I'm fairly sure it's not silicone.

Second that.. Closely related to Neoprene.

I did some research into these pink sleeves not long ago, as you find them all over the English Electric Lightning aircraft cable looms. Earliest date I found for "Helsyn" sleeves (HELlermann / SYNthetic) was 1944: https://www.scottbouch.com/mcfs/lightningt5t55/wiring-helsyn.htm

Hellermann also could supply them with text printed, as cable markers, I've still not found a way to achieve this myself at home, think it needs a heat process.

Cheers, Scott

[ortek_service]

Originally Posted by ScottBouch

>>
>>
Hellermann also could supply them with text printed, as cable markers, I've still not found a way to achieve this myself at home, think it needs a heat process.

Cheers, Scott

Dymo & Brother etc. produce Heatshrink Cartridges for their (Thermal) Labelling Machines
- at least on their more-'professional' >=19mm tape width supporting ones, as may sometimes need capability to print text in vertical direction
And you can get these in a variety of widths / diameters & colours, that you could either use direct or shrink over existing Hellermann etc. sleeves.

You can also get labelling cartridges for these, that produce wrap-round labels, intending for applying to computer network cables etc. - after they have had their connectors attached (so can't use standard slide-over heatshrink that easily).

[FrankB]

I have run into numerous cases where the HT anode lead on a TV has silicone degradation and running out the end if the outer wiring sheath.

[Uncle Bulgaria]

Do you think that is down to a particularity of its use in a television?