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Old 7th Sep 2016, 10:40 pm   #21
kalee20
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Default Re: Plastics that deteriorate

PVC sleeving does silly things if in contact with polystyrene, that's why power leads for appliances are packed in a polythene bag, and not put in direct contact with polystyrene packing foam.

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Old 7th Sep 2016, 10:45 pm   #22
G0HZU_JMR
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Default Re: Plastics that deteriorate

Funnily enough, I saw this youtube video a few weeks back....
It covers and explains this issue. It's also worth reading the 'show more' text below the video on youtube.

Sadly, I think he threw the tools away because it looks like there is no cure for this issue.

https://www.youtube.com/watch?v=KTxkunC0Lo8
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Old 8th Sep 2016, 12:09 am   #23
Lucien Nunes
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Default Re: Plastics that deteriorate

The 'green goo' exudate from PVC cables is not always significantly conductive as the video text implies. The phthalate plasticiser released from the PVC itself is insulative, I think the breakdown voltage falls somewhat according to how much copper oxide it picks up as it reacts with the conductor. I have certainly tested affected cables where it did not measurably reduce the insulation resistance even where the terminations were coated with goo, IIRC they all exceeded 200MΩ at 500V DC. However, where there is goo, there is cable insulation lacking its plasticiser that will become embrittled and at risk of cracking away from the conductors if disturbed, so replacement is indicated. Only cables from around the late 60s are involved, as the suspect PVC formulation was phased out.

The related problem of plasticiser being leached from PVC by contact with polystyrene of course can affect cables of any age. When expanded polystyrene foam was widely used as cheap thermal insulation e.g. on boats, and PVC cables were run in contact with it before the problem was realised, the resulting damage in some cases necessitated a rewire. As with green goo, the warmer a cable runs the more rapid the deterioration.

Small clear plastic parts drawers and trays are often made of polystyrene. PVC parts stored in these will react with and stick to, even eat through, the bottom of the tray. I had a PVC cable grommet kit from RS supplied in a polystyrene 18-compartment box, into which the grommets started to embed until I transferred them to a similar tray made of HDPE.

I'm not sure if binder hydrolysis in magnetic tapes (aka sticky shed) qualifies for this thread as it's only the coating, not the tape base, that is breaking down. It is the degenerative mechanism that I have had most cause to study as my best and favourite recordings, from when I was learning the art and craft of recording, are badly affected. Most were made on second-hand Ampex 406 and 456 from the early 1980s, which is a consistent offender.

Have we covered urethane foam yet? I have an electromechanical voltmeter to restore that contains uniselectors, the noise of which was muffled by lining the case with foam. It has deteriorated into blobs of sticky goop that have got into the uniselectors and stuck all over the contacts and moving parts. Nasty!
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Old 8th Sep 2016, 2:02 am   #24
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Default Re: Plastics that deteriorate

On the subject of PVC, is there anything you can apply to it to keep it soft, or soften it if it's starting to harden? I've had this problem with Beyer DT100 headphones; the ear pads go hard and the headband padding cracks.

Those Ampex tapes were awful. I tried to tell the technical guy that when I was a film student, and he got annoyed with me. I mean, I only used the stuff, he was the one who'd approved it! I prefer the 60s pink-backed BASF tapes, they last for ever.
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Old 8th Sep 2016, 8:59 am   #25
Goldie99
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Default Re: Plastics that deteriorate

Quote:
Originally Posted by Lucien Nunes View Post
Have we covered urethane foam yet? I have an electromechanical voltmeter to restore that contains uniselectors, the noise of which was muffled by lining the case with foam. It has deteriorated into blobs of sticky goop that have got into the uniselectors and stuck all over the contacts and moving parts. Nasty!
That doesn't really sound like a typical polyether polyurethane foam degradation - do you know roughly what age the voltmeter is ? just wondering if it could be a polyester based product from the early days of PU foams (e.g., ca. mid '50's), in which case almost anything's possible....
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Old 9th Sep 2016, 1:53 am   #26
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Default Re: Plastics that deteriorate

Quote:
Originally Posted by Lucien Nunes View Post
Have we covered urethane foam yet? I have an electromechanical voltmeter to restore that contains uniselectors, the noise of which was muffled by lining the case with foam. It has deteriorated into blobs of sticky goop that have got into the uniselectors and stuck all over the contacts and moving parts. Nasty!
Lycra (aka elastane, spandex) is polyurethane, spun into fibres. I've seen old leggings where the elastomer fibres had degraded in a similar fashion.
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Old 14th Sep 2016, 7:48 am   #27
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Default Re: Plastics that deteriorate

Quote:
Originally Posted by Lucien Nunes View Post
The only item I have which I know is subject to this process at the moment is a mechanical calculator, on which the keys are slowly shrinking and distorting. It is quarantined but in truth having recorded it photographically I should probably just remove the keys, turn it upside down and use it for a display of the mechanism as they will eventually be just lumps of goo.
Or maybe use the keys as models to get some new ones made on a 3D printer before they become too distorted.
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Old 14th Sep 2016, 8:49 am   #28
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Default Re: Plastics that deteriorate

I was going to mention PVC grommets (and sleeving) too til Lucien mentioned it. The popular 'Raaco' component drawers were polystyrene (brittle & clear) but have changed now to IMHO polythene (flexible & opaque). So, if you have problems with plastic bits in old drawers, just order replacement drawers from rs/farnell/cpc, as they are cheap.
I keep my grommets in a linbin these days.
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