10mF gooey rubber seal - is it possible to revert the effects of a plasticizer? - Page 1
Forum section: electronics / general

author: RoGeorge
author_rank: Super Contributor
timestamp: November 03, 2019, 11:00:33 am
content: Just found out in a nasty way that sugar is a plasticizer.A month ago or so, spilled by mistake about a spoon of instant coffee made with lots of sugar (no milk) in a cardboard storage box full with big capacitors, then immediately clean the capacitors from those drops of coffee. Today, 2 of the 10 000 uF/40V SPRAGUE from 1980 have a gooey rubber seal instead of a clean healthy rubber like it was before the spill.Searched if sugar can do that, and it seems it canhttps://duckduckgo.com/?q=sugar+plasticizerThe capacitors are still good, but I don't know for how long.Is it possible to "revert" a plasticizer, and get back the healthy rubber?

author: SeanB
author_rank: Super Contributor
timestamp: November 03, 2019, 12:04:47 pm
content: Short reply is no, the rubber probably was already degrading, the extra just drove it over the edge. Got some NOS electrolytics as well that are starting to do that, bit of a pity, as the electrolyte coming out is rather corrosive as well, so will destroy the leads of the rest in short order.R32

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 03, 2019, 12:52:05 pm
content: You could try smearing some electronics safe silicone over it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 03, 2019, 11:27:28 pm
content: From 1980? They ain't worth anything, throw 'em out.Or get off the pot and build something with them and use them until they dry up, then replace them.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: November 04, 2019, 11:44:52 am
content: Quote from: T3sl4co1l on November 03, 2019, 11:27:28 pmMost reasonable adviceThrowing them away is not so appealing, they are about $40 each at Mouser.They still show 10mF when measured. I found a datasheet with the expected losses, will measure them just out of curiosity. I bet they are still in spec. Unfortunately, I couldn't find any ESR specifications for them, also no max current specs, the datasheet says "High ripple current" whatever "high" would mean. This is the closest datasheet I could find:https://www.mouser.com/datasheet/2/427/36ddedx-239977.pdf- Anybody have the datasheet for SPRAGUE 27D series Aluminum capacitors, please?- What to build with 3x10mF/40V? The closest transformer I have is 2x22Vrms (aprox. 2x30Vdc) at about 300-500W (22.5 square centimeters E+I core). A 10mF would give about 1V ripple for a 1A load at 100Hz.Or maybe use them for something else, other than filtering after a bridge, any ideas?

author: tom66
author_rank: Super Contributor
timestamp: November 04, 2019, 01:27:12 pm
content: It's not as if equivalent capacitors are that expensive:https://www.mouser.co.uk/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/_/N-75hqt?P=1z0wqs8Z1yx4avvUse them while you can, but after that, they have had a good long 39 year life, I don't think anyone will be too upset if they died this way.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2019, 04:16:31 am
content: Back in the days I would expect "high ripple current" to be a matter of magnitude, i.e. you're putting 5A or so into them on account of their size, not so much because of what you're doing with them?That transformer sounds like a good fit. Maybe make a classic linear bench supply or something? Audio amp? Both?I had such a combination from some hardware I took apart, so I made a +/-20V 10A bench supply. It's actually an audio amp circuit. With the Darlington output transistors rated for tons of amperes, it would only be useful at its ratings with very low impedance (~1 ohm) speakers, so I've basically never used it for that purpose. I do have a BNC jack on it for input, should I need to do so. Its input is switched between BNC and an internal reference (TL431 and divider, with a pot). Mostly I use it for 12V or such at the bench, but I can also hook up the function generator and use it for voltage sweep or step testing.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: November 05, 2019, 09:44:17 am
content: A draft measurement for DC leakage, just out of curiosity. All in spec.Not sure why that sudden drop in the brown trace. It happens that C1 was the most coffee spilled one. The rubber seals for C1 and C2 were rinsed with tap water then left to sit a few hours for the water to evaporate prior to measurement. Maybe C1 still had some water on it, IDK.After the yesterday measurement, they were all left charged at 50V. Over night they loosed some charge, measuring now 25.8V, 24.5V 30.3V and 22.4V respectively.

author: tooki
author_rank: Super Contributor
timestamp: November 07, 2019, 12:41:32 pm
content: Quote from: RoGeorge on November 03, 2019, 11:00:33 amDid you read any of those results? They talk about sugar as a plasticizerin starch- or gelatin-based films. Nothing at all to do with rubber or plastics, and given that rubber is often used as a stopper for sugary substances, almost guaranteed to not be the issue. (Any baker can tell you that sugar is a plasticizer in baked goods, as it binds water.)Are you sure it’s not just sugar residue on the rubber?! Or even that the actual cleaning merely exposed rubber degradation that had already occurred under the surface? I find it hard to believe that any component in coffee could cause such a reaction from such brief contact.

author: soldar
author_rank: Super Contributor
timestamp: November 07, 2019, 12:57:14 pm
content: I have kept and sometimes used old electrolytics. Mostly they still work OK although some are no good. For their value they are huge compared to today's sizes.I would not use them in anything that required high reliability but for projects and playing around I would certainly use them.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 07, 2019, 02:52:36 pm
content: I don't know what made the rubber gooey, maybe it was the instant coffee, or the combination of them, but for sure before the spill all capacitors were just fine, and only those 2 touched by the liquid are now in bad shape. Or maybe it's just a coincidence that after 39 years, 2 of them decided to go south together during a single month, IDK.As a curiosity today, two days after letting them charged at 50V, they still retain about 20-25V.Meanwhile I found a datasheet from another similar SPRAGUE size capacitors, this time they have specified ESR (28 m\$\Omega\$ at 120 Hz) and max I ripple (6.84 A at 120 Hz) valueshttps://ro.mouser.com/datasheet/2/427/36dy-111434.pdf.Maybe I'll try to measure ESR too, just as a curiosity, but measuring 0.028at 120 Hz and with 30V or so DC bias won't be easy.Another fun fact, it seems that the former DC leakage chart might have values much bigger than in reality, because I didn't wait long enough for the DC leakage current to settle, and it was all measured with long leads and in a noisy environment anyway.The manual of this very old SPRAGUE capacitor tester recommends to wait up to 30 minutes for the leakage current to stabilize (see page 9 paragraph 4.3 -http://www.mcmlv.org/Archive/TestEquipment/Sprague_TO6A.pdf).

author: RoGeorge
author_rank: Super Contributor
timestamp: November 09, 2019, 02:49:25 pm
content: After leaving them for one night staying upside down, the damaged capacitors showed some new spill.The bottom left one still has a good rubber seal. The moment I heated the terminals of a damaged one with a soldering iron, the vent whole started bubblingQuote from: NiHaoMike on November 03, 2019, 12:52:05 pmI'll do that just out of curiosity, just to see how much will it keep them going, thank you for the idea.Quote from: T3sl4co1l on November 05, 2019, 04:16:31 amGreat idea, that it'll be, thanks! A power amp for the DDS generator. So far it happened that I needed such a thing more than once!Side note again, 3 days later after letting them charged at 50V, the bubbling one from the pic is completely discharged, the other two still retained about 20V each, after more than 3 days!

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2019, 05:45:22 pm
content: Quote from: RoGeorge on November 09, 2019, 02:49:25 pmThanks for the pictures. I’m pretty sure those caps’ rubber had already degraded, and all the coffee did was rinse off the dust that had stuck to it.