--And then something weird happened! (PVA conductivity and possible capacitance) - Page 1
Forum section: electronics / projects

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 16, 2014, 11:48:30 pm
content: So there I was - fixing some home-grown PCB when I needed to gob something down: and being out of hot-glue and super-glue, I reached for the next best thing: PVA.Apparently this particular brand conducts when it's wet: because I got a little shock from a button cell mounted on the PCB.But that wasn't all!Having found something that apparently conducts when its a liquid* I decided to do some investigation: so I made a little line of glue on some perspex and got out the multimeter. Sure enough, pretty much no resistance when wet: but that wasn't all I noticed... after checking the resistance I (actually accidentally) looked at the voltage reading on the multimeter; and there was a faint reading (started at 500mV) which slowly decreased down to zero.What I assumed was happening is that I was "charging" it by checking it on the Ohms range: and then draining that charge as I looked at it on the millivolt range.Any ideas?Steps to reproduce the experiment:1 - make a small line of PVA glue on any surface (I've tried it on paper, perspex and aluminium)2 - "Charge" it by putting a low voltage across it3 - Measure the voltage (likely millivolts) [voltage decreases with measurements: possibly with time as well]4 - Repeatable* I've tested a different brand (a cheaper brand) of PVA and it doesnt conduct when its a liquid: but it does show the same millivolt charge after looking at it on the ohm range - or indeed putting a 9V battery across it--Edit-- water does it as well apparently: though not anywhere near as much (c.100 millivolts to start with). Is this a common property? Is there something n00by I'm missing? (Or is mybusted)?

author: Nerull
author_rank: Frequent Contributor
timestamp: January 17, 2014, 10:15:53 pm
content: There is no way you got shocked by a button cell - that sounds like ESD.

author: c4757p
author_rank: Super Contributor
timestamp: January 17, 2014, 10:30:29 pm
content: Unlikely that the glue is charged. More likely themeter's input capacitance is charged, and being slowly discharged through what remains of the glue's conductance.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 18, 2014, 06:15:01 am
content: PVA glue is corrosive, don't use it on electronics.

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 18, 2014, 07:05:30 am
content: ESD - good call - I did think it odd! Never actually had a static shock with any electronics before (not shock i could feel anyway!) that will teach me for not wearing my wriststrap xDI dont usually use PVA - Ive since got more glue in. It was to actually to hold a dip switch to its mounting plate while I soldered it - I got the shock/discharge when some of it fell on the board and I wiped it off with my finger.As for multimeter-capacitance: I did try shorting the probes together to get a 0v reading - still got the same readings off the PVA though. Is that still likely to be the capacitor?Is there a simple way to rule out the multimeter? I dont actually see how the glue can be storing anything as it should be an amorphous liquid wit little to no potential difference...Ps - thanks for the replies xDPps - looking at the structure of PVA (Im a chemist by trade xD) there may be some breakdown within the pva (which according to my calculations should be an insulator at most common voltages...). Since my particular brand is pretty conductive there must be something else in it (such as an acid or anything that generates free ions xD). The investigation continues. Ill fire it throug the HPLC or GC atthe lab in a week or too and see if I can get an inkling of whats actually in it.

author: SeanB
author_rank: Super Contributor
timestamp: January 18, 2014, 08:40:47 am
content: PVA = Vinyl Acetate = weak organic acid. Thus with the probes possibly being either worn or with the one being deeper than the other and having thus a lower dissolved oxygen content = electrochemical cell. Same as any other liquid containing polar compounds. even pure alcohol conducts electricity, and can be used in an electrochemical cell.

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 18, 2014, 12:55:31 pm
content: I could be wrong, but im not entirely sure that fits with my observations - will check tomorrow when im back at the workshop (at the Missus' house at the mo). Voltage (Ill measure the current as well) decreases with time - and if you remove the probes, place a voltage across the glue, and then re-test the voltage its up again - also increases with 'charge' time - where it the probes, I would have thought the reading would have been unaffected (though i could be wrong!)If it is forming an electrochemical cell with the probes acting as electrodes would switching the probes round with the meter invert to a negative voltage (at least for a little while before the probes redox back to the way they currently are)I cant remember (when I say im a chemist - I specialise in biochemistry-immunology which means I haven't touched inorganic chem in five or six years:/) - but would the differrnce in electronegativity between probes be large enough to give that voltage from just an oxidised probe? I would assume that the other probe (the anode) has also been reduced? Wouldnt that balance out any difference in delta e-neg caused by oxidation, negating any potential difference that would otherwise form - or is the common usage of the probes to measure current caused bias in the reduction/oxidation rates?It really has left me puzzled! Ideally PVA glue is made by making an emulsion with water: but why one brand is opaque white when wet (and non-conductive according to my meter) - and why the other brand is clear and has practically no resistance and yet third brand i have is also white but IS conductive... makes me wonder if theyve added a surfactant to the mixture to act as a detergent - but meh Im stumped for the nonce!Thanks for the idea btw! Ill see if I cant measure the conductivity in the lab with some platinum electrodes and some better equipment

author: SeanB
author_rank: Super Contributor
timestamp: January 18, 2014, 03:02:06 pm
content: Opacity when wet is done with the fillers used. Zinc oxide as a filler makes it higher resistance per volume, and keeps it white as well. Using gelatine or another gel will make it clear and quite conductive. The most expensive filler it rutile ( TiO) which makes it brilliant white.

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 18, 2014, 03:52:09 pm
content: Excellent!!! I was thinking along the lines of increased diffraction of light in it due to high micelles density in the emulsion or something along those lines!Thanks for that

author: wobblycogs
author_rank: Contributor
timestamp: January 18, 2014, 07:35:56 pm
content: I'm not saying for sure that there isn't filler in PVA but I've applied a lot of PVA over the last few years doing up our house and I don't ever remember it being anything other than clear once dry. If there was some sort of inorganic filler in it you would expect the dry product to be at least slightly white and have at some opacity. Even thinking about where I've spilt PVA and not noticed the final lump looks more like a glue stick or clear silicon, certainly not what I'd expect from something containing fillers.My guess would be it's down to the fact it's an emulsion and diffraction / scattering of the light (can't think how I'd test for that though). If you can run the sample through a GC-MS you should be able to identify every component easily as I'm sure they will all be common chemicals and in the MS library. I'd be interested to know what's in it actually - don't forget you'll likely have a lot of impurities in there as I can't imagine they care too much about the purity of such a bulk product.If you are wondering, yes, I too was a chemist once upon a time.

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 21, 2014, 12:17:59 pm
content: Apparently I'm not allowed to waste company solvents (what... you mean water? You know it comes out the taps, right?) or time (about 15 minutes per run?) on running a GC for " own amusement".::Sigh:: I've resorted to emailing a materials scientist in the local university to see what their opinions are.Im raging at my supervisor's attitude towards SCIENCETMthoughI might see if I can put it through a microcentrifuge at some point to see if there's a filler that might separate out from the main mixture.I'll get back to you all soon xD

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 25, 2014, 10:19:49 pm
content: An update!Platinum electrodes (tested against several controls (water, acetone, THF, and electrophoresis gel (agarose)) using several different meters have all measured a current across liquid PVA glue emulsion of around 10-20 mA for a short while at c.500 mV (both decreasing to 0). Of the controls, only water and acetone did this; and only 1-5mA at c.100 mV in both cases.I really dont know what to make of itThe platinum electrodes are used for hundreds of other things and shouldn't oxidise: giving reliable readings in even the smallest things (no, seriously, I've measured electrical properties of cells with this pair before (not individual cells mind: good luck getting probes onto one or two cells)I think the capacitor-in-the-meter explanation is probably the best one - but I still havent been able to figure out why putting a voltage across the PVA seems to raise the reading even if the meters haven't been near it. I'll have to check with some more lab equipment (insofar I've used my Fluke179 and some Cheapie multimeter I picked up for a tenner; both give similar readings - though I can't vouch for accuracy as the Fluke hasn't been calibrated since I got it second hand... and the Cheapie is.... well....)Any ideas how I can rule out the meters themselves? I really am confuddled and bemused by this xD