Fun trick with epoxy drying resistance

[BiOzZ]

Ok so a bit of history im working on a 5A 32v adjustable powersupply and it uses an LM338 and a mosfet (for current regulation) so i had to glue 2 heatsincs together using epoxy and i used my multiplier to make sure they were really not touching but close enough to exchange a wee bit of heat and have the fan blow in to them both without much loss

when i glued them together (between each fin about 4 inches long and 5 fins) the meter showed 5 Mohm and went up slowly and when the 5 minute set time was reached it was at 20 Mohm after the 30 minute harden time it was 55Mohm and it maxed out my 75Mohm meter at the 1 hour complete time

not EXACTLY useful but if your making a rig to set epoxy, want to monitor your epoxy, want to have fun showing friends, making the most useless resistor, or whathaveyou

i think its atleast worth sharing and the resistance depends fully on the surface area and other factors but it might come to use some time in your life!

EDIT: they were about a mm apart

EDIT2: if it helps ANYONE it was "devcon (damn i wish i thought of that name first ...) home 5 minute clear binary epoxy"
... it also says "5 minute" is a registered trademark ... huh?

EDIT3: its been like 2 hours and i put the 36v of my supply across it and its not passing a single uA so i think its totally isolated now

[Rerouter]

still interesting that the resistance increased by 15 times, which should be relatively constant no matter the thickness of the epoxy or the metal

[BiOzZ]

still interesting that the resistance increased by 15 times, which should be relatively constant no matter the thickness of the epoxy or the metal

yeah it increases apparently when the epoxy hardens to be a fairly good insulator from a fairly poor one, found it a real interesting reaction ... might be worth some looking in to or it might be a "no shit" moment to someone who knows how binary epoxy works ...

[hiddensoul]

still interesting that the resistance increased by 15 times, which should be relatively constant no matter the thickness of the epoxy or the metal

yeah it increases apparently when the epoxy hardens to be a fairly good insulator from a fairly poor one, found it a real interesting reaction ... might be worth some looking in to or it might be a "no shit" moment to someone who knows how binary epoxy works ...

I am by no means a binary epoxy adhesive expert but it really makes perfect sense, while in a liquid/semi liquid state it has more free moving electrons as it hardens the movement of the electrons and in turn the flow of electrons decreases. Pretty much normal thermodynamics behaviour but I could just be blowing out my arse as I dont know if there is some underlying chemistry at play

[robrenz]

Some filled epoxies like JB weld are "non conductive" but in reality are when used in a very thin bond line they do conduct well.

[SeanB]

I often use a quickset steel epoxy to hold heatsinks to devices, like very hot chips. Thin layer on clean smooth surfaces and spread it out and leave to cure. I have checked them for insulation resistance and still will not allow high voltage across them, though they are fine under 50v in many cases.

[babysitter]

3 thoughts:
a) Did you do some damage by flowing a current through the liquid, electrolytically separating stuff that should be together to bond ? Is a epoxi exposed to a current during curing (it rhymes!) different to one that doesnt see current ?

b) how compares the temperature to resistance behaviour to the temperature-to-curing speed behaviour ?

c) homemade electret !

[calexanian]

cool observation.