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View Full Version : Solved Needed: some way to seal electronics inside heat shrink tubing up to 250 degrees F.



ElectricAye
01-05-2012, 03:48 PM
I'm in the process of making a probe, approximately 1.5 inch long and about 0.3 inches in diameter. The probe has a few fine wires sticking out of it and contains only a few small electronic components and the whole thing must slide along inside a glass tube less than 0.5 inch in diameter partially filled with aqueous solution. I want the wall thickness of this probe to be as thin as possible, so I've been playing with heat shrink tubing that contains a heat-activated adhesive, which melts and seals up the ends so water can't sneak into the probe. Everything looks good until I must autoclave the probe (at one atmosphere, roughly 250 degrees F), at which time the lovely adhesive starts to remelt somewhat and drool somewhat and not look so pretty anymore. Upon cooling, the adhesive seems to turn back to the solid state but I'm afraid I can't control its deformation while it's in the autoclave.

Anybody know of a substitute for such an adhesive, one that could be melted with a heat gun but stay in place during autoclaving? Or perhaps inserting a thin piece of plastic that would thoroughly melt and bond well with the heat shrink tubing yet stay in place at 250 F?

I suppose another possibility would be to dip the assembled electronics into some sort of viscous epoxy-like solution. That would be great so long as I know it bonds well with the PVC wire insulation and won't form too many nooks and crannies around the electronics and yet still results in a fairly thin skin between the electronics and the fluid outside. But that might be too much to ask of a single epoxy, I'm guessing.

Any suggestions - or lessons learned on what NOT to try - will be greatly appreciated.

Thanks.

Phil Pilgrim (PhiPi)
01-05-2012, 04:28 PM
You could dip the probe in heat-conductive RTV silicone, pull it out, and let it cure. It would be best to create a vacuum on the dipped assembly to remove any air pockets before pulling it out.

-Phil

RDL2004
01-05-2012, 04:58 PM
This might work...

Permatex 80050 Clear RTV Silicone (Amazon.com link) (http://www.amazon.com/gp/product/B0002UEPVI/)

Phil Pilgrim (PhiPi)
01-05-2012, 05:07 PM
I was thinking more of a fairly thin two-part silicone. Many of the one-part RTVs cure with acetic acid, which also corrodes electronics.

-Phil

Mike Cook
01-05-2012, 05:20 PM
You might want to check out http://www.plastidip.com/industrial_solutions/Liquid_Tape

They might have something to offer.

ElectricAye
01-05-2012, 05:33 PM
This might work...

Permatex 80050 Clear RTV Silicone (Amazon.com link) (http://www.amazon.com/gp/product/B0002UEPVI/)

Thanks. I'll need to check out if this is compatible with things.



I was thinking more of a fairly thin two-part silicone. Many of the one-part RTVs cure with acetic acid, which also corrodes electronics.

-Phil

Whoa. That's an interesting point, of which I almost forgot. This probe moves around in a realm of microbes that can feed off acetic acid. In fact, they love the stuff. So acetic acids and such are a definite no-no. Some rubbery things like to cure with some heavy metals, too, so I'll have to check on the biocompatibility of that as well. I guess that's why I was kinda hoping for a hot melt plastic approach so there might be less likelihood of unmixed uncured byproducts getting into the food chain of these microbes. They must be kept on the microbial version of WeightWatchers, so I can't allow them to cheat by nibbling on potting compound when nobody's watching.

ElectricAye
01-05-2012, 05:35 PM
You might want to check out http://www.plastidip.com/industrial_solutions/Liquid_Tape

....

Thanks for the idea, Mike, but I tried that stuff a couple years ago for a different project and I just couldn't get it to work very well for smaller components. Maybe I used it wrong, but it just gave me all kinds of problems.

WBA Consulting
01-05-2012, 06:52 PM
Look into using a vinyl cap (http://www.caplugs.com/productdetails.aspx?id=1000047&itemno=VC+SERIES) from Caplug and then filling it with 3M Scotch Weld (http://solutions.3m.com/wps/portal/3M/en_US/Oil-Gas/Home/Prod_Info/Prod_Catalog/?PC_7_RJH9U5230GE3E02LECFTDQO0L7000000_nid=GS1R74K KZRgsFTXPWQH1VCglLF8LWCMKNGbl) Epoxy Potting Compound like a 3m DP-270 (http://www.alliedelec.com/search/productdetail.aspx?SKU=70113956#tab=overview). Some thermistors (http://www.ussensor.com/h3686.html) we use are done that way by the manufacturer, US Sensor.

RDL2004
01-05-2012, 06:59 PM
Just look for RTV silicone that is "neutral cure". I thought the one I linked to above was, but I don't see it mentioned. These are usually moisture cure, no heavy metals. I think Jameco, Newark and a few others sell this. You might also check what's available at a local auto parts store.


GC Electronics Silicone Sealant (Amazon.com link) (http://www.amazon.com/GC-Electronic-Silicone-Sealant-Adhesive/dp/B004SPJN6K)

ElectricAye
01-05-2012, 08:17 PM
Look into using a vinyl cap....

That's an outstanding idea! And knowing that it bonds to the DP-270 is excellent news, too. Awesome. Thanks, Andrew!




Just look for RTV silicone that is "neutral cure"....

Okay, I'll have to remember that one. Lots of heavy metal compounds are toxic to microbes, even ones that don't bother humans very much like tin and copper. So it's great to know there are gooey things out there that won't necessarily poison our innocent nanoscopic friends. Thanks!

Franklin
01-05-2012, 08:49 PM
Perhaps http://www.techspray.com/product-info.php?pId=63&cId=4 ?

Lawson
01-05-2012, 11:36 PM
Sounds like you have a couple requirements to meet. No melting at 250F (121 C), thermo-set or a theroplastic that melts before tin-lead solder (183 C or 361.4 F), and chemical compatibility. I did a quick search to see if Polyethylene (PE) or Polypropylene (PP) would be suitable as they are easy to work with and chemically compatible with most anything. Wikipedia says high density PE melts at about 120-130C. I'd expect a welded on PE cover to distort at 120C. The close relative PP has a bit higher melting range of 160-166C with highly crystaline grades melting sooner near 130C. You could make the whole capsule out of PE and/or PP or a thin sheet of PE could be used instead of the current hot-melt adhesive. (i.e. a section cut from the side of a milk jug) Also, assuming this is for a university, proffessors in the polymer science or chemistry department might be able to find someone with more specialized knowledge.

I'll also second the suggestion to just dip it in epoxy. Epoxy should stick to PVC just fine. I would suggest cooking the probe in the sterilization oven for several hours before use. This will evaporate out any traces of un-cured epoxy.

Lawson

Tracy Allen
01-05-2012, 11:48 PM
Type ES2000 tubing from Raychem is a semi-rigid dual-wall that is rated for operation up to 266F (130C). Full recovery temperature is 275F. That is higher than many other dual wall tubing types and it might hold up okay in the autoclave at 250F. The adhesive and wall might be thicker than you want, but adhesive is gooey and bonds well even to plasticized PVC cables. I've used it successfully for water and soil temperature sensors that have been deployed for years. I don't know about its specific bio-compatibility though.

ElectricAye
01-06-2012, 03:12 PM
Perhaps http://www.techspray.com/product-info.php?pId=63&cId=4 ?

Interesting idea. I never thought about a spray-on coating. And I guess if it doesn't work out, I could always sell the unused portion to Ravenkallen for hairspray. Thanks, Franklin.


.... I did a quick search to see if Polyethylene (PE) or Polypropylene (PP) would be suitable as they are easy to work with and chemically compatible with most anything. ...Epoxy should stick to PVC just fine. I would suggest cooking the probe in the sterilization oven for several hours before use. This will evaporate out any traces of un-cured epoxy.
...

You're right about Polyethylene. I learned the hard way that it comes out of the autoclave looking like an alien life form. Polypropylene is nice because it withstands autoclaves and can be used with bleach. So the eternal questions are always what will bond and seal around the wires. I like your idea about cooking the epoxy to get rid of volatiles. Thanks, Lawson.




Type ES2000 tubing from Raychem... bonds well even to plasticized PVC cables. I've used it successfully for water and soil temperature sensors that have been deployed for years. I don't know about its specific bio-compatibility though.

Okay, I've put this Raychem tubing on my list, too. It sounds like a winner. Perhaps Andrew's idea of a vinyl cap combined with something like this would be the fast and furious way to go. One thing I don't like about the potting compounds is that they seem to require some highfalutin two-component dispenser, which can be kinda pricey. Thanks, Tracy. Much appreciated.

Thanks everyone! I'm marking this thread as SOLVED while I try out some of your most excellent ideas. :-)