welding Polycarbonate cracks 2

[AYahya]

Hi
i am reposting this question in the polymer section with some updates.
i have a clear polycarbonate camera housing ( the one you put a professional camera in it and take it deep underwater. its rated to 60m depth ). i found 3 tiny surface cracks each of 2-3 mm long around one of the control glands penetrating the housing wall. i know they're surface cracks because they are not leaking water, yet.
i suspect that the housing got a bang in airport during flight or something or may be its some kind of stress cracks.
i read about using Methylene Chloride in welding polycarbonate but i don't know anyone who actually tried it in similar cases like mien. some people also suggested cyanoacrylate super glue.
what do you suggest to use on these tiny cracks that will fill and weld them permanently and does not introduce more stress cracks over time under seawater pressure ?

i asked the customer support of eplastics.com and he replied by : "We carry a glue called IPS Weldon #3 which works to melt the material together as strong as before. The only problem is that over time you will get more and more stress cracks. The only way to stop that is to drill a very tiny hole at the end of the cracks to relieve the stress, but I know that is not possible in your application. Best bet would be to just glue using the weld on and hoping for the best"

any suggestions ? here is a close picture:

 

[AYahya]

any suggestions ?

 

[Demon3]

Yes, there are several adhesives approved for use on polycarbonate. Do a Google search for Loctite and polycarbonate for examples. Some are crazy glue (super glue in the UK) types made of cyanoacrylates but the one I would choose for your application is this epoxy because it's rated for continuous water immersion and is impact resistant as well.

Loctite 1363118 0.85-Ounce Plastic Syringe Epoxy Plastic Bonder available for $5.48 at Amazon.

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500:

http://www.phantomplastics.com

Webinars on plastics, fillers & impact modification:

http://www.plastictraining.com

 

[AYahya]

Thanks a lot
But I guess the epoxy is not water thin so it won't penetrate the crack area. I think I'll try the super glue first. What about the stress crack ? Are they going to stop?

 

[Demon3]

As I mentioned, the crazy glue types are also approved for polycarbonate and they are "water thin".

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500:

http://www.phantomplastics.com

Webinars on plastics, fillers & impact modification:

http://www.plastictraining.com

 

[Demon3]

Check out page 38-39 here on adhesives for polycarbonate. Look for the ones with lowest viscosity.

http://www.exdron.com/Specs/Loctite Plastics.pdf

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500:

http://www.phantomplastics.com

Webinars on plastics, fillers & impact modification:

http://www.plastictraining.com

 

[rickfischer51]

I think it likely the cracks will keep growing. They didn't start for no reason. The picture looks like a bulkhead fitting with an O-ring. The fitting is tightened to compress the o-ring, which is a source of stress, plus the stress from up to 85 psi, then add miscellaneous banging around. The life of a part has two phases: crack initiation and crack growth. You are now in the second phase. The crack will grow slowly until it gets to a critical size, then bang! The conditions at the crack tip (plastic zone at the tip, plastic constraint on the adjacent non-yielded material, high tensile hydrostatic stress, etc) causes crazing, and this leads to crack growth. I don't think you can rely on adhesive to wick completely into the crack and bond it up. The crack tip will still be there. That's what was behind the suggestion to drill out the crack tips. If this were mine, I would remove the fitting, completely machine away the existing hole (there may be some aspect of how the hole was created that led to the cracking) and then glue a patch to the outside with a new carefully machined hole for the fitting.

How old is this housing? Polycarbonate is prone to hydrolytic degradation. I spent a lot of time working on failures in telecommunications components that failed after time in hot steel outdoor cabinets in humid environments because the moisture reacted with the PC and degraded it. PC can be used in moist applications if it is blended with a stabilizer, but the stabilizers are consumed over time, and don't prevent hydrolysis, they just postpone it. If this housing is old, you might want to replace it, especially considering the cost of what's inside. Chris, your thoughts?

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[AYahya]

thanks Rick,
the housing is 2 years old and has been in water many times. drilling small holes at the end of the cracks is not really an option for me. but i can take the fittings out and apply the adhesive if this might release the stress and help solve the problem.
what do you think ?

 

[Demon3]

Look at it this way. Your device is destined to break soon in its present state. Gluing will plug the cracks, maybe completely and maybe just partially but in either case it will relieve the stress at the crack tip and help extend the life of the product. As the glue is inexpensive I'd suggest you try it. No-one here can promise you a certain outcome in advance. Theory only takes us so far. It allows a best guess as to the result. Then we must proceed based on that.

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500:

http://www.phantomplastics.com

Webinars on plastics, fillers & impact modification:

http://www.plastictraining.com