Polycarbonate Cracks

[Mich]

I have some problems with a polycarbonate product.
It’s cylindrical and sometimes in the thinner areas some cracks appears.
We observe cracks in proximity of possible welding lines areas and near the snap fit opening, they usually appear after few days since the snap-fit assembly.
Before the assembly they are exposed to UV for 2 or 3 minutes to activate a UV adhesive (and PC doesn’t have anti-UV additives), and sometimes we use cyanoacrylates in the snap-fit opening.
Do you have any ideas of crack causes?
Mold Temperature to low?
UV?
Bad design?
Cyanoacrylates?


Thanks

S.

 

[Demon3]

Polycarbonate is famous for that. It is called environmental stress cracking and basically it means that solvents cause the material to crack due to built in stresses in the polycarbonate. The glue is causing it. You need another glue.

Motor cycle helmets are made of polycarbonate. Put one sticker on them or attach something with superglue and the whole helmet is worthless.


Chris DeArmitt

 

[Demon3]

Contact someone reputable like Henkel / Loctite and get recommedations. There should be information on their web site. Last time I checked they had great info there.


Chris DeArmitt

 

[Mich]

Thanks a lot for your quick answer!
So, do you think that the guilty one is Cyanoacrylate?
Not the UV glue?

 

[patprimmer]

he guilt will e in the solvent in the glue. We do not know the individual formulations.

The solvents which cause most problems are halogenated hydrocarbons and aromatics, even in very low concentrations.

I have seen the plasticisers migrating out of PVC cables cause this problem in PC used for electrical housings and appliances.

PC even unstabilised has reasonable UV resistance. It will take many hours of exposure to cause significant damage.

It is solvent stress cracking. If it is not the glue, it is another solvent in the environment. PC spectacle frames fail due to contact with things as common as sun tan lotion and personal insect repelant.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Demon3]

I agree that solvents attack the PC and it's not the UV as the time is too short. I feel that the cyanoacrylate monomer in the superglue may act the same as a solvent.

To test which adhesive or solvent is causing ESCR you normally bend a test bar by a fixed amount and apply the solvent. If it's senstive then it will crack immediately or within a fairly short time. That's how you can work out where the problem lies and screen glues that won't cause cracking.


Chris DeArmitt

 

[Mich]

In the enviroment there is also an epoxy resin, but it is not in direct contact with PC.

 

[patprimmer]

It does not have to be in direct contact. Fumes from a solvent can cause the problem.

Like Demon3 says, bend some samples an expose them to any suspect substance in the potential environment.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Mich]

Thanks a lot for your answers!

Do you now something for remove/clean liquid epoxy resin (not cured) from polycarbonate without damaging it (ESCR)?

 

[Demon3]

Sure, you can find info on the chemical resistance of polymers to solvents here...

http://www.ides.com/resources/plastic-chemical-resistance.asp

Polycarbonate is well covered.

also...

http://www.millerplastics.net/chemical_resistance_chart.html

and...

http://www.rtpcompany.com/info/guide/resistance.htm

Looks like ethanol is worth a try.


Chris DeArmitt