Bonding/Cracking problem with ABS

[ittam]

I am bonding a 1 m long black ABS molding (air-mold) with PU glue on a steel surface using primer and activator. The glue and the system are approved by glue manufacturer. The application is on a vehicle. The ABS is cracking after a period of several months. The material is acc. to scans ok. Does anyone have experience of the possible factors or materials which could cause the kind of cracks.

Thanks in advance

 

[patprimmer]

Stress from differential expansion between the ABS and the steel. The ABS will expand in the order of 10 times the rate for steel. This will create enough stress to induce sovent stress cracking.

Regards
Pat
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[CoryPad]

Where on the vehicle? ABS can be degraded by UV (among other things), so it may not have anything to do with the adhesive.

Regards,

Cory

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[ittam]

we have tested the application in extreme temperatures and quick changes before use - no problem. The application is on the vehicle roof, but not fully exposed for sun. The short period of exposure should from my earlier experience of 25 y not be the case for this.

 

[CoryPad]

Can you share a photograph of the part on the vehicle? Also one of the part showing the cracks?

Regards,

Cory

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[Demon3]

ABS is a family of materials that has better chemical resistance than HIPS because of the acrylonitrile in the ABS (HIPS is ABS with no acrylonitrile comonomer).

The more acrylonitrile, the better the chemical resistance and not all ABS types are the same. I would suggest that you ask your supplier for an ABS grade with high acrylonitrile levels.

ABS is intrinsically very unstable to UV light, the polybutadiene rubber gets degraded and the material becomes brittle. If they accidentally sent you regular ABS without UV stabilizer then that would explain what you're seeing. If they did UV stabilize it (using additives) then you either need a better stabilizer or to use ASA instread of ABS. ASA retains properties and color for decades outdoors but is more expensive than ABS.


Chris DeArmitt

 

[patprimmer]

ASA is also more sensitive to solvent stress crack.

Does stress increase susceptibility to UV degradation. I imagine it might.

Just because it does not crack wit one thermal cycle fresh after assembly does not mean it will not stress crack after repeated cycles, and exposure to chemicals and UV light

Regards
Pat
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[Demon3]

Hi Pat, my experience has been the opposite, probably because not all ABS or ASA are the same. The ASA I worked with has a very high acrylonitrile (AN) level. For example ABS normally has 24-25% AN because that's an azeotropic mixture between AN and styrene so it's easy to make it that way. As well as that AN is more expensive that styrene so manufacturers like to use less if they can. Some ASA grades have 30-35% AN and those are much more resistanct to environmental stress cracking than an ABS with 24% AN.

As you know, the ASA will have much better UV resistance than any ABS because ASA has a rubber that is more or less impervious to UV (polybutylacrylate).


Chris DeArmitt

 

[ittam]

in the attached picture is a section of the profile. The gas filled chamber is half cut away ( on top). The glue used can be Sika 221 or Betaseal 1001. Both are very flexible. In the groove of the profile is mounted a EPDM sealing.

 

[CoryPad]

The sharp nature of the crack, coupled with the lack of any apparent plasticity/deformation/strain, indicates a degradation mechanism. Some possibilities have been mentioned already (solvent, UV). What is the dark patch at the lower edge of the crack? Why is some of the part dark and glossy, while other parts are gray and dull?

Regards,

Cory

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[Pud]

It's not by any chance multi-point gated and this is at a weld line? Just a thought at 1m long...


Cheers

Harry

 

[patprimmer]

Good call Harry.

It's also right on the edge of the date stamp which may be a stress riser.

Do those adheives contain any solvents. Are the surfaces prepared with solvents.

Are there any plasticised plastics or rubbers in the vacinity.



Regards
Pat
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[Pud]

Hi Pat,

Agree re stress raiser (although this would suggest marginal design criteria)- also solvents. Many years ago we made some hollow ABS balls with the two halves glued. Tried some activator (Loctite) which we sprayed on the joint. Cured off instantly, but the balls fell apart about a month later! Not on the glue line - the rest of it, due to residual solvents! Activator "approved" by Loctite!!!

I wonder if the op is a bit misguided using adhesive "approved" by the adhesive supplier. They would probably approve anything!. If the curing agents are not water based, and after extended inhalation you get "high", they are no good for ABS!

Could the op indicate whether the cracking is in generally the same position, or if it's random along the length? Pat may have a very valid point. ABS, like most amorphous polymers are very notch sensitive, and the sharp edges at the bottom of the date stamp may be the cause.

Also, is the ABS part glued so that stresses are "built in" - e.g. straight ABS part glued to a curved steel part?


Cheers


Harry

 

[ittam]

The application is glued stress free and it remains also in normal conditions stress free. It might be under stress when vibrating on the street, but it is not known for us yet. The cracking happen in random position and we've seen no connection to any external nor internal notches.

 

[patprimmer]

I will say it again.

Plastics typically have about 10 times the co-efficient of thermal expansion of steel.

Car bodies go through considerable thermal cycling.

I have actually seen a car where the OEM engineers attached a glass filled PBT cowl grill with 3 self tapping screws. One screw in the middle and one at each end. The screws went through the moulding directly into the sheet metal on the firewall.The thermal expansio in normal use sheared the outboard off or pulled them out of the sheet metal.

To solve the problem, they still used a fixed screw in the middle, but moulded in slots and used shoulder bolts on the outboard attachments. These shoulder bolts and slots allowed about 3mm or 1/8" movement at each end, or 6mm or 1/4inch over the entire length which was about 1 mere.

Glass filled PBT has about half the coefficient of expansion of ABS. 6mm of strain over 1 metre in ABS equates to considerable stress.



Regards
Pat
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