viscosity of polyamide after heat-aging

[hacki72]

I have noticed a "strange" behaviour of a molded PA12 GF30 part.
The viscosity of the material after heat storage 1000h/150°C is higher than the original value.
Normally the molecules should be damaged and as a result of this effect the viscosity should be lower.
The viscosity was measured with an Ubbelohde-viscometer.
Has someone an idea what happened?

Thanks
Hacki

 

[Demon3]

Nylon is very sensitive to water uptake. When dry as molded it has high moldulus and low impact resistance. When left in open atmosphere it takes up water and becomes plasticised to give much lower modulus (i.e. 50% lower) and high impact resistance.

Your 1000h at 150°C probably removes a lot of the water and that may affect the viscosity measurements. In effect your measurements before heat treatment are done in solvent plus water (from the nylon) whereas your measurements after ageing are done in pure solvent. So your effective solvent is not the same in both cases, the affinity of the nylon will be different in both cases and so the radius of gyration of the solvated nylon and its viscosity may be different.

Alternatively it may be that the nylon cross-links lightly on ageing. Not all polymers undergo chain scission on heat ageing, some cross-link. From memory for example PP undergoes chain scission but PE cross-links.


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[patprimmer]

1000 hours at 150C should cause oxidation and cross-linking and substantial embrittlement.
I would expect that effect to me somewhat greater than the drying out with nylon 12 as nylon 12 has somewhat less affinity for water than more common nylons.



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[Demon3]

Yes, I missed that it's nylon 12. You're right of course that it's much less water sensitive than nylon 6 or nylon 6,6.

Have you tried adding stabilisers? I think copper iodide and hindered phenolics are usual for nylons.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

Copper iodide is a very effective and commonly used antioxidant for nylon, so long as a murky green natural colour is not an issue.

Regards

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[hacki72]

Thanks to you for your answers and ideas.
I think the "crosslink and oxidation -thing" is a
possible answer.
The water absorbtion is definetly not the reason because we
measured the amount of volatile components in the material before the "Visco-Test".
(Before visco-testing the original material and before visco-testing the conditioned material.)
We also found that the aged material was not completely
solved in the solvent. We found small amounts of "non-soluble" nylon.

Hacki

 

[patprimmer]

There can be non soluble (in acid) additives in nylon compounds, but I guess you tested non aged vs aged.

This is starting to sound a bit like a student post.


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[hacki72]

No, no student post.
"Material-lab tester for initial samples-post"
We must make such tests according to customer-specifications
in the automotive industry.
Though we are a traditional elastomeric components company
we have more and more plastics in our products and sometimes we must ask these questions.
I could have asked some of our specialists at the companies that selled us our testing equipement but then I wait for days for an answer...
And this forum is a good place for a quick and qualified answer!


thanks
Hacki