oxidative attack of GF nylon components

[dom253]

Is it possible that the removal of a lubricating oil containing antioxidant additives may give rise to the nylon components within the system suffering oxidative attack. What i'm trying to say is........do the antioxidant additives in lubricant oil also act as an antioxidant film that protects components from oxidative attack..? Also, would the free radicals contributing to oxidation be neutralised by the additive , thus reducing potential for oxidative attack of the nylon components.

 

[Demon3]

Yes, the same antioxidant types are used in oils and in polymers (hindered phenols mainly). So, removing an oil rich in antioxidants will reduce the oxidative stability of your nylon.

Some oils have huge concentrations of antioxidants. You can dissolve much more antioxidant in an oil than you can in a polymer. So, that may also contribute. The oil could be a reservoir for antioxidant.

Chris DeArmitt

http://www.phantomplastics.com

Consulting to the plastics industry

 

[dom253]

That sounds logical Demon3. The problem i have is that 30% glass filled nylon components are becoming so degraded they can easily crumbled between your fingers. The temp is no higher than 65 degrees c. The components are only in service 6 months. There is water present. These components have worked fine for years. Now we have an occasion where when used in a system where the oil is removed and replaced with RO water, the components are being attacked. The RO pH is fairly neutral, around 6--7. There are no obvious chemicals present that would attack the Nylon.

 

[patprimmer]

Glass filled nylon should last well past 5000 hours at 65 deg C

Water at 65 deg C will be worse than dry as the nylon is hydrolysed and the sizing agent on the fibres is attacked, but 5000 hours still seems a low temperature to get severe degradation.

Regards
Pat
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[Demon3]

The glass fibers can provide a path for the water to wick right into your material faster than it would have without fiber type fillers. I am still surprised that you are getting a crumbly material though. Can you replace it with glass filled PP? That would be not only cheaper but more resistant to water.

I think your selection of nylon (which is high polarity) was good when in contact with oil (which is low polarity).

Now that you intend contact with water (high polarity), nylon (high polarity) is not a good choice because it will be swollen by the water and get soft. In contrast polypropylene (low polarity) is ideal because it resists water. It's used for washing machines and dishwasher internals for just that reason. Contact the PP supplier and get a grade with appropriate antioxidants in it to resist the temperature. PP in washing machine parts can survive oven ageing at 150C for 8000 hours.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[dom253]

The components are still required to be used with oil. A change in material type is last option due to different shrinkage rates, resulting in expensive tool mods. What could attack nylon so badly. Am i overlooking something? Apparantely no ozone present. There is copper in the system which doesn't like RO water. No one has seen anything like this before.

 

[dom253]

There are also natural coloured unfilled nylon components in the system. These do not degrade like the GF nylon, but the surface cracks up a lot like crazy paving, and turns GREEN

 

[Pud]

and turns GREEN

Is not green the colour of copper salts? This would indicate water take-up.

Is it PA6 or PA6.6? The latter is somewhat better for hydrolysis. Much depends on the processing conditions also. Is the surface crystalline - correct tool temp?

There should be alternatives available without tool mods as most semi-crystalline materials have a similar shrinkage. (e.g. Acetal, PP as mentioned, PA12, mPPO, etc.)

I have assumed it's not coloured white with TiO2...

H

http://www.tynevalleyplastics.co.uk

 

[Pud]

Oops - scrub the mPPO from above. It's amorphous.

H

http://www.tynevalleyplastics.co.uk

 

[dom253]

I don't know what the green is Pud!! It's PA66. Why would green indicate water take up? There is a lot of copper in the system so could the green be from the copper. The surface is not so much crystaline, we are going to visit moulders to go through their process settings.

 

[Demon3]

As mentioned, PP should work in the same mold as nylon as they are both semi-crystalline and therefore have similar shrinkage.

Nylon is stabilized with copper salts so that could in theory cause green. If you have copper in solution in your system that could be the cause of failure. Any transition metals (iron, copper, chromium, vanadium etc) accelerate the polymer degradation hugely even at ppm levels.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

The green will be the copper iodide heat stabiliser.

Nylons are particularly susceptible to attack from zinc chloride from water treatment and galvinised plumbing fittings or even poor quality brass.

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

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[dom253]

not much of any metals in analysed condensate. 8ppm pottasium, is that high? copper 0.07 ppm..!!

 

[Plasman10]

There may be a few things going on... oil may also be inhibiting the oxidation by means of displacing some of the oxygen. there are some alternatives which do very well in water and agind such as the hydrolysis resistance grades available or switching to a partially aromatic resins HTN PPA. Many traditional nylon components are migrating to the PPA type products for better thermal aging and chemical resistance.

 

[patprimmer]

For any type of Nylon 65 deg C dry in air, 5000 hours is not a severe test. Something else is going on to catalyse the degradation.

Regards
Pat
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