Nylon absorption and water loss over time causing it to become brittle 1

[NateUMF]

My company is buying a molded part made of Nylon 6 and we're having a problem with brittle fracture. I understand that we should condition the part in order to increase the elongation and reduce the chances of brittle fracture, but if the part is in use in a very dry environment(aka New Mexico, Phoenix, etc), will it lose the moisture? If so, how quickly will it lose the moisture?

If this is the case, we'll most likely need to change resins. What's the best resin to use to ensure we have toughness but no chance of brittle fracture?

 

[Demon3]

Freshly molded parts tend to be brittle due to low moisture then they take up moisture over some days. It can take weeks to get to equilibrium for thicker parts. Sometimes people add water to make the water uptake faster. I.e. zip ties are brittle when made so I know of one company that adds a drop of water to each bag of them shipped to make sure they are tough when they arrive.

I am not sure whether low humidity will make the nylon lose too much water. Pat Primmer and Harry Rice probably do know.

To find a plastic with good toughness check

http://www.matweb.com

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First guess would be polycarbonate if you can tolerate it's bad chemical resistance. If not then a PP copolymer could work.

Chris DeArmitt PhD FRSC CChem

 

[NateUMF]

Chris,

How do I get in touch with Pat or Harry? I'm brand new on here.

As I said, I know the parts we're making will be used in very arid climates as well as very humid climates, and I want to make sure that this variation won't cause problems. The part is supposed to last up to 5 years with sporadic loading.

Nate

 

[Demon3]

Pat and Harry (alias Pud) are never far away. They're members here and will chime in soon I am sure!

Welcome to the best site on the net!

Chris

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

Alternatives depends on why nylon was originally chosen. Nylon has a very good overall balance of properties, but changes due to water absorption is its Achilles heel.

Because of the other good overall properties you should be careful in choosing a replacement.

Nylon naturally attracts water. Water rapidly attaches to or detaches from its surface until it is in equilibrium or balance with it's surroundings. With nylon 6 that is about 3% moisture at 55% RH. that goes up to 9% at 100% RH and down to zero at zero RH.

Although the moisture equilibrium at the surface is very rapid, that applies to only the depth of a water molecule or two.

The water will try to reach equilibrium throughout the thickness of the moulding, but diffusion or rate of travel through the moulding part the surface is very slow. It is very dependent on both concentration gradient and temperature. Treating in boiling water can reduce conditioning times from months to hours for thick moulding, however this still over conditions the surface and you still need to wait until the water is evenly distributed if that is your aim.

Having said all this, the most important thing is to condition the surface and to a moderate depth below it as this is where cracks start.

Even in dry climate, the nylon retains some water and also takes some weeks to dry out during a dry spell, but also some time to reabsorb during wet periods.

I am not sure just how dry New Mexico and Arizona are and how long between wet and dry spells and just what toughness you require and how thick your moulding is.

A moderately impact modified nylon 6 or even a supertough nylon 6.6 might have the properties you require, I say might as you do not actually disclose required properties or application.

Then again, maybe PP or acetal or polyester might have the properties to match those of nylon that you require.

A change of material will also impact on mould shrinkage andmaybe filling and ejection from the mould.

Regards
Pat
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[NateUMF]

We need a material that has good toughness characteristics (high yield strength) but also has at least reasonable flexibility (lower flex mod). The key for us is that the thick plastic piece, roughly 3/8" thick at it's thickest point, does not break under significant bending load (up to 400lbf). However, it also must be stiff enough to resist significant yielding. Finally, it must be able to maintain these characteristics in warm, cold, dry, and arid climates as our product is sold around the world.

We're looking into some acetals right now as well as some copolymers such as PPO, but would also like to think that an impact-modified nylon would work. However, if it is guaranteed that the nylon will dry out significantly over time in a very arid climate, this may not be a good solution. Thoughts?

 

[patprimmer]

Toughened nylons are not quite so dependent on moisture content to be tough, but they generally lose some modulus.

Supertough grades are generally ductile even dry as moulded.

Acetal is rather poor to UV.

Without a hint to application or environment of use we could continue to recommend by guesswork forever.

Regards
Pat
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[Pud]

Assuming the parts have been moulded correctly - most importantly correct material pre-drying, and there is still a problem, I would agree with Pat and use a pre-toughened nylon.

IIRC, nylon will not dry out too much in normal (i.e. less than 100ºC) temps anywhere in the world.

My first assumption would be incorrect processing as above (or inbuilt stresses - I would bet that due to the thick parts, the tool is being run at too low a temp - should be 60ºC min for critical parts - 80ºC better)

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Another option to retain flex mod but get a significant increase in toughness in thick section is to use extrusion grade.

This is an issue with rail track insulators that need good creep resistance to tightly retain the track clamp, but need to be tough enough so the retainer can be inserted with a sledge hammer. Tracks in Malaysia and central Australia see very different humidity conditions. They use extrusion grade nylon 6.6. They are constantly looking for alternatives, but the extrusion grade nylon still has the best overall balance for rail track insulators.

Conditioning technique is important on thick mouldings and the mouldings never fully dry out, they just stabilise around a higher or lower level of water.

The climate that seems to dry them out worst is arctic.

Regards
Pat
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[NateUMF]

Thanks for all of your responses. I really appreciate the help. Right now we've decided to test impact-modified nylon 6, and the initial results have been very encouraging. If that doesn't work in the long run, we'll look into the extrusion grade Nylon 6,6.

As for the processing, I do have one question. As a relative newcomer to plastics processing, should I be concerned if the cushion is very inconsistent but the part weight is stable? To me, the cushion is tracking out of control, but if the part weight and shot size are in control, the cushion shouldn't make a difference, right?

 

[Pud]

Nylon always gives erratic screwback as it's self lubricating and screw slip occurs. Minimum back pressure is the order of the day!

As long as you have a minimum cushion of say 3mm (1/8") it should be ok.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

To big a cushion is a lot better than to small. 3mm to 6mm is normally about perfect.Once you get a significant portion of the next shot left in the cushion it can lead to thermal degradation.

So long as shot weight is consistent the mouldings should be OK, however variations in cushion indicates something is worn or parameters are not ideal.

Pud is correct about very low back pressure, normally about 10% is enough.

Other causes of inconsistent cushion are poor temperature control or worn check valve or inconsistent melt viscosity or inconsistent moisture content.

Regards
Pat
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