Nylon 6/6 mechanical properties?? 2

[MASSEY]

How are the mechanical properties of Nylon 6/6 affected due to moisture absorbtion.

I understand a typical value may be 3% moisture absorbtion by volume. How does this affect the tensile, yield, compression strengths?

Conversely, if in a very dry environment will the Nylon become more brittle due to lack of moisture?

Is the rigidity affected due to too much or too little moisture?

I'm kinda shooting in the dark here, but the suppliers I have contacted seem to be as clueless as I am.

 

[patprimmer]

You must be contacting the wrong suppliers, as I have worked for quite a few over the years, and all the reputable ones had publications which would exactly answer your questions.

Such suppliers could be Bayer, DSM, DuPont, Solucia? (used to be Monsanto), Celenese. I am sure there will be more.

The figures I have for the water absorption Nylon 6.6 are from ICI publications which I acquired in about 1980.

These are:-

Equilibrium at 100% RH 8.1%
Equilibrium at 65% RH 3%

The nylon will absorb, or desorb water in an attempt to reach equilibrium, but it happens quite slowly.

Time taken to reach equilibrium
6.4mm thick 100% RH, 20 deg C >900 days
6.4mm thick 100% RH, 100 deg C 100 hours
3.2mm thick 100% RH, 20 deg C 150 days
3.2mm thick 100% RH, 100 deg C 30 hours
1.6mm thick 100% RH, 20 deg C 45 days
1.6mm thick 100% RH, 100 deg C 6 hours
0.8mm thick 100% RH, 100 deg C 2 hours

The moisture content of nylon can have a dramatic effect on it's properties, such as:-
3.0% water content will swell the nylon by about 0.8%
Tensile strength at break is hardly effected.
Elongation at break is increased from 50% to 200% after moisture conditioning.
Flex Mod is is decreased from 3.2 to 1.3 GPa
Compressive strength goes from 90 to 50 MPa

The mouldings will be brittle if the moisture content is to low, which will occur if the mouldings are tested soon after moulding, or if they are allowed to dry out by being stored in a dry environment. One very rough test for this is to rattle several mouldings together. If they are dry they tinkle, if they are conditioned, they have a dull sound.

As seen by the figures above, the rigidity is effected by a power of nearly 3 by moisturising.


Regards
pat

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[MASSEY]

Thank you Pat, this information is very helpful.

When I mentioned 'suppliers' I should have said 'distributors'. My mistake.

 

[patprimmer]

I forgot to mention a few points to consider, these being:-

That the surface layer of nylon conditions very quickly, The long time taken to reach equilibrium is because the moisture travels very slowly through the nylon.
The moisture content of the surface is all that really matters re brittleness, as the surface is where cracks start. They never start in the centre.
The surface moisture content also has much more effect on the beam flex mod than does moisture in the matrix. "I" beams or sandwich construction vs a solid bar is a good analogy.
Treating the parts with boiling water, accelerates conditioning, but the nylon will swell a lot at the surface, before it swells in the matrix. This can cause internal stresses due to the differential expansion
Treating the parts with boiling water causes some hydrolysis of the nylon, resulting in a lower final strength than would be obtained with cooler conditioning.
Treating the parts at over 60 deg c in water, causes the nylon to anneal as well as to swell. This has a significant effect on the final size. Note, the nylon anneals at about 180 deg in dry heat, but at 60 deg in water.


There are a number of factors in the shrinkage of plastics. These are:-
Change of phase from liquid to solid.
Thermal contraction during the cooling from solidification to room temp.
Post mould shrinkage, which can be measurable for up to a week with some materials.
Crystallisation. The rate and degree of crystallisation as well as the size and nature of the crystals formed can have a significant effect on final size
Crystallisation during and after that phase change



Regards
pat

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