Aging a PBT-10%glass material

[Leyla83]

To simulate ageing and degradation of a plastic material PBT containing 10% of glass in the lab, I am trying to run humidity, wetting, and drying processes. From samples
gathered from the field this material have had cracks appearing in the finished assemblies. This material is suspected to have shrunken over time.

Basically my question is in order to age and degrade this material severely and quickly what should be the sequence in the cycle? And how can I test the material shrinking theory?

Option #1
Humidity @35°C
Drying @60°C
Wetting@25°C



Option #2
Wetting @25°C
Humidity @35°C
Drying @60°C

Or any other test sequence suggestions. In our lab I also have the ability to run thermal, corrosion, and various mechanical test.

Regards

 

[patprimmer]

Have the parts failed.

If so, how long before it failed.

What is size after moulding, say 5 min, 1 hour, 1 day, 1 week and one month.

What is environment of storage and use.

PBT can crack some time after moulding from differential post moulding shrinkage causing a stress build up that exceeds the elongation of break.

It can also degrade , especially when in a hot wet environment.

Regards

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[Leyla83]

Patprimmer,

Regardless of the environment and use conditions. The question is not to know the reasons of degradation of the polymer. I just want to know what are the "correct" mechanical tests to perform in order to degrade the material severely and quickly.

I also want to know the theory of shrinking of a PBT/glass composite.

Thank you for your help

 

[patprimmer]

Why.

Is this work related.

If you want to degrade it really rapidly, put a flame under it.

It shrinks because as it cools, the kinetic energy in it's molecules reduces, this decreases mobility in the molecules and micro structure which allows stronger bonds to form between molecules. These bonds may take a more ordered structure and pack closely in a semi crystalline pattern.

Sorry to be flippant, but your question is so broad, and details of your background are so sparse, that you might require the most basic answer or you might require a thesis on the subject. It is a complicated subject if studied in detail.

Regards

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[Leyla83]

I already gave two options and ask about the appropriate sequence (method) to use!
There is 3 possibilities and the answer would be: option 1 - option 2 or other.

I ve never ask about a thesis on the subject...nor the reasons of shrinking that I already know.

Sorry, i read my question again and I think that it is clear: What are the acceptable methods (sequences) to degrade a PBT/glass composite by using the wetting and drying processes?

I will be very thankful if you answer.

Regards

 

[Pud]

It may have not been dry enough when moulded. PBT has to be very dry otherwise you end up with great looking mouldings which are useless mechanically. This, combined with residual stresses are likely.

Imho, correctly processed PBT should withstand your stated tests indefinitely. You will not "dry" it at 60 Deg C.


Cheers


Harry

 

[Demon3]

I would like to point out that it seems to me your cycle 1 and cycle 2 are actually the same once you loop through them. The only difference is the initial state and that will not affect your results.

For "correct mechanical test" I would go for elongation to break.

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

 

[Pud]

Ah! Good point Demon3 - damp as moulded will show a dramatic fall in E before B.

Cheers


Harry

 

[patprimmer]

I am sorry, but it seems English is not your first language and the meaning in your question is far from clear.

Your suggested tests as listed by you make no sense at all.

It is unclear to me why you mention shrinkage and what theory of shrinkage you intend to test.

By your tests I interpret.

Wetting @25 deg C. I interpret this means parts are immersed in water at 25 deg C for an undisclosed time.

Drying @60 deg C. I interpret this to mean heating to 60 deg C. You do not specify if in a frying pan or in a vacuum oven or something in between for an undisclosed time.

Humidity @35 deg C. I have no idea what you mean by this. Humidity level and time are not specified.

You mention samples gathered from the field and shrinking and cracks.

Are you intending to test these samples or are you intending to use new mouldings and trying to degrade them to emulate the samples from the field.

You need to tell us what you are trying to do if you expect useful answers. This is especially true as your English is quite poor.

Student homework posts are not allowed here and if a question sounds unrealistic, it is often suspected to be a student post. As stated, your question sounds secretive and unrealistic. At this stage I am presuming that might be a misunderstanding due to language difficulties, but we need to know more so we know you are not a student cheating on homework.





Regards

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[Leyla83]

Thank you. I understand.

 

[patprimmer]

A wild GUESS at answers to your possible question.

If you treat in water or a very humid environment, your parts will very slowly absorb water up to equilibrium. At the temperatures you mention, this will take a long time. You mention no time. You will get some degradation due to hydrolysis, but this will be very very slow at those temperatures.

If you dry at 60 deg C in normal humid atmosphere, you will never really dry, but you will approach equilibrium with the atmosphere at whatever moisture content it is. This will take a long time. You do not say what time you intend.

If you dry at 60 deg C in an extremely dry atmosphere you will eventually dry the material, but it might take months or even years depending on how dry you need and section thickness of plastic to be dried. You will suffer some thermal degradation during this process

If you dry at 60 deg C in vacuum, this will slowly dry with no damage to polymer.

If this is a preparation to condition parts before a physical test, the sequence depends on what condition you want.

My best GUESS is that you want to establish equilibrium to standard laboratory conditions within a reasonable tolerance in I GUESS a commercially viable time.

The normal method for conditioning is to determine what the equilibrium moisture content is. This takes a long time to accurately test, but can be normally be obtained from existing data.

Once you know this the best method to condition is to treat in water until the surface of your sample is over conditioned, then to dry in normal atmosphere until the centre has picked up moisture to equilibrium and the surface has dried back to equilibrium. The time taken is very dependant on section thickness and accuracy of even distribution required. Moisture absorbs onto the surface very rapidly and reaches equilibrium virtually instantly, but migration of water through the polymer is very slow and to practically approach equilibrium of normal test bar thickness takes a week or two.

If you wish to do tests on freshly moulded parts to test the raw material used for the mouldings you mention, I would suggest that drying 24 hours under vacuum, then normalising overnight then treating in water until a specified weight gain is achieved then normalising 24 hours then do physical testing.

As Demom3 says, elongation is a much better indicator of initial damage than tensile. In fact the tensile might initially increase due to damage and cross linking.

If you actually want to test the mouldings, I would start by weighing some, then drying under vacuum then reweighing until no practical loss in weight. I would the either dry or moisture condition until the parts were at a weight to suggest equilibrium was obtained. I would then test the mouldings for elongation.

PBT, especially glass filled will not change properties to any significant degree due to moisture content, so the above is really mostly theoretical.

PBT is very susceptible to post moulding shrinkage. This shrinkage can cause internal stress. Glass fibres help reduce this, but a lot depends on the orientation of the glass fibre in the moulding.

Drying or moisture conditioning might anneal the parts to some extent, depending on temperatures and time.

Premature cracks in PBT are most likely caused by:-

Stress from uneven post moulding shrinkage.

The original moulding being done with material not sufficiently dried.

Exposure to water over 80 deg C for a reasonable time, (from memory, like maybe 1000 hours).

Exposure to UV light for a reasonable amount of time.

The original raw material being to low in molecular weight.

The compound not having the correct formulation re grade of glass, colours, stabilisers, flame retardants or whatever.

End of beginning of thesis.

End of trying to GUESS.

I sincerely hope you really are starting to understand.




Regards

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[rodneybennet]

Hi Leyla83,

Especially for mechanical degradation (ageing) in humid and/or corrosive environments, you may want to take a look at the approach that is presented here:

http://www.composite-agency.com/package.htm

They have a lot of experience with these sorts of tests and simulations.

Regards