thermal set response of plastics(ABS) 1

[EngForm78]

I am working on a problem concerning the compression yield of plastics due to expansion from heating. Does anyone has thoughts or information on the response of plastics in this type of case?

Here is brief description of the problem:
If I were to constrain a bar at room temp so that it could not expand but only contract. Then heat it to some point were it has taken a compression set and return to room temperture. I will see that the part is shorter than at first because of the yielding. Now reheat it to a higher temperature, and then return to room temperature and measure the gap. Then contiue this cycle with successviely higher temps and measuring the gap each time.

Will the bar in question at some points not take a set because the previous set is large enough that the bar cannot expand to the constraints again? I see a response like this in some of my tests. Could it be something else?

Thanks

EngForm78

 

[patprimmer]

It depends on the temperature range, but what you say makes sense.

It will creep under compressive load.

The higher the temperature, the higher the load and the more the resultant creep.

Each repeat will reduce the load due to previous creep.

Once you get past the softening point, the material will actually mould to the shape of the restraints, then shrink on cooling to the point that it looses contact with the restraints, and will not expand enough to touch them until you get back to the softening temp at which stage it has very little if any resistance to deformation.

ABS does not have a clear melting point, so this will be a transitional range rather than a definable point.

ABS like all thermoplastics and lead, glass and tar, is really a super viscous liquid and does flow at room temperature, it is just that the viscosity is so high it appears to all practical purposes to be a solid.

Regards

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[Demon3]

Interesting. ABS is composed of an SAN (styrene-acrylonitrile statistical copolymer) with around 30 weight % of polybutadiene rubber particles in it to give impact resistance. When you compress the sample you will compress more or less only the rubber because it has a lower modulus by far compared to the SAN. Then when cool it the rubber will remain compressed. There are several factors here to consider.

The compressed rubber is likely to have a different Tg than the uncompresed so if you do DSC you may see a shift in Tg. That rubber normally has a Tg of around -80°C and that may increase for the compressed rubber.

Another important fact is that polybutadiene contains loads of double bonds and is not temperature stable. The consequence is that your experiment with repeated heated cylces will eventually oxidise and destroy the rubber. The rubber will cross-link and cease to be a rubber at all. This means that you compress the rubber but it will no longer spring back because you've cross-linked it in its compressed form.

You might want to check the papers by Prof. Clive Bucknall because he has looked at the effects of compressing or expansing the ABS on the rubber.

Hope this helps.

 

[proscow]

I assume that each successive soak temperature is higher so if the bar does not expand thermally to fill the gap it can only be because it is "shrinking" either due to overall weight loss on ageing or due to stress relief of internal (moulding) stresses. You do not say temperature or duration of each dwell. You should use a control sample exposed to the same cycles but unrestrained and measure its length change.

 

[Demon3]

I had an idea for you. If you want to investigate this and be sure that the effects you see are not due to degradation / oxidation of the rubber then I suggest that you do the experiments on ASA instead of ABS. ASA is very much like ABS but uses a very stable rubber (polybutylacrylate) instead of polybutadiene. In consequence the ASA does not degrade at higher temperatures or under UV light. ASA is available from BASF, LG and some other companies.