Thermal Stability of HDPE Pipe?

[ram0608]

Dear Sirs,

Is Thermal Stability required more than 150 minutes for a HDPE Pipe at 200 Deg.C? ASTM D 3895 - 07 mentioned Oxidatin Induction Time (OIT) for HDPE material / pipe is more than 150 minutes at 200 Deg.C, Whereas all other internaltional standards ISO 4427, EN 12232 etc, giving OIT for HDPE shall be > 20 min at 200 Deg.C is sufficient. We are under impression & according to information from sources, that the value mentioned in ASTM have a typographic error, is it correct? or it's really required that many minutes of thermal stability? Any one can explain in detail? Thanks in advance - RAM

 

[patprimmer]

As it melts at about 135 deg C I wonder what the importance of stability at 200 deg C is other than processing conditions when moulding.

150 min seems an extraordinary long time to require it to be held above its normal melt processing temperature.

Regards
Pat
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[Demon3]

The thermal stability you need depends on the use of the pipe. PE hot water pipes have to last 50 years(from memory). That will require more antioxidant than a pipe used at room temperature and for shorter periods. So, there is no absolute amount of stability needed, it depends on use.

The second point is that there is very poor correlation between stability by OIT and actual stability in use. The correlation between oven ageing and real use is better but still not good.

See for example the paper:

Extrapolation of accelerated aging data : Arrhenius or erroneous ?

The only think OIT is really useful for is as a quality control test. For example, you have a formulation that works in the application (has enough stability) and you want to be able to tell whether you material has the specified amount of antioxidant. You can use OIT to measure the antioxidant concentration. It cannot be used to compare one antioxidant to another.

Chris DeArmitt PhD FRSC CChem

 

[ram0608]

Dear Pat,

The test used to know the oxidation stability of material at process parameters (190 - 220 Deg.C). During processing at that temperaure material exposed to oxygen with ambient atmosphere at different phases. So,it should not get oxidized easily. For that antioxidants used by material supplier.
For Pipe, oxidation stability is required becuase it exposed to atmosphere at 200 - 220 Deg.C during joining by butt-fusion technique. Even a thicker (70mm)pipe will expose not more than 10-12 mins. at this tempreature so, as ISO, EN standrds mentioned that the oxidation stability for pipe is >20 mins is sufficient. But, in what basis ASTM speicification mentioned the OIT value >150 min?

 

[Demon3]

There are two things here. You are saying that the pipe has to be stable at high temperature for 20 minutes. That's fine but does not include the amount of stability needed for the pipe in use for 20, 30 or 50 years. That is the larger number of 150 minutes.

As mentioned, that is not accurate because OIT does not correlate with long term stability (it's a rough guide).

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

My problem here is pretty much the same as Chris.

120 min is far in excess of realistic requirements as a processing stabiliser.

120 min at much higher than in use temperature is virtually useless as an indicator of in use stability.

Regards
Pat
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[Demon3]

BTW if you want to contact an expert on polyolefin stability and antioxidants then try Svein Jamtvedt at Norner.no

Chris DeArmitt PhD FRSC CChem