Polyethylene Antioxidant levels

[dav363]

I've been told of a past instance where an MDPE pipeline jacket became brittle and subsequently cracked, requiring the whole jacket to be removed and re-extruded. The cause was said to be related to the antioxidant levels in the plastic. Can someone please provide a quick lesson on antioxidants in PE's and how their effect relates to material embrittlement. As you might guess polymers aren't my strong point.

Thanks

 

[SJones]

The embrittlement would come under the generic description of 'ageing' (or aging in some parts of the world). Try a Google search on ageing of polyethylene (prevent the word "electrical" in the search string).

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Compositepro]

Aging of plastics is primarily caused by oxidation of the polymer. Oxidation does not just mean reaction with oxygen but in chemistry refers to electron transfer between molecules. Oxidation is accelerated at high temperatures, by sunlight, and by oxygen and ozone. Free-radicals are created by oxidation and can cause lots of damage by chain reation. Anti-oxidants stop the chain reaction. Oxidation breaks the backbone of polymers and reduces their molecular weight. This will make them weak and brittle. Almost all things made from plastic contain anti-oxidants.

 

[rconner]

Notmy strong popint either but I think Compositepro provided some good information -- actually I think it is reported by many references "oxidation" can even be promoted by agents other than oxygen and ozone (e.g. see quote at

http://www.janalab.com/expertise_testing_chemical.php?selected=2

, "At the levels found in potable water in North America, the chlorine used to disinfect the water can promote oxidation of piping materials. “

 

[doig]

Day363, Details, details, details: Where IS the P/L? Underwater or in air or buried? What is the service? How deep? This could be several cause: Cathodic disbondment comes to mind? Was this the sole coating? I believe you said it was 'extruded onto the pipe'? New to me but why?

Was the pipe external environment hot? Cold? The P/L product hot? cold? What length? Diameter? Wall thickness?

Also, why MDPE? HDPE is more forgiving and stable.

This could be as simple as a bad-batch of MDPE. If there's real $$ at stake: get a forensic chemist. If your just satisfying your curiosity: Get into the original spec and see if there's anything in the conditions that would cause this.

Again: Why MDPE? Or HDPE?

Best,

ima nemisis doig