Are you comparing oxidative reactivity at melt temperatures or at room temperature? I'm addressing room temperature or near room temperature comparison in my comment below. Seems to me that if you compared the two at high enough temperature to melt out the crystallinity in both UHMWPE and HDPE, then oxidative rates would be very similar.

Antioxidants are usually added to commercial grades of HDPE by dispersing them in the polymer melt. Because UHMWPE doesn't become a liquid when it melts, it could be a problem to get stabilizers into it. Maybe it could be done with a low-enough MW stabilizer that diffused well, dispersed on the UHMWPE powder before sintering the powder, but I've not heard of this. I would try a simple monomeric hindered phenolic, like butylated hydroxy phenol. Otherwise, try getting the a/o added in the gel state, with the polymer network highly swollen with a solvent, as in preparing ultra-drawn fiber.

And yes, I would agree that other things being equal, because UHMWPE is less crystalline than HDPE (as usually prepared) its oxidative reactivity in solid state would be expected to be greater. In uranyl oxide-stained TEM microphotographs of lightly oxidized HDPE, we saw most intense staining in the amorphous regions, indicating oxidation there, but sparing the crystalline regions. The stainable regions spread like tooth decay as oxidation progressed. There isn't much room in the crystallites for oxygen to diffuse in.

It would be interesting to compare unstabilized UHMWPE and PE oxidative rates at equal solid densities. I'm seeing a specific gravity of 0.944 for UHMWPE quoted in one online source, but 0.97 for gel-spun fiber.

Thanks, Uwehelmut. Note that I'm not the question poster, though.

That all sounds reasonable to me. I didn't get into copolymer type, prior oxidative exposure in manufacturing, or the situation in random branching LDPE. Oxidative stability is a big issue and too poorly understood by most who use PE.

Thank you, Mr. Steve Nazar and Mr. Uwehelmut Wolfmeier, for your valuable comments.

However, in some literature, it is reported that " HDPE with

higher branching than UHMWPE, therefore, gives rise to

the higher T10% values and results in higher thermal stability

than the UHMWPE."

10.1515/polyeng-2012-0142

But how has HDPE branching increased thermal stability?

Prajesh, that link takes me to a J. Polymer Engineering article that is paywalled. I can only see the abstract. I agree the statement you quoted doesn't make sense on the face of it, but perhaps it could make more sense if we knew the materials used, which are perhaps stated deeper in the paper. I wonder if the HDPE used contained stabilizer, but the UHMWPE did not. This may not have been known to the authors.

HDPE has only a limited amount of short chain branching, which does mean there are some tertiary carbons that are more susceptible to hydrogen abstraction. This should increase oxidative reactivity, all other things being equal. But as Uwehelmut has pointed out, the effect is going to be small. It could easily be overwhelmed by other factors such as prior thermal exposure or simply air ageing.

But similar behavior has been reported by another researcher, and I have also found the same results in my experiment. Here we are discussing the thermal analysis under a nitrogen atmosphere while performing TGA analysis. In my study, I am using Blow molded grade HDPE, but I don't know whether it contains a stabilizer or not.

Hi Prajesh -- Blowmoulding grades would normally be well-stabilized, as sold. ...Or they should be! In blowmoulding plants it is usual to regrind and re-use the trim from tops and tails of the parison. So a fairly large proportion of moulding feed has already had one thermal history, and some has two, and a lesser proportion has three...and so on. But even the small proportions having multiple melt exposures do matter, because oxidation is so "autocatalytic"...at least in the absence of stabilization.

You are doing TGA to measure stability, or for some other reason? Usually stability is measured by oxidation induction time (OIT) after conditioning in a nitrogen flow. Oxygen or air is then switched for the nitrogen, and temperature is either constant or ramped. This is usually done in a differential scanning calorimeter, but I suppose a TGA could be used. It won't be as sensitive.

Watch out for inhomogeneity of oxidative stability in the polymer at all size scales. A/O packages are sometimes difficult to meter into pelletizing extruders uniformly because of caking on additive feeders. Maybe since I left the business, polymer plants have eliminated this old problem? Pellets may also have different thermal histories depending on their varying residence times and paths in extrusion equipment. One pellet of HDPE might act very different from another in the same lot.

Some, but not all, of the hindered phenolic antioxidants generate thermal degradation products that fluoresce under a UV lamp, with color varying from blue through to yellow as oxidation increases. Examination of a bin of pellets under UV can quickly show a wide range of pellet-to-pellet variation.

Prajesh; the UHMWPE has a much lower kinetic energy, and will degrade faster at the same temperature than HDPE.

The molecular structure of polymeric materials leads to temperature dependency and critical thermal transitions. This temperature dependency within plastics is directly attributable to the viscoelastic nature of polymeric materials. As the temperature is increased, the polymer chains are further apart, there is more free volume and kinetic energy, and the molecules are more mobile and can slide past one another and disentangle more easily.

Emilio Malaguti is this related to the amorphous and crystalline structure of materials?

No;

this is related to the higher molecular weight (chain length) of the UHMWPE.

Are we forgetting that there are catalyst residues in commercial polymers? Small amounts of Metal oxides and TiO 2 in particular have been used to accelerate the photodegradation of polyethylene for many years.

Ultrahigh-molecular-weight polyethylene (UHMWPE) has been the material of choice for the polymeric component for joining other component because of its excellent combination of wear resistance, structural strength and biocompatibility in polymerization. This may cause a foreign-body response, leading to extensive bone resorption and gross loosening of the implants. Also, it involve thermal treatment as the fibers are made up of extremely long chains of polyethylene, which are aligned in the same direction in polyethylene.

Thank you, everyone, for helping me with this question by providing the best possible answer. I appreciate the time and effort you have put into helping me.