Tuesday, July 28, 2015

Real World UV Degradation of PET

Long-time readers of this blog are well aware of my ongoing complaints (1, 2, 3, 4, 5, 6, 7, 8, 9 and 10) about researchers running poorly designed UV exposure tests on polymers. So when PlasticsNews yesterday highlighted a new report from Plastic Technologies, Inc., on how UV exposure leads to yellowing in recycled polyethylene terephthalate (PET), I figured it would make for some terrific blogging fodder for tomorrow.

I was wrong. Or at least my expectations were wrong. Here's what changed my mind completely:
"Two-liter PET bottles produced using a commercial grade of PET were used for this study. This resin did not contain any ultraviolet absorbing additives. These virgin bottles were crushed, stacked five to six deep, and placed in uncovered open sided crates to afford maximum exposure to the elements. These crates of bottles were then placed on the roof of Plastic Technologies, Inc. building in early January. Every three months, the bottles in the crates were agitated so that those on the bottom had a chance over time to move to the top or outside edges. Another set of bottles was stored indoors approximately 18-inches under a fluorescent light source. This light source was left on continuously, exposing the bottles for two months. A third set of bottles was stored and protected from light exposure for one year for use as a control."
I can't believe it. Sure, they ran the mandatory expose-it-to-a-continuous-UV-light-source-to-scorch-it-beyond-all-reason, but they also had material exposed to REAL WORLD CONDITIONS. Industrial researchers got right what so many academic researchers have gotten wrong!

The results of the testing were pretty interesting as well. While the sunlight did yellow the PET a little, putting the exposed PET through an extruder (once again, duplicating REAL WORLD CONDITIONS) drastically increase the amount of yellow. While the authors did not offer an explanation, I would guess that the sunlight initiated a degradation reaction (perhaps along the lines of a Photo-Fries reaction) and the additional thermal cycle really allowed for the reaction to run wild.

But this graph on the right is especially telling. The y-axis is the "b*" values from a Hunter L*a*b* spectrophotometer. I won't go into all the details, but positive b* values are a measure of yellowness. The plot is for test plaques, meaning the PET has already gone through the extruder. The blue diamond is the yellowness for the samples exposed to a fluorescent lamp for 2 continuous months. That point is well off the curve, and shows that sunlight is more more aggressive about degrading PET. This also re-emphasizes my point to ALWAYS run real world exposure controls. Accelerated aging is not just a matter of counting photons - it is far more complicated. Woe unto those who think otherwise.

My hats off to Dr. Schloss and Ms. Brown for getting the research right. You can simulate real world conditions all you want, but nothing beats using the real world conditions. Is it really that difficult of a concept?

Previous Years

July 28, 2014 - The Failed Dow Chemical/Kuwaiti JV: Is it finally over?


milkshake said...

A hypothesis, shooting from the hip

PET has two building blocks: terepthalic acid, which absorbs UV very nicely but it is really stable towards photodegradation, and ethylene glycol, which does not absorb UV much but has reactive methylenes. I would expect photodegradation to lead to polymer chain breaks with aldehyde and carboxyl ends. Alpha alkoxyester-acetaldehydes are very reactive, and their polycondensation products are probably the souce of discoloration. Reactive as they are, the aldehydes are colorless on their own, and they are stuck in the matrix and cannot come in contact one with another. That is, until you heat up and melt the stuff during the extrusion...

Anonymous said...

"You can simulate real world conditions all you want, but nothing beats using the real world conditions."

This is a really well-written sentence that underlines so much of what goes wrong with experimental research and drawing conclusions from data that is not reflective of conditions in reality. It is a particular problem with research pertaining to the "environmental sciences."

James Blish once wrote (admittedly in a very different context) that "the only substitute for a sharp sword is a sharp sword."