Olefin metathesis - as a degradation route for PE?

Normally when I hear of olefin metathesis in connection with polymers, it is regarding polymerization (such as ring-opening metathesis polymerization (ROMP)). So I was surprised the other day to read a paper where metathesis was used to depolymerize a polymer, and not just any polymer, but a polymer lacking in olefin groups - polyethylene.

The report appears in Science Advances (open access) and shows off some clever tricks. The polyethylene is dissolved in a light alkane (naptha or similar) and then a dehydrogenation is carried out on both the polyethylene and the solvent. After that, the metathesis can happen.

A metathesis reaction is one where there a cross-exchange between two different chemicals. An example would be A-X + B-Y → A-Y + B-X. For olefin metathesis, the A and Y are on either side of a double bond, as are the B and X. And the same is true for the A-Y and the B-X, so the reaction is A=X + B=Y → A=Y + B=X. where A=X is the partially dehydrogenated polyethylene and B=Y is the partially hydrogenated light alkane. If the double bond is near the center of the PE molecule, you are able to pretty much cut the molecular weight in half in just one reaction. Since the molecules are dehydrogenated in multiple locations, the PE can quickly be reduced to very short chains by allowing the reaction to repeatedly occur. And all the while, the degradation products are completely soluble in the solvent.

The technique works for the whole spectrum of PE, from M_w = 3350 daltons to ultra-high molecular weight (M_v = 1.7 x 10⁶ daltons), as well as LDPE and LLDPE (no surprise there, but glad they checked) as well as on PE that had antioxidants compounded into it. Would it work for polypropylene? Polystyrene? PVC? Inquiring minds want to know!

The statistics of this reaction are intriguing to think about. This degradation reaction is actually more akin to a condensation reaction (run in reverse, of course) than the addition reaction that created the PE, but there is so much more. Is there an optimal level of dehydrogenation (as a function of MW, MWD, branching...)? Is there is an optimal light alkane mix? Would having alkenes already in it help or hurt? Modeling this could be quite a bit of fun.

The use of the end product as a fuel is suggested by the authors, who strongly believe that it is an economically feasible route, much better than anaerobic pyrolysis. No numbers are provided however. I won't get into criticizing them at this point, since this is just a first discovery and with just 56% yield, there is a lot of work ahead for someone. But this process and its simplicity seems promising and I would encourage the researchers to push on.

Previous Years

June 23, 2011 - Older workers

June 23, 2010 - Skewing the results - heavily

June 23, 2009 - If anybody dares quote Paul Simon...

LA to Feds: Thanks everyone! We had a ball! (or 96 million)

Back in August to much hoopla, Los Angeles placed 96 million black HDPE balls on top of their water reservoirs. Plastics News is now reporting that the balls on three of the four reservoirs are being removed due to federal regulations.

Reaction to the balls has varied from the start and not surprisingly, some people were very suspicious going so far as to establish a Reddit community rife with conspiracy theories. That some in that community believe the balls leach BPA is hardly surprising.

It's not mentioned what will happen to the used balls. A massive ball pit is always a possibility, but most ball pits that I've seen use multicolored balls and not just black ones. Recycling options are somewhat limited as black HDPE isn't all that common. (Black is common in film form, but I suspect the melt index for these molded balls is far too high for film formation.) I think the most likely outcome will be a classified ad in Plastics News:

For sale: 96 million used black HDPE balls. 4" diameter. Available for immediate pickup in Los Angeles California. Cash only (We're still flirting with backruptcy!). Special consideration will be given to businesses in the St. Louis, Missouri area (we feel really bad about taking back the Ram -- we really do. NOT!)

Previous Years

January 19, 2012 - A Bad Day in the Operating Room

January 19, 2011 - What's in a Name?

January 19, 2010 - What's In My Inbox

Plastics to Aid in Fighting the California Drought

The State of California is suffering through a 5-year drought with no end in sight. Ironically, as much as Californians love to rage against plastics (instituting endless bag bans and regulating countless other chemicals that are added to plastics or used in their production), there are more and more examples of plastics being used to help the state in this time of need.

In just the past few days, the city of Los Angeles placed 96 million black, hollow polyethylene balls in a water reservoir. The balls float on top of the water reducing evaporation, keep away birds and provide other benefits.

That would have been fun to help install. And if the reservoir ever runs dry, then LA will have the worlds largest ball pit.

At the same time two, plastic pipe manufacturers are using the drought to help sell their products. Nationwide, about 16% of all purified water is lost due to leaks and broken pipes with most of the pipes being ductile iron. Large volume plastic pipes were not available when most of these water systems were originally installed but they are available now. PVC pipe is ridiculously easy to join. As many homeowners know, you brush on the primer and adhesive and then push the two pieces together. Voila - a solvent weld that is most likely stronger than the original pipe. Doing this with a 12" OD pipe or larger is a little more difficult due to the weight of the pieces, but the heavy equipment needed to lift them is readily available. HDPE pipes are also available. HDPE cannot be solvent welded however, so more specialized equipment is needed to heat up the end sections to allow them to adhere, but again, this is being done more and more in new installations.

The unfortunate part of this is that California is 5 years into the drought and only now are these steps being taken. Neither step will eliminate the drought, but they will allow existing water supplies to last longer. Had these steps been taken 4 years ago, the Hollywood movies stars might still be able to water their lawns. It's just another reminder of our inability to focus on long-term problems until they reach a crisis state.

Previous Years

August 14, 2014 - Oh Brave New World!

August 14, 2013 - The Pitch Drop Experiment for the Impatient

August 14, 2012 - Polymerizing Antioxidants

Fairlife Followup

Last week I wrote the bottle that contains Fairlife chocolate milk and in particular about how confusing the recycling code at the bottom is. It's the number 7, but instead of saying "other" as it should, it says "PETE" (which corresponds to the number 1 code). I contacted Fairlife about this and received the following email:

"Hi John!

Thanks so much for reaching out; we REALLY appreciate your taking the time to get in touch with us.

Our bottles are primarily made from PET, which is #1 plastic. We add a very small percentage of white colorant to protect the milk from UV and visible light (UV and visible light impact the integrity of the milk and the vitamins present in it). The addition of the white colorant is what makes the plastic bottle #7. PET plastic and the white colorant are both approved by the FDA as safe for packaging food products. There is no BPA in any of our packaging.

If you have any other questions or comments, please do not hesitate to give us a shout!

All the best,

Brooke
Consumer Affairs

fairlife� ultra-filtered milk"

(Glad to see that the exclamation point key on their computer works so well!)

This just doesn't add up. Adding white colorant to PET doesn't make it a number 7, just as adding any other pigment to any other plastic doesn't change the base polymer or its recyclability.

But I question the need for a pigment at all. While Brooke is correct that the white plastic will "protect the milk from UV and visible light", a white pigment isn't needed. The bottle is already mostly covered in a brown-colored overwrap film which will block light. Besides, "normal" milk is packed in high-density polyethylene (HDPE) which is a hazy white without any white additives. The whiteness arises from the crystals in the material scattering light (which coincidentally is also why milk appears white).

(As an aside, UV absorbers have been added to polyethylene milk bottles, but understandably, consumers are put off by the yellow color. This is hardly new technology, having been around since at least 1993.)

My guess: there is a barrier layer in the package which makes the whole mess incompatible with PET, and that the white pigment story is just a red herring.

Previous Years

July 22, 2011 - The Heat Index and Jenson's Inequality

July 22, 2010 - A Bio-based Acrylic

Polyethylene Production coming to North Dakota

While my homestate of Minnesota remains a frack-free zone (due to Mother Nature's choice to stock us with iron, copper and other minerals rather than even a drop of petroleum), our neighboring state of North Dakota is pretty much ground zero for the effort. And we hear about it a lot since many people have taken the day's drive out there for the good paying jobs. Western North Dakota is not highly populated, so manpower is short. If you can pass a drug test and supply your own housing, you can get a job. (And many hiring managers will say under their breath that they might let one of those conditions slide). Minnesota is also affected by the transportation of the flammable, petroleum liquids along the rail lines in our state. And the sudden increase in demand for the trains causes other logistic nightmares for anyone shipping anything else by rail.

Now comes a report from Plastemart that North Dakota will soon have their own world class polyethylene production facility - a $4 billion dollar investment. $1.5 million metric tonnes a year of HDPE - from North Dakota. That is pretty impressive.

In my mind, it was just a matter of time before this happened, although I've not heard anything previous about it unlike the plant being built in West Virginia to take advantage of the Marcellus shale production. At the same time, I bet that this plant will be far more expensive than the original estimate for many of the reasons I already discussed. The labor to build the plant will need to be imported - there aren't too many pipefitters out there and they are already kept busy with the existing fracking operations. And the housing shortage will only increase. While transporting polyethylene by train is much less risky than transporting petroleum liquids, it is not as efficient. The bulk density of polyethylene is about 0.5 g/cm³, a good fraction less than any hydrocarbon liquids. So that means more strain on the train network.

Looking at the very-long-term picture, at some point fracking production will dry up, and so the question be what happens to the facility. Will it be abandoned or will it continue to operate, albeit with a biobased source of ethylene, such as that produced by dehydration of ethanol? North Dakota isn't a very large corn producer, but over the coming decades, alternative biofeedstocks for ethanol will be developed, including some that could be raised in the dry regions of North Dakota. Either way, I'm prety sure that I will not see that future. It's too many decades down the road. (Yes, fracking will go on that long.)

Previous Years

October 15, 2013 - Turning Plastic Bags into Carbon Nanotubes