biodegradable plastics

=chemistry =explanation =material science

 

 

We've all seen stories about plastic waste in the ocean. That's been the main reason for bans of plastic bags and straws.

Relatively little of the plastic waste in the ocean comes from the areas implementing such bans. Those bans seem to be designed for maximum salience, not environmental benefit. They are a silly way for misinformed people to feel like they're contributing to solving environmental problems.

But while I don't approve of banning plastic bags and straws, I do think it's very reasonable to make them out of biodegradable plastics.


To be biodegradable, plastics must have bonds that enzymes can break by hydrolysis. The options are hemiacetals and esters. The esters must be aliphatic, so PET isn't biodegradable.

Any biodegradable polyester will also degrade gradually when exposed to water, especially hot or acidic water. PET can be used for soda bottles, but not water pipes. Biodegradable polyesters are less resistant to hydrolysis than PET, and I don't think they'll be usable for either. But handling shorter-term exposure (like drink cups) isn't a big problem.


thermoplastic starch / TPS

The most common biodegradable plastic is thermoplastic starch, which is normal starch mixed with some stuff to make it meltable. Starch is water-soluble and unsurprisingly TPS also absorbs a lot of water. It's also very weak. TPS is cheap, but as a plastic it's quite bad.


poly(lactic acid) / PLA

The 2nd-most common biodegradable plastic is poly(lactic acid). PLA is relatively strong, and can be >10x as strong as TPS. It's made from lactic acid, which is particularly easy to ferment things to.

But PLA does have some disadvantages:
 • It's brittle. This can be fixed by blending it with a bit of other stuff, so this isn't a serious problem, but blending it does reduce its strength.
 • Its heat deflection temperature is low, meaning it loses most of its strength at temperatures some hot foods are at. PLA is a poor choice for containers for hot drinks or soups.
 • Molding it has to be done relatively slowly.
 • It's not a very good gas barrier.

Lactic acid for PLA is made by fermentation of sugar or starch. Because it's very acidic and this is bad for most microorganisms, calcium carbonate was added during fermentation, producing precipitated calcium lactate, which was then acidified with sulfuric acid, producing a lot of calcium sulfate as waste.

But there are microorganisms that can survive very acidic environments, and lactic acid fermentation is easy and produces lots of excess ATP that can be used to maintain intracellular pH. Since 2010, companies have been moving to fermentation with yeasts that can produce >10% concentrations of lactic acid in water. This reduces costs significantly.


poly(butylene succinate) / PBS

This is my personal favorite. It has a good balance of strength, flexibility, and heat resistance. Overall, it's similar to polypropylene.


PBS

 

Poly(butylene succinate) is better than poly(ethylene succinate). With these aliphatic even-numbered polyesters, melting point and strength are better when the diol and diacid are the same length; if you imagine the charge density, you can see how the diacid segment fits together with the diol segment.

That's a bit unfortunate, because butanediol is ~$2.50/kg, more than 2x the cost of ethylene glycol. The cost of butanediol is perhaps the biggest problem with PBS. But a plastic grocery bag is ~6 grams, so +$1/kg is about an extra 0.6 cents.

The other reason why PLA has been favored over PBS is that companies can say PLA is "100% renewable" but I'm personally not very concerned about that: it doesn't matter if your carbon comes from biomass, what matters is the net impact of production, and if you're concerned about running out of oil and natural gas, well, that's still a ways off and you can always make syngas from biomass.

Succinic acid can be made by fermentation, but it's expensive to separate. Currently, it's usually made by butane -> maleic anhydride -> succinic anhydride.

Butanediol is made by a variety of routes, none of which are very efficient. It's fairly expensive for a molecule of its complexity.

The focus has been on succinic acid via fermentation and butanediol from petrochemicals, but I actually think that should be reversed.

Succinic acid can be made by double carbonylation of ethylene oxide. This should be cheaper than the maleic anhydride and fermentation routes.

It's now possible to get >10% concentrations of butanediol by fermentation. This should be cheaper than current routes.


polyurethane

Polyurethanes just need some long stuff with multiple hydroxyl groups. It's very possible to use biodegradable polyesters for the soft segments of polyurethane. Poly(ethylene adipate) is a reasonable choice for this, as is poly(caprolactone) which is more expensive but has lower dispersity. Natural oil polyols are also biodegradable.

While these polyurethanes can be thermoplastic, they're not usually included in lists of biodegradable plastics, because they're elastomers - rubbery stuff that returns to its original shape well but is weak unless it's stretched a lot. But of course, that's useful too.

 

 



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