=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.
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.