Machine Design

How "green" are green plastics?

Biodegradable plastics are easier on the environment than those based on petroleum, right?

How "green" are green plastics?

Leland Teschler, Editor


Careful, it's a trick question, at least according to a study from Athena Institute International, a nonprofit that does R&D on sustainable building practices. Recently it compared the environmental burdens of consumer packaging made from common plastics such as PET and polypropylene (PP) to those associated with polylactide (PLA), a widely used corn-based plastic.

Athena's study looked strictly at five packaging items: 16-oz cold-drink cups, two-piece deli containers, envelope window film, foam meat trays, and 12-oz water bottles. It calculated the total energy consumed, the amount of postconsumer solid waste generated, and the amount of greenhouse gases created both with ordinary packaging and from versions made from PLA. When the final tallies were in, packaging made from PLA wasn't necessarily better than that from PET, PP, or high-impact and general-purpose polystyrene (HIPS and GPPS).

The reasons why are interesting. It takes about as much energy to form packaging made from PLA as from petroleum-based resins. This stands to reason: An operation that turns PLA pellets into packages looks a lot like one using pellets of ordinary resins.

Surprisingly, PLA packaging also creates about as much solid waste as the ordinary kind. The main reason is that the PLA packages in the study weighed more than all but one of the petroleum-based packages. There is just more PLA needed for a given package. This is a big reason PLA packaging consumes more energy from fabrication-to-grave. But a contributing factor is an extra drying step necessary for PLA that ordinary plastics don't need.

One might also think the disposal options for PLA would be "greener" than those for conventional plastic. But claims about how easily bioplastics break down are only conjecture at this point, says the report. To back up its claim, Athena points to evidence that PLA degrades only with difficulty and not at all in home-composting setups. That means any effort to reduce PLA into simpler compounds would have to be a commercial effort.

And burning the stuff doesn't help. Incinerating PLA gives off the same by-products as incinerating conventional plastics: The greenhouse gas CO2, water, and other compounds get released into the atmosphere. Of course, PLA is theoretically recyclable, but there is a catch here, too. For one thing, the material can't be mixed with other plastics for recycling. In fact, mixing PLA bottles with PET bottles could harm the existing infrastructure for reusing PET.

It's likely that newer bioplastics will have the same kinds of disadvantages as PLA. So whether these materials are a better way of deploying resources than ordinary plastics is debatable. After all, land used to grow crops for bioplastics can't be used for producing food or ethanol. And even if we completely eliminated the use of hydrocarbon-based polymers, the result would only be a 3% drop in the production of oil and natural gas.


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