PLA as a replacement of PC? or HDPE? or PP? Are you Nuts?

The use of bio-based plastics continues to grow, which raises the question of whether or not they are created with less energy (or greenhouse gases or environmental damage or...) than petroleum-based plastics. These are all good questions to ask, and while answering them can be tricky, with countless further questions about the methodologies used, at the very least, it is critical that the comparisons be made on an apples-to-apples basis. A recently published study fails on this basis.

The nova-Institute GMBH released the results of a meta-study on March 5 (the original study is in German, but an English summary is also available). The study found that polyhydroxyalkanoate (PHA) and polylactic acid (PLA) used less petroleum resources than other petroleum-based plastics did, albeit, as you can see from the plot below, there is such scatter in the data that the results are not statistically sound, but lets overlook that completely today.

To me, the more embarrassing failure of the study is the other plastics which they compared the PHA and the PLA to, specifically the polypropylene, polyethylene, polystyrene and polycarbonate.

In my mind, only the PET comparison has any validity. If I was designing a product that could potentially use PET, PHA and PLA could possibly be acceptable alternatives, although the low glass transition temperature of PLA is always a concern. A large industrial application for PET is packaging materials, an arena that is also an early foothold for PLA, so comparing PET to PLA is justified. But if the product I was designing was more likely to be made from PP, PE, PS and especially PC, it would quite foolish to consider PHA and PLA as alternative materials. PP, PE and PS are far to cost effective to consider replacing them with a more expensive alternative, even if the mechanical properties would be there (and they aren't). PC has outstanding optical clarity and is used almost exclusively because of that property, as it is otherwise too expensive to be used for much else.

No. Despite the apparent good news here, the news is really not that good at all.