Poly lactic acid is another type of aliphatic polyester that was first synthesised by Carothers and his team at Du Pont in 1932. (He then largely ignored it!) It has short methyl side chains and a low hydrocarbon-to-CO2 ratio which yields a stiffer and stronger polymer compared to PCL. However, due to the increase in ester groups, PLA is more susceptible to hydrolysis, and over time, atmospheric exposure can cause degradation through polymer chain-scission. (3D printers are all too familiar with this issue.) PLA is often purified, modified, and blended with other materials to mitigate this problem, but ongoing embrittlement of the base polymer makes it increasingly likely to fragment. In the open environment, the resulting PLA “microplastic” pieces can take a long time to be digested by the available microbial species.
Over the last 30 years, extensive research has been conducted on aliphatic polyesters such as PCL and PLA. The general conclusions are that unbranched, low-molecular weight (i.e., short) polymers with low crystallinity are more easily consumed by microbes in the environment, and especially those with a low glass-transition temperature (Tg, as it's known).
With a Tg of 60C, this is a problem for PLA, since below this temperature the polymer chains are essentially frozen together and difficult for microbes to assimilate. No open-environment study of PLA has yet reported any rapid microbial digestion at ambient temperatures. Accordingly, PLA is considered more suitable for semi-durable items that still need to biodegrade eventually, but typically only on a decadal time scale.
For short-duration items, PLA is only suitable for bio-disposal by hot composting at temperatures approaching 58C, as specified by standards EN13432/14995 and ASTM D-6400.
In the test shown here, a PLA test stick (2mm x 10mm cross-section) was placed in moist garden compost at 15-25C, and no degradation was observed after 6 months. By contrast, a similar sample of polycaprolactone (PCL) showed obvious microbial attack after just 3 months.
Essentially then, in the much cooler conditions typically found in nature, PLA biodegrades very slowly. Without the assistance of hot composting, full bio-assimilation is estimated to take many years or even decades.
In some situations, this can be advantageous, as it allows production of semi-durable plastic items that will - eventually - biodegrade. But for shorter lifespan items that might be discarded into the environment, more rapid consumption is desirable, and other aliphatic polyesters such as PCL or P4HB are regarded as a more appropriate choice.
For this reason, rapstrap did not compromise, and we decided to use a more expensive material that's much harder to work with. So if you're looking for a rapidly biodegradable cable tie, please contact use for a sample and try the experiment yourself!