Polylactic acid (PLA) is a linear aliphatic polyester produced by poly-condensation of naturally produced lactic acid or by the catalytic ring opening of the lactide group. Lactic acid is produced (via starch fermentation) as a co-product of corn wet milling. The ester linkages in PLA are sensitive to both chemical hydrolysis and enzymatic chain cleavage.
PLA is often blended with starch to increase biodegradability and reduce costs. However, the brittleness of the starch-PLA blend is a major drawback in many applications. To remedy this limitation, a number of low molecular weight plasticisers such as glycerol, sorbitol and triethyl citrate are used.
PLA does not have full food contact approval due to its fermentation manufacturing method.
Developments
A number of companies produce PLA, such as Cargill Dow LLC. PLA produced by Cargill Dow was originally sold under the name EcoPLA, but now is known as NatureWorks PLA, which is actually a family of PLA polymers that can be used alone or blended with other natural-based polymers.
Table 3.1 details some of the other PLA biodegradable plastics that are commercially available.
| Trade-name | Supplier | Origin |
|---|---|---|
| Lacea | Mitsui Toatsu | Japan |
| Lucty | Shimazu | Japan |
| NatureWorks | Cargill Dow | USA |
Applications
The applications for PLA are thermoformed products such as drink cups, take-away food trays, containers and planter boxes. The material has good rigidity characteristics, allowing it to replace polystryene and PET in some applications.
Degradation Mechanisms and Properties
PLA is fully biodegradable when composted in a large-scale operation with temperatures of 60°C and above. The first stage of degradation of PLA (two weeks) is via hydrolysis to water soluble compounds and lactic acid. Rapid metabolisation of these products into CO2, water and biomass by a variety of micro-organisms occurs after hydrolysis.
PLA does not biodegrade readily at temperatures less than 60°C due to its 'glass transition' temperature being close to 60°C.
