Chemistry
The Polymer Chemistry Behind Biodegradable Plastics from Lactic Acid
Quick fact
Polylactic acid (PLA) is one of the most common biodegradable plastics, but it will not degrade in your backyard compost or the ocean; it requires industrial composting at temperatures above 58°C (136°F).
Why this is interesting
Your single-use coffee cup might be made from corn instead of oil, but is it truly biodegradable? Let's explore the chemistry behind this seemingly 'green' material.
Read the full explanation
Understanding The Polymer Chemistry Behind Biodegradable Plastics from Lactic Acid
Lactic acid is a small molecule that contains both a hydroxyl group (-OH) and a carboxyl group (-COOH). These two functional groups can react with each other to form ester bonds, linking many lactic acid molecules into a long chain called polylactic acid (PLA). This process is similar to linking beads on a necklace, where each bead is a lactic acid molecule. The key challenge is that as water is released during the reaction, it can also break the growing chain, so you need special techniques to drive the polymerization to high chain lengths.
A deeper explanation
There are two primary ways to make PLA. The direct condensation of lactic acid is simple but yields low-molecular-weight polymer because water removal is difficult. A more efficient method is ring-opening polymerization of lactide, a cyclic dimer of lactic acid. First, lactic acid is converted into lactide through controlled condensation, then lactide is reacted in the presence of a catalyst, such as tin(II) octanoate, to open the ring and form long chains. This route gives precise control over polymer architecture, resulting in high-molecular-weight PLA with consistent properties. PLA's biodegradability comes from its ester bonds, which can be hydrolyzed by moisture and then further broken down by microorganisms. However, hydrolysis is slow at ambient temperatures because the polymer is hydrophobic and relatively crystalline. Industrial composter conditions—higher temperature and humidity—accelerate hydrolysis, making the process practical. This understanding shows that 'biodegradable' is not a simple label; the environment determines the rate.