Polylactic Acid (PLA) is a plant-based plastic derived from fermented starches, commonly sourced from corn or sugarcane. This material has gained widespread popularity in packaging and 3D printing as an alternative to traditional petroleum-based polymers. Despite its “bio-plastic” label, PLA does not dissolve in cold or room-temperature water. A PLA coffee cup or 3D-printed object will remain structurally intact if submerged.
The Chemistry of PLA and Water Stability
Polylactic Acid is chemically classified as an aliphatic polyester, a type of polymer chain linked by ester bonds. PLA resists dissolution due to its inherent hydrophobic nature, meaning it repels water molecules.
The long polymer chains of PLA are tightly packed together, and water molecules at ambient temperatures lack the energy to penetrate and break these internal forces. Water can chemically break a polyester only through hydrolysis, where a water molecule physically cleaves an ester bond. At room temperature, this hydrolysis reaction is so slow that any degradation is purely superficial and would take centuries.
The polymer’s glass transition temperature, typically around 60 to 65 degrees Celsius, marks the point where the material softens and water diffusion significantly increases. Below this temperature, the polymer chains are stiff and highly resistant to water infiltration, which keeps the material stable in everyday use. PLA products maintain their structural integrity when exposed to cold drinks, rain, or tap water over extended periods.
Accelerated Breakdown: When PLA Degrades Rapidly
While PLA is stable in cold water, it rapidly breaks down under specific conditions of accelerated hydrolysis. This breakdown is primarily triggered by high temperatures, typically above 60 degrees Celsius (140 degrees Fahrenheit). The elevated thermal energy allows water molecules to attack the ester bonds, causing the long polymer chains to fracture into smaller, soluble fragments.
This chemical scission releases lactic acid molecules, which act as internal catalysts for the reaction. The presence of these acidic fragments lowers the local pH within the plastic matrix, accelerating the remaining hydrolysis in a self-sustaining process. In extremely hot water, such as 90 degrees Celsius, a thin sample of PLA can be fully chemically degraded in as little as two days.
The rate of breakdown can be further increased by exposing the material to solutions with highly acidic or alkaline pH levels. Strong acids tend to cause chain cleavage preferentially at the ends of the polymer, while strong bases promote random scission along the backbone of the chains. The polymer’s molecular weight drops significantly, leading to a loss of structure and transformation into smaller chemical components.
The Reality of PLA Biodegradation and Composting
The concept of “biodegradation” for PLA is often misunderstood, as it is a two-step process requiring highly specific conditions not met in nature. The initial step is the abiotic hydrolysis of the polymer chains into small oligomers, requiring high heat and moisture. Only once the chains are sufficiently fractured can the second stage, true biodegradation, begin.
Biodegradation involves the consumption of these smaller molecular fragments by specialized microorganisms and their enzymes. This process is limited to controlled industrial composting facilities. These facilities sustain temperatures between 55 and 60 degrees Celsius (131–140 degrees Fahrenheit), combined with controlled humidity and oxygen levels.
Under these managed conditions, PLA can fully convert into carbon dioxide, water, and biomass within 60 to 180 days. If a PLA product is placed in a home compost bin, a landfill, or left in a freshwater environment, the temperature and microbial activity are insufficient. In these cold, anaerobic environments, PLA will persist for a very long time, much like conventional plastics.