Polylactic Acid, or PLA, is a versatile bioplastic derived from renewable resources such as corn starch or sugarcane. It has found widespread application across various industries, including packaging for food and beverages, materials for 3D printing, and even medical devices like sutures and implants. Given its increasing prevalence as an alternative to traditional plastics, understanding how PLA behaves when exposed to water, particularly regarding its degradation, is a relevant inquiry. This material does undergo a form of breakdown in aqueous environments, although the conditions and extent of this process are nuanced.
How PLA Breaks Down in Water
The primary mechanism by which Polylactic Acid degrades in water is hydrolysis. This process involves water molecules interacting directly with the polymer chains. Water molecules attack and break the ester bonds that link the individual lactic acid units within the PLA polymer chains. This chemical scission fragments the large polymer molecules into smaller molecular units, called oligomers, and eventually into their original monomer, lactic acid.
The initial stages of degradation often occur on the surface of the material, with water molecules diffusing into the polymer matrix. As the process continues, these smaller fragments can diffuse out, allowing further water penetration. This chemical transformation reduces the material’s molecular weight and can lead to changes in its physical and mechanical properties.
Conditions Influencing PLA Degradation
Several environmental factors influence the rate and extent of PLA’s hydrolytic degradation in water. Temperature is a prominent factor, with higher temperatures accelerating the hydrolysis process. Increased temperatures also make the polymer chains more flexible, which enhances water absorption and promotes hydrolysis.
The pH level of the surrounding water also plays a role. Both acidic and alkaline conditions can accelerate the hydrolysis of PLA. While neutral pH allows for some hydrolysis, extreme pH values catalyze the breaking of the ester bonds more efficiently. Additionally, the presence of specific enzymes or microbial activity can influence the degradation, as certain microorganisms can secrete enzymes that break down PLA, or they can consume the lactic acid produced by hydrolysis.
PLA Degradation and Environmental Impact
While PLA undergoes hydrolysis in water, this chemical breakdown does not equate to complete biodegradation in natural aquatic environments like rivers, lakes, or oceans. True biodegradation involves microorganisms converting the material entirely into carbon dioxide, water, and biomass. In natural water bodies, ambient temperatures are often too low, and specific microbial communities required for complete conversion are generally not present in sufficient numbers.
Studies show that PLA submerged in marine environments exhibited no significant signs of degradation even after prolonged periods. This indicates that while hydrolysis may initiate, it does not lead to the rapid and complete breakdown often associated with “biodegradable” claims in these settings. For PLA to fully biodegrade, specific conditions are necessary, typically found in industrial composting facilities.
These facilities maintain high temperatures, often around 50°C to 60°C, and possess active, specialized microbial populations designed to break down organic materials efficiently. Under these controlled conditions, PLA can be converted into constituent components. Therefore, PLA’s “biodegradability” is highly dependent on disposal environment, and its behavior in natural water bodies differs considerably from its performance in industrial composting settings.