Polylactic Acid (PLA) is a widely used bioplastic, found in 3D printing filaments, packaging, and disposable consumer goods. Derived from renewable resources like corn starch or sugarcane, its interaction with water is a common point of inquiry. This article clarifies PLA’s behavior when exposed to water, addressing its solubility and degradation processes.
PLA’s Interaction with Water
PLA does not dissolve in water under typical environmental conditions. Its chemical structure, characterized by long polymer chains linked by ester bonds, makes it inherently hydrophobic. Water molecules are unable to effectively surround and pull apart these large, non-polar polymer chains. Unlike salt or sugar, which readily disperse in water, PLA molecules do not separate in this manner.
Water can slowly diffuse into the polymer matrix, but significant swelling or dissolution is negligible due to PLA’s hydrophobic nature. The robust ester linkages resist direct attack by water molecules under neutral pH and ambient temperatures. This makes PLA water-resistant, similar to many conventional plastics, ensuring its stability in liquid contact applications. For instance, a PLA object placed in water at room temperature can remain intact for several years.
While PLA does not dissolve, some surface degradation can occur under extreme conditions. Very high temperatures, approaching or exceeding PLA’s glass transition temperature (50-70°C), or exposure to strong chemical solutions like concentrated alkaline solutions, can induce breakdown. This process is a chemical degradation where ester bonds are cleaved, causing the material to break into smaller fragments rather than disappearing into a solution.
How PLA Breaks Down
Although PLA does not dissolve in water, it undergoes degradation, a gradual breakdown of its polymer structure. This process primarily involves two mechanisms: hydrolysis and subsequent biodegradation. Hydrolysis is the initial chemical reaction where water molecules slowly attack and cleave the ester bonds within the PLA polymer chains. This chemical breakdown leads to the formation of shorter polymer chains, known as oligomers.
This hydrolytic breakdown is slow at ambient temperatures but significantly accelerates with increased heat and moisture. Temperatures near or above PLA’s glass transition temperature (50-70°C) make the polymer chains more flexible and accessible to water, increasing the degradation rate. As the polymer chains fragment into smaller oligomers and ultimately into lactic acid monomers, the material’s molecular weight decreases.
Once these smaller fragments form, they become accessible for microbial activity, initiating biodegradation. Microorganisms consume these broken-down PLA molecules. Through their metabolic processes, these microbes convert the organic material into natural compounds like carbon dioxide, water, and biomass. This complete breakdown requires specific conditions found in industrial composting facilities.
Industrial composting environments provide the necessary high temperatures (50-60°C), controlled moisture levels, and active microbes for efficient degradation. Without these controlled conditions, such as in home composts, landfills, or natural water bodies, PLA degrades very slowly, often over several years. Therefore, while PLA is biodegradable, it does not simply dissolve in water or degrade quickly in uncontrolled natural settings.