Polylactic Acid (PLA) is a popular bioplastic, but its designation as “compostable” is often misunderstood. The decomposition timeline depends almost entirely on the specific disposal conditions, which are rarely met outside of specialized facilities. This leads to the incorrect assumption that PLA will break down rapidly in a typical home compost pile or a landfill.
Understanding PLA The Bioplastic Context
Polylactic Acid is an aliphatic polyester derived from renewable resources like corn, sugarcane, or cassava root, distinguishing it from traditional fossil fuel-based plastics. The production process ferments plant sugars to create lactic acid, which is then chemically processed and polymerized into long chains, forming the final plastic resin. PLA is commonly used in applications such as food packaging, disposable cutlery, and 3D printing filament.
Industrial Composting The Required Environment
PLA is engineered to be compostable only within the intensely controlled environment of an industrial composting facility. These facilities maintain the precise, sustained conditions necessary to initiate the polymer’s breakdown, requiring elevated temperatures between 55 to 60 degrees Celsius (131 to 140 degrees Fahrenheit).
The Decomposition Process
The decomposition begins with hydrolysis, where water molecules break down the long PLA polymer chains into smaller fragments and lactic acid. This initial step is accelerated by the high heat. Once the polymer’s molecular weight decreases, specialized microbes consume the remaining lactic acid and fragments, converting them into carbon dioxide, water, and biomass.
Certification Standards
Certification standards, such as ASTM D6400 or EN 13432, require that a PLA product achieves at least 90% disintegration within a 12-week period. These standards also mandate that the material must be fully mineralized—converted to carbon dioxide—within six months under controlled, aerobic conditions.
Decomposition in Ambient Settings
When PLA products are disposed of in ambient, uncontrolled environments, their decomposition rate slows dramatically, often becoming negligible.
Landfills and Home Composting
A municipal landfill lacks oxygen, has low temperatures, and minimal moisture—the opposite of the conditions needed for PLA breakdown. In these anaerobic environments, PLA is highly stable and can take decades, or even hundreds of years, to degrade. Similarly, a home compost bin does not generate the sustained high heat required for initial hydrolysis. At the lower temperatures typical of home composting or natural soil, PLA exhibits very limited or no significant biodegradation.
Aquatic Environments
In aquatic environments, such as oceans or lakes, the low temperature and lack of specific microbial enzymes mean that PLA will persist for long periods. While the material may fragment into microplastics due to prolonged exposure to water and sun, it will not fully biodegrade back into its base components. Discarding PLA outside of an industrial composting facility results in its persistence, similar to conventional plastics.
Factors That Accelerate or Hinder Breakdown
The rate at which PLA breaks down is governed by several interconnected factors that influence the speed of its initial hydrolysis and subsequent microbial action. Temperature is the most influential variable, directly controlling the rate of the polymer’s ester bond breakdown; higher temperatures drastically increase this speed. Moisture is also significant, as water molecules are chemically required to break the polymer chains during hydrolysis.
Material Structure and Geometry
The material’s inherent structure, specifically its degree of crystallinity, plays a crucial role in determining its susceptibility to degradation. Amorphous regions are attacked first, meaning highly crystalline PLA is more resistant to breakdown. The molecular weight is another determinant, as longer polymer chains resist the chain scission process, leading to a slower degradation rate. Finally, the object’s geometry affects the rate, as a thin film has a greater surface area exposed to water and microbes than a thick block.