Plastic, composed of long-chain polymer molecules, is ubiquitous due to its durability and low cost. Conventional plastics resist natural decomposition for centuries, creating the challenge of plastic waste. To make plastic biodegradable, the material must be engineered to break down completely into natural components like water, carbon dioxide, and biomass through the action of microorganisms. This requires altering the polymer structure so organisms can recognize and consume the material as a food source. Two primary strategies exist: manufacturing new polymers from renewable sources or chemically modifying existing petroleum-based polymers.
Creating Biodegradable Polymers
The most direct approach involves constructing the polymer from renewable biomass sources, resulting in materials often referred to as bioplastics. These materials replace petroleum feedstock with resources such as starches, sugars, and plant oils. The resulting polymer structure is inherently designed to be susceptible to microbial consumption once discarded.
Polylactic Acid (PLA) is one prominent example, produced through the fermentation of sugars derived from corn starch, sugarcane, or cassava. In this process, plant starch is converted into dextrose, which is then fermented by microorganisms, typically Lactobacillus bacteria, to produce lactic acid. The purified lactic acid monomers are then chemically linked together to form the long chains of PLA.
Another family of materials is Polyhydroxyalkanoates (PHAs), which are polyesters naturally produced and stored by bacteria. Microorganisms synthesize PHAs as an energy and carbon reserve when fed an excess of a carbon source but starved of other nutrients. The bacteria accumulate the PHA within their cells as granules, which are then extracted and purified for use as a thermoplastic material. PHAs are appealing because they can biodegrade in many natural environments, including soil and water. Starch-based polymers, which blend natural starch with plasticizers, are also used to facilitate the breakdown of the entire structure.
Enhancing Degradation Through Chemical Additives
An alternative strategy modifies conventional, petroleum-derived plastics, such as polyethylene (PE) or polypropylene (PP), by incorporating pro-degradant additives. This method introduces elements that accelerate the initial breakdown of the existing plastic structure, often described by a mechanism called oxo-degradation.
These additives typically consist of metal salts, such as those containing cobalt, manganese, or iron, which act as catalysts. When the plastic is exposed to heat and ultraviolet (UV) light, these metal ions accelerate an oxidation process called chain scission. This chemical reaction breaks the long, stable polymer chains into much smaller fragments.
This fragmentation is a necessary first step because the massive size of the original plastic molecule prevents microbes from accessing it. Once the molecular weight of the polymer drops below a certain threshold, the resulting small fragments become small enough to be consumed by microorganisms. This two-stage process relies on an initial abiotic chemical breakdown, followed by the biological action of microbes.
The Process of Microbial Breakdown
Regardless of the plastic’s source, the final and most crucial step is complete decomposition through microbial action. This biological process, known as mineralization, distinguishes true biodegradation from simple fragmentation.
Microorganisms, primarily bacteria and fungi, achieve this by secreting specialized extracellular enzymes onto the plastic’s surface. These enzymes are biological catalysts that target and sever the chemical bonds within the polymer chain, such as the ester bonds in polyesters like PLA or PHA.
Once the enzymes break the polymer down into small oligomers and monomers, these molecules are absorbed into the microbial cell. The microorganisms then metabolize these fragments for carbon and energy, converting the material into carbon dioxide, water, and new biomass. This full conversion to harmless natural substances defines biodegradation.
The rate of this process depends highly on the environmental conditions surrounding the plastic, requiring specific levels of temperature, moisture, and oxygen. Industrial composting facilities maintain high temperatures (often above 50°C) and controlled oxygen levels, providing the ideal environment for rapid decomposition in a matter of weeks. In contrast, decomposition in natural environments like soil or seawater proceeds much more slowly or may not complete at all.
Standardization and Testing Methods
To verify that a plastic is truly biodegradable, manufacturers rely on rigorous, standardized testing protocols. These methods quantify the extent and rate of microbial consumption under specific, controlled conditions. The most common approach involves respirometric tests, which measure the carbon dioxide (CO2) evolved during decomposition.
In a typical aerobic test, the plastic sample is incubated with a microbial inoculum in a contained system. The CO2 released is collected and measured over a defined period. The percentage of CO2 evolved, relative to the total organic carbon, provides a direct measure of how much material has been mineralized by the microbes.
It is important to distinguish between the general term “biodegradable” and the more specific term “compostable,” as defined by standards like ASTM D6400 or EN 13432. A material labeled “compostable” must meet strict requirements, including physical disintegration and achieving 90% biodegradation within a specific timeframe, such as 180 days, under industrial composting conditions. The designation “biodegradable” is a broader claim that does not imply a specific environment, time frame, or end result, making verified “compostability” a more reliable indicator.