Polylactic Acid (PLA), commonly known as corn plastic, is a bioplastic derived from renewable sources rather than fossil fuels. It is a thermoplastic polymer produced from the sugars found in plants, most often the starch from corn kernels. This material was developed as an alternative to traditional petroleum-based plastics, offering a reduced reliance on finite resources. PLA represents a significant shift toward materials sourced from agriculture.
Converting Corn Starch to Polylactic Acid
The production of polylactic acid begins with converting corn starch into simple sugars, such as dextrose. This initial step involves wet milling, where the starch is separated and then hydrolyzed using acid or enzymes. The resulting dextrose solution becomes the feedstock for the next stage, a biological process similar to brewing or making yogurt.
The sugar is then subjected to fermentation, typically using specific strains of bacteria like Lactobacillus, which metabolize the dextrose. The metabolic byproduct of this fermentation is lactic acid, the same organic compound found in sour milk and muscle tissue. The final step involves polymerization, a chemical reaction where individual lactic acid molecules are linked together to form long chains. This is often achieved by converting lactic acid into a cyclic intermediate called a lactide, which is then polymerized to create the final PLA polymer.
Practical Uses and Material Characteristics
PLA’s material characteristics make it suitable for a wide array of applications, particularly in single-use items and specialized manufacturing. Its clarity and rigidity are similar to polystyrene and polyethylene terephthalate (PET), leading to its use in clear food containers, beverage cups, and disposable cutlery. The polymer is also widely utilized as the primary filament material in desktop 3D printing due to its ease of processing and relatively low melting temperature.
The physical properties of PLA impose certain constraints on its use. It exhibits a low glass transition temperature, ranging from about 60 to 65 degrees Celsius. This heat sensitivity means PLA products can soften or lose their shape when exposed to hot liquids or high-temperature environments, preventing its use for items like coffee cups or microwavable containers. Although it possesses good stiffness and strength at room temperature, it is a relatively brittle material with low resistance to impact compared to traditional plastics like high-density polyethylene (HDPE).
Environmental Fate and Proper Disposal
The question of whether corn plastic is truly biodegradable is complex, as it hinges entirely on the disposal environment. Polylactic acid is classified as compostable, meaning it is designed to break down into carbon dioxide, water, and biomass. However, this process requires specific, controlled conditions that are rarely met outside of industrial facilities.
For PLA to break down efficiently, it must be processed in a commercial composting facility that maintains high temperatures, typically exceeding 60 degrees Celsius, along with controlled moisture and microbial activity. This is the standard outlined by specifications like ASTM D6400, which requires the material to disintegrate and biodegrade within a specific timeframe. Without these elevated temperatures and specialized conditions, the degradation process is extremely slow.
PLA does not readily decompose in standard home compost piles, which do not reach the necessary sustained high heat. More significantly, it is not designed to break down in landfills or marine environments. Studies indicate that PLA products in a typical anaerobic landfill environment can remain intact for decades, degrading at a rate of only around one percent over a century. Placing PLA in a standard recycling bin can contaminate the stream of petroleum-based plastics like PET and HDPE, as the two materials are chemically incompatible and cannot be melted down together.
PLA production often requires less non-renewable energy and can result in lower greenhouse gas emissions compared to some conventional plastics. However, this environmental advantage is heavily dependent on the end-of-life scenario. If PLA does not reach an industrial composting facility, it behaves much like traditional plastic waste, leading to a disconnect between the material’s renewable origin and its actual environmental impact upon disposal.