Nature Works: Polylactic Acid
How a bioplastic derived from fermented plant starch became the centre of hopes for a post- petroleum materials industry
A. Polylactic acid, commonly known as PLA, is a biodegradable thermoplastic derived from fermented plant starch — typically maize, sugarcane, or cassava — that has attracted significant commercial and scientific interest as a sustainable alternative to petroleum-derived plastics. Unlike most conventional plastics, which are synthesised from fossil fuel feedstocks and resist biological degradation for centuries, PLA is produced from renewable plant material and is capable of breaking down under appropriate conditions to produce lactic acid, water, and carbon dioxide. Since its commercial production was pioneered by NatureWorks LLC in the late 1990s, PLA has found applications in packaging, disposable tableware, textiles, and medical devices.
B. The production of PLA begins with the fermentation of plant sugars by lactic acid bacteria, which convert glucose derived from the plant feedstock into lactic acid. The lactic acid is then polymerised — joined together into long chain molecules — through a process that produces a relatively pure polymer with predictable physical properties. The key physical properties of PLA — its transparency, stiffness, and moderate barrier properties against oxygen and moisture — make it particularly suitable for packaging applications including food containers, bottles, and films. Its glass transition temperature and melting point are lower than those of commonly used petroleum-derived plastics such as polyethylene terephthalate, which limits some applications but is not a problem in most ambient-temperature packaging contexts.
C. The degradability of PLA is its most commercially and environmentally significant attribute, but it is also one of the most frequently misunderstood. PLA does not degrade rapidly in the natural environment: in ambient soil, ocean water, or domestic compost bins, it breaks down very slowly — over years to decades — because the temperatures required for rapid hydrolysis of the polymer chains are rarely reached outside industrial facilities. Meaningful biodegradation requires conditions that are only reliably achieved in industrial composting facilities, where sustained temperatures above 60°C, controlled moisture levels, and appropriate microbial populations can break down PLA within a few weeks. PLA that enters the general waste stream — as landfill or litter — degrades at rates comparable to conventional plastics.
D. The lifecycle carbon footprint of PLA compared to petroleum-derived plastics depends critically on accounting assumptions and particularly on how land use change is treated. The production of PLA from maize or sugarcane requires agricultural land, water, and energy inputs — including synthetic fertilisers whose production is energy-intensive — that are not required for fossil fuel feedstock extraction. Under some accounting frameworks, PLA's carbon footprint per unit mass is broadly similar to that of equivalent petroleum-derived plastics; under others, the biogenic origin of its carbon — which is incorporated from atmospheric carbon dioxide during plant growth — gives it a substantially lower net greenhouse gas contribution. The debate over this accounting question has not been fully resolved.
E. The medical applications of PLA are among its most technically demanding and economically valuable. PLA is biocompatible — it does not provoke significant immune reactions when implanted in living tissue — and its in-body degradation products (lactic acid) are metabolised normally by human cells. These properties make it suitable for absorbable sutures, drug delivery capsules, bone screws and scaffolds used in orthopaedic surgery, and tissue engineering applications where a temporary structural support is required that will gradually be replaced by living tissue as it degrades. The rate of degradation can be adjusted by modifying the molecular weight of the polymer or by blending PLA with other biodegradable polymers.
F. The commercial future of PLA is closely linked to the continuing development of industrial composting infrastructure and to the ability of producers to reduce production costs relative to conventional plastics. Investment in industrial composting and waste sorting systems — which would allow PLA packaging to be reliably collected and composted rather than landfilled — has been slow in most countries, limiting the actual environmental benefit of the material relative to its theoretical potential. Second-generation PLA production, using agricultural residues rather than food crop feedstocks and employing more energy-efficient fermentation and polymerisation processes, promises to improve the economics and sustainability profile of the material, but remains at a relatively early commercial stage.