Biodegradable polymers are materials that can be broken down by living things. Unlike most polymers, which are made to last a long time, biodegradable polymers are not. These polymers can be made from natural materials, oil-based substances, or a mix of both.

Polymers are the main part of most plastics, so talking about biodegradable plastics and polymers is closely connected. The words "bioplastic" and "biodegradable polymer" are similar but not the same. Some bioplastics (plastics made from plants or other living things) are not biodegradable, and some biodegradable plastics are made entirely from oil. As more companies try to appear environmentally friendly, they are exploring and using bioplastics more often. However, the exact meaning of "bioplastic" is still being discussed. The term is often used for many different products that may be made from living things, break down naturally, or both. This can mean that some plastics made from oil might be called "bioplastics" even if they have no parts from living things. Some people are unsure if bioplastics will solve environmental problems as expected.

History

Early research on biodegradable materials happened before the time when synthetic polymers, which need petrochemicals, were developed. This early research focused on natural polymers or their related substances. One of the first medical uses of a biodegradable polymer was the catgut suture, which was used as early as 100 AD. The first catgut sutures were made from sheep intestines, but today’s catgut sutures are made from purified collagen taken from the small intestines of cattle, sheep, or goats.

In the 1830s, scientists changed cellulose into gun cotton (cellulose nitrate) and later into cellulose acetates, which are likely the first biodegradable (semi-synthetic) polymers. Early studies on the biopolymer polyhydroxyalkanoate (PHA) helped prepare the way for its commercial use. Later attempts by W.R. Grace & Co. (USA) did not succeed. In 1973, when OPEC stopped exporting oil to the United States to raise global oil prices, efforts by Imperial Chemical Industries (ICI UK) to produce polyhydroxybutyrate (PHB) using the bacteria Alcaligenes latus also failed. The PHA made during this time was a type called short-chain-length polyhydroxyalkanoate (scl-PHA). Research on PHA continues. A material related to PHA is polylactic acid (PLA). Studies on how to make PLA from lactic acid and its related substances began at DuPont in the 1930s. In the 1970s, a combination of PLA and polyglycolic acid led to the creation of Vicryl, a type of surgical thread that dissolves in the body.

The idea of synthetic biodegradable plastics and polymers was first introduced in the 1980s. In 1992, an international meeting brought together experts on biodegradable polymers to discuss a definition, standard, and testing method for these materials. Organizations like the American Society for Testing of Materials (ASTM) and the International Standards Organization (ISO) were also created to oversee these efforts. In the late 2010s, some clothing and grocery store companies began using biodegradable bags.

Large-scale industrial production of biodegradable polymers started in the late 1990s.

Types of biodegradable polymers

Most biodegradable polymers are polyesters. The ester group (RC(O)OR') can break down when exposed to water or through the action of enzymes. It is also important to consider how well the additives used with polymers can break down.

Biologically made polymers come from natural sources like plants, animals, or microorganisms.

Polyhydroxyalkanoates (PHAs) are a type of biodegradable plastic naturally made by certain microorganisms, such as Cupriavidus necator. Examples of PHAs include poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV), and polyhydroxyhexanoate (PHH). Microorganisms produce PHA when they are deprived of certain nutrients, like phosphorus, nitrogen, or oxygen, and given too much carbon. PHA granules are then removed by breaking open the microorganisms.

PHAs can be divided into two groups:
– scl-PHA, made from short-chain hydroxy fatty acids (3 to 5 carbon atoms), produced by bacteria like Cupriavidus necator and Alcaligenes latus (PHB).
– mcl-PHA, made from medium-chain hydroxy fatty acids (6 to 14 carbon atoms), produced by bacteria like Pseudomonas putida.

Synthetic biology is helping scientists find better ways to increase PHA production.

Polylactic acid (PLA) is a thermoplastic polyester made from renewable plant materials, such as corn, cassava, sugarcane, or sugar beet pulp. In 2010, PLA was the second most widely used bioplastic globally.

PLA can break down in composting conditions, but it is not considered biodegradable under American and European standards because it does not break down outside of artificial composting environments.

Starch blends are made by mixing starch with plasticizers. Starch alone is brittle at room temperature, so plasticizers are added during a process called starch gelatinization to improve its structure. While starch is biodegradable, not all plasticizers are. The biodegradability of the plasticizer determines if the starch blend can break down.

Examples of biodegradable starch blends include starch/polylactic acid, starch/polycaprolactone, and starch/polybutylene-adipate-co-terephthalate. Blends like starch/polyolefin are not biodegradable.

Cellulose bioplastics are mainly made from cellulose esters, such as cellulose acetate and nitrocellulose, and their derivatives, like celluloid. Cellulose can become thermoplastic when modified extensively.

Common non-biodegradable petroleum-based plastics include polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). These plastics are valued for their strength and durability. For example, PVC is often used in sewage systems because it resists corrosion. Some petrochemicals are also used to make biodegradable polymers.

Polyglycolic acid (PGA) is a thermoplastic polymer made from glycolic acid. PGA is used in medical applications, like surgical sutures, because it can break down safely in the body. The ester bonds in PGA make it unstable in water, allowing it to break down into glycolic acid. This breakdown can be sped up by enzymes called esterases. Glycolic acid can then enter the body’s energy cycle and be excreted as water and carbon dioxide. PGA is closely related to polylactic acid (PLA) and PLA-PGA blends. Since lactic acid is naturally produced, it is considered a bio-derived biodegradable material.

Polybutylene succinate (PBS) is made from succinic acid and 1,4-butanediol. It is a thermoplastic used in food and cosmetic packaging films. In agriculture, PBS is used as a biodegradable mulching film. PBS can be broken down by bacteria like Amycolatopsis sp. HT-6 and Penicillium sp. strain 14-3. Other bacteria, such as Microbispora rosea, Excellospora japonica, and Excellospora viridilutea, can also break down PBS.

Polycaprolactone (PCL) is made through a process called ring-opening polymerization of caprolactone. It is used as an implantable biomaterial. Bacteria like Bacillota and Pseudomonadota can break down PCL. Penicillium sp. strain 26-1 can break down high-density PCL, but not as quickly as Aspergillus sp. strain ST-01. Some Clostridium species can break down PCL in environments without oxygen.

Polybutylene adipate terephthalate (PBAT) is a biodegradable copolymer made from butanediol and two acids: adipic acid and terephthalic acid.

A major type of biodegradable polyester not listed is polyethylene terephthalate (PET), which is produced in large amounts globally. Some bacteria and their enzymes can break down PET, but the process is very slow.

Poly(vinyl alcohol) (PVA) is a rare biodegradable vinyl polymer that dissolves in water. Because water is a safe and inexpensive solvent, PVA is used in many applications, including 3D printing, food packaging, textile coatings, paper coatings, and healthcare products.

Many bio-derived and fully synthetic polymers have been developed for medical use, with varying levels of biodegradation. Examples include polyanhydrides, polyacetals, poly(ortho esters), polyurethanes, polycarbonates, and polyamides.

Biodegradation pathways and mechanisms

Most biodegradable polymers are polyesters. These materials break down through a process called hydrolysis, which means they split using water to form a carboxylic acid (RCO2H) and an alcohol (ROH):

In the case of polyesters made from hydroxyl carboxylic acids (such as PLA, PCA, and PHB), the alcohol and carboxylic acid parts come from the same monomer. This simplifies the hydrolysis equation:

At neutral pH, the carboxylic acid exists as a carboxylate ion.

Hydrolysis can occur through chemical methods (without enzymes) or with the help of enzymes. These enzymes may come from outside a cell or be released when cells break apart. Polymers are too large to enter cells. Chemical hydrolysis is often slow, but acids, bases, and mineral surfaces can speed up the process. Once polyesters are fully broken down, the resulting monomers can enter cells, where they are further broken down. Microbial degradation is sometimes described as a three-step process. In the end, biodegradation produces water (H2O) and carbon dioxide (CO2).

Biodegradability is a "system property." This means whether a plastic item breaks down depends not only on the material itself but also on the environment it ends up in. The speed of biodegradation depends on factors like temperature, the item’s size, and the presence of specific microorganisms. Synthetic polyolefins, such as polyethylene, are among the least degradable materials.

Lower plastic recovery rates often occur because conventional plastics mix with organic waste (like food scraps, wet paper, and liquids), leading to increased waste in landfills and natural areas. Composting mixed organic materials (food scraps, yard trimmings, and wet, non-recyclable paper) is a way to recover large amounts of waste and improve recycling efforts. As of 2015, food scraps and wet, non-recyclable paper accounted for 39.6 million and 67.9 million tons of municipal solid waste, respectively.

Applications and uses

Biodegradable polymers are important in medicine, agriculture, and packaging. One area of research is using these polymers to deliver medicine in a controlled way. For a biodegradable polymer to be used in medicine, it must meet these conditions:

• It must not be harmful to the body to avoid immune reactions;
• The time it takes to break down must match the time needed for treatment;
• The materials left after breaking down must not harm cells and must leave the body easily;
• The material must be easy to shape to fit the task’s needs;
• It must be easy to clean and sterilize;
• It must last long enough to be stored and used safely.

Biodegradable polymers are also used in tissue engineering, which helps grow new tissues or organs using artificial materials. These materials can help grow tissues in labs or support new structures in labs. A biodegradable scaffold is preferred because it reduces the risk of the body rejecting the material. While some systems are not yet ready for human use, research on animals has shown success, such as growing rat muscle tissue on a scaffold made of polycaprolactone and polylactide. Future research may allow these materials to help repair or replace human tissues and organs. Creating organs like kidneys requires a scaffold to guide growth, after which the scaffold breaks down and leaves the body safely. Scientists have used materials like polyglycolic acid and polylactic acid to build blood vessels for heart repair. These scaffolds help create healthy arteries and veins.

In orthopedic uses, such as replacing bones or joints, both biodegradable and non-biodegradable polymers are used. Non-biodegradable materials like silicone rubber, polyethylene, and acrylic resins have been used as biocompatible cements to fix prostheses. Biodegradable materials like polyglycolide, polylactide, and chitosan are used to repair cartilage, ligaments, and tendons. For example, poly(L-lactide) (PLA) is used to make screws and tools for repairing knee cartilage and is sold under the name Clearfix Meniscal Dart/Screw. PLA takes more than two years to break down in the body.

Biodegradable plastics can replace traditional plastics that stay in landfills and cause pollution. In 2010, the U.S. Environmental Protection Agency (EPA) reported that the U.S. had 31 million tons of plastic waste, which was 12.4% of all household and business waste. Only 2.55 million tons were recovered, which was 8.2% of the total waste. This is much lower than the 34.1% recovery rate for all waste.

Biodegradable plastics can help reduce waste by allowing composting of materials that would otherwise go to landfills. Compostable plastics are lightweight, strong, and inexpensive, like regular plastics, but they can fully break down in industrial composting facilities. Supporters say certified biodegradable plastics can be mixed with organic waste, making it easier to compost large amounts of waste that are hard to recycle.

Using biodegradable plastics allows more waste to be recovered through composting, which is better than landfilling or burning.

In addition to medicine, biodegradable polymers help reduce waste in packaging. Efforts are being made to replace petroleum-based materials with biodegradable ones. Polylactic acid (PLA) is a common polymer used for packaging, such as films, wraps, and containers. In 2002, the FDA said PLA is safe for food packaging. BASF sells a product called ecovio®, which combines a compostable polyester (ecoflex®) with PLA. This material is used for plastic films like shopping bags and organic waste bags. It can also be used for molded products, coated paper, and foam-like items.

Regulations/standards

To ensure that products labeled as "biodegradable" are accurate, the following rules have been created:

The Biodegradable Products Institute (BPI) is the main group in the United States that certifies biodegradable products. ASTM International sets methods to test if plastics are biodegradable in environments with or without oxygen, as well as in marine settings. A group called Committee D20.96, which focuses on environmentally degradable plastics and bio-based products, oversees these rules. The current ASTM standards include two types: specifications and test methods. Specifications determine if a product meets requirements, while test methods describe how to conduct tests, including time limits and how harmful the tests might be.

Under anaerobic conditions (without oxygen), a product must have at least 70% of its material broken down within 30 days (ASTM D5511-18) or within the time set by the test (ASTM D5526-18) to be considered biodegradable. Test methods explain how to perform tests but do not decide if the results are acceptable or not.

For aerobic composting (with oxygen), plastics must break down completely into carbon dioxide, with at least 90% of the material turned into carbon dioxide within 180 days (about 6 months). Specifications include rules that determine if a product passes or fails.

European standards also require that 90% of a polymer break down into carbon dioxide within 6 months.

In November 2022, the European Commission proposed new rules to replace an older law from 1994 about packaging waste. These rules also aim to clarify how to label products as biobased, biodegradable, or compostable.

In October 2020, British Standards introduced new rules for biodegradable plastics. To meet these rules, biodegradable plastics must break down into a wax that contains no microplastics or nanoplastics within two years. This breakdown can be caused by sunlight, air, or water. Niall Dunne, the chief executive of Polymateria, stated that his company developed polyethylene film that degraded in 226 days and plastic cups that broke down in 336 days.