Biodegradable polymers are polymers that can be decomposed by the action of living organisms. Whereas most polymers are designed for longevity, biodegradable polymers are not. Biodegradable polymers can be derived from renewable raw materials, petrochemicals, or combinations thereof.
Polymers are the majority component of most plastics, so the topics of biodegradable plastics and biodegradable polymers are intimately related. While the words "bioplastic" and "biodegradable polymer" are similar, they are not synonymous. Bioplastics are composed of biologically synthesized polymers, i.e., derived partly or entirely from biomass. Some are biodegradable. Definitions are debated. Biodegradable plastics are composed of polymers that can be petroleum-based, biologically derived, or a mixture thereof.
History Very early work on biodegradable materials necessarily preceded the era of synthetic polymers, which require petrochemicals. This early work focused on natural polymers or their derivatives. One of the first medicinal uses of a biodegradable polymer was the catgut suture, which dates back to at least 100 AD. The first catgut sutures were made from the intestines of sheep, but modern catgut sutures are made from purified collagen extracted from the small intestines of cattle, sheep, or goats.
In the 1830's, cellulose was converted to gun cotton (cellulose nitrate) and then cellulose acetates, which are probably the first biodegradable (semi-synthetic) polymers. Early studies on the biopolymers polyhydroxyalkanoate (PHA) provided the groundwork for its commercial production. Follow-up efforts by W.R. Grace & Co. (USA) failed. When OPEC halted oil exports to the US to boost global oil prices in 1973, efforts to produce polyhydroxybutyrate (PHB) using the strain Alcaligenes latus by Imperial Chemical Industries (ICI UK) also collapsed. The specific PHA produced in this instance was a short-chain-length polyhydroxyalkanoate (scl-PHA). Efforts continue. Related to PHA is polylactic acid (PLA). Studies on its polymerization of lactic acid and its derivatives began at DuPont in the 1930's. In the 1970's, a copoiymer of PLA and polyglycolic acid led to the commercialization of Vicryl, resorbable suturing material.
The concept of synthetic biodegradable plastics and polymers was first introduced in the 1980s. In 1992, an international meeting was called where leaders in biodegradable polymers met to discuss a definition, standard, and testing protocol for biodegradable polymers. Also, oversight organizations such as American Society for Testing of Materials (ASTM) and the International Standards Organization (ISO) were created. Some clothing and grocery store chains have pushed to utilize biodegradable bags in the late 2010s.
Industrial production of biodegradable polymers commenced on scale in the late 1990's.
Types of biodegradable polymers Most biodegradable polymers are polyesters. The ester group (RC(O)OR') is susceptible to hydrolysis by both chemical (i.e. simply exposure to water) and enzymatic action. In addition to polymers, the biodegradability of additives requires attention.
Bio-based polymers
Polyhydroxyalkanoates (PHAs) Polyhydroxyalkanoates are a class of biodegradable plastic naturally produced by various micro-organisms (example: Cuprividus necator). Specific types of PHAs include poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and polyhydroxyhexanoate (PHH). The biosynthesis of PHA is usually driven by depriving organisms of certain nutrients (e.g. lack of macro elements such as phosphorus, nitrogen, or oxygen) and supplying an excess of carbon sources. PHA granules are then recovered by rupturing the micro-organisms.
PHA can be further classified into two types:
scl-PHA from hydroxy fatty acids with short chain lengths including three to five carbon atoms are synthesized by numerous bacteria, including Cupriavidus necator and Alcaligenes latus (PHB). mcl-PHA from hydroxy fatty acids with medium chain lengths including six to 14 carbon atoms, can be made for example, by Pseudomonas putida.
Synthetic biology is defining ways to improve yields of PHA's.
Polylactic acid (PLA)
Polylactic acid is thermoplastic aliphatic polyester synthesized from renewable biomass, typically from fermented plant starch such as from maize, cassava, sugarcane or sugar beet pulp. In 2010, PLA had the second-highest consumption volume of any bioplastic of the world.
PLA is compostable, but non-biodegradable according to American and European standards because it does not biodegrade outside of artificial composting conditions (see ).
Starch blends Starch blends are thermoplastic polymers produced by blending starch with plasticizers. Because starch polymers on their own are brittle at room temperature, plasticizers are added in a process called starch gelatinization to augment its crystallization. While all starches are biodegradable, not all plasticizers are. Thus, the biodegradability of the plasticizer determines the biodegradability of the starch blend.
Biodegradable starch blends include starch/polylactic acid, starch/polycaprolactone, and starch/polybutylene-adipate-co-terephthalate.
Others blends such as starch/polyolefin are not biodegradable.
Cellulose-based plastics Cellulose bioplastics are mainly the cellulose esters, (including cellulose acetate and nitrocellulose) and their derivatives, including celluloid. Cellulose can become thermoplastic when extensively modified.
Petroleum-based plastics Many of the most widely used petroleum-based plastics resist biodegradation. These recalcitrant materials include polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC), representing 58% of synthetic polymers. To some extent, these materials are desirable for their resilience. For example, PVC plumbing is used extensively for sewage, which is corrosive.
Polyesters Polyesters, such as polyethylene terephthalate (PET), representing 6.2% of synthetic petroleum-derived polymers are susceptible to biodegradation by the action of esterase enzymes. This conversion has yet to be implemented on an industrial scale.
A few specialized polyesters are prone toward biodegradation. This attractive feature does not address the problem of plastics pollution because these more labile polyesters represent a tiny fraction of synthetic polymers. Polyglycolic acid (PGA) is a thermoplastic polymer derived from the hydroxycarboxylic acid glycolic acid. PGA is often used in medical applications such as PGA sutures for its biodegradability. The ester linkage in the backbone of polyglycolic acid gives it hydrolytic instability. Thus polyglycolic acid can degrade into its nontoxic monomer, glycolic acid, through hydrolysis, This process can be expedited with esterases (enzymes that facilitate hydrolysis of esters). In the body, glycolic acid can enter the tricarboxylic acid cycle, after which can be excreted as water and carbon dioxide. Closely related to PGA is polylactic acid (and PLA-PGA blends). Since lactic acid is obtained biologically, it is discussed as a bio-derived biodegradable material.
Polybutylene succinate (PBS) is derived from succinic acid and 1,4-butanediol. It a thermoplastic polymer resin that is used in packaging films for food and cosmetics. In the agricultural field, PBS is used as a biodegradable mulching film. PBS can be degraded by Amycolatopsis sp. HT-6 and Penicillium sp. strain 14-3. In addition, Microbispora rosea, Excellospora japonica and Excellospora viridilutea have been shown to consume samples of emulsified PBS.
Polycaprolactone (PCL) is obtained by ring-opening polymerization of the monomer caprolactone. It is a prominent implantable biomaterial. It has been shown that Bacillota and Pseudomonadota can degrade PCL. Penicillium sp. strain 26-1 can degrade high density PCL, although not as quickly as thermotolerant Aspergillus sp. strain ST-01. Species of clostridium can degrade PCL under anaerobic conditions.
Polybutylene adipate terephthalate (PBAT) is another biodegradable copolymer. It is derived from butanediol and two kinds of dicarboxylic acids, adipic acid and terephthalic acid.
A prominent omission from the list of biodegradable polyesters is polyethylene terephthalate (PET), of which >80M tons/y are produced. Bacteria and their associated enzymes that degrade PET have been identified, but the conversions are slow.
Poly(vinyl alcohol) (PVA, PVOH) Poly(vinyl alcohol) is one of the few biodegradable vinyl polymers that is soluble in water. Due to its solubility in water (an inexpensive and harmless solvent), PVA has a wide range of applications including 3d printing, food packaging, textiles coating, paper coating, and healthcare products.
Other biodegradable polymers Motivated significantly by medical applications, many bio-derived and totally synthetic polymers have been developed with varying degrees of biodegradation. Some of these polymers are polyanhydrides, polyacetals, poly(ortho esters), polyurethanes, polycarbonates, and polyamides.
Biodegradation pathways and mechanisms Most biodegradable polymers are polyesters. They break down by hydrolysis, i.e., a cleavage (lysis) with water to give a carboxylic acid (RCO2H) and an alcohol (ROH):
The cases of polyesters derived from hydroxyl carboxylic acids (PLA, PCA, PHB), the alcohol and the carboxylic acid are part of the same monomer, so the equation for hydrolysis is simplified:
To be even more precise, at neutral pH, the carboxylic acid exists as the carboxylate:
Hydrolysis can induced by "chemical" routes (no enzymes) or enzyme-catalyzed. These enzymes are exported from a cell or result from the rupture of some cell. Polymers are too large to enter cells. Chemical hydrolysis can be very slow, but the presence of acids, bases, and mineral surfaces promote the process. Once the polyesters are fully hydrolyzed, the monomers are suited for complete degradation by entering the cellular environment and being metabolized. Microbial degradation is sometimes considered as a 3-step process. Ultimately the biodegradation affords H2O and CO2.
Biodegradability is a "system property". That is, whether a particular plastic item will biodegrade depends not only on the intrinsic properties of the item, but also on the conditions in the environment in which it ends up. The rate at which plastic biodegrades depends on a wide range of environmental conditions, including temperature, its physical size, and the presence of specific microorganisms. Synthetic polyolefins (polyethylene, etc) are some of the least degradable.
Depressed plastics recovery rates can be attributed to conventional plastics are often commingled with organic wastes (food scraps, wet paper, and liquids), leading to accumulation of waste in landfills and natural habitats. On the other hand, composting of these mixed organics (food scraps, yard trimmings, and wet, non-recyclable paper) is a potential strategy for recovering large quantities of waste and dramatically increasing community recycling goals. As of 2015, food scraps and wet, non-recyclable paper respectively comprise 39.6 million and 67.9 million tons of municipal solid waste.
Applications and uses
Biodegradable polymers are of significant interest to medicine, agriculture, and packaging.
Medical One of the most active areas of research in biodegradable polymer is in controlled drug delivery. In order for a biodegradable polymer to be used as a therapeutic, it must meet several criteria:
Be non-toxic in order to eliminate foreign body response; The time it takes for the polymer to degrade is proportional to the time required for therapy; The products resulting from biodegradation are not cytotoxic and are readily eliminated from the body; The material must be easily processed in order to tailor the mechanical properties for the required task; Be easily sterilized; Have acceptable shelf life.